Resources | Tech-Talks Bregenz | Research | Christoph Wächter | Micro-Optics | Aug 19, 2020

TechTalks BREGENZ - Dr. Christoph Wächter, Senior Scientist at Fraunhofer IOF, Germany

Research topics have become more important with LEDs and LED lighting micro optics and freeform optics. A new approach based on an IP for a fly's eye condenser from 1928, filed by Zeiss Ikon AG that extends possibilities and allows for new applications was presented ‚at LpS 2019. The jury was so impressed with the paper that it won the Scientific Award 2019. Dr. Guenther Sejkora and Arno Grabher-Meyer from LED professional talked to Dr. Christoph Wächer, co-author and presenter of the paper about his approach, his career and work. Dr. Christoph Wächter is a senior scientist at Fraunhofer IOF.

LED professional: First off, we'd like to congratulate you and Peter Schreiber on winning the LpS 2019 Scientific Award.

Christoph Wächter: Thank you very much.

LED professional: What did you think when you heard that you won?

Christoph Wächter: We weren't expecting it at all, so we were very surprised. It was a great feeling – a great honor. What was presented in the paper was an irregular fly's eye condenser and now we have the means to extend the field of applications that we can address with those micro-optics for illumination and potentially for imaging tasks. So, it was an honor and a surprise and it encourages us to go further in this direction. A great day!

LED professional: The award is mainly for the authors, but it is also meant for the whole team. I believe you worked with a big team, so do you have an idea of how you're going to celebrate with them?

Christoph Wächter: I imagine that we'll go into the lab and show them the award and the certificate and tell them, "We did it!" Peter said yesterday that we'd get a big tent and celebrate together. And you're right – it was a very big team that contributed to the contents of this paper – so many, that I can't list them all. There were people that contributed the central idea, did numerics, those that made sure the technology was feasible, someone made the ray tracing, I contributed parts for generating masks if needed, a group of technicians were responsible for the reflow master. Then there was the person responsible for the UV-LED-lithography and all the engineers that were responsible for all the modifications of the exposure regime, the design engineers, and so on and so on. There are just too many to name them individually.

LED professional: What impressed the jury was the entirety of the work you did, starting with the analysis of the state-of-the-art and then formulating the research question of what you wanted to achieve. You then went on with the theoretical calculations, the simulations, mastering, production of the demonstrators and finally the measurements.  It was a complete work, and one of the reasons the award went to your team. Today, it seems, that people only see one aspect of scientific work and you saw the entire work. Fraunhofer is well known but IOF isn't. Could you tell us what it is and what the main topics of IOF are?

Christoph Wächter: IOF is one of about 70 Fraunhofer institutes. Fraunhofer IOF, the Institute for Applied Optics and Precision Engineering was founded in 1992 – not long after the reunification of Germany. The idea was to keep knowledge and competences of colleagues from university, academy of sciences, Zeiss R&D and to group them together in Jena.  The founding consortium, under the guidance of Professor Karthe, spoke with Fraunhofer in Munich, and they decided to start it as an institution, and not an institute, giving us five years to get our finances in accordance with the Fraunhofer rules – to earn our living by carrying out projects with not much basic funding. Three years later we were able to change our standing from institution to institute, and since then we have been constantly and rapidly expanding. We started with sixty people and now we are two hundred and fifty. Our facilities were expanded in 2002 and recently we had to enlarge them again.

We are active in quite a few different fields, and as the name says: applied optics and precision engineering, it is really widespread. But it offers the chance, on the one hand, to look at micro-optics, and on the other hand, at complex assemblies which may contain micro-optics or could be larger telescope elements. So the range we are looking at meanwhile expands from space applications down to micro-optic or nano-optic elements that are used in telecom or for sensors. Other applications for microlenses are cameras or miniaturized microscopes. It's a very broad range that is backed up by a lot of different technologies. So we have technologies for a nano-fabrication; we have a very specific tool for electron beam lithography; we have precision manufacturing: single point diamond turning, a construction department that is able to make lightweight assemblies, we are active in the field of 3D structure generation by metal printing or printing polymeric materials. The range of technologies we have in house is really wide; I just can't name all of them here.

UV-LED-Lithography-tool for master generation. © Fraunhofer IOFUV-LED-Lithography-tool for master generation (© Fraunhofer IOF)

LED professional: I wasn't aware of the fact that Fraunhofer manufactures a lot of different optics. I know Fraunhofer more as an institute that does research. How much manufacturing do you do?  

