In recent years, the significance of terahertz (THz) and (sub-)THz communications has grown substantially due to its promising trade-off between higher capacity compared to microwave-based communication and better resilience against weather dependent influences (e.g., fog and rain). While electronic and optoelectronic techniques have been extensively explored, each offering distinct advantages and limitations, they have predominantly been demonstrated and discussed as individual experiments, making performance comparison challenging. This paper addresses this gap by systematically benchmarking electronic and optoelectronic signal generation approaches under comparable conditions. Our experiments incorporate various receiver types, revealing that best performance is achieved by combining optoelectronic signal generation techniques at the transmitter in combination with an all-electric intradyne receiver. This results in a remarkable line rate of 200 Gbit/s over a distance of 52 m. To our knowledge, this represents the highest line rate achieved for technically relevant transmission distances for indoor access or outdoor small cell networks.

We report on the successful implementation of a photonic-based beam steering approach for point-to-multipoint terahertz (THz) communications. The frequency-agile radiation properties of a THz leaky-wave antenna connected to a photodiode are utilized in the remote transmitter for generating multiple individually steerable THz beams. The approach benefits from the ultra-wide frequency tunability of optical heterodyne systems for frequency-agile THz beam steering. Moreover, the approach allows to exploit high performance and mature optical modulation techniques for generating THz beams with a high data capacity. In the THz receiver, a carrier frequency insensitive envelope THz detector based upon a single Schottky-barrier diode is used. In detail, THz communications in the 300 GHz band (WR3.4) with a maximum steering angle up to 90° is reported. Experimentally, for single steerable THz beam operation, a record high data rate of 35 Gbit/s at a wireless distance of 30 cm is achieved using two-level pulse amplitude modulation. Also, longer wireless distances up to 110 cm with 5 Gbit/s are demonstrated. Furthermore, point-to-multipoint THz communications is reported using two individually steerable THz beams carrying 10 Gbit/s and 5 Gbit/s over 70 cm and 50 cm, respectively.

The generation and transmission of millimeter-wave signals for 5G applications require the use of broadband and high output power photodetectors to bridge from the optical and electronic domains. Therefore, the deep knowledge on the equivalent circuit characteristics of these devices is vital. This study reviews and analyzes de-embedding techniques contributing to the characterization of the physical aspects within the active region of uni-traveling carrier photodiodes. De-embedding methods analytically remove the parasitic effects of the electrical transmission lines connected to their active area allowing the extraction of their series resistance and junction capacitance toward the synthesis of an equivalent circuit with lumped elements. The open-short technique is examined and a systematic error introduced by this process underlines the vulnerability of the method on removing parasitics with higher complexity. This error is quantified leading to the implementation of a corrected equation converging with the characteristic features of an $S$-parameter-based de-embedding. These characteristics are also analyzed through simulation approaches showing minimal equivalent inaccuracies on eliminating more complex symmetrical parasitics. A thorough comparison between these three methods is conducted through the calculation of lumped components corresponding to the active region of diodes with different sizes.

Microwave holography technique reconstructs a target image using recorded amplitudes and phases of the signals reflected from the target with Fast Fourier Transform (FFT)-based algorithms. The reconstruction algorithms have two or more steps of two- and three-dimensional Fourier transforms, which have a high computational load. In this paper, by neglecting the impact of target depth on image reconstruction, an efficient Fresnel-based algorithm is proposed, involving only one-step FFT for both single- and multi-frequency microwave imaging. Numerous tests have been performed to show the effectiveness of the proposed algorithm including planar and non-planar targets, using the raw data gathered by means of a scanner operating in X-band. Finally, a low-cost and high-speed hardware architecture based on fixed-point arithmetic is introduced which reconstructs the planar targets. This pipeline architecture was tested on field programmable gate arrays operating at 200 MHz clock frequency, which illustrates more than 30 times improvement in computation time compared with a computer.

This paper presents a new optically controlled reconfigurable ultra-wideband antenna using reconfigurable optical router with photonic crystal substrate. The proposed antenna has three optical switches. The optical switches are made by placing silicon wafers over three slots etched on the resonator. The coplanar fed microstrip antenna can work at eight modes using optically controlled switches. This design proposes triple narrow notched bands at center frequencies 3.5 GHz “WiMAX”, 5.5 GHz “WLAN” and 8.4 GHz “X-band satellite communication”. The proposed antenna satisfies the voltage standing wave ratio requirement of <2 in the frequency band between 2.6 and 11.8 GHz except for the three rejected bands. According to the incident light, the physical properties of these switches can be changed from an insulator state (OFF state) to a near-conducting state (ON state). The incident light is coupled to the optical switches using a reconfigurable optical router. The proposed antenna provides high gain, and high efficiency all over the frequency band excluding the rejected bands.

