Optical Signal-to-noise Ratio Degradation Research Articles

Graph wavelets for fault localization in optical mesh networks

• Real-time fault detection and localization mechanism in Elastic Optical Networks (EONs). • Alarm correlation strategies fail in the face of static threshold settings. • Detects and localizes silent network failures. • Multi-resolution analysis based on graph wavelets to address fault detection and localization problems. • Exceeds alarm based engines for fault detection and localization accuracy. EONs (Elastic Optical Networks) form the backbone of web-scale cloud infrastructure. These networks should deal with network faults and restoration in real-time with high accuracy and scalability. Thus, fault management functions to detect and localize such failures, play a critical role in modern day EONs. Network impairments are complex in nature in optical networks such as fiber losses, NLI (Non Linear Impairments) and ASE (Amplified Stimulated Emission). Currently, pre-engineered thresholds are used to detect failures in optical networks. Traditional alarm correlation strategies fail in the face of these static threshold settings. In this work, we introduce a real-time fault detection and localization mechanism, which is agnostic to such pre-engineered threshold settings. We apply multi-resolution analysis based on graph wavelets to address fault detection and localization problems. Thus, we propose to use spectrograms and wavelet coefficients to identify the irregularities in an otherwise smooth graph signal. We present a real-time fault localization analysis based on a novel Wavelet Coefficients Trail (WCT) algorithm to root-cause a client signal failure due to OSNR (Optical Signal to Noise Ratio) degradation. We apply the proposed techniques and algorithms to simulated and actual network data. Our results indicate that the proposed methods can root-cause OSNR degradation to major/minor deviations in received power on optical spans. In conclusion, we present a fault localization framework based on graph wavelets in optical mesh networks.

In-Band OSNR Measurement Method for All-Optical Regenerators in Optical Domain

We propose an in-band measurement method of optical signal-to-noise ratio (OSNR) output from an all-optical regeneration system with a nonlinear power transfer function (PTF) according to the fact that there are different average gains of signal and noise. For the all-optical quadrature phase-shift keying (QPSK) regenerator as an example, the output OSNR is derived from the input OSNR and the total gain of the degraded QPSK signal. Our simulation shows that the OSNR results obtained by this method are in agreement with those calculated from the error vector magnitude (EVM) formula. The method presented here has good applicability for different data rates but is also useful for analyzing the OSNR degradation of other nonlinear devices in optical communication links.

A WDM Network Controller With Real-Time Updates of the Physical Layer Abstraction

This paper describes the implementation of a wavelength division multiplexing network controller that accounts for the varying conditions of the network physical layer in real time. A newly added software module is used to estimate the physical layer quality of transmission (QoT-E) in the form of a single transmission figure of merit for every line span in the network, i.e., the optical signal-to-noise ratio (OSNR) degradation. The estimated OSNR degradation is used to verify lightpath feasibility in the network, and produce optimal routing and wavelength assignment solutions in real time, without the need for a separate network planning tool. The OSNR degradation is estimated without requiring detailed knowledge of the optical components deployed in the network, and consequently the approach is vendor agnostic. The accuracy of the estimated OSNR degradation is assessed experimentally, and its impact on two network-wide performance indicators is studied through simulation.

Self-Learning Monitoring On-Demand Strategy for Optical Networks

In current dynamic optical networks with cascaded filters and amplifiers, the optical signal-to-noise ratio (OSNR) can vary significantly from channel to channel. Under such uncertainty, OSNR prediction for unestab-lished channels becomes indispensable but remains a big challenge. For protective network planning purposes such as margin threshold setting or wavelength assignment, it is desirable to evaluate the worst OSNR performance of each network link. Such exploration will unavoidably employ active monitoring probes, which may cause interruptions to the network. An efficient active monitoring strategy that optimizes the choice of probes or monitoring trials is needed. We propose a self-learning monitoring strategy integrated at intermediate nodes. Our method can intelligently select the channel to be monitored in order to search for the target global maxima of OSNR degradation for a specific link. Our monitoring scheme detects intermediate node OSNR in the linear regime. It is shown that our method can predict the target OSNR value with only 0.5 dB error while reducing the required monitoring data by up to 91% compared to conventional methods.

