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1.3 WAVELENGTH DIVISION MULTIPLEXING

Theoretically, fiber has extremely high bandwidth (about 25 THz [terahertz], i.e., 25 million MHz [megahertz] or 25 × 1012 Hz [Hertz]!) in the 1.55 low-attenuation band, and this is 1,000 times the total bandwidth of radio on the planet Earth [29]. However, only speeds (data rates) of a few gigabits per second are achieved because the rate at which an end user (for example, a workstation) can access the network is limited by electronic speed, which is a few gigabits per second. Hence it is extremely difficult to exploit all of the huge bandwidth of a single fiber using a single high-capacity wavelength channel due to optical-electronic bandwidth mismatch or "electronic bottleneck." The recent breakthroughs (Tb/s) are the result of two major developments: wavelength division multiplexing (WDM), which is a method of sending many light beams of different wavelengths simultaneously down the core of an optical fiber; and the erbium-doped fiber amplifier (EDFA), which amplifies signals at many different wavelengths simultaneously, regardless of their modulation scheme or speed.

WDM is conceptually similar to frequency division multiplexing (FDM), in which multiple information signals (each corresponding to an end user operating at electronic speed) modulate optical signals at different wavelengths, and the resulting signals are combined and transmitted simultaneously over the same optical fiber as shown in Fig. 1.5. Prisms and diffraction gratings can be used to combine (multiplex) or split (demultiplex) different color (wavelength) signals. FDM is

Figure 1.5 Wavelength division multiplexing.

used to carry many radio channels over the air or several simultaneous TV channels over cable. A WDM optical system using a diffraction grating is completely passive, unlike electrical FDM, and thus is highly reliable. Further, a carrier wave of each WDM optical channel is higher than that of an FDM channel by a million times in frequency (THz versus MHz). Within each WDM channel, it is possible to have FDM where the channel bandwidth is subdivided into many radio frequency channels, each at a different frequency. This is called subcarrier multiplexing. A wavelength can also be shared among many nodes in the network by electronic time division multiplexing. Note that WDM eliminates the electronic bottleneck by dividing the optical transmission spectrum (1.55-micron band) into a number of nonoverlapping wavelength channels, which coexist on a single fiber, with each wavelength supporting a single communication channel operating at peak electronic speed. The attraction of WDM is that a huge increase in available bandwidth can be obtained without the huge investment necessary to deploy additional optical fiber. To transmit 40 Gb/s over 600 km using a traditional system, 16 separate fiber pairs and 224 ([600/40 - 1]16) regenerators are required, as regenerators are placed every 40 km. On the other hand, a 16-channel WDM system requires only one fiber pair and 4 (600/120 - 1) optical amplifiers, as amplifiers are placed every 120 km. WDM has been used to upgrade the capacity of installed point-to-point transmission systems, typically by adding two, three, or four additional wavelengths. WDM systems that use 16 wavelengths at OC-48 and 32 wavelengths at OC-192 to provide aggregate rates up to 40 and 320 Gb/s, respectively, are available. The dense WDM (DWDM) technique effectively increases the total number of channels in a fiber by using very narrowly spaced (or densely packed) channels. Typical channel spacings range from 0.4 nm (1 nanometer is 10-9 meter) to 4 nm (50 GHz to 500 GHz).

As we have seen earlier, when the attenuation problem was solved, dispersion effects became significant. Similarly, when the dispersion problem was solved, nonlinear effects in fiber became dominant. The nonlinear effects—stimulated Raman scattering, stimulated Brillouin scattering, self- and cross-phase modulation, and four-wave mixing—may potentially limit the performance (maximum transmission rate) of WDM communication systems. Stimulated Raman scattering is caused by the interaction of the optical signal with silica molecules in the fiber. This interaction can lead to transfer of power from lower-wavelength channels to higher-wavelength channels. Stimulated Brillouin scattering is caused by the interaction between the optical signal and acoustic waves in the fiber. This interaction can make the power from the optical signal be scattered back toward the transmitter. Self- and cross-phase modulation and four-wave mixing are caused because, in optical fiber, the index of refraction depends on the optical intensity of signals propagating through the fiber. Self-phase modulation is caused by variations in the power of an optical signal and results in variations in the phase of the signal. Due to dispersion in fiber, phase shifts get transformed to signal distortion. By contrast, cross-phase modulation is due to a change in intensity of a signal propagating at a different wavelength. Four-wave mixing occurs when two or more optical signals (wavelengths) mix in such a way that they produce new optical frequencies called sidebands, which can cause interference if they overlap with frequencies used for data transmission. The above nonlinearities in fiber can be controlled by careful choice of channel power and channel spacing.

The advent of EDFA enabled commercial development of WDM systems by providing a way to amplify all the wavelengths at the same time, regardless of their individual bit rates, modulation scheme, or power levels. Before the invention of EDFAs, the effects of optical loss were compensated every few tens of kilometers by an electronic regenerator, which required that the optical signals be converted to electrical signals and then back again to optical ones. Most important, electronic regenerators work only for the designated bit rate at only one wavelength. The EDFA amplifier contains several meters of silica glass fiber that have been doped with ions of erbium, a rare-earth metal. An optical pump laser is then used to energize the erbium ions, which boost or amplify the optical signals that are passing through (see Fig. 1.6). It is a wonderful coincidence of nature that this amplification band (1.53–1.56 microns), with a gain spectrum of 0.03–0.04 microns, coincides with the 1.55-micron band of optical fibers.

Figure 1.6 (a) EDFA. (b) EDFA gain profile.

There are three types of signal regenerators available in practice: The 3R regenerator (a regenerator executing reshaping and reclocking operations; reshaping of the signal reproduces the original pulse shape of each bit, eliminating much of the noise, and reclocking of the signal synchronizes the signal to its original bit timing pattern and bit rate), the 2R regenerator (a regenerator executing only the reshaping operation), and the 1R regenerator (a regenerator, without reshaping and retiming operations, carrying out simple amplification using EDFAs or other amplifiers). A network element which combines an optical receiver, some degree of regeneration, and an optical transmitter is called a transponder. If a transponder transmits on a wavelength that is different from that of the received signal, it is also carrying out wavelength conversion operation as a by-product of regeneration.

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