Recording or replay device and method for recognition of a focussing status

摘要

The method identifies the focussing state of a scanning light beam (14) incident on a data carrier (11). Astigmatism is produced in a light beam (15) reflected by the data carrier (11). The reflected light beam is split by a birefringent element (23-24) into an ordinary and an extraordinary fractional beam (25,26). The focus state of the scanning light beam (14) is inferred from the cross-sectional state of the extraordinary light beam (26). A recording or play-back device for reading from or writing to optical information carriers uses this method. The device has a light source (12) which illuminates the data carrier. The birefringent element (23-24) produces astigmatism in the reflected light beam (15).