太赫兹宽带分谐波混频器设计

太赫兹分谐波混频器的变频损耗、噪声系数等指标与基波混频器相近,且本振频率为射频频率的一半,大大 降低了本振源的设计难度和制作成本,是高性能太赫兹接收前端的关键部件。本文介绍了一种覆盖全波导带宽的太赫 兹宽带分谐波混频器的设计,对电路中射频波导至悬置带线过渡结构和本振中频双工器进行仿真和优化设计。并以 0.14~0.22THz 分谐波混频器为例进行设计和制作,测试结果表明0.14 ~0.22THz 分谐波混频器在全波导频段内最大变频 损耗低于15dB,中频3dB 带宽大于20GHz。     

Enhancement of CW THz Wave Power Under Noncollinear Phase-Matching Conditions in Difference Frequency Generation

We investigated methods for improvement of continuous-wave (CW) terahertz (THz) output power by laser diode (LD) pumping in noncollinear phase-matched difference frequency generation (DFG). The effects of interaction length and beam spot size of input lasers (near-IR) in GaP crystals were studied. The THz wave power dependence on various sizes of GaP crystals was investigated and it was observed that an output power of 4 nW was obtained with a 20 mm long GaP crystal at 1.5 THz. Also, the THz wave absorption coefficient was dominant for longer GaP crystals at high frequencies (above 2.5 THz). The THz wave power dependence on beam spot size (1.2 mm–300 $mu{rm m}$) of near-IR lasers at 1.62 THz was studied, and an improvement of THz wave power being seen with a 500 $mu{rm m}$ beam spot size, while the beam divergence effect was dominant for beam spot sizes below 500 $mu{rm m}$.      

Terahertz 2-D Photonic Crystal Waveguides

A 2-D photonic crystal waveguide with strong passbands in the THz frequency range is demonstrated. The 2-D metallic photonic crystal waveguide structure is bounded by parallel metal plates in the vertical direction and by a square lattice of metallized cylinders on the horizontal axis. The attenuation of the photonic crystal waveguide is experimentally determined and compared to the fundamental mode of a comparably sized rectangular waveguide. The measured dispersion characteristics of the waveguide are discussed, and the output is compared to a finite-difference time-domain simulation.     

0.41 THz折叠波导慢波结构分析与设计

在0.14 THz,0.22 THz和0.34 THz折叠波导行波管研制的基础上,讨论了0.41 THz折叠波导行波管慢波结构设计与加工的可行性,分析研究了折叠波导慢波结构弯曲处直角弯曲与半圈弯曲、方形电子注通道与圆形电子注通道对色散特性、耦合阻抗、带宽、冷损耗和增益的影响。考虑了慢波结构中增加理想衰减器对该行波管带宽和增益的影响,得到了0.41 THz折叠波导行波管慢波结构的初步设计方案,为太赫兹折叠波导行波管的继续发展打下了一定基础。     

Two-Directional CW THz Wave Generation System by Pumping With a Single Fiber Amplifier of Near-IR Lasers

We constructed a two-directional continuous-wave (CW) terahertz (THz) wave generation system with laser diode pumping. The generation method is based on difference frequency generation (DFG) of two near-infrared (NIR) lasers by excitation of phonon polaritons in a GaP crystal. The two NIR lasers for DFG are amplified simultaneously using one ytterbium-doped fiber amplifier (FA). Efficient beam overlapping of the NIR lasers was achieved on the GaP crystal surface. This system consists of a simple optical design with a single FA. Narrow linewidth CW THz waves, which are generated in two directions, can be applied to high-resolution spectroscopy.      

Low-loss Transmission Lines for High-power Terahertz Radiation

Applications of high-power Terahertz (THz) sources require low-loss transmission lines to minimize loss, prevent overheating and preserve the purity of the transmission mode. Concepts for THz transmission lines are reviewed with special emphasis on overmoded, metallic, corrugated transmission lines. Using the fundamental HE₁₁ mode, these transmission lines have been successfully implemented with very low-loss at high average power levels on plasma heating experiments and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. Loss in these lines occurs directly, due to ohmic loss in the fundamental mode, and indirectly, due to mode conversion into high order modes whose ohmic loss increases as the square of the mode index. An analytic expression is derived for ohmic loss in the modes of a corrugated, metallic waveguide, including loss on both the waveguide inner surfaces and grooves. Simulations of loss with the numerical code HFSS are in good agreement with the analytic expression. Experimental tests were conducted to determine the loss of the HE₁₁ mode in a 19 mm diameter, helically-tapped, three meter long brass waveguide with a design frequency of 330 GHz. The measured loss at 250 GHz was 0.029 ± 0.009 dB/m using a vector network analyzer approach and 0.047 ± 0.01 dB/m using a radiometer. The experimental results are in reasonable agreement with theory. These values of loss, amounting to about 1% or less per meter, are acceptable for the DNP NMR application. Loss in a practical transmission line may be much higher than the loss calculated for the HE₁₁ mode due to mode conversion to higher order modes caused by waveguide imperfections or miter bends.     

