SPR Based Fiber Optic Sensor

Fiber optic sensor based on surface plasmon resonance, employing thin film of nickel is presented analytically. Increase in thickness of nickel film results in the enhancement of sensitivity of the sensor. SPR Sensor supported by large thickness of nickel film possesses maximum sensitivity.
Introduction
Surface plasmon resonance i.e. SPR principle has been an important sensing method since last thirty years. In the beginning, chemical sensing utilizing SPR is demonstrated by Liedberg et al. [1].

Collective resonating oscillations of free electrons survive on metal layer. It produces charge density wave moving along the metal layer. This charge density wave is transverse wave in nature and is identified as surface plasmon wave. Surface plasmon wave is excited by incident p-polarized light. For examining surface plasmon resonance, Kretschmann geometry is exercised [2, 3]. Optical fiber based SPR sensors offer many advantages than prism based SPR sensors [4-6].
In the past, lot of research is conducted on optical fiber based SPR sensors [7-10]. In recent times, nickel (Ni) is shown to exhibit sensing relevance because of its excellent magneto optical merits [11]. Apart from this, Ni is chemically inactive and the cost of Ni is lower than that of noble metals. Hence, the use of Ni instead of noble metals decreases the price of SPR sensor. Current study discusses a SPR based fiber optic sensor utilizing thin film of Ni. Effect of thickness of Ni film on the sensitivity of SPR sensor is illustrated. Sensitivity is enhanced with the increase in the thickness of Ni film.
Theory
Sensing system of the sensor contains fiber core-Ni layer-sample medium. Plastic cladding about the core from the central part of step index multimode PCS fiber is eradicated and is covered with thin layer of Ni. This layer of Ni is ultimately enclosed by the sample medium. Incident light from a white light source is allowed to enter into one end of the optical fiber and the transmitted light is noticed at the opposite end of the optical fiber.
The core of optical fiber is formed by fused silica. Refractive index of fused silica alters with wavelength as, 23 22322 22221 22111b ab ab a) ( n? +? +? + =??????? (1) Here, ? is the wavelength of incident light in µm and a1, a2, a3, b1, b2 and b3 are Sellmeier coefficients. The values of coefficients, used in (1) are specified as, a1 = 0.6961663, a2 = 0.4079426, a3 = 0.8974794, b1 = 0.0684043 µm, b2 = 0.1162414 µm and b3 = 9.896161 µm [12].
The dielectric constant of a metal can be mentioned as, ) ( 1 ) (22? ? ?? ?? ? ? ?i ic pcmi mr m+ ? = + = (2)
Where, ?p and ?c are plasma and collision wavelengths of the metal respectively.
For, Ni: p?= 2.5381 x 10-7 m andc?= 2.8409 x 10-5 m.
Also, the dielectric constant of sample medium is written as,2s sn =?
where, sn is refractive index of the sample medium. Resonance condition for the surface plasmon wave is written as, } K Re{ sin nsp=???12 (3) Here, 2 22s ms ms ms mspn nc K+ =+ =? ?? ?? ?? ?? is the wave vector of surface plasmon wave and c is the velocity of light in vacuum. Reflection coefficient of p-polarized light is calculated by using matrix method [13].
Normalized transmitted power from the sensor is computed as [14]. Further, the sensitivity of sensor can be described as change in resonance wavelength per unit change in refractive index of sample medium [15].
Results and Discussion
For simulation, refractive index of sample medium is presumed to be altered from 1.33 to 1.37. Values of various parameters used are mentioned as; fiber’s numerical aperture = 0.24, core diameter of fiber = 600 µm and exposed sensing region length = 15 mm.
Transmitted power from the sensor is measured for different thicknesses (20 nm-80 nm) of Ni layer and consequent resonance wavelengths are measured. Resonance wavelengths for different thicknesses increase linearly with increase in the refractive index of the sample medium.
20 40 60 80 0 15003000 4500 6000 7500
Sensitivity (nm/RIU)|Thickness of Ni layer (nm)
Figure
1. Variation of sensitivity with thickness of Ni layer. Figure 1 represents the variation of sensitivity with Ni layer thickness.
Ni layer thickness is increased from 20 nm to 80 nm. Sensitivity is enlarged with increase in Ni layer thickness. The reason for this enhancement in sensitivity is ascribed to high value of real part of dielectric constant of Ni. Therefore for a fixed change in refractive index of sample medium, Ni enhances the shift between resonance wavelengths. This results in enhanced sensitivity of sensor with increase in Ni layer thickness.
Thus, large Ni layer thickness leads in high sensitivity of SPR based sensor.
Conclusions
Theoretical analysis of SPR based fiber optic sensor with thin layer of Ni is carried out. Sensitivity of SPR based sensor is enlarged with increase in Ni layer thickness. In order to achieve highest sensitivity of the sensor, large thickness of Ni layer is advised.
Acknowledgments
Navneet K. Sharma wishes to thank Defence Research; Development Organization (DRDO), India for the financial grant provided through the project number ERIP/ER/DG-ECS/990116205/M/01/1687.
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