  We present an innovative method to enhance Long Period Optical Fiber Gratings
(LPGs) for refractive index sensing using microsphere-assisted superlens laser
nanostructuring. This technique involves self-assembling a silica microsphere
monolayer on LPGs' outer surface, followed by pulsed laser irradiation to
generate nanoholes (300-500 nm) forming nanohole-structured LPGs (NS-LPGs). In
experiments, two nanohole densities were compared for their impact on sensing
performance in sucrose and glycerin solutions. The nanostructured NS-LPGs
showed improved sensitivity by 16.08% and 19.57% compared to regular LPGs, with
higher nanohole density yielding greater enhancement. Importantly, the
permanent nanohole structures ensure durability in harsh environments,
surpassing conventional surface-coating-based LPGs. Further improvements can be
achieved by refining nanostructuring density and controlling nanohole size and
depth. Our work represents a notable advancement in LPG sensor engineering,
prioritizing surface nanostructuring over nano-coating, promising enhanced
refractive index sensing applications.
