Demonstration of a spectral profile multiplexing algorithm to demodulate fiber bragg grating strain sensors with a narrowband light source

Vertical cavity surface emitting laser (VCSEL) light sources are cheap, lightweight lasers with the potential to serve as light sources in fiber bragg grating (FBG) sensing systems. VCSEL sources are ideal for aerospace nondestructive evaluation applications, where available power may be limited. The drawback to using VCSEL sources for multiplexed sensor arrays is the narrow bandwidth of light that they produce. Currently, this narrow bandwidth limits the number of FBGs that can be multiplexed onto a single fiber. A new signal processing algorithm has been developed using spectral profile multiplexing. The algorithm identifies the wavelength location of individual sensors based on their unique spectral profiles and an evolutionary algorithm used to correlate the measured array spectrum with a predicted one. This work demonstrates the multiplexing of four FBG sensors using a single 0.5 mW VCSEL with modulated current to sweep a 6 nm bandwidth of light. Strain sensing results are also compared with those collected from a commercial high-power interrogator to provide the upper performance limit. The goal of this research was not to outperform the commercial interrogator, but instead to determine how much performance is lost in order to achieve a low-power system. The experimental results demonstrate the potential to use this approach when collecting data from a low-power strain sensor array for material testing; however, they also highlight the importance of noise levels on the quality of strain data obtained and the improved signal processing that would be required to achieve sufficient wavelength location accuracy.