Laser Communication Research Articles

Coherent beam combining of wavelength-division multiplexed telecom signals: toward high-power and high-bit-rate free-space optical telecommunications

This work presents a coherent beam combining system achieving constructive interferences of 8 arms seeded by 4 wavelength-division multiplexed (WDM) channels modulated at 10 Gbit/s each. The whole spectrum spans over 2.4 THz (19 nm) in the C-band. Phase-locking of one channel, combined with an accurate equalization process of the optical path differences (OPD) of the arms, allows us to maximize the combining efficiency of all the channels simultaneously. Each channel achieves a combining efficiency higher than 95%. Supported by an analytical description of the system, tolerancing rules underline the need for precise OPD equalization accuracy: a 5 µm OPD mismatch results in a 1% combining efficiency loss. Bit error rate and eye diagram analysis demonstrate the reliability of the system to transmit data without penalties. This marks a step toward the development of innovative WDM laser sources for high-power, high-bit-rate free-space optical telecommunications.

Comparative Test of Novel Fluorescent Optical Antennas for LED‐ and Laser‐Based Optical Wireless Communications

AbstractFluorescent Concentrators (FCs) are set to revolutionize Optical Wireless Communications (OWC), offering significant advantages over traditional receivers, such as large Field of View (FoV) and enhanced optical gain (OG). In this study, three different Optical Antennas are presented and evaluated as OWC receivers, exploiting FCs incorporating different fluorophores. The performances of DQ1 and Lumogen Red 305F (LR305), and a newly synthesized fluorophore (H2) are comparatively tested for solar energy harvesting, never proposed before for OWC. The devices are tested in high bit rate, laser‐based OWC configuration, and for indoor LED‐based Visible Light Communication (VLC) by presenting a realistic indoor VLC transmitter‐receiver (TX‐RX) system embedding high‐power white LEDs as transmitter. The results demonstrate that all the proposed OAs exhibit greater angular stability than traditional detectors, potentially offering greater resilience to misalignment and beam wandering effects, particularly relevant also in Free Space Optical (FSO) configurations. Furthermore, the intrinsic properties of H2 and LR305 make them particularly suited for white‐light VLC applications. Based on a thorough experimental characterization, the work is the first to propose and evaluate in detail the performance of various new OAs, depending on the specific application, highlighting the efficiency of such devices across various OWC fields.

Performance investigation of an OAM-SK optical communication system based on a PCNN-LDPC under atmospheric turbulence

This paper proposes a probabilistic convolutional neural network-low density parity check (PCNN-LDPC) demodulation scheme for orbital angular momentum shift keying free-space optical (OAM-SK-FSO) communication systems, with a focus on its bit error rate (BER) performance. Initially, the original information sequences were encoded by a low-density parity check (LDPC) and transformed into superposition state Laguerre Gaussian (LG) beams using a 16-Ary mapping scheme. The transmission of LG beams through atmospheric turbulence was then simulated using the power spectral inversion method, and the dataset was constructed and trained using a convolutional neural network (CNN). During demodulation, the CNN adaptive demodulator was integrated with the LDPC decoding algorithm. Classification probabilities from the CNN adaptive demodulator were used in the LDPC decoding process to enhance the channel estimation credibility. The computational results demonstrate that the BER of the PCNN-LDPC significantly outperforms both the CNN adaptive demodulator and CNN-LDPC demodulation strategy. Additionally, the performances of three LDPC decoding algorithms—min-sum (MS), normalized min-sum (NMS), and offset min-sum (OMS)—are compared, revealing that the probabilistic convolutional neural network-normalized min-sum (PCNN-NMS) achieves the best BER performance, highest convergence efficiency, and greatest decoding stability. These findings provide theoretical references and technical support for studying the BER performance of OAM-SK-FSO communication systems.

