Amplifier for optical fiber telecommunication lines and optical fiber telecommunication lines incorporating said amplifier

An optical amplifier (3) in accordance with the invention consists of the assembly of a length of active-core optical fiber (8), single-mode to both pumping and signal optical radiations, and a dichroic coupler (6) which includes two optical fiber lengths (9, 10), both single-mode to pumping and signal optical radiations, coupled to each other over one portion (11) by fusion of the respective claddings and substantial setting in common of the respective cores in the fused portion (11).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an amplifier of the so-called active-core 
type for optical fiber telecommunication lines and to the optical fiber 
telecommunication lines incorporating said amplifiers. 
2. Description of the Prior Art 
So-called active-core amplifiers consist of a length of active-core optical 
fiber which will be defined in the description that follows and a source 
of optical pumping radiation which will be defined in the following 
description as well. 
An active-core optical fiber is an optical fiber the core of which, made of 
silica glass, contains active dopants as hereinafter defined, in addition 
to the dopants necessary to make said core have a refractive index higher 
than that of the cladding, that is the radially outermost layer made of 
silica glass as well. 
The above-mentioned active dopants are formed with substances such as 
rare-earths, erbium and the like for example, which, when excited by 
optical radiation, known as optical pumping radiation, the wavelength of 
which depends on the particular dopants selected, have the property of 
emitting optical radiation, known as emission radiation which has a 
different wavelength depending however on the particular dopant selected. 
Another feature of the active dopants referred to hereinbefore is the fact 
that, once excited by the optical pumping radiation, they are capable of 
emitting said optical emission radiation when optical radiation of the 
same wavelength as the latter impinges on them. 
The source of optical pumping radiation is generally a laser and in 
particular a laser diode capable of emitting optical radiation of the same 
wavelength as the one required and necessary for exciting the active 
dopants present in the optical fiber core having an active core as 
previously defined. 
Active-core amplifiers for optical fiber telecommunication lines are 
already known in the art. 
An optical fiber amplifier having an active core for optical fiber 
telecommunication lines comprises a source of optical pumping radiation 
optically connected to a dichroic coupler to which one portion of the 
optical fiber of the transmission or telecommunication line is optically 
coupled. 
In turn the dichroic coupler is directly connected to the optical fiber 
length having an active core and this first length is in turn connected 
again to a second optical fiber portion of the transmission or 
telecommunication line. 
In the above known amplifier the source of optical pumping radiation, 
through the dichronic coupler, sends its own radiation to the length of 
active-core optical fiber where it causes the excitation of the active 
dopants present therein. 
Also sent to the length of the active-core optical fiber through the 
dichronic coupler, are the signals to be amplified coming from the first 
optical fiber portion of the telecommunication line which necessarily must 
have a wavelength identical to the emission wavelength of the active 
dopants present in the length of active-core optical fiber. 
At the moment at which an optical signal enters the length of the 
active-core optical fiber it encounters the dopants in an excited 
condition due to the optical pumping radiation and, for the reason set 
forth before, a massive emission of optical radiation occurs which has the 
same wavelength as that of the signal, which brings about, as a result, 
the amplification of said signal. 
In the amplifiers in question and previously described, there is the 
problem of increasing their yield, defined as the ratio between the 
obtainable amplification gain and the applied pumping radiation power, 
while at the same time achieving the benefits of amplifiers on a large 
scale, capable of offering sufficient reliability so as to enable them to 
be used in practical applications, such as for example, their easy and 
safe insertion in optical fiber telecommunication lines. 
In the publication "Fourteenth European Conference on Optical 
Communication" of Sep. 11-15, 1988, on pages 25 to 28, there are the gain 
values of known amplifiers, which range between 0.14 and 0.31 dB/mW. For 
the purpose of improving the gain of optical amplifiers, set forth in the 
same publication are the experimental results of the gain obtained with an 
optical amplifier made in the laboratory in which the length of the 
active-core optical fiber, single-mode both to signal and pumping optical 
radiations, contains erbium as an active dopant. The source of optical 
pumping radiation used has a wavelength of 980 nm and the optical signal 
radiation used has a wavelength of 1536 nm. 
