| dc.description.abstract | In recent decades, gaining full control over the spatial degrees of freedom of light has become a much sought-after capability. Applications abound in both classical and quantum optics, and include imaging, optical communications and optical computing. The development of systems capable of freely manipulating spatial states of light is fueled by the fact that encoding information into the spatial components, alongside with spectral and polarisation degrees of freedom of light, promises to substantially enhance data transmission capacity of the current communication systems and contribute to revolutionising imaging techniques. Thus the presented research is concentrated on the design and experimental implementation of high-dimensional multi-plane light converter (MPLC) devices performing arbitrary spatial basis transformations on monochromatic optical fields. While building on the recent advancements in the field, the few-plane prototypes realised in this work, functioning in the limit N≫M are the first examples of high-dimensional systems operating on generalised spatial bases, where N is the number of spatial modes in the basis, and M is the number of MPLC planes.
These pre-programmed multi-plane light converter devices comprise series of 2D diffractive elements separated by free space. In this work, a new technique for numerical optimisation of these diffractive planes is developed based on a gradient ascent, which is a more flexible generalisation of the current state-of-the-art MPLC design method. All the MPLCs optimised with this algorithm are benchmarked using the spatial mode sorting and transforming operations on the bases of arbitrary speckles, rendering these devices as general as possible by not capitalising on any underlying spatial symmetry. With the average cross-talk representing an average error in sorting photons based on their transverse spatial state, an enhancement of its levels up to 10 times for particular designs was shown using the newly derived optimisation technique, which was proven to provide better control over the light in a low-plane MPLC limit due to carefully tailored objective functions. The optimised systems achieved high average transformation fidelity levels f≳96% of linear optical circuits operating on N=28 random speckles in M=5 MPLC planes, while the low-cross-talk solutions for the arbitrary spatial N=55 mode sorters are unlocked starting from as little as M=2 planes.
Series of high-dimensional few-plane MPLC prototypes operating on arbitrary speckle, Zernike, Bessel mode bases and orbital angular momentum (OAM) states of light were experimentally implemented using a single experimental setup, emphasising the versatility of this generalised approach. Experimental spatial mode sorters operating on bases of random speckles ranging from N=3 to N=55 with M=5 MPLC planes were shown. First published prototypes of spatial mode sorters operating on N=3-36 Zernike modes were realised, with the potential applications ranging from the improvement of super-resolution microscopy schemes to coronagraphy in astronomy. Two methods of optimising the misalignments of the experimental multi-plane light converters were developed and presented. One of them, performing in situ genetic algorithm-based optimisation, was shown capable of consistently and efficiently deriving the misalignment parameter values that lay near the global minimum of the problem in just ∼30-60 minutes of operation. The second misalignment optimisation method based on the diffractive neural network layered model was shown linking the implementation of the MPLCs directly to the concept of artificial neural networks that learn the parameters through the process of error back-propagation.
The applications of the generalised high-dimensional MPLC devices as the all-optical tools to invert the transmission matrices (TMs) of the forward scattering media are discussed in this work from the point of view of unscrambling light when imaging through such disordered media. The low-resolution, M=5-plane MPLC-based inverters of the step-index multi-mode optical fibres (MMFs) allowing single-shot incoherent imaging and data transmission through fibres were designed, yielding the theoretical average fidelity of f≈98%. As such prototype devices are all-optical and passive, they are capable of inverting the scrambling introduced by the MMF at the speed of light and are easily reconfigurable in the case its bending configuration has changed. The discretised devices with a channel count of N=19 and N=30 which dictates the resolution of such imaging systems were experimentally implemented, being the first all-optical MMF inverters ever presented. As the multimode fibres hold promise for the potential use as short-range interconnects in data centres and ultra-thin endoscopes, the designed all-optical inverters strongly contribute to facilitating their inevitable adoption.
Furthermore, the high-resolution model of the all-optical MMF inverter operating on N=403 eigenmodes simultaneously using the total number of M=31 MPLC planes is presented. It allows imaging not only through ideal and flexible fibres, but also when one does not have access to the distal facet of the MMF - as is the case when imaging deep inside the body. The high-resolution incoherent imaging through step-index multimode fibres with fidelity levels up to f≈68% depending on the degree of inter-modal coupling between the eigenmodes of the flexible MMF is shown. The prospects of future research in the area comprise the potential ultra-compact implementations of these high-dimensional multi-plane light conversion devices by means of direct laser writing technology. | en_GB |
