Sensitivity Enhancement in Low Cutoff Wavelength Long-Period Fiber Gratings by Cladding Diameter Reduction

Abstract

The diameter of long-period fiber gratings (LPFGs) fabricated in optical fibers with a low cutoff wavelength was be reduced by hydrofluoric acid etching, enhancing the sensitivity to refractive index by more than a factor of 3, to 2611 nm/refractive index unit in the range from 1.333 to 1.4278. The grating period selected for the LPFGs allowed access to the dispersion turning point at wavelengths close to the visible range of the optical spectrum, where optical equipment is less expensive. As an example of an application, a pH sensor based on the deposition of a polymeric coating was analyzed in two situations: with an LPFG without diameter reduction and with an LPFG with diameter reduction. Again, a sensitivity increase of a factor of near 3 was obtained, demonstrating the ability of this method to enhance the sensitivity of thin-film-coated LPFG chemical sensors.

Optical Fiber Temperature Sensors and Their Biomedical Applications

“…Functional materials can be deposited on the side-or end-face of fibers with different techniques, such as spin-coating, dip-coating, thermal evaporation, or sputtering [90]. The use of polymeric sensitive materials in optical fiber sensors has the advantage of enhanced response time with better sensitivity and selectivity [6,91]. Table 2 reports the main configurations for temperature parameter using MMI sensors reported over the last five years.…”

Refractive Index Sensor Based on Graphene Oxide-Coated Long-Period Fiber Grating Inscribed in a Two-Mode Fiber

“…To overcome the intrinsic limitations in common sensors and further optimize the sensing performance, a series of techniques have been proposed and demonstrated. A typical approach is to utilize a special structure or fiber as the sensing region of LPFGs through means such as cladding etching [10], side polishing [11], fiber tapering [12], and combination with a few-mode fiber [13]. Among these schemes, the LPFG inscribed in a few-mode fiber (LPFG-FMF) is of significant interest due to its several advantages such as a good mechanical stability, low cost, and easy fabrication.…”

Temperature Stability and Spectral Tuning of Long Period Fiber Gratings Fabricated by Femtosecond Laser Direct Writing

“…Several methods have been explored for the fabrication of LPFGs, including exposure to UV and CO 2 laser radiation [ 14 , 15 , 16 , 17 ], electric arc discharge [ 18 , 19 ], and mechanical pressure [ 1 ]. However, post-processing techniques such as taper tuning [ 10 ], cladding etching [ 20 ], and overlay coating [ 10 , 11 ] are necessary for the fabrication and tuning of an LPFG at the TAP. These fabrication processes can weaken the optical fiber and render its implementation difficult [ 1 ].…”