Photonically-driven Schottky diode based 0.3 THz heterodyne receiver.

Photonics-based technologies are key players in a number of emerging applications in the terahertz (THz) field. These solutions exploit the well-known advantages of optical devices, such as ultra-wide tuneability and direct integration with fiber networks. However, THz receivers are mainly implemented by fully electronic solutions, where Schottky barrier diodes (SBD) are the preferred option as detectors and mixers due to their excellent response within the THz range at room temperature, and technological maturity. Here, we demonstrate an SBD-based subharmonic mixer (SHM) at 300 GHz pumped with a photonic local oscillator. The Schottky mixer is a prototype designed and manufactured by ACST GmbH, operating at 270-320 GHz. The local oscillator is generated by photomixing on a high-frequency and high-power uni-travelling-carrier photodiode (UTC-PD), providing enough power to saturate conversion loss. Minimum single-side-band conversion loss of 14.4 dB and a peak dynamic range of 130 dB have been measured. Finally, as a proof of concept we realize an all-photonics-based 5 Gbps wireless bridge, utilizing the optically-pumped SBD mixer. With this work, we prove the feasibility of high-performance hybrid Schottky-photonic THz receivers, incorporating the best of both worlds.

High-power UTC-photodiodes for an optically pumped subharmonic terahertz receiver.

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 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 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.