Wavelength-change characteristics of semiconductor lasers and their application to holographic contouring

Iterative ripple error suppression algorithm for the dynamic interferometry

In dynamic interferometry, the retardation error of quarter-wave plate (QWP) causes inconsistency of the background intensity and the modulation of the four phase shift interferograms, which makes the periodic ripple error in the measurement results. In this paper, an iterative algorithm is proposed to solve this problem. Both simulations and experiments validated that the algorithm can effectively eliminate the ripple error in the measurement results with stable and quick convergence, therefore the measurement accuracy of the dynamic interferometer can be improved without any extra manual operation.

Large step-height measurements using multiple-wavelength holographic interferometry with tunable laser diodes

Accurate measurement of large step heights using multiple-wavelength holographic interferometry is realized using laser diodes. Due to the high-resolution wavelength tunability of such lasers, a pair of holograms with a wavelength difference of less than 0.01 nm is recorded and used to extract a phase difference having a large synthetic wavelength. Phase differences with synthetic wavelengths ranging from 2.5 to 73 mm are extracted by using pairs of holograms with wavelength differences between 0.3 and 0.01 nm. By combining the phase differences, measurements with a step height of 18 mm and an rms error of 0.04 mm could be achieved. The requirements for performing the phase unwrapping are discussed. Precise knowledge of the recording wavelengths is required to correctly perform this unwrapping.

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.

Automatic deformation analysis by a TV speckle interferometer using a laser diode

A phase-shifting TV speckle interferometer is developed by using the frequency modulation capability of a laser diode and is applied to automatic deformation measurements. Temperature modulation is used for generating the frequency shift of a laser diode. We store four speckle interferograms with relative phase differences of pi/2 in a frame grabber by monitoring a phase-shift signal, which is obtained by integrating the fringe intensity over a local window. The optimum size of the local window is examined experimentally. Then, the optimum processing conditions for reducing the speckle noise in interferograms,including an averaging window size, are found. Experimental results of the automatic deformation analysis indicate a displacement accuracy of better than lambda/30.

Compact holographic camera without vibration insulation using a 670-nm semiconductor laser

A compact camera for holography and holographic interferometry is presented. Owing to its simple and rigid construction, vibration insulation is not necessary. The use of a semiconductor laser with a wavelength of 670 nm permits the use of common holographic recording materials. The only optical components of this camera are a beam splitter and a deflection mirror. As an example of the application of this camera, holographic surface contouring is demonstrated.

Denisyuk hologram recording with a laser diode

Making Denisyuk holograms in the near IR with a laser diode is reported, tuning reconstruction to the visible.

Heterodyne interferometry with a frequency-modulated laser diode

A digital phase measuring interferometer with a frequency-modulated laser diode using the integratedbucket technique is described. The injection current is continuously changed to introduce a time- varying phase difference between the two beams of an unbalanced Twyman-Green interferometer. The intensity of the interference patterns is integrated with a CCD array sensor for intervals of one-quarter period of the fringe. Using the intensity data a microcomputer calculates the phase to be detected. Some experimental results with the interferometer are presented; the rms repeatability obtained was lambda/80.

Absolute distance measurement by wavelength shift interferometry with a laser diode: some systematic error sources

A method for measuring absolute distance by the wavelength shift of laser diode light has previously been proposed. In this work three serious systematic error sources for the method are discussed and some of the discussion is confirmed by experiment. The error sources are optical feedback effect, longitudinal mode distribution of laser light, and unwanted light reflected from optical devices (coherent noise). The optical feedback effect influences the wavelength shift of the emitted light. The mode distribution causes the periodic error dependent on the measured distance, and the maximum error is determined by the change in the intensity ratio of the submodes to the main mode. Coherent noise causes the periodic error also dependent on the distance, and the maximum error is determined by the amplitude ratio of the measuring lightwave to the noise. These systematic errors are observed in some demonstrative experiments.

Interferometer for measuring displacement and distance

A simple interferometer for measuring both relative displacement and absolute distance is fabricated that uses a laser diode. The sign of the displacement is detected by means of a lambda/8 plate, and the distance is measured by a FM radar technique of modulating the laser-diode frequency. Measurement accuracies of 0.02 microm for displacement and 100 microm for distance are obtained over a range of a few meters.

Digital phase-measuring interferometry with a tunable laser diode

A digital phase-measuring interferometer with a laser-diode source has been developed that is based on a fringe-scanning technique with a stepwise wavelength change by variation of the laser injection current. The phase is changed to produce a relative phase difference between the beams in the two arms of the interferometer. Calibrated phase shifts used for a phase-extraction algorithm are derived from one-dimensional least-squares fits to cosine fringe functions to achieve accurate results. Experimental results are presented.