Spectral enrichment of optical microresonator combs by means of a frequency-shifting loop

A fairly large number of optical frequency comb (OFC) generation methods have been proposed and practically implemented to date. All of them have their own advantages and disadvantages. For example, the application of bulk whispering gallery modes or on-chip optical ring microresonators allows one to obtain soliton Kerr combs in the spectral band achieving tens of nm and containing hundreds of lines. However, the spectral spacing between these lines will be determined by the dimensions of the microresonator and, as a rule, ranges from tens to hundreds of GHz. On the other hand, the application of frequency-shifting loop (FSL) systems with an acousto-optic (AO) device as the frequency shifter allows one to obtain OFCs with a frequency spacing of tens of MHz, but the spectral band of the AO FSL OFCs containing several thousand lines will hardly exceed 0.5 nm. In this paper, we propose a method to expand the AO FSL OFCs' spectral band applying multifrequency optical seeding. As a source of such seeding we propose to use lines of a soliton comb spectrum generated with an optical microresonator. This approach can also be considered as a method for enriching the optical spectrum of the microresonator OFC by adding spectral components generated in AO FSLs with a frequency spacing that is a multiple of the seeding soliton microcomb lines' frequency spacing. The paper presents the results of an experiment that confirms the possibility of implementation of such a concept.