EN
This paper presents the results of measurements of the refractive index and thickness of the waveguide layer SU-8. The mode sensitivity has been calculated as a function of the thickness in a bimodal structure. The differential interference was analyzed concerning modes of the same types TE_0-TE_1 and TM_0-TM_1 and modes of the same order (TE_0-TM_0, TE_1-TM_1). The thickness of the layer was determined when the interferometer is most sensitive to changes of the refractive index. It has been proved that the sensitivity of the structure can be increased by adding a nanometric layer with a high refraction index (n_{A}=1.975) on the waveguide layer.