Coherent Communication Research Articles

Optimization of the coupling efficiency under self-injection locking in a narrow linewidth hybrid wavelength tunable laser diode

In this study, a fiber Fabry–Perot (FFP) interferometer was connected to a hybrid wavelength-tunable laser diode consisting of a semiconductor optical amplifier chip and Si photonics chip, and spectral linewidth narrowing was achieved using the self-injection locking effect. The hybrid wavelength-tunable laser was equipped with a variable-reflectivity mirror, which allowed the optimization of κ, the coupling efficiency between the laser cavity and FFP interferometer. By optimizing κ, self-injection locking stabilized, and the linewidth was controlled between 4 and 300 kHz. This laser can be used to evaluate the linewidth required for communication systems such as digital coherent communication and the accuracy required for distance measurement.

Coherent Chaotic Communication Using Generalized Runge–Kutta Method

Computer simulation of continuous chaotic systems is usually performed using numerical methods. The discretization may introduce new properties into finite-difference models compared to their continuous prototypes and can therefore lead to new types of dynamical behavior exhibited by discrete chaotic systems. It is known that one can control the dynamics of a discrete system using a special class of integration methods. One of the applications of such a phenomenon is chaos-based communication systems, which have recently attracted attention due to their high covertness and broadband transmission capability. Proper modulation of chaotic carrier signals is one of the key problems in chaos-based communication system design. It is challenging to modulate and demodulate a chaotic signal in the same way as a conventional signal due to its noise-like shape and broadband characteristics. Therefore, the development of new modulation–demodulation techniques is of great interest in the field. One possible approach here is to use adaptive numerical integration, which allows control of the properties of the finite-difference chaotic model. In this study, we describe a novel modulation technique for chaos-based communication systems based on generalized explicit second-order Runge–Kutta methods. We use a specially designed test bench to evaluate the efficiency of the proposed modulation method and compare it with state-of-the-art solutions. Experimental results show that the proposed modulation technique outperforms the conventional parametric modulation method in both coverage and noise immunity. The obtained results can be efficiently applied to the design of advanced chaos-based communication systems as well as being used to improve existing architectures.

Self-injection locked low-noise Brillouin random fiber laser via dynamic fiber grating for QAM coherent communication

A low-noise Brillouin random fiber laser (BRFL) based on dynamic fiber grating (DFG)-assisted self-injection locking (SIL) as a laser carrier for coherent communication is proposed and experimentally demonstrated. The utilization of the DFG-based SIL basically guarantees an innovative purification of ultra-narrow-linewidth laser radiation after the removal of residual random modes from the gain competition, enabling unprecedented long-term frequency-stabilized coherent lasing resonance over a record of 30 s. Consequently, the relative intensity noise of the generated Stokes random laser is significantly suppressed by ∼20 dB, and the frequency/phase noise imposed by random mode hopping is additionally mitigated. Meanwhile, stimulated Brillouin scattering and randomly distributed Rayleigh scattering along the kilometer-long single-mode fiber further suppress laser frequency/phase noise, benefiting the ultra-narrow laser linewidth of 450 Hz. As a proof-of-concept, an 8-Gbaud (32 Gb/s) 16-quadrature amplitude modulation transmission based on the proposed self-injection locked low-noise BRFL as the laser carrier is demonstrated, achieving a low bit error rate of 3.02×10−5. The impact of the laser noise on coherent communication is systematically investigated, highlighting the potential in high-capacity coherent communication.

