Before describing the technologies used to advance the state of the art of fiber-optic communication systems, it is useful to look at the block diagram of a generic communication system in Fig. 8.3a. It consists of an optical transmitter and an optical receiver connected to the two ends of a communication channel that can be a coaxial cable (or simply air) for electric communication systems but takes the form of an optical fiber for all fiber-optic communication systems.
8.2.1 Optical Transmitters and Receivers
The role of optical transmitters is to convert the information available in an electrical form into an optical form, and to launch the resulting optical signal into a communication channel. Figure 8.3b shows the block diagram of an optical transmitter consisting of an optical source, a data modulator, and electronic circuitry used to derive them. Semiconductor lasers are commonly used as optical sources, although light-emitting diodes (LEDs) may also be used for some less-demanding applications. In both cases, the source output is in the form of an electromagnetic wave of constant amplitude. The role of the modulator is to impose the electrical data on this carrier wave by changing its amplitude, or phase, or both of them. In the case of some less-demanding applications, the current injected into a semiconductor laser itself is modulated directly, alleviating the need of an expensive modulator.
The role of optical receivers is to recover the original electrical data from the optical signal received at the output end of the communication channel. Figure 8.3c shows the block diagram of an optical receiver. It consists of a photodetector and a demodulator, together with the electronic circuitry used to derive them. Semiconductor photodiodes are used as detectors because of their compact size and low cost. The design of the demodulator depends on the modulation scheme used at the transmitter. Many optical communication systems employ a binary scheme referred to as intensity modulation with direct detection. Demodulation in this case is done by a decision circuit that identifies incoming bits as 1 or 0, depending on the amplitude of the electric signal. All optical receivers make some errors because of degradation of any optical signal during its transmission and detection, shot noise being the most fundamental source of noise. The performance of a digital lightwave system is characterized through the bit-error rate. It is customary to define it as the average probability of identifying a bit incorrectly. The error-correction codes are sometimes used to improve the raw bit-error rate of an optical communication system.
8.2.2 Optical Fibers and Cables
Most people are aware from listening to radios or watching televisions that electromagnetic waves can be transmitted through air. However, optical communication systems require electromagnetic waves whose frequencies lie in the visible or near-infrared region. Although such waves can propagate through air over short distances in good weather conditions, this approach is not suitable for making optical communication networks spanning the whole world. Optical fibers solve this problem and transmit light over long distances, irrespective of weather conditions, by confining the optical wave to the vicinity of a microscopic cylindrical glass core through a phenomenon known as total internal reflection.
Figure 8.4 shows the structure of an optical fiber designed to support a single spatial mode by reducing its core diameter to below 10 μm. In the case of a graded-index multimode fiber the core diameter is typically 50 μm. The core is made of silica glass and is doped with germania to enhance its refractive index slightly (by about 0.5 %) compared to the surrounding cladding that is also made of silica glass. A buffer layer is added on top of the cladding before putting a plastic jacket. The outer diameter of the entire structure, only a fraction of a millimeter, is so small that the fiber is barely visible. Before it can be used to transmit information, one or more optical fibers are enclosed inside a cable whose diameter may vary from 1 to 20 mm, depending on the intended application.
What happens to an optical signal transmitted through an optical fiber? Ideally, it should not be modified by the fiber at all. In practice, it becomes weaker because of unavoidable losses and is distorted through the phenomena such as chromatic dispersion and the Kerr nonlinearity . As discussed earlier, losses were the limiting factor until 1970 when a fiber with manageable losses was first produced . Losses were reduced further during the decade of 1970s, and by 1979 they have been reduced to a level as low as 0.2 dB/km at wavelengths near 1.55 μm. Figure 8.5 shows the wavelength dependence of power losses measured for such a fiber . Multiple peaks in the experimental curve are due to the presence of residual water vapors. The dashed line, marked Rayleigh scattering, indicates that, beside water vapors, most of the loss can be attributed to the fundamental phenomenon of Rayleigh scattering, the same one responsible for the blue color of our sky. Indeed, although water peaks have nearly disappeared in modern fibers, their losses have not changed much as they are still limited by Rayleigh scattering.
8.2.3 Modulations Formats
The first step in the design of any optical communication system is to decide how the electrical binary data would be converted into an optical bit stream. As mentioned earlier, an electro-optic modulator is used for this purpose. The simplest technique employs optical pulses such that the presence of a pulse in the time slot of a bit corresponds to 1, and its absence indicates a 0 bit. This is referred to as on–off keying since the optical signal is either “off” or “on” depending on whether a 0 or 1 bit is being transmitted.
