Search Results (620)

We propose an external-cavity laser that combines wide tunability with narrow linewidth. The design utilizes a low-loss Si₃N₄ waveguide and a thermally tuned cascaded triple-ring resonator to enable continuous wavelength tuning. The numerical simulations indicate that the proposed laser exhibits a tuning range of 64 nm with a sub-kHz linewidth, an SMSR of more than 80 dB, an output power of 24 mW and a linewidth of 193 Hz at 1550 nm. Furthermore, we perform comparative system-level simulations using QPSK and 16QAM coherent optical fiber links at 50 Gbaud over 100 km. Under identical conditions, when the laser linewidth is reduced from 1 MHz level to 193 Hz, the BER of 16QAM decreases from 1.5 × 10⁻³ to 5.3 × 10⁻⁵. These results indicate that a narrow linewidth effectively mitigates phase noise degradation in high-order modulation formats. With its narrow linewidth, wide tuning range, high SMSR, and high output power, this laser serves as a promising on-chip light source for high-resolution sensing and coherent optical communications. Full article

Optical Fiber Composite Overhead Ground Wire (OPGW) cables serve dual functions in power systems, lightning protection and critical communication infrastructure for real-time grid monitoring. Accurate OPGW identification during UAV inspections is essential to prevent miscuts and maintain power-communication functionality. However, detecting small, twisted OPGW segments among visually similar ground wires is challenging, particularly given the computational and energy constraints of edge-based UAV platforms. We propose OPGW-DETR, a lightweight detector based on the D-FINE framework, optimized for low-power operation to enable reliable detection. The model incorporates two key innovations: multi-scale convolutional global average pooling (MC-GAP), which fuses spatial features across multiple receptive fields and integrates spectrally motivated features for enhanced fine-grained representation, and a hybrid gating mechanism that dynamically balances global and spatial features while preserving original information through residual connections. By enabling real-time inference with minimal energy consumption, OPGW-DETR addresses UAV battery and bandwidth limitations while ensuring continuous detection capability. Evaluated on a custom OPGW dataset, the S-scale model achieves 3.9% improvement in average precision (AP) and 2.5% improvement in AP₅₀ over the baseline. By mitigating misidentification risks, these gains improve communication reliability. As a result, uninterrupted grid monitoring becomes feasible in low-power UAV inspection scenarios, where accurate detection is essential to ensure communication integrity and safeguard the power grid. Full article

Microwave photonics has recently come to the forefront as a valuable approach to generating, processing, and measuring signals in high-performance domains such as communication, radar, and timing systems. Recent studies have introduced a range of photonics-based phase-noise analyzers (PNAs) that utilize a variety of architectures, including phase detection, frequency discrimination, and hybrid mechanisms that combine optical with electronic processing. This review focuses on microwave photonic techniques for phase-noise measurement based on the fiber-optic delay-line method, by exploring their fundamental principles, system design frameworks, and performance indicators. The fiber-optic delay-line method is examined as the core architecture, due to the exceptionally low loss and wide bandwidth of the optical fiber, which enable long delays and high measurement sensitivity. Through the integration of insights garnered from recent publications, our objective is to deliver a comprehensive understanding of the strengths and limitations associated with fiber-optic delay-line-based PNAs and to pinpoint new and promising areas for advancing research in the field of oscillator metrology. Full article

With the increasing demand for high throughput and ultra-dense small cell deployment in the next-generation communication networks, spectrum resources are becoming increasingly strained. At the same time, the security risks posed by eavesdropping remain a significant concern, particularly due to the broadcast-access property of optical fronthaul networks. To address these challenges, we propose a high-security, high-spectrum efficiency radio-over-fiber (RoF) system in this paper, which leverages compressive sensing (CS)-based algorithms and chaotic encryption. An 8 Gbit/s RoF system is experimentally demonstrated, with 10 km optical fiber transmission and 20 GHz radio frequency (RF) transmission. In our experiment, spectrum efficiency is enhanced by compressing transmission data and reducing the quantization bit requirements, while security is maintained with minimal degradation in signal quality. The system could recover the signal correctly after dequantization with 6-bit fronthaul quantization, achieving a structural similarity index (SSIM) of 0.952 for the legitimate receiver (Bob) at a compression ratio of 0.75. In contrast, the SSIM for the unauthorized receiver (Eve) is only 0.073, highlighting the effectiveness of the proposed security approach. Full article

Quantum noise fundamentally limits the performance of fiber-optic systems beyond the standard quantum limit (SQL), restricting long-distance quantum key distribution, quantum communication, and precision quantum sensing. To overcome these limitations, quantum-squeezed states enable quadrature-dependent noise suppression, yet their benefits rapidly degrade under fiber attenuation, necessitating low-noise amplification. Since conventional phase-insensitive amplifiers (PIAs) impose a minimum 3 dB noise figure (NF) penalty and disrupt quantum correlations, phase-sensitive amplification (PSA) becomes essential. In this work, we propose a PSA based on dual-pump frequency-degenerate four-wave mixing (FWM) to amplify weak coherent squeezed states. Here, the PSA is seeded by an information-carrying single-mode squeezed state, where the information is encoded in the displacement degree of freedom, rather than in the squeezing itself. By optimizing the relative phases among the squeezed state, pump fields, and weak signal, the scheme maintains proper squeezing alignment and preserves the encoded quantum correlations during propagation. Under low-loss conditions, it is shown that the effective NF reaches −7.787 dB, demonstrating that the scheme enables quantum-limited amplification suitable for long-haul transmission and offering a viable path toward scalable fiber-based quantum technologies. Full article

