Chapter 8 Multichannel Systems In principle, the capacity of optical communication systems can exceed 10 Tb/s be- cause of a large frequency associated with the optical carrier. In practice, however, the bit rate was limited to 10 Gb/s or less until 1995 because of the limitations imposed by the dispersive and nonlinear effects and by the speed of electronic components. Since then, transmission of multiple optical channels over the same fiber has provided a sim- ple way for extending the system capacity to beyond 1 Tb/s. Channel multiplexing can be done in the time or the frequency domain through time-division multiplexing (TDM) and frequency-division multiplexing (FDM), respectively. The TDM and FDM techniques can also be used in the electrical domain (see Section 1.2.2). To make the distinction explicit, it is common to refer to the two optical-domain techniques as op- tical TDM (OTDM) and wavelength-division multiplexing (WDM), respectively. The development of such multichannel systems attracted considerable attention during the 1990s. In fact, WDM lightwave systems were available commercially by 1996. This chapter is organized as follows. Sections 8.1–8.3 are devoted to WDM light- wave systems by considering in different sections the architectural aspects of such sys- tems, the optical components needed for their implementation, and the performance issues such as interchannel crosstalk. In Section 8.4 we focus on the basic concepts behind OTDM systems and issues related to their practical implementation. Subcarrier multiplexing, a scheme in which FDM is implemented in the microwave domain, is discussed in Section 8.5. The technique of code-division multiplexing is the focus of Section 8.6. 8.1 WDM Lightwave Systems WDM corresponds to the scheme in which multiple optical carriers at different wave- lengths are modulated by using independent electrical bit streams (which may them- selves use TDM and FDM techniques in the electrical domain) and are then transmitted over the same fiber. The optical signal at the receiver is demultiplexed into separate channels by using an optical technique. WDM has the potential for exploiting the large bandwidth offered by optical fibers. For example, hundreds of 10-Gb/s channels can 330 Fiber-Optic Communications Systems, Third Edition. Govind P. Agrawal Copyright  2002 John Wiley & Sons, Inc. ISBNs: 0-471-21571-6 (Hardback); 0-471-22114-7 (Electronic) 8.1. WDM LIGHTWAVE SYSTEMS 331 Figure 8.1: Low-loss transmission windows of silica fibers in the wavelength regions near 1.3 and 1.55 µ m. The inset shows the WDM technique schematically. be transmitted over the same fiber when channel spacing is reduced to below 100 GHz. Figure 8.1 shows the low-loss transmission windows of optical fibers centered near 1.3 and 1.55 µ m. If the OH peak can be eliminated using “dry” fibers, the total capacity of a WDM system can ultimately exceed 30 Tb/s. The concept of WDM has been pursued since the first commercial lightwave sys- tem became available in 1980. In its simplest form, WDM was used to transmit two channels in different transmission windows of an optical fiber. For example, an ex- isting 1.3- µ m lightwave system can be upgraded in capacity by adding another chan- nel near 1.55 µ m, resulting in a channel spacing of 250 nm. Considerable attention was directed during the 1980s toward reducing the channel spacing, and multichannel systems with a channel spacing of less than 0.1 nm had been demonstrated by 1990 [1]–[4]. However, it was during the decade of the 1990s that WDM systems were de- veloped most aggressively [5]–[12]. Commercial WDM systems first appeared around 1995, and their total capacity exceeded 1.6 Tb/s by the year 2000. Several laboratory experiments demonstrated in 2001 a system capacity of more than 10 Tb/s although the transmission distance was limited to below 200 km. Clearly, the advent of WDM has led to a virtual revolution in designing lightwave systems. This section focuses on WDM systems by classifying them into three categories introduced in Section 5.1. 8.1.1 High-Capacity Point-to-Point Links For long-haul fiber links forming the backbone or the core of a telecommunication network, the role of WDM is simply to increase the total bit rate [14]. Figure 8.2 shows schematically such a point-to-point, high-capacity, WDM link. The output of several transmitters, each operating at its own carrier frequency (or wavelength), is multiplexed together. The multiplexed signal is launched into the optical fiber for transmission to the other end, where a demultiplexer sends each channel to its own receiver. When N 332 CHAPTER 8. MULTICHANNEL SYSTEMS Tx Tx Tx Figure 8.2: Multichannel point-to-point fiber link. Separate transmitter-receiver pairs are used to send and receive the signal at different wavelengths channels at bit rates B 1 , B 2 , , and B N are transmitted simultaneously over a fiber of length L, the total bit rate–distance product, BL, becomes BL =(B 1 + B 2 + ···+ B N )L. (8.1.1) For equal bit rates, the