Shunji Ohara

New Optical Erasable Medium Using Tellurium Suboxide Thin Film

A suboxide thin film of TeOx ( x is smaller than the stoich.Lometric value ) containing a small amount of additives of such metals and semimetals as Sn and Ge has been found to have the property of showing a reversible change in its optical constants when the film was exposed in turn to two diode laser spots with different sizes, and this property can be utilized for an erasable disc. For recording, a TeOx thin film deposited on a PMMA substrate is irradiated by a diffraction limited laser spot, 0.8 pm in dia., modulated in response to an input electrical signal. This causes a micro-sized area of the film to be suddenly heated and rapidly quenched, resulting in a decrease in optical reflectivity. This recorded bit can be erased at real time by the irradiation of a diffused laser spot to an elliptical shape, 1 x 10 pm, by which the bit recovers its optical properties. An optical head having the two laser beams, one for recording/playback and one for erasing, has been developed. Real time erasing and recording at the same time has been achieved for a video signal. In excess of a million record/erase cycles have been demonstrated with no significant degradation in playback signal quality or erasability. Carrier-to-noise ratio, C/N, was more than 55 dB at 5 MHz, 30 kHz bandwidth.

High-density mark edge recording on a phase-change rewritable disk by a 680-nm laser diode

This paper reports a study of mark edge recording on the phase change rewritable disk. With the optical head using 680-nm laser diode and lens NA 0.6, 55 dB or higher carrier-to-noise ratio is obtained for signals whose recording mark length is 0.53 micrometers . In mark edge recording it is important to record all marks in their proper lengths. Hence, we first study shift behavior of the beginning and ending edges with regard to all marks, and then propose a new writing compensation method to make up for shifts. In the new compensation method the laser power modulation pattern consists of the first pulse, the multi-pulse chain and the last pulse. Adjusting the first and last pulse positions so that all marks are recorded in their proper lengths makes it possible to accomplish mark edge recording of the RLL (1,7) code with the linear density 0.4 micrometers /bit, jitter (sigma) /T(omega) to 6.5%. For the linear density of 0.35 micrometers /bit as well, a clear eye-pattern is obtained.

Self-Sharpening Effect of Phase Change Erasable Media

The characteristics of single-beam one-pass overwrite of SbTeGe phase change erasable (PCE) optical disks have been studied from a drive design point of view. In the recording process, an amorphous mark surrounded by a crystal ring (erase state) is recorded.The amorphous mark always has a clear edge even if erasability is relatively low in the low erase power area. We call this phenomenon the self-sharpening effect (SSE) which gives several advantages to the performance and design of the drive. SSE realizes wide power tolerance in a practical drive, which results in excellent interchangeability in terms of write/erase laser power. SSE also achieves a good off-track performance of less than +/- 0.1 µm of the optical beam and enables pulse edge recording even if overwrite erasability is relatively low.

Optical Video Recorder Using Tellurium Sub-Oxide Thin Film Disk

A compact optical still video recorder using Te0x(x÷1) thin film disc and a laser diode as the light source has been developed for video signal recording and playback in a real time mode. The recorder is capable of storing up to 15,000 still pictures on one side of a pregrooved disc, 200 mm in diameter with an average random access time of 0.5 seconds for any desired frame. The reflectivity of TeOx thin film recording layer which is deposited on a pregrooved plastic substrate by evaporating Te and TeO2 compound varies with absorption of irradiated thermal energy of focused laser beam. Carrier to noise ratio greater than 55 dB were achieved at incident power level of 7-9 mW, which was focused by an objective lens having numerical aperture (N.A.) of 0.5. Signal to noise ratio of more than 43 dB and resolution of more than 260 TV lines of playback video signal were obtained.

