Fig. 1: Non-degenerate response of the photonic lantern wavefront sensor to focal plane phase.

a Schematics of a multi-core photonic lantern showing how the phase and intensity of the input field into the multimode fibre end-face evolve into an array of uncoupled single-mode cores with different intensities. b The results of three RSoft simulations demonstrating the concept of the photonic lantern wavefront sensor, and its ability to measure both amplitude and phase. The first column shows the phase of the wavefront, and the second and third columns show the intensity and phase of the resulting PSF respectively. The fourth column shows the intensities of the 19 single-mode outputs of the photonic lantern, when the corresponding PSF is injected. In the first example (first row) a flat wavefront is used. In the second and third rows, astigmatism with an amplitude of 0.8 radians, but with opposite signs, is introduced. This results in identical intensity structure in the image plane (2nd column), and so could not be distinguished with an imaging sensor. However the (usually un-measured) phase in the focal plane (3rd column) shows the difference between the two astigmatism terms, which is successfully measured by the photonic lantern (as shown by the different set of outputs from the lantern, in the 4th column). Simulations are performed at a wavelength of 1550 nm. Intensities are plotted with a square-root stretch to better show faint detail.