Microresonator-based optical frequency comb generation has attracted much interest in the past years. While there have been extensive studies on the frequency-domain behavior of these combs, it remained unclear whether these devices generate short optical pulses in the time domain. The groups of Alexander Gaeta and Michal Lipson at Cornell University studied the time-domain properties of microresonator-based frequency combs and show that these combs can transition into operating regimes where they generate sub-200-femtosecond pulses.

Optical frequency comb generation in optical microresonators can be described by cascaded four-wave mixing processes that convert light from a pump laser into optical sidebands. In particular, the high finesse and corresponding extremely high circulating light intensities in these resonators enable the generation of broadband optical combs. Unlike in femtosecond-laser-based frequency comb generators, in which the comb generation process is usually explained by a fast saturable absorber mode-locking effect, the comb generation in microresonators has been mostly described as a pure frequency-mixing effect. In order to understand the time-domain representation of the generated optical spectra one has to know the relative phases of all the generated comb modes. However, the interaction and mixing of all the different comb modes make it difficult to theoretically predict their phases. Thus, it is exciting to see Saha et al.'s measurement demonstrating the transition into a regime that generates short optical pulses, which indicates that there is indeed a mode-locking mechanism that preserves the phase differences between all the generated optical sidebands.

Understanding pulse generation dynamics in whispering gallery mode resonators has attracted interest in many research institutes around the world, including groups from EPFL in Switzerland, Purdue University and NIST in the USA, and INRS in Canada. The results presented by Saha et al. suggest that the comb-generation process can be described by soliton formation in the time domain. As the authors state, other effects might lead to mode locking as well, for example a self-focusing effect similar to conventional Kerr-lens mode locking that would confine the light towards the center of the resonator (away from the lossy boundary). Developing a physical background for the generation of ultrashort pulses in microresonators is one of the most challenging tasks in this research area, and the work by Saha and colleagues is an important step towards a more detailed understanding.

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