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Design and performance of a multipoint high resolution vibrometer

We have adapted a Mach-Zender type laser velocimeter for making simultaneous measurements at several, arbitrarily chosen points. The optical design is based on a single laser source and two acousto-optic modulators, which generate a common heterodyne carrier frequency in all measurement channels. By using two modulators, one to increase the frequency of the reference beam with 80 MHz, and a second one which decreases the frequency with a bit more than 80 MHz, we can adjust the heterodyne beat signal to any frequency of choice, thus adapting it to the input of commercially available FM demodulation chips or avoiding noisy parts of the laser noise spectrum. The apparatus is intended for simultaneous vibration measurements at several points of instable objects, such as biological specimens or MEMS, so that instantaneous phase relationships and amplitude ratios can be determined. So far, instrument performance has been tested with measurements on two points of interest. In a multipoint system, especially when it uses a single laser source and a common heterodyne frequency, channel cross talk is a major point of concern. The optical design ensures very good channel separation and allows expansion to at least four channels. We present channel separation measurements obtained with one channel focused on a vibrating membrane or a vibrating plate and the other focused on an adjacent solid object. Even with these object points less than 0.4 mm apart, channel separation was found to be better than 78 dB at all frequencies. The velocity calibration of the system is determined by the laser wavelength and the sensitivity of the FM detection circuitry only. With a maximal velocity amplitude of 52 mm/s, the velocity resolution and the detection limit of our system were determined to be 2.6 square root(bandwidth) μm.s-1Hz-1/2 in both channels.