Research on Manufacturing Technology for Multi-Layer Diffractive Optical Element and Applications in Camera Lenses

Masamichi Saito, Production Engineering Research Laboratory, Canon Inc.
Masaaki Nakabayashi, Production Engineering Research Laboratory, Canon Inc.
Toru Imanari, Production Engineering Center, Canon Inc.
Yuichi Miyoshi, Plastic Injection Molds Technology Center, Canon Inc.
Akihiko Yokoyama, Production Engineering Research Laboratory, Canon Inc.

1. Abstract
Diffractive optical elements have traditionally been regarded as unsuitable for use as photographic optical elements due to the undesirable diffraction flare generated. However, a multi-layer structure, in which two appropriate diffractive optical layers control the undesirable diffraction light and thus achieve ideal diffraction throughout the entire visible light range, has been developed. The result is the Multi-Layer Diffractive Optical Element, the world's first diffraction-type optical element applied in photographic lenses.
Such optical elements were made possible by integrating various technologies, including a multi-layer structure diffraction grating for controlling the unwanted diffraction flare, metal mold processing equipment, metal mold processing technology, molding technology, and other technologies required to implement the design.

2. Outline of the Technology
Figure 1 is a schematic diagram of the Multi-Layer Diffractive Optical Element. On the surface of each glass lens, which is the base material, there is an element containing a layer of diffraction grating formed by ultraviolet-curing resin. The grating pitch, grating height, and grating spacing must precisely match the shape and width specified in the design.
The mold for this diffractive optical element is machined with a high-precision free-form machine tool as shown in Figure 2. This mold is then used directly as an optical-element mold, without polishing of the cutting surface. In consideration of future development of optical molds, this machine tool is designed to be capable of not only the turning used here but also 5-axis synchronous machining by an all-axis non-contacting direct drive. The tool also has a high dynamic stiffness while maintaining a large machining area (310 x 170 mm) through such methods as FEM analysis. For cost-efficiency, the machine tool has a speed of 7 mm/s while achieving sufficiently accurate shape and surface roughness for optical elements.
The shape of this mold is transfer-molded onto the glass lens using ultraviolet-curing resin. The curing reaction of the resin is strictly controlled so that retraction due to hardening is kept to the nanometer level; such strict management enables the shape of the mold to be compensated by feedback.

3. Conclusion
As shown in Figure 3, this Multi-Layer Diffractive Optical Element has made it possible to create a more compact, lighter EF400mm f/4 lens while maintaining the same high image quality as in a design consisting only of refraction lenses; the total length is reduced by about 27% (from 317 mm to 232.7 mm) and the mass by about 31% (from 3000 g to 2080 g). (The lens with refraction optical elements is a conceptual design example of our company.)


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