Long Range Surface Plasmon Resonance Based Taper Fiber Optic Sensor with Enhanced Sensitivity using Au Nano-Layer through Radially Polarized Light

A new theoretical approach towards the sensitivity enhancement of tapered fiber optic sensor based on the long range surface plasmon (LRSP) resonance technique in Teflonmetal coated tapered fiber structure using radially polarized is proposed, designed and simulated within Kretschmann-Raether geometry. The configuration comprising a fiber core coated with 690 nm and 70 nm thin Teflon layer and Au layer respectively where the uniform taper waist having diameter 330 µm and 350 µm with taper ratio of 1.7 (NA: 0.25, 10 mm long waist region) and a sensing layer having varying refractive index from 1.333 to 1.353. With the increase in refractive index the observed results indicates a 1.7 times better sensitive tapered fiber sensor as compared to the existing LRSP based fibre optic sensors using intensity interrogation technique. To the best of our knowledge several articles have been devoted in the field of LRSP based fibre optic sensor with p-polarized light whereas no such article has yet been reported with Teflonused as a dielectric between metal coated taper fiber optic sensors using radially polarized light with better sensitivity. Here sensitivity also analysed w.r.t wavelength interrogation technique where the sensitivity enhancement is about 1.7 times than the existing fiber optic sensors.


INTRODUCTION
It is well known that surface plasmon is a coherent oscillation of electromagnetic wave at the metal-dielectric interface [1]. Only TM polarized light can excite them where the associated field has a maxima at the interface and decays exponentially in the metal and dielectric respectively [2]. For the elevation of surface plasmon in surface plasmon resonance (SPR) based sensor structures, Kretschmann's configuration based on impaired total internal reflection is used in the preferred configuration [3]. In this method, over the base of high index coupling prism, a thin layer of metal is deposited and evanescent wave is produced which travel through the prism-metal interface and elevate SPR [4]. In case of SPR based fiber optic sensor, the cladding is removed from the middle part of the fiber and coated with required layer over the unclad core. When a light beam from a respective source (polychromatic) is incident on the one end of the fiber, the transmitted light spectrum is recorded at the other end. The SPR at the metal-dielectric interface is excited by evanescent wave created at core-metal interface [5]. At resonance condition, when the propagation constant of evanescent wave equal to the propagation constant of surface plasmon then SPR excitation occurs.In case of wavelength int resonance wavelength a dip in the transmitted wavelength is maximum transfer of energy from evanescent wave to surface plasmon resonance wavelength [6], [7]. The resonance wavelength depends on the refract which leads to a change in theresonance wavelength. The refractive index of the d medium can be recognized by evaluating the movement [11].By using the symmetric configuration of the radially polarized beam (RPB), in fiber also SPR cab be excited more efficiently. RPB has a polarization ordered on the optical axis. Computer generated hologram are one of the method used to obtain radially polarized beam to the TM polarized light in flat bulk system.

