Importance

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Abstract

The movement of a predetermined zone of a surface of a vibrating object is measured by separating a laser beam having a predetermined coherence length into a probe beam and a reference beam. The reference beam is then delayed relative to the probe beam by a predetermined time interval sufficient for the probe beam to travel through a predetermined distance at least corresponding to the coherence length, before being combined with the probe beam to form a combination beam. The combination beam is then subdivided into a plurality of substantially equal component beams each of which is conducted along a separate path to a location spaced from the predetermined zone substantially by one-half of the distance traveled by light in the predetermined time interval between the above location and the predetermined zone. A predetermined portion of each component beam is reflected at the respective location back into the respective path, while the remainder of each component beam is directed to the predetermined zone for return therefrom at least predominantly back toward the respective location and into the respective path with attendant coherent interference of the returned probe beam part with the reflected reference beam part of the same component beam. The return light beam are demodulated to obtain an indication of the movement of the predetermined zone.

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First Claim