Potential Multiparameter Sensor Based on Thin-Cladding Fiber Helical Long-Period Fiber Gratings

In this article, we propose and demonstrate a potential multiparameter sensor based on helical long-period fiber gratings (HLPFGs) in thin-cladding fiber (TCF) using CO2 laser inscription. The fabricated TCF-HLPFGs with a grating period of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$132~\mu $ </tex-math></inline-formula>m and a short length of ~2 cm exhibit two distinct resonance dips (dip1 and dip2) in the transmission spectrum with large wavelength separation of 241.2 nm, corresponding to the coupling from fundamental core mode to the LP16 and LP17 cladding modes, respectively. Then, a comparative investigation of the bending, refractive index (RI), and temperature sensing based on both dual dips is demonstrated. Within the curvature range of 1.47–5.05 m<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula>, the bending sensitivity of −6.75 and 9.32 nm/m<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula> could be achieved for dip1 and dip2, respectively. In surrounding RI (SRI) region of 1.3839–1.443, the sensitivities of dip1 and dip2 are measured to be −150.8 and −647.75 nm/RIU on wavelength and 31.93 and −200.77 dB/RIU on transmission depth, respectively. And the temperature sensitivities are 48 and 78 pm/°C, for dip1 and dip2, respectively. The results present that dip2 outperforms dip1 with higher sensitivities, due to the effect of higher order cladding modes. With the superior performance and good repeatability in the bending reaction of the proposed TCF-HLPFGs, the simultaneous measurement of bending and temperature could be achieved. Therefore, the proposed TCF-HLPFGs offer great potential in the field of multiparameter sensing measurement.