PROBLEM TO BE SOLVED: To provide a device showing a high extinction ratio and suitable for digital operation.
SOLUTION: First and second electrodes 7, 8 are formed on both sides of a waveguide structure, and a carrier injected region 3a and a non-carrier injected region 3b are formed in the waveguide structure by the electrodes. When a large amt. of carriers are accumulated in the carrier injected region 3a, the refractive index in this region relatively decreases than the refractive index in the adjacent non-carrier injected region. In this state, even when light waves of a low energy level enter, the amt. of the carrier consumed by the light waves is small so that the refractive index of this region is still maintained low than the refractive index in the non-carrier injected region and that the incident signal light is emitted to the outside. When light waves of a high energy level propagate, the amt. of the carrier consumed by the light waves increases so that the refractive index in the region 3a relatively increases than the refractive index in the non-carrier injected region 3b and that a photo-induced waveguide is formed from the incident face to the exit face. As a result, photoamplified signal light by the synergistic effect of the guiding effect and photo-amplification effect can be emitted.