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Abstract
A flexible metallic waveguide with elliptical core that achieves single-polarization single-mode (SPSM) propagation at millimeter wave was designed, fabricated, and characterized. In order to achieve SPSM propagation, optimization of the lengths of major/minor axes of elliptical core was conducted to cut off one of the two orthogonally polarized fundamental modes and all high-order modes. A one-meter long hollow elliptical waveguide (HEW) with major/minor axis length of 1.5/2.7 mm was fabricated. The substrate tube was a flexible elliptical polycarbonate (PC) tube, which was fabricated through glass-draw technique. Silver film was then coated on the inner surface of the tube. Simulation results show that the 1.5/2.7 mm HEW maintains SPSM propagation in the frequency band from 66.5 to 114 GHz. The SPSM operation was experimentally discussed in detail at 100 GHz. The measured loss of 2.58 dB/m and the output polarization ratio of 99.9% was obtained after propagating one meter. Furthermore, the waveguide was robust to bending and twisting. The additional loss was as small as 0.2 dB/m even when the waveguide was coiled into a circle. The potential application of HEWs as polarizers was demonstrated by using a 10 cm long waveguide for polarization detection and extinction ratio of 22.3 dB was achieved at 100 GHz.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Millimeter and terahertz (THz) waves have attracted more and more attention in the past few years owing to the extensive application in the fields of communication [1], spectroscopy [2], imaging [3] and sensing [4]. However, most of the Millimeter wave and THz systems are relying on free-space transmission resulting in bulky and difficult to manipulate [5,6]. Meanwhile, polarization preservation is essential for polarization sensitive systems, such as THz time-domain spectroscopy [7] and new generation THz communication technology [8]. Therefore, there is an urgent need for flexible, polarization-maintaining (PM) and low-loss waveguides for THz propagation.
High birefringence and single-polarization single-mode (SPSM) waveguide are two main types of PM waveguide. Various forms of waveguides, such as photonic crystal fiber (PCF) [9–12], polymer optical waveguides [13–15], Bragg fiber [16], and elliptical hollow fiber, have been proposed for achieving high birefringence. Introducing an asymmetric structure in these waveguides is the most common scheme to achieve high birefringence.
Yani Zhang et al. [17] proposed a porous-core PCF achieving low-loss and polarization THz transmission in a wide frequency range of 1.0–2.0 THz. A THz grating based on a subwavelength rectangular polymer waveguide was proposed by Haisu Li et al., [18] and realized over a 20.9 dB extinction ratio (ER). Previous researches of our team demonstrated the polarization-maintaining performance of hollow elliptical waveguide (HEW). Numerical analyses were made for attenuation constants, group velocity, modal birefringence, and modal power fraction in the air core, and optimization of the fiber geometry [19]. A silver-coated HEW was fabricated [20], and an ER of 90% and a loss of 0.79 dB/m at 0.65 THz were realized. An elliptical hollow fiber with silver and dielectric inner-coatings [21] was proposed to achieve both low loss and good polarization-maintaining performance.
However, the PM ability of high birefringence fibers is influenced by polarization crosstalk and polarization mode dispersion [22,23]. SPSM waveguide is a potential candidate to solve this defect, owing to that only one polarized mode can be propagated. SPSM operating in millimeter or THz wave has been investigated by few researchers. Yu Hou et al. [24] simulated a hollow core fiber that realized the confinement loss of 0.004 dB/m at 1.675 THz, and achieved SPSM operation in the range of 1.67–1.8 THz. Tianyu Yang et al. [23] designed a SPSM PCF from 1.10 to 1.74 THz and the losses of unwanted modes are 30 dB larger than that of the wanted mode after propagating in a 3.2 cm length of the PCF.
