Yi Jiang

Sapphire Fabry-Perot interferometers are useful sensors for realizing high-precision strain measurement under ultrahigh temperature. The low fineness of sapphire Fabry-Perot cavities, due to the relatively low endface reflectivity, limits the accuracy of the cavity length demodulation and hence the strain measurement. Here we propose a multi-order optimal cavity length estimation algorithm to increase the demodulation accuracy of sapphire Fabry-Perot cavities especially under high temperature. The algorithm is designed to evaluate the optimal cavity length by exploiting all valleys in the reflection spectrum and to determine possible cavity lengths of each interference order, then obtaining the optimal cavity length with the minimum deviation between the valleys. Using the algorithm, the repeatability of the cavity length demodulation can reach 1 nm, corresponding to a strain measurement repeatability of 0.1 με. The zero-drift and the repeatability precision of the cavity demodulation is measured as 0.1% (FS) and higher than 0.5% (FS) respectively. As an application, the method is exploited on the thermal wind tunnel testing experiment. Results show that the strain measurement error is less than 5% (FS) at 1000 °C, fulfilling the requirements for strain monitoring of wind tunnel balances.

A high-sensitivity sapphire extrinsic Fabry-Perot interferometer (EFPI) is proposed for the high-resolution pressure measurement in high temperature vacuum environment. A composite Fabry-Perot (FP) cavity structure is fabricated by directly bonding three sapphire wafers with different thickness to simultaneously measure temperature and pressure, which can effectively address the cross-sensitivity problem from temperature to pressure. Experimental results show that the pressure can be accurately measured from 0 to 100 kPa within the temperature range of 29 °C to 800 °C. The optical cavity length (OCL) of the sealed air cavity varies linearly with pressure, and the pressure sensitivity increases with temperature, resulting in a maximum error of 4.3 kPa. The pressure sensitivities are 56.22 nm/kPa and 63.24 nm/kPa at room temperature and 800 °C, and the resolutions of pressure measurement are 17.79 Pa and 15.81 Pa, respectively. Based on its excellent performance, the proposed high-sensitivity EFPI is a prospective sensor for measuring the vacuum pressure in high temperature environment.

A high-accuracy white light interferometry (WLI) demodulation algorithm based on the frequency shift technique is proposed and demonstrated for interrogating fiber-optic dual-cavity Fabry-Perot interferometric (DFPI) sensors, in which the intensity of the reflection spectrum generated by the short FP cavity is relatively weak. The long FP cavity’s optical path difference (OPD) is demodulated directly from the DPFI’s reflection spectrum using the maximum likelihood estimation (MLE) algorithm. A stronger reflection spectrum of the short FP cavity is acquired by shifting the frequency of the combined FP cavity’s reflection spectrum toward a lower frequency direction, and the shift value is equal to the frequency of the long FP cavity’s reflection spectrum. Finally, the short FP cavity’s OPD is demodulated by the multi-extrema-tracing technique. The frequency shift technique can effectively improve the demodulation accuracy of the short FP cavity’s OPD by obtaining a reflection spectrum with a higher signal-to-noise ratio (SNR). Simulation results show that the demodulation errors for both cavities’ OPDs are within ± 0.5 nm. Furthermore, this proposed algorithm has demonstrated excellent demodulation performance for a sapphire DFPI fiber-optic temperature and pressure hybrid sensor. The proposed demodulation algorithm has the advantages of high accuracy, resistance to demodulation jumps, large dynamic range, and absolute measurement, which has wide application potential for interrogating DFPI sensors with a weak reflection component of the shot FP cavity.

An external cavity wavelength-fiber ring laser (ECWTFL) based on a semiconductor optical amplifier and a combined wavelength scanning filter in the Littrow configuration is proposed and experimentally demonstrated. With the benefit of the combination of an external cavity wavelength filter and a Lyot filter, the laser achieves a single-mode narrow linewidth output with a linewidth of 1.75 kHz. The wavelength tuning range reaches 133 nm, covering the entire S + C band. The proposed ECWTFL is used for demodulation of a fiber EFPI sensor; the result shows that the proposed ECWTFL has the ability to demodulate the small cavity-length FPI sensor.

