An ultra-sensitive and compact refractive index sensor based on intense cladding-modulation

This paper proposes an ultrasensitive and compact refractive index sensor based on an intensely cladding-modulation long-period fiber grating (ICM-LPFG), fabricated solely by periodic symmetric deep D-shaped grooves on both sides of a single-mode fiber using a CO₂ laser. The sensor demonstrates ultrahigh sensitivity up to 6358.96 nm/RIU within the refractive index range of 1.3858–1.4270, features a low temperature sensitivity of 50.92 pm/°C, possesses a minimal sensing length of merely 2.06 mm, and exhibits a low temperature cross-sensitivity of only 8.01 × 10^-6 RIU/℃. This sensor offers significant advantages in fabrication simplicity, structural compactness, and low-temperature cross-sensitivity. To our knowledge, it achieves ultrahigh sensitivity among reported all-fiber LPFG-based RI sensors requiring neither expensive equipment nor specialized processes. It demonstrates promising potential for applications in biochemical and environmental monitoring fields demanding miniaturization and cost-effective solutions under complex environmental conditions.