Yun-Jiang Rao

We demonstrate an all-fiber optical Fabry–Perot interferometer (FPI) strain sensor whose cavity is a microscopic air bubble. The bubble is formed by fusion splicing together two sections of single-mode fibers (SMFs) with cleaved flat tip and arc fusion induced hemispherical tip, respectively. The fabricated interferometers are with bubble diameters of typically $\sim 100\mathrm{\mu m}$. Strain and temperature sensitivities of fabricated interferometers are studied experimentally; a strain sensitivity of over $4\mathrm{Pm}/\mathrm{\mu \epsilon }$ and a thermal sensitivity of less than $0.9\mathrm{Pm}/°\mathrm{C}$ is obtained.

We have constructed a novel refractive index (RI) sensor based on a fiber optic Fabry–Perot interferometer (FPI) by splicing a section of hollow core fiber between a single-mode fiber and a photonic crystal fiber (PCF). Owing to the air holes in the cladding of the PCF, various substances, such as liquids and gases with different RI, can enter or leave the in-fiber air cavity, which makes the device usable as a refractometer. In this paper, the fiber optic FPI sensor has been used to monitor the RI changes of air with different pressures, and the experimental results show that such a sensor has an RI sensitivity of $805.1\mathrm{\mu m}/\mathrm{RIU}$, and hysteresis is not observed. Moreover, the easy fabrication method gives the in-fiber refractometer many potential applications in the sensing field.

A novel fiber-optic in-line etalon formed by splicing a section of hollow-core photonic crystal fiber (HCPCF) in between two single-mode fibers is proposed and demonstrated, for the first time to our knowledge. Such a HCPCF-based etalon acts as an excellent optical waveguide to form a Fabry–Perot interferometer and hence allows the cavity length to be as long as several centimeters with good visibility as the transmission loss of the HCPCF is much smaller than that of a hollow core fiber; this offers great potential to generate a practical dense fiber-optic sensor network with spatial frequency division-multiplexing. This novel etalon is demonstrated for strain measurement, and the experimental results show that a good visibility of 0.3 and a strain accuracy of better than $\pm 5\mu \epsilon$ are achieved.

A demodulation algorithm for spatial-frequency-division-multiplexed fiber-optic Fizeau strain sensor networks with a large number of sensors is proposed to effectively reduce the cross talk between any two adjacent sensors and hence substantially enhance the multiplexing capability of the network, which is based on the Pisarenko algorithm. The cross talk between two fiber-optic Fizeau sensors is investigated experimentally. The experimental results show that a strain accuracy of better than $\pm 10\mu ϵ$ can be achieved even when the cavity length difference is $\sim 100\mu \mathrm{m}$. It was demonstrated that the multiplexing capability of the spatial-frequency-division-multiplexed fiber-optic Fizeau sensor network can be $\sim 5$ times greater than that of the conventional fast-Fourier-transform algorithm, and this can lead to achievement of a sensing network with a multiplexing capability of as many as 1000 Fizeau sensors.