system capacity is enhanced by a factor of N. An early experi- ment in 1985 demonstrated the BL product of 1.37 (Tb/s)-km by transmitting 10 chan- nels at 2 Gb/s over 68.3 km of standard fiber with a channel spacing of 1.35 nm [3]. The ultimate capacity of WDM fiber links depends on how closely channels can be packed in the wavelength domain. The minimum channel spacing is limited by interchannel crosstalk, an issue covered in Section 8.3. Typically, channel spacing ∆ ν ch should exceed 2B at the bit rate B. This requirement wastes considerable bandwidth. It is common to introduce a measure of the spectral efficiency of a WDM system as η s = B/∆ ν ch . Attempts are made to make η s as large as possible. The channel frequencies (or wavelengths) of WDM systems have been standard- ized by the International Telecommunication Union (ITU) on a 100-GHz grid in the frequency range 186–196 THz (covering the C and L bands in the wavelength range 1530–1612 nm). For this reason, channel spacing for most commercial WDM systems is 100 GHz (0.8 nm at 1552 nm). This value leads to only 10% spectral efficiency at the bit rate of 10 Gb/s. More recently, ITU has specified WDM channels with a frequency spacing of 50 GHz. The use of this channel spacing in combination with the bit rate of 40 Gb/s has the potential of increasing the spectral efficiency to 80%. WDM systems were moving in that direction in 2001. What is the ultimate capacity of WDM systems? The low-loss region of the state- of-the-art “dry” fibers (e.g, fibers with reduced OH-absorption near 1.4 µ m) extends over 300 nm in the wavelength region covering 1.3–1.6 µ m (see Fig. 8.1). The min- imum channel spacing can be as small as 50 GHz or 0.4 nm for 40-Gb/s channels. Since 750 channels can be accommodated over the 300-nm bandwidth, the resulting effective bit rate can be as large as 30 Tb/s. If we assume that the WDM signal can be transmitted over 1000 km by using optical amplifiers with dispersion management, the effective BL product may exceed 30,000 (Tb/s)-km with the use of WDM technology. 8.1. WDM LIGHTWAVE SYSTEMS 333 Table 8.1 High-capacity WDM transmission experiments Channels Bit Rate Capacity Distance NBL Product N B (Gb/s) NB (Tb/s) L (km) [(Pb/s)-km] 120 20 2.40 6200 14.88 132 20 2.64 120 0.317 160 20 3.20 1500 4.80 82 40 3.28 300 0.984 256 40 10.24 100 1.024 273 40 10.92 117 1.278 This should be contrasted with the third-generation commercial lightwave systems, which transmitted a single channel over 80 km or so at a bit rate of up to 2.5 Gb/s, resulting in BL values of at most 0.2 (Tb/s)-km. Clearly, the use of WDM has the po- tential of improving the performance of modern lightwave systems by a factor of more than 100,000. In practice, many factors limit the use of the entire low-loss window. As seen in Chapter 6, most optical amplifiers have a finite bandwidth. The number of channels is often limited by the bandwidth over which amplifiers can provide nearly uniform gain. The bandwidth of erbium-doped fiber amplifiers is limited to 40 nm even with the use of gain-flattening techniques (see Section 6.4). The use of Raman amplification has extended the bandwidth to near 100 nm. Among other factors that limit the number of channels are (i) stability and tunability of distributed feedback (DFB) semiconductor lasers, (ii) signal degradation during transmission because of various nonlinear effects, and (iii) interchannel crosstalk during demultiplexing. High-capacity WDM fiber links require many high-performance components, such as transmitters integrating multiple DFB lasers, channel multiplexers and demultiplexers with add-drop capability, and large-bandwidth constant-gain amplifiers. Experimental results on WDM systems can be divided into two groups based on whether the transmission distance is ∼100 km or exceeds 1000 km. Since the 1985 experiment in which ten 2-Gb/s channels were transmitted over 68 km [3], both the number of channels and the bit rate of individual channels have increased considerably. A capacity of 340 Gb/s was demonstrated in 1995 by transmitting 17 channels, each operating at 20 Gb/s, over 150 km [15]. This was followed within a year by several experiments that realized a capacity of 1 Tb/s. By 2001, the capacity of WDM systems exceeded 10 Tb/s in several laboratory experiments. In one experiment, 273 channels, spaced 0.4-nm apart and each operating at 40 Gb/s, were transmitted over 117 km using three in-line amplifiers, resulting in a total bit rate of 11 Tb/s and a BL product of 1300 (Tb/s)-km [16]. Table 8.1 lists several WDM transmission experiments in which the system capacity exceeded 2 Tb/s. The second group of WDM experiments is concerned with transmission distance of more than 5000 km for submarine applications. In a 1996 experiment, 100-Gb/s transmission (20 channels at 5 Gb/s) over 9100 km was realized using the polarization- scrambling and forward-error-correction techniques [17]. The number of channels was 334 CHAPTER 8. MULTICHANNEL SYSTEMS later increased