Optical coupling characteristics of the $LiNbO_3$ wavwguide device and the optical fiber are investigated for various annealing conditions of the waveguide fabrication process. The waveguide device that we used in the experiment is an integrated optical ac high voltage sensor based on a Z-cut $LiNbO_3$ cutoff modulator is fabricated by the annealed proton-exchange method. Its sensing voltage range exceeds ac 1400 $V_{p-p}$, and its thermal characteristics are stable within ±5% in the temperature range from 10℃ to 60℃. To find the fabrication condition for minimizing the insertion loss, the waveguide mode size and the waveguide propagation loss are measured for various annealing times. The input and output end of the waveguide are coupled to a single-mode and a multi-mode fiber, respectively. The obtained insertion loss is 6.4dB including misalignment loss of about 3dB. The optical fibers are fixed by using a UV-curable epoxy without significant additional loss.