This paper presents a novel method for the precise localization of remote radio-signal sources using a formation of unmanned aerial vehicles (UAVs). The approach is based on time-difference-of-arrival (TDoA) measurements and the geometric analysis of hyperbolas formed by pairs of UAVs. By studying the asymptotic intersections of these hyperbolas, the method ensures unique determination of the source position, even in the presence of multiple intersection points. Theoretical analysis confirms that the correct intersection point is located at a significantly larger distance from the UAV formation center compared to spurious intersections, providing a rigorous criterion for resolving localization ambiguity. The proposed framework also addresses secure inter-UAV communication via optical-fiber links and supports expansion of UAV groups with directional antennas and low-power signal relays. Additionally, the study discusses practical UAV configurations, including hybrid propulsion and jet-assisted kamikaze platforms, demonstrating the applicability of the method in contested environments. The results indicate that this approach provides a robust mathematical basis for unambiguous emitter localization and enables scalable, secure, and resilient multi-UAV systems, with potential applications in electronic-warfare scenarios, surveillance, and tactical operations. Full article

Nyquist pulses are critical in optical communication networks and signal processing systems. We present, to our best knowledge, the first demonstration of all-optical Nyquist pulse generation using phase-modulated fiber Bragg gratings (PM-FBGs) in transmission. PM-FBGs are a class of fiber gratings that have a nearly uniform coupling strength and a spatially varying grating period. As examples, we have designed and numerically simulated photonic Nyquist pulses with roll-off factors of 0.9, 0.5, and 0.1, respectively. The grating profiles are obtained employing numerical optimization algorithms. Numerical simulations confirm that the generated pulses are in good agreement with ideal Nyquist pulses over a 500 GHz bandwidth and have a good tolerance to the variations in the input pulse width. Full article

In optical receiver mode (ORM) division multiplexing optical communication systems, which can ultimately achieve a very high spectral efficiency, an accurate evaluation of the ORMs is crucial. Conventionally, to find the mode functions and the eigenvalues of ORMs, we have to solve an integral equation numerically. Here, we introduce a new method that solves a Schrödinger equation instead. This method assumes that the optical receiver uses an optical Fabry–Perot filter to select an optical channel from the received optical channels. The time-reversed impulse response of the optical receiver’s electrical filter is proportional to the potential in the Schrödinger equation. We show two potential cases that have exact solutions. One is the square-well potential case and the other is the exponential-well potential case. Full article

Direct detection (DD) is a straightforward, cost-effective receiving scheme for medium- and short-range fiber-optic communication systems, yet directly accessing phase information presents inherent challenges. The temporal transport-of-intensity equation (T-TIE) enables phase recovery from intensity data, but the accuracy of this phase-retrieval method is constrained by finite difference approximation errors of intensity derivatives and electrical noise interference. In this paper, we propose a 4th-order central difference method for calculating intensity derivatives to enhance approximation accuracy and implement multiple intensity measurements to further mitigate electrical noise interference. The proposed method is validated in a 28 GBaud single-carrier 16-quadrature amplitude modulation (16QAM) direct detection system. The research results indicate that, under conditions of 10 nA dark current and 20 pA/Hz^^1/2 thermal noise, our method achieves a receiver sensitivity gain of 14.85 dB compared with the 1st-order forward difference method and 8.47 dB compared with the 2nd-order central difference method at the 7% hard decision forward error correction (7% HD-FEC) threshold. Full article

This paper presents the construction of exact wave solutions for the generalized nonlinear Schrödinger equation (NLSE) with second-order spatiotemporal dispersion using the modified exponential rational function method (mERFM). The NLSE plays a vital role in various fields such as quantum mechanics, oceanography, transmission lines, and optical fiber communications, particularly in modeling pulse dynamics extending beyond the traditional slowly varying envelope estimation. By incorporating higher-order dispersion and nonlinear effects, including cubic–quintic nonlinearities, this generalized model provides a more accurate representation of ultrashort pulse propagation in optical fibers and oceanic environments. A wide range of soliton solutions is obtained, including bright and dark solitons, as well as trigonometric, hyperbolic, rational, exponential, and singular forms. These solutions offer valuable insights into nonlinear wave dynamics and multi-soliton interactions relevant to shallow- and deep-water wave propagation. Conservation laws associated with the model are also derived, reinforcing the physical consistency of the system. The stability of the obtained solutions is investigated through the analysis of modulation instability (MI), confirming their robustness and physical relevance. Graphical representations based on specific parameter selections further illustrate the complex dynamics governed by the model. Overall, the study demonstrates the effectiveness of mERFM in solving higher-order nonlinear evolution equations and highlights its applicability across various domains of physics and engineering. Full article

