CA2042987C - Unit for amplifying light signals in optical fiber transmission lines - Google Patents
Unit for amplifying light signals in optical fiber transmission linesInfo
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- CA2042987C CA2042987C CA 2042987 CA2042987A CA2042987C CA 2042987 C CA2042987 C CA 2042987C CA 2042987 CA2042987 CA 2042987 CA 2042987 A CA2042987 A CA 2042987A CA 2042987 C CA2042987 C CA 2042987C
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Abstract
Active-fiber optical amplifier connected to optical fiber lines or a telecommunication signal source through energy reflection limiting means, such as optical isolators, for limiting the reflected energy, due to Rayleigh scattering, which reaches the active fiber, to permit increased amplifier gain.
The input of the active fiber is connected to a dichroic coupler which is also connected to a pumping energy source by an optical fiber having a bevelled end surface or an antireflection coating at the energy source end to limit reflection of energy to the active fiber. Also, a telecommunication system including one or more of such protected amplifiers.
The input of the active fiber is connected to a dichroic coupler which is also connected to a pumping energy source by an optical fiber having a bevelled end surface or an antireflection coating at the energy source end to limit reflection of energy to the active fiber. Also, a telecommunication system including one or more of such protected amplifiers.
Description
- 2~42g87 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 B ~-~2987 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 B' ~2~8~
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 B
2(~42987 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 B
~0~2987 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 - 6a -B ~
;~ 12~87 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 - 6b -~' 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.
- 6c -'E~?
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 ~,.
;~04~87 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 - 7a -G
'~ 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 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 , 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 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 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 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.
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 B ~-~2987 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 B' ~2~8~
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 B
2(~42987 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 B
~0~2987 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 - 6a -B ~
;~ 12~87 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 - 6b -~' 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.
- 6c -'E~?
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 ~,.
;~04~87 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 - 7a -G
'~ 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 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 , 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 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 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 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.
Claims (14)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An optical fiber telecommunication system, comprising an optical signal transmission station having an optical signal source for transmitting optical signals at a predetermined wavelength and an optical signal receiving station and an opti-cal fiber line interconnecting said transmission station and said receiving station, said optical fiber line including at least one optical amplifier intermediate and interconnecting portions of said line for amplifying optical signals received by said amplifier and thereby providing amplifier gain without converting said optical signals to another form, said optical amplifier including an active fiber. with a core doped with a flourescing substance and a pumping source coupled thereto by means of a further optical fiber for providing energy to said active fiber, said active fiber having an input end coupled to the portion of said optical fiber line which is coupled to said transmission station and an output end coupled at least to another portion of said fiber line which is coupled to said receiving station, at least one said active fiber end being coupled to said further optical fiber, the improvement compri-sing at least energy reflection limiting means for reducing the energy reflected toward said active fiber by way of said por-tions of said optical fiber line and said further optical fiber, said energy reflection limiting means including a first energy reflection limiting means intermediate said output end of said active fiber and said another portion of said optical fiber line, a second energy reflection limiting means inter-mediate said optical signal source and said input end of said active fiber and at least in the event that said further opti-cal fiber is coupled to a said end of said active fiber without an energy reflection limiting means intermediate the point of coupling of said further optical fiber to the last-mentioned said end, said further-optical fiber includes energy reflection limiting means which includes an end surface thereof spaced from said point of coupling and extending at an oblique angle to the optical axis of said further optical fiber, and each of said energy reflection-limiting means has an energy reflectivi-ty toward said active fiber which is at least 10 dB lower than the energy reflectivity of said optical fiber line due to Rayleigh scattering at said predetermined wavelength.
