Note: Descriptions are shown in the official language in which they were submitted.
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67487-430
UNIT FOR AMPLIFYING LIGHT SIGNALS IN OPTICAL
FIBER TRANSMISSION LINES
Fleld of the Invention
The present lnventlon relates to an optlcal flber
telecommunlcatlon llne, provlded wlth actlve-flber optlcal
ampllflers ln whlch the reflectlons towards the ampllflers are
contalned below a predetermlned value.
Background of the Inventlon
It ls known that optlcal flbers havlng a doped core
ln whlch the doplng ls carrled out wlth the use of partlcular
substances, such as rare earth lons, have stlmulated emlsslon
features and are sultable for use ln optical amplifiers in
optical fiber telecommunlcatlon llnes for clvll telephony.
Such ampllfiers are referred to in the European
patent application No. 90112920.5 of the assignee of this
application.
By "fiber optical amplifiers", also referred to as
"active fiber optlcal amplifiers", is meant amplifiers ln whlch
the optical transmission signal is amplified as such, while
keeping its optical form without a conversion of the same to
another form, such as, for example, an electronic conversion in
which there is conversion of the optical signals to electrical
signals amplification of the electrical signals and a conver-
sion of the electrlcal slgnals to the optlcal form. In fiber
optical amplifiers, the amplifying element consists of a por-
tlon of optical fiber of the type described having a predeter-
mined length, connected ln serles between two lengths of optl-
cal llne ~
2~42~87
fiber and provided with feeding means to feed the optical pumpingsignal .
Fiber optical amplifiers have particular advantages for use
in telecommunication lines as they offer high gains when they are
utilized as line amplifiers, and such gains can be brought to the
desired value by suitably selecting the active fiber length
and/or dopant content or, should they be used as power
amplifiers, they offer a high amplification efficiency.
Particularly detrimental to such amplifiers are the signal
reflections which occur at the ends of the fiber itself.
~ rom Japanese patents 52-155901 and 63-219186 and from
"BLECTRONICS LETTERSn, vol. 24, no. 1, 7th January 1988, pages
36-38, it is known that, in a laser or optical semiconductor
amplifier, there is the risk of instability and arising of
oscillations due to the reflections at the amplifier ends.
In the above patents and article, in order to eliminate
these reflections, it is broadly taught to couple an optical
isolator to the semiconductor laser, which prevents the light
reflected by the coupling surfaces between the line fibers and
these devices from reaching the lasers themselves.
In an active-fiber amplifier, no interface surfaces are
present between the line fibers and the amplifier because the
line fibers are directly welded to the active fiber of the
amplifier. Therefore, the reflection phenomena are not generally
expec ted .
E~owever, it has been discovered that in an active-fiber
amplifier, in the absence of means limiting reflections toward
the active fiber, it is impossible to reach high amplification
gains due to the occurrence of noise of the interferometric type
as a result of beats between the direct signal and reflected
signals in the line fibers themselves and, at all events,
directed toward the active fiber The presence of
~042987
lnterferometrlc nolse ls of llttle lmportance ln a
semlconductor ampllfler whlch has low galns and small
construction slzes, whereas lt becomes partlcularly lmportant
ln an actlve-flber ampllfler capable of reachlng very hlgh
galns and havlng an actlve flber of slgnlflcant length,
generally ln the range of some tens of meters and much greater
than the coherence dlstance of the slgnal generatlng laser.
Therefore, ln an ampllfler of the actlve core flber
type, the problem arlses of protectlng the actlve flber wlth
respect to such nolse source and keeplng each form of
reflectlon toward the actlve flber ltself below critlcal
values so as not to ~eopardlze the transmlsslon quality while
malntalnlng hlgh values of the ampllflcatlon galn.
The above-clted European patent appllcatlon No.
90112920.5 teaches the lntroductlon of optlcal lsolators ln
flber optlcal ampllflers, sald lsolators havlng a reflectlvlty
llmlted to below a crltlcal value.
