Depth distribution of birefringence in magneto-optical recording disk substrates

Abstract

Polycarbonate substrates are the industry standard for optical storage. Optical anisotropy is frozen into the substrate as polymer orientation during mold flow and thermal stress during cooling. The substrate optical anisotropy degrades the readback of the data [carrier-to-noise ratio (CNR)] by increasing the noise level on Kerr rotation measurement. There are also synergistic effects of the disk optical properties with focus and tracking optics. For optimum CNR, the beam is focused on the magneto-optical (MO) film. For optimum tracking, the beam is focused to maximize track error signal (TES). For the disks considered the difference between the offset for optimum CNR and that for optimum TES was between 300 and 600 μm. This results in a compromise between optimum CNR and TES, which could limit MO storage density and performance. Conventional methods of bulk optical anisotropy measurement are unsuccessful in explaining this variance. These give an average or effective optical property. The optical properties are also known to vary with the depth in the substrate and especially near the mold walls as determined by optical polarizing microscope. A scanning micropolarimeter instrument is presented to rapidly measure the depth distribution of the optical properties. The interaction between the obliquely incident polarized beam with the microscopic variation in the index of refraction throughout the depth of the substrate is proposed as the source of the difference between the optimum focus offsets.