Laser Offset Frequency Locking Based on Digital Frequency Division and Time-Interval Detection

Laser offset frequency locking systems are essential for achieving high stability and precision in applications such as optical communications and continuous-variable quantum key distribution (CV-QKD). Conventional approaches typically rely on direct frequency measurement to determine the offset between the laser frequency and a target frequency, followed by feedback control to stabilize the output. However, the performance of such systems is often limited by the precision of the frequency measurement. In this work, we propose an offset frequency locking technique based on digital frequency division and time-interval measurement. The core of the technique is low-jitter frequency division combined with high-resolution time-interval measurement to extract the offset frequency deviation. It employs a field-programmable gate array (FPGA) to perform digital frequency division, incorporates a D flip-flop to reduce frequency-division jitter, and uses a high-precision time-to-digital converter (TDC) for time-interval detection. Experimental results show that the proposed system stabilizes the laser offset frequency to a peak-to-peak value of 288 kHz and a jitter of 46 kHz, corresponding to reductions of 64% and 49%, respectively, compared with commercial instruments. These results demonstrate the potential of the method for high-precision applications.