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.
Industrial applications increasingly demand femtosecond and picosecond pulse duration laser systems with a range of wavelengths from the near infrared to the ultraviolet. This paper reports a dual function thin slab femtosecond amplifier system operating at a range of wavelengths via frequency conversion. A Yb:YAG thin slab amplifier was demonstrated that incorporates both the pre-amplifier and power amplifier functions in a single crystal. To ensure efficient, homogenous pumping of the thin slab a novel diode bar imaging technique is used. The combination of the homogenous pumping and the unique amplifier architecture enables average output powers < 200 W to be maintained across a range of pulse repetition frequencies from 1 MHz to 40 MHz with a pulse duration of 50 ps at a wavelength of 1030 nm and M2 ~ 1.2. The pulse was compressed to a duration of 900 ± 100 fs with a compressor efficiency of < 90 %. A modified LXR100 system which is based on this technology was used as the fundamental 1030 nm source, with output powers up to 130 W, from 1 MHz – 10 MHz and pulse durations in the 900 fs regime. Second harmonic generation of 515 nm was achieved by coupling the beam into a type-1 critically phase matched LBO crystal to enable efficient conversion whilst avoiding damage. Second harmonic generation of 515 nm to < 95 W at a conversion efficiency of ~74 % and pulse energy of < 96 μJ and M2 < 1.3 was achieved. Further wavelength conversion was undertaken by coupling the 515 nm and unconverted 1030 nm output into a second LBO crystal for third harmonic generation. Type-1 and type-2 phasematching configurations for third harmonic generation were investigated, with an output power of < 20 W and pulse energy of < 20 μJ achieved at 343 nm with a conversion efficiency with respect to incident 1030 nm of < 21 % for both configurations.