APPARATUS AND PROCESS FOR THE FABRICATION OF OPTICAL FIBRES

Abstract: The apparatus is composed of two coaxial crucibles (3, 4) the outer one (4) presenting a low-softening temperature, so that it might be drawn with the raw materials. Thus a fibre is obtained with an outer coating which protects it against atmospheric agents. According to this process raw materials are used allowing the fabrication of fibres which may be used to transmit optical radiations with wavelengths longer than 5 µm, with an attenuation minimum lower that 10 -3 dB/km.

Description: Apparatus and Process for due vahrication of optical Fibres Description The present invention relates to the fabrication of phy sical carriers for optical-telecommunitations systems and in party cular it refers to an apparatus. and process for the fabrication of opticaL fibre Optical transmissions over very long distances are known to be limited. by the optical properties of the materials used for the fabrication of physical carriers; in particular, the attenuation and the dispersion determine the wavelength range usable for the transmis sion. The raw material present1 used in. optical fibre fabrica tion is- the silica, SiO1, differently doped in function of the wanted refractive index and dispersion properties. It.presents a minimum dispersion value at a wavelength- o± 1. iL7m, but with a suitable dopant, compensations are effected, to approach it to the minimum .attenuation value and to obtain. in this way an optimal transmis sion window. Minimum. attenuation depends also on the Rayleigh scattering losses and is expressed, in function of the wavelength > and of the material characteristics, by the following relationship: EMI1.1 where d is the scattering coefficient. Its value is: EMI1.2 where p is the mean photo elastic constant K is the Boltzmann S constant TG is the glass transition temperature ssT is the isothermal compressibility The magnitudes n, p, TG e ssT are peculiar constants of the material. The attenuation due to scattering has a theoretical mi nimum value of about 0.2 dB/km at the wavelength of 1.6 m for silica and it is not possible to go down a-s at longer wavelengths the attenuation tail c'abs of the stretching vibration of the lattice bond EMI2.1 intervenes This latter has a behaviour given by the expression: EMI2.2 where A and & are constants and #1 is the stretching fundamental- vibration frequency of the bond. Si-O4 and depends on the force constant [f] anon the reduced mass SiO of the silicon-oxygen bonds given by: mSi.mO SiO = mSi + 2 mO where mSi and mO are silicon and oxygen atomic masses respectively. The relationship which expresses the value of V1 is the following: #1 = 2#(f/ SiO)1/2 ShouLd the silica be replaced by material with lower, that is with a lower force constant f and/or a higher reduced mass the the minimum attenuation region could be shifted towards longer wavelengths, where scattering losses decrease. In silica the parameter which makes V1 value relatively high is the reduced mass of the group Si-O, which is conditioned by the small value of the oxygen mass. Therefore, in order to obtain the wanted result,alternative materials to oxides are to be found. These requirements are partly satisfied by halides. In fact, the compounds of F, Clb Br, 3 have greater reduced-masses. :- In the optical fibre field a number of materials have already been proposed, suck as fluorides; but strious problems are encountered relating to' their use Particularly: high reactivity to gases. composing the atmosphere; - toxicity; - low stability with: easy devitrification of the produced. glass; - lack of a suited technology to the fibre fabrication; - low value of the minimum o± optical dispersion, with consequent difficult optimization o± minimum attenuation and dispersion con ditions The disadvantages above are: overcome by the present apparatus and process for the fabrication of optical fibres, which, thanks to. the use of suitable. materials, allow the transmission of radiation with wavelengths- longer than 5 m, present a minimum attenuation lower than 1 c-3 dB/km and are particularly protected against the natural agencies The present invention provides an apparatus for the production of optical fibres consisting o± a first. crucible terminating in a nozzle and of a second crucible surrounding the first, wherein the first crucible has a softening temperature- higher than that of the raw materials the opticaLfibres. are made of' and presents at the bottom a meniscus, with the geometric. profile: which can be obtained e. by drawing and the the second crucible is made of low-melting glass so as. to have a softening temperature similar to that of the raw materials and a'-nxeni'scus'. directly obtained during fibre drawing, mechanicaL devices been provided to. feed the first crucible.causing the translation. of. a'rod. made- of the materials forming the fibre core and the. translation ofv the; second crucible together with the materials forming-the-fibre cladding inserted between the- two crucibles, first furnace for the vitrification of the materials contained in the crucibles and a second furnace for the drawing being also provided. The foregoing and other characteristics of the present invention will be made clearer by the following description of a pre ferred embodiment thereof given by way of example and not in a limiting sense, and by the- annexed drawing showing an apparatus for the production of optical fibres. To obtain a remarkable shift of the fundamental-vibra- tion: frequency with respect to that of. silica, chlorides are to be used in the raw material production Besides avoiding the above listed fluoride disadsrantages a f' & ther reduction of the attenuation can be achieved thanks to the chlorine mass which is greater than that of fluorine. Moreover, chlorides present