We present a frequency conversion setup using three type I phase-matched LBO crystals for the generation of high-power deep ultraviolet (DUV) radiation by frequency conversion of pulsed infrared laser radiation. The presented approach allows more independent control of the three individual conversion processes compared to LBO-based approaches presented to date. An average output power of 33 W at a wavelength of 257 nm was demonstrated. The duration of the DUV pulses was 3.4 ps, the pulse repetition rate was 400 kHz, and the beam propagation factor was measured to be M 2 = 1.99 at the highest power.
In this paper, we present a novel method, to the best of our knowledge, for achieving high-power second-harmonic generation (SHG) of continuous-wave light. By employing lithium triborate-based resonant cavity with moderate finesse, we propose an approach for generating efficient SHG in the visible and ultraviolet spectral ranges without requiring an active electronic servo loop. As a proof of concept, we demonstrate frequency doubling of a high-power 1064 nm laser to 532 nm. To the best of our knowledge, this is the first implementation of such a cavity featuring a highly reflective coating on one end of this crystal. This quasi-monolithic design enables simultaneous control of phase matching and cavity resonance solely through temperature adjustment of the nonlinear crystal.
We report on high average-power, high-energy picosecond fourth-harmonic generation in LBO. The first stages of a Yb:YAG laser chain operating at 1 kHz repetition rate generate few-picosecond 220 mJ chirped pulses at 1030 nm fundamental wavelength. They are frequency-converted in a cascade of three LBO crystals to generate the second-, third-, and fourth-harmonics at 515 nm, 343 nm and 257 nm respectively. Crystals thicknesses and angular phase-matching detuning were calculated as a function of pulse duration through broadband nonlinear optical numerical simulations. Last crystal is both conduction-cooled on edge and surface-cooled at center through forced-air flow to mitigate heating due to nonlinear absorption in the deep-UV and reduce temperature gradients. Chirped-pulse duration was experimentally adjusted to achieve stable 20% overall conversion efficiency. Near-field beam profiles were continuously recorded at 10 Hz, for all four wavelengths involved, together with corresponding energies, showing no significant beam degradation over 50 hours. Temperatures of the two last crystals were monitored, and will help optimize surface cooling for future power ramping-up.
We report on an analysis of the nonlinear absorption in lithium triborate (LBO) used for second and third harmonic generation of ultrashort laser pulses at average powers in the order of kW and with sub-picosecond pulse duration. Thermographic imaging of the LBO crystals together with a simple analytical model revealed the presence of nonlinear absorption in both harmonic generation processes. Subsequent processing with a numerical model considering the nonlinear mixing, the absorption, and the heat conduction was used to estimate the absorption coefficients. Average powers exceeding 100 W in the ultraviolet and 400 W in the visible spectral range were obtained while maintaining a good beam quality by avoiding excessive nonlinear absorption.
We report Second Harmonic Generation (SHG) and Third Harmonic Generation (THG) energy conversion efficiencies up to 59% and 27%, respectively, for laser pulses simultaneously delivering high peak power in the sub-TW range and average powers in the sub-kW range. No damage or efficiency decrease is observed after more than 100 h operation time. The resulting high-energy visible and near-UV pulses are suitable for applications, such as lightning control, material analysis and machining, or OPCPA pumping.
