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9 Analog Optical Fiber Channels
optical amplifier gives rise to ASE noise. In principle, the three dominant factors that
cause signal impairments in a fiber link are shot noise, optical amplifier noise, and
laser clipping.
Most other degradation effects can be sufficiently reduced or eliminated.
This section examines a simple single-channel amplitude-modulated signal sent
at baseband frequencies. Section 9.3 addresses multichannel systems in which
intermodulation noise becomes important.
9.2.1 Carrier Power
To find the carrier power, first consider the signal generated at the transmitter. As
shown in Fig. 9.2, the drive current through the optical source is the sum of the fixed
bias current and a time-varying sinusoid. The source acts as a square-law device, so
that the envelope of the output optical power P(t) has the same form as the input
drive current. If the time-varying analog drive signal is s(t), then
P(t) = P t [1 + ms(t)]
(9.2)
Fig. 9.2 Biasing conditions of a laser diode and its response to analog signal modulation