Illuminating the lantern: coherent, spectro-polarimetric characterisation of a multimode converter

TL;DR

This paper introduces a novel spectro-polarimetric measurement system for photonic lanterns, providing detailed mode transfer matrices across wavelengths and polarizations, which enhances understanding and design of multimode photonic devices.

Contribution

It presents the first multi-wavelength, polarization-resolved characterization of photonic lantern modes using digital holography, revealing wavelength-dependent modal evolution and dispersion.

Findings

Measured mode transfer matrices across 73 nm wavelength range

Uncovered wavelength scale of modal evolution and dispersion differences

Validated system performance on single-mode and multicore fiber lanterns

Abstract

While photonic lanterns 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 constrained at the design stage due to manufacturing imperfections. Accurate knowledge of the mapping enables complex sensing or beam control applications that leverage multimode conversion. In this work, we present 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. We provide the first multi-wavelength, polarisation decomposed characterisation of the principal modes of a photonic lantern. Performance of our testbed is validated on a single-mode fibre then harnessed to characterise a 19-port, multicore fibre fed photonic lantern. We uncover the typical wavelength scale at which the modal mapping evolves and measure the relative dispersion in the device, finding significant differences with idealised simulations. In addition to detailing the system, we also share the empirical mode transfer matrices, enabling future work in astrophotonic design, computational imaging, device fabrication feedback loops and beam shaping.