- Polymer-based micro-optics can withstand the power levels in a laser
University of Stuttgart Researchers have demonstrated for the first time that 3D printed polymer-based micro-optics can withstand the heat and power levels found inside a laser. This effectively created a hybrid laser by 3D printing optics on fibres. The breakthrough enables low-cost, compact, and stable laser sources that can be used in a variety of applications, including lidar systems used in autonomous vehicles.
“We significantly reduced the size of a laser by using 3D printing to fabricate high-quality micro-optics directly on glass fibers used inside of lasers. This is the first implementation of such 3D-printed optics in a real-world laser, highlighting their high damage threshold and stability.”– Simon Angstenberger, Research team leader from the 4th Physics Institute at the University of Stuttgart, Germany
Creating hybrid laser by 3D printing optics on fibers
In the journal Optics Letters, the researchers describe how they 3D printed microscale optics directly onto optical fibers to combine fibers and laser crystals inside a single laser oscillator in a compact way. The resulting hybrid laser was stable at output powers of more than 20 mW at 1063.4 nm, with a maximum output power of 37 mW.
The new laser combines the advantages of fiber-based lasers, such as compactness, robustness, and low cost, with the advantages of crystal-based solid-state lasers, which can have a wide range of properties such as different powers and colours.
Angstenberger added, “Until now, 3D-printed optics have primarily been used for low-power applications such as endoscopy. 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 University of Stuttgart’s 4th Physics Institute has a long history of developing 3D-printed micro-optics, particularly the ability to print them directly on fibres. They employ two-photon polymerization, a 3D printing technique that focuses an infrared laser onto a UV-sensitive photoresist.
Two infrared photons will be absorbed simultaneously in the laser’s focal region, hardening the UV resistance. Moving the focus allows for the creation of various shapes with high precision. This method can be used to create miniaturised optics as well as novel functionalities such as free-form optics and complex lens systems.
“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,” said Angstenberger. “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 new study, the researchers used a Nanoscribe 3D printer to print lenses with a 0.25 mm diameter and an 80 micron height onto the end of a fibre with the same diameter using two-photon polymerization. This entailed creating an optical element using commercial software, inserting the fibre into the 3D printer, and printing the small structure on the end of the fibre. This process must be extremely precise in terms of aligning the printing to the fibre and printing accuracy.
Creating a hybrid laser
Following the completion of the printing, the researchers assembled the laser and laser cavity. Rather than using a crystal inside a bulky and expensive laser cavity made of mirrors, they used fibres to form part of the cavity, resulting in a hybrid fiber-crystal laser. The lenses printed on the ends of the fibres focus and collect light into and out of the laser crystal. The fibres were then glued into a mount to make the laser system more stable and less susceptible to air turbulence. The crystal and printed lenses were only 5 X 5 cm2.
The continuous recording of laser power over several hours confirmed that the printed optics inside the system did not deteriorate or affect the laser’s long-term properties. Scanning electron microscopy images of the optics after use in the laser cavity revealed no visible damage.
“We found that the printed optics were more stable than the commercial fiber Bragg grating we used, which ended up limiting our maximum power,” said Angstenberger.
The researchers are now working to improve the printed optics’ efficiency. Larger fibres with optimised freeform and aspherical lens designs, or a combination of lenses printed directly onto the fibre, may aid in increasing output power. They also want to show off different crystals in the laser, which could allow the output to be tailored to specific applications.
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