This paper proposes a novel cholesteric liquid crystal (CLC) film–based dual-probe fiber sensor to monitor volatile organic compound (VOC) gas. The sensor consists of a 2×2 multimode fiber coupler, in which the two output fiber ends are covered by two types of CLC films (CLCF) with different pitches. It can be observed that the reflection peak wavelengths of CLCs shift to the red side as the VOC gas concentration increases. The sensitivities of the two CLCFs are 8.435 nm·L/mmol and 14.867 nm·L/mmol to acetone, 14.586 nm·L/mmol and 29.303 nm·L/mmol to ethanol, respectively. In addition, the dependence of the peak wavelength shift of CLCF on the total concentration of the acetone and ethanol mixed gas at different mixing ratios is measured. The linear relationships between the peak shift of CLCFs, the total mixed gas concentration and acetone/ethanol ratio are calculated using the least-squares method. Therefore, this proposed dual-probe fiber optic sensor can distinguish the concentrations of acetone and ethanol in a mixed gas of acetone and ethanol.

An investigation into a novel in-vivo PMMA (polymethyl methacrylate) plastic fiber-optic dosimeter for monitoring low doses of ionizing radiotherapy radiation in real time and for integrating measurements is presented. The fabricated optical fiber tip possessed an embedded structure. A scintillation material, terbium-doped gadolinium oxysulfide (Gd₂O₂S:Tb), capable of emitting visible light at around 545 nm which is ideal for transmission through the PMMA when exposed to ionizing radiation was embedded in the PMMA plastic fiber. The dose rate of incident ionizing radiation is measured by analyzing the signal intensity emitted from the scintillation material which propagates through the fiber to a distal MPPC (multi-pixel photon counter). The dosimeter exhibits good repeatability with an excellent linear relationship between the fiber-optic dosimeter output and the absorbed radiation dose with an outstanding isotropic response in its radial angular dependence.

A novel sensor structure has been proposed and experimentally investigated for simultaneous strain and temperature measurement. The structure is fabricated by weak power modulation of CO2 laser exposure on tapered long period fiber grating (LPFG). Compared with the transmission spectrum of the tapered LPFG, two peaks appear in the transmission spectrum of the novel structure. These resonance peaks exhibit different sensitivity responses; thus, simultaneous measurement of strain and temperature is realized by monitoring the wavelength shift of the two peaks. Experiment results indicate that strain sensitivities of the two peaks are 1.82 pm/μE and 8.17 pm/μE, and temperature sensitivities are 47.9 pm/°C and 65 pm/°C, respectively.

This review focuses on the preparation methods and wide applications of optic fiber sensors based on core-offset structure. Firstly, the classification and preparation methods of core-offset structures are introduced. Then, the applications of different types of sensors based on the core-offset structure are investigated, including the measurement of physical parameters such as temperature, strain, refractive index (RI), bending, magnetic field by Mach-Zehnder interferometer (MZI), cascaded sensor, long period fiber grating (LPFG), fiber Bragg grating (FBG), Fabry-Perot interferometer (FPI) and Surface plasmon resonance (SPR). Compared with the traditional fiber sensor, core-offset structure sensor with a non-axisymmetric characteristic has excellent sensing characteristics, especially in the recognition of bending direction and high RI sensitivity. Furthermore, the design scheme, working principle and sensing characteristics of different core-offset sensors are also briefly described. Finally, the existing challenges and development direction of optic fiber sensors based on core-offset structure is discussed and prospected.

A multimode fiber chirped long-period grating (MCLPG) is proposed and studied as a sensor for simultaneous measurement of strain and temperature. This sensor consists of linear chirped multimode fiber and fixed-length single-mode fiber. The numerical analyses of the transmission spectrum and coupling mode are carried out to obtain appropriate period arrangement. The experimental results are in good agreement with the theoretical results. The chirped period arrangement induces a transmission spectrum with double peaks. Therefore, the simultaneous measurement of strain and temperature is realized by observing the wavelength shift of the two peaks. In addition, the high sensitivity of two resonance peaks provides a good resolution for the sensor.

A long period fiber grating prepared by multimode fibers (MMF-LPFG) is proposed and experimentally demonstrated as a sensitivity-enhanced strain sensor. The core diameter mismatch between the MMFs excites the high order cladding mode to obtain a compact sensing structure (3.6 mm). The strain sensitivity of the MMF-LPFG structure is enhanced to −15.06 pm/ $\mu \varepsilon $ when the grating period is closer to the dispersion turning point (DTP). Meanwhile, the novel fiber structure also has an excellent mechanical property to achieve a large dynamic range ( $\text{0-4508} ~\mu \varepsilon $ ) in strain measurement.