Two-port polarization independent electro-optically tunable wavelength filter in lithium niobate

Two-port polarization independent electro-optically wavelength tunable filters based on asymmetric Mach-Zehnder structure in LiNbO3 substrate have been developed for 1.55μm application. The operation principle is based on Mach-Zehnder interference and TE↔TM mode conversion. Fabrication parameters for channel waveguides, interferometers and mode converters have been optimized. 7μm wide single mode straight channel waveguides were produced by diffusing 1050-1100A thick Ti into LiNbO3 substrate. Insertion loss of 3.6dB was achieved for both TE and TM polarization. Mach-Zehnder interferometer performance was optimized by testing the Y-branch on samples cut in half length compared to complete device. Best results were obtained from samples that were produced by diffusion at 1025 degC for 11 hours of 1050A thick Ti film, and by diffusion at 1025 degC for 12 hours of 1090A thick Ti film. Metal electrodes were added to one arm of asymmetric Mach-Zehnder interferometers to evaluate electro-optic modulation. Modulation depth as high as 99.6% for TE mode and 98.9% for TM mode were obtained. TE↔TM mode conversion was demonstrated on straight channel waveguides with conversion efficiency greater than 96% utilizing 500 strain inducing SiO2 grating pads with 21μm spatial period. Two-port polarization independent electro-optically tunable wavelength filters were produced based on the optimized parameters described above. The -3dB bandwidth of the filter is 2.4nm. The nearest side lobe to the main peak is more than 13dB below the central lobe for both TE polarization and TM polarization. A thermal tuning rate of -0.765nm/degC is obtained. An electrical tuning range of 12.8nm and a tuning rate of 0.08nm/V were achieved