PROCESS OF AND APPARATUS FOR MANUFACTURING OPTICAL FIBRES FOR OPERATION IN THE MEDIUM INFRARED TRANSMISSION RANGE

Abstract: The process of manufacturing optical fibres for transmis­ sion in the medium infrared employs metal fluorides and chlorides obtained by the vapour-phase reaction, started at ambient temperature by UV radiation, between an organo­ metallic compound and a fluorine or chlorine containing reactant. The apparatus for carrying out the process is also provided.

Description: Process of and Apparatus for Manufacturing Optical Fibres for Operation in the Medium Infrared Transmission Range Description The present invention relates to the industrial production of physical carriers for optical telecommunication systems and more particularly it concerns a process for the fabrication of optical fibres for operation in the medium infrared transmission range. Various materials exist which exhibit a minimum of intrinsic attenuation of the order of 10-2 to 10-4 dB/Km in the spectral region comprised between 2 and 12 pm. They are therefore considered to be suited to the fabrication of extremely low loss optical fibres to be used for transmission systems with widely spaced repeaters, operating in such a wavelength range and more particularly in the range 2 to 8 Im Materials used to manufacture optical fibres must have various characteristics not only of the optical type, namely high mechanical resistance, chemical and structural stability, low reactivity with the environment. Among the various materials suitable for fibres for operation in the medium infrared transmission range, halide glasses and more particularly fluoride and chloride glasses better satisfy the above requirements. Fluorides are suited to the spectral region about 3 jia and chlorides to that about 6 pm. To obtain such materials, organometallic compounds of the elements involved are preferred as base reactants, because many of these compounds are liquid or gaseous at ambient temperature and hence are suited to vapour-phase deposition. Organo-metallic compounds which can be used have the general formula MRn or n-m fim where M is the metal involved, with valence n, R is an alkyl group and X is a halogen, in particular F or C1. Some examples ,are A1(CH3)3 (trimethylaluminum) having melting point Tf a 0 and boiling point Te = 1300C; Al(CH)2C1 (dimethylaluminum chloride) having Tf - -500C and Te = ,340C (at 26.6 KPa); cd(CH3)2 (timethylcadmium), having Tf = -4.50C and Te = 105.50C. For the deposition of the mixtures of halides (fluorides or chlorides) which are to originate the preform, the use of techniques similar to Organo-metallic Chemical Vapour Deposition, already used in semiconductor technology, has been suggested e.g. in European Patent Application No. 84111019.0, wherein halides are obtained by thermallyactivated synthesis reactions. Some disadvantages of this proposal are the presence of intermediate reaction products and even of stable compounds which can interact in destructive way -in the vitreous matrix to be obtained (e.g. carbon particles or metal halides can become formed). Besides, the temperature affects the deposition as to initial and final concentration ratios among the various glass components: thus, in case of thermal activation, the reaction phase limits the degree of freedom of the deposition, since deposition temperature is determined by the reaction starting temperature. Since vitreous stability condition is critical and depends both on the thermal evolution of the glass forming process and on element concentrations, the invention aims to provide a process wherein the reaction and deposition phases are thermally separated and wherein the reaction phase is started at a temperature much lower than that of formation of the vitreous state. The invention provides a process of manufacturing optical fibres for operation in the medium infrared transmission range, wherein a preform is drawn which is made from glasses based on metal halides obtained by vapour phase reaction between an organometallic compound of the element whose halide is to be obtained and a gaseous reactant containing the halogen, characterized in that the reaction is started at ambient temperature by ultraviolet radiation, and the halides produced undergo a thermal treatment so as to meet thermophoretic deposition conditions and to obtain the subsequent matrix vitrification and consolidation. This process exploits the property of radical decomposition of the compounds involved in the reaction when they are exposed to W radiation. An example is given by the following sequence: EMI3.1 Such chain reactions allow the continuous formation of X groups capable of replacing alkyl groups in compound MRn. In these reactions M, R, X, have the meaning already disclosed, and X., R denote halogen and organic radical respectively. Radiation W starts a chain reaction yielding the metal halide according to the following reactions: EMI3.2 depending on the starting organo-metallic compound. By a suitable choice of alkyl groups, compound R is a volatile compound and can be easily separated from the solid metal halide deposited. More particularly methyl (cm3) and ethyl (C2H5), whose chlorides and fluorides are gaseous at ambient temperature and easy to remove, are suitable alkyl groups. Such reactions prevent formation of compounds such as those formed in case of thermal activation, since the reaction is solely oriented to the formation of the metal halide without parallel reactions, and hence the above danger of producing impurities which might interact in destructive way with the vitreous matrix, is avoided. Besides, OH-group formation, which might increase fibre attenuation, is intrinsically avoided. The invention provides also an apparatus for carrying out the process comprising: - a first chamber (reaction chamber), at