Toshihiro Kubota

Parallel two-step phase-shifting digital holography

We propose a parallel two-step phase-shifting digital holography technique capable of instantaneous measurement of three-dimensional objects, with a view toward measurement of dynamically moving objects. The technique is based on phase-shifting interferometry. The proposed technique carries out the two-step phase-shifting method at one time and can be optically implemented by using a phase-shifting array device located in the reference beam. The array device has a periodic two-step phase distribution, and its configuration is simplified compared with that required for three-step and four-step parallel phase-shifting digital holographies. Therefore the optical system of the proposed technique is more suitable for the realization of a parallel phase-shifting digital holography system. We conduct both a numerical simulation and a preliminary experiment in the proposed technique. The results of the simulation and the experiment agree well with those of sequential phase-shifting digital holography, and results are superior to those obtained by conventional digital holography using the Fresnel transform alone. Thus the effectiveness of the proposed technique is verified.

High-speed phase imaging by parallel phase-shifting digital holography.

Parallel phase-shifting digital holography can obtain three-dimensional information of a dynamically moving object with high accuracy by using space-division multiplexing of multiple holograms required for phase-shifting interferometry. We demonstrated high-speed parallel phase-shifting digital holography and obtained images of the phase variation of air caused by a compressed gas flow sprayed from a nozzle. In particular, we found the interesting phenomenon of periodic phase distributions. Reconstructed images were obtained at frame rates of 20,000 and 180,000 frames per second.

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.

Hologram formation with red light in methylene blue sensitized dichromated gelatin.

The spectral sensitization of hardened dichromated gelatin to red light is carried out using methylene blue as sensitizer. The sensitized dichromated gelatin plate can be stored for over 1 month in an ammonia atmosphere without loss of photosensitivity. Holograms recorded in the sensitized dichromated gelatin using a He-Ne laser have high diffraction efficiency and low noise if they are swelled at a temperature near the gelatin gel melting point during development processing. Maximum diffraction efficiency has been found to be as high as 88%. The energy required to obtain 80% diffraction efficiency is 150-400 mJ/cm(2).

Improvement of color reproduction in color digital holography by using spectral estimation technique

We propose a color digital holography by using spectral estimation technique to improve the color reproduction of objects. In conventional color digital holography, there is insufficient spectral information in holograms, and the color of the reconstructed images depend on only reflectances at three discrete wavelengths used in the recording of holograms. Therefore the color-composite image of the three reconstructed images is not accurate in color reproduction. However, in our proposed method, the spectral estimation technique was applied, which has been reported in multispectral imaging. According to the spectral estimation technique, the continuous spectrum of object can be estimated and the color reproduction is improved. The effectiveness of the proposed method was confirmed by a numerical simulation and an experiment, and, in the results, the average color differences are decreased from 35.81 to 7.88 and from 43.60 to 25.28, respectively.

Single-shot femtosecond-pulsed phase-shifting digital holography

Parallel phase-shifting digital holography is capable of three-dimensional measurement of a dynamically moving object with a single-shot recording. In this letter, we demonstrated a parallel phase-shifting digital holography using a single femtosecond light pulse whose central wavelength and temporal duration were 800 nm and 96 fs, respectively. As an object, we set spark discharge in atmospheric pressure air induced by applying a high voltage to between two electrodes. The instantaneous change in phase caused by the spark discharge was clearly reconstructed. The reconstructed phase image shows the change of refractive index of air was -3.7 × 10(-4).

High-Speed Three-Dimensional Microscope for Dynamically Moving Biological Objects Based on Parallel Phase-Shifting Digital Holographic Microscopy

We succeeded in 3-D motion-picture imaging for dynamic motion of biological objects swimming at different depths by parallel phase-shifting digital holographic microscopy (PPSDHM). PPSDHM is a technique for high-quality instantaneous 3-D imaging of specimens in the microscopic field of view by single-shot recording of multiple phase-shifted holograms. We constructed a PPSDHM for recording high-speed dynamic phenomena and then motions of biological objects in water were recorded at 20 000 and 150 000 frames/s, respectively, which is the fastest among the previous reports on 3-D imaging of biological objects.

Four-Wavelength Color Digital Holography

We propose a color digital holography using four recording wavelengths to improve the color reproduction of three-dimensional imaging of objects. In conventional color digital holography, three-wavelength light beams corresponding to red, green, and blue are used for recording holograms. In this paper, a laser-emitting yellow light is added to the conventional color digital holography. Also spectral estimation technique is introduced to the four-wavelength recording technique to improve the color reproduction of color digital holography. The effectiveness of the proposed method was numerically confirmed. Also the proposed technique was experimentally demonstrated using four lasers operated at 473, 532, 561, and 633 nm, respectively. When Macbeth color chart consisting of 24 color patches was used as objects, in comparison to conventional technique, the average color differences between the objects and the reconstructed images by the proposed technique for the 24 colors are decreased from 7.88 to 6.24 in numerical simulation and from 10.02 to 8.63 in experiment, respectively.

The Bending of Interference Fringes inside a Hologram

The bending of interference fringes inside a hologram due to a prestress in the photographic emulsion is investigated. The influence of the bending on the diffraction efficiency and the angular sensitivity is explained qualitatively in terms of the Fresnel-Kirchhoff diffraction theory. The quantitative analysis is carried out using the coupled wave theory with the attenuated grating model. Experimental results obtained using a Kodak 649 F plate are compared with the theory. The bending is found to be zero at the emulsion surface and its shape was represented approximately by a quadratic curve in the direction of the emulsion thickness.

Lippmann color holograms recorded in methylene-blue-sensitized dichromated gelatin

Experimental techniques are described for recording Lippmann color holograms in methylene-blue-sensitized dichromated gelatin. 458-, 488-, and 514.5-nm light from an argon laser and 633-nm light from a He-Ne laser are used to record the holograms. Data on the diffraction efficiency and wavelength selectivity are presented.