| In recent years, light guiding devices with longitudinal modulations have attracted a lot of attention in many application areas, such as fiber optics, optics communications, and the lighting industry. In this thesis, three types of light guiding devices with longitudinal modulations are thoroughly investigated, which include long period gratings in photonic nanostructures, multi-to-single fiber mode converter, and light uniformers for color mixing. The study in this thesis provides helpful guidelines for the corresponding application areas.; First of all, long period gratings in photonic nanostructures including fiber and waveguide configurations are numerically analyzed with the Finite-Difference Time-Domain (FDTD) method. A detailed quantitative study on the relationship between the spectral response shift of long period gratings and the geometrical and physical parameters of photonic nanostructures is performed. It is found that the unique photonic nanostructures can substantially enhance the tuning capability of the spectral response of long period gratings.; Then, a novel multi-to-single fiber mode converter based on a tapered multimode fiber is theoretically studied and experimentally implemented. Experimental results have confirmed theoretical predictions. The tapered multimode fiber has a multimode input end and a single mode output end, which is fabricated by a reproducible chemical etching process. This unique mode converter has a number of advantages, such as high conversion efficiency, compact size, and low cost.; Finally, an experimental study is conducted to investigate the light uniformers based on liquid guide with longitudinal modulations for their possible use as color mixing elements. In this study, three types of light uniformers are systematically analyzed, which include a straight light guide, a tapered light guide, and a U-shaped light guide. This study consists of two parts: a computer simulation using a commercial ray tracing software package LightTools; and an experimental study verifying the results obtained from the simulation. Beam uniformity, in terms of illuminance and color, is compared between different light guides. The experimentally observed results agree well with the simulated results. It is found in this study that the U-shaped light uniformer has high light transmission efficiency and the best color mixing effect among the three investigated light guides. |
