We numerically investigate the dynamics of Kerr comb generation in a calcium fluoride whispering gallery mode (WGM) microcavity taking thermal effects into account. Soliton pulse generation is investigated in the presence of thermooptic (TO) and thermal expansion (TE) effects and particularly when the TO effect is negative. We found that, with optimized cavity parameters, self-induced cavity resonance shifts resulting from a negative TO effect allow the system to generate a soliton pulse with no wavelength scanning or dynamic power optimization, which are usually needed to achieve a soliton pulse with silica and other WGM microcavity systems. A negative TO effect automatically tunes the cavity resonance so that the cavity will achieve a soliton state. The competition between the TO and TE effects is also investigated in detail.
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