Multimode interference in a single-mode–multimode–single-mode (SMS) fibre structure leads to thegeneration of transmission bands and attenuation bands in the optical spectrum. In this work, the pro-gressive deposition, with layer-by-layer assembly technique, of a high refractive index thin-film onto theSMS structure permits to observe the evolution of the bands as a function of the coating thickness. Theresults show that it is possible to increase the sensitivity of the device to variations of the coating thick-ness, what indicates the adequateness of this structure as a sensing platform. The experimental resultsare corroborated with a numerical method based on coupled-mode theory. As a practical application, asensitivity improvement has been obtained a pH sensor by increasing the coating thickness.