Effect of Wet Etching of Arc-Induced Long-Period Gratings on Their Refractive Index Sensitivity

The paper presents highly refractive index sensitive operation of long-period gratings enhanced by wet etching of the fiber cladding. Long-period gratings with period of 283 μm were induced in Corning SMF28 optical fibre using electric arc technique. Then etching in hydrofluoric (HF) acid solution has been applied to improve operation of the sensor. Sensitivity improvement effect induced by fiber cladding reduction has been numerically simulated. The developed numerical model enables quick estimation of cladding thickness reduction essential to achieve dispersion turning point. At this point long-period gratings reaches maximum sensitivity to external medium refractive index. Simulation showed that dispersion turning point for this long-period grating is achieved by cladding thickness reduction of 1.7 μm. We obtained good simulation match with experimental results, where approximately the same thickness was etched. The refractive index sensitivity of the long-period grating-based sensor has been improved by factor of 5. Since long-period grating period is limited when using electric arc fabrication method, this physical limitation makes difficult achievement of dispersion turning point and reaching maximum sensitivity of the long-period grating. We show that sensitivity of electric arc induced long-period grating can be significantly improved by post processing techniques such as wet etching.