High Gain Solid-State Amplifiers for Picosecond Pulses

Picosecond solid-state lasers are attractive for many industrial and scientific applications, such as precision material processing (Dausinger et al., 2003; Breitling et al., 2004), nonlinear optics (McCarthy & Hanna 1993; Ruffling et al., 2001; Sun et al., 2007) and laser spectroscopy (Mani et al., 2001). In contrast with traditional laser processing performed with 10-100 ns multi-kHz sources, picosecond laser-matter interaction is basically non-thermal, relying on multi-photon ionisation and photo-ablation processes that allow cleaner and much higher spatial definition in laser marking, drilling and cutting. Femtosecond pulses would perform even better in principle, but at the expense of a significant increase in complexity of the laser system that most often is unwelcome in industrial environments. Furthermore, the multi-kW peak-power levels allowed by cw mode-locked picosecond lasers with average power of at least few watts are already sufficient to produce efficient frequency conversion by harmonic, sumor difference-mixing and parametric generation. Semiconductor saturable absorber mirrors (SESAMs) are widely employed for the passive mode-locking of picosecond solid-state lasers (Keller, 2003). SESAMs are very effective and highly reliable when used in low-power oscillators; however, when employed in highpower oscillators, they require a special design as their thermal management becomes a very important issue (Burns et al., 2000; Neuhaus et al., 2008). Indeed, the intense intracavity radiation of this particular operating regime may induce significant optical and thermomechanic stress effects, leading to rapid degradation of their performance. An alternative (and not new) approach to powerful cw picosecond sources is to use a master-oscillator power-amplifier (MOPA) system, in which a seed from a low-power, robust picosecond laser is amplified to the required average power levels through extracavity diode-pumped amplifiers. Some recent results have pointed out the great potential of this approach (Snell et al., 2000; Agnesi et al., 2006a; Nawata et al., 2006; Farrell & Damzen, 2007; McDonagh et al., 2007). To our knowledge, the most powerful cw picosecond source reported to date was a Nd:YVO4 MOPA system longitudinally-pumped with 216 W at 888 nm, where a 60-W cw picosecond mode-locked laser was amplified to 111 W, with 53% amplifier extraction efficiency (McDonagh et al., 2007). Though the master oscillator of this example could be well