New absolute distance measurement technique by self-mixing interferometry in closed loop

We present a method for the measurement of the absolute distance of a remote target based on the laser diode self-mixing interferometry technique, assisted by an electronic feedback loop capable of increasing the measurement accuracy. In self-mixing interferometry, the light emitted by a laser diode is back-scattered by a remote target and re-enters the laser cavity, thus generating an interferometric signal. In the usual self-mixing distance measurement approach, the laser wavelength is linearly modulated, and the target distance is estimated from the count of the number of interferometric fringes. In the new method, we use an electronic feedback loop to generate a wavelength change that exactly corresponds to one single interferometric fringe. This allows to measure the target distance with a higher accuracy, in principle limited only by the detection shot-noise, and not by the fringe quantization error typical of conventional fringe counting approaches. The target distance can be measured with 0.3 mm accuracy, in the 0.2-3 m range.