Abstract
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.
Highlights
The tilted fiber Bragg grating (TFBG) is a device that possesses all the advantages of the well- established fiber Bragg grating (FBG) technology in addition to being able to excite cladding modes resonantly
A TFBG inscribed in multi-mode fiber (MMF) provides a potential solution due to the larger diameter fiber core, which permits multiple guided core modes with different polarization dependence and low transmission loss
Multiple high-order core modes with different polarization dependence can be excited by the tilted gratings inscribed in a MMF and they show a welldefined “comb” of resonances which are spectrally well separated from each other
Summary
The tilted fiber Bragg grating (TFBG) is a device that possesses all the advantages of the well- established fiber Bragg grating (FBG) technology in addition to being able to excite cladding modes resonantly. Multiple high-order core modes with different polarization dependence can be excited by the tilted gratings inscribed in a MMF and they show a welldefined “comb” of resonances which are spectrally well separated from each other (this provides an easy way to isolate them at different wavelengths). RI measurement with sensitivity higher than 500 RIU/nm has been achieved by monitoring the “cut-off” resonance of the cladding modes of 12° MMFTFBG It is the combination of the physical sensing properties (curvature, twist/rotation, lateral pressure) with the clearly distinct biochemical (RI) response that opens up a multitude of opportunities for “multiparameter” sensing with a single kind of MMF-TFBG instead of trying to extract the same information from the complex mode structure of the SMF-TFBG. The mode field profiles associated with these resonances clearly show that the LP1n modes do not possess cylindrical symmetry and that their coupling response to changes in input polarization and twist/rotation will be greatest
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