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

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 characterization 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 what we believe to be the first multi-wavelength, polarization decomposed characterization of the principal modes of a photonic lantern. Performance of our testbed is validated on a single-mode fiber, then harnessed to characterize a 19-port, multicore fiber-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 idealized 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.