In this paper, a hollow elliptical waveguide (HEW) realized SPSM propagation was fabricated and investigated. The elliptical hollow core was introduced and the length of major/minor axis was optimized to cut off X-polarized fundamental mode and high-order modes. Thus, only Y-polarized fundamental mode exists for SPSM propagation. The flexible HEW was fabricated by inner coating silver film on a flexible elliptical polycarbonate (PC) tube. The PC tube was fabricated through glass-draw technique. The HEW realizes SPSM propagation from 66.5 to 114 GHz, and the SPSM performance of the HEW at 100 GHz were verified experimentally. A transmission loss of 2.58 dB/m and polarization ratio of 99.9% at 100 GHz were obtained for the one meter long HEW. The HEW was robust that the loss and polarization-maintaining properties were rarely affected by bending and twisting deformation. Finally, a 10 cm long HEW was used as a polarizer and obtained an extinction ratio of 22.3 dB.
2. Waveguide design and fabrication
A full-vector finite-element method (FEM) was employed to discuss characteristics of HEW. The schematic of the proposed HEW is shown in Fig. 1(a). The X axis was defined as parallel to the major axis and Y axis to the minor axis. The simulated mode fields of TE11X, TE11Y and TE21Y modes are shown and red arrows in the mode field represent the direction of polarization. The X- and Y-polarization are defined according to the direction of polarization. An HEW with 2b = 1.5 mm and 2a = 3 mm (1.5/3-HEW) was simulated as an example to demonstrate loss properties of the HEW. 2a and 2b are the lengths of major and minor axis, and ellipticity η = a/b is 2. Figure 1(b) shows the loss properties of three lowest modes in HEW, TE11X, TE11Y and TE21Y. Logarithmic scales were introduced in the X and Y axes to clearly show the loss properties. As shown in Fig. 1, SPSM band from 2756 to 5030 µm (59.6 to 108.9 GHz) can be realized in the 1.5/3-HEW. It is noted that, after introducing elliptical structure, a large difference of cutoff wavelength λC is generated in the pair of polarized modes TE11X and TE11Y whose loss characteristics are same in a circular waveguide. This is because that the loss characteristics of X- and Y-polarized mode are mainly determined by the length of minor axis and major axis, respectively.
Fig. 1. (a) The schematic of the proposed HEW and the simulated mode fields of TE11X, TE11Y and TE21Y modes. (b) Calculated losses of three lowest modes for a 1.5/3-HEW.
The normalized cutoff wavelength λNC (λC/a) was introduced to discuss the influence of ellipticity. Figure 2(a) shows the λNC for the three lowest modes and Fig. 2(b) is detail of the Fig. 2(a) for ellipticity from 1 to 3. TE11Y is the first order mode. When ellipticity is 1 (Hollow circular core waveguide), the λNC of TE11X and TE11Y are equal. The λNC of TE11X decreases rapidly with the increase of ellipticity. All modes will be eventually cut off as the ellipticity increases. As can be seen, there is little change of the λNC for TE11Y and TE21Y modes with the ellipticity from 1 to 3. It means that, by changing the length of minor axis and keeping the length of major axis unchanged, there is little change of the λC for TE11Y and TE21Y modes. This confirms that the loss characteristics of Y-polarized modes are mainly determined by the length of major axis.
Fig. 2. (a) Normalized cutoff wavelength λNC as a function of ellipticity. (b) Detail of (a) for ellipticity from 1 to 3. The red solid line indicates the SPSM band for 1.5/2.7-HEW (η=1.8). (c) Loss tolerance varies with structural parameters of HEW for SPSM operation at 100 GHz. (d) Simulated losses of each mode for 1.5/2.7-HEW.
It is interesting to find that mode order changes at ellipticity of 1.95. The λNC of TE11X and TE21Y mode are the same at ellipticity of 1.95. With the decrease of ellipticity, TE11X mode becomes the 2nd order mode. It means that TE11X is the 2nd order mode when ellipticity is smaller than 1.95. Otherwise, TE21Y is the 2nd order mode. The conclusion was consistent with the results in Fig. 1 that the TE21Y is the 2nd order mode in the 1.5/3-HEW.
