CHAPTER 2 INTELLIGENT OPTICAL NETWORK AND...

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20 CHAPTER 2 INTELLIGENT OPTICAL NETWORK AND COMPONENTS The explosive growth of internet traffic, audio/video streaming and mobile applications has led to a dramatic increase in the demand for transmission bandwidth, imposing a requirement for high speed broadband networks. Figure 2.1 shows the growth of IP traffic over years. Within a period of 5 to 6 years, it had multiplied more than 10 times. This bandwidth- hungry scenario created by both service providers and end users stimulates the development of novel components and network architectures which should be capable of transmitting data at high bit-rates. The physical layer of such network should be capable of providing bandwidth on-demand, and, since the traffic may change in time, the provision of the bandwidth should be made reconfigurable. Figure 2.1 Growth of IP traffic (CISCO)

Transcript of CHAPTER 2 INTELLIGENT OPTICAL NETWORK AND...

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CHAPTER 2

INTELLIGENT OPTICAL NETWORK AND

COMPONENTS

The explosive growth of internet traffic, audio/video streaming and

mobile applications has led to a dramatic increase in the demand for

transmission bandwidth, imposing a requirement for high speed broadband

networks. Figure 2.1 shows the growth of IP traffic over years. Within a

period of 5 to 6 years, it had multiplied more than 10 times. This bandwidth-

hungry scenario created by both service providers and end users stimulates

the development of novel components and network architectures which

should be capable of transmitting data at high bit-rates. The physical layer of

such network should be capable of providing bandwidth on-demand, and,

since the traffic may change in time, the provision of the bandwidth should be

made reconfigurable.

Figure 2.1 Growth of IP traffic (CISCO)

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The growing popularity of fiber as an efficient transmission

medium in terms of achievable data rate and high speed connectivity made

Optical networks and its related technologies as the underlying

infrastructure for transporting all these high bandwidth services.

This chapter gives an overview of first generation network to third

generation intelligent optical network. It also discusses about the wavelength

division multiplexing technology of concern for the next generation optical

network. The various technologies related to the design of optical switches

and reconfigurable add drop multiplexers which are the key elements in an

intelligent optical network are presented.

2.1 INTELLIGENT OPTICAL NETWORKS

2.1.1 First Generation Optical Network

First generation optical networks use optical fiber as the

transmission medium. It is just a replacement for copper cable for

transmission at higher bit rates over longer distances. They are single

wavelength systems where all the switching, processing, and routing

functions are performed using electronic equipment. First-generation optical

networks include synchronous optical network (SONET), synchronous digital

hierarchy (SDH) networks, and asynchronous transfer mode (ATM) networks.

In early 1988, synchronous optical networking (SONET) and

synchronous digital hierarchy (SDH) were the emerging backbone fiber

standards of all future telecommunications networks. Both SONET and SDH

were seen as the solution for carriers in developing multiplexing standards

and techniques to support the network. These form the heart of the

telecommunications infrastructure in North America, Europe, and Japan,

respectively. The architecture of SONET is shown in Figure 2.2 (Rajiv

Ramaswami et al 2010).

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Figure 2.2 First generation SONET architecture (Rajiv Ramaswami et al 2010)

First-generation networks also include a variety of enterprise

networks, such as enterprise system connection (ESCON), high-performance

parallel interface (HIPPI) networks, and fiber channel networks, which are

used for computer interconnections with other computers and peripheral

systems. First-generation optical networks also include fiber distributed data

interface (FDDI) networks, widely deployed in local area networks (LANs),

and metropolitan area networks (MANs). The main drawback of these

systems is that the switching and routing functions that were performed by

electronics which is a bottleneck to the speed of the network. Further,

SONET/SDH uses time division multiplexing (TDM) which limits the

capacity of the network. This is the motivation for moving to second

generation wavelength routing network (Bates 2001).

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2.1.2 Second Generation Optical Network

SONET and SDH standards were designed originally for the TDM

systems prevalent in the 1980s. Using TDM, a data stream at a higher bit rate

is generated directly by multiplexing lower-bit-rate channels. High-capacity

TDM systems operate at levels up to OC-192, or 10 Gbps. The problem

comes with moving to higher bandwidth speeds at OC-768 and above.

