German researchers have developed a 3D printer capable of producing micron-scale optics with high performance and reproducibility. The method, known as femtosecond laser writing, can be used to create almost any type of integrated optical element on a micron or smaller scale, which could help miniaturise optical instruments for numerous applications.
The 3D printer was created by Nanoscribe, a spin-off from the Karlsruhe Institute of Technology, and researchers from the University of Stuttgart. As reported in journal Optica, the team demonstrated direct fabrication of optical elements as small as 4.4 microns onto the exact centre of an optical fibre only 125 microns in diameter. It was shown that the performance of the optical elements closely matched simulations.
Femtosecond laser writing uses a laser that emits very short pulses of light to selectively harden a light-sensitive material. The material hardens only in the small 3D area where the laser light is focused, and any unhardened material can then be washed away, revealing the 3D structure that has been created.
Although many labs around the world have made their own 3D laser writing systems, these home-made systems are sensitive to environmental conditions and variances in laser power and thus can’t reliably create high quality micro-optics. To overcome these problems, the researchers used a commercially available two-photon 3D laser lithography system designed to write nanometre-sized structures.
‘Although femtosecond laser writing has been demonstrated in the lab, we have shown that it can be used to make high performance micro-optics in a manner that is highly repeatable and reliable,’ said Harald Giessen, chair for ultrafast nano-optics, University of Stuttgart, who led the research team. ‘We believe our approach can be scaled up for volume manufacturing and used to directly print almost any type of optical element on a tiny scale, opening up a new era of integrated micro- and nano-optics.’
The researchers used the 3D laser writing system to create phase masks, optical elements which offer a way to shape the light coming out of the end of a fibre as opposed to using large, bulky lenses.
Normally, light leaving the end of a fibre is Gaussian, or bright in the middle and dims towards the edges. The researchers created phase masks directly on the end of the optical fibre that shaped the light into either a flat-top profile that is equally bright across the illuminated area or a doughnut-shape that features an empty middle surrounded by a bright ring of light.
‘Because the phase masks are so small and are created directly on the end of the optical fibre, even tiny errors in the centring of the phase mask would cause the doughnut-shaped beam to not look nice and round,’ explained Giessen. ‘We solved one of the hardest problems: repeatedly placing a phase mask with submicron accuracy directly in the centre of the single-mode fibre.’
The direct laser writing approach can be used to create optical elements in numerous ways. The researchers found that creating phase masks ring-by-ring beginning at the centre or layer-by-layer starting from the bottom, both produced high quality structures.
‘Depending on how you write, you might have different optical properties, and also the velocity at which you can write and how fast you can manufacture an optical component depends strongly on your write mode,’ said Timo Gissibl, University of Stuttgart, the paper’s first author.
The phase masks created by the researchers have many potential applications. For example, by replacing large optical lenses, the top-hat phase masks could enable illumination in much smaller endoscopes. In addition, an optical fibre with the phase mask that produces doughnut-shaped light could be placed directly in a liquid and used for optical trapping of particles or cells, for example. Also, photoacoustic endoscopy could benefit from the very compact hollow beam delivery, said Lihong Wang, the inventor of this technology.
The research team is now conducting experiments to see if 3D laser writing can be used to create tiny phase masks that shape an optical fibre’s output into twisted light, which is of great interest for photon entanglement and other applications.
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