Patent Application
20050262876
The invention relates to the field of optical fibers, optical fiber preforms and methods for producing final optical fiber preforms obtained by external plasma deposition around a primary preform.
A conventional optical fiber, for example a single mode fiber (SMF) has a refractive index profile that varies with its radius starting from the center of the fiber which typically is as illustrated in
To improve fiber attenuation, it is known to set out to decrease the amount of germanium in the core 1 of an optical fiber. However, as a difference between the core and cladding refractive indices is determined by the desired propagation characteristics of the optical fiber, this makes it necessary to decrease cladding refractive index, which will now be of silica doped with a dopant that reduces refractive index. This dopant is habitually fluorine. To ensure now that the optical fiber is not too sensitive to microbending and consequently does not exhibit too high microbending losses, this fluorine-doped silica cladding, of refractive index lower than that of silica, needs to be brought up to a critical radius the value of which depends on the core radius. For reasons of preform production casts, beyond this critical radius, the cladding is again in undoped silica, as providing doped silica layers in a preform is more costly than providing undoped silica layers. The optical fiber obtained is illustrated in
The inner cladding 13 is constituted by fluorine-doped silica.
In the first prior art discussed disclosed for example in Patent Abstracts of Japan JP 55100233, this inner cladding 13 can consist of a fluorine-doped silica tube surrounding the doped core of the primary preform. Either the ratio between outer tube radius and outer core radius is relatively high and the primary preform is too expensive, or the ratio between the outer tube radius and outer core radius is fairly small and the optical fiber obtained by pulling the final preform after adding a low quality silica tube by a sleeving operation is too sensitive to microbending and, consequently, exhibits too high microbending losses. This JP 55100233 relates to a method of manufacturing a preform, wherein a doped quartz glass rod as core material is inserted into synthetic quartz glass tube of high purity and they are made solid by heating to a high temperature with an external heat source to obtain a semi-preform rod. The preform thus obtained is further put into a low-purity quartz glass tube such as natural quartz glass tube and made solid by heating to a high temperature with an external heat source, thus producing a preform rod. This Japanese document does not relate to a method for external plasma cladding buildup.
In the second prior art, concerning primary preforms obtained by chemical vapor deposition (CVD) covering MCVD, FCVD deposition and ether depositions of the same type, the inner cladding 13 can be constituted by a fluorine-doped silica tube inside of which an optical cladding of the CVD type bas been deposited, the outer cladding 12 being constituted from the natural silica grain the refractive index of which is appreciably greater than the refractive index of the fluorine-doped silica tube. A core, generally germanium doped, is deposited inside the optical cladding by CVD-type deposition. Again, like the case above, either the ratio between the outer tube radius and outer core radius is relatively high and the primary preform is too expensive, or the ratio between the outer tube radius and outer core radius is fairly small and the fiber obtained by fiber pulling of the final preform after adding a low quality silica tube by a sleeving operation is too sensitive to microbending and consequently has too high microbending losses.
In the invention, part of the inner cladding 13, that part located externally of and on the outside of the fluorine-doped silica tube is obtained by deposition from synthetic grain in fluorine-doped silica, which is distinctly less costly than providing layers by CVD or the use of a fluorine-doped silica tube of appreciable cross-section, making it possible to provide an inner cladding 13 of outer radius sufficiently great compared to the outer radius of the primary preform core to obtain an optical fiber that is relatively insensitive to microbending while keeping production casts at a reasonable level. Regarding the outer cladding 12, this is obtained by overcladding from natural silica grain deposition, as using fluorine-doped synthetic silica grain for the complete final preform would be too expensive. It is the fact of overcladding the primary preform in a two-stage operation, first using fluorine-doped synthetic silica grain followed by natural silica grain, which allows an optical fiber simultaneously having low and consequently good attenuation and low sensitivity to microbending to be obtained, and which is simultaneously relatively inexpensive to produce.
