Gain clamping in dual-stage L-band EDFA by recycling a backward ASE

Gain clamping in dual-stage long-wavelength (L-band) erbium-doped fiber amplifier (EDFA) is demonstrated. It uses two circulators and a broadband fiber Bragg grating (FBG) to route a wasted backward conventional band (C-band) amplified spontaneous emission (ASE) from the second stage and launch it back into the input end of the first stage. The amplifier gain is clamped at 16.5 dB with variation less than 0.2 dB and the dynamic range up to -7.5 dBm for the VOA loss of 4 dB. The clamped gain level also can be controlled from 15.0 to 22.9 dB by increasing the VOA loss from 0 to 32 dB without much variation in noise figure.