Motofumi Fujii

Experimental demonstration of parallel two-step phase-shifting digital holography

Parallel two-step phase-shifting digital holography is a technique for single-shot implementation of phase-shifting interferometry and requires only the intensity distribution of the reference wave and spatial two phase-shifted holograms. We constructed a system of parallel two-step phase-shifting digital holography and experimentally demonstrated the technique, for the first time. The system uses an originally fabricated image sensor having an array of 2 × 1 micro polarizers. Each micro polarizer was attached on pixel by pixel. In the experiment, the unwanted images, the zero-order diffraction wave and the conjugate image, are removed from the reconstructed image of objects by the system, while the images superimpose on the image of objects reconstructed by Fresnel transform alone. Also the capability of single-shot and three-dimensional imaging is demonstrated by the system.

Parallel phase-shifting digital holographic microscopy

We propose parallel phase-shifting digital holographic microscopy (PPSDHM) which has the ability of three-dimensional (3-D) motion measurement using space-division multiplexing technique. By the PPSDHM, instantaneous information of both the 3-D structure and the phase distributions of specimens can be simultaneously acquired with a single-shot exposure. We constructed a parallel phase-shifting digital holographic microscope consisting of an optical interferometer and an image sensor on which micro polarizers are attached pixel by pixel. The validity of the PPSDHM was experimentally verified by demonstrating the single-shot 3-D imaging and phase-imaging ability of the constructed microscope.

Removing the Residual Zeroth-Order Diffraction Wave in Polarization-Based Parallel Phase-Shifting Digital Holography System

Although we have constructed parallel phase-shifting digital holography systems using an array of micropolarizers, we found that the zeroth-order diffraction wave was not removed completely. Then, we report the reason why the zeroth-order diffraction wave is not always removed completely and propose an algorithm to remove the residual zeroth-order diffraction wave. In the algorithm, phase-shifted holograms obtained by parallel phase-shifting digital holography and the intensity distribution of the reference wave are divided into segments and the pixel values in the segments are multiplied by proper correction constants. The effectiveness of the proposed algorithm was experimentally verified.

Comparative evaluation of the image-reconstruction algorithms of single-shot phase-shifting digital holography

We comparatively evaluated two types of image-reconstruction algorithms in two types of single-shot phase-shifting digital holography techniques to investigate which algorithm is more suitable for high-quality and instantaneous three-dimensional imaging. Two types of image-reconstruction algorithms were proposed for each single-shot phase-shifting interferometry so far. One generates multiple phase-shifted holograms from the recorded single hologram by utilizing interpolation and derives the complex amplitude of the object wave by applying the calculation of phase-shifting interferometry to the multiple phase-shifted holograms. The other does not use interpolation but applies the calculation of phase-shifting interferometry to the neighboring pixels in the recorded hologram to derive the complex amplitude. We experimentally evaluated the qualities of the image reconstructed by the algorithms in each single-shot phase-shifting technique. It was clarified that the former algorithm is more effective for high-quality imaging free from the unwanted images in single-shot technique using an array of optical elements. In single-shot technique using spatial carrier, although the latter algorithm is able to reconstruct a brighter image for wide area than the former, spatial-filtering technique is needed for removing the conjugate image.

Compensation algorithm for the phase-shift error of polarization-based parallel two-step phase-shifting digital holography.

We propose an algorithm for compensating the phase-shift error of polarization-based parallel two-step phase-shifting digital holography, which is a technique for recording a spatial two-step phase-shifted hologram. Although a polarization-based system of the technique has been experimentally demonstrated, there had been the problem that the phase difference of two phase-shifted holograms had been changed by the extinction ratio of the micropolarizer array attached to the image sensor used in the system. To improve the performance of the system, we established and formulated an algorithm for compensating the phase-shift error. Accurate spatial phase-shifting interferometry in the system can be conducted by the algorithm regardless of phase-shift error due to the extinction ratio. By the numerical simulation, the proposed algorithm was capable of reducing the root mean square errors of the reconstructed image by 1/4 and 1/5 in amplitude and phase, respectively. Also, the algorithm was experimentally demonstrated, and the experimental results showed that the system employing the proposed algorithm suppressed the conjugate image, which slightly appeared in the image reconstructed by the system not employing the algorithm, even when the extinction ratio was 10:1. Thus, the effectiveness of the proposed algorithm was numerically and experimentally verified.

Construction of a portable parallel phase-shifting digital holography system

We propose a portable parallel phase-shifting digital holography system. Parallel phase-shifting digital holography is a technique capable of instantaneous and three-dimensional measurement of objects. This technique has been verified by an experiment on anti-vibration optical tables, but had not been realized as a system for practical use yet. Then, we designed a portable system of parallel phase-shifting digital holography for the purpose of practical use. The size and weight of the system are 450 mm (L)×250 mm (W)×200 mm (H) and less than 7 kg, respectively. We designed the configuration of the system in a way which can be easily changed to implement two types of parallel phase-shifting digital holographies. The system was constructed by the use of ready-made optical components. To confirm the validity of the system, the capability of the three-dimensional imaging free from the superimposition of the unwanted images, which was the problem accompanied with conventional in-line digital holography, with a single-shot exposure was successfully demonstrated.

262500-Frames-Per-Second Phase-Shifting Digital Holography

Thanks to parallel phase-shifting digital holography, we succeeded in 262500-frames-per-second phase-shifting digital holography. Dynamic phase change of air caused by focusing of a femtosecond light pulse was observed by the technique.

A4-Sized Parallel Phase-Shifting Digital Holography System

An A4-sized parallel phase-shifting digital holography system was constructed and experimentally demonstrated. Parallel phase-shifting digital holography is a technique being capable of instantaneous recording of three-dimensional (3-D) image of moving object. Aiming at a 3-D motion picture measurement of moving object system that can be carried on sites where the measurement system is needed, we improved a previous portable parallel phase-shifting digital holography system. The size and weight of the improved portable system are 300 mm × 210 mm × 160 mm and 6 kg, respectively. Also, we experimentally recorded a series of holograms of a moving object and reconstructed motion pictures, which show the same event but are focused at each depth, from the holograms. Thus, the 3-D motion picture recording and reconstruction capability of the constructed system were successfully demonstrated. A 3-D motion picture at 10 frames/s was obtained by the system.

Light-in-Flight Recording by Parallel Phase-Shifting Digital Holography

We propose and demonstrate light-in-flight recording by parallel phase-shifting digital holography to obtain a motion picture of ultrashort light pulse propagation free from unwanted images, which are a zeroth-order diffraction image and a conjugate image, by phase-shifting interferometry with a single-shot exposure. We developed a polarization-imaging camera to record the propagation of a femtosecond light pulse whose center wavelength and temporal duration were 800 nm and 96 fs, respectively, and observed the propagation of the pulse without the unwanted images for 10 ps.

Portable parallel phase-shifting digital holography systems

We have constructed two portable parallel phase-shifting digital holography systems. One is an A3-sized parallel phase-shifting digital holography system and the other is an A4-sized parallel phase-shifting digital holography system. The size and weight of the A3-sized system are 450 mm (L) × 250 mm (W) × 200 mm (H) and 7kg, respectively. The system was constructed by use of ready-made optical components. To reduce the size and weight of the A3-sized system, the A4-sized system was designed and constructed. The size and weight of the A4-sized system are 300 mm (L) × 210 mm (W) × 160 mm (H) and 6 kg, respectively. The 3D motion picture recording and reconstruction capability of the constructed system were successfully demonstrated at 10 frames/second.