Telecommunication line with optical fiber

TELECOMMUNICATION LINE WITH OPTICAL V

Technical field

The invention relates to an optical fiber telecommunication line provided with active fiber optic amplifiers in which the reflections towards the amplifiers are below a predetermined value.

BACKGROUND OF THE INVENTION

It is known that optical fibers having a doped core in which doping is performed using special agents such as rare earth ions have stimulated emission capability and are suitable for use as optical amplifiers in civil telephony lines.

Such a type of amplifier is described in European Patent Application No. 9CI1292O.5 by the same inventor.

By optical fiber amplifiers, also referred to as active optical fiber amplifiers, are meant amplifiers in which the optical transmitted signal is amplified as such, while retaining its optical form without requiring it to be converted to another form, e.g. electronic, its amplification in this new form and its re-transformation into the optical form, and wherein the amplifying element consists of a piece of optical fiber of the type described above having a predetermined length and connected in series between two fiber sections of the optical line and provided with a corresponding supply means for supplying the optical pumping signal.

Amplifiers of this kind have particular advantages when used in telecommunications lines since they give high gains when used as line amplifiers, which gains can be brought to the desired value by appropriate selection of active fiber length and / or dopant content, or used as power amplifiers. amplifiers and then have high gain efficiency.

Reflections of signals that occur at the ends of the optical fiber are particularly dangerous for amplifiers.

Japanese Patent Specifications Nos. 52-155901 and 63-219186 and Electronics Letters Volume 24, No. 1 of 7.1. 1988, pages 36-38, it is known that in a laser or an optical semiconductor amplifier there is a risk of instability and oscillations due to reflections at the ends of the amplifier fiber.

The aforementioned patents and treaties disclose

And the provision of connecting an optical insulator to a semiconductor laser, thereby preventing light reflected by the bonding surfaces between the line fibers and these devices from impacting the lasers.

In the active fiber amplifier, there are no transition surfaces between the line fibers and the amplifier because the line fibers are directly welded to the active fiber of the amplifier. For this reason, reflective phenomena are generally not expected.

However, it has been found that in an active-fiber amplifier without a means of reducing the reflection towards the active-fiber, it is not possible to achieve high gain gains due to interferometric type noise as a result of collisions between the direct signal and reflected signals in the line fibers alone. fiber. The presence of interferometric noise is of little importance in a semiconductor amplifier having low gains and small design dimensions, while becoming particularly significant in an active fiber amplifier capable of achieving very high gains and having an active fiber of considerable length over several tens of meters, much greater than the coherence distance. a laser generating a signal.

In the active-core fiber amplifier, there is a problem of protecting the active fiber against a source of such noise and keeping each form of reflection towards the active fiber below critical values ​​so as not to compromise transmission quality while maintaining high gain gain values.

The above-mentioned European patent application No. 90112920.5 proposes the introduction of optical insulators in fiber optic amplifiers, which amplifiers would have a reflectance limited below the critical value.

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical fiber telecommunication line comprising active fiber optic amplifiers in which the amplifiers are protected against all disadvantages resulting from reflections, in accordance with European Patent Application No. 90112920.5. optical insulation characteristics.

The invention therefore provides an optical fiber telecommunication line in which the optical signal is transmitted from one end to the other

-3the second end of the line without regeneration, and along which at least one line amplifier provided with an active fiber having a core doped with a fluorescent substance is arranged, characterized in that all fibers connected to or associated with the active amplifier fiber have means for reducing reflection whose reflection, seen from the front end of the active fiber is at least 10 dB lower than the reflectance corresponding to the Rayleigh scattering in the fiber at the transmission wavelength.

According to a preferred embodiment of the invention, the reflection limiting means provides a reflectivity of the fibers converging to the active fiber that are equal to or less than 15 dB than the reflectance corresponding to the Rayleigh scattering in the fiber at the transmission wavelength.

According to a further preferred embodiment of the invention, all fibers attached to or belonging to each amplifier active fiber are provided with a reflection limiting means having a reflectance as seen from the front end of the active fiber whose absolute value is at least 10 dB, preferably 15 dB higher than expected gain of the amplifier.

According to another preferred embodiment of the invention, the reflection limiting means comprises optical polarization control insulators located upstream and downstream of the active amplifier fiber.

According to a further preferred embodiment of the invention, the optical insulators are of the type which is insensitive to the polarization of the transmitted signal.

