DEVICE AND METHOD FOR MEASURING BIREFRINGENCE IN AN OPTICAL DATA CARRIER

摘要

A device for measuring birefringence in an optical data carrier (20) has a laser light source (21) with polarization modifying means (22), a beamsplitter (23), a polarizer (24), a photodetector (25), and a controller (26) operatively connected to the photodetector. The laser light source illuminates various spots on the optical data carrier by means of a laser beam. The photodetector receives a reflected laser beam, converts it into an electric signal and supplies this signal to the controller, which determines a value for the birefringence in the illuminated spot on the optical data carrier. Furthermore, the device is provided with a transparent reference element (30) with known birefringence properties, a mirror (31), and means (32) for directing the laser beam towards the reference element (30) in a first calibration position; directly towards the mirror (31) in a second calibration position; and towards the optical data carrier (20) during non-calibration time. In the first calibration position the controller (26) calculates a first calibration value from the signal received from the photodetector, said first calibration value corresponding to a given birefringence in the optical data carrier, and in the second calibration position the controller calculates a second calibration value, corresponding to a situation essentially without any birefringence at all in the optical data carrier. These calibration values are used for determining the birefringence in a given spot on the optical data carrier.