50W 343nm frequency-tripled operation of a high-repetition-rate, novel architecture, femtosecond thin slab amplifier

There is a growing demand for MHz-repetition rate industrial ultrafast laser sources that operate from the ultraviolet to the near IR. This paper discusses third harmonic generation (THG) of a high repetition-rate, femtosecond dual function Yb:YAG thin slab amplifier. Both pre-amplifier and power amplifier functions take place in a single crystal and homogeneous pumping of the thin slab is achieved by means of novel diode bar imaging. Fundamental output powers of <130 W at a wavelength of 1030 nm are maintained across a range of pulse repetition frequencies from 1 MHz to 40 MHz with a pulse duration of 900 ± 100 fs and M2 ~ 1.2. Second harmonic generation (SHG) at 515 nm with a conversion efficiency of 76% with respect to the incident fundamental is achieved using type-1 critically phase-matched LBO resulting in average powers of <90 W, a pulse energy of <90 µJ, with an M2< 1.2 and pulse durations of 800 ± 100 fs. The fundamental and second harmonic outputs were frequency mixed in a critically phase matched LBO crystal to produce third harmonic generation (THG) at 343 nm. Several lengths of type-2 THG LBO were investigated alongside type-1 THG LBO, with <50 W of UV produced at single-pass conversion efficiencies of <40% with respect to incident fundamental power at the SHG stage, corresponding to pulse energies of <50 µJ. Beam quality and pulse durations at 343 nm were investigated across 1–40 MHz.