Optical Communication Techniques Research Articles

Autoencoder assisted subcarrier optimization for nonlinear frequency division multiplexing.

Nonlinear frequency division multiplexing (NFDM) is a novel optical communication technique that can achieve nonlinear free transmission. However, current design of NFDM is analogous to orthogonal frequency division multiplexing (OFDM), where sinc function is utilized as subcarriers, which may not be optimal for nonlinear spectrums. In this paper, we propose an auto-encoder (AE) assisted subcarrier optimization scheme for dual-polarized (DP) NFDM systems. Numerical verifications show that our scheme can improve the Q-factor by 1.54 dB and 0.62 dB compared to sinc subcarrier and linear minimum mean square error (LMMSE) equalization, respectively, in a 960 km transmission scenario. We also analyze the characteristics of the optimized subcarriers and discuss how they enhance the performance. Furthermore, we demonstrate the robustness of the optimized subcarriers to different modulation formats, transmission distances and bandwidth. Our work provides a new idea in subcarrier design for NFDM.

Analysis of atmospheric attenuation of a FSO–WDM system for long-range communication

Abstract An FSO system can provide a solution to the problem of last-mile connectivity while also offering a high degree of security and a large capacity for high-speed information transmission, making it a versatile and powerful option for data communication. Despite the above-mentioned advantages, some factors have to be considered when establishing long-range communication, one such factor is atmospheric turbulence (haze, light fog, moderate fog) which degrades the ideal characteristics of the FSO channel and causes a loss in the signal’s power. This loss of power during transmission can be improved by using suitable amplifiers (hybrid optical amplifier, semiconductor optical amplifier) to provide better performance. The next-generation networks must be capable of supporting a significant amount of backhaul data traffic, accommodating a larger number of users, and increasing channel capacity to meet the demands of modern data communication. By implementing wavelength division multiplexing (WDM) and free space optical (FSO) communication techniques, the capacity of the channel can be increased, and more users can be accommodated. The maximum link length offered by the FSO–WDM system has been investigated with hybrid optical amplifier considering the minimum bit error rate, which can provide reliable and long-distance communication.

Generation of Photon Orbital Angular Momentum and Its Application in Space Division Multiplexing

In the last three decades, light’s orbital angular momentum (OAM) has been of great interest because it has unique characteristics that make it sought after in many research fields, especially in optical communications. To address the exponentially increasing demands for higher data rates and capacity in optical communication systems, OAM has emerged as an additional degree of freedom for multiplexing and transmitting multiple independent data streams within a single spatial mode using the spatial division multiplexing (SDM) technology. Innumerable research findings have proven to scale up the channel capacity of communication links by a very high order of magnitude, allowing it to circumvent the reaching of optical fiber’s non-linear Shannon limit. This review paper provides a background and overview of OAM beams, covering the fundamental concepts, the various OAM generators, and the recent experimental and commercial applications of the OAM-SDM multiplexing technique in optical communications.

Underwater optical wireless communication system performance improvement using convolutional neural networks

Many applications that could benefit from the underwater optical wireless communication technique face challenges in using this technology due to the substantial, varying attenuation that affects optical signal transmission through waterbodies. This research demonstrated that convolutional neural networks (CNNs) could readily address these problems. A modified CNN model was proposed to recover the original data of a non-return to zero on–off keying modulated signal transmitted optically through a tank full of Gulf seawater. A comparison between the proposed CNN model and a conventional fixed-threshold decoder (FTD) demonstrates the excellent performance of the proposed CNN model, which improved the bit error ratio (BER), signal-to-noise ratio (SNR), and effective channel length. The BER of the optical signals that are transmitted at powers of 24, 26, and 27 dBm and a bit rate of 10 Mbit/s at a distance of 3 m from the transmitter when FTD is used is 7.826 × 10−7, 5.049 × 10−8, and 8.38 × 10−10, respectively. When the CNN decoder is used at the same distance and powers, the BER is 6.23 × 10−14, 1.44 × 10−16, and 2.69 × 10−18, respectively. In conclusion, the BER decreased by about seven orders of magnitude, the effective channel length increased by four times, and the SNR decreased by about 20 dB. The simplicity of the proposed CNN decoder is independent of the prior knowledge of the channel conditions. Furthermore, the magnificent obtained results make the proposed CNN decoder an ideal substitute for ordinary underwater optical wireless communication decoders.

Digital-filter-aided crosstalk-mitigation for a high spatial resolution AWGR-based 2D IR beam-steered indoor optical wireless communication system.

