  When light is passed through aberrated optical systems, the resulting
degradation in amplitude and phase has deleterious effects, for example, on
resolution in imaging, spot sizes in focussing, and the beam quality factor of
the output beam. Traditionally this is either pre- or post-corrected by
adaptive optics or phase conjugation. Here we consider the medium as a complex
channel and search for the eigenmodes of the channel, the modes that propagate
through this system without alteration. We employ a quantum-inspired approach
and apply it to the tilted lens as our example channel, a highly astigmatic
system that is routined used as a desired distortion inducer to measure orbital
angular momentum. We find the eigenmodes analytically, show their robustness in
a practical experiment, and outline how this approach may be extended to
arbitrary astigmatic systems.
