Applications of the Planar Fiber Optic Chip

Abstract

The planar fiber-optic chip (FOC) technology combines the sensitivity of an attenuated total reflection (ATR) element with the ease of use of fiber-optic based spectrometers and light sources to create an improved platform for spectroscopic analysis of molecular adsorbates. A multi-mode optical fiber mounted in a V-groove block was side-polished to create a planar platform that allows access to the evanescent field escaping from the fiber core and has been previously applied to absorbance and spectroelectrochemical measurements of molecular thinfilms. Light generated in a surface-confined thin molecular film can be back-coupled into the FOC platform when the conditions for light propagation within the waveguide are met. In this chapter the current applications of the FOC platform will be presented including spectroelectrochemical measurements, fluorescence detection of a bioassay, a broadband fiber optic light source, and Raman interrogation of molecular adsorbates. In recent years, both planar waveguide-based and fiber-optic-based chemical sensors and biosensors have been developed in an attempt to meet the need for miniature, multifunctional, and sensitive sensor platforms. (Bradshaw et al., 2005; Kuswandi et al., 2001; Monk & Walt, 2004; Plowman et al., 1998; Potyrailo et al., 1998; Reichert, 1989; Tien, 1971; Wolfbeis, 2006) The benefits of fiber optic platforms have led several manufactures of analytical instrumentation to develop inexpensive fiber compatible equipment such as readily available fiber-coupled light sources and spectrometers with standard distal end fiber coupling schemes. Fiber coupled sensing architectures, utilizing the fiber as the optical signal transduction platform, have been developed for various geometries including distal end, tapered, de-clad cylindrical core, U-shape de-clad cylindrical core, and biconical tapered optical fibers. (Leung et al., 2007; McDonagh et al., 2008) Simple distal end fiber optic sensors are commercially available where the exposed core on a cleaved and polished end of a fiber is used as the sensing platform. However, the distal end geometry is limited by low sensitivity due to the small interaction area, analogous to the single-pass transmission absorbance measurement. A second more fragile distal end sensor geometry uses a tapered fiber where the fiber core is etched with HF into a point. The tapered fiber increases the evanescent field amplitude and penetration depth, thus increasing the sensitivity of the platform. Tapered fiber optic sensors are primarily used as fluorescence detection platforms in biochemical and clinical applications. (Anderson et al., 1993; Anderson et al., 1994; Anderson et al., 1994; Golden et al., 1992; Grant & Glass, 1997; Maragos & Thompson, 1999; Thompson & Maragos, 1996; Wiejata et al., 2003; Zhou et al., 1997)