Passively Q-switched Tm:YLF laser at 2.3 µm: instability analysis and stabilization

We report on a diode-pumped passively Q-switched Tm:YLF laser emitting at 2.3 µm. To promote emission at this wavelength, a cascade laser scheme is implemented, combining the consecutive ³H₄ → ³H₅ and ³F₄ → ³H₆ transitions in the Tm³⁺-doped material. In this configuration, laser operation at 1.9 µm is deliberately isolated from the Cr:ZnSe saturable absorber avoiding undesired saturation of the absorber. The 1.9-µm laser then runs alone in continuous-wave (cw) regime and the 2.3 µm laser in passively Q-switched regime. The Q-switch laser dynamics reveal intrinsic instabilities, which are further analyzed using Poincaré maps to uncover the underlying structures of an atypical Q-switched behavior. A detailed study demonstrates that these instabilities originate from the slightly multimode nature of the beam. Therefore, a stabilization process is settled by spatial filtering of the laser mode to reach a stable regime. In this stable regime, we obtain 1.5 µs pulses with an energy of 4 µJ with a repetition rate up to 5 kHz. Finally, the dynamics correlation between the two lasers of the cascade is also studied.