Enhancing Free Space Optical System Performance through Fog and Atmospheric Turbulence using Power Optimization

Free Space Optical (FSO) communication is gaining traction as a pivotal technology for next-generation communication systems, offering extremely high data rates, unlicensed bandwidth, and rapid transmission capabilities. However, its performance is significantly hindered by atmospheric factors such as turbulence and fog. This paper presents a comprehensive model for FSO communication designed to optimize performance under various atmospheric conditions. We assess channel capacity across a spectrum of weak to strong atmospheric turbulence using a Gamma-Gamma channel distribution. To mitigate channel losses, our system employs Wavelength Division Multiplexing (WDM) and multi-beam Multiple Input Multiple Output (MIMO) technologies. Results indicate that the integration of diversity techniques and WDM substantially enhances system performance in adverse weather conditions. Furthermore, power optimization is achieved through the implementation of optical amplifiers and feedback mechanisms from the receiver to the transmitter to adjust the transmitter power in accordance with received Bit Error Rate (BER). The proposed power-optimized WDM MIMO system demonstrates a remarkable BER of 1.48884e-15, while extending the transmission link distance to 2500 meters with a Q factor of 21.5 even under strong atmospheric turbulence conditions.

LDPC-coded OAM shift-keying FSO communication system with dual-pattern CNN demodulator

An LDPC-coded orbital angular momentum shift-keying (OAM-SK) free space optical (FSO) communication system with a dual-pattern convolutional neural network (CNN) demodulator is put forward to resist the adverse effect of atmospheric turbulence (AT). Diverging from the standard single-pattern CNN demodulator, the proposed method inputs a dual OAM light pattern into the demodulator. The first pattern stems from the Laguerre-Gaussian OAM-SK light focused by a convex lens, while the second is acquired post-cylindrical lens modulation. The dual-pattern CNN demodulator can extract richer features from the incoming light, enhancing the recognition accuracy for OAM-SK modes. This advancement notably enables the recognition of OAM-SK modes with opposite orbital quantum numbers, a challenging task for standard single-pattern CNN demodulators. We have refined communication reliability by integrating LDPC coding with OAM-SK, and LDPC decoding has also been explicitly designed for OAM-SK channels. Simulations validate our proposed method, achieving a recognition accuracy 0.869 under strong AT (Cn2=5×10−14m−2/3). We use the image transmission as a benchmark; the dual-pattern CNN demodulator enhances the LDPC-coded OAM-SK FSO link, achieving a 30 dB boost in the image’s PSNR compared to the traditional CNN demodulation. The bit error rate (BER) drops to 3.8e−5, achieving significant advancement in comparison to the single-pattern CNN demodulators.

Single-shot diffractive spectrometer for photonic orbital angular momentum

Vortex beams carrying orbital angular momentum (OAM) provide an infinite degree-of-freedom and hold high potential in various applications, from high-capacity optical communication to diagnosis of materials with chirality. Quantitative spatial spectrum analysis of OAM modes is essential for these applications, yet it is still a challenge to obtain the OAM spectrum under short-wavelength systems such as extreme ultraviolet (EUV) and x-ray. Here, we introduce a simple single-shot diffractive method that can reconstruct arbitrary helical wavefront and quantitatively decompose individual OAM modes. There is no need to conduct any calibration associated with the beam to be measured; the only prior knowledge required is the transmission function of a random diffusing wavefront modulator. Experimental results show that this method can retrieve the spectrum of arbitrary OAM modes with intermodal crosstalk lower than −16.91 dB for topological charge greater than 50. The proof-of-concept visible light experiments of multiplexing and demultiplexing of OAM modes showed its potential applications in laser communication and metrology. Given the simplicity of lens-less system setup, the single-shot capability, and its suitability for arbitrary OAM modes, we envision it setting up a brand-new diffractive solution for structured wavefront analysis over a broad spectral range, from visible light to EUV, x-ray, and even electron beam.

Photon Communication Systems: "Exploring New Frontiers of Radio Frequency Connectivity"

Photon communication is an optical free-space communication. Speculate to be the upcoming generation wireless communication technology due to its latent features like high data rate increased bandwidth unregulated spectrum immunity to interference etc. Laser communication offers a viable alternative for inter-satellite links and other applications to radio frequency (RF) communication, where high - performance links are necessary. In defense, laser communication facilitates reliable satellite communications (SATCOM), secure drone operations, and high-speed data exchange between naval vessels, aircraft, and ground units

240 Gbps SI-FMF/FSO communication system based on LP modes and PV code with the impact of foggy weather in Alexandria and Setif

