Interrogation of FBG-based strain sensors by means of laser radio-frequency modulation techniques

We report on novel, highly-sensitive methods for interrogation of fibre Bragg gratings (FBGs) as well as high-finesse fibre resonators. Basically, the strain detection technique relies on radio-frequency modulation of a telecom distributed-feedback diode laser with phase-sensitive detection of the sensor-reflected signals. In a first set-up, the optical power from a fibre grating is demodulated at multiples of the sideband frequency and a dispersive signal, which monitors thermal and mechanical stress on the FBG, is generated. A fast Fourier transform analysis of this signal revealed the possibility of detecting dynamic strains up to 20 kHz, this limit being set only by the bandwidth of the test device. Minimum detectable strain levels below 200 ne Hz(-1/2), in the quasi-static domain (0.5-2 Hz), and between 1 and 4 n epsilon Hz(-1/2) in the 0.4-1 kHz range, were achieved. A different approach is based on an in-fibre Fabry-Perot cavity, made of an FBG pair with very high peak reflectivity (>99%). In this scheme, the diode laser was actively frequency-locked to the FBG cavity, using the Pound-Drever-Hall technique. The resulting error signal was used as a monitor of the strain suffered by the intra-cavity fibre. We demonstrated that a sensitivity gain of at least one order of magnitude could be obtained by this system in a very compact design. Analysis and quantification of the main limiting factors were also carried out in both cases.