Laser - Self - Mixing interferometry for multi degrees-of-freedoom sensors

The development of a no-contact optical sensor based on the Laser-Self-Mixing (LSM) effect for the detection of linear and angular degrees-of-freedom of the motion of a reflective target is presented and demonstrated via the implementation of a prototype experimental setup. This issue is addressed by differential measurements of couples of identical LSM interferometers, where an innovative target consisting of three reciprocally tilted mirrors attached to the moving object is exploited. Besides, the plane wave approximation is demonstrated to be unable to correctly model the operation of a LSM displacement sensors when the laser beam is not perfectly collimated. To counter this problem, a simulation model of the laser/mirror interaction in the Gaussian beam approximation is developed. In this way, it is demonstrated that a diverging laser beam allows for 1) the preservation of the LSM signal even in presence of target wobbling, and 2) the increase of the dynamic range for linear measurement up to 2 meters without any external optical attenuator along the external cavity, thus leading to a simplification and an increased robustness of the experimental arrangement.