Even if it has not been specified in the aforementioned publication the 
dichroic coupler adopted in the optical amplifier hereinbefore described 
is inevitably a dichroic coupler of the so-called microoptics type, that 
is a dichroic coupler in which lenses are used in order to be able to 
introduce the optical pumping and signal radiations into the length of 
active-core optical fiber. This is due to the fact that dichroic couplers 
of the other existing types are unable to operate in a satisfactory manner 
with the particular length of active-core optical fiber in question. 
By adopting this known solution a yield of 2.2 dB/mW, intended as the ratio 
between the gain and the pumping power used, can be reached, which value 
is interesting. However, that amplifier has the drawback that it is not 
reliable for use on a large scale due to the particular dichroic coupler 
adopted therein. 
In fact a dichroic coupler of the microoptics type is very delicate in 
itself and it can hardly be inserted in an optical fiber telecommunication 
line, which results in unreliability of the telecommunication lines which 
incorporate such amplifiers. 
SUMMARY OF THE INVENTION 
The present invention aims at providing optical amplifiers with higher 
yield than known ones and in particular with a yield up to 4.5 dB/mW, 
which amplifiers are not only reliable for large scale industrial use, but 
are also capable of simplifying to the maximum degree the operations 
necessary for their introduction into the optical fiber telecommunication 
lines, also making the latter more reliable. 
It is therefore an object of the present invention to provide an amplifier 
for signal-transmitting optical fiber telecommunication lines which can be 
interposed between one portion of the optical fiber of the line and a 
second portion of the optical fiber of the line, comprising a source of 
optical pumping radiation, a dichroic coupler adapted to be connected to 
the first portion of the line and connected to the source of optical 
pumping radiation and a length of active-core optical fiber, single-mode 
to both signal and pumping optical radiations, having one end connected to 
the dichroic coupler and the other end susceptible of being connected to 
said second optical fiber portion of the line, said dichroic coupler is of 
the type comprising two lengths of nonactive-core optical fiber disposed 
in side by side relation over a portion of their length where they are 
optically coupled to each other by fusion of the respective claddings and 
substantial setting in common of the respective cores by stretching the 
lengths themselves, both said optical fiber lengths forming the dichroic 
coupler being single-mode both for the optical signal radiation and 
optical pumping radiation. 
It is another object of the present invention to provide an optical fiber 
telecommunication line comprising at least one signal-transmitting optical 
fiber portion and at least a second signal-transmitting optical fiber 
portion optically connected to each other through an optical amplifier 
interposed therebetween, said optical amplifier comprising a source of 
optical pumping radiation, a dichroic coupler optically associated with 
the source of optical pumping radiation and with the first 
signal-transmitting optical fiber portion of the line, a length of 
active-core optical fiber, single-mode both to signal and pumping optical 
radiations downstream of the dichroic coupler and optically connected 
thereto, said length of active-core optical fiber being also optically 
connected to the second signal-transmitting optical fiber portion of the 
line, said dichroic coupler is of the type comprising two lengths of 
nonactive-core optical fiber disposed in side by side relation over one 
portion of their length where they are optically coupled to each other by 
fusion of the respective claddings and substantial setting in common of 
the respective cores by stretching the lengths themselves, both optical 
fiber lengths of the dichroic coupler being single-mode both for signal 
and pumping optical radiations.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Shown in FIG. 1 is an optical fiber telecommunication line which includes 
an optical amplifier in accordance with the invention, and therefore an 
optical fiber telecommunication line in accordance with the invention. 
As viewed in FIG. 1, said line comprises a transmitter 1 of any known type, 
capable of sending optical signals to a signal-transmitting optical fiber, 
which therefore will not be described herein. 
Transmitter 1 has the particular features of incorporating a source of 
signal-emitting optical radiation the wavelength of which is adapted for 
the operation of an optical amplifier such as for example a DFB laser 
diode capable of emitting an optical radiation having a wavelength of 1536 
nm. 
However, the above mentioned example of source of optical signal radiation 
which is the one usually adopted in the optical fiber telecommunication 
field, is not intended in a limiting sense as regards the scope of the 
present invention. 
