Search Results (55)

We propose and experimentally demonstrate an integrated photonic terahertz (THz) frequency-hopping (FH)/dehopping system using electro-optically wavelength-tunable lasers to enhance physical-layer security in terahertz wireless links. The FH THz signal is generated around the 300 GHz band by photomixing a fixed-wavelength laser and an electro-optically tunable laser, while a synchronized FH local oscillator (LO) signal is generated at the receiver for secure photonic dehopping. By utilizing a waveguide-integrated THz combiner and THz detector, the FH LO remains internally coupled and unexposed to potential eavesdroppers. A signal with a 40 GHz hopping span around the 300 GHz band and a 50 Mhops/s FH rate is dehopped to a 5 GHz intermediate frequency. This system simplifies the receiver complexity while enabling secure, high-speed physical-layer terahertz communications. Full article

Wireless communication above $300\mathrm{GHz}$ requires highly sophisticated analog circuit design due to severe frequency dependent ohmic losses. The complexity of such electronic hardware motivates exploring wireless quantum optical communication approaches even for the 6G “terahertz (THz) range” $\left[300\mathrm{GHz},10\mathrm{THz}\right]$. In this work, the classical radio frequency (RF)-based inner physical layer (PHY) transceiver blocks of channel coded wireless communication systems are replaced by wireless quantum optical transceiver blocks. Short range concepts employing LEDs as transmitters are particularly attractive, owing to their low implementation cost and practical simplicity. In contrast to laser based wireless quantum optical transmission over multipath channels, the quantum mechanical density operator ${\underline{\mathit{\rho }}}_{\mathrm{RX},[{\mathfrak{s}}_i,b_i]}$ and the transition probability $\gamma ({\mathfrak{s}}_i,{\mathfrak{s}}_{i+1})$ required by the quantum data detection must be revised accordingly. Furthermore, the novel interpretation introduced here, in which the extrinsic information is treated as a diversity branch rather than as an estimate of the a priori information, facilitates turbo equalization that still can accomodate varying a priori information. However, due to the limited uncoded transmission performance achievable with such systems, the incorporation of sophisticated channel coding schemes appears imperative. The authors therefore investigate the combination of sophisticated channel coding techniques, such as polar coding, with LED based wireless quantum optical transmission technologies. All numerical results assume a cryogenically cooled receiver front-end (approximately 10 K), yielding thermal noise levels. Operation at room temperature in the 6G THz range $\left[300\mathrm{GHz},10\mathrm{THz}\right]$ would require an average number ${\overline{N}}_{\alpha }$ of thermal noise photon values of approximately 5 to 20, which is beyond the scope of this feasibility study. The results show that the proposed paradigm enables simple, robust, and practically viable wireless quantum optical communication systems with favorable transmission performance. Additional gains are achieved through iterative turbo equalization. The results also suggest that the proposed approach can pave the way toward robust and economically viable future communication solutions. Full article

The terahertz (THz) frequency band has emerged as a promising frontier for next-generation wireless communication systems targeting ultra-high data rates, ultra-low latency, and spectrum expansion beyond conventional millimeter-wave regimes. Realizing practical THz communication links, however, critically depends on stable, tunable, and integrable signal sources capable of delivering sufficient output power while maintaining spectral purity and energy efficiency. Among the various THz generation approaches, optoelectronic techniques offer unique advantages, including large bandwidth, wide frequency tunability and compatibility with fiber-optic infrastructures. This review provides a technology-focused assessment of key optoelectronic THz source technologies, namely photoconductive antennas, quantum cascade lasers, and unitraveling carrier photodiode (UTC-PD)-based photomixers, with particular emphasis on UTC-PD photomixers due to their strong suitability for continuous-wave THz generation and fiber-compatible architectures. The implications of optoelectronic THz sources for system-level architectures, including THz-over-fiber links, coherent detection schemes, and phased-array integration, are further examined. Finally, critical challenges and emerging research directions toward monolithic photonic–terahertz integration and deployable high-capacity wireless front-ends are discussed. This review aims to provide a structured perspective on the state of optoelectronic THz source technologies and their role in enabling practical next-generation communication systems. Full article

