Polarization-independent electrooptically tunable add-drop filter in titanium:lithium niobium oxide at 1.55 micrometer-wavelength regime

A novel polarization independent rapidly tunable add/drop filter based on passive polarization beam splitters and strain-induced phase-matched polarization mode converters in LiNbO₃ has been developed in the 1.55 μm wavelength regime. Optimum fabrication parameters for channel waveguides, polarization beam splitters and polarization mode converters are identified. Single mode Ti-indiffused channel waveguides for both TE and TM polarizations have been produced in 7μm wide waveguides. The propagation losses of the waveguides are 0.08dB/cm for TE polarization and 0.17 dB/cm for TM polarization. High extinction polarization splitters of 25dB for both TE and TM polarization were achieved using a two-mode interference directional coupler configuration with a 0.55° splitting angle. The extinction ratio of the splitters is insensitive to wavelength in the tunable regime of the filter, and to temperature changes of the substrate over tested thermo-optic tuning range. Tunable TE $\leftrightarrow$ TM polarization mode converters with more than 99% conversion efficiency were produced with a total of 500 strain inducing grating pads of 21 μm spatial period using a surface SiO₂ film between 1.30 pm to 1.45 pm thick deposited at 360°C and patterned at room temperature. Thermal and electrooptic tuning of the polarization mode converters were demonstrated. Polarization-independent electro-optically tunable add/drop filters that are based on the above described splitters and tunable TE $\leftrightarrow$ TM mode converters have been produced. The −3dB bandwidth for drop and add channel is 2.3 nm, which is in close agreement with the expected value 2.4nm, and the express channel isolation is 20.2dB. The nearest side lobes to the main peak are approximately 11.5dB below the central lobe. A maximum tuning range of 24 nm, tuning speed of 1.28 Å/ns and linear tuning rate of 0.1nm/V were obtained. Fiber-to-fiber insertion loss is <5.4 dB for both express and drop cases, and the polarization dependent loss is 0.24 dB. Both losses are wavelength insensitive.