Long-haul Fiber Links Research Articles

A 300 km fiber channel mapping using neural networks for Gb/s physical-layer key distribution.

Physical-layer secure key distribution (PLSKD) generally acquires highly correlated entropy sources via bidirectional transmission to share the channel reciprocity. For long-haul fiber links, the non-negligible backscattering noise (BSN) and the challenge of bidirectional optical amplification degrade the key generation performances. Since the channel reciprocity can be precisely mapped using neural networks (NNs), unidirectional PLSKD provides a feasible PLSKD for longer fiber links. Here, a final error-free key generation rate (KGR) in unidirectional PLSKD of 3.07 Gb/s is demonstrated over a 300 km fiber link using NNs. Moreover, the channel mapping is analyzed in terms of fiber distance, chromatic dispersion, the nonlinearity of random source, and BSN.

Controlling of the bidirectional amplifier chain for optical frequency distribution based on a two-dimensional noise detector.

In the paper we present a solution for controlling the chain of bidirectional optical amplifiers, intended for long-haul fiber links used to distribute signals produced by optical atomic clocks. The solution is based on a dedicated two-channel noise detector, which allows independent measurement of the noise contributions related to interferometric signal fading and additive wideband noise. New signal quality metrics, based on two-dimensional noise detector, allows to distribute properly the needed gain among the chained amplifiers. Experimental results performed both in laboratory conditions and on a real 600 km-long link, confirming proper operation of proposed solutions, are presented.

Multi-span long-haul fiber transmission model based on cascaded neural networks with multi-head attention mechanism

In this paper, a novelty fiber transmission model consisting of cascaded neural networks with the multi-head attention mechanism has been put forward to solve signal transmission prediction problems in multi-span long-haul fiber link. After appropriately training the model with collected data by the gradient descent method, it can gradually handle the rules of signals' changes over each span and predict the signal transmission results with notably low time cost compared with traditional split-step Fourier method based long-haul model. Through numerical demonstration, this new model can predict 1000km fiber link with symbol rate up to 40GBaud in the QAM modulation with extremely low predicting error.

Statistical properties of nonlinear distortion of a polarization-multiplexed OFDM signal in long-haul fiber links.

We investigate nonlinear transmission regimes of a polarization-multiplexed 16-quadrature amplitude modulation (PDM-16-QAM) orthogonal frequency-division multiplexed (OFDM) signal in a long-haul optical link. We study the dependence between the strength of nonlinear distortion and statistical properties of a PDM-OFDM signal. We also consider the constellation shaping-based solutions that allow to significantly reduce the bit error rate (up to three to 12 times at the cost of 5%-20% signal redundancy) and propose a method to analytically optimize the symbol distribution for higher-order modulation formats using only the initial signal.

Dispersion independent long-haul photon-counting optical time-domain reflectometry

Photon-counting optical time-domain reflectometry (PC-OTDR) based on single photon detection is an effective scheme to attain the high spatial resolution for optical fiber fault monitoring. Currently, due to the spatial resolution of PC-OTDR being proportional to the pulse width of a laser beam, short laser pulses are essential for a high spatial resolution. However, short laser pulses have a large bandwidth, which would be widened by the dispersion of fiber, causing inevitable deterioration in the spatial resolution, especially for long-haul fiber links. In this Letter, we propose a scheme of dispersion independent PC-OTDR based on an infinite backscatter technique. Our experimental results-with more than 45 km long fiber-show that the spatial resolution of the PC-OTDR system is independent with the total dispersion of the fiber under test. Our method provides an avenue for developing long-haul PC-OTDR with high performance.

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.

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.

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.

Design of AWG Devices for All-Optical Time-Frequency Packing

We present the design guidelines for novel passive arrayed waveguide grating (AWG) devices to all-optically implement time-frequency packing techniques in orthogonal frequency division multiplexing (OFDM) systems. The proposed approach can increase the spectral efficiency in long-haul fiber links. We analyze the system performances, evaluating the maximum achievable information rate and spectral efficiency, and we found the optimal AWG layout to maximize the overall system capacity. We demonstrate, through simulations, that an overall 1.4 Tb/s transmission over 200 GHz bandwidth can be reached, using two polarizations, quadrature phase shift keying modulation, and digital feedback equalization.

