Abstract:
Doped silica glass can be manufactured by reacting gaseous vapors of silica-forming compounds and dopant-forming compounds. Increased fluorine dopant can be provided with less fluorine dopant-forming compound, when the fluorine dopant-forming compound is hexafluoroethane (C 2  F 6 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method of manufacturing fluorine-doped optical fibers. 
     The use of fluorine as a refractive index decreasing dopant in the manufacture of optical fibers on the basis of fused silica is known from the following patents and publications: 
     (1) A. Muhlich, K. Rau, F. Simmat, N. Treber, 1st ECOC, IEE, London 1975 
     (2) K. Abe, 2nd ECOC, IEE, Paris 1976 
     (3) DE-PS No. 25 38 313 (corresponding to U.S. Pat. No. 4,045,198) 
     (4) D. Kuppers, J. Koenings, H. Wilson, 3rd ECOC, Munich 1977 
     (5) D. Kuppers, J. Koenings, H. Wilson, J. Electrochem. Soc. 125 (1978) 1298 
     (6) A. Muhlich, K. Rau, N. Treber, 3rd ECOC, Munich 1977 
     (7) K. Rau, A. Muhlich, N. Treber, Topical Meeting on Fiber Transmission, IEEE, Williamsburg 1977 
     (8) DE-OS No. 29 31 092 (corresponding to U.S. Pat. No. 4,221,825) 
     (9) B. J. Ainslie, C. R. Day, P. W. France, K. J. Beales, G. R. Newns, Electron. Lett. 15 (1979) 411 
     (10) J. W. Fleming, V. R. Raju, Electron. Lett. 17 (1981) 867. 
     In the manufacture of fused silica optical fibers according to both the thermally activated MCVD method (publications 2, 3, 9) and in plasma-activated manufacturing processes (publications 4, 5, 6, 7, 10) fluorine may be used as a dopant. The following compounds may serve as fluorine sources: SiF 4  (2, 4, 5), NF 3 , SF 6  (8), CCl 2  F 2  (3,8) and CF 4  (2, 9). By using the MCVD method it is possible to produce glass layers with refractive index differences of approximately 0.5% by fluorine doping (2, 3). However, relatively large quantities of fluorine compound have to be consumed to reach these values. According to (2), a difference in refractive index of 0.5% is reached only with a SiF 4  /SiCl 4  ratio of 12:1. The large excess of SiF 4 , however, degrades the deposition from the gas phase. Therefore, in the MCVD process fluorine is usually used only together with other dopants (9, 10). 
     The use of plasma activation under normal pressure (6, 8) and at low pressure (4, 5) permits the production of glass layers with differences in refractive index of 1% (7) and 1.3% (5), respectively. In these multimode optical fibers doped with fluorine only, optical attenuations of 2.2 dB/km at 1060 nm were realized. Monomode optical fibers have so far not been manufactured in this manner. A high fluorine compound concentration, as compared to the SiCl 4 , in the gaseous phase is also required in this process. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to produce glass layers having increased differences in refractive index, while reducing the quantity of fluorine compound to be used in the production of such layers. 
     According to the invention this object is achieved in that hexafluoroethane (C 2  F 6 ) is used as a fluorine source in a CVD method. 
     As a CVD method, preferably the low-pressure PCVD method is used. This method is described in the publication by P. Geittner, D. Kuppers and H. Lydtin entitled &#34;Low-loss optical fibers prepared by plasma-activated chemical vapor deposition (CVD)&#34; (Applied Phys. Lett., Volume 28, No. 11, pages 645-646, June 1976). and in the DE-PS No. 24 44 100 corresponding to U.S. Pat. No. Re. 30,635). The contents of these publications is hereby incorporated as a reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a graph showing the differences in refractive index between doped and undoped silica glass layers as a function of the concentration, in the gaseous phase, of the fluorine compound used to make the doped layers. Curves are shown for various methods and for various fluorine dopant-providing compounds. 
