ABSTRACT
Biosensors have been attracting much attention in a variety of applications for bio-marker detection in medical diagnostics and pathogen or toxin detection in food and water. Conventional biosensors based on the fluoroimmunoassay technique should exploit the fluorescence labeling of the antigen or target DNA and always require additional reagents [1]. Ordinary biosensors have many disadvantages, such as high cost, a complicated configuration, and the impossibility of real-time detection. To overcome these drawbacks, many methods have been proposed for developing label-free detection biosensors [2,3]. In particular, fiber-optic biosensors are promising devices for high-quality, label-free detection because they can overcome the limitations of bulk refractometers through their high sensitivity, fast response, convenience for in situ and remote sensing over long distances, and immunity to electromagnetic interference [4–8]. Various fiber-optic biosensors using the surface plasmon resonance (SPR) phenomenon have been proposed [1,9,10]. However, it is necessary to precisely design and fabricate the physical structure of SPR biosensors because SPR properties are highly sensitive to metal content, thickness, and biomolecules. Recently, biosensors based on fiber gratings have been widely investigated [11–15]. Most fiber grating–based biosensors have utilized long-period gratings (LPGs) with radiation mode coupling at resonance wavelengths that are very sensitive to variation in the external medium [16]. One simple technique for producing fiber grating–based index sensors is to use LPGs based on cladding mode coupling [4]. To improve the sensitivity of LPG-based biosensors, a variety of devices including colloidal gold modified LPGs [13], LPGs with etched cladding [14], and LPGs with an nanostructured overlayer [15] have been proposed. However, modifying LPGs in these ways is difficult because of the delicate and hazardous procedures and the requirement for additional materials. A more effective method to induce evanescent field coupling is based on using a D-shaped fiber. If part of the cladding region is removed, the external index change can directly interact with the evanescent field of the fundamental mode [17]. To enhance the performance of the D-shaped fiber-based biosensor, it was proposed that the surface LPG be inscribed in the D-shaped fiber to induce resonant coupling [18–22].
