针对基于非线性偏振旋转(NPR)原理的被动锁模光纤激光器稳定性差和不能自行进入锁模状态的问题，本文设计了自动锁模电路,用于对激光器进行实时调控以获得稳定的锁模输出。该项设计利用高速光电探测器(PD)将NPR被动锁模光纤激光器输出的光信号转化为电脉冲信号，经过线性放大、整形处理后输出至单片机进行计数；单片机根据快速锁模判定算法判断该激光器的输出状态，并自动反馈调节加装在光纤激光器上的电控偏振控制器(PC)，从而实现激光器的锁模稳定状态输出。实验结果表明，对于重复频率为6.238 MHz的NPR被动锁模激光器，自动锁模电路能够在6 ms内检测到失锁状态，在最长10.24 s内自动搜索达到稳定锁模工作状态。对于重复频率在16 MHz以下的NPR被动锁模激光器，自动锁模电路都能够快速地实现自行启动或将启动后因故失锁的状态调节回锁模状态，达到预先的设计要求，具有结构简单、成本低、功耗低及性能稳定等优点。

As passively mode-locked fiber lasers based on Nonlinear Polarization Rotation(NPR) have the problem of low stability and difficulty to self-start into a mode-locking state during operation, an online automatic mode-lock circuit was designed to adjust the laser in real time to get a stable mode-locked output. In the design, the signal from NPR passively mode-locked laser was transformed to an electrical pulse signal with a high speed photoelectric detector, and it was amplified and shaped linearly to be received by the Micro Computer Unit(MCU) to count. Then the MCU distinguished the state of the laser based on quick mode-lock judge algorithm and adjusted the electric controlled polarization controller (PC) on the fiber laser automatically. The experiment results show that for the NPR mode-locked fiber laser with a repetition rate of 6.238 MHz ,the automatic mode-lock circuit can distinguish the losing lock in 6 ms, and can find and get into the mode-lock state in 10.24 s automatically. For NPR passive mode-locked fiber lasers with repetition rate lower than 16 MHz, this automatic mode-lock circuit can get into a stable mode-locked state quickly or restart immediately if the mode-lock state is lost for some reasons，which reaches the design requirements and shows advantages of simple structures, low costs, low powers, and high reliability.