Study on damage mitigation for dielectric mirrors by using femtosecond laser micromachining

Electric field distribution, in the wavelength range 1053 nm and 0° high reflection coatings, with different truncated conical pits has been estimated by using the finite difference time domain method (FDTD). Results of simulations indicate that the smaller the angle between the pit’s edge and the normal line, the higher the damage threshold of the mitigation pit. In the experimental process, the dimension of this angle mainly depends on two factors, i.e. the influencing area of the focal spot and the depth of mitigation pits. Because the ratio between them is the angle’s tangent, decreasing the influencing area of the focal spot and increasing the depth of the machined area could yield a mitigation pit with a smaller angle. By optimizing the focal spot size, pulse energy, step size and the number of machining passes of femtosecond laser micromachining, a pit with an angle of 25° and a depth of 14 μm is obtained. The typical damage threshold of the mitigation pit is about 21 J/cm2, which is 2.3 times greater than the fluence-limited defect. Moreover, the laser damage testing results of 50 mitigation pits show that the mitigation process has a good repeatability. The correlation between the cone angle and the damage threshold is also examined, the simulations are in agreement with the experimental results. The ratio of the maximum intensification between 45° and 25° cone angles is ～2.5 and that of the damage threshold between the two angles is ～0.5. At the same time, the relationship between the micromachining pulse width and the damage threshold is also estimated: if other process parameters are kept constant, a longer pulse length tends to produce lower laser-resistant mitigation pits. Compared to the result of 260 fs laser pulse, the truncated conical pit created by 6 ps laser pulse has a smaller depth, which implies that more thermal effect occurs during the miromachining process. However, cracks are not found around the pit. Thus, thermal damage is not the major reason for the decrease of damage threshold. Meanwhile, smaller depth also indicates that the pit has a large cone angle. According to the result of former FDTD simulation, the decrease of damage threshold is mainly caused by electric field enhancement in a pit with a large cone angle.