Breakthroughs in ultra precision manufacturing technology of large-aperture high-power laser optics

To construct the largest, most complex and systematic high-power solid-state laser facility by far, which will be extensively used for inertial confinement fusion (ICF) research, there is an urgent need for high-quality laser optics with high manufacturing capability, which include neodymium-doped phosphate glass magnifiers, plane mirrors, aspheric focusing lenses, diffractive elements, and large-aperture nonlinear laser crystals. Such optics need to be manufactured with extreme requirements on precision control and defects density. In order to achieve  ideal beam quality and stable operation under high-throughput conditions with laser optics that are group-able, module-able, and compact, the manufacturing approaches need to  progrmmable highly efficient, mass-producible, and programmable,  and will be the focus of the research, which requires major breakthroughs.

Therefore, the Advanced Optical Manufacturing Team of Research Centre of Laser Fusion, China Academy of Engineering Physics proposed a detailed method covering the whole manufacturing process for high-power laser optics with ultra-precision and determinacy as its primary focus. In response to the manufacturing requirements of soft and brittle laser crystal components, the team made major breakthrough on the single-point ultra-precision diamond fly-cutting technology. In response to the manufacturing requirements of hard and brittle fused silica components, the team made major breakthrough on the key technologies such as aspheric ultra-precision CNC grinding and deterministic polishing.

The article summarizes the main research progresses of ultra-precision manufacturing technology of large-aperture and high-power laser optics in recent years.

disambiguation Single-point diamond fly-cutting

KDP (Potassium dihydrogen phosphate) crystal, which is currently the only large-aperture crystal component suitable for high-power laser devices, is susceptible to deliquescence, sensitive to temperature changes, and easy to crack, which makes it difficult to process. In response to the batch production requirements of large-aperture KDP crystal components, the research team established a constitutive model of KDP crystal based on the basic mechanical properties of the crystal material. Through comprehensive theoretical analysis and experimental results, the conditions of generating smooth surfaces under the brittle-plastic mixed cutting mode are defined, and the design method of ultra-precision fly-cutting machine based on frequency domain error analysis is discovered. The development of single-point ultra-precision diamond fly-cutting technology and equipment has enabled high-precision batch production of large-aperture soft and brittle KDP crystal components.

Fig. 1 Simulation results of the brittle-plastic transition depth of KDP crystals. (a) 0° direction simulation result; (b) 45° direction simulation result.

Aspheric ultra-precision CNC grinding

The aspheric focusing lens required for high-power laser devices is usually a high-order, freeform, and non-rotationally symmetric curved surface. The research team developed an ultra-precision CNC grinding machine for large-aperture aspheric optical components through the cooperation of many top Chinese scholars. This machine grinds off-axis aspheric components with an aperture of 500mm×500mm, and its surface error PV value reaches 3.3μm. The depth of subsurface defects is within 5μm. In the research process of ultra-precision CNC grinding of aspheric surfaces, a breakthrough has been made in the precision dressing technology of diamond grinding wheels, which enbales the high-precision, automated batch production of aspheric components.

Fig. 2  Results of off-axis aspheric optics after grinding. (a) Result of form error; (b) Result of sub-surface crack depth.

Ultra-precision deterministic polishing

In order to meet the demand of ultra-precision deterministic polishing, the primary research focus is the development of technologies and equipments of high-efficiency Bonnet polishing, deterministic full-aperture polishing, low-modulation small tool CNC polishing, and magnetorheological finishing (MRF) etc.When compared with traditional processing techniques, these technologies have solved the key technical problems such as surface precision deterministic control and surface defect control, and have achieved high-efficiency convergence of the full frequency spectrum accuracy of large-aperture, high-power laser optical components.

Fig. 3 The actual picture of aspherical element airbag polishing and magnetorheological polishing. (a) Physical image of bonnet polishing; (b) Physical image of magnetorheological polishing.

About The Group

The Advanced Optical Manufacturing Team of Research Centre of Laser Fusion, China Academy of Engineering Physics has long been committed to the research of ultra-precision manufacturing technology for high-power laser optics. The team has received supports including National Science and Technology Major Project , 863 plan, national defense basic scientific research, national defense foundation strengthening plan, national natural science foundation, etc., and has won more than 20 provincial and ministerial science and technology awards, including 4 first prizes for military scientific and technological progress, 1 first prize for national defense science and technology, 1 first prize for provincial technological invention. In recent years, the Advanced Optical Manufacturing Team of Research Centre of Laser Fusion, China Academy of Engineering Physics has published more than 200 SCI papers in international academic journals such as International Journal of Machine Tools & Manufacurte, Journal of the American Ceramic Society, Optics Letters、Optics Express, Applied Surface Science, Laser Physics Letters, Applied Optics , etc.

Article

Fan Fei, Xu Xi, Xu Qiao, et al. Progress on ultra precision manufacturing technology of large-aperture high-power laser optics[J]. Opto-Electronic Engineering, 2020, 47(8): 200135.

DOI:10.12086/oee.2020.200135