Long-haul Links Research Articles

Stable frequency dissemination over multi-access fiber loop link with optical comb.

An optical comb based stable frequency dissemination system is proposed and experimentally demonstrated over a multi-access optical fiber loop link. In the system, a new technique based on optical-microwave phase locking loop is designed for phase compensation. In the experiment, a mode-locked fiber laser at a repetition rate of 100 MHz is used to provide an optical source at local site, then it transmits along a 150 km fiber loop link. To testify the proposed system, two accessing nodes are measured in the loop link. The dissemination frequency instability is measured at 3.65 × 10-15/1 s and 7.8 × 10-18/1000 s at the intermediate node. The similar performance is shown at the other node. Hence, the system has the potential application in high-precision frequency transmission system via a long-haul multi-access loop link.

Realization of a Long-haul Optical Link with Erbium Doped Fiber Amplifier

AbstractThe need for high capacity and bandwidth in broadband communication systems increased rapidly in a few past years. Optical fiber is now the major transmission medium for fast and reliable communication replacing the old copper-based connections. However, with the deployment of optical networks, number of problems arise. The main problem of optical networks is the amplification in the long-distance transmission. Erbium doped fiber amplifier (EDFA) is the leading technology in the field of optical amplifiers. It uses erbium doped fiber to amplify optical signal. The importance of amplification in optical domain is relevant in long-haul and high-speed transmission systems. In this paper the study of the EDFA is presented. Based on an analytical study, the simulation model of the EDFA is created. The main aim is to determine the optimal parameters of the EDFA for a long-haul 16-channel DWDM (Dense Wavelength Division Multiplexing) system. The performance of the proposed DWDM system is mathematically analyzed using BER (Bit Error Rate) and Q factor.

Long-haul optical transmission link using low-noise phase-sensitive amplifiers

The capacity and reach of long-haul fiber optical communication systems is limited by in-line amplifier noise and fiber nonlinearities. Phase-sensitive amplifiers add 6 dB less noise than conventional phase-insensitive amplifiers, such as erbium-doped fiber amplifiers, and they can provide nonlinearity mitigation after each span. Realizing a long-haul transmission link with in-line phase-sensitive amplifiers providing simultaneous low-noise amplification and nonlinearity mitigation is challenging and to date no such transmission link has been demonstrated. Here, we demonstrate a multi-channel-compatible and modulation-format-independent long-haul transmission link with in-line phase-sensitive amplifiers. Compared to a link amplified by conventional erbium-doped fiber amplifiers, we demonstrate a reach improvement of 5.6 times at optimal launch powers with the phase-sensitively amplified link operating at a total accumulated nonlinear phase shift of 6.2 rad. The phase-sensitively amplified link transmits two data-carrying waves, thus occupying twice the bandwidth and propagating twice the total power compared to the phase-insensitively amplified link.

Invited Article: Visualisation of extreme value events in optical communications

Fluctuations of a temporal signal propagating along long-haul transoceanic scale fiber links can be visualised in the spatio-temporal domain drawing visual analogy with ocean waves. Substantial overlapping of information symbols or use of multi-frequency signals leads to strong statistical deviations of local peak power from an average signal power level. We consider long-haul optical communication systems from this unusual angle, treating them as physical systems with a huge number of random statistical events, including extreme value fluctuations that potentially might affect the quality of data transmission. We apply the well-established concepts of adaptive wavefront shaping used in imaging through turbid medium to detect the detrimental phase modulated sequences in optical communications that can cause extreme power outages (rare optical waves of ultra-high amplitude) during propagation down the ultra-long fiber line. We illustrate the concept by a theoretical analysis of rare events of high-intensity fluctuations—optical freak waves, taking as an example an increasingly popular optical frequency division multiplexing data format where the problem of high peak to average power ratio is the most acute. We also show how such short living extreme value spikes in the optical data streams are affected by nonlinearity and demonstrate the negative impact of such events on the system performance.

