Normal-mode small-signal analysis of traveling-wave directional couplers

The small-signal frequency responses of the 1*2 and 2*2 directional coupler traveling-wave modulators are analyzed in closed forms including the effect of both the optical/microwave velocity mismatch and microwave loss. When the microwave loss is negligible, the small-signal bandwidths of 1*2 and 2*2 directional coupler traveling-wave modulators are 20 and 28%, respectively, larger than that of an interferometric modulator with the same electrode length. However, when the microwave loss is large, the 1*2 coupler has a larger bandwidth.<
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