Semiconductor laser's on-line coherence calibration and testing of frequency stability

One of the main constituent parts of optical coherent measuring apparatus is laser as source with stable performance of frequency, radiation intensity, and light beam uniformity. At present time semiconductor lasers are rather attractive devices in view of there low prices, small size, serviceability. Progress in its quality leads to including them not only in lightheads, but as lighting unit in measuring apparatus. In order to guarantee accuracy of measuring instruments, all parts of them must have stable performance, and in this respect semiconductor laser demand stabilization more that one characteristic quantity at once. And frequency stability on the one hand is overwhelmingly important for constancy of optical measuring instruments, on the other hand our investigations show that its regulatory control is very arduous task. Both holographic methods and phase modulated speckle interferometry clearly recognize smooth frequency shift and frequency jumping depending on pumping current and temperature. And for repeatability it's required to return both of them. So it is necessary laser frequency testing during working. For interferometric comparison circuit it is frequency variation that exerts influence on fringes pattern generation, so just this parameter should be traced in the course of measuring. Specially prepared test object, introduced in holographic scheme, allows to uncover frequency variation, if they had have place, and to reproduce coherence function of laser source. Complicated coherence function of semi conductor lasers can destroy interference pattern or foul the interpretation of it. So this coherence calibration is also very useful for results validity. Phase modulated speckle interferometry method allows to build phase correlation portraits. analogical to interferograms, hence multiwavelength contour generation masks the picture of intrinsic object information too. Both real wavelength change and nonresolution area, when coherence length is less then path-length difference, may be obtained with the help of known wedge incline near the measured object. Since measured and testing image will be entered at the same time, test results will be applied in interpretation of measurement results. Thus on-line testing of laser characteristic quantities allows using unstable sources and increasing of measurement precision.