Investigation of Costas loop phase deviation effect on BER performance of space downlink coherent communication system with DPSK scheme

The Costas loop is an important technology for synchronous demodulation in space coherent communication. However, due to the limitation of the loop device and processing technology, it is difficult to achieve high-precision locking, which results in the Costas loop phase deviation and further affects the quality of the demodulation signal. In this paper, a closed-form bit error rate (BER) model is derived to evaluate the Costas loop phase deviation effect on BER performance in the space differential phase shift keying (DPSK) modulation and heterodyne detection communication system, considering laser phase noise, atmospheric turbulence, and detector noise. Based on the BER model, analytical solution simulation is conducted. The results show that the BER performance deteriorates varying with the increase of the Costas loop phase deviation. In addition, we research how to better balance the laser linewidth and Costas loop accuracy in the system design to ensure good communication status. We find that the wider the laser linewidth, the more stringent the Costas loop accuracy. Furthermore, the BER performance versus the Costas loop phase deviation under different satellite-to-ground communication system parameter settings including divergence angle, receiving aperture, signal laser power, and data rate are analyzed. This work provides valuable references for space optical coherent communication systems design.

High-order-mode Brillouin random fiber laser based on distributed Rayleigh scattering in all-few mode fiber half-open cavity

A high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on distributed Rayleigh scattering in an all-few-mode fiber random cavity is proposed and experimentally demonstrated. Compared to traditional BRFLs oscillating in the single-mode-fiber cavity, the few-mode-fiber random cavity allows the high-order spatial mode Stokes beams to be activated and amplified via intra-mode stimulated Brillouin scattering for sufficient gain. Besides random feedback of distributed Rayleigh scattering along few-mode fibers, a few-mode fiber Bragg grating (FM-FBG) is employed to provide the one-end transverse mode-dependent reflection, which facilitates a high-purity oscillating random laser mode switchable between LP01 and LP11 mode by readily tunning the working wavelength of the Brillouin pump. Consequently, the all few-mode fiber-based BRFL emits a 600-Hz narrow-linewidth LP11 mode with a high mode purity of ∼91.3 %, which paves the way for spatial multiplexing coherent communication, high-resolution imaging, highly sensitive sensing, etc.

Chaotic Communication Systems with Signal Modulation Based on Controlled Symmetry of Semi-Implicit Finite-Difference Models

The article is devoted to investigation coherent communication system model with a new method of signal modulation based on variable symmetry of finite-difference schemes with subsequent experimental analysis of the effectiveness of different modulation techniques. The aim of the study is to investigate a computer model of chaotic communication system with signal modulation based on variable symmetry of semi-implicit finite-difference schemes. Novelty: elements of scientific novelty have finite-difference models of receivers/transmitters, allowing to realize a new method of modulation of chaotic signals. Result: obtaining a simulation model of coherent chaotic communication systems with tools for covertness and noise immunity analyses. Practical relevance: The simulation model of chaotic communication system is a necessary tool for analyzing the performance of the system before its physical implementation.

Bi-GRU Enhanced Cost-Effective Memory-Aware End-to-End Learning for Geometric Constellation Shaping in Optical Coherent Communications

Bi-GRU Enhanced Cost-Effective Memory-Aware End-to-End Learning for Geometric Constellation Shaping in Optical Coherent Communications

International migration and the NGOs working in the field of migration in Turkey

AbstractThis study aims to analyse the activities and discourses of the non‐governmental organizations (NGOs) working in the field of migration in Turkey. The research explores how the NGOs frame the issue of migration differently and how they define and comprehend migration, migrants, and refugees. The article discusses whether the NGOs view refugees as subjects needing help, support, and protection or as actors who have rights and must be empowered and argues that there is tension between the two approaches. These points are discussed through a field study, conducted in 2020 (between February and April), comprising in‐depth interviews with representatives of 13 NGOs working in the field of migration. The study points to structural, historical, and conjectural causes and aspects of the weakness of rights‐based attitudes and activities among the NGOs working in the field of migration in Turkey. The article finds out that the actions of the NGOs working in the field of migration are concentrated on the basic needs of refugees, and structural limitations, such as political pressure, political polarization, and capacity problems, push the NGOs to evaluate the issue of migration within relatively narrow frames. The article also stresses the importance of an enabling political and legal environment and a coherent and structured communication strategy to promote a rights‐based migration agenda.

