FR2798023A1 - Transmission system optical wave division multiplexing power signal command having optical transmission channel with attenuators individually optical drive signal fed/optical sub system feeding - Google Patents

Abstract

The system of individual command of power signals has optical transmission channels (wd1..wdn) with optical attenuators (VoA) feeding an optical sub system. Each optical attenuator can be commanded separately. The individual drive signals (ps1..psn) are detected inside the optical sub system and each optical attenuator is set as a function of the drive signals.

Description

In optical transmission systems, the use of the wavelength multiplex transmission method, also known as the Wavelength DeMultiplex (WDM) transmission method, makes it possible to obtain efficient operation. transmission resources offered by transmission medium constituted by "optical glass fibers". In the transmission method, a plurality of individual WDM signals, each representing a transmission channel, are joined or multiplexed to give a WDM signal, to be transmitted by means of an optical fiberglass. Current optical transmission systems, such as, for example, WDM communication networks, consist of optical transmission and reception units, intermediate amplifiers (for example electric regenerators) and purely optical subsystems.

Such an optical subsystem, arranged in a WDM communication network, generally comprises an optical multiplexer, used for the optical multiplexing of several individual WDM signals to give a WDM signal, a plurality of optical amplifiers (for example erbium doped fiber amplifiers). (EDFA)), used for the optical amplification of the WDM signal, several sections of optical fiberglass or optical waveguides and at least one optical demultiplexer for demultiplexing the WDM signal to give the corresponding individual WDM signals. It may furthermore be provided, in an optical subsystem, optical filters, serving for the suppression of distortion and the filtering of WDM signals introduced into the subsystem, and filters performing "Add / Drop" functions, also called "Add / Drop Multiplexers". use of Add / Drop multiplexers allows the coupling or decoupling of individual wavelengths or individual WDM signals determined respectively to or from the WDM signal. Since all the optical transmission channels offered by the WDM signal or all individual WDM signals have the same source location and the same target location, additional Add / Drop multiplexers are arranged in the optical subsystems.

For performing optical communication functions, it is possible to have, in the optical subsystems, additional optical components for performing spatial and wavelength conversion of the individual WDM signals. Such optical components for performing a combination from multiplexed spatial and frequency multiplexed multiplex communication and wavelength multiplexed communication are also referred to as "optical cross-connections" (optical interconnects).

The individual WDM signals introduced are amplified differently by means of the optical amplifiers arranged in the optical subsystems, whereas, during the passage of individual WDM signals in the optical subsystem, the optical signal-to-noise ratio of each individual WDM signal is submitted. to a different influence and thus the individual signals present at the output of the optical subsystem each have a different signal power level. In current WDM communication networks, there are different blanks for processing individual WDM signals having different power levels.

A rough idea is to have no effect on the power level of the respective individual WDM signals. This results in a low exploitation of the transmission resources offered by the WDM communication network, since the entire system must be designed to take account of the worst-case transmission channel. The maximum transmission line is therefore limited.

As another draft solution, US Pat. No. 5,513,029 for example describes a method of monitoring the transmission quality - also called performance - of an optical WDM transmission system, in which individual WDM signals are each modulated by means of a respective low frequency pilot signal at known depth of modulation via a respective optical transmitter. In order to monitor transmission quality of the WDM transmission system the channel specific pilot signals, which are modulated on individual WDM signals, are detected and for each the respective signal-to-noise ratio is determined and respective optical transmitters are controlled according to the signal ratios. -noise determined.

 detects dynamic signal-to-noise ratios of individual WDM signals routed by means of the optical transmission system to react flexibly to aging phenomena of the WDM communication system or to other disturbances, access to the transmitters sending the signals respective individual WDMs is still necessary. The disadvantage is that the described method can rarely be used in current WDM communication systems, since these are often composed of partial sections, or functional units, which are specific to different manufacturers and by which, due to a lack of homogeneous interfaces, no control access on optical transmitters sending the individual WDM signals is possible. As another disadvantage, the method described can not be used in purely optical communication networks having, therein, add / drop multiplexers or optical cross-connections, since these optical components indicated allow, for example, to introduce into the optical subsystem, or to extract from them individual WDM signals on whose sources, optical transceivers from which no control access can take place.