For further investigating SPSM operation at 100 GHz and designing the waveguide, loss tolerance of the HEW varied with structural parameters was calculated and shown in Fig. 2(c). The HEW maintained single TE11Y mode propagation only when the length of major axis 2a is limited to 1.79–3.34 mm. TE11Y mode will be cut off when 2a is smaller than 1.79 mm and TE21Y will be excited when 2a is larger than 3.34 mm. The loss of the HEW (TE11Y mode) increases with the increase of ellipticity. Several loss tolerant curves (3, 5, 10 dB/m) are given.
A 1.5/2.7-HEW (Ellipticity of 1.8) was mainly discussed in this study. As indicated by the red solid line in Fig. 2(b), the SPSM wavelength band for 1.5/2.7-HEW corresponds to λNC from 1.95a to 3.34a, that is, wavelength of 2630 to 4509 µm or frequency of 66.5 to 114 GHz. The 1.5/2.7-HEW was also marked in Fig. 2(c). It locates in the loss area of less than 3 dB/m. The simulation loss is 2.31 dB/m. For 1.5/2.7-HEW, the loss of each mode as a function of wavelength are given in Fig. 2(d). It further confirms that the second order mode is TE11X mode, which is different from 1.5/3-HEW in Fig. 1. The SPSM wavelength band is from 2630 to 4509 µm, which is consistent with the conclusion in Fig. 2(b).
3. Waveguide characterization
Based on the simulation results, a 1.5/2.7-HEW with one-meter length was fabricated. The substrate material of the HEW is polycarbonate (PC). The substrate tube was manufactured by glass-draw technique [25]. By controlling the heating temperature and drawing speed, an elliptical tube was achieved. The silver (Ag) film was inner-coated on the tube through a liquid-phase chemistry process [26,27]. The thickness of the Ag film was around 350 nm, which was much thicker than the skin depth. A no-SPSM 3/5-HEW was also fabricated for comparison. The fabricated HEWs and enlarged cross section of 1.5/2.7-HEW are shown in Fig. 3(a).
Fig. 3. (a) Fabricated 1.5/2.7-HEW and 3/5-HEW and enlarged cross section of 1.5/2.7-HEW. (b) Experimental setup, inset is source WR10 output and its simulated output field. Normalized polar images for (c) WR10 source and (d) 1.5/2.7-HEW output.
3.1 Experimental configuration
The transmission losses and polarization characteristics were experimentally investigated. Figure 3(b) shows the experimental setup. The output end of the 100 GHz source is a standard WR10 rectangular waveguide as shown in inset. Therefore, the output is linearly polarized along short side of the WR10. The simulated mode field is also shown in inset and red arrow on the mode field represents the direction of polarization.
Since the size of WR10 cross section is well matched with that of the 1.5/2.7-HEW, the 100 GHz radiation was directly coupled into the HEW. A rotating wire-grid polarizer, with extinction ratio (ER) about 30 dB, was used to detect the power at various polarization angles. A TPX lens with a focal length of 32 mm was put after the wire-grid polarizer to focus the beam into a power meter. Cutback method was employed to measure transmission loss. The measured HEW was purged with N2 gas to remove air and water vapor.
3.2 Polarization characteristic
Polarization-maintaining ability of the waveguides was evaluated by polarization ratio V
where Pmax and Pmin are the maximum and minimum measured output power by rotating the polarizer for one cycle. The polarization-rotating property of the 100 GHz source and the output of 1.5/2.7-HEW are shown in Fig. 3(c) and (d). The rectangular and elliptical frames represent the shape of output ends for WR10 source and the measured waveguide, respectively. It can be seen that the polarization ratio of 1.5/2.7-HEW output is the same as the source of 99.9%, which shows a good polarization-maintaining performance for the fabricated waveguide. On the other hand, the measured polarization ratio for the 3/5-HEW output was 98.5%, which is lower than the 1.5/2.7-HEW.3.3 Flexible propagation
Bending and twisting for the waveguide are usually unavoidable in flexible propagation [28,29]. Structural deformation may occur since the cross-section of the elliptical waveguide is non-circularly symmetric. Therefore, the robustness of waveguide to the bending and twisting was experimentally characterized.
