PURPOSE: To enable compensation for an excessive loss in a passive star coupler and attenuation in a transmission filter by coupling an optical frequency-division multiplex signal, analog amplitude-modulated signal, or frequency-modulated signal with the input port of the passive star coupler and using an adjustable optical filter at the place of output. CONSTITUTION: The output of a Fabri-Perot filter 40 is applied to a detector 46 through an isolator 44, an amplifier 42, and a fixed optical filter 43. The optical amplifier 42 functions as a low-noise preamplifier having its gain adjusted so that its signal-initiated noise is a main noise term. The amplifier 42 is placed on the downstream side of the Fabri-Perot filter 40 and then the power level applied to the amplifier becomes substantially 1/2N (n: number of channels) of transmitter power and the saturation of the amplifier is minimized as a restrainer. Thus, the division loss in the start coupler or a splitter limits the power margin greatly, but those characteristic loss and excessive loss can be compensated by an optical amplifier which gives a gain through an FDM band to some limit.
FREQUENCY DIVISION MULTIACCESS NET