Laser Communication Research Articles

Mitigating the Impact of Lognormal Atmospheric Turbulence Channel Estimation on FSO Communication Systems Using Advanced Deep Learning Modules

ABSTRACTCurrently, researchers and commercial entities worldwide are highly interested in optical wireless communication links operating in terrestrial environments. These links offer the distinct advantage of enabling high‐speed data transmission while keeping operational and installation costs low, all without requiring licensing. Free‐space optical (FSO) communication has established itself as a reliable method for delivering ultra‐high data‐rate services. However, the presence of atmospheric turbulence‐induced fading poses a significant challenge for FSO communication systems, particularly when operating over channels affected by time‐varying turbulence. The effectiveness of FSO systems is greatly influenced by atmospheric conditions in a specific area because the laser beam propagates through the atmosphere. As a consequence, this can lead to a significant degradation such as high complexity, high estimation error, and high bit error rate (BER) in the performance of FSO systems. This paper presents a stacked dual attention LSTM network with layered neural Turing machine (SDALSTM‐LaNTM) for FSO channel estimation over lognormal turbulence‐induced fading channels. In this approach, the stacked structure ensures a robust feature extraction capacity of SDALSTM and enhances its ability to capture intricate relationships among multiple atmospheric variables. After employing the stacked LSTM networks, the extracted features are combined with the augmented features obtained through the dual attention (DAttn) mechanism by concatenation. Then the LaNTM architecture used external memory to solve the channel parameter estimation problem and update new set of hidden features for estimation task. The ability to access and update memory enables the model to capture long‐term dependencies and context, making it beneficial for channel prediction in FSO systems. Presently, we are in the process of verifying the channel estimation model using measured FSO channel data. Simulation results clearly demonstrate the outstanding estimation performance of the SDALSTM‐LaNTM model in FSO systems, specifically when dealing with lognormal turbulence‐induced fading channels. Moreover, we highlight the effectiveness of the SDALSTM‐LaNTM model by conducting a comprehensive comparison with existing models, considering factors such as average capacity performance, outage probability estimation, and BER.

In-orbit demonstration of acquisition and tracking on OSIRIS4CubeSat

OSIRIS4CubeSat is the smallest commercially available laser communication terminal within the confines of 0.3-units. It was launched as PIXL-1 inside of a 3-unit CubeSat into space to demonstrate optical direct to earth links. This work primarily focuses on the commissioning phase, particularly emphasizing the performance of the active fine pointing and tracking mechanism. This steering mechanism, with a 1 degree radius field of regard, plays a decisive role in compensating for pointing inaccuracies from the satellite’s attitude determination and control system. The design and validation of open-loop acquisition and closed-loop tracking are presented. The feedback sensor adapts to changing beacon power levels, that occur during a direct to earth link, by an adaptive gain control. Subsequent evaluation of satellite passes, with telemetry recordings obtained from the deployed CubeSat, showcases superior performance that meets the pointing requirements of the link budget. By using the developed control-loop logic, OSIRIS4CubeSat achieves a mean tracking error of 71 µrad, with a 3σ deviation of 140 µrad down to low power levels of 238 pW. These results serve as validation of OSIRIS4CubeSat, demonstrating its capability to enable high-speed data transmission from low Earth orbit to the Earth’s surface at data rates of up to 100 Mbit/s.

Generation of partially coherent full Poincaré beam arrays and their Stokes scintillations in turbulent media

A type of vector beam arrays, called partially coherent full Poincaré (PCFP) beam arrays, is introduced and experimentally synthesized using modal-vector-decomposition method. Our experimental system involves a digital micro-mirror device, which can generate such beam arrays with controllable spatial coherence and array structure in almost real-time, enabling to test the impact of atmospheric turbulence on them. Furthermore, we experimentally examine the scintillations of four Stokes parameters (denoted as S0, S1, S2, and S3) of PCFP beam arrays propagating through lab-simulated turbulence. The results indicate that the Stokes scintillations decrease as the number of beamlets increase or spatial coherence decreases. In contrast to S0 scintillation (intensity scintillation), S1, S2, or S3 scintillation is less affected by the turbulence under the same conditions. Our experimental results show that S2 and S3 scintillations could reduce by 67.2% and 52.4% compared to the intensity scintillation in strong turbulence. Our findings have potential applications in free-space optical communication when the Stokes parameter S2 or S3 is served as an information carrier.

