Abstract

An optical head which uses a single diffraction grating to eliminate the need for both a beam-splitter and a biprism lens. This invention allows the placement of all the elements except the photodetector along a single optical axis with the photodetector immediately adjacent to the optical axis. This arrangement reduces alignment problems and vibrational errors. A semiconductor laser is provided which produces a laser beam which impinges upon a movable objective lens. The objective lens focuses the laser beam onto an information medium. A holographic diffraction grating is placed between the laser and the objective lens. The diffracted beams on the forward path from the laser to the objective lens are not used, but on the return path, one of the diffracted beams is focused onto a photodetector. In a first embodiment, the photodetector is adjacent the semiconductor laser in substantially the same plane. In a second embodiment, an additional lens is used to produce an additional focus point intermediate the laser and the medium, with the photodetector being in substantially the same plane as this additional focal point. By using an appropriate fringe pattern for the hologram lens, the return beam can be focused on the photodetector as desired.