Magneto-Optic Compensator for MO Disk Substrate Birefringence

Injection-molding techniques employed in the fabrication of polymer substrates used in magneto-optic disks introduce stress-induced birefringence. This birefringence, which varies from point to point on the disk, results in a low-frequency noise component to the signal available in the detection of magneto-optically stored data. This paper describes a method, based on the use of a variable-angle Faraday rotator, for compensating for the substrate birefringence. It should be pointed out that the following method is predicated on the assumptions that a) vertical birefingence can be ignored, and b) the in-plane birefringence axes are oriented along the disk radial and tangential axes at each point on the disk. Although it is realized that the vertical birefringence component cannot in general be ignored,1 the compensation presently described should improve the signal-to-noise ratio.