Efficient generation of femtosecond deep-ultraviolet pulses by single-focus cascaded second-harmonic conversion

We demonstrate an ultrafast deep-ultraviolet source based on single-focus fourth-harmonic (FH) generation of a 1008 kHz Yb laser (1026 nm, 215 fs, 100 nJ-level) using two stages of second harmonic generation. Two critically phase-matched crystals, BiBO for the first stage (1026 nm to 512 nm) and BBO for the second stage (512 nm to 256 nm), are placed within the Rayleigh range of a single focusing lens, yielding a compact, low-loss geometry. We achieve 20.8 ± 0.8% conversion from the fundamental to the FH at 256 nm, with a FH pulse duration of 246 fs measured by cross-correlation. The FH beam profile is smooth and near-circular with <i>M</i><i>x</i>2=1.69±0.07, <i>M</i><i>y</i>2=1.03±0.05. After thermalization and a small crystal-angle adjustment, the FH pulse energy stability is 0.7% relative standard deviation. By solving the unidirectional pulse propagation equation the experimental data is reproduced with remaining discrepancies attributable to the assumption that the input pulse is transform limited and uncertainties in nonlinear crystal data. To our knowledge, this represents the highest FH conversion efficiency for this type of fundamental laser and establishes the single-focus geometry as a robust route to deep-ultraviolet output for high-repetition-rate, low-pulse-energy laser systems.