Effect of localized doping in microknot fiber resonators for resonance-shift based sensing

On-fiber microstructures are favorable platforms for sensing applications. We have studied analytically the mechanical bending strength in undoped and doped micro-tapers, and numerically the photonic transmission in 3D resonating microknots defined on optical tapered fibers as functions of radius and doping. In favor of sensing applications, we deduced the microstructure sensitivities to localized refractive index variation in terms of local resonance phase shifts, slopes, dynamical range and refractive-index range of sensitivities. The spectral response in both microknots (MKRs) and microloops (MLRs) have been studied in terms of refractive index variation profile. Based on the analysis of the transmission characteristics in MKRs with varying ambient index variations, we observed a linear response to small refractive index gradients in the NIR wavelength range. We propose that microstructures of 50-micron radii and below, with moderate doping, can exhibit improved resonant-shift based sensing capabilities.