This work presents the first realization of a photonics-based two-dimensional (2D) array architecture concept that combines a three-dimensional (3D) dielectric rod waveguide (DRW) antenna with a $4 \times 4 \text{InP}$-based photodiode (PD) array. Simulations demonstrate that the proposed approach supports 2D beam-steering at carrier frequencies of at least up to 210 GHz. Preliminary characterization results of the monolithically integrated $4 \times 4 \text{InP}$ PD array, which incorporates semiconductor optical amplifiers (SOAs), confirm wideband THz generation. Furthermore, the proposed architecture mitigates microscale assembly challenges through a novel alignment concept for chiplevel integration. This paves the way towards broadband THz sources for next-generation communication systems.

Integrated sensing and communication has been considered a focal 6G technology, enabling simultaneous high capacity transmission and mm-scale resolution. Applications such as metrology, radar or imaging which require both high resolution and high frequency stability will potentially benefit from utilizing the sub-THz frequency band. Electronics has limitations in this range of the spectrum, while optical methods using free-running independent lasers exhibit high phase noise in generated signals and high frequency drift due to lack of phase correlation between them. Optical heterodyning based on optical frequency combs can ensure phase correlation while representing a straightforward approach for sub-THz signal generation. A frequency comb centered on a sub-THz carrier frequency with a line spacing of several lines of MHz can advantage sensing applications and assist with channel estimation in the THz spectrum. We demonstrate the generation of a 50 MHz free spectral range sub-THz frequency comb centered at 126 GHz (D-Band), expanding across 3 GHz with 5 dB flatness. A cascaded modulator architecture is used to generate an electro-optical frequency comb with excellent phase correlation, and a second comb generation stage using pulse modulation achieves the free spectral range in the 10's MHz range. Finally, a waveguide integrated PIN photodiode helped to achieve the sub-THz comb in the electrical domain. We demonstrated RF linewidths in the Hz range for the sub-THz comb and the coherence between its lines. Our approach, with integration potential, enables low size, weight and power consumption characteristics, together with Hz order frequency stability and low phase noise for multiple frequencies simultaneously.

We analyze the weather-related link availabilities of high-capacity THz-wireless transmission systems, focusing on fiber-integrated point-to-point links in the lower THz frequency range around 300 GHz. First, we discuss latest component technologies for electronic and photonic THz generation, showing that there is a good basis for the implementation of wideband THz-wireless links. Then, we review the application of a THz-wireless fiber extender and identify challenges for its integration into future 6G optical networks, being mainly the linear optical-to-THz conversion and the weather-dependent THz link loss. Both aspects will be addressed in the following: We describe the concept and implementation of a fiber-integrated THz outdoor unit prototype based on electronic THz components and provide link budget calculations for a data rate of 100 Gbit/s. Then, we estimate the THz link loss from theoretical models and real weather data, showing that high link availabilities above 99.999% are possible for link distances of 500 m with state-of-the-art components. Estimates for a longer distance of 1 km are given as well. Finally, we describe our outdoor testbed built with our prototypes and present measured data on the link attenuation over 500 m in comparison to the theoretical expectations, achieving a good correlation.

Photomixers, which convert optical signals into high-frequency electrical signals, are promising sources and detectors for terahertz (THz) wireless communications due to their broad tunability, high bandwidth, and easy integration with fiber-optic networks. Photodiode (PD)-based THz emitters are already the state-of-the-art for highest data rate THz wireless links. Photonic THz receivers, such as photoconductive antennas (PCAs), have the same benefits of high THz bandwidth and potentially the same very low phase-noise as PD emitters. However, PCAs have not yet demonstrated competitive receiver performance compared to electronic mixers. This limitation arises from the restricted conversion gain and intermediate frequency (IF) bandwidth of the top-illuminated PCAs used in current systems. In this work, we present a novel photomixing heterodyne THz receiver based on waveguide-integrated (win) PCAs, which offers a 25 dB increase in conversion gain due to benefits arising from the optical waveguide coupling. We design and optimize a high-frequency package for the win-PCAs, achieving a record 3- and 6-dB IF bandwidth of 25 and 40 GHz, respectively. With this receiver, we now attain gross data rates of up to 84 Gbit/s, which is a new record for photonic wireless links with PCA receivers. At the same time, we demonstrate the ultra-broadband operation capabilities of the win-PCA, enabling data transmission at carrier frequencies from 100 to 600 GHz with the same receiver.

We present a photonic integrated circuit (PIC) designed for high-speed terahertz (THz) spectroscopy, operating at kilohertz repetition rates. This PIC integrates all components to generate and modulate the optical beating signals, enabling continuous wave THz generation and coherent detection. We implement an innovative sparse sensing approach with a discrete frequency sweep across 735 GHz in less than 1 ms and achieve a peak dynamic range exceeding 90 dB within a measurement time of less than 1 second. We apply this approach to a multi-layer thickness evaluation and validate the acquired amplitude and phase information, yielding a standard deviation below 2 µm across all three layers. Our experimental evaluation is the first demonstration of spectroscopic THz measurements utilizing a PIC operated at kilohertz repetition rates. This innovative spectrometer is ideally suited for industrial applications, including in-line monitoring and material characterization, showcasing the potential of photonic integration to advance THz spectroscopy.

