Dual-parameter sensing based on symmetrically chirped long-period fiber grating

<sec>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 gratings (LPFGs) with the same length and average period but opposite chirp coefficients, forming an in-fiber Mach-Zehnder interferometer (MZI). Due to the chirping effect of the LPFG, the core mode at different wavelength couples to the cladding modes at different positions within the positively chirped LPFG. Integrated with the symmetry of the SCLPFG, the stimulated cladding mode recouples to 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 increasing. 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, for a specific measurand, the sensitivities interrogated by the dips at different wavelengths in the fringe pattern are inherently different, which leads to the possibility of multi-parameter sensing through a differential modulation method.</sec><sec>The fringe characteristics and sensing mechanism are systematically investigated through theoretical analysis and numerical simulation. In the experimental section, the SCLPFG structure is engraved on a Corning single-mode fiber by irradiating photosensitive core with point-by-point UV pulsed laser. 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 is conducted, verifying the differential response of distinct fringe dips to SRI and temperature variations. A 2×2 sensitivity coefficient matrix is established by linearly fitting the SRI and temperature response data, which are obtained by interrogating two dips at different wavelengths. Thus, the variations of SRI and temperature are determined by multiplying the inverse sensitivity coefficient matrix with the wavelength shift array. Furthermore, temperature sensitivities are corrected by considering the thermal effect on the refractive index of the liquid. Finally, the maximum sensitivity of the sensor to SRI is –95.316 nm/RIU and a maximum sensitivity to temperature is 0.0849 nm/℃, both of which have excellent linearity. This sensing scheme features a compact structure, high sensitivity, and the ability to measure multiple parameters. Moreover, the multi-channel nonuniform fringe characteristics enable the sensor configuration to be extended for simultaneous measurement of three or more parameters, thus providing a promising lab-on-fiber platform for multi-parameter sensing applications.</sec>