Objective  In laser ranging processes, Single-Photon Avalanche Diode (SPAD) is commonly used as a detector. However, this type of detector exhibits a time-walk effect, where different input energies result in different photon detection times. In such cases, it is necessary to monitor the laser power in real-time to analyze the variations in laser energy and the impact of the detector itself on ranging accuracy. Furthermore, due to the complexity of satellite laser ranging systems, troubleshooting typically requires a significant amount of time. Real-time monitoring of laser power allows for quick identification and troubleshooting of laser transmitter energy, reducing the time required for identifying system faults. Therefore, obtaining real-time laser emission power data serves as a crucial basis for subsequent analysis of data accuracy and troubleshooting of laser ranging system faults.

  Methods  To address the limitations of traditional real-time laser power monitoring techniques, such as laser energy attenuation, susceptibility to introducing optical axis deviation, and difficulties in practical application, a real-time laser power monitoring method is proposed for laser ranging systems. Here is the method: Before ranging, insert laser power meter II into the optical path and adjust the laser diode current to obtain multiple sets of different laser emission powers. Use laser power meters I and II to measure the transmitted light and reflected light from the reflector respectively, establishing the corresponding relationship between transmitted and reflected light (Fig.4). During ranging, remove laser power meter II from the optical path, and laser power meter I continuously measures the transmitted light from the reflector in the laser emission path (Fig.3). Utilize the previously established corresponding relationship between transmitted and reflected light to obtain real-time reflected light power through relative measurement. This achieves the effect of real-time monitoring of laser emission power. Validate the method by constructing an experimental platform based on the 53 cm dual-tube telescope at Yunnan Observatory.

  Results and Discussions   By adjusting the laser diode current to change the laser power, multiple sets of data for the transmitted laser power and reflected laser power were measured. The data was then used to perform a linear fit using the least squares method. The significance of the regression equation was evaluated using the F-test, yielding an F-value of 3 171.039 5. Consulting the F-distribution table revealed that the regression was highly significant, indicating a strong linear relationship between the reflected and transmitted laser powers. The residual standard deviation (σ) of the regression equation was found to be 0.007 3. The maximum deviation between the measured values of reflected laser power and the fitted results was 1.49% of the current measurement, demonstrating that the regression line accuracy meets the requirements for laser ranging (Fig.5). The proposed method was subjected to intermittent measurements over a duration of 7 hours. The F-value obtained from the F-test was 1057.7779, which means the regression was still highly significant. The residual standard deviation (σ) was calculated to be 0.0165, and the maximum deviation value of the reflected laser power measurement from the fitted result is 3.75% of the current measurement value. This meets the accuracy requirements, demonstrating that the proposed method can maintain long-term stability and fulfill the needs of long-time satellite laser ranging (Fig.6).

  Conclusions  The experimental results indicate that the proposed method of real-time laser power monitoring can accurately obtain the laser emission power without loss in the laser emission path. The reflected laser power and transmitted laser power exhibit a strong linear relationship, with a Spearman correlation coefficient of 0.999 1. This linear relationship remains stable and reliable during long-duration laser ranging experiments. The feasibility of this method has been verified, meeting the power measurement requirements for laser ranging of various spatial targets. Therefore, this method can be applied to the real-time monitoring of laser power for various spatial objects laser ranging.