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Page 1: DWDM principle

DWDM Principle

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Contents

Section 1 DWDM Overview 1-1

1.1 DWDM Technology Background 1-1

1.2 DWDM Principles Overview 1-2

1.3 DWDM Equipment Operating Modes 1-3

1.3.1 Two-fiber bi-directional transmission 1-3

1.3.2 Single fiber bi-directional transmission 1-4

1.3.3 Add and drop of optical signals 1-5

1.4 Application Modes of DWDM 1-5

1.5 Advantages of DWDM 1-5

Section 2 DWDM Transmission Media 2-1

2.1 Optical Fiber Structures 2-1

2.2 Types of Optical Fiber 2-2

2.3 Basic Features of Optical Fiber 2-3

2.3.1 Physical Dimension (Mode field diameter) 2-3

2.3.2 Mode Field Concentricity Error 2-4

2.3.3 Bend Loss 2-4

2.3.4 Attenuation Constant 2-4

2.3.5 Dispersion Coefficient 2-5

2.3.6 Cutoff Wavelength 2-6

2.4 Types and Properties of Optical Fiber Cable 2-6

2.4.1 Types of Optical Fiber Cable 2-6

2.4.2 Properties of Optical Fiber Cable 2-6

Section 3 DWDM Key Technologies 3-1

3.1 Lasers 3-1

3.1.1 Laser Modulation Modes 3-1

3.1.2 Wavelength Stability and Control of Laser 3-4

3.2 Erbium-doped Optical Fiber Amplifier (EDFA) 3-5

3.2.1 EDFA Operating Principle 3-5

3.2.2 Applications of EDFA 3-7

3.2.3 Gain Control of EDFA 3-9

3.2.4 Limitations of EDFA 3-13

3.3 DWDM Components 3-14

3.3.1 Optical Grating Type DWDM Component 3-15

3.3.2 Dielectric Film Type DWDM Component 3-16

3.3.3 Fused Conical Type DWDM Component 3-17

3.3.4 Integrated Optical Waveguide Type DWDM Component 3-17

3.3.5 Performances of DWDM Components 3-18

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Section 4 DWDM Networking Design 4-1

4.1 Some Network Element Types of DWDM 4-1

4.1.1 Optical Terminal Equipment (OTM) 4-1

4.1.2 Optical Line Amplification Unit (OLA) 4-2

4.1.3 Optical Add/drop Multiplexing Unit (OADM) 4-3

4.1.4 Electrical Regeneration Unit (REG) 4-4

4.2 General Constitution of DWDM network 4-5

4.2.1 Point-to-point Networking 4-5

4.2.2 Chain Type Networking 4-6

4.2.3 Ring Type Networking 4-6

4.2.4 Network Management Information Channel Backup and Interconnection Capability

4-7

4.3 Factors To Be Considered in DWDM Networking 4-9

4.3.1 Dispersion Limited Distance 4-9

4.3.2 Power 4-10

4.3.3 Optical Signal-to-Noise Ratio 4-11

4.3.4 Other Factors 4-14

4.4 DWDM Network Protection 4-23

4.4.1 Protection Based on single Wavelength 4-23

4.4.2 Optical Multiplex Section (OMSP) Protection 4-25

4.4.3 Applications in Ring Networks 4-26

4.5 Analysis to The Examples 4-27

4.5.1 Networking Diagram (Physical Network Stations) 4-27

4.5.2 Networking Diagram (considering the dispersion limited distance of the lasers to divide the regenerator sections of the network)

4-27

4.5.3 Networking Diagram (considering the power of optical amplifiers to divide the optical regenerator sections)

4-28

4.5.4 Networking Diagram (considering OSNR) 4-29

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DWDM Principle Section 1 DWDM Overview

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Section 1 DWDM Overview

Objectives:

To master the concepts of DWDM.

To know the background and technology characteristics of DWDM.

1.1 DWDM Technology Background

With the dramatic increase of voice services and emergence of various new services, especially the quick change of IP technology, network capacity will inevitably be faced with critical challenge. Traditional methods for transmission network capacity expansion adopt space division multiplexing (SDM) or time division multiplexing (TDM).

1. Space Division Multiplexing (SDM)

Space division multiplexing linearly expands the transmission capacity by adding fibers, and the transmission equipment is also linearly added.

At present, fiber manufacture technology is quite mature. Ribbon optical fiber cables with tens of cores are rather prevalent and advanced connection technique for optical fiber simplifies cable construction. However, the increment of fibers brings much inconvenience to the construction and circuit maintenance in the future. Additionally, if the existing optical fiber cable lines have no sufficient fibers and require to lay new fiber cables for capacity expansion, engineering cost will increase in duplication. Moreover, this method doesn't sufficiently utilize the transmission bandwidth of the optical fiber and wastes the bandwidth resources. It's not always possible to lay new optical fibers to expand the capacity during the construction of communication networks. Actually, in the initial stage of the project, it is hard to predict the ever-growing service demands and to plan the number of fibers to lay. Hence, SDM method for capacity expansion is quite limited.

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2. Time Division Multiplexing (TDM)

TDM is a commonly used method for capacity expansion, e.g. multiplexing of the primary group to the fourth group of the traditional PDH, and STM-1, STM-4, STM-16 and STM-64 of current SDH. TDM technology can enhance the capacity of optical transmission information in duplication and greatly reduce the circuit cost in equipment and line. Moreover, it is easy to extract specific digital signals from the data stream via this multiplexing method. It is especially suitable for networks requiring the protection strategy of self-healing rings.

However, TDM method has two disadvantages. Firstly, it affects services. An overall upgrade to higher rate levels requires to replace the network interfaces and equipment completely. Thus the equipment in operation must be interrupted during the upgrade process. Secondly, rate upgrade lacks of flexibility. Let's take SDH as an example, when a system with a line rate of 155Mbit/s is required to provide two 155Mbit/s channels, the only way is to upgrade the system to 622Mbit/s even though two 155Mbit/s are unused.

For TDM equipment of higher rate, the cost is relatively high. Furthermore, 40Gbit/s TDM equipment has already reached the rate limitation of electronic devices. Even the nonlinear effects of 10Gbit/s rate in different fiber types will set various limitations to transmission.

Currently, TDM is a commonly used capacity expansion method. It can implement capacity expansion via continuous system rate upgrade. When certain rate level is reached, other solutions must be found because of characteristic limitations of devices, lines, etc.

All the basic transmission networks, whether using SDM or TDM to expand the capacity, adopt traditional PDH or SDH technology, i.e. utilizing optical signals on a single wavelength for transmission. This transmission method is a great waste of optical capacity because the bandwidth of optical fiber is almost infinite when compared to the single wavelength channel we currently use. We are worrying about the jam of networks, on the other hand huge network resources are being wasted.

DWDM technology emerged under this background. It greatly increases the network capacity, makes full use of the bandwidth resources of optical fibers and cuts down the waste of network resources.

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1.2 DWDM Theory Overview

DWDM technology utilizes the bandwidth and low attenuation characteristics of single mode optical fiber, adopts multiple wavelengths as carriers and allows them to transmit in the fiber simultaneously.

When compared to common single channel systems, dense-WDM (DWDM) greatly increases the network capacity and makes full use of the bandwidth resources of optical fibers. Moreover, DWDM has many advantages such as simple capacity expansion and reliable performances. Especially, it can access various types of services and this gives it a bright prospective of application.

In analog carrier communication systems, the frequency division multiplexing (FDM) method is often adopted to make full use of the bandwidth resources of cables and enhance the transmission capacity of the system, i.e. transmitting several channels of signals simultaneously in a single cable and, at the receiver end, utilizing band-pass filters to filter the signal on each channel according to the frequency differences among the carriers.

Similarly, in optical fiber communication systems, optical frequency division multiplexing method can also be used to enhance the transmission capacity of the systems. In fact, this multiplexing method is very effective in optical communication systems. Unlike the frequency division multiplexing in analog carrier communication systems, optical fiber communication systems utilize optical wavelengths as signal carriers, divide the low attenuation window of optical fibers into several channels according to the frequency (or wavelength) difference of each wavelength channel and implement multiplexing transmission of multi-hannel optical signals in a single fiber.

Since some optical components (such as narrow-bandwidth optical filters and coherent lasers) are currently not mature, it is difficult to implement ultra-dense optical frequency division multiplexing (coherent optical communication technology) of optical channels. However, alternate-channel optical frequency division multiplexing can be implemented based on current component technical level. Usually, multiplexing with a larger channel spacing (even in different windows of optical fibers) is called optical wavelength division multiplexing (WDM), and WDM in the same window with smaller channel spacing is called dense wavelength division multiplexing (DWDM). With the progress of technologies, nanometer level multiplexing can be implemented by using modern technologies. Even sub-nanometer level multiplexing can be implemented but merely with stricter component technical requirements. Hence, multiplexing of 8, 16, 32 or more wavelengths with smaller wavelength spacing is called DWDM.

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The diagram of DWDM system structure and optical spectrum is shown in Figure 1-1. At the transmit end, optical transmitters output optical signals of different wavelengths whose accuracy and stability meet certain requirements. These signals are multiplexed via an optical wavelength multiplexer and sent to an erbium-doped optical fiber power amplifier (it is mainly used to compensate the power loss aroused by the multiplexer and enhance the launched power of the optical signals). After amplification, this multi-channel optical signal is sent to the optical fiber for transmitting. In the midway optical line amplifiers can be installed or not according to practical conditions. At the receiver end, the signals are amplified by the optical pre-amplifier (it is mainly used to enhance receiving sensitivity and prolong transmission distance) and sent to the optical wavelength de-multiplexer which separates them into the original multi-channel optical signals.

Figure 1-1 The diagram of DWDM system structure and spectrum

MUX

DMUX

Optical boosteramplifier

Wavelength

Singlechannel

Optical lineamplifier

Optical lineamplifier Optical pre-amplifier

Opticalspectrum

Opticalspectrum

Wavelength

OTU

OTU

O TU

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1.3 DWDM Equipment Operating Modes

1.3.1 Two-fiber Bi-directional Transmission

As shown in Figure 1-2, a single optical fiber implements only one directional transmission of optical signals. Hence the same wavelengths can be reused in two directions.

Figure 1-2 Two-fiber bi-directional transmission DWDM system

This kind of DWDM system can effectively exploit the huge bandwidth resources of optical fiber and expand the transmission capacity of a single optical fiber in several or tens of times. In long-haul networks, capacity can be expanded by adding wavelengths gradually according to the demands of practical traffic, which is very flexible. This is, under the condition that the actual fiber dispersion isn't known, also an approach to use multiple 2.5Gbit/s systems to implement ultra-large capacity transmission, avoiding adopting ultrahigh speed optical systems.

1.3.2 Single fiber Bi-directional Transmission

As shown in Figure 1-3, a single fiber transmits optical signals of two directions simultaneously, and the signals in the two different directions should be assigned on different wavelengths.

1

1

Optical sourceN

NWDM

1

NN

Optical source1

WDM

Detector1

N+1

DetectorN

2NWDM WDM

1 N 2N

N+1

Detector1

Detector1

DetectorN

Optical source1

Optical sourceN

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Figure 1-3 The DWDM system which adopts single fiber bi-directional transmission

Single fiber bi-directional transmission allows a single fiber to carry full duplex channels and, generally, saves one half of the fiber components of unidirectional transmission. Since signals transmitted in the two directions do not interact and create FWM (Four-Wave Mixing) products, its total FWM products are much less than two-fiber unidirectional transmission. However, the disadvantage of this system is that it requires a special measure to deal with the light reflection (including discrete reflection resulted by optical connectors and Rayleigh backward reflection of the fiber) to avoid multi-path interference. When the optical signal needs to be amplified to elongate prolong transmission distance, components such as bi-directional optical fiber amplifier and optical circulator must be adopted, but their noise factor is a little worse.

1.3.3 Add and Drop of Optical Signals

1

1

Optical sourceN

N

WDM

Detector1

DetectorN

1

N

N

Optical source1

WDM

DetectorN+1

N+1

Detector2N

2N

N+1 2N

Optical source2N

Optical sourceN+1

2N

N+1

A single optical fiber

1 N

OADM OADM

Detector 2Optical source

1Detector 1

Optical source2

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Figure 1-4 Optical add and drop transmission

By utilizing optical add/drop multiplexer (OADM), optical signals of the wavelengths can be added and dropped in the intermediate stations, i.e. implementing add/drop of optical paths. This method can be used to implement ring type networking of DWDM systems. At present, OADM can only be made as fixed wavelength add/drop device (as shown in Figure 1-4) and thus the flexibility of this operating mode is limited.

1.4 Application Modes of DWDM

Generally, DWDM has two application modes:

⌧ Open DWDM

⌧ Integrated DWDM

The feature of open DWDM system is that it has no special requirements for multiplex terminal optical interfaces as long as they meet the optical interface standards defined in ITU-T G.957. The DWDM system adopts wavelength conversion technology to convert the optical signal of multiplex terminal into specific wavelength. Optical signals from different terminal equipment are converted into different wavelengths meeting the ITU-T recommendation, then multiplexed.

