1引言在光数据存储领域，光盘衬底和它对系统性能的影响一直被广泛研究[～12]。众所周知，衬底双折射使磁光数据存储系统产生诸多问题。为测量垂直观折射和横向（面内）双折射[1～9]，以及摘清衬底双折射如何影响这种系统的性能[10～12]已作了很多研究。特别是，衬底双折射使磁 1 Introduction In the field of optical data storage, the influence of optical disc substrate and its performance on the system has been extensively studied [~ 12]. It is well-known that substrate birefringence causes many problems with magneto-optical data storage systems. In order to measure the vertical and horizontal (in-plane) birefringence [1 ~ 9], and how to extract the birefringence of the substrate, the performance of this system [10-12] has been studied. In particular, the birefringence of the substrate causes magnetism