We report on the design, fabrication, and characterization of a low-loss birefringent semi-tube anti-resonant hollow-core fiber (AR-HCF). By optimizing the structure design and the stack-and-draw fabrication technique, a transmission loss of 4.8 dB/km at 1522 nm, a 25 cm bending radius. Combined with the single mode and low dispersion features, the developed semi-tube AR-HCF may find a variety of applications in frequency metrology, interferometric fiber gyroscopes, and long-baseline stellar interferometry.

Terahertz waves possess unique electromagnetic properties, such as penetration, high capacity, and non-destructive testing capabilities, making the study of their absorption characteristics highly significant. Building on previous narrowband and broadband research, this paper introduces an absorber capable of switching between narrowband and broadband modes. This absorber leverages the tunability of graphene and the phase transition properties of vanadium dioxide (VO₂) to achieve adjustable and switchable absorption characteristics. When VO₂ is in its insulating state, the absorption spectrum exhibits dual narrowband absorption with an absorption rate approaching 100%. As the temperature increases, VO₂ transitions to a conductive state, resulting in ultra-wideband absorption with a bandwidth of 2.24 THz. Research has shown that the broadband mode is robust with respect to relaxation time and the thickness of VO₂ (within a certain range). The broadband mode is also insensitive to the angle of incidence. The narrow-to-wideband switching absorber demonstrated in this paper offers new insights for research in related fields.

A novel fiber-interface directional waveguide coupler was inscribed on the surface of a coreless fiber by femtosecond laser, and was successfully applied to highly sensitive refractive index (RI) measurements. The primary arm was first inscribed to couple light from a lead-in single mode fiber to the fiber interface, then back to a lead-out single mode fiber. A side arm was inscribed parallel and in close proximity to the primary arm. Light propagating in the primary arm could then be efficiently coupled into the side arm when a phase-matching condition was met, which produced a dramatic spectral dip at the coupling wavelength. The proposed device achieved a sensitivity as high as ∼8249 nm/RIU over an RI range of 1.44-1.45, due to strong evanescent fields excited in fiber-interface waveguides. The proposed in-fiber directional coupler exhibits high mechanical strength, a compact configuration, and excellent RI sensitivity. As such, it has significant potential for practical applications in biochemical sensing.

A new approach to conquer the thermal phase drift of an optical fiber Fabry-Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. By employing a hollow-core anti-resonant fiber (HC-ARF) and optimizing the fusion splicing (includes mode field adaptation) between the lead-in single-mode fiber (SMF) and the HC-ARF, a high spectral resolution (λ/Δλ ≈ 3.8 × 10⁴) optical fiber air-cavity FPI sensor with a fringe visibility higher than 7 dB is constructed. To eliminate the thermal phase drift (i.e. temperature crosstalk) of the sensor that originates from the intrinsic thermal expansion effect of the silica material of the HC-ARF, the FPI air cavity is connected to the external environments, by which the effect of air expelling from the cavity with temperature increasing can well compensate the temperature-induced cavity elongation. As a result, the thermal phase drift of the FPI is reduced to zero at a temperature range of ∼ 80–110 °C and within the temperature range of 40–80 °C, the thermal phase drift is still halved compared with the sealed FPI cavity. The nearly zero thermal phase drift of a FPI at such a temperature range has never been achieved before, to our best knowledge. As a proof of concept, a temperature-immune fiber-optic strain sensor is demonstrated. This work offers a new and efficient approach to eliminate the thermal phase drift (i.e. temperature crosstalk) of a fiber-optic device, which may significantly improve the measurement accuracy and detection limit of fiber-optic FPI sensors. Furthermore, the principle and schema can be generalized to a wide variety of fiber-optic devices.

A wave-band-tunable optical fiber broadband orbital angular momentum (OAM) mode converter based on a helical long-period fiber grating (HLPFG) and dispersion turning point (DTP) tuning technique is demonstrated both theoretically and experimentally. The DTP tuning is achieved by thinning the optical fiber during the HLPFG inscription. As a proof of concept, the DTP wavelength of the LP_1,5 mode is successfully tuned from the original ∼2.4 µm to ∼2.0 µm and ∼1.7 µm. With the help of the HLPFG, broadband OAM mode conversion (LP_0,1→LP_1,5) is demonstrated near the 2.0 µm and 1.7 µm wave bands. This work addresses a longstanding problem that the broadband mode conversion is limited by the intrinsic DTP wavelength of the modes and provides a new, to the best of our knowledge, alternative for broadband OAM mode conversion at the desired wave bands.

High-performance interconnection between hollow-core fiber and conventional solid-core fiber is of great significance for a lot of promising applications of hollow-core fibers. The current problems for high-performance hollow-core fiber/solid-core fiber interconnection mainly involve mode field mismatch of the two fibers and the complex splicing process. Subject of study. We study fusion splicing of anti-resonant hollow-core fiber with low loss (0.52 dB) and conventional single-mode fiber using a convenient graded-index bridge fiber. Purpose of the work. We aim to eliminate the mode field diameter mismatch between anti-resonant hollow-core fiber and single-mode fiber and simplify the mode field diameter adaptation method and process. Method. We employ a convenient graded-index bridge fiber approach, where a gradient index fiber is first spliced with the single-mode fiber, and the mode interference within the graded-index bridge fiber causes periodic enlargement and shrinkage of the mode field diameter. By precise cutting of the graded-index bridge fiber, the mode field diameter can be well controlled and made approximately equal to the mode field diameter of the anti-resonant hollow-core fiber. After mode field diameter adaptation, an optimized arc discharge fusion splicing procedure is applied for the anti-resonant hollow-core fiber/single-mode fiber fusion splicing. Main results. A fusion splicing loss of about 0.52 dB is achieved, which involves nearly 0.42 dB mode-field-diameter-mismatch-induced loss and about 0.1 dB fusion splicing loss that is caused by the deformation of the anti-resonant hollow-core fiber. Practical significance. The developed approach is convenient and cost-effective, which can benefit a lot of applications of anti-resonant hollow-core fibers in future telecommunications, fiber gas lasers as well as fiber-optic sensing.