Integrated DWDM system, without adopting wavelength conversion technology, requires that the optical signal wavelengths of the multiplex terminal meets DWDM system specifications. Different multiplex terminal transmits different wavelengths meeting the ITU-T recommendation. Thus, when connected to the multiplexer, these wavelengths occupy different channels and multiplexing is implemented.

Different application modes can be adopted according to the demands of engineering. In practical applications, open DWDM and integrate DWDM can be mixed.

1.5 Advantages of DWDM

The capacity of optical fiber is huge. However, traditional optical fiber communication systems, with one optical signal in a single fiber, only exploited a little part of the abundant bandwidth of optical fiber. To effectively use the huge bandwidth resources of optical fiber and increase its transmission capacity, a new

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generation optical fiber communication technology based on dense-WDM (DWDM) has emerged.

DWDM technology has the following features:

1. Ultra-large capacity

The transmittable bandwidth of currently commonly used conventional fiber is very wide, but the utilization ratio is still low. By using DWDM technology, the transmission capacity of a single optical fiber is increased by several, tens of or even hundreds of times when compared to the transmission capacity of single wavelength systems. Recently, NEC Company, Japan, implemented a 132×20Gbit/s experimental DWDM system with a transmission distance of 120km. This system, with a total bandwidth of 35nm (1529nm~1564nm) and a channel spacing of 33GHz, can transmit 40 million telephone calls.

2. Data rate "transparency"

DWDM systems conduct multiplexing and de-multiplexing in terms of optical wavelength differences and are independent to signal rates and modulation modes, i.e. transparent to the data. Hence, they can transmit signals with completely different transmission characteristics and implement combination and separation of various electrical signals, including digital signals and analog signals, and PDH signals and SDH signals.

3. Utmost protection of the existing investment during system upgrade

During the expansion and development of the network, it is an ideal approach to implement capacity expansion without the need to rebuild the optical fiber cables and with the only requirement of replacing the optical transmitters and receivers. This is also a convenient way to introduce broad-band services (such as CATV, HDTV and B-ISDN). Furthermore, any new services or new capacity can be introduced simply by adding an additional wavelength.

4. High flexibility, economy and reliability of networking

When compared to the traditional networks using electrical TDM networks, new communication networks based on DWDM technology are greatly simplified in architecture and have clear network layers. Dispatching of various services can be implemented simply by adjusting the corresponding wavelengths of the optical signals. Because of the simple network architecture, clear layers and convenient service grooming, the flexibility, economy and reliability of networking are obvious.

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5. Compatibility with all optical switching

It is foreseeable that, in the realizable all optical networks in the future, processing such as add/drop and connection of all telecommunication services is implemented by changing and adjusting the optical signal wavelengths. So DWDM technology is one of the key technologies to implement all optical networks. Moreover, DWDM systems can be compatible with future all optical networks. It is possible to implement transparent and highly survivable all optical networks based on the existing DWDM system.

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Section 2 DWDM Transmission Media

Objectives:

To master basic structures and types of optical fibers.

To know basic characteristics of optical fibers.

2.1 Optical Fiber Structures

The kernel of optical fiber used in communication systems consists of a cylindrical glass core and a glass cladding. The outermost layer is a plastic wear-resisting coating. The whole fiber is cylindrical. The typical structure of optical fiber is shown in Figure 2-1.

Figure 2-1 The typical structure of optical fiber

Coating Cladding Core

n2 n1

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Figure 2-2 Three typical types of optical fibers

Thickness of the core and refractive indexes of the core material and cladding material are critical to the properties of the fiber. Figure 2-2 shows three typical optical fibers. As can be seen from this figure, there are two typical refractive index distributions in the fiber core-cladding cross-section. One is that the refractive index radial distributions of the core and the cladding are uniform, and the change of refractive index at the core-cladding boundary is a step function. This fiber is called step-index fiber. The other one is that the refractive index of the core is not a constant. It gradually decreases as the radial coordinate of the core increases until it equals to the index of the cladding. Hence this fiber is called graded-index fiber. The common feature of this two fiber cross-section is that the refractive index of the core n1 is larger than that of the cladding n2. This is also a necessary condition for the optical signal to transmit in the fiber. For a step-index fiber, total internal reflection can occur at the core-cladding boundary and the light wave can propagate along the core. For a graded-index fiber, the continuous refraction occurs to the light wave in the core, forming a light ray similar to the sine-wave through the fiber axis and guiding the light wave to propagate along the core. The tracks of the two light rays are shown in Figure 2-2. With the difference of the diameter size of the core of step-index and graded-index fibers, the number of modes transmitted in the fiber is different. Hence, step-index fiber or graded-index fiber can be classified into single mode fiber and multimode fiber according

2a2b

2a2b

2b2a

n2

n1

n2n1

n(r)n2

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to the number of transmission modes. This is also a classification method of optical fiber. The core diameter of a single mode fiber is very small and, generally, less than 10µm, and the core diameter of a multimode fiber is relatively large and often equal to 50µm. However, there is little difference between the profiles of these two types of fiber. The diameters of fibers with a plastic jacket are less than 1mm.

2.2 Types of Optical Fiber

Since the single-mode optical fiber has advantages of low internal attenuation, large bandwidth, easy upgrade and capacity expansion and low cost, it is internationally agreed that DWDM systems will only utilize single mode fiber as transmission media. At present, ITU-T has defined four types of single mode optical fiber with different design in Recommendations G.652, G.653, G.654 and G.655.

G.652 fiber is currently a single mode fiber for extensive use, called 1310nm property optimal single mode fiber and also called dispersion unshifted fiber. According to the refractive index cross section of the core, it can also be divided into two categories: matched cladding fiber and depressed cladding fiber. They have similar properties. The former is simple in manufacturing but has relatively larger macrobend loss and microbend loss while the later has larger connection loss.

G.653 fiber is called dispersion shifted fiber or 1550nm property optimal fiber. By designing the refractive index cross section, the zero dispersion point of this kind of fiber is shifted to the 1550nm window to match the minimum attenuation window. This makes it possible to implement ultrahigh speed and ultra long distance optical transmission.

G.654 fiber is cut-off wavelength shifted single mode fiber. This kind of fiber is mainly designed to reduce the attenuation at 1550nm. Its zero dispersion point is still near 1310nm. The dispersion at 1550nm is relatively high, up to 18ps/(nm.km). So single longitudinal mode laser must be used to eliminate the affect of the dispersion. G.654 fiber is mainly used for submarine optical fiber communication with very long regenerator section distance.

G.655 fiber, a nonzero dispersion shifted single mode optical fiber, is similar to G.653 fiber and preserves certain dispersion near 1550nm to avoid four-wave mixing phenomenon in DWDM transmission. It is suitable for DWDM system applications.

Except for the above-mentioned four types of standardized fiber, a large effective area fiber suitable for higher capacity and longer distance has emerged. Its zero

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dispersion point is near 1510µm and its effective area is up to 72 square µm. Therefore, it can effectively overcome the nonlinear affects and is especially suitable for DWDM system applications based on 10Gbit/s.

Thinking:

Which type of optical fiber is widely laid at present?

2.3 Basic Features of Optical Fiber

2.3.1 Physical Dimension (Mode Field Diameter)

The fiber core diameter of a single mode fiber is 8~9µm in the same magnitude as the operating wavelength 1.3~1.6µm. Because of the optical diffraction effect, it is not easy to measure the exact value of the fiber cord diameter. In addition, since the field intensity distribution of the fundamental mode LP01 isn't confined within the fiber core, the concept of single mode fiber core diameter is physically meaningless and should be replaced with the concept of mode field diameter. Mode field diameter measures the concentrate level of the fundamental mode field spatial intensity distribution within the fiber.

The nominal mode filed diameter of G.652 fiber at 1310nm wavelength area should be 8.6~9.5µm with a deviation of less than 10%, and the nominal mode filed diameter of G.655 fiber at 1550nm wavelength area should be 8~11µm with a deviation of less than 10%.

The cladding diameter of both types of above-mentioned single mode optical fibers is 125µm.

2.3.2 Mode Field Concentricity Error

Mode field concentricity error refers to the distance between the mode field center and the cladding of the interconnected fibers. Fiber connector loss is in proportion to the square of the mode field concentricity error. So reducing mode field concentricity error is one of the key factors to reduce the fiber connection loss and should be strictly controlled in process. The mode field concentricity error of the two types of single mode optical fibers G.652 and G.655 shouldn't be greater than 1. Generally, it should be less than 0.5.

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2.3.3 Bend Loss

Bend of the optical fiber will cause radiation loss. Actually, bend arises to an optical fiber in two cases. One is that the curvature radius of the bend is much larger than the diameter of the fiber (e.g. this kind of bend may occur when the fiber cable is laid). The other case is microbend. There are many causes for microbend. Microbend, limited to process conditions, may be caused during the production process of the fiber and the cable. Microbends of different curvature radiuses are randomly distributed along the fiber. The bent fiber with larger curvature radius can transmit fewer modes than the straight fiber, and a part of modes are radiated out from the fiber to cause loss. The randomly distributed fiber microbend will result in mode coupling in the fiber and cause energy radiation loss. Bend loss of the fiber is inevitable because it can't be guaranteed that no bend in any form will occur to the fiber and the cable during production or utilization process.

Bend loss is related to the mode field diameter. The bend loss of G.652 fiber shouldn't be larger than 1dB at 1550nm wavelength area, and the bend loss of G.655 fiber shouldn't be larger than 0.5dB at 1550nm area.

2.3.4 Attenuation Constant

Attenuation in optical fiber is mainly determined by three types of loss: absorption loss, scattering loss and bend loss. Bend loss, as described above, has no great effect on the attenuation constant in fiber. So, it is absorption loss and scattering loss that mainly determine the attenuation constant in fiber.

Absorption loss is caused by the fiber material where excessive metal impurity and OH- ion absorb the light to result in loss.

Scattering loss is often caused in the case that a part of optical power is scattered outside the fiber when uneven refractive index distribution local area emerges within the fiber and causes light scattering because of the micro-change in fiber material density and uneven density of compositions such as SiO2, GeO2 and P2O5. Or, scattering loss can be aroused if some defect occurs or some bubbles and gas scabs are remained at the core-cladding boundary. The physical dimension of these structural defects is much larger than the lightwave, causing wavelength independent scattering loss and upward shifting the whole curve of fiber loss spectrum. However, this kind of scattering loss is much less than the former one.

Combining the above losses, the attenuation constant of single mode fiber at 1310nm and 1550nm wavelength areas is 0.3~0.4dB/km (1310nm) and 0.17~0.25dB/km (1550nm), respectively. As defined in ITU-T Recommendation

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G.652, the attenuation constant at 1310nm and 1550nm should be less than 0.5dB/km and 0.4dB/km, respectively.

2.3.5 Dispersion Coefficient

Dispersion in optical fiber refers to a physical phenomenon of signal distortion caused when various modes carrying signal energy or different frequencies of the signal have different group velocity and disperse from each other during propagation. Generally, three kinds of dispersion exist in optical fiber.

1) Modal dispersion: This is caused when the fiber carries multiple modes of the same frequency signal energy and different mode has different time delay during transmission.

2) Material dispersion: Because the refractive index of the fiber core material is a function of the frequency, signal components of different frequency propagate at different velocities along the fiber. This causes dispersion.

3) Waveguide dispersion: In the fiber, for a signal carrying different frequencies in the same mode, dispersion is caused because of different group velocities during propagation.

These three types of dispersion are called chromatic dispersion. ITU-T G.652 defines a zero dispersion wavelength range of 1300nm~1324nm and a maximum dispersion slope of 0.093ps/(nm2.km). In the wavelength range of 1525~1575nm, the dispersion coefficient is approximately 20ps/(nm.km). ITU-T G.653 defines a zero dispersion wavelength 1550nm and a dispersion slope of 0.085ps/(nm2.km) in the wavelength range of 1525~1575nm where the maximum dispersion coefficient is 3.5ps/(nm.km). The absolute value of the dispersion coefficient of G.655 fiber should be within 0.1~6.0 ps/(nm2.km) in the range of 1530~1565nm.

Technical details:

The following figure shows the dispersion characteristics of several types of fiber.

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Dispersion coefficient(ps/nm�km)

G.655 fiber with positivedispersion coefficient

G.653 fiber

Wavelength�(nm)1550

1310

G.652 fiber17

G.655 fiber with negativedispersion coefficient

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2.3.6 Cutoff Wavelength

To avoid modal noise and dispersion penalty, the cutoff wavelength of the shortest optical fiber cable in the system should be less than the shortest operating wavelength of the system. The cutoff wavelength condition can guarantee single mode transmission in the shortest cable and suppress the occurrence of higher order modes or reduce the power penalty of the generated higher order mode noise to an negligible degree. At present, ITU-T has defined three types of cutoff wavelengths.

1) Cutoff wavelength of primary coating fiber in jumper cable shorter than 2m.

2) Cutoff wavelength of 22m cable optical fiber.

3) Cutoff wavelength of 2~20m jumper cable.

For G.652 fiber, the cutoff wavelength is 1260nm in 22m cable, 1260nm in

2~20m jumper cable, and 1250nm in jumper cable shorter than 2m. For G.655

fiber, the cutoff wavelength is 1480nm in 22m cable, 1470nm in primary

coating fiber of jumper cable shorter than 2m, and 1480nm in 2~20m jumper

cable.