Numerical Investigation of the Equalization Enhanced Phase Noise Penalty for M-Quadrature Amplitude Modulation Formats in Short-Haul Few-Mode Fiber Transmission Systems with Time-Domain Equalization

The equalization enhanced phase noise (EEPN), caused by the interaction of the chromatic dispersion (CD) with the phase noise of the local oscillator (LO), has been extensively studied for single-mode optical communication systems. Few-mode fiber (FMF) transmission systems introduce a new channel impairment, the differential mode delay (DMD), which also creates EEPN and hence limits the maximum transmission distance of those systems. In this work, we numerically investigate the optical signal to noise ratio (OSNR) penalties caused by the EEPN in a 3-mode FMF transmission system at 25 GBd for quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 32-QAM and 64-QAM modulation formats when using the blind phase search (BPS) carrier phase recovery (CPR) algorithm, which has been demonstrated to be both robust and suitable for optical communication systems. Our numerical study assumes a short-span of FMF, modeled in the weakly-coupled regime, and includes two cases; the use of ideal mode-selective de/multiplexers at both ends of the FMF span (model A), and the use of ideal non-mode-selective de/multiplexers (model B). The results show that the EEPN has almost no effect in model A. However, EEPN produces a severe penalty in model B with the onset of the OSNR degradation starting for a DMD spread of the impulse response of about 100 symbols for all modulation formats investigated. The distribution ratio of the amount of phase noise between the transmitter and receiver lasers is also assessed for model B and we confirm that the degradation is mainly due to the phase noise of the LO.

OSNR Aware Composition of an Open and Disaggregated Optical Node and Network

A function programmable optical network has been recently proposed to enhance the flexibility of an optical transport based on architecture-on-demand (AoD). The flexible synthesis of optical node architectures provided by AoD enables an open and disaggregated optical layer thanks to the available deep programmability. However, previous studies have focused on how to synthesize a single node out of switching function blocks, thus neglecting the optical signal-to-noise ratio (OSNR) impact, power imbalance effects due to the diverse set of devices traversed per input–output configuration, and networkwide implications. In this work, we present an optical network-wide function synthesis (ONetFuS), which is an algorithm to compose AoD nodes that consider placement and configuration of both switches and amplifiers. ONeFuS minimizes OSNR degradation and deviation across channels and offers enhanced power balance performance. Moreover, ONetFuS addresses multiple-node scenarios to investigate cascading, transmission distance, and networking effects. We compare the number of optical cross-connections computed by our proposal against solutions in the literature. Results in network scenarios, including the number of components, power balance, OSNR variations, and OSNR penalty reductions, prove the suitability of our proposed ONetFuS for open and functional programmable optical networks.

OSNR game optimization with link capacity constraints in general topology WDM networks

This work studies games with coupled constraints in general topology optical wavelength-division multiplexed (WDM) networks towards optimizing channel optical signal-to-noise ratio (OSNR). We first develop a model to describe the network and an OSNR model for each link by investigating the interaction between the network and physical layers. The nonlinear threshold is considered as the link capacity constraint and we study the case in which channel powers are adjustable at optical switching nodes (e.g., optical cross-connects (OXCs)). An OSNR Nash game is formulated with coupled utilities and constraints, in which each player (channel) maximizes its own utility function related to minimizing its individual OSNR degradation. We exploit this OSNR Nash game in three types of network topologies: multi-link topology, quasi-ring topology and general topology. A hierarchical decomposition approach leads to a lower-level game for channels with no coupled constraints and a higher-level optimization problem for the network. Computation of equilibria based on this hierarchical algorithm is discussed and evaluated by simulation for each of the three network topologies.