This paper presents a novel, to the best of our knowledge, high-speed optical communication system achieving a 240 Gbps transmission capacity by integrating step-index few-mode fiber (SI-MMF) and free space optics (FSO). Knowing that Hermite-Gaussian modes (HGMs) are eigenmodes of a free space medium and the linearly polarized modes (LPMs) are eigenmodes of step-index few-mode fiber (SI-FMF), and by considering the similarity between HGM ( and ) and LPM ( and ), respectively, the system employs dual-polarization (DP) and linearly polarized (LP) modes (, , and ), combined with optical code division multiple access (OCDMA) using permutation vector (PV) codes. Each polarization state carries data from three LP modes, with each mode supporting four OCDMA channels, each assigned a unique PV code. The system’s performance is evaluated under varying fog conditions using key metrics such as quality-factor (Q-factor), bit error rate (BER), and eye diagrams, which provide a comprehensive assessment of signal strength and quality. Real-world applicability is assessed using meteorological data from two African cities, Alexandria, Egypt, and Setif, Algeria. System performance is characterized using the Q-factor, bit error rate (BER), and eye diagrams. The results show that, in Alexandria, successful transmission is achieved over 1.41 km. In Setif, the transmission distance is reduced to 1.31 km due to slightly higher atmospheric attenuation. These ranges are observed at a BER below HG00 and a Q-factor exceeding 3.10. Consequently, these findings confirm the viability of the proposed system for high-capacity optical communication in challenging weather conditions, making it suitable for future 6G networks.

Performance Evaluation of Free Space Optical Communication Terminal with Local Weather Considerations

Performance Evaluation of Free Space Optical Communication Terminal with Local Weather Considerations

Dual-wavelength synchronous control method for liquid crystal optical phased array

The liquid crystal optical phased array (LCOPA), as a key beam steering device, has gained increasing significance in the field of space laser communication. With the rapid advancement of space laser communication technology, the demand for precise synchronous control of multi-wavelength beams has significantly increased, particularly in ensuring reliable communication links through synchronized control of signal and beacon beams. The signal beam is primarily utilized for data transmission, while the beacon beam is responsible for path calibration and real-time tracking. However, due to the limitations of natural dispersion effects, conventional LCOPA control methods struggle to achieve synchronized manipulation of beams at different wavelengths, resulting in error accumulation and response delays in communication links, thereby compromising the accuracy and efficiency of information transmission. To address this challenge, this study proposes and validates a dual-wavelength synchronous control method based on LCOPA. The method establishes a phase optimization principle centered on minimizing the least-squares error of complex amplitudes and expands the phase modulation capability of LCOPA hardware, thereby overcoming the natural dispersion governed by the grating equation. Simulation and experimental results demonstrate that this method achieves exceptionally high beam pointing accuracy, meeting the demands of high-precision information transmission in multi-wavelength laser communication. This study provides an innovative technical pathway for the application of LCOPA in multi-wavelength laser communication and establishes a solid theoretical foundation for future experimental research on multi-wavelength control.

Phase control method for generating orbital angular momentum beams using coherent beam combining based on deep learning

In recent years, orbital angular momentum (OAM) beams have shown great potential for applications in laser communication, laser processing, optical imaging, and detection. For free-space optical communication, high-power, high-quality vortex beams with a high signal-to-noise ratio are critical for long-distance communication. Coherent beam combining (CBC) of vortex beams enables the enhancement of power while maintaining high beam quality. Considering the orbital angular momentum spectrum as a new dimension of optical wave resources, achieving rapid phase locking of specific phases is crucial for increasing communication capacity. Traditional phase control methods based on wavefront intensity distribution face limitations in optimization, particularly for centrosymmetric laser phased arrays. To address this, we propose a deep learning-based method using spiral phase modulation. By designing a loss function that eliminates phase periodicity, we establish a nonlinear mapping between the sub-beam phases and the far-field image. To improve the phase prediction accuracy of the deep learning model, we introduce a power-in-the-bucket (PIB) metric for the vortex beam’s main lobe, which mitigates dynamic phase errors caused by thermal and environmental disturbances. This method holds promise for application in high-power vortex beam optical systems with coherent combining of fiber laser phased arrays.