2.4 Types and Properties of Optical Fiber Cable

2.4.1 Types of Optical Fiber Cable

In terms of the structure, optical fiber cable can be classified into four types: loose jacket twist type, skeleton type, central beam nominal type and ribbon optical fiber cable.

According to the laying methods, optical fiber cable can be classified into plow-in optical cable, optical fiber cable for installation in duct, aerial optical cable, submarine optical cable and office optical cable, etc.

According to application situation, traffic demands and capacity expansion demands, the core number of optical fiber cable is classified into 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34 and 36, and can be increased in even number.

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2.4.2 Properties of Optical Fiber Cable

1. Mechanical property: An optical fiber cable should possess certain mechanical property that makes it withstand items including tension, bruise, impulsion, repeated bending, twisting, flexure, hook hang, kink, reeling, etc.

2. Protective property: Optical fiber cable should possess property of moisture proof and water proof. Additionally, it should meet some requirements including protection of termite, rat and insect gnawing, corrosion, lightning, etc.

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Section 3 DWDM Key Technologies

Objectives:

To understand the requirements and solutions of DWDM optical resources.

To understand DWDM optical amplification technology.

To understand DWDM multiplexing and de-multiplexing technology.

3.1 Lasers

Laser, whose function is to generate laser, is an important component of DWDM system. At present, lasers used in DWDM system are semiconductor laser LD (Laser diode).

The operating wavelengths of DWDM systems are relatively dense. Generally, the wavelength spacing is from several nanometer to sub-nanometer. Hence, the laser diode is required to operate in a standard wavelength and possess good stability. On the other hand, the non-electrical regeneration distance of DWDM systems is increased from 50~60km of single SDH system transmission to 500~600km. lasers of the DWDM system are required to adopt lasers more advanced in technology and excellent in performance in order to elongate the dispersion limited distance of the transmission system and overcome fiber nonlinear effects {such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM), cross-phase modulation (XPM), modulation instability and four-wave mixing (FWM)}.

In summary, lasers of DWDM system have two major features:

1. Relatively large dispersion tolerance;

2. Standard and stable wavelength.

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3.1.1 Laser Modulation Modes

At present, optical fiber communication systems for extensive use employing intensity modulation — direct detection system. There are two types of intensity modulation for lasers, i.e. direct modulation and indirect modulation.

1. Direct modulation

Direct modulation: It is also called internal modulation, i.e. directly modulating the laser and changing the launched lightwave intensity by controlling the injection current. LED or LD sources used in traditional PDH and SDH systems of 2.5Gbit/s or below employ this modulation method.

One character of direct modulation is that the launched power is in proportion to the modulation current. It has advantages of simple structure, low loss and low cost. Since it changes the length of the laser resonant cavity, the variation of modulation current will cause a linear variation of the emitting laser wavelength corresponding to the current. This variation, called modulation chirp, is actually a kind of wavelength (frequency) jitter inevitable for direct modulation sources. The chirp broadens the bandwidth of the emitting spectrum of the laser, deteriorates its spectrum characteristics and limits the transmission rate and distance of the system.

Generally, for conventional G.652, the transmission distance is 100km and the

transmission rate 2.5Gbit/s.

For DWDM system without optical line amplifier, direct modulated lasers can be considered to save the cost.

2. Indirect modulation

Indirect modulation: This modulation method is also called external modulation, i.e. modulating the laser indirectly and adding an external modulator in its output path to modulate the lightwave. In fact, this modulator works as a switch, as shown in Figure 3-1.

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Figure 3-1 The structure of external modulated laser

The constant laser is a highly stable source continuously emitting lightwave with fixed wavelength and power. It isn't affected by the electric modulation signal during emitting, so no modulating frequency chirp occurs and the line breadth of its optical spectrum keeps at minimum. According to the electric modulation signal, the optical modulator processes the highly stable light from the constant laser light in a way of either passing through or blocking. During the modulation process, the spectrum characteristics of the lightwave won't be affected. This guarantees the quality of the spectrum.

Lasers adopting indirect modulation are relatively complex with high loss and cost, but its modulating frequency chirp is very low. It can be used in systems whose

transmission rate is 2.5Gbit/s and transmission distance longer than 300km.

Hence, in DWDM systems with optical line amplifiers, the lasers at the transmit end are generally indirectly modulated.

Commonly used external modulators are photoelectric modulator, acoustooptic modulator and waveguide modulator.

The basic operating principle of photoelectric modulator is crystal linear photoelectric effect. Photoelectric effect refers to the phenomenon that electric field causes the variation of the refractive index of a crystal. A crystal that is able to generate the photoelectric effect is called photoelectric crystal.

Acoustooptic modulator is made by utilizing the acoustooptic effect of the dielectric. Acoustooptic effect refers to the phenomenon that the dielectric changes under the pressure of an acoustic wave when it propagates through the dielectric. This change causes the variation of the refractive index of the dielectric and affects the transmission characteristics of the lightwave.

Constantlightsource

Opticalmodulator

Optical signaloutput

Electric modulationsignal input

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Waveguide modulator is manufactured from titanium (Ti) diffused LiNbO2 substrate material on which waveguide is made via photoetching method. It has many advantages such as small in dimension, light in weight and facile for optical integration.

According to the integration and separation conditions of the laser and the external modulator, external modulated lasers can be classified into two categories: integrated external modulated laser and separated external modulated laser.

As a maturing technology, integrated external modulation becomes the development trend of DWDM lasers. The commonly used modulator is electroabsorption modulator which, small and compact and integrated with the laser, meets most application requirements in performances.

Electroabsorption modulator, a kind of loss modulator, operates at the boundary wavelength of the material absorption region. When the modulator isn't biased, the wavelength from the laser is out of the absorption range of the modulator material. Thus the launched power of this wavelength is maximum and the modulator is turned on. When the modulator is biased, the boundary wavelength of the material absorption region shifts and the wavelength from the laser is within this region. Thus the launched power is minimum and the modulator is turned off, as shown in Figure 3-2.

Figure 3-2 Variation of the absorption wavelength of an electroabsorption modulator

Electroabsorption modulator can be manufactured by utilizing the same technical process as semiconductor laser. Therefore, it is easy to integrate the laser and the modulator, suitable for batch production. So its development speed is high. For example, InGaAsP optoelectronic integrated circuit monolithically integrates a

Absorptionregion Absorption

region

Biased

Unbiased

�1 �0 � ��0 �2

�1 is the absorption side wavelength of unbiased modulator�2 is the absorption side wavelength of biased modulator�0 is the operating wavelength of the constant light source

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laser and an electroabsorption modulator on a single chip that is put on a thermoelectric cooler (TEC). This typical optoelectronic integrated circuit is called electroabsorption modulated laser (EML). It can support transmission of 2.5Gbit/s signal over 600km, far exceeding the transmission distance of directly modulated lasers. Its reliability is similar to that of standard DFB lasers with an average life span of 20 years.

Separated external modulated laser generally uses constant output laser (CW) + LiNbO3 Mach-Zehnder external modulator. This modulator separates the light input into two equal signals that, respectively, enter the two branches. These two branches employ electrooptic material whose refractive index changes with the magnitude of the external electrical signal applied to it. Change of the refractive index of the optical branches will result in variation of the signal phases. Hence, when the signals from the two branches recombine at the output end, the combined optical signal is an interfering signal with varying intensity. Via this method, the information of the electrical signal is transferred onto the optical signal. Thus optical intensity modulation is implemented. The frequency chirp of separated external modulated laser can be zero. Moreover, its cost is relatively low when compared to electroabsorption modulated external laser.

3.1.2 Wavelength Stability and Control of Laser

In DWDM system, wavelength stability of lasers is a critical problem. According to ITU-T G.692, deviation of the central wavelengths shouldn't be greater than one fifth (±1/10) of optical channel spacing, i.e. the deviation of the central wavelengths shouldn't be greater than ±20GHz in a system with a channel spacing of 0.8nm.

Because the optical channel spacing is very small (can be as low as 0.8nm), DWDM system has strict requirements to the wavelength stability of the lasers. For example, a 0.5nm variation of wavelength can shift an optical channel to another one. In practical systems, the variation should be controlled within 0.2nm. The specific requirement is determined according to the wavelength spacing, i.e. the smaller the spacing, the higher the requirement. So the lasers should adopt strict wavelength stabilization technology.

Fine tuning of the wavelength of integrated electroabsorption modulated laser is mainly implemented by adjusting the temperature. The temperature sensitivity of

the wavelength is 0.008nm/ . The normal operating temperature is 25 . By

adjusting the chip temperature from 15 to 35 , the EML can be set up to a

specific wavelength with an adjustable range of 1.6nm. The chip temperature is adjusted by changing the drive current of the cooler and using a thermal resistance as feedback. Thus the chip temperature is stabilized and stays at a constant value.

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According to the correspondent characteristics of wavelength and chip temperature, distributed feedback laser (DFB) controls its wavelength by controlling the temperature of the laser chip to achieve wavelength stability. For

DFB laser, the wavelength-temperature coefficient is about 0.02nm/ and its

central wavelength meets the requirement within the range of 15 -35 . This

temperature feedback control method completely depends on the chip temperature of the DFB laser. At present, MWQ-DFB laser technical process can guarantee that the wavelength deviation meets the requirements of DWDM system in the life span (20 years) of the laser.

Except for the temperature, laser drive current can also affect the wavelength. The sensitivity is 0.008nm/mA, smaller than the affect of the temperature in one order. In some cases, its effect is negligible. Additionally, package temperature may also affect the device wavelength (e.g. temperature conduction brought by wires from the package to laser platform and inward radiation from the package shell will also affect the device wavelength). In a well-designed package, its effect can be controlled to minimum.

The above methods can effectively solve the problem of short-term wavelength stability. However, they are incapable of dealing with long-term wavelength variation caused by factors such as laser aging. It is ideal to directly utilize a wavelength sensitive component for wavelength feedback control of the laser. The theory is shown in Figure 3-3. Standard wavelength control of this type of scheme and reference frequency disturbance wavelength control are promising and being developed.

Figure 3-3 Theory for wavelength control

LD

LD control circuitWavelength sensitivecomponent

Optical output

Signalprocessing

For wavelength control For wavelength monitoring

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Thinking:

Why does the DWDM system set strict requirements to the wavelength stability?

3.2 Erbium-doped Optical Fiber Amplifier (EDFA)

As a key component of new generation optical communication systems, erbium doped fiber amplifier (EDFA) has many advantages such as high gain, large output power, wide operating optical bandwidth, polarization independence, low noise factor and amplifying characteristic independent to system bit rate and data format. It is an indispensable key component of high capacity DWDM systems.

3.2.1 EDFA Operating Theory

To amplify optical power, some passive optical components, pump source and erbium-doped fiber are combined together according to specific optical structure. Then EDFA optical amplifier is formed. Figure 3-4 shows a typical optical structure of dual-pumping source erbium-doped optical fiber amplifier.

Figure 3-4 Typical internal light path of EDFA

As shown in Figure 3-4, signal light and pump light from the pumping laser are combined via a DWDM multiplexer, then they are sent to the erbium-doped fiber

WDM

EDF

TAP

ISO

Pumping laser

WDM

ISO

Pumping laser

TAP

Signal input

Signal output

EDF

PD

PD

Optical isolator

Optical splitter Optical coupler

Optical detector

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(EDF). The two pumping lasers form a two-stage pump. Excited by the pumping light, the EDF yields the amplification function. Therefore, the function of amplifying the optical signal is implemented.

1.Erbium-doped optical fiber (EDF)

Erbium-doped optical fiber (EDF), doped with Er3+ of a given density, is the kernel of the optical fiber amplifier. To illustrate its amplification principle, we need to begin with the energy level diagram of Er3+. The outer-shell electrons of Er3+ have three-level structure (E1, E2 and E3 in Figure 3-5), where E1 is ground state, E2 is metastable state and E3 is high level, as shown in Figure 3-5.

Figure 3-5 EDFA energy level diagram

When high energy pumping lasers are used to excite the EDF, lots of bound

electrons of the erbium ions are excited from the ground state to the high level E3.

However, the high level is not stable and erbium ions are soon dropped to the

metastable state E2 via a radiationless decay process (i.e. no photon is released).

E2 level is an metastable energy band on which particles' survival span is relatively

long. Particles excited by the pumping light gather on this level via nonradiative

transition. Thus, population inversion distribution is implemented. When an optical

signal of wavelength 1550nm passes through this erbium-doped fiber, particles in

the metastable state are transited to the ground state via stimulated radiation and

generate photons identical to the photons of the incident signal light. This greatly

increases the quantity of the photons in the signal light, i.e. implementing the

function of continuous amplifying the signal light transmitted in the EDF.

2. Optical coupler (WDM)

PumpE2 metastable state

E3 excited state

1550nm

E1 ground state

1550nm

Decay

light

signal lightsignal light

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Optical coupler, as its name implies, has function of coupling. It couples the signal light and the pumping light and sends them into the erbium-doped fiber. It, also called optical multiplexer, usually employs optical fiber fusible cone multiplexer.

