Development of Velocity Sensor Utilizing Laser Diodes and Avalanche Photodiodes

The system consists of two laser diodes of either the same or different wavelengths in the visible range. Both laser diodes are driven by a single current oscillator at a pre-determined base frequency. This base frequency is divided into two electrical pulse trains at exactly half the original base frequency by a two-stage shift register. One of these electrical pulse trains is applied to the first laser diode to trigger a laser pulse. At a short time later, a second electrical pulse train is applied to the second laser diode to initiate a second laser pulse. These laser pulses are subsequently focused into two measurement volumes by collimator optics, mirrors and focusing lens and resulted in two orthogonal sequentially pulsed fringe systems. The phase different between the first and second laser pulse fringe system can be accurately determined. A receiver system consisted of a single avalanche photodiode and focusing lens is used to detected the scattered Doppler signals from both measurement volumes. The sampling rate of the avalanche photodiode is synchronized with the current oscillator base frequency. Doppler signal from each measurement volume can be de-coupled, from the signal sampled by the photodiode, to obtained two-dimensional velocity information of a particle.