Barium Titanate Photonic Crystal Electro-Optic Modulators for Telecommunication and Data Network Applications

Peter Girouard
2016 unpublished
The microwave, optical, and electro-optic properties of epitaxial barium titanate thin films grown on (100) MgO substrates and photonic crystal electro-optic modulators fabricated on these films were investigated to demonstrate the applicability of these devices for telecommunication and data networks. The electrical and electro-optical properties were characterized up to modulation frequencies of 50 GHz, and the optical properties of photonic crystal waveguides were determined for wavelengths
more » ... panning the optical C band between 1500 and 1580 nm. Microwave scattering parameters were measured on coplanar stripline devices with electrode gap spacings between 5 and 12 µm on barium titanate films with thicknesses between 230 and 680 nm. The microwave index and device characteristic impedance were obtained from the measurements. Larger (lower) microwave indices (impedances) were obtained for devices with narrower electrode gap spacings and on thicker films. Thinner film devices have both lower index mismatch between the co-propagating microwave and 4 optical signals and lower impedance mismatch to a 50 Ω system, resulting in a larger predicted electro-optical 3 dB bandwidth. This was experimentally verified with electrooptical frequency response measurements. These observations were applied to demonstrate a record high 28 GHz electro-optic bandwidth measured for a BaTiO 3 conventional ridge waveguide modulator having 1 mm long electrodes and 12 µm gap spacing on a 260 nm thick film. The half-wave voltage and electro-optic coefficients of barium titanate modulators were measured for films having thicknesses between 260 and 500 nm. The half-wave voltage was directly measured at low frequencies using a polarizer-sample-compensator-analyzer setup by over-driving waveguide integrated modulators beyond their linear response regime. Effective in-device electro-optic coefficients were obtained from the measured half-wave voltages. The effective electro-optic coefficients were found to increase with both applied electrical dc bias and with film thickness. A record low 0.39 V · cm (0.45 V · cm) voltage-length product was measured for barium titanate modulators operating at telecommunication wavelengths on a device with 5 µm electrode gap spacing on a 500 nm thick film modulated at a frequency of 100 Hz (1 MHz). This measured voltage-length product is more than a factor of 5 lower than that reported for state-of-the-art silicon conventional waveguide modulators. The electro-optical characterization of BaTiO 3 films revealed a trade-off that exists for traveling wave BaTiO 3 modulators: lower voltages are obtained in thicker film devices with narrow electrode gap spacing while larger bandwidths are obtained in thinner film devices with wider electrode gap spacing. These findings were supported by calculations of the film thickness dependent half-wave voltage and electro-optic bandwidth. In order to
doi:10.21985/n29t0k fatcat:jdxst2a2dnabnh44cl46he5bne