Six-Mode Seven-Core Fiber for Repeated Dense Space-Division Multiplexing Transmission

We propose the optimum design of high spatial density few-mode multicore fiber (MCF) for repeated space-division multiplexed (SDM) transmission. Numerical analyses reveal that a hexagonally arranged six-mode seven-core structure is potentially better than the other possibility, the six-mode multicore structure, in terms of realizing high spatial density and low mode-dependent loss (MDL) characteristics at the splice point; its benefits will enable us to construct long-haul dense SDM transmission links. We fabricate a 171 μ m cladding six-mode seven-core fiber and successfully demonstrate low loss, differential mode delay (DMD), crosstalk, and splice-induced MDL characteristics while maintaining the relative core multiplicity factor of more than 50. Finally, we demonstrate repeated optical 12 × 12 MIMO transmission of more than 100 km using our six-mode MCF and erbium doped fiber amplifier (EDFA).

Precise Radio Frequency Dissemination via Simulated 3000-km Fiber Re-Circulating Loop

In the long-haul fiber link, the phase noise introduced by erbium-doped fibers amplifiers (EDFAs) cannot be compensated. Its influence is very important and should be considered in long-haul fiber link frequency dissemination. In this letter, we disseminate a high-precision radio frequency signal to a simulated remote site over a long fiber link in the laboratory. The phase noise introduced by EDFA is analyzed. A fiber re-circulating loop with optical switch is constructed to test the performance of frequency dissemination system in the case of cascade using EDFAs. The system can obtain the phase error between the returned signal and the reference signal based on a 3000-km fiber re-circulating loop. The residual frequency instability after phase noise compensation reaches $3.5 \times 10^{\mathrm {-13}}$ at 1 s and $2.0 \times 10^{\mathrm {-15}}$ at 4000 s. Our method provides the basis for long-haul precision frequency dissemination and frequency synchronization between remote sites.

FSO channel estimation for OOK modulation with APD receiver over atmospheric turbulence and pointing errors

In the free-space optical (FSO) links, atmospheric turbulence and pointing errors lead to scintillation in the received signal. Due to its ease of implementation, intensity modulation with direct detection (IM/DD) based on ON–OFF-keying(OOK) is a popular signaling scheme in these systems. For long-haul FSO links, avalanche photo diodes (APDs) are commonly used, which provide an internal gain in photo-detection, allowing larger transmission ranges, as compared with PIN photo-detector (PD) counterparts. Since optimal OOK detection at the receiver requires the knowledge of the instantaneous channel fading coefficient, channel estimation is an important task that can considerably impact the link performance. In this paper, we investigate the channel estimation issue when using an APD at the receiver. Here, optimal signal detection is quite more delicate than in the case of using a PIN PD. In fact, given that APD-based receivers are usually shot-noise limited, the receiver noise will have a different distribution depending on whether the transmitted bit is ‘0’ or ‘1’, and moreover, its statistics are further affected by the scintillation. To deal with this, we first consider minimum mean-square-error (MMSE), maximum a posteriori probability (MAP) and maximum likelihood (ML) channel estimation over an observation window encompassing several consecutive received OOK symbols. Due to the high computational complexity of these methods, in a second step, we propose an ML channel estimator based on the expectation–maximization (EM) algorithm which has a low implementation complexity, making it suitable for high data-rate FSO communications. Numerical results show that for a sufficiently large observation window, by using the proposed EM channel estimator, we can achieve bit error rate performance very close to that with perfect channel state information. We also derive the Cramer–Rao lower bound (CRLB) of MSE of estimation errors and show that for a large enough observation window, this CRLB can be adequately tight.

Dispersion management for nonlinearity mitigation in two-span 28 GBaud QPSK phase-sensitive amplifier links.

We present an investigation of dispersion map optimization for two-span single-channel 28 GBaud QPSK transmission systems with phase-sensitive amplifiers (PSAs). In experiments, when the PSA link is operated in a highly nonlinear regime, a 1.4 dB error vector magnitude (EVM) improvement is achieved compared to a one-span optimized dispersion map link due to improved nonlinearity mitigation. The two-span optimized dispersion map of a PSA link differs from the optimized dispersion map of a dispersion managed phase-insensitive amplifier (PIA) link. Simulations show that the performance of the two-span dispersion map optimized PSA link does not improve by residual dispersion optimization. Further, by using the two-span optimized dispersion maps repeatedly in a long-haul PSA link instead of one-span optimized maps, the maximum transmission reach can be improved 1.5 times.

