Sub-micrometric displacement measurements by an all-fiber laser heterodyne interferometer using digital phase demodulation

An optical/digital method using the laser fiber heterodyne interferometry for monitoring of biological motion (such as otoacoustic emissions on live tympanic membrane) in the range of 5-500 nm is proposed. A frequency-shifted reference beam is compared with an unshifted test beam that is phase modulated by a vibrating surface. The amplitude of the displacement of vibration motion can be measured down to a level of 1.5 nm by using a direct digital phase demodulation scheme. Some experimental results are presented and discussed.