Sensitivity enhancement experimental demonstration using a low cutoff wavelength SMS modified structure coated with a pH sensitive film

•Fiber optic sensors that operate in the visible light spectrum have been developed.•It is possible to increase the sensitivity of these devices by reducing the diameter of the sensible region.•The wavelength shift in the proposed sensors was verified when they were immersed in different refractive indices.•Furthermore, these devices can be applied as pH sensors when are coated with a sensitive film. The multimode interference (MMI) effect in a single mode-multimode-single mode (SMS) can be used for development of wavelength shift detection based sensors. In this work, the focus is centered on obtaining wavelength shifts with low cutoff single mode fibers, which allows exploring the wavelength range from 600 to 1000 nm, where optical sources and detectors are less expensive than at longer wavelengths. In addition, the application of a reduction in the fiber diameter of the SMS structure by means of HF etching, combined with the deposition of a thin-film, enables to enhance the sensitivity of the devices at the same time the objective mentioned before is achieved. In this sense, the effect of the deposition of a pH sensitive thin-film on SMS structures with different diameters allowed attaining a maximum sensitivity of 15 nm per pH unit in the range from pH 4 to pH 6, which improves by a factor of 3 the sensitivity of SMS sensors without etching operating at longer wavelengths.