According to a further preferred embodiment of the invention, the fibers attached to the active fiber of the line amplifier without optical insulators have anti-reflective coatings and an oblique cross-section of the surfaces through which the transmitted signal reflects at wavelength of the transmitted signal lower by at least 10 dB, preferably 15 dB corresponding to the Rayleigh scattering in the fiber at the transmission wavelength.

According to a further preferred embodiment of the invention, the fibers connected to the active fiber of the line amplifier devoid of optical insulators have an anti-reflective coating and / or an oblique section of the surfaces through which the transmitted signal passes, to reflect reflectance at the wavelength of the transmitted signal. advantage by 15 dB than the expected gain of the amplifier.

The invention further provides an optical amplifier for an optical fiber telecommunication line of the type comprising an active optical fiber having a core doped with a fluorescent doping attached at the ends to the optical fibers of the telecommunications line to which the active optical fiber is also attached by a dichroic coupler. to the source of optical pumping signal, whose essence lies in that all the fibers attached to the active optical fiber have a blanket to reduce the reflection causing reflection, seen from the front end of the active optical fiber, by at least 10 dB than the reflectance corresponding to the Rayleigh scattering in the string at the transmission wavelength.

According to a preferred embodiment of the invention, the reflection limiting means causes the reflectivity of the fibers converging to the active optical fiber which are at least 15 dB lower than the reflection caused by the Rayleigh scattering in the fiber.

According to a further preferred embodiment of the invention, all the fibers connected to the active optical fiber have a reflection limiting means having a reflectance as seen from the front end of the active optical fiber whose absolute value is at least 10 dB, preferably at least 15 dB, than the expected gain of the amplifier.

According to a further preferred embodiment of the invention, the reflection limiting means comprises optical polarization control insulators located upstream and downstream of the active optical fiber.

According to a further preferred embodiment of the invention the optical insulators are of the type insensitive to the polarization of the transmitted signal.

According to a further preferred embodiment of the invention, each optical insulator has a reflectance towards the active optical fiber at a wavelength of the transmitted signal lower by 10 dB, preferably by 15 dB than the reflectance given by the Rayleigh scattering in the line fiber.

According to a further preferred embodiment of the invention, each optical insulator has a reflection towards the active optical fiber at the wavelength of the transmitted signal, the absolute value of which is at least 10 dB, preferably 15 dB, higher than the expected gain of the amplifier.

According to a further preferred embodiment of the invention, the lead-in fiber connected to the optical pumping signal source is free of optical insulators and is provided with reflection reduction means at its end connected to the optical pumping signal source

-Containing anti-reflective coating layers and / or oblique cross-section of the surfaces through which the transmitted signal passes, the cdraz limiting means giving a reflectance which is 10 dB, preferably 15 dB less than the reflectance resulting from Rayleigh scattering including attenuations caused by the transmitted signal and a reflected signal by the binding means.

According to a further preferred embodiment of the invention, the lead-in fiber connected to the optical pumping signal source is devoid of optical insulators and provided at its end connected to the optical pumping signal source with reflection reduction means comprising anti-reflective coating layers and / or oblique cross-section of surfaces through which the transmitted signal passes. this reflection limiting means gives a reflectance whose absolute value is at least 10 dB higher, preferably 15 dB, than the expected gain of the amplifier, including attenuation caused by the passage of the transmitted signal and the reflected signal through the coupling means.

According to a further preferred embodiment of the invention, the end of the supply fiber connected to the optical pumping signal source is cut at an angle of 5 ° to 10 ° with respect to a plane perpendicular to the fiber axis.

According to a further preferred embodiment of the invention, the supply fiber connected to the source of the optical pumping signal is connected to the active optical fiber by a dichroic coupler, wherein optical insulators are interposed between the active optical fiber and the line optical fiber at one end thereof and between the line fiber and the dichroic coupler active optical fiber at its other end.

According to a further preferred embodiment of the invention, the supply fiber connected to the source of the optical pumping signal is connected to the active optical fiber by a dichroic coupler and an optical insulator is inserted between the active optical fiber and the diehroic coupler.

According to a further preferred embodiment of the invention, when power amplifiers operating with a power transmitted signal higher than the saturation power of the amplifier and connected directly to the laser transmitting the transmitted signal, provided with the respective optical protection insulator, the optical insulator is positioned only downstream of the active optical fiber.

