The Technical Study Of Terahertz Spectrum Detection Based On Static Fourier Transform

As a great part of the electromagnetic spectrum,terahertz is an important areas where classical mechanics transit to quantum mechanics,and the macroscopic electronics to photonics.Different from traditional optics and microwave,terahertz spectrum can easily show the molecules dynamic characteristics,such as the rotation and vibration level of the molecules,the vibration of the lattice and Plasmon.So terahertz spectrum has a great research value in most fields such as atomic physics,molecular chemistry,medical imaging,astronomy,and ground radar detection and so on.It belongs to the category of weak signal detection in time for a target far distance detecting by terahertz.There are two ways to measure the terahertz spectrum: one is the THz-TDS,and the other one is infrared Fourier spectrometer.Both of them work in the lab for fixed plate.So they do not fit for the target far distance detecting outdoor.According to the needs of the project,we study the design ideas,operating mode,data processing method of the infrared Fourier spectrometer in detail.And then the scheme of terahertz spectrum detection based on static Fourier transform is proposed.Based on the scheme,two key technologies for spectrum detection are explanation in detail.One is the parameters design method for the 3D phase grating,and the other one is the synchronous acquisition method for all of the interference points based on sub-aperture partition.In order to achieve the test for the scheme and 3D phase grating,three kinds of work are finished.One is the modeling,calculation,simulation and experiment verification for the loss of terahertz wave atmospheric transmission.Another is modeling,calculation,simulation and experiment verification for the design of the terahertz lenses and beam collimating.The third is the modeling,calculation,simulation and experiment verification for the terahertz transmission and reflection spectrum for typical materials.The main work and achievements in this paper are described below.The first job is attenuation loss calculations for the THZ wave atmospheric transmission.Base on the HITRAN debase,which is provided by Atomic and Molecular Physics Division,Harvard-Smithsonian Center for Astrophysics,the loss of terahertz wave atmospheric transmission is calculation by line-by-line method.Then atmospheric absorption attenuation characteristic curve is draw from 0.1T-1THz.In the paper,we test the atmospheric absorption attenuation by two different ways.We determine the four characteristic absorption peaks,0.38THz?0.44THz?0.55THz?0.61 THz,respectively.The second job is the design of the terahertz lenses and beam collimating.Compared with visible and infrared lasers,terahertz beams are a kind of Gaussian beam with a big divergence angle and its wave front cannot be simplified to a plane or a spherical wave.All the theoretical derivations from the classical electromagnetic theory and ABCD laws support the following conclusions: firstly,a positive lens converges the terahertz beam in front of the focal plane instead of on the focal plane like,which is quite different from the fact that it makes the visible or infrared lasers converge on the focal plate;secondly,though a positive lens used in front of the detector can improve the energy utilization ratio,it cannot enhance the beam collimation significantly;thirdly,a negative lens which matches the radius of the Gaussian beam's wave front is more appropriate for terahertz beam collimation.The third job is terahertz transmission and reflection spectrum for typical materials.Plates made from Al,C,Si,and HDPE(High Density Polyethylene)in different surface roughness are measured.From the test result,we find that Al is the best material for terahertz mirror,Si is the best material for terahertz beam splitter,and HDPE is the best material for terahertz lenses.The fourth but the most important job is the design of the 3D phase grating.Through in-depth study of the principles of the design ideas,operating mode,data processing method for the infrared Fourier spectrometer,we calculate two important parameters of phase grating,successfully.One is cell number,and the other one is the depth for each groove.The Fraunhofer diffraction for the 3D phase grating is calculated.And then influences for each parameter on the diffraction field distribution are simulated.Finally,the best parameter to the 3D phase grating is determined for given condition.Last is the synchronous acquisition method for all of the interference points and experiment verification.Three ways are presented in the paper,theory of lens imaging,telocentric structure and sub-aperture segment.By comparison,we chose sub-aperture segment combined with Fourier transform technique to achieve the terahertz spectrums.In the experiment,the diffraction efficiency of grating and the frequency spectrum of the terahertz source are measured.The measured results and the actual spectrum of the terahertz source make a good agreement.