Summary of the Inventlon
The present lnventlon aims at provldlng an optlcal
flber telecommunlcatlon llne comprlslng actlve-flber optlcal
ampllflers, as dlstlngulshed from semlconductor ampllflers, ln
whlch the ampllflers are protected agalnst the drawbacks
resultlng from reflections, whlch uses flber optlcal
ampllfiers of the type descrlbed ln European patent
appllcatlon No. 90112920.5 and whlch employs critlcal
parameters permltting one to choose the most sultable optlcal
lsolatlon characterlstlcs for the expected amplifier galn.
Accordlng to a flrst broad aspect, the lnventlon
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provldes an optlcal flber telecommunlcatlon system, comprlslng
an optlcal slgnal transmlsslon station havlng an optlcal
slgnal source for transmlttlng optlcal slgnals at a
predetermlned wavelength and an optlcal slgnal recelvlng
station and an optical flber line interconnecting sald
transmlsslon statlon and sald recelvlng statlon, sald optlcal
flber llne lncludlng at least one optlcal ampllfler
lntermedlate and lnterconnectlng portlons of said llne for
ampllfylng optlcal slgnals received by sald ampllfler and
thereby provldlng ampllfler galn wlthout convertlng sald
optlcal slgnals to another form, sald optlcal ampllfler
lncludlng an actlve flber wlth a core doped wlth a fluoresclng
substance and a pumping source coupled thereto by means of a
further optlcal flber for provldlng energy to sald active
flber, sald actlve flber havlng an lnput end coupled to the
portlon of sald optlcal flber llne which is coupled to sald
transmlsslon statlon and an output end coupled at least to
another portlon of sald flber llne whlch ls coupled to sald
recelvlng statlon, at least one said actlve flber end belng
coupled to sald further optlcal flber, the lmprovement
comprlslng at least energy reflectlon llmltlng means for
reduclng the energy reflected toward sald actlve flber by way
of sald portlons of sald optlcal flber llne and sald further
optlcal flber, sald energy reflectlon llmltlng means lncludlng
a flrst energy reflectlon llmltlng means lntermedlate sald
output end of sald actlve flber and sald another portlon of
sald optlcal flber llne, a second energy reflectlon llmltlng
means lntermedlate sald optlcal slgnal source and sald lnput
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end of sald actlve flber and at least ln the event that sald
further optlcal flber ls coupled to a sald end of sald actlve
flber wlthout an energy reflectlon llmltlng means lntermedlate
the polnt of coupllng of sald further optlcal flber to the
last-mentloned sald end, sald further optlcal flber lncludes
energy reflectlon llmltlng means whlch lncludes an end surface
thereof spaced from sald polnt of coupllng. Accordlng to a
flrst-varlant of thls aspect, the end surface has an antl
reflectlon coatlng thereon.
In a second varlant of thls aspect, the end surface
extends at an obllque angle to the optlcal axls of sald
further optlcal flber, and each of sald energy reflectlon
llmltlng means has an energy reflectlvlty toward sald actlve
flber whlch ls at least 10 dB lower than the energy
reflectlvlty of sald optlcal flber llne due to Raylelgh
scatterlng at sald predetermlned wavelength.
Accordlng to a second broad aspect, the lnventlon
provldes optlcal ampllfylng apparatus for lnterconnectlng an
lnput optlcal flber llne connected to an optlcal
telecommunlcatlon slgnal statlon and carrylng optlcal
telecommunlcatlon slgnals at a predetermlned wavelength wlth
an output optlcal flber llne and dellverlng ampllfled optlcal
telecommunlcatlon slgnals to sald output optlcal flber llne
wlthout converslon of the optlcal telecommunicatlon slgnals to
another form, sald apparatus comprlslng: an actlve-flber
ampllfler havlng a predetermlned ampllflcatlon galn, sald
ampllfler comprlslng an actlve optlcal flber wlth a core doped
wlth a fluoresclng substance and wlth an lnput end and an
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output end, a source of optlcal llght pumplng energy, and
coupllng means coupling sald source to sald input end of said
active optical flber lncludlng an optlcal flber wlth optlcal
energy reflectlon llmitlng means extendlng from sald source of
optlcal llght pumplng energy to sald coupllng means, the
last-mentloned sald optical fiber having a surface at said
source of pumplng energy said surface and the attenuation
caused by the last-mentloned sald optlcal flber provldlng sald
optlcal energy reflectlon llmltlng means for the last
mentloned sald optlcal flber; and optlcal energy reflection
llmitlng means in serles wlth sald lnput end of sald actlve
flber and ln serles wlth said output end of sald active fiber
for limitlng energy reflected to sald actlve flber by optlcal
flbers coupled thereto, the last-mentloned said optical energy
reflectlon limltlng means having a reflectlvlty toward sald
lnput end and sald output end of sald actlve flber at least 10
dB lower than the reflectlvlty of sald optlcal flbers due to
Raylelgh scatterlng at sald predetermlned wavelength.
