In traveling-wave-type optical modulators, the microwave effective index should match the optical group index to realize a maximum bandwidth and a minimum driving voltage. Considering the material dispersion of LiNbO 3 (LN) and the waveguide dispersion of a Ti-diffused optical waveguide, we obtained a waveguide group index of 2.189 at a wavelength of 1550 nm, which is 0.051 larger than the waveguide phase index, from both calculation and experimental results. The effect of the waveguide dispersion on the group index was also found to be much smaller than that of the material dispersion of LN. It was demonstrated experimentally that the index difference induced by index dispersion strongly affects the frequency responses of ultrafast and ultralow-driving-voltage LN modulators.
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Aoki et al. (2006) studied this question.