Loss and polarization properties for waveguide under bending and twisting were measured. In the coupling, the major axis of the measured waveguide was parallel to the long side of WR10 to excite the Y polarization mode. In the bending loss measurement, the 20 cm long input and output ends of the waveguide were kept straight, and the remaining 60 cm waveguide was uniformly bent at various bending angles; In the twisting measurement, the 20 cm long input and output ends were kept straight and untwisted, and the remaining 60 cm waveguide was twisted into different angles.
Figure 4 shows the loss and polarization ratio as a function of bending and twisting angles. Positive and negative angles in Fig. 4(b) represent clockwise and counterclockwise twists. Inset in Fig. 4(b) shows the twisted HEW. The flexible HEW could be bent to 360° or twisted to 180°. Measured loss for straight waveguide was 2.58 dB/m. Bending loss increased from 2.58 to 2.78 dB/m when it was bent to the angle of 360°. The additional loss of bending was as low as 0.2 dB/m. The additional loss of twisting was lower than 0.2 dB/m when it was twisted to the angle of 180°. The polarization ratio was almost constant whether it was bent or twisted. This is because only TE11Y mode exists in the waveguide, and the loss caused by modes coupling in conventional multimode waveguides [30] was greatly weakened. For the same reason, the polarization in the SPSM 1.5/2.7-HEW was rarely affected. These results show that the 1.5/2.7-HEW is robust to bending and twisting at 100 GHz.
Fig. 4. (a) Bend properties of 1.5/2.7-HEW. Inset is the 1.5/2.7-HEW under bending 360°. (b) Twist properties of 1.5/2.7-HEW. Inset indicates the twisted HEW.
3.4 Single-polarization single-mode propagation at 100 GHz
According to the conclusion from Fig. 2(b), the 2nd order mode in 1.5/2.7-HEW is TE11X mode. Thus, when TE11X mode is proved to be cut off, the waveguide realized SPSM operation with single TE11Y mode propagating. The waveguides were placed in several coupling angles θ as shown in Fig. 5(a). The θ is the angle between the waveguide major axis and the source WR10 long side. The red arrow represents the polarization direction of the source.
Fig. 5. Polarization ratio and output power of the HEW for different coupling angles. (a) Coupling angle between waveguide and source, rotating 90° at 15° intervals. Red arrow represents the polarization direction of the WR10 source. (b) Polarization ratio and output power versus coupling angle for 1.5/2.7 and 3/5-HEW at 100 GHz. (c) Polarization ratio and output power versus coupling angle for 1.5/2.7-HEW at 300 GHz.
Figure 5(b) shows the polarization ratio and output power of the 1.5/2.7-HEW and the 3/5-HEW as a function of θ at 100 GHz. As can be seen, the polarization ratio for the 1.5/2.7-HEW remains almost constant. While the polarization ratio for the 3/5-HEW decreases as the θ increases until θ=45°, then it turns to increase. This result indicates that the X-polarized modes can be excited in the 3/5-HEW, but not in the 1.5/2.7-HEW. That is, 3/5-HEW is no-SPSM operation at 100 GHz, while the 1.5/2.7-HEW is, which is consistent with the simulation results shown as points in Fig. 2(b).
The output power for the 1.5/2.7-HEW decreases as the θ increases. When the major axis of the waveguide is perpendicular to the long side of the WR10 source (θ=90°), the output power becomes zero. This is because that the coupling efficiency between the source and the Y-polarized mode decreases as the θ increases, meanwhile, no X-polarized modes can be excited, resulting in a large transmission loss.
The output power for 3/5-HEW decreases slightly with the increase of θ. This means that both X- and Y-polarized modes can be excited. The decrease of output power could be caused by the different loss and coupling efficiency of X- and Y- polarized modes. The simulated losses of TE11X and TE11Y modes at 100 GHZ are 0.75 and 0.65 dB/m, and measured losses of X- and Y- polarized modes are 0.89 and 0.8 dB/m. Thus, the loss difference of X- and Y-polarized modes should be the dominant reason of output power decreasing.
