Focus error detecting element and optical head using the same.

摘要

An optical system for detecting a focus error signal in an optical disc according to the astigmatic method is constituted by a single lens element. This lens element has a first lens surface defined by a toric surface effective to converge the incident rays of light and also to produce an astigmatism. This lens element also has a second lens surface which has a negative power so as to form a telephoto optical system, making it possible to shorten the length of the optical system. <IMAGE>