Der photorefraktive Effekt in Bariumtitanat-Kristallen ist Gegenstand zahlreicher Forschungsarbeiten. Der Grund sind die großen elektrooptischen Koeffizienten dieses Materials, die nur von wenigen, kommerziell noch nicht erhältlichen Kristallen übertroffen werden, z.B. von tetragonalem Kalium-Tantalat-Niobat. Beleuchtet man diese Materialien mit Laserstrahlen, treten einzigartige Effekte auf. So erzeugen z.B. photorefraktive parametrische Verstärkungsprozesse charakteristische kreis- oder linienförmige Streulichtfiguren. Faszinierende Möglichkeiten bieten diese parametrischen Prozesse bei der Verstärkung und Phasenkonjugation von Lichtwellen. In dieser Arbeit werden alle 57 photorefraktiven parametrischen Prozesse, die in den untersuchten Materialien auftreten können, zunächst phänomenologisch analysiert. Das Lösen der gekoppelten Wellengleichungen filtert aus der Vielzahl der Prozesse diejenigen heraus, die eine besonders effiziente Wechselwirkung einfallender Lichtwellen versprechen. Für diese Prozesse werden anschließend explizit Verstärkungsfaktoren berechnet. Durch die Berücksichtigung elastooptischer, piezoelektrischer und raumladungsbegrenzender Effekte können die Einfallswinkel der Lichtstrahlen und die Orientierungen der Kristalle für die Signalstrahlverstärkung und die Phasenkonjugation optimiert werden. Auf der Basis dieser theoretischen Vorarbeiten gelingt im experimentellen Teil dieser Arbeit zum ersten Mal die Verstärkung eines Laserstrahls durch einen photorefraktiven parametrischen Prozeß. Die Verstärkung beträgt in einem 3,2 Millimeter dicken Bariumtitanat-Kristall 9000. Eine Besonderheit der angewandten Verstärkungsprozesse ist die sehr geringe räumliche Bandbreite. Die daraus folgende große Winkelselektivität verspricht viele Anwendungsmöglichkeiten in der optischen Meßtechnik, schränkt den Einsatz zur Bildverstärkung jedoch erheblich ein. Ebenfalls zum ersten Mal wird die Phasenkonjugation durch einen phototrefraktiven parametrischen Prozeß demonstriert. Die Intensitäten der erzeugten phasenkonjugierten Wellen entsprechen denen der verstärkten Signalstrahlen. Es lassen sich also Reflektivitäten bis zu 9000 realisieren. Zudem kann durch eine spezielle Konfiguration mit frequenzverstimmten Signalstrahlen ein prinzipielles Problem der photorefraktiven Wellenmischung gelöst werden: Die Verstärkung sehr schwacher Signalstrahlen. Durch die neu entwickelte Methode der Rauschunterdrückung gelingt es, Lichtwellen mit Leistungen unter einem Nanowatt um mehrere Größenordnungen zu verstärken. Zum Abschluß werden die parametrischen Prozesse noch in Kalium-Tantalat-Niobat untersucht. Bei diesen ersten Untersuchungen gelingt der Nachweis von fünf Prozessen. Die beobachteten ausgeprägten Effekte und die gemessenen riesigen elektrooptischen Koeffizienten zeigen, daß tetragonale Kalium-Tantalat-Niobat-Kristalle ebenfalls sehr interessant für die photorefraktive Wellenmischung sind.

