250 MW peak power ultrafast mid-IR OPCPA

Abstract

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 stretching 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.