Fiber laser can be used for fiber optic communications,laser cutting,industrial manufacture,defense security and many other fields because of its advantages of narrow output linewidth,good reproducibility,etc.However,due to nonlinear and thermal effects,only a limited output power of a single fiber can be obtained with a sharp attenuation of the output beam quality,which obstructs the applications of fiber lasers.Therefore,the research of expanding the power of a fiber laser source while maintaining its beam quality by combining coherent beam has become a hot subject at present.In this field,the performance of phase control of coherent laser beams is a key factor to influence the efficiency of combination.The phase-controlling methods mainly include stochastic parallel gradient descent control algorithm, dithering,and heterodyne detection.In this paper,based on the active phase lock technology,the traditional heterodyne detection method is improved by the use of a fiber electro-optic phase modulator (EOM) rather than an acousto-optic frequency shifter (AOFS) to avoid the complex designs of the RF driver and circuit,which makes the overall experimental setup simple and stable.Moreover,in order to achieve a stable and wide correction range of phase locking,two servo paths are designed by use of piezoelectric transducer (PZT) and EOM1 to correct the optical phase differences.Firstly, a single-frequency narrow-width fiber laser with its central wavelength of 1531 nm is split by a beam splitter to generate a signal and a reference beam,respectively.The reference beam is phase modulated by another EOM2 with a 15 MHz signal.The phase error signal is obtained by demodulating the detected heterodyne signal at the modulation frequency. After that the error signal is divided into two parts,and sent to two PID servos to control PZT and EOM1,respectively. The PZT,used in the slow feedback loop,eliminates the laser phase error induced by the ambient temperature drift, while the EOM1,in the quick feedback loop,can eliminate the influence of high frequency noise.Two PID servos are carefully designed according to the measurements of the dynamic response of the PZT and EOM1.A stable feedback loop with a bandwidth of 220 kHz (limited by the bandwidth of PID controller) is obtained according to the measurement of its phase error signal spectrum,thus a tight lock is expected.As a consequence,the error of phase locking is less than 0.88°,which indicates that the phase control accuracy is λ/400.The long-term stability of the system is assessed by a 2 hour monitoring of the lock error signal.According to the analysis of Allan deviation,the best phase lock value of 0.006° can be obtained for an integration time of 160 s.The overall phase lock experimental setup is simple and easy to operate;moreover the phase lock can be further improved by optimizing the parameters of the PID controller.