Design and fabrication of a shielded‐velocity‐matched Ti:LiNbO₃ optical modulator

This paper describes a shielded‐velocity‐matched Ti:LiNbO 3 optical modulator which realizes a wide bandwidth with the velocity matching between the optical wave and the modulation microwave by placing a shielding conductor close to the coplanar waveguide traveling electrode through an air overlay. In the design of the traveling‐wave electrode, a quasi‐TEM approximation and a spectral domain approach with hybrid mode are used. From numerical analysis it has been found that the effective refractive indices of the microwave and the light wave can be matched by optimally setting the overlay thickness. It has also been found that the increase in the microwave propagation loss or decrease in the characteristic impedance can be suppressed by increasing the thickness of the SiO 2 buffer layer. As for the optical waveguide, the design parameters have been clarified by using a newly developed modified step segment method. Based on these examinations, a Mach‐Zehnder intensity modulator and a phase modulator for the 1.5‐μm wavelength band have been designed and fabricated. As a result of measurement, the optical modulation bandwidth has been improved greatly to 20 GHz. As for the drive voltage, the intensity modulator uses 4.7 V and the phase modulator uses 6 V. The total insertion loss including the connector loss is 2.2 dB for the intensity modulator and 2 dB for the phase modulator. It has also been verified that the modulators have superior ambient stability.