Compatability test for phase-change erasable and WORM media in a multifunction drive

We have presented configuration of the multifunction drive, interchangeability test, and reliability test consisting of temperature test and reading many cycles test. The multifunction drive could be realized by adding laser power control block and the polarity switch of reading amplifier without changing the WORMs head on a current WORM drive. Interchangeability test results showed having good interchangeability among multifunction drives or among WORM drives, and also the multifunction drive had full compatibility with current WORM drive on the current WORM disk. Reliability test results for the multifunction drive using a PCE disk and a WORM disk showed that writing sensitivities of both disks were much less dependent ont he temperature and the drive had stable writing characteristics in environments from 5 degree(s)C to 50 degree(s)C, and furthermore no error bit was observed while reading the most inner track of each disk 10 million cycles with the read power of 1.8 mW being 20% up to normal one at 6.1 m/s of disk velocity.

An Optical Digital Memory Using Tellurium Sub-Oxide Thin Film Disc

A practical optical digital memory using Tellurium sub-oxide [TeOx (x = 1)] thin film disc has been developed for document and digital image storage. In the recording mode, the reflectivity of TeOx thin film recording layer evaporated on a pregrooved PMMA substrate is changed by the absorption of the irradiated thermal energy of focused recording laser beam and the signal is recorded on the disc in the form of a chain of micron-sized flat and non-deformed dots. The optical memory using a laser diode as a light source is capable of storing up to 700 MBytes in user data on one side of a preformatted and pregrooved disc, 200 mm in diameter with an access time below 0.5 seconds. Input NRZ digital data are converted to 4/5 MNRZ code which has less DC component and recorded on the optical disc rotating at 900 rpm. Recording data speed on the disc is 6.8 Mbps and the length of the shortest bits is 0.8 μm The error rate of less than 10-8 has been achieved by the implementation of an error correction scheme with 5.4 % redundancy.

Optimization of mark edge recording waveform for a phase change rewritable optical disk using a 680 nm laser diode

A high density mark edge recording method on a phase change rewritable disk is reported. A carrier-to-noise ratio of 55 dB for a mark length of 0.55,μm is obtained by using a 680 nm, NA 0.6 head and a phase change rewritable disk. In mark edge recording, it is necessary to record each mark in correct length. A new laser power modulation pattern is proposed. This pattern consists of a first-pulse, a multi-pulse chain and a last-pulse. Both positions of the first-ptilse and the last-pulse are movable. By adjusting these positions, a jitter, σ/Tw (Tw is window margin), is improved to 6.5% in high density EFM (Eight to Fourteen Modulation) recording of 0.4 μm/bit.

A High-Density Phase-Change Optical Disk System Possessing Read/Write Compatibility with 90 mm Magneto-Optical Disks

High-density recording characteristics of phase-change optical disks were studied using 2-7 pulse-position-modulation (ppm) in-groove recording. Carrier-to-noise-ratio (CNR) was over 50 dB at a mark length of 0.60 µm and the crosstalk at the track pitch of 1.10 µm was less than -30 dB. In addition, a 256 MB 90 mm overwritable PC optical disk system having read/write compatibility with the ISO 128 MB MO disk has been developed based on the acquired results. The drive is based on the 90 mm MO drive at a wavelength of 780 nm and NA of 0.53. The user capacity of 256 MB was achieved using a mark length of 0.63 µm, track pitch of 1.25 µm, 2-7 ppm in-groove recording, and MCAV with 10 zones. The number of sectors per track was varied from 31 to 49.

Single-beam overwrite optical disk memory using a phase-change medium

A single beam overwrite optical disk memory using a rewritable phase change medium has been developed for digital data storage. The overwrite function was realized by using the same simple optical head as that of write-once memory. An 86mm pregrooved disk has the large capacity of 280MB on both sides. A higher data transfer rate of 6Mbps was achieved. A split type optical head having a 9gr. moving portion realized 42ms seek time with low power consumption. Overwrite operation is done by making amorphous marks and crystallized (erased) area with single laser beam. As the read-out signal is obtained from the difference of their reflectivities in the same way as write-once media, the optical head has become simple and compact. The overwriting temperature of the active layer is much higher than the operating temperature. This optical disk memory can execute reliable write and read operations with a wide tolerance for the operating temperature. Rewritable media were evaluated under practical perating conditions and could be overwritten more than 100,000 cycles on the same physical sectors. Implementing effective defect management, the practical or logical cyclability of the optical disk memory was improved up to one million cycles.