Furthermore, the form "Technical Report, Contributions to the state of the art", page 135 137, Siemens AG July 1999, describes a method of measuring the channel level and measuring OSNR (acronym for Optical English). Signal Noise Rat or optical signal-to-noise ratio) of WDM systems by means of a pilot signal. In the method described, a respective variable optical attenuator is provided on each access input to an optical subsystem, for example upstream of an optical multiplexer or upstream of the Add / Drop multiplexer, in order to allow an adjustment of power channel by channel. Further, a respective low frequency pilot signal or tone, having a determined frequency and modulation depth, is modulated on each of the individual WDM signals sent to the subsystem through the respective optical attenuators. The individual WDM signals are superimposed on each other by means of optical multiplexers. If an OSNR measurement is desired at a given point in the transmission line or the optical subsystem, part of the optical power of the WDM signal is sent, decoupled, to an optical demultiplexer. The individual signals WDM are separated in spatial position by means of the demultiplexer and are each transmitted to a respective photodiode. The electrical signals produced by the photodiodes are analyzed by means of phase-sensitive detectors, the signal part synchronous with the pilot signal and its non-synchronous part with respect thereto being respectively detected. The respective corresponding OSNR values can each be determined from the detected signal portions. The object of the invention is to achieve maximum exploitation of the transmission resources offered by purely optical communication networks, in particular via the WDM communication networks. In particular, in the case of purely optical communication networks, or optical sub-systems, comprising, therein, Add / Drop multiplexers or optical Cross-Connects, it is necessary to obtain dynamic optimization and flexible transmission properties, for example realization of maximum lengths of transmission of optical signals.

The object of the invention is to control the power of individual signals WDM, each representing an optical transmission channel, which are routed to an optical subsystem via at least one optical attenuator which can be controlled, with, for each, a settable channel-to-channel signal power, wherein, by means of said at least one optical attenuator, the individual WDM signals are each modulated by means of a respective channel-specific pilot signal and having a prefixed amplitude and a prefixed frequency, individual WDM signals being multiplexed to give a WDM signal, characterized in that the pilot signals contained in the WDM signal are detected within the optical subsystem, in that the signal powers of respective subsystems of the individual signals WDM are derived from the respective detected pilot signals and that by means of said au In one of the controllable optical attenuators, the respective signal powers, channel-by-channel, of the individual signals WDM are modified or maintained according to respective determined subsystem signal powers.

In the method according to the invention for controlling the power of individual signals WDM each representing a transmission channel, these signals are sent to an optical subsystem via at least one optical attenuator which can be controlled, with, for each, a controllable channel-to-channel signal power. The at least one optical attenuator modulates the individual WDM signals, each by means of a pilot signal having a prefixed amplitude and channel-specific prefixed frequency, and then multiplexing the individual WDM signals to provide a WDM signal. The essential aspect of the method according to the invention lies in the fact that the pilot signals contained in the WDM signal are detected inside the optical subsystem and that the respective subsystem signal powers are derived from the detected signals. . The controllable optical attenuator (s) allows modifying or maintaining the respective signal power, channel by channel, of the individual signals WDM according to the respective subsystem signal strengths of the individual signals WDM that have been determined. The essential advantage of the method according to the invention lies in the fact that each optical subsystem, which transmits one or more WDM signals, can be optimized as an independent unit, a control of the signal power channel by channel being possible for each individual WDM signal introduced into the optical subsystem. Advantageously, purely optical communication networks or optical subsystems comprising, arranged in them, add / drop multiplexers or optical cross-connections can also be optimized with regard to the transmission properties, no complex demultiplexing of the WDM signal within the optical subsystem is necessary and no access to the optical transmitters sending the respective individual WDM signals is necessary.

