PURPOSE: To simultaneously generate synchronous oscillation of many semiconductor lasers with one master laser, by providing a semiconductor laser array, an optical guide layer, and a directional reflector containing a first diffraction grating of refractive index periodical structure and a second diffraction grating of gain periodical structure.

CONSTITUTION: A master laser 1 is connected with the one end of an optical guide layer 2, and a plurality of slave lasers 3 are coupled to the waveguide layer 2. Each of the slave lasers 3 is coupled to an optical guide layer 4, which is coupled to the optical guide layer 2. A directional reflector 6 is set at the coupling part of the optical guide layers 2, 4. The directional reflector 6 has a diffraction grating inclined 45° to the optical path. Each grating of the diffraction grating consists of a first diffraction grating 8 having periodical structure of refractive index and a second grating 10 having phase difference to the first diffraction grating and periodical structure of gain. The periodical structure of refractive index can be formed by using a high refractive index medium or a low refractive index medium. The periodical structure of gain can be formed by using a gain medium.