Optical Links Research Articles

Optimal Scanning Pattern for Initial Free-Space Optical-Link Alignment

Since free-space optical links (especially fully photonic ones) are very challenging to accurately align; scanning algorithms are used for the initial search and alignment of the transceivers. The initial alignment aims to intercept the optical beam so that it hits a position-sensitive detector. However, this operation can be very time-consuming (depending on the system parameters, such as transceiver parameters, distance between transceivers, divergence of the transmitter, angle of view of the receiver, etc.). A spiral scan is used as the most widespread pattern for scanning. This article examines the effects of system parameters (e.g., global navigation satellite systems and compass accuracy) on the angular area of uncertainty that must be scanned to find the optical beam. Furthermore, several types of spiral pattern are compared depending on the time of the scan execution and the required number of points for scanning the given uncertainty area. The cut hexagonal spiral scan achieved the best results as it required 18.1% less time than the common spiral scan for the presented transceiver.

Experimental demonstration of positioning the cooperation terminal for underwater wireless optical communication based on visual navigation

Underwater wireless optical communication (UWOC) is a useful way to transmit a large volume of data in ocean engineering. In application, one of the UWOC terminals is usually carried on an underwater robot to actively adjust its orientation and position to establish an optical link, which requires the mobile terminal to locate the cooperation partner. At a short communication distance, visual navigation is a preferred method to measure the position. In this method, a camera is installed on the mobile terminal while several light-emitting diodes (LEDs) are in the cooperation one. In the image of the camera, by capturing the pattern formed of LEDs light spots, the relative position and orientation between terminals could be calculated. However, in a UWOC system, visual navigation is interfered with by the UWOC transmitter since it forms an additional light spot in the camera’s image. To solve this issue, we present a UWOC-compatible visual navigation method, in which the UWOC light spot is distinguished with the hue, saturation, and value (HSV) color model. Then, we experimentally demonstrate the effectiveness of the method in a pool, applying signals with different bitrates and power on the transmitter. The results show that, while the distance between terminals is within 350 cm and the yaw angle is within 5°, the measured errors are within 1.77 cm and 0.92°, respectively, which is accurate enough for most UWOC applications. The method presented in this paper is simple, pragmatic, and useful for establishing a UWOC link.

Mobile networks expanding to space: Overview of the seranis beyond 5G testbed

In recent years, Non-Terrestrial Networks (NTNs) have gained increased attention from the 3rd Generation Partnership Project (3GPP) due to their potential to enhance cellular networks by improving coverage, resiliency, and reliability, especially in rural areas and disaster scenarios. The upcoming Sixth-Generation (6G) cellular networks aim to establish layers of cells at different altitudes with Base Stations (BSs) on Earth and in space, providing a seamless user experience. Testing and experimentation are crucial for realizing this ambitious vision. University of the Bundeswehr Munich (UniBw M) is deploying a Beyond 5G (B5G) and 6G testbed comprising both space and ground segments. The space segment of the testbed is composed of the ATHENE-1 satellite that is going to be launched in 2025. The ground segment includes the satellite ground station with several full motion antennas for the radiofrequency links and an optical ground station for the free-space optical link based on laser technology, while the Terrestrial Network (TN) component is deployed using a Fifth-Generation (5G) Non-Public Network (NPN) with multiple gNodeBs (gNBs) and multiple 5G core solutions. In its current state, the testbed includes various measurement equipment and emulators in the 5G Lab, a Gnb with two active cells, a core network, a satellite ground station, and a mobile 5G on-the-move solution. This testbed allows various experiments including interference management between existing networks, positioning and localization, Public Protection and Disaster Recovery (PPDR) using Multi-Access Edge Computing (MEC) and NTN. This paper describes the components of the Seamless Radio Access Networks for Internet of Space (SeRANIS) B5G testbed, the current status, future deployment plan, preliminary test results, and planned tests.

Analysis of the Compressed Video with HEVC under Optical Link Transmission

Abstract We study the feasibility of video transmission over optical fibre to optimise bandwidth with the implementation of HEVC codec features. We use simulation (Matlab and the OptiSystem software). Different values of the CRF are used to evaluate its impact on the visual quality and the size of the encoded file, as well as its influence on the video transmission performance. The simulation results show that by adjusting the CRF, the encoders can optimise the compression of the video data to reduce the file size while preserving an acceptable visual quality. This makes it possible to adapt the transmission to the bandwidth constraints of the optical fibre, by choosing higher CRF values to reduce the size of the files and save bandwidth, or lower values to maintain optimal quality when the bandwidth is sufficient. In addition, from the optical fibre point of view, the dispersion weakens and the eye opens, and it is observed that the length of the fibres is inversely proportional to the signal transmission quality. Thus, the judicious use of different CRF values can contribute to efficient and high-quality video transmission via optical fibre.

