OPTICAL TRANSMISSION SYSTEM WITH RAMAN AMPLIFIERS COMPRISING A SUPERVISORY SYSTEM

Abstract: An optical transmission system comprising a supervisory system of devices disposed along an optical link is disclosed. Counter-propagating Raman amplifiers are disposed along the optical link. Low-frequency supervisory signals are superimposed on the optical signals traveling on the optical link by suitable modulators in the devices disposed along the line. Counter-propagating Raman amplifiers amplify optical signals and do not introduce a substantial amount of attenuation on the low-frequency supervisory signals.

Description: OPTICAL TRANSMISSION SYSTEM WITH RAMAN AMPLIFIERS COMPRISING A SUPERVISORY SYSTEM The present invention relates an optical transmission system with Raman amplifiers comprising a supervisory system. The present invention also relates to an optical repeater. In an optical transmission system, especially in a submarine optical transmission system, there is the very felt need of supervising the operating state of devices disposed along the optical line, such as for example optical repeaters, branching units and so on. In the particular case of a submarine system, such supervising has to be performed remotely. In the present description and claims, the expression "supervisory signal" is used to indicate either command informations suitable to set predetermined system parameters (such as, for example, the gain and the output power of an optical amplifier), or query informations suitable to check the operating state of a device, or informations on the operating state of said device, or communications between the maintenance and/or supervisory personnel operating in intermediate points of the optical line or in terminal stations of the system. For example, the supervising of the operating state of an optical repeater typically comprises control functions, such as for example the regulation of the value of the current furnished to the pump lasers, or the switching between a working pump laser and a spare pump laser, and so on. Further, the supervising of the operating state of an optical repeater typically also comprises monitor functions, such as for example the monitoring of the operating temperature of the repeater, or the monitoring of the current furnished to the pump lasers, or the monitoring of the input power of the optical signal entering into the repeater and/or the monitoring of the output power of the optical signal exiting from the repeater, and so on. For the purpose of supervising, many kinds of supervisory systems in optical transmission systems have been proposed. For example, EP0504777 discloses a transmission system for transmitting over an optical link an auxiliary signal that is auxiliary relative to a traffic signal which is transmitted simultaneously over said link, the system including at least one equipment including optical amplification means. For the purpose of enabling said equipment to emit the auxiliary signal, the system includes means for modulating the gain of the optical amplification means of said equipment by said auxiliary signal. Optical amplification means disclosed in 777 patent are rare-earth doped fiber amplifiers and semiconductor amplifiers. EP0675610 teaches to modulate the pump radiation of an erbium doped fiber amplifier through a modulating signal carrying supervisory informations. Such modulating signal has a high modulation frequency, that is, a modulation period that is less than the fluorescence time of erbium ions, so as not to affect the gain of the erbium doped fiber amplifier. In this way, the supervisory informations are sent using as optical carrier the excess pump radiation that does not contribute to the erbium doped fiber amplifier pumping. US5625481 teaches to modulate the spontaneous emission of an erbium-doped optical fiber amplifier with a supervisory signal through a band pass optical filter whose transmission characteristic is changed in function of the supervisory signal. US 6111687 teaches to use a band pass optical filter for modulating an optical signal in output from an optical amplifier with such amplitude and frequency as to not disturb the data transmission performed by the optical signal. Such modulation allows the optical amplifier to transmit supervisory messages. EP0751635 describes a supervisory system for a WDM optical communication system for transmitting a command signal from a terminal station to an erbium- doped optical fibre amplifier and response signals from an erbium-doped optical fibre amplifier to the terminal station. A first method described for transmitting the command signal consists in using the same command signal to directly modulate, one by one, a plurality of optical sources that generate laser beams at different wavelengths. The laser beams at different wavelengths are then externally modulated by the respective main signals to be transmitted along the system and thus, wavelength multiplexed. According to a second method, the laser beams at different wavelengths are first externally modulated by the respective main signals, then they are wavelength multiplexed in a single WDM optical signal; afterwards, the latter is externally modulated in function of the command signal through a lithium niobate modulator (LiNbO3). On the other hand, as regards the response signals sent by the erbium-doped optical amplifiers to the terminal stations, they are transmitted by directly modulating the pump source of the optical amplifiers in function of the response signal to be transmitted so as to modulate the gain of the erbium-doped optical amplifiers. The command signals have a frequency in the range of 10 MHz whereas response signals have a frequency in the range of KHz. Fiber Raman amplifiers have been attracting a great attention, because of their capability to increase the transmission capacity and/or repeaterless span lengths. Raman amplification is an amplification by which energy is transferred from an electromagnetic pump wave to a lower frequency signal wave via a molecular vibration. The responsible mechanism is stimulated Raman scattering (SRS). Raman amplifiers offer several advantages, such as a low noise, a greater flexibility in choosing the signal wavelength and a broad gain bandwidth. The greater flexibility in choosing the signal wavelength mainly depends on the fact that the Raman peak of a material, exploited for the amplification of the signal, is dependent practically only on the pump wavelength, differently from what happens for example in erbium-doped fiber amplifiers, in which the choice of the signal wavelength is restricted by the stimulated emission cross-section of the erbium. The broad gain bandwidth of Raman amplifiers can be much enlarged, for example by using multiple pump sources. Such a broad gain bandwidth may represent the possibility to extend the usable optical bandwidth outside the conventional C-band and the extended L-band of the erbium-doped fiber amplifiers. The Applicant has faced the problem of implementing a supervisory system in an optical system comprising Raman amplifiers. It is known that the gain of a Raman amplifier may be modulated according to a supervisory signal to be sent on optical link. For example, US6188508 discloses a control signal superimposer for superimposing a control signal on a signal light, comprising: a pumping light source for generating a pumping light with intensity fluctuation in accordance with the control signal; a Raman amplification medium pumped by the pumping light from the pumping light source for Raman-amplifying the signal light; a combiner for combining the pumping light output from the pumping light source and the signal light to be Raman-amplified and then supplying them to the Raman amplification medium; and an optical filter for extracting the signal light component from the output light of the Raman amplification medium and terminating the pumping light component. In the '508 patent it is disclosed that in order to superimpose the control signal on the 1.5 μm band signal light through the Raman amplification, it is necessary to propagate the 1.5 μm band signal light desired to be amplified and the pumping light in the same direction. A control signal superimposer of the kind disclosed in '508 patent is disclosed also in the article of H. Maeda et al., "Remote Supervisory System of FSA-WDM System", NTT Review, vol.12, n.4 (2000), pages 25-31. A control command corresponding to a repeater supervision item is issued. The terminal equipment converts the received command to a supervisory signal of low-speed subcarrier and superimposes the supervisory signal on the WDM signal for transmission along a transmission line with repeaters. A repeater sends the response signal to the terminal equipment at both ends of the transmission line, or performs repeater state control. Raman amplification is exploited for supervisory signal transmission from the terminal equipment. The configuration of the supervisory signal modulator uses dispersion shifted fiber (DSF) for Raman amplification. A subcarrier-ASK signal is used to intensity-modulate the 1.48 μm pumping source. The resulting signal co-propagates along the DSF with the WDM signal and Raman-amplifies the WDM signal according to the subcarrier-ASK signal. This WDM signal is then sent to the submarine transmission line. A repeater response signal from the submarine optical repeater is superimposed on the amplified spontaneous emission (ASE) light produced by the repeaters when the system has failed or is out-of-service. EP1122898 discloses an optical repeater monitoring system comprising an oscillating source, a reference signal transmitter for transmitting a reference signal of a predetermined frequency generated by an output of the oscillating source to a first optical fiber, and an optical repeater. The optical repeater has a first photodetector for converting light from the first optical fiber into an electrical signal, a reference signal extractor for extracting a component of the reference signal from an output of the first photodetector, a carrier generator for generating a carrier from an output of the reference signal extractor, a monitor signal modulator for modulating the carrier generated by the carrier generator with a monitor signal showing an operating state of the optical repeater, a transmitter