Page 419 - Fiber Bragg Gratings

396 Chapter 8 Fiber Grating Lasers and Amplifiers

Figure 8.34: Cross-talk experienced by a contradirectionally propagating
small signal 1560-nm probe to a modulated 1550-nm signal. Probe amplified using
gain control (solid) and without gain control (dashed) [134].

Determination of the minimum required pump power to maintain
gain control in the presence of signals of known magnitude is more in-
volved. Approximate amplifier analyses (e.g., [135]) can be employed, but
values so obtained substantially underestimate the actual power require-
ments. Principally, this is because no account is taken of pair induced
quenching effects that degrade power conversion efficiencies even in low
3+
Er ion concentration fibers [136].

8.8.4 Cavity stability
The gain stability of the amplifier is determined by the stability of the
control laser wavelength and the laser cavity loss. The laser wavelength is
fixed by the narrow-linewidth grating reflectors that have a temperature
sensitivity of ~0.01 nm/°C. To avoid changes in cavity loss if drifting
should occur, the use of one narrow- and one broader-band reflector is
preferable. Reflections at the laser wavelength from other parts of the
transmission system will alter the effective cavity loss, as will polarization
dependence combined with birefringence in the fiber. The use of high-
reflection gratings with an associated intracavity side-tap attenuator, as