to 32, resulting in a 160-Gb/s transmission over 9300 km [18]. In a 2001 experiment, a 2.4-Tb/s WDM signal (120 channels, each operating at 20 Gb/s) was transmitted over 6200 km, resulting in a NBL product of almost 15 (Pb/s)-km (see Table 8.1). This should be compared with the first fiber-optic cable laid across the Atlantic ocean (TAT-8); it operated at 0.27 Gb/s with NBL ≈1.5 (Tb/s)-km. The use of WDM had improved the capacity of undersea systems by a factor of 10,000 by 2001. On the commercial side, WDM systems with a capacity of 40 Gb/s (16 channels at 2.5 Gb/s or 4 channels at 10 Gb/s) were available in 1996. The 16-channel system cov- ered a wavelength range of about 12 nm in the 1.55- µ m region with a channel spacing of 0.8 nm. WDM fiber links operating at 160 Gb/s (16 channels at 10 Gb/s) appeared in 1998. By 2001, WDM systems with a capacity of 1.6 Tb/s (realized by multiplexing 160 channels, each operating at 10 Gb/s) were available. Moreover, systems with a 6.4- Tb/s capacity were in the development stage (160 channels at 40 Gb/s). This should be contrasted with the 10-Gb/s capacity of the third-generation systems available before the advent of the WDM technique. The use of WDM had improved the capacity of commercial terrestrial systems by a factor of more than 6000 by 2001. 8.1.2 Wide-Area and Metro-Area Networks Optical networks, as discussed in Section 5.1, are used to connect a large group of users spread over a geographical area. They can be classified as a local-area network (LAN), metropolitan-area network (MAN), or a wide-area network (WAN) depending on the area they cover [6]–[11]. All three types of networks can benefit from the WDM technology. They can be designed using the hub, ring, or star topology. A ring topology is most practical for MANs and WANs, while the star topology is commonly used for LANs. At the LAN level, a broadcast star is used to combine multiple channels. At the next level, several LANs are connected to a MAN by using passive wavelength routing. At the highest level, several MANs connect to a WAN whose nodes are interconnected in a mesh topology. At the WAN level, the network makes extensive use of switches and wavelength-shifting devices so that it is dynamically configurable. Consider first a WAN covering a wide area (e.g., a country). Historically, telecom- munication and computer networks (such as the Internet) occupying the entire U.S. ge- ographical region have used a hub topology shown schematically in Fig. 8.3. Such net- works are often called mesh networks [19]. Hubs or nodes located in large metropoli- tan areas house electronic switches, which connect any two nodes either by creating a “virtual circuit” between them or by using packet switching through protocols such as TCP/IP (transmission control protocol/Internet protocol) and asynchronous transfer mode (ATM). With the advent of WDM during the 1990s, the nodes were connected through point-to-point WDM links, but the switching was being done electronically even in 2001. Such transport networks are termed “opaque” networks because they require optical-to-electronic conversion. As a result, neither the bit rate nor the modu- lation format can be changed without changing the switching equipment. An all-optical network in which a WDM signal can pass through multiple nodes (possibly modified by adding or dropping certain channels) is called optically “trans- parent.” Transparent WDM networks are desirable as they do not require demultiplex- ing and optical-to-electronic conversion of all WDM channels. As a result, they are 8.1. WDM LIGHTWAVE SYSTEMS 335 Figure 8.3: An example of a wide-area network in the form of several interconnected SONET rings. (After Ref. [19]; c 2000 IEEE; reproduced with permission.) not limited by the electronic-speed bottleneck and may help in reducing the cost of installing and maintaining the network. The nodes in a transparent WDM network (see Fig. 8.3) switch channels using optical cross-connects. Such devices were still in their infancy in 2001. An alternative topology implements a regional WDM network in the form of sev- eral interconnected rings. Figure 8.4 shows such a scheme schematically [20]. The feeder ring connects to the backbone of the network through an egress node. This ring employs four fibers to ensure robustness. Two of the fibers are used to route the data in the clockwise and counterclockwise directions. The other two fibers are called protec- tion fibers and are used in case a point-to-point link fails (self-healing). The feeder ring supplies data to several other rings through access nodes. An add–drop multiplexer can be used at all nodes to drop and to add individual WDM channels. Dropped channels can be distributed to users using bus, tree, or ring networks. Notice that nodes are not always directly connected and require data transfer at multiple hubs. Such networks are called multihop networks. Metro networks or MANs connect several central offices within a metropolitan area. The ring topology is