FR2787954A1 - Non soliton fibre optic transmission repeaters system having Raman effect distributed pre amplifier feeding main amplifier. - Google Patents

Abstract

The non soliton fibre optic transmission system has fibre optic sections (6i-1,6i) and repeaters (5-i). The repeater has a discrete amplifier (7) and a pre amplifier (8,9). The pre amplifier is a distributed Raman effect pump and coupler. The pre amplifier reduces the noise input level to the main amplifier.

Description

l

  QUASI-DISTRIBUTED AMPLIFICATION IN A

FIBER OPTIC TRANSMISSION

  The subject of the present invention is a fiber optic transmission system with non-soliton signals, as well as a transmission method in such a system, which make it possible to improve performance in terms of signal to noise ratio, or even in terms possible lengths for the system. It relates to

  also a repeater for such a transmission system.

  The invention applies in particular to fiber optic transmission systems, using wavelength multiplexing (in English "wavelength

multiplexing or WDM division).

  One of the known solutions for the transmission of non-soliton signals in optical fibers consists in having optical amplifiers, and in particular optical fiber amplifiers doped with erbium, at regular intervals along the transmission system. Such a solution is for example described in Bergano, Long haul WDM transmission using optimum channel modulation: 32 x 5 Gbit / s 9300 km demonstration, OFC'97 post deadline 16. The transmission distances in such a system are limited by the signal ratio on noise, and in particular by the presence of spontaneous amplification noise (ASE, acronym from the English "amplified

  spontaneous emission ") generated in the amplifiers.

  It was also proposed in an article by Morten Nissov and others, 100 Gb / s (1 Oxl 0 Gb / s) WDM transmission over 7200 km using distribution Raman amplification, OFC'97, post deadline paper, to use in a system transmission only amplification distributed by Raman effect for amplification at regular intervals of the signal. This solution improves the signal-to-noise ratio by around 2 dB compared to a comparable solution using only discrete amplifiers. It has the drawback of requiring high pump powers to obtain sufficient pumping; in the proposed article, the pump powers are of the order of 300 mW. Another disadvantage for the industrial implementation of such a solution is that it is difficult with Raman pumping in a direction opposite to the direction

  signal propagation to control the power of each amplification stage.

  A description of the Raman effect is given in the work of G.P.

  Agrawal, Nonlinear Fiber Optics, Academic Press 1980.

  Furthermore, for fiber optic transmission systems with wavelength multiplexing, there is the problem of gain flatness. The amplifiers used in such systems to compensate for line loss at regular intervals are associated with filters, so as to present a gain as flat as possible over the entire wavelength range used in the transmission system. The flatness of the gain, or even the limitation of the gain excursion is obtained during the installation of the line by an adequate optimization of the

  amplifiers and associated filters.

  One of the problems encountered for such optical fiber transmission systems is the aging of the components. The attenuation of the line fibers or amplifiers increases so as to decrease the power applied to the input of each amplifier. As the output power of amplifiers is generally controlled so as to have a fixed value, this reduction in input power causes an increase in the gain of the amplifier, and a variation in the shape of the gain of the amplifier. In this case, the shorter wavelengths are more amplified. This shift of the gain peak towards the low wavelengths causes a variation of the gain over the wavelength range of the multiplex; the gain variation as a function of the wavelength thus looks like a line of decreasing slope. This variation in gain over time causes an amplification imbalance between the different channels of the multiplex in wavelengths. This problem is mentioned in O. Gautheron et al., Robustness of WDM submarine transmission to degraded conditions, OFC'97 Technical Digest, p. 61. This presentation proposes a solution consisting in readjusting the power pre-emphasis in the transmission terminal, so as to compensate for the shift in gain towards short wavelengths; in other words, it is proposed to increase the transmission power of the longer wavelength channels, compared to the shorter wavelength channels, so as to compensate for the relative increase in gain. for shorter wavelength channels; this allows you to adjust

  the signal to noise ratio for the different channels.

  This solution leads to new problems. The relative power of the different channels cannot be increased indefinitely: the readjustment of the pre-emphasis is limited; in fact, the signal to noise ratio at the output of the link is a decreasing function of the pre-emphasis; a pre-emphasis of 7 dB thus induces a loss of the order of 0.5 dB in the signal to noise ratio at the link output, the loss being of the order of 1.2 dB for a pre-emphasis of 10 dB; in practice, the pre-emphasis is therefore most often limited to 7 dB. Increasing the pre-emphasis is also all the more difficult as the channels are numerous and the links long. Limiting the pre-emphasis at the end of the life of a link to 7 dB leads to specifications of the amplifiers and associated filters which make their

difficult and expensive to manufacture.