Creation of Bessel–Gaussian Beams from Necklace Beams via Second-Harmonic Generation

The interest in (quasi-)nondiffracting beams is rooted in applications spanning from secure sharing cryptographic keys real-world free-space optical communications and high-order harmonic generation to high-aspect-ratio nanochannel machining, photopolymerization, and nanopatterning, just to mention a few. In this work, we explore the robustness of the approach for generating Bessel–Gaussian beams by Fourier transforming ring-shaped beams and push the limits further. Here, instead of ring-shaped beams, we use strongly azimuthally modulated necklace beams. Necklace structures are generated by interference of OV beams that carry equal topological charges of opposite signs. In order to effectively account for the azimuthal π-phase jumps in the necklace beams, we first generate their second harmonic, thereafter focusing (i.e., Fourier transforming) them with a thin lens. In this way, we successfully create Bessel–Gaussian beams in the second harmonic of a pump beam with strong azimuthal modulation. The experimental data presented are in good agreement with the developed analytical model.

Demonstration of atom interrogation using photonic integrated circuits anodically bonded to ultra-high vacuum envelopes for epoxy-free scalable quantum sensors

Reliable integration of photonic integrated circuits (PICs) into quantum sensors has the potential to drastically reduce sensor size, ease manufacturing scalability, and improve performance in applications where the sensor is subject to high accelerations, vibrations, and temperature changes. In a traditional quantum sensor assembly, free-space optics are subject to pointing inaccuracies and temperature-dependent misalignment. Moreover, the use of epoxy or sealants for affixing either free-space optics or PICs within a sensor vacuum envelope leads to sensor vacuum degradation and is difficult to scale. In this paper, we describe the hermetic integration of a PIC with a vacuum envelope via anodic bonding. We demonstrate utility of this assembly with two proof-of-concept atom-interrogation experiments: (i) spectroscopy of a cold-atom sample using a grating-emitted probe; (ii) spectroscopy of alkali atoms using an evanescent field from an exposed ridge waveguide. This work shows a key process step on a path to quantum sensor manufacturing scalability.

Correlation of instantaneous fading in bidirectional channels of atmospheric laser communication

The instantaneous fading correlation of the bidirectional channel is an important characteristic of the atmospheric random channel. The instantaneous channel state of the transmitting end can be directly obtained through the instantaneous channel state of the receiving end, reducing the overhead of the additional feedback branch. In this paper, an autoregressive (AR) model and an inverse transformation method are used to generate time-dependent optical signals obeying an APD exponential Weibull distribution. Based on the channel reciprocity function, a channel cross-correlation model on the laser communication transmission path is established, and the influence of factors such as transmitting and receiving apertures, zenith angle, transmission distance, altitude, wavelength, and detector responsivity on the cross-correlation coefficient is studied. Based on theoretical research, an experimental system was built to verify the heterogeneous reciprocity model, and the experimental results were basically consistent with the simulation results. A channel state information prediction and evaluation are achieved through a relevant reciprocity theory, providing a basis for the development of adaptive technology in atmospheric laser communications.

Topology Control of Low-Connection UAV Laser Network with Virtual Nodes

Space laser communication technology has advantages such as high bandwidth and low interference, but the limitation of the load capacity of an unmanned aerial vehicle (UAV) makes it difficult for UAV clusters to have sufficient communication links to build a stable communication network. Aiming to address the problem of the low upper limit of the node degree of the self-organized network after the introduction of space laser communication technology for UAVs, a virtual node topology control (VNTC) algorithm is proposed for constructing an efficient and stable communication network structure. The algorithm is based on the unit disc graph model and achieves a lower-node-degree upper bound while maintaining connectivity through node pairing and virtual graph construction. The theoretical analysis and experimental results of the algorithm show that the algorithm can effectively deal with the topology control problem in UAV laser networks. In contrast to the XTC algorithm (exceptional topology control), the VNTC algorithm maintains the connectivity and most of the planarizability of the network, and, at the same time, it performs better in reducing the node degree and number of edges, thus providing an effective solution for low-connectivity topology control.

