Birefringence distribution in magneto-optical disk substrate fabricated by injection compression molding

It is necessary to improve optical and mechanical properties in optical disk substrates as information storage devices with high storage density using short-wavelength lasers are being developed. Injection compression molding is regarded as the most suitable process to manufacture optical disk substrates with high dimensional accuracy, low residual stress, and superb optical properties. Polycarbonate magnetooptical disk substrates are fabricated by injection compression molding, and the birefringence, regarded as one of the most important optical properties for optical disk, is measured. The effects of various processing conditions on the development of birefringence distribution are examined experimentally. It is found that the values of the birefringence distribution are very sensitive to the mold wall temperature history. By controlling the compression process, the overall birefringence value can be reduced.