Abstract

Previous efforts have concentrated on adjusting the Ti:LiNbO₃ fabrication parameters to achieve optimal coupling to single-mode fibers. Such waveguides exhibit fairly large mode size and are not suitable for efficient modulator applications. By methodical investigation, we have achieved small-mode size low-loss waveguides at 1.3 μm. A series of 2-cm long single-mode waveguides with titanium thicknesses of 500, 600, 700, 800, 900 and 1100Å and widths ranging from 2 to 4 μm were fabricated on the z surface of LiNbO₃ by diffusing in a wet O₂ environment for 5 and 6 h at 1025°C. The waveguides were then polished and characterized at 1.3-μm wavelength by measuring the fiber-waveguide throughput loss and the optical near-field patterns. The propagation loss in the guides was isolated from the other loss mechanisms by subtracting the mode mismatch loss between the channel waveguide and the single-mode fiber (which was evaluated numerically) and the Fresnel losses. The results indicate that the smallest possible waveguide (Ti strip width of 4 μm and thickness 800 Å, t = 6 h) at 1.3 μm has a 1/e intensity full width and depth of 3.8 and 2.7 μm, respectively, for the TM mode. The TE mode profile is considerably larger. This guide exhibits a propagation loss of 0.2 dB/cm for both TE and TM modes and a mode mismatch loss with a Corning single-mode fiber (7-μm 1/e intensity diameter) of 1.81 dB/face for TM modes. It is expected that this large mode mismatch loss can be eliminated by using a suitable tapered transition.

© 1986 Optical Society of America