A semiconductor light amplifier is driven by a drive device, and selectively amplifies one of a TM mode component and a TE mode component of input light. For example, TE mode light is selectively amplified by making thickness of the active layer of the semiconductor light amplifier not more than 0.05 .mu.m, or by tilting at least one of the entrance and exit faces of the semiconductor light amplifier by 10 to 16 degrees. An another type of light amplifying polarizer, input signal light and excitation light are combined by a half mirror or an optical coupler, and combined light is input to a rare-earth-element doped, polarization maintaining optical fiber.