Next generation optical communication systems will be characterized by increasing data rates, high signal powers and dense wavelength division multiplexing (DWDM). In future-optical networks channels will be routed through complex, meshed networks (ASTN, Automatically Switched Transport Networks, ITU-T Recommendation G.808 0/Y.1304). These networks will be able to setup transparent optical paths without converting the optical signals to electrical signals. In all-optical networks the physical impairments and degradation effects play an important role. There is a multitude of degradation effects like dispersion, noise, crosstalk, fiber nonlinearities, polarization dependent loss, etc. To enable a fast setup and the best choice of one of the available paths, the signal quality along the whole transmission distance has to be evaluated very fast.
Ideally, only a single figure of merit (FOM), e.g. the bit error rate (BER), will be computed, which incorporates all degradation effects. Therefore it is important to characterize the different physical impairments analytically. Signal distortions can be measured by an eye opening penalty (EOP) and degradation effects due to noise by the optical signal-to-noise ratio (OSNR). The goal is to find and calculate these impairments from the signal parameters (modulation format, data rate, duty cycle, channel spacing, etc.) as well as the route parameters (fiber lengths and parameters, EDFA powers, etc.). Due to the need of fast routing algorithms, time-consuming numerical methods or a complete system simulation are not practical. In addition, it is not possible to linearly accumulate the different degradation effects.
The focus of this work is to find analytical or heuristic formulas for each degradation effect. These approximation formulas are compared to the results obtained from a complete simulation of a reference system with the help of the simulation tool PHOTOSS.
Table of Contents
1 INTRODUCTION
1.1 Introduction
1.2 Optical communications systems
1.3 Aims and objectives
1.4 Structure of this thesis
2 THEORY
2.1 Chapter overview
2.2 Maxwell’s equations
2.3 Reflection and refraction
2.4 Dispersion and loss
2.5 Effective length and area
2.6 Nonlinear Schrödinger equation (NSE)
3 COMPONENTS
3.1 Chapter overview
3.2 Modulation format
3.3 Filter
3.4 Assessment of the signal quality
3.4.1 Eye opening penalty
3.4.2 Q-factor
4 LINEAR DEGRADATION EFFECTS
4.1 Chapter overview
4.2 Inter-channel crosstalk
4.2.1 Continuous-wave (CW) case
4.2.2 Non-return to zero (NRZ) case
4.2.3 Return to zero (RZ) case
4.3 Narrow-band spectral filtering
4.4 Optical demux filter optimization
4.5 Dispersion
4.5.1 Group velocity dispersion (GVD)
4.5.2 Third-order dispersion (TOD)
5 NONLINEAR DEGRADATION EFFECTS
5.1 Chapter overview
5.2 Four-wave mixing (FWM)
5.2.1 Approximation of the signal-to-crosstalk ratio
5.2.2 Simulations of the NRZ case
5.2.3 Simulations of the RZ case
5.3 Self-phase modulation (SPM)
5.4 Stimulated Raman Scattering (SRS)
5.4.1 Theoretical considerations
5.4.2 Continuous-wave (CW) case
5.4.3 Non-return to zero (NRZ) case
5.4.4 Effects of group-velocity dispersion (GVD)
5.4.5 Simulations of multi-span systems with GVD
6 EXAMPLES OF NETWORK PLANNING
6.1 Chapter overview
6.2 Variation of the channel input power
6.3 Variation of the number of WDM channels
6.4 Variation of the channel spacing
7 CONCLUSION AND OUTLOOK
Research Objective and Scope
This thesis aims to analytically characterize the physical impairments affecting optical signals in transparent, automatically switched transport networks (ASTN). The research focuses on developing analytical or heuristic approximation formulas for various linear and nonlinear degradation effects to enable rapid evaluation of signal quality within routing algorithms, avoiding the computational intensity of full-scale system simulations.
- Analysis of linear degradation effects including inter-channel crosstalk, narrow-band spectral filtering, and dispersion.
- Investigation of nonlinear degradation effects such as four-wave mixing (FWM), self-phase modulation (SPM), and stimulated Raman scattering (SRS).
- Development of fast, analytical approximation formulas for signal impairments to replace complex numerical methods.
- Comparison of analytical results with full-scale simulations using the PHOTOSS software package.
