${\rm Ti}{:}{\rm Er}{:}{\rm LiNbO}_{3}$ Strip Waveguide With High Diffusion-Doped ${\rm Er}^{3+}$

We report a Ti:Er:LiNbO₃ strip waveguide with high diffusion-doped surface Er³⁺ concentration. The waveguide was fabricated with a technological process in sequence of preparation of noncongruent, Li-deficient LiNbO₃ substrate by performing Li-poor vapor transport equilibration treatment on a congruent Z-cut LiNbO₃ plate, diffusion of 40-nm-thick Er metal film, and fabrication of 8-μm-wide Ti-diffused strip waveguide. The waveguide retains the LiNbO₃ phase and shows the waveguiding characteristics similar to the conventional Ti:LiNbO₃ waveguide. Secondary ion mass spectrometry study shows that the Er³⁺ diffusion reservoir was exhausted and the profile is Gaussian with a surface concentration two times larger than that of the conventional Ti:Er:LiNbO₃ waveguide. The waveguide shows stable 1547-nm small-signal enhancement under the 1480-nm pumping without serious optical damage observed, and a 5-dB signal enhancement is obtained for the available coupled pump power of only 90 mW. A saturated net gain as much as 5 dB/cm is predicted theoretically.