6-mJ, 4-ns Pulse Generation at 2.09 $μ$m from a Diode-Pumped Ho:YAG Thin-Disk Laser

A holmium-doped yttrium aluminum garnet (Ho:YAG) thin-disk was experimentally investigated under Q-switching and cavity-dumping operation schemes, pumped by a 1.9 $μ$m laser-diode (LD). The laser generated pulses at 2090 nm with energies more than 6 mJ and pulse duration down to 3.8 ns, corresponding to a peak power of 1.6 MW with near-diffraction-limited beam quality. The compact and robust system was used for laser-induced breakdown spectroscopy (LIBS) experiments, demonstrating its practical usability. These results represent, to the best of our knowledge, the first demonstration of a Ho:YAG thin-disk laser providing MW peak-power in nanosecond regime.

💡 Research Summary

This paper reports the development and experimental characterization of a holmium‑doped yttrium aluminum garnet (Ho:YAG) thin‑disk laser operating around 2.09 µm, pumped directly by a 1.9 µm diode laser. The authors investigated two pulsed operation schemes—conventional Q‑switching and cavity‑dumping—using a rubidium titanate phosphate (RTP) Pockels cell as the fast optical switch. The thin‑disk crystal (400 µm thick, 1.5 at.% Ho) was mounted on a silicon‑carbide (SiC) heat sink and placed in a 72‑pass pump geometry, receiving up to 50 W of continuous‑wave pump power from an un‑stabilized 1.91 µm laser diode (LD) with a 12 nm full‑width at half‑maximum spectral width.

In Q‑switching mode, the long upper‑state lifetime of Ho:YAG (~7 ms) allowed energy storage in the gain medium. At repetition rates of 1 kHz and below, pulse energies exceeding 5 mJ were obtained, with the shortest Q‑switched pulse duration of 292 ns, yielding a maximum peak power of ~18 kW. However, as pump power increased, the output exhibited roll‑over in pulse energy, attributed to the broadband, temperature‑sensitive LD spectrum, heating of the disk and the RTP crystal (≈100 °C measured), and detrimental processes such as energy‑transfer up‑conversion (ETU) and excited‑state absorption (ESA). The limited net gain inherent to the thin‑disk geometry prevented further shortening of the Q‑switched pulses.

Cavity‑dumping was implemented by replacing the output coupler with a high‑reflectivity mirror and using the Pockels cell to abruptly switch the cavity from low‑loss to high‑loss, dumping the stored intracavity energy in a single round‑trip. The optical cavity length (≈114 cm) dictated a pulse duration of 3.8 ns, independent of repetition rate. This dramatic reduction in pulse width increased the peak power by two orders of magnitude, reaching 1.6 MW while maintaining pulse energies above 6 mJ. Beam quality measurements gave M² values of 1.7 (horizontal) and 1.5 (vertical), indicating near‑diffraction‑limited performance. The spectral output centered at 2090 nm with a sub‑nanometer bandwidth.

The authors discuss four pathways for further energy scaling: (1) replacing the SiC heat sink with a chemical‑vapor‑deposited (CVD) diamond substrate to exploit its three‑fold higher thermal conductivity, thereby reducing crystal temperature and suppressing ETU; (2) employing a wavelength‑stabilized high‑power 1.9 µm diode or, more realistically, a 1907 nm thulium‑fiber laser pumped by mature 793 nm diodes, which would improve pump absorption and lower thermal load; (3) increasing the number of passes through the thin‑disk per round‑trip (e.g., from 4 to 8) to boost gain, noting that in cavity‑dumping this would lengthen the cavity and thus the pulse duration; and (4) optimizing the Ho concentration and disk thickness to balance gain against thermal effects.

Practical applicability was demonstrated by using the cavity‑dumped source for laser‑induced breakdown spectroscopy (LIBS) of lunar‑regolith‑grown plants, showing that the high‑intensity, short‑duration pulses can generate a stable plasma suitable for spectroscopic analysis. The authors conclude that cavity‑dumped Ho:YAG thin‑disk lasers provide a compact, robust platform for MW‑peak‑power nanosecond pulses at 2 µm, filling a gap between Yb‑based thin‑disk systems and longer‑wavelength OPA/OPA drivers. Future work will integrate the thin‑disk as the main amplifier in a chirped‑pulse‑amplification (CPA) chain to achieve picosecond pulses with millijoule‑level energies. The research was funded by the MERIT programme (EU) and the Czech state budget under project LasApp.