Theses in the Working Group for Optical Technologies

We always offer exciting topics for Bachelor's and Master's theses. The research questions listed below are current examples and are, with appropriate adaptations, suitable for both Bachelor's and Master's degrees.

In our group the theses are integrated into current research, so that the specific task can change or be adapted according to interest.

Get in touch with us, either with the Ph.D. students or with Prof. Carsten Fallnich, so that we can discuss together what your collaboration in the Optical Technologies working group could look like. We look forward to seeing you!

In the research field of transverse modes, we specifically generate spatio(-temporally) structured light distributions in laser resonators. You will expand your knowledge of the fundamentals of photonics, particularly in regard to optical resonators, their modes and Gaussian beams. In addition to data evaluation, many experiments are also accompanied by numerical calculations or simulations in order to check the experimental results.

Your work will be supervised by Ph. D. students Jan Wichmann and Michael Zwilich. Possible Bachelor's and Master's theses are:

• Propagation of transverse eigenmodes in GRIN fibers

  In free space resonators modes of the Hermite Gaussian mode set are easy to excite. These can be converted into the Laguerre-Gauss mode set, which are also eigenmodes of GRIN fibers (GRIN=gradient index). The aim of this work is to investigate how efficiently this conversion and propagation works. This allows to combine the advantages of free beam resonators (simple generation) and fibers (simple beam transport and amplification).

• Selective transverse mode excitation through a deformable mirror

  Space telescopes use deformable mirrors to compensate for wavefront disturbances. Here we want to investigate whether this technique can be used to selectively excite transverse modes in laser resonators. By selectively deforming one resonator mirror, the self-replicating field distribution is influenced so that the beam profile of the laser output can be dynamically altered.

In the field of nonlinear microscopy, we are working on the improved excitation and detection of coherent Raman processes in order to suppress undesired background signals and noise influences and thus obtain more meaningful microscopy images. In addition to general knowledge of photonics, you will therefore also learn how to analyze and, if necessary, process images and prepare samples as part of your thesis. In the laboratory, the focus is not only on setting up your own experiment but also on operating existing microscopes. Knowledge of electronics and signal processing is also acquired, particularly for work on detection mechanisms.

The theses are supervised by Kristin Wallmeier and Nick Lemberger. Currently we offer the following topics for Bachelor's and Master's theses:

• Spatial beam shaping in coherent Raman scattering for imaging below the diffraction limit

  So-called "donut beams" have already been used in fluorescence microscopy to resolve structures below the diffraction limit. In this work, the generation of such beams using different mechanisms and their application for Raman imaging will be investigated.

• Imaging by using polarization-resolved Raman scattering to determine molecular symmetries

  Excitation beams with different polarization states can be used to distinguish between symmetries of molecular vibrational states or the orientation of molecules in anisotropic samples. In this work, the symmetry properties will be combined with imaging to obtain even more information about the sample using Raman microscopy.

In our group, we use integrated silicon nitride waveguides (Si₃N₄), tantalum pentoxide waveguides (Ta₂O₅) as well as photonic crystal fibers for nonlinear light conversion. We investigate how the waveguide geometry influences four-wave mixing process as well as supercontinuum generation. These integrated Si₃N₄ and Ta₂O₅ waveguides then find their application in chip based optical parametric oscillators that we develop in our group. In addition to our experimental activities, we also work on simulations, for example, the propagation of nonlinear pulses using the multi-mode nonlinear Schrödinger equation.

The theses will be supervised by Ph.D. students Maximilian Timmerkamp and Ming Gao. Currently, there are the following represantative topics for Bachelor and Master students:

• Investigation of four-wave-mixing in novel waveguide structures

  In waveguides the nonlinear gain provided by four-wave-mixing depends on the geometry of the waveguide, because it affects the modal dispersion. In contrast to commonly used straight waveguides, that can only be modified in their height and width, arrayed or tapered waveguides offer a larger design space. In this work, four-wave-mixing gain will be measured experimentally, ideally backed with theoretical calculations or simulations.

• Investigation of inverted tapers for efficient input coupling in waveguides

  The input coupling efficiency to a submicron-wide waveguide is limited due to its small mode field diameter. However, by using inverted tapers structure on the input and output facets of the waveguides, the input coupling efficiency could dramatically increase. In this work, we design and numerically simulate different structures of inverted tapers to investigate their influence of input coupling efficiency to waveguides.