In 1997, wavelength-division multiplexing (WDM) became the

technology of the future which has the benefit that multiple wavelengths

could add to the capacity of fiber-based networks. Many wavelengths of light

increase the capacity of the installed fiber to 320 Gbps and reached 2.6

terabits per second (Tbps) and beyond at present. In future it will be extended

to 100 Tbps. WDM can carry multiple data bit rates, enabling multiple

channels to be carried on a single fiber. The technique uses different

wavelengths of light down the same fiber to carry different channels of

information, which are then separated out at the receiver that identifies each

wavelength.

The drawback of OEO conversion required in the nodes of the first

generation optical network is alleviated by the introduction of optical layer.

The International Telecommunication Union (ITU) has defined a new layer

called the optical layer that provides lightpaths to the higher layers.

A lightpath is an end-to-end connection established across an optical network.

The optical layer is a server layer that provides services to other client layers.

This optical layer provides lightpaths to a variety of client layers such as

include IP, Ethernet, and SONET/SDH, as well as other possible protocols

such as Fiber Channel.

Second-generation optical networks have routing, switching, and

intelligence in the optical layer. Optical networks based on this paradigm are

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now being deployed. The architecture of such a network is shown in the

following Figure 2.3.

Figure 2.3 Second generation wavelength-routing network

(Rajiv Ramaswami et al 2010)

The key network elements that enable optical networking are

optical line terminals (OLTs), optical add/drop multiplexers (OADMs), and

optical crossconnects (OXCs). However these components perform switching

and routing in optical domain, they tend to have fixed wavelength

provisioning and routing and the network is not flexible because of the non

availability of tunable all optical components in the network.

2.1.3 Third Generation Intelligent Optical Network

The drawback of static wavelength provisioning and routing in

second generation optical network is overcome with emergence of

reconfigurable components which offers flexibility to the third generation

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optical network. This makes the third generation optical network more

intelligent. These networks are commonly referred to as intelligent optical

networks.

In the past, transport networks with centrally planned and

provisioned circuits, required databases with perfect knowledge of network

and available bandwidth. But this approach required too much time to provide

new service, which is unacceptable to the new bandwidth on demand services

that are being deployed. A new type of transport network is required, one

based on intelligent, dynamic network elements (Cortez 2002).

As the next generation intelligent optical networking emerges, it

evolves from the existing fixed point-to-point optical links to a dynamic

network, with all-optical switches, ROADMs, OXCs and a new level of

flexibility available at the optical layer.

Intelligent optical network (ION), enabled by high capacity cross

connects with software intelligence, grants new methods for managing high

capacity core optical network. IONs will also provide on-demand service

deployment, allowing for a greater flexibity.

Provisioning of new services in an intelligent optical network needs

integration of reconfigurable add drop multiplexers and control layer

signalling and routing protocol.

The introduction of ROADM technology makes the optical

wavelengths be dynamically switched within an optical network (Figure 2.4).

All traffic no longer needed to pass through an electronic switch fabric to

enable local add/drop as in first generation network or wavelength add drop is

not fixed in nature as in second generation network. Any wavelength can be

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added or dropped depending upon the service requirement which is the major

benefit of third generation intelligent optical network.

Figure 2.4 Third generation Intelligent Optical Network

2.2 WAVELENGTH DIVISION MULTIPLEXING

Wavelength division multiplexing is an optical technique that

combines multiple, unique optical signals at different wavelengths onto a

single fiber. At the receiving end, these optical signals are demultiplexed into

separate fibers. The bandwidth capacity of the fiber is multiplied by the

number of wavelengths multiplexed onto the fiber. Figure 2.5 shows the

WDM technique. The variations of WDM that are commonly used are Coarse

Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division

Multiplexing (DWDM).

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Figure 2.5 Wavelength Division Multiplexing

2.2.1 Coarse Wavelength Division Multiplexing

Coarse Wavelength Division Multiplexing is a standardized WDM

technology that uses up to 20 different wavelengths for data transmission over

a single fiber. The channel spacing is 20nm including C-band, O-Band, E-

Band and S+L–Band as shown in Figure 2.6. CWDM uses a ‘coarse’

wavelength grid, so the underlying optical component technology is simpler.

This makes CWDM systems very cost-effective, but also limits them in terms

of total capacity. CWDM system is widely used in the metro and access

network. The CWDM system is scalable, but the scalability is limited.

