Theoretical investigation for two cascaded SPR fiber optic sensors

To measure simultaneously two kinds of chemical substance by using wavelength division multiplexing technology, we report a theoretical investigation for two cascaded surface plasmon resonance fiber optic sensors (SPRFOSs). First, the effects of kinds and thicknesses of metal layers on the transmitted spectrum of two cascaded SPRFOSs have been discussed, and the calculations show that the cascaded SPRFOSs should adopt different kinds and moderate thicknesses of metal layers to produce two resonance wavelengths with a greater difference. For example, the 30 nm thickness of silver layer and the 50 nm thickness of gold layer may give a transmitted spectrum with two obvious dips apart from about 130 nm. Another method is that, the two SPRFOSs contain the same metal layer, and the metal layer of one sensor is coated by a dielectric overlayer with moderate thickness, such method can also produce two obvious dips whose difference of resonance wavelengths is dependent on the refractive index (RI) and thickness of dielectric overlayer. For example, a 20 nm Ta2O5 dielectric overlayer will give two dips apart from about 150 nm.