Research Of Brillouin Distributed Optical Fiber Sensor Based On Coded Pulses

Brillouin optical fiber sensor (BOFS) is one of fully distributed optical fiber sensor. It can be used to obtain the temperature and strain along the fiber distrubtedly. Thus, it is superior to other optical fiber sensor in the field of health monitoring for large civil structures such as bridges, dams, tunnels and submarine cables. Brillouin optical time domain reflectometry (BOTDR) and Brillouin optical time domain analysis (BOTDA) are two main technologies of Brillouin scattering based distributed optical fiber sensors. However, their performance is limited by the fiber loss and nonlinear effect in the fiber. It makes them cannot satisfy for many requirements of engineering applications, in which high resolution and long sensing range are required simutaneously. In this thesis, principle of BOFS is researched, and Features of spontaneous and stimulated Brillouin scattering are analysed. It is proposed to use coded probe pulses method for the improvement of the system signal-to-noise ratio (SNR). The spatial resolution, frequency accuracy and sensing length with different pulse shapes and sequence lengthes are studied. The decoded method is optimized according to the capability of the detector. It allows achieving long sensing length and high frequency accuracy without reducing the spatial resolution of the system. The primary research works are listed as follows:1. According to the signal character of coded BOTDR and BOTDA sensing system, a resample method based decoding technique was proposed to solve the time trace distortion causing by the difference of the pulsewidth and sampling time duration. Thus, the correlation based coding technique can be used to improve the SNR without signal distortion.2. The Hadamard sequences, which has the highest coding gain was introduced to modulate the probe pulses of BOTDR. Since it reduces the requirement for the power of the laer and the performance of the optical amplifier, the amplified spontaneous emissions (ASE) noise can be avoid and the cost of the system can be reduced as well.3. The return-to-zero (RZ) coding method was proposed to instead the non-return-to-zero (NRZ) coding method in the BOTDA system to solve the signal distortion problem causing by the stimulate Brillouin scattering (SBS) effect. The spatial resolution and frequency accuracy of the BOTDA systems using RZ and NRZ coded pulse was compared. The experimental results show that the signal distortion introduced by the nonlinear amplification of the SBS can be avoid when using RZ coded pulse method.4. RZ coded pulses technique combined with the differential pulse pair (DPP) method was prosed to futher enhance the spatial resolution, frequency accuracy, and dynamic range of sensing system. The acheived spatial resolution over 50km fiber breaks the limitation of the intrinsic spatial resolution of 1m for the first time.