The growing demand for wireless connectivity is attracting interest in optical wireless communication (OWC) technique. In this paper, a filter-aided crosstalk mitigation scheme, employing digital Nyquist filters, is proposed to eliminate the trade-off between the spatial resolution and the channel capacity for the AWGR-based 2D infrared beam-steered indoor OWC system. By shaping the transmitted signal for narrow spectral occupancy, the inter-channel crosstalk resulting from the imperfect AWGR filtering can be avoided, which enables a denser AWGR grid. In addition, the spectral-efficient signal reduces the bandwidth requirement of the AWGR, which allows a low-complexity AWGR design. Thirdly, the proposed method is not sensitive to the wavelength misalignment between AWGRs and lasers, which relaxes the design of high wavelength stability lasers. Moreover, the proposed method is cost-efficient as we can make use of the mature DSP technique without additional optical components. The 20-Gbit/s data rate OWC capacity using PAM4 format has been experimentally demonstrated over a 6-GHz bandwidth-limited AWGR-based 1.1-m free-space link. The experimental results show the feasibility and effectiveness of the proposed method. By combining our proposed method with the polarization orthogonality technique, a promising capacity per beam of 40 Gbit/s is potentially attainable.

On the performance of a hybrid frequency–phase-keying-based MIMO hybrid RF/FSO communication link

A hybrid technique in optical wireless communications is an effective solution to counter impairments and to improve link availability. In this work, a hybrid frequency phase keying (FPK) modulation scheme, based on frequency shift keying (FSK) and phase shift keying (PSK), for a hybrid radio frequency (RF)/free space optical (FSO) communication system is proposed. The hybrid FPK modulation scheme combines advantageous features of FSK and PSK and has the possibility to provide improved link performance in adverse channel conditions. A transmit aperture selection (TAS) and maximal ratio combining (MRC) diversity-based multiple-input–multiple-output (MIMO) hybrid RF/FSO system is considered to mitigate adverse fading effects. Analytical closed-form expressions are derived for the average symbol error probability to trace the performance of the TAS-MRC-based MIMO hybrid RF/FSO system with FPK modulation over fading and pointing error conditions. A comparative analysis of average symbol error performance is presented. The results clearly indicate that the proposed FPK modulation scheme with the MIMO hybrid RF/FSO communication system link is advantageous, as it exploits the complementary properties of both RF and FSO channel characteristics and PSK and FSK modulation schemes. In addition, the proposed MIMO hybrid RF/FSO system with FPK modulation offers superior performance, compared with MIMO RF, FSO, or hybrid RF/FSO systems applied with FSK and PSK modulation schemes. An optimum beam waist is derived analytically, and it is further verified with a numerical optimization technique. All results are validated using Monte Carlo simulation.

GMM based low-complexity adaptive machine-learning equalizers for optical fiber communication

The demand for high-speed data transmission has increased rapidly over the past few years, leading to advanced optical communication techniques. However, most of machine learning based equalizers which are trained with offline dataset mainly focus on achieving low BER, neglecting the generalization ability when the properties of optical link change. In this paper, we propose Gaussian Mixture model (GMM) based low-complexity adaptive machine-learning equalizers. The proposed online training strategy can fine-tune the parameters in GMM with a small amount of training sequence by introducing Maximum a posteriori probability (MAP) algorithm and sliding window. The size of online training sequence can be reduced by utilizing the parameters trained from offline data as the priori information to reduce the size of online training sequence. The proposed adaptive GMM based equalizers can not only show an excellent capability of mitigating nonlinear distortions but also show the ability to update parameters automatically according to the channel conditions. In addition, experimental results show that by introducing MAP algorithm and sliding window, the convergence speed of the proposed equalizers have been accelerated by 2 times, compared with the case that the traditional Expectation Maximization (EM) algorithm is used for online parameter update. On the other hand, there is no significant increase in computational complexity during the online stage.

Dual‐Band Organic Photodetectors for Dual‐Channel Optical Communications

AbstractThe dual‐band organic photodetectors (OPDs) detect incoming light selectively in two different bands. This work demonstrates a bias‐switchable dual‐band OPD, achieving a fast‐switching speed of over 1000 times per second and a −3 dB cutoff frequency of > 100 kHz for high‐speed optical communications in two distinct bands, e.g., near‐infrared (NIR) and visible light. Visible‐blind NIR detection with a high responsivity of 435 mA W–1 is realized for the OPDs operated under a small reverse bias of −1.0 V. NIR‐blind visible light detection with a responsivity of 325 mA W–1 is obtained for the OPDs operated under a low forward bias of 1.0 V. Dual‐channel NIR/visible optical communication technique has been demonstrated, providing an opportunity for a plethora of applications in information electronics and the Internet‐of‐Things.

