Elsevier

Biosensors and Bioelectronics

Volume 20, Issue 7, 15 January 2005, Pages 1312-1319
Biosensors and Bioelectronics

Effect of taper geometries and launch angle on evanescent wave penetration depth in optical fibers

https://doi.org/10.1016/j.bios.2004.04.026Get rights and content

Abstract

A large penetration depth of an evanescent wave is the key to success for developing an ultra high-resolution fiber-based evanescent wave biosensor. Tapering the fiber and launching light at an angle has the potential of increasing the penetration depth of evanescent wave manifolds. The effects of tapering, launch angle and taper length of the fiber have been explored in detail using a ray-tracing model to calculate the highest possible penetration depth of the evanescent field. Evanescent wave penetration depths of the order of the size of living cells have been achieved by optimizing the parameters relating geometry of tapered fibers.

Introduction

Biological systems (such as tissues, micro-organisms, enzymes, antibodies, nucleic acids, etc.) when combined with a physico-chemical transducer (optical, electrochemical, thermometric, piezoelectric) form a biosensor. Recently, optical fibers have become an important part of sensor technology (Love et al., 1991, Love et al., 1991, Willer et al., 2002, Axelrod, 2001). Their use as a probe or as a sensing element is increasing in clinical, pharmaceutical, industrial and military applications. Excellent light delivery, long interaction length, low cost and ability not only to excite the target molecules but also to capture the emitted light from the targets are the main points in favor of the use of optical fibers in biosensors.

Fluorescence techniques provide sensitive detection of biomolecules. Furthermore, since fluorescence intensity is proportional to the excitation intensity, even weak signals can be observed. In last decade reagentless fiber-based biosensors have been developed (Love et al., 1991, Love et al., 1991). These biosensors are capable of detecting changes in cell behavior, metabolism and cell death when exposed to toxic agents.

A large signal-to-noise ratio in evanescent wave sensors compared to distal end sensors is a potential advantage of evanescent wave-based sensors. The evanescent wave configuration is an effective method of detecting various drugs of abuse, toxic materials, chemical and biological agents. Sensors based on evanescent waves allow highly sensitive monitoring and measuring of various physical and chemical parameters (Willer et al., 2002). Use of evanescent waves makes it possible to excite the cells near to the solid surface and provides quantitative information about the position, composition and motion of even a single cell (Axelrod, 2001).

In an optical fiber, if the refractive index of the surrounding medium (ns) is less than the core refractive index (n1) then the power in the fiber core is insensitive to the environmental changes in the surroundings. On the other hand, if (nsn1) and the fiber cladding is only of few wavelengths (<5), then the power in the core decreases exponentially and will penetrate into the surroundings (Wang et al., 1995). However, in fibers where the refractive indexes are not equal, an evanescent wave (EW) effect is still experienced with a penetration depth of the order of wavelength.

The evanescent wave exponentially decays as a function of radius of the fiber (Love et al., 1991, Love et al., 1991). The strength of the evanescent field (energy per unit area per second) depends on several factors, refractive index of the core (n1), refractive index of the aqueous surrounding medium (naq), the core radius (R0) and operating wavelength (λ). There are two main difficulties faced in fiber-based evanescent wave biosensors. Firstly, in comparison to distal end biosensors, only a small amount of power is available in the evanescent wave sensors for generating a fluorescence signal (Snyder and Love, 1983). Secondly, in signal acquisition, there is only a low coupling efficiency of the fluorescence signal back to the fiber itself (Thompson and Kondracki, 1995). Thus, there is a critical need to optimize the design of the fiber-based optical sensor leading to high excitations and a high level of fluorescence signal acquisition at the output end of the fiber.

If the fiber is made to operate in an evanescent mode then it senses a thin region around the fiber core and all along the length of the fiber. Earlier work has shown dependence of the strong evanescent wave absorption in the cladding region, which depends on the length of the fiber, fiber radius, launch angle and the wavelength of light used (Love et al., 1991, Love et al., 1991). Prior studies found that the coupling of the laser light into the fiber would produce more evanescent wave absorption if it enters into a tilted fiber rather than a non-tilted fiber (Love et al., 1991, Love et al., 1991). Another way of enhancing the EW absorption is tapering the fiber. Tapering the optical fiber can increase the strength of the evanescent waves in the cladding region, which increases the excitation events in the cladding (Messica et al., 1996). From a biological point of view, increasing the penetration depth is critical as this increases the emission (fluorescence) or scattering process (Raman) inside the biological cells. Both fluorescence and Raman scattering give useful structural information of the cells but only if the penetration depth extends into the cell.

The aim of this paper is to establish the design conditions for a high EW penetration depth so that sufficient amounts of power can be transmitted to create fluorescence photons from the target molecules in the surrounding medium and return to the core of the fiber. This paper describes the theoretical results for variation of penetration depths of evanescent waves as a function of taper ratios of tapered fiber in various taper geometries, taper length and launch angles. Large penetration depths of the order of the size of living cells have been achieved.

Section snippets

Theory

We propose a new design of biosensor based on detecting the intrinsic fluorescence of living cells like nicotinamide adenine dinucleotide (NADH) of refractive index 1.33. To increase the intensity of the EW, efforts are made to increase the penetration depth in the region surrounded by the core of the fiber. The cladding is removed to enhance the penetration depth of the EW and the fiber is placed in an aqueous medium, which acts as the cladding of the fiber.

When light is incident on the

Results and discussion

In our calculations, the incident light of wavelength 350 nm from a laser diode is used and the taper length is assumed to be 1.0 cm and the core radius is 100 μm. The refractive index of the core is given by the numerical aperture of the fiber. The refractive index of the cell living on the fiber is considered to be 1.33. The programs were written in Matcad version 11. While using a tapered design the fiber cladding has been removed. The incident light is sent into the fiber at various angles

Conclusions

Commercially available low NA silica fiber can be used as a sensing element if the design characteristics of taper ratio, taper length and the launch angle have been optimized to produce a large penetration depth of the evanescent wave. All three taper geometries, linear, parabolic and an exponential, give the same useful results of high penetration depths of an evanescent wave. The V-number mismatch restriction reduces the fiber core radius to a minimum radius to reduce the fluorescent losses.

Acknowledgements

The authors acknowledge partial financial support of the UK Engineering and Physics Research Council (MA) and the US DARPA (LH). The kind assistance of Professor Terry King (Laser Photonics, University of Manchester) is appreciated.

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