基于多模LD直接调制和瑞利与布里渊自外差的温度和应变分布同时测量方法研究

To solve the problem that the existing schemes are too complicated and expensive, and to enhance the system spatial resolution，a new method，which bases on direct intensity modulation of multimode laser diode(LD) and self heterodyne detection of Rayleigh scattering and Brillouin scattering in a fiber，is proposed to realize the fast and simultaneous measurement of temperature and strain along the fiber with high accuracy and high spatial resolution. This project aims at putting forward some new methods as follows. With direct intensity modulation of multimode LD，the measurement method for output amplitude and frequency of self heterodyne detection of Rayleigh and Brillouin scattering signals with multimode and wide bandwidth transimitted along the fiber, and the method for calculation and improvement of system signal to noise ratio. Evaluating and improving methods for spatial resolution and measurement accuracy through introducing the theory of white light interferometer into BOTDR system. Evaluating, improving and amending methods for the influence of fiber dispersion，nonlinearity，and wavelength dependence of Brillouin spectrum on system performance by mathematical modeling. Adoption of new signal processing technologies. Calibration method for temperature and strain information with high accuracy, and so on. Through online monitoring the temperature and strain distribution of power cables and large engineering structures, the system can locate the fault points rapidly with high spatial resolution, and has wide application prospects. By adopting direct intensity modulation of multimode LD, and self heterodyne detection of Rayleigh and Brillouin scattering together, the system can eliminate the influence of frequency drift and chirp, polarization state of light source, and Rayleigh coherent noise, and it can operate without external modulator and polarization controller. The proposed method can simplify the system, decrease its costs and improve its performance greatly. The topic is first proposed in the project, and is of great significance in simplification, cost reduction and localized production of Brillouin sensing system.

针对现有方案复杂、昂贵和空间分辨率低的问题，提出一种基于多模LD直接调制、同一光纤中瑞利与布里渊自外差的全新的温度与应变分布同时测量方法，实现沿光纤温度与应变的高精度、高空间分辨率快速、准确测量。主要内容：LD直接调制时，经光纤传输的多模宽谱瑞利与布里渊自外差输出幅度和频率检测、系统信噪比计算和改善方法；引入白光干涉理论，提出空间分辨率和测量精度的建模评价和提高方法；光纤色散、非线性、布里渊谱波长依赖性对系统性能的影响和建模校正方法；新型信号处理技术采用，温度和应变信息标定等。该方法通过对电力线缆和大型工程结构等温度和应变分布的在线监测，可实现故障快速准确定位，应用前景广阔。采用LD直接调制、瑞利与布里渊自外差，无需考虑光源频率漂移、啁啾、偏振和瑞利相干噪声影响，可省去外调制和偏振控制，大大简化系统结构、降低成本、提高系统性能。本课题为项目首次提出，对系统简单化、低成本化和国产化意义重大。

由于受激布里渊散射(SBS)阈值是限制布里渊光时域反射(BOTDR)系统入纤光功率和信噪比的主要因素，本项目理论解析了单端SBS耦合波方程并提出了光纤SBS阈值测量新方法；通过对基于多模法布里-珀罗激光器(FP-LD)的外差检测BOTDR系统的建模分析与计算及光源频率不稳定性对外差检测布里渊信号影响的理论和实验研究，搭建了基于多模FP-LD、宽带LD和多波长光源的自外差检测BOTDR系统；为了解决系统性能指标相互制约的问题，研究了基于单波长、多波长和多模FP-LD的雪崩光电二极管本地外差检测格雷编码BOTDR系统的最佳编码长度，并搭建了本地外差和自外差检测格雷编码BOTDR温度测量系统进行验证，提出了基于时域和谱域整形的单波长和多波长外差检测BOTDR温度测量方法。为了准确实时地提取布里渊散射谱，提出了利用Pseudo-Voigt基函数和L-M算法的特征提取方法及脉冲光布里渊散射谱拟合方法，针对单峰和多峰的布里渊散射谱形态，研究了布里渊散射谱特征参数快速高精度提取方法；提出了一种应变和温度同时标定的新方法；为了解决用BOTDR监测光电复合海缆时的温度应变区分问题，研究了基于BOTDR的已敷设传感光纤测量数据的分析与处理方法，并对已敷设光纤的温度和应变进行了区分测量。提出了基于多频相干光时域反射计(COTDR)的分布式光纤测量方法，研究了单模光纤色散对脉冲宽度及COTDR系统性能的影响，提出了利用BOTDR和COTDR相结合的传感系统实现温度和应变同时测量的方法。针对BOTDR系统中被测信号弱的问题，搭建了瑞利布里渊光时域分析(BOTDA)温度传感系统，并采用多波长光源技术实现了信噪比的提高；为了降低相干瑞利噪声(CRN)对系统性能的影响，提出了采用波长扫描平均技术减小CRN的方法，为本项目CRN的降低提供了理论依据；将脉冲编码和脉冲预泵浦技术引入瑞利BOTDA系统，实现了信噪比、空间分辨率和测量精度的同时提高。考虑到多模石英光纤的布里渊散射也存在多个传输模式，其物理机理、理论和技术问题类似于基于多模FP-LD的布里渊分布式传感系统，研究了多模石英光纤的布里渊散射及其温度和应变响应特性。