Siegmund Schröter

Data transmission with twisted light through a free-space to fiber optical communication link

Mode division multiplexing (MDM), where information is transmitted in the spatial modes of light, is mooted as a future technology with which to transmit large bits of information. However, one of the key issues in optical communication lies in connecting free-space to optical fiber networks, otherwise known as the last mile problem. This is particularly problematic for MDM as the eigenmodes of free-space and fibers are in general not the same. Here we demonstrate a data transmission scheme across a free-space and fiber link using twisted light in the form of Laguerre–Gaussian (LG) azimuthal modes. As a proof-of-principle we design and implement a custom fiber where the supported LG modes can be grouped into five non-degenerate sets, and successfully transmit a gray-scale image across the composite link using one mode from each group, thereby ensuring minimal crosstalk.

Preparation of arsenic sulfide thin films for integrated optical elements by spiral bar coating

We report on the preparation of thin As35S65 films from their amine based solutions by a spiral bar coating technique on flexible PET substrates and the direct writing of subwavelength surface corrugated gratings with a period of 350 nm into these films by excimer laser interference lithography. The structural and optical properties of bar coated As35S65 glass are investigated in detail and compared with those of samples fabricated by vacuum thermal evaporation. The polarization dependent spectral filtering properties of the guided mode resonance devices built by the waveguiding thin films and the surface relief gratings written by a single laser pulse are presented finally.

Wave-optical modeling beyond the thin-element-approximation

The optical design and analysis of modern micro-optical elements with high index contrasts and large numerical apertures is still challenging, as fast and accurate wave-optical simulations beyond the thin-element-approximation are required. We introduce a modified formulation of the wave-propagation-method and assess its performance in comparison to different beam-propagation-methods with respect to accuracy, required sampling densities, and computational performance. For typical micro-optical components, the wave-propagation-method is found to be considerably faster and more accurate at even lower sampling densities compared to the different beam-propagation-methods. This enables realistic wave-optical simulations beyond the thin-element-approximation for micro-optical components. As an example, the modified wave-propagation-method is applied for in-line holographic measurements of strongly diffracting objects. From a direct comparison of experimental results and corresponding simulations, the geometric parameters of a test object could be retrieved with high accuracy.

Mode Coupling in Few-Mode Fibers Induced by Mechanical Stress

We investigate mode coupling in few-mode fibers induced by homogeneously and periodically applied mechanical stress. To view the power transfer between individual modes, a modal decomposition is performed at the end of the fiber using computer-generated holograms. Coupling between polarization and angular degenerated modes as well as between non-degenerated modes is confirmed experimentally and coupling parameters are inferred. The presented studies pave the way to detailed investigations of mode coupling in mode-multiplexed telecommunication systems and high-power high beam quality fiber lasers.

Measurement of effective refractive index differences in multimode optical fibers based on modal decomposition.

We demonstrate the nondestructive measurement of the effective refractive index difference of two arbitrary modes within a multimode optical fiber by utilizing a tunable fiber grating. We use a mechanical grating of variable period to couple the respective modes and measure the mode content at the fiber output based on the correlation filter technique. From the dependence of the coupling efficiency on the grating period, the effective index difference of the modes can be extracted with high accuracy.

Detection of mode conversion effects in passive LMA fibers by means of optical correlation analysis

To qualify passive fibers for (high power) laser beam delivery, different experimental approaches (interferometric, heterodyn, M2, ...for beam characterization at fiber output are under test in the community. Measurement of the individual strength of different components (eigenmodes) contained in the superposition at the fiber output in dependence for example on bending radius seems to be very promising. This can be done by means of optical correlation filters based on DOEs. For a standard telecommunication fiber SMF-28, operated at 633 nm, this could be demonstrated earlier1 by us. Here we present experimental results for quantitative proof of LP modes in LMA fibers as well as in SMF-28 fibers by means of such correlation filters, and demonstrate potential and limitations of this approach.

Noninvasive characterization of optical fibers

Capillary optical fibers with hole diameters of several micrometers are important for novel plasmonic applications and medical diagnosis. In order to ensure the optical functionality of these fibers, the diameter of the capillary hole needs to be realized with high accuracy. Here, we introduce a novel and noninvasive methodology to characterize optical fibers and discuss it for the assessment of capillaries. In this method, the fiber is side-illuminated by a coherent beam, and the resulting diffraction pattern is analyzed. This corresponds to an in-line holographic measurement in the presence of strong scattering. A numerical parameter retrieval allows us to characterize the capillary hole diameter with an accuracy of approximately 100xa0nm for radii between several hundreds of nanometers and several tens of micrometers.

Laser-beam characterization by means of modal decomposition versus M2 method

Modal decomposition by means of correlation filters has been proved as a key for real online laser beam analysis. To compare that method with the standard M2 method, we generated series of different laser beams (1064 nm), applied both methods to one and the same beam and evaluated achieved results. An adjustable Nd:YAG laser served as transversal mode generator, delivering diverse pure Gauss-Hermite modes and superpositions of modes, respectively. In the case of incoherent superposition of modes, their particular contribution to the general M2 value should be proportional to their relative strength, whereas in the case of coherent superposition M2 is distinctly influenced by phase differences between the discrete modal components. Achieved experimental findings are well confirmed by computer simulation.