MESA+ University of Twente
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Imran Akca (promotion date: 5 December 2012)

Spectral-domain Optical Coherence Tomography on a Silicon Chip

Promotion date: December 5

Promotor: Prof.dr. Markus Pollnau

Assistant promotor: Dr. ir. René M. de Ridder

Optical coherence tomography (OCT) is a three-dimensional optical imaging method that offers non-invasive, close-to-histology-level image quality. Based on broadband spectral interferometry, OCT has enabled clinical applications ranging from ophthalmology to cardiology, that radically altered the view of in vivo medical diagnostics.

Current implementations are based on bulk or fiber optics which require accurate optical alignment and additionally increase their cost, size, and mechanical instability. To make OCT widely available these limitations have to be overcome by radically different approaches. Integrated optics can drastically reduce manufacturing costs, increase stability, and enables quasi maintenance-free OCT systems.

In this project an important step towards a cheap and small spectral-domain OCT system is performed by integrating its key components - a broadband spectrometer and a beam splitter - on a silicon chip. The spectrometer is formed by an arrayed-waveguide grating with a large free spectral range of 136 nm and a high wavelength resolution of 0.21 nm. The integrated beam splitter is realized by a non-uniform adiabatic coupler with a 3-dB splitting ratio that is almost constant over a large range of 150 nm. The overall device size is only 3.0×1.2×0.1 cm3, which is multiple orders of magnitude smaller than the bulky counterparts.

In vivo performance of the partially integrated OCT system is demonstrated in human skin with a penetration range of up to 1.4 mm, an axial resolution of 7.5 µm, and a signal-to-noise ratio of 74 dB close by the zero delay. Considering the high performance of this early OCT on-a-chip system and the anticipated improvements in this technology, a completely different range of devices and new fields of applications become feasible.

Was your thesis work application oriented?

Yes, it was. Integrating components of an optical coherence tomography system on a single chip is a very promising approach for making such bulky and expensive devices cheap, small, portable, quasi maintenance-free, and most importantly accessible to wider groups, notably in developing countries. This final goal - a portable and widely accessible tool - kept me motivated for four years.

We aimed at two applications; namely skin and retina imaging. The design parameters were defined by the requirements of these specific applications. The final device which has an on-chip spectrometer and a beam splitter were designed by taking into account the requirements. The parameters which define the OCT imaging quality were defined in order to reach the imaging quality of a commercial OCT system. For example in the latest measurements we achieved a resolution of 7.5 µm and a penetration depth of 1.4 mm which are comparable to the commercial OCT system performance.

Though applications are on the horizon, theoretical calculations and optical simulations were performed before designing each component. Especially in the coupler design stage I spent quite a long time for optimization of the device which necessitated extensive simulations and theoretical calculations.

Was there a special moment during your thesis project?

Due to some unexpected reasons we had to change the group that we collaborated with in the second year of my PhD. We were so lucky to initiate a collaboration with Prof. Wolfgang Drexler’s group (Medical University of Vienna, Austria) in a very short time. I will always remember this collaboration as the most productive and coherent collaboration I have ever had in my life. With the help of smart and experienced scientists, Boris Považay and Aneesh Alex, I was able to get some amazing images of human skin using the on-chip system that I developed. I will cherish the results and the good friends gained there until the end of my life.

Were your findings published in scientific magazines?

I had publications in Optics Letters, Optics Express, IEEE Journal of Selected Topics in Quantum Electronics, and IEEE Photonics Technology Letters. I am still working on a paper and hopefully I will get more results after my defense which could result in an additional publication as well. Also I attended several conferences in the US as well as in Europe as a speaker.

What are your future plans?

The beauty of discovery keeps me in academics, therefore continuing my career in science is my goal. Currently I am searching for a research group in the US as well as in Europe, and hopefully after having 2-3 years of post-doc experience, I will go back to Turkey in order to set up my own research group and accomplish my dream of combining biomedical science with optics, which is a missing research field in Turkey for the time being.

What is important for Mesa+ to stay successful in future?

Mesa+ has several good scientists, including my promoter René M. de Ridder. I learnt a great deal by the kind criticism he transferred. He also was very interested in my specific field of research, and I had fruitful discussions with him which helped me to improve my optical designs in a much better way.

Thanks to the wonderful Mesa+ technicians, I always felt secure with their kind help and advices. For the sake of Mesa+ future, they should be protected and treated very wellJ.

There is a great opportunity in Mesa+ to collaborate with several research groups in a very nice harmony. However to avoid probable problems, the PhD promoters should be very careful in choosing the right group and in case of problems they should step in if action is needed. The PhD’s are dependent on good external relations and don’t possess the power to overcome this kind of difficulties by themselves.