Figure 1.

Illustration of the double-clad optical fiber combining interferometric and plasmonic sensor modalities with $n_{\mathrm{core}}=1.46208$, $n_{1\mathrm{st}\mathrm{cladding}}=1.45713$, and $n_{2\mathrm{nd}\mathrm{cladding}}=1.44344$. Light in the range of 1500–1600 nm (${\lambda }_{\mathrm{I}}$) is confined as single transverse mode both in the fiber and in the hydrogel volume, with reflection at the OF–hydrogel interface and hydrogel–solution interface illustrated with red color. Multi-mode with light in the range of 450–850 nm (${\lambda }_{\mathrm{II}}$) is guided in the first cladding with numerical aperture illustrated with green on fiber-end face. The FP interference is measured with ${\lambda }_{\mathrm{I}}$, while the LSPR signal from gold nanorods (GNR) is measured with ${\lambda }_{\mathrm{II}}$.