Pupil plane filtering for optical pickup heads with effective numerical aperture of 1.1 and 2.0

A pupil plane filtering technique is applied to data signal detection in an optical data storage system that uses a solid immersion lens (SIL) and a quadrilayer phase change recording medium. Two systems that have the effective numerical aperture (NA_EFF) of 1.1 and 2.0 are investigated. A new filter is designed for the NA_EFF = 2.0 system on the basis of this study. We have confirmed by numerical calculations that the technique improves signal contrast, which is defined by the ratio of the difference signal to the sum signal between the homogeneous crystalline area and the homogeneous amorphous area. We have also confirmed that the technique makes the contrast less sensitive to the gap width between the SIL and the recording medium. The vectorial feature of the light is considered in the calculations. The calculated irradiance distributions and signal levels for the system with NA_EFF = 1.1 are confirmed by experiments.