Abstract: A plurality of optical attenuators are connected in series between a light input end and a light output end. Light attenuations capacities of the optical attenuators increase successively from the light input end towards the light output end. The optical attenuators attenuate an input light, but in this instance, light consumption power by the respective optical attenuators are almost equalized.

Abstract: An optical fiber amplifier is free from optical oscillation, if operated for high output power level and can be manufactured at low cost. It comprises an input optical fiber 11 for receiving signal light, a first isolator 12 for blocking any return light of the optical signal entered into the input optical fiber 11, an erbium-doped optical fiber 13 for amplifying the entered optical signal, a pumping source 14, a wavelength division multiplexer 15 for coupling the signal light and the excited light from the pumping source 14, an optical signal output optical fiber 16 connected to the wavelength division multiplexer 15 to produce an amplified optical signal, a second isolator 17 arranged on the optical signal output optical fiber 16 and a excited light input optical fiber 19 for connecting the wavelength division multiplexer 15 and the pumping source 14 and the wavelength division multiplexer 15 has a dielectric multilayer filter 18 that reflects signal light and transmits excited light.