Patent ID: 7917030

Claim:
An optical transmission system comprising: an input optical fiber carrying a first sub-band of WDM client optical signals in a first wavelength band along an optical transmission link; a booster EDFA coupled to the input optical fiber; and an in-band transponder operative to emit an in-band (IB) control signal launched into the input fiber upstream from the booster EDFA and propagating in a second sub-band of the first band, wherein the booster EDFA receives the WDM client and IB control signals and outputs a composite signal carried by an output fiber along the optical transmission link, the booster EDFA being operative to switch the optical transmission system between: a nominal mode corresponding to an automatic gain-control (AGC) regime of the booster EDFA in which the EDFA outputs the composite signal having a high power, and an automatic power reduction mode corresponding to an automatic power control (APC) regime of the booster EDFA in which the EDFA outputs the composite signal upon a loss of the optical link, wherein the composite signal in the APC regime has a low power smaller than the high power of the composite signal in AGC regime, and a second transponder operative to emit an out-of band (OB) control signal in a second wavelength band launched into the output fiber, the first and second transponders being operative to detect the respective IB and OB control signals in the AGG regime of the booster EDFA, the booster EDFA operating in the APC regime when both the IB and OB control signals are not detected by the respective first and second transponders.