Fiber Optic Sensors Based on Surface Plasmon Resonance B.D. Gupta Physics Department, Indian Institute of Technology Delhi New Delhi ��� 110016, India e-mail address:bdgupta@physics.iitd.ernet.in Abstract: Surface plasmon resonance based fiber optic sensors with different probe designs are presented. The modeling of each probe is carried out using ray optics. The performance of each probe is evaluated in terms of sensitivity. ��2009 Optical Society of America OCIS codes: 0.60.0060, 0.60.2370 1. Introduction Surface plasmon resonance (SPR) is one of the most promising optical techniques that find applications in different fields. The first sensing application of SPR technique was reported in 1983 [1]. Since then, a numerous SPR sensing structures for chemical and biochemical sensing have been reported. In SPR technique, a TM or p-polarized light causes the excitation of electron density oscillations at the metal-dielectric interface. When the energy as well as the momentum of both, the incident light and SPW, match a resonance occurs which results in a sharp dip in the reflected light intensity. The resonance condition depends on the angle of incidence, wavelength of the light beam and the dielectric functions of both the metal as well as the dielectric. The resonance parameter (angle or wavelength) depends on the refractive index of the dielectric medium. Change in refractive index changes the value of the resonance parameter. Surface plasmon resonance based optical fiber sensors have drawn lot of attention due to miniaturization and remote sensing. The performance of these sensors is, generally, evaluated in terms of sensitivity and signal to noise ratio. In this article we focus on various designs of the fiber optic SPR probe that have been studied to enhance the performance of the sensor. 2. Probe designs Sensitivity, detection accuracy, reproducibility and operating range of a sensor are the important parameters to compare with other sensors. A best sensor is the one that has high sensitivity, detection accuracy and operating range in addition to giving reproducible results. To achieve this various modifications have been proposed for the SPR probe. For metallic coating on fiber core either silver or gold is used. Gold demonstrates a higher shift of resonance parameter to change in refractive index of sensing layer and is chemically stable. Silver, on the other hand, displays a narrower width of the SPR curve causing a higher SNR or detection accuracy. The chemical stability of silver is poor due to its oxidation. Therefore, the treatment of silver surface by a thin and dense cover is required. A new structure of resonant metal film based on bimetallic layers (gold as outer one) on the core of the optical fiber with spectral interrogation method is reported [2]. The sensitivity and SNR were evaluated numerically for different ratios of the thickness of silver and gold layers. The capability of other metals such as copper (Cu) and aluminium (Al) in addition to their bimetallic combinations with Ag and Au were theoretically investigated for the design of fiber-based sensors [3]. The sensitivity of the fiber optic SPR sensor can be increased on adding dopants in the material of the fiber core [4]. This is because the SPR condition depends upon the refractive index of the material of the fiber core. Several groups have worked on the improvement of the sensitivity of a fiber optic SPR sensor by changing the shape or the geometry of SPR probe. Tapering the fiber optic SPR probe was one of the modifications [5]. The use of dual-tapered and tetra-tapered fiber optic SPR probes for gas and liquid sensing has also been reported [6]. Changing the profile of the tapered SPR probe also affects the sensitivity of the sensor [7]. Theoretical analysis predicts an increase in the sensitivity with the increase in the taper ratio. The increase in sensitivity occurs because of the decrease in the angle of incidence of the guided rays with the normal to the core-cladding interface in the tapered region. To further enhance the sensitivity, a SPR probe of uniform core (with metallic coating) sandwiched between two unclad tapered fiber regions has been proposed [8]. The angle of incidence of the ray with the normal to the core-cladding interface can also be brought close to the critical angle by using a U-shaped probe. A SPR based fiber optic sensor with uniform semi-metal coated U-shaped probe has been analyzed using a bi-dimensional model a214_1.pdf FWG4.pdf �� 2009 OSA/FiO/LS/AO/AIOM/COSI/LM/SRS 2009 FWG4.pdf

