Scientists from ETH Zurich have created a laser that generates the most powerful ultra-short laser pulses to date. In the future, these high-power pulses may be used for materials processing or precise measurements.
A peek inside the record-breaking laser. The image shows the round amplifier disk, through which the laser beam passes several times (bright spot at the center). Image Credit: Moritz Seidel/ETH Zurich
The word "laser" often brings to mind a sharply focused, continuous beam of light. While lasers that produce such beams are widely used, science and industry also frequently require extremely short, powerful pulses of laser light. These pulses can be employed for precision material processing or to generate high harmonic frequencies, such as X-rays, which enable the observation of ultrafast processes in the attosecond range (a billionth of a billionth of a second).
A team of researchers at ETH Zurich, led by Ursula Keller, professor at the Institute for Quantum Electronics, has now set a new benchmark for such laser pulses. Their laser pulses achieve an average power of 550 watts, exceeding the previous record by more than 50 %, making them the strongest ever generated by a laser oscillator.
Despite their power, these pulses are incredibly brief, lasting less than a picosecond (a millionth of a millionth of a second) and emitted in a regular sequence at a rate of five million pulses per second. These short bursts of energy reach peak powers of 100 megawatts—enough, in theory, to power 100,000 vacuum cleaners momentarily.
For the past 25 years, Keller's research group has focused on refining short-pulsed disk lasers, where the laser material is a thin disk, just 100 micrometers thick, made of a crystal doped with ytterbium atoms.
Throughout their work, Keller and her team frequently encountered new challenges that initially hindered further power increases. These obstacles sometimes led to dramatic incidents, including the destruction of various components within the laser system. However, overcoming these challenges provided valuable insights, enabling the team to make short-pulsed lasers, which are widely used in industrial applications, more reliable and efficient.
The combination of even higher power and pulse rates of 5.5 MHz, which we have now achieved, is based on two innovations.
Moritz Seidel, Ph.D. Student, ETH Zurich
One key innovation was a specialized mirror arrangement that directs the laser light through the disk multiple times before it exits through an outcoupling mirror.
“This arrangement allows us to amplify the light extremely without the laser becoming instable”, said Seidel.
The second innovation involves the heart of the pulsed laser: a specialized mirror made from semiconductor material, known as SESAM (Semiconductor Saturable Absorber Mirror), which was invented by Keller thirty years ago. Unlike conventional mirrors, SESAM’s reflectivity changes based on the intensity of the light striking it. This unique property enables precise control over the laser pulses, contributing to the system’s enhanced performance and stability.
Pulses Thanks to SESAM
By using the SESAM, the researchers were able to manipulate their laser into emitting short pulses instead of a continuous beam. Pulses have a higher intensity because the light energy is concentrated over a shorter duration.
For the laser to emit light at all, the internal light intensity must exceed a specific threshold. This is where the SESAM plays a crucial role: it reflects light, which has passed through the amplifying disk multiple times, with higher efficiency when the light intensity is strong. As a result, the laser naturally shifts into pulsed mode, producing high-intensity bursts.
Pulses with powers comparable to the ones we have now achieved could, up to now, only be achieved by sending weaker laser pulses through several separate amplifiers outside the laser.
Moritz Seidel, Ph.D. Student, ETH Zurich
The challenge with this approach is that the amplification also increases noise, leading to power fluctuations, which can pose issues, particularly in precision measurements.
To generate high power directly within the laser oscillator, the researchers had to overcome several complex technical hurdles. One such challenge was how to attach a thin sapphire window to the semiconductor layer of the SESAM mirror. This sapphire window significantly enhances the mirror's properties, but integrating it required careful engineering to maintain the system's overall stability and performance.
“When it finally worked and we watched how the laser created pulses that was really cool,” said Seidel.
Alternative to Amplifiers
The support by ETH Zurich over the years and the reliable funding of my research by the Swiss National Fund has helped me and my collaborators reach this great outcome. We now also expect to be able to shorten these pulses very efficiently to the regime of a few cycles, which is very important for creating attosecond pulses.
Ursula Keller, Professor, Institute for Quantum Electronics, ETH Zurich
According to Keller, the fast and powerful pulses enabled by the new laser could also be used in the development of new frequency combs in the ultraviolet to X-ray range. These frequency combs have the potential to create even more precise clocks, advancing the accuracy of timekeeping and various other precision measurement applications.
“A dream would be to show, one day, that the natural constants are not constant after all,” said Keller.
Additionally, the laser can generate terahertz radiation, which has a much longer wavelength than visible or infrared light. This type of radiation can be used in various applications, such as testing and analyzing materials.
“All in all, one can say that with our pulses lasers, we have shown that laser oscillators are a good alternative to amplifier-based laser systems and that they enable new and better measurement,” Keller summarized.
Journal Reference:
Seidel, M., et al. (2024) Ultrafast 550-W average-power thin-disk laser oscillator. Optica. doi.org/10.1364/optica.529185.