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

This paper reports the first demonstration of a diode-pumped Ho:YAG thin-disk laser operating in Q-switching and cavity-dumping modes to generate 6-mJ, 4-ns pulses at 2.09 μm with 1.6 MW peak power, which was successfully applied to laser-induced breakdown spectroscopy.

Original authors: Y. Koshiba, J. Mužík, M. Smrž, M. Dvořák, S. Kudělková, A. Fajstavr, Tomáš Mocek

Published 2026-01-28

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CC BY 4.0

Original authors: Y. Koshiba, J. Mužík, M. Smrž, M. Dvořák, S. Kudělková, A. Fajstavr, Tomáš Mocek

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a very powerful flashlight, but instead of just shining a steady beam, you want it to fire off incredibly bright, super-short bursts of light. This is exactly what the scientists in this paper achieved, but with a laser that glows in the "near-infrared" (a color our eyes can't see, but which is great for things like analyzing materials).

Here is the story of how they built this "light cannon," explained simply:

The Engine: A Thin Slice of Crystal

Think of the heart of their laser as a tiny, 400-micron-thick slice of a special crystal called Ho:YAG (doped with Holmium). It's so thin it's almost like a piece of glass you could hold between your fingers.

The Problem: Thin slices are great at cooling down (like a thin slice of pizza cools faster than a deep-dish), but they are hard to "fill up" with energy because light passes right through them too quickly.
The Fix: The scientists made the crystal slightly thicker than usual and pumped it with a laser diode (like a high-powered LED) that shines at a specific color (1.9 µm). They bounced this pump light back and forth through the crystal 72 times to make sure they squeezed every bit of energy out of it.

The Two Modes of Operation

The team tested two different ways to release this stored energy, using a special "shutter" (called a Pockels cell) that acts like a high-speed gate.

1. The "Q-Switch" Mode (The Slow Leak)

Imagine filling a bucket with a hose, but the hole at the bottom is plugged. You let the water build up pressure, then you suddenly unplug the hole.

What happened: They let energy build up in the crystal and then released it.
The Result: They got big bursts of energy (over 5 millijoules), but the bursts were a bit "long" in time (about 292 nanoseconds).
The Limit: Because the burst lasted a relatively long time, the "peak power" (how hard the punch was at its hardest point) was only about 18 kilowatts. It was strong, but not a "sledgehammer" yet.

2. The "Cavity-Dumping" Mode (The Sudden Dump)

Now, imagine that same bucket, but instead of just opening a hole, you flip the whole bucket upside down instantly. All the water dumps out in one single, massive splash.

What happened: They let the energy build up inside the laser cavity, and then they flipped a switch that forced all the light to shoot out in a single trip around the loop.
The Result: This was the big breakthrough. Because the light came out so fast (in just 3.8 nanoseconds), the energy was compressed into a tiny moment.
The Power: Even though the total energy was similar to the first mode, the speed made the peak power skyrocket to 1.6 Megawatts. That's like a lightning bolt in a box!

Why This Matters (According to the Paper)

A New Record: This is the first time anyone has made a "thin-disk" laser of this specific type (Ho:YAG) hit the "Megawatt" power level in such a short burst.
Real-World Use: They didn't just build it and leave it on a shelf. They actually used this laser to perform LIBS experiments (Laser-Induced Breakdown Spectroscopy). Think of this as using the laser to zap a tiny spot on a material, creating a tiny spark of plasma, and then reading the light from that spark to figure out what the material is made of. This proves the laser is robust and useful for real science.
Quality: The beam was very clean and focused (near-perfect), meaning it could be used for precise tasks.

What's Next? (The "To-Do" List)

The scientists admit their current setup has some limits, mostly because the crystal gets hot (like a car engine on a summer day). They suggest a few upgrades for the future:

Better Cooling: Swap the current cooling block for one made of diamond (which conducts heat three times better than what they used).
Better Pump: Use a more stable, precise light source to feed the laser, rather than their current "broad" one.
More Bounces: Change the mirrors so the light bounces through the crystal more times to get even more energy.

In a nutshell: The team built a compact, robust laser that can fire off incredibly powerful, nanosecond-long bursts of infrared light. By using a clever "dumping" technique, they turned a standard laser pulse into a megawatt-level punch, proving it's ready for serious scientific work like analyzing materials.

