🔬 optics

Comparative study of second harmonic generation at 1030 nm in BiBO and LBO crystals using a 100 W-class picosecond laser

This study presents a systematic experimental comparison demonstrating that both BiBO and LBO crystals achieve an identical record-breaking 32 W of second-harmonic generation at 515 nm (56% efficiency) when driven by a high-power 1030 nm picosecond laser, providing critical benchmarks for selecting nonlinear crystals in high-average-power frequency conversion applications.

Original authors: Huzefa Aliasger, Šimon Šatra, Ondřej Novák, Jiří Mužík, Michal Jelínek, Martin Smrž, Tomáš Mocek

Published 2026-05-19

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Huzefa Aliasger, Šimon Šatra, Ondřej Novák, Jiří Mužík, Michal Jelínek, Martin Smrž, 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 powerful laser beam that is invisible to the human eye, glowing in the near-infrared spectrum (like a very deep red that we can't quite see). Scientists want to turn this invisible beam into a bright, visible green light, similar to the green laser pointers used in presentations, but much more powerful.

To do this, they need to pass the invisible light through special "magic crystals" that act like translators, converting the color of the light. This paper is a head-to-head race between two of these crystals: BiBO (Bismuth Triborate) and LBO (Lithium Triborate).

Here is the breakdown of their experiment and findings in simple terms:

The Setup: A High-Speed Race

The scientists used a very fast, powerful laser (100 watts of power) that fires tiny, ultra-short pulses of light. They wanted to see which crystal could turn the most of this invisible light into green light (515 nm) without breaking or losing efficiency.

The Contenders:
- BiBO: Think of this as the sprinter. It is very good at converting light quickly, so you only need a tiny piece of it (1.5 mm long) to get the job done. It's like a high-performance sports car that gets great gas mileage but has a very sensitive engine.
- LBO: Think of this as the marathon runner. It's not as "fast" at converting light per inch, so you need a longer piece (6 mm long) to get the same result. However, it is built like a tank; it's very sturdy and doesn't overheat easily.

The Results: A Tie on Power, But Different Personalities

When they pushed both crystals to their limits with the full 57-watt laser, they both won the power race.

Both crystals produced 32 Watts of bright green light.
Both achieved a 56% conversion efficiency (meaning more than half of the invisible light became green light).
This is a record-breaking amount of green light generated using the BiBO crystal.

However, while they finished with the same score, they got there in very different ways:

1. The Heat Problem (Thermal Stability)

BiBO (The Sprinter): Because it is so efficient at converting light, it also absorbs a lot of heat in the process. As the laser ran for an hour, the BiBO crystal got hot (about 7°C warmer). This heat acted like a "warping" force, slightly changing the crystal's shape and causing the green light output to drop by about 8% over time. To fix this, the scientists had to physically tilt the crystal back and forth to "re-tune" it, like adjusting a radio to find the clearest signal again.
LBO (The Marathon Runner): This crystal stayed much cooler. It absorbed very little heat, so its output remained incredibly stable. It didn't need any adjustments and kept the green light steady for the whole hour.

2. The Size and Speed

BiBO: Because it is so powerful, it can do the job in a crystal that is four times shorter than the LBO. This is great for making compact, small devices. It also produced a slightly shorter pulse of light and a slightly wider range of colors (spectrum), which can be useful for specific scientific tasks.
LBO: It needed to be longer to catch up, but it handled the "traffic" of the light beam more smoothly, keeping the beam shape very clean.

3. The "Tolerance" (Angular Acceptance)
Imagine trying to shine a flashlight through a keyhole.

Usually, shorter crystals are harder to align (the keyhole is smaller).
Surprisingly, even though BiBO was much shorter, it was just as easy to align as the longer LBO crystal. This is because BiBO is so "strong" that it compensates for its small size.

The Bottom Line

The paper concludes that both crystals are excellent choices for turning powerful invisible lasers into green light, but they serve different needs:

Choose BiBO if: You need a compact, small device and want the highest possible efficiency in a short space. However, you must be prepared to manage the heat (perhaps by putting the crystal in a temperature-controlled oven) to keep the light stable.
Choose LBO if: You need a system that runs for a long time without anyone touching it. It is more "forgiving" and stable, making it the safer, more reliable choice for industrial or long-term use, even though it requires a larger crystal.

In short, it's a choice between a high-performance, compact engine that needs careful tuning (BiBO) versus a reliable, steady engine that just keeps running (LBO). Both can get you to the destination (32 Watts of green light), but the journey is different.

