Objective  The demand for on-line and real-time monitoring of frequency-stabilized semiconductor laser is very high and urgent. Especially, the continuous wave lidar developed in recent years usually takes a single frequency semiconductor laser as a seed source, and obtains the frequency of the radar signal through coherent detection, so as to obtain the distance of the target. It results in the frequency accuracy of the seed light source directly determing the ranging accuracy. Therefore, new requirements are proposed for the characterization of the frequency stabilization of the light source. More attention is paid to short-term (in coherent time of sub-microsecond up to milliseconds) frequency change patterns, rather than the absolute frequency accuracy in a long duration of minutes, even up to 24 hours; At the same time, the frequency monitoring system is required to have functions of on-line and real-time monitoring.

  Methods  Aiming at these requirements, based on the principle of delay self-heterodyne, this paper proposes a method for characterizing the frequency of frequency stabilized laser. By deriving the principle rigorously and programming the algorithm, the monitoring system not only has a simple structure (Fig.1), but also realizes the functions of online and real-time monitoring.

  Results and Discussions  The frequency variation curve of a frequency-stabilized distributed feedback semiconductor laser (DFB-LD) using hydrogen cyanide (H¹³C¹⁴N) gas absorption spectrum based on side frequency locking technique is measured. The result is that the maximum frequency range of the stabilized laser is about 25 MHz in 10 ms, and it is clearly observed that the frequency changes of the laser are not in one-way drift pattern (Fig.2). In order to further verify the accuracy of this method, the mainstream femtosecond optical frequency comb beat method is adopted to measure the frequency change of the same frequency-stabilized DFB-LD offline (Fig.3-4). The experimental results show that the frequency range is about 30 MHz within 50 minutes (Fig.5).

  Conclusions  The measurement results of the two methods are in the same magnitude order of MHz, which proves that the method is a fast and reliable way for optical frequency analysis, and can be used to adjust a servo-loop system of frequency stabilized laser in real time and on-line in application systems.