The transmission and polarization characteristics of the 1.5/2.7-HEW at 300 GHz were also measured by using a linearly polarized 300 GHz source with a WR3 waveguide output. Output power of the source is about 15 mW. The above experiments at different θ were carried out again at 300 GHz for 1.5/2.7-HEW. As shown in Fig. 5(c), the polarization ratio decreases with θ when θ < 45° and increases with θ when θ > 45°. The output power slightly increases with θ. This is mainly because that the transmission loss for TE11Y is higher than that of the TE11X at 300 GHz. The simulated losses of TE11X and TE11Y modes are 0.83 and 1.23 dB/m respectively. And measured transmission losses of X- and Y- polarized modes are 0.95 and 1.34 dB/m. The results are similar to that of 3/5-HEW at 100 GHz, which show that the 1.5/2.7-HEW is no longer SPSM operation at 300 GHz frequency.
3.5 Polarizer application
Considering a miniaturized polarizer, the extinction performance of a short 1.5/2.7-HEW at 100 GHz was tested. A 10 cm long 1.5/2.7-HEW was directly coupled to the source WR10. The output power was measured by rotating the waveguide to change the coupling angle. Figure 6 shows output power of the waveguide with rotation angle. The maximum and minimum output power of 85.3 and 0.5 mW were measured when rotation angle was 0 and 90°, respectively. The extinction ratio is 22.3 dB. Inset shows the polar image. This shows that a short 1.5/2.7-HEW could be used as a polarizer with polarization direction along minor axis.
Fig. 6. Extinction performance of a 10 cm long 1.5/2.7-HEW. Inset is the corresponding output polar image.
4. Conclusion
In summary, a flexible metallic waveguide that realized SPSM propagation in a wide millimeter wavelength band was fabricated and characterized. The waveguide is only composted of an elliptical hollow core. The simple structure simplifies the fabricate process, and improves flexibility of the waveguide. The calculation results showed that the fundamental TE11X mode and all high order modes could be cut off by properly designing the length of major and minor axes. After optimization, the hollow elliptical waveguide with a minor/major axis of 1.5/2.7 mm was designed to achieve SPSM propagation around 100 GHz. The waveguide was fabricated by inner coating silver film in an elliptical core polycarbonate tube. Simulations showed that the SPSM propagation range is from 66.5 to 114 GHz. The measured loss of 2.58 dB/m, polarization ratio of 99.9% and additional bending loss as low as 0.2 dB/m at 360 degrees bending were obtained by using a 100 GHz linear polarized source. The experimental results showed that for 1.5/2.7-HEW, the X-polarized mode was cut off at 100 GHz, and only the TE11Y mode existed. While both X- and Y-polarized modes existed for 3/5-HEW at 100 GHz and for 1.5/2.7-HEW at 300 GHz. A 10 cm long 1.5/2.7-HEW can be used as a polarizer at 100 GHz. The minor axis direction is the polarization direction, and the extinction ratio of 22.3 dB was achieved. The discussed SPSM waveguide has potential applications in various millimeter/THz systems, including next generation communication, polarization-sensitive device and imaging.
