METHOD FOR THE FABRICATION OF LENSES WITH A VARIABLE REFRACTION INDEX

Abstract: The invention presents a method for the fabrication or production of three-dimensional lenses with a variable refraction index by wrapping a material with a given refraction index. It is preferred, that this material has the shape of a thread, which might be cylindrical. The preferred shape of the lens to be produced is spherical or semi-spherical, which can be achieved by an appropiate wrapping process or by cutting the spherical shape. By the inventive method it is possible to produce the said lenses with a smooth varying of the refraction index. It is preferred to use the produced lenses as part of a microwave antenna system.

Description: -5-ϊ£-12---_.Ϊ2£_l_l-ϊ_£S-.£i£-iii2Q_2Ϊ_k-ϊD--§-i_i_it h a variabLe Refraction Index The invention reLates to a method for the fabrication of three-di mensiona L Lenses with a variabLe refraction index. Lenses with a variabLe refraction index, such as a Luneburg Lens or a Eaton-Lippmann Lens, are weLL known- It is aLso known, e.g. from US 4 288 337, that Lenses with variabLe refraction indexes can be used as radar refLectors or, as is known from E. F. BuckLey; "Stepped-Index Luneburg Lenses"; ELectronic Design, Apri L, 13, 1960, as part of an antenna system. As BuckLey has described in said articLe, it is a known meth¬ od for the fabrication of Luneburg Lenses to use a hemispheri- caL-sheLL construction with a given number of Layers. According to said US patent the Layers for the fabrication of Luneburg and Eaton-Lippmann Lenses can be produced by mixed dieLectrics. Such a mixed dieLectric can be obtained by ix¬ ing expanded particLes seLected from the group consisting of expanded poLystoroLs, expanded po Lyethy Lenes, expanded po Lyuret hanes, gLass baLLoons and si Lica baLLoons, with met¬ al-coated particLes consisting of said expanded particLes, surfaces of which have been coated with a thin fi Lm seLected from the group of chromium, aLuminium, copper, nickeL, goLd, si Lver, and magnesium in proper proportions to obtain a de¬ sired dieLectric constant then forming the same to the de¬ si red shape by the use of a binder. From the article "A multiple-beam mult ple-frequency spheri¬ cal Lens Antenna System providing hemispherical Coverage" of M. A. Mitchel et al.; 6. International Conference on Antennas and Propagation CICAP) 1989, Part 1, pp. 394 - 398 it is known that the relative dieLectric constant, and by this the refraction index, of a dielectric material, such as polysterene, can be modified by a variation of density of said material. Thereby hemispericaL shells with given refraction indexes may be produced. From US 3 307 196 a method is known, which allows the produc¬ tion of a two-dimensional dielectric Lens, such as a disk, by winding a ribbon, shut or strip type of module. In the US patent 3 307 196 it is proposed to fabricate a three-dimensional dielectric Lens by the individual prepara¬ tion of wound disks, which are superposed upon one another. The superposed disks are of successively different diameters and dieLec-tric profi le and could be formed, starting with individual substantially strip or ribbon modules, by cutting successively different lengths away from the high dieLectric constant ends of said different strips, and then rewinding the resultant successively different length strips. There are two disadvantages in the known methods for the fabrication of three-dimensional Lenses. It is either possi¬ ble just to approximate the variation of the refractive index required, which is dependent on the dielectric constant. Or it is necessary to carry out a large number of steps. That means no easy and practical method for smoothLy varying the refractive index has been achieved. By using shells with different dielectric constants and there¬ by with different refraction indexes, reflection losses occur by which power is reflected from the dielectric boundaries. It is an object of the nvention, to present an easy method for the fabrication of lenses with a variabLe refraction index, which overcomes the deficiencies of the prior art. This can be realized by a method according to claim 1. The method according to the invention allows to produce three-dimensional lenses with a variable refraction index n by wrapping a material with a given refraction index, e.g such as the known materials from US 4 288 337, into the final shape of the Lens to be produced. It is an advantage of the invention to present a method for the fabrication with a reduced number of steps. It is a further advantage of the invention to produce lenses with a better aperture efficiency by avoiding surface waves, which are set up at the spherical boundaries, and by achiev¬ ing a more exact phase of the coLliminated rays at the feed points, which makes the lens less frequency dependent. If the material with the given refraction ndex s shaped as a thread, the method for the fabrication can be executed more ea si Iy. The present invention wi ll be better understood with the aid of the following description and accompanying drawings, where- i n Fig 1 shows a known Luneburg lens radar reflector, Fig 2 shows a known Luneburg lens antenna, Fig 3, shows a preferred embodiment, Fig 4a, b show possible shapes of thread used. Prior to the detai led description should be mentioned, that in the preferred embodiment the Lenses to be produced are able to refract electromagnetic waves, preferably microwaves. In this case the material with a given refraction index n is a dielectric material and the refrac ion index n is given by the expression E = n 2 , where E is the relative dielectric constant. Though the preferred embodiment is shown with lenses for electromagnet c waves, it should be kept in mind, that the invention is not limited to such Lenses. By using a material with an appropriate refraction index even lenses, which are able to refract any other waves, e.g. sound waves, may be produced. Fig. 1 shows a three-dimensional Luneburg lens 10, which works as radar reflector and as is state of the art. An incom¬ ing wave 11 is focussed by the lens 10 in such a way that the wave is focussed on a focus point 12. The wave is reflected by a reflector 13, whereby the reflected wave 14 is generat¬ ed, which is led by the lens 10 in such a way, that it leaves the Lens 10 in the same direction as the incoming wave 11 came from. For leading the incoming wave 11 and the refLected wave 14 in the desired manner, it is necessary, that the relationship be¬ tween the relative dielectric constant ECr ) and the normalized radius r/a is given by (1) where r is the distance from the center point, a is the radius of the lens 10, and r/a = 1.0 at the outer surface of the Lens. Fig. 2 shows another application of the Luneburg lens 10. The difference between this embodiment and the embodiment of fig. 1 is, that here an incoming wave, such as 11a is led to a first focus point 12a and received by a first feeder horn 20a. In the same manner incoming waves 11b and 11c are led to focus points 12b, 12c and received by feeder horns 20 b, 20c respectively. The signals received by the feeder horns 20a, 20b, 20c are led to not shown receivers. Of course the system according to fig. 2 can also work as transmitter antenna, if transmitters are connected to the feeder horns 20a, 20b, 20c. According to the invention the three-di ensiona L Lens 10 is produced by wrapping a dieLectric material, preferably shaped as a thread. This is in principle shown in fig.3. Starting at the center point of a Lens 10' to be produced, a dieLectric thread 21 is wrapped around the center point. Said thread has at Least initially a relative dielectric constant E = 2.0. With an increasing distance from the center point the effective relative dieLectric constant E(r) of the lens 10 1 to be produced decreases according to the formula (1) . The effective relative dielectric constant may be varied by a var ation of the relative dielectric constant E of the thread. This could be achieved e.g. by a variation of the che ical composition or by a variation of the density of said thread with length. A variation of density with Length could be achieved e.g. by a variation of pressure, proceeded by a press arranged before the Lens 10' to be produced. Another possibi lity of variation of the relative dieLectric constant E may be achieved by a thread, created by severa l strands, whereby the number and/or the relative dielectric constant E of said strands may vary with Length. It is sti ll another possibi lity to vary the effective dieLectric constant E by a variation of the amount of trapped air CE = 1 ) . This might be realized e.g. by a variation of the thickness of the thread, whereby the amount of trapped air s increased and thereby the effective relative dieLectric constant is decreased. It s another possibi lity to use a crimped thread, e.g. like it is shown in fig. 4a or 4b, which might be stretched by a variation of a stretching force used. The dielectric constant of the thread may also be varied along the length with the aid of a metallic paint. In this case a low density dieLectric thread of constant dielectric constant is used and as it is wrapped into the shape of the Lens to be produced small areas of the thread are painted at a separation necessary to give the correct dielectric con¬ stant profi le. That means for a desired value of the effec¬ tive refraction index the thread used is painted with a