Physical Layer Security of Multimode Optical Fiber Transmission Systems

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Singlemode optical fibre networks have facilitated the emergence of the modern internet. Sophisticated multiplexing techniques applied to several domains (time, polarisation, wavelength) enable fast data modulation. In the era of data centres, which require a continuous increase of network capacities, multimode fibres (MMF) offer the potential to increase possible data rates by additional exploiting the spatial domain. In addition, it is possible to exploit the complex light propagation within MMF by suitable transmitter-side wavefront shaping to increase information security. This approach is called Physical Layer Security (PLS) and was first investigated experimentally on MMFs by our lab. In contrast to classical cryptographic techniques on higher layers, PLS generates codes are based on physical channel properties. Further, PLS is compatible with classical components of optical data transmission (amplifiers, repeaters). If the communication channel between two nodes (Alice and Bob) is known, for example, by measuring the transmission property (transmission matrix, TM), an SNR advantage over a potential eavesdropper (Eve) can be achieved by subsequent inverse precoding. While the channel is calibrated to Bob, Eve suffers mode-dependent losses that allow Alice to degrade Eve's channel quality to achieve an advantage for Bob via suitable coding algorithms (artificial noise, "wiretap coding"). The first fundamental investigations towards PLS demonstrated the potential of the approach on 10 m step-index fibres. These exhibit strong mode mixing. However, in conventional MMF communication networks, graded index (GRIN) MMFs with comparatively less mixing are used over much longer distances. At long distances, time variance occurs, which is why calibration and communication must occur within the finite coherence time of the channel. Therefore, we aim for a real-time calibration based on a neural network to investigate PLS in MMF at usual transmission distances. We are also investigating the influence of polarisation on PLS and whether manipulation of polarisation crosstalk can lead to an increase in information security. Using a second light path and copula theory, statistical models will be created to investigate stochastic dependencies between orthogonal polarisation states. This can be used to determine the increase in secrecy, e.g. through "secret key generation".