There exist several methods of manufacturing optical fibers. However, the procedure always includes the following:
preparing a preform, i.e. a rod of vitreous silica of very high purity having the desired refractive index profile; and then
drawing the fiber from the preform.
To manufacture optical fibers for telecommunications purposes, the common practice is to use vapor reaction methods which enable very high degrees of purity to be obtained. The base material is pure silica. Dopants, called "primary" dopants, are added to modify the refractive index of the silica. The dopants selected depend on the desired index difference. For example, boron and fluorine can be used to reduce the refractive index of silica, or phosphorous and germanium can be used to increase it.
Active optical fibers have also been known for some time which are obtained by doping using rare earth elements, e.g. erbium. These dopants are called "secondary" dopants. They are generally used together with another dopant which is a primary dopant, namely alumina, for the purpose of avoiding the phenomenon of rare earth ions "clustering".
To obtain better efficiency in active optical fibers, it has been found necessary to improve interaction between light and matter. This can be done in two ways. Firstly, by increasing energy density in the core of the optical fiber by reducing its radius, e.g. to the m range. This makes it necessary to have a large index difference (about 30 to 50.times.10.sup.-3) and a high concentration of primary dopant. Alternatively, interaction between light and matter can be improved by confining the active species (e.g. erbium) in the most central region of the core.
A problem then arises of the primary and the secondary dopants diffusing which makes it difficult to obtain the two above-mentioned objectives.
Such diffusion takes place during all stages of the manufacturing method in which heating to a high temperature is required, namely during collapsing and possible stretching of the preform and also during fiber-drawing. The temperature is normally governed by the refractory nature of the non-doped outer cladding. This problem due to diffusion occurs as soon as the preform is made, e.g. using the well known technique of internal modified chemical vapor deposition (MCVD). For spectroscopically-favorable core compositions, including an Al.sub.2 O.sub.3 content of about 10% and a GeO.sub.2 content of a few tens of %, this diffusion problem manifests itself in the extreme difficulty of simultaneously obtaining:
.DELTA.n&gt;30.times.10.sup.-3
2A&gt;1 mm (where A is the radius of the core in the preform); and
the radius in which the secondary dopant is confined is significantly different from A.
There also arises a problem of thermomechanical stresses due to the large difference in expansion coefficient between the core of the fiber having a high content of primary dopant and its outer cladding which is normally undoped.
In order to mitigate these drawbacks, the present invention proposes making preforms and fibers in which the outer cladding is doped, making it possible:
to maintain its refractive index at a level slightly greater than that of the optical cladding;
reducing its softening temperature to a value close to that of the optical cladding, thereby making it possible to provide preforms and fibers under temperature conditions that are lower, thereby reducing diffusion phenomena; and
to reduce the mean thermomechanical stresses by moving the expansion coefficients or the softening temperatures closer together.
In internal deposition techniques, it is the deposition tube that must include the doping. The invention is applicable, in particular, to the techniques of internal deposition in a tube known by the terms MCVD, plasma chemical vapor deposition (PCVD), surface wave plasma chemical vapor deposition (SPCVD), etc. The techniques used for rare earth doping may be impregnation, nebulization, or the organometallic method.