Patent ID: 7508852

Claim:
A passive mode-locked semiconductor laser diode which includes a gain region in which a population inversion is formed, and a saturable absorption region with a function modulating light intensity, and in which said gain region and said saturable absorption region are arranged in series, wherein in said gain region, an optical gain for polarization with the plane of polarization in a direction in parallel with the width direction of an optical waveguide which exists in said gain region is larger than an optical gain for polarization with the plane of polarization in a direction in parallel with the thickness direction of the optical waveguide which exists in said gain region, and wherein in said saturable absorption region, the absorption saturation energy for the polarization with the plane of polarization in a direction in parallel with the thickness direction of the optical waveguide which exists in said gain region is smaller than the absorption saturation energy for the polarization with the plane of polarization in a direction in parallel with the width direction of the optical waveguide which exists in said saturable absorption region, and, a modulation factor for polarization with the plane of polarization in a direction in parallel with the width direction of said optical waveguide which exists in said saturable absorption region is equal to a modulation factor for polarization with the plane of polarization in a direction in parallel with the thickness direction of the optical waveguide which exists in said gain region.