Dual heterodyne and light-amplifying fiber Mach-Zehnder interferometers for on-line displacement measurement of low reflectivity objects

Abstract

A heterodyne interferometric system, which includes dual optical fiber Mach-Zehnder interferometers and has light-amplifying capability for on-line measurement of low reflectivity objects, is presented and experimented. Two fiber Bragg gratings with the same Bragg wavelength are employed to construct two overlapped and independent optical fiber Mach-Zehnder interferometers that have almost identical optical paths. One of the interferometers is used to sense environmental disturbances while the other one performs measurement. By subtracting the outputs of the two interferometers, the influences resulting from the environmental disturbances can be eliminated effectively. Two acoustic-optical modulators connected in series are incorporated in the reference arm of the interferometers to configure heterodyne interference, while a length of erbium-doped fiber pumped with a 980 nm laser is embedded in the measurement arm to amplify the reflected light from the measured object for improving the fringe visibility of the interferometric signal. Hence the system is suitable for on-line measurement of low reflectivity objects. Displacement measurement experiments of 100 μm have been carried out for ten times and the standard deviation of the repeated measurements is 6 nm.