Quantentechnologie

In this thesis, I report on the experimental investigation and the computer simulation of optical microfibre-based modal interferometers. An optical microfibre (OMF) can be produced from a commercial single-mode optical fibre by a tapering process consisting in simultaneous heating and pulling the fibre. OMFs have attracted much attention in the recent years due to high light concentration, a strong evanescent field around the OMF waist, and convenience of use thanks to their fibre-coupled nature. It makes them a promising element for both basic research and sensing applications. Interferometers based on OMFs extend possible application areas to dispersive sensing. In a single-OMF modal interferometer (SOMMI), the two interferometer arms share the same path, and interference occurs between two transverse modes excited in the down-taper and recombined in the up-taper. During my work, I have produced OMF samples, characterized them, and used them as SOMMIs. To verify the OMF shape, different approaches have been implemented, including a light scattering method and a newly developed optical harmonic generation-based diameter measurement method [1]. For actual verification of the SOMMI performance, a simple post-production procedure, based on the stretch-interferometry, was realized. In this stretch-test, the experimental samples showed high contrast and very good signal-to-noise ratio making them suitable for sensing applications. Additionally, they were tested using spectral interferometry in air. Furthermore, I have designed and produced SOMMI samples specifically for interferometry in liquids and tested them as a refractive index sensor. Exhibiting a characteristic achromatic fringe, SOMMIs are a promising tool for the absolute refractive index measurement. In this experiment, a sensitivity of 3000 to 4000 nm per refractive index unit was measured. This is the highest sensitivity observed in non-birefringent OMF-based sensors so far. I have also developed a computer model of OMFs and SOMMIs. While the calculation methods for light propagation simulation in usual optical fibres are well established, simulation of OMFs demands many questions to be answered. The main challenge here is the calculation of the taper regions, where the fibre diameter varies from the standard diameter of a commercial fibre of 125 um to the diameter of the OMF waist of several hundred nanometres. Together with the diameter, the light-guidance regime changes from the weak guidance in the untapered fibre to the strong guidance in the waist, requiring different models to be combined. To the best of my knowledge, I have created the first reliably working software code for automatic calculation of all guided modes supported by tapered fibres [2]. I have then used this code to create computer models for stretch- and spectral-interference in SOMMIs. The experimental results confirm the validity of these models.

We review our recent progress in the production and characterization of tapered optical fibers with a sub-wavelength diameter waist. Such fibers exhibit a pronounced evanescent field and are therefore a useful tool for highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist or of the medium surrounding. We use a carefully designed flame pulling process that allows us to realize preset fiber diameter profiles. In order to determine the waist diameter and to verify the fiber profile, we employ scanning electron microscope measurements and a novel accurate in situ optical method based on harmonic generation. We use our fibers for linear and non-linear absorption and fluorescence spectroscopy of surface-adsorbed organic molecules and investigate their agglomeration dynamics. Furthermore, we apply our spectroscopic method to quantum dots on the surface of the fiber waist and to caesium vapor surrounding the fiber. Finally, towards dispersive measurements, we present our first results on building and testing a single-fiber bi-modal interferometer.

Applications of subwavelength-diameter optical fibres in nonlinear optics require precise knowledge of the submicrometre fibre waist diameter. We demonstrate a new technique for optical measurement of the diameter based on second- and third-harmonic generation with an accuracy of better than 2%. To generate the harmonic light, inter-modal phase matching must be achieved. We find that the phase-matching condition allows us to unambiguously deduce the fibre diameter from the wavelength of the harmonic light. High-resolution scanning electron microscope imaging is used to verify the results.