3. Optical isolator (ISO)

Optical isolator (ISO), a kind of component utilizing Faraday magnetooptical effect, allows only unidirectional light transmission. Along the light path, the functions of the two isolators are as follows: The input isolator can block the backward ASE in the EDF, keep it from interfering the transmitters of the system and from generating larger noise when it is reflected at the input end and reenter the EDF. The output isolator prevents the amplified optical signal, when reflected at the output end, from reentering the EDF, consuming particles and affecting the amplification characteristics of the EDF.

4. Pumping laser (PUMP)

Pumping laser, the energy source of EDFA, provides energy for amplifying the optical signal. Generally, it is a semiconductor laser with output wavelength of 980nm or 1480nm. When passing through the EDF, the pumping light pumps the erbium ions from low level to high level. Thus population inversion is formed. When the signal light passes through, the energy will be transferred to it. Hence, optical amplification is implemented.

5. Optical splitter (TAP)

The optical splitter used in the EDFA is a one by two component. Its function is to tap off a small part of the optical signal for monitoring the optical power of the main channel.

6. Optical detector (PD)

The PD is an optical power detector. Its function is to convert the received optical power into photocurrent via photoelectric conversion. Hence, it monitors the input and output optical power of the EDFA module.

3.2.2 Applications of EDFA

According to its location in the DWDM optical transmission network, EDFA can be classified into booster amplifier (BA), line amplifier (LA) and preamplifier (PA).

1. Booster amplifier (BA)

Booster amplifier is installed behind the transmitters of terminal equipment or regeneration equipment, as shown in Figure 3-6. The major function of the booster

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amplifier is to boost the launched power and elongate transmission distance by enhancing the optical power injected into the fiber (generally above 10dBm). So in some documents, it is also named as power booster amplifier. Here, its noise characteristic requirement is not high. The major requirement is linear power amplification characteristic. Generally, booster amplifier works in the saturation range of gain or input power in order to enhance the conversion efficiency from pumping source power to optical signal power.

Figure 3-6 Location of the amplifier in the regenerator section

2. Line amplifier (LA)

Line amplifier is located in the middle of the whole regenerator section, as shown in Figure 3-7. This is an application form to insert the EDFA into the optical fiber transmission link and amplify the signal directly. A regenerator section can be configured with multiple line amplifiers according to the demands. Line amplifier is mainly applied in long-haul communication or CATV distribution networks. Here, the EDFA is required to have high small-signal gain and low noise factor.

Repeating section

BA

Optical fiber connector

DWDMequipment

DWDMe q u i p

m e n t

Repeating section

LA

Optical fiber connector

DWDMequipment

DWDMequipment

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Figure 3-7 Location of the line amplifier in the regenerator section

3. Pre-amplifier (PA)

Pre-amplifier is located at the end of the regenerator section but in front of the optical receiving equipment, as shown in Figure 3-8. The main function of this amplifier is to amplify the small signal attenuated along the link and enhance the receiving sensitivity of the optical receiver. Here the main problem is noise. The main noise in EDFA is amplified spontaneous emission (ASE). This noise makes the optoelectronic detector output three noise components, i.e. extra shot noise due to the increase of optical power, signal-ASE beat noise and ASE-ASE beat noise. By using a narrow-band optical filter (1nm bandwidth), most ASE-ASE beat noise can be filtered and extra shot noise can be reduced. But the signal-ASE beat noise can't be filtered. Despite of this, the noise characteristic of EDFA is greatly improved by adopting the optical filter. The pre-amplifier greatly improves the sensitivity of receivers employing direct detection. For example, the sensitivity of an EDFA receiver of 2.5Gbit/s can be up to -43.3dBm. An improvement of about 10dB is achieved when compared to the receivers employing direct detection without EDFA.

Figure 3-8 Location of the pre-amplifier in the regenerator section

Β Tricks:

R e p e a t in g s e c tio n

P A

O p tic a l f ib e r c o n n e c to r

DWDMe q u ipm e n t

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BA, PA and LA differ from each other in that their locations in the DWDM network are different. However, the most important difference lies in their input optical power and gain:

BA: relatively high input optical power and low gain;

PA: relatively low optical power and low gain, similar to BA;

LA: relatively low input optical power, similar to PA, but its gain larger than BA.

3.2.3 Gain Control of EDFA

1. EDFA gain flatness control

In DWDM systems, the more the optical channels multiplexed, the more the optical amplifiers needed in cascading. This requires that a single amplifier occupies a wider and wider bandwidth.

However, EDFA based on ordinary pure silicon optical fiber has a very narrow flat gain range between 1549 and 1561nm, a range of approximately 12nm. And the gain fluctuation between 1530 and 1542nm is very large, up to about 8dB. When the channel arrangement of the DWDM system exceeds the flat gain range, channels near 1540nm will suffer severe signal-to-noise degradation and normal signal output can't be guaranteed.

To solve the above-mentioned problem and adapt to the development of DWDM systems, a gain flattened EDFA based on aluminum-doped silicon optical fiber is developed. It greatly improves the operating wavelength bandwidth of the EDFA and suppresses gain fluctuation. The up-to-date mature technology can achieve 1dB gain flattened range which almost expands to the whole erbium pass-band (1525nm~1560nm). Basically, it has solved the problem of gain unflatness of ordinary EDFA. Figure 3-9 compares the gain curves of non-aluminum-doped EDFA and aluminum-doped EDFA.

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Figure 3-9 Improvement of EDFA gain curve flatness

Technically, the range of 1525nm~1540nm in EDFA gain curve is called blue band area and the range of 1540nm ~1565nm is called red band area. Generally, red band area is preferential when the transmission capacity is less than 40Gbit/s.

1525nm-1565nm non-aluminum-doped EDFA

Gain

1525nm-1565nmaluminum-doped EDFA

Gain

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Technical details:

Performance comparison of EDFA gain unflatness and flatness is given in Figure 3-10.

Figure 3-10 Diagram of EDFA gain flatness

2. EDFA gain-locking

EDFA gain-locking is an important problem because the WDM system is a multi-wavelength working system. When certain wavelengths are dropped, their energy will be transferred to those undropped signals due to gain competition. Thus the power of other wavelengths increases. At the receive end, abrupt increment of the electrical level is possible to cause error. In limiting case, if seven wavelengths of eight wavelengths are dropped, all the energy will concentrate to the one wavelength left and its power may be up to about 17dBm. This will result in strong nonlinear effects or receiving power overload of the receiver, and this will also cause lots of errors.

There are many gain-locking technologies for EDFA. One typical method is to control the gain of pumping laser. The internal monitoring electric circuit of the EDFA controls the output of the pumping source by monitoring the input-output power ratio. When some signals of the input wavelengths are dropped, the input power will decrease and the output-input power ratio will increase. Via the

Cascading amplification of amplifier gain unflatness

Cascading amplification of amplifier gain flatness

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feedback circuit, the output power of the pumping source will be reduced in order to keep the gain (output/input) of the EDFA. Hence, the total output power of the EDFA is reduced and the output signal power is kept stable. The process is shown in Figure 3-11.

Figure 3-11 Gain-locking technology of controlling the pumping laser

Another method is saturation wavelength. At the transmit end, except for the eight operating wavelengths, system sends another wavelength as saturation wavelength. In normal cases, the output power of this wavelength is very small. When some line signals are dropped, the output power of the saturation wavelength will automatically increase in order to compensate the energy of the lost wavelengths and maintain the output power and gain of the EDFA. When the multi-wavelength line signals are restored, the output power of the saturation wavelength will correspondingly decrease. This method directly controls the output of the saturation wavelength laser, so its speed is faster than controlling the pumping source.

OUTPUTINPUT

Non-linear control

PIN PIN

TAPTAP P U M P

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Technical details:

Figure 3-12 NO Gain-locking when EDFA falling wavelength and adding wavelength

Figure 3-13 Having Gain-locking when EDFA falling wavelength and adding wavelength

3.2.4 Limitations of EDFA

EDFA solves the problem of line attenuation in DWDM systems. However it also brings some new problems.

addingwavelength

fallingwavelength >1dB

>1dB

Falling wavelength

adding wavelength

Falling wavelength

adding wavelength

<0.5dB

<0.5dB

<0.5dB

<0.5dB

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1. Non-linearity problem

Although enhanced by adopting EDFA, the optical power is not the higher the better. When it reaches a certain level, the optical fiber will generate nonlinear effects (including Raman scattering and Brillouin scattering). Especially, EDFA has greater affect to stimulated Brillouin scattering (SBS). Nonlinear effects greatly limits the amplification performance of the EDFA and the implementation of long distance repeaterless transmission.

2. Optical surge problem

EDFA can enhance the input optical power rapidly. However, since its dynamic gain variation is slow, optical surge will occur at the moment when the input signal power jumps, i.e. a peak occurs to the output optical power. The optical surge phenomenon is especially obvious in the case of EDFA cascading. The peak power can be up to a few watts and is possible to damage the O/E converter and the end surface of the optical connector.

3. Dispersion problem

Although the problem of attenuation limited repeaterless long haul transmission is solved after adopting EDFA, the total dispersion increases as the distance becomes longer. Thus the former attenuation limited system turns into dispersion limited system.

3.3 DWDM Components

In a DWDM system, DWDM components are classified into two types: multiplexer and de-multiplexer, as shown in Figure 3-14. The main function of the multiplexer is to combine multiple signal wavelengths into a single optical fiber for transmission. The main function of the de-multiplexer is to separate the multiple signal wavelengths transmitted in a single optical fiber. The key to the performance of a DWDM system is DWDM component whose requirements are enough multiplexing channels, low insertion loss, large crosstalk attenuation, wide pass-band, etc. Multiplexer and de-multiplexer are the same in principle and only need to change the input and output directions. The performances of the DWDM components used in DWDM systems should meet the requirements defined in ITU-T G.671 and other related recommendations.

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Figure 3-14 DWDM components

There are many methods to manufacture DWDM components each of which has its own features. At present, there are four types of widespread commercial DWDM components: interference light filter type, optical fiber coupler type, optical grating type and arrayed waveguide grating (AWG) type.

3.3.1 Optical Grating Type DWDM Component

Optical grating type DWDM component, a kind of angular dispersion type component, employs the angular dispersion component to separate and combine optical signals of different wavelengths. The most prevalent diffraction grating is made by depositing epoxy resin on a glass substrate and then fabricating grating lines on the epoxy resin to form a so-called reflective-type blazed diffraction grating. When the incident light reaches the optical grating, the optical signals with different wavelengths are reflected in different angles due to the angular dispersion function of the grating. Then these signals are converged to different output optical fibers via lenses in order to implement wavelength selection function. The inverse process is also right, as shown in Figure 3-15. The advantage of the blazed diffraction grating is high-resolution wavelength selection function which can separate most energy of specific wavelength from other wavelengths in centralized directions.

WDM �1��2��n��

�1

�2

�n

(a) Multiplexer

WDM�1��2��n��

�1

�2

�n

(b) De-multiplexer

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Figure 3-15 Principle of blazed optical grating type DWDM component

The blazed grating type filter has excellent wavelength selectivity and can reduce the wavelength spacing to about 0.5nm. Moreover, the grating type component is parallel operated and its insertion loss doesn't increase with the number of wavelengths multiplexed. Hence large number of multiplexing channels can be achieved. At present, multiplexing of 131 wavelengths with a spacing of 0.5nm is implemented and the isolation is good. For a wavelength spacing of 1nm, the isolation is up to 5dB. The disadvantage of blazed grating is relatively large insertion loss, generally 3~8dB. Moreover, it is very sensitive to polarization and its optical channel bandwidth-to-spacing ratio isn't ideal. So the optical spectrum utilization ratio isn't high enough. And the wavelength fault-tolerance requirement for the laser and DWDM component is relatively high. Additionally, its temperature drift varies with the thermal expansion coefficient and refractive index of the material. Typically, the component temperature shift is relatively high,

approximately 0.012nm / . If temperature control measures are adopted, the

temperature shift can be reduced to 0.0004nm / . So temperature control

measures are feasible and necessary.

This optical grating requires high manufacturing accuracy and is not suitable for mass production. Hence, it is generally applied in experimental scientific research.

Except for the above-mentioned optical fiber component, the manufacturing technology for optical fiber Bragg grating filter is gradually maturing. It is manufactured employing the interference of high power ultraviolet light beams to form periodic variation of refractive index at the optical fiber core. The accuracy can be up to 10000 lines per centimeter, as shown in Figure 3-16. Fiber Bragg grating can be feasibly designed and manufactured with low cost. It has very low

�1�2�3�4�5��n output (in)

�1�2���n input (out)

Diffractiongrating

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insertion loss and stable temperature characteristic. Its intraband filtering characteristic is flat and out-of-band is very steep ( rolling slope is better than 150dB/nm and out-of-band suppress ratio is up to 50dB). This component can be directly melted with the optical fiber of the system. So it can be fabricated into band-pass or band-stop filter with small channel spacing. At present, it is extensively applied in DWDM system. However this kind of optical fiber grating has relatively narrow wavelength range, only applicable to single wavelength. The benefit it brings in is that the filters can be added or removed according to the number of wavelength used. So the application is flexible.