  A final problem encountered for fiber optic transmission systems having repeaters is that of tuning the repeaters to the installation, depending on the characteristics of the fiber in a section of the transmission system between two repeaters. In fact, in practice, line losses on the sections of the connection between the different repeaters have different values from the set value; the repeaters then receive different powers, so that the performance of the system is improved if one adapts each repeater according to the line loss of the fiber section which precedes it, to ensure a flat gain

  despite variations in the power applied to the input.

  To achieve this adaptation, it has been proposed to give an amplifier a gain greater than the theoretically necessary gain, and to associate it with a variable optical attenuator making it possible to reduce the gain of the amplifier attenuator assembly to the necessary value, depending on the online loss on the section concerned. In this solution, the loss of each optical attenuator is adapted so that the sum of the line loss on the section concerned and the loss of the attenuator is constant. This solution degrades the performance of the link in terms of noise: we choose for the gain value of the amplifier the value

  corresponding to the maximum losses online on the section concerned.

  Another solution for carrying out this adaptation of the amplifier consists in intentionally varying the length of doped optical fiber according to the

  amplifiers. This solution is complex to implement and expensive.

  An article by Kyo Inoue et al., Tunable Gain Equalization Using a Mach Zender Optical Fiber in Multistage Fiber Amplifiers, IEEE Photonics Technology Letters, vol. 3 no 8, (1991) suggests using a Mach Zender optical filter to adapt the gain of a three-stage EDFA (fiber amplifier doped with erbium),

  in a 29-channel wavelength multiplex transmission system.

  Mach Zender filter transmittance slope compensates for gain variations

  induced on the multiplex band by three stages of EDFA amplifiers.

  Application FR-A-2 773 425 proposes to use, for the gain adaptation of an optical fiber amplifier doped with erbium, a filter having a bell transfer function, the central wavelength of which is tunable; the filter is tunable over a sufficient wavelength range for use

  one side or the other of the filter to compensate for variations in slope.

  M. Takeda et al., Active gain equalization by preferentially 1.43 Itm or 1.48 pm pumped Raman amplification, 1 0'h Optical Amplifiers and their Applications 1999, Nara Japan, paper 76 / ThA3-1 uses Raman preamplification upstream of an amplifier with optical fiber doped with erbium for gain equalization. To this end, this article proposes to use for the Raman preamplification two separate pumps, at wavelengths of 1430 and 1480 nm. In the wavelength range of the multiplex, the Raman gain has a negative slope for the pump at 1430 nm, and a positive slope for the pump at 1480 nm. Adjusting the power of the pumps allows gain equalization. This solution has the disadvantage of requiring two pumps; in addition, for the 1430 nm pump, the Raman gain is not linear in dB as a function of the wavelength over the 25 nm range (1535-1560 nm) typically used for the multiplex. The equalization is in fact limited to a band of 15 nm. The subject of the invention is a solution to

  signal-to-noise problem in a non-signal transmission system

  solitons, presenting discrete amplifiers. It makes it possible to limit the effects of spontaneous amplification noise, or to increase the length of the

  transmission for a given signal-to-noise ratio.

  The invention also proposes in certain embodiments a solution to the problem of the aging of the components of the transmission system. It also offers a solution to the problem of gain adaptation of repeaters in

transmission systems.

  More specifically, the invention provides a fiber optic transmission system with non-soliton signals, comprising a plurality of fiber sections and repeaters, characterized in that at least one repeater comprises discrete amplification means and means distributed preamplification of the signals in the line fiber upstream of the repeater in the direction of propagation of the signals. Advantageously, the distributed preamplification means include

  line fiber pumping means.

  These line fiber pumping means may include a

pump at 1.44 pm.

  Preferably, the power pumped through the fiber is less than 100 mW.

  In one embodiment, the preamplification means comprise

  pumping means by stimulated Raman effect of the line fiber.

  It is possible that the distributed preamplification means are upstream of the discrete amplification means in the repeater, or on the contrary downstream of these. In another embodiment, the discrete amplification means comprise a section of fiber strongly doped with erbium, and means for pumping this fiber. In this case, the pumping means include

  discrete amplification means include a 0.98 or 1.48 p.m. pump

  Advantageously, each repeater comprises means for

  distributed preamplification and discrete amplification means.

  In one embodiment, the distributed preamplification means

  have a gain which is between 0.5 and 2 dB.

  In yet another embodiment, the distributed preamplification means are controlled as a function of the input power of the means

  discrete amplification. They can also be controlled remotely.

  In yet another embodiment, the means for pumping the line fiber comprises a pump between 1470 and 1480 nm, the power of which is preferably controlled remotely. In this case, it is preferable that the repeater includes a filter with a negative slope transfer function. Can also

  provide that the discrete amplification means have a negative slope gain.

