Analysis of some intrinsic limitations of a laser range finder using the self-mixing interference

For sensing applications like absolute distance measurement using the self-mixing effect, the laser diode is modulated by a triangular signal in order to produce an excursion of the optical frequency. The optical beam back-scattered by a noncooperative target into the laser diode active cavity causes strong variations of the optical output power, these variations being counted in order to determine distances. In this paper, some intrinsic limitations of a feedback interferometric absolute distance measurement system are analyzed. The spectral characteristics, the wavelength or the optical power of the laser diode are disturbed by the optical feedback. These changes modify parameters like the coherence length (i.e. the maximum range of the sensor) or the value of the injection current to be modulated. So, laser diodes have to be characterized in terms of spectral linewidth. In the case of strong feedback, the optical power is also disturbed by the hysteresis. The influence of the thermal effects is analyzed and a solution is proposed to improve the accuracy of the sensor.