An optical amplifier includes at least a first wavelength multiplexing unit for multiplexing a signal light and a pumping light source, a first optical amplifying medium for amplifying the signal light by utilizing the pumping light and providing an output of amplified signal light, a first demultiplexing unit, a first dispersion compensation unit for receiving the amplified signal light from the first demultiplexing unit and modifying a waveform of the amplified signal light so as to compensate for waveform distortion of the signal light along a transmission path of the signal light and providing an output of a waveform modified signal light, a second wavelength multiplexing unit, a second optical amplifying medium, a second demultiplexing unit, and a second dispersion compensation unit. The optical amplifier also forms a part of an optical transmitter and an optical receiver.