2. An optical fiber telecommunication system, comprising an optical signal transmission station having an optical-signal source for transmitting optical signals at a predetermined wavelength and an optical signal receiving station and an opti-cal fiber line interconnecting said transmission station and said receiving station, said optical fiber line including at least one optical amplifier intermediate and interconnecting portions of said line for amplifying optical signals received by said amplifier and thereby providing amplifier gain without converting said optical signals to another form, said optical amplifier including an active fiber with a core doped with a fluorescing substance and a pumping source coupled thereto by means of a further optical fiber for providing energy to said active fiber, said active fiber having an input end coupled to the portion of said optical fiber line which is coupled to said transmission station and an output end coupled at least to another portion of said fiber line which is coupled to said receiving station, at least one said active fiber end being coupled to said further optical fiber, the improvement com-prising at least energy reflection limiting means for reducing the energy reflected toward said active fiber by way of said portions of said optical fiber line and said further optical fiber, said energy reflection limiting means including a first energy reflection limiting means intermediate said output end of said active fiber and said another portion of said optical fiber line, a second energy reflection limiting means inter-mediate said optical signal source and said input end of said active fiber and at least in the event that said further opti-cal fiber is coupled to a said end of said active fiber without an energy reflection limiting means intermediate the point of coupling of said further optical fiber to the last-mentioned said end, said further optical fiber includes energy reflection limiting means which includes an end surface thereof spaced from said point of coupling which has an anti-reflection coat-ing thereon.
3. Optical amplifying apparatus for interconnecting an input optical fiber line connected to an optical telecommunica-tion signal station and carrying optical telecommunication signals at a predetermined wavelength with an output optical fiber line and delivering amplified optical telecommunication signals to said output optical fiber line without conversion of the optical telecommunication signals to another form, said apparatus comprising:
an active-fiber amplifier having a predetermined amplification gain, said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber having a surface at said source of pumping energy which extends at an oblique angle with respect to the optical axis of the last-mentioned said optical fiber, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber, and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 10 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelength.
an active-fiber amplifier having a predetermined amplification gain, said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber having a surface at said source of pumping energy which extends at an oblique angle with respect to the optical axis of the last-mentioned said optical fiber, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber, and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 10 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelength.
4 Optical amplifying apparatus as set forth in claim 3 wherein said surface extends at an angle between 5° and 10° to a plane perpendicular to said axis.
5. Optical amplifying apparatus as set forth in claim 3 wherein said coupling means is a dichroic coupler and wherein said optical energy reflection limiting means comprises a first optical isolator interconnecting said dichroic coupler with said input end of said active fiber and a second optical isola-tor connected to said output end of said active fiber.
6. Optical amplifying apparatus for interconnecting an input optical fiber line connected to an optical telecommunica-tion signal station and carrying optical telecommunication i signals at a predetermined wavelength with an output optical fiber line and delivering amplified optical telecommunication signals to said output optical fiber line without conversion of the optical telecommunication signals to another form, said apparatus comprising:
an active-fiber amplifier having a predetermined amplification gain, said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber having a surface at said source of pumping energy having anti-reflection coatings at said surface, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber; and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 10 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelength.
an active-fiber amplifier having a predetermined amplification gain, said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber having a surface at said source of pumping energy having anti-reflection coatings at said surface, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber; and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 10 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelength.
7. Optical amplifying apparatus for interconnecting an input optical fiber line connected to an optical telecommunica-tion signal station and carrying optical telecommunication signals at a predetermined wavelength with an output optical fiber line and delivering amplified optical telecommunication signals to said output optical fiber line without conversion of the optical telecommunication signals to another form, said apparatus comprising:
an active-fiber amplifier having a predetermined amplification gain, said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber attenuating said tele-communication signals and having a surface at said source of pumping energy which extends at an oblique angle with respect to the optical axis of the last-mentioned said optical fiber, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber; and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 15 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelength.
an active-fiber amplifier having a predetermined amplification gain, said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber attenuating said tele-communication signals and having a surface at said source of pumping energy which extends at an oblique angle with respect to the optical axis of the last-mentioned said optical fiber, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber; and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 15 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelength.
8 Optical amplifying apparatus for interconnecting an input optical fiber line connected to an optical telecommunica-tion signal station and carrying optical telecommunication signals at a predetermined wavelength with an output optical fiber line and delivering amplified optical telecommunication signals to said output optical fiber line without conversion of the optical telecommunication signals to another form, said apparatus comprising:
an active-fiber amplifier having a predetermined amplification gain said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber attenuating said tele-communication signals and having a surface at said source of pumping energy having anti-reflection coatings at said surface, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber; and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 15 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelengths.
