A traveling-wave Z-cut $LiNbO_3$ Mach-Zehnder interferometric modulator by using the proton exchange method has been designed and fabricated. Mode profiles and insertion losses of the channel waveguides and the Y-branch costituting the modulator are characterized for various annealing times. The characteristic impedance, the microwave effective index and the loss of the asymmetric coplanar strip type electrode are examined theoretically and experimentally for hot electrode width, gap between hot and ground electrodes, electrode thickness, and buffer layer thickness. From these results the modulator is designed to have switching voltage ($V_\pi$) of 10.2 V, 3 dB small-signal bandwidth of 12.5 GHz for a 14.3 mm long interaction length at 1.5 μm. The fabricated modulator with 14.3 mm long electrode has been found to have $V_\pi$ of 5.2 V and the extinction ratio of 24.8 dB. The bandwidth is measured to be 9.5 GHz using the swept frequency technique. The reason for discrepancies between the design and the measurement are discussed.