Accordlng to a flrst varlant of this aspect, the surface at
said source of pumplng energy has an antl-reflectlve coatlng
thereon.
In a second varlant of thls aspect, the surface
extends at an obllque angle wlth respect to the optlcal axls
of the last-mentloned sald optlcal flber.
Accordlng to a thlrd broad aspect, the lnventlon
provides an optical signal transmission system for
transmitting optical signals at a predetermlned wavelength
from a transmltter to a recelver of such optlcal signals at a
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long dlstance from sald transmltter, sald system comprlslng:
a transmltter of optlcal slgnals at sald predetermlned
wavelength; an actlve flber ampllfler for ampllfylng slgnals
at sald predetermlned wavelength havlng an lnput and an output
and comprlslng an actlve flber of predetermlned
length connectlng sald amplifler lnput and output, sald
ampllfler havlng a galn greater than 15 dB; a recelver of
optlcal slgnals at sald predetermlned wavelength;
a flrst optlcal transmlsslon llne flber havlng a
flrst llne flber lnput at one end thereof connected to
sald transmltter of optlcal slgnals and a flrst llne flber
output at the other end thereof; flrst lnterconnectlng means
lnterconnectlng sald flrst llne flber output wlth sald actlve
flber ampllfler at sald lnput of the latter;
a second optlcal transmlsslon llne flber havlng a
second llne flber output at one end thereof connected to
sald recelver of optlcal slgnals and havlng a second llne
flber lnput at the other end thereof; and second
lnterconnectlng means lnterconnectlng sald output of sald
actlve flber ampllfler wlth sald second line flber input; at
least one of sald flrst optlcal transmlsslon llne flber and
second optlcal transmlsslon llne flber havlng a length between
the lnput and output thereof greater than sald predetermlned
length of sald actlve flber and such that optical slgnals
applled to the lnput thereof are slgnlflcantly attenuated ln
travelllng from the lnput to the output thereof and havlng a
length such that a portlon of optlcal slgnals at sald
predetermlned wavelength applled to the other end of sald one
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of sald flrst optlcal transmlsslon llne flber and sald second
optlcal transmlsslon llne flber are reflected back toward sald
actlve flber ampllfler, due to Raylelgh scatterlng, ln an
amount sufflclent to reduce the slgnal-to-nolse ratlo at sald
recelver; the one of sald flrst and second lnterconnectlng
means lnterconnectlng sald one of sald flrst optlcal
transmlsslon llne flber and sald second optlcal transmlsslon
llne flber wlth sald actlve flber comprlslng a unldlrectlonal
optlcal lsolator whlch substantlally prevents optlcal slgnals
due to Raylelgh scatterlng from enterlng sald ampllfler whlle
transmlttlng optlcal slgnals at sald predetermlned wavelength;
and sald actlve flber ampllfler also comprlslng a pumplng
slgnal source, and a further optlcal flber connectlng sald
pumplng slgnal source and sald actlve flber, sald further
optlcal flber havlng an energy reflectlon llmltlng means.
Accordlng to a flrst varlant of thls aspect, sald
unldlrectlonal optlcal lsolator and sald energy reflectlon
llmltlng means of sald further optlcal flber havlng a
reflectlvlty toward sald actlve flber at sald slgnal
wavelength lower by at least 10 dB than the reflectlvlty due
to Raylelgh scatterlng ln any of sald flrst or second optlcal
transmlsslon llne flbers whereby reflected optlcal slgnals
lncludlng at least optlcal slgnals reflected ln sald one of
sald flrst optlcal transmlsslon llne flber and ln sald second
optlcal transmlsslon llne flber and ln sald further flber, are
substantlally prevented from reachlng sald actlve flber.
