Refractive Index Sensors Based on Long-Period Grating in a Negative Curvature Hollow-Core Fiber.

Hanna Izabela Stawska,Maciej Andrzej Popenda

doi:10.3390/s21051803

Abstract

Long-period optical fiber gratings (LPGs) are one of the widely used concepts for the sensing of refractive index (RI) changes. Negative curvature hollow-core fibers (NCHCFs), with their relatively large internal diameters that are easy to fill with liquids, appear as a very interesting medium to combine with the idea of LPGs and use for RI sensing. However, to date, there has been no investigation of the RI sensing capabilities of the NCHCF-based LPGs. The results presented in the paper do not only address this matter, but also compare the RI sensitivities of the NCHCFs alone and the gratings. By modeling two revolver-type fibers, with their internal diameters reflecting the results of the possible LPG-inscription process, the authors show that the fibers’ transmission windows shift in response to the RI change, resulting in changes in RI sensitivities as high as −4411 nm/RIU. On the contrary, the shift in the transmission dip of the NCHCF-based LPGs corresponds to a sensitivity of −658 nm/RIU. A general confirmation of these results was ensured by comparing the analytical formulas describing the sensitivities of the NCHCFs and the NCHCF-based LPGs.

Highlights

Optical fibers are considered an interesting solution for the design of refractive index sensors (RISs) due to their compact size, immunity to electromagnetic interference, rapid response and high sensitivity
The presented results constitute one of the first numerical analyses of the topic of longperiod gratings written on negative curvature hollow-core fibers, addressing the matter of their possible use in sensing the refractive index changes in liquid samples
Negative curvature hollow-core fibers (NCHCFs) are, in general, considered a good medium to fill with liquids, because of their unique optical properties, and due to the ease of the procedure, as the diameters of their core and capillaries are relatively large and do not pose a challenge in the filling procedure

Summary

Introduction

Optical fibers are considered an interesting solution for the design of refractive index sensors (RISs) due to their compact size, immunity to electromagnetic interference, rapid response and high sensitivity. One can find a relatively recent report on a conventional, SMF-based (single-mode fiber) LPG with sensitivities of approximately 800 nm/RIU in the RI range of 1.33–1.4, while the same LPG for the RI above 1.4 achieves sensitivities in the order of thousands [8]. To 2400 nm/RIU [19] at RI below 1.4 Another important type of PCF—the hollow-core photonic bandgap fibers (HCPBFs)—has been used as an RI sensor, exhibiting a blue wavelength shift (towards shorter wavelengths) of 280 nm when the refractive indices changed from 1.33 to 1.39, resulting in an approximate sensitivity (absolute value) of. The authors of the presented paper conducted an experiment with the Kagomé-style NCHCFs, showing that the transmission windows shift in response to the different refractive index of the liquid filling the fibers, with the measured sensitivities (absolute values) being approximately 2870 nm/RIU [28]. LPG alone basedisona an with its intended RI sensitivity higher than the sensitivity of the fiber alone is a very difficult task

NCHCF and NCHCF-Based LPG—Models and Diameters

NCHCF—Optical

Summary and Conclusions