Christoph Wächter: The usage of English words can be misleading in this case. When I say manufacture, it usually means the fabrication of demonstrators. We develop a technology, we develop the means to fabricate micro-optics but we aren't capable of doing any type of mass production. And, as a matter of fact, Fraunhofer is eligible for basic funding and to produce commercially would be in contradiction to that. It wouldn't be compatible with any competition in the worlds of R&D and technology development. But in certain cases we develop machines or technologies running on machines – either machines developed by us, or machines being available on the market – and bring those machines or technologies to maturity. In consequence we transfer the process or the machine plus technology to customers. If they are interested in entering a new field of technology and we can provide appropriate technologies developed by Fraunhofer, we establish a spin-off.  

Many years ago we made a lot of effort in the field of graded index lenses. When we started in the 90's we had some activities running in the field of telecommunication. And with ion exchange you can make nicely graded index waveguides buried beneath the surface. You can use them for splitters, for arrayed waveguide gratings and the like. The technology was mature at the end of the '90's. Further basic research work or basic technological development was not seen. It worked and we were able to think about making a spin-off company. The company makes graded index rod lenses, some wave-guide components if necessary, and it's really nice. A few weeks ago I had a call from a former customer of ours asking if he could have the same components that he had in 1994. After looking up what we had done for him back then, I called him back and told him we were not able to produce the same components anymore but our spin-off company could do it. This is an example of long-lasting success stories with spin-offs.   

We have spin-offs in the field of layer deposition, e.g. for interference layer systems in the extreme UV at 13 nanometers for use in microelectronics production tools. Also for this technology we did the development in house.

We do demonstrators; we develop technologies and do some production in case nobody else can do it. In the field of applied micro-optics we have a transfer directly to the industry with equipment that's available on the market. With our special knowledge we help them to bring those technologies to mass production on their own premises.

LED professional: About twenty years ago I saw the first optical microstructures at the research fair. At that time, practically nobody knew what micro-optics was. Today they are much more important. In the final selection for the LpS Scientific Paper Award, four of the nine papers dealt with optical microstructures. Do you think that the increasing importance of optical microstructures is because of the technology we have today to generate microstructures or is it really an advantage for certain applications?

Christoph Waechter: They definitely go hand in hand. We have the capabilities to produce micro-optical structures, and we have possibilities with micro-optical structures, which would rarely work with bulk optics. A simple example of what we were talking about yesterday in the lecture: If I'm going to shape light distributions – first to collimate, then to make it efficient, then to look for higher brightness of the sources. You can scale it either on 3D, but then it gets bulky. Or you can have enlarged power by only enlarging it in 2 dimensions with some kind of micro-optics plus fly's eyes condensers. New possibilities are based on what we have from, basically, the semiconductor equipment industry, lithographic tools, mask aligners for replication etc. Nowadays we also have nicely evolving tools for 3D printing in qualities that are required for optical applications, and of course, the possibilities arise with the design capabilities, too. If you do micro- or nanostructures you have to have in mind that it isn't just standard, simple ray tracing that you can apply. You have to have more tools. You have to mind diffractive effects; you must be capable of using wave optical and numerical design methods, methods to combine them and to look at the structures as a whole. If you are capable of this it's a nice evolving thing. The thing that optics could profit from, especially in the field of waveguide optics, are developments that basically stem from the semiconductor industry. Getting more precision in the preparation of tiny structures can be used nicely nowadays for micro- and nano-optics as well. With the e-beam lithography we have in the house we can lamellar slices and gratings with 100 or 150 nanometers structure size and suitable depths. If we didn't have the large development field from the semiconductors, optics would not have had the force to drive it. Fortunately, though, we have the possibilities, and people are more and more aware of the fact that with the new principles they can get better and cheaper components, at least on a mass production scale, and they can get new features for the optics.

Segmented high-beam (© Fraunhofer IOF)Segmented high-beam (© Fraunhofer IOF)

LED professional: When I look at the other papers that describe microstructure optics, the authors take a very direct approach using the micro-optics as a secondary lens and just calculating for the individual beam that they want going in a certain direction then mainly calculating the angle of the surface and then forming a complete optic. You use your irregular fly's eye condenser differently, using the micro-optics as tertiary optics. Could you tell our readers what you see as the advantages for your approach?

Christoph Waechter: Basically, I guess the approaches are not that different. Not all authors use the same nomenclature naming primary, secondary and tertiary optics. Of course there are different approaches, especially as we saw yesterday in a lecture, with diffractive elements we usually don't use collimation in between. You aren't really forced to have collimation in between but you may have it. What we intended to have was a clear development after what we had before. In fact the paper or presentation we made was a kind of historical review.