  The invention also relates to a method for transmitting non-signal signals.

  solitons in a line optical fiber transmission system having at least one repeater, the method comprising - at least one step of distributed preamplification of the signals in the line fiber, and

  - at least one step of discrete amplification of the signals in the repeater.

  Preferably, the distributed preamplification step includes pumping

line fiber.

  The distributed preamplification step is advantageously carried out by effect

Raman stimulated.

  Advantageously, the distributed preamplification step is carried out as a function of the power of the signals undergoing the discrete amplification step. We can also provide that the distributed preamplification step is carried out with a gain

remote control.

  The invention finally relates to a repeater for a fiber optic transmission system with non-soliton signals, comprising discrete amplification means.

  signals and pumping means for a distributed preamplification.

  Advantageously, the pumping means for a preamplification

  distributed include at least one pump at 1.44 p.m.

  In one embodiment, the pumping means for a distributed preamplification comprise at least one pump for pumping by

  Raman effect stimulated by a line fiber.

  Preferably, the discrete amplification means comprise a section of fiber strongly doped with erbium, and means for pumping this fiber. These

  pumping means may include a pump with 0.98 or 1.48 tam.

  In the repeater, the pumping means for a distributed preamplification can be upstream of the discrete amplification means or downstream from them. Advantageously, the pumping means for a distributed preamplification are controlled as a function of the input power of the means

  discrete amplification. They can also be controlled remotely.

  In one embodiment, the means for pumping the line fiber comprises a pump between 1470 and 1480 nm, the power of which is preferably controlled remotely. In this case, it is advantageous that the repeater includes a filter with a negative slope transfer function, or that

  the discrete amplification means have a negative slope gain.

  Other characteristics and advantages of the invention will appear on reading

  of the following description of embodiments of the invention, given as

  example and with reference to the accompanying drawings which show: - Figure 1, a block diagram of a transmission system in which the invention can be implemented; - Figure 2, a block diagram of a repeater of a transmission system according to the invention; - Figure 3, a more detailed block diagram of the repeater of Figure 2; - Figure 4, a block diagram of another embodiment of a 1 0 repeater according to the invention; - Figure 5, a block diagram of a repeater according to the invention, in which the distributed amplification pump passes through the discrete amplifier; - Figure 6, a diagram of an embodiment of the invention, with a control of the power of the distributed amplification pump; - Figure 7, a graph of the gain variations as a function of the wavelength, in the embodiment of Figure 6 - Figure 8, a block diagram of an embodiment of the invention, adapted to the gain equalization, at the installation of the link; - Figure 9, a graph of the Raman gain as a function of the wavelength, for different pumping powers - Figures 10 to 12, gain and loss graphs for the repeater of the

figure 8.

  The invention applies to any fiber optic transmission system and non-soliton signals, single channel or wavelength multiplexing. It applies to all types of non-soliton signals, and for example to RZ signals

  (return to zero) or NRZ (not return to zero).

  The invention proposes, in order to improve the signal-to-noise ratio in the discrete amplifiers, to provide, upstream of these amplifiers, a preamplification by Raman effect, i.e. to combine discrete amplifiers and distributed amplifiers. The presence of the distributed amplifiers makes it possible to preamplify the input signal of the discrete amplifiers, and consequently limit the noise generated in these amplifiers. In fact, with a higher level input signal, the spontaneous amplification noise in discrete amplifiers of the erbium doped fiber amplifier type is lower. In addition, the presence of discrete amplifiers allows better gain control on a stage of the transmission system, and in particular allows the application of known gain control solutions based on feedback. In addition, this discrete amplifier makes it possible to provide the essential of the amplification gain according to the invention. A low preamplification gain - for example of the order of 1 dB for distributed preamplification before discrete amplification - is enough to increase the signal-to-noise ratio throughout the system. Indeed, most of the amplification, in the preferred embodiments of the invention, is provided by the discrete amplifiers; conversely, the solution of the aforementioned patent application essentially proposes to ensure the amplification of the signals by the distributed amplifiers, so as to avoid as much as possible the asymmetry of the

collisions between solitons.

  Figure 1 shows a block diagram of a transmission system in which the invention can be implemented. The system includes a Tx transmitter

  1, sending signals in a first line fiber 2, and an Rx receiver 3.

  Between the end of the first line fiber 2 and the receiver 3 are provided a plurality of sections or sections 4i, i being an integer. Each section comprises a repeater 5i, and line fiber 6 ;. The repeater 5i of a section 4i amplifies the signals it receives from the line fiber 6i-1 of the previous section and transmits the amplified signals on the line fiber 6i of its section. In a manner known per se, the transmission system can comprise filters, as well as insulators or other components, which are not described here. The invention proposes to carry out a preamplification distributed by Raman effect, on the line fiber preceding a repeater, so that the power at the input of the discrete amplifier

  arranged in the repeater is superior.