An overview of the figures in the drawings

The invention is illustrated in the drawings, wherein Fig. 1 is a diagram

Fig. 2 is a diagram of an active optical fiber optical amplifier according to a preferred embodiment of the invention; Fig. 3 is a diagram of an active optical fiber optical amplifier according to an alternative embodiment of the invention; and Fig. 4 is a diagram of a fiber optic optical line equipped with line amplifiers and power amplifiers. an active optical fiber amplifier according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen from FIG. 1, a fiber optic telecommunications line generally comprises a transmitting station 1 and a receiving station 2, which are spaced one hundred or thousands of kilometers apart, and between them is a line optical fiber 2 having suitable transmission characteristics. The line optical fiber 2 transmits the signal from the transmitting station 1 to the receiving station 2.

In order to bridge the desired total distance between the transmitting station 1 and the receiving station 2, it is necessary to transmit a signal of sufficient power and then compensate signal attenuation along the line fiber 2 · 2. into the line filament 2 a signal of a higher power than that which can normally be generated by the laser 4. In addition, after a certain length of the line filament 2, for example after several hundred kilometers, there is provided a first line amplifier 6a, shown in FIG. 1, adapted to bring the transmitted signal again to a sufficiently high level. This is followed by further sections of the line fiber 2^and further line amplifiers 6b, 6c and thus the total line length is covered.

Amplifiers are usually optical amplifiers which are particularly advantageous for the present application, since the signal transmitted in them retains the optical form and therefore does not need to be read and converted to electronic form and processing and amplification and re-conversion to optical form for transmission over telecommunications line.

The above operations would in fact reduce the transmission capacity of the telecommunications line, in particular as regards the transmission rate, which is conditioned by the processing speed of the transmitted signal in the electronic devices used.

In contrast, in an optical amplifier, the signal still retains the optical form and thus is not subject to transmission rate limitations or other undesirable effects.

-7These optical amplifiers make it possible to achieve particularly good telecommunication line characteristics in terms of profitability and efficiency.

FIG. 2 schematically illustrates the structure of an optical fiber amplifier. The line filament 2> through which the transmitted signal of wavelength λ, which must be amplified, is connected to the dichroic coupler 2 »^ve wherein the transmitted signal is combined in a single output fiber 8 with a pumping signal of a wavelength λp produced by the pumping optical signal source 6, which is a laser. The active optical fiber 10 connected to the output fiber 8 of the dichroic coupler 2 forms the actual amplification element of the transmitted signal reintroduced into the line fiber 2 to be transferred to its destination.

A silicon-based optical fiber, the core of which is doped with a fluorescent substance, which, in the presence of an optical pumping signal of wavelength, is capable of producing stimulated emission coherent with the signal at the transmission wavelength X, is used to form the active optical fiber 10 the signal is greatly amplified by the dero rise to the input signal.

It is known that in any amplifier the gain G is dependent on the reflectances R R, R.₂ measured at its ends according to:

G (dB) <- 1/2 JJ (dB) + R₂(dB) J, 1) where reflectance! R ^, R.₂ are defined as:

R (dB) = 10 In_r/ P_t) where P_t is the transmitted power and P_r is the reflected power.

The above relationships essentially indicate that achieving high gains in the amplifier is limited by the reflection characteristics at the ends of the amplifier itself, or otherwise, to achieve large gain gains it is necessary to have high reflectances R R and R ^.₂·

Indeed, if one part of the light signal present by the amplifier is reflected back to its end, that part is amplified, partially rebound at the opposite end and reintroduced into the amplifier, the cycle repeated several times. When these reflections and gain reach an overall high value, it is possible to achieve an oscillation condition that prevents the amplifier from operating properly and which requires a reduction in the maximum gain gain to prevent this phenomenon.

In addition to this phenomenon, the back reflection of the transmitted signal in the amplifier by the reflective elements behind the amplifier, e.g.

With the filament 2, said reflection is again amplified and further reflected by reflection elements placed in front of the amplifier causing clashes between direct and reflected signals, referred to as interferometric noise.

This interferometric noise is particularly significant in the case of active-fiber amplifiers having a length of the amplifying element, i.e. fibers, greater than the length corresponding to the coherence time of the laser that has developed the signal. Under these conditions, the coherence between the direct and reflected signals is lost, the reflected signal is translated relative to the direct signal, and when it is of sufficient intensity, it interferes with the transmission quality.

