This paper presents the results of a new approach to the analysis of interference patterns in the modulator output waveguide in a Mach-Zehnder interferometer configuration based on lithium niobate thin films. Interference patterns that occur in the output waveguide have been demonstrated. An approach of analysis of the patterns has been described. The method allows us to obtain information about the value of voltage applied to the arm of the interferometer and to double upper limit of measuring electric field strength using MZI based electro-optic modulator.