A Mach–Zehnder interferometric magnetic field sensor based on a photonic crystal fiber (PCF) and magnetic fluid (MF) was designed and experimentally demonstrated. The sensing probe consists of a single-mode-(SM)-multimode-PCF-SM fiber structure through arc fusion splicing. It was then laser engrave notched with the femtosecond laser so that the PCF cladding was selectively infilled MF. A well-defined interference pattern was obtained on account of the tunable refractive index of the MF infilled PCF cladding. The transmission spectra of the proposed sensor under different magnetic field intensities have been measured and theoretically analyzed. The results show that the sensitivity of the proposed sensor can reach $-0.13\mathrm{dB}/\mathrm{mT}$ and 0.07334 nm/mT in the magnetic field intensity from 1 mT to 20 mT and 2 mT to 20 mT, respectively.

A compact and simple double F-P interference optical fiber optic gas pressure sensor based on suspended core structure is proposed and demonstrated. The sensor is formed by splicing a Section of thin-walled grapefruit photonic crystal fiber (G-PCF) with single-mode fiber (SMF). By adjusting the special fusion parameters, a short air-cavity region is formed in the G-PCF next to the splicing point with the SMF segment, and the other end of the G-PCF segment is open to the atmosphere. The output interference spectrum of the sensor comprises of two spectral parameters that have different sensitivities to temperature and gas pressure. One of them is the wideband envelope that is associated with the air-cavity and it is only sensitive to the gas pressure but insensitive to the change of ambient temperature. This property is particularly useful for addressing the complex temperature-pressure cross sensitivity of both spectral parameters. The research findings indicate that the sensor can perform an accurate measurement for gas pressure in dynamic range of 0-1.0 MPa under the high temperature condition up to 500 °C.

The work demonstrates for the first time a thermal regenerated grating (RG) operating at an ultra-high temperature up to 1400°C. A new class of photosensitive optical fiber based on erbium-doped yttrium stabilized zirconia-calcium-alumina-phospho silica (Er-YZCAPS) glass is fabricated using modified chemical vapor deposition (MCVD) process, followed by solution doping technique and conventional fiber drawing. A type-I seed grating inscribed in this fiber is thermal regenerated based on the conventional thermal annealing technique. The investigation result indicates that the produced RG has an ultrahigh temperature sustainability up to 1400°C. The measured temperature sensitivities are 14.1 and 15.1 pm/°C for the temperature ranges of 25°C–1000°C and 1000°C–1400°C, respectively.

In this work, we present a discriminative optical fiber sensor for temperature and strain measurement. The sensor comprises of two cascaded thermal regenerated Fiber Bragg gratings (RFBGs) incorporated with two fused silica capillary tubes. The RFBG ₂ is loosely enclosed in a fine fused silica tube and made solely sensitivity to temperature whereas the RFBG ₁ sensor still retains its sensitivity to both strain and temperature. These properties have made the discriminative measurement accurate and directive. The experimental results indicate that temperature response is linear in the range of 100 – 1000°C with the sensitivity of ~15.7 pm/°C. Besides, it presents good repeatability in strain detection at high temperatures (300 °C – 900 °C). The incorporation of the two fine glass tubes has enhanced the modified RFBG’s strain sensitivity to as high as ~5.46 pm/ $\mu \varepsilon $ in the measurement range of $0~\mu \varepsilon $ to 120 $\mu \varepsilon $ at 600 °C, which is about five times higher than that of common RFBG strain sensors. The sensitivity can be further enhanced by manipulating the parameters of the sensor’s structure.

A sensitivity-improved fiber-optic strain sensor using an etched and regenerated fiber Bragg grating (ER-FBG) suitable for a large range of temperature measurements has been proposed and experimentally demonstrated. The process of chemical etching (from 125 µm to 60 µm) provides regenerated gratings (at a temperature of 680 °C) with a stronger reflective intensity (from 43.7% to 69.8%), together with an improved and linear strain sensitivity (from 0.9 pm/με to 4.5 pm/με) over a large temperature range (from room temperature to 800 °C), making it a useful strain sensor for high temperature environments.

We have experimentally demonstrated an optical fiber Mach-Zehnder interferometer (MZI) structure formed by a few-mode photonic crystal fiber (PCF) for curvature measurement and inscribed a fiber Bragg grating (FBG) in the PCF for the purpose of simultaneously measuring temperature. The structure consists of a PCF sandwiched between two multi-mode fibers (MMFs). Bending experimental results show that the proposed sensor has a sensitivity of −1.03 nm/m⁻¹ at a curvature range from 10 m⁻¹ to 22.4 m⁻¹, and the curvature sensitivity of the embedded FBG was −0.003 nm/m⁻¹. Temperature response experimental results showed that the MZI’s wavelength, λ_a, has a sensitivity of 60.3 pm/°C, and the FBG’s Bragg wavelength, λ_b, has sensitivity of 9.2 pm/°C in the temperature range of 8 to 100 °C. As such, it can be used for simultaneous measurement of curvature and temperature over ranges of 10 m⁻¹ to 22.4 m⁻¹ and 8 °C to 100 °C, respectively. The results show that the embedded FBG can be a good indicator to compensate the varying ambient temperature during a curvature measurement.