Multi-pass resonator design for energy scaling of mode-locked thin-disk lasers K. Schuhmann* a,b , K. Kirch a,b , A. Antognini a,b a Institute for Particle Physics, ETH, 8093 Zurich, Switzerland b Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland ABSTRACT We present a novel multi-pass resonator architecture paving the way for pulse energy scaling of mode-locked thin-disk lasers. It consists of a concatenation of nearly identical optical segments, each segment corresponding to a round-trip in an optically stable cavity containing an active medium exhibiting soft aperture effects. Contrarily to state-of-the-art multi-pass oscillators based on imaging schemes, the stability region (for variations of the active medium thermal lens) of the here disclosed resonator architecture does not shrink with the number of passes. As a result, the proposed architecture enables the realization of an arbitrarily large number of passes at the thin-disk at a given pump power without the reduction of output power seen in the imaging-based state-of-the-art multi-pass oscillators. Keywords: Thin-disk lasers, Laser resonators, Mode-locked lasers, Thermal lens effect, Ultrafast lasers, Energy scaling, Soft aperture 1. MOTIVATION Ultra-short laser pulse sources [1], [2] enable a large variety of fundamental physics investigations, as well as technological and industrial applications. Many applications in industry and strong-field physics will tremendously benefit from the increase of the pulse energy in the mJ regime at few MHz repetition rates [3]: on one hand production throughput and material compendium extension especially for materials where non-linear multi-photon absorption is required, and on the other hand, reduced measurement times, increased signal to noise, and new scientific possibilities. Mode-locked thin-disk lasers [4], [5] are widely used in research laboratories and in industry because of their power scaling and high pulse energy capabilities [6], [7], [8], [9], [10], [11]. The output pulse energy E of a mode-locked thin-disk laser can be increased, at a given average output power P avg , by reducing the laser repetition rate f rep , given the simple relation E = P avg / f rep from energy conservation. Smaller repetition rates can be achieved simply by increasing the oscillator cavity length. One successful way to increment the resonator length was obtained by inserting into the cavity a Herriott cell [12], [13]. However, the elevated intra-cavity pulse energy achieved in this way required operation of the oscillator in an evacuated environment to avoid detrimental non-linear effects in air [13]. The cavity length can be also increased by folding the laser beam at the active medium (thin-disk) several times per round-trip [6], [14]. The large gain per round-trip achievable with such an active multi-pass cell enables large output coupling, which brings along a reduction of the intra-cavity power. Hence, this scheme providing a long cavity and decreased intra-cavity intensity is twofold advantageous and qualifies this resonator architecture for industrial applications as it allows operation in air. Another important feature of a multi-pass resonator scheme is the reduction of Q-switching instabilities due to a linear decrease of the gain saturation fluence with the number of reflections at the thin- disk [15], [16]. The multi-pass active cells realized to date [6], [14] are based on relay 4f-imaging: 4f optical segments are used to image the thin-disk from pass to pass at the active medium so that the beam propagation in the active multi-pass cell proceeds following the scheme disk-4f-disk-4f-disk-4f… . The 4f propagation from the optical point of view corresponds to an effective propagation of zero length and it does not provide stability for misalignment or variation of the thin-disk focal strength. Hence, to realize a stable laser operation, the 4f multi-pass cell has to be embedded in a stable optical resonator [6]. *skarsten@phys.ethz.ch; phone: +41446332013 Solid State Lasers XXVI: Technology and Devices, edited by W. Andrew Clarkson, Ramesh K. Shori, Proc. of SPIE Vol. 10082, 100820J · © 2017 SPIE · CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2251913 Proc. of SPIE Vol. 10082 100820J-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 19 Apr 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use