CRE: A novel QoT-aware routing and wavelength assignment algorithm in all-optical networks

In a transparent wavelength routed optical network, physical impairments degrade optical signal to noise ratio (OSNR) of a transmitted signal. Therefore, the quality of a lightpath must be checked during connection setup procedure. Since a lightpath setup may inject crosstalk on all previously established lightpaths with which it shares links or nodes, OSNR for those lightpaths should be checked as well as the OSNR of the tentative lightpath. On the other hand, by increasing the number of hops for a lightpath, the amount of OSNR degradation increases due to crosstalk. Therefore, long paths are subject to high quality of transmission (QoT) degradation due to crosstalk. In this paper, we enhance OSNR degradation for different lightpaths with different hop numbers in such a way that OSNR degradation for long-hop lightpaths becomes closer to short-hop lightpaths, i.e., a fair OSNR degradation. We provide a mechanism called Crosstalk Risk Estimation (CRE) by which we can find the nodes with positive crosstalk risk (i.e., the node in which generated crosstalk noise violates its crosstalk tolerance bound). Then, after establishing a lightpath, a rerouting mechanism is employed to reduce crosstalk risks of these nodes by migrating some of the lightpaths that pass through them. Actually, each node that is positive crosstalk risk decides to reroute some lightpaths traversing from itself. This algorithm tries to minimize the impact of establishing a new lightpath on the already established lightpaths. Our performance evaluations show that the CRE algorithm can provide better QoT and network access fairness because it can reduce the crosstalk risk and successfully mitigate crosstalk effects.

A Distributed Optimization Approach to Constrained OSNR Problem

This paper studies constrained optical signal-to-noise ratio (OSNR) problem via a distributed optimization approach. In multi-channel optical systems, the signal over an optical link can be regarded as an interfering noise for others, which leads to OSNR degradation. Regulating the input optical power at the Source (transmitter) aims to achieve satisfactory OSNR level at the Destination (receiver) for each channel. Moreover, because all wavelength-multiplexed channels in a link share the same optical fiber, the total input power in a link has to be below the nonlinearity threshold, which corresponds to a link capacity constraint. We formulate the OSNR optimization problem as one of utility maximization with the objectives of achieving an OSNR target level for each channel while minimizing the interference and also satisfying the link capacity constraint. We derive conditions for the existence of a unique optimal solution, leading to a basis for an admission control scheme. By using a Lagrangian relaxation approach we propose two distributed update algorithms: a primal algorithm and a dual algorithm, and study their convergence properties both theoretically and numerically.

A Nested Noncooperative OSNR Game in Distributed WDM Optical Links

This paper develops a Nash game formulation for optical signal-to-noise ratio (OSNR) in distributed optical links. The starting point is a recent network OSNR model developed for optically amplified links whereby channel powers are adjusted independently only at transmitter sites. A more general case is considered here where channel powers are also adjustable at intermediary dynamic sites, specific to optical networks. For this inherent distributed configuration a nested Nash game is formulated towards minimizing channel OSNR degradation along the link. Existence and uniqueness of the Nash equilibrium solution is shown and a recursive procedure for constructing it is given. Based on this, an iterative algorithm that is distributed with respect to both channels and -spans is proposed.

Study of phase-noise properties and timing jitter of 40-GHz all-optical clock recovery using self-pulsating semiconductor lasers

This paper reports on timing-jitter analysis of an all-optical clock-recovery scheme at 40 GHz using self-pulsating (SP) lasers. Based on the analogy with injection locking of oscillators, theoretical investigations on phase-noise properties of the recovered clock lead to the demonstration of a filtering function with slope that is compliant with the International Telecommunications Union (ITU) standards and allow us to underline the dependence of the cutoff frequency of the filtering transfer function on the spectral linewidth of the free running SP laser. From this phase-noise analysis, an analytical expression of the timing jitter of the recovered clock is derived, including the optical signal-to-noise ratio (OSNR) of the injected signal. A set of experiments on all-optical clock recovery at 40 GHz is then presented and demonstrates the crucial role of the spectral linewidth on the timing-jitter-filtering function of the SP laser. In good agreement with theoretical results, the impact of the OSNR degradation of the injected signal on the timing jitter is also demonstrated. Finally, the all-optical clock-recovery operation using a quantum-dot SP laser is shown to be standard compliant in terms of timing jitter, even for highly degraded OSNR.

