ИСТИНА

Kerr frequency combs are generated by coupling a cw laser and High-Q microresonator that converts the initial pump frequency into a broadband frequency comb by cascaded four-wave mixing processes [1]. Microresonator-based frequency combs are widely employed in various applications where high power per comb line and high repetition rate are important, e.g.in precision frequency metrology, highly multiplexed spectroscopy, fiber telecommunications and many others [2]. Pump threshold power is determined by Q-factor and can be reduced to sub-mW level. It was shown that the integrated group velocity dispersion (GVD) of the microresonator plays an important role in the possibility of comb generation, because phase-matching condition for four-wave mixing processes impose severe restrictions on the microresonator dispersion [3]. Generation of frequency combs in the normal GVD region has been actively explored in recent years and has been demonstrated experimentally, but has been less well studied. New way to comb generation in normal GVD was opened due to the interplay between Kerr and Raman effects [4]. Stimulated Raman scattering and four-wave mixing process are the most important effects associated with the cubic nonlinearity of the resonator. However, the influence of Raman effect on frequency combs characteristics was studied mostly for anomalous GVD. In this work, we offer theoretical approach based on coupled-mode equation to study interaction between Kerr and Raman effects in whispering-gallery mode microresonators and experimentally demonstrated the generation of an optical frequency comb in the Raman scattering band (Fig 1). The proposed approach allows to describe the dynamics of the frequency comb generation in the Raman scattering band and to take the characteristics of the resonator dispersion into account. We demonstrated numerically that stimulated Raman scattering may provide generation of optical frequency comb in normal GVD region. The relevance of the theoretical model is confirmed by the experimental results.