Sub-picosecond mode-locked laser operation in a femtosecond-laser-inscribed Yb:CaF₂ channel waveguide

We present the successful demonstration of both Q-switched mode-locked (QSML) and continuous-wave mode-locked (CWML) operation in a femtosecond-laser-inscribed Yb:CaF 2 double-track waveguide (WG) structure. A semiconductor saturable absorber output coupler (SOC) was used as the mode locker with fine tuning of the intracavity group delay dispersion (GDD) achieved through the Gires-Tournois interference (GTI) effect induced by an air gap. To ensure sufficient fluence on the saturable absorber, the cavity was extended to 50 cm by inserting two lenses between the SOC and WG, resulting in a repetition frequency of ∼300 MHz. In the QSML regime, the laser exhibited an amplitude modulation period of 65 kHz within Q-switched pulses of 3-µs duration. Notably, in the purely CWML regime, the laser generated a maximum output power of 51 mW near 1036 nm with a pulse width of 979 fs.