Optical Fiber Link Simulation Tutorial

This tutorial demonstrates how to simulate a nonlinear optical fiber communication system using comnumpy. You will learn how to:

• Build a simulation chain with QAM modulation, pulse shaping, and fiber propagation.

• Visualize received signals before and after linear and nonlinear equalization.

• Apply digital back-propagation (DBP) for nonlinear compensation.

• Compute Symbol Error Rate (SER) to quantify performance.

This tutorial is suited for engineers and students interested in optical communications and nonlinear fiber effects.

Introduction

Prerequisites

Ensure you have the following Python libraries installed:

numpy
matplotlib
comnumpy

Note that the simulation can be computationally intensive and may take some time to run depending on your machine.

Simulation Setup

Import Libraries

First, import the necessary libraries and comnumpy components:

Define System Parameters

Set key parameters such as modulation order, oversampling factors, fiber link properties, and noise figure:

Create Communication Chain

Build a processing chain consisting of symbol generation, mapping, pulse shaping (SRRC filter), amplification, fiber propagation via FiberLink, and matched filtering with downsampling:

This simulates the full transmission over an optical fiber with nonlinear effects and noise.

The optical channel (Fiber_Link) is modeled as a concatenation of N_span spans of standard single-mode fiber (SMF). Each span has a fixed length L_span (typically 80 km) and is followed by an Erbium-Doped Fiber Amplifier (EDFA), which compensates for fiber loss while introducing amplified spontaneous emission (ASE) noise (characterized by the noise figure NF_dB).

Within each span, the signal undergoes chromatic dispersion and nonlinear Kerr effects, simulated using the Split-Step Fourier Method (SSFM). The interplay between dispersion and nonlinearity distorts the signal amplitude and phase, motivating the use of advanced digital signal processing techniques at the receiver.

Run Simulation and Extract Signals

Execute the chain and extract the transmitted and received signals. Then, plot the received constellation diagram to inspect signal quality at the receiver.

Received Signal

Linear and Nonlinear Compensation

We apply two compensation strategies:

• Linear equalization: compensates for linear impairments such as chromatic dispersion and attenuation.

• Nonlinear equalization (DBP): performs full Digital Back-Propagation, which mitigates nonlinear fiber effects by numerically inverting the propagation using a reversed Split-Step Fourier Method.

After compensation, the received signal may exhibit a residual phase rotation. Therefore, phase correction is applied before computing the Symbol Error Rate (SER).

Conclusion

This tutorial showed how to:

• Model an optical fiber communication system with nonlinear effects.

• Use SRRC filtering and oversampling to shape signals.

• Simulate fiber propagation with noise and nonlinearities.

• Apply linear and nonlinear compensation techniques (DBP).

• Quantify performance improvements via SER metrics.

This simulation illustrates how comnumpy can be used to study advanced fiber-optic communication design and compensation techniques.