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An optical system of off-axis digital holography for imaging the Jones vector of an object wave is improved, and a Faraday rotator for the reference wave is also newly constructed. To evaluate the accuracy of the polarization analysis, quarter- and half-wave plates are used as the object, and the distribution of the polarization state of the transmitted light is analyzed for various orientations of the wave plates. The polarization analysis is also simulated, and the effect of a finite value of the extinction ratio and the modulation error for the reference wave is investigated numerically.

A closed-loop phase-shifting Fizeau-type interferometer was constructed that uses direct frequency modulation of a laser diode. The interferometer is servo controlled entirely in the phase domain, where optical phases are detected by two-frequency optical heterodyning. A detailed study of stabilization of the interferometer under feedback control was conducted both experimentally and theoretically. The interferometer showed good stability against vibration up to 200 Hz. The stabilization factors obtained experimentally are in good agreement with the theoretical calculations. The phase-shifting experiment was accomplished with high precision as well as with high stability against external disturbances. The profile measurement of a mirror surface was made with a phase-shifting analysis algorithm, and good measurement reproducibility of lambda/60 in the root-mean-square value was obtained for ten measurements within a period of 20 min.

We describe data compression in phase-shifting digital holography. We demonstrate by experimentation that an image of a diffusely reflecting object can be reconstructed only by phase data of the derived complex amplitude. It is shown that reduction of the bit depth of the phase data does not seriously damage the image even down to 1 bit. We observe enhancement of halo in the image with low bit depths. This tendency is verified quantitatively by a one-dimensional simulation. Our procedure for smoothing the images that result from the data-compression methods is shown to be effective.

A new scheme of the Faraday effect current sensor of a magnetic ring-core type is reported. With this scheme a uniaxial magnetic garnet film is obliquely inserted into a very narrow gap in the ring core and a probe light beam is passed through the garnet film in the transverse direction to the core. It is experimentally demonstrated that this current sensor shows good sensitivity as well as good isolation from surrounding currents, particularly because of the very narrow core gap allowed. This transverse scheme is much simpler and less expensive than the known longitudinal scheme and can offer a practical optical current sensor of a ring-core type.

To stabilize the phase-shifting Fizeau-type interferometer against environmental disturbances (namely, vibration and temperature variations), the feedback scheme that uses the current-induced frequency modulation of a laser diode (lambda = 633 nm) and the two-frequency optical heterodyne method has been investigated, with particular attention to improvement of the achievable stabilization. It is demonstrated that introduction of the proportional-integral control into the feedback system improves stabilization against the proportional control case; e.g., stabilization is improved 5 times for 100-nm(p-p) vibration at the frequency range at 30 Hz. The surface profile measurement for a sample mirror was conducted with a reproducibility of 6.8 nm in the root mean square under the subwavelength-amplitude vibration at 100 Hz.

A new type of Faraday effect optical current transformer has been developed that uses a single block of square flint glass with dielectric-coated total reflection surfaces as the sensing element. Numerical calculation has shown that the coating of two dielectric layers of 45.59-nm-thick Ta2O5 and 448.35-nm-thick SiO2 films on a flint glass surface produces zero retardation total reflection for the 45 degrees incident angle of light at lambda = 840 nm with great incident angle, wavelength, and film thickness tolerances. A fiber-linked current transformer has been constructed and experimentally demonstrated to exhibit high isolation from surrounding currents as well as high stability against mechanical disturbances.

Surface contouring by phase-shifting digital holography is proposed and verified by experiments and numerical simulations. Digital holograms are recorded before and after mode hopping of a laser diode subject to current tuning, and the difference of the reconstructed phases at each wavelength is computed to deliver surface contours of a diffusely reflecting surface. Since normal incidence on the object is employed, the method does not need the removal of the tilt component and is free from the shadowing effect as advantages over the dual-incidence method proposed before by the first author.

A two-dimensional noncontact optical fiber water droplet sensor that uses the fluorescence from a laser-diode-pumped Tm3+:YAG and that uses a single fiber bundle for the probe fiber has been constructed. The sensor is based on absorption spectroscopy but has the significant advantage that the residual pump light works for the reference signal. The water depth can be evaluated from the minimum intensity ratio between the fluorescence signal and the reference signal when the probe is scanned in plane. The sensor can also estimate both the size and the position of a water droplet. A high degree of discrimination of a water droplet from other perturbations such as oil and surface irregularities has been demonstrated. An approximate calculation in a hemispherical model of a water droplet was carried out and is shown to be in good agreement with experimental results.