Measurement and analysis of a piezoelectric mirror shifter impulse response using self-correcting synthetic-heterodyne demodulation based Michelson interferometer vibrometer with gain and phase control feedback

Abstract

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 significantly less sensitive to the received optical power. The system, which has a frequency range of 0.2-9 kHz, is used to measure the impulse response of a piezoelectric mirror shifter. The vibration signal is analyzed using Hilbert transform techniques to determine the shifter resonant frequencies and decay rates.