for transmitting an output of the monitor signal modulator to a second optical fiber. Optical amplifiers (e.g. optical amplifiers using erbium-doped optical fiber), comprised in the optical repeater, are pumped by a pumping circuit and then optically amplify signal lights from the first and the second optical fiber. The output of the monitor signal modulator is applied to the pumping circuit. The pumping circuit weakly modulates the intensity of the pumping light to be transmitted to the optical amplifiers according to the output from the monitor signal modulator. The gain of the optical amplifiers is thus modulated by the output of the monitor signal modulator in order to transmit the monitor signal to a terminal station. In the '898 patent application, it is disclosed that such technique is also applicable to Raman amplification: in such case, pumping light for leading the Raman amplification within the wavelength band of the signal light is applied to the optical fiber line and the intensity of the pumping light is modulated with the output of the monitor signal modulator. As a result, the gain of the signal light propagating on the optical fiber line fluctuates according to the output of the monitor signal modulator and thus brings the same effect with the case in which the gain of the optical amplifiers is fluctuated. However, the Applicant has found that a signal light modulated with a low frequency subcarrier entering in a Raman amplifier behaves in different manners if the pump radiation co-propagates with the signal light (that is, propagates in the same direction of the signal light) or counter-propagates with respect to the signal light (that is, propagates in the opposite direction with respect to the signal light). In fact, if the signal light and the pump radiation counter-propagate in the Raman amplifying medium, the low frequency subcarrier superimposed to the signal light is substantially left unchanged if the low frequency subcarrier lays in a band of frequencies higher than some kHz, that is, in a typical band of frequencies for a supervisory signal. On the other hand, if the signal light and the pump radiation co- propagate in the Raman amplifying medium, the low frequency subcarrier superimposed to the signal light may be attenuated in a substantial manner. This may cause the loss of the supervisory signal at the end of a chain of co- propagating Raman amplifiers. Herein and in the following, by "co-propagating Raman amplifier" it is to be intended an amplifier in which the pump radiation and the light to be amplified propagate in the Raman medium (e.g. an optical fiber) in the same direction; by "counter-propagating Raman amplifier" it is to be intended an amplifier in which the pump radiation and the light to be amplified propagate in the Raman medium in opposite directions. As disclosed in the US patent 6188508 cited above, a low frequency modulation of the pump source providing the pumping radiation of a Raman amplifier superimposes such frequency modulation to a signal light amplified in the Raman amplifier if the pump radiation and the signal light co-propagate in the Raman medium. On the other hand, the Applicant has verified that if the pump radiation and the signal light counter-propagate in the Raman medium, no superposition of the low frequency modulation on the signal light practically occurs at frequencies of the order of one kHz and above. In such conditions, a supervisory signal carrying a sufficient quantity of informations could hardly be superimposed to a signal light by modulation of the gain of a counter-propagating Raman amplifier. Summarizing what said above, the Applicant has found that in an optical transmission system comprising a chain of Raman amplifiers, the following problems may arise, for the implementation of a supervisory system in the optical transmission system: a) if the Raman amplifiers are co-propagating, each of them could be gain- modulated in order to superimpose a supervisory signal to a signal light propagating through the chain. However, each Raman amplifier would substantially attenuate a supervisory signal superimposed on the signal light by a previous Raman amplifier of the chain or Raman amplifiers or by a terminal station, so that such supervisory signal may be lost at the end of the chain of Raman amplifiers; b) if the Raman amplifiers are counter-propagating, a supervisory signal superimposed to a propagating signal light may pass through the chain of Raman amplifiers with substantially no attenuation. However, the counter- propagating Raman amplifiers may hardly be gain-modulated in order to provide such supervisory signals by superposition on the signal light. The Applicant has found that such problems may be solved by implementing a supervisory system in which: a) the Raman amplifiers disposed along the optical line are counter- propagating; and b) a suitable modulator, other than the pump source for Raman amplification, is provided in the optical repeaters, that is in the places of the optical line in which the pumping radiation for Raman amplification is added to the line, (and/or in other devices disposed along the optical line necessitating monitoring, such as for example in branching units) in order to superimpose a supervisory signal to the signal light. In a first aspect thereof, the invention relates an optical transmission system comprising at least a first and a second terminal station, optically connected with each other by an optical link, said first or said second terminal station being adapted to send on said optical link at least a first optical signal having a first direction, said system further comprising: - a first supervisory unit associated to a device disposed along said optical link, said first supervisory unit being adapted for generating a first supervisory signal, said first supervisory unit being further associated to at least one modulator being adapted to superimpose on said first optical signal said first supervisory signal; characterized in that said system further comprises: - at least one pump source disposed along said optical link, said pump source being adapted to send on said optical link a pump radiation in a second direction opposite to said first direction, so as to cause Raman amplification of said first optical signal and of said first supervisory signal superimposed on said first optical signal. In a second aspect thereof, the invention relates an optical repeater comprising at least a first optical fiber adapted to carry a first optical signal in a first direction, at least a first modulator connected to said first optical fiber and a supervisory unit associated to said first modulator, said supervisory unit being adapted to generate a first supervisory signal, said first modulator being adapted to superimpose said first supervisory signal on said first optical signal. Said optical repeater further comprises at least a first pump source connected to said first optical fiber, said first pump source being adapted to send on said first optical fiber a pump radiation in a second direction opposite to said first direction, said pump radiation being adapted to cause Raman amplification of said first optical signal on said first optical fiber. In a third aspect thereof, the invention relates to a method for supervising an optical transmission system comprising an optical link between at least a first and a second terminal station, said method comprising: transmitting a first optical signal on said optical link in a first direction; superimposing a first supervisory signal on said first optical signal; characterized in that said method further comprises: - sending on said optical link a pump radiation in a second direction opposite to said first direction, so as to cause Raman amplification of said first optical signal and of said first supervisory signal superimposed on said first optical signal. Further features and advantages of the present invention will be better illustrated by the following detailed description, herein given with reference to the enclosed drawings, in which: - Figure 1 schematically shows an optical transmission system according to the invention; - Figure 2 schematically shows a preferred embodiment of a repeater according to the invention; - Figure 3 schematically shows an exemplary supervisory unit suitable for a repeater according to the invention; - Figure 4 schematically shows an exemplary supervisory unit suitable for a terminal station of an optical system according to the invention; - Figure 5 schematically shows a first experimental setup used by the Applicant for evaluating how efficiently a counter-propagating Raman amplifier may allow the passage of a low-frequency supervisory signal superimposed on an optical signal; - Figures 6 and 7 show the results of two measures made with the experimental setup of fig.5; - Figure 8 schematically shows a second experimental setup used by the Applicant for evaluating how efficiently a co-propagating Raman amplifier may allow the passage of a low-frequency supervisory signal superimposed on an optical signal; - Figure 9 shows the result of one measure made with the experimental setup of fig.8; - Figure 10 schematically shows a third experimental setup used by the Applicant for evaluating how efficiently a counter-propagating Raman amplifier may generate a low-frequency supervisory signal by modulation of its gain; - Figure 11 shows the result of one measure made with the experimental setup of fig.10; - Figure 12 schematically shows a third experimental setup used by the Applicant for evaluating how efficiently a co-propagating Raman amplifier may generate a low-frequency supervisory signal by modulation of its gain; - Figure 13 shows the result of one measure made with the experimental setup of fig.12; - Figures 14 and 15 show plots of the attenuation introduced on the modulation depth index at the exit of a counter-propagating Raman amplifier with modulation of its gain (squares) and of a co-propagating Raman amplifier with modulation of its gain (dots), respectively with an input power of the optical signal of 15 dBm and 5 dBm. The Applicant has performed a series of experiments in order to understand: a) how a Raman amplifier may allow the passage-through of a low-frequency supervisory signal superimposed on an optical signal; b) how a Raman amplifier