An optical head includes a light emitter/detector to emit a laser light towards
an optical disc and to detect a return light from the optical disc, a diffraction
grating to split the laser light into three beams, and a light converging optical
system to focus the laser light on a recording surface of the optical disc. The
diffraction grating has formed therein a plurality of slits formed whose depth
is selected so that the efficiency of diffraction of first-order light when a first
light beam is incident upon the diffraction grating is higher than that when a
second light beam having a longer wavelength than the first light beam is incident
upon the diffraction grating.