This study investigates trellis shaping (TS) for mitigating nonlinear impairments in photonic terahertz (THz) communication systems. A parallel shaping architecture integrating hierarchical modulation (HM) with adjustable power allocation is proposed. The scheme applies TS to structure the data stream hierarchically and employs superposition coding (SC) to generate non-uniformly distributed TS-16QAM signals, enhancing transmission performance. Experimental validation over a 5-m link at 320 GHz demonstrates that the proposed scheme achieves a 1.65 dB improvement in input power sensitivity and a 1.39 dB gain in Q-factor compared to the Maxwell–Boltzmann (MB) distribution. Furthermore, the system provides enhanced power allocation flexibility, expanding the tunable range of the hierarchical power ratio by a factor of 1.37. Full article

All-optical logic gates have emerged as a critical technology for enabling broadband, low-loss, and high-speed communication systems, addressing the inherent bandwidth limitations of electronic counterparts. Here, we propose a Y-shaped structure leveraging unidirectional modes in the terahertz regime, which enables the realization of multifunctional all-optical logic gates within the lower- and upper-frequency bandwidth regions, including, but not limited to, AND, OR, NOT, and XNOR gates. Numerical simulations and theoretical analyses confirm that the proposed logic gates exhibit robust one-way propagation characteristics, with electromagnetic signals demonstrating complete immunity to backscattering even in the presence of structural defects. Furthermore, nonlocal effects are found to have a negligible impact on the operational bandwidths of our design. Building upon this Y-shaped configuration, we further develop an all-optical digital logic system (AODLS) capable of supporting bifrequency multi-input and multi-output logic operations. When lower- and upper-frequency signals are injected into separate input ports, their corresponding output signals remain fully independent, eliminating cross-talk and enabling true parallel computation. This dual-band parallel processing capability represents a significant advance over conventional single-band all-optical logic systems, opening new avenues for high-throughput all-optical computing and integrated photonic circuits. Full article

Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) have gained significant attention over the past two decades due to their versatility in a wide range of photonic applications. This review focuses on VECSEL configurations for dual-wavelength emission, highlighting their use in high-resolution spectroscopy, terahertz (THz) generation, and advanced optical communication. We explore recent developments in VECSEL designs, including systems utilizing birefringent crystals for polarization-based frequency separation and configurations with dual-VECSEL chips or dual-gain regions within a single cavity. These two-wavelength VECSELs enable diverse operation modes, including narrow-linewidth, pulsed, multimode, and frequency-converted emission, with high-brightness output, excellent beam quality, and tunable wavelengths. Additionally, the review discusses advancements in dual-frequency VECSELs, with applications in LIDAR systems for environmental monitoring, highly stable optical clocks, and fiber sensors. We examine improvements in cavity design, semiconductor structures, and power stabilization, which have enhanced frequency stability and spectral purity, making VECSELs suitable for precision metrology and sensing applications. Full article

The rapid development of 6G wireless networks requires ultra-high data rates that traditional microwave frequencies cannot support. Photonics-assisted terahertz (THz) technologies offer a promising solution by combining high-capacity optical fibers with wideband wireless transmission. However, as bandwidth expands, sampling frequency offset (SFO) becomes a critical issue that degrades signal quality in single-carrier systems. This paper evaluates the performance of two main compensation methods within a photonics-assisted THz system operating at 320 GHz. We compare the Gardner clock recovery algorithm and the Digital Interpolation Compensation Algorithm (DICA) across various modulation formats and offset levels. Our findings indicate that the Gardner algorithm is effective for low-order modulation when the SFO is below 100 ppm, but its performance fails outside this range. Conversely, the DICA provides robust compensation up to 1000 ppm regardless of the modulation format, provided that the exact offset value is known. Without proper compensation, the system BER increases significantly as the SFO grows. These results demonstrate the complementary nature of these two algorithms and provide a practical guide for selecting compensation strategies in future high-speed THz communication links. Full article

Terahertz (THz) communication has become a key enabling technology for the future sixth generation (6G) due to its rich spectrum resources, supporting emerging applications such as holographic communication and ultra-wideband transmission. This article provides a comprehensive review of recent advances in photonic THz communications, covering device, system, and antenna technologies. First, the electronic bottlenecks in conventional THz systems, including limited bandwidth and severe phase noise generated by frequency doubling, are discussed, emphasizing the advantages of photonic methods in ultra-wideband signal generation and seamless integration with fiber-optic networks. Then, the effects of the carrier transit time, absorber layer thickness, and saturation effects on the bandwidth efficiency performance in single-row carrier photodiodes are reviewed, as well as multi-parameter co-optimization strategies for an enhanced performance. In addition, the latest progress in high spectral efficiency (SE) multi-dimensional multiplexing, lightweight high-gain lens antennas and multi-antenna MIMO transmission mechanisms in multi-antenna THz systems are also summarized. Finally, the prospects and challenges of photonic THz communications in long-distance links and space applications are discussed. Full article

