3D Printing Creates New Optical Possibilities

HANK HOGAN, CONTRIBUTING EDITOR

Emerging 3D printing technology is transforming once-impossible designs of optical components into optimized elements that can improve medical instruments, research tools, communications systems, and consumer devices.

In 3D printing, an additive process prints an entire optical component one tiny particle at a time, enabling designs that combine multiple functions in a single lens or pre-align a system by creating optics, mounts, and baffles together. The new optical designs can reduce a product’s footprint, improve technical performance, and facilitate commercial scale-up.

Australian and German researchers recently 3D-printed a single 330-μmdiameter lens, for example, that simultaneously met high- and low-numerical aperture requirements with a lens-in-lens design¹. Impossible to fabricate conventionally, the lens enabled both fluorescence and optical coherence tomography with a 0.52-mm-diameter probe. In their paper, the researchers reported a better than 10× improvement in fluorescence contrast compared to a more traditional fiber-optic design.

“3D printing actually shines in high miniaturization, impossible parts that you cannot do with other methods and also has very nice alignment possibilities,” said Simon Thiele, CTO at 3Dprinting company Printoptix and one of the authors of the paper.

Being able to print an optic along with its mount and other elements saves time and minimizes installation errors. Plus, being able to adjust an optic’s shape and properties on a small scale allows one lens to do the work of several. Medical applications, cellphones, and soldiers in the field could benefit. 3D printing can also reduce the amount of time needed to prototype an optical design from months to days, a significant time savings.

But 3D printing of optical components also faces hurdles, such as limited availability of optically relevant materials, slow manufacturing speed, and high cost, especially for large volumes.

Additive processing options for optics

Industry and researchers are addressing these issues with an array of 3D-printing techniques. Professor Tomasz Tkaczyk of Rice University and his colleagues reviewed the capabilities and performance levels of fused deposition modeling, inkjet printing, stereolithography, and two-photon polymerization².

An integrated 3D-printed 1.1-mm-diameter lens system with a distortion-free 90° field of view for microcamera applications. (Match head used for size comparison.) Made on an optimized Nanoscribe two-photon polymerization (2PP) 3D printer. Courtesy of Printoptix GmbH.

Fused deposition modeling heats a material, extrudes it through a nozzle, and deposits it layer by layer. The researchers reported that the method creates the largest volume structures in the shortest time, but it does so with a feature size of a few hundred nanometers and a surface roughness of about 1000 nm.

In comparison, molded optics have a roughness of about 10 nm. Polished optics are even smoother, with a surface roughness of about 1 nm. Surface roughness and feature size are important because they determine optical performance. They need to be comparable to or below the wavelength of interest for feature size, and well below it for roughness.

Two-photon polymerization (2PP) uses a pulsed laser to induce a multiphoton process that occurs only in a small focal region, and the photons alter the material. Moving the focal point in three dimensions creates structures, layer by layer. 2PP printed the finest features, the researchers said, resulting in feature sizes of <0.1 μm and a surface roughness of 10 nm. However, the technique was the slowest to make a large-volume object. Inkjet printing (which deposits ink drops for 3D printing) and stereolithography (which 3D-prints using photolithography) fell in between the other techniques in terms of speed, feature size, and surface roughness. Inkjet printing allowed for smaller and smoother features.

An image sensor that includes a complex lens system is 3D-printed directly onto an imaging sensor matrix. Courtesy of Andrea Toulouse/Nils Fahrbach.

Various technologies are in commercial production. Luxexcel uses inkjet printing followed by UV curing of a polymer to make 3D-printed lenses, for instance. The company won a 2022 Prism Award for a solution that builds smart eyewear with prescription lenses.

Multi-index elements

Startup NanoVox, which is transitioning its name to Vadient Optics, also uses inkjet 3D printing, but it does so with a twist — offering volumetric index of refraction gradient optics (VIRGO).

“We’re not just printing polymers as they are,” said James Field, the company’s vice president of business development. “We’re using high and low index of refraction monomers that are infused with nanoparticles that sculpt the light in the bulk [material] instead of having to do it with surface curvature.”

NanoVox achieves this optical performance by doping the ink with nanoparticles smaller than 10 nm. The company’s pilot production machine, a third-generation system, can have as many as eight printheads, each loaded with a different ink formulation. The machine adjusts ink dispensing to create optics with changes in index of refraction from one small volume to the next. The technology can fabricate a single flat lens that outperforms a combination of multiple curved ones.

The potential market for this technique is large, according to Field. Cameras in phones use a stack of eight or nine lenses, for instance, to correct for aberrations and distortions. Using NanoVox’s technology, the lens count could drop to four, which would save phone-makers money and possibly space.

NanoVox is making its own machines while developing the nanoparticle-infused inks. The company will be a service bureau, Field said. Thus, it will make products for others and not sell its own. Another 3D-printing startup is Printoptix, which uses 2PP systems from printmaker Nanoscribe. Printoptix is currently a service bureau that sells printing and prototyping services. It is transitioning to selling its own parts and eventually to licensing its processes, according to managing director Nils Fahrbach. He said the company’s optimizations accelerate optics processing.

