Compact and low-cost devices are needed for autonomous driving to image and measure distances to objects 360-degree around. We have been developing an omnidirectional stereo camera exploiting two hyperbolic mirrors and a single set of a lens and sensor, which makes this camera cost efficient. This paper presents a new calibration method for this camera. Based on the original calibration method by Mei and Rives, we improve the calibration accuracy by considering higherorder radial distortion, detailed tangential distortion, an image sensor tilt, and a lens-mirror offset. Our method is applied to our prototype and reduces the root mean square of the calibration accuracy by 1.2 times and 2.2 times for the upper- and lower-view images, respectively. The distance error is less than 8% up to objects 14 meters apart, which is improved more than seven times compared to the original method, although the error is still larger than the target value of 5%. We consider that the remaining calibration error is due to distortion of the glass cylinder and a degraded optical resolution. As future work, we plan to make further improvements in the calibration and optical resolution of the prototype. In addition, a rectification method for cylindrically expanded images needs to be developed.
Lensless light-field imaging with Fresnel zone aperture (FZA) is a technology that enables lensless cameras with beneficial features such as light calculation load in the image reconstruction processing, and refocusing, i.e. post-capturing focus adjustment. The ability to refocus indicates that the lensless camera s 3D information of the scene and the depth information can be extracted through comparison of contrasts among reconstructed images at different focus distances. This feature is promising since monocular 3D sensing can be achieved without using special light sources. However, since exhaustive search is needed to extract depth information, application of the contrast method is unsuitable for purposes that require real-time depth sensing. To overcome this limitation, we aimed to establish a new fast 3D sensing algorithm for lensless cameras with FZA. The proposed algorithm is based on our findings that reconstructed images of the lensless camera are complex images that have values in real and imaginary parts, and the sign of the imaginary part is reversed when the focus position of the lensless camera crosses the in-focus position. Since this "zero-crossing point" can be calculated through simple interpolations, 3D sensing can be achieved without exhaustively searching over numbers of reconstructed images with various focus positions. We have verified the effectiveness of the algorithm through simulation and experiments with developed prototypes. The algorithm enables real-time compact 3D sensors suitable for various applications, e.g. smartphones, robots, and vehicles.
KEYWORDS: Objectives, Multiplexing, Holographic data storage systems, Holography, Holograms, Signal to noise ratio, Spatial light modulators, Mirrors, Laser systems engineering, Data storage
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.
Optical signal amplification by a homodyne detection scheme is proposed and experimentally
demonstrated. We estimated that this scheme improved the signal-to-noise ratio of an 8-layer 3x
read-speed Blu-ray Disc (BD) by more than 20 dB.
In high-density optical disc systems that use high-NA objective lenses and multi-layered discs, it is important to reduce spherical aberration (SA). With high-NA lenses, SA is induced by errors in substrate thickness. The distance between layers in multi-layered discs also leads to SA. To use a type of phase-shifting device as a dynamic compensator for SA, a method of differential focus-error detection has been proposed for the real-time measurement of SA as a spherical aberration signal (SAS). The derivation of an equivalent substrate-thickness variation from the SAS with an accuracy of up to 0.8 mm is demonstrated in this paper. Using the SAS to drive a liquid-crystal phase-shifter, the fluctuation of the SAS caused by the substrate thickness error has been suppressed successfully during real-time disc rotation.
The track pitches of optical discs have become so narrow that it is comparable to the wavelength of laser beam. Finite-difference time-domain (FDTD) simulation, based on vector diffraction analysis, can predict the propagation of light more accurately than scalar analysis, when the size of media texture becomes sub-micron order. The authors applied FDTD simulation to land-and-groove optical disc models, and found out that the effects of 3D geometry is not negligible in analyzing the energy absorption of light inside the land- and-groove multi-layered media. The electromagnetic field in the media does not have the same intensity distribution as the incident beam. Furthermore, the heat conduction inside the media depends on the disc geometry, so the beam spots centered on land and groove makes different effects in heating the recording layers. That is, the spatial and historical profile of temperature requires 3D analysis for both incident light absorption and heat conduction. The difference in temperature profiles is applied to the phase change simulator to see the writing process of the marks in land and groove. We have integrated three simulators: FDTD analysis, heat conduction and phase change simulation. These simulators enabled to evaluate the differences in mark forming process between land and groove.
It is found that there is the region in the exit pupil of the objective lens where waveform polarity of crosstalk from adjacent tracks is inverted from that of total crosstalk. Additionally, the regions that contribute to the total signal of fine mark pattern are separated into two regions with a gap remaining along the radial direction of the disk. Shading or polarity inversion in the region neither this signal region nor inverted crosstalk region reduces the crosstalk from adjacent tracks without reducing the signal of a fine mark patterns.
We have developed an ultra-high-capacity double-sided multi- data-layer ROM that includes additional UV-cured-resin space-layers on two polycarbonate (PC) substrates. This ROM is made using the photo-polymerization (2P) method and has a structure that bonds the two substrates together. In a double-sided dual-data-layer ROM, using the smallest mark size of 0.44 micrometers with a 0.74-micrometer track-pitch for each layer provides a total data capacity of 17 GB using the 8-16 modulation method. Each side of the double-sided dual-sided dual-data-layer ROM consists of a Si-rich silicon nitride semi-reflective layer (layer 0 or 2) on a 570- micrometers-thick PC substrate and an Al reflective layer (layer 1 or 3) on an additional 50-micrometer-thick space layer. We achieved a base jitter of less than 8 percent and a radial tilt margin of 0.7 degree for all four data layers. We also examined the double-sided multi-data-layer ROM disc structure, and demonstrated the feasibility of the double- sided tri-data-layer ROM that includes a total of six layers and has an ultra-high capacity of 25.5 GB.
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