The interferometric cantilever displacement detection in non-contact atomic force microscopy (NC-AFM) is in fundamental aspects explored and optimized. Furthermore, the opto-mechanical interaction of the light field with the cantilever is investigated in detail. Cantilevers are harmonic oscillators that are designed to have a high sensitivity for the detection of minute external forces typically originating from tip-sample interaction. In this work, however, the high sensitivity is used for detailed studies of opto-mechanical forces due to the radiation pressure of the light interacting with the cantilever. The interferometer in the NC-AFM setup consists of an optical cavity working similar to a Fabry-Pérot interferometer in combination with a reference interference arm working similar to a Michelson interferometer combining multi-beam interference with a reference beam resulting in a complex superposition of beams forming the interferometric intensity modulation signal. The character of the interferometer can be adjusted from predominant Michelson to predominant Fabry-Pérot characteristics by the optical loss inside the cavity. A systematic approach for accurate alignment, by using 3D intensity maps and intensity-over-distance curves, as well as the implications of deficient fiber-cantilever configurations are explored and the impact of the interferometer configuration on the detection system noise floor is investigated. A new physical property, namely, the Fabry-Perot enhancement factor is introduced that is a direct measure for the light intensity interacting with the cantilever compared to the reference beam intensity reflected back inside the fiber. The quantification of the optical loss yields an exact knowledge of the amount of light interacting with the cantilever that is crucial to understand opto-mechanical effects. The resulting opto-mechanical force varies sinusoidally during the course of one oscillation cycle. It is a key result of this work that the sinusoidal modification of the cantilever restoring force can be described analogue to the restoring force of a pendulum. This results in an observable amplitude dependent frequency shift of the cantilever oscillation, allowing a calculation of the ratio of the opto-mechanical force relative to the cantilever restoring force and thus allows an in-situ measurement of the cantilever stiffness with remarkable precision. Further investigation of the cantilever oscillation yields that other characteristic properties of the oscillation are significantly modified by the opto-mechanical interaction. The observed effective fundamental mode Q-factor drops significantly while the cantilever amplitude response to a certain excitation voltage increases. A discrete numerical model describing the cantilever as a 1D linear chain of mass points is implemented, yielding that the additional opto-mechanical force results in a partial pinning of the cantilever at the edges of the interferometric fringes. Pinning efficiently shifts energy from the fundamental mode to higher modes and modes of a pinned cantilever, resulting in a complex modal structure.

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No description available.

Wavelength division multiplexing (WDM) is essential for further enhancement of the transmission capacities of optical telecommunication systems. Key devices in WDM networks are multiplexing/demultiplexing components, which enable the combination/separation of several carrier waves with different wavelengths for the purpose of simultaneous transmission through one optical fibre. These components can be realized using Bragg diffraction from volume holographic gratings. Especially reflection holograms provide a pronounced wavelength selectivity which makes them attractive for free-space WDM applications. Holograms can be stored permanently in photorefractive lithium niobate crystals by the method of Thermal Fixing. Heating of the crystal during or after the recording process and subsequent development by homogeneous illumination at room temperature create nonvolatile holograms. The recording and development processes of Thermal Fixing in iron- and copper-doped lithium niobate crystals were investigated. Macroscopic Gaussian-shaped intensity patterns were used to analyse the origin of the fixing mechanism. Spatially resolved absorption measurements were performed to determine the concentration profiles of electron traps (Fe II/III) and protons. Results of computer simulations were compared with experimental results, which showed that protons can be found to work as compensators during hologram recording at temperatures around 180 degree C. Nevertheless thermal fixing without protons was possible, another compensation mechanism stood in. The obtained refractive-index changes were due to the electro-optic effect, other contributions could be neglected. With this detailed knowledge about thermal fixing, a two-channel demultiplexing unit was built by superposition of two thermally fixed reflection holograms in an iron-doped lithium niobate crystal. For this purpose a special two-beam interference setup with precisely adjustable writing angles was arranged in a vacuum chamber to eliminate thermally induced phase disturbances of the holographic recording procedure. Continuous development of the holograms by incoherent light was necessary. In the dark, the enhanced dark conductivity of the crystal used gave rise to a hologram degradation within about one day. Large diffraction efficiencies were attained (intensity losses between 2.3 and 5.2 dB only) uilizing crystals with high-quality polished surfaces. The crosstalk supression of the realized demultiplexer was > 25 dB, which is comparable with the performance of other multiplexing techniques like fibre Bragg gratings or arrayed-waveguide gratings. The low polarization dependence of the demultiplexer can be improved by superposition of two holograms for each channel.