ambient temperature, into which the reactants in vapour state are introduced and which is associated with a system for generating W radiations and con centratig such radiations in the reaction region; - a second chamber (deposition chamber) where the metal halide pro duced by the reaction undergoes the thermal treatments necessary to start and carry out the deposition as well as to consolidate and vitrificate the deposited material, so as to obtain a preform which is then drawn into a fibre. The invention will become more apparent with reference to the annexed drawings, in which: - Fig. 1 is a schematic representation of the process according to the invention; - Fig. 2 is a sectional view of the apparatus according to the inven tion. In Fig. 1, the reactants in vapour state are introduced into a chamber 1 at ambient temperature. Said reactants are a mixture of organo-metallic compounds of the elements forming the glass to be used in the fibre and the halogen (C12 or F2) or a radical-decomposable compound containing the halogen. In correspondence with the end por tion of chamber 1 there is arranged W activating system 2 composed of a W lamp and of the optical system necessary to concentrate the radiations into the reaction zone. Chamber 1 is followed by a second chamber 3, which is associated with heating means 4 keeping within chamber 3 a temperature lower than that of the vitreous transition of the matrix, and is made so as to allow exhausting of volatile products (in particular the alkyl halide). Solid metal halides are collected on a base plate 5, shutting at the bottom chamber 3, and cause on such a plate an axial growth of the vitrifiable matrix. Plate 5 and the matrix are axially displaced and rotated through a furnace 6 at a temperature allowing consolidation and vitrification; here also a treatment with helium and the halogen used for the reaction is carried out, so as to further purify the matrix by eliminating e.g. impurities due to reaction products incompletely volatilized in the preceding phase. Finally, drawing occurs, preceded by preform annealing. Axial growth of the matrix allows continuous fibre production: owing to the very low attenuation of the fibres produced (1043 to 10-4 dB/Km) very long distances could be covered by such fibres without signal regeneration. This continuous production is then necessary to obtain fibre trunks with a length corresponding to such distances, thus eliminating junctions, which introduce each the same attenuation as tens or hundreds of Kms of fibre. The actual apparatus is shown in Fig. 2, for the fabrication of a fibre comprising as usual a core and a cladding with different refractive-index. The Figure shows the drawing already in progress, and hence plate 5 is no longer present. In this Figure, references 11, 21 denote two coaxial reaction chambers, respectively for the formation of the vitreous matrix with highter-refractive index which is to form the core, and of the one with lower refractive index which is to form the cladding. References 12, 22 denote the W activation system corresponding to block 2 of Fig. 1. Chambers 11, 21, communicate at the bottom with a deposition and vitrification chamber 30, where the supporting plate is placed. Chamber 30 presents porous partitions 31, 32 which allow the exhaust of volatile reaction products towards sucking ducts 33 and at the same time define a fixed path for the materials which are being deposited, by leading the core materials towards the central part of the vitreous supporting plate 5 (Figure 1) and those of the cladding towards the plate periphery. The upper part of chamber 30 is at constant and rather high temperature, even if inferior to vitrification temperature, such as to promote exhaust of volatile products and start the deposition. This zone is created by heating means 40. Under this zone, heating means 41 give origin to a zone where the temperature gradually varies from the value starting the deposition at a lower value so as to satisfy a thermophoretic deposition process, and then up to the values necessary to consolidation and to formation of a vitreous matrix. Purification with He and the halogen introduced through a duct 34 takes place during the consolidation phase. A thermostatized chamber 35 for matrix annealing, drawing furnace 36 and fibre winding drum 37, are located downstream chamber 30. Rollers 38 schematize the means causing axial translation of the preform from the matrix 100 while the preform is being formed; the means causing rotation'are not shown; on the other hand they are quite conventional. We will now describe by way of example the manufacturing steps of a PbF2-AlF3 glass in which A1F3 is present in a molar percen- tage from 30% to 60x. Starting organo-metallic compounds are Pb(CH3)4 and A1(CH3)3 which are made to react with F2 in presence of W radiation. Starting-compound percentages are selected so as to give the desired molar percentages of A1F3 and PbFz, taking into account the reaction efficiency. Before deposition there is a zone at about 2000C where exhaust of volatile products is promoted. Fluorides are deposited on the plate according to a thermophoretic process started by the passage of the solid particles through a zone where the temperature passes from 200aC to 1000C (in correspondence with the plate). Then there is a temperature increase up to 1000CC, for mixture melting, followed by a quick cooling to temperatures lower than 0 C. The preform is then passed through an annealing zone at a temperature near the vitreous transition temperature (-300"C) so as to improve mechanical properties of the matrix. Then drawing step occurs. It is clear that what described has been given only by way of non limiting example. Variations and modifications are possible without going out of the scope of the present invention.

