基于科赫曲线分形结构太赫兹带通滤波器

Terahertz Bandpass Filter Based on Koch Curve Fractal Structure

太赫兹滤波器是太赫兹通信、太赫兹成像和太赫兹检测等太赫兹应用系统中不可或缺的功能器件。按照不同的分类方式,滤波器有不同的种类,常见的按照选频功能可分为高通滤波器、低通滤波器、带阻滤波器和带通滤波器。为了实现在太赫兹波段的滤波效果,世界各地的研究人员利用不同的结构、材料和控制方式实现了功能各异的太赫兹滤波器,但是考虑到设计的器件要应用到太赫兹系统中,成本低廉、结构简单、性能优越的太赫兹滤波器一直是研究人员的追求。分形概念自提出以来在很多研究领域都有了快速发展,但是在太赫兹波段的应用还不是很常见,特别是应用于太赫兹功能器件的设计。引入分形中科赫曲线的概念设计并制备了一种新型的太赫兹带通滤波器,该滤波器是在金属薄膜上刻蚀出科赫曲线分形结构,当太赫兹波垂直入射到该滤波器时候实现了在太赫兹波段的窄带滤波。在滤波器的设计过程中,追求理论与实验相结合,首先在电磁仿真软件中建立科赫曲线分形结构滤波器模型进行计算,探究分形结构应用于太赫兹波段进行滤波的可行性,在进行多次计算之后得到优化后的尺寸和结构,然后根据优化后的尺寸加工出科赫曲线分形结构太赫兹滤波器样品,并且将样品放在太赫兹时域光谱系统中进行实验测量,得到实验数据后与仿真结果进行比较。在仿真中利用了时域有限差分法模拟科赫曲线分形结构太赫兹带通滤波器的传输特性,优化后的仿真结果表明:滤波器的谐振频率为0.715 THz,透射系数能够达到0.92,-3 dB带宽为21.9 GHz,将仿真得到的散射参数进行S参数反演得到了太赫兹滤波器样品的电磁参数,这在理论上分析了太赫兹波在谐振点处产生透射增强的原因。利用飞秒激光微加工系统制备了尺寸优化后的科赫曲线分形结构太赫兹带通滤波器样品,然后使用太赫兹时域光谱系统对样品的传输特性进行测试,对实验得到的时域数据进行快速傅里叶变换之后得到频域数据,再把频域数据进行归一化处理后与之前的电磁仿真结果进行对比,发现实验测得的结果与电磁软件仿真得到的结果较为吻合。

Terahertz filter is an indispensable functional device in terahertz communication,terahertz imaging and terahertz detection systems.According to different classification methods,there are different kinds of filters.Common filters can be divided into high-pass filter,low-pass filter,band-stop filter and band-pass filter according to frequency selection function.In order to achieve the filtering effect in terahertz band,researchers around the world use different structures,materials and control methods to achieve terahertz filters with different functions.However,considering that the designed devices are to be applied to terahertz system,the low cost,simple structure and superior performance of terahertz filters have always been the pursuit of researchers.Fractal concept has developed rapidly in many research fields since it was proposed,but its application in terahertz band is not very common,especially in the design of terahertz functional devices.A novel terahertz bandpass filter is designed and fabricated by introducing the concept of Koch curve in fractal.The filter etches the fractal structure of Koch curve on the metal film.When the terahertz wave is incident perpendicularly to the filter,the narrowband filter in terahertz band is realized.In the process of filter design,the combination of theory and experiment is pursued.Firstly,the Koch curve fractal structure filter model is established in electromagnetic simulation software to calculate,and the feasibility of applying fractal structure to terahertz band filtering is explored.After many calculations,the optimized size and structure are obtained.Then,the Koch curve fractal structure is processed according to the optimized size.Sample of terahertz filter is constructed and measured in terahertz time domain spectroscopy system.The experimental data are obtained and compared with the simulation results.The finite difference time domain method is used to simulate the transmission characteristics of THz bandpass filter with Koch curve fractal structure.The optimized simulation results show that the resonant frequency of the filter is 0.715 THz,the transmission coefficient can reach 0.92,and the bandwidth of-3 dB is 21.9 GHz.The electromagnetic parameters of the THz filter sample are obtained by inversion of the simulated scattering parameters with S parameters,which is analyzed theoretically in the reason of transmission enhancement of terahertz wave at resonance point.Sample of THz bandpass filter with fractal structure of Koch curve optimized is fabricatedby femtosecond laser microfabrication system.The transmission characteristics of the sample are tested by THz time domain spectroscopy system.Frequency domain data are obtained after fast Fourier transform of the experimental time domain data.Frequency domain data are normalized and combined with previous electromagnetic simulation results.By comparing the results,it is found that the experimental results are in good agreement with those obtained by electromagnetic software simulation.