Masayuki Yokota

Surface shape measurement by phase-shifting digital holography with a wavelength shift

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.

Image reconstruction only by phase data in phase-shifting digital holography

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.

Digital holographic profilometry of the inner surface of a pipe using a current-induced wavelength change of a laser diode

Phase-shifting digital holography is applied to the measurement of the surface profile of the inner surface of a pipe for the detection of a hole in its wall. For surface contouring of the inner wall, a two-wavelength method involving an injection-current-induced wavelength change of a laser diode is used. To illuminate and obtain information on the inner surface, a cone-shaped mirror is set inside the pipe and moved along in a longitudinal direction. The distribution of a calculated optical path length in an experimental alignment is used to compensate for the distortion due to the misalignment of the mirror in the pipe. Using the proposed method, two pieces of metal sheet pasted on the inner wall of the pipe and a hole in the wall are detected. This shows that the three-dimensional profile of a metal plate on the inner wall of a pipe can be measured using simple image processing.

A compact polarimetric glucose sensor using a high-performance fibre-optic Faraday rotator

A compact high-efficiency Faraday rotator is constructed which uses a single-mode flint glass fibre as the Faraday element and a small-diameter toroidal electric coil as the magnetic system. Using a fibre Faraday rotator as the polarization modulator and/or the polarization compensator, glucose solution in the 10 mm long fused silica cell is detected with a high resolution of 0.01 g dl−1 in the wide measurement range of 0–25 g dl−1.

Mass measuring instrument for use under microgravity conditions

A prototype instrument for measuring astronaut body mass under microgravity conditions has been developed and its performance was evaluated by parabolic flight tests. The instrument, which is the space scale, is applied as follows. Connect the subject astronaut to the space scale with a rubber cord. Use a force transducer to measure the force acting on the subject and an optical interferometer to measure the velocity of the subject. The subjects mass is calculated as the impulse divided by the velocity change, i.e., M=integral Fdt/delta v. Parabolic flight by using a jet aircraft produces a zero-gravity condition lasting approximately 20 s. The performance of the prototype space scale was evaluated during such a flight by measuring the mass of a sample object.

Speckle noise suppression in measurement by phase-shifting digital holography

A new algorithm for fringe phase analysis is proposed for shape and deformation measurement of diffusely reflecting surfaces by digital holography. It derives the phase of the averaged conjugate product of complex amplitudes before and after wavelength change or object deformation, corresponding to the complex coherence factor, and is consistent with a theory on fringe formation in holographic interferometry. By experiments and computer simulations, it is demonstrated that the algorithm is much more immune to speckle noise in phase analysis and unwrapping than conventional methods.

Optical voltage sensor using a pulse-controlled electrooptic quarter waveplate

An optical voltage sensor is presented which can be used for the measurement of dc voltage by use of a Pockels crystal associated with a pulse-controlled electrooptic quarter waveplate. The measurand dc voltage signal is converted into the amplitude of an ac pulse-modulated voltage-sensing signal. Therefore, the output sensing signal is immune to electromagnetic interference owing to the applications of pulse modulation and lock-in amplifying techniques. In addition, the voltage-sensing signal is obtained from a single light path and free from light intensity fluctuations. The dc voltage of /spl plusmn/0.4 to /spl plusmn/180 V has been measured with 1.8% nonlinear error.

Stabilization improvements of laser-diode closed-loop heterodyne phase-shifting interferometer for surface profile measurement

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.

An optical sensor for analysis of soil nutrients by using LED light sources

A compact optical sensor based on photometric detection of soil nutrients is developed. The sensor consists of three light emitting diodes (LEDs) and a simple data I/O circuit using a PIC (peripheral interface controller) device. The wavelength of LEDs is chosen to fit the absorption band of chemical reagents whose colour develops by reaction with soil nutrients. The sensor is applied for detection of six soil nutrients: ammonia nitrogen (NH4–N), nitrate nitrogen (NO3–N), available phosphorus (P2O5), available iron (Fe), exchangeable manganese (Mn) and exchangeable calcium (CaO) from colour changes caused by addition of chemical reagent in a transparent plastic cell. The results are compared with those calculated using absorbance of solutions measured by spectrophotometer and good agreement is observed. The resolution of 1.0–20 mg/100 g has been performed for standard solutions of soil nutrients in a cell of 5.5 mm in path length. The sensor is also applied to an analysis of soil nutrients of various farmlands, and the results are compared with those obtained by a colour chart judgment.

Polarization analysis by off-axis digital holography with an improved optical system and an evaluation of its performance by simulation

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.