an active-fiber amplifier having a predetermined amplification gain said amplifier comprising an active optical fiber with a core doped with a fluorescing sub-stance and with an input end and an output end, a source of optical light pumping energy, and coupling means coup-ling said source to said input end of said active optical fiber including an optical fiber with optical energy re-flection limiting means extending from said source of optical light pumping energy to said coupling means, the last-mentioned said optical fiber attenuating said tele-communication signals and having a surface at said source of pumping energy having anti-reflection coatings at said surface, said surface and the attenuation caused by the last-mentioned said optical fiber providing said optical energy reflection limiting means for the last-mentioned said optical fiber; and optical energy reflection limiting means in series with said input end of said active fiber and in series with said output end of said active fiber for limiting energy reflected to said active fiber by optical fibers coupled thereto, the last-mentioned said optical energy reflection limiting means having a reflectivity toward said input end and said output end of said active fiber at least 15 dB lower than the reflectivity of said optical fibers due to Rayleigh scattering at said predetermined wavelengths.
9. An optical signal transmission system for transmit-ting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals at a long distance from said transmitter, said system comprising:
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said amplifier input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first line fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals applied lo the input thereof are significantly attenuated in travelling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength applied to the other end of said one of said first optical transmission line fiber and said second optical transmis-sion line fiber are reflected back toward said active fiber amplifier, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical transmis-sion line fiber with said active fiber comprising a uni-directional optical ilsolator which substantially prevents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength and said active fiber amplifier also comprising a pumping signal source, and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means;
said unidirectional optical isolator and said energy reflection limiting means of said further optical fiber having a reflectivity toward said active fiber at said signal wavelength lower by at least 10 dB than the reflec-tivity due to Rayleigh scattering in any of said first or second optical transmission line fibers whereby reflected optical signals including at least optical signals reflected in said one of said first optical trans-mission line fiber and in said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching sald active fiber.
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said amplifier input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first line fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals applied lo the input thereof are significantly attenuated in travelling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength applied to the other end of said one of said first optical transmission line fiber and said second optical transmis-sion line fiber are reflected back toward said active fiber amplifier, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical transmis-sion line fiber with said active fiber comprising a uni-directional optical ilsolator which substantially prevents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength and said active fiber amplifier also comprising a pumping signal source, and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means;
said unidirectional optical isolator and said energy reflection limiting means of said further optical fiber having a reflectivity toward said active fiber at said signal wavelength lower by at least 10 dB than the reflec-tivity due to Rayleigh scattering in any of said first or second optical transmission line fibers whereby reflected optical signals including at least optical signals reflected in said one of said first optical trans-mission line fiber and in said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching sald active fiber.
10. An optical signal transmission system as set forth in claim 9 wherein said transmitter of optical signals is connec-ted to said first line fiber input by a said energy reflection limiting means.
ll. An optical signal transmission system as set forth in claim 9 wherein said reflectivity of said unidirectional opti-cal isolator and said energy reflectlng limiting means is lower by at least 15 dB than the reflectivity due to Rayleigh scat-tering.
12. An optical signal transmission system for transmit-ting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals, at a long distance from said transmitter, sold system comprising:
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first line fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals apply to the input thereof are significantly attenuated in traveling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength ap-plied to the other end of said one of said first optical transmission line fiber and said second optical trans-mission line fiber are reflected back toward said active fiber amplifier due to Rayleigh scattering in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical trans-mission line fiber with said active fiber comprising a unidirectional optical isolator which substantially pre-vents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength and said active fiber amplifier comprising a pumping sig-nal source and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means;
the reflectivities of said optical isolator and said energy reflection limiting means of said further optical fiber at said signal wavelength having an absolute value greater by at least 10 dB than said amplifier gain;
whereby reflected optical signals, including at least optical signals reflected in said one of said first optical trans-mission line fiber and said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching said active fiber.
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first line fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals apply to the input thereof are significantly attenuated in traveling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength ap-plied to the other end of said one of said first optical transmission line fiber and said second optical trans-mission line fiber are reflected back toward said active fiber amplifier due to Rayleigh scattering in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical trans-mission line fiber with said active fiber comprising a unidirectional optical isolator which substantially pre-vents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength and said active fiber amplifier comprising a pumping sig-nal source and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means;
the reflectivities of said optical isolator and said energy reflection limiting means of said further optical fiber at said signal wavelength having an absolute value greater by at least 10 dB than said amplifier gain;
whereby reflected optical signals, including at least optical signals reflected in said one of said first optical trans-mission line fiber and said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching said active fiber.
