Polarization-Insensitive Phase Trimming of Silica-Based Waveguide Using 193- and 244-nm UV Irradiation

In this paper, we describe polarization-insensitive phase trimming of silica-based waveguides with both 193- and 244-nm ultraviolet (UV) light and the trimming of both phase error and polarization dependence. A wide beam of 193-nm UV light from an ArF excimer laser is useful for phase trimming multiple waveguides. We utilize stress-release grooves to control birefringence generation while phase trimming, and we reduce the birefringence change during refractive index change by 95%. At the same time, we found that this grooved structure enhances the refractive index increase with UV light. We also utilize 244-nm UV light from a frequency doubled Ar ion laser with high space coherence, which is suitable for trimming the phase error of a single waveguide. We control UV-induced birefringence by controlling the size of the 244-nm laser beam light, and we reduce the birefringence change during phase trimming by 98%. In addition, we investigate the change in the refractive index and stress distribution of our waveguide when UV light is irradiated, and discuss the differences of polarization-insensitive phase trimming between 193- and 244-nm UV light. Finally, we utilize these techniques to eliminate the polarization-dependent phase error of an asymmetric Mach-Zehnder interferometer. We successfully demonstrate the individual trimming of the center wavelength and its polarization dependence.