Abstract

A recently reported¹ extrinsic Fabry-Perot interferometric (EFPI) sensor used a cavity formed by a single-mode fiber (SMF) end and a multimode fiber (MMF) end inserted into a hollow-core fiber (HCF) from its opposite ends. The EFPI lead insensitivity to random phase drifts results in an effective point sensor that is used extensively in strain, vibration, and temperature measurements. However, signal fading² necessitates a stringent requirement of biasing the sensor at the quadrature point. In contrast to a quadrature-phase-shifted scheme,¹ recent spectrum-analysis methods^2,3 of phase detection provide a linear, direct, and self-consistent phase readout without phase bias, source stabilization, or polarization control. Detection by the J₀·J₂ method³ or other passive homodyne methods requires generation of a secondary phase modulation. The EFPI does not permit such a secondary modulation except at the sensor head itself, which is impractical in embedded sensors, or through source frequency modulation, which leads to instabilities. Herein is demonstrated the split-cavity cross-coupled extrinsic fiber interferometer (SCEFI), which eliminates the need for any phase bias, eliminates the effect of intensity variation with the air-gap change, permits secondary phase modulation, detects both static⁴ and dynamic phase shift, and leads to simple two/three sensor multiplexing³ on a single coupler.

© 1993 Optical Society of America