A Novel Hybrid Digital and Analog Laser Synchronization System

Laser synchronization is a technique that locks the wavelength of a free-running laser to that of the reference laser, thereby enabling synchronous changes in the wavelengths of the two lasers. This technique is of crucial importance in both scientific and industrial applications. Conventional synchronization systems, whether digital or analog, have intrinsic limitations in terms of accuracy or bandwidth. The hybrid "digital + analog" system can address this shortcoming. However, all above systems face the challenge of achieving an both high locking accuracy and low structural complexity simultaneously. This paper presents a hybrid "digital + analog" laser synchronization system with low-complexity and high-performance. In the digital part, we proposed a electric intensity locking method based on a band-pass filter, which realizes the fluctuation of frequency offset between a single frequency laser (SFL) and a mode-locked laser (MLL) less than 350 kHz in 24 hours. Following the incorporation of the analog control component, frequency fluctuation is less than 2.5 Hz in 24 hours. By synchronizing two SFLs to a repetition-frequency locked MLL, we achieve indirect synchronization between SFLs with a frequency offset of 10.6 GHz and fluctuation less than 5 Hz in 24 hours, demonstrating robust long- and short-term stability. Since the MLL is employed as a reference, the system can be utilized for cross-band indirect synchronization of multiple lasers. Based on the synchronization system, we propose a photonic-assisted microwave frequency identification scheme, which has detection error of less than 0.6 MHz. The high performance of the synchronization system enables the proposed frequency identification scheme to achieve high measurement accuracy and a theoretically large frequency range.