The Brillouin Scattering Light Characteristics Of A Chaotic Laser In Optical Fiber And Its Application

The distributed optical fiber sensing technique based on Brillouin scattering can accurately measure the information of temperature and strain at any point along the fiber, and it has many significant characteristics, such as anti-electromagnetic interference, broad bandwidth and large dynamic range. Moreover, its measure accuracy, spatial resolution and measure range are all much better than other distributed optical fiber sensing technologies, and has been widely used in structures health monitoring of bridges, tunnels, dams, buildings, and so on. However, it cannot realize the large measurement range and high spatial resolution simultaneously. Considering that the chaotic laser has high stimulated Brillouin scattering(SBS) threshold, broad bandwidth and small coherent length, we introduce the chaotic laser as the optical source of the distributed optical fiber sensing technique based on Brillouin scattering to solve this problem.This paper has studied the Brillouin backscattering light characteristics after the chaotic laser injecting into the optical fiber. On the basis, the experimental system of the distributed optical fiber sensing based on chaotic Brillouin scattering for temperature measurement has been built and its initial results are analyzed carefully. The main works are summarized as follows:1) The experimental setup and principles of the chaotic laser generated by optical feedback are analyzed in detail. Besides, the chaotic laser output characteristics, such as broad bandwidth, flat RF spectrum, tunable central frequency and coherent length, noise-like variation, have been studied.2) The characteristics of the chaotic Brillouin backscattering light generated by injecting a chaotic laser into a single mode fiber have been studied experimentally. The phenomenon of appearing a well in the chaotic Brillouin anti-Stokes light position has been discussed. We have demonstrated theoretically and experimentally that it is because of the chaotic self-similar properties, i.e., the chaotic laser of the central frequency and the anti-Stokes light frequency has the similar properties, which causes that the chaotic laser in the anti-Stokes light frequency and the central frequency light both change into the Stokes light. Owing to the intensity of the anti-Stokes light is so weak, there appears a well after conversion.3) The effect of the chaotic input power and the fiber length on the Brillouin backscattering Stokes light linewidth and the backscattering light power is analyzed. When the chaotic input power is low and manifests as spontaneous Brillouin scattering, the Stokes linewidth is constant along with the increase of the chaotic input power. When the chaotic input power increases continuously and manifests as stimulated Brillouin scattering, the Stokes linewidth decreases gradually and then nearly remains unchanged, and this variation is the same as the variation of the Stokes linewidth along with the fiber length increasing. Backscattering light power increases rapidly and becomes saturated gradually along with the increase of the chaotic input power and the fiber length. Further, the influence factors of the chaotic stimulated Brillouin scattering light threshold also have been studied. The experimental results show that the chaotic stimulated Brillouin scattering light has a high threshold, which is approximately 19 dB larger than that of the conventional continuous-wave laser.4) The distributed optical fiber temperature sensing experimental system based on chaotic Brillouin backscattering is set up. We introduce the chaotic laser as the optical source directly. By adjusting the reference length, the reference light and the Stokes light with the same random states will generate coherent beat frequency, and the coherent peaks in the different positions of the fiber can be obtained to realize the distributed scanning measurement. The last 200 m of the 5.3km optical fibers is put into the thermostat, and the Brillouin frequency shift variation of the beat frequency spectrum is not consistent with the temperature variation. However, this experimental results have demonstrated the feasibility of the system and provide a base for the further experimental study.