2.2.2 Dense Wavelength Division Multiplexing

Dense wavelength division multiplexing combines multiple optical

signal that can be amplified as a group and transported over a single optical

fiber to increase the capacity. The ITU channel grid for DWDM in C-band is

shown in Figure 2.6. Practical deployments of today’s DWDM are spaced at

100 GHz frequencies (or channel spacing of 0.8nm), which allow about 40

wavelengths in the C-band. One key advantage of DWDM is that the gain

region of Erbium-Doped Fiber Amplifiers (EDFAs) is also in the C-band,

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which enables all the wavelengths to be amplified to overcome loss over long

spans of fiber and/or high passive losses. DWDM technology has been used

in long-haul telecommunication systems successfully, which satisfies with the

need of the bandwidth in the backbone (long-haul network). Deployments of

40-channel systems are common, with a few channels being used now and the

rest left dark for future upgrades.

DWDM promises to solve the "fiber exhaust" problem and is

expected to be the central technology in the next generation optical networks

of the future. DWDM and reconfigurable optical add/drop multiplexer

(ROADM) systems are the linchpins of next generation intelligent optical

networks.

Figure 2.6 Wavelength grids of DWDM and CWDM

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2.3 KEY COMPONENTS OF INTELLIGENT OPTICAL NETWORK

2.3.1 High Speed Optical Switch

Optical switches are one of the key components in an optical

network which finds application in protection switching, provisioning of

lightpaths, packet switching applications which requires ultra high speed

switching. As for optical switches, apart from switching speed several other

parameters need to be considered as follows

i. Insertion loss

This is the fraction of signal power that is lost because of the

switch. This loss is usually measured in decibels. The insertion loss should be as small as possible.

ii. Crosstalk

This is the ratio of the power at a specific output from the desired

input to the power from all other inputs. Crosstalk should be as small as

possible.

iii. Extinction ratio

This is the ratio of the output power in the on-state to the output

power in the off-state. This ratio should be as large as possible.

iv. Polarization-dependent loss (PDL)

The switch is said to have polarization-dependent loss, if the loss of

the switch is not equal for both states of polarization of the optical signal. It is desirable that optical switches have low PDL.

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v. Scalability

Scalability refers to the ability to build switches with large port

counts that perform satisfactorily.

vi. Compact size

The switch must have short device length in order to get compact

module size and to implement system on chip, low power consumption and

higher switching speed.

A detailed review on different optical switching technologies is

given below.

a. Opto Mechanical switch

Opto-mechanical switches are the oldest type of optical switch that

are most widely deployed earlier. These devices achieve switching by moving

fiber or other bulk optic elements such as prisms or mirrors to redirect a light

beam to another port. This causes them to be relatively slow with switching

times in the 10-100 ms range. One type of opto mechanical switch inserts and

retracts a reflective surface into a light stream to redirect it to another port and

other architecture redirects the light stream by bending a grating written fiber

(Nagaoka et al 1997, Toshiyoshi and Fujita 1996). Micro mechanical

switches generally have low insertion loss (around 0.6 dB), low channel

crosstalk (around -60dB) and good extinction ratio. However, the

fundamental drawback is the durability of the moving parts.

b. Micro-Electro-Mechanical switch

Micro-Electro-Mechanical System devices (MEMS) can be

considered as a subcategory of optomechanical switches, however, because of

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the fabrication processes and miniature nature of the devices, they have different characteristics, performance and reliability concerns.

MEMS are assemblies of tiny mechanical components fabricated

by depositing layers on a substrate, then etching away selected material using

standard photolithographic technology. A variety of MEMS devices have

been developed for optical switching that share common features. Typically

movable components are suspended on flexible structures above a base layer.

Either electrostatic or magnetic forces between the base and the elevated

components move the structures. Electrostatic forces that create different charges

are easier to control and are more widely used, but magnetic forces are stronger.

There are currently two popular approaches to implement MEMS

optical switches namely 2D MEMS switches and 3D MEMS switches. In 2D

MEMS architecture mirrors are arranged in a crossbar configuration as shown in

Figure 2.7. Each mirror has only two positions and is placed at the intersections of

light paths between the input and output ports. They can be in either the ON

position to reflect light or the OFF position to let light pass uninterrupted ( Behin et

al 1998, Lee et al 1999, Marxer et al 1997, Miller et al 1997).

