Search

Optical correlation systems have been developed as hardware that is useful for high-speed and large-scale data analysis. Among them, our system using the disk-type hologram is promising because it can effectively utilize the control techniques of conventional optical disks. In previous work, however, we used a very simple disk structure consisting of a mirror substrate, photopolymer, and cover glass, and tracking control was not performed. Consequently, it was difficult to stably record a large amount of data in the long range of the circumference since the hologram recording position deviates from the disk circumference due to surface deflection and eccentricity attributed to rotation without tracking controls. In this study, we aim to stabilize the optical correlation system by introducing a new disk structure and conventional servo control. We confirmed that both sufficient correlation signal and servo control signals can be detected with the simple disk structure due to the signal detection principle of the optical correlation.

There are various multiplexing methods in holographic data recording. A larger storage capacity is anticipated when combining two different multiplexing methods and then ensuring crosstalk-free recording. We have proposed a cross-shift multiplexing method that combines shift multiplexing and peristrophic multiplexing. Crosstalk-free conditions for shift multiplexing and peristrophic multiplexing were separately clarified. However, these conditions cannot be individually applied to cross-shift multiplexing. Then, we deduced the analytic formula of crosstalk for cross-shift multiplexing and verified its validity by comparing the theoretical and experimental results. Using this analytic formula, the shift pitch and rotation angle that attain crosstalk-free multiplexing can be determined.

A holographic data storage channel is normally a nonlinear channel; however, it can be made linear. Using coherent addition of DC components in the reproduction process and calculating the square root of intensity, we can retrieve a linearly reproduced signal. Our simulation results revealed that a conventional equalizer works well to suppress interpixel interference so that a higher recording density can be achieved.

An improvement of the recording density in a coaxial holographic recording system is described. We employ a new optical setup into our coaxial system, where the numerical aperture (NA) of an objective lens and the data capacity in one page increase up to 0.85 and 135 kbits, respectively. The recording performances are evaluated experimentally and numerically. The obtained results demonstrate that implemented techniques successfully improve the recording density by a factor of 1.5, achieving a raw data density of 270 Gbit/in.2.

In this paper, we evaluate photopolymer media using a polycarbonate (PC) substrate. In holographic data storage medium, substrates that sandwich the photopolymer material are needed to protect the photopolymer material against exogenous shock and open air. An optical glass such as BK-7 is normally used as a substrate, but a PC substrate has a cost advantage and is easy to fabricate compared with optical glass. For holographic recording and reading, however, the high birefringence and high thermal expansion of a PC substrate are significant problems. First, we analyze the degree of degradation of output power by the polarization change and estimate the threshold value of birefringence to record hologram normally. Next, we estimate the temperature tolerance of hologram readout with polycarbonate substrate hologram medium. These analyses results indicate the possible usage of the PC substrate as holographic recording media.

We investigated the capability and tolerance of the optical collinear system by numerical simulation. Using Fourier optics, the simulation can calculate the hologram pixel image intensity distribution, the corresponding signal-to-noise-ratio and its histogram. For holographic data storage using the optical collinear system, we presented and we discussed the medium position and the medium thickness dependency, the medium displacement in the x-, and the z-directions during the hologram recording process, and the recording wavelength shift.

We have developed a coaxial-type holographic disk system with a small drive. We achieved stable operation without use of a vibration-isolating table, with a 91-Mbps data-transfer rate for recording and retrieving using LDPC code.

We have shown that the signal loss induced by the media temperature change in the coaxial holographic recording can be compensated by tuning the magnification and the wavelength of the reading reference beam appropriately. This technique offers a significant improvement in the temperature tolerance of the coaxial holographic data storage.

We have experimentally evaluated the coherent addition technique, and revealed that the coherent addition can enhance detected data intensity and realize clear three-level data retrieving. Moreover, we have achieved great improvements in the multiplexed performance, which are the SNR gain of +3.5 dB and the data density increase with the factor of 2. We consider that this technique will strongly contribute to future holographic technologies.

We have applied LDPC codes into our coaxial recording system, and revealed its fundamental characteristics. In addition, by applying the E(16, 3, 8) code and optimizing optical conditions, we have successfully improved the recording density, achieving the raw data density of 415 Gbit/in.2.

In this paper, we propose a new analysis model for photopolymer recording processes that calculate the two-dimensional refractive index distribution of multiplexed holograms. For the simulation of the photopolymer medium, time evolution of monomer diffusion and polymerization need to be calculated simultaneously. The distribution of the refractive index inside the medium is induced by these processes. By evaluating the refractive index pattern on each layer, the diffraction beams from the multiplexed hologram can be read out by beam propagation method (BPM). This is the first paper to determine the diffraction beam from a multiplexed hologram in a simulated photopolymer medium process. We analyze the time response of the multiplexed hologram recording processes in the photopolymer, and estimate the degradation of diffraction efficiency with multiplexed recording. This work can greatly contribute to understanding the process of hologram recording.