Claims: Claims 1. Process of manufacturing optical fibres for operation in the medium infrared transmission range, wherein a preform is formed which is made from metal halide glasses obtained by vapour phase reaction between an organo-metallic compound of the element whose halide is to be obtained and a gaseous reactant containing the halogen, characterized in that the ration is started at ambient temperature by ultraviolet radiation, and the halides produced undergo a ther mal treatment so as to meet Nhermophoretic deposition conditions and to obtain the subsequent preform consolidation and vitrifica tion and the annealing thereof. 2. Process according to claim 1, characterized in that both the reac tion producing the fibre core materials and that which produces the fibre cladding materials are started by W radiation. 3. Process according to claim 1 or 2, characterized in that for the thermal treatment the material produced is brought to a first tem perature, which is relatively high but lower than the vitrification temperature, to promote exhaust of the volatile products and to start the deposition; afterwards the material is passed into a zone where the temperature first progressively decreases so as to meet thermophoretic deposition conditions, then increases up to the melting point, and lastly sharply decreases for consolidation and vitreous matrix formation; and finally the material is passed into another zone with constant temperature equal to the vitreous tran sistion temperature for the annealing. 4. Process according to claim Q, characterized in that during the fabrication of the vitreous matrix the deposited material is treated with an inert gas and a halogen to eliminate impurities. 5. Apparatus for manufacturing optical fibres for operation in the medium infrared transmission range, comprising means (36,38) for drawing a preform obtained from a matrix (100) composed of metal halide glasses obtained by vapour-phase reaction between an organo-metallic com pound of the element whose halide is to be obtained and a gaseous reactant containing the halogen, characterized in that it comprises, upstream the drawing means (36-38): - a reaction chamber (1; 11, 21) at ambient temperature, into o which vapour-phase reactants are introduced, and which is associated with a system (2; 12, 22) for generating W radiations and concentrating same in the reaction chamber; - a deposition and vitrification chamber (3; 6, 30) associated with means (4; 40, 41) for subjecting the halide produced by the reaction to the thermal treatments necessary to start and carry out the axial deposition and the consolidation and vitrification of the deposited material; - a chamber (35) for annealing the preform resulting from vitrification. 6. Apparatus according to claim 5, characterized in that said reaction chamber comprises two coaxial rooms (11, 21) for the reactions intended to produce fibre-core and cladding material, each room being equipped with its own system (12, 22) for starting the reaction by W radiation, and said second chamber (30) comprises two coaxial rooms separated by porous partitions (31, 32) defining two fixed paths for core and cladding material deposition and letting reaction volatile products pass to a sucking conduit (33). 7. Apparatus according to claim 5 or 6, characterized in that the means (40, 41) for thermal treatment define inside said deposition chamber (30) two axially subsequent zones, the first being at high and constant temperature, even though lower than vitrification temperature, while in the second chamber the temperature first gradually decreases to meet thermophoretic deposition conditions, hence it increases again up to the values corresponding to melting point and finally presents a sharp decrease down to values which can be lower than 0 C. 8. Apparatus according to claim 7, characterized in that in correspon dence with said second zone the deposition chamber comprises a conduit (34) for introducing inert gas and halogen to eliminate impurities in the deposited material.

Inventors: PARISI, GIUSEPPE, ROBA, GIACOMO
Applicants: CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1986
Main classification: C03B37
IPC codes: C03B37, G02B6