may generate a low-frequency supervisory signal by modulation of its gain. Experiments for both points a) and b) have been carried out both with co- propagating and counter-propagating configurations of Raman amplifiers. Experiment 1 Fig.5 shows the experimental setup used for point a) with a counter-propagating configuration of Raman amplifier. A wide bandwidth laser source 50 (about 30 nm) emitting an optical continuous wave signal around 1550 nm was connected to a low-frequency modulator 51 and then to 55 km of a standard single mode (SM) optical fiber 52, used as Raman medium. The wide bandwidth laser source 50 was used in order to simulate a WDM signal comprising different wavelength channels. Low-frequency modulator 51 was a magneto-optical attenuator of the YS-500 type, produced by FDK Corporation. A laser source 53 was provided for emitting a pump radiation having a wavelength of 1450 nm and coupled to the optical fiber 52, through a WDM coupler 54, on the opposite side with respect to the wide bandwidth laser source 50. A photodiode 55 was coupled to a second end of the WDM coupler 54. A network analyzer 56 was connected to the low- frequency modulator 51 and to the photodiode 55. Variable attenuators 57, 58, 59 were provided for regulating the optical power entering into the Raman medium 52 and into the photodiode 55. The network analyzer 56 applied to the low-frequency modulator 51 signals having frequencies comprised between 100 Hz and 1 MHz and evaluated the response at the photodiode. The measure was conducted by previously measuring the response with the pump source switched off, in order to calibrate the network analyzer 56, and then by switching on the pump source. The network analyzer thus gave the difference between the response with the pump source switched on and the response with pump source switched off, that is the attenuation on the low-frequency signal introduced by the Raman amplification. In order to avoid any non-linear behavior of the photodiode 55 when the pump was switched off and when the pump was switched on, the attenuator 59 was regulated so that in both cases the same power was fed to the photodiode 55. The measure was first carried out with an input power of the signal of +5 dBm, roughly corresponding to the optical power of 10 WDM channels having a power- per-channel of -5 dBm. The pump power was regulated in order to obtain a Raman gain of 6.15 dB. Substantially no attenuation for the low-frequency signal was observed in the whole range of frequencies between 100 Hz and 1 Mhz. Fig.6 shows the result visualized by the network analyzer. In the x-axis the frequency range between 100 Hz and 1 MHz is shown. In the y-axis the attenuation on the low-frequency signal is shown. Each division in the y-axis corresponds to an attenuation of 2 dB. Marker 60 is positioned at 10 kHz and highlights a value of 0.0075 dB. The measure was then earned out with an input power of +15 dBm, roughly corresponding to the optical power of 100 WDM channels having a power-per- channel of -5 dBm. The pump power was regulated in order to obtain a Raman gain of 5.94 dB. Even in this case no substantial attenuation was observed for the low-frequency signal at frequencies higher than about 1 kHz. For frequencies lower than about 1 kHz, an attenuation of about 0.5 dB was observed. The Applicant has verified that such attenuation becomes higher with the growing of the gain. In another measurement, made with 40 km of SM fiber and a more powerful pump source in a setup according to fig.5, an input power of 17.95 dBm was launched in the SM fiber, together with a counter-propagating pump power regulated for obtaining a Raman gain of 11 dB. Fig.7 shows the result visualized by the network analyzer (in this case the x-axis ranges from 10 Hz to 1 MHz). A high-pass behavior of the counter-propagating Raman amplifier is well visible. At frequencies higher than about 10 kHz the attenuation is substantially negligible. At frequencies below 1 kHz (e.g. at the position of the marker 70 at about 200 Hz) the attenuation is about 5 dB, so that the low-frequency signal can be lost. However, it has to be noticed that a threshold frequency of 5-10 kHz is definitely acceptable for a supervisory signal. The oscillations in the lower frequency band visible in fig.7 are due to measure errors caused by the network analyzer. In conclusion, this first series of measures shows that a counter-propagating Raman amplifier does not introduce a significant amount of attenuation on a low- frequency signal superimposed on an optical signal at typical frequencies of supervisory signals. With high input power and high gains, the Raman amplifier has a high-pass behavior: however, the threshold frequency is sufficiently low to allow the implementation of a supervisory system. Experiment 2 Fig.8 shows the experimental setup used for point a) with a co-propagating configuration of Raman amplifier. The same reference numbers of fig.5 are used for corresponding elements. A further WDM coupler 54' was added in order to extract and terminate with a connector having low back-reflection the residual pump power radiation at the output of the fiber 52. The following table 1 shows the results obtained in different measures made with different lengths of Raman fiber, different input powers of the optical signal and different Raman gains. Table 1 As it can be seen, an attenuation of more than 1.7 dB is introduced on the low- frequency signal in all cases. Further, the higher the gain, the higher the attenuation. Fig.9 shows the result visualized by the network analyzer in the worst case (second row in table 1): the frequency range is from 100 Hz to 1 MHz. As it can be seen, almost the same attenuation is introduced in the whole range of frequencies. The marker 90, positioned at 10 kHz, highlights the attenuation value of 3.49 dB reported in table 1. In conclusion, the second series of measures shows that a co-propagating Raman amplifier introduces a significant amount of attenuation on a low-frequency signal superimposed on an optical signal at typical frequencies for supervisory signals. This may cause the loss of the supervisory signal in a chain of co-propagating Raman amplifiers comprising even few amplifiers. Experiment 3 Fig.10 shows the experimental setup used for point b) with a counter-propagating configuration of a Raman amplifier. The same reference numbers of fig.5 are used for corresponding elements. For this series of measures, the low-frequency modulator 51 was connected to the Raman pump source, in order to modulate the Raman gain. Fig.11 shows the result visualized at the network analyzer in a frequency range between 10 Hz and 1 MHz, for an input power of the optical signal in the Raman medium of 18 dBm, a Raman gain of 0.7 dB and a modulation depth applied by the low-frequency modulator 51 of 17.7%. Each division in the y- axis corresponds to an attenuation of 10 dB. The length of the Raman fiber was 40 km. As it can be seen, a low-pass behavior is shown, with a threshold of about 1 kHz, definitely unacceptable for supervisory purposes. Thus, a counter-propagating Raman amplifier does not allow the generation of a supervisory signal of typical frequencies by modulation of its gain. Experiment 4 Fig.12 shows the experimental setup used for point b) with a co-propagating configuration of a Raman amplifier. The same reference numbers of fig.8 are used for corresponding elements. Fig.13 shows the result visualized at the network analyzer in a frequency range between 10 Hz and 1 MHz, for an input power of the optical signal in the Raman medium of 18 dBm, a Raman gain of 1 dB and a modulation depth applied by the low-frequency modulator of 17.7%. The length of the Raman fiber was 40 km. As it can be seen, no low-pass behavior is shown this time. Thus, a co-propagating Raman amplifier allows the generation of a supervisory signal of typical frequencies by modulation of its gain. Experiment 5 A further series of measures similar to those shown with reference to experiments 3 and 4 was done in order to evaluate the attenuation introduced on the modulation depth index m, both in counter-propagating and co-propagating configurations. The pump laser was modulated with different frequencies and with different modulation depths in a range comprised between 10 Hz and 1 MHz: for each frequency and modulation depth, the attenuation introduced on the modulation depth index m at the exit of the Raman medium was evaluated. Table 2 resumes the obtained results with an input power of the optical signal in the Raman medium of 15 dBm and a Raman gain of 3 dB, in co-propagating configuration. Table 2 Table 3 resumes the obtained results with an input power of the optical signal in the Raman medium of 15 dBm and a Raman gain of 2.5 dB, in counter- propagating configuration. As it can be seen, at frequencies over 1 kHz the attenuation becomes higher and higher, so that it is not possible to obtain, at the exit of the Raman medium, an acceptable modulation depth. Table 3 Fig.14 shows the obtained results in a plot having the frequency in the x-axis and the attenuation on m in the y-axis. The curve with dots represents the behavior of the co-propagating Raman amplifier with gain modulation, the curve with squares represents the behavior of counter-propagating Raman amplifier with gain modulation. Fig.15 shows the results of a measure obtained according to what stated above, obtained by reducing the input power in the Raman medium to 5 dBm. As it can be seen, similar behaviors are obtained with respect to fig.14 for the co-propagating and counter-propagating configurations. Implementation of the Supervisory System Fig.1 schematically shows an optical transmission system 10 according to the invention, comprising two terminal stations 11a, 11b, typically adapted to transmit and receive optical signals over an optical link 12. For these purposes, terminal stations 11a, 11b each comprise at least one transmitter 13a, 13b and at least one receiver 14a, 14b. For WDM transmission, terminal stations 11a, 11b comprise a plurality of transmitters and receivers, for example twenty or thirty-two or sixty-four or one hundred transmitters and receivers. Typically, each transmitter 13a, 13b comprises a laser source, adapted to emit a continuous wave optical signal having a predetermined wavelength, and