Modulation Format Identification in a Satellite to Ground Optical Wireless Communication Systems Using a Convolution Neural Network

The satellite-to-ground communication system is a significant part of future space communication networks. The free-space optical (FSO) communication technique is a prospective solution for satellite-to-ground communication. However, atmospheric optical turbulence is a major impairment in FSO communication systems. In this paper, to improve the performance and flexibility of a satellite-to-ground laser communication system, we put forward a novel modulation format identification (MFI) technique for an FSO communication system based on a convolution neural network (CNN). The results indicate that our CNN model can blindly and accurately identify the modulation format with classification accuracy up to 99.98% for random channel condition, including the strength of turbulence and signal-to-noise ratio (SNR) of additive Gaussian white noise (AWGN) ranging from 10dB to 30dB. Moreover, the CNN demonstrated robustness against atmospheric optical turbulence and suggested immunity to additive noise. Therefore, the proposed methodology proved to be a viable solution in the application of an FSO communication simulation channel, which can easily deal with the scene of fast modulation format switching and accurate identification to satisfy system requirements. Therefore, we hope this scheme can find a practical implementation in satellite-to-ground optical wireless systems.

Dissipative Kerr soliton microcombs for FEC-free optical communications over 100 channels

The demand for high-speed and highly efficient optical communication techniques has been rapidly growing due to the ever-increasing volume of data traffic. As well as the digital coherent communication used for core and metro networks, intensity modulation and direct detection (IM-DD) are still promising schemes in intra/inter data centers thanks to their low latency, high reliability, and good cost performance. In this work, we study a microresonator-based frequency comb as a potential light source for future IM-DD optical systems where applications may include replacing individual stabilized lasers with a continuous laser driven microresonator. Regarding comb line powers and spectral intervals, we compare a modulation instability comb and a soliton microcomb and provide a quantitative analysis with regard to telecom applications. Our experimental demonstration achieved a forward error correction (FEC) free operation of bit-error rate (BER) <10-9 with a 1.45 Tbps capacity using a total of 145 lines over the entire C-band and revealed the possibility of soliton microcomb-based ultra-dense wavelength division multiplexing (WDM) with a simple, cost-effective IM-DD scheme, with a view to future practical use in data centers.

Mass-Manufactured Beam-Steering Metasurfaces for High-Speed Full-Duplex Optical Wireless-Broadcasting Communications.

Beam-steering devices, which are at the heart of optical wireless-broadcasting communication links, play an important role in data allocation and exchange. An ideal beam-steering device features large steering angles, arbitrary channel numbers, reconfigurability, and ultracompactness. However, these criteria have been achieved only partially with conventional beam-steering devices based on waveguides, micro-electricalmechanical systems, spatial light modulators, and gratings, which will substantially limit the application of optical wireless-broadcasting communication techniques. In this study, an ultracompact full-duplex metabroadcasting communication system is designed and experimentally demonstrated, which exhibits beam steering angles up to ±40°, 14 broadcasting channels with capacity for downstream and upstream links up to 100 and 10Gbps for each user channel, three operating modes for flexible signal switching, and metadevice dimensions as small as 2mm×2mm. In particular, the beam-steering metadevices are mass-manufactured by a complementary metal-oxide-semiconductor (CMOS) processing platform, which shows their potential for large-scale commercial applications. The demonstrated metabroadcasting communication system merges optical wireless-broadcasting communications and metasurfaces, which reduces the complexity of beam-steering devices while significantly increasing their performance, opening up a new avenue for high-quality optical wireless-broadcasting communications.

Optical camera communications: practical constraints, applications, potential challenges, and future directions

Mobile wireless communications mainly rely on radio frequency to transmit data. However, its congested spectrum cannot meet the growing requirements for future high data rate applications. Recently, optical wireless communication techniques have opened up a vast optical spectrum and emerged as a cost-effective and practical alternative to congested radio frequency wireless technologies. As a supplementary technique of high-speed optical wireless communication, optical camera communication has become very attractive due to recent developments in communications hinging on camera or image sensors in smart devices with low implementation complexity. In conjunction with its ubiquity in consumer electronics such as pads and smartphones, optical camera communication takes advantage of an image sensor or camera in areas such as intelligent transportation, vehicular communication, motion capture, indoor localization, and the Internet of Things. Optical camera communication and visible light communication are considered in IEEE 802.15.7m standardization. Optical camera communication based on camera and organic/inorganic light emitting diodes offers localization and low-rate transmission in indoor as well as outdoor applications. This paper presents a comprehensive survey of optical camera communication, principles, and recent standardization activities. This survey covers multiple aspects of optical camera communication such as transceivers, multiple-input and multiple-output, and diversity. It outlines various applications of optical camera communication. It also addresses some practical constraints, modulation schemes, and error correction coding techniques for optical camera communication systems. Finally, in the last section of this survey, potential challenges and future research directions are presented.

