A terahertz polymer waveguide containing a cylindrical dielectric support tube, the support tube is made of a polymer material, another tube of the same material, the walls of which have at least three equidistant semicircular corrugations in the direction longitudinal to the central axis, is inserted inside the support tube, and the pointed parts of the boundary between two adjacent corrugations are tightly abutted to the inner surface of the pipe, where the corrugated shape of the pipe walls creates a waveguide core in its inner part with a boundary, which is a surface with negative curvature, characterized in that the waveguide has an external coating of nylon, a material that does not transmit THz radiation, while the geometric dimensions of the waveguide in cross–section are selected in such a way that a single mode is formed inside the waveguide in the region of the central axis, which provides low losses and dispersion for the THz signal along the length transfers.

The invention relates to the field of terahertz optics, in particular to means of waveguide transmission of THz radiation, and can be used in such areas as information transmission at THz frequencies, express methods for analyzing gases, including atmospheric pollution or the composition of human exhaled air, which can be used to assess the state of health, safety and passenger screening in transport as a tool for the identification of prohibited substances.

The invention relates to waveguide methods for transmitting electromagnetic radiation in the frequency range 0.1-10 THz, which occupies an intermediate position between the microwave and infrared regions of the electromagnetic wave scale. Due to a number of specific properties of THz, the range is attractive for conducting a wide range of fundamental and applied research in the field of physics and chemistry.

The main requirements for practical applications of THz waveguides are maximum radiation transmission efficiency over a wide spectral range, low dispersion for a THz pulse over the transmission length, and flexibility (low bending losses of radiation power, mechanical possibility of reversible bending).

The fundamental problem in the development of THz waveguides is the lack of sufficiently transparent elastic materials for this spectral range. Therefore, waveguide designs are the most promising, where the maximum fraction of radiation is transmitted in air (or another gas), and the minimum fraction is distributed in a solid material that restricts the waveguide channel.

Hollow-core waveguides using the photonic crystal concept to hold and transmit radiation through a waveguide air core are promising. The waveguide properties in this case are due to the fact that the reflecting shell in cross-section is a regular two-dimensional periodic structure (analogous to a Bragg lattice) with a period of the order of the radiation wavelength. The scattering of radiation on the periodic structure creates exclusion conditions for the wavelengths corresponding to the transparency windows of the waveguide. As a result, localized radiation modes are formed inside the central air region, which propagate along the air core and cannot leave it.

A porous THc-FVC is known, which is a cylinder made of polymer, parallel to the central axis, which has longitudinal air channels that form a hexagonal porous array of holes in the cross section. The diameter of the channels and the distances between them are less than the wavelength of THz radiation. The meaning of the periodic air-polymer structure is that the resulting effective refractive index is formed within the hexagonal array, which allows the transmission of appropriate radiation modes along this array [A. Hassani, A. Dupuis, M. Skorobogatiy. Low loss porous terahertz fibers containing multiple subwavelength holes. – Applied Physics Letters 92, 071101 (2008)].

A photonic crystal waveguide (PCM) with a hollow core is known, where the reflective shell consists of alternating layers of polymers with different refractive indices [US patent US 2009/0097809 A1, published 04/16/2009].

The disadvantage of such waveguides is the narrow bandwidth and rather large loss of THz radiation power at the output due to the strong absorption of THz radiation in the polymers used, as well as due to the propagation of the transmitted field over several reflective layers in a structured shell, where the radiation is absorbed.

A sapphire terahertz photonic crystal waveguide is known, which is a dielectric body made of monocrystalline sapphire, in which there are channels parallel to the central longitudinal axis arranged in a hexagonal structure, while the minimum cross-section size of the waveguide channels is equal to or greater than the wavelength of the transmitted THz radiation. The use of monocrystalline sapphire as a waveguide material made it possible to reduce the power loss of transmitted THz radiation, since sapphire has a significantly lower absorption of THz electromagnetic radiation compared to polymer media and glasses [RF Patent No. 2601770 C1 publ.10.11.2016].

