Optical information reading apparatus with multi film elements to compensate for wavelength fluctuation

摘要

An apparatus for optically reading information out of an opto-magnetic disk including a semiconductor laser emitting a polarized laser light beam which is made incident upon the opto-magnetic disk via beam splitter, reflection surface and objective lens. The light beam reflected by the opto-magnetic disk is made incident upon a photodetector by the objective lens and beam splitter. In order to compensate for the phase difference of P polarized and S polarized light components of the light beam reflected by the opto-magnetic disk within a wavelength over which the wavelength of the light beam might fluctuate due to the temperature variation, multi film constructions are provided in the beam splitter and reflection surface, the multi film constructions having such characteristics that the wavelength dependencies of the phase difference of the beam splitter and reflection surface can be made substantially a constant value such as zero and 90 degrees.