Page 420 - Fiber Bragg Gratings

8.8 Gain-flattening and clamping in fiber amplifiers 397

opposed to reflectors of lower reflectivity, offers greater resilience to stray
light from other parts of the transmission system.

8.8.5 Noise figure

The signal-spontaneous beat-noise figure for an amplifier is often given
as

where P^ is the spontaneous emission power at frequency v, measured
in a bandwidth 8v. Over a length of fiber in which the population inversion
is constant, the noise figure for that inversion can be calculated using

It can be seen that for a given wavelength, the best noise figure is
obtained for the highest possible inversion i.e., maximum n 2/rii.
In a full-length amplifier, whether or not it is gain controlled, the
local population inversion varies along the fiber length. The overall noise
figure of the amplifier is predominantly determined at the signal input
end [137], as spontaneous emission generated at the input experiences
the full amplifier gain before being detected within the bandwidth of the
signal receiver.
In an AGC amplifier, the evolution of the population inversion is, in
part, determined by the laser power. For good overall noise performance,
it is beneficial to minimize the laser power at the input in order to ensure
that, as far as possible, the input inversion is determined by the shorter-
wavelength pump source. This involves implementing as asymmetric a
cavity configuration as possible. This is achieved by locating the bulk of
the cavity loss at the signal output end of the amplifier and by choosing
the wavelength of the control laser to be in a high-gain region of the
spectrum requiring a correspondingly high cavity loss.
Figure 8.35 shows the measured gain and noise figures of an AGC
amplifier with a linear control laser cavity at 1530 nm [138]. The pump
and signal powers are referenced to the input and output of the doped
fiber and the measured noise figures are compared with data obtained
using a full numerical amplifier model [139]. In one case the cavity loss