Research On Terahertz Coherent Measurement Technology

Research on terahertz science and technology is currently a hot topic.As a kind of electromagnetic wave,terahertz has a wavelength between 30-3000 microns.Because of its good coherence,we can use the interference effect to study the thickness and optical parameters of the material.Information,used to measure information such as the amplitude,frequency and phase of terahertz waves.Compared to the optical band,there are many methods and equipment for measuring the optical parameters of materials such as dielectric constant and refractive index,such as ellipsometers,spectrometers,etc.These equipment invert the material by reflecting or transmitting the incident light.Optical parameters.However,the performance of the current terahertz source and detector is far from the performance of the optical band source and detector,so the measurement by reflection or transmission is currently only suitable for measuring the optical parameters of the optical band of the material,in the terahertz band There are still difficulties in realization.Although terahertz time-domain spectrometers based on expensive femtosecond lasers are available,the terahertz source in this measurement system is pulsed rather than a continuous terahertz source.This pulsed terahertz system has a complicated optical path and uses many optical components.The system debugging takes a long time,has low efficiency,and high cost.Therefore,the development of a measurement system based on a continuous terahertz source has significant advantages.In addition,there are many methods for measuring the dielectric constant of materials in the microwave band,such as the rectangular waveguide method and the quasi-optical coupling method.These rectangular waveguide systems and quasi-optical components are large in the microwave band and easy to machine,but in the terahertz band,As the wavelength becomes smaller,the corresponding measurement system component size also decreases accordingly,and the processing accuracy requires more than 10 microns,which brings great difficulties to mechanical processing.Therefore,in the field of terahertz measurement,characterizing material parameters with high efficiency and high accuracy is still one of the difficulties in current research.This paper proposes to build a simple optical path system consisting of a terahertz source,a detector and the material to be measured,and analyze the spatial coupling or interference caused by the terahertz wave in the free path.The following results have been achieved:First,make comparative measurements through different optical path scanning methods,and use Fourier transform to analyze the detector response waveform so that you obtain the frequency properties of the terahertz source;Second,by segmenting the optical path,the dielectric material is inserted at half of the scanning path,and the signal phase jump caused by the inserted material is captured,and then the refractive index of the dielectric material is calculated.The materials such as A4 paper,plastic film and glass sheet were measured.In the final experiment,the refractive index of A4 paper at 216 GHz was 1.632,and the refractive indices of glass sheet and plastic film were 1.549 and 2.04,respectively.The measurement results show that the method of using this coherent measurement system and method to extract the refractive index of the material has an accuracy rate of more than 98%.