Enhanced sensitivity optical fibre sensors using subcarrier gratings

Volanthen, M., Geiger, H. and Dakin, J.P. (1997) Enhanced sensitivity optical fibre sensors using subcarrier gratings. IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring.

Abstract

Fibre Bragg gratings (FBGs) have been widely used to sense strain and temperature since both parameters change the resonant frequency of the grating. The grating is suited to sensing applications since the resonance is encoded in the reflected wavelength, which is unaffected by amplitude fluctuations in the interrogation system. Recently sensors have been made using long period gratings (LPGs), gratings with a pitch of the order of 250µm (resonant at 400GHz). Similar to the FBG, physical fields effect the resonant frequency, however harmonics of this frequency are interrogated, rather than the fundamental resonance. We propose the first subcarrier fibre grating (SFG), a very long period fibre grating. The SFG is resonant at RF frequencies, determined by the spacing of the reflectors. The resonant frequency may be interrogated by observing the interference of RF modulation carried on a low-coherence optical signal. This interrogation method using subcarrier interferometry is similar to interrogation systems previously used with long-gauge-length optical fibre sensors. In such sensors the value of the measurand is encoded in the subcarrier frequency, a parameter that is also independent of amplitude fluctuations and which may be measured simply, rapidly and accurately.
Previous sensors interrogated by subcarrier interferometry have defined the sensing section by a single reflector at each end. The SFG is effectively one such sensing section, but with additional reflectors periodically located between the two end reflectors. Compared with previous subcarrier sensors, the SFG exhibits enhanced sensitivity and provides limited information on the spatial uniformity of the measurand.

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