Spectral-mismatch-induced resolution limit of interferometric fiber Fabry-Perot sensor system

The mismatch of fiber Bragg gratings (FBGs) in spectral profiles can lead to a severe degraded resolution of the constructed fiber Fabry-Perot (FFP) sensor system through its effect on the fringe visibility. The variation of visibility induced by spectral mismatch and the corresponding phase resolution limit are analyzed theoretically and experimentally. Theoretical analyses are based on the approximation of Gaussian profiles to the reflection spectra of weak FBGs, especially with consideration of side lobes. The investigation provides an insight into the evolution of the fringe visibility caused by spectral mismatch, and shows good agreement with experimental results. An optimum phase resolution of about 55 μrad/Hz^1/2 above 100 Hz is achieved for a nearly 4 m-long FFP sensor by matching spectral profiles of the gratings and balancing path length differences of the tandem interferometers.