[9]. The advantage of U-shaped probe is that it can be used as a point sensor and is less fragile as compared to tapered probe. The SPR probes reported above use multimode fibers. A side polished single mode optical fiber with thin metal over layer has been used as a fiber optic SPR probe [10]. In this configuration, the guided mode propagating in the fiber excites the surface plasmon wave at the interface between the metal and a sensing medium. The resonance occurs if the two modes are closely phase matched. Such single-mode fiber based SPR sensor is more sensitive and more accurate in comparison to those with multi-mode fibers. However, their fabrication is much more complex and sophisticated compared with those that use multi-mode fibers. The advantage of side polished half block SPR sensor is that it requires a very small amount of sample for measuring the refractive index. Recently SPR based side polished multimode fiber sensor has been reported 11]. Apart from side polished single mode fiber, D-type single mode optical fibers have been used for sensing applications utilizing SPR technique [12]. These fibers also improve the sensitivity of the SPR sensor. Most of the theoretical studies on fiber optic SPR sensors using ray optics and reported in the literature do not consider skew rays in the analysis. These studies consider the propagation of meridional rays only in the fiber which makes the analysis simpler. Recently, the effect of skew rays on the sensitivity and the SNR of a fiber optic SPR sensor have been studied using spectral interrogation method [13]. Both the sensitivity and the SNR decrease as the value of skewness parameter increases irrespective of the metal used for coating. The other designs for SPR based fiber optic sensor includes SPR probe at one of the ends of the fiber with the reflecting end face [14,15] and a fiber tip [16,17]. The photonic bandgap fiber based SPR sensors have also been reported very recently [18]. 3. Summary In the present talk we shall present various designs of the fiber optic SPR sensors that have been used to enhance the performance of the sensor. 4. References [1] B. Liedberg, C. Nylander, and I. Sundstrom, ���Surface plasmon resonance for gas detection and biosensing���, Sensors and Actuators 4, 299-304 (1983). [2] A.K. Sharma and B.D. Gupta, ���On the sensitivity and signal to noise ratio of a step-index fiber optic surface plasmon resonance sensor with bimetallic layers���, Optics Communications 245, 159-169 (2005). [3] A.K. Sharma and B.D. Gupta, ���On the performance of different bimetallic combinations in surface plasmon resonance based fiber optic sensors���, Journal of Applied Physics 101, 093111 (2005). [4] K. Sharma, Rajan and B.D. Gupta, ���Influence of dopants on the performance of a fiber optic surface plasmon resonance sensor���, Optics Communications 274, 320-326 (2007). [5] Y. Kim, W. Peng, S. Banerji, and K. S. Booksh, "Tapered fiber optic surface plasmon resonance sensor for analyses of vapor and liquid phases", Optics Letters 30, 2218-2220 (2005). [6] Grunwald and G. Holst, ���Fibre optic refractive index microsensor based on white-light SPR excitation���, Sensors and Actuators A 113, 174 180 (2004). [7] R.K. Verma, A.K. Sharma, and B.D. Gupta, ���Surface plasmon resonance based tapered fiber optic sensor with different taper profiles���, Optics Communications 281, 1486-1491 (2008). [8] R.K. Verma, A.K. Sharma, and B.D. Gupta, ���Modeling of tapered fiber-optic surface plasmon resonance sensor with enhanced sensitivity���, IEEE Photonics Technology Letters 19, 1786-1788 (2007). [9] R.K. Verma and B.D. Gupta, ���Theoretical modeling of a bi-dimensional U-shaped surface plasmon resonance based fibre optic sensor for sensitivity enhancement���, J. Phys. D: Appl. Phys. 41, 095106 (2008). [10] J. Homola and R. Slavik, ���Fibre-optic sensor based on surface plasmon resonance���, Electronics Letters 32, 480-482 (1996). [11] H.Y. Lin, W.H. Tsai, Y.C. Tsao and B.C. Sheu, ���Side-polished multimode fiber biosensor based on surface plasmon resonance with halogen light���, Applied Optics 46, 800-806 (2007). [12] M.H. Chiu, C.H. Shih and M.H. Chi, ���Optimum sensitivity of single-mode D-type optical fiber sensor in the intensity measurement���, Sensors and Actuators B 123, 1120-1124 (2007). [13] Y.S. Dwivedi, A.K. Sharma and B.D. Gupta, ���Influence of skew rays on the sensitivity and signal-to-noise ratio of a fiber optic surface-plasmon-resonance sensor: a theoretical study���, Applied Optics 46, 4563-4569 (2006). [14] D.J. Gentleman and K.S. Booksh, ���Determining salinity using a multimode fiber optic surface plasmon resonance dip-probe���, Talanta 68, 504-515, (2006). [15] H. Suzuki, M. Sugimoto, Y. Matsui and J. Kondoh, ���Effects of gold film thickness on spectrum profile and sensitivity of a multimode-optical-fiber SPR sensor���, Sensors and Actuators B 132, 26-33 (2008). [16] K. Kurihara, H. Ohkawa, Y. Iwasaki, O. Niwa, T. Tobita and K. Suzuki, ���Fiber-optic conical microsensors for surface plasmon resonance using chemically etched single-mode fiber���, Analytica Chimica Acta 523, 165-170 (2004). [17] T. Abrahamyan and Kh. Nerkararyan, ���Surface plasmon resonance on vicinity of gold-coated fiber tip���, Physics Letters A 364, 494- 496 (2007). [18] Gauvreau, A. Hassani, M.F. Fehri, A. Kabashin and M. Skorobogatiy, ���Photonic bandgap fiber-based surface plasmon resonance sensors���, Optics Express 15, 11413-11426 (2007). a214_1.pdf FWG4.pdf �� 2009 OSA/FiO/LS/AO/AIOM/COSI/LM/SRS 2009 FWG4.pdf