Regular arrangement of doublet lens directly fabricated on a CMOS image sensor.

Researchers at the University of Stuttgart in Germany have used ultra-short laser pulses, in combination with optical photoresist, to create optical lenses barely larger than a human hair.

The femtosecond laser, with pulse durations smaller than 100 femtoseconds, is being focused in a microscope into liquid photoresist which rests on a glass substrate or an optical fiber. Two photons of the red laser beam with a wavelength of 785nm are being absorbed simultaneously in the focus and expose the photoresist. This crosslinks the polymer and hardens it. The laser beam is directed with a scanner or by moving the substrate. After exposure, the unexposed photoresist is washed away with a solvent. Only the hardened transparent polymer remains and forms the optical element.

Using this method, optical free-form surfaces can be created with sub-micrometer accuracy.

The precision of the 3D laser writing allows for construction of common spherical lenses as well as more ideal surfaces – such as paraboloids or aspheres of higher order. Optical systems with two or more lenses can be realized for the first time with this method, opening the door to aberration correction and micro-optical imaging systems with precision quality.

Ph.D. student Timo Gissibl in the group of Prof. Harald Giessen at the 4th Physics Institute printed micro-objectives with a diameter and height of 125µm, even on optical fibers, opening the path for construction of novel and extremely small endoscopes suited for small body openings or inspecting machine parts. The optical design was realized in the Stuttgart Research Center for Photonics Engineering by Ph.D. student Simon Thiele from the group of Prof. Alois Herkommer at the Institute of Technical Optics in Stuttgart.

Image of a multi-lens system with a diameter of 600µm next to a doublet lens with a diameter of 120µm.

Gissibl also printed optical free-form surfaces and miniature objectives directly onto complementary metal-oxide semiconductor (CMOS) image chips, creating an extremely compact sensor. Using such optics, small cameras for drones that are no larger than a bee are possible as well as small sensors for autonomous cars, robots, and body sensors; and surround-cameras for cell phones.

The researchers were also able to combine their optics with illumination systems. The optics on an LED concentrate light in a narrow direction. Illumination of ring-shaped areas, triangles, or elongated rectangles can be obtained with miniature, free-form optics.

The Stuttgart researchers believe that 3D printing of optics will open a new era of optics manufacturing.

“The time from the idea, the optics design, a CAD model, to the finished, 3D printed micro-objectives is going to be less than a day,” Giessen states. “We are going to open potentials just like computer-aided design and computer-integrated manufacturing did in mechanical engineering a few years ago.”

The project was supported by Baden-Württemberg Stiftung within the project Spitzenforschung. The Karlsruhe startup Nanoscribe, founded by researchers from The Karlsruhe Institute of Technology, constructed the highly precise femtosecond 3D printer. Carl Zeiss AG in Oberkochen regularly gives the team advice. And some of the world’s leading endoscopy manufacturers can also be found in the state of Baden-Württemberg.

For additional information contact Prof. Dr. Harald Giessen, giessen@physik.uni-stuttgart.de or 0711.6856.5111.