Radiation hardness and quality validation of the on-detector electronics for the CMS Drift Tubes upgrade

In view of the High Luminosity LHC upgrade, the so-called Phase 2 upgrade, the electronics of the Drift Tubes (DT) subdetector of CMS will undergo a complete innovation. The requirements in terms of trigger rate will exceed the capabilities of the present electronics. Thus, all the on-detector electronics together with the associated back-end need to be replaced. Phase-2 on-detector electronics for DT consist of about 800 FPGAs (Field Programmable Gate Array) based boards called OBDT (On-detector Board for Drift Tubes). These boards are sub-divided in two different categories: 600 OBDT ϕ and 200 OBDT θ, targeting respectively the readout of DT wires parallel and normal to the LHC beams. Each OBDT ϕ is able to time-digitize 240 channels with sub-nanosecond resolution and upstream to the back end using multiple high-speed optical links running at 10.24 Gb/s. The choice of components known to have good resistance to radiation was a requirement in the design of the OBDT. The main component, the FPGA, is a flash-based PolarFire from Microsemi, already qualified in different facilities for radiation hardness tests. As a validation step, a campaign of radiation tests was carried out at the INFN-TIFPA Protontherapy Centre in Trento, Italy, using proton beams. The behavior of an OBDT ϕ board was evaluated during radiation exposure with a total dose much higher than expected to be integrated during 10 years of HL-LHC, which is 0.5 Gy.

Geometric Implications of Photodiode Arrays on Received Power Distribution in Mobile Underwater Optical Wireless Communication.

Underwater optical wireless communication (UOWC) has gained interest in recent years with the introduction of autonomous and remotely operated mobile systems in blue economic ventures such as offshore food production and energy generation. Here, we devised a model for estimating the received power distribution of diffused line-of-sight mobile optical links, accommodating irregular intensity distributions beyond the beam-spread angle of the emitter. We then used this model to conduct a spatial analysis investigating the parametric influence of the placement, orientation, and angular spread of photodiodes in array-based receivers on the mobile UOWC links in different Jerlov seawater types. It revealed that flat arrays were best for links where strict alignment could be maintained, whereas curved arrays performed better spatially but were not always optimal. Furthermore, utilizing two or more spectrally distinct wavelengths and more bandwidth-efficient modulation may be preferred for received-signal intensity-based localization and improving link range in clearer oceans, respectively. Considering the geometric implications of the array of receiver photodiodes for mobile UOWCs, we recommend the use of dynamically shape-shifting array geometries.

Experimental feasibility analysis of quantum/classical coexistence over fibre and free space links

AbstractThe authors present a novel approach to Quantum Key Distribution (QKD) research, emphasising cost‐effectiveness and practicality using a single photon polarisation‐encoded system employing mainly commercial off‐the‐shelf components. This study diverges from previous high‐cost, high‐end setups by exploring the viability of QKD in more accessible and realistic settings. Our approach focuses on practical measurements of the signal‐to‐noise ratio by analysing polarisation‐encoded photonic qubits over various transmission scenarios. The authors introduce a simplified evaluation method that incorporates experimental measurements, such as noise sources and losses, into a semi‐empirical theoretical framework. This framework simulates the standard DS‐BB84 protocol to estimate Secure Key Rates (SKRs), offering an alternative approach on the evaluation of the practical implementation of QKD. Specifically, the authors examine the feasibility of QKD over a 2.2 km intra‐campus fibre link in coexistence scenarios, identifying optimal Wavelength‐Division Multiplexing allocations to minimise Raman noise, achieving an expected SKR of up to 300 bps. Additionally, the authors’ study extends to 40 m indoor and 100 m outdoor Free‐Space Optical (FSO) links using low‐cost components, where the authors recorded Quantum Bit Error Rate (QBER) values below 3.2%, allowing for possible SKRs up to 600 bps even in daylight operation. The converged fibre/FSO scenario demonstrated robust performance, with QBER values below 3.7% and an expected SKR of over 200 bps. Our research bridges the gap between high‐end and economical QKD solutions, providing valuable insights into the feasibility of QKD in everyday scenarios, especially within metropolitan fibre based and FSO links. By leveraging cost‐effective components and a simplified single photon exchange setup, the authors work paves the way for the effortless characterisation of deployed infrastructure, highlighting its potential in diverse settings and its accessibility for widespread implementation.

