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Quantum technologies

Dieter Meschede's research group
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Theses - Optical microfibres
 

2012

  • J. E. Hartung
    Surface protection of optical microfibres for caesium spectroscopy, (2012), Diplom thesisBibTeXPDF
    ABSTRACT »
    In this thesis I present experiments done with optical fibres tapered down to submicrometre diameters. In such an optical microfibre, OMF, the light is strongly confined over a large length with a strong evanescent field propagating outside of the OMF. With this high light intensity in the surrounding medium of the OMF over a length of several millimetres, OMFs offer excellent conditions for light-matter interaction experiments. Linear absorption spectroscopy of hot caesium vapour using OMFs has already been done in our group [1]. The goal of my work was to investigate the possibilities of further spectroscopic methods. After presenting the considerations about pump - probe spectroscopy of caesium vapour using OMFs, I will introduce the optical setup I built. It offers the possibility for saturated absorption and polarisation spectroscopy using OMFs with a great flexibility. In first experiments the setup proofed to work, but transmission losses due to chemical reactions of the caesium with the surface of the OMFs prevented further investigations. To overcome this problem, the surface of the OMF had to be protected. I will present the surface passivation method which fulfils all of our demands and the developed treatment procedure of an OMF. This treatment procedure can now be applied and an OMF with a chemically passivated surface can be connected to the ready to use optical setup for pump - probe spectroscopy of hot atmoic caesium vapour.
  • K. Karapetyan
    Single optical microfibre-based modal interferometer, (2012), PhD thesisBibTeXPDF
    ABSTRACT »
    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.

2011

  • F. Bruse
    Harmonic Generation with Optical Microfibres under Controlled Atmospheres, (2011), Diplom thesisBibTeXPDF
  • U. Wiedemann
    Control of photochromic molecules adsorbed to optical microfibres, (2011), PhD thesisBibTeXPDF
    ABSTRACT »
    The high light intensity in an optical microfibre and the resulting nonlinear effects were applied to develop a new method to precisely determine the microfibre diameter. The evanescent field of these optical microfibres was then used to control the internal state of surface-adsorbed photochromic molecules. I start with a brief sketch of the mathematical description of light propagation in step-index optical fibres. From the results the main properties of optical microfibres are derived. Then, I describe the fabrication of optical microfibres with special requirements for the experiments presented later in the thesis. A new technique to measure the submicrometre diameter of optical microfibres with an accuracy of better than 2 % is presented. This method is based on second- and third-harmonic generation. It is found that the fibre diameter can be unambiguously deduced from the peak wavelength of the harmonic light. High-resolution scanning electron microscope imaging is used to verify the results. In the following, the experimental basics for the switching of photochromic molecules adsorbed to optical microfibres are described. I present the technique to deposit and detect the molecules and show their basic behaviour due to light exposure. The internal state of the molecules is measured via their state-dependent light absorption. Repeated switching between the states is achieved by exposure to the evanescent field of a few nanowatts of light guided in the microfibre. The photochromic processes are then quantitatively analysed. Time-resolved photoswitching dynamics are measured and mathematically modelled with a rate equation model. By adjusting the microfibre evanescent field strength the dynamic equilibrium state of the molecules is controlled. I also study how many times the photochromic system can be switched before undergoing significant photochemical degradation.

2010

  • D. Pritzkau
    Measurement of submicrometer diameters of tapered optical fibres using scanning electron microscopy, (2010), Diplom thesisBibTeXPDF

2008

  • G. Sagué
    Cold atom physics using ultra-thin optical fibres, (2008), PhD thesisBibTeXPDF
    ABSTRACT »
    In this thesis I present experiments concerning the investigation and manipulation of cold neutral atoms using ultra-thin optical fibres with a diameter smaller than the wavelength of the guided light. In such a fibre-field configuration the guided light exhibits a large evanescent field that penetrates into the free-space surrounding the fibre thus enabling to couple laser cooled atoms to the fibre mode. By trapping and cooling caesium atoms in a magneto-optical trap formed around the fibre I investigated the interaction of the atoms with the evanescent field at sub-micrometre distances from the fibre surface. Chapters 1 and 2 provide the theoretical foundations of this work. Chapter 1 describes the propagation of light in optical fibres. The general solution of the Maxwell’s equations in the fibre that complements the description is provided in Appendix A. In Chapter 2, the theory of the interaction of atoms with time-varying electric fields is described. In Chapter 3 the resonant interaction of laser cooled caesium atoms with the evanescent field of a probe laser launched through a 500-nm diameter fibre is studied. A detailed analysis of the atomic absorption at sub-micrometre distances from the fibre surface is given. I have performed Monte-Carlo simulations of atomic trajectories inside the cold atom cloud surrounding the fibre. From the simulations, the atomic density at the vicinity of the fibre is deduced and the absorbance profiles of the atoms measured during the experiments can be modelled. By carefully investigating the linewidths of these profiles, clear evidence of dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms is found. The atomic spontaneous emission into the guided mode of a 500-nm diameter optical fibre is the focus of Chapter 4. Here, I show that the fibre can be used as an efficient tool to collect and guide the spontaneous emission of the atoms. The dipole force induced by the evanescent field on the atoms is the central idea of the experiments performed in Chapter 5. I have built a new version of the experimental setup that opens the route towards atom trapping in the evanescent field in an array of surface microtraps around the fibre. Such traps are created by the combination of two laser fields with opposite sign of the detuning with respect to the excitation frequency of the atoms. The first experimental results reporting the influence of the two-colour evanescent field on the spectral properties of the atoms are presented.

2007

  • F. Warken
    Ultra thin glass fibers as a tool for coupling light and matter, (2007), PhD thesisBibTeXPDF
    ABSTRACT »
    This thesis presents an examination of ultrathin glass fibers as a novel tool for coupling light and matter. As a basic concept, matter, i.e. atoms, molecules, etc., will be coupled to the evanescent field in the vicinity of the fiber surface, which contains a large portion of the power of the guided light. Here, the effects of forming and microstructuring of these fibers on the field strength at the surface relative to the field strength in the center of the fiber are studied. Chapter 1 reports on the construction and optimization of a pulling system for glass fibers, which can be used to reproducibly manufacture subwavelength diameter fibers of centimeters length from standard glass fibers with high accuracy. The transmission through the whole structure is measured to be up to 97 %. The properties of the evanescent field and its potential for coupling of light and matter are analyzed in chapter 2 by spectroscopy of thin surface adsorbed films of organic molecules (PTCDA). It is theoretically and experimentally shown that the spectroscopic sensitivity can be increased by several orders of magnitude with respect to conventional techniques. This method allows for the first time the observation of sub-monolayer dynamics of structural changes of PTCDA on glass at ambient conditions. In chapter 3 two types of resonators, which can be built from glass fibers, are investigated. Firstly, a Bragg-mirror is integrated into an ultrathin glass fiber and the reflectivity is measured qualitatively. The second resonator type can be formed from a 16 µm thick fiber. Selective coupling of light from an ultrathin fiber into a whispering-gallery mode of the resonator is realized with an efficiency of 99,3% and tuning of the resonance frequency by more than one free spectral range of order 100 GHz is demonstrated. The quality factor of the excited modes has been measured to be of order 105 and limitations are discussed. Finally, a method is developed to determine the quantum numbers of a resonator mode. Thereby, this thesis makes a contribution towards the utilization of ultrathin and structured glass fibers to couple light and matter and opens the route to fiber-based quantum optics experiments.

2005

  • E. Vetsch
    Evaneszente-Wellen-Spektroskopie mit ultradünnen optischen Glasfasern, (2005), Diplom thesisBibTeXPDF