Characterization of 1/spl times/N broadcast and 2/spl times/N multicast packet switching using active-vertical-coupler-based optical crosspoint switch

This paper reports the expansion of a novel optical multicast scheme to 1-to-4 and 2-to-4 configurations using an active-vertical-coupler (AVC)-based optical crosspoint switch (OXS) matrix. A 1-to-4 broadcast experiment has been carried out first, with only 0.5 dB excess loss per signal split. Input signals with two wavelengths are then fed into one row of OXS to investigate the effect of wavelength-division multiplexing on the 1-to-4 multicast switching. Finally, combining both, the expanded 2-to-N multicast is also achieved with an N value up to 4. The switching characteristics, switched signal quality, and optical signal-to-noise ratio (OSNR) have been investigated in different configurations. Power penalties of less than 4.5 dB are found in worst case switched signals in the 2-to-4 configuration. Slow OSNR degradation in line with previous prediction is also observed. The measured results confirm the excellent multicast switching characteristics nearly free from power splitting loss in this device.

Waveband converters based on four-wave mixing in SOAs

Four wave mixing (FWM) is distinguished from other wavelength conversion techniques by its ability to simultaneously convert a number of input wavelength channels. In this case, optical signal-to-noise ratio (OSNR) is insufficient to describe the performance of the device as many effects are involved. A multiwavelength FWM model is used here to simulate a waveband converter (WBC). The numerical model predicts the waveform of the FWM product. Based on that output, the Q factor of the signal and the power penalty induced to the signal can be calculated to evaluate the performance of such a device. Meanwhile, an analytical model is used for the calculation of the signal power levels and the standard deviation of the fluctuation; hence, it describes the constituent effects-namely, the extinction ratio (ER) degradation, the OSNR degradation, the gain modulation (GM) related crosstalk, and interference. The model's validity is tested against the numerical results. To the best of the authors' knowledge, this is the first time that a numerical model and an analytical model are used to systematically investigate a WBC and to identify the specific effects and derive the design rules. These rules are tested in the experiment. Finally, a tunable WBC (TWBC) based on the dual-pump configuration is described and implemented experimentally

2-to-4 optical multicast using active vertical coupler optical crosspoint switch matrix

A novel optical multicast switching scheme using an active vertical coupler (AVC)-based optical crosspoint switch (OXS) matrix is expanded from a 1-to-N broadcast to 2-to-N multicast. The switching characteristics, switched signal quality, and optical signal-to-noise ratio (OSNR) have been investigated in different multicasting configurations. All configurations are shown to be nearly free from power splitting loss. Power penalties of less than 4.5 dB are found in switched signals in the 2-to-4 configuration. Slow OSNR degradation in line with previous prediction is also observed.

Design and experimental characterization of EDFA-based WDM ring networks with free ASE light recirculation and link control for network survivability

We theoretically and experimentally investigate the performance of erbium-doped fiber amplifier (EDFA)-based WDM ring networks with free amplified spontaneous emission (ASE) light recirculation. We show that, with proper network and amplifier design, the lasing light generated by free ASE recirculation within the looped network provides an effective gain clamping technique, ensuring limited signal power excursions under WDM channels add-drop operations. Considering a ring network composed of eight fiber sections and eight EDFAs, maximum signal power overshoots below 2.5dB have been measured under 23/24 WDM channels drop. Optical signal-to-noise ratio (OSNR) analysis and bit-error rate (BER) measurement at 10 Gb/s confirm acceptable performances and negligible penalties due to polarization effects and relative intensity noise transfer from laser light to WDM signals. We also propose and demonstrate a new link control technique which overcomes the main limiting factors of such networks, respectively, related to OSNR degradation, stability and survivability to fiber and EDFA breakages.