The self-mixing interference inside a laser diode: application to displacement, velocity and distance measurement

A low-cost compact self-aligned optical sensor using the self-mixing effect inside a cw single-mode near infrared laser diode has been developed for distance, velocity and displacement measurements. The optical beam back-scattered by a rough target into the laser diode cavity causes strong variations of the optical output power, the laser and the external target acting as a 3-mirror Fabry-Perot cavity. As each period of the optical power variations of the sawtoothlike self-mixing interference is due to a target displacement of a half-wavelength lambda / 2 (similar to 400 nm), the motion law of this target can be represented by the addition of such small displacements. This sensor has also been used for velocity measurement up to several meters per second by applying a FFT directly on the self-mixing signal to reduce the influence of the speckle. By modifying the shape of the injection current, this sensor can be used as a laser range finder to measure distances from 1 m to 2 m with an accuracy of +/- 1.5 mm. In order to improve the accuracy of the range finder, the choice of the optical source is also discussed.