Figure 3-16 Optical fiber Bragg grating filter

3.3.2 Dielectric Film Type DWDM Component

Dielectric film filter type DWDM component is a kind of interactive DWDM component consists of dielectric films (DTF). DTF interference filter is composed of tens of dielectric films of different material, different refractive index and different thickness combined according to design requirements. Each layer is 1/4 wavelength in thickness. Layers of high refractive index and low refractive index are alternatively overlapped. When the light incidents on the high refractive layer, the reflected light has no phase shift. However, when the light incidents on the low refractive layer, the reflected light undergoes a 1800 phase shift. Since the layer thickness is 1/4 wavelength (900), the light reflected by the low refractive layer undergoes a 3600 phase shift and in-phase superposes with the light reflected by the high refractive layer. Thus, reflected lights of the layers superpose near the central wavelengths and form intensive reflected light at the front-end surface of the filter. In the highly backward reflecting area, the reflected light suddenly decreases and most light becomes transmitted light. Accordingly, the film

Periodic variation of the refractive index (grating)

Ultraviole light interference

�1�2�3 �2

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interference type filter can be made to band pass certain wavelength range and band stop the other wavelength range, forming the required filter characteristics. The structural principle of the film interference type filter is shown in Figure 3-17.

The main features of dielectric film filter DWDM component are as follows: miniaturization and structural stability of the component can be implemented via design, the signal pass-band is flat and polarization-independent, and its insertion loss is low and channel isolation is good. The disadvantage is that the number of channels can't be large. The specific characteristics are related to its structure. For instance, if the film filter type DWDM component utilizes soft material, its wavelength may be changed under the environmental influence because the filter can easily absorb moisture. When employing hard dielectric film material, the

temperature stability is better than 0.0005nm/ . Additionally, this kind of

component has relatively long design and manufacturing process and low volume of production. And if epoxy resin is used along the light path, it is not easy to achieve high isolation and narrow bandwidth.

In DWDM systems, when only 4 to 16 wavelengths are involved, this type of DWDM component is relatively ideal.

Figure 3-17 Principle of film interference filter type de-multiplexer

3.3.3 Fused Conical Type DWDM Component

There are two types of optical fiber coupler. The extensively used one is fused biconical tapered coupler, i.e. drawing multiple fibers under hot-melt condition to

�1-4

�4

�2

3

Self-focusing lens

�1 filter

�3 filter

Glass

�1

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form a cone and slightly twisting them to fuse them together. Because the cores of different fibers are extremely close, the required coupling power can be obtained via evanescent wave coupling on the conical region. The second type of coupler employs grinding and polishing methods to remove part of cladding of the optical fiber so that only a thin cladding layer is left. Then two optical fibers processed via the same method are butt jointed and coated a layer of index matched solution between them. Thus the two fibers can couple via the evanescent wave in the cladding and obtain the demanded coupling power. Fused conical type DWDM component is simple to manufacture and is extensively applied.

3.3.4 Integrated Optical Waveguide Type DWDM Component

Integrated optical waveguide type DWDM component is a plane waveguide component based on optical integration technology. The typical manufacturing process is to deposit a thin layer of silica glass on the silicon substrate, form the demanded pattern by utilizing photetch and etch. This component supports integration manufacture and has great application prospective in future access networks. Moreover, except for DWDM component, it can be fabricated into matrix structure to add/drop optical signal channels (OADM). This is a preferred scheme for implementing optical switching in future optical transport networks.

A typical component which uses integrated optical waveguide DWDM is arrayed waveguide grating (AWG) optical multiplexer/de-multiplexer manufactured by NTT Company, Japan. It has many advantages, including small wavelength spacing, large number of channels and flat pass-band. So it is especially suitable for ultrahigh-speed and large capacity DWDM systems. Its structural diagram is shown in Figure 3-18.

1 2��

Waveguide

grating

Free space

Fan-like

waveguide

Fan-like

waveguide

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Figure 3-18 Principle of AWG DWDM component

3.3.5 Performances of DWDM Components

Table 3-1 Comparison of various DWDM components Componen

t type Mechanis

m Mass

production

Channel spacing

(nm)

Number of

channels

Crosstalk (dB) Insertion loss (dB)

Main disadvant

ages

Refractive grating type

Angular diffraction

Average 0.5~10 131 -30 3~6 Temperature sensitive

Dielectric film type

Interference/absorption

Average 1~100 2~32 -25 2~6 Small number of channels

Fusible cone type

Wavelength dependent

Relatively easy

10~100 2~6 - 10~45) 0.2~1.5 Small number of channels

Integrated optical waveguide type

Plane waveguide

Easy 1~5 4~32 -25 6~11 Large insertion loss

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Section 4 DWDM Networking Design

Objectives:

To understand the basic concepts of DWDM networking.

To master the configuration of different network elements of DWDM.

To master some factors to consider during DWDM network design.

To know general protection mechanisms of DWDM networks.

4.1 Some Network Element Types of DWDM

In terms of usage, DWDM equipment is generally classified into four types: optical terminal equipment, optical line amplifier equipment, optical add/drop multiplexing equipment and electrical regeneration equipment. Now we take the 16

32 -wavelength equipment of DWDM system as an example to explain the

functions of these network element types in the network.

4.1.1 Optical Terminal unit (OTM)

At the transmitter end, OTM multiplexes the STM-16 signals of 16 wavelengths λ1~λ16 (λ32)into a DWDM main optical channel via the multiplexer, amplifies the optical power of the main channel, and then adds an optical supervisory channel

雜.

At the receiver end, firstly OTM extracts the optical supervisory channel 雜. Then

the DWDM main optical channel is amplified and de-multiplexed into STM-16 signals of the 16/32 different wavelengths.

The signal flow of OTM is shown in Figure 4-1.

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Figure 4-1 OTM signal flow

4.1.2 Optical Line Amplifier Unit (OLA)

OptiX BWS 320G optical regeneration equipment is configured with an optical line amplifier in each transmission direction. OLA of each direction firstly extracts the optical supervisory channel (OSC) and processes it, then amplifies the main optical channel signals, multiplexes them with the optical supervisory channel and sends them onto the fiber. The signal flow of OLA is shown in Figure 4-2.

SD

H

RWC

D16

WPA

S

C

A

SC1

WBA

M16

TWC

MM

MM

AA

ARI

TO

RO

TI

RM

TM

D32

M32

EastWest

WPA

S

C

A

SC2

WBA

M

M

A

A A

S

C

A

WBA

AWPAM

M

RI RO

RM1

TM1 RM2

TM2

TI TO

RIROTO TI

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Figure 4-2 OLA signal flow

The whole equipment is installed in a subrack. In the figure, each direction employs a pair of WPA+WBA to conduct optical line amplification. It can also use single WLA or WBA to conduct unidirectional optical line amplification.

4.1.3 Optical Add/Drop Multiplexing Unit (OADM)

The optical add/drop multiplexing unit (OADM) of DWDM system operates in two modes, i.e. a board uses static OADM to add/drop wavelengths or two OTMs adopts back-to-back mode to form an OADM equipment which can add/drop wavelengths.

1. Static add/drop multiplexing equipment of DWDM system

In HUAWEI OptiX BWS 320G DWDM system, optical add/drop multiplexer equipment can use a board to implement static add/drop of wavelengths. Each OADM equipment is capable of adding/dropping 1 to 8 wavelengths in order to meet the practical demands of various projects.

After receiving the line optical signal, the OADM equipment firstly extracts the optical supervisory channel and then uses a WPA to pre-amplify the main channel. Via the ADD/DROP unit, a given number of signals are dropped from the optical signal with 16/32 STM-16 according to wavelengths. The other wavelengths are directly inserted into the main channel via the ADD/DROP unit. After power amplification, the main channel is combined with the local optical supervisory channel and sent to the remote end. The main channel between ADD/DROP units is configured with a variable attenuator to adjust optical power equalization between pass-through channels and ADD channels. Channels dropped at the local station are required connect to the SDH equipment via RWC, and those added at the local station are required to connect to the SDH equipment via TWC.

For example of an OADM (add/drop four wavelengths), its signal flow is shown in Figure 4-3.

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RI

SCA

WPA

WBA

SCA

WPA

MR2 MR2

SC2

SCC OHP

RM1

TM1

TM2

RM2

WPA

RI

TO RO

TI

TO

RO

TI

RWC

RWC

TWC

TWC

λ1 λ2 λ1 λ2

TWC

TWC

RWC

RWC

λ1 λ2λ1 λ2

WBA

Figure 4-3 Static OADM signal flow

2. OADM equipment consists of two back-to-back OTMs

Two back-to-back OTMs are used to form an OADM equipment which can add/drop wavelengths. This mode is more flexible when compared to the static OADM which uses a board to conduct wavelength conversion. It can add/drop any of wavelengths from 1 to 16/32, more feasible for networking. If a signal channel isn't added/dropped at this station, it can directly access the TWC of the same wavelength via the D16/D32 output port and then enter the M16/M32 board in the other direction.

The signal flow of the OADM consisting of two back-to-back OTMs is shown in Figure 4-4.

M

SD

H

RWC

D16

WPA

S

C

A

SC

WBA

6M1

TWC

MM

M

AA

A

east

SD

H

S

C

A

TWC

A

WBA

A RWC D

16

WPA

A

west M

M

MM

RI RO

TO TI

RM

TM

TM

RM

TI TO

RI

RO

2/1

TM

RM

D32

M32

6M1

M32

D32

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Figure 4-4 The signal flow of the OADM consisting of two back-to-back OTMs

4.1.4 Electrical Regeneration Unit (REG)

For projects adopting regenerator section cascading, electric regenerator (REG) is required. The electric regeneration equipment has no services to add/drop and is merely used to elongate dispersion limited transmission distance. The signal flow of the electrical regeneration equipment is shown in Figure 4-5.

Figure 4-5 The signal flow of the electrical regeneration equipment REG

Note:

M

D16

WPA

S

C

A

SC

WBA

6M1

TWC

MM

M

AA

A

east

S

C

A

TWC 6

M1

A

WBA

AD16

WPA

A

west M

M

MM

RI RO

TO TI

RM

TM

TM

RM

TI TO

RI

RO

2/1

TM

RM

D32

D32

M32

M32

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Other basic network unit types of DWDM equipment are included in these types, with similar functions and same position in the network. They only differ in names. The following contents related to the network element or board are described by using OptiX BWS 320G equipment configuration and board.

4.2 General Constitution of DWDM Network

Basic network modes of DWDM system are point-to-point , chain and ring . Other complex network forms can be combined by using these three modes. When application together with STM-16 equipment, they can form very complex optical transmission network.

4.2.1 Point-to-point Networking

Figure 4-6 Schematic diagram of WDM point-to-point networking

SDH OTM

SDH OTM

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4.2.2 Chain Networking

Figure 4-7 Schematic diagram of WDM chain type networking

4.2.3 Ring Networking

In local area network especially metropolitan network applications, DWDM optical add/drop multiplexers can be used to form ring networks according to user demands. Generally in ring networks, path protection ring and multiplex section protection are provided by SDH equipment itself, so it is not necessary for the DWDM equipment to provide other protection methods. But wavelength protection can be provided according to user requirements. The ring networking is shown in Figure 4-8.

SDH OTM

SDH SDHOTM OADM

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Figure 4-8 Schematic diagram of DWDM ring network

4.2.4 Network Management Information Channel Backup and Interconnection Capability

High reliability is required by optical transmission networks adopting DWDM. In a transmission network, network management information is transmitted via a supervisory channel which generally uses the same physical channel as the main channel. Thus, the supervisory channel will also fail when the main channel fails. So network management information backup channel is required.

In ring networking, when certain section fails (e.g. fiber cable damage), network management information can be automatically switched to the supervisory channel in the other direction of the ring. So the management of the whole network won't be affected. Figure 4-9 illustrates an automatic backup approach of the network management information for ring networking.

OADM

OADM OADM

OADM

1~8

1~8

1~8

1~8

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Figure 4-9 Network management information channel backup in ring networking (when certain transmission section fails)

However, when both of the two ends of certain office in a fiber section fail or certain transmission section in point-to-point and chain networking fails, network management information channel will fail. Consequently, network management administrators won't be able to get the supervisory information of failed office and do some operation with it.. To avoid this circumstance, network management information should use the backup channel. The SDH network elements can provide backup network management information channel by using data communication network.

Between two network elements in need of protection, a network management information backup channel can be established by accessing the data communication network via routers. When the network is normal, network management information is transmitted by the main supervisory channel, as shown in Figure 4-10.

NMS

GNEManagement information

Management information

NE

NE

NE

NE

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Figure 4-10 Network management information channel backup (in normal case)

When the main channel fails, network elements automatically switch the management information to the backup channel to guarantee that the network management system can supervise and operate the entire network. The whole switching process is conducted automatically, not requiring manual intervention. A network management information channel backup is illustrated in Figure 4-11.

NMS

Main channel/Supervisory channel

GNE

Management information

Backup supervisory channel

Router Router

NE

DCN

NMS

Main channel/Supervisory channel

GNE

Management information

Router Router

NE

DCN

Backup supervisory channel

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Figure 4-11 Network management information channel backup (in case of main channel failure)

Attention: In network planning, different routes should be selected respectively for backup supervisory channel and main channel. Otherwise backup function won't be implemented.

