In-series multimode interference sensors and Fabry-Perot interferometers for enhanced wavelength shift resolving capabilities

• Refractive index sensor obtained by integrating a SMS structure with a FPI in an in-series configuration. • The FPI modulates the SMS signature with interferometric fringes, reducing the width of the spectral features monitored. • Achieved a sensitivity-to-FWHM ratio displaying a 33-fold improvement over the SMS structure alone. • Two SMS-FPI configurations: one with an tunable air–gap FPI and another with a polymer film drop-casted onto a fiber tip. We report on the development of a refractive index sensor obtained by using a singlemode-multimode-singlemode (SMS) structure and a Fabry-Perot interferometer (FPI) set into an in-series configuration. Due to the self-imaging phenomenon, the SMS structure, formed by splicing a no-core fiber between two singlemode fibers, provides a broad spectral peak whose central wavelength position is sensitive to variations in the refractive index of the medium surrounding the fiber. In turn, thanks to the in-series SMS-FPI configuration, the sensor’s reflection spectrum exhibits the SMS spectral signature modulated by FPI fringes. This readily allows for reducing the width of the spectral features monitored during the sensing measurements, thus enhancing the capabilities of adequately resolving the corresponding spectral shifts. The FPIs reported in this investigation have been fabricated by using two different methods, namely by forming an air–gap FPI between the cleaved ends of two singlemode optical fibers, and by casting a polymeric film onto a connectorized fiber end tip. In the first configuration, the distance between the two cleaved fiber ends could be varied to tune the FPI’s free spectral range, hence allowing for tailoring the widths of the spectral oscillations to be monitored during the sensing measurements. Alternatively, the second configuration, while avoiding the use of motorized translation stages, provides a more versatile option for applications. Thus, we understand that our work expands the application of multimode interference and FPIs in sensing scenarios, providing new opportunities for probing physical and chemical parameters by exploring their combined response.