Study on time-frequency domain characteristics and cross-media communication of laser acoustic signals based on optical breakdown mechanism

Abstract Cross-media communication is a hot research issue at present. The use of sound waves generated by laser induced sound mechanism for cross-media communication is a new topic proposed in recent years. It is of great significance to study the characteristics of laser acoustic signal and the feasibility study for cross-media detection. The time-frequency characteristics and directivity of the laser induced sound signal are studied on the self-built experimental platform. The results show that the pulse duration of the laser acoustic signal is about 0.2ms, and the peak frequency is about 14.89kHz. The directivity is anisotropy, the signal strength in horizontal direction is the strongest, and the signal strength perpendicular to the water surface is the weakest, so it has a good horizontal distance transmission ability. This paper verifies the detection system of trans-air-sea interface communication and trans-air-sea interface communication based on laser sound and conducts the communication experiment of laser acoustic signal. By modulating laser energy, the underwater acoustic signal is modulated in time domain, and the signal communication with 0-bit error rate of 8bps within the meter is realized. The detection threshold of acoustic signal is calculated to be 44.93dB after horizontal transmission of 1km. The analysis and simulation of water surface micro-motion based on laser induced sound are carried out. The results show that the wave crest of water surface generated by laser acoustic signal through underwater reflection reaches the order of micron and can be detected by radar. The surface wave intensity is little affected by the depth of detection, so it could work in deep water. This study provides a certain experimental and theoretical basis for laser communication across the air and sea interface.

Performance Analysis of Photon-Limited Free-Space Optical Communications with Practical Photon-Counting Receivers

Performance Analysis of Photon-Limited Free-Space Optical Communications with Practical Photon-Counting Receivers

Propagation Properties of Partially Coherent Flat-Topped Beam Rectangular Arrays in Plasma and Atmospheric Turbulence

Propagation properties represent a critical aspect of laser beams utilized in free space optical (FSO) communications. We examined the evolution characteristics of the electric field associated with partially coherent flat-topped beam rectangular arrays propagating bidirectionally through the turbulent atmosphere and plasma links. Utilizing the optical transmission matrix, alongside the second moment theory and Wigner distribution functions, we derived analytical expressions for both the intensity distribution and propagation factors of the partially coherent flat-topped beam rectangular arrays affected by the atmospheric turbulence and plasma disturbances. The numerical results indicate that appropriately selecting parameters such as beam order, transverse spatial coherence width, and beam width can effectively mitigate the adverse effects on propagation properties caused by the turbulent atmosphere and plasma. Our results have significant implications for FSO communications within specific environmental contexts.

Enhanced Neural Architecture for Real-Time Deep Learning Wavefront Sensing.

To achieve real-time deep learning wavefront sensing (DLWFS) of dynamic random wavefront distortions induced by atmospheric turbulence, this study proposes an enhanced wavefront sensing neural network (WFSNet) based on convolutional neural networks (CNN). We introduce a novel multi-objective neural architecture search (MNAS) method designed to attain Pareto optimality in terms of error and floating-point operations (FLOPs) for the WFSNet. Utilizing EfficientNet-B0 prototypes, we propose a WFSNet with enhanced neural architecture which significantly reduces computational costs by 80% while improving wavefront sensing accuracy by 22%. Indoor experiments substantiate this effectiveness. This study offers a novel approach to real-time DLWFS and proposes a potential solution for high-speed, cost-effective wavefront sensing in the adaptive optical systems of satellite-to-ground laser communication (SGLC) terminals.

Low-complexity 24-h 29-km inter-island real-time FSO communication under strong turbulences.

Free space optical communication (FSOC) technology can be used for data transmission between ocean islands as backup wireless communication networks to cope with traffic surges and emergencies. In this paper, we experimentally demonstrate the results of a 24-h real-time single-wavelength 2.5-Gbps FSOC between two islands 29 km apart at a low altitude with low complexity. On-off keying signals generated by field-programmable gate array (FPGA) board and commercial low-cost SFP + optical modules are employed as transceiver signals. At the receiving end, a 250-mm large-aperture Cassegrain telescope and 105-µm multi-mode fibers (MMFs) are utilized to reduce the scintillation index from 1.5 to 0.48 and from 0.55 to 0.25 under strong and weak fluctuations, respectively. The averaged receiving optical power, scintillation index, and average bit rate error (BER) are experimentally observed over a 24-hour period with an averaged transmitting power of 36 dBm. The experimental results show that the proportions reach 93.73% and 99.9% for BER below 1 × 10-6 and 1 × 10-3, respectively.