an external optical modulator, for example a lithium niobate modulator, adapted to superimpose on the continuous wave optical signal emitted by the laser source a traffic signal at a predetermined high frequency or bit rate, such as for example 2.5 Gbit s, 10 Gbit s or 40 Gbit s. Alternatively, the laser source may be directly modulated with the traffic signal. A preferred wavelength range for the optical signal is between about 1460 nm and about 1650 nm. A more preferred wavelength range for the optical signal is between about 1520 nm and about 1630 nm. Typically, in case of WDM transmission each transmitter may also comprise a variable optical attenuator, adapted to set a predetermined power level for each signal wavelength (pre-emphasis level). Preferably, the traffic signal may be coded with a FEC (Forward Error Correction) code in order to maintain good transmission characteristics of the optical signal (e.g. low bit-error-rate and/or signal-to-noise- ratio). In case of WDM transmission, the different signal wavelengths emitted by the plurality of transmitters 13a, 13b are multiplexed by a suitable multiplexing device 15a, 15b on a single optical path. Such multiplexing device can be any kind of multiplexing device (or combination of multiplexing devices), such as a fused fiber or planar optics coupler, a Mach-Zehnder device, an AWG (Arrayed Waveguide Grating), an interferential filter, a micro-optics filter and the like. Each receiver 14a, 14b is adapted to convert an incoming optical signal in an electrical signal. Typically, this task may be provided by a photodetector. The receiver may also discriminate the traffic signal from the electrical signal. For a WDM transmission, a plurality of photodetectors is provided. A demultiplexing device 16a, 16b allows to separate the different signal wavelengths from a single optical path to a plurality of optical paths, each terminating with a receiver. The multiplexing device can be any kind of demultiplexing device (or combination of demultiplexing devices), such as a fused fiber or planar optics coupler, a Mach- Zehnder device, an AWG (Arrayed Waveguide Grating), an interferential filter, a micro-optics filter and the like. At least one supervisory unit 17a, 17b is provided in the terminal stations 11a, 11b. The supervisory units 17a, 17b of the terminal stations are adapted at least to receive a portion of an optical signal coming from the optical link 12 and to discriminate from said portion of optical signal a supervisory signal superimposed by devices disposed along the optical link 12 on the optical signal, such superposition being carried out by techniques explained in the following. Typically, the supervisory units 17a, 17b of the terminal stations also provide for the generation of a supervisory signal to be superimposed on an optical signal before sending of the same on the optical link 12 by the terminal stations 11a, 11b. The terminal stations 11a, 11b, typically comprise also a transmitter amplifier, preferably an optical amplifier (not shown), for amplifying the optical signal before sending of the same on the optical link 12. The transmitter amplifier is adapted to set the power of the optical signal to a suitable level, in order to counteract the attenuation introduced by a first portion of optical link 12. For a WDM transmission, the transmitter amplifier is typically disposed downstream the multiplexing device 15a, 15b. The terminal stations 11a, 11b, typically also comprise a receiver amplifier, preferably an optical amplifier (not shown), for amplifying the optical signal coming from the optical link 12 before sending the same toward the receiver 14a, 14b. The receiver amplifier is adapted to set the power of the optical signal to a suitable level, for example in order to counteract the attenuation introduced on the optical signal by the last portion of the optical link 12 and/or by the optical devices comprised in the receiver 14a, 14b. For a WDM transmission, the receiver amplifier is typically disposed upstream the demultiplexing device 16a, 16b. The optical link 12 typically comprises optical fibers 18. More particularly, the optical link 12 typically comprises a first optical fiber connecting the transmitter (or transmitters) 13a of the terminal station 11a to the receiver (or receivers) 14b of the terminal station 11 b, and a second optical fiber connecting the transmitter (or transmitters) 13b of the terminal station 11b to the receiver 14a (or receivers) of the terminal station 11a. Optical fibers used in the optical link 12 are typically single mode fibers. For example, they can be standard single mode optical fibers (SMF), having chromatic dispersion lying between approximately +16 ps/(nm-km) and +20 ps/(nrτvkm) at a wavelength of 1550 nm, or dispersion-shifted fibers (DSF), having a dispersion approaching zero at a wavelength of 1550 nm, or NZD (non-zero dispersion) fibers, with dispersion of between approximately 1 ps/(nrτvkm) and 4.5 ps/(nnrvkm), in absolute value, at a wavelength of 1550 nm, or fibers of the HDS (half-dispersion-shifted) type having a positive dispersion which is intermediate between that of an NZD type fiber and a standard single-mode fiber. In order to counteract four-wave-mixing possibly occurring in WDM transmission, the fibers comprised in the optical link 12 may be disposed according to so-called "dispersion maps", that may include spans including NZD fibers and/or SMF fibers and/or HDF fibers having a positive dispersion, alternating with spans including dispersion compensating (DCF) fibers or NZD fibers having a negative dispersion. DCF fibers having a reversed dispersion slope may be used in the transmission system to reduce the dependence of the average dispersion on wavelength. In order to reduce the occurrence of FWM, the optical fibers included in the optical link 12 may have a dispersion which is greater than or equal to approximately 1 ps/(nm km) in absolute value at a wavelength of 1550 nm. Dispersion compensating gratings may be also used for dispersion compensation, preferably fiber gratings. The optical link 12 comprises at least one repeater 19 comprising at least one pump source coupled to the optical link 12 in order to send on the optical link 12 a pump radiation in a direction opposite to the direction of the optical signal traveling on the optical link 12, so as to cause counter-propagating Raman amplification of the optical signal on at least a portion of the optical link 12 upstream the pump source. Fig.2 schematically shows a preferred embodiment of a repeater 19 according to the invention, being adapted to be disposed along an optical link comprising a first fiber 20a and a second fiber 20b. The first fiber 20a is adapted for transmitting optical signals λ' SιS up in a forward direction (from terminal station 11a to terminal station 11b, using the reference numbers of fig.1), while the second fiber 20b is adapted for transmitting optical signals λ" s , sup in a backward direction (from terminal station 11b to terminal station 11a, using the reference numbers of fig.1). The terms "forward" and "backward" are used only for exemplary purposes. In fig.2 optical connections have been represented with continuous lines, while electrical connections have been represented by dashed lines. The repeater 19 comprises at least a first pump source 21a, adapted for emitting a pump radiation having a wavelength λ p . The pump radiation emitted by the first pump source 21a is coupled on the first optical fiber 20a through a WDM coupler 22a, in a direction opposite to the direction of an optical signal traveling on the first optical fiber 20a, so as to cause counter-propagating Raman amplification of the same on the portion of the first fiber 20a upstream the pump source 21a. The wavelength λp of the pump radiation emitted by the pump source 21a is shifted in a lower wavelength region with respect to the wavelength of the optical signal of a quantity approximately equal to the Raman-Stokes frequency shift (see G.P Agrawal, "Nonlinear fiber optics", Academic Press Inc. (1995), pag.317-9) of the core material of the portion of first optical fiber 20a upstream the pump source 21a. For example, for silica-germania fibers the Raman-Stokes frequency shift is approximately equal to 13.2 THz. This corresponds, in a wavelength region around 1500-1600 nm, to a wavelength shift at lower wavelengths of about 100 nm. Thus, if the optical signal has a wavelength lying in the 1.5 μm band, a pump wavelength lying in the 1.4 μm band may be used, for example a pump wavelength of 1450 nm or even 1480 nm. The first pump source 21a may comprise a single pump laser or multiple pump lasers, according to the needs. Multiple pump lasers having different wavelengths may be used, for example, in order to obtain a wide and flat gain bandwidth, which is advantageous for a WDM transmission. In order to reduce polarization effects, preferably two pump lasers having substantially the same wavelength emission and substantially the same pump power are used for each pump wavelength, such two pump lasers being coupled together through a polarization beam splitter, so that a polarization independent pump radiation is outputted by the polarization beam splitter. The pump power emitted by each pump laser is typically higher than 200 mW. Preferably, the repeater 19 further comprises a second pump source 21b, coupled to the second optical fiber 20b through a WDM coupler 22b, adapted for sending a pump radiation on the second optical fiber 20b in a direction opposite to the direction of an optical signal traveling on the second optical fiber 20b, so as to cause counter-propagating Raman amplification of the same on the portion of the second fiber 20b upstream the pump source 21b. As far as the pump wavelength or wavelengths, the pump power, the number of pump lasers comprised in the pump source and the coupling of the pump lasers of the second pump source 21 b, reference is made to what stated for the first pump source 21a. In order to enhance the Raman effect in the portion of the first optical fiber 20a upstream the first pump source 21a and/or in the portion of the second optical fiber 20b upstream the second pump source 21 b, the effective area of the fiber portion (or portions) may be preferably lower than or equal to about 80 μm 2 , more preferably lower