Bi-Tapered Optical Fibers: Signal Analysis for Sensing Applications
Optical fibers have a silica core surrounded by a cladding; the refractive index allows for total internal reflection of light through the core. Depending on the thickness of the cladding, a portion of the electric field permeates the core/cladding interface and decays as a function of distance from the interface resulting in an evanescent wave. Such waves are the basis for the surface plasmon resonance biosensing technique. Bi-tapering of single mode fibers results in a fiber with a much thinned cladding and allows the the field to exist outside the tapered region. Coating the tapered region with biomolecular recognition elements provides a biosensing capability on the fibers: binding of analyte to the recognition layer results in a molecular conformational change that is detected as a change in the light propagation pattern, that is, the refractive index (RI) [2-4]. In this study we focus on the characterization of the fiber signal as influenced by the refractive index of the surrounding aqueous solution.
Single mode fibers were tapered biconically to two different waist lengths (L; 5 and 10 mm), and two different waist diameters (d; 10 and 14 microns). Previous experiments found that these taper dimensions result in total phase changes of less than 2Π for the entire range of refractive indices tested. Optical fibers with 9/125µm core/cladding diameter and pigtail connectors were used. The ends of the fibers were cleaved at a 90° angle with the Vytran LDC-200-G optical fiber cleaving system. These cleaved fibers were spliced with the Vytran GPX-3000 graphite filament, fusion fire-polishing system. Once spliced, the GPX-3000 was used to taper the fiber to the appropriate waist length and diameter, as mentioned above. Fibers were mounted in in a custom Teflon flow cell and exposed to varying concentrations of aqueous glycerol solutions. Refractive index of glycerol solutions was measured using an ABBE-3L refractometer; the solution refractive indices ranged from 1.3330 to 1.3405. The signal was generated using tunable laser that scans across the wavelength spectrum from 1480 to 1550 nm. The light enters into the SMF fiber, passes down-taper to the fiber waist region, re-enters the up-taper region, and is eventually received at the photodetector.
We applied a novel signal analysis of our transmission data to extract a phase shift related to detection of solution refractive index changes around the bi-tapered fiber. The tapered fibers work by the interaction of (at least) two modes in the taper region: a high order mode (HOM) associated with the cladding and the fundamental core mode. These modes generate an oscillatory response of the optical fiber sensor as the wavelength is scanned. Here we analyzed smaller refractive index variations (~0.0005 RIU) and found that a nearly linear wavelength shift of the interference patterns occurs. The main new element is that the data analysis can resolve extremely small variations in the refractive index. In our analysis we decompose the signal data into Fourier components and use the dominant oscillation frequency for further analysis. The signal analysis enables improved high-resolution detection and this protocol will now be applied to bio-functionalized bi-tapered fibers for the detection of real-world analytes in aqueous solutions.
 B.J. King, I. Idehenre, P.E. Powers, A.M. Sarangan, J.W. Haus and K.M. Hansen, "Tapered optical fibers for aqueous and gaseous phase biosensing applications," Proc. of the SPIE 2013, 85700G-85700G-85710.
 J. Wen Bin, L. Huan Huan, T. Swee Chuan, C. Kin Kee and A. Lim, "Ultrahigh Sensitivity Refractive Index Sensor Based on Optical Microfiber," Photonics Technology Letters, IEEE, 24(2012) 1872-1874.
 S. Lacroix, F. Gonthier, R.J. Black and J. Bures, "Tapered-fiber interferometric wavelength response: the achromatic fringe," Opt. Lett., 13(1988) 395-397.
 G. Salceda-Delgado, D. Monzon-Hernandez, A. Martinez-Rios, G.A. Cardenas-Sevilla and J. Villatoro, "Optical microfiber mode interferometer for temperature-independent refractometric sensing," Opt. Lett., 37 (2012) 1974-1976.