Numerical investigation of cladding mode overlap of evanescent waves with air channels in index-guiding photonic crystal fiber long-period gratings

The array of axially aligned air channels and the robust waveguide characteristics of index-guiding photonic crystal fibers (IG-PCFs) integrated with long-period gratings (LPGs) make them a powerful platform for chemical sensing and detection. Compared to their conventional all-solid fiber counterpart, the IG-PCFs are a particularly attractive sensing device as they are both a waveguide and a vapor/aqueous transmission cell, permitting light intensity-analyte interaction over long path length without the removal of fiber cladding. While the fundamental core-mode in the IG-PCF has been utilized for evanescent field based sensing, there exist two inherent limitations: (1) only short distance extended by evanescent waves from the guiding core to the surrounding PCF cladding air channels to restrict the probing of an analyte only in the inner most ring of the air channels in cladding, and (2) less than 1% power of the core-mode overlap with the surrounding air channels leading to weak light intensity-analyte interactions due to the localization of the core-mode in the fiber core area. Should a cladding-mode with maximum overlap in air channels be excited by an LPG, it would fundamentally increase the evanescent field sensitivity. In this work, we present the simulation for the mode properties of selected IG-PCF for optimization of mode field distribution and light power overlap with air channels in fiber cladding. The numerical calculation reveals that if the optimized cladding-mode is selectively coupled, the evanescent wave overlap (at wavelength of 1550 nm) with cladding air channels of the round and hexagonal structures can be increased from 0.11% and 0.13% up to 4.01% and 6.54%, respectively.