Mie Scattering-Enhanced Fiber-Optic Refractometer

Abstract

A kind of biconically-tapered optical fiber (BTOF) modified by silica nanospheres is proposed to implement detection of refractive index (RI), especially low RI, with strong nonlinearity and enhanced sensitivity; this idea arose from an interesting experiment with a BTOF whose surface was very rough. The mechanisms of the nonlinear characters are investigated and explained by the coupling effects of Mie scattering and multimode propagation, and this phenomenon is repeated in several experiments with different fibers. Measured sensitivity magnification of more than 100 from a 2.8-µm-thick and 14-mm-long BTOF after modification by 400-nm silica nanospheres is shown in this letter.

Silica nanospheres for filtering higher-order optical fiber modes

“…Coupling between modes due to the nonuniform coating of nanospheres would contribute to a stronger power exchange between HE 11 and HE 12 and thus larger oscillatory amplitude in the experimental spectrum. The slight difference in periodicity between the experiments and theory may be attributed to the simplifications in modeling the TOF [6,13]. Despite these discrepancies, the theory qualitatively reflects the experiments and well explains the phenomenon of filtering higher-order modes by silica nanospheres.…”

“…The process of scattering losses is similar to the absorbance of an optical-fiber absorbing sensor [6], i.e., scattering and absorbance have the same effect because they both cause power loss through the attenuation of the evanescent wave. Therefore, the remaining power of a certain mode propagating through the waist region of the MTOF can be modeled according to the absorbing fiber-optic sensor as follows [6,10]:…”

“…3(d). As a result, while silica nanospheres help increase the sensitivity of a TOF when used for RI detection [6], they make the TOF more immune to the absorbing surrounding medium. In some practical situations, such as measuring the RI of biomolecules, this MTOF may be a promising tool to extract the RI information with little interference from the absorbance properties of the biomolecules.…”

“…In a previous work from our group [6], a tapered optical fiber (TOF) refractometer was enhanced by modifying the waist region with a coating of SiO 2 nanospheres. The combination of nanosphere-induced scattering losses and multimode propagation of the tapered fiber gave rise to a broadened detection range and improved sensitivity for RI sensing.…”

Tuning response range of a transmission-based fiber-optic refractometer through LP_11 mode

“…The main reason may be that the fundamental mode LP 01 transmitted in the nanofiber doesn't have a cut-off frequency, which suggests it's impossible to alter the fact that the largest evanescent field and thus highest sensitivity occur at high RI close to that of fiber, although the diameter is shrunk to a sub-wavelength scale. In one previous work of our group [12], we added a coating of silica nanospheres to the surface of a tapered fiber; this kind of refractometer combined the scattering effect of nanospheres and mode propagation of the tapered fiber; thus higher sensitivity and a wider detection range were achieved. Yet this nanoparticle modified fiber has lower sensitivity at low RI than at high RI, and it's practically difficult to control the nanosphere coating.…”