Downstream of the transmitter 1 the line comprises one portion 2 of signal 
transmitting optical fiber which is optically connected at one end thereof 
to said transmitter. 
The first optical fiber portion 2 has the other end optically connected to 
an optical amplifier 3 according to the invention the characteristics of 
which will be described in the following. 
Downstream of the amplifier 3 and optically coupled thereto there is a 
second optical fiber portion 4 the characteristics of which are the same 
as those of the first optical fiber portion 2. 
The optical fiber portion 4, one end of which is optically connected to the 
optical amplifier 3, has the other end optically connected to an optical 
receiver 5 of any known type, which therefore will not be described. 
The optical amplifier 3 in accordance with the invention used in the 
previously described optical fiber telecommunication line in accordance 
with the invention is diagrammatically shown in FIG. 2. 
As viewed in FIG. 2, the optical amplifier includes a particular dichroic 
coupler 6, which will be described in detail in the description that 
follows, to which a source 7 of optical pumping radiation is optically 
coupled as well as a length of active-core optical fiber 8 located 
downstream of the dichroic coupler 6. 
In an optical amplifier in accordance with the invention the length of 
active-core optical fiber is of the single-mode type both for optical 
signal radiation and optical pumping radiation. 
For example in an optical amplifier according to the invention the length 
of active-core optical fiber, still containing trivalent erbium uniformly 
dispersed in said core as an active dopant, has a core diameter of 5.4 
.mu.m. In general the core diameter in the length of the active-core 
optical fiber is in the range of 5.2 to 5.6 .mu.m. 
On the contrary the cladding outer diameter of the length of active-core 
optical fiber is for example of 125 .mu.m, as it is usual in optical 
fibers. 
In the active-core optical fiber the ratio relating to the difference 
between the refractive index of the core and of the cladding with respect 
to the refractive index of the cladding, which is generally included 
between 0.0051 and 0.0058, is 0.0056 for example. 
In the above example and when the length of the active-core optical fiber 
has a cladding made of silica glass and devoid of any dopant and therefore 
a refractive index of 1.450, the core of said active-core optical fiber 
has a refractive index of 1.458. 
The dichroic coupler 6 of the amplifier 3 in accordance with the invention, 
as shown in FIG. 3 on an enlarged scale, consists of two optical fiber 
lengths 9 and 10, which are both made of silica glass and devoid of active 
dopants, parallel to each other and coupled by fusion of the respective 
claddings and stretched immediately afterwards, so that after the 
operation in the area in which the claddings are joined together by 
fusion, the cores become substantially coincident over a portion 11 (FIG. 
3) thereby practically giving rise to a sole core. 
In particular one of the characteristics of a dichroic coupler in an 
amplifier of the invention is that at the coupling area of the two optical 
fiber lengths constituting it, where the respective cores are set in 
common giving rise to a substantially sole core, the diameter of the sole 
core is smaller than the diameter of the core of each optical fiber length 
at the ends thereof and in particular that the ratio between the diameter 
of the substantially common core of the two optical fiber lengths and the 
diameter of the core of the latter at their ends is from 0.3 to 0.5. 
The diameter of the substantially common core of the two optical fiber 
lengths constituting the dichroic coupler is generally selected of such a 
value as to cause therein an optical power loss not higher than 1 dB. 
Value ranges generally valid for any dichroic coupler of an optical 
amplifier in accordance with the invention cannot be reported, since in 
determining the diameter value of the substantially common core adapted 
for the purposes of the present invention, the particular wavelengths of 
the optical signal and pumping radiations adopted are factors, which can 
remarkably vary from one another. However, a technician skilled in the art 
and only in possession of the above disclosure limiting the power loss not 
higher than 1 dB, is capable of at least experimentally determining the 
diameter value of the substantially common core of the two optical fiber 
lengths in the dichroic coupler. 
For example in the case of a 980-nm wavelength of the optical pumping 
radiation and a 1536-nm wavelength of the optical signal radiation, the 
diameter of the substantially common core of the two optical fiber lengths 
is in the range of 1.56 to 2.8.mu.. 