  Figures 2 to 4 show examples of repeaters which can be used in a transmission system according to the invention. In each of the embodiments described with reference to these figures, a discrete amplification is provided, in combination with a distributed amplification, which preamplifies on the fiber of

  line the signal before it enters the discrete amplifier.

  Figure 2 shows a block diagram of a repeater according to the invention.

  There is shown in the figure a repeater 5i, the line fiber 6i-1 of the section

  above, and the line fiber 6i through which the repeater transmits its signals.

  The repeater comprises a discrete amplifier 7, for example an amplifier with a fiber doped with erbium and a pump. We can also use another type

  amplifier, for example an opto-electronic amplifier.

  Upstream of the discrete amplifier in the signal propagation direction is provided a coupler 8, for example an optical multiplexer. This coupler makes it possible to couple in the line fiber 6i-1, in a direction opposite to the direction of propagation of the signal, a pump signal supplied by a pump 9. The pump 9 provides a signal adapted to cause preamplification in the fiber line, and by

  example a preamplification by stimulated Raman effect.

  For a usual line fiber of the DSF type (acronym for “dispersion shifted fiber”), the pump can for example be a semiconductor laser pump, providing a signal at 1.44 jIm. Such

  signal is adapted to cause a preamplification of the signal in the line fiber.

  One can use a pump power coupled in the fiber of the order of mW. This power depends both on the power of the pump 9 and on the efficiency of the coupler 8. Such pump power injected into the fiber

  provides a preamplification gain by Raman effect of around 1.7dB.

  As explained above, such gain values are enough to reduce noise

  generated by the discrete amplifier 7.

  Thus, for a discrete amplifier consisting of a fiber coil heavily doped with erbium, with a 1.48 ptm pump, the noise generated is substantially proportional to the gain, within the range of use of the amplifier. If the gain of the amplifier is reduced by approximately 1 dB, for a constant output power, which can be obtained by means of a preamplification according to the invention, the noise generated by the discrete amplifier decreases by approximately 1 dB . For comparison, the noise generated in the amplifier distributed by Raman effect, for a preamplification gain of the order of 1 dB, is less than 0.2 dB. The invention thus provides, for the same total gain on the line fiber and in the discrete amplifier, a

  improvement of the noise factor of the section of the order of 0.8 dB.

  The precise choice of preamplification gain depends on the transmission system. Advantageously, the preamplification gain is greater than or equal to 0.5 dB. This lower limit corresponds to the preamplification gain necessary to compensate for the losses induced by the insertion into the transmission system of the coupler of the pump used for the distributed amplification. The preamplification gain is preferably less than or equal to 2 dB. This upper limit corresponds in practice to the gain obtained for pumps usually usable in a repeater, typically for a pump power of 80 mW. We can in any case use pump powers lower than 100 mW For comparison, the gain of the discrete amplifier is for example

close to 10 dB.

  This range of values allows a good reduction of the noise generated by the discrete amplifier, while avoiding the use of high power preamplification pumps. The overall improvement in the signal-to-noise ratio over a section can be estimated at around 0.8 times the Raman preamplification gain. Thus, for a preamplification gain of the order of 2 dB, an improvement in the

  signal to noise ratio of the order of 1.6 dB.

  Figure 3 shows a block diagram of another embodiment of a repeater according to the invention; this figure shows the various components of the discrete amplifier 7. This comprises, in the direction of propagation of the signal, an optical coupler 11, for example an optical multiplexer, which couples in the fiber the signal supplied by a pump 12. The coupler is followed by an isolator 13, which prevents the spontaneous amplification noise from rising in the opposite direction to the direction of propagation of the signal. The insulator 13 is followed by a section 14 of erbium-doped fiber. The pump is preferably made up of a semiconductor laser providing a signal at 0.98 or 1.48 tam, and the fiber is a fiber strongly doped with erbium. One can in particular use for the discrete amplifier the fibers known per se for this purpose. It will be noted that the words "heavily doped", in the context of amplification in optical signal fibers are perfectly clear to those skilled in the art, and correspond to a technical reality. Heavily doped fibers allow amplification over a fiber length of a few tens of meters. An amplifier made from a coil of such a fiber can

  traditionally be contained in a box, and is called a discrete amplifier.

  Conversely, distributed amplification is called amplification in the line fiber of the transmission system. Thus, the skilled person clearly understands the

  distinction between "discrete amplification" and "distributed amplification".

  Figure 4 shows a block diagram of yet another embodiment of a repeater according to the invention; the embodiment of Figure 4 is similar to that of Figure 3, except that the isolator 13 is disposed not downstream of the coupler 11 of the discrete amplifier 7, but upstream thereof in the direction of signal propagation. This position of the isolator separates the discrete amplifier from the distributed preamplifier, and avoids the reflection of the signal.

  pumping of the discrete amplifier towards the line fiber 6i 1.