13. An optical signal transmission system as set forth in claim 12 wherein said transmitter of optical signals is conec-ted to said first line fiber input by a said energy reflection limiting means.
14. An optical signal transmission system as set forth in claim 9 wherein said reflectivity of said optical isolator and said energy reflecting limiting means is lower by at least 15 dB than the reflectivity due to Rayleigh scattering.
15. An optical signal transmission system for trans-mitting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals, at a long distance from said transmitter, said system comprising a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first line fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals applied to the input thereof are significantly attenuated in travelling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength applied to the other end of said one of said first optical transmission line fiber and said-second optical trans-mission line fiber are reflected back toward said active fiber amplifier, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical trans-mission line fiber with said active fiber comprising a unidirectional optical isolator which substantially prevents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength; and said active fiber amplifier comprising a pumping signal source, a further optical fiber connecting said pumping signal source and said active fiber, and an energy reflection limiting means interposed between said pumping signal source and said active fiber;
the reflectivities of said optical isolator and said energy reflection limiting means of said further optical fiber at said signal wavelength having an absolute value greater by at least 10 dB than said amplifier gain;
whereby reflected optical signals, including at least optical signals reflected in said one of staid first optical trans-mission line fiber and said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching said active fiber.
16. An optical signal transmission system as set forth in claim 15 wherein said energy reflection limiting means is . . .
interposed between said further optical fiber and said active fiber.
17. An optical signal transmission system as set forth in claim 15 wherein said energy reflection limiting means is interposed between said pumping signal source and said further optical fiber.
18. An optical signal transmission system as set forth in claim 15 wherein said absolute value is higher than said ampli-fication gain by at least 15 dB.
19. An optical signal transmission system for transmit-ting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals at a long distance from said transmitter, said system comprising;
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said amplifier input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first fine fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals applied to the input thereof are significantly attenuated in travelling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength applied to the other end of said one of said first optical transmission line fiber and said second optical trans-mission line fiber are reflected back toward said active fiber amplifier, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical trans-mission line fiber with said active fiber comprising a unidirectional optical isolator which substantially pre-vents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength, and said active fiber amplifier also comprising a pumping signal source, a further optical fiber connecting said pumping signal source and said active fiber and an energy reflection limiting means interposed between said pumping signal source and said active fiber;
said unidirectional optical isolator and said energy reflection limiting means of said further optical fiber having a reflectivity toward said active fiber at said signal wavelength lower by at least 10 dB than the reflec-tivity due to Rayleigh scattering in any of said first or second optical transmission line fibers;
whereby reflected optical signals, including at least optical signals reflected in said one of said first optical trans-mission line fiber and in said second optical transmission line fiber and in said further fiber, are substantially prevented from roaching said active fiber.
20. An optical signal transmission system as set forth in claim 19 wherein said energy reflection limiting means is interposed between said further optical fiber and said active fiber.
21. An optical signal transmission system as set forth in claim 19 wherein said energy reflection limiting means is interposed between said pumping signal source and said further optical fiber.
22. An optical signal transmission system as set forth in claim 19 wherein said reflectivity of said unidirectional opti-cal isolator and said energy reflecting limiting means is lower by at least 15 dB than the reflectivity due to Rayleigh scat-tering.
23. An optical signal transmission system as set forth in claim 19 wherein said unidirectional isolator is a polarization control optical isolator.
24. An optical signal transmission system as set forth in claim 23 wherein said polarization control optical isolator is insensitive to the polarization of the optical signal transmit-ted by said transmitter.
