Low-drive-voltage Ti: LiNbO₃ Mach-Zehnder modulator using a coupled line

A modulator that uses a coupled line as a traveling-wave electrode to reduce the drive voltage is discussed. For modulation, complementary signals with equal amplitude and 180° out of phase are applied to the coupled line. The modulator was fabricated on a z-cut LiNbO₃ wafer for use at 1.55 μm. Wavegiudes were formed by diffusing 90-nm-thick, 7-μm-wide Ti stripes for 8 h 1050°C in wet O₂. The 500-nm-thick SiO₂ buffer layer was formed by electron-beam evaporation. The coupled line used plated gold electrodes 9 μm wide, 5 μm thick, and 20 mm long. The two lines were 12 μm apart. The drive voltage is 27 V at 100 Hz. The insertion loss of the microwave energy increases slowly as the frequency rises. It is -3 dB at 2.5 GHz and -6 dB at 10 GHz. The 3-dB bandwidth of the modulation response exceeds 5 GHz. The bandwidth is limited by the loss of microwave energy and velocity mismatch. It can be expanded by optimizing the coupled linear structure and using a thicker buffer layer and electrodes. This modulator shows promise for use in broadband and low-drive-voltage modulation.