PURPOSE: An automatically gain controlled optical fiber amplifier is provided to improve an operating speed by respectively controlling gains of each channel in real time. CONSTITUTION: An automatically gain controlled optical fiber amplifier comprises a first optical tap(301) partially dividing an input signal, a second optical tap(302) partially dividing an output signal, an 1x2 optical coupler(303) distributing the output of the first optical tap(301) to a signal processor(330) and a first wavelength selecting filter(321), the first wavelength selecting filter(321) separating an arbitrary channel signal from a partial signal of the output signal of the 1x2 optical coupler(303), a second wavelength selecting filter(322) separating an arbitrary channel signal from a partial signal of the output of the second optical tap(302), the signal processor(330) measuring all input signal intensity and the number of input channel by receiving signals from the 1x2 optical coupler(303) and the wavelength selecting filters(321,322), a controller(340) generating controlling signals according to the result of the signal processor(330), an optical amplifying and attenuating part(310) supplying signals to the second optical tap(302) according to the controller(340).
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