Reflections that may occur in the amplifier may be due to the presence of contact surfaces at its ends as a result of, but without, the presence of known reflections, as in the case of fiber amplifiers, where the amplifier element consists of active optical fiber 10 directly welded to dichroic coupler 2 ^ak the line fiber 2> scattering inside the line fiber 2 before and after the amplifier, known as Rayleigh scattering, causes reflection of light energy.

Rayleigh scattering, which occurs throughout the fiber, has been found to produce a reflectance of about -30 dB.

Other forms of reflection may arise when high light output is transmitted due to a phenomenon known as Brillouin scattering.

According to the present invention, the limitation of the maximum gain attainable in the line amplifier resulting from the above described reflection phenomena can be avoided by arranging the optical insulators 11 before and after the optical active fiber 10. In particular, one optical insulator 11a is placed before the dichroic coupler 2 immediately after the line fiber 2 ^and the second optical insulator 11b is located downstream of the active optical fiber 10 before the next section of line fiber 2;

Optical insulators 11a, 11b are devices adapted to allow unidirectional light transmission. For the purposes of the present invention, the optical insulators 11a, 11b are required to be of the polarized signal independent type and have an insulation degree of at least greater than 20 dB and a low reflectance of at least 10 dB than the reflectance given by Rayleigh scattering. filament of endless length and preferably lower by at least 15 dB than the aforementioned value.

Indeed, it has been found that the presence of optical insulators having the characteristics described above ensures that the active amplifier element, that is, the doped optically active fiber 10, can operate under conditions sufficiently distant from those conditions in which the noise resulting from reflections of different origins described above, in the presence of gain gains usually obtainable by active fiber amplifiers, which are about 30 dB, may arise, which basically corresponds to the absolute reflectance value! given by Rayleigh scattering in a fiber of infinite length.

In order to achieve higher gains, a correspondingly low reflectance value is required, whereas the reflectance according to the present invention in FIG

it must in all cases have an absolute value higher by at least 10 dB, preferably by at least 15 dB than the expected gain of the amplifier.

Accordingly, to achieve a gain of 40 dB, it is desired that the reflectance toward the active optical fiber 10 be at least below -50 dB in each fiber connected thereto, and preferably below -55 dB for the transmission wavelength.

The prescribed reflectance characteristics of the optical insulators 11a, 11b can be achieved by known means, such as multilayer coatings, surfaces through which the transmitted signal passes obliquely to the direction of signal travel, and the like.

In order to avoid reflection noise, according to the present invention, the feeder fiber 12, which transmits the optical pumping signal to the dichroic coupler 7 and hence to the optically active fiber 10, must have limited reflectance towards the optically active fiber 10. In fact, a fraction of light output the transmission wavelength that progresses back to the dichroic coupler 2 is transmitted in the feeder thread 12. since the dichroic couplers 2 generally used for this purpose in two bundled branches do not have an absolute separation between the two wavelengths for which the dichroic couplers 2 are designed. As a result of this non-absolute decoupling, a significant proportion of the light output having a transmission wavelength in the range of, for example, several percent is transmitted by coupling to the branch of the dichroic coupler 2 that carries the optical pumping signal.

If this fraction of the light transmission power of the transmission wavelength is at the end of the supply fiber 12 where it is connected

The optical pumping signal source 6, reflected, is again sent via the dichroic coupler 2 to the interior of the active optical fiber 10 and contributes significantly to the above-described phenomena generating interferometric noise.

Accordingly, it is desired that the lead fiber 12 have a reflectance value lower by 10 dB, preferably by 15 dB, than the value corresponding to the Rayleigh scattering in the filament without twice the attenuation value given by the transmission wavelength signal input in the optical pumping signal branch of the dichroic coupling signal. member 2 ·

In other words, it is desired that at the end of the active optical fiber 10 connected to the output fiber 8, the reflectance of any fiber connected to it is generally lower by at least 10 dB, preferably 15 dB, than that corresponding to Rayleigh scattering in the filament, or value is higher than expected profit. Similarly, the reflectance at the other end of the active optical fiber 10 must be limited.