Figure 2.7 Schematic of 2-D MEMS switch (Ming C. Wu et al 2006)

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In 3D MEMS a connection path is established by tilting two

mirrors independently to direct the light from an input port to a selected

output port as shown in Figure 2.8. The drawback of this approach is that a

complex and very expensive feedback system is required to maintain the

position of the mirrors during external disturbances or drift ( Yamamoto et al

2003, Zheng et al 2003) .

Figure 2.8 Schematic of 3-D MEMS switch(Ming C. Wu et al 2006)

Several works based on MEMS 2-D switch (2 × 2) has been

reported (Miller et al 1997, Marxer et al 1997, Marxer et al 1999). There are

different approaches for the actuation of the micromirror. The first method is

based on the rotation of the micromirror. The micromirror is initially parallel

to the substrate (OFF position). The micromirror can be actuated to on state

by rotating the mirror to the vertical position (Toshiyoshi and Fujita 1996, Lin

et al 1998). The second approach moves the vertical micromirrors in and out

of the optical paths without changing the mirror angle (Miller et al 1997,

Marxer et al 1997, Marxer et al 1999, Kuo et al 2004).

Even though MEMS switches are in micro scale size, have moving

parts controlled by electronics. Hence reliability of MEMS optical switches is

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serious issue. Further, large amount of electrical connections for the

micromirrors are required which leads to more crosstalk. Also, the switching

time is in the order microseconds only.

c. Liquid Crystal Switch

Liquid crystals are more widely used in displays. It also finds

application in optical switching. Liquid crystal is a thin layer between a pair

of parallel glass plates. Liquid crystals switches works by processing

polarization states of light (Riza et al 1999, Yang et al 2003). A liquid crystal

switch is shown in Figure 2.9.

Figure 2.9 Liquid crystal switch

The working principle of a liquid crystal optical switch is based on

the change of polarization state of incident light by a liquid crystal by the

application of an electric field over the liquid crystal. The change of

polarization in combination with polarization selective beam splitters allows

optical space switching. In order to make the devices polarization insensitive,

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some kind of polarization diversity must be implemented, which makes the

technology more complex.

d. Thermo-optic Switches

The operation of thermo optic switch is based on the thermo optic

effect. The refractive index of the material is varied, due to the temperature

variation of the material itself (Diemeer et al 1989, Suijten et al 1994,

Tapalian et al 2002). There are two main categories of thermo optical

switches: interferometric and digital optical switches. In interferometric

switches, the refractive index of the waveguide arms on the substrate use

temperature control to change index of refraction properties of Interferometer

based waveguide arms on the substrate.

The structure of an interferometric thermo optic switch is shown

in Figure 2.10. The light is processed by waveguide interaction and is guided

through the appropriate path to the desired port. The relative phase of the light

in the two parallel guides determines from which output port the light

emerges. Signals are switched or modulated by varying the refractive indices

of the two parallel guides relative to each other, either by changing the

temperature of one guide while the other remains constant, or by changing the

two simultaneously in opposite directions. Interference is constructive or

destructive, as the power alternate outputs is minimized or maximized,

respectively. The output port is thus selected.

Thermo optic switch suffers from low operating speed due to the

intrinsic heating mechanism. Further the switching speed of thermo optic

switch is in terms of milliseconds, which hinders its operation in many high

speed applications.

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Figure 2.10 Interferometric thermo optic switch

e. Semiconductor optical amplifier switch

Optical switching can be performed using semiconductor optical

amplifiers (SOAs) combined with a passive waveguiding network. The

optical amplifiers serve dual purposes, gating the signal and amplifying the

signal. Amplification is needed in order to offset the losses associated with

the passive waveguide elements as well as the losses from component

misalignments in the switch module (Gustavsson et al 1992, Ehrhardt et al

1993, Dorgeuille et al 1996).

Most of the optical switches using SOAs have focused on making

monolithic switch in the same material containing the SOA. This approach

can help alleviate the misalignment between the passive waveguide routing

part of the devices and amplifiers. However these devices often require

complicated multistep epitaxial growth and complex processing (Fan and

Hooker 2000). In a 2X2 switch structure, four SOAs are needed to perform

the gating and the amplification of the optical signal if enough current is

applied to the SOA. Figure 2.11 shows a SOA switch configuration. An

interesting characteristic of SOA switches is that, they allow amplification of

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the travelling signals, thus making possible to restore signal level besides

routing.