Funding
National Natural Science Foundation of China (61775060, 61975034); National Defense Science and Technology Innovation Special Zone.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. E. A. Kittlaus, D. Eliyahu, S. Ganji, S. Williams, A. B. Matsko, K. B. Cooper, and S. Forouhar, “A low-noise photonic heterodyne synthesizer and its application to millimeter-wave radar,” Nat. Commun. 12(1), 4397 (2021). [CrossRef]
2. Y. Deng, L. Jing, and Q. Sun, “Traceable Measurements of CW and Pulse Terahertz Power With Terahertz Radiometer,” IEEE J. Sel. Top. Quantum Electron. 23(4), 1–6 (2017). [CrossRef]
3. M. He, J. Zeng, X. Zhang, X. Zhu, and Y. Shi, “Transmission and imaging characteristics of flexible gradually tapered waveguide at 0.3 THz,” Opt. Express 29(6), 8430–8440 (2021). [CrossRef]
4. X. Han, S. Yan, Z. Zang, D. Wei, and C. Du, “Label-free protein detection using terahertz time-domain spectroscopy,” Biomed. Opt. Express 9(3), 994–1005 (2018). [CrossRef]
5. M. Navarro-Cía, M. S. Vitiello, C. M. Bledt, J. E. Melzer, and O. Mitrofanov, “Terahertz wave transmission in flexible polystyrene-lined hollow metallic waveguides for the 2.5-5 THz band,” Opt. Express 21(20), 23748–23755 (2013). [CrossRef]
6. D. Wang, “Guided propagation of terahertz pulses on metal wires,” J. Opt. Soc. Am. B 22(9), 2001–2008 (2005). [CrossRef]
7. M. B. Byrne, M. U. Shaukat, J. E. Cunningham, E. H. Linfield, and A. G. Davies, “Simultaneous measurement of orthogonal components of polarization in a free-space propagating terahertz signal using electro-optic detection,” Appl. Phys. Lett. 98(15), 151104 (2011). [CrossRef]
8. T. Yang, C. Ding, R. W. Ziolkowski, and Y. Guo, “A Terahertz (THz) Single-Polarization-Single-Mode (SPSM) Photonic Crystal Fiber (PCF),” Materials 12(15), 2442 (2019). [CrossRef]
9. X. Zhang, Y. Liu, Z. Wang, J. Yu, and H. Zhang, “LP01-LP11a mode converters based on long-period fiber gratings in a two-mode polarization-maintaining photonic crystal fiber,” Opt. Express 26(6), 7013–7021 (2018). [CrossRef]
10. F. Li, M. He, X. Zhang, M. Chang, Z. Wu, Z. Liu, and H. Chen, “Elliptical As2Se3 filled core ultra-high-nonlinearity and polarization- maintaining photonic crystal fiber with double hexagonal lattice cladding,” Opt. Mater. 79, 137–146 (2018). [CrossRef]
11. A. Aming, M. Uthman, R. Chitaree, W. Mohammed, and B. M. A. Rahman, “Design and Characterization of Porous Core Polarization Maintaining Photonic Crystal Fiber for THz Guidance,” J. Lightwave Technol. 34(23), 5583–5590 (2016). [CrossRef]
12. R. Islam, M. S. Habib, G. K. M. Hasanuzzaman, S. Rana, and M. A. Sadath, “Novel porous fiber based on dual-asymmetry for low-loss polarization maintaining THz wave guidance,” Opt. Lett. 41(3), 440–443 (2016). [CrossRef]
13. Y. Morimoto, H. Matsui, M. Hikita, and T. Ishigure, “Polarization dependence of optical properties of single-mode polymer optical waveguides fabricated under different processes at 1310/1550 nm,” J. Lightwave Technol. 38(14), 3670–3676 (2020). [CrossRef]
14. G. Ren, Y. Gong, P. Shum, X. Yu, J. Hu, G. Wang, M. O. L. Chuen, and V. Paulose, “Low-loss air-core polarization maintaining terahertz fiber,” Opt. Express 16(18), 13593–13598 (2008). [CrossRef]
