Eriko Tatsu

Experimental Demonstration of Optical Phase Multilevel Recording in Microhologram

The microhologram is one of the most promising candidates for the next generation optical disc. It can achieve a huge data capacity because it is suitable for multilayer recording. However, it cannot increase the data transfer rate because of its comparable areal recording density with conventional optical discs. Moreover, the signal level in the readout process of this scheme is in general very low, which prevents its practical use. Recently, an optical phase multilevel scheme that overcomes the above drawbacks of the microhologram has been proposed. The scheme uses an optical phase as stored information, which enables data readout with an extremely high signal-to-noise ratio and multilevel modulation. In this report, recording and readout processes of the proposed scheme are demonstrated experimentally. Four-level phase modulation was successfully regenerated from weak 30 nW microholograms with errors of +7.0/-12.2°, suggesting that a further increase in the number of levels is possible.

Simultaneous Multi-Bit Recording in Fused Silica for Permanent Storage

In recent years, optical discs and hard disc drives have been widely used as storage media. However, the lifetime of recorded data in these media is about 100 years. On the other hand, a permanent storage system that can store data for more than 1,000 years is strongly required, especially for historically valuable data. One candidate system for permanent storage is a system using fused silica, which is thermally and chemically stable. In this paper, we reported simultaneous multi-bit recording in fused silica with a femtosecond laser and a spatial light modulator. The recording quality was evaluated using signal-to-noise ratio with an optical microscope. We recorded a four-layer sample with a dot pitch of 2.8 µm and obtained a signal-to-noise ratio greater than 15 dB. Furthermore, we confirmed that the sample had good thermal resistance at 1,000 °C for 120 min, which indicates a lifetime of over 319 million years.

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.

A Driveless Read System for Permanently Recorded Data in Fused Silica

A driveless, multilayer read system was demonstrated as applicable to a permanent digital-storage using fused silica. The combination of a low-magnification microscope and signal processing were simple enough to be emulated in the distant future. Test data were recorded in a 2-mm-thick fused-silica plate by a femtosecond laser to form four layers with dot pitch of 2.8 µm and interlayer distance of 60 µm. The total recording density was 40 Mbytes/in.2, which is as high as that of a conventional compact disc. This system achieved a bit-error rate in the order of 10-3 when reading the test data (without error-correction code) from images taken at a 19.5-times-magnification. Signal processing using unsharp masking and subtraction of images at different focal points effectively contribute to read data from blurred image with the low bit-error rate.

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.

Experimental Demonstration of Reducing Interlayer Crosstalk of Multilayer Disc in a Three Beam Optical Disc Tester Using Polarizing Device

We demonstrated interlayer crosstalk reduction of multilayer disc by using a polarizing device in a three-beam optical disc tester experimentally. Application of the device to a dual-layer BD with 5 μm layer spacing showed satisfactory effect, roughly halving DPP signal fluctuation.

Reducing Interlayer Crosstalk in Reading Multilayer Disc for Compact Optical Pickup Using Polarizing Filter

We proposed an interlayer crosstalk reduction device that can be used in an optical pickup based on the astigmatic aberration method. The device has two wave plates covering the half plane at different positions on the optical axis and an analyzer, and is placed in the beam of astigmatic aberration. We can make the polarization state of the beam after transmission of the two wave plates different according to whether the focal line position is situated between the two wave plates. We measured the device transmission and confirmed that the optical characteristics agree well with the simulation.

A Multilayered Blu-Ray Disc readout technique with an optical longitudinal filter for reducing interlayer crosstalk in a three-beam optical system

A Multilayered Blu-ray Disc (BD) readout technique—using an optical longitudinal filter (OLF) and differential push-pull (DPP)—to reduce interlayer crosstalk was demonstrated. DPP signal fluctuation was suppressed to one-third (6% to 2%), and satisfactory readout jitters (about 8%) were obtained for a triple-layered BD.