The disadvantage of such a waveguide is its short length (several tens of cm), lack of physical bending capability for practical applications of delivering radiation to hard-to-reach places, and relative high cost.

The closest in technical essence to the claimed invention is a microstructured fiber optic fiber with a hollow core for localization and transmission of high optical power radiation in the spectral frequency range from visible to infrared radiation [RF Patent No. 2563555 C1 publ.20.09.2015].

The features of the analogue, which coincide with the essential features of the claimed invention, are: the presence of a support tube made of a dielectric material; the presence of a waveguide hollow core formed by an area near the axis of the waveguide, while the boundaries of the core have a surface with negative curvature and an external protective coating.

However, this waveguide does not contain structural elements in the form of thin-walled tubes made of dielectric material attached to the inner surface of the support tube, which can either touch each other or be located separately. In addition, the technical result of the prototype is to localize high-power optical radiation in the spectral frequency range from visible to infrared radiation, while the purpose of the claimed invention relates to the THz region of the spectrum, where the wavelength is significantly longer, which imposes new restrictions on the diameter of the waveguide.

Thus, the closest analogue of the claimed invention is a device with a hollow waveguide core surrounded by a reflective shell in the form of a single layer of cylindrical capillaries in the amount of three or more.

This type of construction was first called waveguides with a negative curvature of the core –shell boundary [A.D. Pryamikov, A.S. Biriukov, A.F. Kosolapov, V.G. Plotnichenko et.al.Demonstration of a waveguide regime for a silicon hollow — core microstructured optical fiber with a negative curvature of the core boundary in the spectral region > 3.5 µm. Opt.Express. 2011/ 19. 1441-1448]

The fundamental difference between this approach and known devices using the concept of a photonic crystal as a reflecting shell is that the waveguide properties are formed by the reflection of radiation from the first core–shell interface. The defining parameters that determine the waveguide properties and spectral transmission range are the diameter of the hollow waveguide core, the wall thickness of the capillaries, the degree of negative curvature of the interface, the THz refraction of the material, as well as the stability of these parameters along the length of the waveguide. In particular, an increase in the diameter of the waveguide cavity, a decrease in the thickness of the capillary walls, and an increase in the degree of negative curvature of the core-shell boundary each individually leads to a decrease in radiation losses. However, these parameters are interconnected with each other, limited by their external size and mechanical strength. For example, in an analog design where the shell consists of 8 capillaries, an increase in the diameter of the hollow core will lead to an increase in the diameter of the capillaries, which will eventually increase the overall diameter of the waveguide and, consequently, impair flexibility. On the other hand, in order to increase the negative curvature of the core–shell boundary, it is necessary to reduce the number of capillaries, which will lead to a decrease in the diameter of the waveguide cavity. Obviously, an optimal ratio between these three parameters is required to achieve an acceptable efficiency of transmitting THz radiation by a waveguide.

The direct transfer of the waveguide configuration with a negative curvature of the core-shell boundary of the THz band faces problems, the main of which is the significant diameter of the fiber cross–section (several cm, since the wavelength of THz radiation has a cm scale), which negatively affects its flexibility. In addition, the most acceptable dielectric materials for the THz band in terms of optical transparency are polymers (polyethylene, polypropylene, polymethylpentene). The manufacture of waveguides with a negative curvature of the core–shell boundary from polymers is associated with even greater difficulties compared to the case of their manufacture from glasses. They are mainly related to maintaining the stability of the geometric shape and cross-sectional dimensions along the length of the waveguide, which negatively affects its waveguide properties.

The objective of the invention is to create a THz waveguide with a hollow core and a single–row reflective shell that provides an increase in the diameter of the hollow waveguide core at a fixed value of the total diameter of the waveguide, which ultimately leads to an increase in the flexibility of the waveguide and to a decrease in the specific losses of THz radiation transmitted by it.