We propose a new type of lensless camera enabling light-field imaging for focusing after image capture and show its feasibilities with some prototyping. The camera basically consists only of an image sensor and Fresnel zone aperture (FZA). Point sources making up the subjects to be captured cast overlapping shadows of the FZA on the sensor, which result in overlapping straight moiré fringes due to multiplication of another virtual FZA in the computer. The fringes generate a captured image by two-dimensional fast Fourier transform. Refocusing is possible by adjusting the size of the virtual FZA. We found this imaging principle is quite analogous to a coherent hologram. Not only the functions of still cameras but also of video cameras are confirmed experimentally by using the prototyped cameras.
The distribution of the optical near-field generated by a probe with a wedge-shaped metallic plate was calculated using a finite difference time domain method. The dependence of the distribution on the size and the material of the metallic plate was calculated, and it was shown that a strong optical near-field was generated at the apex of the metallic plate when the size and the material were optimized so that a surface plasmon was excited in the metallic plate. The influence of the recording medium was also calculated. The resonance wavelength shifted toward a longer wavelength and the decay length of the optical near-field increased when the recording medium was placed near the probe. The spot size calculated on the surface of the recording medium was 30 nm, and the efficiency (defined as the ratio between the power of the optical near-field at the surface of the recording medium and that of the incident light) was about 20% when the spacing between the probe and the recording medium was 10 nm. The near-field distribution for a probe with two metallic plates was also calculated, and it was shown that a strong optical near-field was generated between the apices of the metallic plates when the plasmon was excited in the metallic plates.
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.
We have applied the blue-violet laser in developing several technologies for the next-generation digital versatile disk (DVD). We are convinced that four items, i.e., large capacity, high data-transfer-rate, cyclability, and portability, are equally important. We have set targets for capacity and data-transfer-rate. Separate development processes have been carried out in the fields of optical heads, systems (servo-mechanisms, read/write channel and signal processing) and basic subjects and combined the results to achieve our targets. Basic subjects that cross these technical fields and have previously been neglected were studied to find solutions to new problems that arise with next-generation requirements.
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.
Wavefront coding (WFC) with a cubic phase mask (CPM) for extended depth of focus (EDOF) has been studied since 1995. However, the asymmetric surface shape of a CPM has several disadvantages, such as necessity of a square aperture, directional artifacts in images, rotational adjustment during assembly, defocus-dependent lateral image displacement, and difficulty in fabrication. To overcome these disadvantages, we propose an annular phase mask (APM) with a rotationally symmetric form. An APM adds the phase of the parabolic profile in plural annuluses and phase steps between annuluses to the penetrating light. We confirmed an EDOF effect comparable to that of a CPM in our APM design by simulation. The depth of focus of a WFC system with the APM was estimated to be about 6.8 times larger than that of a conventional system. The effect observed in the experiments using a fabricated APM is consistent with the simulation result.
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