Glass for a fibre-optical element
United States Patent 11 1 Hartman GLASS FOR A-FIBRE-OPTICAL ELEMENT [75] Inventor: Christianus Johannes Maria Hartman, Emmasingel. Eindhoven. Netherlands [73] Assignee: U.S. Philips Corporation, New  
 York, NY.  
 [22] Filed: Mar. 27, 1973 [21] Appl. No.: 345,443  
 [30] Foreign Application Priority Data Apr. 5, 1972 Netherlands 7204501 [52] U.S. C1. 106/54; 106/47 Q; 106/50 [51] Int. Cl C03c 3/08; C030 13/00 [58] Field of Search 106/50, 54, 47 Q, 47 R [56] References Cited UNITED STATES PATENTS 3,460.954 8/1969 Young 106/50 [451 Apr. 22, 1975 Young 106/50 Connclly 106/50 Primary limminer-Robert V. Hines Aszvixlum E.\&#39;uminerThomas A. Waltz Attorney. Agent, or F irm-Frank R. Trifari [57] ABSTRACT A core glass for fibre optics having a composition between the following limits in percent by weight:  
 2 Claims, N0 Drawings GLASS FOR A FIBRE-OPTICAL ELEMENT The invention relates to glass having a high refractive index which is suitable as a core of glass fibres bundled in a fibre-optical element.  
  Such elements which are known, for example, from UK. Patent Specification 1,108,509 have frequently been used in recent years where images having a very weak brightness are transported and where loss of definition due to dispersion must not occur. They are used, for example, in image intensifiers and television camera tubes. In these fibre-optical elements a bundle of a large number of fibres having a very small diameter is used.  
  The operation of such a fibre is such that a beam of light which impinges on one side on the end of the fibre remains substantially within the fibre by means of total reflection when it passes through this fibre and reaches the other end with substantially the same intensity. In order to realise this such a fibre consists of a cylindrical core of transparent material having a high refractive index (n,) which is concentrically surrounded by a cladding of glass having a low refractive index (11 If necessary a layer of another material may be provided on the outer side of the cladding in order to absorb unwanted light in the fibre plate and to obtain an optimum transmission of the image. The aim is to make the value of n as large as possible and that of n as small as possible in order that the angle 9 which may be constituted between the beam of light impinging upon the fibre-optical element and the normal on the end face of the fibre without escaping from the fibre is as large as possible. There is the following relation between this critical angle, the refractive indeces of the two kinds of glass and the refractive index of the ambiance (n,,):  
 n sin \l n, n  
  It is to be noted that in this respect cylindrical is to be understood to mean any closed form, thus not only a circular but also rectangular or polygonal.  
  Glass for the fibre cores must satisfy a number of requirements. To obviate cathode poisoning during use in an image intensifier or a similar device PbO, CdO and halogens must be absent. Due to the risk of discolouration in the presence of traces of iron, TiO must be absent. In connection with safety, ThO and other radioactive materials are to be prevented. The use of expensive materials such as GeO ln O and Y O is to be limited as much as possible. The B 0 content must not be too high because this promotes striae and devitrification during melting and moulding. As regards expansion and viscosity the glass must be adapted to the commonly used cladding glasses. The glass must not contain stones, crystalline products and the like and during the drawing process devitrification must not occur. Sources for dispersion phenomena due to diffusion, crystallisation and phase separation at the operating temperatures must not be produced at the boundary surface between the core and the cladding glass.  
  For the core material having a refractive index of approximately 1.80 borosilicate glasses are known which comprise lanthanum oxide, barium oxide, zirconium oxide, tantalum oxide and niobium oxide such as are described in the German published Patent Application No. 2,010,672 and have the following composition in percent by weight:  
 SiO 0 20 Zr0 0 10 B 0 8 30} Ta O Nb O 0 25 Geo: 0 l5 together 20 TiO 0 5 La O l5 30 B210 CaO 5 25 A1 0 0 5 or those which have been described in the German published Patent Application No. 1,596,867 having the following composition in percent by weight:  
 SiO O 10 B210 0 47 B 0 19 26 Ta O 0 3 ZrO; 0 5 Geo O 2 1.11 0, 12 31 Nb O W0 TiO l3 l7. CdO 0 32 It has been found in practice that these glasses have some drawbacks. In the first place it was found that TiO can easily give rise to unwanted colouring in connection with iron impurities. Furthermore a larger quantity of B 0 of 20 percent of more, is less preferable because a part sublimates therefrom during melting. Expensive materials such as GeO and Ta O should rather be replaced by cheaper materials.  