Demodulating Optical Wireless Communication of FBG Sensing with Turbulence-Caused Noise by Stacked Denoising Autoencoders and the Deep Belief Network

Free-space optics communication (FSO) can be used as a transmission medium for fiber optic sensing signals to make fiber optic sensing easier to implement; however, interference with the sensing signals caused by the optical turbulence and scattering of airborne particles in the FSO path is a potential problem. This work aims to deep denoise sensed signals from fiber Bragg grating (FBG) sensors based on FSO link transmission using advanced denoising deep learning techniques, such as stacked denoising autoencoders (SDAE). Furthermore, it will demodulate the sensed wavelength of FBGs by applying the deep belief network (DBN) technique. This is the first time the real FBG sensing experiment has utilized the actual noise interference caused by the environmental turbulence from an FSO link rather than adding noise through numerical processing. Consequently, the spectrum of the FBG sensors is clearly modulated by the noise and the issue with peak power variation. This complicates the determination of the center wavelengths of multiple stacked FBG spectra, requiring the use of machine learning techniques to predict these wavelengths. The results indicate that SDAE is efficient in denoising from the FBG spectrum, and DBN is effective in demodulating the central wavelength of the overlapped FBG spectrum. Thus, it is beneficial to implement an FSO link-based FBG sensing system in adverse weather conditions or atmospheric turbulence.

Spatiotemporal evolution of dispersion-induced pulse broadening in underwater laser communication

Spatiotemporal evolution of dispersion-induced pulse broadening in underwater laser communication

Enhancing the efficiency of wavelength-shift fiber-based detectors for optical wireless communications

A wavelength-shift fiber-based optical detector promises to revolutionize the deployment of optical wireless communication (OWC) due to its inherent advantages over traditional receivers. These advantages include a flexible structure, a wide field of view (FOV), and a large active area. Despite progress in previous studies, there remains a gap in optimizing the re-utilization of unabsorbed light within wavelength-shift fiber (WSF) and maximizing the efficiency of light focusing onto photodetectors. To address these challenges, this study explores three novel, to the best of our knowledge, approaches to enhance the light conversion and detection efficiency of WSF-based optical detectors. First, a reflective mirror is employed behind the WSF array to increase the light absorption and re-emission probability. Second, a reflective mirror is placed at one end of the WSF array to direct the light toward the opposite end. Third, a tightly bundled WSF array configuration focuses the emitted light onto the photodetector’s active area. Experimental results demonstrate that each approach significantly improves the peak-to-peak voltage. This work presents an optical detector design featuring a large active area of 0.4cm×20cm, based on a blue-to-green color-converting WSF and achieving a high 3-dB bandwidth of up to 48 MHz. This design enables real-time data transmission at rates of 275 Mbps using non-return-to-zero on-off keying (NRZ-OOK) modulation over a distance of 1 m. Additionally, the transmission link operates at over 250 Mbps, with bit error rates (BERs) below the forward error correction (FEC) limit, under a wide FOV of 60°. This work opens exciting possibilities for revolutionizing photodetection schemes in non-line-of-sight free-space optical communications.

Performance comparison of different intensity modulation formats for FSO systems

Abstract Intensity modulation (IM) formats are the legacy formats which are continuing to act as modulation formats of interest in fiber optic communication systems and optical networks. The IM formats have become formats of interest in free space optic communication (FSO) systems due to its simple generation and detection principle, thus making FSO system economical and easier to handle/operate under adverse and random characteristics of atmospheric channel. This paper compares the performance of different IM formats (with memory and without memory or memoryless). The IM formats considered here are non return to zero (NRZ), return to zero (RZ), carrier suppressed RZ (CSRZ), vestigial side band CSRZ (VSB-CSRZ), duobinary RZ (DRZ) and modified duobinary RZ (MDRZ). The comparative analysis investigates the effect of launch power and length of FSO channel at different data rates of 10 Gb/s and 40 Gb/s for single channel FSO system, with the help of simulations. The results discussed, depicts that VSB-CSRZ outperforms other modulation formats at 10 Gb/s and duobinary format has shown lesser degradation in system performance when bit rate increases from 10 Gb/s to 40 Gb/s.

