A dual-parameter sensor based on a symmetrically chirped long-period fiber grating (SCLPFG) is proposed and demonstrated. The SCLPFG consists of two segments of long-period fiber grating (LPFG) of identical length and average period, but with opposite chirp coefficients, resulting in the formation of an in-fiber Mach-Zehnder interferometer (MZI). Due to the chirping effect of the LPFG, the core mode at different wavelengths is coupled to the cladding mode at varying positions within the positively chirped LPFG. Integrated with the symmetry of the SCLPFG, the stimulated cladding mode is recoupled into the core at the symmetrical position in the negatively chirped LPFG. Consequently, in this MZI configuration, the effective length of the interference arm is not fixed but varies with wavelength. As a result, the transmission spectrum of the SCLPFG is characterized by a nonuniform fringe pattern where the free spectrum range (FSR) increases with wavelength. For the MZI-based fiber sensor, the phase difference between the core and cladding modes, influenced by environmental parameters, plays a crucial role in determining sensitivity, as this phase difference is directly proportional to the length of the interference arm. Therefore, the sensitivities interrogated by the dips at different wavelengths in the fringe pattern are inherently different for a specific measurand, leading to the potential for multi-parameter sensing through a differential modulation method.
The fringe characteristics and sensing mechanism are systematically investigated through theoretical analysis and numerical simulation. In the experimental section, the SCLPFG structure was inscribed into a Corning single-mode fiber via point-by-point UV pulse laser irradiation on the photosensitive core. The grating exhibits an average period of 321 μm and a chirping coefficient of ±21.9 μm/cm, with the total length of the symmetrically chirped grating determined to be 4.34 cm. Experimental implementation of simultaneous dual-parameter sensing for surrounding refractive index (SRI) and temperature was conducted, verifying the differential response of distinct fringe dips to SRI and temperature variations. A 2×2 sensitivity coefficient matrix was established by linearly fitting the SRI and temperature response data, which were obtained by interrogating two dips at different wavelengths. Thus, the variations of SRI and temperature were determined based on the inverse sensitivity coefficient matrix multiplied by the wavelength shift array. Furthermore, temperature sensitivities were corrected by accounting for the thermal effect on the liquid refractive index. Finally, the sensor achieved maximum sensitivities of -95.316 nm/RIU for SRI and 0.0849 nm/℃ for temperature, both with excellent linearity. This sensing scheme features a compact structure, high sensitivity, and multi-parameter measurement capability. Moreover, the multi-channel nonuniform fringe characteristics enable the sensor configuration to be extended for simultaneous measurement of three or more parameters, providing a promising lab-on-fiber platform for multi-parameter sensing applications.