A unique coaxial optical system which enables us to record and retrieve the holographic data on a continuously rotating disc is proposed. The system supports an im age-stabilizing technique and several servo mechanisms. Keywords : Holographic data recording, optical system, image-stabilizing technique, servo mechanism. 1. INTRODUCTION Holographic data storage is an attractive candidate for next generation high-density optical memories. Among several holographic recording systems 1) , the combination of a coaxial recording method with a shift multiplexing technique is most promising because the optical system has a simple structure and simple co mponents such an optical disc pick-up. Also, holographic disc rotating at a constant velocity is similar to the optical disc. From these characteristics, one can expect to realize a very compact, inexpensive holographic disc drive. In order to perform shift multiplexing on a rotating disc, however, a short-pulse laser with considerable power at its peak must be used for recording/retrieving. Generally, a solid-state Q-switch laser is used for this application, but it is expensive and space-consuming.

107 Mbps and 92 Mbps data-transfer rates for recording and retrieving, respectively, have been achieved by improving the coaxial holographic disk system using the IS technique.

A coaxial type holographic data storage evaluation system, capable of 500-fps-consecutive writing and reading, has been developed, whose light source is a semiconductor blue laser diode. This is realized by an image-stabilizing technique.

The first numerical simulator for holographic data storage using the optical collinear system was developed. The dependency of the Signal to Noise Ratio for media displacement and wavelength shift were calculated and were discussed.

A new multiplexing technique for optical collinear holography is introduced. The result shows that 12x12 at 16µm pitch multiplexing were attained. And a possibility to the user bit density of 42Gbits/inch2 was implied.

We developed one dimensional data pattern hologram writer using GLV device as a spatial light modulator. The following advantages were expected: easier pixel matching, simpler signal processing, low height drive and high transfer rate. 171 angular multiplex recording holograms on a lmm thickness polymer media, corresponding to 6.22 pixels/μm 2 in density were demonstrated. With a Bit Error Rate (BER) of less than 2 x 10 -4 , 101 multiplexing holograms were successfully stored. Then, considerable error factors were discussed and solutions were given for higher multiplex recording.

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.

PSR (premastered optical disk by super-resolution) is a super-resolution technique used to increase the recording density of ROM optical disks. Our PSR system uses a novel disk with a nonlinear optical layer, a second-harmonic-generation green laser ((lambda) equals 532 nm), 0.52NA objective lens and eight-to-fourteen modulation coding. By using PSR, a high C/N ration of 51dB was obtained for a pit length of 0.2 micrometers which is beyond the conventional optical limit. Furthermore, we obtained a bit error rate of less than 10-5 for a linear density of 0.185 micrometers /bit and a track pitch of 0.8 micrometers , thus demonstrating the feasibility of our PSR system with six times the areal density compared to the current CD.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The transition jitters of a Double-masked Rear Aperture Detection (D-RAD) readout werecalculated as a function of defocusing, disk tilt and recording mark width. The modelling resultssuccessfully explain our experimental results. Large jitter tolerances and a low byte error rate were obtained using 0.3 tm bit length recording. 1. INTRODUCTIONMagnetically induced super resolution (MSR) in magneto-optical (MO) disks using exchange- coupled magnetic multi-layer film have been previously reported'6. A portion of the focused spot is optically masked for the readout of the MO signal. The effective spot size after masking is much smallerthan that of the focused spot. Therefore super-resolution is realized.Various types of the MSR methods have been developed to achieve super-resolution. Among thesemethods, D-RAD has an advantage as it achieves a higher linear-density readout, because the width of theeffective spot in the tangential direction is significantly reduced by double masks. Using this method the

The cut-off frequency was extended by a factor of more than two using a magneto-optical disk with thermally induced mask.

A super resolution has been achieved in the magneto-optical disk using an exchange-coupled multilayer film. Two types of magnetically induced super resolution, front aperture detection and rear aperture detection have been successfully investigated. A high C/N of more than 42 dB is obtained experimentally for a mark length of 0.3 micrometers which is much shorter than the resolution limit expected from the conventional diffraction theory.

Holographic memory surpasses all other optical memories in both data density and data-transfer rate-but its development has lagged behind disk technologies. The coaxial recording method may enable holography to become an integral part of the optical storage marketplace of the future.

We describe a coaxial holographic recording system for achieving high recording density. We implement several techniques, such as an objective lens with high numerical aperture (NA), high capacity page data format, a random binary phase mask, and an optical noise reduction element. Our system successfully realizes a hologram recording/retrieving at a low diffraction efficiency less than 2.0 x 10(-3) and achieves a raw data density of 180 Gbit/in.(2), thus demonstrating the potential of a coaxial holographic system for high-density optical storage systems.

