Focusing lens created for laser Megajoule project
Roland Roux
A recent report from the French atomic energy commission (CEA) and the REOSC SFIM Grou¥(Massy, France) concludes that the component considered most challenging to fabricate for the French Laser Megajoule project (LMJ) for inertial confinement fusion has been successfully demonstrated by REOSC (Saint Pierre du Perray, France). The company used its expertise in fine polishing of very large mirrors to produce a prototype large, square focusing lens for the LMJ (see photo).
The Megajoule project (see Laser Focus World, Feb. 1997, p. 42) has been designed to produce 1.8 MJ pulses. To achieve this output the laser system will use 4320 laser amplifiers (42 ¥ 76 cm), 240 polarizers (42 ¥ 76 cm), 1200 mirrors (40 ¥ 40 cm and 40 ¥ 56 cm), 960 square, moderately aspheric spatial filtering lenses, and 240 square, highly aspheric focusing lenses.
To meet specifications, these lenses, made from fused silica, must focus the 40 ¥ 40-cm laser beam at a distance of 7 m. In addition, they must also give it a permanent deviation angle of 2.4°. The transmitted wavefront must be better than l/25 rms at 633 nm. Reduced high-frequency ripple is required as well as low microroughness for high resistance to the laser energy. The design of the proposed focusing lens is a combination of a deviating prism and a plano convex lens that is "superimposed" onto the prism. The entrance face is plano and tilted to produce the deviation, and the beam incidence angle is 7.3° with a 4.9° prism angle and 2.4° deviation angle. The exit surface is convex, and the emerging beam is normal to the convex surface. The resulting aspherization is axisymmetric but with higher amplitude than for other designs.
To demonstrate feasibility of the polishing project, the first trial was conducted on a lens with dimensions smaller than the final component will be. The tested lens was 250 ¥ 250 mm due to limitations of the computer-controlled surfacing equipment. To have the same total aspherical departure of 200 µm as the final 40 ¥ 40 cm lens the relative aperture was increased to f/5. The radius of the convex surface was 640.05 mm and the conic constant -2.12337.
Fabrication of the focusing lens begins with cutting the square contours (future plans are to supply the fused silica raw material in a square shape). After conventional generation of the prism, plano entrance surface, and convex exit surface (best fit radius), the plano surface is polished. Micro-grinding of the aspherical profile is accomplished with an SFIM/ODS MOORE machine, and any subsurface damage is removed by polishing and figuring with REOSC-developed computer-controlled optical surfacing (CCOS) equipment. The CCOS equipment allows polishing of an area that can be reduced to only a few square millimeters and is capable of handling complex nonaxisymmetric surfaces for components sized from 10 to 300 mm.
The final performance tests of the focusing lens showed that the specifications have been reached. Single-pass wavefront quality is 85 nm peak to valley and 8 nm rms, with ripples less than 4 nm rms and microroughness less than 10 Å.
The report concluded that "REOSC has demonstrated its capability to produce the focusing lenses, which are the most difficult optical component of the Megajoule project. Expertise exists, optical performance has been achieved, and internal R&D is continuing on the high-frequency ripples. An industrial team could be constituted within the SFIM grou¥to achieve a production rate of 1 to 2 lenses per week."
In order to better meet the extensive polishing requirements, REOSC decided, one year ago, to implement an ion-beam polishing process. In Sept. 1996, a new facility came on line with 1.4-m-diameter component capacity, material throughput of 20 mm3 per hour, and computer control of the ion-gun motion that is totally compatible with existing REOSC computer-controlled polishing software.