Researchers develop a laser that produces the most powerful ultrashort laser pulses ever

The word laser usually conjures up images of a continuous, intensely focused beam of light. Lasers that produce such light are actually quite common and useful. However, science and industry may also require very short and intense pulses of laser light.

These pulses can be used to process materials and generate harmonic frequencies up to X-rays, making them useful for visualizing very fast processes in the attosecond range (billionths of a second). Helpful.

A research team at ETH Zurich, led by Professor Ursula Keller from the Institute for Quantum Electronics, has set a new record for such laser pulses. With an average power of 550 watts, it exceeded the previous maximum output by more than 50 percent. These are the strongest pulses ever produced by a laser oscillator.

At the same time, they are very short, lasting less than one picosecond, or one millionth of a second. It then exits the laser in a fast, regular sequence of 5 million pulses per second. A short pulse can reach a peak power of 100 megawatts (in theory, this is enough to power 100,000 vacuum cleaners for a short period of time).

The researchers recently published their results in the academic journal Optica.

For the past 25 years, Keller’s research group has been working on continued improvements to so-called short-pulse disk lasers. This laser material consists of a thin disk of crystals containing ytterbium atoms, just 100 micrometers thick.

Keller and his colleagues repeatedly encountered new problems that initially prevented them from gaining further power. Frequent incidents occurred in which various parts inside the laser were destroyed. Solving the problem yielded new insights that could improve the reliability of short-pulse lasers, which are also popular in industrial applications.

“The combination of higher power and pulse rate of 5.5 MHz that we have now achieved is based on two innovations,” explains Dr. Moritz Seidel. A student in Keller’s lab. First, he and his colleagues used a special mirror arrangement that sent the light in the laser to the disk several times before exiting the laser through an output-coupling mirror.

“This arrangement allows us to extremely amplify the light without making the laser unstable,” Seidel says.

The second innovation concerns the core of the pulsed laser. A special mirror made of semiconductor material. This mirror was invented by Keller already 30 years ago and has the memorable abbreviation SESAM (Semiconductor Saturable Absorber Mirror). Unlike regular mirrors, the reflectivity of SESAM depends on the intensity of the light that hits it.

Pulse thanks to SESAM

The researchers use SESAM to guide the laser, sending short pulses rather than a continuous beam. The pulse has a higher intensity because the light energy is concentrated over a shorter period of time. For a laser to transmit laser light, the light intensity inside it must exceed a certain threshold.

This is where SESAM comes into play. SESAM efficiently reflects light that has already passed through the amplification disk several times, especially when the light intensity is high. As a result, the laser automatically goes into pulse mode.

“Pulses with power comparable to what we are currently achieving could previously only be achieved by sending weaker laser pulses through several separate amplifiers outside the laser. ” Seidel says.

The drawback is that amplification increases noise with power fluctuations, which is especially problematic for high-precision measurements.

To use laser oscillators directly to generate high power, researchers had to solve a number of difficult technical problems. For example, a thin sapphire window can be attached to the semiconductor layer of a SESAM mirror, which significantly improves the properties of the mirror. .

“When we finally got it working and saw how the laser produced the pulses, it was really cool,” Seidel says.

amplifier replacement

Ursula Keller is also excited about these results, saying, “We expect that…we can very efficiently shorten these pulses to the region of a few cycles. It is very important.”

The fast, powerful pulses made possible by the new laser could also be applied to new so-called frequency combs in the ultraviolet to X-ray range, which could lead to even more accurate clocks, Keller said. There is.

“My dream is to one day prove that the constants of nature are not constant after all,” Keller says. Terahertz radiation, which has wavelengths much longer than visible or infrared light, is also produced by lasers and can be used for things like testing materials.

“Overall, we can say that our pulsed laser shows that laser oscillators are a good alternative to amplifier-based laser systems and enable new and better measurements,” says Keller. Masu.