In a varlant of thls aspect, the reflectlvltles of
sald optlcal lsolator and sald energy reflectlon llmltlng
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20~87
means of sald further optlcal flber at sald slgnal wavelength
havlng an absolute value greater by at least 10 dB than sald
ampllfler galn; whereby reflected optlcal slgnals, lncludlng
at least optlcal slgnals reflected ln sald one of sald flrst
optlcal transmlsslon llne flber and sald second optlcal
transmlsslon llne flber and ln said further flber, are
substantlally prevented from reachlng sald actlve flber.
Preferably sald energy reflectlon llmltlng means ls lnterposed
between sald pumplng slgnal source and sald actlve flber.
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204~987
Brlef Descrlptlon of the Drawlngs
Other ob~ects and advantages of the present
lnventlon wlll be apparent from the followlng detalled
descrlptlon of the presently preferred embodlments thereof,
whlch descrlptlon should be consldered ln con~unctlon wlth the
accompanylng drawlngs ln which:
Flg. 1 ls a schematlc dlagram of an optlcal flber
telecommunlcatlon llne provlded wlth llne and power
ampllflers;
Flg. 2 ls a schematlc dlagram of an actlve-flber
optlcal llne ampllfler, accordlng to a preferred embodlment of
the lnventlon;
Flg. 3 ls a schematlc dlagram of an actlve-flber
optlcal llne ampllfler accordlng to an alternatlve embodlment
thereof; and
Flg. 4 ls a schematlc dlagram of an actlve-flber
optlcal power ampllfler in accordance wlth the lnventlon.
Detalled Descrlptlon of Preferred Embodlments
As shown ln Flg. 1, an optlcal flber
telecommunlcatlon llne generally comprlses a llght slgnal
emlttlng statlon 1, and a receptlon statlon 2, located a
substantlal dlstance from each other, e.g. hundreds or
thousands of kllometers apart, for example. Interposed
between the two statlons is one or more lengths of an optlcal
flber 3, havlng sultable transmlsslon characterlstlcs, through
whlch the slgnal ls gulded from one statlon to the other.
In order to cover the deslred overall dlstance
between statlons 1 and 2, lt ls necessary flrst to send a
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signal of sufflcient power and subsequently to compensate for
the signal attenuation along the fiber. Therefore, the
station 1 comprises, immediately after the laser 4 generating
the optical signal to be
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'~ 2~2987
transmitted, a power amplifier 5 adapted to deliver a signal to
the 1 ine which has a higher power than the one achievable, or
which can be conveniently generated, by the laser 4. In
addition, after a certain fiber length, some hundreds of
kilometers, for example, a first line amplifier 6a is present and
is adapted to bring the signal back to a sufficiently high level
Such amplifier 6a is followed by further fiber lengths 3 and
respective amplifiers 6b, 6c, etc., which are present to the
extent necessary to provide an acceptable signal at the station
2.
Amplifiers 5, 6a, 6b and 6c can conveniently consist of
fiber optical amplifiers. These amplifiers are particularly
adapted to optical fiber telecommunication lines because the
signal maintains the optical form and, therefore, its reading and
conversion to an electronic form as well as electrical processing
and amplification and a new conversion to the optical form for
transmission through the optical fiber line 3 are not required.
In fact, such latter operations restrict the line capacity, in
particular, with respect to transmission speed which is limited
by the processing speed of the electronic apparatus used.
On the contrary, in a fiber optical amplifier, the signal
always remains in optical form, and therefore, it is not
subjected to transmission speed restrictions and the like.
In addition, it is particularly convenient to use optical
amplifiers of the active-core optical fiber type. In fact, these
amplifiers allow particularly good performance to be achieved
both with respect to gain and efficiency.
