Abstract

The wavelength sensitivity and spectral resolution of Mach–Zehnder fiber interferometers obtained through a combination of two identical uncoated and titanium dioxide (TiO₂) coated long period fiber gratings (LPFGs) is presented and compared with single LPFGs-based refractometric sensors. A set of LPFGs were fabricated in single mode fiber with the resonance band having an amplitude of 3 dB in order to split in half the optical power between the core and the specific cladding modes. The separation between the pair of LPFG written in the fiber was varied between 1 and 3 cm and the thickness of the TiO₂ coating around the fiber ranged from 20 to 40 nm. A wavelength shift sensitivity of 216 nm/refractive index units (RIU) was achieved for the device with 3 cm and a 30-nm thick TiO₂ coating, which presented a spectral resolution of 1.1 × 10⁻⁴ RIU. Despite the lower wavelength shift sensitivity of 142 nm/RIU, attained for a 2-cm long device and 30-nm thick TiO₂ coating, a spectral resolution of 1.8 × 10⁻⁵ RIU was measured, which is one order of magnitude lower than a single LPFG.

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