Optical waveguides induced by photorefractive spatial solitons: Fixing mechanism and applications

In this dissertation, we have proposed a simple model to understand the domain flip mechanism. The model is based on the assumption that there are numerous so-called nanopolar regions in the PR crystal, with each individual region having ideal conventional ferroelectric behavior described by five kinds Gibbs free energy curves. Polarstates are distributed in several wells (steady states) in the Gibbs curve and can be transferred from one well to another by overcoming a barrier that is equal to the well's depth. Proper electric field makes some wells shallower even vanish, while some others deeper, therefore changing polarstates distribution. This model can be used to explain domain flip phenomena such as diffraction grating fixing and waveguide fixing which is the focus point of this dissertation. In order to fix highly stable waveguides, a larger external applied field, higher intensity ratio and lower absolute intensity are needed in our experiment. We also investigated the influence of beam intensity, polariziation, wavelength and electric field on the properties of the induced single waveguide. After fixing the waveguide, we investigated the waveguide properties. For practical use, we demonstrated that a single fixed waveguide can be used as a modulator. The half-wave voltage measured from Fig. 3.6 (a) is 110 V. The modulation depth is about 65% and the modulation frequency is about 1.4 KHz. We also fixed a directional coupler by using two mutually incoherent e- or o-polarized solitons. After fixing, we measure the coupling between the waveguides using an 840nm, polarization adjustable probe laser beam. It has been found that when both the fixing beams and the probe beam are e-polarized, the coupling efficiency is ∼33%. It has also been found that the coupling efficiency can be controlled by adjusting the probe beam polarization direction and/or applying proper electric field. The coupling efficiency can be increased by applying a negative electric field while the coupling efficiency can be decreased by applying a positive electric field. And such an active directional coupler can be made into an electro-optic modulator or switch. In order to achieve higher coupling efficiency, the two waveguides should be carefully designed to be identical or nearly identical. And the size of the crystal along propagation direction should equal the coupling length. (Abstract shortened by UMI.)