Technical Summary: Comparative Study of Second Harmonic Generation in BiBO and LBO Crystals

Problem Statement
The rapid advancement of high-average-power picosecond and sub-picosecond laser systems (e.g., thin-disk, fiber, and slab architectures) operating near 1030 nm has created a critical demand for nonlinear frequency-conversion stages capable of delivering high efficiency, excellent beam quality, and long-term stability in the green spectral region. While Lithium Triborate (LBO) has long been the standard for high-power Second Harmonic Generation (SHG) due to its high damage threshold and low walk-off, Bismuth Triborate (BiBO) offers a significantly larger effective nonlinear coefficient, theoretically enabling efficient conversion in much shorter crystals. However, BiBO is known for lower damage thresholds, higher absorption at the second harmonic (SH) wavelength, and larger spatial walk-off. Despite these known trade-offs, systematic experimental comparisons of BiBO and LBO under identical high-average-power (50+ W), picosecond driving conditions have been notably scarce. This study addresses the lack of quantitative benchmarks needed to guide the selection of nonlinear crystals for next-generation harmonic generation modules.

Methodology
The authors conducted a systematic experimental comparison of single-pass SHG in BiBO and LBO crystals using a 1.3 ps, 91 kHz, 57 W average power Yb:YAG thin-disk laser at 1030 nm.

Crystal Configuration: A 1.5 mm long BiBO crystal (cut for Type I $e+e \to o$ in the YZ plane) was compared against a 6 mm long LBO crystal (cut for Type I $o+o \to e$ in the XY plane). The longer LBO length was selected to compensate for its lower nonlinear coefficient, aiming for comparable output. Both crystals featured dual-band anti-reflection coatings for 1030 nm and 515 nm and were operated at ambient temperature without active thermal stabilization.
Characterization: The study measured SH output power dependence, long-term power stability (>1 hour), beam quality ( $M^2$ ), pulse duration, spectral properties, and angular acceptance bandwidth.
Thermal Analysis: Thermal effects were investigated by monitoring phase-matching angle shifts as a function of input power and measuring surface temperature rise using an IR camera. Numerical simulations using mlSNLO software were performed to validate angular acceptance bandwidths and model thermal phase-matching drift.

Key Results

Output Power and Efficiency: Both crystals achieved a maximum SH output of 32 W at 515 nm with a conversion efficiency of 56% at the maximum input power of 57 W. This represents the highest SH output power reported to date for BiBO in the green spectral region using a thin-disk Yb:YAG laser.
Power Dependence: While LBO showed clear efficiency saturation, BiBO efficiency continued to increase with input power, suggesting it had not yet reached its maximum potential. However, further scaling was avoided due to BiBO's lower damage threshold.
Beam Quality: The SH beams exhibited near-diffraction-limited quality ( $M^2 \approx 1.3–1.8$ ). The BiBO-generated beam was approximately 25% smaller in area than the LBO beam, attributed to thermally induced phase-matching variations caused by the Gaussian intensity profile of the fundamental beam.
Pulse Duration and Spectrum: The BiBO crystal produced slightly shorter pulses (1.1 ps vs. 1.2 ps for LBO) and broader spectral bandwidths (0.48 nm vs. 0.42 nm). This is attributed to BiBO's broader spectral acceptance bandwidth and reduced Group Velocity Mismatch (GVM) accumulation due to its shorter crystal length.
Thermal Stability and Phase Matching:
- BiBO: Exhibited significant thermal sensitivity. Over one hour of continuous operation, the SH output dropped by ~8% (2.5 W) due to thermally induced phase mismatch. Re-optimizing the crystal tilt restored stability. IR imaging confirmed a surface temperature rise of ~7°C, corresponding to an internal angular shift of ~2.5 mrad.
- LBO: Demonstrated superior stability with only a ~2.5% power drop over the same period, attributed to its lower absorption at 515 nm.
Angular Acceptance: Despite BiBO's intrinsically narrower acceptance per unit length, the 1.5 mm BiBO crystal exhibited an angular acceptance bandwidth comparable to the 6 mm LBO crystal at low power, and even larger at full power. This confirms that BiBO's high nonlinearity allows for shorter crystals without sacrificing angular tolerance.

Significance and Claims
The paper claims to provide the first direct, quantitative performance benchmarks comparing BiBO and LBO under high-average-power, picosecond conditions. The authors assert that while BiBO enables efficient conversion in a significantly more compact footprint (1.5 mm vs. 6 mm) and offers slightly better pulse/spectral characteristics, it is more susceptible to thermally induced phase-mismatch drift. Conversely, LBO remains the more thermally robust choice for long-term, hands-off operation.

The study concludes that both crystals are viable solutions for generating green light from 100 W-class picosecond lasers near 1030 nm. The optimal choice depends on specific application requirements: BiBO is advantageous for compact, high-efficiency systems (potentially with active temperature control), while LBO is preferred for applications prioritizing long-term thermal stability and minimal power drift. The results serve as a guide for selecting nonlinear crystals for the design of next-generation frequency conversion modules.

The Setup: A High-Speed Race

The Results: A Tie on Power, But Different Personalities

The Bottom Line

Technical Summary: Comparative Study of Second Harmonic Generation in BiBO and LBO Crystals

More like this