Optical: systems and elements – Mirror – With support
Reexamination Certificate
1999-05-25
2001-03-27
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Mirror
With support
C359S846000, C359S896000, C359S900000
Reexamination Certificate
active
06206531
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to lightweight, precision, optical mirror, substrate structures, and, in particular, to such structures comprising a thermally and structurally stable fully encapsulated composite structure including a reticulated structural foam core and an initially separate faceplate, and a method of making such structures.
2. Description of the Prior Art
Previous attempts to produce high precision lightweight mirrors have generally been unsatisfactory for a variety of reasons. For example, Wakugawa et al., U.S. Pat. No. 4,856,887 discloses a monolithic silicon carbide foam core with at least one integrally formed silicon carbide face sheet. The face sheet is integrally formed on the silicon carbide foam by applying a layer of silicon carbide paste to a surface of the foam and then curing that paste to form the face sheet in situ. A reinforcing silicon carbide coating is applied to one or both of the cured integrally formed silicon carbide face sheets by, for example, chemical vapor deposition. It was not appreciated that such integrally formed silicon carbide face sheets have a different coefficient of thermal expansion from those silicon carbide deposits that are formed by chemical vapor deposition. Under vigorous thermal cycling and substantial thermal gradients, micro cracks appear in those chemical vapor deposited silicon carbide mirror surfaces which are supported by such integrally formed face sheets. For this reason, among others, these structures are unsuitable for space-based applications. Glass plates, mounted on foamed silica or glass are generally not robust enough or stable enough under severe thermal and mechanical stress to provide reliable service. Various such structures have been proposed, for example, in Yoshiaki U.S. Pat. No. 5,640,282 (closed cell foamed glass core integrally bonded through a fused silica powder to a glass front plate with an optical surface); Clemino U.S. Pat. No. 4,670,338 (segmented glass foam core bonded by an organic glue to preformed glass plates); Fletcher U.S. Pat. No. 4,035,065 (large diameter cellular glass core bonded by an organic adhesive to a thin glass lamina which bears a reflective coating); and Tatsumasa Nakamura U.S. Pat. No. 5,316,564 (closed cell foamed glass body to which is fused a glass plate with an optical surface). Colarusso et al. U.S. Pat. No. 5,002,378 discloses a mirror cooling system in which a fully enclosed low density porous foam core with a graded pore size is used as a heat pipe for actively cooling a mirror. The purpose of Colarusso et al. is to transfer heat away from the faceplate so as to cool it, rather than to transfer heat away to prevent distortion. A faceplate is bonded, fritted, fused or otherwise mechanically attached to the core. The elements of the actively cooled structure are preferably composed of the same materials. A catalog of materials is listed for use as the face sheet, including, glass, silicon carbide, aluminum, high temperature ceramic, and beryllium. The disclosed substrate is not highly precise. There are a number of deficiencies in the proposed structure and process that, in the aggregate, render the substrate unacceptable for use in high precision mirror applications. The significant advantages to be achieved by fully encapsulating the structure in one monolithic coating were clearly not appreciated. Also, it was not appreciated that providing a graded core would tend to create unequal responses, with resulting physical distortions, to temperature changes and temperature differentials across the substrate. Fully sealing the foam core in a container also creates undesirable pressure gradients in, for example, space-based applications. Such pressure gradients add yet another potential contributing factor to the undesired physical distortion of the mirror surface. During manufacturing, particularly, grinding and polishing, an all silicon carbide face plate, as suggested by Colarusso et al., would require very careful handling because it is more brittle than graphite.
These and other difficulties of the prior art have been overcome according to the present invention.
BRIEF SUMMARY OF THE INVENTION
A preferred embodiment of the lightweight composite optical mirror substrate structure according to the present invention comprises a reticulated structural foam core, an initially separate, thin, dense, prefabricated interlayer rigidly joined to and supported by the reticulated structural core, and a thin optical grade (very fine grained and fully dense) near net shape deposited in situ coating of, for example, silicon carbide, on the exposed surface of the interlayer. The optical grade coating of, for example, silicon carbide, when ground, polished, and coated with various ultra thin optical coatings, which are generally in the order of only a few angstroms thick, serves as a thermally stable precision mirror surface. Such ultra thin optical coatings are generally thin film dielectrics. The prefabricated interlayer serves to provide a smooth uniform surface to receive and support the mirror surface forming deposit. The interlayer and the mirror coating of, for example, silicon carbide, together comprise a mirror faceplate. The reticulated foam provides structural strength and stiffness to the mirror substrate without contributing excessive mass. The present invention particularly contemplates mirror substrates from a centimeter or less to those in excess of one meter or more in diameter.
The inner elements of this optical mirror substrate are all preferably fully encapsulated within a monolithic, structural, formed in situ coating of, for example, silicon carbide. In addition to being capable of being formed into a precision mirror surface, the monolithic coating also exhibits substantial structural strength. The optical mirror substrate thus exhibits primarily the characteristics of the continuous monolithic coating, particularly the strength and thermal characteristics. The configuration of the reticulated structural core is defined, for example, by that of a fragile vitreous carbon skeleton. The fragile skeleton is fully encased in a much stronger coating of, for example, silicon carbide, to provide a reticulated structural foam. The encased skeleton generally provides very little if any of the structural or thermal characteristics of the optical mirror substrate.
The dense prefabricated interlayer, and its bond to the reticulated structural foam, are also generally fully encased by the structural coating so that their characteristics are also substantially dictated by those of the structural coating. It is, however, desirable that at least the thermal characteristics of the interlayer and its bond to the core be closely matched to those of the coating, because the dense interlayer's characteristics do generally influence somewhat the response of the mirror surface to thermal changes and gradients. The continuous monolithic structural coating thus serves to provide the mirror surface, to provide most of the structural strength for the substrate, to define most of the thermal characteristics of the substrate, and to bond the elements of the substrate into one unitary whole.
The mirror substrates according to the present invention are useful in the preparation of close tolerance optical devices which are intended to be used in extreme environments such as those encountered, for example, in space-based applications, other high or low temperature environments, high energy X-ray, chemical or gas dynamic lasers, cyclotrons, and the like. Such mirror substrates can be arcuate, conic, flat, aspheric, higher order aspheric, or of any other configuration, as desired. Tolerances of less than one millionth of an inch are occasionally encountered in such applications. Very high optical quality in the range of from 0.50 to 3.0 wavelength of error can be achieved by mirrors made from the substrates of the present invention. Such accuracy is achievable throughout a wide variety of mirror surface shapes. Very stiff the
Ono Russell M.
Williams Brian E.
Jagger Bruce A.
Nguyen Thong
Ultramet
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