Performance analysis of an underwater wireless optical communication link with Lommel beam

In order to mitigate the stochastic interference of underwater channels and improve the quality of underwater communication systems, it is essential to study the performance of the underwater wireless optical communication (UWOC) links utilizing vortex beams with unique attributes. In this paper, the analytical formulae for the bit error rate (BER) and the average capacity of the UWOC link with diffraction-free Lommel beam are derived under the Rytov theory. Simulation results demonstrate that the system with a long wavelength, a high system signal-to-noise ratio(SNR), a small asymmetric parameter and receiving aperture diameter achieves a high average capacity and a low BER. Furthermore, in the underwater channel with a larger kinetic energy dissipation rate per unit mass of fluid and inner scale, a smaller mean-squared temperature dissipation rate, temperature salinity contribution ratio and transmission distance, the performance of the communication link can be improved. Meanwhile, it is found that the performance of the link with carrier Lommel beam are less sensitive to the topological charge, the scaling factor of the beam and the turbulent outer scale. These findings provide theoretical support for the design and implementation of an UWOC link utilizing the Lommel beam.

Pre-distortion adaptive optics for optical feeder links: simulations and performance analyses

Optical feeder links offer immense utility in meeting future communication demands—however, atmospheric turbulence limits their performance. This work targets this challenge through analyses of a bidirectional free-space optical communication (FSOC) link that incorporates pre-distortion adaptive optics (AO) between the next-generation optical ground station at the German Aerospace Center (DLR) Oberpfaffenhofen and the laser communications terminal on Alphasat—a satellite in geostationary orbit (GEO). The analyses are performed via end-to-end Monte Carlo simulations that provide realistic performance estimates of the bidirectional FSOC link for a GEO feeder link scenario. We find that applying pre-distortion AO reduces the total uplink losses of the bidirectional FSOC link by up to 10 dB and lessens the scintillation at the GEO satellite by an order of magnitude. Moreover, applying pre-distortion AO eases the link budget requirements needed for maintaining 99.9% link uptime by as much as 20-40 dB, while its use with a laser guide star shows an additional performance improvement of up to 8 dB. These findings demonstrate the desirability and feasibility of utilizing pre-distortion AO for the realization of optical feeder links.

Mitigation of atmospheric losses using dual‐polarized 16‐QAM MIMO technique in free‐space optical system in various weather conditions

SummaryThe technique to overcome the challenge while implementing the fiber optical link is investigated using an alternative optical link called a free‐space optical (FSO) link with advanced modulation and signal processing techniques. A dual‐polarized 16‐quadrature amplitude‐modulated signal is transmitted over single and multi‐FSO channels under various atmospheric attenuation conditions. High bandwidth and data rates (up to 10 Gbps), unlicensed spectrum (above 300 GHz), and simplicity of deployment are significant benefits of the FSO system. However, the weather conditions of haze, rain, and fog cause signal attenuation when the optical carrier wave is transmitted over free space, and atmospheric turbulence is a challenge to FSO implementation, which degrades the transmission performance of optical signals for long‐distance communication. For 120 Gbps data transmission, the performance of a multi‐input and multi‐output (MIMO) FSO system is evaluated by observing the bit error rate (BER) and received signal constellation diagram. Optical signal‐to‐noise ratio (OSNR) parameters were iterated to determine the optimal value for back‐to‐back, single‐input, single‐output (SISO), and MIMO systems. Moreover, we examined the proposed system's output with recent articles, including MIMO, multiplexing, and various modulation techniques. The comparison of results shows that the suggested MIMO‐FSO model has enhanced the performance with the lower BER as less than 3.8 × 10−3 (log of BER is −2.42) forward error correction (FEC) limit for 18.7 km link range in clear air with 0.54 dB/km attenuation, 4.8, 4.5, 1.35, and 0.57 km under haze, light fog, medium, and heavy fog with 4.6, 6.9, 28.9, and 75 dB/km attenuations.

Analysis of micro- vs. macro-flows management in QKD-secured edge computing

Quantum Key Distribution (QKD) holds the promise of a secure exchange of cryptographic material between applications that have access to the same network of QKD nodes, interconnected through fiber optic or satellite links. Worldwide several such networks are being deployed at a metropolitan level, where edge computing is already offered by the telco operators to customers as a viable alternative to both cloud and on-premise hosting of computational resources. In this paper, we investigate the implications of enabling QKD for edge-native applications from a practical perspective of resource allocation in the QKD network and the edge infrastructure. Specifically, we consider the dichotomy between aggregating all the applications on the same source–destination path vs. adopting a more flexible micro-flow approach, inspired from Software Defined Networking (SDN) concepts. Our simulation results show that there is a fundamental trade-off between the efficient use of resources and the signaling overhead, which we managed to diminish with the use of suitable hybrid solutions.

