A novel technique for extending the unambiguous measurement range for differential measurements of angular deflections is presented. The technique utilizes a common-path interferometer that simultaneously probes the out-of-plane displacement of three points on the object surface. The system is based on a single laser diode, and all the optical functions of the system are implemented in a dedicated holographic optical element (HOE). The HOE automatically provides spatially phase-stepped interference signals for real-time phase measurement. It is therefore not necessary to employ any polarizing optics or active elements to introduce the phase stepping. The common-path scheme combined with the HOE provides a system that is inherently stable, since the HOE operates as both transmitter and receiver in the system. The system is compact, is robust, and has the potential for being mass-produced at a low cost and is thus well suited for industrial use, such as in commercial vibrometers. The technique is demonstrated in a system for measuring angular deflections of a plane mirror. The technique, however, is not restricted to this use alone and can easily be configured to probe other types of surface displacements, e.g., the deflection of a diaphragm. In the present configuration, the system can measure angular deflections with a sensitivity of 2.5 × 10-7 rad over a measurement range that is approximately 3.5 × 10-3 rad, i.e., a dynamic range of approximately 1:14,000. Furthermore, the system can easily be reconfigured for a desired angular sensitivity and measurement range.
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