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We report a carrier-depletion silicon Mach-Zehnder optical modulator with large optical bandwidth by adopting two symmetric arms. The fiber-to-fiber loss of the device is about 7.2-8 dB in the wavelength range from 1525 to 1565 nm. We have used the truncation method to accurately measure the loss of the optical splitter and combiner, and the on-chip loss is about 3.8 dB. The dynamic extinction ratios at the speed of 40 Gb/s are 4.9-6.4 dB in the wavelength range from 1529 to 1565 nm. By analyzing the dependence of the optical bandwidth on the optical path difference between the two arms, we find that there is an unexpected optical path difference of around 3.3 μm between the two arms, which is considered to originate from the nonuniform morphologies of the waveguide and the nonuniform doping profiles along the two arms and is responsible for the slight wavelength dependence of the static and dynamic response of the silicon Mach-Zehnder optical modulator.