All-optical gain control of in-line erbium-doped fiber amplifiers for hybrid analog/digital WDM systems

Automatic gain control using an all-optical feedback loop in in-line erbium-doped fiber amplifiers (EDFA's) used in hybrid analog/digital wavelength division multiplexing (WDM) systems was studied, It is found that the signal level variation for the digital channels can be maintained within a range less than or equal to 3-dB between the presence and dropout of the analog channel when the narrowband feedback is centered at the amplifide spontaneous emission (ASE) peak (similar to 1532 nm) with loop loss ranging between 13-22 dB, Robust transmission at 2.5 Gb/s without measurable power penalty was obtained for the digital channels when the EDFA was saturated by either the analog or the control lasing signal.