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Terahertz (THz) transmission simulations play an important role in THz technology researches, especially for the structural design of a THz waveguide. Ray model takes into account both structure parameter of waveguide and the divergence angle of beam light and could be an alternative way for THz transmission behavior simulations. In this paper, the ray model is used to calculate the transmission loss of tube waveguide, and the simulated transmission losses are presented to compare with the results calculated by COMSOL. The suitable THz frequency range of ray model is discussed by analyzing the transmission loss spectra of tube waveguides with various core sizes. The credibility of ray model on terahertz transmission simulations is demonstrated based on the experimental results tested by THz-TDS and calculated results.

In recent years, terahertz (THz) waveguides and fibers have been extensively investigated. The dry air is demonstrated to be the most transparent medium in the THz region and can be used for THz transmission. Many types of THz waveguides are designed to transmit THz radiations with dry air medium, including photonic crystal fibers [_{1}) with inner diameter D and a thin cladding (refractive index n_{2}) with thickness t.

Before performing the fabrication procedure of a tube waveguide, the structure of the waveguide needs to be carefully designed to obtain a low transmission loss in theory. The simulation software COMSOL Multiphysics (hereafter referred to as COMSOL), which based on Finite Element Method (FEM), is widely used to the theoretical analysis of the THz transmission. The simulation needs the parameter of D, t, n_{1}, and n_{2}. And the parameter of transmission, including modal distribution, transmission loss in different modes, effective refractive index, and group velocity dispersion, can be obtained with the COMSOL. However, in the practical THz transmission, the transmission loss depends on not only the structure but also the divergence angle of THz beam. And the transmission loss is a combination of transmission losses in multiple modes when the THz radiation is multi-mode transmission.

The ray model [

The transmission loss spectra are calculated by ray model and COMSOL, respectively. The D, t, n_{1}, and n_{2} are 4 mm, 0.2 mm, 1, and 1.4. And the divergence angle of THz beam is 6˚ for ray model. As can be seen in

loss curve calculated by ray model is the black line, and there are four transmission loss curves calculated by COMSOL depending on the four lowest modes (HE_{11}, HE_{21}, TM_{01}, and TE_{01}). In the 0.8 - 2 THz, the transmission losses calculated by ray model and COMSOL are approximated. But when the frequency is lower than 0.65 THz, the transmission loss curves calculated by two methods are divergent. Because the transmission wavelength is close to the inner diameter of waveguide, and the ray model is no longer applicable. Thus the applicable frequency range of the ray model has a lower limit (called limit frequency) for a certain inner diameter of waveguide.

To research the relationship between the limit frequency of ray model and the inner diameter of waveguide, the transmission loss spectra from four tube waveguides, with different inner diameter, are calculated. The D is 2 mm, 4 mm, 6 mm, and 8 mm, respectively, and other parameters are not change. And for convenience, only the fundamental model (HE_{11}) is given in the calculation result of COMSOL. In

In order to clearly indicate the relationship between the limit transmission wavelength and the inner diameter. The limit frequency is translated into limit transmission wavelength. As can be seen in

To prove the applicability of ray model, a Teflon (refractive index 1.4) tube is measured by THz time-domain spectroscopy (THz-TDS) system. The practical D, t, n_{1}, and n_{2} are 3.4 mm, 0.16 mm, 1, and 1.4. And the practical divergence angle of THz beam is 36˚.

and the calculation from ray model. The curves between experiment and calculation are approximately consistent in 0.4 - 3 THz.

In this paper we use ray model to calculation the transmission loss of tube waveguide, and the transmission loss curves are presented to compare with the results calculated by COMSOL. In a range of region, the ray model is an alternative way for the calculation of THz transmission loss. Moreover, the relationship between limit transmission wavelength and inner diameter of waveguide is determined to λ = 0.092D. The experiments testify that the result calculated by ray model is credible.

The authors acknowledge the financial support provided within National Natural Science Foundation of China (NSFC) (61775060, 61275100, and 61761136006).

The authors declare no conflicts of interest regarding the publication of this paper.

Zhong, Y., Lu, X.H., Xie, G.X. and Jing, C.B. (2018) Application of Ray Model for Terahertz Transmission Simulations of Tube Waveguide. Journal of Computer and Communications, 6, 143-147. https://doi.org/10.4236/jcc.2018.612014