Building lenses layer-by-layer
Since its invention, 3D printing has been used to make an increasing range of objects, from prosthetic limbs to fully functioning weapons and vehicles. But what is of particular relevance to the optics industry is the emergence of additive manufacturing processes for making high-quality polymer optics that are light-weight, cheaper and easier to produce than their glass counterparts.
3D printing is also enabling the production of plastic optics that are more complex and much smaller in size – lenses less than 500µm in diameter can now be manufactured and, as a result, are becoming more commonplace in high-end applications such as LEDs, phone cameras, biometrics and, thanks to a project at the University of Stuttgart, soon even in endoscopes.
For most optical applications, glass is the preferred material because of its superior optical properties. However, optics manufacturers are limited in the sizes and shapes they can produce with glass. ‘Glass has very positive aspects for optics functionality. A disadvantage is that we cannot additively manufacture optics in glass directly, but only in photopolymers,’ said Dr Fabian Niesler, head of applications and processes at Nanoscribe. ‘At the scales and dimensions that we are working at, it’s very hard to produce optical components from glass at all,’ he said.
The main advantage of additive manufacturing is that it offers greater levels of design freedom and precision compared with traditional manufacturing processes. ‘3D printing… provides the benefit to produce very high quality polymer micro-optics directly,’ Niesler added. ‘With 3D printing, we have almost no restrictions on the shape of optics, because of the way they are produced. You can fabricate micro-optical components that cannot be fabricated with any other method.’
In addition, the technique can be used to make delicate or complex optical systems that would otherwise be challenging to produce and require multiple fabrication steps. For example, the same printer used to make a single lens can be used to fabricate complex multi-lens systems. ‘When you change something in the design, you don’t have to change the tool. You have the same printer, you have the same machine – you just load a different design, print it and you get out a different optical component from your system,’ Niesler explained.
The material properties of polymers, in combination with the design diversity offered by 3D printing technology, have led to plastic optics becoming the preferred option for certain applications. For example, the low cost of plastic optics makes them practical for single-use disposable medical devices, while the design versatility and precision of 3D printing mean plastic optics are ideal for biometric applications, such as retinal scanners. Plastic optics are also now a common component of smartphones.
Manufacturing micro-optics is a delicate process that cannot be performed by any 3D printer. Although the layer-by-layer building process is universal across all printer models, the way the polymers are processed sets them apart. ‘We [Nanoscribe] are using two-photon polymerisation as the main mechanism,’ explained Niesler. Two-photon polymerisation is a technique used to fabricate structures with resolutions down to 100nm. A femtosecond pulsed laser is focused on a photopolymer, where two photons combine at the focal point and excite the polymer molecules to a higher electron state. The resolution offered by this technique gives delicate micro structures, making it ideal for micro-optics fabrication.
Nanoscribe has also collaborated with the University of Stuttgart, providing equipment and support to researchers at the University’s 4th Physics Institute. The researchers used another 3D printing technique – known as femtosecond laser writing – to fabricate integrated optical elements at a sub-micron scale. Femtosecond laser writing uses very short pulses of light to harden a light-sensitive material selectively. Any unhardened material can then be washed away, revealing the 3D structure that has been created.
With the technique, the researchers developed phaseplates, polarisers, singlet lenses, doublet lenses and triplet objectives. According to Professor Harald Giessen from the 4th Physics Institute, and leader of the research group there, 3D printing is the only way to fabricate such components of this size. ‘There simply aren’t standard methods to produce freeform surfaces with sub-lambda accuracy on a 100µm diameter scale,’ said Giessen. ‘Especially when it comes to doublet and triplet lenses that require an undercut or mounting and alignment; there is no alternative to our method.’
Nanoscribe has recently become more involved in the research conducted at the University of Stuttgart. ‘At the beginning of July we started collaborating with not only Stuttgart but also with an industry partner who is in the endoscope business,’ said Niesler. This is a government funded project called ‘Print Optics’ that aims to demonstrate prototype micro-optics for endoscopes. ‘The goal here is to see how far 3D-printed micro-optics can be driven for an industrial application,’ Niesler continued. The collaboration will develop the two-photon polymerisation process further to produce micro-optics for equipment such as endoscopes. ‘This is really exciting because essentially this focuses on bringing results from academia to industry,’ Niesler said.