Photonic transmitters are a promising approach for future wireless backhauling due to their high bandwidth and compatibility with fiber-optical networks. Additionally, photonic integration allows to create complex structures, e.g. antenna arrays, on a low-footprint chip or chiplets. So far, however, mainly fixed point-to-point links are demonstrated, and point-to-multipoint concepts come with increased effort, e.g. controlling numerous phase shifters to drive optical phased arrays. Our beam-switching approach demonstrates a multi-directional sub-THz transmitter with low operation effort. In a proof-of-concept experiment, we demonstrate 6.4 Gbps net data rate across an angular coverage of 85° in the W-band, specifically at 100 GHz.

We present a fully photonic, ultra-broadband mm-wave and THz wireless link, covering a wide carrier frequency range from 100 to 600 GHz. It employs a PIN photodiode as the emitter and a waveguide-integrated photoconductive antenna (PCA) as the receiver. To the best of our knowledge, this represents the first wireless link capable of transmitting across a 500 GHz frequency range with a single transmitter and receiver. Additionally, we achieve a new record gross data rate of 90 Gbit/s for PCA-based wireless links at a carrier frequency of 120 GHz. These accomplishments are made possible by substantial enhancements in conversion gain and intermediate frequency (IF) bandwidth of the heterodyne receiver, as well as meticulous optimization of operating conditions of the photomixers.

Photonic integration has the potential to revolutionize terahertz (THz) spectroscopy by addressing key limitations of state-of-the-art systems. Currently, these systems are still only infrequently adopted in industrial applications as their large size and low measurement rate limits applications. By utilizing frequency-domain THz systems based on optoelectronic photomixing technology, photonic integrated solutions can offer compact designs and scalable costs. Furthermore, the incorporation of a fast tuneable semiconductor laser and optical phase modulation could significantly enhance the measurement rates, even surpassing the capabilities of existing commercial THz spectrometers. With such photonic integrated spectrometers, a wide range of applications in spectroscopy, non-destructive testing and vector network analysis in the THz range becomes accessible and affordable.

Heterodyne photomixing terahertz (THz) receivers are essential components for photonic THz wireless links and radar applications. In this paper, we present a heterodyne THz receiver based on a waveguide-integrated photoconductive antenna (win-PCA). By designing on-chip electrical guiding structures and an intermediate frequency (IF) interposer, along with their associated transitions, we achieve a usable IF bandwidth of 40 GHz, coupled with a peak conversion gain of -23 dB, both of which are records for heterodyne photoconductive antenna receivers.

The rapid expansion of wireless data communication and integrated sensing systems necessitates the development of advanced antenna technologies capable of operating at higher frequencies and bandwidths with dynamic beam management, specifically directional beam control. This article addresses the challenge by designing a wideband photonic switched-beam antenna consisting of a 1 x 4 array of broadband bowtie antenna elements (AEs) fed by PIN photodiodes (PDs) on an InP substrate. Additional semiconductor optical amplifiers (SOAs) enable selective activation of single elements. Beam switching is realized through a hyperhemispherical lens, where the beam pointing angle is determined by the offset distance of the active AE from the lens axis. Beam pattern measurements confirm clear beam switching behavior with good beam quality up to 300 GHz, and discernible radiation angles up to 2 THz, albeit with degraded beam shapes at the upper end of the spectrum. Our results prove the broadband capabilities of this approach, despite variations in beam quality across different offsets and frequencies. A developed theoretical model, based on subcritical angle incidence at the lens-air interface, accurately predicts the beam pointing angle of the prototype. Simulations have been employed to optimize the design of a 2-D antenna array operating at 100 GHz, providing full 3 dB beam coverage within a +/- 60 degrees range. The presented results highlight the potential of photonic technologies to enable scalable and efficient beam management solutions for applications up to the terahertz frequency range.

We review recent developments in high-capacity THz wireless transmission, focusing on point-to-point links in the lower THz frequency range around 300 GHz. Available technologies and challenges for integration into future 6G optical networks are discussed.

The ultra-broadband operation capabilities of photonic terahertz systems impose stringent requirements on the design of the antenna and often used silicon lens. This work presents a directivity-optimized hyper-hemispherical silicon lens for photonic terahertz transmitters. A single main lobe is maintained from 100 GHz up to 600 GHz, and a peak directivity of 34 dBi is achieved. Compared to previously investigated lens designs, this marks a notable improvement of 7 dBi. The improved radiation characteristic of this lens facilitates the development of more compact systems with fewer additional mirrors or lenses and improves integration with other terahertz components.

