A DFB semiconductor laser device includes a diffraction grating extending
parallel to a laser cavity. The diffraction grating has ununiform
structure wherein some of the corrugation patterns of diffraction grating
are omitted periodically, the diffraction grating has different duty
ratio between the area near the front facet of the laser cavity and the
area near the rear facet, or the length of the diffraction grating is
smaller than the cavity length and the width of the diffraction grating
reduces in the vicinity of the rear end of the diffraction grating down
to zero at the rear end.