RESUMO
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 270â GHz to 320â 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 135â GHz and 160â 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 TE10 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 1â dB bandwidth (BW) of more than 30â GHz (138.8-172.1â GHz) with a corresponding return loss (RL) better than 10â dB, whereas the minimum insertion loss (IL) is 0.65â dB at a frequency of 150â GHz. Experimentally, the 1â dB BW of the fabricated transition is found to be between 140â GHz and 170â GHz, which confirms the numerical results. The minimum measured IL is 2.94â dB, i.e., about 2â 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 (110-170â GHz). Experimentally, at 150â GHz, the output power of the fabricated UTC-PD chip is measured to be +3.38 dBm at a photocurrent of 21â mA. To our knowledge, this is the highest output power ever achieved from a UTC-PD at 150â 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 271â GHz and 321â GHz for a fixed IF at 1â GHz. The achieved results have revealed an inverse relation between the CL and LO power level, where the average minimum CL of 16.8â dB is achieved at the highest applied LO power level, corresponding to a photocurrent of 10â mA. This CL figure is promising and is expected to reach the CL of electronically pumped and commercially available THz mixers (â¼12â 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.