An array of patch-antennas with meandering-gaps on an optical modulator is proposed for wireless millimeter-wave beam-steering through high-speed radio-over-fiber systems. Wireless millimeter-wave can be received by the array of patch-antennas and directly modulated to lightwave by the optical modulator. The wireless millimeter-wave can be steered using the meandering-gaps at the patch-antennas by controlling interaction between millimeter-wave and lightwave electric fields in electro-optic modulation. The basic operation and analysis of the proposed device are discussed. In the experiment, 5 × 5 antenna array in 40 GHz millimeter-wave bands was designed and realized for device characterization and demonstration to wireless millimeter-wave beam-steering. There were five variations of wireless millimeter-wave beam-steering for one-dimensional in xz- or yz-planes that can be obtained with wireless millimeter-wave steerable beams of about ±30°. Additionally, 25 variations of wireless beam-steering can be obtained for two dimension in xyz-space through orthogonal optical modulation. The proposed device is promising to be applied in millimeter-wave/tera-hertz bands for future directional wireless communication and sensing with high-speed and high-resolution operation.

Active electronically scanned array (AESA) radar has large number of transmit/Receive (T/R) modules which require multiple microwave and digital signals. Distribution of these signals through conventional method such as coaxial cable, twisted pair, etc. not only introduces engineering complexities and signal loss but also have limitation of bandwidth, data rate, transmission distance, etc. This paper addresses design and implementation of microwave photonic network for distribution of microwave and digital signals over single optical fiber using wavelength division multiplexing for AESA radars. The design challenge is to limit the variation in output radio frequency power within ±1 dB over full operational band of radar from 2 to 4 GHz and functionality under hostile military environment. Optical amplifiers have been used in all channels to stabilize optical output independent of wavelength with automatic light control. The optical signal is split into 64 identical parts to feed multiplexed signal into different digital receivers physically spread across the antenna array. It is an additional challenge to normalize performance as all 64 receivers show variation in output in spite of identical electronic circuitry. Experimental results validate the feasibility of microwave photonic network for wide branching distribution of multiple microwave and digital signals for AESA radar.

A 10 × 10 μm2 SiGe heterojunction bipolar photo-transistor (HPT) is fabricated using a commercial technological process of 80 GHz SiGe bipolar transistors (HBT). Its technology and structure are first briefly described. Its optimal opto-microwave dynamic performance is then analyzed versus voltage biasing conditions for opto-microwave continuous wave measurements. The optimal biasing points are then chosen in order to maximize the optical transition frequency (fTopt) and the opto-microwave responsivity of the HPT. An opto-microwave scanning near-field optical microscopy (OM-SNOM) is performed using these optimum bias conditions to localize the region of the SiGe HPT with highest frequency response. The OM-SNOM results are key to extract the optical coupling of the probe to the HPT (of 32.3%) and thus the absolute responsivity of the HPT. The effect of the substrate is also observed as it limits the extraction of the intrinsic HPT performance. A maximum optical transition frequency of 4.12 GHz and an absolute low frequency opto-microwave responsivity of 0.805A/W are extracted at 850 nm.

The paper experimentally investigates indoor visible light communication (VLC) channels. The disturbances come from the light bulbs and other VLC systems. The paper demonstrates that the noisiest light bulbs are the fluorescent lamps which emit high frequency disturbances. Different light bulbs are examined to make an optimal carrier choice for the transmitted signal. In the next part of the paper several modulation schemes and two kinds of detector systems are compared. The paper represents also the advantages and disadvantages of receiver constructions with and without a lens. The optimum modulation for different application is investigated.

A novel, photonics-based scheme for the independent and simultaneous beam steering of multiple radio frequency signals at a wideband phased-array antenna is presented. As a proof of concept, a wavelength-selective switch (WSS) is employed both as a wavelength router to feed multiple antenna elements and as a tunable phase shifter to independently control the phase of each signal at any antenna element. In the experiment, two signals at 12.5 and 37.5 GHz are simultaneously fed to the four output ports of the WSS with independent and tunable phase shifts, emulating the independent steering of two signals in a four-element phased-array antenna. The results confirm the precision and flexibility of the proposed scheme, which can be realized both with bulk components or resorting to photonic integrated circuits, especially for wide-band applications. The architecture for a possible integrated implementation of the proposed solution is presented, employing a structure based on micro-ring resonator. Starting from these results, the feasibility of an integrated version of the presented architecture is also considered. The proposed photonic integrated circuit realizing the beam-forming network might be based on tunable true-time delay, as well as on phase shift through micro-ring resonators, and could be conveniently implemented with CMOS-compatible silicon technology.

