Measurement of birefringence of magneto-optical disks

Abstract

Birefringence is the difference in refraction index for different polarization states. Birefringence introduces the phase retardation in the optical system or results in double refraction in the optical path. Injection molded polycarbonate (pc) substrates are widely used as disk substrates for magneto-optical (MO) disks. However, birefringence of the pc disk substrates is still a deficiency for MO recording applications. The resultant phase retardation in the polarization detection system degrades the MO readout signal"2. The double refraction induces additional aberration of astigmatism which both smears the readout of writing pits, and also perturbs the servo signal in servo systems such as astigmatic or knife edge methods. Thus, effective methods applied toward measuring birefringence of pc substrates are essential in controlling the quality of MO disk substrate replications. Conventional methods, e.g. phase compensator or ellipsometer'4 proposed for measuring birefringence of disk substrates do not satisfy all of the requirements. The phase compensator finds it rather difficult in measuring in-plane (normal to the disk thickness direction) birefnngence. The ellipsometric methods are difficult to be set up for measuring inplane birefringence, although it is quite efficient and accurate. Another method,5 using disk tester system to measure both in-plane and normal-to-plane birefringence, is only applicable of using in reflective type disk substrates. The relationship of readout signal and birefnngence of a disk substrate is first derived using Jones matrix in this paper. Both in-plane and normal-to-plane birefnngences ofMO disk substrates are then presented, as measured by the conventional and adaptive method proposed in this study. The data are then applied toward assessing the figure of merits. The implementation as a tool for the fabrication process control of disk substrates is also described.