Two Mach-Zehnder interferometers (MZI), one with Y-junction-couplers and the other with multimode interference (MMI), were presented. Both MZI were developed in silicon-on-insulator (SOI) technology and used plasma dispersion effect for light phase modulation. Fiber-to-fiber insertion losses, absorption coefficients, optical coefficients modulated intensities as function of time and modulation extinction ratios versus signal frequencies were measured for these devices at various wavelengths. Both devices were found to be modulated up to a frequency of 3.5 MHz. Insertion losses at wavelengths equal to 1.3 μm were less than 9.5 dB and excess losses relative to a straight waveguide were less than 2.3 dB.