250 MW peak power ultrafast mid-IR OPCPA

Summary form only given. Laser sources delivering high energy, carrier-envelope phase (CEP) stable ultra-short pulses in the mid-IR spectral range (3 <; λ<; 8 μm) at high repetition rate and featuring excellent long-term stability are in demand for a wide range of experiments in the strong-field physics community. Several sources have been reported over the past few years meeting part of these stringent requirements. In particular laser sources operating at 2 μm wavelength - slightly below the mid-IR spectral range - have been reported delivering outputs in the mJ energy range at kHz repetition rate [1]. Another source delivering mJs of energy at 3.9 μm wavelength has been reported but operates at 20 Hz [2], a repetition rate unsuited for practical applications. Here, we present the highest average power mid-IR laser source reported to date ideally suited for the next generation of high-field physics experiment. The system delivers daily CEP stable pulses with center wavelength at 3.1 μm able to reach the 10<;sup>14<;/sup> W/cm<;sup>2<;/sup> intensity range at 160 kHz.The system relies on an optical parametric chirped pulse amplifier (OPCPA) architecture and is therefore composed of a seed source, a pump source, optical parametric amplifier (OPA) stages and stretcher/compressor assemblies. The pump source is a commercial MOPA system (Lumera Laser GmbH) delivering 6 ps duration pulses with up to 1 mJ output energy at 1064 nm and 160 kHz repetition rate. The seed 3 μm wavelength pulses are obtained via difference frequency generation between femtosecond pulses at 1.5 μm and 1 μm wavelength derived a from a common fiber laser system (Toptica Photonics AG). Parametric amplification is performed in a cascade of 4 MgO-doped periodically poled LiNbO3 crystals. The final OPA stage features large aperture (5x5 mm2) crystals therefore enabling output energy scaling. Pulse - tretching is performed before parametric amplification by propagating the seed pulses through bulk Sapphire and compression is achieved in a Martineztype compressor featuring a deformable mirror for fine adjustment of the spectral phase.The measured output spectrum spans over 600 nm (at 1/e2) and FROG measurements revealed a limited amount of leftover 3rd order dispersion (Fig. 1 - left) yielding pulse duration as short as 65 fs (6 cycles). The output power was measured to be over 6 W before and 3.2 W after compression (corresponding to 20 μJ output energy). Stability, monitored over 4.5 h, revealed power fluctuations <; 1% rms (Fig. 1 - right). CEP fluctuations were measured to be <; 250 mrad over 11 minutes [3]. This system, delivering simultaneously high peak power and high peak intensity, has been successfully used for filamentation studies, photo-ionization studies, electron acceleration experiments and fundamental solid-state physics experiments.