The waveguides and electrodes are in a symmetrical pattern,and there is no apparent difference in the optical wave on the upper waveguide compared to the lower one. However, the EO effect is not necessarily the same. There are 3 electrodes, top, middle and bottom. If the top and bottom electrodes are grounded and potential is applied to the middle one, the EO effect on the upper waveguide is the opposite of the EO effect lower waveguide. The effect, both + and -, gets stronger with increasing potential, leading to constructive or destrtuctive interference in the second coupler.