Characterization of High-Energy Compressed Pulses in the LWIR

Abstract

Continuous advancements of ultrashort laser sources have established them as indispensable tools for scientific research, industrial manufacturing, and medicine. In modern lasers, ultrashort pulse duration in combination with high energy leads to gigawatt-scale peak powers, which makes them a powerful tool for the investigation of ultrafast nonlinear optical and strong field physics phenomena. Conventional solid-state lasers typically operate in the near-IR (0.75-1.4 μm) and short-wave infrared (SWIR) (1.4-3 μm) spectral ranges andfail to cover the entire mid-IR region (3-8 μm) due to a lack of suitable laser materials. Even more problematic is the extension of the laser operation into the long-wave infrared (LWIR) spectral range (8-15 μm). The mid-IR and LWIR spectral regions are especially important, as they are known as the "fingerprint regions" since they cover the majority of molecular vibrations, enabling spectroscopic methods to identify molecular species. Furthermore, instrong field physics, the mid-IR sources serve as drivers to generate high-harmonics and scale cut-off energies towards soft X-rays. To address this spectral gap, the technique of optical parametric (chirped) amplification has emerged as a key technology to generate high-energy ultrashort pulses.This work is devoted to the characterization of the performance of a hybrid optical parametric chirped-pulse amplifier (OPCPA) generating femtosecond millijoule LWIR pulses, which is of importance for the application of the source. For the amplification of LWIR pulses, we employ highly nonlinear ZGP crystals, which are pumped by high-energy picosecond pulses centered in the vicinity of 2.1 μm and originating from another OPCPA, based on KTA nonlinear optical crystals. The front-end for the generation of the 2 μm pump pulses consists of a femtosecond OPA pumped by a NIR Yb:CaF2 laser system generating 200 fs pulses at 1.03 μm wavelength. The KTA OPCPA is pumped by a 100 ps Nd:YAG laser system operating in a burst-mode, which allows for maximizing the energy extraction from the laser amplifier. A combination of the energies of individual pump pulses from the burst into the single intense LWIR pulse is achieved by non-collinearly seeding and pumping KTA and ZGP OPCPA stages, respectively.Spectral and temporal characterization of the output of the hybrid OPCPA system was performed by measuring the spectrum and temporal profile of the compressed pulse. Because of the lack of sensitive detectors and (nonlinear) optical materials that are transparent in the LWIR spectral range, characterization of ultrashort LWIR pulses is complicated. Here, thespectrum of LWIR pulses was recorded by using a scanning monochromator and temporal characterization was performed by using a home-built intensity autocorrelator (AC) and Third Harmonic Generation Frequency Resolved Optical Gating (THG FROG) apparatus. The AC and THG FROG setups were constructed as a part of the project. The measurements revealed that LWIR pulses centered at around 9.5 μm can be compressed to 187.5 fsduration, which is slightly longer than the Fourier transform limit of 176.35 fs.