  The presence of the discrete amplifiers allows gain control, by any feedback mechanism known per se. We can thus take a part of the output signal from the discrete amplifier 7 to control the gain of this discrete amplifier,

  for example by controlling the power of the pump 12.

  The power of the pump 9 used for the preamplification can be controlled, for example by measuring the power of the signal at the input of

the discrete amplifier 7.

  The control means have not been shown in the figure which make it possible, if necessary, to control the discrete or distributed pumping power; we can not only play on the powers of the pumps but also on the coefficients of

coupling of the different couplers.

  FIG. 5 shows a block diagram of a repeater, in which the distributed amplification pump passes through the discrete amplifier; as in the preceding examples, the repeater 5i has a discrete amplifier 7, a pump 9 for distributed amplification and a coupler or multiplexer 8 for injecting into the line fiber the light emitted by the pump 9. As in the modes of previous embodiments, the pumping light for the distributed amplification is counter-propagative, and- is directed towards the fiber located upstream of the repeater. Unlike the embodiments shown in Figures 2 to 4, the coupler 8 is

  located downstream of the discrete amplifier, ie on the output side of this amplifier.

  In other words, the pumping light for distributed amplification passes through

the discrete amplifier.

  The advantage of the solution of FIG. 5, compared to that of the preceding figures, is the compensation for the loss of insertion of the multiplexer 8. For a

  multiplexer at 1450 nm, the insertion loss is typically of the order of 0.6 dB.

  In the assembly of FIG. 5, this loss of insertion can be completely compensated by an increase in the gain of the discrete amplifier. Compared to the assembly of the preceding figures, one avoids having to increase the gain of discrete amplification to compensate for the loss of insertion of the multiplexer. We

  obtains an improved performance of 0.6 dB, in terms of signal-to-noise ratio. The discrete amplifier 7 of FIG. 5 has an optical coupler 11,

  which couples in the direction of propagation of the signals the light provided by a

  pump 12, for example light at 980 nm or close to this wavelength.

  Downstream in the signal propagation direction, the amplifier has a section 14 of erbium-doped fiber. The pump for discrete amplification has a wavelength close to 1450 nm, or close to this wavelength. For such a wavelength, the crossing of the discrete amplifier takes place essentially

without loss.

  Is not provided in the discrete amplifier of FIG. 5 of isolator, so as to let pass upstream the pumping light for the distributed amplification. It remains possible, if necessary to provide an isolator (not shown) downstream of the multiplexer 8 coupling in the fiber the pump light of the distributed amplifier. FIG. 6 shows a diagram of an embodiment of the invention, with a control of the power of the distributed amplification pump. In the embodiment of FIG. 6, the invention provides a solution to the problem of the aging of the components of the transmission system. It reduces the

  gain distortion, and increased losses.

  For this, the invention proposes to control the amplification means

  distributed, as a function of the input power of the discrete amplification means.

  As the output power of the discrete amplification means is generally controlled so as to remain substantially constant, it can thus be ensured that the discrete amplification gain is substantially constant. The constancy of this gain avoids the gain distortions caused in the systems of the prior art by the gain variations. Indeed, for the same gain variation, the gain excursion in a distributed amplifier is approximately four times lower than the excursion

gain in a discrete amplifier.

  In addition, the invention avoids the degradation of the signal to noise ratio encountered in the systems of the prior art, due to the aging of the

  components, and decreasing the input power of discrete amplifiers.

  The assembly of FIG. 6 is identical to that of FIG. 3. However, provision is made for servo-control of the distributed amplification pump 9, depending on the

  input power into the discrete amplifier 7, as symbolized by the arrow 16.

  To this end, it is possible to use all the control means well known per se to those skilled in the art, to ensure that the input power of the discrete amplifier remains substantially constant. One can for example use the same servo device as that which one can use for the servo of the discrete amplifier. Such a device ensures that the input power remains

  constant, with variations of 0.1 dB from the nominal value.

  FIG. 7 shows a graph of the gain variations as a function of the wavelength, in the embodiment of FIG. 6. The wavelength in nanometers, between 1544 and 1564 nm, is plotted on the abscissa, and in ordered the gain variations, in dB. The graph in FIG. 7 corresponds to a simulation of the variations in gain caused by aging, in a chain of 100 repeaters of the kind of that of FIG. 6. At the start, the amplifiers have a flat gain in the length range of wave concerned. The pump power for the distributed amplification is 100 mW, and the distributed amplification gain is 4

  dB. The discrete amplifier is set to a gain of 6 dB.

