Heterodyne Optical Phase-Locked Loop (HOPLL) is an important technology for laser frequency stabilization. In this work, a digital HOPLL system for realizing frequency offset locking of a Distributed Feedback Semiconductor Laser (DFB-SL) is designed and implemented. The system is composed of a Frequency Synthesizer (FS), a passive third-order loop filter and an adjustable gain module. The effectiveness of the system is tested by measuring the frequency offset locking of DFB-SL. The results show that the designed system provides a cost-effective, sensitive, and reliable way to lock the DFB-SL with suitable parameters, and the drift caused by environmental variations can be suppressed effectively.
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