     FIG. 2 shows a Michelson interference-micro-graph of a preform for a monomode optical fiber. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 the differences in refractive index (in per cent) between doped and undoped silica glass layers are plotted against the concentration, ##EQU1## in the gas phase, of fluorine dopant-producing compound. In this equation, Q D  is the gas flow of the fluorine compound, and Q T  is the total gas flow. The individual curves represent values for the following methods, systems, and dopants, respectively: 
     1 PCVD method, system FeO 2  /SiO 2 , 
     2 MCVD method according to Publication (2), 
     3 Method according to Publication (8), fluorine compound CCl 2  F 2 , 
     4 Method according to Publication (8), fluorine compound NF 3 , 
     5 Method according to Publication (8), fluorine compound SF 6 , 
     6 PCVD method, system F/SiO 2  with SiF 4  as a fluorine source, Publication 5, 
     7 Monomode preform manufactured according to the invention. 
     8 PCVD method, system F/SiO 2  with C 2  F 6  as a fluorine source. 
     FIG. 1 shows that it is possible according to the invention by using C 2  F 6  (hexafluoroethane &#34;Freon 116&#34;) to achieve differences in refractive index of more than 2% (See, curve 8). This is an increase of more than 50% as compared to the highest values reached so far in systems with pure fluorine doping and pure germanium dioxide doping. 
     The invention also permits the manufacture of optical fibers with a numerical aperture of more than 0.3% while using only fluorine as a dopant. As appears from FIG. 1, a further advantage of the present invention is that very high differences in refractive index can be achieved with very small concentrations, in the gaseous phase, of the dopant C 2  F 6 . A difference in refractive index of 1% is already reached with a ratio of C 2  F 6  /SiCi 4  of approximately 0.05. hexafluoroethane is therefore to be considered as an extraordinarily active fluorine source when used according to the invention. Also, when C 2  F 6  is used according to the invention, attenuation values of 1.5 dB/km at 1050 nm are achieved. 
     By means of the PCVD method and while using C 2  F 6 , monomode optical fibers consisting of a cladding of silica glass, an intermediate layer whose refractive index has been reduced by fluorine doping, as well as a pure silica core were manufactured. 
     FIG. 2 shows a Michelson-interference micrograph of such a monomode optical fiber. Such a fiber is advantageous because the core consists of pure silica glass and consequently has a smaller Rayleigh scattering than fiber cores with doping. Moreover, during collapsing of inner-coated tubes to form fibers, dopants may evaporate out of the core material. This results into a so-called dip in the refractive index profile. As the inner-coated tubes manufactured according to the invention have only silica on the inner surface, the described dip in the refractive index no longer occurs. 
     Monomode, fibers manufactured according to the invention hence show a lower sensitivity to bending then fibers with a doped core which were also manufactured via an internal coating method. 
     The invention will now be described in greater detail with reference to specific examples. The examples generally correspond to the examples for the PCVD method described in the Publications (4) and (5) in which hexafluoroethane C 2  F 6  was used as a dopant instead of SiF 4 . 
     EXAMPLE 1 
     A constant SiCl 4  flow of 40 sccm with approximately 220 sccm oxygen and 1 sccm C 2  F 6  was passed through a silica tube (length 188 cm, outside diameter 14.2 mm, inside diameter 11.8 mm) for approximately 150 minutes (sccm being the equivalent gas flow Q in cm 3  of gas per minute at 1 bar at 0° C.) The pressure inside the tube is approximately 10 to 14 mbar. During this period of time the outer wall of the tube is heated to approximately 1150° C. A microwave resonator with 200 W power absorption reciprocates along the tube at 3.5 m/minute and induces the deposition of fluorine-containing vitreous silica layers. Finally a few SiO 2  layers without C 2  F 6  addition were deposited. The coated tube was then processed to form a monomode fiber. The core of this fiber as well as the cladding consist of pure silica. the intermediate layer has a refractive index which is reduced by approximately 0.5 to 0.6%. 
     EXAMPLE 2 
     By means of the method described in Example 1, with the variation that the gaseous composition contained between 0.05 and 5.7 sccm C 2  F 6 , differences in refractive index between 0.05% and 2.0% were produced.