paint, which may be metallic. Thickness, density or/and inten¬ sity of this paint may be varied. This is a simple method and wi ll result in a relatively light lens. It s to be said, that electromagnetic scattering by individu¬ al strands of the thread can be made negligible by keeping the radial dimensions of the thread 21 small. Versions of the preferred embodiment may contain at least one of the follow ng variations: - instead of a thread, the material with the given refrac¬ tion index may have any other appropriate shape, e.g. Like a strip, ribbon, or the Like - by using an appropriate dieLectric material, the lens to be produced may be able to refract other electromagnetic waves, such as visible or infrared light. by an appropriate wrapping process, lenses with non- spherical shapes may be produced, - the Lens to be produced may have any des i red. re Lat i on ship between the effecti e dielectric constant ECr) or the refraction index respectively and the normalized radius r/a, e.g. in that way, that the focus point 12 is inside or outs de of the surface of the lens, - the wrapping process may start at the surface of a core, which itself might have a variation of the refraction index and might be located around the center point, several threads may be used, one after the other and/or at the same time, by using a material w th an appropriate refract on index even lenses, which are able to refract any other waves, e.g. acoustic waves, may be produced, a bonding agent may be used, which e.g. ght be wrapped with the dielectr c thread and when cured at an elevated temperature forms a more solid lens. Of course, i might also be possibLe to dip the len.s to be produced nto an appropriate bond ng agent during or/and ' after he wrap¬ ping process. The invention presents a method for the fabr cation or produc¬ tion of three-dimens onal Lenses with a variabLe effective re¬ fraction index by wrapping a material ith a given refract on index, which may be constant or may vary with length. It s preferred, that said material has the shape of a thread, which might be cylindrical. The preferred shapes of the lens to be produced are spherical or semi -spherical. The latter one can be achieved by an appro¬ priate wrapping process or by cutting the spherical shape. By the inventive method it is possibLe to produce the said Lenses with a smooth vary ng of the refraction index.

Claims: C L A I M S 1. Method for the fabrication of three-dimensional lenses with a variabLe refraction index, characterized in that a material with a given refraction index is wrapped into the final shape of the lens to be produced. 2. Method according to claim 1, characterized in that the material with the given refraction index is shaped as a h read . 3. Method according to claim 1 or 2, characterized in that the final shape of he Lens is spherical. 4. Method according to one of the claims 1 to 3, characterized in that the lens to be produced is a Luneburg-type or Eaton-L ppmann-type lens. 5. Method according to one of the claims 1 to 4, characterized in that the material with the g en refrac¬ tion index used has an actual change in the refraction i ndex . 6. Method according to claim 5 characterized in that said thread itself is created by several strands and the actual change in the refraction index is achieved by a variation of the refraction index and/or the number of said strands. Method according to one of the claims 2 to 6, characterized in that the thread used has a changing diameter with Length. 8. Method according to one of the claims 1 to 7, characterized in that the material ith the given refrac¬ tion index is a dielectric material. 9. Method according to one of the claims 1 to 8, characterized in that the material to be wrapped is c ri mped . 10. Method according to claim 9, characterized in that a variation of the refraction index is achieved by stretch¬ ing the crimped material with a given variabLe force. 11. Method according to one of the claims 1 to 10, characterized in that a bonding agent is used, w ich is wrapped with the material with a gi en refraction index. 12. Method according to one of the claims 1 to 11, characterized in that a bonding agent is used, in which the Lens to be produced is dipped in during or/and after wrapping. 13. Method according to one of the claims 1 to 12, characterized in that small areas of the material to be wrapped are painted at a separation necessary to give the correct dieLectric constant profi le.

Inventors: FUJIMOTO MASAHIRO, HARRISON DAVID, MAIER GERHARD, FUJIMOTO, MASAHIRO, HARRISON, DAVID, MAIER, GERHARD
Applicants: THOMSON CONSUMER ELECTRONICS, THOMSON CONSUMER ELECTRONICS S.A.
Application year: 1991
Main classification: H01Q15
IPC codes: H01Q15