Clara Saraceno portrait

[Image: Courtesy of Clara Saraceno]

2019 OSA Ambassador Clara Saraceno, Ruhr Universität Bochum, Germany, delivered a plenary talk this morning at OSA Laser Congress in Vienna, Austria. In case you missed it, Saraceno gave OPN a first look into her presentation, titled “Trends, challenges and applications of high-average-power ultrafast thin-disk lasers.” 

What does your plenary talk at the OSA Laser Congress focus on?

Combining thin-disk technology with ultrafast lasers, which is a great combination, but one that was not obvious for many years because ultrafast also took off not that long ago.

What makes the TDL­–ultrafast combination so powerful?

The general concept of a thin-disk laser (TDL) is that the gain medium in the laser is very thin—but not just thin; thin relative to the laser beam size. If the gain is thin enough, then you can apply bigger and bigger pumping spots. And if you cool the gain medium from the back, then you can extract the heat so efficiently that you can scale the power—infinitely, in principle.

The idea behind it is that you scale the area of your pumping, the area of your laser beam and the cooling area at the same time. Since the disk is very thin, you can actually do this because the heat is one-dimensional. So by increasing the area, you’re increasing the possibility to pump your gain harder.

What are these systems enabling?

The big challenge from the applications point of view is combining very high average power with joule energies. Having one or the other is okay, but you want both, and combining the two is not easy. Which is why people are normally used to working with three orders of magnitude less repetition rate—kilohertz, hertz or megahertz, it’s all the same—than whatever pulse energy they need for their laser.

This what the systems have really enabled. The high-average-power lasers give scientists the capability of doing their experiments with the fixed pulse energy that they need, but tremendously faster—so, many more pulses per second, but the same energy per pulse.

How would you characterize recent progress in this field?

People should be excited about this crazy-fast progress! When you look at what’s been accomplished as a function of time, it really is amazing just how fast things have progressed for some of these systems.

Amplifiers have progressed particularly fast because industry has gotten involved in the development of these lasers. Also, this industry involvement helps academia to develop new ideas, which is another thing that enabled this extraordinary progress.

To give you an example of where things stand now, I would say that in ultrafast amplifier systems, a kilowatt of power is almost the standard. Of course, now we’re starting to move to multi-kilowatts.

Has this opened up new application areas?

The strong push from the industrial side is for machining applications. Especially for special applications where you have really hard materials or materials that are very difficult to process and you need to increase your process speed.

In academia, there are also unusual applications. For example—and this one is kind of funny—there are researchers on top of one of the highest mountains in Switzerland that are trying to use one these crazy kilowatt-joule laser systems to shoot at the sky to guide lightning away from crowded areas when there is a storm. They’re trying to guide it through like a [lightning] rod, but one that you would create with a filament in the sky.

And this is only made possible from these high-repetition-rate, high-energy and high-power lasers. It’s really amazing, and it’s an application that we couldn’t have imagined doing before.

Besides direct applications, like shooting a TDL at the sky, one of the big highlights of this technology is what TDLs allow one to make as secondary sources. A lot of research is focused on using TDLs to cover the whole electromagnetic spectrum—whether you need terahertz or extreme ultraviolet or mid-infrared. Whatever you need, these lasers allow you do it with high power, and that expands the possibilities of what you can do even further.

Are there any challenges to future scaling?

Most of the challenges that people are facing now are components or technical difficulties—mirrors that become damaged, and components that are not large enough aperture.

But some of the remaining challenges have not really been reached yet. Compared with many competing technologies—such as fiber lasers; they reached a physical limitation a long time ago that made researchers combine several lasers and do things like that. Disk lasers are not really there yet. There is still a lot that someone could get out of a single TDL that has not been reached, simply because sometimes progress with components is slower than expected. 

Of course it takes more time to adopt these technologies in different communities. This is still a relatively new technology.