Semiconductor Optical Amplifier

The present invention provides a semiconductor optical amplifier, comprising: a laminated structure, the laminated structure successively comprising a compound semiconductor composed of GaN and having a first conductivity type first compound semiconductor layer, having a light amplification region composed of GaN compound semiconductor, and a third compound semiconductor layer composed of GaN compound semiconductor having a second conductivity type and a second compound semiconductor layer; a second electrode formed on the second compound semiconductor layer; and a first electrode electrically connected to the first compound The semiconductor layer. Said laminated structure having a ridge stripe structure. When each of the width of the ridge stripe structure at the light output end face and the ridge stripe structure on the light incident end face and Wout respectively Win, is satisfied Wout> Win. From the light output end face, along an axis of the semiconductor optical amplifier provided carrier non-injection region in the inner region of the laminated structure. The present invention is for greater light output, and the laser output becomes stable.