The invention concerns production means for the manufacturing of a wave guide, a bundle of wave guides, material consisting of wave guides, a screen consisting of wave guides together with the materials, the screens and the products which are made from these screens, or the above mentioned material.
One of these products is a mirror with a very high reflectance. Mirrors with a somewhat lower reflectance are known form the literature.
A. F. Harvey states on page 236 of xe2x80x9cCoherent Lightxe2x80x9d, published by Wiley-Interscience in London in 1970, that mirrors with a high reflectance can be made by vapour deposition on a insulator of xc2xcxe2x88x92xcex layers of two dielectric media with different refractive indexes. Such multilayer mirrors do have the following 5 disadvantages:
1 ) Their bandwidth is limited. Harvey comments: Let ni be the refractive index of a layer of medium i and xcex94xcex the width of the band, at a distance of the media 1 and 2 of xe2x88x92(xc2xc)xcex, xcex94xcex/xcex=(4/xcfx80)arc sin{(n1xe2x88x92n2)/(n1+n2)} yields. The bandwidth can be enlarged by varying the thickness of the layers. Harvey gives as an example the fact that a multilaser mirror consisting of 35 layers, in the wavelength domain 300 nmxe2x89xa6xcexxe2x89xa6830 nm, can have a reflectance greater than 0.9.
2) The power that these mirrors absorb is too much for some applications e.g. in high power lasers.
3) They do not reflect exclusively according to Snellius"" law and thus scattering of light occurs.
Harvey gives as an example a 15 layer mirror designed for a wavelength of 1.06 xcexcm, which transmits 0.05% reflects 99.34% and absorbs or scatters about 0.6%. Jeffrey W. Griffin et al show in Applied Optics (see vol 25, no.10, page 1532, 1986), that the scattering arises from two sources which cause surface scattering and volume scattering respectively. They state that volume scattering increases with the number of layers and they mention a 15 layer mirror, designed for a wavelength of 0.633 xcexcm having a reflectance of 99% and a volume scattering of 1%. Therefore in practice the reflectance as a function of the number of layers has a maximum which is smaller than 1.
4) They do not have a sharply determinded cutoff wavelength. Therefore it is impossible to use these mirrors e.g. to cut off the spectrum of Nd-YAG laser in such a way that a smaller domain of the spectrum would be available in order to sharpen the focus of the Nd-YAG laser light.
5) Their applicability with respect to short wavelengths is limited due to the fact that at a certain wavelength the reflective ability of the normal reflection decreases as a function of the frequency.
The said invention, in most cases, does not have these disadvantages, or if so, to a lesser degree.
E.g. the wavelength domain having a high reflectance is not limited to the upper side but limits itself to the lowerside. There, the boundary is so sharp that it can be characterised by a cutoff wavelength, xcexc, and can be used e.g. to narrow the spectrum of a Nd-YAG laser. The power P absorbed by the mirror is considerably less than the power Pc absorbed by a conventional multi-layer mirror. For a certain design of the mirror it is possible to estimate the magnitude of P with the help of Pc.
Let Ae be the surface area of the unit cell of the mirror and Ar the surface area of the rim of the unit cell of the mirror: Pxe2x89xa6(Ar/Ae)xc2x7Pc is yielded.
A comparable relation holds also for Ps; the power of the light that has been scattered by the mirror and Pcs the power of the light that has been scattered by a conventional multi layer mirror. In some cases, depending upon the wavelength of the incident photons, it is realized that Ar/Aexe2x89xa610xe2x88x922 holds true. The field of application of the invented mirror is at the short wavelength side less limited than those of the metallic mirror and the multi layer mirror respectively. In the first case, this is caused by the fact that the reflectance is maximum at a 90xc2x0 angle of incidence and in the second case it is caused by the fact that a less normal reflectance is mainly apparent at the outerwall, on the rim of the unit cell. The invention relates to a mirror consisting of various cells which either may or may not be of the same type.
If a mirror consists of a great number of identical cells then such a cell is called a unit cell. In practice a unit cell is built of at least two media whereof at least one corresponds with a wave guide. Such waveguides either may or may not be closed at one side and their length is at least equal to half of their cutoff wavelength xcexc, and preferably much longer than xcexc. The cutoff wavelength is defined as the largest wavelength of the radiation that can be transmitted by the waveguide. In order to prevent that the waves penetrate practically through the material within the wall of a waveguide, such a waveguide must have a length which is at least equal to five times the penetration depth xcex4, of the material of which the wall of the waveguide consists. The penetration depth is defined in a longitudinal section of a wave guide. It is the distance from a point p in the wall of a waveguide to the innersurface of the wall of the waveguide. In point p the absolute value of the wavefunction xcexa8 of a photon transmitted by the waveguide has a value which is (1/e) times its value on the innersurface of the wall of the waveguide.
In a certain kind of unit cell, the simple unit cell medium 1 having refractive index n1 is situated in the centre of a cross section of the cell, while medium 2 having refractive index n2 is situated at the rim of that cross section coinciding exactly with the wall of the unit cell. Two kinds of simple unit cells are distinguished; type 1 being characterized by n1 greater than n2 and type 2 by n2 greater than n3. Composed unit cells are also distinguished.
Such a unit cell has the property of having at least two different waveguides. FIGS. 1A-N shows some and by far not all of the possible examples of cross sections of unit cells. If it is, for whatever reason, important, e.g. to reduce costs, to minimize the amount of material of medium 2 then it is of advantage to make a cross section of both the outer periphery and the innerperiphery of a unit cell, and that the boundaries of medium 2 both should coincide with a regular hexagon.
