An optical fiber is made by drawing a preform on a drawing tower. A preform for example comprises a primary preform consisting of a very high quality glass tube, part of the cladding and the core of the fiber. This primary preform is then overcladded or sleeved to increase its diameter and to form a preform which can be used on a drawing tower. In this context, the term inner cladding is used for the cladding formed inside the tube, and outer cladding for the cladding formed on the outside of the tube. The homothetic fiber drawing operation consists of placing the preform vertically in a tower and drawing a fiber strand from one end of the preform. For this purpose a high temperature is applied locally to one end of the preform until the silica is softened, the fiber drawing speed and temperature then being permanently controlled during the draw operation since they determine the diameter of the fiber.
The geometry of the preform must strictly comply with the ratios of the refractive indexes and diameters of the core and the fiber cladding so that the drawn fiber has the required profile. For optical fibers, the index profile is generally qualified in relation to the graph tracing of the function which associates the refractive index with the radius of the fiber. Conventionally the distance r to the center of the fiber is shown along the abscissa axis, and the difference between the refractive index and the refractive index of the outer cladding of the fiber is shown along the ordinate axis. The index profile is therefore referred to as a “step,” “trapezoidal” or “triangular” index as per the respective graph tracings of step, trapezoidal or triangular shape. These curves generally represent the theoretical or set profile of the fiber, the manufacturing constraints of the fiber possibly leading to a slightly different profile.
An optical fiber conventionally consists of an optical core whose function is to transmit and optionally amplify an optical signal, and of an optical cladding whose function is to confine the optical signal within the core. For this purpose the refractive indexes of the core nc and of the cladding ng are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber decomposes into a fundamental mode guided within the core and into secondary modes guided over a certain distance in the core-cladding assembly, called cladding modes.
As line fiber for optic fiber transmission systems, single mode fibers (SMF) are conventionally used. An SMF fiber conventionally has a core of germanium-doped silica to increase its refractive index, and a cladding of pure silica. To improve attenuation in an optical fiber it is known to reduce the quantity of dopant in the core. However, since the difference in index between the core and the cladding is fixed by the desired propagation properties of the optical fiber, the index of the cladding must then be reduced or at least the index of the inner cladding which is doped with fluorine for example. The condition nc>ng for the refractive indexes of the core nc and cladding ng must be met to ensure guiding of the optical signal along the fiber.
Fibers with pure silica cores are also known, and are called Pure Silica Core Fibers (PSCF). The absence of dopant in the core of a PSCF fiber makes it possible to limit optical losses. A PSCF fiber therefore conventionally has a cladding in silica doped with fluorine to reduce its refractive index. A PSCF fiber can be manufactured from a preform comprising a primary preform consisting of a tube, generally in quartz, in which one or more layers of fluorine-doped silica have been deposited to form an inner cladding, and one or more layers of pure silica have been deposited to form the central core of the fiber. After depositing the layers corresponding to the core and the inner cladding, the tube is closed onto itself during an operation called collapsing. In this way the primary preform is obtained. This primary preform is then overcladded, generally with natural silica particles for cost-related reasons.
A conventional PSCF fiber, or an optical fiber having a central core scarcely doped with germanium with a cladding doped with fluorine, therefore has an index profile with a central core of radius a and index nc corresponding to the index of silica or slightly higher than that of silica, and an inner buried cladding of outer radius b. The inner cladding is said to be buried since it has a refractive index ng that is less than that of the outer cladding ne obtained by the overcladding or sleeving of the primary preform. This outer cladding is generally of pure silica glass and has substantially the same refractive index as the central core in a PSCF fiber.
In the above-described structure, with an outer cladding having substantially the same refractive index as the central core, the fundamental mode is not completely guided and shows additional losses, called leakage. To minimize these leakage losses, the percentage of energy propagating in the outer, pure silica cladding must be reduced. The ratio between the outer radius of the fluorine-doped inner cladding and the radius of the core (b/a) must therefore be sufficiently high; i.e. the inner cladding of doped silica must be extended at least as far as a critical radius b whose value is dependent on the core radius and on the difference Δn=nc−ng between the core index and the index of the inner cladding; for a single mode fiber it is considered that a ratio between the radius of the inner cladding and the radius of the core that is 8 or more (b/a>8) ensures good confinement of the optical signal in the central core and an acceptable level of leakage losses.
To enlarge the outer diameter of the fluorine-doped cladding, document JP 55100233 proposed the use of a tube of fluorine-doped silica to manufacture the primary preform.
Also the capacity of a preform is defined as the quantity of optical fiber length which can be drawn from this preform. The greater the diameter of the preform, the greater this capacity. To reduce manufacturing costs, it is desirable to provide long lengths of linear fibers from one same preform. It is therefore sought to fabricate preforms of large diameter while complying with the above-mentioned constraints regarding the diameters of the central core and the fluorine-doped cladding.
In this context, either the ratio between the outer radius of the primary preform and the radius of the central core is relatively high and the quantity of silica to be deposited inside the tube is high, in which case the primary preform is costly and the method is not very productive; or the ratio between the outer radius of the tube and the radius of the central core is relatively low and the optical fiber obtained by drawing from the final preform does not have good properties and its attenuation is substantially higher.
EP 1 544 175 proposes making a preform with a part of the outer cladding in fluorine-doped silica in order to increase the total diameter of the fluorine-doped cladding without increasing the diameter of the costly primary preform. The primary preform is obtained by successive deposits of layers of doped silica in a tube of fluorine-doped silica, and then this primary preform is overcladded with a first layer of synthetic silica particles doped with fluorine then with a layer of natural silica particles. Overcladding with synthetic silica particles doped with fluorine makes it possible to increase the diameter of the fluorine-doped cladding for one same core diameter, and reduce leakage losses. The overclad of the primary preform using silica particles doped with fluorine is less costly than depositing fluorine-doped silica inside the tube. Nonetheless the fluorine-doped silica particles are synthetic particles which are much more expensive than natural silica particles.
US Patent Application Publication No. 2002/0144521 is directed to a method for manufacturing a preform of large capacity. This document proposes making a primary preform by depositing a large diameter central core inside a tube doped with chlorine and fluorine. The tube is doped with fluorine to compensate for the increase in refractive index generated by doping with chlorine. The tube is doped with chlorine to limit the presence of OH groups which degrade the optical transmission properties in the central core. The use of said tube doped with chlorine and fluorine makes it possible to reduce the thickness of the inner cladding deposited in the tube in order to produce a primary preform having an enlarged central core diameter. This primary preform is then overcladded to obtain a final preform of large diameter and hence of large capacity. The tube doped with chlorine and fluorine protects the central core against impurities brought by the overcladding process with natural silica particles. However, the refractive index of this tube is substantially the same as that of pure silica.
The compromise between manufacturing a low cost preform having a large drawing capacity for an optical fiber having reduced optical losses with a central core that is not or only scarcely doped and having an inner cladding doped with fluorine still remains to be improved upon.
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