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Principles of laser self-mixing interferometry at the regis-tration of target centimeter displacements


S.G. Gurzhin Ph.D. (Eng.), Associate Professor, Department of Information-Measuring and Biomedical Engineering, Ryazan State Radio Engineering University; the Winner of the Lenin Komsomol Prize in Science and Technology, Laureate of the Rya-zan Region on Science and Technology and the Silver Medal Academician V.F. Utkin, Honorary Worker of Higher Profession-al Education of the Russian Federation
K.R. Lovyagin Master Student, Department of Information-Measuring and Biomedical Engineering, Ryazan State Radio Engi-neering University

The self-mixing effect consists in mixing an intracavity electromagnetic wave with an emitted electromagnetic wave, which is re-turned to the laser cavity after interaction with an external reflector (target). This phenomenon is universal and manifests itself in all types of lasers, including gas lasers, plane semiconductor lasers, vertical cavity surface-emitting lasers (VCSEL), infrared lasers, terahertz quantum-cascade lasers (THz QCL), and quantum dot lasers.
All self-mixing systems operate according to a single fundamental principle: the beam is emitted by a laser, propagates in the medium to an external reflector, is partially reflected and returns to the laser cavity, where it interacts with the resonant modes of the laser.
Due to double propagation through the external resonator and the reflection from the target, the reflected beam accumulates in-formation about each of these objects. By mixing inside the cavity, the reflected beam distorts the intracavity electric field, trans-ferring the accumulated information to the outside, where it becomes suitable for measurement through parameters such as the optical power of the radiation, the radiation frequency, and the laser voltage. Changes in optical power are usually monitored with a photodetector; changes in voltage can be measured directly.
When self-mixing system operates on a diffusive target, it becomes necessary to deal with a speckle, which is a random interference pattern that is formed by the mutual interference of coherent waves with random phase shifts and / or a random set of intensities. For this purpose, a technique for bright speckle tracking (BST) has been developed. Its essence lies in the fact that the laser beam by means of moving the lens with the help of piezo-actuators continuously scans the surface of the diffusive target in search of the maximum level of reflected radiation. The amplitude of the beam travel is several micrometers, which is much less than the focusing spot.
However, the BST technique does not expand the dynamic range of displacement measurements, which, when using a diffusive target (for example, human skin), is a fraction of a millimeter. The article considers the possibility of expanding the dynamic range of measurement by using a microprismatic film reflector (flicker). In this case it is possible to record the movement of the target by a value of at least 30 mm.

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