lower reactivity to natural agencies and have tolerable toxicity levels. The obtained-gIass stability can be optimized by duly choosing both the number and percentages of the single components. Since long wavelengths are used the component number canbe increased without increasing too much scattering losses, thus obtaining multi-component chloride-based glasses. Besides to use spectraL regions up to and above 5 us, elements having great atomic mass, suckas Hit Zr, Th, Ba, La, Pb, Bi and generally elements o± the 6th and 7th period are associated with chlorine. The clorides of Hf, Zr, Th can be considered necessary to build the glass matrix, together- or- separately, while the chlorides of the other elements are added to favour the vitrification and shift the dispersion minimum towards the longer- wavelength regions. Chlorides of lighter elements, such as AlCl3, Zn Cl2, etc., can be- used to the same aim. A typical combination of the compounds obtained from the elements above is the following: 50 HE Cl4 - 2.0 Th Cl4 - 10 Zr CL4 - 10 Ba C12 With mixtures of this kind the glass transition temperature TG is comprised within the range 2500 to 500 C. This fact reduces to nearly one third the Rayleigh scattering coefficient The typical silica impurities, such as transition metals and hydroxyl groups do not affect the attenuation of these glasses as the relative absorptiorr bands fall before the 3m wavelength. These compounds are not usable to fabricate fibre pre fotsí according to the traditional CVD techniques, owing to the high boiling temperature o± the basic. reagents. The figure shows an apparatus for the- fabrication of op tical fibres by multi-component glasses based on halides, and more particularly, on chlorides. The apparatus. comprises two furnaces I and 2 and a quart2 crucible a, ending with a nozzle obtained by drawling. Outside the crucible 3- there is- a crucible. 4 consisting of- low-melting oxide bases glass which presents a meniscus- coaxial with the preceding nozzle This meniscus is directly obtained during the fibre drawing. Reference 5 denotes a device for feeding the glass, forming the fibre core inside crucible: 3 Screw devices 6 and 7 advance the glass rod while the- optical- fibre is being drawn. Reference 9: denotes. ananalogous device feeding both the glass forming the outer fibre cladding and the-crucible 4 inside the furnace. Two- screw devices to and 11 advance the material and the crucible 4 The raw materials in powder form, that is the chlorides of the above-cited elements, are first purified by sublimation in inert atmosphere brdulTvarying the pressure and temperature. They are then: mixed so as to attain the. wanted combination with constant percentages at each point and are introduced into a quartz tube. They are the= vitrified in the furnace and the resultant rod 8 is purified by thermal diffusion The materials forming the cladding, similar to the preceding ones, apartfrom percentual variations of a number of components in order to obtain a difference in the refractive indices, are individually purified as in the- previous case and mixed, then they are placed between crucibles 3 and 4, vitrified and purified by thermal diffusion. The materials forming the core and the cladding can be vitrified, as an alternative, directly in the crucibles by using the same furnace 1, which extends over the whole crucible length. Furnace t, placed, in correspondence with the meniscus, generates the temperature. necessary to fibre drawing When choosing low-softening ' oxide-based glass, the coefficients of thermaL expansion, the glass transition temperatures and the density of the chloride--based glasses the fibre is made o± have to: be taken into account. Crucible 4 in fact is drawn together with the fibre, therefore it: acts not only as a crucible, but also as a protective coasting, thus obtaining the insulation from the external environment of the materials forming the fibre-and. a better stability in time Provided the present requirements are satisfied, the coat- does does not exert any thermal: or mechanical stress on the fibre. The thermal expansion: coefficient and glass transition temperature of multi-component chloride-based glasses can vary respectively between 5Q and and 200. 10-7 C-1 and between 200 and 600 C, while their densities. can range from 3 to 8g/cm3. By making the crucible 4 with a. glassy having a density value similar to that of the chlorides, material compenetration with consequent optical fibre contamination is avoided Among those presently on the market it is possible to use. advantageously, for instance, Schott glasses of the kindiSF5D with a density of 4. 1.0 g/cm3, an expansion coefficient of 105 x.l0-7 C-1 and a glass transition temperature of 4060C, or the kind. SF59 with values. of 6.26 g/cm3, 103 . 10-7 C-1 and 3620C of density, expansion coefficient and temperature. After raw: material vitrification, also furnace 2 is acti voted in order to attain the softening. temperature. Then the meniscus is generated in crucible 4, by drawing with due speed such as to obtain the wanted outer fibre diameter, for instance 125 m, accord ing to the present international standard, The core-cladding ratio depends both on the nozzle size of crucible 3 and on the feeding speed of crucible 4. The feeding speed of crucible 4 has to be determined so as to maintain the wanted core-cladding ratio whithin the variability interval allowed by the nozzle size of crucible 3. Devices 5 and 9, designed to supply the glass for the core and the cladding respectively, must present reductions allowing a translation speed in the interval comprised between 1 and 50 cm/h The above description has been given only by way of example and not in a limiting sense. Variations and modifications could be made while remaining within the scope of the invention.

