Unit for amplifying signals of light in optical fiber transmission lines.

The present invention relates to a fiber optic telecommunications line provided with fiber optic active amplifiers in which reflections towards the amplifiers are contained below a predetermined value.

Optical fibers having a doped core in which doping is carried out using particular substances such as rare earth ions are known to have stimulated emission characteristics and are suitable for use as epic fiber amplifiers in the fiber lines. fiber optic telecommunications for civil telephony.

Such types of amplifiers are referred to in European patent application 32. 90112920.5 by the same applicant.

By fiber optic amplifiers, also referred to as optical active fiber amplifiers, is meant amplifiers in which the optical transmission signal is amplified as such, while maintaining its optical form, without requiring reception or conversion thereof. other form, such as electronics, an amplification in said form and a new conversion to optical form, and wherein the amplifier element consists of a described late-type fiber optic portion having a predetermined length, serially connected between two line fiber optic lengths and provided with respective feed means to feed the optical pumping signal. .

Amplifiers of this type have special advantages for use on telecommunication lines as they offer high gains when they are used.

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lysed as line amplifiers, gains that can be brought to the desired value by properly selecting the active fiber length and / or the doper's constituents, or, when they can be used as power amplifiers, they offer a high amplifying power. .

Particularly dangerous for amplifiers are the signal reflections that occur at the ends of the fihira itself.

Japanese Patents 52-155901 and 63-219186 and Slectronics. letters ”, vol. 24 h 21, 7 January 1988, pages 36 and 38, it is known that in a Laser or optical semi-conductor amplifier there is a risk of instability and oscillation due to reflections at the amplifier terminals.

In the aforementioned patents and in order to eliminate these reflections, it is generally taught to attach an optical isolator to the semiconductor laser, which prevents light reflected by the coupling surfaces between the fibers of the line and these devices, reach the lasers themselves.

Hmm. active fiber amplifier there are no interfacial surfaces between the line fibers and the amplifier because the fibers^1;they are directly soldered to the active fiber of the amplifier; therefore reflection phenomena are not generally expected.

It has been found, however, that in an active fiber amplifier, in the absence of reflection limiting mechanisms towards the active fiber, it is impossible to achieve high amplification gains due to the occurrence of interferometric noise as a result of beats between the direct signal and the signals reflected in the thread fibers themselves and in all events directed at the active fiber; the presence of interferometric noise is of little importance in a semiconductor amplifier which

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It has low gains and small frame sizes, while it becomes particularly important in an active fiber amplifier capable of achieving very high gains and having an active fiber of considerable length generally in the range of a few tens of meters, much larger than the distance. coherence of the laser signal generator.

An active-core fiber-type amplifier raises the problem of protecting the active fiber against that noise source and keeping each reflection towards the active fiber itself below critical values ​​so as not to endanger the transmission quality while maintaining high values ​​in the gain of amplification.

At the request of the above European Patent H2. 90112920.5 teaches how to introduce optical isolators into fiber optic amplifiers, said insulators having limited reflectivity below a critical value.

The present invention aims to provide a fiber optic telecommunications line comprising active fiber optic amplifiers in which the amplifiers are protected against all the disadvantages resulting from reflections, in accordance with the teachings of European Patent Application No. 90112920.5, further highlighting the parameters. critical to choose the most suitable optical isolation characteristics for the expected gain of the amplifier.

It is an object of the present invention to provide a fiber optic telecommunication line in which an optical transmission signal is carried from one line terminal to the other unregenerate terminal along which at least one fiber optic amplifier is present comprising a active fiber length with doped core at which doping

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Fluorescent fluorescence, characterized in that all fibers connected to the active fiber of the amplifier or belonging to each amplifier have reflection limiting devices, the reflectivity of which, viewed from the flattened end of the active fiber, is at least 10 dB lower. than reflectivity corresponding to Rayleigh dispersion in fiber at transmission wavelength

Preferably the reflection limiting devices cause fiber reflectivity, which converges towards the active fiber, which is 15 dB or less lower than the reflectivity corresponding to the Rayleigh dispersion in the fiber at the transmission wavelength.

