World's first revolutionary new optical element for a camera lens

Canon Multi-Layer Diffractive Optical Element

This month's Technical Report presents the technical information on the "multi-layer diffractive optical element," an innovative new optical element technology announced last fall. This new optical element is the world's first multi-layer diffractive optical element developed for a camera lens. Canon introduced a prototype of EF 400mm f/4 DO IS USM(DO: an abbreviation of Diffractive Optics), which is a super telephoto lens incorporating this element, at Photokina 2000 in Cologne, Germany last year. The prototype was also displayed at Canon Expo 2000 in New York, Paris and Tokyo, as well as at Photo Expo 2001 this spring. It has been well received on all of these occasions.

Features of the lens equipped with multi-layer diffractive optical element First, given below are the features of a lens with the newly developed multi-layer diffractive optical element. [1] Drastic improvement in imaging performance Drastic improvement in imaging capability can be realized by combining the appropriate refractive optical system with the multi-layer diffractive optical element with its excellent control over chromatic and spherical aberration. Photo 1 EF 400mm f/4 DO IS USM (prototype) and Multi-layer diffractive optical element

[2] Drastic downsizing A drastic reduction in the overall length of the telephoto optical systems is attained by the use of the multi-layer diffractive optical element which has outstanding superiority in color aberration cancellation over that of the conventional fluorite or Ultra Low Dispersion (UD) lens. [3] Drastic weight and length reduction The use of the multi-layer diffractive optical element reduces both the weight of the lens system by application of the low density glass and reduction of the number of lens elements, and the overall length of the optical system. These features are especially effective in super telephoto lenses where chromatic aberration is problematic. Explained next are the technical points of the multi-layer diffractive optical element underlying these features. About Diffraction Diffraction is the tendency for light to spread around and behind the edge of an opening in an opaque obstruction after passing through the opening (see Fig.1). A well-known example of this phenomenon is the flare that occurs when light passes through a camera lens set to a small aperture. Diffraction occurs due to the inherent characteristics of light waves. Another example of diffraction occurs when light passes through two slits set slightly apart. It is the same as when light passes through a small lens aperture. As shown in Fig.2, light waves are generated in "the directions it can easily disperse."

Fig.1 Diffraction occurring with a single slit Fig.2 Diffraction occurring with two slits (amplitude diffraction grating)

The wave motion's combined power goes in the direction where the phases of the light waves spreading from the two slits are in the same direction. Therefore, the wave motion goes in the direction where the wavelength differs by one cycle and in the direction where the wavelength differs by two cycles. The wave motion's combined power therefore goes in multiple directions. The direction in which the overlapping wavelengths differ by only one cycle (1 wavelength) is called the primary diffracted light. This slit structure is called an amplitude diffraction grating. The features of amplitude diffraction gratings are as follows: [1] Changing the spacing (grating interval) between the slits changes the angle of the diffraction. [2] The smaller the grating interval, the greater the diffraction angle. [3] The longer the light's wavelength, the larger the diffraction angle.

Diffractive optical element used as a lens If the diffraction principle is incorporated in a diffraction lens, the incident light's utilization will be very poor due to the amplitude diffraction grating's obstruction (see Fig.2). Also, there will be diffracted light going in various directions. To resolve these problems, Canon developed a sawtooth design in the diffraction grating to create a phase diffraction grating (see Fig.3). A phase diffraction grating used for a lens will have the construction shown in Fig.4.

Fig.3 Saw-toothed shaped diffraction grating (phase diffraction grating) Fig.4 Single-layer diffractive optical element construction

The phase diffraction grating is concentric, with the grating interval getting smaller toward the periphery. The grating's sawtooth shape is also curved. The actual thickness of the grating is only several micrometers(1 micrometer = 1/1,000 millimeters). The grating has over 100 cycles and changes gradually from several millimeters to several tens of micrometers. Features of the diffractive optical element used as a lens are described below: [1] Color Aberration Cancellation In a positive-power diffractive optical element (equivalent to a convex lens), light entering the lens will have dispersion characteristics of image formation according to the wavelengths as follows: • The longer wavelengths will form an image closer to the lens due to the larger diffractive angle. • The shorter wavelengths will form an image farther away from the lens due to the smaller diffractive angle.

In contrast, an ordinary convex lens (refractive optical element), the incident light entering the lens will have the opposite characteristics of image formation as follows: • The longer wavelengths will form an image farther away from the lens due to the smaller angle of refraction. • The shorter wavelengths will form an image closer to the lens due to the larger angle of refraction.