HUAWEI OptiX BWS 320G equipment provides various data interfaces (e.g. RS-232 and Ethernet interface) for management information channel interconnection among different DWDM networks and between DWDM and SDH. This implements unified network management for different transmission equipment. Figure 4-12 illustrates management information channel interconnection among different transmission equipment.

Figure 4-12 Network interconnection among different transmission equipment

ManagementInformation

Channel

ManagementInformation

Channel

ManagementInformation

Channel

NMS

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4.3 Factors To Be Considered in DWDM Networking

4.3.1 Dispersion Limited Distance

1. Dispersion effect description

Dspersion, caused by transmitter optical spectrum characteristic and optical fiber dispersion, is a dominant factor which limits the transmission capacity.

Generally, adding optical amplifier into a system won't remarkably change the total dispersion. As the active gain media in the EDFA, rare-earth-doped optical fiber will cause a little dispersion. The length of this fiber is only in order of several tens or hundreds. The dispersion of rare-earth-doped fiber has little difference with that of fibers defined in ITU-T Recommendations G.652, G.653 and G.655. For a system of tens or hundreds of kilometers, the effect of this dispersion is negligible.

2. Transmission limitation

As the transmission rate in optical fiber communication systems continuously grows and because optical amplifier greatly elongates the no-electrical-regenerator optical transmission distance, the total dispersion and the corresponding dispersion penalty of the whole transmission link may become very large and must be seriously dealt with. Dispersion limitation has currently become the determinant of the regenerator section length. In single mode optical fiber, major dispersion includes material dispersion and waveguide dispersion. It results in different time delay for different frequency components when arriving at the optical receivers after transmitted via the optical fiber. In time domain, it causes broadening of optical pulses, crosstalk among them and degradation of eye patterns, and finally results in the degradation of system error performance.

Different frequency components in the signal are originated from the laser source optical spectrum characteristics, including wavelength, spectrum width, laser chirp, etc. At present, the -20dB optical spectrum width of SLM lasers at 1550nm region can be up to 0.05nm. In this case, laser chirp is the determinant limiting the regenerative length.

3. Method of reducing the effect

Since the presence of optical amplifiers doesn't affect the dispersion effects in the system, it is not required to regulate specific methods of reducing these effects to minimum. However, EDFA, which makes possible long distance no-regenerative-

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regenerator system, will aggravate the impairment caused by the dispersion in the system.

In some subsystems of optical amplifier, a type of passive dispersion compensation device can be assembled with the optical amplifier to form an amplifier subsystem. This subsystem will add limited dispersion to the system. And the dispersion coefficient, inverse to the optical fibers of the system, will reduce the system dispersion. This device can be installed together with an EDFA to compensate the loss related to the passive dispersion compensation function. Additionally, adopting G.655 optical fiber and G.653 optical fiber is favorable for dispersion reduction. If nonlinear impairments are considered thoroughly, G.655 optical fiber has optimal over-all properties in long haul transmission.

4. Consideration in network design

In DWDM network design, firstly the whole network is divided into several regenerator sections, letting the length of each section less than the dispersion limited distance of the laser. Hence, the performance of the whole network can tolerate the effect of dispersion.

Β Tricks:

When we calculate dispersion during DWDM network design, the typical dispersion coefficient at 1550nm window is 17ps/nm.km because optical fibers employed in the world are primarily G.652 fiber. But in engineering design, 20ps/nm.km is adopted for budget.

4.3.2 Power

Long distance transmission of optical signal requires that the signal power is enough to compensate the attenuation of the optical fiber. Generally, the attenuation coefficient of G.652 optical fiber at 1550nm window is about 0.25dB/km. When factors such as optical connectors and optical fiber redundancy are taken into consideration, the combined optical fiber attenuation coefficient is generally less than 0.275dB/km.

During practical calculation, power budget is only conducted for two pieces of adjacent equipment in the transmission network instead of conducting unified power budget for the whole network. The distance (attenuation) between two

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pieces of adjacent equipment in the transmission network is called regeneration distance (attenuation).

Figure 4-13 Schematic diagram of regeneration attenuation

As shown in above figure, S is the transmit reference point of station A, R is the receive reference point of station B and L is the transmission distance between point S and point R. Then:

regeneration distance = Pout - Pin) /a

Pout: channel output power of point S (in dBm). The optical power of point S is related to the configuration of point A.

Pin: permissive channel minimum input power of point R (in dBm).

a: optical fiber cable attenuation per kilometer (dB/km) (using 0.275dB/km according to ITU-T recommendations. It contains the effect of various factors, including connectors and redundancy).

4.3.3 Optical Signal-to-Noise Ratio

1. Generation principle of noise

Optical amplifier creates light around the signal wavelength, i.e. amplified spontaneous emission (ASE). In a transmission system with several cascading EDFAs, ASE noise of optical amplifiers will regenerate a periodic attenuation and amplification process. Because in each optical amplifier the ASE noise input is amplified and superimposed to the ASE generated by that optical amplifier. Hence the total ASE noise power will increase with the number of amplifiers in approximate proportion and the signal power will decrease. The noise power may exceed the signal power.

ASE noise frequency spectrum distribution is expanded with the system length. When the ASE noise from the first optical amplifier is sent to the second one, the gain distribution of the second optical amplifier will change due to the ASE noise

Station A Station BS R

LP Pout in

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caused by gain saturation effect. Similarly, the effective gain distribution of the third optical amplifier will also change. This effect will be transmitted downstream to the next optical amplifier. Even though narrow band filter is used in each optical amplifier, ASE noise will also accumulate. This is because the noise exists in the signal frequency band.

Optical signal-to-noise ratio (OSNR) is defined as:

OSNR = channel optical signal power / channel noise optical power

2. Transmission limitation

ASE noise accumulation affects the system SNR because SNR degradation of the received signal is mainly caused by ASE related beat noise. This type of beat noise linearly increases with the number of optical amplifiers. Hence, error rate degrades as the number of optical amplifiers increases. Besides, the noise is accumulated in exponential form to the gain amplitude of the amplifier.

As the result of optical amplifier gain, the ASE noise frequency spectrum will have a wavelength peak after accumulating in many optical amplifiers. To specially point out, when adopting closed all-optical ring, the ASE noise will infinitely accumulate if cascading infinite number of amplifiers. Although in systems with filters ASE accumulation is remarkably decreased due to the filters, intraband ASE will still increase as the number of optical amplifiers increases. Hence, SNR will degrade with the increase of amplifiers.

3. Methods of ASE reduction

ASE noise accumulation may decrease as the interval of optical amplifiers reduces (when the total gain is equal to the total transmission channel attenuation) because ASE accumulates in exponential form with the increase of the gain amplitude of the amplifier. One of the following filter technologies can further reduce unexpected ASE noise, i.e. adopting ASE noise filter or utilizing self-filtering effect (self-filtering method).

Self-filtering method is suitable for systems with tens of or more optical amplifiers. This method adjusts the signal wavelength to the self-filtering wavelength in order to reduce the ASE noise received by the detector, similar to using a narrow band filter. This is most effective when the approach of reducing optical amplifier interval and employing low gain optical amplifier is used to reduce the initial ASE noise.

If all-optical DWDM closed ring network is adopted, the self-filtering method isn't suitable. In fact, the peak formed in the whole gain frequency spectrum of the optical amplifier may severely affect system performance. In this case, utilizing

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ASE filtering method can utmost reduce ASE noise accumulation. This is achieved via the approach of filtering the DWDM channels which are not sent to the network node before being switched out the node.

For systems with a few optical amplifiers, self-filtering method is not as effective as ASE filtering method. ASE filtering method can flexibly select signal wavelengths and has other advantages. Characteristics of the filter must be carefully selected because cascading filter has narrower pass-band than the signal filter (unless it has a rectangular frequency band).

4. Considerations for OSNR in DWDM network design (Note: If you feel that the contents of this part is a bit abstruse, it is OK to skip it and read next chapter)

For different network applications, OSNR requirements are almost the same, with slight differences as shown in Table 4-1.

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Table 4-1 OSNR comparison Amplifier cascading type Minimum OSNR (dB)

32-channel 8×22dB system (8*80km) 22

32-channel 5×30dB system (5*100km) 20

32-channel 3×33dB system (3*120km) 20

OSNR is one of the most important factors that affect the DWDM system error performance. For a DWDM system with multiple cascading optical line amplifiers, the noise power is dominated by amplified spontaneous emission (ASE) noise.

1)ASE noise accumulation of cascading optical line amplifier

The mathematical model of the ASE noise accumulation in multiple cascading optical line amplifiers is illustrated in Figure 4-14.

Figure 4-14 The mathematical model of the ASE noise accumulation

In Figure 4-14, GN is the gain of EDFAN (in linear unit); LN is the optical fiber cable attenuation of the regenerator section N (in linear unit).

The total ASE noise power = The ASE noise power generated by EDFAN

+ (The ASE noise power generated by EDFAN-1LN-1 GN)

+ …+ (The ASE noise power generated by EDFA2L2 G3 匞N-1 LN-1 GN)

+ (The ASE noise power generated by EDFA1L1 G2 匞N-1 LN-1 GN)

( Equation 4-1)

2)Noise of a single EDFA

G1 G2 GN-1 GN

EDFA1 EDFA2 EDFAN-1 EDFAN

L1 L2 LN-1

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The ASE noise power per unit frequency band generated by a single optical amplifier, PASE is

PASE=2NSP G-1) hν (4-2)

where NSP is the spontaneous noise coefficient of the EDFA;

G is the internal gain of the EDFA;

h is the Planck constant;

v is the optical frequency.

The external noise coefficient of the amplifier, NF is

NF=10Log[2NSP 2NSP 1) /G] ηin

(4-3)

ηIN is the insertion loss of the amplifier (in dB).

3?¨º?Simplified calculation of network OSNR in case of uniform attenuation of the regenerator sections.

Assume that all EDFAs have the same properties and all regenerator sections have uniform attenuation, the total power (including the accumulated ASE power) of each amplifier is the same, and G>>1, where G=L. According to (4-1) and (4-2), (4-3) undergoes a series of processing. Then OSNR is given by:

OSNR=POUT L NF 10LogN 10Log[h ν ν0]

(4-4)

where POUT is the channel output power (in dBm);

L is the attenuation between amplifiers (in dB);

NF is the external noise coefficient (in dB);

N is the number of intervals along the link;

ν 0 is the optical bandwidth;

10Log[h ν ν0]=-58 dBm (1.55μm band zone with 0.1nm bandwidth).

This calculation method can meet the requirements of the general engineering design. However, the following conditions must also be satisfied except for the above-mentioned assumptions.

⌧ The optical de-multiplexers have no periodic characteristic;

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⌧ The optical transmitters have enough extinctivity.

In practical DWDM systems, EDFA gain inequality may cause difference in channel output power and EDFA noise coefficient. So during design, the OSNR of the worst channel should meet the requirements and have sufficient redundancy.

4.3.4 Other Factors

1. Stimulated Brillouin Scattering (SBS)

1) Principle

In the intensity-modulated system employing narrow spectrum line breadth laser, the strong forward transmission signal will convert to backward transmission once the signal optical power exceeds the stimulated Brillouin scattering (SBS) threshold. In SBS, the forward transmission light is scattered in the form of photons. Only the backward scattered light is in single mode optical fiber. The scattered light is shifted from 1550nm by about 11GHz.

SBS effect has a minimum threshold power. However, research indicates that different types of optical fibers and even different optical fibers of the same type have different SBS threshold power. For external modulation systems adopting narrow spectrum line lasers, the typical SBS threshold power is on the order of 20~30mw. Since the effective core area of G.653 fiber is relatively small, the SBS threshold power of systems adopting G.653 fiber is lightly lower than that of systems adopting G.652 fiber. This is true for all the nonlinear effects. SBS threshold power is sensitive to the spectrum line breadth of the laser and the power level.

2) Transmission limitation

SBS greatly limits the optical power transmittable in the fiber. Figure 4-15 describes this effect for narrow band lasers, where all the signal power falls into the Brillouin bandwidth. Then the forward transmission power gradually saturates and the backward scattering power rapidly increases.

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Figure 4-15 SBS threshold of the narrow band laser

3) Methods of reducing the effect

In a system whose laser line breadth is apparently larger than the Brillouin bandwidth or whose signal power smaller than the threshold power, SBS impairment won't occur.

2. Stimulated Raman Scattering (SRS)

1) Principle

SRS is a broadband effect related to the interaction of light with silicon atom vibration modes. SRS makes the signal wavelength works as a Raman pump of the channels of longer wavelength or the Raman-shifted light of spontaneous scattering. In any circumstance, the signals of shorter wavelength will always be weakened by this process. At the same time, the signals of longer wavelength will be enhanced.

2) Transmission limitation

SRS may occur in both single wavelength systems and multi-wavelength systems. In systems with a single wavelength and no line amplifier, the signals may be impaired by this effect when its power is greater than 1W. However, in multi-wavelength systems of relatively wider channel spacing, the channels of shorter wavelength will lose a portion of power to the higher-wavelength channels due to

25

15

5

-5

-15

-25

-35

Scattered power

Input power

15

10

5

0

-5

-10

-15

Output pow

er

-5 0 5 10 15 20 25

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the effect of SRS, leading to a degradation of the signal-to-noise ratio performance. This may limit the total capacity of systems with fixed total number of channels, channel spacing, mean launched optical power and total system length. The SRS dispersion threshold of systems adopting G.653 optical fiber is slightly lower than that of systems employing G.652 fiber because G.653 fiber has smaller equivalent core area. SRS won't cause practical degradation effect for single wavelength systems. However, it may limit the capacity of DWDM systems.