When we first received higher power light emitting diodes directly from Osram in the late 1990's, they told us not to waste the photons. We had small sources with almost Lambertian characteristics. So, how could we get the light out of it: potentially with some reflectors in the ceramics or potentially the dome (which isn't the best version for all cases). Then we had to collimate the light and there we saw that it wasn't that straightforward because it wasn't really a point source and we had some dimensions that we had to adapt, like the sizes of the collimator, etc. Finally it was collimated but not homogenous. So then we had to figure out how to homogenize it. At that point we went back to what had been invented around 1935 in Jena: the fly's eyes condenser, and we were happy with that. Before that we had developments in the field of waveguide optics. Waveguides were made from polymers, and starting from that technology we were able to provide micro lenses with a reflow technique. So we tried the approach of fly's eyes condensers and this gave us a homogenized output of LEDs. In those times we had monochromatic LEDs so the next step was to add all the colors – RGB to make it white. At that time nobody had a white LED. We saw that we could easily homogenize it. Through good luck we saw that we could also use it for imaging as well. We turned it on and saw that more was possible. That's when we got the idea of the chromium mask we could use for structured illumination. One of the shortcomings was that we lost brightness and we weren't supposed to waste the photons. It was a long process and we realized that we could do more if we had more technological possibilities that would allow us to do more than reflow lenses. They are ideally spherical or slightly aspheric but there isn't much more one can do.

Then we came along with laser-lithography with a commercial machine. We learned about what is possible and what could be done better. We had a very clever young man who made a concept. So we developed the first machine for the UV-LED-lithography, which enables us to create particular variable freeform optics. If you define a freeform element it is usually a surface that is not rotational symmetric. And if you use a rectangular piece of a sphere that does not contain the center, it can be considered to be an aspheric element. It's not rotational symmetric – and it is usually hard to fabricate. Using UV-LED-lithography we could fabricate these types of structures and we were free to think about what we could use as the basic elements of the iFEC (irregular fly's eye condenser). As it was presented yesterday, it was an historical review showing how new possibilities and new ideas can expand your mind. If you want to get more into detail, of course it would require another lecture of more than an hour just to get a basic idea of how it works.

Replicated high-beam micro-optics (iFEC structure). © Fraunhofer IOFReplicated high-beam micro-optics (iFEC structure). © Fraunhofer IOF

LED professional: In your presentation you mentioned the BMW Welcome Light. And as the demonstrators you showed the low beam and high beam headlights. We know that the Welcome Light has already been copied by Chinese manufacturers, so is there a specialty in the headlight that can't be copied easily?

Christoph Waechter: First of all, for Fraunhofer it's clear that if you have new inventions, you have the IP as well. If something like the BMW Welcome Light Carpet is copied abroad and brought to market abroad, we can't do that much. On the other hand, there are also Chinese companies coming to us, asking for licenses. So in regards to the headlight, the principle approach was as usual to first secure our IP and then to talk to people about what we could do further and how it would work in future applications.

LED professional: Protection can be taken in different steps. On the one hand, you can protect the idea and on the other hand, it might be manufacturing is more complex and harder to copy and that has to be protected.

Christoph Waechter: Firstly, we protect our ideas as a basic approach. And here it is reasonably easy to detect it if they have been copied.  Secondly, we know how complicated it is to start production and low quality copies wouldn't have very much acceptance on the market. Production tools are not explicitly specific for our application, so we count on the fact that we have some advantage with our technology.

On the other hand, customers will sometimes ask Fraunhofer if we can transfer the technology or equipment over to them as well, which can be the basis for a fruitful cooperation.

Our basic tasks are coming up with new ideas and then developing new technological tools. It's R&D work, so we aren't too worried about IP processes. Of course if something is really successful, IP brings income to Fraunhofer. This income can be spent on new projects in new fields and thus is most welcome.
But of course here I speak for myself. I'm not worried that much – knowing that there is the legal department at the headquarters in Munich.

LED professional: Micro-optics has a lot of advantages – especially the size. According to one of the lectures, yesterday, there are also disadvantages like the stray light generated at the steps between the structure elements. Do you have the same problems with your approach?

Christoph Waechter: Stray light is a severe problem. If we have a smooth lens profile from reflow – if you work with the iFEC you can have lens profiles that have steps in the height – usually we have the next neighbor directly adjacent to it. But to produce sharp vertical edges is really challenging. To get a 90° edge is almost impossible because if you make a UV polymerization there you have some diffusion and some minor divergence as well. You'll only be able to make about 85°.  Light coming on these steps causes stray light. And this is generally a problem.

