Stefan Rothe Bücher

Digital communication has rapidly transformed life, impacting all domains and increasing the risk of cyber attacks, necessitating new information security methods. Optical communication networks are vital as they form the backbone of global digital infrastructure. Multimode fibres, with their spatial paths, can significantly enhance network capacities. This dissertation explores achieving information-theoretic secure data exchange in optical multimode fibres by leveraging physical effects within the fibre channel. The focus is on enabling a transmitter (Alice) to exploit the inherent disorder in multimode fibre, giving a legitimate receiver (Bob) a distinct advantage over an eavesdropper (Eve). This is achieved through modal crosstalk and mode-dependent loss, creating an imbalance among receivers. The technique, known as physical layer security, is experimentally implemented for the first time on multimode fibre. By measuring the optical transmission matrix, Alice and Bob can calibrate their channel and identify optimal spatial paths, utilizing holographic methods and neural networks. They can then apply optical pre-distortion for effective light propagation, while Eve must rely on mathematical processing of her channel. An experiment demonstrated that with special channel coding, secure data exchange of 2 bits per channel use can be achieved, even with Eve having complete channel state information. Future research could enhan