Generation of Fiber-Tip Leaky Mode Resonance for Decoupling Bulk and Surface Properties

Abstract

Fiber-optic surface waves excited by nano-coatings deposited on the surface of optical fibers feature a strong enhanced evanescent field penetrating into external media, which enables the fiber-optic sensors highly sensitive to surrounding environment characterized by a bulk refractive index (RI). However, the enhanced evanescent field also suffers from changings in the surface of fiber-optic sensors, resulting in the bulk RI sensitivity being inevitably disturbed by the surface characteristics, which could lead to a crucial problem that plagues the sensing performance of fiber-optic systems. Herein, a fiber-tip leaky mode resonance (L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sup> MR) probe is proposed, which is capable of decoupling bulk and surface characteristics around the probe without the need of any reference channels. This simple and compact probe consists of a standard multimode optical fiber with the end facet coated with a nanoscale thick titanium dioxide (TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) thin film, which can be used to effectively excite multiple fiber-tip leaky modes with a strong standing wave pattern in surrounding medium. We equated the changing in surrounding RI and the changing in thickness of surface-localized binding layer on the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> thin film to the bulk and surface properties, respectively. The analysis shows that the generation of non-polarized fiber-tip L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sup> MRs corresponds to the energy transfer or mode coupling from fiber core modes to the phased-matched fiber-tip leaky modes supported by the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> thin film, and further modulation of leaky modes is in turn closely related to the thickness of the film. As the results further revealed, the resonance amplitude and wavelength of fiber-tip L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sup> MRs vary linearly with the changes in bulk RI and thickness of surface-localized binding layer, which makes the fiber-tip L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sup> MRs ideal for discriminating between bulk and surface characteristics around the probe. It is worth highlighting that both the highest bulk and surface sensitivities up to 58.5 dB/RIU and 0.595 nm/nm respectively, together with the smallest cross interference between them, can be attained with a single fiber-tip L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sup> MR, which could greatly facilitate the development of ultra-compact lab-on-tip optrodes.