We propose a compact, beaconless inter-satellite laser communication tracking system based on direct fiber control to address the complexity and resource demands of conventional pointing, acquisition, and tracking (PAT) architectures. Unlike traditional sensor-based or beacon-assisted schemes, the proposed method employs a piezoelectric ceramic tube (PCT) to generate high-frequency, small-amplitude nutation of the single-mode fiber (SMF) tip, enabling real-time alignment correction using only the coupled optical power of the communication signal. This fully closed-loop tracking approach operates without position sensors and eliminates the need for beam splitting, external beacon sources, or auxiliary position detectors. A theoretical model is developed to analyze the influence of algorithm parameters and optical spot jitter on dynamic tracking performance. Experimental results show that the closed-loop system reliably converges to the optical spot center, achieving a fine-tracking accuracy of 4.6 $\mathsf{\mu }$rad and a disturbance suppression bandwidth of 200 Hz. By significantly simplifying the terminal architecture, the proposed approach provides an efficient and SWaP-optimized solution for inter-satellite and satellite-to-ground optical communication links. Full article

This study theoretically investigates the coupling efficiency of multi-Gaussian Shell-mode (MGSM) beams in ocean turbulence. The expression for the fiber-coupling efficiency of the MGSM beams propagating through oceanic turbulent media is derived using the cross-spectral density function. Numerical simulations are performed to examine the relationship between fiber-coupling efficiency and the beam order, and the scintillation index of the MGSM beams in ocean turbulence is also examined. In the analysis of transmission efficiency, the effects of the receiving aperture and source coherence on transmission efficiency are investigated, taking into account ocean turbulence induced by salinity and temperature fluctuations. The analysis of the fiber-coupling efficiency for MGSM beams presented in this work provides insights for optimizing the design of free-space optical communication systems. Full article

This study investigates the conformable nonlinear Schrödinger equation (NLSE) with self-phase modulation (SPM) and Kudryashov’s generalized refractive index, crucial for pulse propagation in optical fibers. By applying the modified simplest equation method, we derive several novel soliton solutions and investigate their dynamic behavior within the NLSE framework enhanced with a conformable derivative. The governing conformable NLSE also exhibits symmetry patterns that support the structure and stability of the constructed soliton solutions, linking this work directly with symmetry-based analysis in nonlinear wave models. Furthermore, various graphs are presented through 2D, 3D, and contour plots. These visualizations highlight different soliton profiles, including kink-type, wave, dark, and bell-shaped solitons, showcasing the diverse dynamics achievable under this model, influenced by SPM and Kudryashov’s generalized refractive index. The influence of the conformable parameter and temporal effects on these solitons is also explored. These findings advance the understanding of nonlinear wave propagation and have critical implications for optical fiber communications, where managing pulse distortion and maintaining signal integrity are vital. Full article

We present an experimental demonstration of a daylight-capable Optical Amplify-and-Forward (OAF) relaying system designed to support flexible and high-capacity network topologies. The proposed architecture integrates fiber-based infrastructure with OAF Free Space Optics (FSO) relaying, enabling bidirectional optical communication over 460 m (x2) using SFP-compatible schemes, while addressing Non-Line-of-Sight (NLOS) constraints and fiber disruptions. This work achieves a Bit Error Rate (BER) below the Hard-Decision Forward Error Correction (HD-FEC) limit, validating the feasibility of high-speed urban FSO links. By leveraging low-cost fiber-coupled optical terminals, the system transmits single-carrier 120 Gbps Intensity Modulation/Direct Detection (IM/DD) signals using NRZ (Non-Return-to-Zero) and PAM4 (4-Pulse Amplitude Modulation) modulation formats. Operating entirely in the optical C-Band domain, this approach ensures compatibility with existing infrastructure, supporting scalable mesh FSO deployments and seamless integration with hybrid Radio Frequency (RF)/FSO systems. Full article

With the increasing deployment of multi-wavelength free-space optical (FSO) systems in high-speed satellite data transmission, chromatic aberration has become one of the key factors limiting overall system performance. To address this challenge, this study proposes the integration of an achromatic metalens into a C-band FSO terminal. By employing a genetic algorithm (GA) for phase optimization, inter-wavelength phase compensation and focal consistency are achieved. The optimized metalens reduces the focal drift from 95 μm to 15 μm, representing an 84% reduction in focal deviation. System-level FSO link simulations further demonstrate that the achromatic design reduces the median equivalent BER from 1.2 × 10⁻² to 3.5 × 10⁻³ (a 71% reduction) and increases the FEC-qualified ratio from 25% to 60% (a 35% improvement), confirming its effectiveness in improving multi-wavelength link reliability. These results verify the effectiveness of the proposed approach in enhancing transmission stability and fiber-coupling efficiency in DWDM-FSO systems, providing a promising optical design strategy for high-capacity and broadband space optical communication terminals. Full article