A new analysis method towards highly sensitive plasmonic fiber sensors

The spectral resonance of surface plasmon (SP) carried by a 50 nm gold layer covering a tilted fiber Bragg grating displays a wide sensitivity to the refractive index (RI) of the surrounding medium. That platform is thus conducive to the development of highly sensitive biosensors. Indeed, the p-polarized electromagnetic field shows a characteristic attenuation of its comb- shaped spectrum. One of the most used demodulation techniques so far used follows a singular mode's evolution at the edge of the attenuation area. The sensitivity of such a technique is linear
in a refractive index range limited to 10- 3RIU (refractive index unit). This limitation complicates the analysis if the RI spans over two or more domains. In this paper, a new demodulation based on tracking the crossing point between two envelopes of the p-polarized spectrum in the SP attenuation area is presented. This new technique has a double advantage. Firstly, it has an identical sensitivity independent of the refractive index range. Secondly, the sensitivity is more than 20 times higher than the conventional mode method. Furthermore, it is shown that the sensitivity of this method matches the theoretical SP resonance shift prediction.