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脊波导第一脊到波导底壁距离d1直接影响转换结构的传输性能。对d1
分别取25、35、45、55 μm进行仿真分析。其余结构参数设定如下:第一脊长度L1=145 μm,第二脊长度L2=140 μm,第三脊长度L3=240 μm,第二脊距底壁高度d2=70 μm,第三脊距底壁高度d3=180 μm,脊厚度th=140 μm。 在240~350 GHz频率内,转换结构的仿真结果(S21参数)如图3所示。当d1超过45 μm后传输系数明显下降;d1=25 μm时,高频段传输系数下降;当d1=35 μm时,转换结构会获得较好传输效率和工作带宽。已知共面波导上层金膜距波导底壁高度为40.8 μm,故第一脊要略低于共面波导金膜。对仿真结果进行对比后,文中选取d1为35 μm。
Figure 3. The simulated transmission coefficient of the conversion structure at different first ridge height d1
通过d2、d3进行仿真,结果表明d2在50~70 μm、d3在180~260 μm范围内可使结构获取高传输效率。d2小于70 μm时在低频段损耗较大,为实现长工作带宽本设计选取d2= 70 μm。d3大于180 μm会减小中频段传输损耗但在通频带两端损耗会有增加,故d3
取180 μm。 -
在设定d1为35 μm并保持其他结构参数不变的情况下,改变第一脊的长度L1的数值(分别为95、145、195、245 μm)进行仿真,仿真结果如图4所示。
Figure 4. The simulated transmission coefficient of the conversion structure at different first ridge length L1
从图中可以看出随着L1逐渐增大,转换结构的传输系数逐渐减小,当L1=245 μm时在超过320 GHz的高频段传输系数明显减小,L1=195 μm和L1=145 μm时转换结构均可达到高传输效率,但L1=145 μm 在全频段传输性能更稳定可用带宽更长,文中选取L1=145 μm进行进一步研究。
对L2、L3进行仿真分析, L2取140 μm、L3在180~240 μm范围内不影响结构的高传输效率。L2过小会增加高频段损耗、过大增加低频段损耗,L3取240 μm在高频段损耗更小。
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阶梯脊厚度th在整体转换效果中也起到了重要作用,保持其余结构参数不变,取 th分别为60、100、140、180 μm进行参数扫描,对应的转换结构传输系数如图5所示。
Figure 5. The simulated transmission coefficient of the conversion structure at different step ridge thickness th
分析图5可以得出,随着阶梯脊厚度的增加转换结构在低频传输效率提高,高频传输效率下降。文中选取th =140 μm进行以获取全频段的高传输效率。
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综上,得到THz矩形波导到共面波导转换结构的具体参数如表1所示,由此仿真计算得到转换结构的S参数如图6所示。
Parameter c h L1 L2 L3 d1 d2 d3 th Value/μm 0.4 40 145 140 240 35 70 180 140 Table 1. Conversion structural parameters of terahertz rectangular waveguide to coplanar waveguide
结果表明,该转换结构在240~350 GHz的频带上传输系数高于−3 dB。图7为305 GHz时矩形波导到共面波导的模式转换图,通过图7(a)的电场幅值图可以直观反映电磁波通过转换结构金属脊耦合到共面波导的过程。图7(b)、(c)为电/磁场矢量图,在矩形波导中,可以观察到TE模的电磁场分布,在阶梯脊右侧,只有忽略不计的纵向电磁场分量,表明了TEM模的形成。
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该转换结构应用于THz波段故尺寸在微米尺度,为研究加工误差对结构转换效率的影响,文中对阶梯脊圆倒角半径r及转换结构与共面波导间距s对传输系数的影响进行了仿真如图8所示。
Figure 8. Effect of machining error on transmission coefficient of conversion structure. (a) Circular chamfer r; (b) Spacing between the conversion structure and the coplanar waveguide s
图8(a)为阶梯脊圆倒角对转换结构传输系数的影响。结果表明圆倒角半径r由0 μm增加到80 μm在低频段影响很小,在280 GHz以上传输系数会有明显减小。全频段内r小于40 μm可保证转换结构的良好传输特性。图8(b)为间距s对传输系数的影响。结果表明间距小于3 μm不会影响转换效果。
Design of coupled structure of terahertz rectangular waveguide and coplanar waveguide
doi: 10.3788/IRLA20210733
- Received Date: 2021-11-08
- Rev Recd Date: 2022-01-20
- Publish Date: 2022-08-31
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Key words:
- terahertz coupled structure /
- ridge waveguide /
- schema transformation
Abstract: Terahertz (THz) wavelength lies in between millimeter waves and infrared waves in the electromagnetic spectrum. The existing optical waveguide and microwave millimeter waves waveguide technologies can be applied to the THz band. Because of the strong absorption of THz waves by water vapour and the limitation of manufacturing processes, THz devices were mainly planar structures and rectangular waveguides were commonly used for THz source and transmission. Therefore, the conversion structure between rectangular waveguides and coplanar waveguides has plays an indispensable role in determining the performance of components and systems. In recent research, the ridge waveguide has been used for impedance matching and electromagnetic field mode conversion to accomplish the high-efficiency coupling between THz wave rectangular waveguides and the coplanar waveguides. According to the simulation of CST microwave studio, the results show that the transmission coefficient (S21