Optical traps formed in hollow-core fibers (HCFs) can overcome several limitations of conventional free-space optical tweezers. One of the key issues is to load particles from free space into the hollow core with high efficiency, in which process the capture dynamics of the particles in front of the HCF endface plays an important role. In this work, a comprehensive model of the trapping and capture process of the dielectric particles in front of HCF is established by taking into account the features of the fiber modes and the motional parameters of the particles. Stable capture positions are predicted based on analytical calculations of optical forces, and the dependencies of the equilibrium axial trapping position on the beam numerical aperture, the fiber core and particle diameters are provided. In addition, the trajectories and the capture dynamics of the particles are studied by solving the equation of motion for the particles under the impact of optical forces, predicting feasible parameter ranges of the initial amplitude and direction of particle launch velocity for achieving successful particle capture in front of HCF. The results can provide guidance for further improving the particle-loading efficiencies of the HCF-based optical traps, which may find applications of flying particle sensors and long-range particle binding in HCFs.

A four-wavelength passive demodulation technique for the measurement of static/dynamic composite signals is proposed and experimentally demonstrated. Those previously reported passive demodulation methods for extrinsic Fabry–Perot intererometrics (EFPIs) will collapse if the cavity length changes significantly during signal demodulation. However, the proposed technique can demodulate the measurand even if the cavity length changes significantly. The phase difference between different interferometric signals is firstly calculated by using four interferometric signals. The phase compensation of these interferometric signals is then implemented to eliminate the influence of cavity length variations on the demodulation. The change of the cavity length can then be recovered. Dynamic signals, static signals, and static/dynamic composite signals were interrogated by the demodulation technique with relative errors of ~0.1%. An EFPI with a cavity length ranging from 32.53 μm to 1219.29 μm is detected by the same demodulator. The fast measurement of a cavity length change with an amplitude of ~80 μm is achieved. The technique is applicable to different types of signals applied on EFPIs with an unsteady cavity length.

An all-sapphire fiber extrinsic Fabry-Perot interferometer (EFPI) sensor for the measurement of gas pressure at ultra-high temperature is proposed and experimentally demonstrated. The sensor head is composed of two segments of sapphire fiber and a sapphire capillary. By using the femtosecond (fs) laser micromachining, a through hole is inscribed in the sapphire capillary wall to allow the gas flow. This EFPI is used to measure the pressure in the temperature range of 25-1400 °C by monitoring the optical cavity length of the interferometer. Experiment results show that the sensor exhibits a good linearity within a pressure range from 0 to 5 MPa at a fixed temperature. The pressure sensitivity of the sensor decreases as the temperature increases, and sensitivities are 1.018 μm/MPa and 0.188 μm/MPa at 25 °C and 1400 °C, respectively. The proposed all-sapphire fiber pressure sensor has properties of high temperature resistance, intrinsic safety, small volume, large measuring range, which can be used in high temperature and harsh environments.

An all-sapphire fiber external Fabry–Perot interferometer (EFPI) sensor for measuring gas pressure is proposed and investigated. The sensor head is manufactured from a sapphire fiber ferrule and sapphire tube, and the same material can ensure the stability of the sensor structure at a high temperature. The refractive index of the gas is linearly related to the gas pressure. Therefore, the gas pressure can be measured by studying the optical cavity length of the EFPI. A multi-stage coupled multimode fiber is used to pick up the interference signal of the fiber EFPI. The pressure response of the sensor at different temperatures was measured in the experiment. The experimental results show that the sapphire fiber EFPI can measure 0–5 MPa gas pressure in the environment of 17–1400°C. The sensitivity of the sensor decreases with the increasing temperature, and the maximum sensitivity is 1.1673 µm/MPa (20°C). The sensor is compact and suitable for gas pressure measurement at a high temperature.