Therefore, the diffractive optical element has the image formation characteristics opposite to those of refractive lens, so when you put the two elements together, they tend to cancel out each other's chromatic aberrations (refer to Fig.7 mentioned later). [2] Using as an aspherical surface As described above, the diffractive optical element uses the grating interval to control the light path direction. By varying the interval, the diffractive element can mimic the effect of an aspherical lens. This makes it possible to correct spherical aberrations. Single-layer diffractive optical element Since the diffractive optical element shown in Fig.3 is made of a single diffractive grating, it is called the "single-layer diffractive optical element" in order to distinguish it from the multi-layer diffractive optical element which is mentioned later. Since this single-layer diffractive optical element diffracts light waves of a certain wavelength to achieve the aforementioned effects, it has already been applied to the optics for reading signals in DVD and CD players and other appliances that use a laser beam, which emits a very narrow band of wavelengths. In contrast, a camera lens has to deal with wavelengths ranging from about 400 nanometers to 700 nanometers, so it is impossible to obtain correct diffraction for all wavelengths through the single-layer diffractive optical element. That is why the single-layer element is not used in camera lenses. As shown in the left side of Fig.5, it can perfectly diffract all the light waves within a certain range, but the shorter and longer light waves outside this range do not form an image, resulting in flare.

Fig.5 Single-layer diffractive optical element (left) and Multi-layer diffractive optical element (right)

Multi-layer diffractive optical element Canon succeeded in developing a new diffractive optical element which markedly improves diffraction efficiency for all visible wavelengths and so is applicable to camera lenses. This is the "multi-layer diffractive optical element" used in the EF 400mm f/4 DO IS USM. Construction As depicted in Fig.6, the multi-layer diffractive optical element combines two single-layer diffractive optical elements that have concentric diffraction gratings. By optimizing the grating shape and the grating materials, as well as by very finely adjusting the spacing (micron order) between these two single-layer diffractive optical elements, superfluous diffracted light is suppressed to achieve drastic improvements (almost 100%) in the diffraction efficiency over the entire range of visible wavelengths. This makes it possible to use the element in a camera lens for the first time.

Photo 2 Multi-layer diffractive optical element Fig.6 Multi-layer diffractive optical element construction

Ultra-high precision production technology During the manufacture of the multi-layer diffractive optical element, the precision of the thickness and varying interval of the diffractive grating as well as its positioning must be on a level finer than micrometers. This ultra-high precision has been made possible by Canon-developed three-dimensional, ultra-high precision micromanufacturing technologies, replicated aspherical lens manufacturing and high-precision positioning technologies refined through the production of EF lenses. Correction of chromatic aberrations by the multi-layer diffractive optical element Looking at the focus points of the diffraction grating by wavelength (color), the focus points are evenly spaced along the optical axis(regular dispersion). In the ordinary refractive optical lens, the focus points of the colors will be irregularly spaced. The ordinary lens is designed for the different focal points by wavelength to coincide with each other as much as possible in order to achieve elimination of axial chromatic aberration. The lens with the diffraction grating is designed for the focus points of the refractive optical elements to coincide with the focus points of the diffraction grating in the reverse order. Thus, the combination of these two effects corrects chromatic aberrations (see Figs.7 and 8).

Fig.7 Correction of chromatic aberrations by the multi-layer diffractive optical element Fig.8 Lens downsizing with the multi-layer diffractive optical element

Overview of the prototype EF 400mm f/4 DO IS USM Downsizing and weight reduction As shown in Photo 3, the EF 400mm f/4 DO IS USM is about 26 percent shorter than a comparable lens having a conventional design using a refractive optical element. The weight is also about 36 percent lighter. High image quality As already mentioned, chromatic aberrations caused by the refractive lens group are eliminated by the multi-layer diffractive optical element placed in the front lens group. Chromatic aberrations are thereby suppressed to an absolute minimum. Also, the diffractive optical element's aspherical effect corrects spherical aberrations. This makes it possible to achieve sharp, clear images with superb contrast. Photo 3 Size comparison of lens with multi-layer diffractive optical element (above) and lens with a conventional design (below)

This issue of the Technical Report introduces the new technology of the multi-layer diffractive optical element developed for a camera lens. The multi-layer diffractive optical element is innovative and represents a milestone in that it possesses aspherical characteristics as well as optical characteristics superior to those of the fluorite, UD or aspherical lenses. It is expected that this element will be applied widely to the imaging equipment including not only the future EF lens, but also lenses for digital devices using high-density solid state imaging sensors, and the projection lens of the LCD projectors. Canon is currently exerting its utmost efforts to launch a commercial version of the EF 400mm f/4 DO IS USM in the near future.