Quantitative analysis of optical modes in sensing a medium of the higher refractive index through long‐period fiber grating

Abstract The response of a long‐period fiber grating (LPFG) based refractometric sensor is directly related to the optical modes' effective index of refraction (EIR). In this paper, the quantitative behavior of EIR is investigated with LPFG surrounded dielectric medium's refractive index (RI) higher than cladding RI and cladding radius. To analyze the impact on EIR, the 2‐layer and 3‐layer fiber structure‐based mathematical models have been introduced. In the past literature, the interaction of the ambient medium with the core mode field is rarely investigated. Therefore, such studies have been considered and as a result, a significant impact on core mode EIR has been observed. The influence of ambient‐medium RI (ARI = 1.458–1.738) on EIR is found ascended for its value near to cladding RI at normal cladding radius. However, the range of influencing ARIs has been extended with mode order and cladding radius reduction. This study encourages the measuring of LPFG RI response for the aforesaid ARI range in terms of shift in coupled mode resonance wavelength and improves the precision in sensing ARI.