We report an experimental technique to trigger modelocking
(ML) emission in Frequency-Shifted Feedback
(FSF) lasers. These lasers feature an intracavity modulator
driven by a radio-frequency tone, which shifts the
light spectrum every cavity roundtrip. The technique
consists of the drive of the modulator with a second
tone at the cavity free spectral range (FSR) frequency.
So, in addition to the frequency shift, a weak amplitude
modulation (AM) appears synchronous with the
cavity roundtrip time. The approach is successful as
FSF cavities support chirped modes evenly spaced by
the FSR, whose AM coupling produces convenient seed
pulses for the ML onset. This results in ML emission
at arbitrary frequency shifts and initiation thresholds
lower than in standard, spontaneous FSF laser ML. Simulations
indicate that the role of AM is to trigger the
formation of ML pulses, but the primary mechanism of
pulse buildup is Kerr effect. The technique opens a new
practical route to initiate ML emission in FSF laser
