Long-haul Communication Systems Research Articles

Transmission of 10 Gbps C-band-signal-based Radio Over Fiber for Next Generation Communication Systems

Rapid development of 5G networks encourages researchers to improve the radio-over-fiber (RoF) technique in order to reach 10 Gbps data transmission rates, to increase bandwidth and range, while reducing latency and implementation cost. This paper evaluates an analog radio-over-fiber (ARoF) technique that is compatible with long-distance communication systems. We demonstrate a long distance transmission of a 28 GHz 64 QAM signal via a single mode fiber (SMF) after modulating it with the use of two parallel Mach-Zehnder modulators, without any optical amplifiers. The results show that our prototype solution is capable of transferring data over distances of up to 140 km, via SMF, with a 10 Gbps data rate. The error vector magnitude (EVM) was found to be 7.709%. The proposed system offers exceptional capabilities in terms of supporting high bitrates, while ensuring that EVM remains within the 3GPP limits. Compared to other works, the proposed solution proves to be superior in terms of performance, making it an ideal choice for next generation long-haul communication systems.

Enhancing network performance in a long-haul communication system a wavelength division multiplexing and coherent optical orthogonal frequency division multiplexing approach

Enhancing network performance in a long-haul communication system a wavelength division multiplexing and coherent optical orthogonal frequency division multiplexing approach

Recurrent Neural Networks and Recurrent Optical Spectrum Slicers as Equalizers in High Symbol Rate Optical Transmission Systems

The transition to the edge-cloud era makes ultra-high data rate signals indispensable for covering the immense and increasing traffic demands created. This ecosystem also seeks for power efficient optical modules that will deliver this enormous data load. Optical communication systems and their continuous evolution have responded to the upward trend of capacity needs in all different network ecosystems, scaling from short-reach links serving data center, fiber-to-the-home and 5G/B5G services to metro and long-haul transoceanic cables. Kerr induced non-linearities at long haul and dispersion induced power fading at short reach remain intractable problems that vastly affect high symbol rate systems. So, they must be addressed in a power efficient way in order to cope with the ever-increasing traffic requirements. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, we review our recent work in machine learning and neuromorphic processing in the optical domain for the mitigation of transmission impairments at very high symbol rates. Post-detection techniques based on bidirectional recurrent neural networks for non-linearity compensation and neuromorphic recurrent optical spectrum slicers for power fading mitigation and self-coherent detection emerge as promising solutions for mid-term deployment in long-haul and short-reach communication systems respectively. The current work provides new results in both fields focusing on multi-channel detection in the coherent long-haul domain and on a cost/consumption/performance assessment of neuromorphic photonic processing based on recurrent spectrum slicing in comparison to the state-of-the-art. A thorough analysis of other state-of-the-art techniques in both domains is also provided revealing the merits and shortcomings of recurrent neural networks and neuromorphic photonic processing in high-speed optical communication systems.

Modeling and computational analysis of the combined impact of optical Kerr nonlinearities on the performance of DWDM long-haul communication systems

Modeling and computational analysis of the combined impact of optical Kerr nonlinearities on the performance of DWDM long-haul communication systems

High Spectral Efficiency Photonics-Based Polarization Multiplexing OFDM Signal Delivery at W-Band

We have implemented a large-capacity, long-haul, and high spectral efficiency photonics-based polarization multiplexing communication system at W-band. The signal delivery up to 119.1 Gb/s with spectral efficiency of 9.9 bit/s/Hz is achieved over a 10 km optic-fiber link and 4600 m wireless link. In our study, we utilize an orthogonal frequency division multiplexing (OFDM) scheme to compensate for the signal impairments caused by the dispersion effect and the wireless multipath effect. Based on a pair of antennas, the antenna polarization multiplexing technique is used to realize the polarization-multiplexed mm-wave signal delivery. Also, only one communication link is required, instead of the complex 2×2 line-of-sight multiple-input multiple-output (MIMO) link. Instead of Cassegrain antennas, the combination of plano-convex lens and horn antenna greatly extends the wireless distance. For increasing the spectral efficiency and throughput of the communication system, we employ higher-order modulation formats, probabilistic shaping (PS) techniques, and a series of equalization algorithms. Based on photonics technology, we have created a record-setting distance-rate product of 547.9 Gb/s•km (119.1 Gb/s × 4.6 km) for an independent channel at W-band.

