Kenya Goto

Proposal of Ultrahigh Density Optical Disk System Using a Vertical Cavity Surface Emitting Laser Array

An ultrahigh density optical disk system of Tera bit/inch2 is described. This system utilizes a vertical cavity surface emitting laser (VCSEL) diode array as a parallel optical beam source which can make the disk system more useful at a high data transfer rate and bit density using evanescent waves from each laser cavity. In order to avoid the diffraction limited restriction to the optical beam the near-field optics is adopted to the parallel beam optical head. 1 bit size on the surface of the disk is 10–20 nm in diameter. A polished glass disk substrate is deposited by either phase change medium or photon mode optical medium. The medium thickness of the disk is approximately 10–20 nm. The optical gap size between the surface of the medium and laser output windows of a VCSEL array head is controlled within 20 nm using contact head technology.

Parallel Recording Array Head of Nano-Aperture Flat-Tip Probes for High-Density Near-Field Optical Data Storage

Increasing the memory capacity and data transfer rate of optical data storage to match the market requirements is now a challenging task. To realize a more effective and simple memory system, a parallel near-field optical system has been proposed using vertical cavity surface emitting laser (VCSEL) microprobe array heads. The concept, structure and fabrication process of new flat-tip microprobe arrays have been discussed and realized by the preparation of a silicon probe array in this research. Flat-tip probes are advantageous for improving optical properties since they are prepared from materials of high refractive index and the array shows good structural design for the contact head system with good uniformity of the probe height. We have successfully prepared Si nano-aperture probe arrays with the aperture size of 150 to 500 nm using microfabrication techniques, including photolithography, wet chemical etching, and a newly developed aperture formation process which uses a SiO2 mask layer. The microstructural observation of Si flat-tip probe arrays is in good agreement with our design concepts and supports the strong possibility of their application to actual recording heads. We are now developing a monolithic nano-aperture VCSEL probe to complete our parallel near-field optical system with high memory capacity and fast transfer rates in the near future.

Microoptical Two-Dimensional Devices for the Optical Memory Head of an Ultrahigh Data Transfer Rate and Density Sytem Using a Vertical Cavity Surface Emitting Laser (VCSEL) Array

The parallel optical memory system has important advantages for realizing both a fast data transfer rate and high memory capacity since it is based on multibeam recording and a smaller spot size using the vertical cavity surface emitting laser (VCSEL) and a nanoprobe array. The concept, theoretical analysis and fabrication process for the integrated VCSEL nanoprobe array head are discussed with emphasis on the micro-optical issues such as the improvement of optical efficiency by microlens focusing. The flat-tip nanoprobe structure was successfully prepared with the small metal aperture of 150 nm and 1.25% optical throughput using metal aperture Si or GaP semiconductor nano-probes. To realize better optical throughput in the integrated VCSEL, a special nanoprobe array and a microlens array were developed. The microlens array was prepared using a thermal reflow process of the photoresist and lens shape pattern transfering to the semiconductor nanoprobe bottom face by dry etching. Since this two-dimensional array system requires three-dimensional micro-optical adjustment to focus a very small spot on the recording media, this research can provide guidelines for new micro-optical components in future technology.

Fabrication of Micro-Pyramidal Probe Array with Aperture for Near-Field Optical Memory Applications

A new optical memory system is urgently required to realize high memory capacity and fast data transfer rates in the coming multimedia era. To overcome the current capacity barrier of far-field techniques, a novel near-field optical memory of evanescent wave has been proposed using a vertical cavity surface emitting laser (VCSEL) probe array, consisting of the VCSEL array as a light source and the micro-pyramidal probe array as an evanescent wave exit. The design and fabrication for the aperture probe array were developed in this research. An array of up to 10,000 pieces was prepared successfully using microfabrication processes, including photolithography, silicon wet etching, thermal oxidation and thin film deposition. The pyramidal probes in the array show little variation in size and are sufficiently sharp to apply to the near-field recording head. The probe tip has a small size of 100 nm and the aperture has a diameter of around 150 nm. A smaller aperture can be fabricated with more careful control of the experimental procedures, including the etching process and thermal wet oxidation.

Proposal of Optical Floppy Disk Head and Preliminary Spacing Experiment between Lensless Head and Disk

A new optical floppy disk system using a very simple optical head which consists of a laser diode chip, a photodiode chip and a heatsink is proposed, and fundamental experiments with laser diodes are performed. The system has no optical element like a lens or a prism, and also no focus actuator. Experimental results show that the spacing between the recording film and the light-emitting edge of a laser chip could be designed to be as great as 5-10 µm, which is greater than the expected value. This is avarable solution for improving the dust problem between the head and the disk. As the head is simple and low in weight, a low-cost disk drive with high access time may be possible.

