ETH Zurich creates laser with peak powers of 100 MW

16 Oct 2024

Such laser pulses – with “record” 550 W average power – could be used for precision measurements.

Researchers at ETH Zurich have developed a laser that produces, according to its developers, the strongest ultra-short laser pulses to date: “In the future, such high power pulses could be used for precision measurements or materials processing.”

The researchers, led by Ursula Keller, professor at the Institute for Quantum Electronics, have set a new record for short and strong laser pulses: at 550 watts of average power they surpass the previous maximum by more than 50 percent, which makes them the strongest pulses ever created by a laser oscillator.

At the same time, they are extremely short – they last less than a picosecond – and exit the laser at a rate of five million pulses per second. The short pulses reach peak powers of 100 MW.

The researchers have published their results in Optica.

For the past 25 years, Keller’s research group has been working on the continuous improvement of so-called short pulsed disk lasers, in which the laser material consists of a thin disk, only 100 micrometers thick, of a crystal containing ytterbium atoms.

Frequently, Keller and her team encountered new problems that initially impeded a further increase in power. Quite often, spectacular incidents happened in which different parts inside the laser were destroyed. Solving the problems led to new insights that made short pulsed lasers, which are also popular in industrial applications, more reliable.

“The combination of even higher power and pulse rates of 5.5 megahertz, which we have now achieved, is based on two innovations,” said Moritz Seidel, a PhD student in Keller’s laboratory. For one thing, he and his colleagues used a special arrangement of mirrors that send the light inside the laser through the disk several times before it leaves the laser through an outcoupling mirror. “This arrangement allows us to significantly amplify the light without the laser becoming instable,” he said.

The second innovation regards the centrepiece of the pulsed laser: a special mirror made of semiconductor material, which was invented by Keller 30 years ago , known as SESAM (Semiconductor Saturable Absorber Mirror). Unlike normal mirrors, the reflectivity of a SESAM depends on the strength of the light hitting it.

Pulses thanks to SESAM

Using the SESAM, the researchers coax their laser into sending out short pulses rather than a continuous beam. Pulses have a higher intensity because the light energy is concentrated in a shorter period of time. For a laser to send out laser light at all, the light intensity inside it has to exceed a certain threshold value.

This is where the SESAM comes into play: it reflects the light, which has already passed through the amplifying disk several times, particularly efficiently if the light intensity is high. As a result, the laser automatically goes into pulsed mode.

“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,” said Seidel. The disadvantage of this is that the amplification also leads to more noise, corresponding to fluctuations in the power, which causes problems particularly in precision measurements.

To create the high power directly using the laser oscillator, the researchers had to solve a number of tricky technical problems – for instance, how to attach to the semiconductor layer of the SESAM mirror a thin sapphire window, which strongly improves the properties of the mirror.

Alternative to amplifiers

Prof. Keller is also thrilled by these results and emphasizes: “The support by ETH Zurich over the years and the reliable funding of my research by the Swiss National Fund have 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.”

According to Keller, the fast and strong pulses made possible by the new laser could also see applications in new frequency combs in the ultraviolet to X-ray regime, which could lead to even more precise clocks. “A dream would be to show, one day, that the natural constants aren’t constant after all”, said Keller. Another application is terahertz radiation, which can be created with the laser and then used, for example, to test materials.