15. G. Ren, Y. Gong, S. Ping, Y. Xia, and J. Hu, “Polarization Maintaining Air-Core Bandgap Fibers for Terahertz Wave Guiding,” IEEE J. Quantum Electron. 45(5), 506–513 (2009). [CrossRef]
16. R. Vanvincq, A. Habert, K. Cassez, L. Baudelle, and Bigot, “Polarization-maintaining and single-mode large mode area pixelated Bragg fiber,” Opt. Lett. 45(7), 1946–1949 (2020). [CrossRef]
17. Y. Zhang, X. Jing, D. Qiao, and L. Xue, “Rectangular Porous-Core Photonic-Crystal Fiber With Ultra-Low Flattened Dispersion and High Birefringence for Terahertz Transmission,” Front. Phys. 8, 370 (2020). [CrossRef]
18. H. Li, S. Atakaramians, J. Yuan, H. Xiao, W. Wang, Y. Li, B. Wu, and Z. Han, “Terahertz polarization-maintaining subwavelength filters,” Opt. Express 26(20), 25617–25629 (2018). [CrossRef]
19. X. Tang, B. Sun, and Y. Shi, “Design and optimization of low-loss high-birefringence hollow fiber at terahertz frequency,” Opt. Express 19(25), 24967–24979 (2011). [CrossRef]
20. X. Tang, Y. Jiang, B. Sun, J. Chen, X. Zhu, P. Zhou, D. Wu, and Y. Shi, “Elliptical Hollow Fiber With Inner Silver Coating for Linearly Polarized Terahertz Transmission,” IEEE Photonics Technol. Lett. 25(4), 331–334 (2013). [CrossRef]
21. X. Tang, Z. Yu, X. Tu, J. Chen, A. Argyros, B. T. Kuhlmey, and Y. Shi, “Elliptical metallic hollow fiber inner-coated with non-uniform dielectric layer,” Opt. Express 23(17), 22587–22601 (2015). [CrossRef]
22. H. Chen, W. Hui, H. Hou, and D. Chen, “A terahertz single-polarization single-mode photonic crystal fiber with a rectangular array of micro-holes in the core region,” Opt. Commun. 285(18), 3726–3729 (2012). [CrossRef]
23. T. Yang, C. Ding, R. W. Ziolkowski, and J. Guo, “An Epsilon-Near-Zero (ENZ) Based, Ultra-Wide Bandwidth Terahertz Single-Polarization Single-Mode Photonic Crystal Fiber,” J. Lightwave Technol. 39(1), 223–232 (2021). [CrossRef]
24. Y. Hou, F. Fan, H. Zhang, X. Wang, and S. Chang, “Terahertz Single-Polarization Single-Mode Hollow-Core Fiber Based on Index-Matching Coupling,” IEEE Photonics Technol. Lett. 24(8), 637–639 (2012). [CrossRef]
25. Y. Matsuura, R. Kasahara, T. Katagiri, and M. Miyagi, “Hollow infrared fibers fabricated by glass-drawing technique,” Opt. Express 10(12), 488–492 (2002). [CrossRef]
26. U. Gal, J. Harrington, M. Ben-David, C. Bledt, and I. Gannot, “Coherent hollow-core waveguide bundles for thermal imaging,” Appl. Opt. 49(25), 4700–4709 (2010). [CrossRef]
27. B. Sun, Z. Xuan, K. Iwai, M. Miyagi, C. Nan, and Y. Shi, “Experimental investigation on liquid-phase fabrication techniques for multilayer infrared hollow fiber,” Opt. Fiber Technol. 17(4), 281–285 (2011). [CrossRef]
28. K. Nallappan, Y. Cao, G. Xu, H. Guerboukha, C. Nerguizian, and M. Skorobogatiy, “Dispersion-limited versus power-limited terahertz communication links using solid core subwavelength dielectric fibers,” Photonics Res. 8(11), 1757–1775 (2020). [CrossRef]
29. J. Vaes and P. Reynaert, “Dispersion in Millimeter-wave and THz Dielectric Waveguides: Modeling, Measurement and Performance Limitations,” 50th European Microwave Conference (EuMC), 2021.
30. M. S. Vitiello, J. Xu, F. Beltram, A. Tredicucci, O. Mitrofanov, J. A. Harrington, H. E. Beere, and D. A. Ritchie, “Guiding a terahertz quantum cascade laser into a flexible silver-coated waveguide,” J. Appl. Phy. 110(6), 063112 (2011). [CrossRef]