than or equal to 50 μm 2 . The Raman amplification may occur in a substantial portion of a fiber span between two consecutive repeaters in the optical link or between a repeater and a terminal station (distributed Raman amplification), or in a small portion (not more than 5-10 km) of the same fiber span (lumped Raman amplification). For lumped Raman amplification, the effective area of the fiber portion in which Raman amplification occurs should preferably be lower of the values reported above, for example lower than or equal to 20 μm 2 , in order to avoid use of too high pump powers. In case of a lumped fiber Raman amplifier, the length of fiber acting as Raman medium may be comprised in the repeater 19, for example by winding the same on a suitable drum. Coming back to fig.2, the repeater 19 further comprises a first low frequency modulator 23a connected to the first optical fiber 20a. In fig.2, the first low frequency modulator 23a is connected to the first optical fiber 20a downstream the coupling of the first pump source 21a. In an alternative embodiment, not shown, the first low-frequency modulator 23a may be connected to the first optical fiber 20a upstream the coupling of the pump source 21a. The modulator 23a is adapted to superimpose on the optical signal traveling on the first optical fiber 20a a low- frequency signal, according to a supervisory information. The expression "low- frequency" has to be intended relative to the frequency (or bit rate) of the traffic signal carried by the optical signal: a preferred range of frequencies for the low- frequency supervisory signal is up to about 10 MHz for a bit rate of the traffic signals higher than 1 Gbit s. For higher traffic bit rates (for example 10 or 40 Gbit/s), the low-frequency supervisory signal may also reach higher frequency values, such as 50-100 MHz. In order to superimpose a sufficient quantity of informations, the supervisory signal may be preferably have a frequency higher than or equal to 5 kHz, more preferably higher than or equal to 10 kHz. The modulator 23a may be any kind of device being capable of superimposing a low-frequency signal on the optical signal traveling on the first fiber 20a. The low- frequency supervisory signal is added to the optical signal with a modulation depth preferably not lower than 2%, more preferably not lower than 4%. In order to avoid disturbances on the traffic signal carried by the optical signal, the modulation depth of the supervisory signal may be preferably lower than or equal to 25%, more preferably lower than 20%, even more preferably lower than 10%. For example, the modulator may be a lithium niobate or semiconductor modulator. In an alternative embodiment, the modulator may be a tunable band pass filter, as described in the patent US6111687. In a preferred embodiment, the modulator 23a may be a variable optical attenuator, modulated at a certain modulation depth about an operating point. The operating point of the variable optical attenuator may be chosen in order to regulate one or more operating characteristics of the repeater, such as the output power of the repeater (that is, for Raman amplification, the input power of the optical signal in the fiber span following the repeater itself). More preferably, the variable optical attenuator may be a magneto-optical variable attenuator, which advantageously may be embodied in a very compact device and can be piloted with very low driving currents of the order of tens of mA. Further, magneto-optical attenuators have proven to be very reliable, making them suitable, in particular, for use in submarine systems. A suitable magneto-optical attenuator usable in the repeater according to the invention in order to superimpose on the optical signal a supervisory signal is the model YS-500, produced by FDK Corporation, which can allow supervisory signal modulation up to frequencies of about 300 kHz without substantial variation of the modulation depth over a wavelength range comprised between about 1530 nm and 1560 nm. In a further preferred embodiment, the modulator 23a may be an erbium-doped fiber amplier (EDFA), pumped by a further own pump source (not shown in fig.2), emitting a pump radiation modulated with the low-frequency supervisory signal. The wavelength of the EDFA pumping radiation may be about 1480 nm or 980 nm. Such modulation of the pump source of the EDFA causes a modulation of the gain of the EDFA. In such way, a supervisory signal having a frequency starting from some kHz to about 10-15 kHz can be superimposed on the optical signal. The use of EDFAs in an optical system comprising counter-propagating distributed Raman amplifiers may be advantageous, as it can allow to regulate the optical signal power entering in a fiber span, in order to counteract the attenuation introduced by a first portion of the span, that is in a portion where the power of the counter- propagating pump radiation for Raman amplification is low. This can allow to use lower pump power laser sources for counter-propagating Raman amplification. When using an EDFA for amplification of the optical signal and/or modulation of the supervisory signal, it may be taken into account that an EDFA acts as a high- pass filter on a low-frequency supervisory signal already added to the optical signal traveling on the first optical fiber 20a, for example added by a previous repeater disposed along the optical link or by a terminal station of the optical system. However, the EDFA acts as low-pass filter for the generation of the supervisory signal by gain modulation. This means that supervisory signals having frequencies higher than 10-15 kHz may be hardly generated by modulation of the gain of an EDFA. On the other hand, a supervisory signal having a frequency lower than 10-15 kHz may be attenuated in the passage through an EDFA, as the latter acts, as said, as a high-pass filter. In such case, a regeneration of the supervisory signal in the repeater may be provided. In other words, the already added supervisory signal may be dropped, received and electrically amplified in the repeater and then re-applied for modulating the pump source of the EDFA, as it will be explained in more detail in the following. In a further embodiment, the modulator 23a may be a semiconductor optical amplifier, which driving current is modulated with the supervisory signal. In such way, modulation of the gain of the semiconductor optical amplifier with the supervisory signal is provided, so that the supervisory signal is superimposed to the optical signal. In a further preferred embodiment, the modulator 23a may be a co-propagating Raman amplifier. In other words, the modulator 23a may comprise a further pump source (not shown in fig.2) emitting a pump radiation having a wavelength shifted with respect to the wavelength of the optical signal traveling on the first optical fiber 20a according to the Raman-Stokes frequency shift, being coupled to the first optical fiber 20a, through a suitable WDM coupler, in the same direction of the optical signal, so as to cause co-propagating Raman amplification of the optical signal in at least a portion of the first optical fiber 20a downstream the repeater 19. In order to superimpose the supervisory signal, the pump source is modulated with the supervisory signal, so that the gain of the co-propagating Raman amplifier is modulated with the supervisory signal. Such modulation is then transferred to the optical signal. In such way, supervisory signals having a frequency up to several tens of MHz can be advantageously superimposed on the optical signal, as the Raman effect has a very fast response time. More details on the use of a co- propagating Raman amplifier as supervisory signal superimposer may be found in the above cited US patents 6188508. In order to superimpose the supervisory signal, the co-propagating Raman amplifier need not to be a high gain Raman amplifier: it can even add to the optical signal a small gain lower than about 1 dB. Values of 0.2-0.5 dB may be sufficient to impart a suitable modulation depth to the optical signal according to the supervisory signal. For this purpose, the Raman amplification can be caused with relatively low co-propagating pump power in a relatively small piece of optical fiber, included in the repeater 19, preferably terminated with a filter terminating the co-propagating pump radiation and allowing the passage of the optical signal, such as an isolator, or an interference filter, or a WDM coupler. If the co-propagating Raman amplifier acting as supervisory signal modulator 23a is also used to impart to the optical signal an appreciable higher gain, it may be taken into account of the fact that a co-propagating Raman amplifier may attenuate in substantial manner a low-frequency supervisory signal already added to the optical signal traveling on the first optical fiber 20a, for example added by a previous repeater disposed along the optical link or by a terminal station of the optical system. In such case, a regeneration of the supervisory signal in the repeater may be provided. In other words, the already added supervisory signal can be dropped, received and electrically amplified in the repeater and then re-applied for modulating the pump source of the co- propagating Raman amplifier, as it will be explained in detail in the following. Preferably, the repeater 19 also comprise a second low-frequency modulator 23b connected to the second optical fiber 20b. In fig.2, the second low frequency modulator 23b is connected to the second optical fiber 20b downstream the coupling of the second pump source 21b. In an alternative embodiment, not shown, the second low-frequency modulator 23b may be connected to the second optical fiber 20b upstream the coupling of the pump source 21b. The modulator 23b is adapted to superimpose on the optical signal traveling on the second optical fiber 20a a low-frequency signal, according to a supervisory information. The second low-frequency modulator 23b can be any of the types of modulators already descried with reference to the first modulator 23a. Preferably, the first and the second modulators 23a, 23b are of the same type. The repeater 19 also comprises at least one supervisory unit 24 associated at least to said first low-frequency modulator 23a. The supervisory unit 24 is adapted to generate a first electrical supervisory