Coherent Optical Comb Generation in the Joint Space of Frequency and Topological Charge

Abstract

Coherent photonic states in the frequency domain or optical frequency combs are investigated widely over the past decade. Meanwhile, photonic states based on spatial coherence, in particular optical vortex states, have also been studied extensively. Further exploitation of coherent photonics state in hitherto unexplored higher dimensional spaces can reveal new realms of physics. The on‐chip generation of coherent optical combs in the 2D parameter space of frequency and topological charge is reported. Using a micro‐ring resonator embedded with angular gratings, optical frequency combs are generated by Kerr nonlinearity. The coherent comb lines, with their frequencies inherently matching the cavity resonances, are immediately diffracted by the angular grating to emit an ensemble of coherent spatial photonic states with each state carrying distinctive spin and orbital angular momenta. The mapping between the frequency and the topological charge dimensions leads to the construction of coherent combs in the high‐dimensional frequency‐topological charge (f‐l) space. Operating the device in the normal‐dispersion regime enables the adjustment of comb line spacing from 1 to 11 free spectral ranges by selectively pumping different cavity resonances with high pump‐to‐comb conversion efficiency. Such on‐chip reconfigurable high‐dimensional coherent optical combs open avenues for innovative exploration of physics.