Sodium nitrate sensor based on D-shaped fiber structure

[Display omitted] •A low-cost robust sodium nitrate sensor successfully demonstrated using a D-shaped optical fibers.•The D-shaped fiber structure fabricated by side polishing method.•The graphene coating act as a sensing layer to enhance the sensitivity of the sensing probe.•The sensing probe was embedded onto a fiber ring laser as this setup offers more precise measurement. This paper demonstrates a low-cost robust refractive index (RI) sensor based on a sensing structure realized with graphene-coated D-shaped fiber structure. The proposed sensing structure is of 5-dB insertion loss single mode fibers fabricated by means of side polishing; drop casting is used for the graphene coating that would act as a sensing layer to enhance the sensitivity of the sensing probe. Observations showed that the output power decreased from 7.17 dBm to 5.85 dBm with sensitivity of 0.223%/dBm and linearity of 99% as six different concentrations of sodium nitrate that varies from 1 to 7% were tested on the sensor probe. These results came from the interaction of propagating light in the evanescent wave with the external changing environment. The resonant peak shifted to longer wavelength with increasing refractive index. The sensing structure was embedded onto a fiber ring laser as this combination offers high measurement resolution and increase their remote sensing capability.