Sensitized Distributed Fiber Sensing System Based On Brillouin And Rayleigh Scattering

Distributed optical fiber sensing technology can be widely used in structure health monitoring of large scale, long distance and high dager such as civil engineering, fiber communication, power industry and so on due to its characteristics such as long range/large scale sensing ability, multiple parameters sensitivity and high accuracy. Temperature and vibration which important in various situations, are two typical parameters for static and dynamic parameters respectively. Brillouin optical time domain reflectormetry(BOTDR) and phase sensitive OTDR(Φ-OTDR) based on coherent Rayleigh scattering are two kinds of famous optical fiber sensing technologies. BOTDR can achieve distributed temperature and strain sensing along the fiber; and Φ-OTDR can obtain multiple points sensing for vibration events. The studies about these two techniques used in distributed sensing have been widely reported and have been mature. However, there are still room for improvement. Limited by the weak backscattering Brillouin signal, the temperature accuracy and sensing length are restricted. In addition, in conventional Φ-OTDR whose leaser source with a fixed wavelength, the cohertent Rayleigh signal is only sensible to vibration, thus the sensing parameter is sole, and the frequency response range is limited by the repetition of optical pulses. According to these shortcomings, the main works of this thesis are as follows:1. To obtain distributed temperature sensing with high dynamic range and spatial resolution and high, a distributed Brillouin temperature fiber sensor using a single photon detector has been proposed. Based on single photon detection technique, the problem of weak signal detection in BOTDR can be overcome. In our scheme, the Rayleigh/Anti-Stokes ratio (RASR) is used to measure the temperature information along the sensing fiber. A cascading fiber Bragg grating (FBG) filter is employed to separate Brillouin anti-stokes signals from the backscattering Rayleigh light with a--23dB high rejection ratio. The system exhibits a dynamic range of 18dB (corresponding to 90km sensing length) with a measured temperature error lower than 1.0℃ without any optical amplification. Furthermore, the sensor shows a 1.2m spatial resolution and 2.28℃ temperature error with an 8.5dB dynamic range, which means the sensing length can be extended to 42.5km. This scheme can simultaneously achieve large dynamic range and high spatial resolution.2. To obtain a distributed sensor sensitive to both temperature and vibration, we have proposed and experimentally demonstrated a distributed vibration and temperature sensing system based on the hybrid of Φ-OTDR and fiber Bragg grating (FBG) pair. An interrogation technique has been proposed by treating the low reflectivity Gaussian-shaped FBGs pair as an edge filter that linearly translates the wavelength shift into reflection intensity variation. Closely spaced identical low reflectivity Gaussian-shaped FBGs was embedded into the sensing fiber, so that the same narrow line-width laser used in Φ-OTDR could be employed to pump the FGBs. The experimental results have shown that this designed hybrid sensing system could realize fully distributed vibration sensing along a 20.4km long fiber with 10m spatial resolution, and the perturbation events of 81 Hz can be obviously distinguished with a remarkable signal-to-noise ratio (SNR) of 7.5dB. And multipoint temperature sensing could be obtained at the same time with a resolution superior to 0.12 ℃.3. A hybrid single-end-access Mach-Zehnder interferometer (MZI) and O-OTDR vibration sensing system is proposed and demonstrated experimentally to extend the frequency response range for distributed vibration sensor based on Φ-OTDR. In our system, the narrow optical pulses and the continuous wave are injected into the fiber through the front end of the fiber at the same time. And at the rear end of the fiber, a frequency-shift-mirror (FSM) is designed to back propagate the continuous wave modulated by the external vibration. Thus the Rayleigh backscattering signals (RBS) and the back propagated continuous wave interfere with the reference light at the same end of the sensing fiber and a single-end-access configuration is achieved. The RBS can be successfully separated from the interference signal (IS) through digital signal process due to their different intermediate frequency based on frequency division multiplexing technique. There is no influence between these two schemes. The experimental results show 10m spatial resolution and up to 1.2MHz frequency response along a 6.35km long fiber. This newly designed single-end-access setup can achieve vibration events locating and high frequency events response, which can be widely used in health monitoring for civil infrastructures and transportation.