Laser radio-frequency and cavity-enhanced interrogation techniques for strain sensing by fiber Bragg gratings

We report on the implementation of novel, highly sensitive methods for strain measurements using FBG-based sensors. Basically, the strain detection technique rely on frequency modulation of a 1560-nm pig-tailed diode laser in the radiofrequency range with phase-sensitive detection of the FBG reflected signals. In one set-up, the power directly reflected by the fiber grating is demodulated at multiples ofthe sideband frequency. A different approach is based instead on using as a sensor an in-fiber Fabry-Pérot cavity, made of an FBG pair with very high peak reflectivity (> 99 %). Static and dynamic deformation can be applied to the sensors in a controlled manner thanks to a piezoelectric actuator and a loud speaker. In the first case, a minimum detectable strain of the order of 100 nE/iHz, in the quasi-static domain (0.5÷2 Hz), and 2 nEhJHz around 1 kHz. An FFT analysis of the output signals reveals the possibility oftracing dynamic strains up to 20 kHz, this limit being set only by the test device bandwidth. For the fiber interferometer set-up, similar tests have been performed using an electrical strain gauge as a reference probe. The diode laser, in this case, is actively frequency-locked to the FBG cavity, using the Pound-Drever-Hall technique. The resulting error signal is used as a monitor of strain suffered by the cavity fiber. We show that a sensitivity gain of at least one order of magnitude can be obtained with this scheme.