Yuichi Komatsu

Dual wavelength optical head for 0.6 mm and 1.2 mm substrate thicknesses

A dual wavelength optical head for 0.6 mm and 1.2 mm substrate thicknesses, containing 635 nm and 785 nm laser diodes, has been developed. Holographic optical element (HOE) type optics and variable magnification type optics are described. For both types of optics, high light power efficiency is obtained, which is suitable for random access memory (RAM) applications. Moreover, the use of 785 nm light makes it possible to read compact disc-recordable (CD-R). A readout operation for a quadruple density CD and a conventional CD has been successfully demonstrated.

Three-Dimensional Recording with Electrical Beam Control

A concept of a novel optical storage system has been proposed, which has the large-capacity characteristic because of three-dimensional recording and readout, and high-reliability and low-power-consumption characteristics achieved by controlling a light beam focused in a recording medium electrically instead of mechanically. On the basis of this concept, an optics for microholographic recording was constructed, and electrical beam control elements that vary the position of focused spots in the recording medium in both in-plane and vertical directions were incorporated into the optics. Liquid crystal deflectors and liquid crystal variable-focus lenses were used as the electrical beam control elements for in-plane and vertical beam control, respectively. Plural bits were recorded and read out in both in-plane and vertical directions of the recording medium, and the concept has been experimentally demonstrated.

Land/Groove Signal and Differential Push-Pull Signal Detection for Optical Disks by an Improved 3-Beam Method

In land/groove recording, it is necessary to detect both a land/groove signal and a differential push-pull signal for stabilization of the tracking servo. The former discriminates between land and groove during track pull-in operation, while the latter compensates for offset of the tracking error signal due to shift of the objective lens. This paper proposes an improved 3-beam method for detecting both signals with the same beam configuration. A 4-segment diffraction grating is employed, in which the grating patterns for two diagonal regions are π/2 out of phase. A land/groove signal is obtained from sub-beam signals, while a differential push-pull signal is obtained from the difference between the main beam and sub-beam signals. Features of this method are the applicability to disks with different track pitches and the robustness against disk eccentricity. A land/groove signal was observed, and the compensation of the tracking error signal offset with shift of the objective lens was confirmed experimentally. These results have demonstrated the validity of this method.

Dual-wavelength optical head with a wavelength-selective filter for 0.6- and 1.2-mm-thick-substrate optical disks

A dual-wavelength optical head, believed to be new in design, for 0.6- and 1.2-mm-thick-substrate optical disks was developed by use of 650- and 780-nm-wavelength light and a wavelength-selective filter, which reduces the spherical aberration that is due to the difference in substrate thicknesses and restricts the numerical aperture for 780-nm-wavelength light. According to this configuration, both high light power efficiency and wide image field characteristics are obtained, which are suitable not only for read-only but also for recordable or rewritable optical disk systems. A reading operation for a digital video disc and for a compact disc and a recording operation for a phase change optical disk were successfully demonstrated.

Blue/DVD/CD compatible optical head

Compatibility with Digital Versatile Disc and Compact Disc has been demonstrated for two options of high-density optical disk systems using a blue laser diode. Option 1 (HD DVD) employs a 0.65 numerical aperture objective lens and a 0.6 mm thick protective layer disk, while Option 2 (Blu-ray Disc) employs a 0.85 numerical aperture objective lens and a 0.1 mm thick protective layer disk. An optical head has three laser diodes whose wavelengths are 405 nm, 660 nm, and 785 nm. A spherical aberration caused by the difference in the protective layer thickness of the disk is corrected by an objective lens magnification change, while the numerical aperture of the objective lens is controlled by a wavelength-selective aperture. Experimental results have shown that Option 1 is more preferable, but Option 2 is also acceptable from the viewpoint of the compatibility.

Microholographic Recording Using Single-Sided Optics with Electrical Beam Control

We have proposed a concept of a novel optical storage system for professional use combining three-dimensional recording with electrical beam control[1], which features high-reliability and low-power-consumption characteristic as well as large-capacity characteristic, and demonstrated the concept by using microholographic recording[2]–[5] as three-dimensional recording and liquid crystal elements for electrical beam control.

Optical Head Using Single Objective Lens for Four Types of Optical Disks

An optical head using a single objective lens for Blu-ray disc (BD), high-density digital versatile disc (HD DVD), digital versatile disc (DVD), and compact disc (CD) has been developed, which is suitable for high-speed recording and reading. A relay lens unit, which consists of two convex lenses and one concave lens, corrects the spherical aberration and realizes high light power efficiency for each disk. A numerical aperture (NA) control element, which contains a liquid crystal aperture and a wavelength selective aperture, controls the NA of the objective lens for each disk. A coma aberration caused by the objective lens shift is detected based on differential phase detection signals for read-only disks and push–pull signals for recordable or rewritable disks, and is corrected by moving one of the lenses in the relay lens unit by an actuator. A diffraction-limited focused spot and good readout signal characteristics were obtained for each disk, and the validity of this optical design has been demonstrated.

Radial tilt detection using 3-beam optical head

An optical disk system which uses a high numerical aperture (NA) objective lens is effective for achieving high recording density. However, in such a system, it is necessary to detect and compensate for the radial tilt of the disk, because the coma aberration which deteriorates the read/write characteristics increases and the radial tilt margin becomes narrower. For this purpose, a radial tilt detection method for DVD-RAM disks using the CAPA (Complementary Allocated Pit Address) was reported. This paper proposes a new radial tilt detection method for rewritable optical disks using a 3-beam optical head. This method features high-sensitivity characteristics and is suitable for high-frequency servo operation, because the radial tilt signal is obtained from push-pull signals instead of header signals.

Compact magneto-optical disk head integrated with chip elements

A compact magneto-optical disk head, integrated with chip elements and optical parts, using a hybrid assembly technique, has been developed. Optics and beam splitter characteristics were optimized so as to realize both compact size and sufficient read out characteristics. More than 46 dB C/N ratio and stable operation were achieved, while simple optics and an easy assembly method were used.

Reduction of Interlayer Crosstalk in Dual-Layer Optical Disks Using Patterned Phase Plate

In dual-layer optical disks, light reflected from an adjacent layer overlaps and interferes with light reflected from the target layer on the photodetector. As a result, the signal obtained from outputs of the photodetector fluctuates. In this paper, we propose an optical head which employs a six-segment patterned phase plate in the return path to reduce interlayer crosstalk (fluctuation of a differential push–pull signal) without degrading the readout signal. The patterned phase plate weakens the intensity of the main beam from the adjacent layer which enters the sub-beam detecting sections of the photodetector and restricts the interference between sub-beams from the target layer and the main beam from the adjacent layer. Beam intensity distributions on the photodetector and push–pull signals have been investigated through numerical simulations and experiments. The results demonstrate that interlayer crosstalk can be sufficiently reduced using the patterned phase plate.