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The possibility of an areal density over 50 Gbit/in2 was examined in near-field phase-change recording. The disk structure was optimized to maximize readout signals under the land-and-groove recording condition at a tracking pitch of 160 nm. We also evaluated the signal crosstalk from adjacent tracks. Eye diagrams of 50.4 Gbit/in2 areal density were demonstrated using 1.5 NA optics and a GaN laser diode. The track pitch and linear bit density are 160 nm and 80 nm/bit, respectively. The transmission electron microscope (TEM) micrograph of recorded amorphous marks at an areal density of 50.4 Gbit/in2 is also presented.

Previously we showed substrates without any pit deformation in the case of injection molding a 40 Gbit/inch2 read only memory (ROM) substrate using the single-sided heat insulated mold, in which a heat insulating plate was located below a stamper, and the slow mold opening method. This type of mold is characterized by the fact that pits on the substrates can be replicated at a low mold temperature, to avoid the twisting of substrates. However, it was found that the radial tilt was large. Therefore, we investigated this phenomenon using simulations, and fabricated a double-sided heat insulated mold with a symmetric structure in the thickness direction of the substrates. We showed that performing injection molding using this double-sided mold rendered the radial tilt of substrates controllable, so that it was possible to obtain flat disks. Furthermore, we obtained a 120-mm-diameter 100 Gbit/inch2 ROM substrate, which had no pit deformation over the entire pit area even though the pit wall angle was greater than 80 deg. This density corresponds to approximately 140 Gbyte.

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.

We have investigated high-density magneto-optical recording on a conventionally structured disk with a diameter of 64 millimeters using magnetic field modulation to achieve a capacity of 650 Mbytes. With the groove conditions optimized, the jitter with cross-talk from adjacent tracks was 7.2% and the recording power margin including overwriting and crosswriting was ±20%. As a result of minimizing cross-talk, a sufficient tilt margin of ±0.82 degree and ±0.63 degree in the radial and tangential direction, respectively, was obtained.

SNR of a polycarbonate MO disk with a narrow track of 0.85 μm and with a quadrilayer structure was improved by optimizing and improving the groove geometry, film composition / thickness, and birefringence of the substrate. By using a 690 nm LD, an NA of 0.55, magnetic field modulation, and RLL(1,7) coding, a high areal recording density of 2.5 Gbit/in2 was obtained. Moreover, the PRML detection of PR(1,2,1) increased the density up to 2.9 Gbit/in2.

At present high-density optical discs have narrow track pitch values, and hence it is necessary not only to verify the precision of the track pitch, but also to determine the degree of track pitch variation. Track pitch variation (T P V) is commonly measured using scanning electron, laser scanning and scanning tunneling microscopies. However, these methods are time consuming and it is difficult to take T P V readings from large areas of the disc. We developed a T P V measurement system based on an optical disc player with two readout beams, and using this system we can rapidly measure the T P V over the whole disc surface. In this paper, we describe our T P V measurement system and present the results of T P V measurements taken from a disc containing continuous grooves with a track pitch of 0.84 µm. We found that the T P V of these grooved discs ranged from 30 nm to 100 nm peak-to-peak (pk-pk) with a typical value of 40 nm pk-pk. The measurement resolution of our instrument is about 10 nm pk-pk.