Laser interrogation techniques for high-sensitivity strain sensing by fiber-Bragg-grating structures

Novel interrogation methods for static and dynamic measurements of mechanical deformations by fiber Bragg-gratings (FBGs) structures are presented. The sensor-reflected radiation gives information on suffered strain, with a sensitivity dependent on the interrogation setup. Different approaches have been carried out, based on laser-frequency modulation techniques and near-IR lasers, to measure strain in single-FBG and in resonant high-reflectivity FBG arrays. In particular, for the fiber resonator, the laser frequency is actively locked to the cavity resonances by the Pound-Drever-Hall technique, thus tracking any frequency change due to deformations. The loop error and correction signals fed back to the laser are used as strain monitor. Sensitivity limits vary between 200 n?/?Hz in the quasi-static domain (0.5/2 Hz), and between 1 and 4 n?/?Hz in the 0.4-1 kHz range for the single-FBG scheme, while strain down to 50 p? can be detected by using the laser-cavity-locked method.