Two Distributed Brillouin Scattering Fiber Sensing Systems: Study and Comparison

• Main Authors: YU-CHENG CHOU, 周郁呈
• Other Authors: Shien-Kuei Liaw
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/46089367910045438341

碩士 === 國立臺灣科技大學 === 電子工程系 === 105 === The aim of this thesis is to improve a distributed fiber sensing system based on Stimulated Brillouin scattering (SBS) effect while the improved characteristics include spatial resolution and sensing range. What’s more, we compare the differences between Brillouin optical correlation domain analysis and Brillouin optical time domain analysis. In the experiment, we introduced the measuring principle of distributed fiber sensing system and defined the importance of parameters. While the spatial resolution was affected by the reference frequency and the sideband component suppressed in the probe. Among choosing the reference frequency from 0.125MHz to 5 MHz, we found that the accurate data could be obtained when the reference frequency was set at 0.125 MHz. Therefore, we set the reference frequency of 0.125 MHz as a parameter in this study. Before we purchased the single sideband modulator, the homemade tunable fiber Bragg grating using for obtaining single sideband modulation was discussed. At first, we design and investigate the reflection type (R-type) setups for SSB to improve the data. However, it was not good enough because the difference of optical power between the upper sideband (USB) and lower sideband (LSB) was only about 20 dB by using R-type setup. Finally, the suppression ratio of the other frequency components in the probe was more than 30 dB by combining a single side band modulator (SSBM) with a tunable fiber Bragg grating (FBG). In the original experimental setup, the fiber Fabry-Perot (FFP) tunable filter was used before the lock-in amplifier (LIA) in order to prevent the power from getting too high. We also adjusted the sensitivity of the lock-in amplifier so we could read the data correctly without FFP-tunable filter and optimize the input signal quality. Afterward, we replace the new DFB-LD with better output power and narrower linewidth because it was useful for single side band modulation. The narrow linewidth not only improved the system performance but also made the correlation peak narrower; that is, it was similar to a narrower sensing point. Moreover, by replacing the signal generator, the operation time on the frequency sweep would be faster; while the speed increased more than two times from 11.66 points/min to 24.07 points/min. As for the modulation amplitude, it was selected from the original 100 mV unit as small as 1 mV so the modulation depth of the selection could be more accurate. Finally, we successfully achieved the measurement range to 313 m with 3 m spatial resolution. Finally, we also analyze BOCDA and BOTDA, BOCDA is suitable for the sensing system with dynamic strain measurement and high precision strain measurement because of the high spatial resolution and fast data acquisition. There has been a challenge to achieve commercial products in BOCDA because the measurement range is not long enough. On the other hand, BOTDA is good at measuring long range so there are commercial products based on BOTDA. To understand the BOTDA system, the future researchers may try to use the existing equipment to design BOTDA system.