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.
