WDM Lightwave Systems
In principle, the capacity of an optical communication system can exceed 10 Tb/s because of a large frequency associated with the optical carrier. In practice, however, the bit rate was limited to 10 Gb/s or less until 1990 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 simple 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. To make the distinction explicit, it is common to refer to the two optical-domain techniques as optical TDM (OTDM) and wavelength-division multiplexing (WDM), respectively. The development of multichannel systems attracted considerable attention during the early 1990s, and WDM systems became available commercially by 1996.
WDM corresponds to the scheme in which multiple optical carriers at different wavelengths are modulated by using independent electrical bit streams (which may themselves use TDM and FDM techniques in the electrical domain) and are then transmitted over the same fiber. The optical signal reaching the receiver is then demultiplexed into separate channels by using a suitable optical device. The WDM technique allows us to exploit the large bandwidth offered by optical fibers. For example, hundreds of 40-Gb/s channels can be transmitted over the same fiber when channel spacing is reduce to near 100 GHz. The following figure shows the low-loss transmission windows of standard fibers centered near 1.3 and 1.55 μm. If the OH peak is eliminated using the so-called "dry" fibers, the total capacity of a WDM system may exceed 50 Tb/s.
The concept of WDM has been pursed since the first commercial lightwave system became available in 1980. In its simplest form, WDM was used as early as 1982 to transmit two channels in different transmission windows of an optical fiber. For example, an existing 0.85-μm lightwave system could be upgraded in capacity by adding another channel near 1.3 μm, resulting in a channel spacing of 450 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. However, it was during the decade of the 1990s that WDM systems were developed most aggressively. Commercial WDM systems operating at 20-40 Gb/s first appeared around 1995, but their total capacity exceeded 1.6 Tb/s by the year 2000. Such systems employ hundreds of closely-spaced wavelengths and are referred to as dense WDM systems. Several laboratory experiments demonstrated in 2001 a system capacity of more than 10 Tb/s although their transmission distance was limited to below 200 km. By 2008, capacity of WDM systems approached 30 Tb/s. Clearly, the advent of the WDM technique has led to a virtual revolution in designing lightwave systems.
1. High-Capacity Point-to-Point Links
For long-haul fiber links forming the backbone of a telecommunication network, the role of WDM is simply to increase the total bit rate. The figure below 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 channels at bit rates B1, B2, ..., and BN are transmitted simultaneously over a fiber of length L, the total bit rate-distance product, BL, becomes
BL = (B1 + B2 + ... + BN)L
For equal bit rates, the system capacity is enhanced by a factor of N. An early experiment in 1985 demonstrated the BL product of 1.37 (Tb/s)-km by transmitting 10 channels at 2 Gb/s over 68.3 km of standard fiber with a channel spacing of 1.35 nm.
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. It is common to introduce a measure of the spectral efficiency of a WDM system as
ηs = B/Δνch
where B is the channel bit rate and Δνch is the channel spacing in frequency units. Attempts are made to make ηs as large as possible. For direct-detection systems, channel spacing must be larger than the bit rate B. In practice, spectral efficiency is often < 0.6 b/s/Hz. resulting in waste of considerable channel bandwidth.
The channel frequencies (or wavelengths) of WDM systems were initially standardized 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 a spectral efficiency of only 0.1 b/s/Hz at a 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 can increase spectral efficiency of direct-detection systems to 0.8 b/s/Hz. The use of coherent detection allows ηs > 1 b/s/Hz, and by 2009, values as large as 8 b/s/Hz have been realized.
What is the ultimate capacity of WDM systems? The low-loss region of the state-of-the-art dry fibers (i.e., fibers with reduced OH-absorption near 1.4 μm) extends over 300 nm in the wavelength region covering 1.3-1.6 μm. The minimum channel spacing can be 25 GHz (0.2 nm) or less for 100-Gb/s channels if coherent detection is employed. Since 1500 channels with 0.2-nm spacing can be accommodated over a 300-nm bandwidth, the resulting capacity can be as large as 150 Tb/s. If we assume that such a WDM signal can be transmitted over 4000 km by using optical amplifiers with dispersion management, the effective BL product may eventually exceed 600 (Pb/s)-km with the use of WDM technology. 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 potential of improving the performance of modern lightwave systems by a factor of more than one million.