also used for such networks. The main difference from the ring shown in Fig. 8.4 stems from the scaling and cost considerations. The traffic flows in a metro ring at a modest bit rate compared with a WAN ring forming the backbone of a nationwide network. Typically, each channel operates at 2.5 Gb/s. To reduce the cost, a coarse WDM technique is used (in place of dense WDM common in the back- bone rings) by using a channel spacing in the 2- to 10-nm range. Moreover, often just two fibers are used inside the ring, one for carrying the data and the other for pro- tecting against a failure. Most metro networks were using electrical switching in 2001 although optical switching is the ultimate goal. In a test-bed implementation of an opti- cally switched metro network, called the multiwavelength optical network (MONET), several sites within the Washington, DC, area of the United States were connected us- 336 CHAPTER 8. MULTICHANNEL SYSTEMS Figure 8.4: A WDM network with a feeder ring connected to several local distribution networks. (After Ref. [20]; c 1999 IEEE; reproduced with permission.) ing a set of eight standard wavelengths in the 1.55- µ m region with a channel spacing of 200 GHz [21]. MONET incorporated diverse switching technologies [synchronous digital hierarchy (SDH), asynchronous transfer mode (ATM), etc.] into an all-optical ring network using cross-connect switches based on the LiNbO 3 technology. 8.1.3 Multiple-Access WDM Networks Multiple-access networks offer a random bidirectional access to each subscriber. Each user can receive and transmit information to any other user of the network at all times. Telephone networks provide one example; they are known as subscriber loop, local- loop, or access networks. Another example is provided by the Internet used for con- necting multiple computers. In 2001, both the local-loop and computer networks were using electrical techniques to provide bidirectional access through circuit or packet switching. The main limitation of such techniques is that each node on the network must be capable of processing the entire network traffic. Since it is difficult to achieve electronic processing speeds in excess of 10 Gb/s, such networks are inherently limited by the electronics. The use of WDM permits a novel approach in which the channel wavelength itself can be used for switching, routing, or distributing each channel to its destination, re- sulting in an all-optical network. Since wavelength is used for multiple access, such a WDM approach is referred to as wavelength-division multiple access (WDMA). A considerable amount of research and development work was done during the 1990s for developing WDMA networks [22]–[26]. Broadly speaking, WDMA networks can be classified into two categories, called single-hop and multihop all-optical networks [6]. Every node is directly connected to all other nodes in a single-hop network, resulting in a fully connected network. In contrast, multihop networks are only partially con- 8.1. WDM LIGHTWAVE SYSTEMS 337 Figure 8.5: Schematic of the Lambdanet with N nodes. Each node consists of one transmitter and N receivers. (After Ref. [28]; c 1990 IEEE; reprinted with permission.) nected such that an optical signal sent by one node may require several hops through intermediate nodes before reaching its destination. In each category, transmitters and receivers can have their operating frequencies either fixed or tunable. Several architectures can be used for all-optical multihop networks [6]–[11]. Hy- percube architecture provides one example—it has been used for interconnecting mul- tiple processors in a supercomputer [27]. The hypercube configuration can be easily visualized in three dimensions such that eight nodes are located at eight corners of a simple cube. In general, the number of nodes N must be of the form 2 m , where m is the dimensionality of the hypercube. Each node is connected to m different nodes. The maximum number of hops is limited to m, while the average number of hops is about m/2 for large N. Each node requires m receivers. The number of receivers can be reduced by using a variant, known as the deBruijn network, but it requires more than m/2 hops on average. Another example of a multihop WDM network is provided by the shuffle network or its bidirectional equivalent—the Banyan network. Figure 8.5 shows an example of the single-hop WDM network based on the use of a broadcast star. This network, called the Lambdanet [28], is an example of the broadcast-and-select network. The new feature of the Lambdanet is that each node is equipped with one transmitter emitting at a unique wavelength and N receivers op- erating at the N wavelengths, where N is the number of nodes. The output of all transmitters is combined in a passive star and distributed to all receivers equally. Each node receives the entire traffic flowing across the network. A tunable optical filter can be used to select the desired channel. In the case of the Lambdanet, each node uses a bank of receivers in