Abstract: The invention is directed to a failure detection system and method for detecting malfunction of an optical amplifier module with one or multiple transmission channels. The failure detection system comprises a unit for measuring a performance parameter of the module; a unit for providing an expected performance parameter; and a comparator unit for receiving the performance parameter and the expected performance parameter and producing an error signal when the performance parameter substantially departs from the expected performance parameter. The system also includes a display/alarm unit for receiving the error signal and accordingly signaling failure of the module. The performance parameter is an output value of the module; a correspondence between an output value and an input value for a transmission channel; a figure of merit (FOM.sup.t=t); a set of gains (g) for all transmission channels; and the dynamic range of the amplifier module.

Abstract: An add/drop multiplexer/demultiplexer (ADM) for switching, modulating and attenuating optical signals in a fiber optic network employing wavelength division multiplexing (WDM) is disclosed. The ADM is equipped an optical multiplexer for splitting an input WDM signal into individual optical signals, leading to respective 2.times.2 switches. Each switch has another input originating from a plurality of "add lines", and selects one of its inputs to be dropped and the other to continue along a main signal path. The retained signals may be modulated and attenuated prior to being tapped and finally multiplexed together by a WDM multiplexer. The tapped signals are optoelectronically converted and fed back to a controller, preferably a digital signal processor running a software algorithm, which controls the switching, modulation and attenuation.

Abstract: A digital controller for an injection laser diode. The controller maintains the laser diode average and peak power levels constant in spite of temperature and/or aging effects. A pseudo-random sequence is superimposed on the `0` and/or `1` current levels to the laser diode and the optical output is detected by a back facet monitor. The detected signal is compared with preset references and a feedback signal is used to control both the bias current and the modulation current.