Nonlinearity of a channel-waveguide phase modulator

The phase velocity of light in a channel waveguide using a LiNbO₃ crystal is analyzed as a function of the voltage applied to the crystal. A refinement of the method of an effective refractive index is proposed. This refinement makes it possible to use the method near the cutoff for a waveguide mode. At voltages on the order of 10 V, the nonlinearity of the phase characteristic amounts to ~5 · 10^{– 4} of the linear phase shift.