Laser-induced damage to optical components seriously hinders the output ability and operation ability of high-power laser systems, particularly UV laser-induced damage. Various studies on the damage generation and growth of fused silica have been conducted, and it is recognized that laser damage is caused by several kinds of defects, including subsurface cracks, organic pollution on the optical component surface, and so on. Advanced mitigation processes are employed to eliminate the defects and considerably enhance the damage performance of fused silica components. However, the laser damage performances of vacuum windows which are a special kind of fused silica optical component employed in high-power laser systems, is substantially worse than those of other fused silica components. The rear surface of the vacuum window withstands tens of MPa tensile stress to isolate the internal vacuum environment in the target chamber from the external atmospheric environment outside the target chamber, which may influence the damage generation and growth characteristics of the vacuum window. To solve the damage problem of vacuum window in high-power laser systems, it is necessary to investigate the impact of external stress on damage generation and growth of fused silica.
We employ the Nd∶YAG laser delivering a Gaussian pulse width of 5 ns at 351 nm. Uncoated fused silica samples are investigated in the experiments. First, the sample is placed in a specially made clamping structure (Fig.1) to apply uniform external stress to the sample. Simultaneously, the laser displacement sensor is employed to measure the surface displacement of fused silica sample. Driven by a one-dimensional stepper motor, the surface displacement in the middle area of the sample is sampled. The finite element simulation is conducted using ANSYS software, and the relationship between surface stress and surface displacement is deduced. Next, the 1-on-1 approach is employed to test the laser damage threshold, and the R-on-1 approach is employed to test the growth threshold of the initial damage. For each sample, the damage test is conducted under the condition of external stress, and then the stress is removed for further testing. Simultaneously, to observe the damage phenomenon under each given laser fluence, a CCD camera is employed to monitor the damage after each laser radiation, and an optical microscope with a resolution of 6 μm is used to measure the size of the damage crater ex situ off-line.
The experimental findings reveal that in the case of the tensile stress of 10-50 MPa, the difference in the damage threshold of fused silica with and without stress is less than 4.1% of the minimum (Fig.3), and the average difference between the initial damage point sizes is less than 5.5% of the minimum (Fig.5), indicating that the external tensile stress does not affect the damage threshold and the size of initial damage. The average damage growth threshold of fused silica with and without stress is 3.84 J/cm2 and 3.88 J/cm2, respectively (Fig.6), demonstrating that the external stress does not affect the damage growth threshold of fused silica. This is because the damage to fused silica under ultraviolet laser irradiation is primarily caused by the defect absorbing laser energy, causing the temperatures of the defect and its surrounding local area to rise, and finally resulting in "microexplosion" .The pressure of the shock wave produced by microexplosion can reach 30 GPa, while the external stress applied in the experiment is less than 50 MPa, making it difficult to influence the laser damage process of the material.
Damage threshold and damage growth threshold of fused silica under the external tensile stress of 10-50 MPa are experimentally investigated. The findings reveal that the difference in the damage threshold of fused silica with and without stress is less than 4.1% of the minimum, and the average difference in the initial damage size is less than 5.5% of the minimum. In the experiments, no impact of stress on the damage threshold is observed. In addition, the average damage growth thresholds of fused silica with and without stress are 3.84 J/cm2 and 3.88 J/cm2, respectively. No substantial effect of the stress on the damage growth threshold is found. In this study, the effect of external stress on the damage performance of fused silica is studied and the results rule out the effect of external stress on the damage performance of fused silica, providing crucial experimental support for solving the damage problem of vacuum window in high-power laser systems.