This paper presents a systematic numerical investigation of a hybrid plasmonic–photonic Panda-ring antenna with an embedded gold grating, designed to enable efficient dual-mode radiation for optical and terahertz communication systems. The proposed structure integrates high-Q whispering-gallery mode (WGM) confinement in a multi-ring dielectric resonator with plasmonic out-coupling at the metal–dielectric interface, allowing controlled conversion of resonantly stored photonic energy into free-space radiation. The electromagnetic behavior is analyzed through a hierarchical structural evolution, progressing from a linear silicon waveguide to single-ring, add–drop, and Panda-ring resonator configurations. Gold is modeled using a dispersive Drude formulation with complex permittivity to accurately capture frequency-dependent plasmonic response at 1.55 µm. Power redistribution within the resonator system is described using coupled-mode theory, with coupling and loss parameters evaluated consistently from full-wave numerical simulations. Full-wave simulations using OptiFDTD and CST Studio Suite demonstrate that purely photonic resonators exhibit strong WGM confinement but negligible radiation, while plasmonic gratings alone suffer from low efficiency due to the absence of coherent photonic excitation. In contrast, the proposed hybrid Panda-ring antenna achieves stable and directive far-field radiation under WGM excitation, with a realized gain of approximately 8.05 dBi at 193.5 THz. The performance enhancement originates from synergistic hybrid SPP–WGM coupling, establishing a WGM-driven radiation mechanism suitable for Li-Fi and terahertz wireless applications. Full article

To address the phase noise issue in terahertz OFDM system, this paper proposes a dual-branch deep learning phase noise compensation network named AdaPhaseNet. The Transformer branch of this network leverages the powerful modeling capability of Transformers for long-range dependencies to achieve long-range phase noise estimation and compensation, while the CNN branch is employed for local signal enhancement. Finally, an optimized signal is output through a confidence-driven adaptive fusion module. For experimental validation of the algorithm, we constructed a photonic terahertz communication system comprising 10 km of fiber and 5 m of wireless transmission. Experimental results show that, compared with multiple baseline models, AdaPhaseNet achieves relative BER reductions ranging from 37.0% to 57.9% and EVM gains ranging from 1.4 dB to 3.2 dB. Full article

Chiral metasurfaces exhibit enormous potential in optical applications, and their integration with phase-change material vanadium dioxide (VO₂) provides a novel pathway for dynamic regulation. In this study, a chiral absorptive metasurface based on VO₂ is designed. By tuning the VO₂ conductivity around the operating frequency of 2.81 THz, the circular dichroism (CD) can be continuously adjusted from 0.06 to 0.95, realizing a high-contrast chiral switch. On this basis, the Pancharatnam–Berry (PB) phase is introduced to construct a chirality-dependent phase gradient: when the VO₂ conductivity is 200,000 S/m, only the left-handed circularly polarized (LCP) wave is subjected to periodic phase modulation, enabling controllable deflection of the reflected beam, while the right-handed circularly polarized (RCP) wave is selectively absorbed. This “chiral phase encoding” strategy simultaneously achieves absorptive CD tuning and reflective beam shaping on a single metasurface, significantly enhancing the flexible manipulation capability of circular polarization states in the terahertz band. It provides a compact and efficient solution for reconfigurable imaging, unidirectional communication, and integrated photonics systems. Full article