signal, that can be applied to a driving circuit of the modulator 23a in order to superimpose the supervisory signal on the optical signal. Preferably, the supervisory unit 24 may be further associated to the first pump source 21a, in order to sense and preferably regulate the pump power emission of the same. Further, the supervisory unit 24 may be also preferably associated to the second low-frequency modulator 23b. A second electrical supervisory signal may be generated in the supervisory unit 24, to be applied to a driving circuit to the second low-frequency modulator 23b. Further, the supervisory unit may be also preferably associated to the second pump power source 21 b, in order to sense and preferably regulate the pump power emission of the same. A preferred embodiment of a supervisory unit suitable for use in the repeater 19 will be described in the following. Preferably, the repeater 19 further comprises at least a first photodetector 25a. This is connected to a first optical power splitter 26a, that splits a portion of optical signal traveling on the first optical fiber 20a (for example 5%-10% of the power of the optical signal) and sends such portion of optical signal towards the photodetector 25a. The photodetector 25a converts the portion of optical signal in an electrical signal. Such electrical signal may be preferably furnished to the supervisory unit 24. Preferably, the repeater 19 further comprises at least a second photodetector 25b. This is connected to a second optical power splitter 26b, that splits a portion of optical signal traveling on the second optical fiber 20b (for example 5%-10% of the power of the optical signal) and sends such portion of optical signal towards the photodetector 25b. The photodetector 25b converts the portion of optical signal in an electrical signal. Such electrical signal may be preferably furnished to the supervisory unit 24. The functioning of the repeater 19 will be explained hereinafter. A first optical signal carrying a high frequency traffic signal and possibly a low-frequency supervisory signal λ' s , S up travels on the first optical fiber 20a in a forward direction. For example, the supervisory signal carried by the first optical signal comprises an interrogation signal on the operating state of the repeater 19, e.g., its environment temperature, or the operating temperature of the pump sources 21a, 21b, or the pump power of the pump sources 21a, 21b. Such interrogation signal was typically superimposed on the first optical signal in a first terminal station. The counter- propagating pump power furnished by the first pump source 21a allows Raman amplification of the first optical signal, together with substantially no attenuation of the low-frequency supervisory signal, superimposed on the first optical signal. The power splitter 26a splits a portion of the first optical signal and sends it towards the photodetector 25a, which transforms the same in an electrical signal. Such electrical signal is fed to the supervisory unit 24, which discriminates the interrogation signal and may elaborate the same in order to understand if such interrogation signal is addressed to the repeater 19 or to a subsequent repeater. In the latter case, typically nothing is done. In the former case, typically an electrical response supervisory signal is generated in the supervisory unit 24, comprising the informations on the operating state of the repeater requested. The supervisory unit 24 then may apply the supervisory response signal to the driving circuit of the second low-frequency modulator 23b, that superimposes the supervisory response signal to a second optical signal carrying a high frequency traffic signal and possibly another low-frequency supervisory signal λ" s , Su p, traveling on the second optical fiber 20b in a backward direction. In a preferred embodiment, the supervisory response signal is also applied to the driving circuit of the first low- frequency modulator 23a, so that the supervisory response signal is superimposed on the optical signals traveling in both forward and backward directions on the optical link. According to an alternative embodiment, the supervisory response signal is only superimposed by the first low-frequency modulator 23a in a forward direction toward a second terminal station. Such second terminal station then may send the response signal to the first terminal station by using an external line, for example a public switched telephone (PSTN) line. In place of or combined with the interrogation signal, the supervisory signal carried by the first optical signal λ' s , Su p may comprise a command signal in order to regulate one operating parameter of the repeater 19, such as for example the pump power of the Raman pump sources 21a, 21b or the operating point of the low-frequency modulators 23a, 23b. Clearly, an analogous functioning of the repeater 19 may be described with reference to a second optical signal λ" SιSup traveling on the second optical fiber 20b in backward direction, possibly carrying a low-frequency supervisory signal. Coming back to fig.1, the optical transmission system 10 according to the invention preferably has a plurality of repeaters 19 comprising pump sources coupling on the optical link a pump radiation counter-propagating with respect to the optical signal, so as to cause Raman amplification of the optical signal. Further, counter-propagating pump radiation does not attenuate low-frequency supervisory signals superimposed on the optical signal, i.e. also the low-frequency supervisory signals carried by the optical signal maintain an acceptable strength due to the counter-propagating Raman amplification. The repeaters 19 may be disposed at distances of about 50-100 km with each other. The pump radiation power may be set so as to cause a Raman gain in the range of between 10-25 dB. At the input of each span between two consecutive repeaters, the input power of the optical signal (i.e. the output power of the repeater) may be for example set in a range between 5dBm and 18 dBm, in dependence of the number of wavelengths used for the optical signal. Counter-propagating Raman amplification is particularly advantageous in submarine optical transmission systems, as it allows to maintain a suitable amplification of the optical signal together with a low noise figure. A modulator suitable for superimposing on an optical signal a low-frequency supervisory signal, associated to a suitable supervisory unit, may also be provided in devices other than repeaters, disposed along the optical link 12. For example, it can be provided in a branching unit disposed in a point of a submarine optical link, such branching unit being suitable for routing all or part of the traffic and/or supervisory signals toward a third terminal station disposed at the end of a spare optical link. The branching unit may also comprise a photodetector connected through an optical power splitter to the optical link, adapted for receiving a portion of optical signal possibly carrying a low-frequency supervisory signal and associated to the supervisory unit. Preferably, the branching unit may comprise two low-frequency modulators, being adapted to superimpose a supervisory signal generated in the supervisory unit on an optical link in both forward and backward directions. Preferably, the branching unit may comprise two photodetectors for receiving portions of optical signal possibly carrying supervisory signals coming from both forward and backward directions, the photodetectors being associated with the supervisory unit. A third low-frequency modulator may be provided for superimposing a supervisory signal on an optical signal sent on the spare link. A third photodetector may be provided for receiving portions of optical signal possibly carrying supervisory signals coming from the spare link, the third photodetector being associated with the supervisory unit. Low-frequency supervisory signals may be superimposed to the optical signal at the terminal stations in any suitable manner. For example, the supervisory signal may be added at the transmitter, by modulating the driving current of the laser diode emitting the optical signal with the supervisory signal. This technique may be disadvantageous in WDM systems, wherein multiple laser diodes are present, so that the driving current of each laser diode should be modulated. This may require a proper control electronics for the direct modulation of each laser source, and may imply an increase in complexity of electric connections and wiring, in costs and in dimensions. Moreover, it may need a complex calibration of the modulation depth for each laser source. Preferably, the superposition of the supervisory signal at the terminal station may be performed downstream the transmitter by a suitable low-frequency modulator. This technique may be used also for WDM transmission: in such case the low- frequency modulator may be advantageously disposed downstream the multiplexing device, so that the supervisory signal may be superimposed on the whole WDM optical signal with a single electrical connection. As far as the low- frequency modulator that may be used in the terminal stations, it may be any of the low-frequency modulators described above with reference to the optical repeater of fig.2. For example, the low-frequency modulator may be the transmitter optical amplifier, which gain may be modulated with the supervisory signal. Such transmitter optical amplifier may be for example a co-propagating Raman amplifier, i.e. a pump source may be disposed in the terminal station coupled to the input of the optical link, so as to couple a pump radiation on the optical link in the same direction of the optical signal. The pump source may be modulated with the supervisory signal, so that the supervisory signal is superimposed on the optical signal during Raman amplification of the same. In another preferred embodiment, the low-frequency modulator may be an optical variable attenuator, more preferably a magneto-optical variable attenuator. A suitable magneto-optical variable attenuator is the model YS-500, produced by FDK Corporation. Fig.3 schematically shows an exemplary supervisory unit 24 that can be used in the repeater of fig.2. The supervisory unit 24 comprises at least one filtering circuit 241a, 241b, at least one amplifying circuit 