We can suppress stray light to some order with clever designs and clever technology. If we do the iFEC design process we can formulate boundary conditions and minimize jumps. Developing the high beam for the headlight we did two attempts. For the first attempt we did not use the jump minimization because it seemed to be easier for the definition of the master and the production process.  But we saw a lot of stray light where we were more or less sure that it was caused by the steps. So we made the next attempt. It was a learning by doing process. We don't like a chromium mask beneath the structure for stray light suppression. But if we have an area that is prone to stray light then we can reduce this with some minor stripes of chromium.  

Here it doesn't really matter if we lose some efficiency by the chromium. If we shaped it accordingly we would lose about the same as we miss due to the stray light, about 1.5% in total.

Stray light always has an impact on optical functionality. But with a clever combination of design and technology one can avoid such unwanted effects.

LED professional: I have two questions about that: The problem starts with an incomplete collimation, is that right? The second question is: Can you create the same effect with a gradient optical refractive index instead of geometric structures?

Christoph Waechter: In answer to your first question: Yes. This is the basis of the fly's eyes condenser. In practice, the input to the fly's eyes condenser is a bundle with limited divergence. A perfect collimation would require complete parallelism, i.e. due to etendue-conservation it would require to have very large diameters. The acceptance angle for such fly's eyes condensers depends on the distance between the lens arrays, the focal length, and the lens height. From this we get the numerical aperture – what is accepted and what is leaving the exit lens array of the fly's eyes condenser regularly. And of course, if we have some light that isn't sufficiently collimated we have the problem with stray light generated already at the entrance lens array.

The second question: shaping the refractive index in the dependence of the local coordinates of the surface is a very nice idea but is hard to achieve. Usually you have one and the same material and you make the refraction due to the bended surface. Of course you can do refraction by using a locally varying refractive index but how can it be generated? Using slightly different materials at different positions we dealt with before at Fraunhofer with the graded index media fabricated by ion exchange. But with this technology we cannot achieve the same effects as with different surface radii as we use, for instance, for the microlenses.

What we can do with gradient index technology are other things. It's suited for gradient index rod lenses or buried waveguides or something like that, but we wouldn't have any idea how to transfer it to generate iFEC structures.  

Furthermore, to realize a locally varying effective index we need lithographic structure for each sample, which is not that cost effective.

Functional test of low beam characteristics (© Fraunhofer IOF)Functional test of low beam characteristics (© Fraunhofer IOF)

LED professional: In your lecture you talked about automotive applications of the iFEC technology. Do you already have ideas or are you even transferring this technology into general lighting?

Christoph Waechter: This was one of the basic reasons that we came to the LpS. We just finished a spotlight project, which was funded by Fraunhofer internally and we had good results.  Now we are looking for follow-up projects together with industrial partners and we want to show what we could achieve. Of course, when we started the project we had a lot of different applications in mind, ranging from shop lighting to individual lighting, general lighting or advertising, even applications in the field of health – like luminaires for operating rooms. Now we can show potential customers what is possible and also tell them that we are willing to serve real world applications where our ideas can be used fruitfully.

LED professional: Yesterday I saw that you had a good discussion with a potential customer about all the options.

Christoph Waechter: That's for sure! We have good contacts and projects within the field of the automotive industry. But we are here at the conference to make other contacts in other industries as well. The Welcome Light Carpet is a fancy product. Other carmakers could make something similar. But we feel like we could do more in other application fields. So a conference like this one is just the location for the right audience. We get to know other people with other ideas that can be adapted to other applications. It's also great to be able to talk to other lecturers or individuals at their booths so we can see what they can do and vice versa.
We hope to be able to expand our field of applications by far.

A number of members of the iFEC-team (© Fraunhofer IOF)A number of members of the iFEC-team (© Fraunhofer IOF)

LED professional: In that case we can only hope that the Scientific Award will help you to become more well-known and bring you into new fields of application and new projects. Good Luck!

Christoph Waechter: Thank you!

Dr. Christoph Wächter
Dr. Christoph Wächter received his degree in physics and the title of Dr.rer.nat in Theoretical Physics in 1981 and 1987, respectively, from the Friedrich-Schiller-University in Jena. He has been active in the field of photonics since 1983 when he joined the Physics Dept. at Jena University and started his research activities in the area of mulitlayer and waveguide optics, with particular emphasis on non-linear guided wave phenomena. In 1992 he joined the micro-optics division of the Fraunhofer Institute for Applied Optics and Precision Engineering, Jena, where he is concerned with the design of integrated optical micro-optical devices.

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