An industrial ultrashort pulse thin slab laser amplifier.
Fulford, Benjamin S.
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A laser amplifier has been designed and constructed for use in a Master Oscillator – Power Amplifier configuration for the amplification of the output from a low average power, high beam quality ultrashort pulse oscillator. The aim of the project was to design and construct an amplifier module that could provide a technology platform for products that would satisfy the growing market requirements for high average laser systems beyond 50 W of average output power with pulse duration of 0.5-5 picoseconds. The design was based around a Yb:YAG thin slab crystal gain medium that utilised a novel pump scheme delivering up to 321 W of ~ 940 nm wavelength pump light. At maximum pump power, the small-signal gain coefficient of the amplifier was measured to be ~ 1.6 cm-1 . Measurements of the pump-induced lens strength of the amplifier at various pump powers provided the necessary optical data which enabled a multi-fold mirror arrangement to be designed that folded the seed beam through the thin slab in order to maximise extracted power. This design included the novel implementation of both the pre-amplifier and power amplifier functions from a single Yb:YAG crystal gain cell. This approach facilitated the input seed power to be as low as 12 mW without the onset of parasitic oscillations. Integrating a 3.5 W, 55 MHz ultrashort pulse oscillator to the amplifier resulted in a maximum output power of 91 W with pulse durations of 710 fs, whilst the inclusion of a pulse-picker resulted in an output power of 87.5 W with pulse durations of 905 fs at 4.5 MHz with 316 mW of input power. This demonstrated that the novel use of the pre-amplifier yielded a unique feature; that at high pump powers, there is only a small percentage change in output power for a large percentage change in average input power. In essence, the average output was near constant over a large range of pulse-picked frequencies maximising the dynamic range of pulse energy and pulse frequency combinations. The beam quality in both regimes was measured to give an M2 value of ~ 1.1 in the y-axis and ~ 1.5 in the x-axis, reducing to ~ 1.2 in the x-axis after spatial filtering with a loss of ~10% - 15% of average output power. Power scalability was demonstrated by cascading two additional amplifier modules and resulted in a maximum output power of 230 W at 4 MHz, yielding pulse energies in excess of 57 µJ.