This thesis represents the partial fulfilment of the requirements for the danish ph.d. degree. I have been involved in both basic research of UV induced refractive index changes in silica glasses and in concrete applications. I have performed work on the thermal stability of UV-induced index changes in silica glasses where a new continuous isochronal annealing method was introduced. The method was applied to gratings written in D2-loaded fibers and non-loaded fibers. For the non-loaded fibers the obtained results are in good agreement with what has previously been observed. For the D2-loaded fibers two separate engergy distributions are resolved indicating that two different defect types are present. The influence of core concentricity error on the asymmetric directional bend induced resonance splitting of a long period fiber grating was investigated. A qualitiative model to describe the asymmetry showed good agreement with the obeserved data. The results were used to make a direction sensitive bend sensor of only one fiber. The sensor has further the advantage that it is insensitve to cross sensitivity from temperature, strin, and other external factors. Finally, an investigation of Nragg gratings for analysing the mode structure of UV-written waveguides was performed. the analysed waveguides were all found to be multimode. This was due to poor optical quality of the UV bema used for the inscription, which therefore had a larger focus width than expected. To improve the optical quality, a pinhole was inserted at the focus point in a beam expander and with this improved setup a focus of the expected focus width was obtained. This allowed for fabrication of low loss single mode UV-written waveguides.