Two-mode fiber-optic sensors using a white-light spectral interferometric technique

Abstract

We analyze theoretically fiber-optic sensor configurations working in the time domain or in the spectral domain and utilizing interference between two spatial or polarization modes. We show for example that a two-mode fiber interferometer in which the phase difference between the two modes is large and the intermodal group optical path difference (OPD) is zero at a specific wavelength of a light source of a short coherence length can be used without losing both time-domain and spectral-domain interference. Similarly, we analyze theoretically a tandem configuration of two interferometers with one of the interferometers as a receiving interferometer and the other one as a sensing two-mode fiber interferometer. We show that time-domain or spectral-domain interference lost due to the OPD between the two modes exceeding the coherence length can be restored in the configuration. We analyze both theoretically and experimentally a tandem configuration of the receiving interferometer and the sensing two-mode fiber interferometer in which spectral interference between two polarization modes is detected by a low-resolution spectrometer. The configuration is characterized by suitable adjusting of the measuring sensitivity over a broad wavelength range. We present some preliminary experimental results regarding sensing of strain using an elliptical-core optical fiber.