Effect of disk birefringence on a differential magneto-optic readout

The performance of a magneto-optic readout system can be degraded by a phase shift between two basic polarization components of a readout beam. One source of this phase shift is undesired birefringence in the substrate of an optical disk. The effect of disk birefringence on the signal-to-noise ratio of a differential magneto-optic readout is evaluated. It is shown that this effect may be represented by two parameters J(2) and J(3) that depend only on the birefringence and the numerical aperture of a beam illuminating the disk. Previous papers on this subject either presented involved, open-form expressions for the differential magneto-optic readout or assumed that the beam illuminating the disk is collimated. Here simple closed-form formulas are derived by using parameters J(2) and J(3). The effects of system conditions that can interact with the disk birefringence, such as an imbalance between the differential data detectors, are also considered.