An approach for reflectivity measurement of ultra-weak fiber Bragg gratings is demonstrated. The scheme uses a referenced high-reflectivity FBG to combine with the measured FBG. They have different center wavelengths. By using two referenced FBGs, the reflectivity of one measured FBG is calculated to be 0.298% and 0.287% respectively. For reflectivity measurement of ultra-weak FBG, to eliminate the influence of the reflection spectra side lobe for the referenced FBG, we get the reflection spectrum of the referenced FBG and measured FBG respectively under the same input intensity. The reflectivity of an ultra-weak FBG is measured to be 0.00916% (-40.38dB) and 0.00803%(-40.95dB) based on the two referenced FBGs. The results presents that the method is feasible.
As one of the main factors of restricting the fiber Bragg grating based Fabry–Perot (FBG-based F-P) sensitivity enhancement system, the intensity will be more and more unbalanced and weakened following the improvement of sensitivity enhancing ability. In this paper, intensity unbalance of the pulse train is analyzed. A secondary wave chopping, amplifying and filtering modified scheme is proposed to solve it. A four centimeters diameter compliant cylinder fiber optic accelerometer is fabricated. An experiment is set to validate the improved system. The phase sensitivity calibration results are consistent with the theory well and truly by phase-generated carrier (PGC) scheme. This system achieves 20 times (26 dB) sensitivity enhancement and 10 dB noise floor improvement at the same time successfully.