Ichirou Yamaguchi

Measurement of surface shape and deformation by phase-shifting image digital holography

Surface shape and deformation measurement by phase-shifting digital holography using an imaging setup is presented. The differences of the reconstructed phases before and after tilt of the illumination beams and object deformation provide the contour lines of the surface height and deformation, respectively. Suppression of speckle noise and comparisons between experiments and calculation are discussed.

Laser speckle rotary encoder

A new rotary encoder is proposed which detects displacement of speckle arising in the diffraction field of a cylinder illuminated by a narrow laser beam. The displacement is detected from the peak position of the cross-correlation between the outputs of a linear image sensor. The speckle displacement due to small rotation of the cylinder is derived first. In experiments correlation is computed on a microcomputer for investigating the sensitivity and repeatability. A He-Ne laser and a laser diode are used for metal and paper surfaces. Acceleration and extension of the measurement range has also been possible by developing a real-time correlator.

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.

Surface Contouring by Phase-Shifting Digital Holography and Noise Reduction

Surface contouring by phase-shifting digital holography is proposed that derives surface height from the difference of reconstructed object phases for different incident angles at an object. It directly delivers surface shape without fringe phase analysis. Its sensitivity depends on the difference of incident angles. By proper selection of the angles we can derive surface height without phase unwrapping but with low sensitivity. Higher sensitivity can be attained by increasing the difference of incident angles. Then we need phase unwrapping that is sensitive to noise due to laser speckles in the reconstructed images. This noise could be suppressed by selecting phase values at points of maximum intensity in data reduction from 1,024x1,024 to 512x512 and by selecting paths for phase unwrapping considering the intensity distribution.

Linear And Rotary Encoders Using Electronic Speckle Correlation

Speckle patterns can be used as natural surface markings which are printed by high coherence of laser light. Speckle displacement caused by linear or rotary motion of a surface is detected with a linear image sensor whose outputs are analyzed by a correlator. By using an image sensor of the pitch 15 pm and 1024 elements, resolution of a micrometer and 1/1000° have been attained. Main advantages of the encoders are extreme simplicity in optical configurations and wide ranges of measurement.

Height gauging by wavelength-scanning interferometry with phase detection

We have realized an accurate step height measurement by using the wavelength scanning interferometry combined with phase- shifting technique. It is based on detection of phase variation slope along the wavenumber axis at each pixel of a CCD camera taking a number of interferograms at different wavelengths and phases. By using a tunable laser diode with wavelength-scanning range of approximately 8 nm, a height deviation of sub-micrometer could be measured by using a PZT phase-shifter. This technique was applied to a height gauging of the gold bump array.

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.