Nonlinear Photonics and Signal Processing

Photonics - Statistics of complex systems
Statistics of complex systems

Though nonlinear physics has a rather long history, beginning with the works of Newton and Huygens, science and technologies of the 19th and most of the 20th century had been dominated by linear mathematical models and linear physical phenomena. 

Over the last decades there has been growing recognition of the importance of physical systems in which nonlinearity introduces a rich variety of fundamentally new properties - properties that can never be observed in linear models or implemented in linear devices. 

The understanding and mastering of nonlinear physical systems has the potential to enable a new generation of engineering concepts. Our research is at the interface between fundamental nonlinear science and practical applications in fibre optics and photonics.

Key research areas   

  • Ultrafast nonlinear phenomena in optical fibre 
  • Nonlinear wave theory 
  • Optical solitons 
  • Statistics of complex systems 
  • Nonlinear optical pulse shaping & light manipulation 
  • Fundamentals of fibre lasers 
  • Shannon capacity of nonlinear systems 
  • Quasi-lossless transmission systems 
  • Nonlinear structures for information transmission 
  • All-optical regeneration and signal processing   

Possible applications

  • Parametric amplifiers

  • Nonlinear communications

  • Optical switching

  • Optical computing

  • Fibre lasers

  • Raman amplifiers

Since Shannon derived the seminal formula for the capacity of the additive linear white Gaussian noise (AWGN) channel, it has commonly been interpreted as the ultimate limit of error-free information transmission rate. However, the capacity above the corresponding linear channel limit can be achieved when noise is suppressed using nonlinear elements. Regeneration is a fundamental concept that extends from biology to optical communications. All optical regeneration of coherent signal has attracted particular attention. Surprisingly,the quantitative impact of regeneration on the Shannon capacity has remained unstudied.We proposed a new method of designing regenerative transmission systems with capacity that is higher than the corresponding linear AWGN channel, and illustrated it by the regenerative Fourier transform (RFT) for efficient regeneration of multilevel multidimensional signals. The regenerative Shannon limit – the upper bound of regeneration efficiency – was derived.

Figure below shows gain (above linear AWGN channel) for the different number of regenerators. The analytical results shown by black lines demonstrate an excellent agreement with numerics (solid coloured lines).The inset shows mutual information gain for M2 - rectangular constellations approaching capacity gain.

Shannon capacity
This work, which is a part of the research on the use of nonlinear phenomena in optical fibres for the generation and shaping of optical pulses conducted in collaboration with the University of Bourogne (Prof. C. Finot), presents a detailed experimental characterization of the adiabatic transition process of an initially low-energy Gaussian pulse to the asymptotic self-similar parabolic solution in optical fibre amplifiers operating in the normal dispersion regime. The experimental study coupled with a numerical analysis highlights the various stages of the nonlinear reshaping. The impact of saturation of the gain in the amplifier is also clearly shown, requiring the inclusion of a varying gain along the fibre in the numerical model. 

The results presented demonstrate that even though optical amplifier similaritons were highlighted as soon as 2000, there are still some aspects of the nonlinear pulse dynamics that are not fully explored. Many applications, such as optical signal processing and modelocked fibre lasers, may benefit from this better understanding.

An analytical method for optimizing phase sensitive amplifiers (PSAs) for regeneration in multilevel phase encoded transmission systems. The model accurately predicts the optimum transfer function characteristics and identifies operating tolerances for different signal constellations and transmission scenarios. The results demonstrate the scalability of the scheme and show the significance of having simultaneous optimization of the transfer function and the signal alphabet. The model is general and can be applied to any regenerative system. 

We also investigated the transmission performance of advanced modulation formats in nonlinear regenerative channels based on cascaded phase sensitive amplifiers. We identified the impact of amplitude and phase noise dynamics along the transmission line and showed that after a cascade of regenerators, densely packed single ring PSK constellations outperform multi-ring constellations. The results of this study will greatly simplify the design of future nonlinear regenerative channels for ultra-high capacity transmission.