Mitigation of nonlinearities in long-haul DWDM soliton-based communication system

Abstract The ongoing challenge of research and development is focusing on increasing the overall capacity of the transmission medium, achieving a high bit rate, and designing long-haul optical communication systems. The main requirement of the transmission system in supporting high bit rates in optical communication systems is their tolerance to nonlinear fiber impairments. Optical soliton pulses that preserve their shape over long distances are the straightforward solution to this problem. In this paper, DWDM soliton-based optical communication system is designed to mitigate the nonlinear effects at a data rate of 10 Gbps with channel spacing of 100 GHZ by using a microring resonator (MRR) device which acts as an add/drop filter that generates a filter narrowband wavelength from a wider optical spectrum. The transmission link of the optical fiber has a total length of 180 km and MRR is inserted at the end of the loop which provides the required bandwidth. The high bit rate signals can be transmitted through this MRR system to get the filtered signal which is less affected by the nonlinearities. This filtered signal is observed at a total transmission distance of 54,000 km. Also, by using dispersion compensation techniques, the impact of four-wave mixing (FWM) is mitigated. The performance of the designed system is evaluated based on the quality factor (Q-factor) along with the bit error rate (BER) obtained.

Master–slave carrier phase recovery using optical phase conjugation for frequency comb-based long-haul coherent communication systems

Taking optical frequency combs as WDM laser sources for coherent optical communication systems has great potential in low-cost, low-complexity, and large-capacity communications. Master–slave carrier phase recovery (MS-CPR) utilizes the phase coherence of received signals to reduce the computational complexity (CC) in frequency comb-based communication systems. In long-haul systems, chromatic dispersion (CD) weakens the phase coherence and makes MS-CPR ineffective. Besides, nonlinear effects greatly degrade the quality of received signals. A master–slave carrier phase recovery method using optical phase conjugation (OPC) is proposed for frequency comb-based long-haul coherent optical communication systems. By placing the OPC module in the middle of the link, not only the weakening of phase coherence caused by CD is solved, but also nonlinear effects are mitigated. The simulation results show that the proposed method has better performance than conventional MS-CPR and independent carrier phase recovery (ICPR). In the 1000 km long system with 16 quadrature amplitude modulation (QAM) signals, the BER of proposed method is 9.3% of conventional MS-CPR and even 40% lower than ICPR, and the BER of all channels is below the threshold for 7% overhead hard-decision forward error correction (7% HD-FEC).

Study of Chromatic Dispersion in Single-Mode Optical Fiber

In the optical communication system, optical fibers are used to transmit information in a long-haul communication system at a very high speed. However, attenuation causes loss in signal transmission. Chromatic dispersion is an ultimate limiting factor for attenuation in high-speed long-distance communication. The chromatic dispersion causes a broadening of the incident pulse while traversing along the fiber length. This broadening reduces the information-carrying capacity of the transmitted signal. Chromatic dispersion is caused by material and waveguide dispersion. We used COMSOL Multiphysics to evaluate chromatic dispersion for the different refractive indexes of silica glass by varying wavelengths from 1300 (nm) to 1600 (nm). We have modelled a 2D optical fiber using the Electromagnetic Wave, Frequency Domain. The effective mode index was computed using Mode Analysis and total dispersion is calculated theoretically. The computed values of chromatic dispersion are within 7% of theoretical values.

Over 100 nm Bandwidth Orbital Angular Momentum Modes Amplification For MDM and WDM Transmission With a Ring-Core Bi/Er Co-Doped Fiber

We designed and fabricated a ring-core Bi/Er co-doped fiber (RC-BEDF), and thus investigated the broad-spectrum amplification of orbital angular momentum (OAM) modes. The deviation from the average gain of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> modes enabled gains of <1.8 dB and <1.9 dB respectively, and the differential mode gain (DMG) between the two modes was <1.0 dB over 100 nm bandwidth (1525–1626 nm). In particular, the 3 dB bandwidth of the gain spectra exhibited 74 nm (1552–1626 nm) and 69 nm (1557–1626 nm), respectively. Furthermore, four beams of different wavelengths, i.e., 1525, 1560, 1590, and 1626 nm, were separately extracted from the broad spectra to detect the amplified modes, by which the interferograms of first- and second-order OAM modes were obtained. The amplified OAM mode purities of the wavelengths were all above 95%. Moreover, dual-channel combination of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> mode at 1550 nm and OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mode at 1600 nm was amplified, which indicated realization of different-OAM-modes amplification with wavelength-division multiplexing (WDM). Herein, for the first time, we extended the C+L band OAM modes amplification to 1626 nm and achieved OAM multiplexed amplification of WDM signals. This ultra-broad-spectrum and ultra-low-DMG OAM amplification has been widely considered as a promising technology for long-haul mode division multiplexing and WDM fiber-optics communication systems and networks.

