Highly sensitive polarization-dependent salinity sensing with CO2-written polarization-maintaining and single-mode microcrack long-period fiber gratings

Salinity sensing is vital for monitoring food processes such as fermentation and brining, where reliable measurements are needed in the 0–15% NaCl concentration range. It is enabled by the refractive index (RI) change in the surrounding liquid, which modifies the cladding mode effective indices, impacting the coupling conditions through the evanescent field at the cladding-environment interface. CO₂-written long-period fiber gratings (LPFGs) with asymmetric microcrack structures formed by localized heating of the germanium-doped core in single-mode fibers (SMF) and boron-doped stress rods in polarization-maintaining fibers (PMF) were explored to measure the change in polarization through the phase shift at different wavelengths using dispersion and polarization properties linked to salinity changes. These structures act as Fabry-Pérot (FP) cavities, enhancing higher-order mode coupling and light interaction with the surrounding medium due to the wavelength-dependent RIs of different modes. Due to their asymmetric structure, the devices sensitivity becomes polarization-dependent since the asymmetric cladding modes provide a non-uniform transverse distribution, favouring certain axes where the evanescent field has increased interactions with the environment. With increasing NaCl concentrations, the change in surrounding RI becomes wavelength- and SOP-dependent, leading to corresponding phase shifts and amplitude changes in the LPFG response. We characterize the gratings using a single-polarization analysis to compensate for spectral broadening caused by mixed or unpolarized input light. Under a single-polarization input, the SM-LPFG achieves the highest sensitivity, up to 0.395 dB/%, while the fast-axis PM-LPFG exhibits lower temperature cross-sensitivity, higher stability and lower limit of detection (LOD), improving the sensitivity and detection reliability over SMF-based sensors. • Polarization-dependent salinity sensing reduces spectral broadening • Microcrack cavities from single-sided inscription with CO 2 laser enhances coupling • Microcrack cavities increase interaction between light and external medium • SMF- and PMF-inscribed microcrack LPFGs along two different axes