Doppler velocity measurement with self-mixing effect of direct modulated laser diode enabling velocity direction

Doppler Laser Velocity measurement by self-mixing effect using direct modulation laser diode is proposed to distinguish the direction of the object movement. To distinguish the movement direction in conventional way, it is needed to assemble a frequency shift device such as an acoustic optic cell in the optical system. However, the self-mixing scheme cannot satisfy this condition this due to its simple system. Therefore, we applied triangular-wave modulation current with repetition frequency of 250Hz directly to the laser diode emitting at a wavelength of 780nm. Target speed of 0.1 - 0.4 cm/s is measured and its movement direction forward and/or backward can be distinguished successfully. Both the velocity and moving direction of the target are obtained.