Figure 2.11 2×2 Hybrid SOA switch (Fan and Hooker 2000)

f. Acousto-optic Switch

The acousto-optic switching effect consists in the variation of the

refractive index of a medium, caused by the mechanical strains accompanying

the transit of a surface acoustic wave (Briks et al 1994, Dai Enguang et al

2000, Sapriel et al 2002). This wave can set up a diffraction grating within the

medium. The grating pace can be such to modify the polarization of an optical

signal traveling through the medium. A 2 × 2 switch is obtained using a

polarizing beam splitter, which separates the TE and TM components that are

then routed through two distinct waveguides (Simth et al 1996). If there are

no resonance phenomena along the waveguides, the polarization of light is

unchanged and the signals are recombined at the first output port (bar state). If

an acoustic wave is present, TE and TM components vary their polarization

and the signal is directed to the second output port (cross state), as can be seen

in Figure 2.12. If the incoming signal is multi-wavelength, it is even possible

to switch selectively different wavelengths off the beam, as it is possible to

have several acoustic waves in the material, having different frequencies, at

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the same time. This technology can be used to build also large-size switching

fabrics.

Figure 2.12 Acousto-optic switch (Simth et al 1996)

g. Electro Optic Switch

Figure 2.13 shows an electro-optic switch which use highly

birefringent substrate material and electrical fields to redirect light from one

port to another. The switching is done by the change of refractive index

induced in a waveguide via the electro optic effect when an electric field is

applied across it (Lee et al 1997, Lee et al 2001, Thackara et al 1995). This

effect has been observed in inorganic materials such as LiNbO3, InP, GaAsP

and LiTaO3 as well as organic materials like polymers ( Blistanov et al 1988,

Bachmann et al 1993, Lee et al 2002, Lin et al 2007, Lee et al 2002). The

change in the index of refraction manipulates the light through the appropriate

waveguide path to the desired port. An electro-optic switch is capable of

changing its state extremely rapidly, typically in less than a nanosecond.

Larger switches can be realized by integrating several 2x2 switches on a

single substrate.

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Figure 2.13 Electro-optic switch

2.3.2 Reconfigurable Optical Add Drop Multiplexer

An optical add-drop multiplexer (OADM) is a device used

in wavelength-division multiplexing systems for selectively removing (drops)

a wavelength from a multiplicity of wavelengths in a fiber and then adds in the

same wavelength, but with different data content into the fiber. "Add" and "drop"

here refer to the capability of the device to add one or more new wavelength

channels to an existing multi-wavelength WDM signal, and/or to drop (remove)

one or more channels, passing those signals to another network path.

OADMs are classified as fixed-wavelength and reconfigurable

OADMs. In fixed-wavelength OADM, the wavelength has been selected and

remains the same until human intervention changes it. In dynamically selectable

OADM or reconfigurable OADM, the wavelengths may be dynamically added

or dropped from the fiber through some switching mechanism.

There are several technologies to realize an OADM including thin

film filters, fiber Bragg gratings with optical circulators, free space grating

devices and integrated planar arrayed waveguide gratings. The switching or

reconfiguration functions range from the manual fiber patch panel to a variety

of switching technologies including MEMS, liquid crystal and thermo-optic

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switches in planar waveguide circuits. Some of the optical add drop

technologies are discussed below.

a. Mach-Zehnder Interferometer based OADM

The Mach-Zehnder interferometer based OADM shown in Figure

2.14 comprises two fused 3 dB couplers with the two arms of the Mach-

Zehnder ideally equal. Two identical Bragg gratings are then written

symmetrically in the arms of the Mach-Zehnder. The component is therefore

perfectly balanced. The Bragg gratings are written with a resonant wavelength

equal to 3. The unaffected wavelengths ( 1, 2 and 4) therefore ideally see a

normal Mach-Zehnder interferometer, which splits the light at the first

coupler equally and recombines the light in the second 3 dB coupler. If the

Mach-Zehnder is perfectly balanced, no light will emerge at port C. 3 on the

other hand is also split by the first 3dB coupler but is reflected by the two

identical Bragg gratings. On reaching the first coupler, coherent

recombination occurs and 3 exits the dropped port (Bilodeau et al 1995). The

symmetry of the component lends itself to add 3 to the remaining

wavelengths. By adding more matched gratings with different resonant

wavelengths, several different wavelengths can be added or dropped.