place of a tunable filter. This feature creates a nonblocking net- work whose capacity and connectivity can be reconfigured electronically depending on the application. The network is also transparent to the bit rate or the modulation format. Different users can transmit data at different bit rates with different modulation formats. The flexibility of the Lambdanet makes it suitable for many applications. The main drawback of the Lambdanet is that the number of users is limited by the number 338 CHAPTER 8. MULTICHANNEL SYSTEMS Figure 8.6: Passive photonic loop for local-loop applications. (After Ref. [31]; c 1988 IEE; reprinted with permission.) of available wavelengths. Moreover, each node requires many receivers (equal to the number of nodes), resulting in a considerable investment in hardware costs. A tunable receiver can reduce the cost and complexity of the Lambdanet. This is the approach adopted for the Rainbow network [29]. This network can support up to 32 nodes, each of which can transmit 1-Gb/s signals over 10–20 km. It makes use of a central passive star (see Fig. 8.5) together with the high-performance parallel interface for connecting multiple computers. A tunable optical filter is used to select the unique wavelength associated with each node. The main shortcoming of the Rainbow network is that tuning of receivers is a relatively slow process, making it difficult to use packet switching. An example of the WDM network that uses packet switching is provided by the Starnet. It can transmit data at bit rates of up to 1.25 Gb/s per node over a 10-km diameter while maintaining a signal-to-noise ratio (SNR) close to 24 dB [30]. WDM networks making use of a passive star coupler are often called passive op- tical networks (PONs) because they avoid active switching. PONs have the potential for bringing optical fibers to the home (or at least to the curb). In one scheme, called a passive photonic loop [31], multiple wavelengths are used for routing signals in the local loop. Figure 8.6 shows a block diagram of such a network. The central office contains N transmitters emitting at wavelengths λ 1 , λ 2 , , λ N and N receivers operat- ing at wavelengths λ N+1 , , λ 2N for a network of N subscribers. The signals to each subscriber are carried on separate wavelengths in each direction. A remote node mul- tiplexes signals from the subscribers to send the combined signal to the central office. It also demultiplexes signals for individual subscribers. The remote node is passive and requires little maintenance if passive WDM components are used. A switch at the central office routes signals depending on their wavelengths. The design of access networks for telecommunication applications was still evolv- ing in 2001 [26]. The goal is to provide broadband access to each user and to deliver audio, video, and data channels on demand, while keeping the cost down. Indeed, many low-cost WDM components are being developed for this purpose. A technique known as spectral slicing uses the broad emission spectrum of an LED to provide mul- tiple WDM channels inexpensively. A waveguide-grating router (WGR) can be used for wavelength routing. Spectral slicing and WGR devices are discussed in the next section devoted to WDM components. 8.2. WDM COMPONENTS 339 Figure 8.7: Channel selection through a tunable optical filter. 8.2 WDM Components The implementation of WDM technology for fiber-optic communication systems re- quires several new optical components. Among them are multiplexers, which combine the output of several transmitters and launch it into an optical fiber (see Fig. 8.2); demultiplexers which split the received multichannel signal into individual channels destined to different receivers; star couplers which mix the output of several transmit- ters and broadcast the mixed signal to multiple receivers (see Fig. 8.5); tunable optical filters which filter out one channel at a specific wavelength that can be changed by tuning the passband of the optical filter; multiwavelength optical transmitters whose wavelength can be tuned over a few nanometers; add–drop multiplexers and WGRs which can distribute the WDM signal to different ports; and wavelength shifters which switch the channel wavelength. This section focuses on all such WDM components. 8.2.1 Tunable Optical Filters It is instructive to consider optical filters first since they are often the building blocks of more complex WDM components. The role of a tunable optical filter in a WDM system is to select a desired channel at the receiver. Figure 8.7 shows the selection mechanism schematically. The filter bandwidth must be large enough to transmit the desired channel but, at the same time, small enough to block the neighboring channels. All optical filters require a wavelength-selective mechanism and can be classified into two broad categories depending on whether optical interference or diffraction is the underlying physical mechanism. Each category can be further subdivided accord- ing to the scheme adopted. In this section we consider four kinds of optical filters; Fig. 8.8 shows an example of each kind. The desirable properties of a tunable opti- cal filter include: (1) wide tuning range to maximize the number of channels that can be selected, (2) negligible crosstalk to avoid interference from adjacent channels, (3) fast tuning speed to minimize the access time, (4) small insertion loss, (5) polariza- tion insensitivity, (6) stability against environmental changes (humidity, temperature, vibrations, etc.), and (7) last but not the least, low cost. [...]