There are still two choices for the format of the resulting optical bit stream. These are shown in Fig. 8.6 and are known as the return-to-zero (RZ) and nonreturn-to-zero (NRZ) formats. In the RZ format, each optical pulse representing bit 1 is shorter than the bit slot, and its amplitude returns to zero before the bit duration is over. In the NRZ format, the optical pulse remains on throughout the bit slot, and its amplitude does not drop to zero between two or more successive 1 bits. As a result, temporal width of pulses varies depending on the bit pattern, whereas it remains the same in the case of RZ format. An advantage of the NRZ format is that the bandwidth associated with the bit stream is smaller by about a factor of 2 simply because on–off transitions occur fewer times. Electrical communication systems employed the NRZ format for this reason in view of their limited bandwidth. The bandwidth of optical communication systems is large enough that the RZ format can be used without much concern. However, the NRZ format was employed initially. The switch to the RZ format was made only after 1999 when it was found that its use helps in designing high-capacity lightwave systems. By now, the RZ format is use almost exclusively for WDM systems whose individual channels are designed to operate at bit rates exceeding 10 Gbit/s.
8.2.4 Channel Multiplexing
Before the advent of the Internet, telephones were used most often for communicating information. When an analog electric signal representing human voice is digitized, the resulting digital signal contains 64,000 bits over each one second duration. The bit rate of such an optical bit stream is clearly 64 kbit/s. Since fiber-optic communication systems are capable of transmitting at bit rates of up to 40 Gbit/s, it would be a huge waste of bandwidth if a single telephone call was sent over an optical fiber. To utilize the system capacity fully, it is necessary to transmit many voice channels simultaneously through multiplexing. This can be accomplished through time-division multiplexing (TDM) or WDM. In the case of TDM, bits associated with different channels are interleaved in the time domain to form a composite bit stream. For example, the bit slot is about 15 μs for a single voice channel operating at 64 kb/s. Five such channels can be multiplexed through TDM if the bit streams of successive channels are delayed by 3 μs. Figure 8.7a shows the resulting bit stream schematically at a composite bit rate of 320 kb/s. In the case of WDM, the channels are spaced apart in the frequency domain. Each channel is carried by its own carrier wave. The carrier frequencies are spaced more than the channel bandwidth so that the channel spectra do not overlap, as seen in Fig. 8.7b. WDM is suitable for both analog and digital signals and is used in broadcasting of radio and television channels. TDM is readily implemented for digital signals and is commonly used for telecommunication networks.
The concept of TDM has been used to form digital hierarchies. In North America and Japan, the first level corresponds to multiplexing of 24 voice channels with a composite bit rate of 1.544 Mb/s (hierarchy DS-1), whereas in Europe 30 voice channels are multiplexed, resulting in a composite bit rate of 2.048 Mb/s. The bit rate of the multiplexed signal is slightly larger than the simple product of 64 kb/s with the number of channels because of extra control bits that are added for separating channels at the receiver end. The second-level hierarchy is obtained by multiplexing four DS-1 channels. This results in a bit rate of 6.312 Mb/s (hierarchy DS-2) for North America and 8.448 Mb/s for Europe. This procedure is continued to obtain higher-level hierarchies.
The lack of an international standard in the telecommunication industry during the 1980s led to the advent of a new standard, first called the synchronous optical network (SONET) and later termed the synchronous digital hierarchy (SDH). It defines a synchronous frame structure for transmitting TDM digital signals. The basic building block of the SONET has a bit rate of 51.84 Mbit/s. The corresponding optical signal is referred to as OC-1, where OC stands for optical carrier. The basic building block of the SDH has a bit rate of 155.52 Mbit/s and is referred to as STM-1, where STM stands for a synchronous transport module. A useful feature of the SONET and SDH is that higher levels have a bit rate that is an exact multiple of the basic bit rate. Table 8.1 lists the correspondence between SONET and SDH bit rates for several levels. Commercial STM-256 (OC-768) systems operating near 40 Gbit/s became available by 2002. One such optical channel transmits more than half million telephone conversations over a single optical fiber. If the WDM technique is employed to transmit 100 channels at different wavelengths, one fiber can transport more than 50 million telephone conversations at the same time.