Tatsuro Ide

New optical architecture for holographic data storage system compatible with Blu-ray Disc™ system

Abstract. A new optical architecture for holographic data storage system which is compatible with a Blu-ray Disc™ (BD) system is proposed. In the architecture, both signal and reference beams pass through a single objective lens with numerical aperture (NA) 0.85 for realizing angularly multiplexed recording. The geometry of the architecture brings a high affinity with an optical architecture in the BD system because the objective lens can be placed parallel to a holographic medium. Through the comparison of experimental results with theory, the validity of the optical architecture was verified and demonstrated that the conventional objective lens motion technique in the BD system is available for angularly multiplexed recording. The test-bed composed of a blue laser system and an objective lens of the NA 0.85 was designed. The feasibility of its compatibility with BD is examined through the designed test-bed.

Amplification of Optical Disk Readout Signals by Homodyne Detection

We experimentally demonstrated the amplification of optical disk readout signals by homodyne detection. This technique uses optical interference to amplify the signals. We further applied phase-diversity detection to reliably obtain the amplified readout signal. The optical system was carefully designed so that a sufficiently amplified readout signal can be obtained. In particular, we applied a corner cube prism as a reflection mirror to achieve sufficient stability of the interferometric optical system. We experimentally demonstrated a 3.6 times amplification of a Blu-ray Disc readout signal. The estimated signal-to-noise ratio (SNR) improvement for an assumed eight-layer optical disk readout signals by applying homodyne detection on the basis of the observed amplification was +7.9 dB, which significantly enables reliable readout of recorded signals. The present technique will be essential for the real commercialization of next-generation multilayer optical disk because of its outstanding ability of SNR improvement.

Formularization and simulation of Bragg selectivity of readout signals in angular-multiplexing holographic data storage

Bragg selectivity of readout signals in angular-multiplexing holographic data storage was investigated. The effects of degrading factors, namely, volume change, refractive-index change, and positional change (tilt and rotation) of a hologram, and the effects of compensating variables, namely, wavelength shift and reference-beam-angle shift, on Bragg selectivity were evaluated. Deviation of wave vectors of recovered pixels of a hologram from the Bragg condition under degrading factors and compensating variables, namely, Bragg mismatch, Δσ, was mathematically derived. Approximating Δσ by using the first-order Maclaurin series with respect to degrading factors and compensating variables revealed their effects on Bragg selectivity. The extent to which wavelength and angle of reference beam should be shifted to compensate for the degrading factors were determined. Then, readout images were simulated under multiple degrading factors and compensating variables. These simulated images were found to agree well with the experimentally obtained ones, which reveals the validity of the formalization of Bragg selectivity.

Interlayer crosstalk reduction of a multilayer Blu-ray Disc using a grating in a three beam optical system.

In multilayer optical discs, light reflected by out-of-focus layers, which we call interlayer crosstalk, causes the tracking error signal to fluctuate, making the readout signal unstable. We previously proposed a novel method to use a grating along the optical axis in the return path of a pickup to suppress the fluctuation of a differential push-pull (DPP) signal. We develop a pickup and evaluate its performance to stabilize the DPP signal experimentally. DPP signal fluctuation is suppressed to one-third (6% to 2%), and also satisfactory readout jitters (about 8%) are obtained for a triple-layer Blu-ray Disc (BD), which demonstrate the validity of this method to reduce interlayer crosstalk of multilayer optical discs.

Signal-Readout System for Optical Pickup with Homodyne Detection Scheme

We developed a signal-readout system suitable for optical pickups with a homodyne detection scheme, which is used to amplify signal lights using optical interference. The system consists of an optical-signal detection circuit and a readout-signal generator. The optical-signal detection circuit, which contains pairs of photodiodes connected in series, successfully cancels out large DC components arising from a reference light. This made it possible to avoid the output signal saturation of current-to-voltage amplifiers and raise the upper limit of signal gain. The readout-signal generator generates a readout signal on the basis of the phase-diversity method, which stabilizes readout signal amplitude. By using this system, a signal gain of 3.6 was obtained on a commercially available disc.

