Measuring distribution of the ellipsoid of birefringence through the thickness of optical disk substrates

Simple recurrence matrix relations for multilayer anisotropic thin films

Generalized Abelès relations for one anisotropic thin film [E. Cojocaru, Appl. Opt. 36, 2825-2829 (1997)] are developed for light propagation from an isotropic medium of incidence (with refractive index n(0)) within a multilayer anisotropic thin film coated onto an anisotropic substrate. An immersion model is used for which it is assumed that each layer is imaginatively embedded between isotropic gaps of zero thickness and refractive index n(0). This model leads to simple expressions for the resultant transmitted and reflected electric field amplitudes at interfaces. They parallel the Abelès recurrence relations for layered isotropic media. These matrix relations include multiple reflections while they deal with total fields. They can be applied directly to complex stacks of isotropic and anisotropic thin films.

Subtle effects of the substrate in optical disk data storage systems

Much attention has been focused on the effects of the disk substrate in optical data storage. In particular, substrate birefringence has been studied extensively because it causes significant problems in magneto-optic systems. We investigate certain subtle effects of the substrate, such as feedback into the laser diode, in compact disk and phase-change systems. Our analysis of the compact disk system led us to discover a new technique for the rapid measurement of the substrate birefringence. We also address the question of how focusing the laser beam through the substrate will affect the depth of focus.

Temperature dependence of the birefringence of optical-disk substrates

Injection-molded polycarbonate substrates are used predominantly in read-only, write-once, phase-change and magneto-optic disks for data storage. The in-plane and vertical birefringences of these substrates adversely affect the performance of optical data-storage systems. The disks are typically expected to operate in the ambient temperature range of 5-50 °C. We have investigated the behavior of the in-plane and vertical birefringences of a polycarbonate disk substrate in this temperature range using a custom-built ellipsometer. This study reveals that the in-plane birefringence changes dramatically within the investigated range of temperatures, whereas the vertical birefringence remains essentially constant. We suspect that the change in birefringence is due primarily to thermally induced stress in the substrate.

Simple birefringence measurement method for coated optical disks with a fixed incident angle ellipsometer

In a coated and laminated optical disk, only a reflectance method is available for oblique incidence retardation measurements to evaluate the birefringence in a polycarbonate substrate. I propose a simple measurement method that simultaneously measures both the lateral and vertical birefringence in a coated substrate. In this reflectance method, only two oblique incidence retardation values with a fixed incident angle are measured from the radial and the circumferential directions of the disk. Using the sum and the difference of these two retardation values, we can calculate both birefringences without any cumbersome curve-fitting procedure. This method can easily be introduced into the routine inspection for optical disk manufacturing.

Retroreflecting ellipsometer for measuring the birefringence of optical disk substrates

A retroreflecting ellipsometer has been constructed for measuring the birefringence of optical disk substrates. In contrast to conventional ellipsometers with two mechanical arms, this system has only one arm along which both the incident and reflected beams travel. This construction eliminates the mechanical limitations of conventional ellipsometers, thereby permitting normal incidence on the sample. In addition, the single arm is adjustable in two dimensions, with the polar incident angle, θ(inc), varying from 0° to 70°, and the azimuthal incident angle, Φ(inc), varying from 0° to 360°. The condition of normal incidence permits accurate measurement of in-plane birefringence. The adjustability of both θ(inc) and Φ(inc) is necessary for the measurement of possible tilts of the index ellipsoid, and also for the variation of birefringence through the substrate thickness. Measurement results showing the useful features of the equipment are presented. The optics of the hemispherical assembly used for retroreflection as well as for the elimination of undesirable refractions are also studied by use of the ZEMAX lens design program.