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Telecommunications makes wide use of optical techniques in which the carrier wave belongs to the classical optical domain. The wave modulation allows transmission of analog or digital signals up to a few gigahertz or gigabits per second on a carrier of very high frequency, typically 186 to 196 THz. In fact, the bit rate can be increased further, using several carrier waves that are propagating without significant interaction on the same fiber. It is obvious that each frequency corresponds to a different wavelength. This technique is called frequency division multiplexing (FDM) or wavelength division multiplexing (WDM). The latter term is currently preferred in most cases. DWDM is reserved for very close frequency spacing (typically less than 100 GHz corresponding to 0.8 nm at wavelengths near 1.5 Î¼m). The term ''frequency division multiplexing" is used in a few cases, such as multiplexing with optical frequency shift keying and coherent detection. But the terminology is not completely stabilized.
With WDM, it is possible to couple sources emitting at different wavelengths- Î»1, Î»2, Î»j, ... Î»n-into the same optical fiber. After transmission on the fiber, the Î»1, Î»2, ... Î»n signals can be separated towards different detectors at the fiber extremity (Figure 2.1). The component at the entrance must inject the signals coming from the different sources into the fiber with minimum losses: This is the multiplexer. The component separating the wavelengths is the demultiplexer. A simple optical coupler may replace the multiplexer, but losses will increase. Obviously, when the light propagation
Figure 2.1: Wave Division Multiplexing
is reversed, the multiplexer becomes the demultiplexer, and the reverse is also true. It is important to note, however, that the coupling efficiency is not necessarily preserved in reverse operation. For example, if the multiplexer uses single-mode (SM) entrance fibers and a multimode output fiber, the coupling losses would be excessive in the reversed usage. Multiplexers designed with identical input and output fibers are usually reversible. Simultaneous multiplexing of input channels and demultiplexing of output channels can be performed by the same component, the multi/demultiplexer.
2.2 BRIEF HISTORY OF WDM.
The optical multiplexing concept is not new. To our knowledge, it dates back to at least 1958 [1, 2]. Perhaps we can say that the idea of sending multiple signals, as shown in Figure 2.1, was straightforward, as it was a transposition of techniques used in classical telecommunications with electronics signals. But the technical problems to be solved were very difficult, and it took experts a great deal of time to solve them. About 20 years later, the first practical components for multiplexing were proposed primarily in the United States, Japan, and Europe. In 1977, the first grating-WDM passive component was developed by Tomlinson and Aumiller .
2.3 WDM AND TDM.
Is it easier to multiplex the signal in the electronic domain-time division multiplexing (TDM), or in the optical domain (FDM or WDM)? The answer to this question is not easy and the optimum solution is generally found in the association of the different techniques.
For low bit-rate services (< 2 Mbps), it is generally better to use only TDM techniques. For uncompressed, high-definition television (HDTV) broadcasting, WDM is highly recommended. The video compression techniques minimize the bandwidth requirement. However, at the time of this writing, CATV and HDTV still require 4 Mbps and 25 Mbps, respectively. Applications such as video networks linking workstations, television studio center signal routing systems, video conference networks, interactive video training systems, bank information service networks, and data-transfer networks between computers, integrated service digital networks (ISDN), teledistribution, and generally all broadband networks increasingly use both time and wavelength multiplexed optical lines. Today, the predicted demand per subscriber in 2010 is on the order of 100 Mbps. That will not be possible without the deployment of DWDM optical fiber networks.
It is understood that a practical network is very often made up of an association of architectures that constitute the physical medium of the network between stations. The topology is called ''virtual" when it is concerned only with logical connections between stations. One example of an optical-multiplexing application is to create virtual topologies on request. The network configuration can be modified independently of its physical topology by changing the emitted or received optical frequencies. In these architectures WDM cross-connectors, WDM routers, and WDM add/drops become more and more important.