Terahertz (THz) wireless communication is a key enabler for 6G networks, offering ultra-high data rates and broad bandwidth scalability. Photonic generation of THz signals using photodiodes has emerged as a promising approach, yet a systematic performance comparison between different architectures remains largely unexplored. This paper presents an experimental investigation of two distinct THz transmitter architectures. One is based on a WR3.4 waveguide-coupled photodiode, while the other employs a quasi-optically coupled photodiode with a silicon lens. Both architectures are evaluated using probabilistic constellation shaping with high-order quadrature amplitude modulation (QAM). We evaluate output power, bit error rate (BER), and system linearity across both photodiode-based architectures under standardized conditions. The results reveal key trade-offs between transmitter design choices, nonlinearities, and achievable performance, offering critical insights for optimizing next-generation THz wireless systems.

Planar transmitting antennas with a direct photodiode (PD) connection for 50 GHz fiber-fed signal radiation are presented. The short asymmetrical inductive dipole (SAID) antenna enables independent adjustment of either the real and imaginary part of the input impedance through specific geometrical dimensions, allowing for flexible conjugate impedance match with the PD, maximizing the radiated power. Measured results confirm excellent antenna efficiency and radiation patterns, demonstrating the effectiveness of the approach.

Recent advancements in hybrid photonic integrated circuits (PICs) for wireless communications are reviewed, with a focus on innovations developed at Fraunhofer HHI. This work leverages hybrid integration technology, which combines indium phosphide (InP) active elements, silicon nitride (Si3N4) low-loss waveguides, and high-efficient thermal-optical tunable polymers with micro-optical functions to achieve fully integrated wireless transceivers. Key contributions include (1) On-chip optical injection locking for generating phase-locked optical beat notes at 45 GHz, enabled by cascaded InP phase modulators and hybrid InP/polymer tunable lasers with a 3.8 GHz locking range. (2) Waveguide-integrated THz emitters and receivers, featuring photoconductive antennas (PCAs) with a 22× improved photoresponse compared to top-illuminated designs, alongside scalable 1 × 4 PIN-PD and PCA arrays for enhanced power and directivity. (3) Beam steering at 300 GHz using a polymer-based optical phased array (OPA) integrated with an InP antenna array, achieving continuous steering across 20° and a 10.6 dB increase in output power. (4) Demonstration of fully integrated hybrid wireless transceiver PICs combining InP, Si3N4, and polymer material platforms, validated through key component characterization, on-chip optical frequency comb generation, and coherent beat note generation at 45 GHz. These advancements result in compact form factors, reduced power consumption, and enhanced scalability, positioning PICs as an enabling technology for future high-speed wireless networks.

Terahertz wireless communications is an increasingly interesting research topic due to the high demand for un-allocated channels and high data rates. Photonic solutions have shown great potential in this field. However, most photonics assisted THz links so far have employed optoelectronics only on the transmit side. Thus, the full potential of photonic THz communication has not been utilized yet. Here, we introduce optoelectronics also on the receive side by using a photoconductive antenna based heterodyne THz detector. This allows down-conversion of data signals from the W-, D-, and THz-band to the baseband using a laser beat signal as local oscillator. Using electromagnetic modeling, we designed passive radio frequency structures and a receiver package to handle high intermediate frequency output signals. In a homodyne spectroscopic setup, the receiver shows a frequency response superior to state-of-the-art photoconductive antennas due to an improved photoconductive material. In a heterodyne testbed, the receiver exhibits a large intermediate frequency bandwidth of 11 GHz and a conversion gain of -47 dB. This enabled us to employ the receiver in a fully photonic wireless link at sub-terahertz and terahertz frequencies together with a PIN photodiode emitter. We achieved error-free transmission of 4-QAM signals with gross data rates up to 12 Gbit/s at carrier frequencies up to 320 GHz. This work shows the huge potential of optoelectronic receivers for THz wireless communications and enables the exploration of full photonic THz links.

Broadband terahertz spectroscopy is a valuable analytical tool in science and a promising technology for industrial non-destructive, non-contact testing, e.g. thickness measurements of thin dielectric layers. Optoelectronic conversion using photomixers is a widespread approach for coherent terahertz spectroscopy. State-of-the-art spectrometers consist of discrete, fiber-based components, leading to complex and costly setups. In cost-sensitive applications, this prevents the use of these spectrometers. We developed a terahertz spectrometer based on a dedicated photonic integrated circuit and commercial electronic integrated circuits to overcome these limitations. The photonic subsystem can be connected to commercial tunable lasers and provides the optical signal processing to drive the photoconductive antennas. The electronic subsystem includes the required drivers, analog signal processing, and data acquisition. Combined, the system measures $10 \times 16 \times 7.5$ cm3 only. We compare both subsystems individually and as a whole to state-of-the-art lab equipment in terms of spectral performance and measurement speed. Due to the flexibility in measurement modes, the integrated system can be adapted to specific measurement tasks, e.g. 2.8 THz-wide spectra within 0.5 s for high-speed, or 3.6 THz bandwidth with >80 dB dynamic range in less than 3 minutes for high-precision. This is the first realization of a terahertz spectrometer based on photonic and electronic integration rivaling state-of-the-art and non-integrated commercial spectrometers. This approach paves the way for compact and economic terahertz systems, providing access to terahertz technology for cost-sensitive sectors in research and industry.