Specialty fibers have enabled a wide range of sensing applications. Particularly, with the recent advancement of anti-resonant effects, specialty fibers with hollow structures offer a unique sensing platform to achieve highly accurate and ultra-compact fiber optic sensors with large measurement ranges. This review presents an overview of recent progress in anti-resonant hollow-core fibers for sensing applications. Both regular and irregular-shaped fibers and their performance in various sensing scenarios are summarized. Finally, the challenges and possible solutions are briefly presented with some perspectives toward the future development of anti-resonant hollow-core fibers for advanced sensing.

A novel scheme is proposed to mitigate the atmospheric turbulence effect in free space optical (FSO) communication employing orbital angular momentum (OAM) multiplexing. In this scheme, the Gaussian beam is used as an auxiliary light with a common-path to obtain the distortion information caused by atmospheric turbulence. After turbulence, the heterodyne coherent detection technology is demonstrated to realize the turbulence mitigation. With the same turbulence distortion, the OAM beams and the Gaussian beam are respectively utilized as the signal light and the local oscillation light. Then the turbulence distortion is counteracted to a large extent. Meanwhile, a phase matching method is proposed to select the specific OAM mode. The discrimination between the neighboring OAM modes is obviously improved by detecting the output photocurrent. Moreover, two methods of beam size adjustment have been analyzed to achieve better performance for turbulence mitigation. Numerical results show that the system bit error rate (BER) can reach 10⁻⁵ under strong turbulence in simulation situation.

An ultrasensitive optical fiber sensor based on the Fabry-Perot interferometer (FPI) and the optical Vernier effect formed by an anti-resonant Bragg hollow core fiber (BHCF) and a single mode fiber (SMF) cascade structure is proposed for measuring ultra-high temperatures. This compact all-fiber structure can be easily fabricated by splicing one end of the BHCF with the SMF used as a light guide, and the other end with the SMF used as a sensor and cut to a specific length. The Vernier effect is formed by the reflection of three beams of light from the SMF-BHCF fusion interface and the two air/glass interfaces at both ends of the sensing SMF. When the length of the two optical fibers meets certain conditions, high-order harmonics Vernier effect can be formed, and the temperature sensitivity can be improved. For the harmonic order i = 1, the temperature sensitivity of the proposed fiber optic sensor is 0.2977 nm/°C in the temperature range of 795-830°C, and the magnification M factor is about 27.59. The fabrication of the optic fiber sensor is straightforward, reproducible, stable, small size, and low-cost, which exhibits potential applications in various industrial processes.

In this study, the orbital angular momentum (OAM) sidebands of Laguerre-Gauss beams reflecting on graphene metasurfaces are investigated. Upon reflection, vortex beams carrying orbital angular momentum will acquire sidebands, whose relative intensity varies depending on the Fermi energy, the external magnetic field, and/or the wave frequency. The relative intensity of the sideband OAM modes locally has a small trough for s-polarized beams at the topological transition point between the hyperbolic and elliptic topology. Energy can transfer from the central mode to the neighboring OAM modes increasing the topologic charge l. When the electric field of the incident s-polarized light occurs along the low energy dissipation direction of the graphene metasurfaces, it is helpful for the mode transformation of vortex beams. When the electric field of the incident s-polarized light occurs along the high energy dissipation direction, it is beneficial to suppressing crosstalk of different sidemodes in terahertz communication.

The power-boosted laser transmitted via a fiber tip sinusoidally excites the target trace molecules, generating a photothermal (PT) effect. The Gaussian-distributed plane wave heats the medium adjacent to the Fresnel reflection surface. Meanwhile, a continuous probe light traverses the heating field, and the periodic temperature change then modulates the phase of the probe light. A multi-beam interferometer formed by aligning the Fresnel reflection surface to a gold-coated high-reflection surface possesses high resolution and sensitivity, significantly enhancing the detection performance. A well-established theoretical model of instantaneous PT transduction and optical phase modulation is employed to obtain the optimized interferometric configuration. To validate the effective interferometric phase transformation within the cavity, a comparison is made between the simple silicon cavity and the gold-coated cavity. The limitation of detection of such extrinsic fiber-tip PT sensors indicates one magnitude lower as compared to the conventional PT interferometric gas sensor with such robust and compact sensing designs. This work lays a solid foundation for future research on gas laser phase modulators and nonlinear laser–matter interactions.

Wide-bandgap semiconductor-based X-ray detectors provide a number of advantages. As presented in this letter, (Mg)ZnO photoconductive direct-conversion X-ray detectors with various Mg contents were fabricated and demonstrated. The signal-to-noise ratio (SNR) and response time of the devices under 6 MV hard X-ray irradiation were significantly improved after doping with Mg. The Ag-Mg _0.19 Zn _0.81 O-Ag photoconductive detector with optimal Mg contents exhibited a sensitivity of 3.03 $\mu \text{C}$ /(mGy $_{\mathrm {air}}\cdot $ cm ³ ), a short response time of approximately 0.2 s (rise time) and 0.3 s (decay time), and a large SNR of 278 with a bias of 30 V under hard X-ray irradiation (100 mGy _air /s). The results indicate great application prospects for wide-bandgap MgZnO in direct-conversion X-ray detectors.