Simulation And Experiment Of Terahertz Digital Holography Based On Quantum Cascade Laser

This paper consists of part work of "National Major Instrument Development Project" which the author has participated in.All the experiment is carried out in Research Centre of Laser Fusion,China Academy of Engineering Physics,including measurement and improvement of beam profile for terahertz quantum cascade laser,continuous-wave terahertz in-line and off-axis digital holography.Besides,Pro.Wang in Beijing University of Technology is responsible for reconstruction of holographic imaging.In order to improve the resolution of terahertz digital holography,and apply this technology to the study of cell transplantation security,the author set up an experimental system of holographic imaging with high resolution,digital and portability.It could be also used in the probing field of optical properties,dielectric constant,thickness and density of space distribution of film(diamond film,solar film)and ceramic sample.Simulation and adjustment of the whole system is carried out by the author.The validity and reliability of the transmission system has been verified using THz imager step by step.Terahertz Quantum Cascade Laser in continuous wave mode is utilized in our experiment.As the divergence of the laser beam generated by Quantum Cascade Laser is relative large,which is not conducive to the process of experiment and the acquisition of the desirable results.Considering this,the author utilized convex lens to optimize the laser beam.Before this,Zemax is used to simulate.Beam profile and irradiance distribution at different plane is obtained along the propagation in simulation and experiment.It tends to be that the results in simulation and experiment are well matched.They prove that beam quality has been enhanced a lot due to the aim of convex lens.What's more,characteristic parameters of the focused and collimated laser beam have been interpreted in detail as well.On this basis,the author established the off-axis digital holographic imaging system.To validate the correctness of this optical system,holograms with no object located in the object beam path were recorded sequentially in air and vacuum conditions.Comparison between the two holograms illustrated that hologram retrieved in air exhibits high quality and contrast.In addition to the off-axis holographic imaging system,the author also established in-line digital holographic imaging system.The extrapolation method with a modified padding was proposed to successfully to simultaneously reconstruct the absorption and phase-shifting distributions of various objects from this single in-line terahertz hologram.Our experiment represents an important step towards using quantum cascade laser in terahertz in-line digital holography as a complementary imaging technique.