tEchnology
DiFFrActiVE optics
welding powders, using a beam shape modified by a DOE as opposed to fusing them, results in a fully dense part with improved functionality.’ Lloyd adds that, in terms of welding metal, the
grain structure across the width of the weld can be controlled with a DOE to produce an even weld structure, which matches the parent material better than a conventional weld.
computer-generated holograms While industrial laser cutting and welding benefit from beam shapers, optics manufacturers are using the beam-shaping properties of computer- generated hologram (CGH) nulls, a class of 3D diffraction grating, as part of an interferometric measurement device. The CGH nulls can be used with an interferometer to measure the surface profile of aspheres to high degrees of accuracy. In the optics world, fabrication has to be carried out to optical tolerances, which can be fractions of a wavelength. This is particularly difficult for aspheres and, according to Dr Chunyu Zhao at the College of Optical Sciences, University of Arizona, there are not many other metrology options for these optics apart from CGHs. ‘Refractive or reflective nulls, made up of spherical surfaces, can be designed for some axi-symmetric
A diffractive optic for co2 optical Engineering
lasers from laser
aspheric mirrors, but these are expensive and take a relatively long time to produce. Also, it is multiple elements assembled together, so the accuracy of these refractive nulls is not great either,’ he says. A CGH null, on the other hand, can create almost any shape of wavefront, can be fabricated quickly and accurately and measure large wavefront slopes with the wavefront accuracy and spatial resolution of an interferometer. The downside, as Steve Arnold, president of US CGH company Diffraction International, points out is that these large wavefront slopes must be close to their design values, requiring a custom CGH null for each asphere design.
The University of Arizona houses two
facilities for large optics fabrication: the Steward Observatory Mirror Laboratory, which produces mostly 6-8m telescope mirrors, and a second optics shop in the College of Optical Sciences fabricating metre-class optics. The university is fabricating the first primary mirror segment for the Giant Magellan Telescope (GMT), one of the three extremely large telescope projects currently ongoing. The primary mirror is made up of six off-axis ellipsoidal segments, each one measuring 8.4m in diameter. It has also produced some very large CGH nulls, the largest of which was 1.8m in diameter on a curved surface and was used to test a large convex secondary mirror for a telescope. In terms of fabricating these telescope mirrors,
Dr Zhao comments: ‘There is really no other competing interferometric test for measuring large aspheres and off-axis aspheres other than using the CGHs.’
The interferometric measurement technique
works on the principle of transforming the wavefront from spherical to aspheric. The measurement beam exits the null perpendicular to the test asphere surface, reflects and retraces its path, and is converted back from aspheric to spherical. Therefore, as Arnold of Diffraction ➤
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