Design and fabrication of short-period long period gratings for refractive index sensing

Fiber optic sensors offer high sensitivity and remote sensing capabilities. This study explores short-period long period gratings (SP-LPGs), fabricated via femtosecond laser, as compact and highly sensitive alternatives to conventional LPGs for refractive index (RI) sensing. Gratings with different periods were modeled and tested using a microfluidic setup. The shortest period (Λ = 13 μm) showed high sensitivity and RI resolution (1.1 × 10^-6 RIU), while the longest provided better linearity. An intermediate-period grating (Λ = 14.25 μm) offered optimal performance, demonstrating that period selection allows for tailoring sensitivity and linearity. These results make SP-LPGs promising for applications in medical diagnostics and environmental monitoring.