Kentaro Osawa

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.

Low cost, high resolution optical coherence tomography utilizing a narrowband laser diode

We developed a low cost, high resolution optical coherence tomography system utilizing a narrowband laser diode (LD), which is usually used in optical pickup for compact disc. To achieve high axial resolution even with the narrow bandwidth of the LD, we have constructed a free space interferometer including a phase-diversity detection system and a high numerical aperture (NA) objective. The axial and lateral resolution in the air was about 2.6 μm and 1 μm, respectively. The tomographic imaging of biological tissue was demonstrated, and the results showed that our OCT system enabled cellular-level imaging.

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.

Spheroid imaging of phase-diversity homodyne OCT

Non-invasive 3D imaging technique is essential for regenerative tissues evaluation. Optical coherence tomography (OCT) is one of 3D imaging tools with no staining and is used extensively for fundus examination. We have developed Phase-Diversity Homodyne OCT which enables cell imaging because of high resolution, whereas conventional OCT was not used for cell imaging because of low resolution. We demonstrated non-invasive imaging inside living spheroids with Phase-Diversity Homodyne OCT. Spheroids are spheroidal cell aggregates and used as regenerative tissues. Cartilage cells were cultured in low-adhesion 96-well plates and spheroids were manufactured. Cell membrane and cytoplasm of spheroid were imaged with OCT.

Cell sheets image validation of phase-diversity homodyne OCT and effect of the light irradiation on cells

Optical coherence tomography (OCT) is one of powerful 3D tissue imaging tools with no fluorescence staining. We have reported that Phase-Diversity Homodyne OCT developed in Hitachi could be useful for non-invasive regeneration tissue evaluation test. The OCT enables cell imaging because of high resolution (axial resolution; ~2.6 μm, lateral resolution; ~1 μm, in the air), whereas conventional OCT was not used for cell imaging because of low resolution (10~20 μm). Furthermore, the OCT has advantage over other 3D imaging devices in cost because the light source and the objective were originally used as an optical pickup of compact disc. In this report, we aimed to assess effectiveness and safety of Phase-Diversity Homodyne OCT cell imaging. Effectiveness of OCT was evaluated by imaging a living cell sheet of human oral mucosal epithelial cells. OCT images were compared with reflection confocal microscopy (RCM) images, because confocal optical system is the highest resolution (<1 μm) 3D in vivo imaging technique. Similar nuclei images were confirmed with OCT and RCM, which suggested the OCT has enough resolution to image nuclei inside a cell sheet. Degree of differentiation could be estimated using OCT images, which becomes possible because the size of cells depends on distribution of differentiation. Effect of the OCT light irradiation on cells was studied using NIH/3T3 cells. Light irradiation, the exposure amount of which is equivalent to OCT, had no impact on cell shape, cell viability, and proliferation rate. It suggested that the light irradiation has no cell damage under the condition.

In vivo optical interferometric imaging of human skin utilizing monochromatic light source.

We have demonstrated tomographic imaging of in vivo human skin with an optical interferometric imaging technique using a monochromatic light source. The axial resolution of this method is determined by the center wavelength and the NA of the objective and is irrelevant to the bandwidth of the light source in contrast to optical coherence tomography. Our imaging system is constructed with low-priced and small-sized compact disk optical pickup components, a laser diode, a high NA objective, and a voice coil actuator. In spite of its low cost and small size, our imaging system can visualize the structure of human skin as clearly as a commercial reflectance confocal microscope.

In-vivo human skin imaging by monochromatic source optical coherence tomography

We demonstrated imaging of in-vivo human skin with a low cost and high resolution optical coherence tomography (OCT) using monochromatic light source. Our OCT system is based on a free-space interferometer constructed with low cost optical pickup components, laser diode, high numerical aperture objective, and actuator. Even with narrow bandwidth of the light source, we achieved the axial resolution of 2.6 μm due to the wavefront mismatch between the signal and reference light. OCT imaging of human skin showed skin inner structure as clearly as imaging with a commercial reflectance confocal microscope.

Human oral mucosal epithelial cell sheets imaging with high-resolution phase-diversity homodyne OCT

There is a need for development of non-invasive technique to evaluate regenerative tissues such as cell sheets for transplantation. We demonstrated non-invasive imaging inside living cell sheets of human oral mucosal epithelial cells by phase-diversity homodyne optical coherence tomography (OCT). The new method OCT developed in Hitachi enables cell imaging because of high resolution (axial resolution; ~2.6 μm, lateral resolution; ~1 μm, in the air). Nuclei inside cell sheets were imaged with sufficient spatial resolution to identify each cell. It suggested that the new method OCT could be useful for non-invasive cell sheet evaluation test.