25. An optical signal transmission system for transmit-tong optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals, at a long distance from said transmitter, said system comprising:
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an out-put and comprising an active fiber of a predetermined length connecting said input and output, said amplifier having a gain greater than 15 dB;
a first optical transmission line fiber, having a first line fiber input connected to said transmitter of optical signals at said predetermined wavelength and a first line fiber output, said first optical transmission fine fiber having a length between its input and output greater than said predetermined length of said active flyer and such that optical signals at said predetermined wavelength applied to said first fine fiber input are significantly attenuated in travelling from said first line fiber input to said first fine fiber output and having a length such that a portion of optical signals applied to said first fine fiber output are reflected back toward said first fine fiber output, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver, a receiver of optical signals at said predetermined wavelength;
a second optical transmission fine fiber having a second line fiber input and a second line fiber output connected to said receiver of optical signals at said pre-determined wavelength, said second optical transmission line fiber having a length greater than said predetermined length of said active fiber and such that a portion of optical signals applied to sold second line fiber input are reflected back toward said second line fiber input, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
first connecting means interconnecting said first line fiber output with said input of said active fiber amplifier and comprising a first optical isolator opti-cally connected to said first line fiber output and to said input of said active fiber amplifier for supplying optical signals at said first line fiber output to said input of said active fiber amplifier, said first optical isolator being unidirectional for transmitting optical signals substantially only from said first line fiber output to said input of said active fiber amplifier, and said first interconnecting means having a reflectivity lower by at least 10 dB than the reflectivity to Rayleigh scattering of said first optical transmission line fiber, second interconnecting means interconnecting said output of said active fiber amplifier with said second fine fiber input, comprising a second optical isolator optically connected to said output of said active fiber amplifier and to said second line fiber input for sup-plying optical signals at said output of said active fiber amplifier to said second line fiber input, said second optical isolator being unidirectional for transmitting optical signals substantially only from said output of said active fiber amplifier to said second line fiber input, and said second interconnecting means having a reflectivity lower by at least 10 dB than the reflectivity due to Rayleigh scattering of said second optical trans-mission line fiber; and said active fiber amplifier also comprising a pumping signal source, and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means having a reflectivity toward said active fiber at said signal wavelength lower by at least 10 dB than reflectivi-ty due to Raylieigh scattering in any of said first or second optical transmission line fiber;
whereby reflected optical signals, including optical signals reflected in said first optical transmission line fiber, in said second optical transmission line fiber and in said further fiber, are substantially prevented from roaching said active fiber.
26. An optical signal transmission system for transmit-ting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals, at a long distance from said transmitter, said system comprising:
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said input and- output, said amplifier having a gain greater than 15 dB;
a first optical transmission line fiber having a first line fiber input connected to said transmitter of optical signals at said predetermined wavelength and a first line fiber output, said first optical transmission line fiber having a length between its input and output greater than said predetermined length of said active fiber and such that optical signals at said predetermined wavelength applied to said first line fiber input are significantly attenuated in traveling from said first line fiber input to said first line fiber output and having a length such that a portion of optical signals applied to said first line fiber output are reflected back toward said first line fiber output, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
a receiver of optical signals at said predetermined wavelength;
a second optical transmission line fiber having a second line fiber input and a second line fiber output connected to said receiver of optical signals at said predetermined wavelength, said second optical transmission line fiber having a length greater than said predetermined length of said active fiber and such that a portion of optical signals applied to said second line fiber input are reflected back toward said second line fiber input, due to Rayleigh scattering in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
first interconnecting means interconnecting said first line fiber output with said input of said active fiber amplifier and comprising a first optical isolator optically connected to said first line fiber output and to said input of said active fiber amplifier for supplying optical signals at said first line fiber output to said input of said active fiber amplifier, said first optical isolator being unidirectional for transmitting optical signals substantially only from said first line fiber output to said input of said active fiber amplifier;
second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input and comprising a second optical isolator optically connected to said output of said active fiber amplifier and to said second line fiber input, for supply-ing optical signals at said output of said active fiber amplifier to said second line fiber input, said second optical isolator being unidirectional for transmitting optical signals substantially only from said output of said active fiber amplifier to said second line fiber input; and said active fiber amplifier also comprising a pumping signal source, and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means;
the reflectivities of said first optical isolator, said second optical isolator and said energy reflection limiting means toward said active fiber at said signal wavelength having an absolute value greater by at least 10 dB than said amplifier gain;
whereby reflected optical signals, including optical signals reflected in said first optical transmission line fiber, in said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching said active fiber.
27. An optical signal transmission system as set forth in claims 25 or 26 wherein said first optical isolator and said second optical isolator are polarization control isolators.
