Modeling and Characterization of N onlinear Materials for Protection of Optical Sensors

Sammanfattning: This thesis concerns nonlinear materials and experimental techniques for optical power limiting applications. Theoretical models to analyze and simulate related measurements were studied. Two different numerical modeling tools for propagation of optical radiation through optically nonlinear media were developed. The first, relying on a Gaussian decomposition technique, was developed for multi-mode Gaussian beams and extended to allow for optically thick samples. The method relies on a multi-layer approach, where all layers are treated as independent, and may have different linear and nonlinear optical properties. Hereby the model allows simulation of concentration gradients and several samples in succession, each having arbitrary material properties. The second approach takes into account time-dependent nonlinear systems. A computer program based on the Cranck-Nicholson scheme was written in Matlab code and used to simulate the propagation of irradiance distribution for various temporal pulse-shapes. The differential equations associated with 3- and 5-level systems were solved numerically in each point over the active time-space grid. Instantaneous two-photon absorption and dynamic multi-level systems were used, one at a time, or in combination, to model material responses to optical pulses.In addition, several materials were characterized using the Z-scan technique and related techniques. Chloro-aluminium phthalocyanine, CAP, was examined using several different pulse-lengths in the nanosecond regime for both thin and thick samples. A test-bed for investigating the capability of various materials to protect an optical sensor from hazardous laser radiation was constructed. This set-up used a transversal top-hat laser beam focused on 2 mm thick samples in an f/5 optical system, and was used to characterize materials for optical power limiting.