PURPOSE: To assure the focusing with high accuracy without deteriorating the features of an astigmatism method and at the same time to realize a compact and lightweight structure of an optical pickup, by using a zone plate of stripe structure in place of an expensive cylindrical lens.
CONSTITUTION: The reflected light given from a disk 8 is separated by a beam splitter 2 and led to a photodetector 6 to produce a focusing error signal. A stripe-shaped zone plate 14 having a condensing or dispersing function is used in place of a cylindrical lens which generates astigmatism. The parallel luminous fluxes incident to the plate 14 are focused in the direction of an axis X and not focused in the direction of an axis Y to be converted into cylindrical waves in parallel as they are. Then a focal line 20 is formed at a place of distance (f). Thus a focusing error signal can be obtained from the detector 6.
SAEKI RIICHI
TAKEI TOSHIO
