Performance analysis of cladding radius on spectral behaviour and sensitivity of LPFG with ambient refractive index higher than cladding region

For ambient refractive index (ARI) higher than cladding region, the refractive index (RI) sensitivity of long-period fiber grating (LPFG) measured in terms of the shift in the cladding mode resonance wavelength (CMRW) is very low. This may be due to the occurrence of a small CMRW shift with a change in ARI. This paper emphasizes the effect of cladding radius reduction as well as cladding mode-order over the spectral behavior, RI response and figure of merit of LPFG for such ARIs. Therefore, mathematical approaches designed on three-layer and two-layer fiber geometries are employed on single-mode optical fiber (SMF28e) to estimate optical modes. The influence of cladding radius reduction (from 62.5 to 32.5 µm) on spectral profile and RI sensitivity is analyzed for three cladding modes (HE13, HE14 and HE15). The spectral behavior of all three modes for the ARI range (1.458–1.738) has described the shifting of CMRW towards longer wavelengths. From the analysis, the larger shifts in CMRW with ARIs have been shown by HE15 cladding mode at a cladding radius of 32.5 µm as compared to normal cladding LPFG. Moreover, the spectral bands of ARIs near to cladding index have shown very low differentiation in attenuation spectrum depths at reduced cladding radius.