Keller-Led Team Posts an Ultra-Short Pulse Record

Researchers at ETH Zurich have developed a laser that can produce extremely short pulses with peak powers up to 100 MW and 550 W of average power. The researchers, led by Ursula Keller, a professor at the Institute for Quantum Electronics, report that the achievement marks a record — surpassing the previous reported maximum by more than 50%. The demonstrated pulses are the strongest pulses ever created by a laser oscillator.

In demonstration, the pulses, lasting less than a picosecond, exited the laser in a regular sequence at a high rate of five million pulses per second.

The interior of the record-breaking pulsed laser. The laser has an average power of 550 W and reached a peak power of 100 MW. Courtesy of Moritz Seidel/ETH Zurich.

“Pulses with powers comparable to the ones we have now could, up to now, only be achieved by sending weaker laser pulses through several separate amplifiers outside the laser,” said Moritz Seidel, a PhD student in Keller’s research group. “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 attach a thin sapphire window to the semiconductor layer of the semiconductor saturable absorber mirror (SESAM), which improved its properties.

According to Keller, the team expects to soon be able to shorten the pulses to a few cycles for the creation of attosecond pulses. The pulses could see use in new frequency comb designs in the UV to X-ray regime, which could lead to more precise clocks. Terahertz radiation can also be created with the laser, the researchers said, and then used to test materials.

For the past 25 years, Keller has been working on the advancement of short pulsed disk lasers, in which the laser material consists of a thin disk (100 µm thick) of a crystal containing ytterbium atoms. According to Seidel, the recent achievement was based on two factors. The first was an 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 amplify the light extremely without the laser becoming instable,” Seidel said.

The researchers’ pulsed laser includes an array of mirrors that send light through the lasers disk several times and a central semiconductor mirror with a thin sapphire window to automatically create the laser pulses. Courtesy of Heidi Hofstetter/ETH Zurich.

The second was the addition of a mirror acting as the “centerpiece” of the pulsed laser, which was fabricated from semiconductor material. The reflectivity of this SESAM depends on the strength of the light hitting it. The researches used SESAM to make the laser automatically produce pulses by reflecting high intensity light, which had already made many passes through an amplifying disk.

“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 said.

The research was published in Optica (www.doi.org/10.1364/OPTICA.529185).