  An object of the invention is to provide a glass composition which does not have these drawbacks and whose physical properties are such that they can be used in combination with the common cladding glasses.  
  A commonly used cladding glass, is, for example, the following (in percent by weight):  
 SiO 67.6 Nu o 0.4 B 0 19.0 K 0 8 .4 Li O 0.9 AI O 3.6  
 As O 0.1  
 SiO- 2 3 A1 0; Na O having a refractive index m, 1.50, a coefficient of expansion between 30 and 300C of 66.2 X 10&#39; per C, an annealing point (log n 13.4) of 540C and a softening point (log 1 7.65) of 696C. In order to be compatible therewith the coefficient of expansion of the core glass must be slightly higher so that compressive stress is present in the cladding glass and the softening point if 60 to C higher which, as experience shows, leads to optimum results during the drawing process.  
  The glass composition according to the invention is characterized in that it is located within the following limits given in percent by weight:  
 B 0; 4 l6 ZnO 2 6 SiO; l0 20 combined 14 26 ZrO; 4 8 AI O l 4 Nb O S 13 1.21 0, 3| 35 Ta O, 2 10 B210 12 l6 and preferably within the following limits:  
 13,0 4 16 Zn() 3 5 S l0 21)}comhined l4 26 Zr() 6 8 A1 0 1 4 Nb- O 8 ll L:1 32 35 Til- 0;, 4 6. B110 12 15 A 1 0,- Nu O SiO- 2 0 The filled ampoule was evacuated, sealed, and heated for one-half to 1 hour at a temperature of 700725C. Sheet-like fibre-optical elements were sawn from the product obtained and they were finally polished. The optical quality of these sheets is very satisfactory.  
 What is claimed is:  
  1. Glass having a high refractive index which is particularly suitable as a core of glass fibres bundled in a fibreoptical element and which consists essentially of the following constituents having between the following limits in percent by weight:  
 Composition in 7! by weight 2 3 8 0 11.1 8.1 6.1 10.1 12.1 510 14.1 16.1 18.1 14.1 12.1 A1 0 2.2 2.2 2.2 2.2 2.2 Lu o 33.0 340 34.0 34.0 340 8:10 13.7 13.7 13.7 13.7 13.7 ZnO 3.7 3.7 3.7 3.7 3.7 zro 7.0 7.0 7.0 7.0 7.0 N6 0, 10.2 10.2 10.2 10.2 10.2 T11 0, 5.0 5.0 5.0 5.0 5.0 n,, 1.802 1.808 1.808 1.808 1.808 UC I()&#34;(-300C) 72.0 72.4 72.4 72.4 72.4 T 7.65&#34;&#34; 784 784 796 775 764 A glass tube was manufactured consisting of one of 82 11 4 l6 Z110 2 the two above-mentioned cladding glasses having a wall Sio m 20 14 7 26 zrol 4 8 2 thlekness of from 1 to 1.5 mms, an external d1ameter N20 4 Nhzos 5 &#39;3 of 17.5 mms and a length of 300 mms. 161%), 11,205 2 H) A fitting ground cylinder was placed therein which consisted of glass having one of the five abovementioncd compositions.  
  The combinations thus obtained were first drawn out to fibres having a diameter of approximately 300 u at a temperature of 770-780C. These fibres were bounded to a diameter of 12 mms and these bundles were again drawn out to 300 p. so that the original fibre had a diameter of approximately 6 a. The obtained composed fibres were cut to lengths of 100 mms and bundled in a dense packing in an ampoule having a diameter of 25 mms of a borosilieate glass having a coefficient of expansion adapted to the fibre plate such as that of the following composition in percent by weight:  
 B 0, 4 16 ZnO 3 5 combined 14 26 510-, 10 20 ZrO 6 11 A1 0,, 1 4 Nb O 8 ll La O 32 35 111 0,, 1 4 6 B 12 15