太赫兹辐射源与波导耦合研究新进展

对国际上关于半导体太赫兹辐射源和太赫兹传输波导之间的耦合问题的研究进展和最新动态进行了较详细的分析和归纳总结.太赫兹频段是一个非常有科学价值电磁波频段,它在信息科学,天文学、医学、环境科学、国防等领域都有广阔的应用前景和应用价值.太赫兹的应用除了太赫兹辐射源,太赫兹传输器件,太赫兹探测器等关键器件之外,还必须解决太赫兹辐射源和太赫兹传输器件之间的耦合问题.     

Widely Frequency-Tunable Terahertz Wave Generation and Spectroscopic Application

The THz-wave generator based on the resonance of phonon-polaritons in GaP enabled THz spectral measurements of organic molecules like DNA/RNA-related molecules in wide frequency range (0.6–5.8 THz). High spectral purity of the generated THz-wave enables even the detection of defect structures of organic compounds, and will give a new tool for molecular sciences. We demonstrated THz imaging of a liver cancer. The THz-wave generator can be made small-size, light-weight system.     

折叠波导结构的THz振荡辐射源研究

 以折叠波导为基本结构的真空电子器件,具有大功率、宽频带和制造成本低的优点,本文通过深入研究折叠波导振荡器的冷腔结构、色散方程和耦合阻抗,给出了可以产生THz的折叠波导振荡器的基本结构.并利用三维PIC粒子模拟软件进行了三维模拟计算,结果显示,在输入电子注电压为20.6kV,电流为0.12A,磁场为3000高斯时,可得到频率为0.1THz,平均输出功率174W,互作用效率大于7%.     

Characterizing the beam properties of terahertz quantum-cascade lasers

Terahertz quantum-cascade lasers (QCLs) are very promising radiation sources for many scientific and commercial applications. Shaping and characterizing the beam profile of a QCL is crucial for any of these applications. Usually the beam profile should be as close as possible to a fundamental Gaussian TEM₀₀ mode. In order to completely characterize the laser beam the power and the wavefront have to be measured. We describe methods for characterizing the beam properties of QCLs. Several QCLs with single-plasmon waveguide and emission frequencies between 2 and 5 THz are investigated. The beam profiles of these lasers are shaped into almost fundamental Gaussian modes using dedicated lenses. The beam propagation factor M² is as low as 1.2. The wavefront is measured along the axis of propagation with a THz Hartmann sensor. Its curvature behaves as expected for a Gaussian beam. The applied methods can be transferred to any other THz beam.     

太赫兹矩形波导与共面波导耦合结构设计

太赫兹（THz）波位于微波与红外光波之间,现有微波和光波段波导技术应用正在向THz波段拓展。但是,由于水汽对THz波的强吸收及制造工艺等原因,THz器件主要是平面结构,而THz源及其传输需要用矩形波导。因此,矩形波导与共面波导之间的转换结构成为决定元件和系统性能的关键部分。该设计利用脊波导进行阻抗匹配及电磁场模式转换,实现THz波矩形波导到共面波导的高效率耦合。结果表明,在0.2~0.4 THz频段内,该转换结构的传输系数(S₂₁)高于?3 dB,可以对THz电磁场进行高效率转换。该结果可用于太赫兹分子探测、太赫兹通信等领域,为0.2 THz以上太赫兹的模式转换提供了一种可行方案。     

0.22 THz Waveguide Bandpass Filter with High Stop-Band Suppression

A terahertz (THz) waveguide band-pass filter using an iris inductive window coupled structure was designed and fabricated. The filter was designed at 0.22 THz with a pass band of 20 GHz. The measured results show that the center frequency is 0.218 THz with a pass band of 0.205 THz to 0.231 THz, the minimum insertion loss is 1.26 dB at 0.224 THz, and the return loss is less than 13.1 dB. The stop-band suppression is 65.6 dB at 0.193 THz and 51.8 dB at 0.243 THz, respectively, which means a good performance of high stop-band suppression. A good agreement exists between the measured S-parameters and the simulated ones, especially in the upper band. The proposed THz waveguide filter has potential applications in THz communications.     

太赫兹过模传输线损耗研究

太赫兹波的频率较高,在波导传输中损耗较大。本文采用微扰法对太赫兹波在矩形波导和圆波导中传输时的损耗进行了理论分析,并对矩形波导与圆波导中不同频率,不同尺寸以及不同模式的衰减特性进行了比较,结果表明采用圆波导TEo。模传输可获得最小的衰减。     

Raman spectra of GaPAs–GaP heterostructure waveguides

A GaPAs waveguide with GaP cladding layers, has been fabricated in only two steps of LPE growth. One-way optical loss of the fabricated GaPAs–GaP waveguide is 12% mm⁻¹ (α=1.9 cm⁻¹). The free carrier concentration of the GaPAs layer is 1×10¹⁶ cm⁻³ (α=0.05 cm⁻¹). The backward spontaneous Raman spectrum from the GaPAs waveguide shows a GaP-like longitudinal optical (LO) phonon line. The LO band is intensified and shifted to a higher frequency compared to the LO phonon of GaP bulk crystal.     

