PURPOSE: To form an optical element such as a semiconductor photoamplifier or the like whose noise characteristic is excellent by providing the following: a medium which guides and amplifies light; and a saturable absorption medium which photodetects the amplified light from the medium.

CONSTITUTION: Exciation light 55 is introduced into an Er-doped optical fiber 52 through a lens 54 and a fiber coupler 51 from a semiconductor laser 53, while the Er-doped optical fiber 52 is being excited, signal light 50 is introduced into the Er-doped optical fiber 52 and is amplified in this state; a semiconductor multilayer film 59 is irradiated with amplified signal light 58 to improve an S/N ratio of the signal light. For the semiconductor multilayer film 59, a saturable absorption layer (multiple quantum well layer) 18 composed of undoped InP 17 and undoped InGaAsP 16 is grown on an undoped InP substrate 60 by an MOVPE method, and, lastly, SiO2 films 61, 62,2200 thick, are formed on the saturable absorption layer 18 and below the InP substrate 60 as reflectionless coating films by an ECR plasma CVD method. Since an influence by a noise can be reduced in this case, a reproduction and relay distance can be extended.