Full-duplex lightwave transport systems based on long-haul SMF and optical free-space transmissions

A full-duplex lightwave transport system employing wavelength-division-multiplexing (WDM) and optical add-drop multiplexing techniques, as well as optical free-space transmission scheme is proposed and experimentally demonstrated. Over an 80-km single-mode fiber (SMF) and 2.4 m optical free-space transmissions, impressive bit error rate (BER) performance is obtained for long-haul fiber link and finite free-space transmission distance. Such a full-duplex lightwave transport system based on long-haul SMF and optical free-space transmissions has been successfully demonstrated, which cannot only present its advancement in lightwave application, but also reveal its simplicity and convenience for the real implementation. Our proposed systems are suitable for the lightwave communication systems in wired and wireless transmissions.

Noise performance of optical fiber transmission links that use non-degenerate cascaded phase-sensitive amplifiers

Based on semi-classical theory, the noise performance of a multi-span fiber optical transmission system employing a cascaded phase-insensitive amplifier (PIA) and phase-sensitive amplifiers (PSAs) is investigated. Compared with the pure-PIA and pure-PSA based in-line amplification schemes, the copier + PSA scheme is found to improve the system NF by up to 6 and 3 dB, respectively, in an optimized long-haul fiber link. In addition, this cascaded configuration will significantly relax the requirement for accurate phase- and wavelength-locking which is rigorously needed in the pure-PSA configuration. This scheme is also modulation-format independent. As a proof of concept, the NF of a fiber parametric amplifier based copier + PSA cascade with inter-stage attenuation representing the fiber link is measured, which shows a 1.8-dB total NF improvement over the conventional EDFA cascade.

Improving the chromatic dispersion tolerance in long-haul fibre links using the coherent optical orthogonal frequency division multiplexing

Numerical simulations of the coherent optical orthogonal frequency division multiplexing modems are undertaken to investigate the effect of the adaptive modulation, the number of sub-carriers, the cyclic prefix (CP) length, the clipping ratio, quantisation bit resolution and the sampling speed of analogue-to-digital converters (ADCs) on the chromatic dispersion (CD) of a single mode fibre (SMF) at data rates up to 80 Gbps. The use of a large number of sub-carriers is more effective in combating fibre dispersion than employing a long CP; moreover, the optimum number of sub-carriers in the presence of both SMF non-linearities and CD has been identified. The authors show that using a high bit resolution ADC with a high clipping ratio, the transmission distance can be increased at specific data rates. Furthermore, it is shown that ADCs with a low sampling speed also improve the system tolerance to the fibre CD. In addition, simulation results show that the use of adaptive modulation schemes improves spectrum usage efficiency, thus resulting in higher tolerance to the CD when compared with the case in which identical modulation formats are adopted across all sub-carriers.

Phased-array cancellation of nonlinear FWM in coherent OFDM dispersive multi-span links

We develop an analytic model of Coherent Optical Orthogonal Frequency Division Multiplexing (OFDM) propagation and detection over multi-span long-haul fiber links, comprehensively and rigorously analyzing the impairments due the combined effects of FWM, Dispersion and ASE noise. Consistent with prior work of Innoe and Schadt in the WDM context, our new closed-form expressions for the total FWM received power fluctuations in the wake of dispersive phase mismatch in OFDM transmission, indicate that the FWM contributions of the multitude of spans build-up on a phased-array basis. For particular ultra-long haul link designs, the effectiveness of dispersion in reducing FWM is far greater than previously assumed in OFDM system analysis. The key is having the dominant FWM intermodulation products due to the multiple spans, destructively interfere, mutually cancelling their FWM intermodulation products, analogous to operating at the null of a phased-array antenna system. By applying the new analysis tools, this mode of effectively mitigating the FWM impairment, is shown under specific dispersion and spectral management conditions, to substantially suppress the FWM power fluctuations. Accounting for the phased-array concept and applying the compact OFDM design formulas developed here, we analyzed system performance of a 40 Gbps coherent OFDM system, over standard G.652 fiber, with cyclic prefix based electronic dispersion compensation but no optical compensation along the link. The transmission range for 10-3 target BER is almost tripled from 2560 km to 6960 km, relative to a reference system performing optical dispersion compensation in every span (ideally accounting for FWM and ASE noise and the cyclic prefix overhead, but excluding additional impairments).

