Microresonator combs have revolutionized the field of frequency metrology, replacing bulky mode-locked laser setups with chip-scale devices. While these novel comb sources have demonstrated their usefulness in numerous applications the exact nature of the mode-locking process in these devices appears to be not fully understood as it is commonly believed that synchronization between laser modes can only be achieved in the presence of an effective saturable absorber inside cavity. In the absence of saturable absorption, one would therefore expect that soliton solutions of the Haus master equation are not unconditionally stable against residual third-order dispersion. Consequently, deviations from a perfect equidistance may arise. Here we show that four-wave mixing processes can, to some extent, take over the role of saturable absorption and lead to a synchronization of modes. Within a certain range of dispersions, stable soliton solutions can be found not only in the anomalous dispersion regime, but also for zero, normal, or third-order dispersion. However, if the soliton conditions are not exactly matched, breather solutions form, and the resulting combs show deviations from equidistance.
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