Abstract: An optical information processing apparatus comprising a diffraction grating for producing a zero-order beam which is a beam emitted by a light source and allowed to pass therethrough and two plus and minus first-order sub-beams respectively having positive and negative astigmatisms, radiated in directions at slight angles different from the main beam, wherein said two sub-beams are focused at opposite positions across said main beam, wherein focusing control is performed by a focusing error signal detected from a difference of signal amplitudes obtained from the respective reflected light quantities of said two sub-beams and wherein tracking control is performed by an off-track signal obtained from a signal according to a reflected light quantity of said main beam.

Abstract: Disclosed herein is a compact, adjustment-free, unitary optical head wherein diffraction grating made of a birefrigence material and having convexities and concavities of grating grooves so set that the difference of their optical path lengths is some multiples and a half of the wavelength for an ordinary beam and an extraordinary beam, a diffraction grating providing astigmatisms to .+-.1 order beams, a quarter wavelength plate, and other optical components, are interposed between a semiconductor laser and a focussing lens and are stored inside an optical head body filled with a dry nitrogen gas. A focus error detection signal is obtained from the difference of the degrees of modulation of the .+-.1 order reflected beams by pre-wobbled pits, and adjustment of the optical beam can be carried out by driving the body by a two-directional head actuator.