Ho:YAG Thin-Disk Laser with 230 W Multimode and 150 W Single-Mode Output

This paper reports a continuous-wave Ho:YAG thin-disk laser that achieves record-breaking output powers of 230 W in multimode and 152.3 W in single-mode operation, demonstrating high efficiency and excellent beam quality.

The Gist

Imagine you are trying to build a powerful water hose that shoots a stream of water so precisely it can cut through steel, but you want the water to be a very specific color (in this case, an invisible infrared color used for medical and industrial tasks).

This paper describes a team of scientists who built a new, super-efficient "laser hose" called a Ho:YAG Thin-Disk Laser. Here is the story of how they did it, explained simply.

The Problem: The "Hot Potato"

Lasers work by pumping energy into a crystal to make it glow. But when you pump a lot of energy in, the crystal gets hot.

• The Old Way (Rod Lasers): Imagine trying to cool a thick, hot log of wood by blowing air on just one side. The inside gets scorching hot, the wood warps, and the beam of light gets messy. This is what happens with traditional rod-shaped lasers at high power.
• The New Way (Thin-Disk): The scientists used a "Thin-Disk" design. Think of this like a pizza crust instead of a log. It is very thin (about the width of a human hair) but wide. Because it's so thin, heat can escape easily from both sides, just like a pizza cooling down quickly on a metal tray. This keeps the laser "cool" and the beam sharp.

The Challenge: The "Traffic Jam"

Even with the thin disk, there were two big problems stopping them from getting more power:

1. The Pump Source: The "engine" pushing energy into the laser (a Tm fiber laser) was like a hose with a wobbly, uneven spray. It wasn't hitting the disk evenly.
2. The Crystal's Limit: The crystal (Ho:YAG) is like a sponge that can only hold so much water before it starts leaking or breaking. If you make the sponge too "spongy" (dope it heavily), it actually starts to waste energy.

The Solution: Smoothing the Flow and Widening the Net

The team fixed these issues with two clever tricks:

1. The "Flat-Top" Spray: They took their wobbly pump laser and ran it through a special fiber-optic "strainer." This smoothed out the spray into a perfect, flat circle (like a flat-top hat). Now, the energy hits the disk evenly, like rain falling on a flat roof, rather than a concentrated jet that burns a hole in the middle.
2. The "48-Pass" Dance: Instead of hitting the disk once, they bounced the pump light back and forth 48 times inside the system before it finally hit the disk. Imagine a game of ping-pong where the ball bounces 48 times to make sure it hits the target perfectly. This ensured almost all the energy was absorbed.

The Results: Breaking Records

By using these tricks, they achieved two amazing things:

• The "Multimode" Beast (230 Watts): They turned the laser up to maximum power. It's like opening the firehose all the way. It shoots out 230 Watts of power. It's not perfectly focused (the beam is a bit wide), but it's incredibly strong. This is the most powerful 2-micron thin-disk laser ever made.
• The "Single-Mode" Sniper (152 Watts): They adjusted the mirrors to make the beam super-tight and precise (like a laser pointer). Even with this strict focus, they still got 152 Watts of power. That is like having a laser pointer as strong as a high-powered industrial heater, but it stays perfectly straight and doesn't wobble.

Why Does This Matter?

Think of this laser as a new kind of "scalpel" or "welder" for the future.

• Medical: It can be used for delicate eye surgeries or treating blood vessels because that specific color of light is absorbed well by water in the body.
• Industry: It can cut or weld materials that are hard to process with other lasers.
• Science: It helps scientists study the atmosphere and detect pollution.

The Future

The scientists admit they hit a "ceiling" because their mirrors weren't perfect and the crystal could only absorb so much. But they have a roadmap for the future:

• Better Mirrors: To stop losing energy.
• Smarter Crystal: To absorb more energy without breaking.
• More Bounces: To squeeze even more power out of the same setup.

In a nutshell: They took a tricky, heat-sensitive laser crystal, gave it a "cooling system" (the thin disk), smoothed out its fuel supply, and managed to make it the strongest and most efficient of its kind in the world. It's a major step toward making high-power lasers that are smaller, cooler, and more useful for everyone.

Technical Summary

1. Problem Statement

While lasers operating in the 2 μm spectral region are critical for environmental monitoring, scientific research, and material processing, high-power solid-state lasers in this band face significant challenges.