Technical Summary: 6-mJ, 4-ns Pulse Generation at 2.09 µm from a Diode-Pumped Ho:YAG Thin-Disk Laser

Problem and Motivation
While Yb-doped thin-disk lasers operating near 1 µm have achieved significant success in high-power and high-energy applications, extending this architecture to other wavelengths remains a challenge. Current methods for generating longer wavelengths, such as the short-wave infrared (SWIR) region around 2 µm, often rely on nonlinear conversion techniques (e.g., harmonic generation or optical parametric oscillation/amplification) driven by Yb lasers. These approaches suffer from limited conversion efficiency and require complex, meticulous alignment. Consequently, there is a growing interest in developing thin-disk lasers using gain media that can directly generate SWIR wavelengths for applications in spectroscopy, remote sensing, material processing, and as drivers for advanced scientific sources like high-harmonic generation and laser-Compton scattering. Ho:YAG is a candidate for direct generation at 2.09 µm, but its implementation in the thin-disk geometry faces challenges related to low gain, heat management, and energy-transfer upconversion (ETU).

Methodology
The study experimentally investigated a Ho:YAG thin-disk laser under two distinct operation schemes: Q-switching and cavity-dumping.

Gain Medium: A 400 µm thick Ho:YAG crystal with a 1.5 at.% doping concentration was used. This doping level was selected to be higher than typical rod lasers to compensate for the low gain inherent to the thin-disk geometry, despite the risk of increased heat generation and ETU.
Thermal Management: The crystal was mounted on a silicon carbide (SiC) heat sink, chosen as a cost-effective alternative to chemical vapor deposited (CVD) diamond, though with lower thermal conductivity.
Pumping: The system was pumped by a 1.9 µm laser diode (LD) delivering up to 50 W of continuous-wave power in a 72-pass pumping head. The pump source was not wavelength-stabilized, resulting in a broadband emission (12 nm FWHM).
Switching Mechanism: A Pockels cell (PC) composed of a pair of rubidium titanyle phosphate (RTP) crystals, driven by a high-voltage driver with a rise time below 5 ns, was employed for both Q-switching and cavity-dumping.
Experimental Setup: The resonator included an output coupler (OC) for Q-switching (tested at 2.6%, 4.6%, 5.8%, and 7.1% transmittance) and a high-reflective (HR) mirror for cavity-dumping.

Key Results

Q-Switching Operation: At repetition rates of 1 kHz or lower, the laser generated pulse energies exceeding 5 mJ. However, the pulse duration remained longer than 292 ns, limiting the peak power to a maximum of 18.3 kW. The pulse duration was limited by the net gain available in the thin-disk geometry.
Cavity-Dumping Operation: By replacing the OC with an HR mirror and dumping the intracavity energy within a single round-trip, the system achieved significantly shorter pulses. The pulse duration was approximately 3.8 ns, corresponding to the 114 cm cavity round-trip time.
Performance Metrics: The cavity-dumped system produced pulses with energies greater than 6 mJ and a peak power of 1.6 MW. The beam quality was near-diffraction-limited, with measured $M^2$ factors of approximately 1.7 (horizontal) and 1.5 (vertical) at 0.5 kHz repetition rate.
Practical Demonstration: The compact and robust system was successfully utilized for laser-induced breakdown spectroscopy (LIBS) experiments, validating its practical usability.

Significance and Claims
To the best of the authors' knowledge, this work represents the first demonstration of a Ho:YAG thin-disk laser capable of delivering megawatt-level peak power in the nanosecond regime. The study confirms that cavity-dumping is a superior approach for generating high-peak-power nanosecond pulses in thin-disk lasers compared to standard Q-switching, as it overcomes the gain limitations that restrict pulse shortening in Q-switched configurations.

Future Directions and Limitations
The authors identify several avenues for further energy scaling and performance improvement, noting that the current system is limited by the SiC heat sink, the non-stabilized pump spectrum, and the specific doping/thickness balance. Proposed improvements include:

Replacing the SiC heat sink with CVD diamond to better manage thermal loading and reduce ETU.
Utilizing a narrow-linewidth, wavelength-stabilized pump source, potentially a Tm-fiber laser tuned to 1907 nm, to enhance pump absorption and efficiency.
Increasing the number of passes through the gain medium per round-trip.
Optimizing the trade-off between doping concentration and crystal thickness.

The authors state that, encouraged by these results, they plan to utilize the Ho:YAG thin-disk as a main amplifier in a chirped-pulse amplification (CPA) scheme to aim for picosecond pulses with millijoule-level energies.