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Aachen laser builders hunt for invisible gravitational waves

Researchers at the Fraunhofer Institute for Laser Technology ILT in Aachen are working on the laser at the heart of the Einstein Telescope.

Laser specialists Patrick Baer and Melina Reiter (ILT, Aachen) are working on a possible solution for the Einstein Telescope’s lasers. The gravitational waves the telescope will look for are themselves invisible, but to detect them you still need light. Laser light to be precise, because that does not fan out like the torch in your mobile phone. A laser stays together in a tight beam without weakening. Researchers want to bounce lasers back and forth in the 10-kilometre-long tunnels of the Einstein Telescope, looking for minute changes in the distance between detectors at the vertices.

From the beginning of their project, Baer and Reiter have involved companies sought spin-offs: “No work has yet been done for this particular wavelength and technology. Therefore, when we publish our results, we expect a lot of interest from outside basic research,” says Patrick Baer.

Patrick Baer and Melina Reiter - credit: De Limburger
Patrick Baer and Melina Reiter – credit: De Limburger
How did you get started on laser research?

MR: “A few years ago, I deliberately applied for a job as a student assistant at the Fraunhofer Institute; before that, I’d never seen the inside of a laboratory. I thought it would be interesting to put my theoretical knowledge into practice. For example, using computer models to predict the performance of our laser system and checking that in the lab with the real components.”

PB: “When I studied physics, I was always interested in research that can be used immediately. The Fraunhofer Institutes’ mission is to further develop technology from universities and research institutes and bring it to industry, and to drive spin-offs. That suited me perfectly.”

Een 'elegant breadboard', oftewel een laboratoriumopstelling van de fiberlaser die ILT Fraunhofer heeft ontwikkeld voor de Einstein Telescope. Foto gebruikt met toestemming van ILT Fraunhofer; alle rechten voorbehouden
An ‘elegant breadboard’, or laboratory setup, of the fibre laser that ILT Fraunhofer has developed for the Einstein Telescope. Photo used with permission from ILT Fraunhofer; all rights reserved
How challenging is the laser for the Einstein Telescope?

PB: “We need a laser beam that has a lot of power and at the same time is very stable in all kinds of aspects: intensity, polarisation, direction, and more. Technology with those specifications does already exist, but mainly at a wavelength of a micrometre. That is inconvenient because that wavelength isn’t reflected by the Einstein Telescope’s silicon mirrors. We therefore had to build from scratch a new laser that works around two micrometres.”

MR: “The technology we chose is a fibre laser based on glass fibre. This can produce and amplify varying wavelengths of light, depending on the materials that have been mixed through the glass fibre as doping. Our system uses holmium. It was sometimes quite a challenge to find the right components for that, but the basic system now works and we look forward to testing it at the ETpathfinder facility in Maastricht.”

That sounds like custom work for the Einstein Telescope. Do you also expect interest from companies in this technology?

MR: “Jazeker! Nu we hebben laten zien dat je hoog-vermogen lasers kunt maken op deze nieuwe golflengte, komen er al snel toepassingen in de metrologie of quantumtechnologie in beeld. Die zijn misschien geïnteresseerd in iets andere laserparameters, maar de basistechnologie is nu bewezen.”

Laboratoriumversie van de Einstein Telescope laser die is ontwikkeld door ILT Fraunhofer. Foto gebruikt met toestemming van ILT Fraunhofer; alle rechten voorbehouden
Laboratory version of the Einstein Telescope laser developed by ILT Fraunhofer. Photo used with permission from ILT Fraunhofer; all rights reserved
What makes the Einstein Telescope so exciting to work on?

PB: “It’s fantastic to work on this challenging technology with the prospect that it could end up in the Einstein Telescope. Especially if we can get companies to work with us to further improve the system. I’m passionate about this technology and getting companies involved in it. That’s crucial to getting everyone ready to create technology for the Einstein Telescope, but also to encourage spin-offs.”

MR: “Apart from the technical challenge and the idea that we’re contributing to such an international research facility for astrophysical research? How regional this project is. I think it’s an exciting idea that in a few years’ time, we could drive through the hills here with our families and explain that our handiwork is looking at the stars from underground.”

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