Distributed feedback semiconductor laser with monitor

In a distributed feedback semiconductor laser with a monitor, having a substrate, a DFB region on the substrate comprising a light emitting layer, periodic corrugations extending in the direction of travel of light and a laser DFB region pn junction, the improvement comprising a window region provided on said substrate on at least one end of the light emitting layer with a semiconductor material having an energy gap greater than the energy gap of the light emitting layer, and a cap layer of first conductivity type provided on the DFB region and the window region having an energy gap smaller than the energy gap of the light emitting layer and a monitor pn junction electrically isolated from the DFB regoin pn junction so that a part of the light output emitted from the light emitting layer reaches the monitor pn junction by Fresnel diffraction of the light output in the window region and an electrical output for monitoring the light output is obtained through the monitor pn junction in the cap layer, the monitor pn junction being formed in the cap layer of first conductivity type by providing a second conductivity type region in the cap layer, and electrodes on the cap layer on the DFB region for effecting laser oscillation and others disposed for monitoring.