PROPOSED SCHEME
The fiction and characterization of fiber optic senor based on SPR is reported in this article where a thin layer of Teflon AF over it to intensify the sensitivity. A detailed theoretical study of sensing enhancement by modeling the SPR sensor with taper probe has been transition region of the taper is taken in account to study the sole effect of the taper profile sensitivity. A plastic clad silica taper fiber of core diameter 330 µm and numerical apertur 0.25 was used to invent the taper 1.333 to 1.353. Here around 10 m The unclad portion was cleaned by high tension cleaned by acetone.Then, using thermal evaporation technique the unclad portion of taper fiber was coated with Teflon layer of 690 nm thickness in vacuum coating unit at 5×10 -6 mbar of pressure. Using the same technique, an additional layer of gold (Au) was coated over the Teflon of thickness 70 nm to case of wavelength interrogation method used in fiber senso n the transmitted wavelength is obtained because maximum transfer of energy from evanescent wave to surface plasmon which occurs The resonance wavelength depends on the refract to a change in theresonance wavelength. The refractive index of the d evaluating the movement in the resonance wavelength [8 symmetric configuration of the radially polarized beam (RPB), in fiber also SPR cab be excited more efficiently. Ascribed to the rotational symmetry of the fiber, the RPB has a polarization ordered on the optical axis. Computer generated hologram are one of the method used to obtain radially polarized beam. SPR excitation can be possible only due bulk system. and characterization of fiber optic senor based on SPR is reported in this article where a thin layer of Teflon AF-1600 coated over the fiber. A nanometer thin coated gold the sensitivity. A detailed theoretical study of sensing enhancement by modeling the SPR sensor with taper probe has been shown here. As an active SPR region, the transition region of the taper is taken in account to study the sole effect of the taper profile A plastic clad silica taper fiber of core diameter 330 µm and numerical apertur taper fiber SPR probe forsensing the refractive index 10 mmof length of fiber is removed from the middle portion.
LRSP based Teflon-Au coated tapered multi-mode fiber with radially polarized he unclad portion was cleaned by high tension bombardment on vacuum chamber after being , using thermal evaporation technique the unclad portion of taper Teflon layer of 690 nm thickness in vacuum coating unit mbar of pressure. Using the same technique, an additional layer of gold (Au) was coated over the Teflon of thickness 70 nm to intensify the sensitivity of the sensor in fiber sensor, at obtained because of the occurs only at The resonance wavelength depends on the refractive index to a change in theresonance wavelength. The refractive index of the dielectric n the resonance wavelength [8]symmetric configuration of the radially polarized beam (RPB), in fiber also to the rotational symmetry of the fiber, the RPB has a polarization ordered on the optical axis. Computer generated hologram are one few . SPR excitation can be possible only due and characterization of fiber optic senor based on SPR is reported in this article 1600 coated over the fiber. A nanometer thin coated gold the sensitivity. A detailed theoretical study of sensing enhancement by here. As an active SPR region, the transition region of the taper is taken in account to study the sole effect of the taper profile A plastic clad silica taper fiber of core diameter 330 µm and numerical aperture of active index varies from oved from the middle portion. shown in Fig. 1. The several output from the proposed arrangement cab be identify using the following points.

Transmitted power:
Here it isassumed that most of the light is transmitted through the meridian plane that wasintroduced into the sensing fiber at the central. Then the effective light intensity to SPR excitation is defined as [12], [13]- Where I is the light intensity, Ф is defined as the angle between the polarization direction and a meridian plane. ɑ istheeffective intensity coefficient. Therefore the normalized transmitted power is modified to - Here R ,is the reflection coefficient, the value of the effective intensity coefficient is 1 and ½ for p-polarized beam [13], [14].

Sensitivity Analysis:
The propagation constant of the surface plasmon rises significantly due to a charge in the refractive index,

3.
RESULTS AND DISCUSSION By analytical method the simulated the transmitted power vs. wavelength spectra having 690 nm of Teflon layer thickness, 70 nm of gold layer thickness with 330 µm and 350 µm taper waist diameter have been represent in Fig. 2 and Fig. 3 respectively. With the increment of in refractive index from 1.333 to 1.353 the output power also increases.Analysis of the performance of the sensor can be done through Different taper waist diameter. Fig. 4 indicates that the resonance wavelength linearly increase with the increased refractive index for 330 µm and 350 µm taper waist diameter of the taper fiber.
With more increased sensitivity using 330 µm taper waist region Fig. 5 shows the output power vs refractive index graph for sensing medium using radially polarized beam and ppolarized beam respectively. The analysis of sensitivity between radially polarized beam and p-polarized beam clearly depicts that using the radially polarized beam 1.7 times better sensitivity can be achieved as compared to p-polarized light beam.

CONCLUSION
With the radially polarized light the sensitivity analysis is investigated for wavelength as well as intensity interrogation technique on taper waist region (waist diameter of 350 µm and 330 µm) and Teflon-Au interface in proposed sensing probe. By wavelengthand intensity interrogation technique with radially polarized light, the taper fiber probe increases the sensitivity 1.7 times as compared to the symmetrical fiber probe. This sensitivity enhancement is achieved owing to the radial field distribution of radially polarized light at the fiber surface. Moreover several articles have been reported to detect the refractive index variation through different fibre structures, but no such article has yet been reported with better sensitivity with radially polarized light for detecting the variation of refractive index of the sensing layer. Thus with the enhanced sensitivity the presented article is a unique SPR based taper fiber sensing probe to detect the refractive index variation using with radially polarized light where the sensitivity investigation is in wavelength and intensity interrogation techniques.