Abstract
In this paper, we demonstrate a fiber Bragg grating (FBG) with a wide range and a comb with continuous cladding mode resonances inscribed in non-photosensitive single mode fibers using a femtosecond laser and a phase mask. The FBG is inscribed in the core and cladding, exciting a series of cladding modes in transmission. The birefringence induced by this FBG structure offers significant polarization-dependence for cladding modes, thus allowing the vector fiber twist to be perceived. By measuring the peak-to-peak differential intensity of orthogonally polarized cladding mode resonances, the proposed sensor presents totally opposite intensity response in the anticlockwise direction for the torsion angle ranging from −45° to 45°. The cladding mode comb approximately covers wavelengths over the O-, E-, S-, and C-bands in transmission. The cutoff cladding mode of air can be observed in the spectrum. Thus, the sensible refractive index range is estimated to be from 1.00 to 1.44. Temperature responsivity of the grating is also characterized. The proposed device potentially provides new solutions to the various challenges of physical vector and bio-chemical parameters sensing.
Highlights
Fiber cladding mode-based sensors have attracted widespread attention due to their unique sensing properties
Two orthogonal polarizations of cladding modes respond to both orientation and amplitude of these applied vector parameters, such as fiber Bragg grating (FBG) written in the polarization-maintained fiber for vibration [23], Tilted fiber Bragg grating (TFBG) with excessive angle of 81◦ for twist [24,25], re-grown TFBG with split cladding mode for bending [26], and TFBG with metallic oxide coating for twist [27]
A CMFBG was inscribed in the fiber core and cladding by a femtosecond laser and phase mask
Summary
Fiber cladding mode-based sensors have attracted widespread attention due to their unique sensing properties. Two orthogonal polarizations of cladding modes respond to both orientation and amplitude of these applied vector parameters, such as FBG written in the polarization-maintained fiber for vibration [23], TFBG with excessive angle of 81◦ for twist [24,25], re-grown TFBG with split cladding mode for bending [26], and TFBG with metallic oxide coating for twist [27] In these studies, the researchers fully utilize the strain response difference of two orthogonal polarized resonances (including p- and s-polarization) to achieve vector measurements. The wavelength range and amount of cladding modes are highly relevant, for RI measurement These can be modified by designed grating inscription, such as off-axis TFBG fabricated by femtosecond laser line-by-line technique [19], multi-angle TFBG fabricated by a scanned phase-mask technique [28], moiré TFBG fabricated by using a direct phased mask with two-stage. The proposed sensor enables vector twist measurement and wide-range RI measurement
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