Enhancing Underwater Wireless Sensor Networks With Flexible Communication and Positioning

An innovative approach centred around underwater wireless communication and positioning in Underwater Wireless Sensor Networks, addressing limited coverage by proposing a dual-hop system combining optical fiber and wireless links. Time frame design and Code Division Multiplexing Access enhance multi-user signal transmission efficiency. Proof-of-concept experiments validate feasibility, with the introduction of the hybrid fish eye routing protocol further enhancing performance by intelligently combining fish-eye routing and traditional protocols for improved coverage and reliability. Moreover, the hybrid fish eye protocol incorporates adaptive routing algorithms that dynamically adjust to changes in underwater conditions, ensuring robustness and adaptability in challenging environments. Additionally, the hybrid fish eye protocol optimizes energy consumption by minimizing unnecessary transmissions, thereby prolonging the lifespan of underwater sensor networks. With full-duplex communication, precise positioning, and an extended transmission range, convergent system demonstrates significant potential for supporting high-capacity transmission among mobile nodes in forthcoming underwater scenarios

Total losses and dispersion effects management and upgrading fiber reach in ultra-high optical transmission system based on hybrid amplification system

Abstract This work demonstrated the total losses and dispersion effects management and upgrading the fiber reach in ultra-high optical transmission system based on hybrid amplification system. Based on the requirements of the system, a number of trade-offs must be made between the numerous interconnected performance characteristics of each component in the design of a high quality transmission link. The transmission distance is the greatest distance that an optical signal has to travel. It affects the choice of optical component, fiber type, and signal amplification techniques. Fiber amplifiers or signal regeneration may be required to compensate for signal loss over greater distances. Reviewing these crucial aspects helps when selecting the best optical components and system layout. This ensures the optical link performs as expected. Hybrid amplification system unit (EDFA/Raman/EDFA), (EDFA/Raman), (BFA/PDFA) and (Raman/EDFA) is employed in the transmission system to strength the signal and upgrade the system performance operation efficiency. Space division multiplexing scheme is also employed with the propagation scheme in order to upgrade and control of the total losses and dispersion effects. The optimum total losses/dispersion/repeater spacing are studied with the variations of the total number of links per fiber core channel.

High-Speed Low-Complexity Optical PAM Links With a High-Slope-Efficiency Optical Modulator: Analysis and Demonstration

High-Speed Low-Complexity Optical PAM Links With a High-Slope-Efficiency Optical Modulator: Analysis and Demonstration

Transferring time and frequency signal through phase compensated telecommunication fibres

Precise time and frequency signal transfer over White Rabbit Precision Time Protocol (WRPTP) based underground optical fibre link has been experimentally demonstrated for its potential enhancement and better signal integrity. In order to prove its advantages in practical applications, the time synchronization performance of WRPTP-based underground telecommunication optical fibre links have been examined in controlled environment conditions as well as in varying ambient conditions. Under controlled environment conditions, time synchronization has been achieved within ± 100 ps uncertainty but variation in ambient conditions degrades the stability as well accuracy of the time transfer link. The introduction of active phase variation compensation enhances the accuracy by ∼ 66 % of the WRPTP-based underground time transfer link. The link’s instability in terms of Modified Allan deviation reaches to 2.0 E -15 from 7.5 E -15 at 4096 s of integration time which indicate that dynamic phase variation compensation improves the stability of WRPTP-based time transfer link under varying ambient conditions.

Optically synchronized unidirectional optical amplifier-based coherent optical fiber links.

We report on the realization of a unidirectional transmission-based bidirectional erbium-doped fiber amplifier (UTB-EDFA) for the coherent optical fiber links. By applying an optical phase-locked loop (OPLL) between the two unidirectional EDFA (Ui-EDFA) paths, the annoying uncorrelated phase noise between the two paths can be largely suppressed. Promisingly, we can independently optimize the gains of the UTB-EDFAs for bidirectional transmissions, resulting in higher net gain acquired compared with the conventional single-path bidirectional EDFA (SPBA)-based ones. We demonstrate that the fractional frequency instability of the UTB-EDFA-based scheme can be decreased by 26.3% over the most asymmetrical 100 km two-way optical frequency comparison (TWC) system compared with the SPBA-based ones and, more importantly, can acquire higher net gain for unevenly distributed sub-links over ultra-long fiber links, such as 1000 km, by independently optimizing the gains. This technique paves the way for the applications of large-scale fiber networks.

