  Anomalous microresonator dispersion is mandatory for Kerr soliton microcomb
formation, which depends critically on the geometry of the microresonator and
can hardly be tuned after the structure is made. To date, cavity-based
microcombs have only been generated with fundamental whispering gallery modes
(WGMs) of anomalous dispersion in microresonators. Moreover, microcomb
generation in highly Raman-active platforms such as lithium niobate (LN)
microresonators frequently suffers from stimulated Raman scattering and mode
crossing due to the existence of multiple families of high-order WGMs. Here, we
reveal a unique Kerr soliton microcomb generation mechanism through mode
trimming in a weakly perturbed LN microdisk resonator. Remarkably, the soliton
comb is generated with fundamental WGMs of normal dispersion and free from the
mode crossing and Raman scattering effects. A robust soliton with a spectrum
spanning from 1450 nm to 1620 nm at an on-chip pump power of 35 mW. Our
discovery offers a powerful solution to circumvent the stringent requirements
on high-precision dispersion engineering and termination of Raman excitation
for soliton generation in the high-Q microdisk.
