The short, regular pulses of femtosecond lasers are put to effective use in numerous applications, including laser surgery, micromachining, microscopy, and spectroscopy. Free-space optics offers substantial freedom in femtosecond laser design — but this comes at the cost of limited miniaturization and manufacturability. A new approach, developed at École Polytechnique Fédérale de Lausanne (EPFL), makes the free-space optical components of a femtosecond laser easier to align, thus making the laser easier to manufacture. “Going through the exercise of painful complex optical alignments makes you dream of simpler and more reliable ways to align complex optics,” said professor Yves Bellouard, who led the EPFL team. Bellouard heads the Galatea Lab at EPFL, which uses femtosecond lasers to investigate the nonlinear properties of materials. EPFL scientists showed that it is possible to make a femtosecond laser that fits in the palm of one’s hand using a single glass substrate. Courtesy of EPFL/Jamani Caillet-CC-BY-SA 4.0. The researchers used a commercial femtosecond laser to make their new laser — in essence, they used one laser to make another. In the EPFL-designed laser, all the optical functions are integrated in a single glass substrate, including the functions that require active tuning or electro-optics effects. The EPFL approach relies on femtosecond laser machining combined with post-processing methods to fabricate the optical cavity and perform the finely calibrated alignment required for proper laser operation. Using the commercial femtosecond laser, the researchers etched grooves in the glass substrate. The etchings enabled the team to precisely place the essential components of the EPFL laser. The researchers placed the components of the laser cavity in the pre-formed glass substrate and aligned the components in a contactless manner via laser-matter interaction. One of the laser’s mirrors sat in a groove with micromechanical flexures that were engineered to locally stir the mirror when exposed to femtosecond laser light. This enabled the researchers to use the commercial femtosecond laser to align the mirrors to create a stable, small-scale femtosecond laser. The built-in, embedded flexural elements could be fine-tuned remotely using the commercial femtosecond laser to achieve ultra-accurate positioning, with subnanometer and subradian angular resolutions. “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 subnanometers,” Bellouard said. The single substrate provided a reference coordinate frame that was shared by all components, which was necessary to position and align the components accurately with respect to each other. The use of femtosecond laser exposure with etching, combined with precision engineering design methods, allowed a positioning accuracy, before adjustment, of close to 1 µm, because all the reference surfaces were manufactured at once using a single manufacturing platform. Scientists from EPFL’s Galatea Lab made gigafemto lasers on a glass substrate. Courtesy of EPFL/Jamani Caillet-CC-BY-SA 4.0. The fused silica that was used for the substrate provided superior thermal stability against temperature fluctuations. “We want to make stable lasers, so we use glass because it has a lower thermal expansion than conventional substrates, it is a stable material, and it is transparent for the laser light we use,” Bellouard said. Despite its small size, the femtosecond laser from EPFL can reach approximately 1 kW of peak power and emit pulses of less than 200 fs. The laser has a repetition rate in the gigahertz range and is packaged in a small form factor, with a footprint about the size of a credit card, and passive cooling. Ongoing research programs at the Galatea Lab will explore the use of this technology in the context of quantum optical system assembly and will push the limits of what is currently achievable in terms of miniaturization and alignment accuracy. The alignment process is still supervised by a human operator, and with practice can take a few hours to achieve, the team said. The manufacturing concept for the EPFL femtosecond laser is generic and could be expanded to other types of miniature cavities. The researchers believe that their approach to designing and manufacturing femtosecond lasers could enable a paradigm shift to free-space laser-cavity manufacturing, and open a pathway to highly integrated, palm-size, yet stable laser sources with excellent beam quality and high peak-power pulses. The EPFL femtosecond laser technology is to be spun off by Cassio-P, a company to be headed by Antoine Delgoffe at Galatea Lab, who joined the project with the mission of finalizing the proof-of-concept into a future commercial device. The research was published in Optica (www.doi.org/10.1364/OPTICA.496503).
