Circulator-Free Brillouin Photonic Planar Circuit

On-chip stimulated Brillouin scattering (SBS) that results from enhanced light–sound interactions, has recently demonstrated great applicability and versatility for signal generation and manipulation. Progress toward realizing fully integrated Brillouin photonic circuits is promising; however, the control and routing of on-chip optical waves requires non-reciprocal elements such as circulators, which is challenging and adds complexity. Here, a circulator-free Brillouin photonic planar waveguide circuit which eliminates the need for separate non-reciprocal elements is demonstrated. Backward inter-modal Brillouin scattering (BIBS) is used in a planar photonic integrated circuit, for the first time, to provide Brillouin processing capability in a multi-modal waveguide, conveniently allowing for the separation of counter-propagating pump and signal using passive integrated mode-selective filters. Using an As₂S₃–Si hybrid multi-modal photonic circuit, inter-optical-mode energy transfer with a Brillouin gain coefficient of 280 m⁻¹W⁻¹ is experimentally demonstrated, representing two orders of magnitude enhancement compared to conventional optical fibers. This on-chip BIBS circuit allows for independent routing of pump and signal waves, showing resilience to the cross talk. The experimental demonstrations provide an important basis for achieving fully integrated Brillouin photonic circuits without requiring on-chip circulators and fine spectral filtering, opening a new dimension for studying spatial-dependent photon–phonon nonlinear dynamics.