针对电力变压器内部局部放电产生的超声波信号的检测需求，利用半导体激光器的自混合干涉效应，结合Sagnac干涉技术，提出一种结构简单、构思新颖的光纤超声波传感系统。在弱反馈的条件下，通过Sagnac干涉仪对放电超声波扰动传感光纤所引起的激光器振荡频率变化的测量，可以检测出局部放电。探讨了该系统的工作原理，并开展了实验研究。对铅笔芯折断或金属针自由掉落以及压电陶瓷片等多种方式产生的超声波信号进行检测，检验了该实验系统的传感性能。用平行电极的高压放电模拟电力系统的放电现象，包括电晕放电和局部放电。分别在不同的湿度和高压条件下对其进行检测，均获得了良好的效果。实验结果表明，这种传感方式具有检测灵敏度高和高频响应特性好等优点，能够检测高达300 kHz的超声波信号，可满足电力系统对局部放电检测的需求。

Aiming at the detection requirements for ultrasonic waves resulted from partial discharges in power transformers, a fiber-optic ultrasonic wave sensor system with a simple and novel structure utilizing the self-mixing interference effect in a semiconductor laser combined with Sagnac interferometric techniques is proposed. Under the condition of weak external optical feedback, the partial discharges can be determined by measuring the changes of oscillating frequency of the laser diode induced by the disturbances of sensing fiber by discharge-induced ultrasonic waves with the Sagnac interferometer. The operation principles of the system are explored and experimental researches are carried out. The ultrasonic waves produced by various ways such as pencil broken, metal pin free falling and piezoelectric ceramics are detected. The sensing performances of the experimental system are tested. The discharge phenomena in a power system are simulated with high-voltage discharges occurring between two parallel copper wires, including the corona discharges and partial discharges. They are detected under different humidity levels and voltages. The satisfied results are obtained. All the experimental results show that the proposed sensor system has high sensitivities and good high frequency responses as well as a capacity to detect weak ultrasonic waves up to 300 kHz. It can be concluded that the proposed system can meet the requirements for partial discharge detection in the power industry.