Research progress of laser-induced damage of nodular defects

In near infrared high reflectors, nodular defects are the main factor inducing the laser damage of optical coatings. In order to improve the laser-induced damage threshold (LIDT) of the optical coatings, it is necessary to study the damage characteristics of nodules. In this paper, the studies on laser-induced damage of nodular defects is reviewed from two aspects: the real nodules and the artificial nodules. For real nodules, the geometries of nodular defects were established and the preliminary understanding of the damage mechanisms of nodular defects was achieved. The finite-difference time-domain (FDTD) technique has been successfully used to simulate the electric-field intensity enhancement in nodules, which helps to explain the damage mechanism of nodules. The experimental methods of eliminating the seeds and laser conditioning have been proposed to control nodular defects and to improve LIDT. However, the properties of real nodules, such as the diameters, absorption and lodging depth of seeds, are quite diverse and very difficult to predict, control and reproduce. This makes the systematical and quantitative study of nodular damage quite challenging, and the understanding of the nodular damage mechanisms is still insufficient. Artificial nodules offered the opportunity to study their damage behaviors systematically, quantitatively or even in a single factor manner, which greatly improved the efficiency and reliability of the experimental studies. In addition, a direct comparison between the experimental results and theoretical simulation results could be achieved using the well-controlled artificial nodules. The damage morphologies of artificial nodules were almost the same with the FDTD simulated electric-field intensity distributions, which not only demonstrates the correctness of the simulated results but also further illustrates that the electric field enhancement is the main mechanism of inducing nodular damage. After having a deeper understanding of the nodular damage mechanisms, the broadband coating and planarization technique have been used to suppress the electric field enhancement and to improve the LIDT of nodules. This extends the ideas and methods dealing with nodular defects, from solely removing nodular defects to controlling nodular defects, which opens up new directions for improving the LIDT of laser coatings.