Wireless domestic applications involving high data rates are required to work on millimeter wave band. Signal propagation at this frequency range is affected by walls and oxygen absorption which limits communication distances to few meters in one room. Radio coverage can be extended to other rooms by optical links. Performances of such photonic systems are dependent on optoelectronic devices, electrical driving, and receiver circuits. In this paper, radio-over-fiber (RoF) links based on the intensity modulation and direct detection technique are investigated for transmission of a broadband OFDM signal. Direct and external modulations are exploited to analyze system performances according to the ultra wideband (UWB) millimeter-band standard. To avoid component tolerances at high frequencies, an intermediate frequency modulation of the optical transducers is chosen. Optoelectronic and optical components of RoF links are modeled by equivalent electrical circuits with consideration of noise and nonlinearities. These models are validated in system simulation by error vector magnitude evaluation with a measurement setup according to the UWB centimeter-band standard.

Two types of optoelectronic oscillators delivering high spectral purity microwave signals are presented in this paper. These oscillators use the Pound–Drever–Hall laser stabilization technique to lock the laser carrier onto two different types of passive optical resonators featuring high-quality factors: a fiber ring resonator (FRR) and a whispering gallery mode monocrystalline disk-shaped micro-resonator. The different noise processes occurring inside these oscillators are discussed. Particular attention is given to the conversion of the laser's amplitude and frequency noise into RF phase noise via the laser stabilization loop and the resonator, and via the photodetector nonlinearity as well. A modeling approach using CAD software is also proposed to qualitatively evaluate laser noise conversion through the optical resonator. Moreover, different contributions of nonlinear optical scattering noise are discussed, mainly in the case of the FRR-based oscillator. When controlling these nonlinear optical effects in the case of the FRR, low-phase noise operation of the oscillator has been achieved, with a −40 dBc/Hz noise level at 10 Hz offset frequency from a 10.2 GHz RF carrier.

Millimeter-wave wireless space-division-multiplexing signals were discriminated by a newly developed electro-optic modulator using an array of antenna-coupled electrodes and polarization-reversed structures of ferroelectric optical crystals. Discrimination with channel isolation over 13 dB was obtained for two 38 GHz wireless signals with an irradiation angle difference of 15° by use of a fabricated device 30 mm in length.

In this paper we propose a photonic technique for a reconfigurable microwave signal generation based on the beating of two laser modes coming from a regenerative fiber mode-locked laser (FMLL) into a photodiode. The excellent performance of this kind of pulsed laser guarantees high stability on the directly generated microwave signal even at ultrahigh frequencies (up to W band). Therefore, by using the proposed architecture, the performance of a reconfigurable full digital coherent radar system can be enhanced for example in terms of moving target indicator (MTI) improvement factor and analog to digital converter maximum signal to noise ratio. Differently from the conventional radar oscillators, whose performance strongly deteriorate with increasing frequencies, the photonic radio frequency (RF) generation always shows an excellent spectral purity. Moreover, thanks to the achievable high repetition rates and the coherence properties of the FMLL, this laser scheme has also been proposed for digitizing, directly at RF, the received signal by electro-optical sampling. Thus the advantage of using just one device for signal generation in both the transmitter and receiver chain, makes the proposed solution a cost-effective architecture for microwave signal generation.

In this article, analog fiber optic links (radio-over-fiber, RoF, links) are presented as a flexible, low-cost solution for in-house distribution of millimeter-wave (mmw) signals. Mode-locked laser diodes (MLLD) serve as inexpensive mmw sources for the downlink distribution of mmw signals across an optical fiber link. We compare the robustness of direct and external RF modulation for such MLLD-based RoF systems, whereas the error vector magnitude (EVM) of the received symbols serves as a figure of merit. On the eve of 60 GHz WLAN standardization, we experimentally investigate the transmission of narrowband WLAN (IEEE 802.11a) signals in the millimetric range at moderate data rates. We also demonstrate broadband transmission of multi-band orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) european computer manufacturers association (ECMA 368) signals in the 60 GHz band for data rates of up to 480 Mbps.

The operation of a microwave photonic link is thoroughly investigated both theoretically and experimentally. To this aim, we have developed a simulation tool based on an accurate physical model embedded in a radio frequency (RF) chain simulator. The theoretical predictions are tested on an intensity modulation-direct detection (IMDD) link we have specifically developed to this purpose. Our simulation tool takes into account both optical and electrical characteristics of the link components including the laser dynamics and impedance matching networks. It thus enables an accurate understanding of the different physical and electrical phenomena governing the link's performances even under unusual operation conditions. Specifically, we were able to isolate an unusual behavior and to confirm it experimentally. It is thereby clear that the noise figure of a microwave optical link can be lower than the electrical losses, such as a mismatched output passive electrical network. This state is reached when the optical losses are high enough and when the link's output impedance is mismatched, too.