Illuminating the photonic lantern: coherent spectro-polarimetric characterisation for high angular resolution astronomy

Abstract

Photonic technologies offer an enticing means to reach exquisite detail with high angular resolution measurements of astrophysical scenes through precision measurement and control. Over the past decade, photonic lanterns have become key components for applications requiring mode sorting ranging from astronomical imaging to free-space communications. In these applications, knowledge of the electric field needs to be inferred from intensity measurements alone or beam-shape control is needed. While these devices can efficiently and uniquely map a set of input modes to single-mode outputs (or vice versa), the optical mode transfer matrix of any particular fabricated device cannot be fully specified at the design stage due to manufacturing limitations. This thesis presents a characterisation system to directly measure the electric field from a photonic lantern using digital off-axis holography, following its evolution over a 73 nm range near 1550 nm and in two orthogonal, linear polarisations. Performance of the testbed is validated on a single-mode fibre and applied to a 19-port, multicore fibre based photonic lantern. The first broadband multi-wavelength, polarisation decomposed characterisation of the mode transfer matrix of a photonic lantern is presented. As part of this characterisation, the system measures the position of zero path difference between different ports and polarisations, reveals the typical wavelength scale over which the modal mapping evolves and quantifies the mode dispersion within the device itself. In addition to detailing the system, empirical mode transfer matrices, raw data and post-processing code are shared, enabling future work in astrophotonics to understand where photonic lanterns fit in the wider picture of high angular resolution astronomical instrumentation.