Highly sensitive static strain fiber-Bragg grating sensor using an interrogating laser locked to stable atomic lines

Due to their fiber-based, highly localized and wavelength cncoded operation, fiber Bragg gratings (FBG) offer attractive sensing possibilities, especially in strain and temperature embedded sensing of smart structures. In this paper we present a novel, passive interrogation method for FBG static strain measurements, capable of very high sensitivity and high resolution. The obtained sensitivity is comparable to the best results obtained using other interrogation techniques. In order to detect the static strain-induced wavelength shift in the reflection signal of an FBG, a stable wavelength reference should be used. Possible frequency references include another isolated FBG [1], an interferometric reference - e.g. a Fabry-Pcrot cavity, or an atomic or molecular transition. The highest long-Term stability and accuracy is achieved by locking to atomic/molecular references. These absorption lines arc very stable with respect to the operating conditions. For example, semiconductor lasers locked to Rubidium (Rb) Doppler-broadencd lines have negligible temperature-induced frequency shift ( lMHz/°C) and very small intensity induced shift (-5 MHz/(mW/cm2) [2J. For comparison, the center reflection frequency of an FBG has a temperature-induced frequency shift of -1.2 GHz/'C. Even a thermally-compensated FBG has a much larger shift than that of atomic or molecular lines.