In a dichroic coupler which can be used with an amplifier in accordance 
with the invention the length of the portion over which the cores of the 
two optical fiber lengths are substantially in common also depends on the 
particular wavelengths of the adopted pumping and signal radiations. 
Should for example said pumping and signal wavelengths be 980 and 1536 nm 
respectively, the length of a said portion will be comprised between 0.9 
and 1.2 cm. A general standard for determining the length of the above 
portion is that said length must cause both pumping and signal radiations 
to be entirely and exclusively conveyed towards only one of the two 
terminals of the dichroic coupler facing the length of active-core optical 
fiber. By this sole indication a technician skilled in the art is capable 
of creating a dichroic coupler having such a characteristic during the 
construction of the dichroic coupler itself. 
In fact, by coupling one of the ends of the two optical fiber lengths to 
the sources of optical pumping radiation and optical signal radiation 
respectively, before melting the claddings in the portion in which they 
are disposed in side by side relation it is possible to stop the 
stretching applied to said lengths during the fusion of their claddings at 
the moment at which no optical radiation issues from one of the two other 
ends, whereas both signal and pumping optical radiations issue from the 
other of the two other ends. 
Another characteristic of a dichroic coupler 6 for an amplifier in 
accordance with the invention is that the two optical fiber lengths 9 and 
10 forming it must be both single-mode to the signal and pumping optical 
radiations used in the line. 
Still another characteristic of a dichroic coupler for an amplifier in 
accordance with the invention is that the distribution of the nonactive 
dopants in the core and cladding of the optical fiber component portions 
must be substantially identical to the distribution of the nonactive 
dopants present in the length of active-core optical fiber. 
As previously indicated, the inventive optical amplifier consists of the 
assembly of one length of active-core optical fiber having the above 
stated characteristics and a dichroic coupler in series with the length of 
active-core optical fiber and as previously described as regards the 
characteristics thereof. 
Preferably in an optical amplifier in accordance with the invention, for 
the purpose both of achieving an easy and safe coupling to the optical 
fiber portions of the signal transmitting line and enabling an easy and 
safe coupling between the two components of the amplifier, there is also 
the characteristic hereinafter set forth. 
The mode diameter to the wavelength of the transmission signals, defined 
and detectable according to CCITT Rule G 652 of 1986, relating to the 
optical fiber lengths forming the amplifier and therefore to the length of 
active-core optical fiber and to the two lengths of optical fibers forming 
the dichroic coupler is substantially identical to the mode diameter to 
said signal wavelength of the optical fiber portions of the 
telecommunication line to which the amplifier is designed to be coupled. 
With the above stated features of the components of an amplifier in 
accordance with the invention, the junction of said components to one 
another and of them all together to the optical fiber portions of the line 
is carried out by mere butt melting, that is the ends of the different 
types of optical fibers involved are joined without practically incurring 
losses during the couplings. 
In particular, as shown in FIG. 2, the coupling between the end of the 
signal transmitting optical fiber portion 2 of the line is joined by butt 
welding to one end of the optical fiber length 9 of the dichroic coupler 
3. 
In turn the other end of the optical fiber length 9 of the dichroic coupler 
3 is joined by butt welding to one end of the length of active-core 
optical fiber 8 the other end of which is joined by butt welding to the 
portion of the transmission optical fiber 4 of the telecommunication line. 
Finally, the source 7 of optical pumping radiation (consisting for example 
of a In-Ga-As laser diode known per se and capable of emitting a 980-nm 
optical radiation which is the one used in the case in which the 
active-core optical fiber is doped with trivalent erbium and the signal is 
a 1536-nm wavelength optical radiation) is optically coupled to one end of 
the optical fiber length 10 of the dichroic coupler. 
Operation of an amplifier in accordance with the invention and of a line in 
accordance with the invention incorporating said amplifiers, will be now 
described with reference to the figures of the accompanying drawing sheet 
concerning particular embodiments of the invention. 
The transmitter 1 of a type known per se and commonly used in optical fiber 
telecommunication lines emits signals using an optical radiation having a 
wavelength of substantially 1536 nm which, as is known, is the wavelength 
for transmission signals enabling the minimum attenuation to be achieved 
within the transmission optical fibers of the line, identified by 
reference numerals 2 and 4 in FIG. 1. 