Combined impact of SRS, FWM and ASE noise in UDWDM/DWDM long-haul communication systems using EDFAs

In this paper, the combined impact of SRS, FWM and ASE noise in center channels of 7-, 15-, 31- and 63-channel UDWDM/DWDM long-haul multi-span optical fiber communication systems using different numbers of EDFAs has been analyzed with simulations. UDWDM/DWDM systems with varying system parameters, i.e. channel input powers, channel spacing values, multi-span fiber lengths and numbers of EDFAs, have been taken into account in simulations. Comparative OSNR simulation results satisfying a minimum 23 dB OSNR criterion under the combined impact of SRS, FWM and ASE noise have been presented and evaluated. OSNR vs channel input power simulation results show that FWM becomes the most dominant effect with respect to SRS and ASE noise in UDWDM systems. Therefore, OSNR values decrease as channel input powers increase in multi-span UDWDM systems having 3.125 GHz and 6.25 GHz channel spacing values. However, both ASE noise and SRS become more dominant with respect to FWM in DWDM systems with 100 GHz channel spacing values. OSNR values increase as the channel input powers increase in those multi-span DWDM systems. Simulation results for OSNR vs channel spacing values show that OSNR values obtained in multi-span UDWDM systems increase as channel spacing values increase in the 3.125 GHz–6.25 GHz range. This is due to the decrease in FWM crosstalk, which has a dominant effect in UDWDM systems, with increasing channel spacing values in those UDWDM systems. However, OSNR values obtained in multi-span DWDM systems decrease as channel spacing values increase in the 12.5 GHz–100 GHz range. This is due to the increase in ASE noise, which has a dominant effect in DWDM systems, with increasing optical filter bandwidths of those DWDM systems. OSNR vs channel length simulation results show that OSNR values generally decrease with increasing total fiber lengths and inter-amplifier fiber lengths in both UDWDM and DWDM multi-span systems. This is due to the FWM crosstalk and the ASE noise, which become more dominant with respect to the SRS as the total fiber length and inter-amplifier fiber lengths increase. Simulation results for OSNR vs number of EDFAs show that OSNR values obtained in multi-span UDWDM systems having 3.125 GHz channel spacing values decrease with increasing numbers of EDFAs. However, OSNR values in multi-span UDWDM/DWDM systems having 6.25 GHz-100 GHz channel spacing range increase with increasing numbers of EDFAs. Simulation results provide important clues for the design and implementation of reliable long-haul UDWDM/DWDM systems.

Increase of Side Band Powers in Parallel Phase Modulation by Lithium Niobate–based Electro-optic Crystal

An electro-optic Pockels cell can show very good optical switching behavior for its very good first-order non-linear effect. The response time or the switching time of this type of cell is also very fast. When an electric field is applied to it in a proper way, then it changes the propagation of a polarized light beam passing through it, for which there is found a change in refractive index occurring in it. The change in refractive index is linearly proportional to the applied electric field. This causes a change in optical path inside the material for the propagating light signal, which results in a phase modulation of the light beam. Electro-optic crystals were seen for massive uses in phase and intensity modulation–based all-optical long-haul communication systems. In integrated optics, this effect also has strong uses for processing of optical data. In this paper, the authors propose a new scheme where a single biasing electrical field is parallelly applied in two orthogonal directions at a time on a specific light signal, instead of one biasing signal on the light beam. This parallel application of biasing signal enhances the depth of modulation.

