In a groundbreaking development, researchers have demonstrated the capability of 3D-printed polymer-based micro-optics to withstand the heat and power levels generated within a laser. This breakthrough paves the way for cost-effective, compact, and stable laser sources, with potential applications in lidar systems for autonomous vehicles.
The research team, led by Simon Angstenberger from the 4th Physics Institute at the University of Stuttgart in Germany, successfully reduced the size of a laser by utilizing 3D printing to create high-quality micro-optics directly on glass fibers used within lasers. This marks the inaugural implementation of 3D-printed optics in a real-world laser, showcasing their high damage threshold and stability.
Published in Optics Letters, the study details how microscale optics were 3D printed directly onto optical fibers, seamlessly integrating fibers and laser crystals within a single laser oscillator in a compact manner. The resultant hybrid laser demonstrated stable operation at output powers exceeding 20 mW at 1063.4 nm, with a maximum output power of 37 mW.
This innovative laser design combines the compactness, robustness, and affordability of fiber-based lasers with the diverse properties offered by crystal-based solid-state lasers, including different powers and colors.
“Until now, 3D-printed optics have primarily been used for low-power applications such as endoscopy,” highlighted Angstenberger. “The ability to use them with high-power applications could be useful for lithography and laser marking, for example. We showed that these 3D micro-optics printed onto fibers can be used to focus large amounts of light down to a single point, which could be useful for medical applications such as precisely destroying cancerous tissue.”
Taking the heat
The 4th Physics Institute at the University of Stuttgart has a rich history of pioneering 3D-printed micro-optics, particularly the ability to directly print them onto fibers. Employing the two-photon polymerization 3D printing approach, this method utilizes an infrared laser focused into a UV-sensitive photoresist.
Within the laser's focal region, simultaneous absorption of two infrared photons occurs, leading to the hardening of the UV resistance. Precision is achieved by moving the focus, allowing the creation of various shapes and introducing novel functionalities such as free-form optics or intricate lens systems.
Simon Angstenberger noted, “Because these 3D-printed elements are made of polymers, it was unclear whether they could withstand the significant amount of heat load and optical power that occurs inside a laser cavity. We found that they are surprisingly stable, and we were not able to observe any kind of damage on the lenses even after several hours of running the laser.”
For the recent study, the researchers utilized a 3D printer by Nanoscribe to craft lenses with a 0.25 mm diameter and a height of 80 microns directly onto the end of a fiber with the same diameter using two-photon polymerization.
This intricate process involves designing an optical element with commercial software, inserting the fiber into the 3D printer, and then precisely printing the small structure onto the end of the fiber. The precision required extends to aligning the printing with the fiber and ensuring accuracy throughout the printing process.
Creating a hybrid laser
Upon completing the 3D printing process, the researchers meticulously assembled the laser and its cavity. Departing from the conventional use of crystals within a laser cavity made of bulky and expensive mirrors, they ingeniously employed fibers as part of the cavity, giving rise to a hybrid fiber-crystal laser. The lenses, intricately printed at the end of the fibers, serve to focus and couple light into and out of the laser crystal.
To enhance stability and reduce susceptibility to air turbulence, the fibers were affixed to a mount, compacting the laser system. The crystal and the printed lenses, measuring just 5 X 5 cm², demonstrated the efficiency of this innovative approach.
Continuous monitoring of laser power over several hours affirmed that the printed optics within the system remained resilient without deterioration, preserving the long-term properties of the laser. Moreover, scanning electron microscopy images of the optics post-use in the laser cavity revealed no visible damage. Simon Angstenberger noted, “Interestingly, we found that the printed optics were more stable than the commercial fiber Bragg grating we used, which ended up limiting our maximum power.”
The researchers are actively pursuing further enhancements in the efficiency of the printed optics. This involves exploring larger fibers with optimized freeform and aspherical lens designs or a combination of lenses printed directly onto the fiber, aiming to enhance output power. Additionally, they aspire to demonstrate the integration of different crystals in the laser, offering customization for specific applications.