The structure of a fiber optical amplifier is
diagrammatically shown in Fig. 2. The line fiber 3 in which a
transmission signal which must be amplified travels at a
wavelength,~ 5 is connected to a dichroic coupler 7 in which the
transmission signal is joined on a single outgoing fiber 8 with a
20~2987
pumping signal of wavelength ,~ p, generated by a pumping laser
emitter 9. An active fiber 10, connected to the fiber 8 coming
out of the coupler constitutes the signal amplifying element
which amplified signal introduced into the outgoing line fiber 3
and transmitted toward its destination.
In order to make the active fiber 10 forming the amplifying
element of the assembly, a silicon-based optical fiber is used,
and the core of which is doped with a fluorescing substance
which, in the presence of luminous pumping light at the
wavelength ~ p, is capable of generating a stimulated emission
coherent with the signal at the transmission wavelength ,~ 5 so
that the outgoing signal appears greatly amplified relative to
the incoming signal.
It is known that in any fiber amplifier the gain G is
related to the reflectivities Rl, R2 measured at the ends thereof
by the relation:
G(dB) - 1/2 (Rl(dB) + R2(dB) ),
in which reflectivities Rl, R2 are defined as:
R(dB) = 10 loge(pr/pt)
where Pt is the transmitted power, whereas Pr is the ref lected
powe r .
Substantially, the foregoing means that the achievement of
high gains in the amplifier is limited by the reflection
characteristics at the ends of the amplifier fiber itself, or, in
other words, that in order to achieve high amplification gains,
it is necessary to have high reflectivities Rl and R2.
In fact, if one part of the light signal present within the
amplifier fiber is reflected back to the end thereof, said part
is amplified, partly re~lected again at the opposite end and
introduced again into the amplifier fiber, the cycle being
repeated several times. When said reflections and
amplifications, in total, take a high value, it is possible to
2042987
reach an oscillation condition which makes the correct operation
of the amplifier impossible which dictates that the maximum
amplification gain must be limited in order to avoid the
occurrence of this phenomenon.
In addition to this phenomenon, the reflection back within
the amplifier of the transmission signal itself by the reflecting
elements downstream of the amplifier (the line fiber itself for
example), where said reflection is amplified again and further
reflected by reflecting elements located upstream of the
amplifier, gives rise to a beat phenomenon between the direct and
reflected signals, referred to as interferometric noise.
This interferometric noise becomes particularly important in
the case of active-fiber amplifiers, which have a length of the
amplifying element, that is the active fiber, greater than the
length corresponding to the coherence time of the laser which has
generated the signals. In fact, under these conditions, the
coherence between the direct and the reflected signals is lost,
the reflected signal becomes offset relative to the direct signal
and, if it has sufficient intensity, it becomes detrimental to
the transmission quality.
The reflections which can take place in the amplifier can be
due to the presence of interface surfaces at the ends thereof as
a result of the well known refraction phenomena, but also in the
absence of these surfaces, as in the case of active fiber
amplifiers, in which the amplifying element consists of an active
fiber 10 directly welded to the coupler 7 and the line fibers, it
is ~The scattering at the inside of the line fibers upstream and
downstream of the amplifier (known as "Rayleigh scattering")
which produces a reflection of the luminous power.
In fact, it has been noted that the Rayleigh scattering
which occurs in the whole fiber, produces a reflectivity the
value of which is about -30 dB.
I î 2Q~2987
Further reflection forms can be produced when strong
luminous powers are transmitted due to the phenomenon known as
"Brillouin scatteringR.
~ ccording to the present invention, the limitations to the
maximum gain achievable in a line amplifier resulting from the
above described reflection phenomena can be eliminated by
arranging optical isolators lla and llb upstream and downstream
of the amplifying fiber 10 as shown in Fig. 3. In particular, an
optical isolator lla is located upstream of the coupler 7,
immediately after the incoming line fiber 3 and an optical
isolator llb is located downstream of the fiber 10 before the
next length of line fiber 3.
Optical isolators are known and are devices adapted to allow
the unidirectional passage of light. For the purposes of the
present invention, the optical isolators are required to be of
the type independent of the transmission signal polarization, to
have an isolation degree at least higher than 20 dB and to
exhibit a low reflectivity, at least lower by 10 dB than the
reflectivity value given by the Rayleigh scattering in a fiber of
an inf inite length and preferably, lower by at least 15 dl3 than
such value.
