Interferometric measurements of nonlinear optical properties for all optical switching applications in dielectric waveguides.

摘要

The successful implementation of nonlinear devices, for example for all-optical switching, depends critically on the availability of appropriate nonlinear optical materials. Most of the currently used methods to measure optical nonlinearities of materials are either indirect or inadequate for separating the fast electronic effects from slow thermo-optic processes. The motivation of this Ph.D. research was to develop a direct and accurate measurement method to evaluate the nonlinear optical properties of various, recently available waveguide materials for all-optical switching applications. A pulse modulated Mach-Zehnder scanning interferometer was built and revised to obtain a resolution of π/100 for nonlinear phase measurements. The evolution of this instrument included the development of single pulse extraction from a mode-locked pulse train, intensity modulation of single pulses, numerical Hilbert transformation of fringe data set, mode profile calculation inside waveguides with a numerical Fourier method, and a careful study of pulse breakup effect associated with instantaneous nonlinear phase shift. Electronic and thermal nonlinear refractive indices of various newly developed materials, especially DANS channel waveguides, DAN single crystal fibers, LiNbO₃ channel waveguide were examined with this method at the 1.32 μm wavelength. For the DAN single crystal cored fibers, the physical origin of the exceptionally large nonlinear phase changes in single crystal fibers was identified to be the cascading of two second order nonlinear processes. In the LiNbO₃ waveguide, cascaded nonlinear phase changes near the second harmonic phase matching temperature were demonstrated for the first time. Based on the results above, single crystal organic fibers appear very promising for ultrafast all optical switching applications. This demonstrates that the interferometric measurement method based on a scanning pulse modulated Mach-Zehnder Interferometer has proven to be one of the best methods for identifying nonlinear materials for all-optical switching applications at the 1.32 μm communications wavelength.