A practical all-fibre laser vibrometer

Since the advent of the laser, optical metrology is providing mechanical engineers with data which they previously have considered to be unobtainable. Vibration measurement in situations where the target surface is hot, light or rotating precludes the use of contacting transducers such as accelerometers and, as a result, laser vibrometers have been developed to solve this problem. Their principle of operation relies on the detection of the Doppler shift in the scattered light from the target surface by heterodyne with frequency-shifted reference light from the same laser source. The frequency shift in the reference light provides a carrier frequency in the photodetector output which the Doppler shift from the target surface then frequency modulates. In this way the ambiguity in the direction of the target motion is removed. The idea of incorporating an open air path-laser vibrometer into an all-fiber device is attractive since this would allow for remote, passive sensing of vibration in situations where direct optical access is precluded. Unfortunately, the need to frequency shift the reference light means that with commercially available frequency shifters, such as Bragg cells, the light must leave the fiber and therefore traverse a different optical path to the target light. Environmental disturbance of either target or reference optical path, in this case, is seen as spurious vibrations included in the detector output. Environmental effects such as temperature changes, acoustic pressure fluctuations or vibration of the fiber itself may change the refractive index of either light path to cause this noise. In order to provide a practical engineering instrument for on-site use these problems must be avoided. An all-fiber device will be described which was first reported in Electronics Letter