The modulators where based on the Mach-Zehnder interferometer, where the ligth coming from a waveguide is split and directed into the two arms of the interferometer and then recombined in the output waveguide. If the optical path is the same in both arms, the two signals recombine in phase and a maximum of output light intensity occurs. Otherwise, if they are out of phase for half wave, a minimum - virtually null - output intensity results. Thanks to the electrooptical properties of lithium niobate, switching between both situations can be induced externally by applying an electric field across one of the two arms.