In this work, the thermo-optic coefficient of silica fiber is analyzed and measured within a wide temperature range of 0-1200 °C for the first time. To obtain the thermo-optic coefficient, experiments were carried out by using fiber optic extrinsic Fabry-Perot interferometers (EFPIs) embedded in silica fiber. The optical cavity lengths of the EFPIs were measured with white-light interferometry. The experimental results show that the first and second orders of the thermo-optic coefficient are 1.090×10-5/°C and 1.611×10-9/°C2, respectively. The quadratic model for the temperature dependence of the basic optical parameter, the refractive index, is rigorous within a wide temperature range. Further, our work also provides a temperature calibration method for extrinsic Fabry-Perot interferometric temperature sensors. The temperatures measured by the sensors are in excellent agreement with those measured by the reference thermocouple. Hence, this calibration method is promising for temperature sensors with a wide working temperature range.

A self-temperature-calibrated gas pressure sensor with a sandwich structure made of single-mode fiber (SMF)-hollow core fiber (HCF)-SMF is proposed and experimentally demonstrated. A Fabry-Perot interferometer (FPI) is formed by the SMF-HCF-SMF structure along the axial direction, and an antiresonant reflecting optical waveguide (ARROW) is formed by the ring-cladding of the HCF along the radial direction. A micro-channel is drilled on the ring-cladding of the HCF using a femtosecond laser to facilitate air entering/exiting the HCF. The FPI functions as the pressure sensor, and the ARROW functions as the temperature sensor. The initial wavelength and pressure sensitivity of the FPI can be calibrated from the temperature obtained by measuring the optical thickness of the ARROW. The experimental results show that the ARROW exhibits a temperature sensitivity of ~0.584 nm/°C, and the pressure sensitivity of the FPI ranges from 3.884 to 0.919 nm/MPa, within the temperature range of 37-1007 °C. The simplicity and durability of the sensor make it suitable for reliable gas pressure measurement in high-temperature environments.

A fiber-optic dual-cavity Fabry-Perot interferometric sensor for the simultaneous measurement of temperature and pressure is proposed and experimentally demonstrated. The sensor cascades a fiber-optic intrinsic Fabry-Perot interferometer (IFPI) and an extrinsic Fabry-Perot interferometer (EFPI). The IFPI is used as the temperature sensor and the EFPI is used as the pressure sensor. The pressure sensor is based on a diaphragm-free structure and can work stably at high temperature. Two optical path differences are interrogated simultaneously by using the fast Fourier transform-based white-light interferometry and the peak-to-peak method. Experimental results show that the proposed sensor exhibits a temperature sensitivity of 29.63 nm/°C within the temperature range of 40 °C-1100 °C and a pressure sensitivity of 1465.8 nm/MPa within the pressure range of 0-10 MPa at the room temperature.

A fiber-optic hybrid sensor based on dual-cavity Fabry-Perot interferometers is proposed and demonstrated for the simultaneous measurement of high temperature and high pressure. The proposed sensor is fabricated by fusion splicing of two single-mode fibers (SMFs), a hollow core fiber (HCF), and a coreless fiber (CF), forming a four-section structure of leading SMF-SMF-HCF-CF. A micro-hole is fabricated at the end of the leading SMF using a femtosecond laser, forming an air gap between two SMFs. The CF is thinned and roughened by the femtosecond laser to form a diaphragm. The SMF-based cavity is used as the temperature sensor and the HCF-based cavity as the pressure sensor. Lengths of two cavities are interrogated simultaneously by using the fast Fourier transform-based white-light interferometry. Experimental results show that the proposed sensor exhibits a temperature sensitivity of 19.8 nm/°C and a pressure sensitivity of ~98 nm/MPa, within the temperature range of 20 °C-800 °C and the pressure range of 0-10 MPa. The maximum measurement errors of temperature and pressure are 5 °C and 0.2 MPa, respectively. As far as we know, it is the first time to detect the temperature of 800 °C and the pressure of 10 MPa simultaneously.

A three-wavelength passive demodulation technique to interrogate extrinsic Fabry-Perot interferometric (EFPI) sensors with arbitrary cavity length has been developed. The DC component is obtained online, then the applied dynamic signal is recovered by using three new signals without the DC component. The performance of the technique is demonstrated by simulations and experiments. The demodulation technique can extract dynamic signals, regardless of whether the phase modulation is larger than 2π. Theoretically, EFPI sensors with arbitrary cavity length can be demodulated by the demodulation technique, and EFPI sensors with cavity lengths in the 22.96-1002.3 μm range are detected successfully by the same demodulator in experiments. The technique is robust with respect to the bending loss of the leading fiber. The demodulation technique provides a robust and accurate solution to measure dynamic signals for EFPI sensors. It has the properties of high frequency, a large dynamic range, and high sensitivity. The paper demonstrates this technique’s potential for measuring dynamic signals.

A dual-wavelength DC compensation technique for the demodulation of extrinsic Fabry-Perot interferometric (EFPI) fiber sensors is developed. The technique can overcome two problems in the measurement: the cavity length of the EFPI and wavelengths of light sources must be matched, and the DC component of the interferometric fringe must be eliminated. The proposed demodulation technique is verified by demodulating EFPI sensors with different cavity lengths and different phase modulations. Experimental results show that the demodulation technique can interrogate an EFPI sensor with any cavity length and any phase modulation. Besides, the technique has properties of high frequency, large dynamic range, and high sensitivity.