28. An optical signal transmission system as set forth in claim 25 or 26 wherein said first interconnecting means further comprises a dichroic coupler having two inputs and an output, one of said inputs of said dichroic coupler being connected to said first line fiber output, said output of said dichroic coupler being connected to said first optical isolator and the other of said inputs of said dichroic coupler being connected to said further optical fiber.
29. An optical signal transmission system as set forth in claim 11 wherein each said optical isolator is insensitive to the polarization of said optical signals.
30. An optical signal transmission system as set forth in claim 25 wherein said reflectivities of said reflection limit-in means, said first optical isolator, and said second optical isolator is 15 dB lower than said reflectivity due to Rayleigh scattering.
31. An optical signal transmission system as set forth in
14. An optical signal transmission system as set forth in claim 9 wherein said reflectivity of said optical isolator and said energy reflecting limiting means is lower by at least 15 dB than the reflectivity due to Rayleigh scattering.
15. An optical signal transmission system for trans-mitting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals, at a long distance from said transmitter, said system comprising a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first line fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals applied to the input thereof are significantly attenuated in travelling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength applied to the other end of said one of said first optical transmission line fiber and said-second optical trans-mission line fiber are reflected back toward said active fiber amplifier, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical trans-mission line fiber with said active fiber comprising a unidirectional optical isolator which substantially prevents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength; and said active fiber amplifier comprising a pumping signal source, a further optical fiber connecting said pumping signal source and said active fiber, and an energy reflection limiting means interposed between said pumping signal source and said active fiber;
the reflectivities of said optical isolator and said energy reflection limiting means of said further optical fiber at said signal wavelength having an absolute value greater by at least 10 dB than said amplifier gain;
whereby reflected optical signals, including at least optical signals reflected in said one of staid first optical trans-mission line fiber and said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching said active fiber.
16. An optical signal transmission system as set forth in claim 15 wherein said energy reflection limiting means is . . .
interposed between said further optical fiber and said active fiber.
17. An optical signal transmission system as set forth in claim 15 wherein said energy reflection limiting means is interposed between said pumping signal source and said further optical fiber.
18. An optical signal transmission system as set forth in claim 15 wherein said absolute value is higher than said ampli-fication gain by at least 15 dB.
19. An optical signal transmission system for transmit-ting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals at a long distance from said transmitter, said system comprising;
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said amplifier input and output, said amplifier having a gain greater than 15 dB;
a receiver of optical signals at said predetermined wavelength;
a first optical transmission line fiber having a first line fiber input at one end thereof connected to said transmitter of optical signals and a first fine fiber output at the other end thereof;
first interconnecting means interconnecting said first line fiber output with said active fiber amplifier at said input of the latter;
a second optical transmission line fiber having a second line fiber output at one end thereof connected to said receiver of optical signals and having a second line fiber input at the other end thereof; and second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input;
at least one of said first optical transmission line fiber and second optical transmission line fiber having a length between the input and output thereof greater than said predetermined length of said active fiber and such that optical signals applied to the input thereof are significantly attenuated in travelling from the input to the output thereof and having a length such that a portion of optical signals at said predetermined wavelength applied to the other end of said one of said first optical transmission line fiber and said second optical trans-mission line fiber are reflected back toward said active fiber amplifier, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
the one of said first and second interconnecting means interconnecting said one of said first optical transmission line fiber and said second optical trans-mission line fiber with said active fiber comprising a unidirectional optical isolator which substantially pre-vents optical signals due to Rayleigh scattering from entering said amplifier while transmitting optical signals at said predetermined wavelength, and said active fiber amplifier also comprising a pumping signal source, a further optical fiber connecting said pumping signal source and said active fiber and an energy reflection limiting means interposed between said pumping signal source and said active fiber;
said unidirectional optical isolator and said energy reflection limiting means of said further optical fiber having a reflectivity toward said active fiber at said signal wavelength lower by at least 10 dB than the reflec-tivity due to Rayleigh scattering in any of said first or second optical transmission line fibers;
whereby reflected optical signals, including at least optical signals reflected in said one of said first optical trans-mission line fiber and in said second optical transmission line fiber and in said further fiber, are substantially prevented from roaching said active fiber.
