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The output level of a high-power laser system is closely related to the ability of a thin film element to resist laser damage. The ability of the thin-film component to resist laser (nanosecond pulsed laser) damage is mainly limited to various types of defects in the thin film. Defects include defects originating from the substrate, the film layer, and after coating. With the progress of the coating process, defects originating in the film layer have been effectively suppressed.
Defects originating from the substrate play an increasingly prominent role in the process of laser-induced damage of thin-film devices. They have become the key factors restricting the enhancement of the damage threshold of Nd:YAG lasers. However, there are few research reports about the effect of these defects on the laser thin film, and most of them are qualitative research. The mechanism of substrate defects affecting the damage threshold of thin film components is not yet clear. Substrate defects include structural defects such as absorptive impurity defects, surface scratches, and pits. The Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Key Laboratory of High-power Laser Materials, Chinese Academy of Sciences, published an original paper on Opt. Lett. Lett. 40, 3731-3734 (2015)], [Opt. Lett. 40, 1330-1333 (2015)] proposed the use of femtosecond laser machining pits to reveal structural defects originating from the surface of the substrate and inducing laser damage to thin film components. Mechanisms.
The Shanghai Institute of Optics, Chinese Academy of Sciences High-power Laser Materials Key Laboratory uses a femtosecond laser micro-machining platform to produce specific-sized pit defects (length: ~7um, width: ~3um, depth: ~1um) on a quartz substrate. The laser-induced damage behavior of anti-reflection coating and high-reflective coating deposited on femtosecond laser processing pits and conventional substrates was investigated and compared.
The results of the study indicate that absorptive impurity defects play an extremely important role in the laser induced damage mechanism for anti-reflection coatings. Substrate surface/subsurface absorptive impurity defects migrate to the substrate surface during film preparation and accumulate into larger size impurity particles, which in turn couples with the film layer, inducing antireflection film elements to have a lower energy flow density than the film layer. Injury under laser irradiation with an intrinsic laser damage threshold. Through the corresponding technical means (reducing the coating temperature, substrate pickling before plating, etc.), the coupling between the substrate defects and the film layer can be effectively suppressed, thereby improving the anti-laser damage ability of the anti-reflection film.
However, the performance of high-reflective films is very different. Structural defects play a more important role in the laser-induced damage mechanism of high-reflection films. This is mainly attributed to two points: First, the film deposited on the high-reflection film. Cracks occur in the layer, resulting in changes in the mechanical properties of the film; Second, structural defects lead to enhanced electric field at the interface between the film and the film.
The recognition of the mechanism of laser damage induced by defects on the surface of the substrate by the Shanghai Institute of Optics and Lasers, Chinese Academy of Sciences, breaks through the traditional perspective of only focusing on the performance of the film itself, and proposes new ways to further improve the laser-induced damage resistance of the film element. direction.
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