Ultracompact and broadband mid-infrared polarization beam splitter based on an asymmetric directional coupler consisting of GaAs–CaF[formula omitted] hybrid plasmonic waveguide and GaAs nanowire

In this paper, an asymmetric directional coupler (DC) based on a GaAs–CaF2 hybrid plasmonic waveguide (HPW) and a GaAs nanowire (NW) is designed for the mid-infrared polarization beam splitter (PBS). The mode characteristics of the GaAs–CaF2 HPW and GaAs NW-based PBS are simulated by using the finite element method (FEM). With the appropriate widths of the HPW and NW, the TE polarization mode in the HPW and NW is phase-matched, whereas the TM polarization mode is phase mismatch. Hence, the TE mode undergoes a strong coupling to the GaAs NW while the TM mode pass through the GaAs–CaF2 HPW directly. The results show that the PBS achieves the extinction ratio (ER) of 19.78 dB and insertion loss (IL) of 1.64 dB for the TE mode, ER of 7.78 dB and IL of 2.64 dB for the TM mode at the operating wavelength of 3.5 μm. The PBS based on an asymmetric DC paves the way to achieve integrated waveplates, which has potential applications in the detection and analysis of airborne molecules and the optical communication techniques with large signal transmission capacity.

Non-yellowish and heat-resistant adhesive for a transparent heat sinking film

The development of optical communication techniques and optical components is rapidly progressing in the era of high-speed information communication. In this area, many studies have been carried out to investigate highly transparent optical adhesives. UV-curing adhesives, which rely on irradiating ultraviolet rays, are a major candidate for use in this field. However, they suffer from thermal degradation and discoloration due to long-term UV irradiation. Thus, in this study, we present a transparent adhesive by replacing the photo-initiator to improve the yellowish discoloration phenomenon, which affects aging and low surface quality. By replacing 2-benzyl-2-(dimethylamino)-4′-mopholinobutyrophenone, which causes yellow changes due to a red-shift and donation electron groups, with α -hydroxycyclohexylphenyl ketone, this approach was successful to improve the yellowish discoloration problem. By taking advantage of the excellent thermal and mechanical stabilities in addition to resistance to long-term UV irradiation, a transparent heat dissipation film formed by grid-patterned boron nitride nanosheets was manufactured by a transfer printing method. The film can be utilized as a major component in electrical and optical films which require thermal resistivity and optical transparency.

Distributed Optical Fiber Sensing Assisted by Optical Communication Techniques

The development of optical fiber technology has facilitated the technological innovation in optical fiber communication and sensing systems over the past decades. Among all the fiber sensing technologies, distributed optical fiber sensing (DOFS) has attracted great research interests and has been extensively investigated. In optical fiber communication systems, optical parameters such as intensity, phase, polarization, or frequency are modulated to realize data transmission, while for DOFS they are employed to measure the distributed physical parameter variation along the sensing fiber. Due to the similarities shared by optical fiber communication systems and DOFS systems, optical communication techniques, including coherent detection, polarization diversity and multicarrier signaling have been widely used in the DOFS to improve the sensing performance. In this article, principles of coherent detection, polarization diversity and multicarrier signaling are introduced and their applications in phase sensitive optical time domain reflectometry and Brillouin optical time domain analyzer are reviewed. A newly proposed unidirectional forward transmission based DOFS is also presented.

PAPR REDUCTION TECHNIQUES AND IMAGE QUALITY ASSESSMENTIN IMAGE BASED MMS VLC SYSTEM

The radiofrequency method of transmission suffers from the problem of the limited licensed radio frequency spectrum, &amp; security, wide EMI (electromagnetic interference), high multipath fading &amp; power consumption for short-range applications. The solution to these problems can be achieved by using optical wireless communication technique called the Visible Light Communication system where the data is sent through regulated emission of light. The transmission of digital image information productively in the present optical wireless communication framework is an uphill task as these systems often suffer from problems like frequency selective fading propagation losses and inter-symbol interferences. In order to conquer such difficulties we hereby propose a robust method to transfer the digital image through a Multi-Carrier Modulation Scheme-MMS. In this paper, an Image-Based Multicarrier modulation VLC system is designed through which is sent a corrupted Digital Image with additive white Gaussian noise and explored the overall execution of the Image based Multicarrier modulation VLC system at distinct SNR values. The paper also addresses the issue of high Peak to Average Power Ratio (PAPR) in Multicarrier modulation and reduced it by using different PAPR reduction techniques. The received image quality of the proposed system is obtained under different signal to noise ratios at different modulation schemes..