20. An optical signal transmission system as set forth in claim 19 wherein said energy reflection limiting means is interposed between said further optical fiber and said active fiber.
21. An optical signal transmission system as set forth in claim 19 wherein said energy reflection limiting means is interposed between said pumping signal source and said further optical fiber.
22. An optical signal transmission system as set forth in claim 19 wherein said reflectivity of said unidirectional opti-cal isolator and said energy reflecting limiting means is lower by at least 15 dB than the reflectivity due to Rayleigh scat-tering.
23. An optical signal transmission system as set forth in claim 19 wherein said unidirectional isolator is a polarization control optical isolator.
24. An optical signal transmission system as set forth in claim 23 wherein said polarization control optical isolator is insensitive to the polarization of the optical signal transmit-ted by said transmitter.
25. An optical signal transmission system for transmit-tong optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals, at a long distance from said transmitter, said system comprising:
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an out-put and comprising an active fiber of a predetermined length connecting said input and output, said amplifier having a gain greater than 15 dB;
a first optical transmission line fiber, having a first line fiber input connected to said transmitter of optical signals at said predetermined wavelength and a first line fiber output, said first optical transmission fine fiber having a length between its input and output greater than said predetermined length of said active flyer and such that optical signals at said predetermined wavelength applied to said first fine fiber input are significantly attenuated in travelling from said first line fiber input to said first fine fiber output and having a length such that a portion of optical signals applied to said first fine fiber output are reflected back toward said first fine fiber output, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver, a receiver of optical signals at said predetermined wavelength;
a second optical transmission fine fiber having a second line fiber input and a second line fiber output connected to said receiver of optical signals at said pre-determined wavelength, said second optical transmission line fiber having a length greater than said predetermined length of said active fiber and such that a portion of optical signals applied to sold second line fiber input are reflected back toward said second line fiber input, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
first connecting means interconnecting said first line fiber output with said input of said active fiber amplifier and comprising a first optical isolator opti-cally connected to said first line fiber output and to said input of said active fiber amplifier for supplying optical signals at said first line fiber output to said input of said active fiber amplifier, said first optical isolator being unidirectional for transmitting optical signals substantially only from said first line fiber output to said input of said active fiber amplifier, and said first interconnecting means having a reflectivity lower by at least 10 dB than the reflectivity to Rayleigh scattering of said first optical transmission line fiber, second interconnecting means interconnecting said output of said active fiber amplifier with said second fine fiber input, comprising a second optical isolator optically connected to said output of said active fiber amplifier and to said second line fiber input for sup-plying optical signals at said output of said active fiber amplifier to said second line fiber input, said second optical isolator being unidirectional for transmitting optical signals substantially only from said output of said active fiber amplifier to said second line fiber input, and said second interconnecting means having a reflectivity lower by at least 10 dB than the reflectivity due to Rayleigh scattering of said second optical trans-mission line fiber; and said active fiber amplifier also comprising a pumping signal source, and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means having a reflectivity toward said active fiber at said signal wavelength lower by at least 10 dB than reflectivi-ty due to Raylieigh scattering in any of said first or second optical transmission line fiber;
whereby reflected optical signals, including optical signals reflected in said first optical transmission line fiber, in said second optical transmission line fiber and in said further fiber, are substantially prevented from roaching said active fiber.
