They break a record in laser pulses

The word laser usually conjures up the image of a highly concentrated and continuous beam of light. Lasers that produce this type of light are, in fact, very common and useful. However, science and industry also often need very short, powerful pulses of laser light. These pulses can be used to machining materials or to create high harmonic frequencies, all the way to X-rays, which can help make extremely fast processes visible in the attosecond range. (An attosecond is one billionth of a billionth of a second.)

A team of researchers from the ETH Zurich in Switzerland, consisting of Ursula Keller, Moritz Seidel, Lukas Lang and Christopher R. Phillips, has now set a new record for laser pulses of this type: with 550 watts of average power they surpass by more than 50 percent the previous maximum, making them the most powerful pulses yet created by a laser oscillator.

It is also striking how extremely short they are: they last less than a picosecond (a millionth of a millionth of a second) and leave the laser cannon at a rate of more than five million pulses per second.

The short pulses reach maximum powers of 100 megawatts, which, in theory, would be enough to power 100,000 vacuum cleaners for a very short period of time.

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

A look inside the record-breaking laser generator. The image shows the amplifier disk, through which the laser beam passes several times (bright spot in the center). (Photo: Moritz Seidel / ETH Zurich)

Time and time again, Keller and his collaborators encountered new problems that, in principle, prevented further increases in power. There were often spectacular incidents in which various parts inside the laser generator were destroyed. Resolution of the problems led to new insights that made short-pulse lasers more reliable, paving the way for broader use of such lasers in industrial applications.

The combination of even higher power and a pulse frequency of 5.5 megahertz, which has now been achieved, is based on two innovations.

One of these innovations is a special mirror arrangement that sends light inside the laser through the disk several times before it exits the laser cannon through a decoupling mirror. This configuration allows the light to be greatly amplified without the laser becoming unstable.

The second innovation is the use of a mirror of a special type, characterized, among other things, by being made of semiconductor material. Unlike what happens in normal mirrors, the reflectivity of a mirror of this kind depends on the intensity of the light that falls on it.

Using this unique mirror, the laser generator can be caused to emit short pulses instead of a continuous beam.

The pulses have greater intensity because the light energy is concentrated in a shorter period of time.

Keller and his collaborators present the technical details of their achievement in the academic journal Optica, under the title “Ultrafast 550-W average-power thin-disk laser oscillator.” (Fountain: NCYT by Amazings)