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A prototype 10-channel Vibration Sensor has been developed which features (1) a single HeNe laser and a single photodetector for simultaneous, noncontact, and remote sensing of ten independent vibratory targets, (2) frequency response from dc to beyond 60 Hz, (3) 0.08 pm amplitude resolution and refinable to better than 0.02 pm, (4) signal delay of the sensor output less than 2 psec to the actual vibration, (5) digital signal processing and digital output for computational convenience, and (6) expandibility for more channels. Although the maximum sensible amplitude and frequency product is presently limited to 0.1 Hz (or a maximum range rate of 63 cm/sec) for a 4 MHz electronic bandwidth, this sensor is still nearly perfect for measuring the dynamics of structure and sensing vibrations for active control of structures. The sensor employs a 0.7 mW HeNe laser, an avalanche photodiode, and two Bragg cells, one to provide the target sensing beam with an offset frequency relative to the optical local oscillator (which is split off from the laser out-put) and the other for dividing the target sensing beam into multiple beams, electronically. It is possible to simultaneously sample and hence monitor a large number of targets to which corner cubes have been affixed. Optical path variations due to the vibratory motion of each target is measured by comparing zero-crossings of the heterodyne signal from each target with those of each reference signal, identifiable by a specific Bragg frequency. Each zero-crossing count difference corresponds to an optical path variation of one-half of a laser wavelength or 0.32 pm. The resolution of this sensor has been refined by measuring the timing of zero-crossings of the heterodyne signal from each target and a 0.08 pm amplitude resolution has been demonstrated with an optical head 0.03 cubic meters in volume.
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