Low-coherence Mach-Zehnder interrogation of fibre-Fabry-Perot sensor incorporating balanced detection

In-fibre Bragg gratings are currently being evaluated as sensors for monitoring various parameters including the structural health of bridges, strain in advanced composites, and high frequency (MHz) acoustic fields. These devices are attractive because they offer the benefits of ease of multiplexing, the simultaneous measurement of temperature, and a potentially low cost. In this paper, the core of our scheme comprises a fibre-Fabry-Perot sensor which is defined by Bragg-grating miners. Dynamic strains applied to the sensor change the optical path-length between the grating-mirrors and are encoded as a change in signal-phase. A low-coherence Mach-Zehnder processing interferometer, with a high-frequency phase-carrier in one arm, is used to interrogate the sensor. Using the fact that the two outputs of the interferometer are in anti-phase, we extend the approach by applying balanced detection to the Mach-Zehnder outputs in order to eliminate intensity-based optical noise. A noise-rejection better than 40dBV is demonstrated.