Expansion of spatial measurement range by use of vernier effect in multiplexed fibre Bragg grating strain sensor with synthesis of optical coherence function

Abstract

Adopting a technique named synthesis of optical coherence function (SOCF), we have recently published a system of multiplexed fibre optic sensors with fibre Bragg gratings (FBGs) which may have similar Bragg wavelengths to each other (Hotate et al 2004 Meas. Sci. Technol. 15 148–53, Hotate and Enyama 2003 Proc. 16th Int. Conf. Optical Fiber Sensors (Nara, Japan) pp 522–5). In that system, the coherence function is synthesized into a series of periodical peaks that can be scanned along the fibre. By using one of the peaks as a measurement window, and sweeping it along a string of FBGs, the FBGs can be spatially resolved. To avoid the error signal from coherence peaks other than the designated measurement window, the string of FBGs has to be arranged within one period of the coherence peaks, i.e., the spatial measurement range of the system is limited to one period of the coherence function. In this paper, we propose a novel scheme that uses multiple coherence peaks to detect multiple FBGs. The coherence peaks and FBGs are arranged with the vernier effect, which means the period of the coherence function is slightly different from the interval of the FBGs, so at any moment, only one FBG can meet a coherence peak. In this way, the limitation to the measurement range is solved.