Reduced sampling rate Kalman filters for carrier phase and frequency offset tracking in 200\xa0Gbps 16 QAM coherent communication system

We propose 1 state and 2 state multi-step Kalman filters (MKFs) to estimate and compensate CFO, LPN and NLPN in long-haul coherent fiber-optic communication systems. The proposed filters generate state estimates once every m symbols and therefore operate at a reduced sampling rate compared to conventional KFs that perform symbol by symbol processing. No computations are performed to obtain phase estimates of the intermediate m-1 samples; instead, the present and previous estimates are averaged and used to derotate the intermediate m-1 samples which are then demodulated to recover the transmitted symbols. This reduces the computational load on the receiver DSP. Further, in order to improve estimation accuracy, we adaptively vary the process noise covariance Q. Simulation results of 200 Gbps PDM 16 QAM system over 12 spans shows that the proposed 1 state MKF can reduce the sampling rate requirement by a factor of m = 20 with Q-factor degradation of 1.32 dB compared to single-step KF at linewidth of 100 kHz. The 2 state MKF tracks PN and CFO with a maximum step size of m=10 for a CFO of 100 MHz at linewidth of 100 kHz. We also study the dynamic performance of the proposed algorithms by applying step change to CFO. The 2 state MKF with adaptive Q is able to track a step change of 400 MHz of CFO with m = 1 and 3 with high estimation accuracy but slower convergence time compared to the non-adaptive 2 state MKF. Finally, we study the computational requirements of the proposed MKFs and show that they offer significant reduction in computations compared to single-step KF thus making the proposed filters suitable for hardware implementation.

Superconducting qubit to optical photon transduction.

Conversion of electrical and optical signals lies at the foundation of the global internet. Such converters are used to extend the reach of long-haul fibre-optic communication systems and within data centres for high-speed optical networking of computers. Likewise, coherent microwave-to-optical conversion of single photons would enable the exchange of quantum states between remotely connected superconducting quantum processors1. Despite the prospects of quantum networking2, maintaining the fragile quantum state in such a conversion process with superconducting qubits has not yet been achieved. Here we demonstrate the conversion of a microwave-frequency excitation of a transmon-a type of superconducting qubit-into an optical photon. We achieve this by using an intermediary nanomechanical resonator that converts the electrical excitation of the qubit into a single phonon by means of a piezoelectric interaction3 and subsequently converts the phonon to an optical photon by means of radiation pressure4. We demonstrate optical photon generation from the qubit by recording quantum Rabi oscillations of the qubit through single-photon detection of the emitted light over an optical fibre. With proposed improvements in the device and external measurement set-up, such quantum transducers might be used to realize new hybrid quantum networks2,5 and, ultimately, distributed quantum computers6,7.

Health monitoring of long-haul fiber communication system using chaotic OTDR

A novel chaotic optical time-domain reflectometry (OTDR)-based approach was proposed for monitoring long-haul fiber communication systems with multiple fiber segments. The self-phase modulation and group velocity dispersion effects of the optical cable was considered in demonstrating the proof-of-concept experiment and simulation. In experiments, the correlation peaks are clearly obtained from the correlation trace between the reference and reflected (or scattered) light signals propagating in three optical-fiber segments. The technique affords a high spatial resolution of 2 m, and further long-haul fiber simulations indicate that the sensing distance can be more than 3300 km. Thus, the new proposed technique can be effectively applied for health monitoring of long-haul fiber communication systems.

Nonlinear depolarization of light in optical communication fiber

We report the experimental observation of a novel transmission phenomenon in optical long-haul communication systems. Unpolarized ASE depolarizes via nonlinear fiber interactions a cw laser light during their copropagation which leads to small but measurable ultrafast polarization state fluctuations at the fiber output. We provide a phenomenological approach and a theory that qualitatively corroborates our experimental results. One of our major findings suggests that the applicability of the often used Manakov equation needs to be scrutinized for highly accurate studies of nonlinear polarization state evolutions in noisy environments. The described phenomenon leads to a qualitatively different microscopic understanding of nonlinear light propagation in fiber and can contribute toward an explanation for today’s commonly perceived gap between simulated and experimentally obtained system performance in optical data transmission.

Low Dark Current 1.55 Micrometer InAs Quantum Dash Waveguide Photodiodes

Photodiodes and integrated optical receivers operating at 1.55 micrometer (μm) wavelength are crucial for long-haul communication and data transfer systems. In this paper, we report C-band InAs quantum dash (Qdash) waveguide photodiodes (PDs) with a record-low dark current of 5 pA, a responsivity of 0.26 A/W at 1.55 μm, and open eye diagrams up to 10 Gb/s. These Qdash-based PDs leverage the same epitaxial layers and processing steps as Qdash lasers and can thus be integrated with laser sources for power monitors or amplifiers for preamplified receivers, manifesting themselves as a promising alternative to their InGaAs and Ge counterparts in low-power optical communication links.