Enhancing the Secure Transmission of Data Over Optical Fiber Networks from Source to Destination

ABSTRACT The quantum key distribution (QKD) method allows for the safe creation of encryption keys between trusted entities for secure communication. The suggested system is designed to enable secure communication between multiple parties (a single sender, Alice, and multiple receivers, Bobs) connected via a public channel susceptible to interception. It enables Alice to privately share encryption keys with each Bob over dedicated quantum channels, which can then be used to decrypt messages sent over the public network. The procedure involves Alice generating optical signals with randomly assigned properties such as intensity, phase, polarization, and frequencies. Alice randomly sends either signal states carrying information or decoy states with different frequencies to the receivers (Bobs) over the quantum channels. She also sends a synchronization signal for timing alignment with the quantum signals over the optical links. At the receiving end, the synchronization pulse timestamps the incoming quantum signals. Each Bob then randomly alters the frequencies of the received quantum signals and randomly measures their phase or polarization. The key is created from the frequency, phase, or polarization information of the quantum signals, resulting in longer keys. Key management agents manage the securely QKD-generated keys to encrypt information before sending it to the intended receivers. Essentially, this framework establishes authenticated, private communication by using QKD over dedicated ultra-secure quantum links to distribute decryption keys, ensuring that only intended users can access the content. The approach is designed to ensure end-to-end secure communication across the network.

Correlation and polarization singularities of a radially polarized Gaussian Schell-model vortex beam propagating in oceanic turbulence

The correlation and polarization singularities as the important parameters of a radially polarized Gaussian Schell-model vortex beam propagating in oceanic turbulence have been investigated in detail. On the one hand, the correlation singularity of the beam will first split, and then generate new correlation singularities, and finally vanish in pairs. The longer the propagating distance, the larger the rate of dissipation of mean-square temperature, and the lower initial correlation lengths reduce the stability of correlation singularities. On the other hand, polarization singularities also split during transmission. The different initial correlation lengths cause the uneven distribution of polarization singularities, and the high order topological charge leads to the generation of new polarization singularities at short distances. Our numerical findings may be of great significance for detection and imaging of the oceanic optical telecommunication links.

Performance Analysis of Gaussian Apodized Fiber Bragg Grating (GA-FBG) Dispersion Compensator for a Long Haul Optical Fiber Link

This examination gives a complete analysis of the GA-FBG, a generally involved grating for dispersion compensation across a 150 kilometer optical link, from each point. The essential objective of this study is to distinguish how long the GA-FBG can work as an independent dissemination reimbursement gadget and how well it acts in various compensation settings. In this review, the GA-FBG is utilized as an option in contrast to the exorbitant Dispersion-Reimbursing Fiber inside the association to give a more safe optical link. The investigation delves into three specific compensation methods and studies the GA-FBG grating boundaries in light of the relative abundance of real imperatives associated with a Fiber Bragg Grating (FBG) in the design phase. Using a Piece Blunder Rate analyzer to examine the streamlined links' exhibition highlights allows us to evaluate their productivity. Results show that the GA-FBG exhibits a common dispersion compensation adequacy when the link's channel boundaries are tuned. Using gearbox over distances easily exceeding 500 km, the balanced compensation strategy delivers a remarkable Q-factor respect. Utilizing OptiSystem programming, we validate the proposed system's functional viability. The results of our study indicate that the GA-FBG is capable of efficiently counteracting dispersion, leading to an optical link that is both more economical and effective, with significant potential for extended transmission ranges.

Dissemination of UTC(NIST) over 20 km of commercial optical fiber with active phase stabilization.

We demonstrate the transfer of a cesium frequency standard steered to UTC(NIST) over 20 km of dark telecom optical fiber. Our dissemination scheme uses an active stabilization technique with a phase-locked voltage-controlled oscillator. Out-of-loop characterization of the optical fiber link performance is done with dual-fiber and single-fiber transfer schemes. We observe a fractional frequency instability of 1.5 × 10-12 and 2 × 10-15 at averaging intervals of 1 s and 105 s, respectively, for the link. Both schemes are sufficient to transfer the cesium clock reference without degrading the signal, with nearly an order of magnitude lower fractional frequency instability than the cesium clocks over all time scales. The simplicity of the two-fiber technique may be useful in future long-distance applications where higher stability requirements are not paramount, as it avoids technical complications involved with the single-fiber scheme.