Claims: Patent Claims 1Apparatus for the production of optical fibres consisting of a first crucible (3) terminating in a nozzle- and of a second cruci ble (4) surrounding the first, characterized in that the first cru cible has a softening temperature higher than that of the row material the optical fibres; are made o± and presents at the bot tom t meniscus, obtained by drawing and the second crucible is made of low- melting, glass so as to have a softening temperature similar to that of the raw- materials and a meniscus directly ob tained during the fibre drawing, mechanical devices (5, 9) being provided to feed the first crucible causing the translation of a rod (8) made of thecmaterials forming the fibre core and the translation of the second crucible together with the materials forming the fibre cladding inserted between the two crucibles, a first furnace (1) for the vitrification of the materials contained in the crucibles-and a second furnace (2) for the drawing being also provided 1Apparatus as ia claim 1, characterized in that said mechanical devices (f, 9) comprise screw devices designed to reduce the translation speed to such a value that the material quantity ar riving at the drawing region compensates for that used in opti cal fibre drawing 3. Apparatus as in claim 1 or 2, characterized in that said scco;lti cible (4) is made of glass having, glass transition temperature, density and thermal-expansion coefficient similar to those of the material the core and cladding, are composed of. 4. Process for the fabrication,of optical fibres; utilizing the appa- ratus of any of the preceding claims. characterized in that lile terials forming the fibre core and the materials forming the fibre dadding are placed. z that first crucible (3) and between said first and second crucibles (4) respectively, after being purified and mixed to obtain a homogeneous composition at each point, then unvitrified materials are vitrified and those already vitrified are further purified by thermal diffusion, fibre draw ing being then started by, raising the region with nozzle and me niscue of said crucibles to the spftening temperature- of the raw- materials an & of the second crucible and then compensating for the consumption of material, used in the drawn fibre, by trans lating the second nozzle, the raw materials forming the cladd ing and the raw materials forming the core towards the same fibre drawing direction and at a speed adequate to the wanted ratio between the- core. diameter- and the cladding diameter. 5 Process as in claim 4, characterized in that, said materials forming the core are introduced into said first nozzle as powder and are then vitrified.. 6. Process as in claim 4, chasacterized in that said materials forming the core are- introduced into said first nozzle already vitrified and in the shape of a rod T Process as in any of claims 4 to 6, characterized in that said raw materials forming the fibre core and cladding are halides. 8.. Process. as in any of claims 4 to 7, characteriZed in that sale raw materials forming the fibre core and cladding comprise clor- ides. 9; Process as in claim 8, characterized in that said raw mater als composing the fibre core and cladding comprise chlorides of elements having high atomic mass for the construction of the glass matrix and to a lower amount chlorides of elements with lower atomic mass to increase the matrix stability and to obtain optical fibres with a large-r- bandwidth. 10. Process as in claim- 9, characterized in that said high atomic mass elements comprise Hf, Zr, Tht Ba, La, Pb and Bi 11. Process an in claim a or 10, characterized in that said lower atomic mass elements comprise Al and Zn. I OpticaL fibres fabricated according to the process- of any of claims L to Ii characterized in that they comprise an inner core and a cladding surrounding said core which are composed of said raw materials and a coat surrounding said cladding which ia composed of the glass said second crucible (4) is made of.

Inventors: MODONE, EROS, ROBA, GIACOMO
Applicants: CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1982
Main classification: C03B37
IPC codes: C03B37, G02B6