PURPOSE: To obtain a far-field pattern of single peak by applying thin films which offer phase shift of λ/2 on every other oscillation edge planes.

CONSTITUTION: On an edge plane 4 of phase-locked array laser, thin films 5 which causes phase shift of λ/2 are applied on every other light emitting points to make a phase 6 of a wave front of a projected beam flat. For example, there is a method in which SiO2 is applied by sputtering and this is easy. The thickness is determined so that an optical path length, i.e. a refractive index multiplied by the thickness becomes equal to λ/2. If the phase becomes flat by such a method, a far-field pattern 7 of the projected beam becomes what has a single peak. If this is applied to an optical system in a laser beam printer or an optical disc, the beam of extremely high output can be obtained with high efficiency.