CHAPTER 2 INTELLIGENT OPTICAL NETWORK AND...
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.