PURPOSE: To generate a crossover mode and a bistable property in optional light output by generating a crossover mode with which the emitted light distribution on both edge faces makes a mirror image with each other by the simultational oscillation of two resonance modes.

CONSTITUTION: WG1 to WG4 are formed by dividing an active waveguide region into four parts, and the power outputted from the WG1 to WG4 is formed into P1 to P4. When a carrier (carrier density N2 to N4) has oblique distribution, the light pattern emitted from both edge faces make a crossover mode and a mirror image is formed. When the carrier has an oblique distribution as above-mentioned, the light distribution of a resonant transverse mode, which does not change its shape even after making a round-trip, also has an oblique distribution. The shape of the two resonance modes is almost determined by the distribution of refractive index, and the selection of either of the above- mentioned modes (or both of them) is determined by gain distribution. A crossover mode can be generated against the light output in a wide range by making a calculation taking into consideration of the simultaneous oscillation of two modes.