3D printing primes fiber ends so different wavelengths focus precisely

26 Oct 2022

Researchers from Germany, Australia, and UK cut aberration to create “razor-sharp images”.

An interdisciplinary team of researchers from Korea, Australia, Great Britain, and Germany, with the participation of Leibniz Institute of Photonic Technology (Leibniz IPHT), has optimized an optical glass fiber such that light of different wavelengths can be focused extremely precisely. The accuracy is achieved by 3D nanoprinting of an optical lens, which is applied to the end of the fiber.

The team says that this achievement “opens up new possibilities for applications in microscopy and endoscopy as well as in laser therapy and sensor technology”. The work is described in Nature Communications.

Lenses at the end faces of optical fibers currently used in endoscopy for medical diagnostics have the disadvantage of chromatic aberration. This imaging error of optics, caused by the fact that light of different wavelengths is shaped and refracted differently, leading to a shift in the focal point and thus to blurring in imaging over a wide range of wavelengths. Achromatic lenses, which can minimize these optical aberrations, provide a remedy.

Metalens for precise focus

Such an achromatic lens, a so-called meta lens, which is attached to the end of an optical fiber and allows focusing and imaging of minute details with depth-of-field imaging, has now been realized for the first time by an international team.

“For ideal light shaping and achromatic focusing, we realized an ultrathin polymer-based lens, which consists of a complex design of geometric structures in the form of nanopillars,” said Prof. Dr. Markus Schmidt, head of the Fiber Photonics Department at Leibniz IPHT, who co-developed the optical lens.

“This structure was printed directly on the tip of a 3D-printed hollow tower structure on one of the end faces of a commercial optical fiber. In this way, optical fibers can be functionalized in such a way that light can be focused very efficiently on a focal point and images with high resolution can be generated,” Dr. Schmidt said.

The meta lens realized by the researchers has a lens diameter of 100 µm and a numerical aperture of 0.2, significantly higher than in previously used achromatic lenses, on the end of fiber end faces, and thus achieves better resolution. The lens allows optical aberrations to be corrected and light in a spectral bandwidth of 400 nm in the infrared range to be focused precisely.

Dr. Schmidt added, “It is remarkable that the individual nanopillars have different heights ranging from 8.5 µm to 13.5 µm. This allows the different wavelengths of light to be focused on a single focal point.”

In experiments, the researchers have demonstrated the lens and focusing efficiency of the new fiber optics using fiber-based confocal scanning imaging as an example. Using a fiber with achromatic meta-optics, they achieved “convincing image quality with high image acquisition efficiency and high image contrast at different wavelengths”. The focus positions remained almost constant even at different wavelengths.

Optimal light shaping

Dr. Schmidt added, “Since our nanostructured meta lens is extremely small and flat, a fiber optic design with achromatic optics at the top offers the potential to further advance miniaturized and flexible endoscopic imaging systems based on fiber technology and to enable even more gentle minimally invasive examinations.”

In addition to this main area of application, the researchers see further areas of application in the field of laser-assisted therapy and surgery, in fiber-optic communication and fiber sensor technology.