An antiresonant hollow core effective single-mode terahertz fiber is proposed and successfully fabricated with a photosensitive resin (SomosEvoLVe 128) by a 3D printer. The single mode characteristics of the fiber are found to be related to the area of the semielliptical tubes in the cladding. By optimizing the cladding structure parameters, a Higher Order Mode Extinction Ratio (HOMER) of above 6 can be achieved. A 30 cm long sample with optimized structural parameters is obtained by cascading two 15 cm long fiber samples fabricated by a 3D printer. The transmission loss and the electric field distribution are measured by a terahertz time domain spectroscopy (TDS) system. The electric field distribution measured at the output end of the fiber sample has the Gaussian profile, which accords with single-mode field distribution. Experimental results show that the average loss is 0.048 cm ^-1 within the frequency range from 0.2 to 1 THz, and the minimum loss is obtained as low as 0.009 cm ^-1 at the frequency of 0.82 THz.

A novel high-birefringence and low-loss hollow-core (HC) antiresonant (AR) THz waveguide is proposed and experimental demonstrated. The waveguide is a circular tube embedded with two pairs of symmetrical parallel slabs. With the support of a circular tube, one pair of slabs near the waveguide center is used to form a rectangular-like core to achieve high birefringence, and another pair of slabs is placed in the cladding to further reduce the waveguide loss. Simulation results indicate that the birefringence reaches the magnitude order of 10 ⁻³ within the transmission window from 0.38 to 0.50 THz. The highest birefringence reaches 3.42 × 10 ⁻³ at 0.38 THz with a loss of 0.09 cm ⁻¹ . We manufacture the HC-AR THz waveguide with photosensitive resin by using three-dimensional printing technology. And the waveguide properties are investigated by THz time-domain spectroscopy system. Experimental results demonstrate that the birefringence variation of the HC-AR THz waveguide is within the range of 1.57 × 10 ⁻³ to 4.77 × 10 ⁻³ in the 0.39 to 0.51 THz transmission window. The highest birefringence is up to 4.77 × 10 ⁻³ at 0.39 THz and the corresponding loss is 0.12 cm ⁻¹ . To the best of authors’ knowledge, the proposed HC-AR THz waveguide achieves the highest birefringence and relatively low loss in current HC-AR THz waveguides.

An ultra-wide single-mode (SM) frequency bandwidth and low-flattened dispersion hollow-core negative-curvature (HC-NC) terahertz (THz) waveguide is proposed. SM guidance is achieved by introducing 5-tube nested semi-elliptical tubes (SETs) in the cladding to couple high order modes (HOMs) and cladding modes (CMs). By rotating the nested SETs at the appropriate angle, the HC-NC THz waveguide can achieve excellent SM guidance with HOM extinction ratio (HOMER) >100 within an ultra-wide bandwidth of 460 GHz from the frequency of 0.70 THz to 1.16 THz. The low transmission loss of fundamental mode (FM) (<0.12 dB/m) and low-flattened dispersion (−0.085 ± 0.08 ps/THz/cm) can be obtained simultaneously. To the best of our knowledge, our proposed THz waveguide creates a record of ultra-wide SM frequency bandwidth among the current hollow-core anti-resonant (HC-AR) THz waveguides. Besides, an ultra-high HOMER of 380 and FM loss of only 0.05 dB/m can be achieved at 0.90 THz. In addition, the waveguide also exhibits a good fabrication tolerance. These excellent performances make the proposed HC-NC THz waveguide a great application potential for THz transmission.

A compact sensor integrated in a single hollow core Bragg fiber is proposed and demonstrated for simultaneous measurement of strain and temperature. The sensor is manufactured by splicing a 3 mm long homemade Hollow Core Bragg Fiber (HCBF) sandwiched between two segments of Single Mode Fibers (SMFs) with a lateral offset at one end. Experimental results demonstrate that both Mach-Zehnder Interferometer (MZI) and Bragg resonant mechanism are coexisting in the HCBF-based sensor. Two sensing mechanisms have different strain and temperature sensitivities with good linearity. The axial strain sensitivities of -0.4 pm/με and 0.33 pm/με and the temperature sensitivities of 40.8 pm/ °C and 26.5 pm /°C are obtained by monitoring the wavelength shifts with axial strain and temperature in temperature range of 60-80°C. Therefore, simultaneous measurement of temperature and strain can be achieved by using a characteristic matrix approach. Due to the large refractive index difference between the core and cladding of the Bragg fiber, the length of HCBF for forming mode interference in the sensor could be greatly reduced. This makes the sensor more compact than most previous reports for simultaneous measurement of temperature and strain.

This contribution demonstrates the interaction between the fiber Bragg grating (FBG) and the ultrasound. When the grating length is much longer than the ultrasonic wavelength, the conventional wavelength-modulation method is invalid. In order to find out an effective sensing method in this case, a novel sidelobe-modulation has been proposed and investigated. When grating length is much longer than the ultrasonic wavelength, the Bragg wavelength is not shifted, however, the reflectivity of sidelobe is modulated by ultrasound, periodically, which can be utilized as an effective ultrasound sensing method. In order to optimize the performances of this method, the characteristics of the sidelobes are further analyzed for different ultrasonic displacement amplitude, frequency and grating length, respectively. The analysis results indicate that a proper choice of the grating length would enable the sidelobe-modulation to be utilized for detecting the ultrasound with appropriate range of ultrasonic frequency and amplitude. This method is preliminarily validated in the experiment, and the experimental results show that our proposed sidelobe-modulation method is effective to detect the 7.8 MHz ultrasonic wave by a simple intensity-demodulation using a 5 mm FBG, where the wavelength-modulation is invalid.

An ultra-broadband 3-dB coupler based on a polymer dual-hollow-core anti-resonant fiber (DHC-ARF) is designed to work in the E + S + C + L + U communication band. By incorporating two elliptical-like cores and modulating the air gap between the two cores, the wavelength and polarization dependence of the DHC-ARF-based coupler is reduced effectively. The feasibility of using a 1.46 cm long DHC-ARF as the ultra-broadband coupler for the operating bandwidth of 400 nm in the range between 1.33 µm and 1.73 µm is demonstrated theoretically. The coupling ratio of each polarized mode stabilizes at 50 ± 2% and the coupling ratio difference between the two polarized modes changes within ±0.6%. This DHC-ARF coupler which is made of a polymer can be fabricated by high-resolution 3D printing. Compared to a silica-based DHC-ARF coupler, the polymer-based DHC-ARF coupler is easier to manufacture and the total loss of the latter is only 0.041 ± 0.006 dB in the operating bandwidth. The polymer hollow-core fiber coupler boasting an ultra-broadband, short component length, and low loss is very promising in next-generation, high-speed, and large-capacity hollow-core fiber communication systems.