According to a particular embodiment, above all in order to achieve high amplification gains, all fibers connected to the amplifier's active fiber or belonging to each amplifier are provided with reflection limiting devices having a reflectivity, as seen from above. flattened end of the active fibron, the absolute value of which is higher by at least 10 dB and preferably by at least 15 dB than the expected amplification gain.

The reflection limiting mechanism comprises polarization control optical isolators located downstream and upstream of the active optical fiber of the amplifier.

Optical isolators are of a type insensitive to transmission signal bias.

The fibers attached to the active fiber of the amplifier, devoid of optical isolators, have anti-reflection coatings and / or an oblique cut of the surface (s) traversed by the transmission signal giving rise to wavelength reflectivity. of the lowest transmission signal at least 10 dB and preferably at least 15 dB than the reflectivity corresponding to Rayleigh dispersion in the fiber at the wavelength of the braid4,

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mission, or the absolute value of which is greater by at least 10 dB and preferably by at least 15 dB than the expected amplifier gain.

It is also an object of the present invention to provide an optical fiber-optic fiber optic telecommunication line amplifier comprising an active fiber optic having a fluorescent doped core, attached at respective ends to the optical fibers of a fiber line. which the active optical fiber is also connected by means of their coupling devices, at least one optical fiber attached to a source of an optical pumping signal, characterized in that all fibers attached to the active fiber exhibit reflection limiting mechanisms causing a reflectivity, at least 10 dB, of the active fiber end view and preferably at least 15 dB, than the reflectivity corresponding to the Rayleigh dispersion in the fiber at the transmission wavelength, or the absolute value of which is at least 10 dB higher and preferably at least 15 dB higher than the expected amplification gain »

In the above amplifier the reflection limiting mechanism comprises polarization control optical isolators located in the same and opposite direction of the active fiber optic current; Optical isolators are polarization insensitive.

Each optical isolator has a fiber-active reflectivity active at the wavelength of the transmission signal that is less than 10 dB and preferably 15 dB dc than the reflectivity given by the Rqyleigh dispersion in the line fiber, or the absolute value of which is higher in at least 10 dB and preferably at least 15 dB than the expected amplifier gain.

The optical fiber is connected to the optical pumping source and is devoid of optical isolators and at the terminal¹-5

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The respondent connected to said source is provided with a reflection limiting mechanism comprising anti-reflection coating layers and / or an oblique cut of the surface (s) traversed by the transmission signal, said reflection limiting mechanism providing a reflectivity.

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which is less than 10 dB and preferably 15 dB than the reflectivity resulting from the Rayleigh scatter, or the absolute value ds which is greater by at least 10 dB and preferably at least 15 dB than the expected amplification gain. , including attenuations due to the transmission signal and reflected signal passing through the optical fiber terminal coupling mechanism connected to the pumping signal source is cut at an angle ranging from 52 to 102 ° from the plane normal to the fiber axis .

The fiber connected to the optical pumping signal source is connected to the active fiber via a dichroic coupler and the optical isolators are placed between the active fiber and the optical fiber of the line at a respective terminal, and between the fiber of the line and the coupler. Optical immediately adjacent to the active fiber at the other end of the active fiber itself, The fiber connected to the optical pumping signal source is joined to the active fiber via a dichroic coupler and an optical isolator is interposed between the active fiber and the dichroic coupler.

In the case of power amplifiers operating with a transmit signal having a power greater than the saturation power of the amplifier and connected at a position adjacent to the transmitting signal laser equipped with its protective optical isolator, an optical isolator is present. only at the downstream end of the fiber acting in the direction of the transmission signal current.

Further details will become clear from the following description of the invention hereinafter with reference to the accompanying drawings, in which:

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- Figure 1 is a diagram of a fiber optic telecommunications line provided with a power amplifier and line,

Figure 2 is a structural diagram of an active fiber optic line amplifier according to a preferred embodiment of the invention;

Figure 3 is a structural diagram of an active fiber optic line amplifier according to an alternative embodiment;

Figure 4 is a structural diagram of an active fiber optic power amplifier according to the invention.

As shown in Figure 1, a fiber optic telecommunications line generally comprises a broadcasting station 1, and a receiving station 2, hundreds or thousands of kilometers apart, for example; Interposed between the two stations is an optical fiber 3 having suitable transmission characteristics through which the signal is carried from one station to another.