Fabrication of Micro-Aperture Surface Emitting Laser for Near Field Optical Data Storage

We fabricated a micro-aperture vertical cavity surface emitting laser (VCSEL) for use in near field optical data storage. A composite mirror of Au and a 11-pair semiconductor distributed Bragg reflector (DBR) provides a reflectivity of more than 99%. Fabricated VCSEL devices exhibited sub-mA low threshold operation. A micro-aperture with a diameter below 400 nm was introduced in Au film using focused ion beam etching. A localized near field at a 400 nm aperture was observed and the output power from the aperture was estimated to be 10 µW.

Spherical Grating Objective Lenses for Optical Disk Pick-ups

Plane grating collimator lenses [GCLs] and spherical grating objective lenses [SGLs] have been developed and installed in different optical pick-ups for CD players. The relief profile for the grating lenses have been designed to obtain high diffraction power efficiency of about 80% for the GCLs and about 65% for the SGLs. The blazed grating profile has been engraved by diamond turning lathes. GCLs were mass-produced by the plastic injection molding method and installed in more than 80 thousand optical pick-ups for portable and general audio use. SGLs have a much higher NA and can be used as objectives for the optical pick-ups. A thin plastic relief grating profile for the SGLs is formed by the photo-polymerization method on a flat face of a piano-spherical glass substrate. The field angle for the SGLs is more than ±0.5 degree, within the Marechel Criterion.

High-Throughput GaP Microprobe Array having Uniform Aperture Size Distribution for Near-Field Optical Memory

The GaP metal-aperture microprobe array has been studied for application to the parallel near-field optical array head of high memory capacity and fast data transfer rate. The fabrication of such an array having uniform aperture size distribution was realized using a low-temperature etching solution of HF:HNO3 and the careful control of ion-milling parameters. After the flat-tip formation by the wet etching method and the aperture formation by the ion-milling process, GaP microprobes of more than 90% in the array show an acceptable size distribution in the range of 150±30 nm. It was observed from the near field scanning optical microscope (NSOM) observation that tiny apertures are formed on the top of the microprobes and light comes out only from the aperture. The light penetrated through the 200 nm aperture shows approximately 170 nm beam size in full width half maximum (FWHM). Due to the high refractive index, the cut-off diameter of GaP microprobes decreases to approximately 200 nm, resulting in improved optical throughput with more than 100 times increment compared with that of the conventional optical fiber probe. For the microprobe having 150 nm aperture, 1.25% optical throughput was realized in this research. The design concept for the semiconductor microprobe array having high optical throughput with uniform aperture size distribution has been proved experimentally.

The Development of a Novel Optical Floppy Disk Drive Using a Phase Change Optical Medium and a Quasi-Near-Field Optical Head

The optical memory system still has some shortcomings such as slow tracking access speed and high fabrication cost due to its relatively heavy head weight and complex optical elements. To overcome these limitations, a new lensless optical floppy disk drive has been developed using a phase change optical floppy disk and an external cavity semiconductor laser with a good antireflection (AR) coating on the emitting facet. The main components of this novel system are a contact-type head of quasi-near-field optics and a phase change medium prepared on a flexible polyimide substrate. The AR-coated semiconductor laser oscillates externally with the disk and its laser emission output shows sufficient variance to detect the phase difference on the recording layer. In our primitive drive, the simple optical head shows good writing performance and the recorded marks present sub-micrometer size, which indicate the high possibility of the development of a new, inexpensive floppy system of above 500 MB data capacity.

Numerical Simulation of Readout Using Optical Feedback in the Integrated Vertical Cavity Surface Emithing Laser Microprobe Head

The vertical cavity surface emitting laser (VCSEL) characteristics of voltage change in the integrated VCSEL microprobe head have been analyzed using the finite difference time domain (FDTD) method and laser rate equations. The reflectance difference of 9.7% from the phase change media was calculated with the GaP microprobe having a 150 nm aperture, resulting in the voltage change of 2.9 mV in the VCSEL. The 30 dB CNR of the readout signal was evaluated with the relative noise of VCSEL and it was found that the CNR could increase with further improvement of the VCSEL structure and the reflectance difference in the media.