242a, 242b, at least one threshold circuit 243a, 243b and a processing unit 244. A first electrical signal coming from photodiode 25a (see fig.2) is inputted in the supervisory unit 24. A second electrical signal coming from photodiode 25b may be also inputted in the supervisory unit 24. Filtering circuit 241a, 241b allows to discriminate the supervisory signal low-frequency band from the traffic signal high-frequency band. Amplifying circuit 242a, 242b allows amplification of the electrical signal. Downstream the filtering and amplifying circuits a threshold circuit 243a, 243b, typically comprising a peak detector and a comparator circuit, determines the presence or the absence of a supervisory signal, for example coming from a terminal station. In presence of a supervisory signal, the threshold circuit feeds the signal to the processing unit 244, which is suitable to process the supervisory signal, in order to check whether there are command and/or interrogation signals addressed to the repeater. In this case, the processing unit 244 may execute the commands contained in such signals (for example, it regulates the pump power of the pump source or sources 21a, 21b for Raman amplification) and/or may generate response signals (for example, comprising informations on the operating status of the repeater). Such response signal may be fed by the processing unit 244 to the driving circuit of at least one of modulators 23a, 23b (see fig.2). Typically, the response signal is sent in both directions on the optical link: for this purpose, the response signal is fed to both modulators 23a, 23b. The processing unit 244 may also generate supervisory signals to be fed to at least one of the modulators 23a, 23b independently of a specific interrogation signal coming from the terminal stations, for example by continuously sending informations on the operating state of the repeater for monitoring purposes. Further, if the supervisory signal is not addressed to the repeater (for example because such supervisory signal was sent by a previous repeater for response to an interrogation signal), the processing unit 244 may also simply re-transmit such supervisory signal coming from the line to the subsequent repeater. In such case, the processing unit may simply feed to the driving circuits of one of the low-frequency modulators 23a, 23b the discriminated supervisory signal, for re-transmission to the subsequent repeater. The re-transmission of the supervisory signal in at least one of the repeaters of the optical link may be used, for example, when a co-propagating Raman amplifier is used in the repeater as low-frequency modulator for superimposing the supervisory signal, or when an EDFA is used in the repeater as low-frequency modulator for superimposing a supervisory signal having a subcarrier frequency of less than 10-15 kHz. In these cases, the supervisory signals traveling on the optical link may be attenuated in the passage through the co-propagating Raman amplifier or the EDFA: this may cause the loss of such supervisory signal after the passage through even a few number of repeaters, if the supervisory signal is not regenerated. Circuits used in the supervisory unit 24 may be realized with ordinary components. Processing unit 244 may be for example a conventional processing unit of the ASIC (Application Specific Integrated Circuit) or of the FPGA (Field Programmable Gate Array) type. Fig.4 schematically shows an exemplary supervisory unit 17 that can be used in a terminal station of the optical transmission system according to the invention. The supervisory unit is connected to a photodetector 175 which transforms a portion of optical signal coming from a first optical fiber 20b, possibly carrying a supervisory signal coming from a device disposed along the optical link (e.g. a repeater), in an electrical signal. An optical power splitter 176 (e.g. a 90/10 coupler), coupled to the first fiber 20b, may be used for splitting the portion of optical signal. The electrical signal is fed to a filtering circuit 171 and an amplifying circuit 172 of the supervisory unit 17. The filtering circuit 171 discriminates the low-frequency supervisory signal from the high frequency traffic signal. The amplifying circuit 172 amplifies the electrical signal. Downstream the filtering and amplifying circuits an analog/digital converter 173 may convert the electrical analog signal in an electrical digital signal. Such digital signal is then passed to a processing unit 174. For example, the processing unit 174 may be a conventional processing unit of the DSP (Digital Signal Processor) type, suitable to perform a peak detection of the electrical signal and an operation of comparison with a predetermined threshold for determining the presence or absence of the supervisory signal. The processing unit 174 may also generate a supervisory signal to be sent on the optical link by superposition on the optical signal. Such supervisory signal is then passed, for example, to the driving circuit of a low-frequency modulator 177, connected to a second optical fiber 20a.

Claims: CLAIMS 1. An optical transmission system comprising at least a first and a second terminal station, optically connected with each other by an optical link, said first or said second terminal station being adapted to send on said optical link at least a first optical signal having a first direction, said system further comprising: - a first supervisory unit associated to a device disposed along said optical link, said first supervisory unit being adapted for generating a first supervisory signal, said first supervisory unit being further associated to at least one modulator being adapted to superimpose on said first optical signal said first supervisory signal; characterized in that said system further comprises: - at least one pump source disposed along said optical link, said pump source being adapted to send on said optical link a pump radiation in a second direction opposite to said first direction, so as to cause Raman amplification of said first optical signal and of said first supervisory signal superimposed on said first optical signal. 2. An optical transmission system according to claim 1 , characterized in that said first supervisory signal comprises at least an information signal on an operating state of said device. 3. An optical transmission system according to claim 1 or 2, characterized in that said device disposed along said optical link is a repeater and said at least one pump source is comprised in said repeater. 4. An optical transmission system according to claim 3, characterized in that said first supervisory unit is associated to said pump source. 5. An optical transmission system according to any one of the preceding claims, characterized in that said modulator is a lithium niobate modulator. 6. An optical transmission system according to any one of claims 1 to 4, characterized in that said modulator is a semiconductor modulator. 7. An optical transmission system according to any one of claims 1 to 4, characterized in that said modulator is a tunable band-pass filter. 8. An optical transmission system according to any one of claims 1 to 4, characterized in that said modulator is a variable optical attenuator. 9. An optical transmission system according to claim 8, characterized in that said variable optical attenuator is a magneto-optical variable attenuator. 10. An optical transmission system according to any one of claims 1 to 4, characterized in that said 'modulator comprises an optical amplifier, and in that said first supervisory signal is adapted to modulate a gain of said optical amplifier. 11. An optical transmission system according to claim 10, characterized in that said optical amplifier is an erbium-doped fiber amplifier. 12. An optical transmission system according to claim 11, characterized in that said erbium doped fiber amplifier comprises at least one erbium-doped fiber and at least a further pump source adapted for emission of a pumping radiation for said erbium doped fiber, said first supervisory unit being associated to said further pump source, so that said first supervisory signal is adapted to modulate said pumping radiation for said erbium-doped fiber. 13. An optical transmission system according to claim 10, characterized in that said optical amplifier is a semiconductor amplifier 14. An optical transmission system according to claim 10, characterized in that said optical amplifier is a co-propagating Raman amplifier. 15. An optical transmission system according to claim 14, characterized in that said co-propagating Raman amplifier comprises at least a further pump source being adapted to send on said optical link a pumping radiation in said first direction, so as to cause Raman amplification of said first optical signal, said first supervisory unit being associated to said further pump source, so that said first supervisory signal is adapted to modulate said pumping radiation for Raman amplification. 16. An optical transmission system according to any one of the preceding claims, characterized in that said device further comprises at least one photodetector, being adapted to receive at least a portion of said first optical signal and transform said portion of first optical signal in an electrical signal. 17. An optical transmission system according to claim 16, characterized in that said first supervisory unit is associated to said photodetector and is adapted to discriminate a second supervisory signal carried by said first optical signal. 18. An optical transmission system according to claim 17, characterized in that said first supervisory unit is adapted to feed the discriminated second supervisory signal to said at least one modulator. 19. An optical transmission system according to any one of the preceding claims, characterized in that said optical link comprises at least a first optical fiber and a second optical fiber, said first optical fiber being adapted to carry said first optical signal in said first direction and said second optical fiber being adapted to carry a second optical signal in said second direction. 20. An optical transmission system according to claim 19, characterized in that said device comprises at least a second pump source, said second Raman .. pump source being adapted to send on said second optical fiber a pump radiation in said first direction, opposite to the direction of said second optical signal, so as to cause Raman amplification of said second optical signal. 