Generating high fidelity structured light fields through an ultrathin multimode fibre using phase retrieval

Light transmission through a multimode fiber (MMF) has gained major importance for imaging and manipulation. The majority of phase retrieval algorithms used for a MMF implicitly assume light propagation to be described by a unitary operation, yet the transmission matrix of a multimode fiber is inherently non-unitary. It is demonstrated that this erroneous assumption can impede the performance of many commonly used MMF phase retrieval algorithms and demonstrate that the weighted Yang–Gu algorithm outperforms other phase retrieval algorithms in this scenario. Once accounted for, the non-unitary property of the transmission matrix can be leveraged to generate intricate intensity and phase patterns at the output of the fiber, and shape specific output fields. This is experimentally demonstrated by generating Laguerre–Gaussian beams that carry orbital angular momentum, and by forming images in planes offset from the distal end of the fiber facet.