MDM‐Incorporated Quad Donut Modes OCDMA‐FSO Scheme for Secure High‐Altitude Platforms‐To‐Satellite Scenarios

ABSTRACTThe next generation of communication networks is envisioned to be driven by high‐altitude platform (HAP)‐to‐satellite systems. License‐free narrow beam free space optics (FSO) can provide uninterrupted connectivity in satellite‐enabled Internet of Things scenarios. This work proposes a HAP‐to‐satellite hybrid mode division multiplexing–optical code division multiple access (MDM‐OCDMA) scheme employing zero cross‐correlation (ZCC) and multidiagonal (MD) codes. In high‐speed satellite communications, a 12 × 10‐Gbps hybrid MDM‐OCDMA scheme carrying four donut modes (0, 1, 2, and 3) enables high channel capacity, sufficient spectral efficiency, and data security under severe link conditions. Based on simulations, the hybrid MDM‐OCDMA scheme using the ZCC code achieves a greater FSO link distance of 420 m compared with when using the MD code at a 10e‐9 error correction limit under clear air and weak turbulent scenario. The reliable transmission distance of 250 m is achievable with the 120‐Gbps proposed system, assuming pointing errors of 2 mrad, 1‐dB receiver, and 2.5‐dB transmitter losses. The results also show that the system can sustain an additional loss of 1.5–2 dB over 250 m for all donut modes. Additionally, the system using ZCC code contributes to low power consumption over MD and variable weight ZCC code and thus requires a minimum received power of −4.02 dBm. It also offers high optical signal to noise of 42–52 dB, −11.58 to −22.20 dB of gain, 11.58–22.20 dB of noise figure, and −48.11 to −58.73 dBm of signal power at output over 200–700 m range up to 2 dB of additional loss. Comparative analysis indicates that the proposed system is more efficient, less complex, adequately distance‐friendly (=420 m), and capable of higher data rates (=120 Gbps) compared with other works. This makes it a promising solution for future high‐speed satellite communications considering the impact of link losses and atmospheric conditions.

Scintillation index and outage probability of vortex Gaussian beams for horizontal links in weak atmospheric turbulence

Abstract By using Rytov method, for the Gaussian vortex beam, off-axis scintillation index of is examined in horizontal laser communication links operating in weakly turbulent atmosphere. Performances of laser communication systems, defined by the outage probability in this study, is evaluated by the use of lognormal distributed intensity in finding the scintillation index. Off-axis scintillation index of vortex Gaussian beams is analytically derived and evaluated in horizontal atmospheric link. The scintillation index obtained from the figures drawn versus the source size and propagation length is used to calculate the outage probability. It is found that turbulence affects vortex Gaussian beam less when compared with Gaussian beams that are non-vortex. Our important finding is that the scintillation index reduces when the topological charge increases.

Retraction Note: Homodyne detection based multiple-beam WDM FSO communication system under various environmental conditions

Retraction Note: Homodyne detection based multiple-beam WDM FSO communication system under various environmental conditions

Research on Tracking Control Technology Based on Fuzzy PID in Underwater Optical Communication

In order to realize effective laser communication on underwater mobile platforms, the active tracking and alignment technology of underwater laser communication (UWLC) is studied. Firstly, the servo control principle of the UWLC system is analyzed. Secondly, aiming at underwater disturbance, an adaptive fuzzy PID controller is designed to realize parameter self-tuning, thereby improving the system’s anti-disturbance ability. The designed algorithm was simulated, and the simulation results show that the adaptive fuzzy PID algorithm has better anti-disturbance ability and tracking performance than the traditional PID. Finally, an experimental platform was built for dynamic tracking experiments, and the results show that the dynamic tracking accuracy of the designed control algorithm was improved by 30.29% compared with the traditional control algorithm, which provides a certain reference for the development of laser communication on underwater moving platforms.

