A laser interferometer for measuring linear vibration

A reference-beam type interferometer for the measurement of vibration is described. In the technique used, laser light reflected from a vibrating object is shifted in frequency (Doppler shift) because of the velocity of the object. The reflected light is focussed on to a photodetector where it mixes with a frequency-shifted reference beam. A frequency tracker converts the photodetector signal to a voltage signal proportional to the object's velocity. The system offers a method for measuring vibration in situations where the use of conventional transducers would be impractical. When the system was calibrated against a velocity transducer and an accelerometer, the frequency was flat from virtually d.c. to 2.5 kHz.