A novel 3-D multilayer packaging technology for integrating an array of indium phosphide (InP)-based terahertz photodiodes (THz-PDs) with a rectangular waveguide power combiner (WR-PC) is proposed. The packaging concept is based on a vertical integration of an InP THz-PDs array with a multilayered WR-PC made of metallized glass-reinforced epoxy FR4 laminates using direct wafer bonding. The key motivation of this work is to develop a low-cost packaging technology for coherent power combining in the THz regime. The proposed multilayered packaging technology is generic, i.e., in principle, it would allow the integration of different and multiple planar arrays featuring photonic or electronic devices. To our knowledge, this is the first 3-D packaging concept for the THz frequency range that enables the integration of 2-D arrays of photonic or electronic devices. As a proof of concept, we here report on the design, fabrication, and experimental characterization of a straight hollow rectangular waveguide (WR-waveguide) and a 2 x 1 WR-PC for the WR3-band (220-320 GHz). Both packages feature standard cross-sectional hollow WR3-waveguides (862 μm x 431 μm) and corresponding standard UG-387/U-M flanges. They are fabricated using a stack of vertically bonded unit cells, each consisting of a 50-μm-thick glass-reinforced epoxy FR4 laminate with a 23-μm-thick top and bottom metallization. The size of a single quadratic FR4 unit cell is 24 mm x 24 mm. The fabricated straight WR3-waveguide consists of a stack of 57 FR4 unit cells of a total length of 5.4 mm. The measured transmission loss is less than 0.3 dB/mm and the return loss (RL) is less than 10 dB, within the entire WR3-band. Next, a multilayered FR4 2 x 1 WR3-PC is reported. Its design is based on that of a T-junction in H-plane where the inputs are modified to be on the same plane for facilitating subsequent integration with the planar InP chips of THz-PDs. The length of the WR3-PC is intentionally reduced for compactness and its impedance is gradually matched to achieve a low insertion loss (IL) over the entire WR3-band. The fabricated multilayered WR3-PC consists of 41 FR4 unit-cells resulting in a total thickness of only 3.9 mm. For the frequency range from 240 to 320 GHz, the simulated IL, minimum isolation, and RL are 0.16, 4.2, and 6.5 dB, respectively. For the experimental characterization using a WR3-band vector network analyzer (VNA), two multilayered FR4 2 x 1 WR3-PCs are connected back-to-back (B2B). The measured average IL and RL within the frequency range from 240 to 320 GHz are found to be 3.6 dB and below 5.2 dB, respectively. The measured average IL for a single WR3-PC is 1.8 dB, which is higher than expected because of imperfect fabrication and misalignment losses during the integration. An improved fabrication and packaging process will allow mitigating the losses in future runs.

A periodic leaky-wave antenna (LWA) design based on low loss substrate-integrated waveguide (SIW) technology with inset half-wave microstrip antennas is presented. The developed LWA operates in the V-band between 50 and 70 GHz and has been fabricated using standard printed circuit board (PCB) technology. The presented LWA is highly functional and very compact supporting 1D beam steering and multibeam operation with only a single radio frequency (RF) feeding port. Within the operational 50–70 GHz bandwidth, the LWA scans through broadside, providing over 40° H-plane beam steering. When operated within the 57–66 GHz band, the maximum steering angle is 18.2°. The maximum gain of the fabricated LWAs is 15.4 dBi with only a small gain variation of +/−1.5 dB across the operational bandwidth. The beam steering and multibeam capability of the fabricated LWA is further utilized to support mobile users in a 60 GHz hot-spot. For a single user, a maximum wireless on-off keying (OOK) data rate of 2.5 Gbit/s is demonstrated. Multibeam operation is achieved using the LWA in combination with multiple dense wavelength division multiplexing (WDM) channels and remote optical heterodyning. Experimentally, multibeam operation supporting three users within a 57–66 GHz hot-spot with a total wireless cell capacity of 3 Gbit/s is achieved.