In fact, it has been found that the presence of isolators
having the above characteristics ensures that the active element
of the amplifier, that is the doped fiber, can operate under
conditions which are far enough from those in which noises
resulting from reflections of various nature, as above described,
may occur in the presence of amplification gains usually
achievable with fiber amplifiers and which are of about 30 dB,
which value substantially corresponds to the absolute value of
the reflectivity given by the Rayleigh scattering in a fiber of
infinite length.
In order to achieve higher gains, a correspondingly low
11
, a 2042987
reflectivity value is required, which reflectivity in accordance
with the invention must at all events have an absolute value
higher by at least 10 dB, and preferably at least 15 dB, than the
expected amplifier gain value.
For example, the foregoing means that in order to achieve a
gain of 40 dB, the reflectivity towards the active fiber in each
fiber connected to the active fiber itself is required to be at
least lower than -50 dB and preferably lower than -55 dB for the
transmission wavelength.
The prescribed reflectivity characteristics of the isolators
can be achieved by known means, such as multilayer coatings,
surfaces passed through by the transmission signal oblique to the
propagation direction of the signal itself and the like. Such
means is generally well known in the art, and therefore, it will
not be further described.
In addition, in order to avoid noises given by reflections,
in accordance with the present invention, the fiber 12, which
transmits the luminous pumping power to the coupler 7, and
therefore, to the active fiber 10, must also have a limited
reflectivity towards the active fiber itself. In fact, a
luminous power fraction at the transmission wavelength which
propagates back to the coupler 7 is sent within the fiber 12,
since couplers commonly used for the purpose in the two coupled
branches do not have an absolute separation between the two
wavelengths for which the coupler themselves are intended. Due
to this lack of absolute separation, a not negligible percentage
of luminous power at the transmission wavelength, in the range of
some percent for example, is coupled on the coupler branch
carrying the pumping power.
If this light fraction at the transmission wavelength, at
the end of the fiber 12 where it is optically connected to the
pumping laser 9, encounters a reflecting surface, it will be sent
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1 3 20~.2987
again, through the coupler 7, to the inside of the active fiber
and it will contribute as well for the above described phenomena
of generating interferometric noise.
Therefore, for the fiber 12, a reflectivity value lower by
10 dB, and preferably by 15 dB, than the value corresponding to
the Rayleigh scattering in an infinite fiber less twice the
attenuation value given by the passage of the transmission
wavelength in the coupler ' s pumping branch is required.
In other words, it is desired that, at the end of the active
fiber 10 connected to the fiber 8, the reflectivity on any fiber
connected thereto be, in total, lower by at least 10 dB, and pre-
ferably by 15 dB, than that corresponding to the Rayleigh scat-
tering in an infinite ~iber, or correspondingly, the absolute
value of which is higher than the expected gain. Also, the refle-
ctivity at the opposite end of the fiber 10 must be restricted.
The prescribed reflectivity characteristics of fiber 12 can
be achieved by expedients known in the art, such as multilayer
coatings or oblique surfaces. In particular, an oblique cut of
the end surface 13 of fiber 12 coupled to laser 9, at an angle,
preferably in the range of 5 to 10, to a plane normal to the
fiber axis ensures a reflectivity lower than -15 dB which, added
to the attenuations due to the passage through the coupler 7 of
about -20 dB for each passage for example, gives an overall
reflectivity seen from the end of fiber 10 of -55 dB, lower by
about 15 dB than the reflectivity given by the Rayleigh
scattering (about -30 dB).
The reflection phenomena in fiber 12 could also be
eliminated by disposing the optical isolator lla downstream of
the coupler 7, immediately before the active fiber 10, as shown
in Fig. 3. This solution, which allows to avoid the use of
antireflection expedients at the end of fiber 12, can be adopted in
the case in which the loss of pumping power which takes place
13
l~ ~n~2~
while the isolator is being crossed is not detrimental to the
good operation of the amplifier.
