Mode rejection and counter-propagating all-optical gratings in multimode waveguides

In this report, we demonstrate two novel fundamental notions in the counter-propagating nonlinear interaction scheme, namely mode rejection and all-optical gratings. We consider a probe Forward Signal (FS) and a counter-propagating Backward Control Beam (BCB) that are launched at the opposite ends of a multimode and multicore fibre. When both the FS and the CB are in a strong nonlinear regime, a FS with arbitrary modal state undergoes the suppression of a specific spatial mode that is fixed by the CB, which we name mode rejection. When the FS and the CB are respectively in a weak and a strong nonlinear regime, the CB plays the role of an all-optical grating for the FS. This dynamic is exploited to achieve key-all optical operations for the multimode platform, including tunable power splitting, power combining, and power switching.