Influence of atmospheric turbulence on coherent detection performance of space coherent optical communication

Space coherent optical communication technology is considered to be an important way to overcome the bottleneck in current high-speed space communication. However, atmospheric turbulence seriously limits its realization. Based on the Huygens-Fresnel principle and the low-frequency compensation power spectrum inversion method, this work first investigates the random distribution characteristics of the amplitude and phase of a Gaussian beam after it has been transmitted through atmospheric turbulence. Then, using the coherent mixing efficiency and communication bit error rate model, the influence of atmospheric turbulence on the performance of spatial coherent optical communication systems is obtained. Finally, a laser heterodyne detection experimental system is built to quantitatively study the influence of atmospheric turbulence on the coherent detection performance of spatial coherent optical communication. The conclusions drawn from this study are as follows. 1) The spatial phase distortion caused by the weak turbulence channel is relatively small and will hardly affect the light intensity distribution characteristics of the Gaussian beam. In the case of weak turbulence, the influence of weak turbulence on the performance of coherent optical communication system is almost negligible. The communication bit error rate will decrease rapidly with the increase of the number of single bit data photons. The communication signal-to-noise ratio can be better than 10<sup>–5</sup> when the number of single-bit photons is greater than 10. 2) Moderate turbulence will change the intensity distribution characteristics of the Gaussian beam, but will not cause a serious shift in the center of the spot. Under moderate turbulence conditions, the coherent mixing efficiency decreases rapidly as the turbulence intensity continues to increase, but the communication bit error rate still decreases rapidly with the increase of the number of single bit data photons. At this time, increasing the number of single-bit photons can suppress the negative influence of moderate intensity turbulence on the performance of coherent optical communication systems. 3) Strong turbulence will cause severe spatial phase distortion of the beam, destroy the consistency of the light intensity distribution, and cause a serious shift in the center of the spot. Under strong turbulence conditions, the coherent mixing efficiency of coherent optical communication systems approaches zero, and increasing the number of single bit data photons cannot significantly reduce the bit error rate, seriously affecting the quality of coherent optical communication. Atmospheric turbulence is an important limiting factor for developing space coherent optical communication. This study can provide useful references for evaluating the performance of space coherent optical communication systems.

Multifunctionality of discourse marker portanto in Braga’s speech

"Multifunctionality of discourse marker portanto in Braga’s Speech. This paper explores the multifunctionality and usage patterns of the discourse marker portanto in Braga’s speech. Our main goal is to propose a revised description of portanto, while acknowledging the validity of previous research on the topic. The investigation focused on the co-occurrence of functions associated with portanto, emphasizing its pivotal role in speech construction. The frequency distribution of portanto usage was found to be consistent with earlier studies, affirming its most prevalent application as a filler (marker). Our study also highlighted that an excessive use of discourse markers can create the illusion of coherence, rather than genuinely ensuring it, especially in contexts characterized by frequent pauses, verbal hesitations, and syntactic interruptions. Furthermore, this research examined the influence of discourse genre on portanto’s usage patterns. Notably, the two predominant uses observed in Braga’s speech were linked to specific aspects of sociolinguistic interviews, particularly the recollection of life stories and features of spontaneous speech. The findings enrich our understanding of this discourse marker's multifaceted role in facilitating coherent communication and its significance in various speech contexts. Keywords: Braga’s speech, colloquial register, discourse markers, European Portuguese, multifunctionality."

Same-Wavelength Bidirectional Coherent Fiber Communication Scheme With OSNR Improved by Raman Amplifiers

Same-Wavelength Bidirectional Coherent Fiber Communication Scheme With OSNR Improved by Raman Amplifiers

Adaptive-optics-based turbulence mitigation in a 400 Gbit/s free-space optical link by multiplexing Laguerre–Gaussian modes varying both radial and azimuthal spatial indices

