Opt Express. 2022 Nov 21;30(24):43798-43814. doi: 10.1364/OE.470375.
In this work, we present an optically subharmonic pumped WR3-mixer for enabling photonic coherent frequency-domain terahertz (THz) imaging and spectroscopy systems in the future. The studied mixer operates within the upper range of the WR3-band from 270 GHz to 320 GHz. High-power uni-travelling carrier photodiodes (UTC-PDs) are developed for providing the subharmonic local oscillator (LO) signal within the corresponding WR6-band in the range between 135 GHz and 160 GHz. The proposed THz mixer module consists of a gallium arsenide (GaAs)-based low barrier Schottky diodes (LBSDs) chip and an indium phosphide (InP)-based UTC-PD chip. For integrating the UTC-PD with the WR6 at the mixer's LO input, an E-plane transition and a stepped-impedance microstrip line low pass filter (MSL-LPF) are developed and monolithically integrated with the UTC-PD chip on a 100 µm thick InP substrate. The E-plane transition converts the quasi-TEM mode of the grounded coplanar waveguide (GCPW) to the dominant TE mode of the WR6 and matches the GCPW's impedance with the WR6's impedance. According to full-wave EM simulations, the transition exhibits a 1 dB bandwidth (BW) of more than 30 GHz (138.8-172.1 GHz) with a corresponding return loss (RL) better than 10 dB, whereas the minimum insertion loss (IL) is 0.65 dB at a frequency of 150 GHz. Experimentally, the 1 dB BW of the fabricated transition is found to be between 140 GHz and 170 GHz, which confirms the numerical results. The minimum measured IL is 2.94 dB, i.e., about 2 dB larger than the simulated value. In order to achieve the required LO power for successfully pumping the mixer in a direct approach (i.e., without an additional LO amplifier), the design of the epitaxial system of the UTC-PD is optimized to provide a high output power within the WR6-band (110-170 GHz). Experimentally, at 150 GHz, the output power of the fabricated UTC-PD chip is measured to be +3.38 dBm at a photocurrent of 21 mA. To our knowledge, this is the highest output power ever achieved from a UTC-PD at 150 GHz. Finally, the developed high-power UTC-PDs are used as LO source to pump the subharmonic WR3-mixer. Experimentally, the conversion loss (CL) is determined in dependency of the LO power levels within the RF frequency range between 271 GHz and 321 GHz for a fixed IF at 1 GHz. The achieved results have revealed an inverse relation between the CL and LO power level, where the average minimum CL of 16.8 dB is achieved at the highest applied LO power level, corresponding to a photocurrent of 10 mA. This CL figure is promising and is expected to reach the CL of electronically pumped and commercially available THz mixers (∼12 dB) after packaging the LO source with the mixer. Furthermore, an average CL of 17.2 dB is measured at a fixed LO frequency of 150 GHz and a tuned RF frequency between 301 GHz and 310 GHz, i.e., IF between 1 GHz and 10 GHz.
在这项工作中，我们提出了一种光亚谐波泵浦 WR3 混频器，以实现未来的光子相干频域太赫兹（THz）成像和光谱系统。研究中的混频器在 WR3 波段的上限范围内工作，频率范围为 270GHz 至 320GHz。我们开发了高功率单载流子光电二极管（UTC-PD），用于在相应的 WR6 波段内提供亚谐波本振（LO）信号，WR6 波段的频率范围在 135GHz 到 160GHz 之间。所提出的 THz 混频器模块由基于砷化镓（GaAs）的低势垒肖特基二极管（LBSD）芯片和基于磷化铟（InP）的 UTC-PD 芯片组成。为了将 UTC-PD 与 WR6 集成在混频器的 LO 输入处，我们开发了 E 面过渡和阶跃阻抗微带线低通滤波器（MSL-LPF），并在 100µm 厚的 InP 衬底上与 UTC-PD 芯片进行了单片集成。E 面过渡将接地共面波导（GCPW）的准 TEM 模式转换为 WR6 的主导 TE 模式，并使 GCPW 的阻抗与 WR6 的阻抗匹配。根据全波电磁仿真，过渡的 1dB 带宽（BW）超过 30GHz（138.8-172.1GHz），相应的回波损耗（RL）优于 10dB，而最小插入损耗（IL）在 150GHz 时为 0.65dB。实验上，制造的过渡的 1dB BW 被发现在 140GHz 和 170GHz 之间，这证实了数值结果。测量到的最小 IL 为 2.94dB，即比模拟值大约大 2dB。为了在直接方法（即不使用附加 LO 放大器）中成功泵浦混频器，需要 LO 功率达到要求，为此优化了 UTC-PD 的外延系统设计，以在 WR6 波段（110-170GHz）内提供高输出功率。实验上，在 150GHz 时，制造的 UTC-PD 芯片的输出功率在 21mA 的光电流下测量为+3.38dBm。据我们所知，这是在 150GHz 时从 UTC-PD 获得的最高输出功率。最后，开发的高功率 UTC-PD 用作 LO 源来泵浦亚谐波 WR3 混频器。实验上，在 RF 频率范围内，在 271GHz 到 321GHz 之间的固定中频（IF）下，确定了 CL 与 LO 功率电平的依赖性。在最高应用 LO 功率电平下，实现了 16.8dB 的平均最小 CL，对应于 10mA 的光电流。这个 CL 数值很有前景，预计在对 LO 源和混频器进行封装后，将达到电子泵浦和商用太赫兹混频器的 CL（约 12dB）。此外，在固定 LO 频率为 150GHz 和调谐 RF 频率在 301GHz 到 310GHz 之间（即 IF 在 1GHz 到 10GHz 之间）的情况下，测量到 17.2dB 的平均 CL。
