  Fiber Kerr resonators are simple driven resonators with desirable wavelength
and repetition rate flexibility for generating ultrashort pulses for
applications including telecommunications, biomedicine, and materials
processing. However, fiber Kerr resonators to date often generate longer pulses
and require more complicated techniques for generating single pulses than would
be desirable for applications. Here we address these limits by demonstrating
robust single-pulse performance with 120-fs pulse durations in fiber Kerr
resonators based on stretched-pulse solitons. Through matching numerical and
experimental studies, stretched-pulse soliton performance is found to strongly
depend on the total cavity length, and the optimum length is found to depend on
the drive, Raman scattering, and the total pulse stretching. By designing the
cavity for this optimum with the described setup, stable stretched-pulse
solitons with 120-fs duration are experimentally observed. In addition, soliton
trapping is demonstrated with a pulsed drive source despite large intracavity
breathing and single-pulse performance is observed. Robust with high
performance single-pulse generation is a critical step toward useful
femtosecond pulse generation.
