22
1 Perspectives on Lightwave Communications
cable-laying ships. Splicing together individual cable sections forms continuous
transmission lines for these long-distance links.
Once the cable is installed, a transmitter can be used to launch a light signal into the
fiber. Chapter 4 describes transmitter configurations and Chap. 5 discusses methods
and devices for connecting sources and other photonic devices to fibers. In general,
the transmitter consists of a light source that is dimensionally compatible with the
fiber core and it contains associated electronic control and modulation circuitry.
Semiconductor light-emitting diodes (LEDs) and laser diodes are suitable sources.
For these devices the light output amplitude can be modulated rapidly by simply
varying the input current at the desired transmission rate, thereby producing a timevarying optical signal. The electric input signals to the transmitter circuitry for driving
the optical source can be either of an analog or digital form. The functions of the
associated transmitter electronics are to set and stabilize the source operating point
and output power level. For high-rate systems (usually greater than about 2.5 Gb/s),
direct modulation of the source can lead to unacceptable optical signal distortion. In
this case, an external modulator is used to vary the amplitude of a continuous light
output from a laser diode source. In the 770-to-910 nm region the light sources are
generally alloys of GaAlAs. At longer wavelengths (1260–1675 nm) an InGaAsP
alloy is the principal optical source material.
After an optical signal is launched into a fiber, it will become progressively attenuated and distorted with increasing distance because of light scattering, absorption,
and dispersion mechanisms in the glass material. As Chap. 6 discusses, at the destination of an optical fiber transmission line, there is a receiving device that interprets
the information contained in the optical signal. Inside the receiver is a photodiode
that detects the weakened and distorted optical signal emerging from the end of an
optical fiber and converts it to an electrical signal (referred to as a photocurrent).
The receiver also contains electronic amplification devices and circuitry to restore
signal fidelity. Silicon photodiodes are used in the 770-to-910 nm region. The primary
material in the 1260-to-1675 nm region is an InGaAs alloy.
The design of an optical receiver is inherently complex and can have rather sophisticated functions because it has to interpret the content of the weakened and degraded
signal received by the photodetector. Chapters 6–8 discuss basic receivers for digital
and analog applications. The principal figure of merit for a receiver is the minimum
optical power necessary at the desired data rate to attain either a given error probability for digital systems or a specified signal-to-noise ratio for an analog system. The
ability of a receiver to achieve a certain performance level depends on the photodetector type, the effects of noise in the system, and the characteristics of the successive
amplification stages in the receiver.
Included in any optical fiber link are various passive and active optical devices that
assist in controlling and guiding the light signals. Chapter 10 describes a variety of
such components. Passive devices are optical components that require no electronic
control for their operation. Among these are optical filters that select only a narrow
spectrum of desired light, optical splitters that divide the power in an optical signal
into a number of different branches, optical multiplexers that combine signals from
1 Perspectives on Lightwave Communications
cable-laying ships. Splicing together individual cable sections forms continuous
transmission lines for these long-distance links.
Once the cable is installed, a transmitter can be used to launch a light signal into the
fiber. Chapter 4 describes transmitter configurations and Chap. 5 discusses methods
and devices for connecting sources and other photonic devices to fibers. In general,
the transmitter consists of a light source that is dimensionally compatible with the
fiber core and it contains associated electronic control and modulation circuitry.
Semiconductor light-emitting diodes (LEDs) and laser diodes are suitable sources.
For these devices the light output amplitude can be modulated rapidly by simply
varying the input current at the desired transmission rate, thereby producing a timevarying optical signal. The electric input signals to the transmitter circuitry for driving
the optical source can be either of an analog or digital form. The functions of the
associated transmitter electronics are to set and stabilize the source operating point
and output power level. For high-rate systems (usually greater than about 2.5 Gb/s),
direct modulation of the source can lead to unacceptable optical signal distortion. In
this case, an external modulator is used to vary the amplitude of a continuous light
output from a laser diode source. In the 770-to-910 nm region the light sources are
generally alloys of GaAlAs. At longer wavelengths (1260–1675 nm) an InGaAsP
alloy is the principal optical source material.
After an optical signal is launched into a fiber, it will become progressively attenuated and distorted with increasing distance because of light scattering, absorption,
and dispersion mechanisms in the glass material. As Chap. 6 discusses, at the destination of an optical fiber transmission line, there is a receiving device that interprets
the information contained in the optical signal. Inside the receiver is a photodiode
that detects the weakened and distorted optical signal emerging from the end of an
optical fiber and converts it to an electrical signal (referred to as a photocurrent).
The receiver also contains electronic amplification devices and circuitry to restore
signal fidelity. Silicon photodiodes are used in the 770-to-910 nm region. The primary
material in the 1260-to-1675 nm region is an InGaAs alloy.
The design of an optical receiver is inherently complex and can have rather sophisticated functions because it has to interpret the content of the weakened and degraded
signal received by the photodetector. Chapters 6–8 discuss basic receivers for digital
and analog applications. The principal figure of merit for a receiver is the minimum
optical power necessary at the desired data rate to attain either a given error probability for digital systems or a specified signal-to-noise ratio for an analog system. The
ability of a receiver to achieve a certain performance level depends on the photodetector type, the effects of noise in the system, and the characteristics of the successive
amplification stages in the receiver.
Included in any optical fiber link are various passive and active optical devices that
assist in controlling and guiding the light signals. Chapter 10 describes a variety of
such components. Passive devices are optical components that require no electronic
control for their operation. Among these are optical filters that select only a narrow
spectrum of desired light, optical splitters that divide the power in an optical signal
into a number of different branches, optical multiplexers that combine signals from