140-μJ, narrow-linewidth, robustly single-transverse mode nanosecond infrared fiber laser platform with fine pulse tailoring capability

Abstract

The work presented in this paper had two main objectives. The first objective was to develop a very stable nanosecond infrared pulsed fiber laser oscillator platform offering a straightforward and accurate control over the pulse characteristics in the time domain. The second objective was to deliver what we call "high quality photons", which means delivering pulses with high energy and excellent beam quality and narrow spectral linewidth, all at the same time and with very good stability. Oscillators with such attributes find applications in material processing fields, for example in memory repair, photovoltaic cell processing or micro-milling, to name just a few. In order to achieve the first objective, an embedded digital platform using high-speed electronics was developed. Using this platform and a computer, pulse shapes have been programmed straightforwardly in the non-volatile memory of the instrument, with an amplitude resolution of 10 bits and a time resolution of 2.5 ns. Optical pulses having tailored temporal profiles, with rise times around 1 ns and pulse energy stability levels better than ± 3% at 3σ, have been generated at high repetition rates (> 100 kHz) at a wavelength of 1064 nm. Achieving the second objective required amplifying the low power master oscillator signal (10-100 mW) to output power levels in the range of 1 to 50 W. A multi-clad, polarization maintaining, Yb-doped large mode area fiber was specially designed to allow for the amplification of high peak power optical pulses, while keeping control over the nonlinear effects and preserving an excellent beam quality. Optical pulses with tailored shapes and pulse energy levels in excess of 140 μJ have been produced for pulse durations in the range of 10 to 80 ns, with 86% of the power emitted in a 0.5-nm bandwidth. The linearly polarized beam M² parameter was smaller than 1.1, with both the astigmatism and the asymmetry below 15%. The pulse energy stability was better than ± 3% at 3σ. We conclude with a discussion about some of the applications of the developed platform.