Model of a double-sided surface plasmon resonance fiber-optic sensor

A model of a surface plasmon resonance fiber-optic sensor with a double-sided metallic layer is presented. Most of such fiber optic sensing configurations are based on a symmetric circular metal layer deposited on a bare fiber core used for excitation of surface plasmon waves. To deposit a homogeneous layer, the fiber sample has to be continually rotated during deposition process, so the deposition chamber has to be equipped with an appropriate positioning device. This difficulty can be avoided when the layer is deposited in two steps without the rotation during the deposition (double-sided deposition). The technique is simpler, but in this case, the layer is not flat and a radial thickness gradient is imposed. Consequently, the sensor starts to be sensitive to polarization of excitation light beam. A theoretical model is used to explain the polarization properties of such a sensing configuration. The analysis is carried out in the frame of optics of layered media. Because the multimode optical fiber with large core diameter is assumed, the eccentricity of the outer metal layer boundary imposed by the thickness gradient is low and the contribution of skew rays in the layer is neglected. The effect of the layer thickness gradient on the performance of the sensor is studied using numerical simulations.