• Thermal Limitations: Traditional rod-shaped Ho:YAG lasers suffer from thermal effects (crystal damage, mode degradation, depolarization) at high power levels, despite the benefits of in-band pumping.
• Thin-Disk Limitations: Although the thin-disk geometry mitigates thermal loading via one-dimensional heat flow, 2 μm thin-disk lasers have historically been limited to ~100 W output. This is significantly lower than Yb:YAG thin-disk lasers (kW level) and even Ho:YAG rod lasers.
• Root Causes: The limitations stem from two main factors:
  1. Lack of high-power flat-top Tm fiber laser pump sources.
  2. Inability to heavily dope Ho:YAG crystals due to up-conversion effects, which restricts the minimum disk thickness and tolerable pump intensity.

2. Methodology

The authors redesigned the pump source and optical cavity to overcome these limitations:

• Gain Medium: A Ho:YAG thin-disk crystal with 1.6 at.% doping, 210 μm thickness, 12 mm diameter, and a 30 m radius of curvature (RoC). It is mounted on a diamond heat sink and water-cooled at 16 °C.
• Pump Source Optimization:
  • Used a 1 kW, 1907 nm Tm fiber laser.
  • Employed tens of meters of mode-adapted fiber to reshape the pump beam into a nearly circular flat-top intensity distribution.
  • Implemented a 48-pass pumping scheme to achieve ~87% total absorption.
  • The pump spot diameter on the disk was enlarged to 4.5 mm to increase pump power handling.
• Cavity Designs:
  • Multimode: A compact V-shaped cavity with a concave mirror (RoC = -1 m) and various output couplers (TOC). The cavity mode (1.8 mm) was significantly smaller than the pump spot, ensuring multimode operation.
  • Single-Mode: An extended cavity with multiple mirrors to improve mode selectivity. The cavity mode diameter was expanded to 3.9 mm, achieving an 87% overlap with the pump mode.
• Constraints: The maximum pump power was limited to 650 W (pump density ~4.1 kW/cm²) to prevent the disk temperature from exceeding the safe operating limit of ~120 °C.

3. Key Contributions

• Pump Reshaping: Successfully adapted a Tm fiber laser to generate a flat-top intensity profile, optimizing energy distribution on the gain medium.
• Power Scaling: Demonstrated a method to scale 2 μm thin-disk laser power beyond the previous ~100 W barrier by optimizing pump spot size and absorption schemes.
• Dual-Mode Achievement: Simultaneously achieved record-breaking output powers in both multimode and single-mode regimes using the same gain medium and pump source.

4. Key Results

• Multimode Operation:
  • Maximum Output Power: 230 W (achieved with 2% output coupler transmittance at 650 W pump power).
  • Efficiency: Slope efficiency of 35.9% and optical-to-optical conversion efficiency of 35.3%.
  • Significance: This represents the highest output power reported to date for a 2 μm thin-disk laser.
• Single-Mode Operation:
  • Maximum Output Power: 152.3 W (achieved with 3% output coupler transmittance).
  • Beam Quality: $M^2$ factors of 1.08 (horizontal) and 1.06 (vertical), indicating near-diffraction-limited performance.
  • Efficiency: Optical-to-optical conversion efficiency of 23.9%.
• Spectral Characteristics: The laser exhibited dual-wavelength oscillation at 2090 nm and 2096 nm. A blue shift in wavelength was observed as output transmittance increased, attributed to high population inversion.
• Thermal Management: A linear relationship was established between disk temperature and pump power ($T = 19 + 0.16 \cdot P$). The system operated safely up to the 650 W pump limit.

5. Significance and Future Outlook

• Record Performance: This work sets a new benchmark for 2 μm thin-disk lasers, bridging the power gap between rod lasers and thin-disk systems while maintaining the thermal advantages of the thin-disk geometry.
• Scalability: The authors identify that further power scaling is possible by:
  1. Reducing pump losses via better mirror coatings.
  2. Optimizing doping concentration (e.g., increasing to 2 at.%) to balance absorption and up-conversion.
  3. Increasing the number of laser passes on the disk to enhance gain length and allow for higher output coupler transmittance.
• Applications: The ability to generate high-power, high-beam-quality 2 μm radiation is crucial for advancing ultrafast laser systems and high-power industrial applications. The pump source used still has a large power margin (up to 1 kW), suggesting significant headroom for future power scaling.