Extended dynamic range detection system for in-fibre Bragg grating strain sensors based on two cascaded interferometric wavelength scanners

A novel interferometric wavelength-shift detection system based on the use of two cascaded two-beam interferometers with different optical path differences (OPDs) is developed for static and quasi-static strain measurement with in-fibre Bragg grating (FBG) sensors. These two interferometers are frequency-division-multiplexed and used to provide two sets of interferometric fringes induced by strain. The optical phase output from interferometer 1 with the larger OPD gives a high-resolution measurement whilst the phase output from interferometer 2 with the shorter OPD is used to determine the number of the fringes obtained with interferometer 1. Compared with the detection scheme using a single interferometric scanner, the unambiguous measurement range has been considerably improved. An experimental system with enhancement factors between 11 and 40 in the unambiguous measurement range has been demonstrated for absolute static strain measurement, A large range to resolution ratio of 4 x 10(4):1 has been achieved. Due to the adjustability of the OPDs in these interferometers, the range to resolution required is also variable, making this system suitable for different applications, On the other hand, a range to resolution of 4 x 10(4):1 makes FBG strain sensors potentially competitive with traditional strain gauges with the well known benefits of fibre-optic sensors. Therefore, it can be anticipated that after further engineering such a system will find important applications in the field of fibre-optic smart structures.