Phase Detector for Laser Phase Perturbation Demodulation

Laser interferometry is a powerful tool for vibration measurement. It makes it especially suited for measurement of small vibrations which would otherwise be interfered with by contact-based methods.This paper is devoted to the design and implementation of a phase detector for LASER phase perturbation demodulation. Based on self-mixing interferometry, the detector achieves vibrations measurements using the fringe counting method. The paper covers design of a LASER driver, a transimpedance amplifier and an output filter. With a bandwidth of over 200 kHz, the detector is designed to measure vibrations larger than half a wavelength of the lasing frequency. This translates to small vibrations as small as a few micrometers for a red LASER diode for example. In the later part of the report, the detector sub circuits are tested and analysed. The complete detector is however not tested for vibration measurements having failed to acquire the appropriate LASER diode in time. Nevertheless, the vibration testing procedure is presented and ways on automating and improving the sensitivity of the detector discussed.