Spatial characterization of nonlinear optical states in microresonators via scattered light imaging

A precise understanding of nonlinear optical states in whispering gallery mode microresonators is crucial for developing future integrated photonic devices, with applications ranging from on-chip biomedical sensors and all-optical network memories to frequency combs for optical clocks. Conventional monitoring relies on spectral and temporal signals transmitted through coupling waveguides. Such approaches, however, lack spatial resolution and cannot provide in situ access to the intracavity field distribution, in particular, when resonators are buried in complex networks of photonic circuits. We present a practical method for the in situ monitoring of nonlinear states by directly imaging scattered light patterns with a short-wave infrared camera. This technique provides spatially resolved information that complements conventional transmission spectroscopy. Through tracking azimuthal and radial patterns along a microresonator, we can characterize transitions between different nonlinear optical states. Direct spatial characterization of microresonator modes facilitates applications such as the optimization of soliton frequency combs, tapless real-time debugging of photonic circuits, and chip-based data switching in telecom networks.