First- and second-order SPP-mode co-induced dual-polarized refractive-index sensor based on photonic crystal fiber and gold film

A dual-polarized photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR) was proposed for measuring the refractive indexes (RIs) of liquids. A gold film and a liquid analyte were successively wrapped around the outer surface of the PCF to stimulate the external SPR effect, which greatly reduced the manufacturing difficulties, compared to depositing the gold film and filling the inner air holes of the PCF with liquid analyte. The performance of the proposed PCF sensor was numerically investigated using the finite-element method. The results show that the proposed PCF sensor can realize dual-polarized detection in the RI range of 1.36–1.42. The x and y polarization (X-P and Y-P) core modes were coupled to the X-P second-order and Y-P first-order surface-plasmon polariton modes, respectively. As the the RI of the analyte increased, the wavelength sensitivity increased and, as a result, maximum values of 15 900 nm RIU−1 for X-P and 16 900 nm RIU−1 for Y-P were achieved. Correspondingly, we obtained a maximum amplitude sensitivity of 2026.05 RIU−1 for X-P and 2102.65 RIU−1 for Y-P. In addition, the fabrication tolerance of the proposed PCF was increased to ±0.4 μm, which made the proposed PCF a favorable candidate for the detection of liquids.