For mobile THz applications, integrated beam steering THz transmitters are essential. Beam steering approaches using leaky-wave antennas (LWAs) are quite attractive in that regard since they do not require complex feeding control circuits because beam steering is simply accomplished by sweeping the operating frequency. To date, only a few THz LWAs have been reported. These LWAs are based on polymer or graphene substrates and thus it is quite impossible to monolithically integrate these antennas with state-of-the-art indium phosphide (InP) based photonic or electronic THz sources and receivers. Therefore, in this paper, we report on an InP-based THz LWA for the first time. The developed and fabricated THz LWA consists of a periodic leaking microstrip line integrated with a grounded coplanar waveguide to microstrip line (GCPW-MSL) transition for future integration with InP-based photodiodes. For fabrication, a substrate-transfer process using silicon as carrier substrate for a 50 m thin InP THz antenna chip has been established. By changing the operating frequency from 230 GHz to 330 GHz, the fabricated antenna allows to sweep the beam direction quasi-linearly from -46 to 42, i.e. the total scanning angle is 88. The measured average realized gain and 3 dB beam width of a 1.5 mm wide InP LWA are ~11 dBi and 10. The paper furthermore discusses the use of the fabricated LWA for THz interconnects.

We report on a novel triple transit region (TTR) layer structure for 1.55 μm waveguide photodiodes (PDs) providing high output power in the millimeter wave (mmW) regime. Basically, the TTR-PD layer structure consists of three transit layers, in which electrons drift at saturation velocity or even at overshoot velocity. Sufficiently strong electric fields (>3000 V/cm) are achieved in all three transit layers even in the undepleted absorber layer and even at very high optical input power levels. This is achieved by incorporating three 10 nm thick p-doped electric field clamp layers. Numerical simulations using the drift-diffusion model (DDM) indicate that for optical intensities up to ~500 kW/cm², no saturation effects occur, i.e. the electric field exceeds the critical electric field in all three transit layers. This fact in conjunction with a high-frequency double-mushroom cross-section of the waveguide TTR-PD ensures high output power levels at mmW frequencies. Fabricated 1.55 µm InGaAs(P)/InP waveguide TTR-PDs exhibit output power levels exceeding 0 dBm (1 mW) and a return loss (RL) up to ~24 dB. Broadband operation with a 3 dB bandwidth beyond 110 GHz is achieved.

In this paper, we report on waveguide-type modified uni-traveling-carrier photodiodes (MUTC-PDs) providing a record high output power level for non-resonant photodiodes in the WR3.4 band. Indium phosphide (InP) based waveguide-type 1.55 µm MUTC-PDs have been fabricated and characterized thoroughly. Maximum output powers of −0.6 dBm and −2.7 dBm were achieved at 240 GHz and 280 GHz, respectively. This has been accomplished by an optimized layer structure and doping profile design that takes transient carrier dynamics into account. An energy-balance model has been developed to study and optimize carrier transport at high optical input intensities. The advantageous THz capabilities of the optimized MUTC layer structure are confirmed by experiments revealing a transit time limited cutoff frequency of 249 GHz and a saturation photocurrent beyond 20 mA in the WR3.4 band. The responsivity for a 16 µm long waveguide-type THz MUTC-PD is found to be 0.25 A/W. In addition, bow-tie antenna integrated waveguide-type MUTC-PDs are fabricated and reported to operate up to 0.7 THz above a received power of −40 dBm.

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.

This paper focuses on the development and characterization of a novel E-band planar bias-tee (BT) circuit featuring a high-speed millimeter wave photodiode (mm-wave PD) module to be integrated in next generation access and mobile networks (5G). The designed bias-tee circuit together with the integrated mm-wave PD chip, i.e., triple transit region photodiode (TTR-PD) allows the development of high-power Radio-over-Fiber (RoF) E-band (70/80 GHz) photonic transmitters (PTs) to be used in wireless extension and mobile backhaul links. The introduced BT circuit provides the protection to the hybrid integrated RF amplifier from being damaged by the PD bias voltage and prevents the leakage of the RF signal through the DC path. The vector network analyzer (VNA) measurements of the BT circuit show that in the frequency range from 70 to 75 GHz, the return loss (RL) is higher than 11 dB, the RF signal suppression level (IS) in the DC path is higher than 30 dB, while the insertion loss (IL) is lower than 2 dB. For optical RF signal generation, two laser sources are used to generate an optical heterodyne signal. Lower dark current levels and a 3-dB bandwidth in the frequency range from 71 to 86 GHz have been demonstrated at the BT output.

This manuscript discusses photonic assisted beam steering and beam switching millimeter-wave and THz antennas. Planar frequency scanning 60 GHz band leaky wave antennas (LWA) providing up to 15.4 dBi gain and scanning angles up to 110° are reported next to 300 GHz band LWAs with a max. directivity of 16 dBi and 72° scanning angle. Furthermore, photonic beam forming networks (OBFN) for true time delay (TTD) and phased array wideband (275-350 GHz) THz antennas are described. Finally, multiple beam antenna arrays for 1D and 2D beam switching are shown. Experimentally, 1D THz beam switching of 18° is demonstrated. Simulation results for 2D beam switching show a scanning angle of 70°.