Influence of strain and pressure to the effective refractive index of the fundamental mode of hollow-core photonic bandgap fibers.

We investigate the phase sensitivity of the fundamental mode of hollow-core photonic bandgap fibers to strain and acoustic pressure. A theoretical model is constructed to analyze the effect of axial strain and acoustic pressure on the effective refractive index of the fundamental mode. Simulation shows that, for the commercial HC-1550-02 fiber, the contribution of mode-index variation to the overall phase sensitivities to axial strain and acoustic pressure are respectively approximately -2% and approximately -17%. The calculated normalized phase-sensitivities of the HC-1550-02 fiber to strain and acoustic pressure are respectively 1 epsilon(-1) and -331.6 dB re microPa(-1) without considering mode-index variation, and 0.9797 epsilon(-1) and -333.1 dB re microPa(-1) when mode-index variation is included in the calculation. The latter matches better with the experimentally measured results.