In general, atmospheric turbulence can degrade the performance of free-space optical (FSO) communication systems by coupling light from one spatial mode to other modes. In this Letter, we experimentally demonstrate a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) FSO mode-division-multiplexing (MDM) coherent communication link through emulated turbulence using four Laguerre Gaussian (LG) modes with different radial and azimuthal indices (LG10, LG11, LG−10, and LG−11). To mitigate turbulence-induced channel cross talk and power loss, we implement an adaptive optics (AO) system at the receiver end. A Gaussian beam at a slightly different wavelength is co-propagated with the data beams as the probe beam. We use a wavefront sensor (WFS) to measure the wavefront distortion of this probe beam, and this information is used to tune a spatial light modulator (SLM) to adaptively correct the four distorted data-beam wavefronts. Using this adaptive-optics approach, the power loss and cross talk are reduced by ∼10 and ∼18 dB, respectively. © 2023 Optical Society of America

Toward RIS-Aided Non-Coherent Communications: A Joint Index Keying M-ary Differential Chaos Shift Keying System

In reconfigurable intelligent surface (RIS)-aided coherent communications, channel state information (CSI) is often assumed to be perfectly estimated at the receiver. However, perfect CSI cannot be available in practice. Furthermore, the complex and ever-changing channel makes the acquisition of accurate CSI unaffordable because of the large overhead in transmitting pilot signals. Motivated by these considerations, a novel non-coherent RIS-aided joint index keying <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary differential chaos shift keying (RIS-JIK-MDCSK) scheme is proposed in this paper, where the receiver can retrieve information bits by performing non-coherent correlation demodulation without requiring CSI, thereby reducing the system complexity. In RIS-JIK-MDCSK, the states of the reference signal, RIS elements, and information-bearing subcarriers are jointly optimized to devise a joint index keying mechanism, where additional information bits are implicitly transmitted by these state indices, thus increasing the throughput and spectral efficiency. Furthermore, an effective joint index keying detection algorithm is proposed to recover information bits. The analytical bit error rate (BER) of RIS-JIK-MDCSK is derived over a Rayleigh fading channel. Other evaluation metrics, including the throughput, spectral efficiency, and system complexity are also analyzed and compared against benchmark schemes. Numerical simulations are performed to evaluate the superiority of RIS-JIK-MDCSK compared to existing schemes.

Flexible Phase Synchronization for Wireless Optical Coherent Communication System With Adaptive Fractionally-Spaced Blind Equalization Combined With Adaptive Kalman Filter

Flexible Phase Synchronization for Wireless Optical Coherent Communication System With Adaptive Fractionally-Spaced Blind Equalization Combined With Adaptive Kalman Filter

Estimating Optimal Synchronization Parameters for Coherent Chaotic Communication Systems in Noisy Conditions

It is known, that coherent chaotic communication systems are more vulnerable to noise in the transmission channel than conventional communications. Among the methods of noise impact reduction, such as extended symbol length and various digital filtering algorithms, the optimization of the synchronization coefficient may appear as a very efficient and simple straightforward approach. However, finding the optimal coefficient for the synchronization of two chaotic oscillators is a challenging task due to the high sensitivity of chaos to any disturbances. In this paper, we propose an algorithm for finding the optimal synchronization parameter K_opt for a coherent chaos-based communication system affected by various noises with different signal-to-noise ratios (SNR). It is shown, that under certain conditions, optimal $K$ provides the lowest possible bit error rate (BER) during the transmission. In addition, we show that various metrics applied to the message demodulation task propose different noise immunity to the overall system. For the experimental part of the study, we simulated and physically prototyped two chaotic communication systems based on well-known Rossler and Lorenz chaotic oscillators. The microcontroller-based prototype of a chaotic communication system was developed to investigate the influence of noise in the real transmission channel. The experimental results obtained using the designed hardware testbench are in good correspondence with the theoretical propositions of the study and simulation results. The suggested evaluation metrics and optimization algorithms can be used in the design of advanced chaos-based communication systems with increased performance.

