CORDIS - EU research results

Open-source freeware for fiber optic communication and sensing simulations

Periodic Reporting for period 1 - SIMFREE (Open-source freeware for fiber optic communication and sensing simulations)

Reporting period: 2017-07-01 to 2019-06-30

The exponential increase of data traffic in optical networks over the past decades facilitated the development of new fibre optic technologies in telecommunication, capable of delivering higher capacities and in distributed sensing applications. In optical communication, spectral efficiency is continuously being increased with advanced modulation formats, reduced channel spacing (Nyquist) and new low noise distributed optical amplification techniques. However, the nonlinear Shannon limit puts the physical restrain on the maximum capacity of the single mode fibre that leads to capacity crunch . The nonlinear interferences with an adjacent channels due to increased spectral power density and relative intensity noise (RIN) from distributed Raman amplifiers are still an issue, hence the optimal design of the fibre optic link is necessary in both, communication and sensing systems.

Majority of long-haul links relay on commercial low-cost Erbium doped amplifiers (EDFA), however, the simulation of an optical transmission based on conventional EDFA is not necessarily the optimal solution for every situation. Low noise Raman amplification can realise a long-term goal of optical communications, as it would bring with it a minimisation of amplified spontaneous emission (ASE) noise build-up. Additionally, average signal power variation control in higher-order advanced Raman links can minimise inter-span power asymmetry that opens new window for a nonlinear impairments compensation using optical phase conjugation (OPC). The Raman technology is also used in distributing sensing for temperature, strain and acoustic data acquisition. One of the drawbacks of distributed Raman amplification is RIN transfer associated with the forward pumping. The inclusion of RIN impairments in the bi-directionally pumped Raman simulations are absolutely necessary for a proper evaluation of a data transmission link, however, they are still absent in the simulation tools developed up to date. Contrary to commercial software, the code in open-source tools can be checked, verified and upgraded by anyone. Moreover, the particular modules can be easily modified to meet requirements of an alternative system design that would, for example, include system specific impairments such as RIN transfer in Raman amplified optical links. Novel modules based on most recent studies on signal power asymmetry control (for OPC) and nonlinear inverse synthesis (NIS) in distributed Raman links that can reduce nonlinear noise in an optical transmission will be implemented and directly compared with alternative methods in a digital domain such as digital backpropagation (DBP) that can compensate for both, linear and nonlinear impairments by solving an inverse nonlinear Schrödinger equation (NLSE) and effectively reduce the impact of nonlinear phase noise (NLPN). Combined with advanced digital coherent detection DBP and NIS allows higher capacity transmission without increasing receivers’ complexity as both the phase and polarisation of the signal can be recovered with digital signal processing (DSP). Finally, digital filtering in DSP enables a receiver to adapt to time-varying impairments and assists use of advanced forward-error correction codes.

The amplification based on advanced distributed Raman schemes can cope with the rapid development of optical telecommunication techniques and can be also applied to distributed sensing technology that enables continuous, real-time measurements along the entire length of a fibre optic cable. The physical layer simulation is an important task in network modeling, helping to optimise the transmission range of individual optical paths.

The target of SIMFREE is to use different simulation platforms to implement advanced modules that can be used in simulation of high-speed optical transmission utilising the advantage of higher order Raman amplification with advanced digital signal processing for nonlinear compensation using digital backpropagation and nonlinear inverse synthesis as well as distributed sensing that enables continuous, real-time measurements along the entire length of a fibre optic cable.
The work performed in the project covers advanced research and complementary training program to the fellow. Training in models and algorithms for several different network scenarios in unrepeatered and long-haul optical transmission systems, modelling, design and optimisation of modern optical communication networks based on Raman amplification considering the quality of service, physical layer impairments, energy efficiency, elastic spectrum assignment, and digital processing that can compensate for the nonlinear impairments were the core focus of the project. Research in the field of Raman based distributed sensing applications and finally theoretical and experimental verification of the models were performed. All these results have been adequately disseminated in 6 scientific publications in journals and relevant conferences, as well as a number of invited talks at different institutions.

The source codes in SciLab are available in a public repository GitHub under link:
Progress beyond state of art includes new advancements in linear and nonlinear compensation improvement utilising simulation results for higher order ultra-long Raman fibre laser based amplification that enabled more accurate power distribution prediction which in turn was used with most advanced transmission modulation formats, namely 64QAM, that requires high precision. New modeling algorithms enabled Relative Intensity Noise (RIN) mitigation and transmission performance enhancement with forward broadband pump where, for the first time, we present a detailed evaluation of the transmission performance in a long-haul 100G DP-QPSK WDM coherent transmission system, using forward-propagated first order pump lasers with different bandwidths/RIN/power levels. We demonstrate that, using a forward-propagated coherent fibre laser with much broader bandwidth and relatively low RIN level can extend the maximum transmission distance. This is mainly due to the ASE noise reduction, and more crucially, the signal distortion transferred from the pump RIN is prevented.

This progress is exemplified in the 6 journal and conference papers published up to date as well as invited talks for a scientific and general public that are listed below.

1. P. Rosa “Granty Europejskie Marie Skłodowska-Curie HORIZON2020,” Uniwersytet
Jagielloński, Kraków, 2019

2. P. Rosa “Granty Europejskie Marie Skłodowska-Curie HORIZON2020” w Strategy of excellene interdisciplinary workshop, Uniwersytet Pedagogiczny, Kraków, 2019

3. P. Rosa “Nonlinearity mitigation in quasi-lossless optical transmission,” Pedagogical University of Cracow, Poland, 2019

4. P. Rosa "Distributed Raman amplification in modern optical transmission," Wroclaw University of Science and Technology, Poland, 2018
Experimental 64QAM transmission results
Experimental setup of a 64QAM transmission line