PURPOSE: To reduce the incident polarization dependence as a whole by providing a region which amplifies a TE mode more intensely and a region which amplifies a TM mode more intensely in the active layer of a semiconductor laser diode type optical amplifier.

CONSTITUTION: An n-type Al0.4Ga0.6As/n-type GaAs multi-quantum well(MQW) buffer layer 102, an n-type InxAl0.4Ga0.6-xAs clad layer 103, an MQW active layer 104, a p-type In0.1Al0.45Ga0.45As intermediate layer 105, a p-type In0.1Al0.36 Ga0.54As clad layer 106 and a p-type In0.1Al0.1Ga0.8 cap layer 107 are built up on an n-type GaAs substrate 101. A TM mode is amplified more in the first region 104a of the active layer 104 and a TE mode is amplified more in the second region 104b of the active layer 104. Thus, a bulk semiconductor or a gain medium with a super-lattice structure without distortion which amplifies the TE mode more and a gain medium with a super-lattice structure with distortion which amplifies the TM mode more are introduced in the thickness direction or light axis direction of the active layer. With this constitution, the incident light polarization dependence of an LD type optical amplifier can be reduced.