German researchers have shown that 3D printing can be used to make highly precise and complex miniature lenses with sizes of just a few microns. The micro­lenses can be used to correct color distor­tion during imaging, enabling small and lightweight cameras that can be designed for a variety of appli­cations. “The ability to 3D print complex micro-optics means that they can be fabri­cated directly onto many different surfaces such as the CCD or CMOS chips used in digital cameras,” said Michael Schmid, a member of the research team from Univer­sity of Stuttgart in Germany. “The micro­-optics can also be printed on the end of optical fibers to create very small medical endoscopes with excellent imaging quality.”

Schmid and colleagues used two-photon litho­graphy to create lenses that combine refrac­tive and diffractive surfaces. They also show that combining different materials can improve the optical performance of these lenses. “3D printing of micro-optics has improved drastically over the past few years and offers a design freedom not available from other methods,” said Schmid. “Our optimized approach for 3D printing complex micro-optics opens many possi­bilities for creating new and inno­vative optical designs that can benefit many research fields and appli­cations.”

Two-photon litho­graphy uses a focused laser beam to polymerize a liquid light-sensitive photo­resist. The two-photon absorption allows cubic micro­meter volumes of the photoresist to be polymerized, which enables fabri­cation of complex optical structures on the micron scale. The research team has been inves­tigating and optimizing micro­optics made with two-photon lithography for the past 10 years. “We noticed that color errors known as chromatic aberrations were present in some of the images created with our micro­-optics, so we set out to design 3D printed lenses with improved optical performance to reduce these errors,” said Schmid.

Chromatic aber­rations occur because the way that light refracts when it enters a lens depends on the color, or wavelength, of the light. This means that without correction, red light will focus to a different spot than blue light, for example, causing fringes or color seams to appear in images. The researchers designed miniature versions of lenses tradi­tionally used to correct for chromatic aberra­tions. They began with an achromatic lens, which combines a refractive and diffrac­tive component to limit the effects of chromatic aberration by bringing two wavelengths into focus on the same plane. The researchers used a commer­cially available two-photon litho­graphy instrument made by Nano­Scribe GmbH to add a diffrac­tive surface to a printed smooth refractive lens in one step.

They then took this a step further by designing an apochromatic lens by combining the refrac­tive-diffractive lens with another lens made from a different photoresist with different optical properties. Topping the two-material lens with the refractive-diffrac­tive surface reduces chromatic aberrations even more, thus improving imaging performance. The design was performed by Simon Thiele from the Institute of Technical Optics in Stuttgart, who recently spun out the company Print­Optics which gives customers access to the entire value chain from design over proto­typing to a series of micro­-optical systems.

To show that the new apo­chromatic lens could reduce chromatic aberration, the researchers measured the focal spot location for three wavelengths and compared them to a simple refractive lens with no color correction. While the reference lens with no chromatic correction showed focal spots separated by many microns, the apo­chromatic lenses exhibited focal spots that aligned within 1 micron. The researchers also used the lenses to acquire images. Images taken using the simple reference lens showed strong color seams. Although the 3D printed achromat reduced these dras­tically, only images taken with the apochromat completely eliminated the color seams.

“Our test results showed that the perfor­mance of 3D printed micro-optics can be improved and that two-photon litho­graphy can be used to combine refractive and diffractive surfaces as well as different photo resists,” said Schmid. The researchers point out that fabri­cation time will become faster in the future, which makes this approach more practical. It currently can take several hours to create one micro­-optical element, depending on size. As the technology continues to mature, the researchers are working to create new lens designs for different appli­cations. (Source: OSA)

Reference: M. Schmid et al.: 3D printed hybrid refractive/diffractive achromat and apochromat for the visible wavelength range, Opt. Lett. 46, 2485 (2021); DOI: 10.1364/OL.423196

Link: Stuttgart Research Center of Photonic Engineering SCoPE, University of Stuttgart, Stuttgart, Germany