SPR optical fiber sensors can be obtained from the above-listed
structures surrounded by a noble metal (most often gold or silver).
Sheaths of thickness ranging between 30 and 70 nm are most
generally used. SPR generation is achieved when the electric field
of the light modes is polarized mostly radially at the surrounding
medium interface. The orthogonal polarization state is not able to
excite the SPR, as the electric field of the light modes is polarized
mostly azimuthally (i.e. tangentially to the metal) at the surrounding
medium interface and thus cannot couple energy to the
surface Plasmon waves. Depending on the configuration, refractometric
sensitivities in the range [102
–105 nm/RIU (refractive
index unit)] have been reported [6].