The unit cells shown in the FIGS. 1A-N are all simple unit cells having the same property; in a cross section of the unit cell the periphery of medium 1 has either precisely 1, or precisely 2 or an infinite amount of points in common with a straight line.
Such unit cells are of importance for applications because in a cross section, the whole of medium 1 can be associated with exactly one cutoff wavelength. These cells are called specific unit cells.
The unit cells in the FIGS. 1B, 1K and 1L are all composed unit cells. The unit cell in FIG. 1B is an example of a unit cell having characteristic dimensions of 0.5xcexci, where xcexci is the cutoff wave length in medium i. With the proper design, a mirror consisting of such unit cells is suitable to reflect all electromagnetic radiation of wavelength xcex provided that xcex greater than xcexci holds.
Conventional multi layer mirrors have structures of characteristic dimensions of 0.25xcexi, where xcexi the wavelength of the light in medium i in the mirror. In practice xcexci≈xcexi holds. Therefore, the said invention can be made with greater precision than the corresponding conventional multi layer mirror, while the lower boundary of the wavelength domain is considerably lower than that of the conventional multi layer mirror. The unit cell shown in FIG. 1K can be used to make mirrors which allow for e.g. fifty percent transmittance. For instance, one waveguide can be used to transmit light while another waveguide is designed to reflect light. This unit cell is an example of a unit cell with more than 1 characteristic dimension. Mirrors consisting of various kinds of unit cells also belong to the invention.
Each cell corresponds with exactly one waveguide.
In FIG. 2 an example of a cross section of such a mirror is shown. This cross section consists of the media 1 and 2. All the waveguides correspond with medium 1. They are so to say inbedded in medium 2. In order to operate as a mirror the condition n1xe2x89xa0n2 must be apparent.
Certainly this example is not the most general one, since medium 2 has a structure in the cross section of the mirror. Each straight line, having at least one point in common with the periphery of a waveguide, has either precisely one, precisely two or an infinite amount of points in common with that periphery. If light of wavelength xcex enters this mirror from a vacuum at normal incidence then the reflectance of the mirror is independent of the space coordinates, provided the following conditions are met:
1) n2xcex greater than xcexc12 where xcexc12 is the greatest cutoff wavelength of a waveguide of the mirror.
2) If in a cross section of the mirror, the boundaries between the medias of 1 and 2 are constant and medium 1 and medium 2 are exchanged then the cutoff wavelength xcexc21 of the new mirror obeys (n1/n2)xc2x7xcexc21 less than xcex.
Mirrors consisting of unit cells which can be considered as variants of the simple unit cell belong also to the invention, provided that medium 2 is replaced by a multi-layer layer of media 2 and 3 and if desired a layer of medium 4, where the thicknesses of the layers meet n2xcex94l2=n3xcex94l3=0.25xcexn3 (xcex is the wavelength in vacuum of the light that has to be reflected). Medium 4 has the function to bear the media 1, 2 and 3. An example of such a unit cell is shown in FIG. 3. It is a variant of the unit cell in FIG. 1.G. Such a unit cell serves as a model for relations of the type Pxe2x89xa6(Ar/Ae)xc2x7Pc which are used to compare both the absorbed power of a mirror designed according to this invention and the scattered power of the mirror with the absorbed power, respectively scattered power of a conventional multi layer mirror. An advantage of the said invention is that a unit cell can be constructed in such a way so that it is closed at one side. This side can be used to remove the heat generated in the media 1, 2, 3 or 4.
Because of the heat transported by radiation it is advantageous to build the closed sides of the waveguides out of material that radiates as a black body or has a very high emissivity. If the mirror is used to reflect monochromatic radiation only. absorbtion will occur only at the surface area Ar of a unit cell.
The quotient Ak/Aa of the surface Ak of the unit cell, located at the closed side of the waveguide that radiates absorbed energy and the surface Aa of the unit cell, located at the open side of the waveguide, that aborbs radiation that was meant to be reflected, yields Ak/Aaxe2x89xa71+Ac/Ar greater than  greater than 1. For both a metal mirror and a multi layer mirror, Ak/Aa=1 yields, causing the latter mirrors to be more sensitive to thermal damage.
The media which are used for the construction of the invention can be solids, liquids, gasses, vapours, plasmas or vacuo.
When liquids are used, e.g. these liquids can be situated, in a container which is transparent. An example of a cross section of a unit cell of such a mirror is shown in FIG. 1.A. Feasible liquids are those liquids which do not mix, have slightly different densities, have a high mutual diffirence in refractive index and absorb a neglectible part of the radiation that is to be reflected. Finally it has been stated that it is possible to pile identical unit cells. One of the piling methods, but certainly not the only one, is a piling according to a so called xe2x80x9cxcex2-brick connectionxe2x80x9d, where xcex2 yields oxe2x89xa6xcex2 less than 1. Cross sections of two waveguide cables are shown in the figures. FIG. 38 shows the case xcex2=o while FIG. 39 shows the case xcex2=1.
There is a special kind of piling when spaces are formed between the piled waveguides, which also can be considered as being waveguides themselves. FIG. 53 shows two examples which are each related to a part of a cross section of a cable that consists of waveguides. There we have generalization of a xe2x80x9cxcex2-brick connectionxe2x80x9d piling. These methods can be used to save material that is of importance in space travel.
A second advantage is that it is possible to choose the refractive index n3 of the clearances equal to 1. That way those clearances can transport information as fast as possible.
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