Substrate Birefringence Measurement Techniques

Abstract

A variety of techniques have recently been published for measuring the bulk average values of the MO disk substrate birefringence(1-4). Interpretation of the bulk average birefringence data has been explored in great detail(5). Birefringence in polycarbonate, PC, magneto-optical, MO, disk substrates arises from a combination of frozen-in polymer chain orientation and a volume change during cooling or thermal stresses (6,7). The historical method for evaluating a subsurface stress is to drill a small shallow hole in the surface and observe the strain in the surface surrounding the hole(8). The stress/strain field around stamped in pits in optical disks may produce local fluctuations in the birefringence if the pits are considered analogous to holes drilled in a stressed surface. However, the hole drilling method provides only the stress near the surface.