2.4 WAVELENGTH DOMAIN AND SEPARATION BETWEEN CHANNELS
With modern, commercially available telecommunication fibers, it is possible to transmit information over a large spectral range (Figure 2.2) with two domains with low attenuation, one around 1.3 Î¼m and another around 1.55 Î¼m. Between these two domains there is generally a high attenuation at 1.39 Î¼m due to the residual OH radical in the fiber. SM silica fibers with minimum loss of about 0.16 dB/km at 1.55 Î¼m are available. Losses < 0.4 dB/Km at 1.5 Î¼m and < 0.5 dB/Km at 1.31 Î¼m are specified in the ITU-T G.652 recommendations. In medium- and long-distance DWDM transmissions, SM fibers are generally used in the domain 1,520 to 1,620 nm (see paragraph 5), due to the availability of efficient optical amplifiers and sources.
Figure 2.2: Typical loss of a low OH content fiber.
For applications such as metropolitan area networks, special SM fibers with very low OH content are commercialized. They can be used from 1,335 to 1,625 nm (for instance, AllWave fiber from Lucent Technologies).
On multimode silica vapor phase axial deposition (VAD) fiber without OH impurity, losses lower than 4 dB on 2.4 km were obtained from 0.65 to 1.9 Î¼m as long ago as 1982 .
On multimode fibers, a graded index design allows a temporal dispersion minimization, but the optimum profile depends greatly on wavelength and material, and the dispersion varies with wavelength.
The separation between channels is now 0.8 nm or more on most of the installed networks using WDM. International Telecommunication Union (ITU) standardization proposes a frequency grid with separations of 100 GHz (about 0.8 nm) with multiples and submultiples. Now the published minimum channel spacing is about 0.1 nm. However, at the beginning of the twenty-first century, nothing lower than 0.2-nm (25 GHz) spacing was commercially available. At first glance, a fiber without OH would allow 1,000 channels at 50-GHz spacing to be multiplexed over its large spectral range!
Of course there are some limitations of WDM (Figure 2.3). The main problem is crosstalk (parasitic light) coming from technical defects in the demultiplexers, but also from physical problems such as wavelength conversion along the transmission fiber by four-wave mixing, Brillouin or Raman effect, or other nonlinear effects. But the theoretical minimum channel spacing is at last related to ''uncertainty" relationships.
Figure 2.3 Principle of multiplexing by diffraction on an optical grating: Wavelengths Î»1, Î»2, Î»3 coming from different directions are diffracted in the same direction into a single-transmission fiber.
2.5 WAVELENGTH ALLOCATION
Thus far, the ITU-T standards recommends 81 channels in the C band with a constant spacing of 50 GHz anchored at 193.1 THz. This range can be extended to the L band (191.4 to 185.9 THz) where sources and amplifiers become available now. This will add 111 channels at 50-GHz spacing.
2.6 OPTICAL WAVELENGTH/OPTICAL FREQUENCY CONVERSION
Depending on one's background (the classical optical field or the microwave field), one generally prefers to represent light vibrations according to their wavelengths in vacuum (the wavelength varies with the medium) or according to their frequencies (invariant of the medium). In order to evaluate the different results given in scientific papers, it is useful to be able to translate quickly.
It is well known that:
where c is the speed of light ( c = 2.9972458 106m/s), Î» is the wavelength in vacuum, and Î½ is the optical frequency.
with Î» Î» Î» in nm and Î½ in GHz
Thus, 100-GHz spacing at 1.55-Î¼m wavelength is equivalent to a spacing of 0.8 nm.
2.7 HOW MANY CHANNELS?
There is actually about 15,000 GHz of optical frequency bandwidth in each 1,300- and 1,550-nm window. With a 10 Gbps bit rate, the uncertainty relationship gives approximately 10 GHz as the limit for optical frequency spacing. This would mean 1,500 channels! But the fiber nonlinearity has always set the limit to a few hundred channels in practical applications.
For the component itself at this limit, the optical crosstalk is the main problem. Acceptable 160-channel grating-WDM components are already manufactured. We believe that many more channels are feasible. Three thousand-channel classical-grating spectrometers are commonly used, why not WDM networks with a few hundred channels? In fact, the main problem is acquiring enough stable fixed or tunable sources. Today, the practical limit is a few tens of sources spaced at 50 GHz. The number of channels will depend on the progress on the sources. It is worth pointing out that an optical frequency chain-generation from a single supercontinuum source with over 1,000 channels at 12.5-GHz spacing has been proposed by H. Takara, et al. in ECOC'2000 (see Chapter 4).