  To simulate the losses caused by aging, a loss of 0.4 dB is induced in each line section. The dotted line in the figure shows the variations in gain induced when these losses are compensated by variations in the gain of the discrete amplifiers. There is an expected increase in gain for the shortest wavelengths, and the corresponding decrease in gain for the longest wavelengths. Over the wavelength range

  retained, the gain difference reaches 12 dB.

  The solid line in FIG. 7 shows the variations in gain which appear when the losses are compensated by an increase in the gain of distributed amplification. In this case, the power of the pump used for the distributed amplification goes from 100 to 1115 mW, which allows the loss of 0.4 dB to be absorbed, and the input power of the discrete amplifier remains constant. . The figure

  shows that the gain variations remain below 4 dB.

  From the point of view of the signal to noise ratio, under the same conditions, the fact of compensating the losses due to aging by variations in the gain of the discrete amplifiers causes a deterioration of the signal to noise ratio of 2 dB for all 1 00 repeaters. Conversely, in the solution of the invention, the

  signal-to-noise ratio does not vary significantly.

  In the embodiment of FIG. 6, the distributed amplification pump is located as in FIG. 3 upstream of the discrete amplifier 7. We could as well control the power of the amplification pump distributed in

the assembly of figure 5.

  In the example of FIG. 7, the gain of distributed amplification of all the repeaters has been controlled. We could vary the gain of distributed amplification only in some of the amplifiers, using the fact that a decrease in the gain of the discrete amplifiers induces a gain variation, by lowering the gain for the

  short wavelengths and increasing the gain of long wavelengths.

  Thus, one could remotely control the power of the distributed amplification pump, in some (or all) repeaters. In this case, to compensate for the aging of the link, it becomes possible to increase the gain of distributed amplification of a repeater; as the output power of the associated discrete amplifier is kept constant, the gain of the discrete amplifier is then lowered. This drop induces a gain variation, in the opposite direction to that caused by the aging of the link. It is thus possible, by controlling at

  distance certain repeaters, to compensate for the effects of aging on the link.

  As an example, one can consider as in the case of FIG. 7 losses of 0.4 dB in each section. An increase in the distributed amplification gain of 4 dB in a repeater out of ten could be enough to induce in a repeater out of ten a variation in gain sufficient to compensate for the variations in gain in

the nine adjacent repeaters.

  In such a configuration, the gain of distributed amplifications is no longer controlled, but is controlled remotely, so as to lower the gain of discrete amplification in the corresponding repeater. You can control the necessary value of the amplification gain distributed in the selected repeaters, depending on the

link performance.

  This solution is slightly more disadvantageous than the previous one in terms of signal-to-noise ratio, but it allows a simplification of the structure of part of the repeaters. It avoids providing servo means in repeaters. Remote control of pump supply voltage is simple

  and can be implemented with means known per se.

  Figure 8 shows a block diagram of an embodiment of the invention, suitable for gain equalization, installation of the link; the repeater of Figure 8 is identical to that of Figure 3; however, the pump for distributed amplification has a wavelength of 1475 nm, and a filter 18 is provided between the coupler 8 and the discrete amplifier 7. This filter has increasing losses as a function of the wavelength on the range of the multiplex, in other words a transfer function with a negative slope. The pumping wavelength for distributed amplification is offset by approximately 75 nm from the central wavelength of the range of the multiplex - 1550 nm for wavelengths from 1535 to 1560 nm in the example. More generally, a pump wavelength between 1470 and 1480 for distributed amplification is suitable. The distributed amplifier therefore has a positive slope gain in the wavelength range of the multiplex. In the repeater of FIG. 8, the power of the amplification pump 9

  distributed is controlled remotely, as symbolized by arrow 20.

  The repeater of FIG. 8 can be used like that of FIG. 6 to compensate for the effects of the aging of the components on the link. In addition, in the embodiment of Figure 8, there is provided a fixed filter, which has a negative slope. In this case, the discrete amplifier 7 has a gain substantially equalized over the wavelength range of the multiplex: the known amplifiers can be used in the repeater of the invention. It is also possible to dispense with the filter 18 having a negative slope. It suffices for this to design the discrete amplifier, with a negative slope, capable of being compensated by the variations in the gain of distributed amplification; this solution involves modifying the amplifier, but it avoids the attenuation induced by the negative slope filter and the insertion losses of this filter. FIG. 9 shows a graph of the Raman gain as a function of the wavelength, for different pumping powers. The wavelengths in nanometers are plotted on the abscissa, and the Raman gain in decibels on the ordinate. The different graphs show the gain, for different powers of the distributed amplification pump; to obtain the results of the figure, a pump was used at 1475 nm, and an amplification fiber with a length of 40 km, with an effective surface of 50 im2. The various graphs in the figure show on the one hand that the gain in dB is an almost linear function over a range of almost 30 nm, whatever the pumping power. The graphs further show that the gain slope can vary, between values which in the figure range from 0.025 dB / nm to

0.2 dB / nm.

