Interferometric method for measuring diffused channel waveguide-index profile.

An interferometric method for measuring the distribution of refractive-index change in diffused channel waveguides is described. It is based on Young's fringes generated by the interference between a point light source and its reflection from the surface of the waveguide substrate and requires the same experimental arrangement as that used for end-fire coupling. The technique allows a nondestructive measurement of the surface-index change and the lateral dependence of the index distribution if the depth diffusion constant and the diffusion time are known. When used in conjunction with successive removal of substrate layers, the method can in theory be used to measure both the depth and lateral dependencies. Experimentally measured surface-index changes and lateral dependencies of index change distributions in 5-, 9-, and 21-microm wide channel waveguides fabricated by diffusion of 195- and 390-A thick Ti films into Z-cut LiNbO(3) for 6 h at 1000 degrees C are given. Maximum surface ordinary index changes were 4.2 x 10(-3) and 8.5 x 10(-3) for the two Ti thicknesses. Measured lateral dependencies agree with the theoretical sum of error function profiles calculated using a lateral diffusion coefficient of 1.0 x 10(-12) cm(2)/sec.