Figure 2.14 Dual Bragg grating Mach-Zehnder interferometer optical

add drop multiplexer (Bilodeau et al 1995)

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b. Arrayed Waveguide Gratings based OADM

For applications requiring high channel counts, Arrayed Waveguide

Grating (AWG) is a very successful filtering device. It is implemented in

integrated planar form, known as Planar Lightwave Circuit (PLC), on the

Silica-on-Silicon platform or alternatively on semiconductor materials (InP).

The configuration of an NxN AWG multiplexer is shown in Figure 2.15.

The device consists of N input/output waveguides, two focusing

slab waveguides and arrayed waveguides with a constant path length

difference between neighbouring waveguides. By launching light in one of

the input waveguides all the arrayed waveguides are excited through the slab

waveguide. After travelling through the arrayed waveguides, the

monochromatic light beam interferes constructively at one focal point in the

second slab. The location of the focal point and consequently the exit

waveguide depends on the wavelength. The main disadvantages of AWG-type

devices are the relatively high insertion loss <-6dB dB and low crosstalk

performance of ~ -25 dB (Hida et al 2000).

Figure 2.15 Schematic of NxN arrayed waveguide grating

(Hida et al 2000)

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c. OADM based on Circulator and Bragg Grating

Different configurations of OADM based on Bragg grating are

available. One of the configurations makes direct use of the reflection

properties of a Bragg grating written in a single mode fiber, with the two

optical circulators as shown in Figure 2.16. This OADM provides low

crosstalk performance and negligible back reflections. However, the main

drawback of this configuration is that the circulators in the OADM suffer

from relatively high insertion loss (~1dB); also it is bulky and expensive and

cannot be easily integrated.

Figure 2.16 Bragg grating based OADM with circulators (Jungho Kim

and Byoungho Lee 2000)

d. OADM based on Fused Coupler and Bragg Grating

This configuration of OADM using selective twin-core coupler and

a single Bragg grating is shown in Figure 2.17. The technique is non

interferometric, and therefore no balancing of optical path length by is

required. In addition, the grating and fused coupler are completely decoupled

and can be made separately. Input channels are fed in from port 2 and coupled

to the core of the twin core (TC) fiber with a high propagation constant before

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seeing the grating. The grating reflects the dropped channel into the core of

the TC fiber with a low propagation constant, heading back to the coupler.

The dropped channel eventually emerges from port 1. The remaining channels

go through the second coupler to emerge at port 4. An added channel can be

fed in from port 3 and is reflected by the same grating to join the transmitted

channels at port 4(Ortega 1998).

Figure 2.17 Fused coupler and fiber Bragg grating OADM (Ortega 1998)

e. Long period fiber grating OADM

Long Period Fiber Grating (LPFG) is a class of fiber grating.

Compared with fiber Bragg grating, the period of grating is more than 100 m.

Long-period fiber grating is capable of coupling light from the guided core

mode of the fiber to selected co-propagating cladding modes at specific

wavelengths. It was first proposed and demonstrated by Vengsarkar et al

(1996), where a periodic index modulation along the core of a single-mode

fiber was induced by UltraViolet (UV) irradiation. The transmission spectrum

of a typical LPFG consists of a series of rejection bands centered at specific

wavelengths, namely the resonance wavelengths, which correspond to

couplings to various cladding modes. LPFGs have been developed into

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couplers by putting two or three parallel identical LPFGs together, which can

be further developed into add/drop multiplexers (Chiang et al 2000, Chiang et

al 2004, Liu et al 2006).

Figure 2.18 Long period fiber grating OADM (Yue-Jing He et al 2006)

Figure 2.18 shows the structure of a LPFG coupler. Although

LPFG offers a number of advantages, including easy fabrication, low

insertion loss and back-reflection, and better wavelength tunability compared

with Fiber Bragg Grating(FBG), it is difficult to fabricate compact devices

suitable for integration and mass production.