21. An optical transmission system according to claim 19 or 20, characterized in that said device comprises at least a second modulator, said second modulator being associated to said first supervisory unit. 22. An optical transmission system according to claim 21 , characterized in that said first supervisory unit is adapted for generating at least a third supervisory signal, and said second modulator is adapted for superimposing said third supervisory signal to said second optical signal. 23. An optical transmission system according to any one of claims 19 to 22, characterized in that said device comprises at least a second photodetector, being adapted to receive at least a portion of said second optical signal and transform said portion of second optical signal in an electrical signal. 24. An optical transmission system according to claim 23, characterized in that said first supervisory unit is associated to said second photodetector and is adapted to discriminate a fourth supervisory signal carried by said second optical signal. 25. An optical transmission system according to any one of the preceding claims, characterized in that said first or said second terminal station comprises a second supervisory unit, being adapted to receive at least a portion of said first or said second optical signal from said optical link and discriminate from said first or said second optical signal said first or said third supervisory signal. 26. An optical transmission system according to claim 25, characterized in that said second supervisory unit is adapted to generate a fifth supervisory signal to be superimposed on said first or said second optical signal. 27. An optical transmission system according to any one of the preceding claims, characterized in that said first or said second optical signal is a WDM optical signal. 28. An optical transmission system according to claim 27, characterized in that said first or said second terminal station comprises a plurality of transmitters being adapted to emit a respective plurality of optical signals having different wavelengths, and a multiplexing device being adapted to multiplex said plurality of optical signal having different wavelengths in said WDM optical signal. 29. An optical transmission system according to claim 28, characterized in that said first or said second terminal station comprises a transmitter optical amplifier. 30. An optical transmission system according to claim 26 and 29, characterized in that said second supervisory unit is associated to said transmitter optical amplifier, so that said fifth supervisory signal is adapted to modulate a gain of said transmitter optical amplifier. 31. An optical transmission system according to claim 30, characterized in that said transmitter optical amplifier is a co-propagating Raman amplifier. 32. An optical transmission system according to any one of claims 26 to 29, characterized in that said first or said second terminal station comprises at least a third modulator, said second supervisory unit being associated to said third modulator in order to superimpose said fifth supervisory signal to said first or said second optical signal. 33. An optical transmission system according to claim 32, characterized in that said third modulator is a variable optical attenuator. 34. An optical transmission system according to claim 33, characterized in that said third modulator is a magneto-optical variable attenuator. 35. An optical repeater comprising at least a first optical fiber adapted to carry a first optical signal in a first direction, at least a first modulator connected to said first optical fiber and a supervisory unit associated to said first modulator, said supervisory unit being adapted to generate a first supervisory signal, said first modulator being adapted to superimpose said first supervisory signal on said first optical signal, characterized in that said optical repeater further comprises at least a first pump source connected to said first optical fiber, said first pump source being adapted to send on said first optical fiber a pump radiation in a second direction opposite to said first direction, said pump radiation being adapted to cause Raman amplification of said first optical signal on said first optical fiber. 36. An optical repeater according to claim 35, characterized in that said supervisory unit is associated to said first pump source. 37. An optical repeater according to claim 35 or 36, characterized in that it further comprises a second optical fiber adapted to carry a second optical signal in a second direction opposite to said first direction and at least a second modulator connected to said second optical fiber. 38. An optical repeater, according to claim 37, characterized in that said supervisory unit is associated to said second modulator and said second modulator is adapted to superimpose said first supervisory signal on said second optical signal. 39. An optical repeater according to any one of claims 37 or 38, characterized in that it further comprises at least a second pump source connected to said second optical fiber, said second pump source being adapted to send on said second optical fiber a pump radiation in a direction opposite to said second direction, said pump radiation being adapted to cause Raman amplification of said second optical signal. 40. An optical repeater according to claim 39, characterized in that said supervisory unit is associated to said second pump source. 41. An optical repeater according to any one of claims 35 to 40, characterized in that said first supervisory signal comprises at least an information signal on the operating state of said first or said second pump source. 42. An optical repeater according to any one of claims 35 to 41 , characterized in that it further comprises a first photodetector connected to said first optical fiber, said first photodetector being adapted to receive at least a portion of said first optical signal and transform said portion of first optical signal in an electrical signal. 43. An optical repeater according to claim 42, characterized in that said supervisory unit is associated to said first photodetector, so as to discriminate a second supervisory signal carried by said first optical signal. 44. An optical repeater according to any one of claims 37 to 43, characterized in that it further comprises a second photodetector connected to said second optical fiber, said second photodetector being adapted to receive at least a portion of said second optical signal and transform said portion of second optical signal in an electrical signal. 45. An optical repeater according to claim 44, characterized in that said supervisory unit is associated to said second photodetector, so as to discriminate a third supervisory signal carried by said second optical signal. 46. An optical repeater according to any one claims 35 to 45, characterized in that said first or said second modulator is a lithium niobate modulator. 47. An optical repeater according to any one of claims 35 to 45, characterized in that said first or said second modulator is a semiconductor modulator. 48. An optical repeater according to any one of claims 35 to 45, characterized in that said first or said second modulator is a tunable band-pass filter. 49. An optical repeater according to any one of claims 35 to 45, characterized in that said first or said second modulator is a variable optical attenuator. 50. An optical repeater according to claim 49, characterized in that said variable optical attenuator is a magneto-optical variable attenuator. 51. An optical repeater according to any one of claims 35 to 45, characterized in that said first or said second modulator comprises an optical amplifier, and in that said first supervisory signal is adapted to modulate a gain of said optical amplifier. 52. An optical repeater according to claim 51 , characterized in that said optical amplifier is an erbium-doped fiber amplifier. 53. An optical repeater according to claim 52, characterized in that said erbium doped fiber amplifier comprises at least one erbium-doped fiber and at least a further pump source adapted for emission of a pumping radiation for said erbium doped fiber, said supervisory unit being associated to said further pump source, so that said first supervisory signal is adapted to modulate said pumping radiation for said erbium-doped fiber. 54. An optical repeater according to claim 51 , characterized in that said optical amplifier is a semiconductor amplifier 55. An optical repeater according to claim 51 , characterized in that said optical amplifier is a co-propagating Raman amplifier. 56. An optical repeater according to claim 55, characterized in that said co- propagating Raman amplifier comprises at least a further pump source being adapted to send on said first optical fiber a pumping radiation in said first direction or on said second optical fiber a pumping radiation in said second direction, said pumping radiation being adapted to cause Raman amplification of said first or of second optical signal, said supervisory unit being associated to said further pump source, so that said first supervisory signal is adapted to modulate said pumping radiation for Raman amplification. 57. A method for supervising an optical transmission system comprising an optical link between at least a first and a second terminal station, said method comprising: - transmitting a first optical signal on said optical link in a first direction; - superimposing a first supervisory signal on said first optical signal; characterized in that said method further comprises: - sending on said optical link a pump radiation in a second direction opposite to said first direction, so as to cause Raman amplification of said first optical signal and of said first supervisory signal superimposed on said first optical signal.

Inventors: AVALLONE ROBERTO, BOIDI CARLO, FREGOSI ANDREA, AVALLONE, ROBERTO, BOIDI, CARLO, FREGOSI, ANDREA
Applicants: AVALLONE ROBERTO, BOIDI CARLO, FREGOSI ANDREA, PIRELLI SUBMARINE TELECOM SYST, AVALLONE, ROBERTO, BOIDI, CARLO, FREGOSI, ANDREA, PIRELLI SUBMARINE TELECOM SYSTEMS ITALIA S.P.A.
Application year: 2001
Main classification: H04B10
IPC codes: H04B10