An All-Digital Optical Phase-Locked Loop Suitable for Satellite Downlinks

The optical signal propagation used in satellite uplinks and downlinks is influenced by absorption, scattering, and changes in the atmospheric refractive index or turbulence, causing optical signal attenuation. A free space optics (FSO) communications system using coherent communication can improve the link sensitivity and reach higher distances. This article proposes a new architecture for the phase detector in an all-digital optical phase-locked loop (OPLL) for coherent optical detection. Firstly, the performance of the proposed phase detector is evaluated under Gaussian noise, where the best operation point is found for the OPLL working with two sample rates: 625 MSa/s and 10 GSa/s. The system analyses also take a non-negligible delay into account. Then, it will be evaluated and compared with an OPLL using an analog phase detector in the presence of atmospheric turbulence. Finally, in three different atmospheric turbulence conditions, the effect of wind speed on communication quality is investigated through the obtained bit error rate (BER) from the recovered data for a bit rate of 20 Gbps. The results show that the proposed digital phase detector can track a signal under longer feedback loop delays and fading signals.

Exploring the Multifaceted Nature of Complex Syntactic Units in Linguistic Texts

This research delves into the intricate realm of complex syntactic units, emphasizing their role as pivotal linguistic and methodological entities. By intertwining the planes of expression and content, the study underscores the structural and semantic markers that define these units. A comprehensive review of existing literature on the subject reveals the depth and breadth of understanding surrounding complex syntactic units. The paper further highlights the primary characteristics of these units, emphasizing their role in bridging individual sentences and facilitating coherent communication. Special focus is given to the concluding sentences within these units, which play a crucial role in connecting and contextualizing disparate sentences.

Modeling of Nonlinear Interference in the Same- Wavelength Bidirectional Coherent Fiber Communication Systems

Modeling of Nonlinear Interference in the Same- Wavelength Bidirectional Coherent Fiber Communication Systems

Continuous variable quantum key distribution based on simultaneous quantum and classical communication in the terahertz band.

Continuous variable quantum key distribution (CVQKD) based on a simultaneous quantum and classical communication (SQCC) protocol can use a single coherent communication system for multiple purposes. In this paper, we propose an inter-satellite SQCC-CVQKD scheme in the terahertz (THz) band. This scheme performs classical modulation at the sending end, followed by quantum Gaussian modulation operations. At the receiving end, an amplifier is used to amplify and demultiplex the received signal, and, finally, a homodyne detector or heterodyne detector is selected for decoding. We have demonstrated the security of the SQCC-CVQKD system in the THz band, and a performance analysis of the scheme is given in the finite-size regime. The simulation results show that this scheme has a higher secret key rate and stronger anti-interference ability in practice. This work provides an effective way to construct wireless quantum communication networks.

Integration of Self-Adaptive Physical-Layer Key Distribution and Encryption in Optical Coherent Communication

We propose and experimentally demonstrate a compatible physical-layer secure optical communication (PLSOC) system that integrates self-adaptive physical-layer key distribution (PLKD) and encryption (PLE) in optical coherent communication. Based on bit error rate difference of QAM signals mapped by asymmetric basis state Y-00 protocol, the secret key can be secretly exchanged over public fiber links without the pre-shared keys. Moreover, we perform a parameter self-adaptive strategy for practical and dynamic PLKD. The security of the key is evaluated in the case of a fiber-tapping attack. A secure hash algorithm, SHA3-512, is used to perform privacy amplification to obtain the virtually secure key. An error-free PLKD rate reaches 39.3 Kbits/s over 300km ultra-low loss fiber. We experimentally enable the integration of the proposed PLKD scheme and quantum noise stream cipher (QNSC) with a single wavelength, same system. Q factor penalty of the integration system compared to the QNSC system is 3.7dB (optical back-to-back) and 4.8dB (300km) respectively. By exploiting a common hardware platform, with the same wavelength, the proposed PLSOC system addresses the problem that PLKD and PLE are separately performed through independent optical fiber links or wavelengths. Since only digital signal processing is used, the scheme does not require extra hardware.