Femtosecond pulse generation with Tm-doped sesquioxides

Summary form only given. High-power ultrashort-pulse lasers that operate around 2μm spectral region are of particular interest in expanding access to broadband coherent source options for the mid-infrared spectral region. For example, such optical frequency comb generators can be utilized for real-time, high-sensitivity and high-resolution detection of absorption features of molecular species present in the so-called “molecular fingerprint” region. Amongst other potential implementations of 2-μm ultrafast lasers are free-space optical communications, high-harmonic generation, infrared pulsed laser ablation of polymer materials and surgery. Tm-doped crystalline gain media could offer an attractive route towards the development of high-power and efficient ultrashort-pulse lasers for the 2-μm spectral region as alternatives to fiber [1] or semiconductor thin disk laser [2] approaches for which only rather limited average powers have been reported to date. For such purposes, Tm-doped crystals having characteristic broadband and relatively homogeneous gain spectra extending beyond 2 μm (that avoid strong water absorption at λ<;1950 nm) are required. Additionally, such media should support efficient cross-relaxation processes in a Tm-Tm manifold for efficient conversion of the pump radiation at 800 nm to laser emission around 2 μm and, as more general requirement, they possess good thermo-mechanical properties.Here we report our recent results on femtosecond mode-locking of Tm-doped sesquioxides (Sc2O3, Lu2O3 and LuScO3) in the 2.0-2.1 μm region. The distinguishing features of these media include strong ground-level Stark splitting that results in the emission spectra that extend beyond 2100 nm and superior thermo-mechanical properties. All the experimental assessments were carried out with a Ti:sapphire pump laser (that produced up to 2.6 W at 796 nm) and an asymmetric, astigmatically-compensated Z-fold resonator which was configured with two high-reflectivity (R > 99.2% from 1770 to 2100 nm) folding mirrors having radii of curvature of 100 mm, an output coupling of 1.2%, 2.3%, or 4% and a SESAM. The SESAMs used for passive mode locking initiation and stabilization are based on an ion-implanted InGaAsSb quantum-well structure and characterised by an initial reflectivity of ~ 99.5-98% in the 1950-2100 nm range. Pairs of IR-grade fused silica or CaF2 prisms were used for the intracavity group velocity dispersion control. The shortest pulse duration of 105 fs was realised with a Tm:LuScO3 crystal operating around 2010 nm (Fig. 1) because it offered the broadest emission features [3]amongst all investigated gain media. The highest average power during mode locking of up to 0.75 W was achieved with the Tm:Lu2O3 ceramic gain medium at 2070 nm for a corresponding pulse duration of 382 fs. Continuous tunability was realized from 2030 nm to 2100 nm during the mode locking of the Tm:Lu2O3 laser where pulses as short as 300 fs were generated with an average power of 430 mW at around 2040 nm. The prospects of further power and energy scaling of femtosecond Tm-doped sesquioxide lasers under highpower laser diode pumping will be discussed where the generation of yet shorter pulses can be expected when optimized SESAM devices or alternative broadband absorbers based on graphene are deployed.