26. An optical signal transmission system for transmit-ting optical signals at a predetermined wavelength from a transmitter to a receiver of such optical signals, at a long distance from said transmitter, said system comprising:
a transmitter of optical signals at said predeter-mined wavelength;
an active fiber amplifier for amplifying signals at said predetermined wavelength having an input and an output and comprising an active fiber of predetermined length connecting said input and- output, said amplifier having a gain greater than 15 dB;
a first optical transmission line fiber having a first line fiber input connected to said transmitter of optical signals at said predetermined wavelength and a first line fiber output, said first optical transmission line fiber having a length between its input and output greater than said predetermined length of said active fiber and such that optical signals at said predetermined wavelength applied to said first line fiber input are significantly attenuated in traveling from said first line fiber input to said first line fiber output and having a length such that a portion of optical signals applied to said first line fiber output are reflected back toward said first line fiber output, due to Rayleigh scattering, in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
a receiver of optical signals at said predetermined wavelength;
a second optical transmission line fiber having a second line fiber input and a second line fiber output connected to said receiver of optical signals at said predetermined wavelength, said second optical transmission line fiber having a length greater than said predetermined length of said active fiber and such that a portion of optical signals applied to said second line fiber input are reflected back toward said second line fiber input, due to Rayleigh scattering in an amount sufficient to reduce the signal-to-noise ratio at said receiver;
first interconnecting means interconnecting said first line fiber output with said input of said active fiber amplifier and comprising a first optical isolator optically connected to said first line fiber output and to said input of said active fiber amplifier for supplying optical signals at said first line fiber output to said input of said active fiber amplifier, said first optical isolator being unidirectional for transmitting optical signals substantially only from said first line fiber output to said input of said active fiber amplifier;
second interconnecting means interconnecting said output of said active fiber amplifier with said second line fiber input and comprising a second optical isolator optically connected to said output of said active fiber amplifier and to said second line fiber input, for supply-ing optical signals at said output of said active fiber amplifier to said second line fiber input, said second optical isolator being unidirectional for transmitting optical signals substantially only from said output of said active fiber amplifier to said second line fiber input; and said active fiber amplifier also comprising a pumping signal source, and a further optical fiber connecting said pumping signal source and said active fiber, said further optical fiber having an energy reflection limiting means;
the reflectivities of said first optical isolator, said second optical isolator and said energy reflection limiting means toward said active fiber at said signal wavelength having an absolute value greater by at least 10 dB than said amplifier gain;
whereby reflected optical signals, including optical signals reflected in said first optical transmission line fiber, in said second optical transmission line fiber and in said further fiber, are substantially prevented from reaching said active fiber.
27. An optical signal transmission system as set forth in claims 25 or 26 wherein said first optical isolator and said second optical isolator are polarization control isolators.
28. An optical signal transmission system as set forth in claim 25 or 26 wherein said first interconnecting means further comprises a dichroic coupler having two inputs and an output, one of said inputs of said dichroic coupler being connected to said first line fiber output, said output of said dichroic coupler being connected to said first optical isolator and the other of said inputs of said dichroic coupler being connected to said further optical fiber.
29. An optical signal transmission system as set forth in claim 11 wherein each said optical isolator is insensitive to the polarization of said optical signals.
30. An optical signal transmission system as set forth in claim 25 wherein said reflectivities of said reflection limit-in means, said first optical isolator, and said second optical isolator is 15 dB lower than said reflectivity due to Rayleigh scattering.
31. An optical signal transmission system as set forth in
claim 14 wherein said absolute value is higher than said ampli-fication gain by at least 15 dB.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IT02043490A IT1248821B (en) | 1990-05-25 | 1990-05-25 | FIBER OPTIC TELECOMMUNICATION LINE WITH ACTIVE FIBER OPTICAL AMPLIFIERS WITH REDUCED REFLECTIONS |
| IT20434 | 1990-05-25 | ||
| US55291890A | 1990-07-16 | 1990-07-16 | |
| US552,918 | 1990-07-16 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2042987A1 CA2042987A1 (en) | 1991-11-26 |
| CA2042987C true CA2042987C (en) | 1997-02-11 |
Family
ID=26327528
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2042987 Expired - Lifetime CA2042987C (en) | 1990-05-25 | 1991-05-21 | Unit for amplifying light signals in optical fiber transmission lines |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2042987C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE10336240B4 (en) * | 2003-08-07 | 2016-06-30 | Xieon Networks S.À.R.L. | Arrangement for reducing reflection interference in a transmission system with at least one connected Raman pump source |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115473503B (en) * | 2022-05-23 | 2024-04-05 | 本源量子计算科技（合肥）股份有限公司 | Characterization method, device, medium and electronic equipment of signal amplifier gain |
-
1991
- 1991-05-21 CA CA 2042987 patent/CA2042987C/en not_active Expired - Lifetime
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE10336240B4 (en) * | 2003-08-07 | 2016-06-30 | Xieon Networks S.À.R.L. | Arrangement for reducing reflection interference in a transmission system with at least one connected Raman pump source |
Also Published As
| Publication number | Publication date |
|---|---|
| CA2042987A1 (en) | 1991-11-26 |
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