Double clad fiber improves the performance of a single-ended optical fiber sensor

Single-ended, amplitude- or wavelength-modulated optical fiber sensors provide an attractive platform for minimally invasive chemical and biological sensing applications. However, in many cases, the sensor performance is limited by the collection efficiency of diffuse scattered and/or reflected light at the distal end of the waveguide, relative to background light generated within the waveguide. Using surface-enhanced Raman scattering (SERS) as an exemplary weak source of diffuse back-scattered light, we have evaluated the performance of single-ended sensing schemes based on hollow-core photonic crystal fiber (PCF) and double-clad fiber (DCF) compared with more conventional single-mode fibers (SMF-28 and S405). The SERS substrates were fabricated by oblique-angle deposition on the optical fiber end faces. In order to generate a comparable SERS substrate at the end face of the hollow-core PCF, a graded-index (GRIN) coupler was spliced onto the cleaved end face. Similar GRIN lens couplers were spliced onto the solid-core fibers for comparison with the PCF, as they are not expected to provide any advantage for coupling diffuse scattered light. The PCF offered no reduction in fiber background signal compared with either the single-mode fiber or the DCF with excitation through the inner core. In contrast, the DCF was found to provide up to 12-fold improvement in the signal-to-background ratio. These results indicate that DCF holds great promise as a single-ended optical fiber sensor platform.