Scaling of peak and average power in femtosecond ytterbium amplifiers

Abstract

Scaling of output parameters of femtosecond laser systems is one of the highly demanded and challenging tasks in modern laser science and engineering. Fundamental science requires ever shorter pulses and higher pulse energies for investigation of ultrafast processes taking place in matter and for new discoveries in the area of strong field physics. Achievable laser intensity has rapidly grown over recent decades with such milestone inventions as chirped pulse amplification (CPA) and implementation of new broadband solid-state laser gain media. Next to fundamental science, laser machining and material processing see increasing adoption in various industries, and benefit from high repetition rate and average power of lasers. Advent of diode-pumped solid-state (DPSS) Yb3+-doped laser technology brought a breakthrough in quantum efficiency and obtainable average power. The efficiency can be further improved by utilizing cryogenic cooling of the gain medium,as it reduces ground state abosption (GSA), considerably improves thermal conductivity and boosts gain and absorption cross-sections of the gain medium. This work is dedicated to peak and average power scaling of cryogenically cooled DPSS Yb3+:CaF2 femtosecond CPA system. Firstly, it aims at upscaling of both peak and average power of a regenerative amplifier (RA) via innovative design of the RA cavity, in particular, by employing a cavity with a dual gain medium and by increasing the mode size on the optical elements. In-depth analysis of the design and experimental verification will be given by considering cavity stability, thermal lensing and thermal management, laser crystal geometry, accumulated nonlinear phase (B-integral), suppression of bifurcations of pulse energy and spectral gain narrowing. Developed CPA system operating at central wavelength of 1030 nm and generating 220-fs pulses with energies exceeding 30 mJ will be presented. The system is capable of delivering up to 24 W of average power at 10-kHz repetition rate. Further upscaling of the peak power to TW-level is achieved by implementing a multipass amplifier (MPA) booster stage after the RA. Here, challenges that are specific to multipass amplifiers (as compared to RAs) will be considered, e.g. compensation of astigmatic aberration and spatial gain narrowing, mitigation of self-phase modulation (SPM) and amplified spontaneous emission (ASE), preservation of the beam quality via repetitive imaging. Other crucial design parameters, such as laser crystal geometry and cooling, pump beamline arrangement, thermal lensing and B-integral will be also discussed. A multistage CPA system producing120-mJ, 225-fs pulses at a repetition rate of 50 Hz with the beam quality parameter M2 less than 1.1 will be introduced. Finally, a nonlinear pulse compression is applied by means of spectral broadening and shifting of the generated 220-fs pulses via cascaded stimulated Raman scattering (SRS) process in a N2-filled stretched hollow core fiber (HCF) and subsequent compression in a set of chirped mirrors. As a result, 14-mJ, 20-fs pulses are obtained, with the central wavelength shifted from 1.03 μm to 1.3 μm, which is crucial for the high harmonic generation (HHG) in the carbon K-edge spectral region.