Simulation of a long-haul fiber optic link with a two-mode optical fiber

Modern telecommunication networks approach the capacity crunch, which is associated with the so-called nonlinear Shannon limit. So, the passage to fiber optic links with few-mode optical fibers is considered as an alternative solution of the described problem concerned with high nonlinearity of conventional commercial single-mode optical fibers. Presently, various designs of few-mode optical fibers have been known, with the recently published works presenting experimental results demonstrating their potentialities for long-haul fiber optic links. A lot of models of long-haul fiber optic links with few-mode optical fibers have been developed based on which features of a few-mode optical fiber transport network were numerically simulated. This work presents the results of simulation of a 6000-km long-haul fiber optic link with a two-mode optical fiber and 100-km-per-span Erbium doped fiber optic amplifiers system under 100 Gbps DP-DQPSK data transmission. We studied the use of particular linearly polarized modes and optical vortices for signal transmission. The computation results were compared with the simulation of the same fiber optic link with a single-mode optical fiber under the identical conditions.

The Compensation for Distorted WDM Signals in the Optical Long-Haul Transmission Link with the Artificially Distributed Lengths of Single Mode Fiber and Residual Dispersion Per Span

The possibility of the flexible configuration of dispersion-managed optical links combined with optical phase conjugation by adopting an artificial distribution of the lengths of single mode fiber (SMF) and residual dispersion per span (RDPS) of the fiber spans has been numerically investigated. We confirmed that ‘DA-DA’ pattern – i.e., the lengths of SMF and RDPS around the front and the rear of OPC are smaller, but the lengths of SMF and RDPS closer to the transmitter and the receiver are larger – is the best artificial pattern among the considered 16 artificial distribution patterns. Furthermore, the system performance in the optical link configured with this pattern is more improved than with the conventional uniform distribution of the SMF lengths and RDPS.

MCRB for timing, phase and frequency estimation in presence of self-phase modulation for low-rate optical communication

In this paper we derive the theoretical lower bound, namely Modified Cramer-Rao bound (MCRB) for symbol timing, phase and frequency offset in presence of nonlinear self-phase modulation (SPM) in a dispersion compensated long-haul coherent fiber link. The system model considers multiple span of fiber each associated with optical amplifier. Dual polarization multilevel quadrature amplitude modulation is opted for data transmission to support the data rate lower than 10 Gigabaud. We find that SPM induces underdamped oscillation on the MCRB bounds depending on the pulse shapes (symmetric and asymmetric) utilized. In presence of realistic low-pass filter at the receiver front end, the MCRB degrades significantly due to SPM. We also show the effect of SPM on symbol error rate degradation. Simulation is carried out with symmetric return-to-zero pulse with duty cycles of 33, 67 % and self-generated asymmetric pulse to verify the theoretical results.

High-Precision Ultralong Distance Time Transfer Using Single-Fiber Bidirectional-Transmission Unidirectional Optical Amplifiers

This paper demonstrates a fiber-optic time transfer over a 6000 km noncalibrated fiber link using proposed single-fiber bidirectional-transmission unidirectional optical amplifiers (SFBT-UOAs). The proposed SFBT-UOA employs a 2 × 2 optical switch to enable the time-division-multiplexed time signals for both directions to pass through the same UOA in different time slots. The bidirectional delay symmetry is guaranteed to the maximum extent by allowing the time signals to be transmitted back and forth using the same wavelength over a single fiber. Meanwhile, the noises suffered in the systems employing single fiber bidirectional optical amplifiers (SF-BOAs), such as double Rayleigh backscattering, are effectively suppressed. Consequently, high-precision fiber-optic time transfer can be implemented over ultralong distance by adopting the proposed SFBT-UOAs. Moreover, the SFBT-UOA can support the conventional network services via wavelength division multiplexings (WDMs) and, therefore, offers the potential to perform high-precision time transfer over long-haul commercial fiber links. Over 6000 km, the measured time deviations of fiber-optic time transfer in the laboratory are less than 190 ps/s and 61 ps/105 s, respectively. The calculated combined uncertainty is not beyond 70 ps, which agrees well with the experimental verification over the noncalibrated fiber link.