To cover the desired total distance between stations 1 and 2 it is first necessary to send a signal of sufficient power and subsequently compensate for signal attenuation along the fiber; therefore, the emitting station comprises, immediately after the laser 4 generating the optical signal to be transmitted, a power amplifier 5 adapted to input a signal into the line which has a higher than achievable power or which may be conveniently generated through laser 4; furthermore, after a certain length of the fiber, a few hundred kilometers for example, a first line 6a amplifier is present, generally shown in the figure, adapted to bring the signal back to a sufficiently high level and followed further by other fiber lengths and amplifiers 6b, 6o, and so forth, which are present here until the entire distance intended

I

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has been traveled *

The amplifiers 5, 6 may advantageously consist of optical amplifiers; These amplifiers are particularly suited to established applications because in them the signal retains the optical form and therefore their reading and conversion to an electronic form as well as processing and amplification and a further conversion to the optical form to run back over the unsound line. needed.

These operations, in fact, restrict the capacity of the line, in particular with respect to the transmission speed which is conditioned by the processing speed of the electronic devices used.

In contrast, in an optical type amplifier the signal always remains in an optical form and is therefore not subject to transmission speed restrictions and the like.

In addition it is particularly convenient to use active core fiber optic type optical amplifiers.

In fact these amplifiers allow particularly good performance in both gain and efficiency.

The structure of a fiber optic amplifier is shown in. diagram in Figure 2; the fiber of line 3, in which the transmission signal to be amplified propagates with a wavelength ^, is connected to the dichroic coupler 7 in which the transmission signal is joined over a single output fiber 8 with a wavelength pumping signal generated by a pumping laser emitter 9; an active fiber 10, attached to fiber 8 exiting the coupler constitutes the signal amplifying element which is therefore reinserted within the fiber of line 3 thus continuing towards its destination.

To produce the active fiber 10 forming he-863488

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amplifier assembly, an optical fiber based on the

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a fluorescent substance which, in the presence of a wavelength beacon light \, is capable of generating a stimulated emission consistent with the broadcasting wavelength signal \ _Q, so that the output signal appears much more amplified than the input signal ·

It is known that in any amplifier the gain

G is bound to reflectivity R1, R1₂ measures at the corresponding ends by the reaction:

G (dB) &lt;- 1/2 0 ^ (dB) + R₂(dB) J, where the reflectivities R-μ R₂ are defined as:

R (dB) = 10 lil (Pi / Pj.) Where is the transmitted force, while P is the reflected force.

Substantially what has been said above means that obtaining high amplifier gains is limited by the reflection characteristics at the terminals of the amplifier itself, or, in other words, to achieve high amplification gains it is necessary to have reflectivities R1 and R4.₂ high.

In fact, if a portion of the light signal present within the amplifier is reflected back to the corresponding terminal, said portion is amplified, partly reflected again at the opposite terminal and introduced back into the amplifier, the cycle being repeated several times. times; when said reflections and amplifications reach a high value globally, it is possible to achieve an oscillation, which condition makes the proper functioning of the amplifier impossible, which dictates that the maximum gain of amplification must be limited to avoid the occurrence of this phenomenon.

In addition to this phenomenon, the reflection of νοίΐε into the amplifier of the transmission signal itself.

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by the reflective elements located downstream of the amplifier (the line fiber itself) where said reflection is amplified again and again reflected by the reflective elements located upstream of the amplifier, gives rise to a beating phenomenon between the direct signals and reflected as interferometric noise.

This interferometric noise becomes particularly important in the case of active fiber amplifiers which have a length of the amplifier element, i.e. fiber, greater than the length corresponding to the coherence time of the laser which generated the signal; Under these de facto conditions, if the coherence between direct and reflected signals is lost, the reflected signal becomes displaced from the direct signal and, if it has sufficient intensity, becomes detrimental to the quality of the transmission.

Reflections that may take place in the amplifier may be due to the presence of interfacial surfaces at its terminals as a result of well-known refractive phenomena, but also in the absence of such surfaces, as in the case of fiber amplifiers, in which the amplifier element consists of. on an active fiber 10 directly welded to the coupler 7 and the line fibers, The dispersion within the fiber line down and up the amplifier (known as S.ayleigh scatter) that produces a reflection of the luminous force.