Analysis of Phase Multilevel Recording on Microholograms

An optical phase multilevel recording technique using a microholographic system and phase-diversity homodyne detection for enhancement of optical disc capacity is investigated. In this technique, multilevel phase signals are stored as the fringe shifts along the optical axis and recovered from the arctangent of two homodyne-detected signals. For comparison, phase signals from Blu-ray Disc read-only memory (BD-ROM) and Blu-ray Disc recordable (BD-R) media obtained by phase-diversity homodyne detection are experimentally evaluated. From the experimental results, we demonstrated that phase-diversity homodyne detection is useful for detecting the phase signal modulation of the signal beam from an optical disc. Furthermore, simulation results on microholograms indicate that phase signals from the microholograms are much more stable despite the variety of their sizes than those from BD-ROM. These results demonstrate the potential of this multilevel recording method.

Reduction of Interlayer Crosstalk of Multilayer Optical Disc by Using Phase-Diversity Homodyne Detection

We theoretically and experimentally studied the effects of phase-diversity homodyne detection on the interlayer crosstalk of a multilayer optical disc by comparison with those of conventional intensity detection. From analytical studies, we clarified the differences in interlayer crosstalk of both detections. Interlayer crosstalk consists of two noises, the intensity of the stray light N1 and the interference between the signal and stray light N2. The noise N1, which is dominant between these two, drastically decreases with layer spacing in phase-diversity homodyne detection owing to mismatch in the phase distribution between reference and stray light compared with that in intensity detection. Simulations and experiments on a dual-layer Blu-ray DiscTM having a layer spacing less than 10 µm demonstrated that phase-diversity homodyne detection provided higher tolerance to interlayer crosstalk than the conventional intensity detection.

Read Data Transfer Rate Estimation in Optical Phase Multilevel Recording

The feasibility of increasing the read data transfer rate (DTR) by introducing optical phase multilevel recording technology was investigated using computer simulations. The signals read back from phase marks suffer from strong intersymbol interference (ISI) when the phase marks are recorded with a linear symbol density comparable to that of current optical disc systems; thus, the partial response most-likely (PRML) method is essential. The increase in the decoder size is a serious problem when applying the PRML method to multilevel signal decoding; however, it was shown that this can be resolved by applying run-length limited (RLL) modulations. With these, it was shown that it is possible to decode 4-ary phase-modulated signals with satisfactory performance using PRML. Therefore, we conclude that it is possible to at least double the read DTR by introducing the optical phase multilevel recording technology.

Use of Grating in Reading Multilayer Discs to Reduce Amount of Interlayer Crosstalk

Crosstalk from adjacent layers affects the tracking error signal of conventional optical pickups when a multilayered disc is read. The tracking error signal fluctuates and makes precise tracking difficult. We propose the use of a grating in the return path to reduce the amount of crosstalk. Using a grating with an uneven plane on the optical axis reduces the amount of stray light causing the crosstalk and allows the target light to pass through. We show that using a grating with an appropriate pitch prevents diffracted light higher than an order of ±1 from going back to detectors. As the shape and depth of the grating are important factors for the zeroth-order light, we chose a deep triangular grating. We measured the distribution of the transmitted intensity through the grating as a focused beam position was scanned and found that the grating can reduce the amount of crosstalk.

Non-contact Deformable Mirror Actuator for Spherical Aberration Compensation

We have proposed and demonstrated a novel non-contact electromagnetic deformable mirror actuator for spherical aberration compensation with simple configurations. The magnetic ring with low stiffness placed on a mirror deformed the mirror into an ideal parabolic shape by applying a magnetic force generated from an electromagnet underneath. We were able to control the deformation of the mirror by adjusting the applied voltage. The maximum deviation from ideal shape was less than 30 nm. In addition, a continuous change of the defocus aberration proportional to the 1.5-λRMS/µm deformation of the mirror was achieved.