In this manuscript, we report on a 1 × 4 optical beam forming network (OBFN) chip using optical phase shifters (OPSs) based on thermo-optically controlled optical ring resonators (ORRs) for 1D beam steering at 0.3 THz. The 1 × 4 OBFN chip consists of four OPSs and is fabricated using TriPleX technology. Each of the four OPSs is realized by two cascaded identical ORRs, to reach a phase shift of 2π. To allow transfer of the optical phase shift to the THz domain by optical heterodyning in high-frequency 1.55 µm modified uni-travelling carrier photodiodes, the ORRs are designed such that one carrier of the optical heterodyne signal is at the ORR’s resonance frequency, whereas the second optical heterodyne signal is at its off-resonance. By adjusting the resonance frequencies of the two ORRs in each OPS synchronously, a relative phase variation between two optical heterodyne carriers of up to 2π with a tuning efficiency of 0.058 W/π, is experimentally demonstrated. Due to the dispersive power transmission loss of the ORRs, phase tuning leads to a power variation of the optical heterodyne-generated signals up to 3.8 dB, which is experimentally characterized at 0.295 THz. It is shown numerically that this power variation only has a minor impact on the steering performance of a 1 × 4 phased array. The determined beam direction deviation and maximum absolute radiation power change are smaller than 1° and 2 dB, respectively. By sweeping the phase difference between two adjacent THz antennas in the 1 × 4 phased array, from −120° to 120°, a beam steering range of ~62° is demonstrated numerically at 0.295 THz.

High-speed detection and identification of mobile terminals located in the vicinity of highly directive base stations is essential for future mm-wave communication systems. We propose a novel optoelectronic frequency modulated continuous wave (FMCW) radar system for detection, localization and identification of multiple mobile terminals. The simultaneous localization and identification of multiple mobile terminals is achieved by using frequency scanning mm-wave leaky-wave antennas (LWAs) and backscattering communications. LWAs provide RF-based beam steering for estimating the direction of arrival of the echoes using a FMCW radar signal. Additionally, the implementation of backscattering technology in the mobile terminals allows the identification of users. Hence, simultaneous user localization and identification without the use of any complicated radar signal post-processing algorithms is possible. Finally, the introduced modulated backscattering reflection shifts the signals of the targets at higher frequencies, which exhibit lower noise floor and thus have higher signal to noise ratio (SNR).

In this paper, we report a phase-sensitive photonic THz two-tone self-mixing imaging system, comprising a self-developed vertical illuminated triple transit region photodiode (TTR-PD) and a commercial square law Schottky barrier diode as THz emitter and THz detector, respectively. Using the two-tone self-mixing approach, the phase information of a device-under-test can be extracted by down-mixing two THz signals inside the SBD, whereas the phase information remains in the output signal. The THz tones are generated by two free-running lasers and the TTR-PD, while one optical signal is externally modulated for double sideband carrier suppression. By means of the self-mixing, the phase noises of the free running lasers are canceled out. Using a second PD for the trigger, lock-in detection allows fast imaging speed, only limited by the integration constant. Beside the imaging system, we present the characteristics of the used vertically illuminated terahertz triple transit region photodiodes with thin depletion zone and small active areas. Numerical analysis by energy-balance model based TCAD simulations show transit-time limitations over 200 GHz, due to the electron field management within the active photodiode layers and the resulting high electron velocities. Therefore, the fabricated TTR-PDs show a flat ± 2dB frequency response within the frequency range from 225 GHz to 305 GHz. By employing the proposed photonic two-tone imaging system with the fabricated TTR-PDs, amplitude and phase difference vector images are taken at 299.5 GHz and 300.5 GHz. Metal and acrylic glass items inside a paper envelope are clearly visible, which demonstrates the potential of the system.

Wireless communications at 300 GHz are expected to play an important role in the deployment of 6G and beyond networks. Multiple electronic and photonic technologies compete in this regard, each bringing its own particular benefits. While purely optoelectronic links are interesting for incorporating photonic advantages in signal transmission and reception, electronic receivers typically based on Schottky mixers are far superior in conversion efficiency. Thus, the link signal-to-noise ratio (SNR) is improved, and higher throughput can be achieved. Here, we demonstrate a fully-optoelectronic 300 GHz band multi-channel link using a novel low-barrier Schottky mixer driven with a photonically generated local oscillator (LO) signal in the receiver using a modified uni-travelling-carrier photodiode (MUTC-PD). This combines the efficient down-conversion of Schottky-based mixers and the advantages of photonic LO signals such as tuneability, remote generation and distribution, and the reuse of coherent technology used in fibre networks. Up to three frequency channels are generated in the transmitter which is also based on a MUTC-PD photomixer. We achieve a maximum aggregate line rate of 180Gbps over a distance of 1.5 meters, utilizing 16-QAM format and optical intensity modulation. The transmission operates within the soft-decision forward error correction (SD-FEC) limit, with each channel being received sequentially.