where nSMF is the core RI of SMF, nHCPCH is the core RI of HCPCF, and
nEX is the RI of external medium, i.e., the analyte. When the sensor
tip is exposed to air (nEX = 1, nSMF = 1.457 and nHCPCH = 1), R1, R2 and R3 are calculated to be 0.036. With such low reflectivity of each surface,
the multiple reflections in the FP cavities can be neglected.