3) Method of reducing the effect

In single wavelength systems, optical filters can be used to filter the unwanted frequency components. However, up to now there are no practical technologies for multi-wavelength systems to eliminate the effect of SRS. The effect of SRS effects can also be released by reducing the signal power. Nevertheless, no apparent SRS limitation has appeared in the carefully-designed DWDM systems implemented at the present time.

3. Self-phase Modulation (SPM)

1) Principle

Because of the Kerr effect, instantaneous variations in the power of an optical signal result in self modulation. This effect is called self-phase modulation. In single wavelength systems, SPM effect will broaden the signal's spectrum when changes in the signal's intensity result in variations in its phase, as shown in Figure 4-16. In the normal dispersion zone of optical fiber, signals propagating along the fiber will experience a longer instantaneous widen once the frequency spectral broadening is caused by SPM due to the dispersion. In the abnormal dispersion zone, the dispersion of optical fiber and SPM may compensate with each other. Thus the signal broadening will be smaller.

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Figure 4-16 Compression and spectrum broadening of the transmission pulse caused by self-phase modulation

2) Transmission limitation

Generally, SPM is relatively apparent only in systems with high accumulated dispersion or ultra long length. Dispersion limited systems may be unable to tolerate SPM effects. In multi-wavelength systems of narrow channel spacing, spectral broadening caused by SPM may lead to interference between adjacent channels.

In G.652 optical fiber, SPM of the low chirp intensity modulated signal leads to compression of the pulse. For G.655 optical fiber of abnormal dispersion characteristic, the SPM effect of the signal is a function of the transmitter power. Pulse compression can suppress the dispersion and provide certain dispersion compensation. However, the maximal dispersion limitation and the corresponding transmission distance limitation still exist.

Figure 4-16 illustrates compression of the transmission pulse caused by SPM of the low chirp intensity modulated signal in G.652 optical fiber, also it can be regarded as spectral broadening.

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3) Method of reducing the effect

To adopt G.653 optical fiber and configure the signal channels near the zero-dispersion zone will benefit the reduction of SPM effects. For systems employing G.652 optical fiber and less than 100km in length, the effects of SPM can be controlled by using dispersion compensation in appropriate intervals. The SPM effects can also be weakened by reducing the input optical power or configuring the system operating wavelengths over the zero dispersion wavelength of G.655 fiber.

4. Cross-phase Modulation (XPM)

1) Principle

In multi-wavelength systems, when variations in light intensity lead to a phase shift, cross-phase modulation will generally broaden the signal spectrum due to the interaction between adjacent channels. The spectral broadening caused by XPM is related to the channel spacing. This is because the dispersion caused by difference in group velocities may lead to the interaction among the pulses which should separately propagate along the optical fiber. In case that XPM results in spectral broadening, the signals will suffer a relatively large instantaneous spectral broadening due to the dispersion effect when propagating along the optical fiber.

2) Transmission limitations

Impairment caused by XPM in G.652 fiber-optic systems is more obvious than that in G.653 and G.655 fiber-optic systems. The broadening, caused by XPM, leads to interference between adjacent channels in multi-wavelength systems.

3) Methods of reducing the effect

XPM can be controlled by selecting appropriate channel spacing. Study shows that the signal distortion caused by XPM in multi-wavelength systems only occurs between adjacent channels. In a 3-channel system, the signal-to-noise ratio (SNR) of the central channel is nearly equal to that of single channel systems. This is because the channel spacing has increased. Hence, the effect of XPM is negligible if signal channels have appropriate spacing. In simulation experiments for systems with a channel power consumption of 5mw, it is approved that a channel spacing of 100GHz is enough for reducing the effects of XPM. The dispersion penalty caused by XPM can be controlled by adopting dispersion compensation in proper intervals along the system.

5. Four-wave Mixing (FWM)

1) Principle

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Four-wave mixing (FWM), also called four-phonon mixing, occurs in the case that two or three lightwaves with different wavelength interact and cause new lightwaves at other wavelengths. These extra wavelengths are so called mixing products or sidebands. This interaction may occur among signals in multi-wavelength systems and EDFA ASE noises and between main modes and side modes.

In case of 3 signals, the mixing products are shown in Figure 4-17.

Figure 4-17 The mixing products caused by 3-wave interaction

When channel spacing is equal, these products will right enter the adjacent signal channels. If the phase matched condition is reached between the sideband and the initial signal, these two lightwaves propagating along the optical fiber will generate highly efficient FWM.

2) Transmission limitation

Occurrence of FWM sidebands may cause remarkable reduction to the signal power. Even more severely, residual interference occurs when the mixing products directly enter the signal channels. This kind of interference is determined by the interaction between the phases of the signals and the sidebands and indicated by the increase and decrease of the signal pulse amplitude.

Residual loss leads to closure of the eye pattern of the receiver and causes bit-error rate (BER) performance degradation. The effect of FWM can be reduced by the breakdown function of frequency spacing and dispersion and the phase matching among lightwaves. G.652 fiber-optic systems suffer less impairment of FWM than those adopting G.653. On the contrary, if a signal is right located at or near the zero-dispersion point, FWM may surge in a relatively short fiber length (i.e. tens of kilometer). Moreover, FWM is sensitive to the channel spacing.

1 2 3

f113

F123,213f112 f223

F132,312f221

F231,321f332 f331

F F F

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Four-wave mixing may cause severe damage to multi-wavelength systems adopting ITU-T G.652 optical fiber because signals can merely tolerate a very small dispersion. In single channel systems, FWM interaction may occur between signals and ASE noises, as well as the main modes and sidemodes of the transmitters. The ASE phase noises accumulated by optical Kerr effect are superimposed to the signal carriers and cause the broadening of the rear part of the signal spectrum.

3) Method of reducing the effect

As mentioned above, FWM band can be suppressed by utilizing the fiber dispersion such as G.655. FWM damage can be released by arranging uneven channel spacing. To lower the power level of G.653 fiber-optic systems can permit multi-wavelength operation, but this will weaken the advantages of the optical amplifiers.

To properly suppress the generation of mixing products, a scheme has been proposed (an existing recommendation or new recommendation for future study) to adopt the optical fiber with a minimum permissible dispersion (non-zero dispersion) in the amplification bandwidth of EDFA. It is also a possible scheme to use the non-zero dispersion optical fiber of inverse dispersion characteristic as replacement section. However, this replacement may encounter difficulties during installation, operation and maintenance because of the introduction of another kind of fiber. Some similar methods are discovered to adopt long fiber sections of limited dispersion and short fiber sections of inverse but relatively greater dispersion (for compensation).

A scheme has been proposed to adopt uneven and relatively large channel spacing to reduce the nonlinear effects and allow to arrange DWDM systems in G.653 fiber to reduce the effect of FWM. To use uneven channel spacing can guarantee that the mixing products caused by three or more channels won't fall into the wavelengths of other channels. However, the power transfer from the signals to the mixing products (i.e. power loss of the signals) keep fixed due to the configuration of uneven channel spacing, and will still lead to remarkable closure phenomenon of the eye pattern. Increase of the channel spacing can also reduce the effect of FWM. This kind of remission technology may be restricted since the gain spectrum will be narrowed due to the cascading of optical amplifiers and the amplification spectrum will be narrowed due to the access of optical amplifiers.

Thinking:

At 1550nm window and among three types of optical fiber: G.652, G.655 and G.653, which fiber has the most severe FWM effect? why?

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6. Polarization Mode Dispersion (PMD)

1) Principle

As we know, the fundamental mode in a circular symmetric dielectric waveguide is dual-degenerate. In a physical optical fiber, this degeneration is separated by birefringence. For polarization-maintaining fibers, birefringence is deliberately introduced. However, for general communication optical fibers, birefringence is an unexpected product which is randomly introduced due to the stress perturbation the fiber suffers.

For birefringent optical fibers, the first term generates a group delay time called polarization dispersion. This kind of polarization dispersion leads to a group delay difference between the orthogonal polarization states, as shown in Figure 4-18.

Figure 4-18 Occurrence of group delay between the orthogonal polarization states

Although PMD effect randomly changes the polarization state of pulses propagating in optical fiber, a pair of orthogonal states or primary states can be determined, i.e. the signal incident to the fiber at the input end keeps its polarization state at the output end. For the first term, these states are independent to the wavelength. However, in some cases the occurrence of the primary states may be related to the wavelength. This, together with the dispersion of the optical fiber, will lead to further degradation.

Birefringence of optical fiber is randomly introduced due to factors such as stress, bending, twisting and temperature. Random birefringence mechanism redetermines the local birefringence axis along the optical fiber and leads to the coupling between polarization modes. The fiber length between this change is called coupling length. The coupling length of an optical fiber refers to the sum of the average value of total local coupling length.

2) Transmission limitation

Slow in propagation

Fast in propagation

Differential group delay

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In digital transmission systems, PMD leads to intersymbol interference. When the total dispersion is equal to 0.4T (T is the bit period), an optical power penalty of about 1dB is introduced. Current study shows that, optical fibers or optical fiber cables are apt to be standardized according to mean PMD, as well as digital transmission systems. It is predicted via computer simulation that the probability for the optical power penalty of a system to exceed 1dB is less than 10-9 if the mean PMD isn't greater than 0.1T.

In a long distance amplified systems employing polarization scrambler (a component which deliberately modulates the polarization state of the laser and makes it work in an unpolarized state), PMD leads to the increase of signal polarization. The interaction between polarization dependent loss and polarization hole-burning causes the degradation of system performances. When additional polarization dependent loss occurs in the system, greater secondary loss will be aroused.

The secondary effect may generate coupling between PMD and dispersion and increase the statistical component of the dispersion. This field is under study.

3) Method of reducing the effect

Since the problem is caused by birefringence, all the efforts for reducing the effects of PMD are related to reducing the birefringence introduced during optical fiber cable manufacturing, such as optimizing optical fiber manufacturing, guaranteeing the concentration of optical fiber, reducing the residual of fiber core and employing accurate cable structure. Typical mean PMD of optical fiber cables is in the following range:

0 < ( ) < 0.5ps/ km

Another method is to add polarization controllers at the input end and the output end. A polarization splitter is connected after the output polarization controller and used to generate an error signal. The output polarization controller searches this error signal and readjusts the polarization controller to minimize the error signal. At the zero-error signal point, the input polarization state is the primary state of the system. This technology has been used to compensate a 5Gbit/s system. Coherent frequency division multiplexing systems also adopt similar technology.

7. Polarization Dependent Loss (PDL)

1) Principle

Polarization dependent loss is caused by dichromatism of optical passive components such as isolator and coupler. When a signal passes through a dichromatic component, its electric field part parallel to the loss axis will suffer

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certain attenuation. Like PMD, the axis direction which determines the PDL is randomly changed.

2) Transmission limitation

In amplified systems, the amplifiers operate in the power conservative mode. The signal and the noise are affected by PDL. However, the signal and the noise suffer different effects because the noise is unpolarized. The noise can be divided into a component parallel to the signal and another one orthogonal to the signal. Optical amplification may increase the component orthogonal to the signal. Additionally, variations of the signal polarization lead to mode dispersion. Thus, magnitude of the orthogonal component of the noise is time-varied. This will reduce the signal-to-noise ratio at the receiver end and cause impairment to the system.

3) Methods of reducing the effects

For PMD, it is important to reduce the polarization mode dependent loss of the components. To be pointed out, the effect of polarization mode dependent loss increases with the number of the amplifiers. For example, this requirement is extremely strict in long distance submarine systems. In short distance systems with only several amplifiers, the effect of polarization mode dependent loss is for further study.

8. Polarization Hole-burning (PHB)

1) Principle

Polarization hole-burning (PHB) is the result of the anisotropic saturation caused by the polarization saturated signal light incident in the erbium-doped optical fiber. This will reduce the options of stimulated states utilizing the polarization field to locate. Hence, the available gain in the orthogonal direction is relatively large. Although erbium ions are randomly distributed in the glass fiber rod material, dipoles related to the erbium ions are anisotropic in the micro level. When the linear polarization saturated signal is equidirectional to the primary axis of the dipoles, the polarization hole-burning has the greatest effect. However, when the polarization state of the saturated signal is elliptical or circular, its effect decreases. Because the total differential gain is the vector sum of these two effects, both the signal laser and the pumping laser will affect the total effect. The degree of hole-burning is in direct proportion to the polarization. Unpolarized saturated signals have no hole-burning problem. This case, as a whole, is similar to the case of circular polarization signal.

2) Transmission limitation

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Because it makes the noise formed along the link larger than the noise budget calculated according to the simple linear theory, PHB will affect the performances of the system. The effects are that the signal-to-noise ratio decreases due to the PHB and that the ultimately measured Q value fluctuates under PMD and PDL. Since there are two factors affecting PHB, there are two ways to affect the system performances. The total effect is in direct proportion to the gain saturation and increases with the saturation.