The prescribed reflectance characteristics of the feed fiber 12 can be achieved by measures known in the art, for example multilayer coatings or inclined surfaces. In particular, the oblique cross-section of the end surface 13 of the lead-in fiber 12 connected to the optical pumping signal source 2 at an angle preferably in the range of 5 ° to 10 ° provides a reflectance of less than -15 dB which adds to the damping effects of the dichroic coupler 2. For example, -20 dB for each pass, gives the total reflectance, seen from the end of the optical active fiber 10, about -55 dB bottom, about 15 dB lower than that given by the Rayleigh scattering of about -30 dB.

Reflection phenomena in the lead fiber 12 could also be avoided by placing the optical insulator 11a behind the dichroic coupler 2 immediately in front of the active optical fiber 10, as shown in Fig. 3. This solution, which allows the use of an anti-reflection measure at the end of the lead fiber 12, can be implemented if the loss of pumping power which occurs when the insulator is omitted does not interfere with the good operation of the amplifier.

In the case of power amplifiers connected directly downstream of the transmitted signal 4, which are powered by an input signal having a high level, higher than the so-called saturation level, above which the transmitted signal power coming from the amplifier depends only on the pumping power supplied and

-11 transmitting high luminous power, for example higher than 4 dBm, in addition to the above-described phenomena, a reflection effect due to Brillouin scattering can occur in which the luminous power sent to the fiber of the fiber optic line exiting the amplifier induces vibrations in the fiber atoms at a wavelength slightly shorter than the wavelength of a direct signal.

This reflected signal can cause a collision with the direct transmission signal, thus producing noise deteriorating the transmission quality and adding to the previously described phenomena.

In the telecommunications line shown schematically in FIG. 1, an optical insulator 15 is arranged immediately after the laser 4 in the signal transmission assembly 14, and the optical insulator 15 protects the laser 4 from reflections that could damage its structure.

According to the invention, the power amplifier 2 which is connected to the assembly 14 is therefore independent of the presence of the optical insulator 11a at its input as shown in Fig. 4, since the reflection elimination function towards the active optical fiber of the amplifier can already be performed existing optical insulator 15.

The other parts of the power amplifier shown in FIG. 4 are similar in terms of graphical representation to those described for line amplifiers and have therefore been given the same reference numerals.

As an example, a telecommunications line was constructed according to the diagram shown in Fig. 1. Directly modulated traditional-type DFB laser having an emission wavelength of 1535 nm was used as the transmitted signal laser. Receiver station 2 consisted of a pin / přijím receiver of known type, followed by broadband amplifiers (not shown).

The line optical fiber 2 consisted of low attenuation and shifted dispersion fibers having zero dispersion close to the transmission wavelength used. The total length of the line was 300 km with attenuation of 60 dB.

The telecommunications line comprised two line amplifiers 6 and a power amplifier 2. These amplifiers were designed as silicon-based silicon-doped active fiber amplifiers 10, doped with germanium and erbium. The optical pumping signal source 2 was a miniaturized double frequency Nd-TAG laser pumped by a diode. The line amplifiers had the structure shown in Fig. 2, the power amplifier had the structure shown in Fig. 4.

-12 Line amplifiers had a total gain every 20 dE. The power amplifier had a saturation power of 9 dBm and an input power of 0 dBm.

The optical insulators 11 were insulators of a polarization control type independent of the transmitted signal and had an isolation greater than 35 dB and a reflectance less than -50 dB. Insulators of this kind are available on the market and therefore their structure will not be described further.

The end 13 of the lead-in fiber 12 connected to the optical pumping signal source 2 has been cut at an angle of 5 °.

The transmission achieved by this structure had a received power of -20 dB and a noise equivalent to -40 dBm.

For comparison, transmission was performed using the same test structure as described above, using commercially available optical isolators 11 having a reflection of -30 dB corresponding to that of Rayleigh scattering in the fiber and adapted to prevent oscillations in the presence of up to 30 dB . Under these conditions, although no oscillations were present, noise having an intensity of -30 dB sufficient to prevent proper reception of the transmitted signal was found, which noise is considered to result in interferometric noise resulting from rayleigh scattering and Brilloulne scattering in active fiber optic amplifiers.

Pichroic couplers 2 are schematically shown in the figures as filament couplers, the use of which is particularly advantageous for forming active optical fiber amplifiers. However, other types of dichroic couplers, such as those used in microoptics, may also be used. For couplers, especially when they are not filament type, the reflectance is less than at least 10 dB than the reflectance given by Rayleigh scattering or an absolute value higher than the gain gain for which the amplifier is considered.

It is understood that many variations can be made without departing from the spirit of the invention.