Wavelength independent chemical sensing using etched thermally regenerated FBG

This paper presents the first time studies on chemical sensing characteristics of HF etched thermally regenerated fiber Bragg grating (FBG). No shift in Bragg wavelength, λBragg, of the FBG during HF etching has been observed for thermally regenerated FBGs. This contrasts with etching of conventional Type-I gratings both with and without annealing at temperatures, T ∼ 600 °C for which the regeneration does not occur. These HF etched FBGs are studied for refractive index sensing of four media i.e. air, methanol, ethanol and ethylene glycol for the chemical sensing applications. The regenerated grating has a variation in reflection intensity with little shift in λBragg, i.e. ΔλBragg < 5 pm. For the non-annealed FBG, both reflected power and peak position of FBGs changed with change in refractive index creating a dual parameter problem for sensing. In this case, the wavelength shifted by more than two orders of magnitude i.e. ΔλBragg > 1 nm. For the annealed FBGs at 600 °C, the change in reflected power of the FBG was higher than the non-annealed FBGs while ΔλBragg was comparable. For chemical sensing based on etched regenerated grating, the removal of the wavelength shift from measurements greatly reduces sensor interrogation complexity allowing for simple amplitude based detection schemes.