1.  Field of the Invention
The present invention relates to an optical fiber telecommunication line, for example, a submarine line, comprising amplifiers for transmitted optical signals and of the type in which lengths of active core optical fibers are used for amplification. 
Also, the present invention relates to optical signal amplifiers for optical fiber telecommunication lines, for example, submarine lines and the like, in which the amplifiers are arranged in positions not easily accessible, of the type making use of lengths of active core optical fiber. 
2.  Description of the Prior Art
As is known, optical fibers of the so-called "active core" type comprise, inside a cladding, at least a core wherein here are doping substances which besides making the refractive index of the core higher than that of the cladding become optical radiation sources having a wavelength .lambda..sub.1 used for the transmission when struck by a radiation having a wavelength .lambda..sub.2 (different from .lambda..sub.1).  The wavelength .lambda..sub.2 is different according to the particular doping substance used. 
Examples of doping substances having this characteristic are erbium and neodymium. 
More particularly, in active core optical fibers the emission of optical radiation having a wavelength .lambda..sub.1 takes place when a radiation having a wavelength .lambda..sub.2, ususally called umping optical radiation, travels in the optical fiber. 
Since the phenomenon of the amplification of the signals made through the so-called active core optical fibers is known a further description will not be given. 
The use of amplifiers provided with active core optical fiber lengths in the optical fiber telecommunication submarine lines will supersede the use of optoelectronic repeaters of the transmitted signals because of a greater reliability in comparison to optoelectronic repeaters due to a lower content of electronic components. 
In fact, as is known, the electronic components present in the optoelectronic repeaters are quite important and are of the type operating at high frequencies.  This is because, in the opto electronic repeaters, the input optical signal modulated at high frequency is transformed into an electric signal necessarily at high frequency, the electric signal is amplified at high frequency and said amplified electric signal is transformed again into an amplified optical signal still at high frequency which is transmitted at the output of the repeaters. 
These high frequency electronic components have proven to be not very reliable over a long length of time due to their failures with the consequent interruption of the line operation. 
This drawback is clearly very undesirable in particular in optical fiber telecommunication submarine lines.  In addition to this, there is difficulty of access to the optoelectronic repeaters for repair purposes and the long time required for putting the line back into service. 
Unlike the optoelectronic repeaters, the known amplifiers provided with active core optical fibers do not contain high frequency electronic components, and the only delicate element they comprise is the pumping optical radiation source constituted in general by a laser, a laser diode or the like. 
However, although the optical fiber telecommunication lines comprising active core optical fiber amplifiers are more reliable with respect to the optoelectronic repeaters because of the lower risk of failures, they are not able to communicate to the terminals of the line, and in case of submarine lines to the land ends, signals corresponding to impropeer operation that can arise during the operation thereof and in case of failure, it is practically impossible to locate the failure along the line. 
For this reason, in the optical fiber telecommunication lines, it is important to provide a plurality of reserve optical channels both in the cables and in the repeaters of in the amplifiers independent of one another to operate when a working optical channel is not utilizable.  Clearly, this constitutes a considerable complexity and a reduction in the effectiveness of the present optical fiber telecommunciation lines.