Design And Investigation Of Mode-field Converter For Sub-wavelength Terahertz Waveguide

Because terahertz wave in the electromagnetic field has some characteristics that other electromagnetic wave segment cannot match,such as low energy,water absorption,strong penetration,so it has an important application value in the field of medical imaging,chemistry,biology,communications and other fields.So far,many terahertz systems have been built on the basis of terahertz waves transmitted in free space.This is a major constraint on the development of terahertz technology,and the terahertz waveguide can promote the integration of terahertz systems.At the same time,broadband terahertz technology requires the terahertz waveguide to achieve low dispersion,low loss,long range broadband transmission.However,the current terahertz waveguide technology is not mature.In addition,the terahertz waveguide has a wide range of applications,such as improving spectral precision,improving imaging resolution,realizing functional devices and sensor detection.So THz waveguide is an important passive device to develop THz technology.The terahertz waveguide is mainly divided into metal terahertz waveguide and polymer terahertz waveguide according to the materials.In recent years,the sub-wavelength terahertz waveguides in polymer waveguides have single mode transmission.The evanescent field is easy to be coupled between multiple waveguides in the outer air envelope.Most of the waves are transmitted in the air envelope to reduce the absorption loss of the material.The advantages of simple structure and easy manufacture make it widely used in the research field of terahertz waveguide.However,its disadvantages mainly include that the transmission of terahertz wave is susceptible to external environment,such as the interference of water absorption and the clamping stress during fixation.The efficiency of terahertz wave coupling directly from free space into sub-wavelength waveguide is low.This is because the sub-wavelength waveguide due to the small size of the waveguide section,the connection with the terahertz source,detector,etc.is difficult,resulting in increased transmission loss of the system.In order to solve the above problems,the design of the low-loss modular fully enclosed sub-wavelength terahertz waveguide is carriedout,and the validity of the structure is verified by simulation.Further,using terahertz time-domain terahertz spectrometer for sub-wavelength waveguide for actual testing,it is found that the coupling efficiency of wave source and waveguide is low at some frequencies.Then,we deduce the reason of low coupling efficiency,and design the sub-wavelength structure of terahertz waveguide mode converter,using 3D printing technology to make samples,finally the experimental results confirm the mode field of converter can increase the transmission efficiency of sub-wavelength terahertz waveguide 5 times.