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In this paper, we describe an application of cross-correlation techniques aimed to the improvement of the sensitivity of a laser interferometric vibration sensor. The interferometer operates remotely from a moving target without requiring retro-reflective mirrors. It is a polarizing interferometer based on a Michelson scheme with a short internal reference path and a long external path which projects to the target through a telescope (Fig. 1). The same telescope collects the laser light scattered back from the target. Any surface is suitable: concrete, metals, bricks, etc. The surface roughness provides the scattered light which is used by the interferometer. The power of the projecting laser light (X = 632.8 nm) is kept below 5 mW for safety reasons. The interferometer operates up to 200 meters away from the target and is optimized for vibration measurements in the range 0.1-150 Hz which is suitable for tests of large civil structures. A detailed description of the interferometer and some examples of field applications are presented in Ref. 1 and 2. In Ref. 1 the sensitivity of the interferometer is discussed in terms of various sources of noise. The atmospheric turbulence gives the largest contribution since the random fluctuation of the refractive index along the propagation direction is detected by the instrument as target vibration. The effect of the beam wandering in the atmosphere due tue turbulence, has been efficiently reduced by an opto-electronic beam steering loop built in the interferometer. The overall average sensitivity in oscillation amplitude measurements performed at 100 meters away from the target is of the order of 0.1 microns. An exem-ple of the frequency domain behaviour of the turbulence effect in real environment is shown in Fig. 2. The data are obtained by aiming the interferometer to a fixed point on a rock 150 meters away. The spectral behaviour is roughly of the type 1/f. The idea described in this paper is to use two identical interferometric vibration sensors pointing to the same target from simmetrical remote positions and cross-correlate the two measured vibration signals. This configuration was experimentally tested in the laboratory on a simulation system and then used for in field measurements. The obtained results show a good reduction of the turbulence background noise allowing an ultimate sensitivity of the order of 0.01 microns.
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