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We describe a method of obtaining an optical air gap control signal for near-field optical disc systems. Because of the total reflection on the surface of a solid immersion lens (SIL), the polarization state of the reflected light differs from that of the incident light. However, when the SIL is close to a disc surface, the polarization state difference between the reflected and the incident light becomes small. We used this difference to obtain an air gap control signal. Both observed and calculated results show that this signal is negligibly affected by the pits on the disc. Thus, our air gap servo system is stable. We show the observed readout signals of a 40 GB ROM disc (with a 12 cm diameter) obtained by an optical head with a 1.4 NA objective lens and a 405nm wavelength GaN LD. Comparisons between observed and calculated results are made.

Land/groove recording in an MO system was demonstrated using MFM recording technology and a 690 nm wavelength diode laser. The CNR of 55 dB and cross-talk of about -35 dB were obtained at a 1.88 micrometers recorded mark length and a 0.7 micrometers yielded a 0.35 micrometers bit length and a 0.7 micrometers track pitch were 8.1% on land and 8.7% on groove. The disk tilt margins were radially +/- 0.42 deg and tangentially +/- 0.45 deg when reading out on land, and +/- 0.40 deg and +/- 0.44 deg on the groove. The recording power margins were estimated to be +/- 8.0% on land and +/- 9.0% on groove considering over-writability and cross-write characteristics.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Near-field optical recording was implemented on a pre-grooved phase-change disk by using a φ1 mm superhemispherical solid immersion lens (SIL) of 1.5 numerical aperture. The SIL unit was mounted on a two-axis actuator for enabling both gap-height and tracking controls. The gap height was precisely adjusted with a capacitance formed between the SIL and disk materials. Groove structures were fabricated on a glass substrate by a reactive-ion-etching (RIE) process and flattened with spin-on-glass (SOG) coating for obtaining reliable push-pull tracking error signals. The phase-change layer stack was optimized by a vector diffraction analysis in order to maximize evanescent coupling at the gap height as small as 50 nm. A bit density of 80 nm was demonstrated on the land of a pre-grooved disk at the wavelength of 407 nm.

We report the results of magneto-optical (MO) detection using an optical phase shifter and a deep land/groove disk. We measured the recording characteristics in the direction of incident linear polarization and found that polarization perpendicular to the groove is best for a deep groove disk. Using perpendicular polarization, with a 693 nm wavelength diode laser and a 0.55-NA objective lens, sufficient readout and recording margins to guard against optical errors were obtained by the experiments at an area density of 3.69 Gbit/in2, which was obtained using a 0.35-µm bit length and a 0.5-µm track pitch.

We report the results of magneto-optical (MO) detection using an optical phase shifter in land/groove recording. We found that the MO detection method improves not only readout carrier level but also cross-talk characteristics against optical errors. Using this method with a 693-nm wavelength diode laser and a 0.55-NA objective lens, sufficient readout margins against optical errors were experimentally obtained at a 3.69-Gbit/in2 areal density, which yielded a 0.35-µm bit length and a 0.5-µm track pitch. Recording with a track density 1.4 times higher was achieved by introducing a 640-nm wavelength diode laser, a 0.7-NA objective lens, and a 0.258-µm bit length and yielded an areal density of 6.95 Gbit/in2.