  As in the embodiment of FIGS. 6 and 7, the amplitude of the correction available depends not only on the gain slopes which the distributed amplification can provide, but also on the gain variations - in the opposite direction - of the discrete amplifier. when the power of the discrete amplification pump decreases. As the distributed amplification gain increases - for example due to remote control of the distributed amplification pump - the discrete amplification gain decreases, so that the output power of the discrete amplifier (and the repeater) remains substantially constant. This drop in discrete amplification gain causes a variation in the gain slope, and decreases the gain for the shortest wavelengths. The gain variations of the discrete amplification and of the distributed amplification therefore go in the same direction, and the invention allows corrections which are greater than the simple gain variations possible thanks to the changes in the pumping power.

for distributed amplification.

  This is clearly shown in Figures 10 to 12, which show gain and loss graphs for the repeater in Figure 8. In each of the figures, we have drawn from left to right: - a Raman gain graph, in dB, in function of the wavelength in nm; - a graph of the losses induced by the fixed filter 18, in dB, as a function of the wavelength in nm; a graph of the discrete amplification gain, in dB, as a function of the wavelength in nm; - a gain graph for the entire repeater, and the amplification

distributed in the fiber upstream.

  Figure 10 shows the case of a pumping power of 200 mW for distributed amplification. This power corresponds to the nominal power setpoint for the repeater, and we see that the amplifier has a flat gain,

  and that the repeater also has a flat gain.

  Figure 1 shows the case of a pumping power of 400 mW for distributed amplification; as explained above, the distributed amplification gain increases, and the slope of this gain also increases. Correspondingly, the gain of the discrete amplifier decreases, and the slope of the gain of the discrete amplifier becomes positive. The gain of the repeater then has a positive slope. This positive slope is capable of compensating for the effects of losses due to aging in the preceding amplifiers. This positive slope can also be used during installation

to adjust the gain of the repeater.

  Figure 12 shows the case of a pumping power of 0 mW for distributed amplification; the amplification gain distributed is zero. Correspondingly, the gain of the discrete amplifier increases compared to the configuration of FIG. 10, and the slope of the gain of the discrete amplifier becomes negative. The gain of the repeater then has a negative slope. This negative slope makes it possible to adapt the gain of the

  repeater when installing the link, especially for terrestrial systems.

  Figures 10 to 12 show that compared to the setpoint configuration of Figure 1 0, the variations of the pump power for the distributed amplification allow to obtain a gain having a negative or positive slope. It is therefore possible, thanks to the invention, to easily adapt the connection, or to compensate for the effects of aging. In the case where the repeater is used only for the adaptation of the link to the installation, it is not essential that the power can be

  remotely controlled, if it can be adjusted during installation.

  The repeater has a slope which can vary by 5 dB over the 25 nm range of the multiplex. By way of comparison, the variations induced on a set of fiber amplifiers doped with erbium by losses of 0.4 dB in the fiber between two repeaters are of the order of 3 dB over a bandwidth of 25 nm . It is therefore sufficient to provide a repeater of the kind of that of FIG. 8 every 20

  repeaters, to be able to largely compensate for the effects of aging.

  The transmission system of FIG. 1 can advantageously have a set of identical repeaters, for example of the type described in FIGS. 2 to 9; it is also possible to provide a combination of distributed amplification and discrete amplification according to the invention only on certain sections, the other sections then implementing known solutions, purely distributed, or purely discrete. Obviously, the invention is not limited to the embodiments described by way of example. The invention can be used not only in a one-way system, but also in a two-way system, in a single-channel system or in a wavelength multiplexing system. It should also be noted that, with respect to the embodiments described, it is possible to add or remove filters or insulators, as required. The type of pumping means used can be adapted to the conditions. You can also use pumps

  redundant, to improve the reliability of repeaters.

  In the examples, wavelengths of 1440 nm or 1450 nm are used for the distributed Raman amplification. Other wavelengths are possible. Similarly, the examples provide for the pumping of the discrete amplifier wavelengths of 980 or 1480 nm. Other wavelengths are possible. In the examples, the pumping in the discrete amplifier is co-propagative. We could also use counter propagative pumping,

  in particular in the case of FIGS. 3 and 4, an isolator 13 is provided.

  Finally, it will be noted that the solutions of FIGS. 6 to 9 apply not only to transmission systems with non-soliton signals, but more generally to transmission systems with signals of any kind, including

soliton signals.

