A novel Laser-Doppler-Velocimeter based on the self-mixing principle for nontactile measurement of velocity and length

A compact near-infrared laser Doppler velocimeter was developed based on laserfeedback interferometry. This very direct method was found to be applicable to moving parts for the measurement of velocity and length. Using a single-mode laser diode operating at 20 mW output power good signal-to-noise ratios and thereby good resolutions could be achieved for over a wide range of materials. We investigated how the spectrum of laser emission is affected by weak optical feedback. The measurements showed that the emission wavelength was stable within 10 (exp -3) if the laser is stabilised in both injection current and temperature. A description of the signal processing technique applied in the frequency domain is given.