Quasi‐matched‐velocity traveling‐wave‐type electrodes for light modulators and their analysis

To increase the bandwidth of the traveling‐wave‐type light modulator, it is necessary to decrease the difference between the velocity of the modulation wave and that of light. The velocity mismatch was improved by incorporating a buffer material with low dielectric constant between the electrodes and substrate so that the modulation efficiency does not deteriorate and the capacitance between the electrodes is minimized. Considering not only the bandwidth but also the ratio between the driving power and the bandwidth (P/Δf), three types of structures have been proposed. For the theoretical analysis, the modulation wave traveling through the electrodes was assumed to be a TEM wave and the model of the two‐dimensional static electric field was adopted. However, since the structure was complicated, a numerical calculation was needed in which the boundary element method was used. The structure with least P/Δf ratio was chosen by using the result of the calculation. As a result of further calculation on the chosen structure with the various thickness of the buffer layer, it was found that the bandwidth is improved drastically and the ratio of P/Δf becomes small by increasing the width of the buffer layer.