Optical Fiber Delivered Ultrafast Plasmonic Optical Switch.

Ultrafast optical switch based on plasmonic nanostructures has large application potentials in optical logic circuits and optical communication systems. Integration of plasmonic optical switching devices with optical fibers is a breakthrough for realizing practical applications in long‐range optical data transmission or communication techniques. Here, the incorporation of plasmonic optical switch devices onto the end facets of optical fibers is reported, so that the switched optical signals are generated by interaction between femtosecond laser pulses and plasmonic nanostructures on one end of the fiber, and are delivered via the fiber waveguide to the other end for detection or decoding. “Quenching” of localized surface plasmon in the gold nanowires by its interaction with band‐edge modulation in gold through strong optical excitation is the responsible photophysics. This work accomplishes for the first time the implementation of ultrafast plasmonic optical switch device on optical fiber tips for logic data transmission.

Network Topologies for Composable Data Centers

Suitable composable data center networks (DCNs) are essential to support the disaggregation of compute components in highly efficient next generation data centers (DCs). However, designing such composable DCNs can be challenging. A composable DCN that adopts a full mesh backplane between disaggregated compute components within a rack and employs dedicated interfaces on each point-to-point link is wasteful and expensive. In this paper, we propose and describe two (i.e., electrical, and electrical-optical) variants of a network for composable DC (NetCoD). NetCoD adopts a targeted design to reduce the number of transceivers required when a mesh physical backplane is deployed between disaggregated compute components in the same rack. The targeted design leverages optical communication techniques and components to achieve this with minimal or no network performance degradation. We formulate a MILP model to evaluate the performance of both variants of NetCoD in rack-scale composable DCs that implement different forms of disaggregation. The electrical-optical variant of NetCoD achieves similar performance as a reference network while utilizing fewer transceivers per compute node. The targeted adoption of optical technologies by both variants of NetCoD achieves greater (4 - 5 times greater) utilization of available network throughput than the reference network which implements a generic design. Under the various forms of disaggregation considered, both variant of NetCoD achieve near-optimal compute energy efficiency in the composable DC while satisfying both compute and network constraints. This is because marginal concession of optimal compute energy efficiency is often required to achieve overall optimal energy efficiency in composable DCs.

AdaNN: Adaptive Neural Network-Based Equalizer via Online Semi-Supervised Learning

The demand for high speed data transmission has increased rapidly, leading to advanced optical communication techniques. In the past few years, multiple equalizers based on neural network (NN) have been proposed to recover signal from nonlinear distortions. However, previous experiments mainly focused on achieving low bit error rate (BER) on certain dataset with an offline-trained NN, neglecting the generalization ability of NN-based equalizer when the properties of optical link change. The development of efficient online training scheme is urgently needed. In this paper, we've proposed an adaptive online training scheme, which can fine-tune parameters of NN-based equalizer without the help of an online training sequence. By introducing data augmentation and virtual adversarial training, the convergence speed has been accelerated by 4.5 times, compared with decision-directed self-training. The proposed adaptive NN-based equalizer is called "AdaNN". Its BER has been evaluated under two scenarios: a 56 Gb/s PAM4-modulated VCSEL-MMF optical link (100-m), and a 32 Gbaud 16QAM-modulated Nyquist-WDM system (960-km SSMF). In our experiments, with the help of AdaNN, BER values can be quickly stabilized below 1e-3 after trained with 10^5 unlabeled symbols. AdaNN shows great performance improvement compared with non-adaptive NN and conventional MLSE.

Optical camera communications: Survey, use cases, challenges, and future trends

Recently, a camera or an image sensor receiver based optical wireless communications (OWC) techniques have attracted particular interest in areas such as the internet of things, indoor localization, motion capture, and intelligent transportation systems. As a supplementary technique of high-speed OWC based on photo-detectors, communications hinging on image sensors as receivers do not need much modification to the current infrastructure, such that the implementation complexity and cost are quite low. Therefore, in this paper, we present a comprehensive survey of optical camera communication (OCC) techniques, and their use in localization, navigation, and motion capture. This survey is distinguishable from the existing reviews on this topic by covering multiple aspects of OCC and its various applications. The first part of the paper focuses on the standardization, channel characterization, modulation, coding, synchronization, and signal processing techniques for OCC systems while the second part of the article presents the literature on OCC based localization, navigation, motion capture, and intelligent transportation systems. Finally, in the last part of the paper, we present the challenges and future research directions of OCC.