Complex dynamics in photonic delay systems: a story of consistency and unpredictability

- Introducció Photonic systems are powerful test-beds for the investigation of complex dynamics emerging from delays in feedback or coupling. The aim of this thesis was to study two relevant properties of photonic delay systems with direct applications in current information processing and encryption systems: consistency and unpredictability. We characterized the ability of laser systems with delay to exhibit, on the one hand, reliable complex dynamics when an external stimulus is applied, and on the other hand, unpredictable complex behavior, depending on the operating conditions. Consistency properties have been studied as a necessary condition for the implementation of Reservoir Computing schemes. The property of unpredictability has been exploited in the application of random bit generation. - Contingut de la investigació For the characterization of the consistency properties, we used three different experiments based on a semiconductor laser systems with delay that followed the drive-response scheme. Through the analysis of the responses to a repeated drive, a consistent or inconsistent behavior can be identified. In the first setup, we investigated consistency of a semiconductor laser to its own time-delayed feedback, so that the drive was the self-generated complex signal, and the response system was the semiconductor laser itself. A high accuracy in the repetition of the drive was achieved with the design of a fiber-optic setup with two feedback loops. This allowed us the extraction of measures like the sub-Lyapunov exponent. We extended the study to the use of a semiconductor laser system with excitability properties subject to electrical input pulse trains. Here, two different pulse trains modulating the pump current of the laser were used as drives, while the semiconductor laser with delayed feedback operating in the chaotic regime of Low Frequency Fluctuations acted as response system. The purpose of this experiment was to study the possibility and requirements to induce a consistent response, particularly the power drop-out, with the injection of a short pulse. To complement the investigations on consistency, the last drive-response scheme used was an electro-optic intensity oscillator driven by 3 scalar signals: an harmonic waveform, a sequence of pulses, and recorded time-traces from the autonomous dynamics. Under certain conditions, the response system can show hysteresis and coexistence of multistable states. We introduced new tools to qua