Energy-Efficient High-Throughput VLSI Architectures for Product-Like Codes

Implementing forward error correction (FEC) for modern long-haul fiber-optic communication systems is a challenge, since these high-throughput systems require FEC circuits that can combine high coding gains and energy-efficient operation. We present very large scale integration (VLSI) decoder architectures for product-like codes for systems with strict throughput and power dissipation requirements. To reduce energy dissipation, our architectures are designed to minimize data transfers in and out of memory blocks, and to use parallel noniterative component decoders. Using a mature 28-nm VLSI process technology node, we showcase different product and staircase decoder implementations that have the capacity to exceed 1-Tb/s information throughputs with energy efficiencies of around 2 pJ/b.

On-chip WDM mode-division multiplexing interconnection with optional demodulation function.

We propose and fabricate a wavelength-division-multiplexing (WDM) compatible and multi-functional mode-division-multiplexing (MDM) integrated circuit, which can perform the mode conversion and multiplexing for the incoming multipath WDM signals, avoiding the wavelength conflict. An phase-to-intensity demodulation function can be optionally applied within the circuit while performing the mode multiplexing. For demonstration, 4 × 10 Gb/s non-return-to-zero differential phase shift keying (NRZ-DPSK) signals are successfully processed, with open and clear eye diagrams. Measured bit error ratio (BER) results show less than 1 dB receive sensitivity variation for three modes and four wavelengths with demodulation. In the case without demodulation, the average power penalties at 4 wavelengths are -1.5, -3 and -3.5 dB for TE₀-TE₀, TE₀-TE₁ and TE₀-TE₂ mode conversions, respectively. The proposed flexible scheme can be used at the interface of long-haul and on-chip communication systems.

Investigation of Four Wave Mixing Effect at Different Transmission Power Levels and Channel Spacing

There is an incredible focus on broadband fiber optical transmission system as it offers many advantages over other communication systems such as large bandwidth and significantly low losses [1]. This high bandwidth in optical communication networks is further increased by using Wavelength Division Multiplexing. In WDM based fiber optical communication systems nonlinearities in fiber are a limiting factor which degrade the system performance and limit the data rate and bandwidth of the system [3]. As we increase the transmission power levels so as to transmit data for long-haul communications systems these non linear effects tend to accumulate. Due to these non linear effects, all input signals may interact with each other in a complicated way which degrades overall performance of optical communication system. In WDM systems, the interaction of input signals with each other adversely affects the performance of the system. Hence it is of great importance to understand fiber non linear effects and its effect on optical fiber communication system. The main objective of this paper is to study non linear effects in fiber optic communication system mainly Four Wave Mixing (FWM) and to analyze to effect of increasing optical transmission powers and channels spacing in a wave division multiplexing system. The results show that as the transmission power is increased, the efficiency of four wave mixing effect increases. Also if the channel spacing in a WDM system is kept uneven, the power levels of mix frequencies produced as a result of FWM effects is reduced to a considerable amount. General Terms Investigation, Four Wave Mixing, Wave Division Multiplexing

Optical Nonlinear Phase Noise Compensation for &lt;inline-formula&gt;&lt;tex-math&gt;$9\times 32$&lt;/tex-math&gt; &lt;/inline-formula&gt;-Gbaud PolDM-16 QAM Transmission Using a Code-Aided Expectation-Maximization Algorithm

Nonlinearity-induced phase noise has become a major obstacle in long-haul coherent fiber-optic communication systems. Such phase noise has been shown to be signal dependent, and correlated over time. We propose a code-aided expectation-maximization algorithm to mitigate such nonlinear phase noise, iteratively utilizing both the time correlation of the nonlinearity-induced impairments and a soft-decision error-control code. Simulation and experimental results show that on a dual-polarization wavelength-division-multiplexed 16 QAM system, launch-power tolerance can be increased by ${1.5}$ dB, and the optical signal-to-noise ratio requirement can be relaxed by ${0.3}$ dB to achieve the same ${Q^2}$ -factor.