PURPOSE: To realize a quasi-continuous wavelength variable operation at the whole gain region of a semiconductor laser by a method wherein the laser and a directional coupler are integrated on the same substrate having a conductivity type and a diffraction grating changed its period to the progressing direction of light is provided on the waveguide on one side of waveguides forming the directional coupler.

CONSTITUTION: A diffraction grating changed its period in the orientation <110> on a (001) substrate is pratially formed on an n-type InP substrate 4 and thereafter, an MQW active layer 1 is selectively grown. That is, an n-type InGaAsP light guide layer 6, a non-doped InGaAsP layer 9, an Inlays well layer 10, 5 periods of a quantum well consisting of an InGaAsP barrier layer 9, a non-doped InGaAsP layer 9 and a p--type InP layer 2 are grown-in order on the InP substrate. Then, the MQW active layer 1 is buried in the layer 2, an Inlays cap layer 7 is grown to form two ridge optical waveguides and the optical waveguides are formed as a directional coupler. Then, an electrode is split into one place on the side of a semiconductor laser and two places on the side of the directional coupler.