In the case of power amplifiers directly connected
downstream of the transmission laser 4 which are fed with an
input signal having a high level, higher than the so-called
n saturation" level beyond which the power of the transmission
signal coming out of the amplifier depends only on the fed
pumping power and which emit a high luminous power (higher than
4 dBm for example), such amplifiers can cause, in addition to the
previously described phenomena, a noise effect given by the
reflection due to the Brillouin scattering. In Brillouin
scattering, the luminous power supplied to the outgoing optical
line fiber from the amplifier excites vibrations in the fiber
atoms, which vibrations in turn give r ise to the generation of a
reflected signal of a wavelength slightly shorter than the direct
signal .
This reflected signal can generate beats with the direct
transmission signal, so as to give origin to a noise damaging the
transmission quality, being added to the previously described
phe nome na .
In the telecommunication line, diagrammatically shown in
Fig. 1, the signal emission assembly, generally identified by the
numeral 14, includes an optical isolator 15 immediately after the
laser 4, which optical isolator performs the function of
protecting the laser itself against reflections which could cause
damage to the structure thereof. According to the invention, a
power amplifier 5 which is contiguous to the assembly 14, can
therefore, omit the optical isolator lla at the input thereo~, as
shown in Fig. 4, because the function of eliminating the
reflections towards the active fiber of the amplifier can, in
this case, be accomplished by the already existing isolator 15.
The remaining parts of the power amplifier shown in Fig. 4
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1~5 20429~7
are similar, as regards the graphic representation, to those
described for line amplifiers, and therefore, they have been
allocated the same re~erence numerals.
By way of example, a telecommunication line has been
constructed in accordance with the diagram shown in Fig. l, by
employing as the transmission laser 4 a directly modulated DFB
laser of the traditional type having an emission wavelength of
1535 nm. The reception station 2 consisted of a receiver of the
pin/HEMT known type, followed by wide band amplifiers not shown.
The line 3 consisted of low attenuation shifted dispersion
fibers having zero dispersion close to the transmission
wavelength used. The overall line length was 300 km and had an
attenuation of 60 dB.
The line comprised two optical line amplifiers 6a and 6b and
a power amplifier 5. These amplifiers were active-fiber
amplifiers and consisted of an active silicon-based fiber 10,
doped with germanium and erbium, pumped with a laser 9 consisting
of a miniaturized Nd-YAG laser, doubled in frequency and diode
pumped. The line amplifiers had the structure shown in Fig. 2,
and the power amplifier had the structure shown in Fig. 4.
Each of the line amplifiers had an overall gain of 20 dB.
The power amplifier had a saturation power equal to 9 dBm and an
input power of 0 dBm.
The optical isolators lla and llb were polarization control
isolators of a type independent of the transmission signal
polarization, having isolation greater than 35 dB and
reflectivity lower than -50 dB. Isolators of this kind are
commercially available, and therefore, their structure will not
be further described.
The end 13 of the fiber 12 connected to the pumping laser
had been cut at an angle of 5 .
The transmission signal achleved by the described structure
I~4 20~87
had a received power of -20 dB and a noise corresponding to
-40 dBm.
For comparison, a transmission line has been constructed
using the same test structure as above described, but in which
commercially available optical isolators lla and llb were
employed. The isolators lla and llb had reflectivity equal to
-30 dB, corresponding to the reflectivity due to the Rayleigh
scattering in the fiber and adapted to avoid the arising of
oscillation in the presence of a gain up to 3 dE~. Under these
conditions, although no oscillations were present, noise having
intensity of -30 dBm was observed, and such noise was sufficient
to prevent the correct transmission reception. Such noise is
considered to be due to the effect of the interferometric noise
resulting from the Rayleigh scattering and Brillouin scattering
within the active-fiber amplifiers.
The optical couplers 7 are diagrammatically shown in the
drawings as fused-fiber couplers, the use of which is
particularly convenient for active-fiber amplifiers. i~owever, it
is also possible to use other types of optic~l couplers, for
example, of the type used in micro-optics. For the couplers, in
particular, when they are not of the fused-fiber type, a
reflectivity lower by at least 10 dB than the ref]ectivity given
by the Rayleigh scattering or with an absolute value higher than
the amplification gain for which the amplifier is intended, is
required .
Although preferred embodiments of the present invention have
been described and illustrated, it will be apparent to those
skilled in the art that various modifications may be made without
departing from the principles of the invention.
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