Raman Turing Patterns in an Optical Microresonator

Microresonator optical frequency combs (OFCs) are of great interest for basic science and high-tech applications. Major attention is usually paid to the OFCs formed near pump frequency. Raman OFCs can also be generated in resonators at Stokes frequencies, but they are much less studied. This work reports a Raman OFC with spectral line spacing equal to a multiple of a free spectral range (FSR) that has not been observed in optical macro- and microresonators before. We experimentally attained a Raman spectrum with tripled FSR line spacing in a silica microsphere. Numerical modeling of the Lugiato-Lefever equation shows that this experimental OFC may correspond to a Turing pattern and provides an insight into the process. A simple analytical model was developed to explain the formation of Turing patterns: Raman lasing first starts in a single Stokes mode closest to the gain maximum; next, if the threshold power is reached and certain dispersion requirements are met, modulation instability (MI) develops for modes with multi-FSR spacing.