- Application of findings to practical network planning scenarios, focusing on parameter variation and system optimization.
Excerpt from the Book
4.2.1 Continuous-wave (CW) case
In this first paragraph the purely linear interference of the signal with a single neighboring channel will be examined analytically. The signals will be described in the complex baseband representation. At the receiver side the photodiode will – seen from the mathematically point of view – square the amplitude of the optical signal M, which means that the optical power is equivalent to the electrical current i.
i = |M1 e^{jω1t} + M2 e^{jω2t}|^2 = M1^2 + M2^2 + 2M1M2 cos((ω1 - ω2)t)
Because the lowest level of a ‘1’ is important for distinguishing between a ‘0’ and a ‘1’ at the decision circuit, this is equivalent to the following formula in the worst case (the minimum of a cosine oscillation is -1).
i_worst-case = M1^2 + M2^2 - 2M1M2
If we define M2 as the interfering signal, a penalty due to the neighboring channel can be defined as the quotient of the signal without crosstalk to the signal including the effects of crosstalk.
Summary of Chapters
1 INTRODUCTION: This chapter provides an introduction to fiber optics and the evolution of optical communication systems, outlining the thesis objectives regarding the analytical modeling of signal impairments.
2 THEORY: This chapter covers the foundational theory of light propagation, including Maxwell’s equations, reflection, refraction, dispersion, and the nonlinear Schrödinger equation (NSE).
3 COMPONENTS: This chapter introduces signal characterization parameters and performance metrics such as modulation formats, Eye Opening Penalty (EOP), and the Q-factor.
4 LINEAR DEGRADATION EFFECTS: This chapter analyzes linear impairments, specifically inter-channel crosstalk, narrow-band spectral filtering, and dispersion, providing analytical approximations for each.
5 NONLINEAR DEGRADATION EFFECTS: This chapter investigates nonlinear effects, including four-wave mixing (FWM), self-phase modulation (SPM), and stimulated Raman scattering (SRS), with a focus on quick system assessment formulas.
6 EXAMPLES OF NETWORK PLANNING: This chapter applies the findings from the previous chapters to network planning scenarios, demonstrating the trade-offs between parameters like power, channel count, and channel spacing.
7 CONCLUSION AND OUTLOOK: This chapter summarizes the project, reviews the achievements, and provides critical insights as well as suggestions for future research in optical networks.
Keywords
Optical communications, ASTN, Physical impairments, Constraint based routing, FWM, SPM, SRS, Dispersion, Crosstalk, Signal quality, EOP, Q-factor, WDM, PHOTOSS, Network planning.
Frequently Asked Questions
What is the core focus of this thesis?
The thesis focuses on the analytical characterization of physical impairments in transparent optical networks to facilitate fast signal quality evaluation for routing and network planning.
What are the primary degradation effects analyzed?
The work covers linear effects like inter-channel crosstalk, narrow-band spectral filtering, and dispersion, as well as nonlinear effects including FWM, SPM, and SRS.
What is the main goal of the research?
The primary goal is to find fast analytical or heuristic formulas for these impairments to avoid time-consuming numerical simulations during the routing process in future automatically switched networks.
Which methodology is used to validate the findings?
The analytical approximation formulas developed are validated by comparing them against results obtained from full-scale numerical simulations using the PHOTOSS software.
What topics are discussed in the main body of the work?
The main body details the theoretical background of light propagation, the impact of various components, specific analytical modeling of linear and nonlinear degradation, and practical network planning examples.
Which keywords best describe this research?
Key terms include Optical communications, ASTN, Physical impairments, FWM, SPM, SRS, Dispersion, and Network planning.
How does this work address the challenges of Four-Wave Mixing (FWM)?
The work provides analytical formulas for FWM crosstalk, investigating how it depends on channel spacing, dispersion, and signal power, and calculates its impact on signal quality in both DSF and NZDSF fiber types.
What role does the PHOTOSS software play in this thesis?
PHOTOSS is used as the simulation platform for full-scale reference systems, allowing the author to verify the accuracy of the proposed analytical approximation formulas.
- Quote paper
- Dr.-Ing. Stephan Pachnicke (Author), 2002, Constraint based routing due to physical impairments in automatically switched transport networks, Munich, GRIN Verlag, https://www.grin.com/document/50146
