Distributed refractive index sensing with cascaded TFBGs via derivative spectrum analysis

Tilted fiber Bragg gratings (TFBGs) are highly sensitive refractometric probes, but their broad cladding-mode spectra have long been considered incompatible with wavelength-division multiplexing. As a result, TFBG refractive-index sensors are generally restricted to single-point operation, despite their inherent advantages. Here, we demonstrate for the first time that multiple bare, uncoated TFBGs (inscribed with distinct grating periods and tilt angles) can be cascaded within a single-mode fiber and independently interrogated over an 80-nm bandwidth. Although the cladding-mode spectra strongly overlap, we show that first-order derivative spectrum analysis isolates the local slope of each cut-off mode, effectively suppressing the envelope distortions typically induced by upstream gratings. This enables reliable and decoupled multipoint refractive-index sensing, with sensitivities of 52-58 nm/RIU that are fully consistent with intrinsic TFBG performance in aqueous media and remain stable after cascading. The derivative method enhances the demodulation accuracy by up to 65% and preserves linearity (R² = 0.94-0.98) while maintaining cross-talk below 0.03 nm. These results overturn the longstanding belief that TFBG refractometers cannot be multiplexed, paving the way for compact, low-cost, and scalable quasi-distributed chemical and biological sensing networks based on TFBG arrays.