To provide a short-pulse train having a desired repetition frequency by forming a distributed feedback semiconductor laser having a diffraction grating designed in such a way as to oscillate in a longitudinal mode of equal intervals and an absorption region integrated in the direction of a resonator thereof.

A diffraction grating 5 is made on the surface of a p-type InGaAsP optical guide layer 4 by an electron beam direct writing method such that the cycle of the diffraction grating is made gradually shorter from 245 to 235 nm from the left side to the right side. The length of one diffraction grating region is 32 μm and the length of a flat region sandwiched by the diffraction region is also 32 μm. A p-side electrode is divided by etching into a distributed feedback region (p) electrode 91 and an absorption region (p) electrode 92. A current of 150 mA is injected through the distributed feedback region of this element and an absorption region is not biased to produce a mode lock pulse train having a repetition frequency of 1 THz.