Polarization-maintaining fiber-optic-grating vector vibroscope

Abstract

A fiber-optic vector vibroscope based on orthogonal polarization cladding-to-core recoupling is demonstrated. A compact structure in which a short section of polarization-maintained (PM) fiber stub containing a straight fiber Bragg grating (FBG) is spliced to another single-mode fiber. Two well-defined orthogonally polarized cladding modes reflected by the PM-FBG are recoupled at the junction and the coupling intensity shows an extremely high sensitivity to bending in the corresponding orthogonal directions. Both the orientation and amplitude of the vibrations can be determined unambiguously via dual-path power detection of these recoupled orthogonal-polarimetric cladding modes (LP 1;12 and LP 1;13 ). Since spectral information is not required, temperature changes do not affect the sensor response, and power fluctuations can be referenced out by monitoring the power in the core mode (LP ). This device has opened up a multitude of opportunities for single-point sensing in hardto-reach spaces, with very controllable cross-sensitivities, absolute and relative measurements of various parameters and offers improved sensitivity to refractive index variations when tapering the fiber to micro-or nano-diameters. However, while early research on PM-FBGs was mainly focused on the core modes properties, the cladding modes of such fibers also have unique mode field shapes and polarization-dependent transmission spectra that can be used for sensing [5,6]. In fact, the polarization dependency of cladding modes has important consequences for sensing even in non-PM fibers. In a prior implementation of a fiber vibroscope [7], we used a tilted FBG to break the cylindrical symmetry of a non-PM multimode fiber and showed that launching linearly polarized light at orthogonal orientations relative to the tilt plane allowed for vectorial vibration information to be retrieved. Whenever working with cladding modes however, a suitable coupling mechanism must be provided to reintroduce the cladding mode light into the upstream core in order to allow remote interrogation with low loss. Any fiber discontinuity can be used for this purpose, as we have shown in [6][7][8][9]. In the new vectorial vibration sensing mechanism presented here, we avoid the uncertainties associated with the reproducibility and spectral quality of tilted gratings in multimode fiber by simply writing a conventional FBG in a short piece of PM-fiber spliced to standard single-mode fiber (Fig. 1). The key to the success of this device lies in the inherent small core mismatch between the two fibers, which allows the coupling of core and cladding modes. It will be shown below that the device thus constructed has far superior spectral qualities that facilitate sensor interrogation while being significantly simpler to fabricate. While the PM-FBG reflection spectrum contains the usual pair of resonances corresponding to the two orthogonal polarizations states of the core mode of the PM fiber, it also shows a group of additional resonances located about 6 nm away (on the short...

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