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Sec. 8–7 Fiber-Optic Systems 619
fiber-optic cable systems are now popular. Digital fiber-optic systems use simple OOK modulation
of an optical source to produce a modulated light beam. The optical source has a
wavelength in the range of 0.85 to 1.6 mm, which is about 190 to 350 THz † . (Analog AM
fiber-optic systems can also be built.)
The optical sources can be classified into two categories: (1) light-emitting diodes
(LED) that produce noncoherent light and (2) solid-state lasers that produce coherent
(single-carrier-frequency) light. The LED source is rugged and inexpensive. It has a relatively
low power output of about -15 dBm (including coupling loss) and a small modulation
bandwidth of about 50 MHz. The laser diode has a relatively high power output of
about +5 dBm (including coupling loss) and a large modulation bandwidth of about
1 GHz. The laser diode is preferred over the LED, since it produces coherent light and
has high output power.
The light source is coupled into the fiber-optic cable, and this cable is the channel transmission
medium. Fiber cables can be classified into two categories: (1) multimodal and
(2) single mode. Multimodal fiber has a core diameter of 50 mm and a cladding diameter of
125 mm. The light is reflected off the core-cladding boundary as it propagates down the fiber
to produce multiple paths of different lengths. This causes pulse dispersion of the OOK signal
at the receiving end and thus severely limits the transmission bit rate that can be accommodated.
The single-mode fiber has a core diameter of approximately 8 mm and propagates a
single wave. Consequently, there is little dispersion of the received pulses of light. Because of
its superior performance, single-mode fiber is preferred.
At the receiving end of the fiber, the receiver consists of a PIN diode, an avalanche
photodiode (APD), or a GaAsMESFES transistor used as an optical detector. In 0.85-mm
systems, any of these devices may be used as a detector. In 1.3- and 1.55-mm systems,
the APD is usually used. These detectors act like a simple envelope detector. As shown in
Fig. 7–14, better performance (lower probability of bit error) could be obtained by using a
coherent detection system that has a product detector. This requires a coherent light source at
the receiver that is mixed with the received OOK signal via the nonlinear action of the photodetector.
It is possible to have coherent PSK and FSK systems [Basch and Brown, 1985].
Dense wavelength division multiplexing (DWDM) systems have a capacity on the order of
1,800 Gbs, as shown in Table 8–2.
Example 8–6 LINK BUDGET FOR A FIBER-OPTIC SYSTEM
Figure 8–28 shows the configuration of a typical fiber-optic system. It consists of two bidirectional
fiber-optic rings with terminals located at service points (such as towns) within a given geographical
area. The ring configuration is used to provide redundant data paths; if a cable is cut
anywhere around the ring, service is still provided to the terminals by the remaining in-service
fiber. The terminals are called adddrop terminals because they adddrop data from the ring for
the subscribers that the terminal serves from its location. As shown in Fig. 8–28, two fibers are required
in the ring for full duplex service. That is, one fiber provides for the transmitted (Tx) data,
and the other fiber provides the simultaneous received (Rx) data.
† One THz is 10 12 Hz.