Sensitivity enhancement in a multimode interference-based SMS fibre structure coated with a thin-film: Theoretical and experimental study

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