A multiparametric fiber optic sensor with a pull-tapered fiber modified by sensitive materials and a microcavity cascade is fabricated, and its strain, temperature, and humidity characteristics are experimentally demonstrated. The proposed microcavity is formed by a femtosecond laser scribing discharge and tapered. The interference peak of the reflection spectrum of the sensor is sensitive to the change in strain, and the experimental results show that the strain sensitivity is 4.8 pm/με, but the structure is insensitive to both temperature and humidity. After the tapered part of the structure is coated with polyvinyl alcohol (PVA) doped with graphene quantum dots (GQDs), the sensitivity of temperature and humidity is significantly improved, with the maximum temperature sensitivity of 20.4 pm/℃ and the maximum relative humidity sensitivity of 14.6 pm/%. Dip1, Dip2, and Dip3 are analyzed, and then the cross-sensitivity is eliminated by using a third-order matrix, so as to simultaneously measure strain, temperature, and humidity. The device is easily fabricated, and it has a small size, excellent linearity, and good application prospects.

Firstly, the femtosecond laser pulse is focused on the single-mode fiber (SMF) with an OLYMPUS objective lens with a numerical aperture of 0.7, and the program is run to write the optical fiber core axially with a controlled writing length of 45 μm, and a fiber fusion splicer is used to discharge the fiber at the center of the inscription. The discharge current and the discharge time of the fusion splicer are set to 20 mA and 1500 ms, respectively, and the bubble is generated inside the fiber with a width of 82 μm and a length of 129 μm. Secondly, the SMF is tapered to obtain the Mach-Zehnder interferometer (MZI). The tapered area of the MZI is stretched to 277 μm, and the MZI is cascaded with a microbubble Fabry-Perot (FP) cavity to develop the required sensor device. The diameter of the tapered fiber after coating is 48 μm, and the coating thickness is 4 μm. 5 mg of GQDs and 100 mg of PVA are used, and they are mixed well, with 100 mL of pure water added. Then, the mixed solution is heated to 95 ℃ and stirred well by a magnetic stirrer for 1 h, so as to obtain the GQDs-PVA solution. After the solvent in the GQDs-PVA solution evaporates, a thin film attached to the tapered part of the sensor can be found, and its main components are PVA and GQDs. The film is observed under a scanning electron microscope, and the size and distribution of GQDs in the PVA film can be known. Finally, the initial spectrum of the sensor after coating GQDs-PVA is measured, and three resonance valleys, namely, Dip1, Dip2, and Dip3 are taken for observation and analysis.

The temperature range of the experimental chamber is 20-100 ℃, with an accuracy of ±0.1 ℃, and the relative humidity measurement range is 10%-95%, with an accuracy of ±0.1%. The broadband light source is a spontaneous radiation source with a spectral range of 1250-1650 nm. The measurement range of the adopted optical spectrum analyzer is 600-1700 nm, with an accuracy of ±0.1 nm. The light is output from the broadband light source and transmitted to the sensor by the coupler. Then, the reflected light is transmitted back to the optical spectrum analyzer by the coupler. Dip1, Dip2, and Dip3 are selected to observe the spectral changes, and the temperature sensitivities are 16.6 pm/℃, 18.5 pm/℃, and 20.4 pm/℃, and the linearities are 0.9857, 0.9859, and 0.9867, respectively. The central wavelengths of Dip1, Dip2, and Dip3 have a positive linear relationship with humidity. The linearities are 0.9889, 0.9652, and 0.9863, and the relative humidity sensitivities are 12.6 pm/%, 13.5 pm/%, and 14.6 pm/%, respectively. The axial strain sensitivities are 4.0 pm/με, 4.3 pm/με, and 4.8 pm/με, and the linearities are 0.9993, 0.9995, and 0.9996, respectively. Finally, a third-order matrix is constructed to eliminate the cross-sensitivity between the three parameters.

Optical fiber sensors cascaded by MZI and FPI are fabricated by femtosecond laser scribing and fiber fusion splicer discharge technology, and their three-parameter sensing characteristics are verified by experiments. The strain sensitivities at the three resonance valleys are 4.0 pm/με, 4.3 pm/με, and 4.8 pm/με, respectively, which are relatively high, with a range of 600 με. When GQDs-PVA is uncoated, the sensitivities of temperature and humidity of the sensor are almost zero. The temperature sensitivities at the three resonance valleys are 16.6 pm/℃, 18.5 pm/℃, and 20.4 pm/℃, and the measurement range of temperature is 24-80 ℃. The relative humidity sensitivities at the three resonance valleys are 12.6 pm/%, 13.5 pm/%, and 14.6 pm/%, and the measurable relative humidity range is 40%-70%. The strain, temperature, and humidity had a positive linear relationship with the corresponding central wavelength. Finally, the cross-sensitivity is eliminated by constructing a third-order matrix, and the strain, temperature, and humidity can be measured simultaneously. The proposed sensor can be easily fabricated, and it has high sensitivity and good application prospects in fields such as medical monitoring, food safety, and environmental detection.