Jun-ichi Kato

Image formation in phase-shifting digital holography and applications to microscopy.

We discuss image formation in phase-shifting digital holography by developing an analytical formulation based on the Fresnel-Kirchhoff diffraction theory. Image-plane position and imaging magnification are derived for general configurations in which a spherical reference is employed. The influences of discrete sampling of the resulting interference patterns by a CCD and numerical reconstruction on qualities of point images are investigated. Dependence of the point images on the ratio of the minimum fringe spacing to pixel pitch of the CCD is numerically analyzed. Two-point resolution and magnification are also investigated as a function of pixel numbers by a simulation using a one-dimensional model. In experiments magnified images of biological objects and a resolution target were reconstructed with the same quality as by conventional microscopy.

Phase-shifting color digital holography.

Digital holography with a three-wavelength laser and a color CCD has been demonstrated. With the phase shifting of the reference beam, in-line holograms for three wavelengths are recorded simultaneously for derivation of the complex amplitude at each wavelength, and then the three monochromatic images are reconstructed and combined into full-color images in the computer. Laser power variation for wavelengths can be compensated for in the reconstruction process. We have compared the images reconstructed by two algorithms using a single Fourier transformation and a convolution with each other by both experiments and numerical simulations. Phase-shifting errors arising at two of the three wavelengths have proved not to cause serious deterioration of the reconstructed images.

Multicolor digital holography with an achromatic phase shifter.

Multiwavelength recording and reconstruction of a three-dimensional object are realized by use of phase-shifting digital holography. Red, green, and blue lines emitted from a white-light He-Cd laser are used for one-step recording of the complex amplitude of the object with a color CCD camera, in which a phase shift is introduced with an achromatic phase shifter based on the geometric phase. Three color images are reconstructed and successfully combined in a computer by use of Fresnel transformation based on a convolution.

Detection of photothermal effect by laser speckle strain gauge

A new method for simple and sensitive detection of photothermal strain caused by laser irradiation of plastic plates has been proposed. In-plane strain of the surface is detected directly by optoelectronic detection of speckle displacements at two symmetrical positions about a probe beam. The initial bending of the plate owing to the temperature gradient along the plate thickness is measured and shows good agreement with one-dimensional calculations.

Active interferometers for shape and deformation measurements

It is generally difficult to perform interferometric measurements of surface shape or deformation outside optical benches because fringes are blurred by external perturbations such as mechanical vibration or air turbulence. For overcoming this we have developed active interferometers, in which fringes are stabilized by detecting their movement induced by the perturbations and feeding back the signal to a piezoelectric mirror of the interferometer. We also extend the system to an active phase-shifting speckle interferometer by which the correlation fringes are shifted under feedback control for automatic analysis. Examples of deformation measurement under air turbulence are presented.

Real-time fringe analyzer and its applications to active optics

A fringe analyzer has been developed which delivers phase distribution from an interference pattern having carrier fringes at the video rate. It is based on the generation of three phase- shifted moire fringe patterns through electronic multiplication with computer generated reference patterns having mutual phase shifts and subsequent phase calculation. After the principle and specifications of the analyzer are presented, applications to feedback control of a liquid crystal spatial phase modulator, measurements of large mirrors using synthetic aperture interferometry, and active phase shifting speckle interferometry are reported.

ESPI vibration fringe enhancement by laser diode wavelength modulation

A novel technique for enhancing the time-average ESPI vibration fringes is presented in this paper. The technique is based on the use of wavelength modulation of a laser diode which is adopted in the interferometer as a coherent laser source. The wavelength modulation of the laser diode gives rise to both the additional phase shift and the speckle decorrelation by which the time-average ESPI vibration fringe quality is improved.

Multidimensional displacement sensor using the optical feedback of laser diodes

A compact optical displacement sensor is reported which is based on the light undulation caused by the optical feedback on a frequency modulated laser diode. For applying this effect to the metrology, the relationship between the feedback rate and the oscillation behavior of the laser diode was investigated experimentally. As the result, it was found that the higher order of the output undulation could be suppressed by keeping the feedback rate low, and displacement measurement was performed with a resolution of 25 nm for the target distance between a few cm and 30 cm. In addition, an improved current modulation scheme for a laser diode as well as a new signal processing techniques for improving the performance is reported. As a further application, this sensor system was extended for precise measurement of two-dimensional displacement by utilizing a spherical retro-reflector.