... MULTICHANNEL SYSTEMS and can be accomplished in a switching time of less than 10 µ s Acousto -optic tunable filters are also suitable for wavelength routing and optical cross-connect applications in dense WDM systems Another category of tunable optical filters operates on the principle of amplification of a selected channel Any amplifier with a gain bandwidth smaller than the channel spacing can be used as an optical... add–drop optical filters, and add–drop multiplexers CHAPTER 8 MULTICHANNEL SYSTEMS 354 Figure 8.17: Schematic of an optical cross-connect based on optical switches 8.2.6 Optical Cross-Connects The development of wide-area WDM networks requires a dynamic wavelength routing scheme that can reconfigure the network while maintaining its nonblocking (transparent) nature This functionality is provided by an optical... act as a 2 × 2 optical switch because the input signal can be directed toward different output ports by changing the delay in one of the arms by a small amount The planar lightwave circuit technology uses the thermo -optic effect to change the refractive index of silica by heating The temperature-induced change in the optical path length provides optical switching As early as 1996, such optical switches... between the optical and acoustic waves and is governed by a phase-matching condition similar to that found for acousto -optic filters As discussed in Section 2.6, SBS occurs only in the backward direction and results in a frequency shift of about 10 GHz in the 1.55-µ m region To use the SBS amplification as a tunable optical filter, a continuous-wave (CW) pump beam is launched at the receiver end of the optical...340 CHAPTER 8 MULTICHANNEL SYSTEMS Figure 8.8: Four kinds of filters based on various interferometric and diffractive devices: (a) Fabry–Perot filter; (b) Mach–Zehnder filter; (c) grating-based Michelson filter; (d) acousto -optic filter The shaded area represents a surface acoustic wave A Fabry–Perot (FP) interferometer—a cavity formed by using two mirrors—can act as a tunable optical filter if its length... formed that acts as an optical filter The bandpass response can be tailored for a multicavity filter formed by using multiple thin-film mirrors separated by several spacer layers Tuning can be realized in several different ways In one approach, an InGaAsP/InP waveguide permits electronic tuning [37] Silicon-based FP CHAPTER 8 MULTICHANNEL SYSTEMS 342 filters can be tuned using thermo -optic tuning [38] Micromechanical... use the phenomenon of optical interference to select 346 CHAPTER 8 MULTICHANNEL SYSTEMS Figure 8.10: Layout of an integrated four-channel waveguide multiplexer based on Mach– Zehnder interferometers (After Ref [69]; c 1988 IEEE; reprinted with permission.) the wavelength [1] Demultiplexers based on the MZ filter have attracted the most attention Similar to the case of a tunable optical filter, several... occur, especially in the case of dense WDM systems with small interchannel spacing Such power leakage is referred to as crosstalk and should be quite small (< −20 dB) for a satisfactory system performance The issue of interchannel crosstalk is discussed in Section 8.3 348 CHAPTER 8 MULTICHANNEL SYSTEMS Figure 8.12: (a) A generic add–drop multiplexer based on optical switches (OS); (b) an add– drop filter... such that an optical amplifier connects each output port of one with the corresponding input port of the another [85] The gain of amplifiers is adjusted such that only the channel to be dropped experiences amplification when passing through the device Such a device is close to the generic add–drop multiplexer shown in Fig 8.12(a) with the only difference that optical switches are replaced with optical amplifiers... planar optical waveguides formed on a silicon substrate [41]–[45] The underlying technology is sometimes called the silicon optical-bench technology [44] Tuning in MZ filters is realized through a chromium heater deposited on one arm of each MZ interferometer (see Fig 7.7) Since the tuning mechanism is thermal, it results in a slow response with a switching time of about 1 ms A separate class of tunable optical . 8 Multichannel Systems In principle, the capacity of optical communication systems can exceed 10 Tb/s be- cause of a large frequency associated with the optical. a tunable optical filter. 8.2 WDM Components The implementation of WDM technology for fiber -optic communication systems re- quires several new optical components.