Self-written waveguides in glass : experimental and numerical investigations
Self-writing is a technique of forming waveguide structures within a material. This thesis is an experimental and theoretical study of the self-writing process in glass, both in bulk materials and in planar layers. The first reported observations of self-writing effects in a bulk glass material are presented, with investigations into both Ce-doped Ga-La-S and Nd-doped BK7 samples. We observe increases in the refractive index of 2.5 x 10−5 in Ce-doped Ga-La-S due to illumination at 1047 nm, leading to the formation of a self-written taper. Corresponding numerical simulations show good agreement with observations, confirming the validity of the numerical model used to simulate these experiments. In the Nd-doped BK7 glass a decrease in index of 7 x 10−5 was observed when exposed at 488 nm, resulting in an increase in the diffraction of the propagating beam during the writing process, and subsequently an enhanced diffraction taper was induced. Here investigations with a Laguerre-Gaussian donut shaped writing beam show that a depressed-index pipe structure can be created by exploiting this negative index change. We have demonstrated both numerically and experimentally that the resulting complex waveguiding structure confines light and can be used as a channel waveguide. Self-written channel waveguides have also been formed in a K+ -ion exchanged Nd-doped BK7 planar glass layer by making use of an index increase of 5.2 x 10−5 induced at a wavelength of 457 nm. A threshold in power, below which photosensitivity does not occur, has been found. This threshold effect has been successfully incorporated in the numerical model, resulting in good agreement between observations and corresponding simulations. The experimental results obtained in the planar layer indicate that this material exhibits a high level of homogeneity and thus is an ideal host for self-writing. Investigations have also been carried out using a two-peaked writing beam to self-write an optical splitter, and both experimental and numerical results show that such a structure can be self-written using this type of writing beam. These results represent the first observations of a complex waveguiding structure being self-written using a single input beam in glass. In conclusion the self-writing fabrication method of waveguides shows great potential for future practical applications and integration with existing devices.