Phase sensitive optical time domain reflectometry (Φ-OTDR) has been widely used in various applications for its distributed measurement capability of dynamic disturbance along the entire length of sensing fiber. In our previous research, the capacity of Φ-OTDR sensing system has been enhanced with ultra-weak fiber Bragg grating (UWFBG) array for high-precision quantitative measurement on the external disturbance. Multipoint nε level dynamic strain variation has been fully captured with 2m spatial resolution. However, only 5 identical UWFBGs were used in the demonstration. For large-scale sensing network, the detectable fiber length would be limited by the signal-to-noise ratio (SNR) and the dynamic range of the reflection signal. In this paper, the performance limit of the UWFGB array enhanced Φ-OTDR sensing system has been investigated. A numerical simulation was performed to illustrate the relationships between the SNR, sensing length, spatial resolution and the UWFBG reflectivity. The simulation results have shown that when the reflectivity is relatively low, the noise is mainly contributed from Rayleigh scattering. On the other hand, the multiple reflections will dominate the overall noise characteristic with the growing of UWFBG reflectivity. The requirement of detection dynamic range was also analyzed. Simulation has shown that lower UWFBG reflectivity will lead to smaller signal dynamic range, making the selection of detector more practicable. The analysis result is quite useful for designing large-scale sensing network based on UWFGB array enhanced Φ-OTDR sensing system in practical situation.
This paper designs and implies a high precision FBG demodulation system which based on F-P etalon. In order to
reduce the influence of the temperature drift effect, the peristaltic effect, and the nonlinear effect of F-P filter in
traditional tunable filter method, F-P etalon is added as dynamical calibration and wavelength reference. Meanwhile
segmentation demodulation which uses ASE spectral characteristics is applied to achieve high accuracy of the center
wavelength of FBG. The experiment shows that the stability, resolution are 0.65pm, 0.23pm, respectively.
Key words: fiber optics; fiber Bragg grating sensor system; tunable Fabry-Perot filter; F-P etalon; spectrum
The leakage light of an electro-optic modulator (EOM) induced by its finite extinction ratio (ER) may degrade the
performance of Brillouin optical time domain reflectometer sensing system, especially for long distance measurement. In
this letter, the configuration of a high ER probe pulse generator assisted by synchronous optical switch has been
presented. A dual pulses interferometric method was also proposed to determine the dynamic ER value (DER) of the
generated probe pulse. Contrast experiments have been performed to verify the effect of the proposed method in a
BOTDR system and the results have shown that the performance of a long distance BOTDR sensing system can be
improved observably with the proposed high ER probe pulse generator. At the end of a 48.5km sensing fiber, the
maximum uncertainty of temperature measurement has been reduced from 5.2℃ to 0.8℃ with 25m spatial resolution
after we improved the extinction ratio of probe pulse from 35dB to 65dB.