LAUSR

Laser Doppler vibrometry is often employed for non-contact vibration measurements. A test beam scattered from a target is interfered with a reference beam on a photodiode and the resulting signal is processed to determine the target vibration. Most commercial vibrometers use a phase modulator in one branch of the interferometer. The requirement for a phase modulator can be removed by the use of synthetic-heterodyne demodulation. The optical source is a sinusoidal current modulated laser diode resulting in a frequency modulation of the input lightwave. The photocurrent has the form of the cosine of a carrier at the modulation frequency plus the dynamic phase difference between the beams, the time differential of which is proportional to the vibration. The detected signal spectrum comprises bands centered at integer multiples of the modulation frequency, which are processed to retrieve the vibration. We describe a new self-correcting synthetic-heterodyne technique employing phase and gain feedback, which is...