The method according to the invention may also have one or more of the following features - the optical signal-to-noise ratios of the individual WDM signals are derived from the detected pilot signals, and the at least one optical attenuator is controlled according to the signal-to-noise ratios. individual signals WDM which have been deduced, - the amplitude of the pilot signals, channel by channel, which have been detected is determined and said at least one optical attenuator is controlled according to the determined amplitudes of the pilot signals, - optical subsystem comprises at least one optical multiplexer, for multiplexing the individual WDM signals, and / or at least one optical amplifier, and / or at least one optical demultiplexer, - optical subsystem, comprises converter means for the conversion of spatial position and / or wavelength conversion and carrier frequency conversion of individual signals WDM, pilot signals that have been modulated on the converted individual WDM signals are detected within the optical subsystem and, by means of the detection result, the association of the transmission channels with the individual WDM signals is verified.

process according to the invention described below in detail with respect to the five figures of the drawings. We see .

FIG. 1 is a purely optical subsystem which is arranged in a WDM communication system or in a WDM communication network and which has disposed therein an Add / Drop multiplexer for coupling individual determined WDM signals. in the optical subsystem or the decoupling of signals therefrom, in FIG. 2, an arrangement for adjusting, channel channel, the power of individual signals WDM sent to the optical subsystem of FIG. FIG. 3 illustrates an exemplary channel-by-channel pilot signal detection arrangement modulated on the individual WDM signals within the optical subsystem. , the optical subsystem FIG. 1 comprising, disposed therein, an additional optical Cross-Connect by means of which a carrier frequency conversion - or a wavelength conversion - e Spatial position conversion of individual WDM signals sent to the optical subsystem are carried out and, in Fig. 5, presented by way of example, an arrangement for verifying the association of transmission channels with the respective individual signals transmitted in the sub-system. system.

FIG. 1 schematically represents, in the form of a block diagram, an arrangement, presented by way of example of an optical subsystem SUB disposed in a WDM communication network. The optical subsystem SUB comprises n optical inputs ESl ... n each of which is sent a respective individual WDM signal wdl ... n. Each individual signal WDM wdl ... n has a carrier frequency or an optical wavelength <B> Il </ B> ... n proper, a respective transmission channel Dl ... n of its own and which serves for the transmission of information being associated with each wavelength Xl ... n.

The n inputs ES1 ... n of the optical sub-system SUB each have a controllable optical attenuator - also called "variable optical attenuator VOA (acronym for the English expression" Variable Optical Attenuator ") -, these attenuators being each connected to a respective input of an optical multiplexer MUX.The n individual signals WDM sent are combined, or multiplexed, giving an optical WDM signal owdm, by means of the optical multiplexer MUX.The multiplexed WDM signal owdm is transmitted by intermediate of an AM output of the optical multiplexer MUX and via a light waveguide, an input EE of a first optical amplifier EDFA1 (acronym for the English expression "Erbium Doped Fiber Amplifier"), for example realized in the form of an erbium-doped fiber amplifier The first optical amplifier EDFA1 is connected, via an AE output and another waveguide 1 at a first input EE of a second optical amplifier EDFA2. The second optical amplifier EDFA2 is connected, via another input EF and an output AF and, for each of them, a respective optical link line, an ADM add / drop multiplexer. The ADM Add / Drop multiplexer is connected, via an EM input and through another controllable VOA optical attenuator, to another ESm input of the optical subsystem SUB. On the other hand, the Add / Drop ADM multiplexer is connected, via another AM output, to an ASm output of the optical subsystem SUB. Another mth individual signal WDM wdm, which has the wavelength 13 and at which a second Dm data channel is associated, is sent to the mth input ESm of the optical sub-system SUB. this exemplary embodiment, the ADM add / drop multiplexer is arranged in such a way that, by means of the latter, the individual signal WDM wd3 sent to the third input ES3 of the optical subsystem SUB is decoupled from the subsystem optical SUB, via the output ASm and, simultaneously, the mth individual signal WDM wdm sent to the <b> the mth ESm input of the subsystem optical SUB is coupled into the optical subsystem, c i.e., is introduced into the WDM signal owdm transmitted therein It should be emphasized that the Add / Drop ADM multiplexer shown in Figure 1 may be connected within the subsystem SUB, directly to the light waveguide, that is to say without any connection with an optical amplifier EDFA1,2.The second optical amplifier EDFA2 is connected via another output AE and the intermediate of another light waveguide to the input ED of an optical demultiplexer DMUX. demultiplexer DMUX makes it possible to perform an optical distribution of the incoming WDM signal owdm giving the corresponding individual WDM signals wd1 ... n, wdm, which are transmitted, via n demultiplexer outputs DMUX, to corresponding outputs ASl ... n of the SUE optical subsystem.