GVD-insensitive stable radio frequency phase dissemination for arbitrary-access loop link

We propose and experimentally demonstrate a stable radio frequency (RF) phase dissemination scheme for a long-haul optical fiber loop link based on frequency mixing. Using a single optical source in both directions of the loop link, additional timing jitter caused by group velocity dispersion (GVD) can be eliminated. Impressive scalability provided by the optical link ensures that arbitrary-access node can obtain an RF signal with a stabilized phase to meet the requirements of multiple users. In our experiment, a 2.4 GHz RF signal is distributed to arbitrary points along a 100 km fiber-optic loop link steadily. Stabilities of the recovered signals from two accessing nodes are recorded. The root-mean-square (RMS) phase jitter of the received signal at either accessing node is reduced from 1.87 rad to no more than 0.027 rad during 1800-second measuring time.

Distributive FIR-Based Chromatic Dispersion Equalization for Coherent Receivers

We propose a novel and efficient multiplierless finite-impulse response (FIR)-based filter architecture for chromatic dispersion equalization (CDE) in coherent optical communication systems. After quantizing the FIR coefficients, we take advantage of the high multiplicity of their real and imaginary parts, employing the distributive property of multiplication over addition to sharply reduce the number of multiplication operations, obtaining the distributive FIR-CDE (D-FIR-CDE). Furthermore, the implementation of multiplication operations with shifts and additions allows us to obtain a multiplierless D-FIR-CDE (MD-FIR-CDE). The proposed equalizers are experimentally validated in a 100G polarization-multiplexed (PM)-QPSK long-haul optical link and compared against benchmark FIR-CDE and frequency-domain (FD)-CDE implementations. We demonstrate computational resources savings of over 99% in number of multiplication operations and 40% in number of additions, relatively to the FIR-CDE implementation. In addition, the D-FIR-CDE is also shown to compare favorably relatively to the most widely used FD-CDE, achieving significant gains both in terms of required chip area and latency: more than 99% and 30% fewer multipliers and additions, respectively, and a latency reduction of over 90%. We have also experimentally demonstrated that the performance penalty imposed by the coefficient quantization tends to decrease with increasing propagation length, rendering it as an attractive solution for efficient and high-performance chromatic dispersion compensation in long-haul optical fiber links.

Staggered Scheduling of Sensor Estimation and Fusion for Tracking Over Long-Haul Links

Networked sensing can be found in a multitude of real-world applications. We focus on the communication-and computation-constrained long-haul sensor networks, where sensors are remotely deployed over a vast geographical area to perform certain tasks. Of special interest is a class of such networks where sensors take measurements of one or more dynamic targets and send their state estimates to a remote fusion center via long-haul satellite links. The severe loss and delay over such links can easily reduce the amount of sensor data received by the fusion center, thereby limiting the potential information fusion gain and resulting in suboptimal tracking performance. In this paper, starting with the temporal-domain staggered estimation for an individual sensor, we explore the impact of the so-called intra-state prediction and retrodiction on estimation errors. We then investigate the effect of such estimation scheduling across different sensors on the spatial-domain fusion performance, where the sensing time epochs across sensors are scheduled in an asynchronous and staggered manner. In particular, the impact of communication delay and loss as well as sensor bias on such scheduling is explored by means of numerical and simulation studies that demonstrate the validity of our analysis.

Chromatic Dispersion Induced Second-Order Distortion in Long-Haul Photonic Links

Closed form expressions for the theoretical photocurrent of a photonic link with a dispersive fiber are derived for multiple external modulation schemes. These expressions are used to derive the link's output power at both the fundamental and second-order frequencies. Expressions for phase modulation (with and without an asymmetric Mach–Zehnder Interferometer), and both X- and Z-cut LiNO3 intensity modulation are presented and compared.

