In many laser micro machining applications, ultra-short pulse (USP) lasers with a dynamical adaptive pulse repetition frequency (PRF) would allow for a significant increase of processing speed. Machining with ultra-fast resonant scanners or new techniques for fast processing narrow beam paths are two examples. These applications require USP lasers which provide a real-time PRF-synchronization to the deflection speed of the laser beam. Moreover, despite the dynamical change of the PRF between single pulses up to 10 MHz the pulse energy shall remain constant for high quality processing. Even today, this seems to be not easily realized. We present a laser system with constant pulse energies for all PRFs from single pulse to 10 MHz with an extremely fine PRF step size. The laser system consists of a mode-locked diode laser, running at a basic PRF of 4:3 GHz and a succeeding ultrafast pulse picker. This leads to a temporal accuracy of 233 ps, 40 times higher than with a typical solidstate mode-locked laser. To guarantee constant pulse energy over the entire PRF-range, the pulsed signal is superimposed with a cw-signal of the same polarization, serving as an inversion control in subsequent fiber and slab-amplifiers. In the end a SHG-stage serves as the desired cw-filter. The output power at 515nm is up to 109W. The pulse duration is 5 ps. With this setup, first experiments were performed showing the advantage of real USP pulse on demand laser systems.
F. Harth, T. Herrmann, and J. A. L'huillier, "High power ultrafast laser with highly dynamic repetition rate and constant pulse energy from single pulse to 10 MHz ," Proc. SPIE 10896, Solid State Lasers XXVIII: Technology and Devices, 108960H (Presented at SPIE LASE: February 06, 2019; Published: 7 March 2019); https://doi.org/10.1117/12.2507821.
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