A Fully Integrated 10-Gb/s Receiver With Adaptive Optical Dispersion Equalizer in 0.13-$\mu{\hbox {m}}$ CMOS

A 10 Gb/s receiver, containing an adaptive equalizer, a clock and data recovery, and a de-multiplexer, is implemented in 0.13-mum CMOS. The chip is intended for long-haul optical fiber links where chromatic and polarization mode dispersions are reach-limiting factors. The equalization is performed by a continuous time filter and a two-tap decision feedback equalizer while automatic threshold and phase adjustments are embedded in the CDR. Use of an analog equalizer with digital adaptation garners total power dissipation of 950 mW. Error-free operation over 200 km of single mode fiber is demonstrated. With 140 km of single mode fiber, optical signal to noise ratio penalty is only 2dB. Differential group delay of 100 ps can also be tolerated

Improving full-duplex radio-on-fiber transport system performance by employing an optical single sideband/vestigial sideband filter

A full-duplex radio-on-fiber (ROF) transport system based on wavelength division multiplexing (WDM) and optical add-drop multiplexing techniques, as well as employing a simple fiber Bragg grating (FBG) filter at the receiving site, is proposed and demonstrated. Over a long-haul fiber link, good transmission performance is obtained in our proposed systems. Since our proposed system does not use a sophisticated optical single sideband (SSB) transmitter, it reveals a prominent alternative with advantages in simplicity and cost. Such a proposed full-duplex ROF transport system is suitable for the long-haul microwave optical link.

Performance enhancement in 10-Gb/s long-haul fiber links with adaptive eye mapping in an integrated 16-bit Si-CMOS transceiver IC

We demonstrate system performance improvement by adaptive eye mapping of a 10-Gb/s silicon complementary metal-oxide-semiconductor 16-bit transceiver integrated circuit (IC). Compared with the conventional receiver with fixed threshold detection, transmission distance has been doubled from 1000 to over 2000 km by adaptively optimizing both the threshold and phase information without data interruption. Monitoring Q and bit-error rate can also be achieved using the same IC.

160 Gbit/s OTDM Long-Haul Transmission with Long-Term Stability Using RZ-DPSK Modulation Format

We describe a transmission system having a data rate of 160 Gbit/s based on the RZ-DPSK modulation format. The 160 Gbit/s single-polarization signal is generated by optical time division multiplexing technology using the base rate of 40Gbit/s. The setup is explained and results are given with a special focus on the stability issue of the transmission system. The pulse source, the optical gate for demultiplexing, the clock recovery and the balanced photo-detector are based on semiconductor components. We present long-term bit error measurements (10 hours) over two different long-haul fiber links. The first link comprises 3 ×106 km standard single mode fiber and uses a PMD mitigation scheme. The other link consists of 4 dispersion managed 80 km fiber spans without the need for an additional PMD compensation. Using EDFA amplification solely and also no FEC, error-free operation was achieved over several hours, only limited by slow drift effects in the laboratory system.

Optical 2R regeneration at 40 Gbit/s using saturable absorber in long-haul dispersion-managed fiber links

The optical regeneration is an attractive method to improve the performance of long-distance data transmission, though its application in high-speed fiber systems requires careful design consideration/optimization. In this letter we investigate 40 Gbit/s dispersion-managed fiber transmission with optical 2R regeneration based on quantum well saturable absorber and highly non-linear fiber. We demonstrate through numerical modeling a feasibility of a single channel transmission over 10,000 km using optimized system design.

Intensity and frequency noise transmission along singlemode fibre at zero-dispersion wavelength

A rigorous small-signal theory for linear and dispersive singlemode fibre communication systems operating at the zero-dispersion wavelength is presented. From this theory, the intensity and frequency noise transmission along the singlemode fibre is assessed. The new theory shows excellent consistency with accurate numerical simulations of intensity and frequency noise spectra after singlemode fibre transmission. Theoretical and simulation results show that fibre transmission at zero-dispersion wavelength leads to intensity and frequency noise spectra at fibre output, similar to intensity and frequency noise spectra, respectively, at the input to the fibre. Accordingly, at zero-dispersion wavelength, second-order dispersion effects on the intensity and frequency noise are irrelevant even for ultra-high bit rates and long-haul fibre links.

Light depolarisation in long fibre links

Light depolarisation in nonsoliton long-haul fibre links is studied. If dispersive effects are absent, the degree of polarisation can be calculated analytically, while in the presence of dispersive effects a numerical analysis is used. Results show how a dispersion management technique enables maintainance of a high degree of polarisation in transoceanic distances.