All-Fiber Bimodal Interferometric Sensor With Inline Femtosecond Laser Ablated Cavity for Refractive Index Sensing

We report a highly sensitive bimodal interferometric sensor fabricated by the removal of a
$500 \mu \text{m}$
long semicylindrical portion from a commercial, single-mode, step-index, circular core fiber (SM-600) using precision femtosecond laser micromachining (FLM). The open hollow extends from the cladding–air interface to the core–cladding interface. This sensing fiber is offset-spliced with single-mode step-index fibers (SM-450) at the input and output ends, so that it facilitates bimodal propagation at 532 nm in the sensing fiber and generates the interference of the two modes in the lead-out fiber. Differential modal phase shift is achieved in the sensing fiber and we demonstrate an experimental refractive index change of
$2.54\times 10^{-{5}}$
RIU with a resolution of
$3\times 10^{-{10}}$
RIU. The conventional phase sensitivity in rad/RIU was
$4010.77 \pi $
rad/RIU at 532 nm. Our results also demonstrate good immunity to temperature-dependent cross-sensitivity. This all-fiber, proof-of-principle sensor can be fabricated for other wavelength ranges and has the potential for miniaturization and further performance enhancement.