Despite 3D printing being around for more than 30 years, the technology is still improving, with greater complexity in the structures and higher printing efficiencies. ‘In recent years, we [Nanoscribe] have improved our product with respect to printing speed and the size of pieces that can be printed – towards larger pieces,’ explained Niesler. ‘At the beginning it was really small pieces, tiny photonic crystals, that demonstrated the superior resolution of the print process. But only this resolution, combined with larger volumes, triggered the industry to see the possibilities of high resolution 3D printing.’ High resolutions are required for producing micro-optics, with precise, smooth surfaces. ‘I think we are on the right track because of the resonance we get from industry,’ Niesler said.
According to Niesler, the optics industry is yet to tap into the full potential of 3D printing. ‘I would say that first of all we are only at the beginning,’ he explained. ‘The challenge for people is to completely understand the possibilities that you have with access to a tool such as 3D printing; you have to re-think your design ideas, how you fabricate these ideas.’ In the short term, the materials, in particular the photopolymers, have to improve. ‘Not only do there need to be more materials compatible with 3D printing, but also materials that provide the optical properties necessary for optical applications,’ he continued.
Giessen from the Stuttgart institute agrees with this, stating that, currently, there is potential for improving the surface roughness, as well as the refractive index homogeneity and losses of the materials. ‘Also, we have not yet produced an achromatic system, as this requires a combination of two or more materials with different dispersion,’ he added. These particular lenses are designed to reduce the effects of chromatic and spherical aberration.
Straight out of the box
3D printing is now at a stage where it can produce ready-to-use lenses that don’t need any further processing. Luxexcel, a 3D printing service provider, specialises in this particular area through manufacturing products that demand a high level of transparency without the need for further modification after production. ‘Other people have solutions where they need post-processing, so they make a lens and still need to polish it, which kind of defeats the purpose,’ claimed Guido Groet, chief commercial officer at Luxexcel. ‘We think that we’re the only [company] able to print a lens that is ready to be used straight out of the printer; it might need some coatings, but it doesn’t need any polishing or any grinding.’
The technology used by Luxexcel in particular has seen advances in recent years. ‘We have been developing the technology for six years,’ said Groet. ‘Initially it was not useable, and today we make products that people actually use in consumer applications in small volumes.’ Back in July 2014, when Electro Optics interviewed one of the company’s founders, Richard van de Vrie, Luxexcel’s 3D printed lenses were not of imaging quality, but were suitable for illumination applications. Since then, with advances in additive manufacturing, the company has developed a full line of prototypes, manufacturing optics for LED lighting, photonics, medical, aerospace, and automotive applications, all usable from the printer.
Prototyping is one of the main benefits of 3D printing. The technology enables quick turnaround times and, in terms of optics, eliminates the need to cast multiple lenses months before a prototype product – an endoscope, for example – is due to be made. According to Groet, 3D printing can be used to produce multiple prototypes in a number of days. ‘Some 3D printers take three days to make a chair; we can print 200 lenses an hour,’ he said.
In recent years, 3D printing has matured to a point where it is used for manufacturing. According to Groet, however, it isn’t a direct threat to companies that operate using large-scale production lines. ‘3D printing is not trying to replace general manufacturing,’ he stated. It’s so much cheaper to have a plant in China that makes you a million lenses, and if you want a million lenses you go to this plant in China.’ Instead, 3D printing is useful in applications where the ability to customise, adjust and experiment is needed. If a manufacturer is unsure of the design of a product, or how many parts they are going to need, then they should use 3D printing, Groet continued.
The other advantage of additive manufacturing is that it enables different materials to be integrated into a lens during production. This means products such as gradient refractive index change (GRIN) lenses can be produced. ‘These look like a flat block but they have the function of a curved lens, which you can do by using different materials at the same time,’ explained Groet. ‘You can also integrate certain functionality within a lens; for example, you can include a sensor in the lens itself to verify the light that goes through.’ This is not something that can be done with traditional manufacturing technology, because it isn’t possible to put something inside the cast when a lens is made. ‘Whereas, we can print something with a sensor in, or a filter, and then keep printing,’ he said. According to Groet, this aspect of 3D printing will certainly be developed over the coming years.
Approximately 70 per cent of the lenses in the world are plastic lenses, according to Groet. To be able to gain further traction in this market, Luxexcel is looking to increase the range of products it is able to offer by expanding the varieties of polymer it uses for its optics. ‘We use an acrylic material that we currently have one refractive index for today; however, we are developing and will introduce other refractive indexes to the market,’ he said. ‘Materials are in constant development to provide better heat, light and UV resistance, along with better compatibility with [optical] coatings.’ Groet explained that improvements in the material are constantly being made, working towards perfect accuracy and transparency. ‘These [properties] are still being tuned and matured every day. We [Luxexcel] print some test lenses every month, and every month I see them improving,’ he said.