Tamiki Takemori

Stabilized and accelerated speckle strain gauge

We have improved the stability and temporal response of the laser speckle strain gauge that detects strain from speckle displacement caused by surface deformation. In the gauge, a narrow laser beam is incident on a diffuse surface, and speckle patterns appearing in the light scattered through the symmetrical angles are detected by a pair of linear image sensors. Speckle displacement is derived by a real-time correlator as the peak of the cross correlation between the current frame and a reference frame of the sensor driven at 1000 frames per second. The reference frame is fixed until the correlation peak becomes lower than a threshold. Using a pair of image sensors, followed by the correlators, and a laser of output power of a few tens of milliwatts, we could measure dynamic strain of polymer films at the frequency between a fraction of hertz and 100 Hz. The resolution of the gauge, which only depends on geometry of the optical system and the sensor pitch, is a few tens of microstrains. The upper limit of the measurement can be extended arbitrarily because small incremental strain is integrated.

Real-time velocity measurement by the use of a speckle-pattern correlation system that incorporates a ferroelectric liquid-crystal spatial light modulator

We describe a technique for noncontact velocity measurement by using double-exposure speckle-pattern techniques with optical signal processing. The two speckle patterns are recorded on a ferroelectric liquid-crystal (FLC) spatial light modulator (SLM), which is a bistable optically addressed SLM, and the composite pattern is then analyzed by an optical system similar to a joint transform correlator, in which another FLC-SLM and a position-sensitive detector are used. We show that the performance of the system can be significantly improved by adjusting the time between exposures using a real-time feedback system that is based on the position of the correlation spot in the output plane.

Real-time profiling of a pure phase object using an auto-Wigner distribution function

Abstract The reconstruction of a pure phase object is investigated on the basis of an auto-Wigner distribution function (AWDF). A cylindrical lens is used as a typical example of the pure phase object and its profile is successfully obtained in real time using its AWDF. The product of the object function and its conjugated one, which is necessary to yield the AWDF, was generated by a Brenner-Lohmann configuration. The Fourier transform of the product is made by a microcomputer without using the holographic method. This paper shows that this method realizes not only a real-time operation but also much improvement in measurement accuracy by thresholding the product obtained optically as the intensity distribution.

Resolution improvement in speckle displacement and strain sensor by correlation interpolation

A new correlator which outputs 48 correlation values around the correlation peak is described, and several interpolation methods for the detection of the displacement of a speckle pattern with less than one pixel resolution of the linear image sensor are experimentally compared. The correlator is capable of computing the correlation function between the current frame and a reference frame. The use of the new correlator in conjunction with interpolation methods makes it possible to obtain the best value of the resolution for the speckle displacement of less than 0.2 pixels.

Optical computing based on a selector logic

Optical logic operations based on a selector logic are proposed. Based on the parallelism of optics, the selector logic is realized by data beam selections through a polarization switching device and a birefringent plate. The optical selector unit proposed here is composed of a pair of 90° twisted nematic liquid crystal cells, a birefringent crystal plate, and a mask. Several logic gates (AND, OR, XOR, and XNOR gates) and a full adder are demonstrated based on the proposed optical selector logic. The parallelism of optics makes it easy to implement the selector logic. As a result, optical parallel logic operations are performed by cascading the selector units and the interconnection problems for a large scale logic circuits can be avoided.

Optical latches based on a selector logic

Optical latches and flip-flops based on an optical selector logic are proposed to realize optical sequencers. The optical selector proposed here consists of a pair of 90° twisted nematic liquid crystal cells, a birefringent crystal plate, and a mask. The optical implementation of the selector logic can be realized by the beam selections through the optical selector unit via a polarization switch, and then the parallel computation can be performed by cascading the optical selector units. An optical SR latch and an optical D flip-flop are demonstrated based on the optical selector logic, and an optical binary counter is designed by using the optical flip-flops.

Generation of a joint pattern for optical speckle JTC by using a liquid crystal cell and a birefringent plate

Abstract A method to form a joint pattern before and after the displacement of a light scattering object is presented by using a spatial shifter consisting of a twisted nematic liquid crystal cell and a birefringent plate. The polarization of the speckle pattern is switched by the 90° twisted nematic liquid crystal and, then, the spatial shift is introduced to the pattern by passing through the birefringent calcite plate depending on its p- or s-polarized state. The generation of the joint pattern is verified by the experiment. It is possible to perform a real-time all optical joint transform correlation (JTC) based on the proposed method.

Displacement meter with ferroelectric liquid crystal spatial light modulator utilizing speckle photography

A speckle photographic technique was employed for the measurement of displacement of diffuse objects. In this method a double exposure is made on an FLC-SLM and the speckle correlation is detected with a position-sensitive detector. Use of this system means easy successive detection of displacement. In addition, the feedback control signals which regulate the FLC-SLM double-writing signals from PSD are expected to expand the measurable range and increase measurement accuracy. This approach has the advantage of measuring displacements of less than 1.0 ms in successive periods of several milliseconds. Experimental results are presented.

Stabilized And Accelerated Speckle Strain Gauge

We have improved stability and temporal response of the laser speckle strain gauge. In the gauge a narrow laser beam is incident on a diffuse surface and speckle patterns appearing in the light scattered to certain directions are detected by linear image sensors. The correlator computes the correlation peak between the current frame and a reference one of the sensor which is driven at 1000 frames per second. The reference frame is fixed until the correlation peak becomes lower than a threshold. By employing a pair of the correlators and a laser of output power of a few tens of mW, we could measure dynamic strain of polymer films, between a fraction of Hertz and one hundred Hertz. The resolution of the gauge which only depends on geometry of the optical system and the sensor pitch, is a few tens of microstrains. The upper limit of the measurement can be extended arbitrarily much because small incremental strain is integrated.

Experimental verification of phase contrast image synthesis system

A new method is presented for synthesizing arbitrary grey level intensity patterns based on phase contrast imaging. The concept is grounded on an extension of the Zernike phase contrast method into the domain of full range phase modulation. By controlling the average value of the input phase function and by choosing appropriate phase retardation at the phase contrast filter, a pure phase to intensity imaging is accomplished. The method presented is also directly applicable in dark field image synthesis. Preliminary experimental results are demonstrated for binary-only phase modulation.