Optical fiber communication systems increasing make use of optical amplifiers to compensate the attenuation of the transmitted signals resulting from propagation along the fiber, in order to avoid the need for optical/electrical conversions, and vice versa, in the repeaters. Those optical amplifiers in general comprise a section of rare earth doped optical fiber which the signal to be amplified and a pump signal, at a different wavelength, are sent. The most commonly used optical amplifiers use as the active fiber a silica fiber, doped for instance with erbium. These amplifiers operate on signals whose wavelength lies in the third transmission window (around 1.55 .mu.m) and require the use of sources at that wavelength. However, silica fibers, which are the most commonly used carriers in optical communication systems, have essentially zero dispersion in the second transmission window (wavelengths around 1.3 .mu.m), while in the third window their dispersion is high (in the order of 15-20 ps/nm.multidot.km). For high bit rate transmissions over long distances, this fact compels introducing into the system means for compensating the chromatic dispersion, which make the communication system complex and costly.
Optical amplifiers operating in the second transmission window have already been proposed: they use fibers made of non-oxide glass, in particular fluoride glasses, aluminium-fluoride glasses or chalcogenide glasses, doped with rare earth metals.
The drawback of optical fibers made of non-oxide glass is that their mechanical and chemical inertia characteristics are worse than those of silica fibers (or, in general, of oxide glass fibers. Moreover, the fabrication process can also cause quality problems, since the mechanical and optical characteristics of those fibers are closely linked with the "thermal history" of the glass from which the fiber is formed and, more particularly, with the number of operations which require heating the glass to a temperature exceeding the glass transition temperature, since such operations may give rise to crystallization or devitrification of the glass matrix.
To avoid the problems connected with the thermal history of the glass, it has been proposed to produce non-oxide fibers by the "double crucible" method. See paragraphs 2.3.2 "Double crucible method" and 2.3.8 "MIR (medium-infrared fibers)" of the book "Fiber Optics Communications Handbook", by the Technical Staff of CSELT, published by TAB Professional and Reference Books, Blue Ridge Summit, Pa., USA, 2nd edition, 1990. This method, however, has problems in controlling the quality of the interface between the cladding and the core glasses, can cause the inclusion of gas bubbles in the fiber and, above all, very difficult to apply in practice when single mode active fibers (whose core diameter must be in the order of 1-2.multidot.mm) are to be obtained, for reasons linked with the control of the geometric dimensions of the output hole of the inner crucible.