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提出了一种基于多模法布里-珀罗激光器外调制的移频本地外差检测布里渊光时域反射系统,分析了系统提高光纤受激布里渊散射阈值和移频本地外差检测的原理;推导了系统信噪比的数学表达式,研究了布里渊叠加谱峰功率和谱宽、系统信噪比和频移测量精度与纵模数的关系,以及实现最优测量精度时纵模数与脉冲宽度的关系,得到了相应的拟合公式。结果表明,选用纵模间隔0.141nm的多模激光器和单纵模峰值功率100mW、脉冲宽度50ns的入纤脉冲时,随着纵模数的增加,25km光纤末端的系统信噪比、温度和应变测量精度均得到了明显提高,且纵模数为20时,系统信噪比相对于单纵模增加11.73dB,温度和应变测量精度分别达到最佳值3.2℃和70.8με;最佳频移测量精度随脉冲宽度的增大先快速提高,后趋近于恒定值2.9 MHz。
A frequency-shifted local heterodyne detector based on multi-mode Fabry-Perot laser external modulation is proposed. The system is analyzed to improve the threshold of fiber-stimulated Brillouin scattering and the local heterodyne The mathematical expressions of signal-to-noise ratio of the system are deduced. The relationship between the power and the spectral width of the Brillouin superposition peak, the measurement accuracy of the signal-to-noise ratio and the frequency shift of the system and the longitudinal mode number, and the realization of the optimal measurement accuracy When the longitudinal mode number and pulse width, the corresponding fitting formula is obtained. The results show that the system SNR, temperature and strain at the end of 25km fiber with the increase of the longitudinal mode number are obtained by using multimode lasers with longitudinal mode separation of 0.141nm and single longitudinal mode power of 100mW and pulse width of 50ns. The accuracy of temperature and strain measurement reached the best 3.2 ℃ and 70.8με, respectively. The best frequency shift measurement Accuracy increases rapidly with increasing pulse width and then approaches 2.9 MHz with a constant value.