Passive Stokes-Based Quadrature Phase Demodulation in Hollow-Core Photonic Crystal Fibers for Strain Sensing

Strain-induced polarization dynamics in a 7-cell hollow-core photonic crystal fiber (HC-PCF) were investigated as a basis for passive phase demodulation in optical strain sensing. The fiber was subjected to controlled longitudinal strain while the output state of polarization was monitored through Stokes parameter measurements at 1550 nm. The polarization evolution was analyzed on the Poincaré sphere, revealing a stable quasi-circular trajectory consistent with a strain-induced differential phase variation between polarization-related propagation states. By exploiting the quadrature relationship between suitable Stokes components, the relative phase was retrieved from the polarization trajectory without active modulation or an external interferometric reference. The reconstructed phase exhibited a linear response to the applied strain, yielding a sensitivity of 5.04 ± 0.08 mrad/με (r² = 0.9991). These results demonstrate the feasibility of a Stokes-based phase interrogation scheme without active optical modulation and highlight the potential of HC-PCFs for compact and passive polarization-based strain sensing.