Signals sent from transmitter 1 to the portion 2 of optical fiber are at 
all events subjected to an attenuation while being transmitted therein and 
they enter the optical fiber length 9 of the dichroic coupler 6 in the 
amplifier 3 with said attenuation. 
Continuously sent to the dichroic coupler 6 and more precisely to the 
optical fiber length 10 of the same is the pumping optical radiation 
emitted from the laser diode 7. 
The optical pumping radiation which, as previously indicated, is selected 
by way of example for an amplifier in accordance with the invention and 
for a 980-nm wavelength line in accordance with the invention, is 
superimposed in the dichroic coupler to the attenuated optical signal 
radiation of 1536-nm wavelength, coming from the transmission optical 
fiber portion 2 of the line. 
In particular, the superimposition of the two signal and pumping optical 
radiations within the dichroic coupler takes place in the region 11 where 
the two cores of the optical fiber component lengths have been made 
substantially coincident, as clearly shown in FIG. 3. 
Since the two optical fiber lengths 9 and 10 forming the dichroic coupler 3 
are both single-mode to both signal and pumping optical radiations, both 
optical radiations issuing from the dichroic coupler are superimposed and 
single-mode. 
In addition, with the use of a dichroic coupler in which the distribution 
of nonactive dopants is identical to that of the nonactive dopants 
existing in the length of active-core optical fiber, losses of optical 
energy in the optical radiations summed to each other in the amplifier do 
not occur. 
In particular, both signal and pumping radiations sent to the input of the 
dichroic coupler come out of the same (due to its above stated 
characteristics) only along the optical fiber length 9 facing the length 
of active-core optical fiber 8 and, since the distribution of nonactive 
dopants is the same in the components of the dichroic coupler and in the 
length of active-core optical fiber, no loss can occur when said 
components are coupled, as no alteration in the single-mode distribution 
of power of the two optical radiations can take place in the junction area 
of said components. 
From the foregoing it results that entering the length end of the 
active-core optical fiber 8 facing the dichroic coupler 3 are 
simultaneously and without losses both the whole optical pumping radiation 
with its utmost power and the optical signal radiation attenuated by 
effect of its passage through the optical fiber portion 2 but with the 
utmost power when the latter comes out of said optical fiber 9. 
Since, as above said, the distribution of the nonactive dopants in the 
optical fiber lengths forming the coupler is substantially identical to 
that of the nonactive dopants present in the length of active-core optical 
fiber, neither does alteration occur in the optical radiations during the 
passage of the same from the dichroic coupler to the length of active-core 
optical fiber. 
In addition, due to the fact that the length of the active-core optical 
fiber is also single-mode both to pumping and signal optical radiations, 
the input and spreading of both radiations therein takes place so that the 
power distribution of said radiations is kept symmetric relative to the 
axis of the length of active-core optical fiber. 
The optical pumping radiation passing through the core of the active-core 
optical fiber length 8 causes the excitation of the active dopant present 
therein. Said active dopant excited by the optical pumping radiation, at 
the instant at which it is hit by the optical signal radiation, emits a 
radiation having the same wavelength, which results in the amplification 
of the optical signal. 
The optical signal so amplified is sent to the portion of transmission 
optical fiber 4 of the line and reaches the receiver 5. 
Experimental tests have been carried out with an amplifier in accordance 
with the invention and the test modalities as well as the results achieved 
will be set forth in the following. 
The particular inventive amplifier submitted to experimental tests has the 
following structure. 
The length of active-core optical fiber adopted is of the step index type 
and is devoid of nonactive dopants in the cladding which is made of silica 
glass and therefore has a 1.45 refractive index, whereas it contains 
germanium in the core as a nonactive dopant, to an amount sufficient to 
give it a 1.458 refractive index; in addition said length of active-core 
optical fiber has a 5.4 .mu.m core diameter and a 125 .mu.m outer diameter 
of the cladding. 
The core of the active-core optical fiber length in question besides 
containing the above stated nonactive dopants, also contains trivalent 
erbium ions as the active dopant, which are uniformly dispersed in said 
core with a concentration of 0.3% by weight, expressed as erbium oxide. 
