We demonstrate an all-solid-state deep-ultraviolet (DUV) laser based on the frequency-quadrupling of a 1 µm, 1.2 ps, Yb: YAG Innoslab solid-state laser at a 10 kHz repetition rate, using LBO and BBO as second-harmonic generation and fourth-harmonic generation crystals, respectively. The DUV laser delivers 20 W, 2.0 mJ, 665 fs, 258 nm DUV pulses, with an overall conversion efficiency of ∼8.7% from 1 µm to DUV. The corresponding peak power of DUV pulses is up to 3 GW, which, to the best of our knowledge, is highest in reported kHz-rate all-solid-state DUV sources driven at 1 µm wavelength.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

High-average-power deep-ultraviolet (DUV) laser sources of below 300 nm with a short pulse duration and a high peak power are attracting extensive attention from researchers due to their applications, such as micromachining in a regime of cold ablations [1], seeding free-electron lasers [2], pumping optical parametric amplification (OPA) and optical parametric chirp-pulse amplification (OPCPA) systems [3], ionizing noble gas and driving coherent soft X-ray radiation through extreme high order harmonic generation (HHG) [4]. In the past decades, with the rapid advances of high-average-power, high-energy 1-µm solid-state and fiber laser technologies, high-average-power, high-energy all-solid-state DUV laser sources with pulse durations from nanosecond (ns) to femtosecond (fs) based on the frequency-quadrupling of those 1-µm lasers have been reported [5–19]. One of the most advanced ns DUV sources was reported by M. Nishioka et al. in 2003 [10], which delivered 40 W average-power DUV pulses at 266 nm, driven by a Nd: YAG laser at a 7 kHz repetition rate. This is the currently reported solid-state DUV laser with the highest average power. In the picosecond (ps) region, a representative high-energy DUV laser was demonstrated by C.-L. Chang et al. in 2015 [7], with a 2.74 mJ pulse energy and a 4.2 ps pulse duration, driven by a ps, 1 kHz, Cryo-Yb: YAG laser. The short pulse duration and high pulse energy make the laser having the highest peak power of 0.56 GW among all reported 1-µm-driven, kHz-rate, all-solid-state DUV lasers. One year later, another ps DUV laser with 6 W average power and an estimated 2 ps pulse duration operating at 100 kHz was reported using a 1030 nm Yb: YAG thin-disk laser as the pump source [13]. The reports of high-average-power DUV sources at sub-ps or fs pulse durations are relatively rare. A recent report is the 4.6 W, 150 fs, 258 nm DUV laser at a high repetition rate of 796 kHz driven by an Yb-doped fiber laser [9]. Despite the laser has a short pulse duration, its peak power is only about 38.5 MW, limited by the relatively low pulse energy. Based on the above literatures, the peak powers of 1-µm-driven, kHz-rate, all-solid-state DUV lasers are wandering around a few tens to a few hundreds of MW either due to the long pulse duration or due to the low pulse energy.

Here, we report a 1-µm-driven, kHz-rate, all-solid-state DUV laser with a peak power beyond 1 GW, simultaneously combined with the characteristics of high average power, high energy and short pulse duration. The DUV laser is based on the frequency-quadrupling of a 1 µm, 1.2 ps, 10 kHz, Yb: YAG Innoslab laser. LBO and BBO crystals are used to implement second-harmonic generation (SHG) and fourth-harmonic generation (FHG), respectively. 20 W, 2.0 mJ, 665 fs, 258 nm DUV pulses are delivered, with a conversion efficiency of ∼8.7% from 1 µm to 258 nm pulses. The corresponding peak power of DUV pulses is up to 3 GW, which is highest among reported kHz-rate all-solid-state DUV lasers driven at 1 µm wavelength. The high peak power of our DUV laser enables researching on nonlinear phenomena under high field which could not be pursued by previously reported kHz-rate, 1-µm-driven, solid-state DUV lasers.

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2. Pump source and experimental setups

The pump source used in our experiment is a commercial Yb: YAG chirped-pulse amplification (CPA) system based on the Innoslab amplification technology (AMPHOS 300). It delivers 270 W, 27 mJ, 1.2 ps, 10 kHz pulses centered at 1030.8 nm. The typical spectrum (0.4 nm resolution) from the pump source is shown in Fig. 1(a), which has a 2.2 nm full width at half-maximum (FWHM) centered at 1030.8 nm. Figures 1(b)–1(d) depict the temporal profile of the pump pulses, measured using a SHG frequency-resolved optical gating (SHG-FROG). The pulse duration is 1.2 ps, corresponding to 1.7 times the transform-limited (TL) pulse duration. Inset in Fig. 1(a) shows the beam profile of the pump laser at 270 W average power, which has an elliptic shape with two distributed sidelobes surrounding the main beam (∼10% energy is stored in the sidelobes). The main beam has a circularity of 80%. These sidelobes are from the residual high-order modes which are not filtered out completely by the spatial filter (beam slit) in the amplifier.

figure: Fig. 1.

Fig. 1. (a) The typical measured and retrieved spectra. Inset shows the beam profile of the pump laser. (b) The retrieved pulse shape and phase. The measured (c) and retrieved (d) FROG traces.

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Figure 2 shows the optical layout of the frequency quadrupling starting from the 1 µm Yb: YAG Innoslab laser. The collimated pump beam is first sent through an energy tuner comprising of a half-wave plate and a thin-film polarizer for controlling the pump energy on SHG crystal. Taking into account of the high peak power of the pump pulses, a telescope is subsequently used to expand the original pump beam to a larger beam size (11.5 mm at 1/e2 intensity along the long axis of the main beam) to avoid crystal damage. After the telescope, a 5-mm-thick LBO with anti-reflection (AR) coatings for 1030 nm and 515 nm on both sides and an aperture of 20×20mm is used and it is cut at