Proton exchanged $LiNbO_3$ waveguide devices such as cutoff modulator, $\Delta$ k directional coupler, and Mach-Zehnder modulator are fabricated so that self-aligned electrodes may be utilized. Then, by using the annealing process of the proton exchange method, it is demonstrated that the characteristics of the guided-wave devices are controllable through the modification of the guided mode profile. Futhermore, a novel fabrication method of $LiNbO_3$ ridge waveguide is proposed and proved experimentally.
First, the cutoff modulator is fabricated in the Y-cut substrate and the modulation characteristics are controlled by annealing. With a short annealing time the intensity modulation is not observed, and with more annealing the linear modulation characteristic is observed in the applied voltage range from 0V to 10V. With far more annealing, the guided mode is observed only when the voltage is applied. Second, $\Delta$ k directional coupler is fabricated with an 1×2 mask pattern and a 2×2 mask pattern in the Z-cut substrates. The voltage-length product of the 1×2 switch is 28 Vmm and the crosstalk is smaller than -20dB. Also the switching voltage variation and the crosstalk variation with annealing is observed. The minimum voltage-length product of the fabricated 2×2 switch is 22.4 Vmm. Also the variation of modulation characteristics with annealing is observed. Finally, the $LiNbO_3$ ridge waveguide is fabricated by wet etching after selective proton exchange in the X-cut substrate and the guided mode is observed with annealing. A Mach-Zehnder modulator employing this ridge waveguide is fabricated. The voltage-length product of the modulator is 12 Vmm and the modulation depth is 96.4%
