Fiber grating sensor interrogation using a double-pass Mach-Zehnder interferometer

We propose an advanced quadrature sampling method for a fiber grating sensor interrogation. A Mach-Zehnder interferometer output is sampled at internal triggers which are generated at the zero-crossing points of a double-pass Mach-Zehnder interferometer output. Due to the phase relationship, the internal trigger signals assure a 90° phase difference between the sampled signals even in the presence of environmental disturbances. Also, no parameter adjustment is required when the frequency and/or shape of the phase modulation signal changes, which makes it a very practical technique for fiber grating sensor demodulation. From the preliminary experiments, we could obtain a measurement resolution of ∼8 pm.