Atmospheric turbulence-distorted OAM mode recognition via multi-scale dilated convolution with multi-level feature fusion

Vortex beams with orbital angular momentum (OAM) significantly enhance system capacity, and high-precision recognition of OAM mode through atmospheric turbulence (AT) channels can markedly improve the information transmission capability of free-space optical communication systems. In this paper, with a cylindrical lens-assisted distinguish between positive and negative OAM, a reliable neural network combining multi-scale dilated convolution (MSDC) unit and multi-level feature fusion (MLFF) module is proposed to detect high order AT-distorted OAM modes. The network fully exploits the features in light-intensity images to achieve a highest recognition accuracy of 99.4% for mode-orders from -20 to +20 in a hybrid ATs dataset (C n 2 = 5×10−16, 5×10−14, 5×10−12 m-2/3), and almost 96% even in strong turbulence. Experimental results on accuracy, efficiency, reliability, and robustness demonstrate that the proposed method excels and provides a trustworthy solution for complex AT-distorted OAM mode recognition.

Deep-learning-assisted optical communication with discretized state space of structured light

Abstract The rich structure of transverse spatial modes of structured light has facilitated their extensive applications in quantum information and optical communication. The Laguerre-Gaussian (LG) modes, which carry a well-defined orbital angular momentum (OAM), consist of a complete orthogonal basis describing the transverse spatial modes of light. The application of OAM in free-space optical communication is restricted due to the experimentally limited OAM numbers and the complex OAM recognition methods. Here, we present a novel method that uses the advanced deep learning technique for LG modes recognition. By discretizing the spatial modes of structured light, we turn the OAM state regression into classification. A proof-of-principle experiment is also performed, showing that our method effectively categorizes OAM states with small training samples and the accuracy exceeds 99% from 3-dimensional to 15-dimensional space. By assigning each category a classical information, we further apply our approach to an image transmission task, achieving a transmission accuracy of 99.58%, which demonstrates the ability to encode large data with low OAM number. This work opens up a new avenue for achieving high-capacity optical communication with low OAM number based on structured light.

Enhanced Performance Analysis of 120 Gbps Single‐Channel IQ Modulation Based PDM‐FSO Transmission System Using 64‐QAM Signals

ABSTRACTThe ever‐augmenting demand of data channel‐bandwidth by the end users has resulted in the exploration of new transmission techniques for wireless transmission networks. In this work, we have demonstrated the modeling of a single‐channel free space optics (FSO) transmission system by employing polarization‐division‐multiplexing (PDM) technique. Further, in‐phase‐quadrature modulation (IQM) has been used to increase the transmission efficiency of the system. To decrease the baud‐rate of the system, we have proposed the use of 64‐level quadrature amplitude modulation (QAM) signals. The net transmission rate of the system is 120 Gbps with a baud‐rate of 10 Gbps. Furthermore, the overcome the signal degradation of the 120 Gbps signal propagating through atmospheric channel, we have reported the use of advanced digital signal processing (DSP) algorithms. The system's performance is investigated for adverse external climate weather conditions and the results reported show reliable 120 Gbps data transmission along range between 870 m and 16.25 km with good performance of BER and EVM 7.5%.

On mitigating reactive jamming with dynamic resource allocation in optical IRS and UAV-assisted FSO-based networks

Free space optics (FSO) offers a promising opportunity to enhance next-generation network’s capacity with its unlicensed spectrum and wide bandwidth. However, jamming attacks, coupled with inherent anomalies in the FSO-based channel, threaten the performance of these networks. This is especially problematic for security-sensitive applications that demand a resilient communication infrastructure. To address this issue, optical intelligent reflecting surfaces (IRS) and unmanned aerial vehicles (UAVs) can provide promising solutions. This work introduces an efficient approach for mirror element assignment in UAV-assisted FSO-based networks, aimed at mitigating reactive jamming attacks while satisfying users’ quality-of-service (QoS) requirements. To ensure network reliability, we formulate an optimization problem that enhances overall network performance by simultaneously allocating resources such as mirror elements with awareness of jamming attacks. The formulated optimization problem is a binary linear programming problem, which is generally NP-hard. To address this, we introduce a batch-based sequential fixing linear programming procedure called the Reactive Jamming-Aware Mirror Element Allocation (RJA-MEA) scheme. This scheme optimally assigns mirror elements to satisfy the users’ rate demands. In this paper, the performance of the RJA-MEA scheme is compared with reference schemes such as Reactive Jamming Unaware-Mirror Element Allocation (RJU-MEA), Reactive Jamming-Aware Equal Mirror Element Allocation (RJA-EMEA), and Reactive Jamming Unaware-Equal Mirror Element Allocation (RJU-EMEA) schemes. The simulation results reveal that the proposed RJA-MEA scheme surpasses existing reference schemes, thereby significantly improving the overall network sumrate performance.