Claims (26)

  1. A fiber optic transmission system with non-soliton signals, comprising a plurality of fiber sections (6i) and repeaters (5i), characterized in that at least one repeater comprises discrete amplification means (7) and means for pre-amplification of the signals in the line fiber (6-1) located upstream of the repeater in the direction of propagation of the signals. 2. Transmission system according to claim 1, characterized in that the distributed preamplification means comprise means (8, 9) for   line fiber pumping (6i-1).   3. Transmission system according to claim 2, characterized in that the line fiber pumping means (6i-1) comprise a pump 1.44 p.m.   4. Transmission system according to claim 3, characterized in that the   power pumped into the fiber is less than 100 mW.   5. Transmission system according to one of claims 1 to 4, characterized in   what the preamplification means include means for   pumping by stimulated Raman effect of the line fiber.   6. Transmission system according to one of claims 1 to 5, characterized in   that the distributed preamplification means (8, 9) are upstream of the   discrete amplification means (7) in the repeater.   7. Transmission system according to one of claims 1 to 5, characterized in   that the distributed preamplification means (8, 9) are downstream of the   discrete amplification means (7) in the repeater.   8. Transmission system according to one of claims 1 to 7, characterized in   that the discrete amplification means comprise a section (14) of fiber strongly doped with erbium, and pumping means (11,12) of this fiber.   9. Transmission system according to claim 8, characterized in that the means for pumping the discrete amplification means comprise a pump at 0.98 or 1.48 ptm.   10. Transmission system according to one of claims 1 to 9, characterized in   what each repeater includes preamplification means   distributed and discrete amplification means.   11. Transmission system according to one of claims 1 to 1 0, characterized   in that the distributed preamplification means have a gain which is between 0.5 and 2 dB.   1 2. Transmission system according to one of claims 1 to 1 1, characterized   in that the distributed preamplification means are controlled as a function of the input power of the discrete amplification means (7)   1 3. Transmission system according to one of claims 1 to 1 1, characterized   in that the distributed preamplification means are controlled remotely.   14. Transmission system according to one of claims 1 to 11, characterized   in that the line fiber pumping means (6i-j) comprise a pump between 1470 and 1480 nm, the power of which is preferably remotely controlled.   15. Transmission system according to claim 14, characterized in that the repeater comprises a filter (18) with a negative slope transfer function. 1 6. Transmission system according to claim 14, characterized in that the   discrete amplification means (7) have a negative slope gain.   17. Method for transmitting non-soliton signals in a line optical fiber transmission system having at least one repeater, the method comprising: - at least one step of distributed preamplification of the signals in the line fiber, and   - at least one step of discrete amplification of the signals in the repeater.   18. The method of claim 17, characterized in that the step of   distributed preamplification includes line fiber pumping.   19. The method of claim 17 or 18, characterized in that the step of   Distributed preamplification takes place by stimulated Raman effect.   20. The method of claim 17, 18 or 19, characterized in that the distributed preamplification step is carried out according to the power of the   signals undergoing the discrete amplification step.   21. The method of claim 17, 18 or 19, characterized in that the step   distributed preamplification is performed with a gain controlled remotely.   22. A repeater (5i) for a fiber optic transmission system with non-soliton signals, comprising discrete amplification means (7) of the signals and pumping means (8, 9) for a distributed preamplification. 23. Repeater according to claim 22, characterized in that the pumping means for a distributed preamplification comprise at least one pump at 1.44 ptm.   24. Repeater according to claim 22 or 23, characterized in that the pumping means for a distributed preamplification comprise at least   a pump for pumping by Raman effect stimulated with a line fiber.   25. Repeater according to claim 22, 23 or 24, characterized in that the discrete amplification means comprise a fiber section (14) heavily doped with erbium, and pumping means (11, 12) of this fiber . 26. Repeater according to claim 25, characterized in that the means for pumping the discrete amplification means comprise a pump 0.98 or 1.48 p.m.   27. Repeater according to one of claims 22 to 26, characterized in that the   pumping means (8, 9) for a distributed preamplification are in   upstream of the discrete amplification means (7).   28. Repeater according to one of claims 22 to 26, characterized in that the   pumping means (8, 9) for a distributed preamplification are in   downstream of the discrete amplification means (7).   29. Repeater according to one of claims 22 to 28, characterized in that the   pumping means (8, 9) for a distributed preamplification are controlled as a function of the input power of the amplification means discreet (7).   30. Repeater according to one of claims 22 to 28, characterized in that the   pumping means (8, 9) for distributed preamplification are remotely controlled.   31. Repeater according to one of claims 22 to 30, characterized in that the   line fiber pumping means (6i-j1) comprise a pump between 1470 and 1480 nm, the power of which is preferably controlled remotely. 32. Repeater according to claim 31, characterized in that the repeater   includes a filter (18) with a negative slope transfer function.   33. Repeater according to claim 31, characterized in that the means   discrete amplification (7) have a negative slope gain.