In this paper, the sensor was proposed by combination of grapefruit photonic crystal fiber (GPCF) and femtosecond laser fabricated fiber Bragg grating (FBG) based on Sagnac interferometer. The GPCF was sandwiched between two single-mode fibers (SMFS) to form a SMF-GPCF-SMF structure, which was based on intermodal interference. The FBG with a central wavelength of 1535.32 nm was inscribed through polyimide coating and cladding by the femtosecond laser point-by-point inscribing method. The spectrum drift data of sensing structure were collected and the dual-parameters matrix of temperature-strain was constructed to realize the simultaneous measurement of temperature and strain. The experimental results showed that the linear range of the strain measurement was from 0 με to 2,000 με at different temperature, the wavelength of FBG and GPCF was red drift and blue drift, respectively, the average strain sensitivity at different temperature was 1.25 pm/με and -2.05 pm/με, all the linearity R2 are higher than 0.999, and has the high repeatability. The linear range of temperature measurement was from 20 ℃ to 450 ℃, the wavelength of FBG and GPCF was red drift and blue drift, respectively, the temperature sensitivity of FBG and GPCF was 15.17 pm/℃ and -8.87 pm/℃, the linearity was 0.99983 and 0.99989, respectively.
We designed and fabricated what we believe to be a novel dual-parameter fiber optic sensor for simultaneous measurement of temperature and strain, which was composed of a femtosecond laser inscribed fiber Bragg grating (FBG), three segments of a single-mode fiber (SMF), and two segments of a multimode fiber (MMF), forming a SMF-MMF-FBG-MMF-SMF structure. The FBG and Mach–Zehnder interferometer (MZI) were present in this structure so that the changes of the temperature and strain parameters can be sensed by the shifts of the reflection center wavelength of the FBG and the interference valley wavelength of the MZI. We simulated the light field distribution of the sensor structure, compared the shapes of the interference spectra formed by the MZI structure with different sensing arm lengths of 25, 35, and 45 mm, and analyzed the spectra in the spatial frequency domain. The simulation results showed that the interference spectrum of the MZI structure with a 25 mm length sensing arm was clearer and more suitable for the experiment. The experimental results showed that the temperature sensitivity of the FBG and MZI was 14.81 and 43.54 pm/°C in the range of 80°C to 240°C, and the strain sensitivity was 1.49 and −2.58 pm/µε in the range of 0 to 1200 µε, with a high linearity and excellent repeatability. The sensor is economical, sensitive, and convenient to fabricate, and exhibits promising applications in the fields of biochemical medical detection and industrial production monitoring.
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