We previously proposed a grating-assisted phase-shifting method for incoherent digital holography (IDH). This method leverages the diffraction of a binary phase grating, making it possible to create four self-interference holograms with different phase shifts in parallel. By simultaneously capturing the holograms with four image sensors, implementing a four-step phase-shifting algorithm and detecting complex amplitude distribution at a single exposure are possible. In this study, we investigated the effect of the axial misalignment of image sensors on the reconstructed images in the grating-assisted phase-shifting method via numerical simulations. The numerical simulations are based on the scalar diffraction theory and allows us to emulate IDH systems. The numerical results provide basic insight and a guideline to design an optical setup for the method within an acceptable tolerance.
We previously proposed a bimodal incoherent digital holography system to implement both three-dimensional and infinite depth-of-field (DOF) imaging. In this paper, we reveal the basic operation of our system in detail to control its imaging properties such as transverse magnification and DOF. We numerically evaluated the effect of rotational shear on these imaging properties, and the results indicate that it is possible to control both properties by simply changing the rotation angle of a light instead of replacing an imaging lens.
KEYWORDS: Signal processing, Signal to noise ratio, Video, Holographic data storage systems, Holograms, Data storage, Distortion, Wavefronts, Video acceleration, Wavefront compensation
We have developed a holographic data storage system that can demonstrate real-time playback of beyond high definition
video signals. In the proposed system, to increase the data-transfer rate of the reproduced data, we focused on improving
the SNR of the reproduced data and on improving the signal processing speed, which the SNR of the reproduced data has
a significant effect on. One of the factors that deteriorate the SNR is shrinkage in the medium. This shrinkage distorts
recorded holograms and degrades the quality of the reproduced data. We investigated wavefront compensation as a
means to improve the SNR of reproduced data degraded by hologram distortion and found that controlling the defocus
component of the reference beam is effective. We have also been developing parallel signal processing to increase the
data-transfer rate. We placed three GPUs in the signal processing unit: one for the reproduced data detection from the
reconstructed image and two for the LDPC decoding for error correction of the reproduced data. The LDPC decoding
required a lot more time than the data detection, so we designed a signal processing in which detected data in the GPU
for the data detection were sent to the two GPUs for the LDPC decoding alternatively. We implemented wavefront
compensation for the defocus component and developed parallel signal processing with three GPUs for our holographic
data storage system. Using this system, we demonstrated real-time playback of beyond high definition video signals with
50 Mbps.
The characteristics of holographic recording in the presence of phase noise caused by air disturbances and by
vibrations were studied. Diffraction efficiencies were calculated using the finite-difference time-domain method. The
simulation results show that the amplitudes of fringes decreases with increasing phase noise, and thus diffraction
efficiencies decrease. A novel method of phase compensation for holographic data storage is proposed. Interference
fringes are incident on a charge coupled device (CCD) camera symmetrically positioned in relation to the recording
media, and the fringes in the media are estimated using the fringes monitored by the camera. The phase information is
calculated using a fast Fourier transform, and a phase modulator is set in the reference beam's path and driven to reduce
the phase error, so that the phase at the CCD camera is stabilized. With this method, the standard deviation of the phase
error is decreased to less than 1/10th that of the conventional method. The diffraction efficiencies of angle multiplex
recording using photopolymer were measured. The diffraction efficiency with phase compensation at 0 degree was
700% larger than without it, and the total relative multiplexing number improved by 50%.
When digital data are recorded, we found that the histogram distribution of the symbol "1" (a white signal) was a
Gaussian density and the distribution of the symbol "0" (a black signal) was a Rayleigh density. We also proposed a
novel SNR measure suitable for holographic data storage. When this phase compensation method was applied to digital
holographic storage in which a spatial light modulator (SLM) was set on reference beam's path, a 5.0-dB improvement
in SNR compared with the value without phase compensation was obtained. As holographic data storage can record
phase, the proposed method can be made into a feasible technology.
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