Finally the length of active-core optical fiber used in the experimental 
test is 8 m long. 
The dichroic coupler used has the two optical fiber component lengths equal 
to each other, of the index step type, made of silica glass and both 
single-mode both to the optical signal radiation and to the optical 
pumping radiation in which the nonactive dopant content and distribution 
is identical to that of the nonactive dopants of the active-core optical 
fiber length. 
In particular in said optical fiber lengths forming the particular dichroic 
coupler used, the diameters and refractive indices for the core and the 
cladding are the same as those for the length of active-core optical 
fiber. In addition, the portion of mutual coupling between the two optical 
fiber lengths where said optical fibers have substantially coincident the 
respective cores is 0.9 cm long and the diameter of the core where said 
core is substantially in common for both optical fiber lengths is 2.1 
.mu.m. 
Finally the ratio between the diameter of the core substantially common to 
the two optical fiber lengths forming the dichroic coupler and the 
diameter of the core of the same lengths at their ends is 0.4. 
The optical pumping radiation source used is an In-Ga-As laser diode 
emitting a continuous optical radiation of 980-nm wavelength and 6-mW 
power. 
In the experimental test two optical fiber portions of the type normally 
used in optical fiber telecommunication lines with a mode diameter to the 
signal radiation equal to that of the optical fiber lengths forming the 
amplifier have been also adopted. 
For the experimental test, used as the source of optical signal radiation 
has been a DFB laser diode emitting an optical radiation of 1536-nm 
wavelength with a 100-mW power and for which the portions of signal 
transmitting optical fibers are single-mode only for the signal optical 
radiation and multimode for the pumping optical radiation. 
The above listed different components have been assembled as follows. 
The source of optical signal radiation has been optically coupled to the 
end of a signal transmitting optical fiber portion so that practically the 
whole radiation issued from the signal source is introduced into said 
optical fiber portion. 
The other end of the above cited optical fiber portion has been joined by 
butt welding to the end of one of the optical fiber lengths of the 
dichroic coupler. 
In addition the source of optical pumping radiation as above specified has 
been coupled to the end of the optical fiber length of the dichroic 
coupler disposed in side by side relation to the length to which the 
portion of signal transmitting optical fiber has been joined so that the 
whole power issued therefrom, which is 6 mW, penetrates into the optical 
fiber portion of the dichroic coupler in question. 
Connected by butt welding to the end of the optical fiber length of the 
dichroic coupler from which both signals are capable of coming out, is one 
end of the active-core optical fiber length the characteristics of which 
have been previously stated. 
Butt welded to the other end of the length of active-core optical fiber is 
one end of the other portion of signal transmitting optical fiber. 
The apparatus for carrying out the experimental tests has been completed by 
coupling the end of the signal transmitting optical fiber portion opposite 
that directly coupled to the length of active-core optical fiber to an 
intensity detector designed to detect the intensity of the radiation 
coming out of said optical fiber portion, in particular to a PIN 
photodiode. 
From the experimental tests carried out it has been possible to notice that 
by using a pump source emitting a 980-nm wavelength pumping radiation with 
a 6-mW power, the obtained gain for the adopted optical signal radiation, 
which is 1536 nm, is 25 dB. 
It results therefore from the experimental tests that the obtained yield, 
expressed as a ratio between the gain and the power of the optical pumping 
radiation used, is 4.1 dB/mW. 
Since the maximum yield which can be reached with the known amplifiers of 
the type in question appears to be 2.2 dB/mW, as can be drawn from the 
relevant literature, the results achieved with the experimental tests 
prove that the present invention has reached the previously stated purpose 
of improving the yield of said amplifiers by 100%. 
Furthermore, from the above description of the particular embodiments it 
also appears that the accomplishment of an amplifier in accordance with 
the invention, as well as its insertion in an optical fiber 
telecommunication line, are easy and very reliable since they can be 
achieved by merely butt welding the involved optical fibers. 
While a particular embodiment of an amplifier and a line in accordance with 
the invention has been described and illustrated, all possible variations 
accessible to a person skilled in the art are intended to be comprised 
within the scope of the claimed invention.