As already explained in the introduction to the description, the WDM signal owdm present at the output AM of the optical multiplexer MUX is amplified, more specifically is negatively influenced, in a different way, or the individual signals WDM wdl ... n , wdm sent to the optical subsystem SUB are amplified, more precisely are negatively influenced, in a different way, during the crossing of the optical subsystem, in particular by passing through the optical amplifiers EDFA1,2 and the optical filter ADM which are arranged in optical sub-system SUB, so that the individual signals WDM wdl ... n, wdm present at the outputs ... n, ASm of the optical sub-system SUB have different optical signal-to-noise ratios.

In order to optimize the optical subsystem transmission properties SUB shown in FIG. 1, for example for the realization of maximum transmission lengths of the individual signals WDM wd1... N, wdm sent to the optical subsystem SUB, it is proceeded to a channel-by-channel setting of the power of the individual WDM signals sent wd1 ... n, wdm, by means of the controllable VOA optical attenuators arranged at the inputs ES1 ... m of the optical subsystem SUB. Each optical attenuator VOA disposed at the respective input ES1 ... m of the optical subsystem SUB comprises additional modulation means by means of which each individual signal WDM wd1 ... n, wdm sent to a respective optical attenuator to be controlled is modulated by means of a pilot signal psl ... m at low frequency, channel by channel.

According to FIG. 2, controllable VOA optical attenuators are connected, each via a respective control input S, to signal generators G1 ... n which are adjustable and serve to produce the pilot signals ps1. ... n which are specific to the channels each have a frequency <B> fi </ B> ... n and a modulation depth specific to the channels. It should be emphasized that in FIG. 2 only the VOA attenuators and the signal generators G1 are shown. .. provided for the individual signals WDM wdl ... n sent to the multiplexer MUX. For the optical attenuators of FIG. 1 which are connected to the Add / Drop ADM multiplexer, there is also provided a not shown signal generator, producing a corresponding pilot signal. The pilot signals produced psl ... n are modulated on the corresponding individual WDM signals wdl ... n, wdm by means of the controllable VOA optical attenuators, while simultaneously, thanks to the respective amplitude of each pilot signal, channel by channel, psl ... n which is produced, the different VOA optical attenuators are controlled and therefore the respective powers of the individual signals WDM sent wdl ... n, wdm are reduced or maintained.

The respective amplitude of each of the channel-specific pilot signals psi ... n which are produced by means of the signal generators Gl ... n, i.e. the degree thus controlled of the attenuation of the powers of the individual signals WDM sent wdl ... n, wdm, is controlled, via a first analysis device ASE1 connected to the signal generators Gl ... n, by means of control signals ssl .. by this device, which is indicated by dashed lines. The first analysis device ASE1 is connected, via a control input SE and a command output SA, to an OSC control channel (acronym for the English term "Optical Supervisory Channal"), also called "optical monitoring channel", which is associated with the optical sub-system By modulating the channel specific signals psl ... n on individual WDM signals wdl ... n, wdm sent to the subsystem optical it is possible that an evaluation of the powers of the individual individual signals WDM wdl ... n, takes place at each point located inside the optical sub-system SUB.

