Patent ID: 8151646

Claim:
A system for measuring vibration comprising: a laser light source that generates a coherent light beam onto a target; at least four detectors that each detects backscattered light from the target by the coherent light beam, each of the plurality of detectors aligned along a plane that is generally normal to the direction of the backscattered light; a differential signal analyzer coupled to each of the plurality of detectors and operable to: receive at least four signals each representative of a magnitude of the backscattered light from each of the at least four detectors; apply a first phase shift to the first signal and a third signal and combine the phase shifted signals with the second signal and a fourth signal to form a horizontal differential signal representative of horizontal vibration of the target; apply a second phase shift to the first and second detectors to form a second phase shifted signal and combine the second phase shifted signal with the third signal and the fourth signal to form a second differential signal, the first phase shift and the second phase shift being essentially 180 degrees; and apply a third phase shift to the second signal, a fourth phase shift to the fourth signal, a fifth phase shift to the third signal, and combine the third phase shifted signal, the fourth phase shifted signal, and the fifth phase shifted signal with the first signal to form the differential signal comprising a tortional vibration mode of the target, the third phase shift being essentially 90 degrees, the fourth phase shift being essentially −90 degrees, and the fifth phase shift being essentially 180 degrees.