438
11 Basics of Optical Amplifiers
EDFAs when they are used in three basic locations. Section 11.7 addresses the operation and use of optical amplifiers based on the Raman scattering mechanism. Finally,
Sect. 11.8 describes wideband optical amplifiers that operate over several wavelength
bands simultaneously.
11.1 Fundamental Optical Amplifier Types
Optical amplifiers have found widespread use in diverse applications ranging from
ultra-long undersea links to short links in access networks. In long-distance undersea
and terrestrial point-to-point links, the traffic patterns are relatively stable, so that
the input power levels to an optical amplifier do not vary significantly. However,
because many closely spaced wavelength channels are being transported over these
links, the amplifier must have a wide spectral response range and be highly reliable.
Usually fewer wavelengths are carried on metro and access network links, but the
traffic patterns can come in bursts and wavelengths often can be added or dropped
randomly depending on customers’ demand for service. Optical amplifiers for these
applications thus need to be able to recover quickly from rapid input power variations
when the number of amplified channels suddenly changes. Although these diverse
applications offer different optical amplifier design challenges, all devices share some
basic operational requirements and performance characteristics, which are given in
this section.
11.1.1 General Applications of Optical Amplifiers
Figure 11.1 shows general applications of the following three classes of optical
amplifiers:
In-line Optical Amplifiers In a single-mode link, the effects of fiber dispersion may
be small so that the main limitation to repeater spacing is fiber attenuation. Because
such a link does not necessarily require a complete regeneration of the signal, simple
amplification of the optical signal is sufficient. Thus an in-line optical amplifier
can be used to compensate for transmission loss and increase the distance between
regenerative repeaters, as Fig. 11.1a illustrates.
Preamplifier Fig. 11.1b shows an optical amplifier being used as a frontend preamplifier for an optical receiver. Thereby, a weak received optical signal is amplified
before photodetection so that the signal-to-noise ratio degradation caused by thermal
noise in the receiver electronics can be suppressed. Compared with other frontend
devices such as avalanche photodiodes or optical heterodyne detectors, an optical
preamplifier provides a larger gain factor and a broader bandwidth.
11 Basics of Optical Amplifiers
EDFAs when they are used in three basic locations. Section 11.7 addresses the operation and use of optical amplifiers based on the Raman scattering mechanism. Finally,
Sect. 11.8 describes wideband optical amplifiers that operate over several wavelength
bands simultaneously.
11.1 Fundamental Optical Amplifier Types
Optical amplifiers have found widespread use in diverse applications ranging from
ultra-long undersea links to short links in access networks. In long-distance undersea
and terrestrial point-to-point links, the traffic patterns are relatively stable, so that
the input power levels to an optical amplifier do not vary significantly. However,
because many closely spaced wavelength channels are being transported over these
links, the amplifier must have a wide spectral response range and be highly reliable.
Usually fewer wavelengths are carried on metro and access network links, but the
traffic patterns can come in bursts and wavelengths often can be added or dropped
randomly depending on customers’ demand for service. Optical amplifiers for these
applications thus need to be able to recover quickly from rapid input power variations
when the number of amplified channels suddenly changes. Although these diverse
applications offer different optical amplifier design challenges, all devices share some
basic operational requirements and performance characteristics, which are given in
this section.
11.1.1 General Applications of Optical Amplifiers
Figure 11.1 shows general applications of the following three classes of optical
amplifiers:
In-line Optical Amplifiers In a single-mode link, the effects of fiber dispersion may
be small so that the main limitation to repeater spacing is fiber attenuation. Because
such a link does not necessarily require a complete regeneration of the signal, simple
amplification of the optical signal is sufficient. Thus an in-line optical amplifier
can be used to compensate for transmission loss and increase the distance between
regenerative repeaters, as Fig. 11.1a illustrates.
Preamplifier Fig. 11.1b shows an optical amplifier being used as a frontend preamplifier for an optical receiver. Thereby, a weak received optical signal is amplified
before photodetection so that the signal-to-noise ratio degradation caused by thermal
noise in the receiver electronics can be suppressed. Compared with other frontend
devices such as avalanche photodiodes or optical heterodyne detectors, an optical
preamplifier provides a larger gain factor and a broader bandwidth.