Polarimetric multi-mode tilted fiber grating sensors.

The polarimetric sensing characteristics of multi-mode-fiber based tilted fiber Bragg gratings (MMF-TFBG) have been analyzed and experimentally demonstrated. The larger diameter fiber core and graded index core/cladding profile enable the tilted gratings to excite multiple high-order core modes with significantly different polarization dependence and to form a well-defined "comb" of spectrally separated resonances at different wavelengths. Orientation-recognized twist/rotation measurements (-90° to 90°) have been achieved with sensitivity of 0.075 dB/deg by using intensity monitoring of two orthogonally polarized odd core-modes (LP11 and LP12). The proposed sensor is compact, works in reflection (a short section of MMF-TFBG spliced with a leading-in single mode fiber without any offset or tapering), is insensitive to temperature (intensity detection instead of wavelength monitoring) and is immune to unwanted intensity fluctuations (differential intensity measurement). Other TFBG sensing modalities, such as lateral pressure and surrounding refractive index are demonstrated separately with the same device configuration and interrogation principles.