High reflective (HR) films with low stress and high damage threshold are deposited on a high-density deformable mirror (HDM) for a high-power laser system. The parameters of the laser system are about 5 J, 6 ns, and 200 Hz @ 1064 nm. The surface figure of the HDM is measured before and after coating, respectively. After coating, the surface error of HDM is only 39-nm rms, and the results show that the residual stress of HR films is very small. Then, a Φ50-mm sample and the HDM are tested successively in the high-power laser system. The temperature rise of the sample and HDM are measured in real-time. Finally, the temperature of HDM has risen by 10°C, while the sample is changed by only 1°C.
In order to reduce the thermal distortion of the deformable mirror(DM) and improve the laser beam quality, a new type of a 19-element water cooling deformable mirror was developed, and the finite element model of DM with water cooling part was built. The deformation, temperature rising and high-order aberrations were calculated after applying the laser irradiation to the mirror. Based on the results, we found the distortions and temperature rising of DM with water cooling part are much lower than those without it. The thermal distortion and temperature rising decreased from 0.316 μm to 0.210 μm and from 30.3°C to 27.1°C respectively, and the corresponding removed low-order Zernike coefficient residual error was reduced from 0.068 μm to 0.043 μm. The high-order aberrations between them were different. The results have practical engineering application for the design of DM in future higher power laser systems
In order to analyze thermal distortions of the deformed mirror for laser systems, the finite element model of a 417-element deformable mirror was built. The deformation, temperature rising and high-order aberrations were calculated after applying the laser irradiation to the mirror. Based on the results, we found the distortions became larger and larger while the absorption of multilayers increasing. When the mirror thickness is 1 mm, the deformation and temperature rising were 0.370 μm and 6.68 ℃ respectively. The PV decreased from 0.370 μm to 0.256 μm when the mirror got thicker from 1 mm to 3 mm, and the corresponding removed low-order Zernike coefficient residual error was reduced from 0.285 μm to 0.145 μm. The high-order aberrations of the deformed mirror were little when the mirror surface thickness was 3 mm. The results have practical engineering application for the design of deformable mirrors for laser systems.
With the development of femtosecond laser system, the laser damage threshold of optical components becomes more important. Meanwhile, in order to obtain better laser beam quality and avoid wavefront distortion caused by optical components, more stringent requirements are put forward for the surface shape of the coated surface of mirrors. HfO2-SiO2 high reflective films were fabricated by e-beam thermal evaporation method. Internal stress of the multilayer dielectrics was modulated by changing the design of films and coating process. Finally, the film with an absolute internal stress less than 100MPa was obtained. The laser damage characteristics of the films at 35fs, 1000Hz were studied, and the damage mechanism was analyzed.
High reflection films for 800nm picoseconds laser system requires broad bandwidth, which is usually about ±50nm, or even to ±70nm, and a high laser damage threshold is needed at the same time. Multilayer dielectrics using three materials Nb2O5/SiO2-HfO2/SiO2 were fabricated by electron beam evaporation. Benefit from its high refractive index of Nb2O5 and the high damage threshold of HfO2 films, the multilayer dielectrics were prepared successfully, which have more than 99.5% reflectance within bandwidth larger than 140nm around the center wavelength of 800 nm. The laser damage characteristics of the films at 150ps, 1Hz were studied, and the damage mechanism was analyzed.
YbF3 was proposed as a substitute for ThF4 in anti-reflection or reflection coatings for the infrared range, and the residual stress of YbF3 thin film using APS plasma ion assisted deposition(PIAD) was studied. From the results, we found the anode voltage of PIAD has a large effect on the residual stress of YbF3 thin film, and the refractive index of YbF3 produced with PIAD was higher than without it, with a possible reason close to packing density. Finally, we produced multi-layer reflection coating on a 260mm diameter mono-crystalline silicon substrate. Its surface contour was approximately 0.240λ (λ＝632.8nm), and the absorption was lower than 200ppm, which can satisfy the practical requirement.