Fig. 3 shows an arrangement according to the invention for determining the channel-by-channel power of the individual signals WDM wd1 ... n, wdm routed within the optical sub-system SUB. For this purpose, a portion of the optical optical WDM signal power owdm is decoupled at any location within the optical subsystem, for example upstream from the ED input of the DMUX optical demultiplexer (see FIG. an optical band filter of an optical derivation, and the decoupled partial signal tos is transmitted to a PD diode photo. The photodiode PD converts the decoupled optical partial signal tos into an electrical signal es, for the determination of the channel-by-channel power portions of the individual signals WDM ... n, wdm multiplexed giving the signal WDM owdm, is sent with n phase sensitive detectors LIVl ... n. The n phase-sensitive detectors can for example be made in the form of phase-locked amplifiers ("Lock-In" amplifiers). For the determination of the channel-by-channel power parts of the individual WDM signals wd1 ... n, wdm, the psl ... m pilot signals contained in the optical WDM signal must be detected by means of the LIVl-phase sensitive detectors. ...not. For this purpose, each detector responsive to the phase LIVl ... n is connected to a reference device RE by means of which n reference signals rsl ... n, each having respectively the frequencies <B> fi </ B>. ..n, fm to record psi ... n, psm lot signals to be detected are generated and are each sent to a reference input R of the respective phase responsive detector LIVl ... n. By an ES command input, the reference device RE is connected to the privileged control channel OSC is thus connected to the first analysis device ASE1. Information about the pilot signals psl ... n, psm modulated on the individual signals WDM wdl. .n, wdm are transmitted by the first analysis device ASE1 to the reference device RE.

Each of the reference frequencies <B> fi </ B> ... n, fm produced makes it possible to record, by means of the detector responsive to the respective phase LIVl ... n, the respective part of the electrical spectrum of the electrical signal each time. sent which is synchronous, in a rigid phase, with the frequency of the reference signal rsl ... n, m (fl ... n, m) each time routed. pilot signals ... n, psm contained in the electrical signal are detected by means of the n detectors sensitive to the phase LIVl ... n, as a function of the reference signals rsl ... n, m ... n, m) sent, then the result of the detection is transmitted to a second analysis device ASE2 connected to the detectors sensitive to the phase LIVl ... n. Analysis means, not shown, which are located in the second analysis device ASE2 make it possible to analyze the detected psl ... n, psm pilot signals. For example, the amplitude of the respective detected psl ... n, psm pilot signals is evaluated and the respective channel-by-channel power portions of the different WDM signals wdl ... n, wdm present at the same time. ED input of the DMUX optical demultiplexer are deduced from the respective evaluation results. As a variant, the respective optical signal-to-noise ratios of the individual signals wd1 ... n, wdm can be deduced, in the second analysis device ASE2, from the detected pilot signals ... n. As a function of the powers deduced, channel by channel, individual signals WDM wdl ... n, present at the input ED of the optical demultiplexer DMUX, control information if appropriate, used to control the VOA optical attenuators located at the inputs ES1. .n, ESm of the optical subsystem SUB, are formed in the second analysis device ASE2 and, via an AS command output of this second analysis device ASE2, they are transmitted to the control channel OSC . The control information if is converted, for example, into the first analysis device ASE1 connected to the optical attenuators VOA to give corresponding control signals ssl ... n for controlling the respective optical attenuators VOA. The sizing of the control signals ... is effected in such a way that, at the input of the optical transmission signal, the individual signals present at the input ED of the optical demultiplexer have the same optical signal-to-noise ratio. .

FIG. 4 shows the optical subsystem SUB described, comprising, therein, an additional OXC optical cross-connect connected to the outputs of the optical demultiplexer. The OXC Optical Cross-Connect allows for free conversion, of spatial position and wavelengths and carrier frequency, of the individual signals wdl ... m sent to the optical subsystem. For the understanding of the spatial position and wavelength conversion, only the wavelengths X 1 ... n and the corresponding associated transmission channels <B> D 1 are shown in FIG. 4. ... n respective individual WDM signals.