Designing metasurface optical interfaces for solid-state qubits using many-body adjoint shape optimization

We present a general strategy for the inverse design of metasurfaces composed of elementary shapes. We use it to design a structure that collects and collimates light from nitrogen-vacancy centers in diamond. Such metasurfaces constitute scalable optical interfaces for solid-state qubits, enabling efficient photon coupling into optical fibers and eliminating free-space collection optics. The many-body shape optimization strategy is a practical alternative to topology optimization that explicitly enforces material and fabrication constraints throughout the optimization, while still achieving high performance. The metasurface is easily adaptable to other solid-state qubits, and the optimization method is broadly applicable to fabrication-constrained photonic design problems.

Continuously-tunable, compact, freespace notch-filter design using an all-dielectric metagrating capped with a low-loss phase change material

Abstract Active metasurfaces utilizing phase change materials (PCMs) are currently under investigation for applications in free-space optical communication, optical signal processing, neuromorphic photonics, quantum photonics, and compact LiDAR. Attention has now turned towards novel PCM like Sb2S3 which exhibit lower optical absorption and reasonable values of refractive-index contrast in comparison to traditional data-storage PCM. We propose and numerically study the class of all-dielectric metagratings capped with low-loss PCM and predict the possibility of continuously tunable resonances whose quality factors degrade gracefully during the amorphous-to-crystalline phase transition of the PCM. Specifically, we consider the CMOS-compatible silicon-nitride on silica substrate material platform for simple and asymmetric metagratings (in particular, the symmetric-broken dimerization) and Sb2S3 capping. Our numerical study predicts that notch-filters operating around the 1550 nm NIR wavelength window can be achieved with tuning range of over 76 nm with Q-factors ranging from 784 (amorphous-phase) to 510 (crystalline-phase) (a degradation in Q of about 35%) and insertion loss of about 0.9 dB. These performance figures are a significant improvement over previously published designs utilizing data-storage PCMs and other traditional notch-filter mechanisms. We examine the influence of grating dimerization and geometrical parameters on performance metrics of the notch-filter and predicts the possibility to trade-off rejection-band and in-band spectral transmission properties. Lastly, we perform a study of all-optical phase change mechanism. Our study is promising for the miniaturization of tunable notch-filter based optical systems.

MEMS fast steering mirror with a large optical aperture and high surface quality for free-space optical communication

This work presents a MEMS fast steering mirror fabricated by an 8-inch AlScN platform, with a large optical aperture of up to 10 mm and high surface quality (RMS < 20 nm, PV < 120 nm), intended for free-space optical communication. The device comprises a high reflective mirror plate, a pillar and four cantilever beams with AlScN thin films. This device is prepared by a wafer-level eutectic bonding process. A reinforcement structure is proposed to increase the resonant frequency and the resistance to deformation. The mechanical performance, including resonant frequency, step response, linearity, and long-term stability are characterized. The maximum tilting angle is 1.38 mrad at 68 V, the settling time is improved from 400 ms in the open-loop control mode to 0.5 ms with the double step algorithm. The relationship of the resonant frequency, surface figure, and the depth of reinforced ribs is studied through characterizing devices with different depths of reinforced ribs. The bonding quality and the mutual melting states of bonding layers interface are investigated.

Scaling wavelength selective switches for multi-band and space-division-multiplexed networks

This paper investigates the practical scalability of wavelength selective switching technology for the emerging multi-band and space-division-multiplexed (SDM) networks. Wavelength selective switching architectures are introduced for multi-band SDM networks. The switching capacity is analyzed for both weakly coupled and strongly coupled SDM networks. Key bottlenecks for scaling up toward multi-band and more spatial modes are identified. Contrary to the conventional view that liquid crystal on silicon (LCOS) was the only technological obstacle, the manufacturability of free-space optics with high numerical apertures and constraints on the optical dimensions also brought significant challenges for the development of highly integrated wavelength selective switches for multi-band SDM networks.

Free space optical communication using OQPSK in the presence of strong atmospheric turbulence and losses

Free space optical communication using OQPSK in the presence of strong atmospheric turbulence and losses