Page 417 - Fiber Bragg Gratings

394 Chapter 8 Fiber Grating Lasers and Amplifiers

Figure 8.32: Gain characteristic of AGC amplifier. Lasing wavelength: 1520
nm; signal wavelength: 1550 nm; signal power: -30 dBm (from: Massicott J. R,
Willson S. D., Wyatt R., Armitage J. R., Kashyap R., and Williams D., "1480nm
pumped erbium doped fibre amplifier with all optical automatic gain control,"
Electron. Lett. 30(12), 962-963, 1994. © IEE 1994, Ref. [134]).

producing a spike before a new equilibrium is reached. In the optical gain-
controlled amplifier, the spike is eliminated.
Additionally, the induced cross-talk is also eliminated, as shown in
Fig. 8.34. A small counterdirectionally propagating probe at 1560 nm is
strongly affected in the uncontrolled amplifier but remains unaffected
with AGC. In the absence of AGC, the CW probe output power more than
doubles when the saturating signal is blocked, whereas in the controlled
case, a change of less than 0.5% in output is seen.
To eliminate the residual laser power at 1520 nm, an additional STG
[122] with a rejection of 30 dB is used. BER measurements performed at
2.5 Gb/sec show no penalty as a result of operating the amplifier in the
optical gain-controlled regime.
A combination of both gain control and gain equalization forms a
highly desirable amplifier. A GEQ filter composed of a concatenated set
of STG filters, (as discussed in Section 8.8.1) added to the AGC amplifier
output shows excellent GEQ-AGC. The flattened spectral shape is main-
tained for as long as the amplifier is operated within the gain-controlled
range.