As has already been explained, the association of the transmission channels D1... N, Dm at the different wavelengths X1 ... n is interchangeable at will by means of filtering units, such as, for example, Add / Drop multiplexers or optical Cross-Connects, which are arranged in the optical subsystem. FIG. 5 shows, in block diagram form, an exemplary arrangement for verifying the association of the transmission channels D1... N, Dm with the individual signals WDM wd1 ... n, wm respective ones or wavelengths Xl ... n, specific to the respective channels. In the case where, at the end of an optical transmission line, information concerning the association of a transmission channel D1 ... m with a determined wavelength λ1 ... n is necessary, for example for controlling successful coupling / decoupling of individual WDM signals by means of an ADM Add / Drop multiplexer, or successful wavelength conversion, performed using an OXC optical cross-connect, this may have in accordance with FIG. 5, directly downstream of an optical demultiplexer DMUX or upstream of an optical multiplexer MUX. In this exemplary embodiment, the arrangement shown in FIG. 5 makes it possible, for example, to check the operation of the Add / Drop multiplexer represented in FIG. 1. For this purpose, the individual signals WDM wdl ... wdm ... wdn present at the outputs of the demultiplexer DMUX are sent, each respectively through an optical branching stage TAP1 ... n, to the input of a respective photodiode PD1 ... n. The photodiodes PD1 ... n are respectively connected respectively to a detector sensitive to the respective phase LIV1 ... n or a respective "Lock-In" amplifier, these detectors or amplifiers being connected to a reference device / analysis RAE. The analysis device ASE is itself connected, via the privileged control channel OSC, to the first analysis device ASE1 associated with the controllable optical attenuators VOA. Reference signals rs (fl ... n) produced by the reference / analysis device RAE, each having a reference frequency <B> fi </ b> ... n determined, are each respectively sent to the different detectors sensitive to the LIV phase ... n. The respective reference frequencies fl ... n are adapted to the frequencies of the pilot signals psl ... n, psm, modulated on the individual signals WDM wdl ... wdm ... wdn, which are expected. The respective pilot signals ps1 ... n, psm, which are modulated on the individual signals WDM, are respectively detected by the detectors sensitive to the phase LIVl ... n, the signal part respecting the sent electrical signal spectrum, which is synchronized, in a rigid manner in phase, with the respective reference signal rs1 ... n (fl ... n) sent, being detected each time. Once the transmitted signal portions are analyzed in the RAE reference / analysis device, the OSC privileged control channel information removed therefrom is transmitted to a preferred, unrepresented control unit.

VOA optical attenuators that can be modulated in a sufficiently fast manner are necessary for carrying out the process according to the invention. For this purpose, it is possible to use variable VOA optical attenuators, for example thermo-optical, which admit modulation frequencies of for example a few kHz.

Claims (5)

A method of controlling the power of individual WDM signals (wdl ... m), each representing an optical transmission channel (Dl ... m), which are routed to an optical subsystem (SUB) by the intermediate of at least one controllable optical attenuator (VOA), each having a channel-specific and controllable signal power, in which, using said at least one optical attenuator (VOA), the individual WDM signals (wdl ... m) are each modulated by means of a respective channel specific pilot signal (psi ... m) and having a prefixed amplitude and a prefixed frequency the individual WDM signals (wdl ... m) being multiplexed to give a signal WDM (owdm), characterized in that the pilot signals (psi ... m) contained in the signal WDM (owdm) are detected inside the optical subsystem (SUB), - this that the respective subsystem signal powers of the individual signals WDM (wdl ... m) are deduced from the respective pilot signals (psl ... m) detected and - by means of said at least one controllable optical attenuator (VOA), the respective signal powers, each specific to a channel, of the individual WDM signals (wdl ... m) are modified or maintained according to the respective subsystem signal powers determined. Method according to Claim 1, characterized in that the optical signal-to-noise ratios of the individual signals WDM (wd1 ... m) are deduced from the detected pilot signals (psi ... m), and - in that said at least one attenuator is controlled according to the signal-to-noise ratios of the individual signals WDM (wd1 ... m) which have been deduced. 3. Method according to claim 1, characterized in that the amplitude of the pilot signals (psl ... m) channel by channel, which have been detected is determined and - in that said at least one optical attenuator is controlled according to amplitudes determined pilot signals (psl ... m). A method according to any one of claims 1 to 3, characterized in that the optical subsystem comprises at least one optical multiplexer for multiplexing the individual WDM signals, and / or at least one optical amplifier, and / or minus an optical demultiplexer. 5. Method according to any one of claims 1 to 4, characterized in that the optical subsystem (SUB) comprises converter means (ADM, OXC) for the conversion of spatial position and / or conversion of lengths. and the carrier frequency conversion of individual signals WDM (wdl ... m), - in that the pilot signals (psl ... m) which have been modulated on the individual signals WDM (wdl ... converted are detected within the subsystem (SUB) and - by means of the detection result the association of the transmission channels with the individual signals WDM (wdl ... m) is verified.