The characteristics of RF parasitic modes and the methods to suppress leakage phenomena in LiNbO3 optical modulators were studied. The dominant zero-cutoff CBCPW modes and several undesired parasitic modes were analyzed with two-dimensional FEM. The effect of parasitic modes on high frequency RF power transmission characteristics were simulated and experimented in the respects of LiNbO3 wafer thickness, the kind of material contacting the back surface of the modulator chip, the gap and width of the CPW electrodes. An appropriate RF electrode geometry, to minimize coupling efficiency between co-planar waveguide and substrate mode, is presented. Experimental results proved that the approaches made in this work are effective for broadening of modulation bandwidth.
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