New range-finding speedometer using a self-mixing laser diode modulated by triangular wave pulse current

A new laser diode range-finding speedometer is proposed, which is modulated by a triangular pulse current superimposed on a dc current instead of a continuous triangular wave employed in the previous sensor. Since a target velocity is directly obtained from a pure Doppler beat frequency measured during the non-modulation period, the new sensor is free from the difficulties due to the critical velocity encountered in the previous sensor. Measurement of a target velocity, 4 mm/s-30 mm/s has been successfully done without the influence of the critical velocity when the target is moving at distances 30 cm-60 cm from the laser diode (LD). The measurement accuracy was 0.5%-1.5% for range and within 2% for velocity. The new sensor will expand the measurable range of velocity in many applications