Modelocking of a Frequency Shifted Feedback Laser triggered by Amplitude Modulation

Abstract

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 lasers