High-resolution GaP Terahertz Spectrometer and Its Application to Detect Defects in Gamma-irradiated Glucose Crystal

We developed high-resolution GaP THz signal generator using Cr:Forsterite lasers with gratings as both a pump and a signal beam for difference-frequency generation. A line width of less than 500 MHz and a wide tunable frequency range (0.6–6.2 THz) provide sufficient resolution for measuring materials with sharp absorption bands using the generator as the light source for a THz spectrometer. This is suitable for materials such as gases or solid samples at low temperatures. We demonstrated the detection of defects in organic materials, as they appear as slight deviations in the absorption frequency in the THz region.     

基于光子晶体多层膜的对称薄膜波导特性研究

研究了一种“光子晶体多层膜+波导层+光子晶体多层膜”对称薄膜波导特性,光子晶体多层膜的第一禁带频率范围为73 THz～99 THz。采用多层介质平板波导理论研究了频率在73 THz～99 THz间的电磁波在波导中的传输特性。结果表明波导传输的是TE0和TM0基模,对于高次模式,不能在波导中传输,另外,频率在73 THz～99 THz间的电磁波在波导层（中心层）的功率约束因子（Γ）在0.99～1之间,即此时电磁波几乎完全局限在波导层内传输,为了比较,处在光子带隙外的频率分别为40 THz和50 THz的电磁波在波导层内的功率约束因子（Γ）分别为0.84和0.86,因此,利用光子带隙特征,由光子晶体多层膜构造的对称薄膜波导具有超低损耗特性。     

Continuous-Wave Frequency-Tunable Terahertz-Wave Generation From GaP

Continuous-wave (CW) single-frequency terahertz (THz) waves were generated using difference-frequency generation via excitation of phonon–polaritons in GaP. The two pump sources were an external cavity laser diode (LD) and an LD-pumped Nd : YAG laser. The power density of the latter beam was enhanced by using a ytterbium-doped fiber amplifier. The two incident beams were focused to near the wavelength of THz waves. This optical alignment enabled us to generate frequency-tunable CW THz waves in the 0.69–2.74 THz range. With a fixed angle between the pump beams, we obtained a frequency bandwidth as large as 600 GHz.     

THz QCL波导结构研究与光束质量分析

THz量子级联激光器是理想的固态THz源,研究波导结构对激射光特性和远场光束质量的影响,是THz QCL设计中的关键。本文采用有限元方法对THz QCL双面和单面金属波导结构的限制和损耗特性进行分析,给出了限制因子、波导损耗和阈值增益随波导结构、激射波长等参数的变化关系。仿真实验结果表明:与单面金属波导相比,双面金属波导对光具有更好的限制作用,损耗也比较小,更适合做有源区的波导限制结构。在计算出波导中光场分布的基础上,又利用矢量衍射理论分析了THz QCL的光束质量,给出了不同波导宽度时出射光束的远场光束宽度和远场发散角,从应用方面为QCL的设计提供了一定参考。     

微加工工艺误差对THz折叠波导行波管性能影响

在太赫兹频段,折叠波导慢波结构主要采用微细加工技术完成。讨论了目前折叠波导慢波结构主要的微加工工艺,分析了主要工艺误差包括波导深度、侧壁垂直度对0.41 THz折叠波导慢波结构高频特性的影响。通过分析比较,a值对折叠波导行波管性能影响很大,需要在工艺中精确控制。在侧壁垂直度为89°范围以内,侧壁垂直度的变化对折叠波导行波管性能影响不大。通过仿真分析,确定了工艺中必须控制加工精确度的工艺步骤,这对0.41 THz折叠波导行波管的研制有非常重要的意义。     

GaP,GaAs和PPLN晶体级联差频产生太赫兹辐射

研究周期极化磷化镓晶体(GaP)、砷化镓晶体(GaAs)和周期极化铌酸锂晶体(PPLN)准相位匹配级联差频产生太赫兹辐射,相较于差频过程,级联过程太赫兹辐射输出功率增大9.5倍。通过分析三波耦合方程,计算并比较晶体的波矢失配量、极化周期和太赫兹功率,结果显示,基于GaP晶体产生的太赫兹功率略大于GaAs晶体输出的功率;GaAs晶体的极化周期最小;PPLN晶体的波矢失配量和极化周期取值范围最小,而输出的太赫兹功率和转换效率最高。建立基于周期极化掺氧化镁铌酸锂晶体(MgO:PPLN)准相位匹配原理的宽调谐激光系统,分析吸收因子对输出太赫兹功率的影响,计算级联差频峰值功率和转换效率。十五阶峰值功率3.72 MW,泵浦光总能量到太赫兹辐射能量的转换效率是3.72%。