A method for measurement of a magnetic field by combining photonic crystal fibers (PCFs) and magnetic fluid is presented and experimentally demonstrated. The magnetic fluid is filled into the air holes of the cladding layer in the PCF. Due to the tunable refractive index property of the magnetic fluid, the refractive index difference between the fiber core and cladding layer is changed with the external magnetic field. The magnetic field can be directly detected by measuring the intensity of the transmission light. A series of magnetic fields with different strengths have been measured with the sensor. The experimental results show that a resolution of up to 0.09 Oe is achieved, and a good repetition is demonstrated experimentally. Compared with other expensive methods, the proposed method possesses high sensitivity and low cost.

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.

An improved peak-to-peak method is developed for interrogating the absolute cavity length of fiber optic extrinsic Fabry–Perot interferometric (EFPI) sensors with high resolution. A fiber Fabry–Perot tunable filter (FFP-TF) is used to scan the optical spectrum of an EFPI, and the problems caused by the nonlinear performance and poor repeatability of the FFP-TF are removed by using a wavelength calibration technique. A linear fitting is used to calculate the wavelength spacing between two adjacent apexes in the optical spectrum, and the cavity length can be retrieved using this wavelength spacing. The experimental results show that the measuring resolution is improved from 25 to $1\mathrm{\mu m}$, and a linear output is also obtained.

A method for measurement of a magnetic field by filling a microhole with magnetic fluid (MF) in a photonic crystal fiber (PCF) is presented and experimentally demonstrated. A microhole is created in the collapsed region between the PCF and the single-mode fiber by using femtosecond laser micromachining, and a PCF-based Mach–Zehnder interferometer is formed. The MF is filled into the microhole. Due to the tunable refractive index property of the MF, the mode field diameter of the propagation light is changed with the external magnetic field, and the magnetic field can be detected by measuring the visibility contrast of the white light interferogram. The experimental results show that sensitivity of up to $0.042\mathrm{dB}/\mathrm{Oe}$ is achieved.

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.

Recent developments in spectral white-light interferometry (WLI) are reviewed. Firstly, the techniques for obtaining optical spectrum are introduced. Secondly, some novel measurement techniques are reviewed, including the improved peak-to-peak WLI, improved wavelength-tracking WLI, Fourier transform WLI, and 3 × 3 coupler based WLI. Furthermore, a hybrid measurement for the intensity-type sensors, interferometric sensors, and fiber Bragg grating sensors is achieved. It is shown that these developments have assisted in the progress of WLI.