Takao Kondo

Gap Servo System for a Biaxial Device Using an Optical Gap Signal in a Near Field Readout System

A readout method using a near field optical head has been proposed as a technique to read out over 30 Gbytes on a 12 cm disc. For the purpose of reading out the RF signal in the system, a gap servo is required to maintain a constant air gap between the solid immersion lens (SIL) and the disc. In this paper, we propose a practical near field readout system with a similar system to an ordinary optical disc, where a biaxial device is used as an optical head actuator and the gap error is detected optically. First, we explain the gap servo system by which it is possible to control the air gap from a far field initial position to a near field target position at a relatively high speed without a head colliding with a disc. Second, we report the result of applying this gap servo system to a 30 Gbyte and a 50 Gbyte ROM disc. Finally, its performance is confirmed by comparing simulated signal with an observed signal.

High-Density Near-Field Optical Disc Recording

We developed a high-density near-field optical recording disc system using a solid immersion lens. The near-field optical pick-up consists of a solid immersion lens with a numerical aperture of 1.84. The laser wavelength for recording is 405 nm. In order to realize the near-field optical recording disc, we used a phase-change recording media and a molded polycarbonate substrate. A clear eye pattern of 112 GB capacity with 160 nm track pitch and 50 nm bit length was observed. The equivalent areal density is 80.6 Gbit/in2. The bottom bit error rate of 3 tracks-write was 4.5×10-5. The readout power margin and the recording power margin were ±30.4% and ±11.2%, respectively.

High-density near-field readout using diamond solid immersion lens

We investigated high-density near-field readout using a diamond solid immersion lens (SIL). A synthetic single-crystal chemical vapor deposition diamond provides a high refractive index and a high transmission for a wide wavelength range. Since the refractive index at a wavelength of 405 nm is 2.458, we could design a solid immersion lens with an effective numerical aperture of 2.34. Using the diamond SIL, we observed the eye pattern of a 150-GB-capacity (104.3 Gbit/in.2) disk with a track pitch of 130 nm and a bit length of 47.6 nm.

High-Density Recording with a Near-Field Optical Disk System Using a Medium with a Top Layer of High Refractive Index

A near-field optical technology using a solid immersion lens (SIL) has been actively studied to expand the storage capacity higher than 100 Gbytes per layer in a 12-cm-sized optical disk. However, the working distance of an objective lens in a near-field optical disk system should only be 25 nm or less. Therefore, from the practical viewpoint, a topcoat layer is required to protect the recording layer when the SIL collides with a disk surface because of disturbances such as dust, shock and vibration. From a mechanical viewpoint, the topcoat should have the mechanical toughness to protect the disk surface. Moreover, from an optical viewpoint, it should have refractive index higher than the numerical aperture of an SIL to achieve a sufficient evanescent couple between the SIL and disk surface. In this study, we describe a topcoat for a near-field optical recording. First, we investigate the topcoat performance from the optical viewpoint. Second, we evaluate the topcoat performance from a mechanical viewpoint. Finally, we report the results of recording experiments for a disk with the topcoat and discuss its performance.

High-Density Near-Field Readout over 50 GB Capacity Using Solid Immersion Lens with High Refractive Index

We have investigated high-density near-field readout using a solid immersion lens with a high refractive index. By using a glass material with a high refractive index of 2.08, we developed an optical pick-up with the effective numerical aperture of 1.8. We could observe a clear eye pattern for a 50 GB capacity disc in 120 mm diameter. We confirmed that the near-field readout system is promising method of realizing a high-density optical disc system.

Readout Method for Read Only Memory Signal and Air Gap Control Signal in a Near Field Optical Disc System

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.

Near-Field Optical Head for Disc Mastering Process

Near-field recording for the disc mastering process was realized. A high-numerical-aperture (NA) objective (NA=1.4) composed of an aspherical lens and a solid immersion lens (SIL) was driven by an air pad and a piezotransducer. The gap between the SIL and the glassmaster was controlled by a new gap detection method using the totally internally reflected beam from the bottom surface of the SIL. In the near-field operation, the gap was maintained at 100 nm with a residual error of ±4.8 nm. In the DC exposure with a laser beam of 413 nm, a narrower groove structure could be formed almost at the inverse rate of NA expansion. In the read-only memory (ROM) exposure with an eight to fourteen modulation plus (EFM+) signal, signal bottom jitter values of 9.8% for an 18 GB disc and 10.9% for a 20 GB disc were obtained.

High-density near-field readout using solid immersion lens made of KTaO3 monocrystal

We developed solid immersion lenses made of a KTaO3 monocrystal. The refractive index of KTaO3 is 2.382 at a wavelength of 405 nm. Using KTaO3 as the raw material of a solid immersion lens, we could design an effective numerical aperture of 2.20. We observed an eye pattern of a 150 GB capacity with a 130 nm track pitch and a 47.6 nm bit length. The areal density is 104.3 Gbit/in.2.

High-density near-field optical disc recording using phase change media and polycarbonate substrate

We developed a high density near field optical recording disc system with a solid immersion lens and two laser sources. In order to realize the near field optical recording, we used a phase change recording media and a molded polycarbonate substrate. The near field optical pick-up consists of a solid immersion lens with numerical aperture of 1.84. The clear eye pattern of 90.2 GB capacity (160nm track pitch and 62 nm per bit) was observed. The jitter using a limit equalizer was 10.0 % without cross-talk. The bit error rate using an adaptive PRML with 8 taps was 3.7e-6 without cross-talk. We confirmed that the near field optical disc system is a promising technology for a next generation high density optical disc system.

Head Analysis in Air Flow on Near-Field Optical Disk System with 2-Axis Actuator

Several near field optical disk systems have been proposed and are expected to be the tools for realizing large-capacity optical disk storage systems. However, there remains a concern as to whether removability can be realized in near-field optical disk systems. To resolve this concern, we analyze a near-field optical head installed onto a 2-axis actuator in an air flow. Finally, we conclude that a conical head and the active servo of a 2-axis actuator contribute to the stability of near-field playability with respect to dust robustness.