All photonic bandgap fiber spectroscopic system for detection of refractive index changes in aqueous analytes

We propose and experimentally demonstrate a sensor of refractive index (RI) of aqueous analytes based on a hollow-core low-refractive-index-contrast Bragg fiber. Variations in refractive index of liquid analytes filling the hollow core of Bragg fiber could modify the resonant condition of the modal confinement, thus leading to both spectral shifts and intensity changes in the fiber transmission. As a proof-of-principle demonstration, we characterize sensor performance using a set of NaCl solutions with different concentrations. The experimental sensitivity of the Bragg fiber sensor is found to be ∼1400 nm/RIU (refractive index unit), which is comparable to those of surface plasmon resonance sensors. Moreover, we demonstrate the integration of a Bragg fiber bundle spectrometer in the liquid-core Bragg fiber sensing system to replace the grating-based monochromator, which could potentially lead to significant cost-saving and increased sensing speed. In summary, we demonstrate an all-photonic-bandgap-fiber RI sensing system that uses a hollow core Bragg fiber to hold and probe the analyte, and a solid-core Bragg fiber bundle for spectral interpretation of the transmitted light.