EPFL builds all-glass femtosecond laser cavity

27 Sep 2023

Miniaturized design could solve alignment challenges for novel optical systems.

A project at Swiss research center EPFL has manufactured a palm-sized all-glass GHz femtosecond laser cavity, potentially offering a novel route to free-space femtosecond lasers.

Conventional femtosecond lasers are made by putting optical components and their mounts on a substrate, an operation where subsequent alignment of the optical components is a demanding condition.

Architectures where light propagates in free space are attractive for several applications, but pose particular challenges for femtosecond sources in terms of miniaturization and the precise alignment of components during manufacture.

Published in Optica, the EPFL breakthrough involves using a commercial femtosecond laser to micromachine both a holder for optical components and separate flexural elements into a glass substrate, and then subsequently use the same commercial source to precisely adjust the position of the mounted components by non-contact means.

"In essence, a femtosecond laser is used to manufacture another femtosecond laser, not only for substrate manufacturing, but also for fine alignment and tuning," commented the team in its published paper.

"We use femtosecond lasers for our research on the non-linear properties of materials and how materials can be modified in their volume," commented Yves Bellouard, head of EPFL's Galatea Lab, which researches ways to tailor material properties locally and inscribe functions in materials via lasers.

"Going through the exercise of painful complex optical alignments makes you dream of simpler and more reliable ways to align complex optics."

Commercialization for interferometers and other systems

The new method builds upon EPFL research during a two-year EU project concluded in 2019, which outlined the principles of the two-step manufacturing operation.

First a commercial femtosecond laser etches slots in a glass substrate for the placement of the essential components of the laser, but not yet at the level of precision needed for the final lasing operation. This initial operation also machines leaf-sprung flexure mechanisms in the substrate at the same time, ready to perform a final positioning operation.

The second stage is triggered by a further exposure to the same commercial femtosecond source, to permanently deform the flexure mechanisms in a controlled manner and move the optical components into the final precise alignment.

EPFL used its technique to demonstrate a low-power GHz repetition femtosecond laser oscillator delivering sub-200 femtosecond pulses, packaged on a credit card-sized footprint and passively cooled. But the same principle could now be applied to the manufacture of free-space optical systems in other contexts, such as interferometers and beam shapers.

The commercial opportunities are being investigated by Cassio-P, an EPFL spin-out headed by Galatea Lab's Antoine Delgoffe. It foresees multiple applications, "from a core component of high-power laser systems to a light source for advanced microscopy, and from a tool to assist the development of new drugs to a core component of time-keeping systems used for satellite navigation," according to Cassio-P.

"This approach to permanently align free-space optical components thanks to laser-matter interaction can be expanded to a broad variety of optical circuits, with extreme alignment resolutions, down to sub-nanometers," said Yves Bellouard.