M.Koch Humidity and temperature can affect wire bound THz communications

Humidity and temperature can affect wire-bound THz communications

M.Taherkhani1,2, R. A. Sadeghzadeh2, J. Taiber1, J. Ornik1, M. Koch1 1Physics department and material science center, Philipps-Universität Marburg, Marburg, Germany 2K. N. Toosi University of Technology, Tehran, Iran, 1631714191

Abstract—We show that the water vapour density in the atmosphere strongly impacts on the transmission of 120 GHz waves through a cladding-free polypropylene fibre. We attribute this effect to the growth of a water film around the fibre which attenuates the evanescent field. Further measurements show that also the temperature of the polymer fibre is important as the absorption of polymers is temperature dependent.

SHORT-range wireless THz communication is likely to become a mass market in the not too far future [1]. Also, THz transmission over polymer wires may find applications, analogous to the glass fibre-based communications at telecom wavelength. As material losses in the THz range in polymers are higher than the losses in glass fibres at telecom wavelengths communication distances will be restricted to a few ten meters. Consequently, in-car communications, or communications between rooms through a ceiling or a wall are conceivable scenarios for wire-bound THz communications. In fact, first THz communications experiments using bare polymer fibres without a cladding have already been performed [2].

Here, we experimentally investigate how the humidity and temperature of the atmosphere affect the transmission of 120 GHz waves through a cladding-free polypropylene fibre. We find that the transmission though this fibre depends on both, the water vapour density (WVD) in the atmosphere and the temperature of the fibre.

We use a commercial round polypropylene fibre with a diameter of 1.75 mm (+-0.05 mm). To determine the transmission through the fibre we employ a CW microwave system from QuinStar Technology operating at 120 GHz. Both, emitter and receiver have a horn antenna attached, into which we stick the dielectric waveguides for end-butt coupling. A part of the modes propagates in air. Hence, not the material absorption is relevant here, but we have to consider an effective absorption coefficient αeff which we determine from experiments with fibres of different lengths at room temperature and very dry air. From these measurements we can determine αeff to be 0.32 m-1 corresponding to 1.39 dB/m.

We first perform an outdoor experiment during which 80% of the polymer fibre with a total length of 433 cm was placed in the open. This first experiment which was carried out over the course of 27 hours in May 2019 illustrated the problem: the WVD in the atmosphere largely influences the power transmitted through the fibre. To have more defined conditions we used a climate chamber from “Nüve” (modell ID300) which was recently modified to allow for THz measurements [3]. This chamber allows us to precisely control humidity and temperature around the fibre. The length of the polymer fibre inside this chamber is 217 cm.

We performed an experiment over a course of 60 hours monitoring the received intensity for varying temperature and relative humidity. From these data we can calculate the WVD and can plot the received intensity versus WVD. This plot is shown in Fig. 1. The values obtained for temperatures of 22oC and 13oC are shown as open and full dots, respectively. It is obvious that the received power strongly depends on the WVD. We attribute this effect to the growth of a water film around the fibre which attenuates the evanescent field. The thickness of this film depends on the WVD. See [4] for more details.

We performed further measurements to determine how the temperature of the fibre impacts the transmission through the fibre. It is well known that the absorption of polymers increases with temperature. We set the relative humidity in the chamber to 5% to reduce the WVD to the lowest possible value which could be achieve without special measures. We find that the power transmitted through the fibre drops more or less exponentially with temperature. See [4] for more details.

Fig. 1. Transmitted signal as a function of the water vapour density for two different temperatures (full dots correspond to 22 °C, open circles correspond to 13 °C).

REFERENCES [1] T. Kürner and S. Priebe, “Towards THz Communications – Status in Research, Standardization and Regulation”, J. Infrared Millim. Terahertz Waves, vol. 35, pp. 53–62, 2014, [2] K. Nallappan, et al, “High Bitrate Data Transmission Using Polypropylene Fiber in Terahertz Frequency Range”, Int. Workshop on Antenna Technology (IWAT), Miami, USA, 2019. [3] J. Ornik, S. Sommer, R. Gente, J.C. Balzer, K. Fey, T. Pillich, M. Koch, „THz Spectroscopy Inside a Climate Chamber“, 43rd International Conference on Infrared, Millimeter, and Terahertz Waves, Nagoya, 2018. [4] M. Taherkhani et al., “The Effect of Humidity and Temperature on Dielectric Fibre–Bound THz Transmission”, J. Infrared Millim. Terahertz Waves, vol. 40, pp. 1092-1102, 2019,

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