In practice, many factors limit the use of the entire low-loss window. 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 (EDFA) is often limited to 40 nm even with the use of gain-flatting techniques. The use of Raman amplification in combination with EDFAs can extend the usable 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. In practice, 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, 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. 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 1.3 (Pb/s)-km. The table below lists several WDM transmission experiments in which system capacity exceeded 10 Tb/s. The record capacity in 2010 occurred for a 69-Tb/s WDM system that transmitted 432 channels at 160 Gb/s over a distance of 240 km. The shift in the bit rate of each channel toward 100 Gb/s after 2007 is due to the 100-Gb/s Ethernet transport standard developed in recent years.
The second group of WDM experiments involves transmission distances 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 polarization scrambling with the forward-error correction (FEC) technique. The pace of rapid development is evident when we note that, by 2001, a 2.4-Tb/s WDM signal (120 channels, each at 20 Gb/s) was transmitted over 6200 km, resulting in a NBL product of 15 (Pb/s)-km. 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 10,000 by 2001. The table below lists several WDM transmission experiments performed since 2001. The record NBL product of 101.8 (Pb/s)-km was realized in a 2010 experiment that transmitted 96 channels at 100Gb/s over a distance of 10,608 km.
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) became available in 1996. Such a 16-channel system covered a wavelength range of about 12 nm in the 1.55-μm region with a channel spacing of 0.8 nm. WDM systems operating at 160 Gb/s (16 channels at 10 Gb/s) appeared in 1998. By 2001, dense WDM systems with a capacity of 1.6 Tb/s (realized by multiplexing 160 channels, each operating at 10 Gb/s) became available. After 2001, bursting of the so-called "telecom bubble" slowed down the demand of new WDM systems considerably. Nevertheless, fourth-generation WDM systems employing Raman amplification of a large number of 40-Gb/s channels had reached the commercial stage by 2003. This should be contrasted with the 10-Gb/s capacity of the third-generation systems available before the advent of the WDM technique. After 2007, commercial WDM systems have also moved toward a bit rate of 100 Gb/s per channel.
2. Wide-Area and Metro-Area Networks
Optical networks 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. 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 WAM covering a wide area (e.gl, a country). Historically, telecommunication and computer networks (such as the Internet) occupying the entire U.S. geographical region have sued a hub topology shown schematically in the figure below.
Such networks are often called mesh networks. Hubs or nodes located in large metropolitan 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 modulation 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 "transparent." Transparent WDM networks are desirable as they do not require demultiplexing and optical-to-electronic conversion of all WDM channels. As a result, they are 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 switch channels using optical cross-connects. Such devices are still in their infancy in 2001.
An alternative topology implements a region WDM network in the form of several interconnected rings. The following figure shows such a scheme schematically.
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 protection 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 the figure above 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 backbone 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 protecting 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 optically switched metro network, called the multiwavelength optical network (MONET), several sites within the Washington, DC, area of the United States were connected using a set of eight standard wavelengths in the 1.55-μm region with a channel spacing of 200 GHz. 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 LiNbO3 technology. Since then, several advances have improved considerably the state of the art of metro networks.
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 use 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 connecting multiple computers. In 2009, 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, resulting 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. Broadly speaking, WDMA networks can be classified into two categories, called single-hop and multihop all-optical networks. 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 connected such that an optical signal sent by one node may require several hops through intermediate nodes before reaching its destination. In each category, transmitter and receivers can have their operating frequencies either fixed or tunable.
Several architectures can be used for all-optical multihop networks. Hypercube architecture provides one example - it has been used for interconnecting multiple processors in a supercomputer. 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 2m, 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 provide by the shuffle network or its bidirectional equivalent - the Banyan network.
The following figure shows an example of the single-hop WDM network based on the use of a broadcast star.
This network, called the Lambdanet, 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 operating 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 network 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 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. 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 the figure above) 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 an SNR close to 24 dB.
WDM networks making use of a passive star coupler are often called passive optical 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, multiple wavelengths are used for routing signal sin the local loop. The following figure shows a block diagram of such a network.
The central office contains N transmitters emitting at wavelengths λ1, λ2, ..., λN and N receivers operating at wavelengths λN+1, ..., λ2N for a network of N subscribers. The signals to each subscriber are carried on separate wavelengths in each direction. A remote node multiplexes 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.
Since 2001, access networks for telecommunication applications have evolved considerably. Proposed architectures include broadband PON (B-PON), Gb/s-PON (G-PON), and Gigabit Ethernet PON (GE-PON). 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. Ineed, many low-cost WDM components are being developed for this purpose. Some of these are covered in the next tutorial devoted to WDM components.