Small-size low-loss Ti:LiNbO3 waveguides for optimum device performance at 1.3 μm

Previous efforts have concentrated on adjusting the Ti:LiNbO 3 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 μ m. 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 μ m were fabricated on the z surface of LiNbO 3 by diffusing in a wet O 2 environment for 5 and 6 h at 1025°C. The waveguides were then polished and characterized at 1.3- μ m 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 μ m and thickness 800 Å, t = 6 h) at 1.3 μ m has a 1/ e intensity full width and depth of 3.8 and 2.7 μ m, 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- μ m 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.