Premastered optical disk by superresolution (PSR) for practical use has been achieved for the first time. The PSR disk has a phase-change material Ge2Sb2Te5 with stoichiometric composition. Superresolution results from a high contrast ratio of reflectance in the light spot between highly reflective crystal region and lowly reflective melted region. A high C/N ratio of 56 dB is obtained at a pit length of 0.4 µm using PSR. The C/N ratio is still as high as 50 dB for a pit length of 0.3 µm. The PSR disk with double linear densities of current CD is successfully read out with a clear eye pattern and a low error rate using current optics.

High-density magneto-optical (MO) recording using a land/groove structured domain wall displacement detection (DWDD) medium has been achieved with a digital versatile disc (DVD) dimensional optical head including a 664-nm wavelength laser diode and a 0.60-NA objective lens. Due to the deep groove depth, easy domain wall displacement is obtained without the use of a track boundary postannealing process. Sufficient readout performance is obtained in experiments at an areal recording density of 11 Gbit/inch2, which is obtained using a 0.13-µm bit length and a 0.45-µm track pitch. This areal density corresponds to a capacity of 15 GB per one 120-mm-diameter disk.

We have examined two unique data error causes for domain wall displacement detection (DWDD) media, dropout and bit shift, using 8–9 modulation and (1, 7)-RLL modulation. We have determined that dropout depends only on the mark length of recorded data, while the deviation of edge shifts, which causes bit shifts, maintains approximately the same value regardless of mark length. Using these characteristics of error causes, we have also shown a prospect of achieving several-tens-nm/bit recordings.

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 adopted mark position recording in order to cancel ghost signals which appear in domain wall displacement detection. We obtained a jitter of 6.8 % at a linear velocity of 1.5 m / sec for a bit length of 0.15 μm and a track pitch of 0.85 μm using a conventional optical pickup. We confirmed that readout and recording power tolerances are ± 22 % and ± 24 %, respectively, at an areal density of 5.0 Gbit / inch2.

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 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.

Premastered optical disk by superresolution (PSR) using rear aperture detection (RAD) has been realized. The PSR disk has a chalcogenide material. Using PSR by RAD with a current CD system, we demonstrated the feasibility of increasing both the linear density and the track density of read only memory (ROM) optical disks. By superresolution in the linear direction, a high C/N of 47 dB was obtained for a pit length of 0.3 µm which is beyond the optical limit. Furthermore, by superresolution in the track direction, a low crosstalk of -38 dB and a wider margin of radial skew angle compared to conventional detection were obtained for a track pitch of 1.1 µm.

In view of the recent progress in visible lasers for next-generation optical disks, we describe the influence of source wavelength, aberration, and noise on eye-diagram jitter, which determines the ultimate disk density. The analysis indicates that the sources used in a readout of a 6× areal density, (4,22) run-length-limited code with a minimum mark length of 0.4 µm must have a wavelength that satisfies the Nyquist condition of relationship between the spot size and the minimum mark length, a wave-front aberration of less than 0.035 rms λ, and relative intensity noise of less than -125 dB/Hz.

Tripled areal density optical recording was realized with the combination of a high power laser-diode pumped intracavity frequency-doubled Nd:YAG laser (SHG green laser), a low noise GdFeCo/TbFeCo double-layered magneto-optical (MO) disk, and high sensitivity photodetectors optimized for the wavelength of the light source. A high carrier-to-noise (C/N) ratio of more than 48 dB and a bit error rate of less than 3×10-6 by (2, 7) run-length-limited (RLL) mark position recording was obtained at a mark length of 0.45 µm (0.6 µm/bit) for a disk with a 0.9 µm track pitch.

We realized magnetically induced super resolution (MSR) using magneto-optical (MO) disks with a thermally formed aperture. The aperture size and position affect the spatial frequency response of the readout signal. Using the principle of superresolution in laser scanning microscopy, we analyze the optical transfer function (OTF) for various apertures. Applying the analysis to MSR readout, OTF of MSR is calculated using scalar diffraction theory. The calculated results agree well with experimental values.

Magnetically induced super resolution (MSR) is a new technology developed by us. It more than doubles the readout resolution limit of a conventional optical disk system without shortening the laser wavelength or increasing the numerical aperture (N.A.) of the objective lens. We examined the characteristics of the readout signals of an MSR-by-RAD (rear aperture detection) disk and obtained a high C/N of more than 49 dB and good stability of the edge position at a mark length of 0.4 µm. We investigated the high-density MO disk system using the MSR-by-RAD disk. Owing to optimization of the writing process and waveform equalization, the system achieved a byte error rate of less than 10-4 at a bit length of 0.3 µm. This bit length corresponds to a linear density more than three times higher than that of the conventional 130-mm MO disk system of the ISO standard.

MSR (Magnetically induced Super Resolution) by FAD (Front Aperture Detection) was demonstrated by the readout waveform of the isolated pit. We calculated the response of the readout signal employing both the thermal analysis and the scaler detection theory. As experiments carried out with the same condition of the simulation, we obtained a favorable agreement between the experimental results and the simulation results.