Firstly, we consider the effect of the polarized pumping laser. To reach the purpose of the discussion, the pumping polarization can be assumed as fixed. Pumping causes differential gain in the orthogonal polarization axis direction. The noise orthogonal to the pump is greater than the noise equidirectional to the pump. However, the polarization axes of the pumping lasers of the amplifiers along the link are incoherent to each other. The accumulation effect shall be similar to a random walk. The pump which results in PHB can be regarded as a related factor to the PDL of an amplifier. Hence, the noise obtained by averaging the number of the amplifiers should be linear, the same as the budget calculated by the simple linear theory.

Signal lasers which cause PHB are slightly different. The lasers are used to propagate signals, so the polarization noise equidirectional to a signal laser obtains the same gain as the signal. However, the noise orthogonal to the signal laser is always orthogonal to the polarization axis of the signal. Hence, the signal increases in a nonlinear mode along the amplified link.

The total differential gain caused by PHB will change with the variations of the signal polarization state along the amplified link (caused by PMD). It changes because the hole-burning effect of the signal is related to the pump effect. When staying in their corresponding polarization states, the signal laser and the pumping laser will change the amplitude of the differential gain variation. Hence, although this makes the total noise increase in a nonlinear form, the noise may be time-varied. As mentioned above, the signal-to-noise ratio will decrease and be time-varied.

3) Method of reducing the effect

There are several methods for reducing the effect of PHB. It is a feasible method to amplify in the small-signal area, but it is not always possible. In many cases, it can meet the demands. Actually, the simplest method is to adopt unpolarized signals which can be generated via many approaches. The most common approach is to adopt polarization scramble to generate signal. If a phase modulator is used, the polarization state will change between the two orthogonal states at all time. Thus, the signal seems to have no polarization.

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This indicates that it's better to arrange the polarization modulation according to a double bit rate because the PDL in the amplifier will be converted from polarization modulation to amplitude modulation. By adopting double bit rate polarization modulation, the amplitude fluctuation stays at the rate above the bandwidth of the detector and is not sensible to the receiver. If this technology is used, the performances of very long distance systems will be improved and reach the expected purpose of high reliability. Polarization modulation has become the standard implementation method for overseas large systems.

However, in long distance amplified systems, PMD will result in secondary polarization and cause PHB which leads to the performance degradation of the systems. This effect improves the complex properties of the interaction of polarization effects in amplified links.

4.4 DWDM Network Protection

Since the load of DWDM systems is large, reliability is especially important.

There are two major protection modes for point-to-point line protection. One, based on single wavelength, is 1+1 or 1:N protection implemented on the SDH layer. The other is based on optical multiplex section protection and protects the multiplexed signals simultaneously in the optical path. This kind of protection is also called optical multiplex section protection (OMSP). In addition, there are other protections based on ring networks.

4.4.1 Protection Based on single Wavelength

1. 1+1 protection based on single wavelength and implemented on SDH layer

Tx1w

Tx1p

Tx2w

Tx2p

Txnw

Txnp

MUX

MUX

LA

LA

WDM systemworking system

WDM systemworking system

Rx1w

Rx1p

Rx2w

Rx2p

Rxnw

Rxnp

DMUX

DMUX

Note: Here SDH equipment is ADM

W: Working channel P: Protection

channel

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Figure 4-19 1+1 protection based single wavelength and implemented on the SDH layer

This protection system mechanism is similar to the 1+1 MSP of SDH system. All the system equipment needs backup, such as SDH terminal, multiplexer/de-multiplexer, optical line amplifier and optical fiber cable line. The SDH signals are permanently bridged to the working system and the protection system. At the receiver end, the status of the SDH signals received by the two DWDM systems are monitored and the more appropriate signal is chosen. This method has high reliability, but its cost is relatively high.

In a DWDM system, switching of each SDH channel isn't related to other channels, i.e. when Tx1 of DWDM system 1 fails and switches to DWDM system 2, Tx2 can still operate in DWDM system 1. Once a switching condition is detected, the protection switching must be completed within 50ms.

2. 1:n protection based on single wavelength and implemented on SDH layer

DWDM system can implement 1:N protection based on single wavelength and implemented on SDH layer. As shown in Figure 4-20, Tx11, Tx21 and Txn1 share a protection section and, together with Txp1, form a 1: n protection relationship. Tx12, Tx22 and Txn2 share a protection section and, together with Txp2, form a 1: n protection relationship. And so on, Tx1m, Tx2m and Txnm share a protection section and, together with Txpm, form a 1: n protection relationship. The SDH multiplex section protection (MSP) monitors and measures the status of received signals and conducts bridge and selection for the appropriate SDH signals from the protection section.

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Figure 4-20 1:n protection based on single wavelength and implemented on SDH layer

In a DWDM system, switching of each SDH channel isn't related to other channels, i.e. when Tx11 of DWDM system 1 fails and switches to DWDM protection system

1, Tx12, Tx13匱 x1m can still operate in DWDM working system 1. Once a

switching condition is detected, the protection switching must be completed within 50ms.

3. 1:n protection based on single wavelength within the same DWDM system

Consider a DWDM line which can carry multiple SDH channels. The idle wavelengths in the same DWDM system can function as protection channels.

Figure 4-21 1:n protection based on single wavelength within the same DWDM system

Tx11

MUX LAWDM working system 1

Tx12

Tx1m

Rx11

DMUX Rx12

Rx1m

Tx21

MUX LATx22

Tx2m

Rx21

DMUX Rx22

Rx2m

Txn1

MUX LAWDM working system n

Txn2

Txnm

Rxn1

DMUX Rxn2

Rxnm

Txp1

MUX LA

WDM working system p

Txp2

Txpm

Rxp1

DMUX Rxp2

Rxpm

N o t e : H e r e t h e S D H

e q u i p m e n t i s A D M

W D M w o r k i n g

s y s t e m 2

Tx1

MUX LA

WDM working system

Tx2

Rx1

DMUX

Rx2

Txn

Txp1

Rxn

Rxp1

Note: Here the SDH equipment is ADM

Working system

Working system

Working system

Working system

Working systemWorking system

Working system

Working system

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Figure 4-21 shows a DWDM system of n+1 wavelength channels with n working wavelengths and 1 protective wavelength as protection system. However, in practical systems, the reliability of optical fibers and optical fiber cables is worse than that of the equipment. So practically, it does not mean much to provide protection only for the system instead of the line.

Once a switching condition is detected, the protection switching must be completed within 50ms.

4.4.2 Optical Multiplex Section Protection (OMSP)

This technology provides 1+1 protection only in the optical path instead of the terminal line. At the transmit end and the receive end, 1×2 optical splitter and switch are used, respectively. Or other approaches are adopted (such as glowing status which refers to a case that the optical amplifier stays at low bias current and the pumping source at low output and that the launched signal is small and can merely detected for monitoring to determine whether it is in normal operating state). At the transmit end, the multiplexed optical signals are separated while at the receive end they are routed. The features of the optical switch are low insertion loss, transparent to the wavelength amplification region of optical fiber, fast in speed. Moreover, it can be highly integrated or miniaturized.

Figure 4-22 Optical multiplex section protection (OMSP)

Figure 4-22 shows an optical multiplex section protection scheme employing optical splitter and optical switch. In this protection system, only the optical fiber cables and DWDM line systems are backed up, and the SDH terminals and multiplexers in the DWDM system terminal stations aren't backed up. In practical systems, an N: 2 coupler can be used to replace the multiplexer and the 1: 2

Tx1

MUXTx2

Txn

1:2 optical splitter

Rx1

DMUXRx2

Rxn

1:2 optical switch

LA

WDM working line 2

Protection system

LAWDM working line 1

Working system

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splitter. When compared to 1+1 protection, this reduces the cost. OMSP has practical meaning only for two independent optical fiber cables.

4.4.3 Applications in Ring Networks

DWDM systems can also be used to form ring networks. One application is to connect the point-to-point DWDM systems based on single wavelength to form a ring, as shown in Figure 4-23. On the SDH layer, 1: n protection is implemented. The SDH system must adopt ADM equipment.

Figure 4-23 A ring formed by point-to-point DWDM systems

In the protection system shown in Figure 4-24, path protection ring and MSP protection ring of the SDH system can be implemented. The DWDM system only provides "virtual" optical fibers. The protection for each wavelength on SDH layer is independent to the protection mode of other wavelengths. This ring can be two-fiber or 4-fiber.

WDM terminal

WDM terminalWDM terminal

WDM terminal

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Figure 4-24 A ring formed by OADMs

To employ OADMs with the add/drop multiplexing capability to form rings is another application mode of DWDM technology in ring networks. At present, ring networks formed by OADMs can be classified into two modes.

One is wavelength path protection based on single wavelength protection, i.e. 1+1 protection of single wavelength which is similar to the path protection of SDH system.

The other is line protection ring which protects the signals of multiplexed wavelengths. When a fiber is cut off, the "loop back" function can be implemented in the two nodes near the fiber cutoff. Thus all the services are protected. This is similar to the MSP of SDH. From the aspect of specific forms, the line protection ring can be divided into two-fiber bi-directional ring and two-fiber unidirectional ring, and four-fiber bi-directional ring can also be formed. In a two-fiber bi-directional ring, half of wavelengths operate as working wavelength and others as protection wavelength.

4.5 Analysis to the Examples

4.5.1 Networking Diagram (Physical Network)

In a practical network shown in the figure below, there are 14 stations: A, B, C, D, E, F, G, H, I, J, K, L, M and N, where A, E and N add/drop services while other stations don't. Distances between the stations are shown in Figure 4-25.

OADM

OADM OADM

OADM

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Figure 4-25 The physical networking diagram of a network

4.5.2 Networking Diagram (considering the dispersion limited distance of the laserlaser to divide the regenerator sections of the network)

The dispersion limited distance of a laser depends on its modulation mode. Generally, the maximum dispersion limited distance of an EA laser can be up to 640km and that of an M-Z external modulated laser can reach 1000km (The line width of M-Z laser is too narrow, unfavorable for overcoming the nonlinear effects of optical fiber). Here we divide the regenerator sections of the network in terms of EA laser which are most commonly used in practical engineering.

We analyze the distance between two adjacent stations which have services to add/drop, as shown in Figure 4-26. The distance between A and E is 386km, meeting the dispersion limited distance requirement of the EA laser. The distance between E and N is 1002km. It doesn't meet the EA laser dispersion limited distance requirement of less than or equal to 640km. So it is necessary to divide the distance between E and N. This distance can be divided into two or three shorter regenerator sections. Here we change the optical amplifier station I between E and N into an electrical regeneration station and divide the regenerator section between E and N into two regenerator sections: E---I and I---N, where the distance of regenerator section E---I is 453km and I---N is 549km. After division, the distances meet the EA laser dispersion limited distance requirement of less than or equal to 640km. The networking is shown in Figure 4-26.

A B C D E FG

H

IJKLMN

54KM 97KM 125KM 110KM 131KM86KM

138KM

98KM176KM60KM110KM75KM128KM

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Figure 4-26 Networking diagram which considers the dispersion limited distance of the lasers

4.5.3 Networking Diagram (considering the power of optical amplifiers to divide the optical regenerator sections)

According to the related ITU-T recommendations, the regenerating distances between adjacent DWDM stations can be specified as four types: 80km, 100km, 120km and 160km. The 160km standard is adopted only when no line amplifier (optical regenerating station) is used. In the cases that line amplifiers are employed,, the recommended distance is generally less than 120km (33dB). However, networking modes greater than 120km (33dB) can still be adopted as long as the specification such as power of optical amplifiers and OSNR meet the requirements. In applications with line amplifiers, the total launched power of the optical amplifiers is generally not greater than +20dBm (for an 16-wavelength system, the power is +8dBm in each channel), the received power of the preamplifiers isn't less than -30dBm in each channel and the distance between two adjacent stations (for an 16-wavelength system) shouldn't less than 139km (38dB/0.275DB/km).

In Figure 4-26, the distance between stations I and J is 176km and exceeds the requirement of the optical amplifier. The distances between other adjacent stations almost meet the requirement. So it is necessary to add an optical regeneratingregeneration station between I and J, dividing I----J into two regenerator sections. As shown in Figure 4-27, the requirement of the amplifier is met after station X is added.

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Figure 4-27 Network diagram considering the power of the optical amplifiers

4.5.4 Networking Diagram (considering OSNR)

Networking modes are related to the OSNR requirement of networks. Via calculation, the OSNR values at the receive ends of the regenerator sections after networking according to Figure 4-26 are, in the direction of A---E-----I-----N, as follows

Station E: OSNR=21.8dB

Station I: OSNR=20.4dB

Station N: OSNR=20.7dB

They meet the OSNR requirement of the network. So the network structure keeps unchanged, as shown in Figure 4-28.

(When the OSNR doesn't meet the network requirement, the regenerator sections should be redivided referring to the dispersion limited distance of the lasers and eventually make the network to meet the requirements of dispersion limited distance, OSNR and optical power budget. Although the nonlinearity of optical fiber is also one of the factors which limit the optical transmission, this limitation can be completely negligible under the application conditions meeting the recommended specifications)

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Figure 4-28 Networking diagram considering OSNR