All-fibre optical sensing system: Bragg grating sensor interrogated by a long-period grating

We report here a simple, low-cost all-fibre grating strain sensing interrogation technique offering high static-strain resolution (0.5 mu epsilon) and large dynamic range (>8100 mu epsilon). A key interrogating component of this technique is a long-period grating which acts as an edge filter converting strain-induced wavelength variation into optical power measurement. The transmission profile of the long-period grating is shown to be nearly linear over a sufficiently wide range, yielding a linear relationship between the resultant intensity and applied strain, and a high resolution achieved by standard lock-in techniques.