Generation and Transmission of High Symbol Rate Single Carrier Electronically Time-Division Multiplexing Signals

We experimentally investigate the generation and transmission of high-symbol-rate single-carrier signals based on the electronically time-division multiplexing (ETDM) method. We have successfully increased the operating bandwidth limit of ETDM-based optoelectronic subsystems to the symbol rate of 128.8 and 138.4 GBaud over a long-haul SMF-28 link. We experimentally demonstrate 8 $\times$ 128.8 Gbaud polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) signals over 2800 km SMF-28 and 4 $\times$ 138.4 GBaud over 1200 km with EDFA-only amplification. To the best of our knowledge, 138.4 GBaud is the highest symbol rate of all-ETDM-based optical coherent system reported so far.

Subband Pairwise Coding for Robust Nyquist-WDM Superchannel Transmission

Nyquist wavelength-division-multiplexed coherent optical superchannel systems closely pack a set of rectangular-shaped channels to realize high-spectral-efficiency transmission. In long-haul fiber links, dividing each of these channels into multiple subbands, each a few-gigahertz wide, can increase tolerance to fiber nonlinearity. However, these low-bandwidth subbands will be sensitive to laser drift, which could cause them to overlap, creating interchannel interference (ICI). In this paper, we propose the application of pairwise precoding across the middle and edge subbands to improve their ICI tolerance. The proposed scheme is verified in superchannel experiments, and shows about 1.5-dB Q 2 improvement after 3840-km transmission for QPSK modulation and 1-dB Q 2 gain with 16-QAM modulation after 800-km transmission.

A Fast Method for Launch Parameter Optimization in Long-Haul Dispersion-Managed Optical Links

A new, fast, and simple methodology for optimizing the launch parameters in a long-haul dispersion-managed links is proposed. With this new, simple method, it is possible to quickly predict both the optimum launch power and optimum predispersion compensation values, as opposed to methods based on the numerical integration of the nonlinear Schrodinger equation, which are complex and computationally demanding. This methodology uses the Gaussian noise model for optimal launch power prediction and it is complemented with an ideal predispersion prediction tool based on the analysis of the dispersion map. Realistic scenarios up to 2423.2 km of propagation were simulated and the results were compared with those of a commercial software tool. The results obtained by the simulations, in this paper, show that this methodology accurately predicts the correct launch parameters in few seconds, drastically reducing computational times when compared with conventional methodologies. A maximum mismatch of 0.9 dB in predicted optimal launch power between both methodologies was obtained, as well as very accurate predictions made the newly developed predispersion prediction tool, which calculates a predispersion value that optimizes the Q factor of the validation scenario.

SMF 길이와 RDPS가 랜덤하게 분포하는 장거리 전송 링크에서의 왜곡된 WDM 채널의 보상

장거리 (<TEX>$50\;fiber\;spans{\times}80km$</TEX>) 전송 링크의 융통적 구성을 위해 단일 모드 광섬유 (SMF; single mode fiber)의 길이와 중계 구간 당 잉여 분산 (RDPS; residual dispersion per span)이 랜덤하게 분포하는 링크에서 광 위상 공액과 결합된 분산 제어 (DM; dispersion management)를 통한 왜곡된 WDM 채널의 보상 특성을 살펴보았다. WDM 채널들의 입사 전력이 0 dBm 이하로 제한된다면 SMF 길이와 RDPS가 랜덤하게 분포하는 링크를 통한 보상 효과가 균일하게 분포하는 링크에서의 보상 효과와 유사하게 나타나는 것을 확인하였다. 이는 제안한 랜덤 분포 링크 구조를 이용하면 장거리 WDM 전송 링크 설계와 구축에서 융통성을 확보할 수 있다는 것을 의미한다. The compensation characteristics of the distorted WDM channels compensated for by dispersion management (DM) and optical phase conjugation in the long-haul (<TEX>$50\;fiber\;spans{\times}80km$</TEX>) transmission link with the randomly distributed single mode fiber (SMF) length and residual dispersion per spans (RDPS) for implementing of the flexible link configuration are investigated. It is confirmed that the compensation effect in the link with the randomly distributed SMF length and RDPS is similar with that in the link with the uniform distribution, when the launch power of WDM channels are restricted within 0 dBm. This result means that the proposed link configuration is useful for designing and deploying the long-haul WDM transmission link.