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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.

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.

This article will describe a series of experiments we conducted for the purpose of pursuing the resolution limit of the PTM process. Disc samples with different track pitches and different linear densities were prepared through the PTM process. The signal properties of these discs were analyzed with the assistance of a near field readout system. The jitter value was 5.6% at the linear density of 70nm/bit and 0.16μm track pitch, which corresponded to 80GB per 12cm diameter disc. Even at a recording density corresponding to a capacity of 100GB we observed jitter values less than 10%.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The areal density of an optical storage disk can be increased by improving the transverse resolution of the optical head which is proportional to λ/NA. In order to improve the resolution, a lot of research has been carried out to develop shorter wavelength light sources such as green or blue SHG lasers1,2) and recently ZnSe or GaN semiconductor blue lasers3). These lasers will soon become commercially available. On the other hand, it is difficult to make a high NA objective using a single molded aspherical element. Furthermore the higher the numerical aperture, the less the tolerance for variations in disk thickness, disk tilt, and focusing which are inversely proportional to the fourth, third, and second power of the numerical aperture, respectively. Other experiments have been carried out to improve the resolution using near-field optics4) such as scanning probes which can improve the areal density drastically, however, such probes seem to be tolerated only for working distances of around 100 nm.

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.

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.

We detected bits sized at 0.237 µm bit length and 0.85 µm track pitch recorded by magnetic field modulation on a magneto-optical disk with a 690-nm laser diode and 0.60-numerical-aperture-objective using a partial-response maximum-likelihood detection method. We confirmed that the optimum partial-response for this high linear density had four coefficients of 1 by analyzing difference metrics of error critical paths. The recorded bits with areal density of 5.6 bit/µm2 were also detected at a low error rate, which combination is 0.237-µm bit length and 0.75-µm track pitch. Its write power tolerance is limited to ±22%.

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.