Investigation On The Fiber Sensing System Based On Rayleigh And Brillouin Scattering Effect

The Rayleigh and Brillouin scattering-base distributed optical fiber sensing technologies can provide the measurement of continuous distribution, long distance, high accuracy for the physical quantities, which are particularly suitable for the structural health monitoring (SHM) of large-scale, long distance and high danger such as civil engineering, fiber communication, petrochemical engineering, power industry and so on, which is also a hot research point of sensing technology at present.Optical time-domain reflectometry (OTDR) and Brillouin optical time-domain analyzer (BOTDA) are the two important optical fiber sensing systems that are both based on the optical time-domain reflectometry technique. The former is based on the Rayleigh scattering while the latter is employed the stimulated Brillouin scattering in fiber. OTDR with the virtue of one-end access is one of the most successful nondestructive diagnostic tools for a fibre link. However, the hight spatial resolution OTDR still has significant research value with the development of the all optical networks and the implementation of the fiber to the home (FTTH). BOTDA can provide high-performance sensing over long fiber range, but the conventional BOTDA based on direct detection requires two-end-access, which makes it less suitable for large scale SHM because the signals become unavailable if there is a break along the fiber under test (FUT). Therefore, the study that how to combine OTDR and BOTDA to overcome the drawback of requiring two-end-access in BOTDA not only have academic value, but also have important social significance. In this dissertation, the principles and characteristics of OTDR and BOTDA are firstly analyzed. Then corresponding investigations are peformed experimentally to improve the sensing performance and to solve the question of the requisite of two-end-access in conventional BOTDA. The main works of this thesis are as follows:1. To enhance the spatial resolution and to increase the sensing length, we have proposed and demonstrated a photon-counting optical time-domain reflectometry (v-OTDR) based on superconducting nanowire single photon detector (SNSPD) after analysis its basic operating principle and performance. Benefiting from low dark count rates, high repetition rate and low jitter of SNSPD, our system achieves a dynamic range of22dB (corresponding to～110km fiber length) with lus probe pulse. Our v-OTDR also exhibits around6.0cm and1.1m of spatial resolution at the end of2km and26km SMF, respectively.2. The factors influencing the stimulated Brillouin gain and the nonlocal effect as well as its suppressing methods are discussed. The priciples of BOTDAs which are respectively based on direct and conherent detection and the SNR enhancement by conherent detection in BOTDA are theoretically analyzed. According to the discussion and analysis above, we have proposed and experimentally demonstrate, for the first time to our knowledge, an improved BOTDA with the capacities of both break interrogation and temperature/strain measurment. In this sensor system, the coherent detection technique is applied to improve the system’s SNR, and the DSB method is used to mitigate nonlocal effects. Without amplification, the sensor achieves a72km sensing range with5m resolution and an estimated temperature uncertainty of1.8°C. When the CW probe input-end of the FUT is disconnected, the sensor system is capable of locating the break along the FUT as COTDR, and achieves a peak dynamic range of36dB with100m spatial resolution. The achieved results show that our BOTDA offers an excellent solution for the requisite of two-end-accesses in conventional BOTDA, which would expand the application of BOTDA. 3. A hybrid single-end-access BOTDA and COTDR sensing system using heterodyne detection is proposed and demonstrated experimentally. In our configuration, the heterodyne detection techniques are applied to improve the SNR of the single-end-access BOTDA. Without adding instruments, our sensor could easily switch between BOTDA and coherent optical time domain reflectometry (COTDR) modes by changing the intermediate frequency of the detected heterodyne signals. We performed the distributed temperature measurement when the sensor works as BOTDA, and a24km sensing range with5m resolution together with a temperature uncertainty of1.0℃are achieved. When it operates as OTDR, a peak dynamic range of12dB is obtained by using50ns probe pulse and the loss coefficient of fiber under test (FUT) is accurately measured. The hybrid sensor not only offers an excellent solution for the requisite of two-end-access in conventional BOTDA and obtains a longer measurement range than that of the existed one-end BOTDA, but also can be used to measure the attenuation of a fiber link or locate the discrete defects on optical fiber systems.