This increase in throughput could be crucial for some applications. The distance between the eyes varies from person to person, for example, and Fahrbach said matching this spacing is critical for high AR/VR performance in smart glasses or goggles. 3D printing offers a way to achieve personalization quickly and, eventually, perhaps at no extra cost. “You just hit a button and the optics are printed individually to the customer at the same cost as any other optic,” he said of such on-the-spot customized AR/VR display components.

Sofía Rodríguez, product marketing manager for Nanoscribe, said the company’s latest 3D printer and polymers offer alignment accuracies down to 100 nm. This enables the printing of freeform optical elements directly onto optical fiber, fiber arrays, and photonic chips.

Expanding material availability

In June 2021, Nanoscribe announced the release of a photoresin of fused silica glass that the company developed with Glassomer. This material allows 3D printing of glass microstructures, giving users a wider range of materials.

Modified inkjet technology is enabling 3D-printed planar lens arrays, such as these 10-mm f/4 gradient index (GRIN) lenses. Current systems can deposit various monomer ink formulations, using up to eight printheads to fabricate flat lenses that meet or exceed the performance of curved optics. Courtesy of NanoVox.

Other researchers are also exploring 3D printing using glass. For example, Rongguang Liang, a professor at the University of Arizona, co-authored an Advanced Science paper this year that introduced a liquid silica resin used in 2PP printing³.

He said some polymers yellow with age, a consequence of being UV light curable. Polymers also absorb water, undergo large expansion and contraction with temperature changes, and are soft compared to glass.

“The optical properties are not as good as glass,” Liang said.

Using a custom-made fused silica resin and a process designed for it, he and his colleagues 3D-printed lenses, a lenslet array, gratings, and freeform optics. They demonstrated the resulting optics’ imaging performance using a standard resolution target, a University of Arizona logo, and biological samples.

Having a wide variety of materials is important for further advancements in 3D printing, Printoptix’s Fahrbach said. One of the strengths of 3D printing is the ability to align the optics, which is easy to achieve when printing optics, baffles, and a mount at the same time. However, if the optic is transmissive, then all of the components — including the baffles — are transparent. As a result, post-processing is necessary, which drives up cost and lowers the overall component throughput. Being able to 3D-print dissimilar materials could overcome this issue and expand the applications.

A lensed fiber array, 3D-printed using Nanoscribe’s 2PP technology, collimates green light into parallel beams. A sample from the research project MiLiQuant. Courtesy of Nanoscribe.

The varying refractive indices that are possible when using NanoVox’s process are one way to achieve this goal of printing different materials. Currently, the change in refractive index is only 0.15, but Field said work is underway to expand and perhaps double this range. He also said the company is working on a glass 3D-printing material and process, but they are not likely to be available for years.

Improving cost and cycle time

To accomplish widespread commercial success, 3D printing must also cost less and/or achieve a faster throughput. Nanoscribe is addressing this challenge, in part, by piloting alternative replication strategies. For example, 3D printing could be used to create an initial component that would then be reproduced in mass quantities using nanoimprint lithography and injection molding, Rodriguez said.

The company Addoptics uses a similar strategy. Marketing manager Meghan Pace said Addoptics 3D-prints a mold that is used to make the optics. Importantly, the components that come out of the mold require little to no postprocessing. This approach reduces the cycle time for a prototype from as much as 18 weeks to six days while also producing a mold suitable for volume production.

Inkjet printing, by its nature, can scale to higher volume production, said Jyrki Saarinen, a professor at the University of Eastern Finland. He and his team have demonstrated and are refining a multimaterial inkjet 3D-printing process for freeform optics.

A dozen planar Alvarez lenses are produced on a NanoVox pilot line with 1-μm drop precision. The line can print lenses measuring in size from 500 μm to 5 in. Scaling volume will require advancements in inkjet equipment, printing materials, and processes. Courtesy of NanoVox.

“The largest printers have tens of printheads attached next to each other,” Saarinen said. “Printing 1 sq cm or 1 sq m, one component versus hundreds of components side by side, takes the same time.”

NanoVox’s third-generation VIRGO pilot production system can churn out 15,000 eyeglass lenses or 300,000 mobile camera lenses per month. This is hundreds of times the throughput of the company’s research system but still small compared to the hundreds of millions of smartphones produced each month.

Further throughput gains require equipment, materials, and process changes. NanoVox’s Field said process throughput and product cost are the most pressing challenges confronting 3D printing of optics.

“They’re just trying to get to the point where they can manufacture it at a cost that makes sense to the buyer,” he said.

References

1. J. Li et al. (2022). 3D-printed micro lensin- lens for in vivo multimodal microendoscopy. Small, Vol. 18, p. 2107032, www.doi.org/10.1002/smll.202107032.

2. G. Berglund et al. (2022). Additive manufacturing for the development of optical/photonic systems and components. Optica, Vol. 9, No. 6, pp. 623-638, www.doi.org/10.1364/optica.451642.

3. Z. Hong et al. (2022). High-precision printing of complex glass imaging optics with precondensed liquid silica resin. Adv Sci, Vol. 9, p. 2105595, www.doi.org/10.1002/advs.202105595.