A system for measuring vibration comprising: a light source that generates a coherent light beam onto a target; a plurality of detectors that each detects backscattered light from the target by the coherent light beam, wherein the plurality of detectors comprises at least four detectors; a differential signal analyzer coupled to each of the plurality of detectors and operable to: receive a plurality of signals each representative of a magnitude of the backscattered light from each of the plurality of detectors; apply a first phase shift to a first signal and a third signal; and combine the first phase shifted signals with a second signal and a fourth signal to form a first differential signal representative of tilt vibration of the target; apply a second phase shift to the first and second signals 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 representative of horizontal vibration of the target, the first phase shift and the second phase shift being essentially 180 degrees; and characterised in that the differential signal analyzer is further operable to: 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 a third differential signal comprising a torsional 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. a light source that generates a coherent light beam onto a target; a plurality of detectors that each detects backscattered light from the target by the coherent light beam, wherein the plurality of detectors comprises at least four detectors; a differential signal analyzer coupled to each of the plurality of detectors and operable to: receive a plurality of signals each representative of a magnitude of the backscattered light from each of the plurality of detectors; apply a first phase shift to a first signal and a third signal; and combine the first phase shifted signals with a second signal and a fourth signal to form a first differential signal representative of tilt vibration of the target; apply a second phase shift to the first and second signals 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 representative of horizontal vibration of the target, the first phase shift and the second phase shift being essentially 180 degrees; and characterised in that the differential signal analyzer is further operable to: 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 a third differential signal comprising a torsional 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. receive a plurality of signals each representative of a magnitude of the backscattered light from each of the plurality of detectors; apply a first phase shift to a first signal and a third signal; and combine the first phase shifted signals with a second signal and a fourth signal to form a first differential signal representative of tilt vibration of the target; apply a second phase shift to the first and second signals 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 representative of horizontal vibration of the target, the first phase shift and the second phase shift being essentially 180 degrees; and characterised in that the differential signal analyzer is further operable to: 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 a third differential signal comprising a torsional 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.