Terahertz wireless communication offers ultra-high bandwidth, enabling an extremely high data rate for next-generation networks. However, it faces challenges including severe propagation loss and atmospheric absorption, which limits the transmission rate and transmission distance. To address the problem, polarization division multiplexing (PDM) and antenna diversity techniques are utilized in this work to increase system capacity without changing the bandwidth of transmitted signals. Meanwhile, duobinary shaping is used to solve the problem of bandwidth limitation of components in the system, and the final duobinary signals are recovered by maximum likelihood sequence detection (MLSD). A Gaussian mixture model (GMM)-enhanced duobinary unsupervised adaptive convolutional neural network (DB-UACNN) is proposed, to further deal with channel noise. Based on the technologies above, a 2 × 2 multiple-input multiple-output (MIMO) photonic-aided terahertz wireless transmission system at 300 GHz is demonstrated. Experimental results have proved that the signal-to-noise ratio (SNR) gain of duobinary shaping is up to 1.87 dB and 1.70 dB in X-polarization and Y-polarization. The proposed GMM-enhanced DB-UACNN also shows extra SNR gain of up to 2.59 dB and 2.63 dB in X-polarization and Y-polarization, compared to the conventional duobinary filter. The high transmission rate of 100 Gbit/s over the distance of 200 m is finally realized under a 7% hard-decision forward error correction (HD-FEC) threshold. Full article

Rydberg atoms are neutral atoms excited to high principal quantum number states, which endows them with exaggerated properties such as large electric dipole moments, long lifetimes, and extreme sensitivity to external electromagnetic fields. These characteristics form the foundation of Rydberg atom-based sensors, an emerging class of quantum devices capable of optically detecting electric fields across frequencies from DC to the terahertz regime. Rydberg-based electrometry operates through both Autler–Townes (AT) splitting of resonant Rydberg transitions and Stark-shift measurements for high-frequency or far-detuned fields, enabling broadband field sensing from DC to the THz regime. Using ladder-type electromagnetically induced transparency (EIT) and AT splitting, these sensors enable non-invasive, SI-traceable measurements of field amplitude, frequency, phase, and polarization. Recent developments have demonstrated broadband electric field probes, voltage calibration standards, and compact RF receivers based on thermal vapor cells and integrated photonic architectures. Furthermore, innovations in multi-photon EIT, superheterodyne readout, and multi wave mixing have expanded the dynamic range and bandwidth of Rydberg-based electrometry. Despite challenges related to environmental perturbations, linewidth broadening, and laser stabilization, ongoing advances in atomic control, hybrid photonic integration, and EIT-based readout promise scalable, chip-compatible sensors. This review summarizes the physical principles, experimental progress, and emerging applications of Rydberg atom-based sensing, emphasizing their potential for next generation quantum metrology, wireless communication, and precision field mapping. Full article

In future 6G systems, terahertz (THz) waves are expected to be essential for enabling extremely high-capacity wireless communication. For such systems, efficient separation of multiple THz waves is required. A diffraction grating enables simple direct detection. Therefore, we designed a blazed grating optimized for THz beam separation. The grating was fabricated using a low-cost 3D printing process with a conductive coating, resulting in a simple structure that enables effective beam separation. Experimental results confirmed high-efficiency beam separation in the 300 GHz band, and 15 Gbit/s wireless communication was successfully demonstrated under these conditions. These results indicate that the proposed blazed grating provides a promising solution as a practical approach for THz wave receivers in future 6G wireless communication networks. Full article

This paper presents a comprehensive review of integrated terahertz (THz) and free-space optical (FSO) communication systems, focusing on their potential to address the escalating demands for high-capacity, long-distance, and ultra-reliable transmission in future six-generation (6G) and space–air–ground integrated networks (SAGIN). The study systematically examines recent advancements in three critical areas: channel modeling, transmission performance, and integrated system architectures. Specifically, it analyzes THz and FSO channel characteristics, including attenuation mechanisms, turbulence effects, pointing errors, and noise sources, and compares their complementary strengths under diverse atmospheric conditions. Key findings reveal that THz communication achieves transmission rates up to several Tbps over distances of several kilometers but is constrained by molecular absorption and weather-induced attenuation, while FSO offers superior bandwidth-distance products yet suffers from turbulence-induced fading, posing significant reliability challenges. The integration of THz and FSO through adaptive switching strategies (e.g., hard and soft switching) demonstrates enhanced reliability and spectral efficiency, with experimental results showing seamless data rates exceeding Tbps in hybrid systems. However, challenges persist in transceiver hardware integration, algorithmic optimization, and dynamic resource allocation. The review concludes by identifying future research directions, including the development of unified channel models, shared architectures, and intelligent switching algorithms to achieve robust integrated communication infrastructures. Full article