It was indeed heated that the Hayleigh dispersion which occurs throughout the fiber produces a reflectivity, the value of which is about -30 dB.

Other forms of reflection may be produced when strong light energies are transmitted due to the phenomenon known as Brillouin scattering.

In accordance with the present invention, limitations on the maximum achievable gain in a line amplifier

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resulting from the reflection phenomena described above can be eliminated by placing optical isolators 11 downstream and upstream of the amplifying fiber; in particular, an optical isolator 11a is located upstream of the coupler 7, immediately after line fiber 3 θ an optical isolator 11b is located downstream of fiber IC before the next line fiber length 3 ·.

Optical isolators are devices adapted to permit unidirectional passage of light; For the purposes of the present invention optimum isolators need to be of the transmission signal bias-independent type, have a degree of isolation at least greater than 20 dB and exhibit a low reflectivity at least 10 dB less than the reflectivity given by the Hayleigh dispersion on a fiber of infinite length and preferably at least 15 dB less than the value behind.

Indeed, it has been found that the presence of insulators having the above characteristics ensures that the active element of the amplifier, which is doped fiber, can operate under conditions which are very far from those in which the noise resulting from immersive reflections as described above. above may occur in the presence of amplification gains usually achievable with fiber amplifiers which are about 30 dB, This value substantially corresponds to the absolute value of reflectivity given by the Sayleigh dispersion in a fiber of infinite length.

In order to obtain higher gains a correspondingly low and required reflectivity value, which reflectivity according to the invention should have is: all points an absolute value greater than at least 10 dB, and preferably at least 15 dB, than the expected value of amplification.

In the preceding mechanisms for example, to obtain a gain of 40 dB, the reflectivity towards the

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The active fiber in each fiber attached to the active fiber itself should be at least less than -50 dB and preferably lower than -55 dB for the transmission wavelength.

The prescribed reflectivity characteristics of insulators can be obtained by known mechanisms such as multi-layer coatings, surfaces crossed by the transmission signal oblique to the direction of propagation of the signal itself and the like. This mechanism is generally well known in the state of the art. and so will not be described anymore.

In addition, to avoid noise generated by reflections in accordance with the present invention, the fiber 12 also, which transmits the light pumping energy to the coupler 7 and therefore to the active fiber 10, must have limited reflectivity towards itself. active fiber; In fact a fraction of the light energy at the transmission wavelength which propagates back to the coupler 7 is sent into the fiber 12, since the couplers generally used for this purpose in the two coupled branches do not have an absolute separation between two. wavelengths to which the couplers themselves are designed; Due to this non-absolute separation, a not insignificant percentage of light energy in the transmission wavelength, for example in the range of a few percent, is coupled over the coupling branch carrying the pumping energy.

If this fraction of light at the transmission wavelength at the end of the fiber 12 where it is optically connected to the pumping laser 9 finds a reflection, it will be sent back through the coupler 7 into the active fiber and will also contribute for the above described phenomena generating the interferometric noise.

For fiber 12, a reflectance value lower by 10 dB, and preferably by 15 dB, than

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that the value corresponding to the Rayleigh dispersion in an infinite fiber minus twice the attenuation value generated by passing the transmission wavelength at the coupler pumping branch is required.

In other words, at the end of the active fiber 10 attached to the fiber 8, the reflectivity on any fiber converging to it must be overall at least 10 dB, and preferably 15 dB, lower than that corresponding to the Rayleigb dispersion in a fiber. infinite fiber (or correspondingly, the absolute value of which is greater than the expected gain); Similarly, the reflectivity at the opposite end of the fiber 10 should be limited.

The prescribed reflectivity characteristics of fiber 12 may be obtained by prior art methods such as multi-layer coatings or oblique surfaces; in particular the oblique sectioning of the final surface 13 of the laser coupled fiber 12 at an angle preferably in the range 52 to 102 ensures a reflectivity of less than -15 dB which, added to the attenuations due to the passage through coupler 7 of about -20 dB for each pass for example yields a total reflectivity seen from the fiber terminal 10 of -55 dB, which is about 15 dB lower than the reflectivity given by the Rayleigh dispersion (about -30 dB).

