Method for measuring laser damage threshold of optical thin film elements based on quantitative damage evaluation

Objective Optical thin film components play a critical role in high-power lasers, and their ability to withstand laser-induced damage is crucial for the overall performance of the laser systems. Accurate measurement of the laser-induced damage threshold (LIDT) of thin film components is of great significance for improving the lifetime and output efficiency of lasers. However, the traditional test method for laser LIDT, based on the scheme outlined in GB/T 16601, involves evaluating the threshold value through damage probability, which requires numerous repetitive experiments and is both cumbersome and time-consuming. Moreover, the evaluation based on whether the damage occurs on the film surface introduces certain errors. The stability and reliability of the damage are influenced by factors such as laser stability, environmental disturbances, coating processes, and internal defects, which cannot be eliminated and affect the measurement results and accuracy. Additionally, probability statistics require testing many samples, resulting in high capital costs. Therefore, it is necessary to develop a fast and efficient method for measuring LIDT.

Methods In this paper a novel method of quantitative evaluation of laser-induced damage degree (QELDD) is presented, for quantitatively assessing the degree of laser-induced damage in thin film components. The method involves analyzing the quantification of laser-induced damage at different energy densities and evaluating the LID threshold (LIDT) through damage trend fitting. To accurately quantify the laser-damaged area, super-resolution white-light interferometric measurement is employed, which ensures nanoscale measurement accuracy. Simulation results demonstrate that the proposed method allows for the three-dimensional reconstruction of nanoscale damage defects with a reconstruction volume error of less than 0.01%. Experimental samples, including laser resonator mirrors and window plates, were measured using this method, without repeating the laser damage experiments. The results were found to be consistent with those obtained using the S-on-1 method, with a deviation not exceeding 0.5 J/cm². The standard deviations of the measurement results were 0.361 J/cm² and 0.064 J/cm², respectively.

Results and Discussions In the simulation results of the laser damage structure model on the surface of the test element, the optimization algorithm achieved a relative error of less than 0.01% for the three-dimensional measurement results (Fig.7). And the reconstruction deviation value is in nanometers order (Fig.8). In the experiment, samples of two laser components are used. The measurement results for the laser resonator mirror are shown (Fig.10, Tab.1). The standard deviation of multiple measurement results is 0.361 J/cm², and the difference from the S-on-1 result is less than 0.5 J/cm². Similarly, the measurement results for the window slice are shown (Fig.11, Tab.2). The standard deviation of the multiple measurement results is 0.064 J/cm², and the difference from the S-on-1 result is less than 0.3 J/cm². The proposed QELDD method is based on single irradiation results of a single sample with different energy densities, eliminating the need for repetitive testing on multiple samples. This ensures good stability and accuracy while maintaining efficiency.

Conclusions In this paper, a new laser damage threshold measurement method for optical films based on quantitative evaluation of the damage degree is proposed. The laser damage area is characterized and quantified with high precision using image super-resolution white light microscopic interferometry. The structural characteristics of the laser damage are also summarized. Based on the quantitative parameters of the laser damage degree, we fit and calculate the laser damage threshold. In the experimental setup, a 1 064 nm laser damage system is utilized, and the laser resonator mirror and window plate are selected as samples for testing. The standard deviations of the two sample results are 0.361 J/cm² and 0.064 J/cm², respectively. When compared with the results obtained using the S-on-1 method, the deviation of the measured results does not exceed 0.5 J/cm², indicating good stability and accuracy of the proposed method. During the quantification and characterization of the laser damage area, the QELDD method effectively distinguishes between valid and invalid damage points based on the damage characteristics, thereby eliminating the influence of invalid damage on the result fitting, and improving measurement efficiency. This method introduces a new approach to LIDT measurement, facilitates further research on the laser damage mechanism of optical components, and provides a theoretical basis for enhancing the manufacturing and coating processes of optical components.