U.S. 2002/0144521 relates to a method of manufacturing an optical fiber preform, the method comprising the following steps:
Such outer deposition can be performed in various different ways, e.g. by plasma deposition, wherein grains of natural silica are deposited by gravity from a feed pipe which is moved in translation parallel to the primary preform, wherein the silica grains are fused and then vitrified at a temperature of about 2300° C. by means of the plasma.
The invention provides an original final preform, an overcladding method making it possible to obtain it, an optical fiber resulting from pulling the said final preform and an optical cable employing several fibers thus obtained.
The invention consequently provides a method for external plasma cladding buildup in which a final optical fiber preform is obtained by overcladding a primary preform with silica grains or crystals and a peripheral layer of the primary preform is constituted by a fluorine-doped silica tube, characterized in that the external plasma cladding buildup method comprises a first step of external plasma overcladding using synthetic fluorine-doped silica grain followed by a second step of overcladding using natural silica grain.
There is also provided a final optical fiber preform comprising a primary preform the peripheral layer of which is constituted by a fluorine-doped silica tube, an external plasma deposited overcladding layer surrounding said primary preform,
characterized in that said deposited overcladding layer comprises a first overcladding sub-layer obtained from fluorine-doped synthetic silica grain surrounding said tube, and a second overcladding sub-layer obtained from natural silica grain and which surrounds said first overcladding sublayer.
The invention will be better understood and other features and advantages will become more apparent from the description that follows taken in conjunction with the attached drawings, provided by way of example. In the drawings:
The plasma overcladding deposition method builds up on a primary preform in order to obtain a final preform by silica grain deposition. The primary preform has an outer peripheral layer constituted by tube 24 in fluorine-doped silica. This peripheral layer of the primary preform is the most outer layer of the primary preform. The primary preform could for example be a cylinder obtained for example by VAD (vapor axial deposition) or OVD (outside vapor deposition) followed by a sleeving operation using a fluorine-doped silica tube. The primary preform is preferably obtained by CVD-type deposition inside a tube 24 in a fluorine-doped silica, like in the case illustrated in
Preferably, in order for the first overcladding sublayer 25 to extend right up to around the critical radius for microbending sensitivity, the outside diameter of the first overcladding sublayer 25 is at least five times greater than the outside diameter of deposited core 21. Advantageously, the outside diameter of first overcladding sublayer 25 is around six times greater than the outside diameter of deposited core 21. Turning back now to
Preferably, the radial thickness of the first overcladding sublayer 25 is greater than half the outside radius of tube 24. This gives a value d-c which is greater than c/2. If the overcladding sub-layer 25 thickness is too small, the quality/price tradeoff gain even if present, will remain fairly small and consequently less valuable.
Preferably, the deposited core 21 is germanium-doped. Other dopants that increase core refractive index could optionally also be used. The whole deposited core 21 is advantageously germanium-doped so that its refractive index remains constantly greater than that of silica, like for example in the case where the final preform is designed to provide a standard single mode fiber after pulling.
The refractive indices of the optical cladding 23, of tube 24 and of the first overcladding sublayer 25 are preferably all fairly close to each other so that the refractive index profile of a fiber obtained after pulling the preform will neither show an annulus nor a dip which could deteriorate optical fiber microbending behavior. On
The invention also concerns the optical fiber obtained by pulling a final preform of the invention. Preferably, the portions of the index profile of the optical fiber respectively obtained from optical cladding 23, tube 24 and the first overcladding sublayer 25 all have the same refractive index, this being below that of silica. The optical fiber preferably concerned by the invention is one which is little sensitive to microbending, while having low attenuation.
The invention also concerns an optical fiber cable comprising several optical fibers according to the invention. Thanks to the absence of dips and, in particular upward steps in the optical fiber refractive index profile, no spurious mode (“mode parasite”) will propagate in these optical fibers.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0314756 | Dec 2003 | FR | national |