The reflection phenomena in fiber 12 may also be eliminated by arranging the optical isolator 11a downstream of the coupler 7 just before the active fiber as shown in Figure 3; This solution, which allows the use of anti-reflective devices to be used at the fiber end 12, can be adapted in case the loss of pumping force that occurs while the insulator is being traversed is not harmful. the proper functioning of the amplifier.

lio case of direct power amplifiers-13-

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downstream of the transmit laser 4 which with an input signal having a level of

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Case Ρ. 615 connected to are powered high, higher than the so-called saturation level beyond which the transmit signal power coming out of the amplifier depends only on the powered pumping power and which emits a high luminous power (greater than 4 dBm for example). ) In addition to the phenomena previously described, the effect of noise arising from reflection due to Brillouin dispersion may occur, in which the light energy admits within the fiber optic line coming out of the amplified! excites vibrations in fiber atoms, which vibrations in turn give rise to the generation of a reflected signal of a wavelength slightly shorter than the direct signal.

This reflected signal may generate a beating with the direct transmission signal, so much so that it may give rise to noise impairing the quality of the transmission by adding to the previously described phenomena.

In a telecommunications line, as shown in the diagram in Pigura 1, the signal emission equipment, generally identified by 14 in Pigura, provides for the presence of an optical isolator 15 immediately after laser 4, which optical isolator performs the function of protect the laser itself from reflections which may cause damage to the laser structure; According to the invention a power amplifier 5 which is adjacent to the unit 14 can therefore dispense with the presence of the optical isolator 11a in the respective input, as shown in Pig. 4, because the function of eliminating reflections towards the active fiber of the amplifier can in this case be attributed to the existing isolator 15.

The remaining parts of the power amplifier shown in Pigura 4 are identical, as regards the graphical representation, to those described for the line amplifiers and therefore assigned to them.

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L / 23 MAf we have reference numbers.

By way of example a telecommunication line has been completed according to the diagram shown in Figure 1, employing as transmitting laser a traditionally modulated DFB laser directly having an emission wavelength of 1535 nm; receiving station 2 consisted of a known pin / HBMT type receiver, followed by broadband amplifiers not shown.

Line 3 consisted of low-attenuated displaced dispersion strands having a zero dispersion close to the transmission wavelength used; the length of the entire line was 300 km to which corresponded a 60 dB attenuation.

The line comprised two line 6 optical amplifiers and one power amplifier 5; these amplifiers were fiber optic amplifiers and consisted of a germanium and orb-doped silicone-based active fiber 10 pumped with a laser 9 consisting of a frequency doubled, miniaturized Nd-YAG laser and diode pumped; the line amplifiers had the structure shown in Figure 2 and the power amplifier had the structure shown in Figure 4.

The line amplifiers each had a total gain of 20 dB. The power amplifier had a saturation power of 9 dBm and an input power of 0 dBm.

Optical isolators 11 were bias control isolators of a type independent of transmit signal bias, having an isolation greater than 35 dB and a reflectivity less than -50 dB; Insulators of this type are available on the market and therefore their structure will not be described in more detail.

fiber terminal 12 connected to the

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The pumping was cut at an angle of 52 °.

The transmission obtained by this structure had a receiving power of -20 dB and a noise corresponding to 40 dBm.

For comparison a transmission was completed using the same test structure as described above, in which commercially available optical isolators 11 were employed which had a reflectivity of -30 dB (corresponding to reflectivity due to Rayleigh dispersion in the fiber and adapted to avoid the emergence of oscillation in the presence of a gain greater than 30 dB); Under these conditions, although there is no oscillation, noise having a intensity of -30 dBm was noted, sufficient to prevent proper transmission reception, which is thought to be due to the effect of interferometric noise resulting from Rayleigh dispersion and dispersion. Brillouin inside the active fiber amplifiers.

Optical amplifiers 7 are shown diagrammatically in the Figures as cast fiber couplers, the use of which is particularly convenient for finishing active fiber amplifiers; however, other types of optimal couplers may also be used, for example of the type used in micro optics; also required for couplers, in particular when they are not of the cast fiber type, a reflectivity less than 10 dB less than the reflectivity given by the Rayleigh dispersion or with an absolute value greater than the amplification gain for which the amplifier is projected.

Many variations and modifications may be introduced without departing from the scope of the spirit of the invention in its general characteristics.