WO2002080426A1

WO2002080426A1 - Wavelength division multiplexing power control system

Info

Publication number: WO2002080426A1
Application number: PCT/GB2002/001112
Authority: WO
Inventors: Andrew George Lauder
Original assignee: Marconi Communications Limited
Priority date: 2001-03-29
Filing date: 2002-03-12
Publication date: 2002-10-10
Also published as: GB0107929D0

Abstract

A method and apparatus (10) are provided for controlling the signal strength of a channel (μ1) in a wavelength division multiplexed signal that provides means to compensate for the wavelength dependent nature of the gain of optical amplifiers (24) and the attenuation of optical components in a telecommunication system. Monitors (30) are positioned after amplifiers (24) in the optical chain (23) to detect the power levels of the wavelength channel within the WDM signal. The value of the detected power is fed back to a power control unit (18) that compares the power level detected on the path to a theoretical value. If the power level is below a threshold value, the power unit increases the signal transmission power until the output achieves the threshold to achieve the necessary optical signal to noise ratio of the received WDM signal.

Description

WAVELENGTH DIVISION MULTIPLEXING POWER CONTROL SYSTEM

The present invention relates to a wavelength division multiplexing (WDM) power control system and method, and to a transmission system including such a control system. The method and system are suitable for controlling the power of the constituent radiation components (usually termed wavelength channels) comprising a WDM optical communication signal.

Wavelength division multiplexing (WDM) is a well known method used to multiplex many signals into a single communication signal. Each channel in the multiplexed signal has a discrete wavelength different to the other channel's wavelengths. In optical communication systems, it is possible to multiplex up to 80 (and theoretically more) separate channels of data into a single WDM signal and transmit it along a single optical fibre.

Losses in the optical signal, for example caused by fibre absorption, limit the length of fibre along which the signal can be transmitted before the power levels of the wavelength channels becomes unacceptable. Optical amplifiers placed along the communication link periodically boost the optical signal strength, and extend the length of the link. Known Erbium doped fibre amplifiers, and the like, provide good amplification of optical signals in fibre communication systems.

However, there are well known problems associated with WDM signals. The optical amplifier gain is wavelength dependent which results in different wavelength channels being amplified by different amounts. Operational parameters of the communication system require a minimum and maximum optical power for each wavelength channel. This necessitates being able to control the optical power of the individual wavelength channels to keep them within operational limits.

In addition, the optical attenuation of optical components in the communications system can vary with wavelength. These variations might be random or known artefacts caused by the manufacture and/or the design of a system.

All these factors build up along the length of the optical system. This results in some wavelength channels being of increased power, whilst others are of reduced power, as the WDM signal propagate through the system. Thus optical powers of the wavelength channels diverge as they propagate along the communication path.

Furthermore, optical receivers have a tolerance to the levels of noise in the received signal. This noise tolerance is a factor in the acceptable optical signal to noise ratio (OSNR) at the receiver. If the OSNR falls below an acceptable limit, the rate of errors in the detected signal can exceed acceptable limits.

Optical amplifiers can introduce noise into signals, and further degrade the OSNR. The extent to which the OSNR is degraded by an amplifier is dependent on the quality of the amplifier system and the power levels of the wavelength channel being amplified. The lower the power of the wavelength channel, the worse the OSNR degradation becomes. It is possible to calculate the degradation of the resulting OSNR depending on optical power levels using known formulae.

In a system that includes several optical amplifiers along its path, the OSNR degradation varies for different channels because each channel sees different attenuation and gain levels.

There are a number of known solutions that alleviate some of these problems intrinsic to WDM systems. The optical powers can be pre-set so that they remain within limits along the optical path and have sufficient power to achieve the required OSNR at the receiver. This system is very simple to implement. However, the transmission powers must be chosen so that the worst and best case tolerances with respect to wavelength channel gain and optical losses remain within an acceptable range of values. The system design is compromised in an effort to reduce these tolerances, resulting in a loss of performance. Normally only a small number of optical amplifiers and a limited range of network configurations can be handled by this type of system.

Optical power levels can be configured manually by setting the power at which each wavelength channel is transmitted. This can be achieved by direct control of the transmission power, attenuating the signal at the transmitter, or by remote control of the network management system. In all cases an unacceptable level of human intervention is required.

The transmission power of the WDM signal can be adjusted, depending on measured received optical power and OSNR, to set the received parameters to acceptable levels. This type of system improves on pre-set power type described above. However, this system does not behave well when the network is changed, for example when additional wavelength channels are configured. Changes to a network node configuration can cause optical power level to change for signals passing through that node. This results in the transmission power of the wavelength channels having to be changed, even though they are not related to the wavelength channels that have been added to or removed from the system when the node was reconfigured. It might be necessary for engineers to visit multiple sites to bring the network back to normal operation once changes have been implemented. The subsequent interruption to service can result in unacceptable system down-time, and the additional cost to the network operator is considerable.

A third method is to automatically control the optical power in the network. This is achieved by de-multiplexing the multiplexed WDM signal, for example at a node or amplifier, selectively attenuating each wavelength channel, re-multiplexing the signal for amplification or transmission, and re-transmitting the signal. This allows the power in each wavelength channel to be controlled at each node or selected nodes on the network. It is possible to keep optical channel powers at the same level for all channels within close tolerances using this method. Such a system will compensate well for the wavelength channels experiencing different gain levels in the amplifiers. The power levels are chosen to meet all system requirements and deliver an acceptable OSNR at the end of each optical channel. This system also allows the network to automatically compensate for changes to its configuration. However, this system is expensive and is not suitable for a low cost WDM system. The additional optical components required are relatively expensive and introduce optical losses in the system. This requires additional amplifiers to compensate for the additional optical losses.

It is an aim of the present invention to ameliorate or eliminate the problems experienced by the prior art systems mentioned above. In its broadest form, this is achieved by feeding back signal strength data from points on the communication path to ascertain the received signal characteristics. The feed back data is compared to a threshold criteria and necessary adjustments to the transmission power for that channel are made to achieve the desired received signal strength for the channel.

More specifically, the present invention provides a wavelength division multiplex (WDM) power control system for use in an optical transmission system, the transmission system comprising, a transmission power controller providing an input to a multiplexer for controlling the power of each wavelength channel in a multiplexed signal, and a signal amplifier on a communication path between the multiplexer and a de-multiplexer for amplifying the multiplexed signal, the WDM power control system comprising: feedback means for feeding back an optical power value for a given transmission wavelength channel to the transmission power controller; a comparator for comparing the optical power value to a threshold value; and a power optimiser for adjusting the transmission power to optimise the received signal in accordance with the result of the comparison.

Also, the present invention provides a transmission system for transmitting a wavelength division multiplexed optical signal, the system comprising: a multiplexer for multiplexing a plurality of signals onto a communication path disposed between a transmitter and a receiver: the transmitter comprising, a plurality of transmitters each for transmitting a signal into a channel of the multiplexed signal; and a controller for controlling the transmission power of the signal; the communication path comprising, an amplifier on the path for amplifying the multiplexed signal, a feedback loop for feeding back an optical power value to a comparator for comparing the optical power value to a threshold value; and a power optimiser for adjusting the transmission power to optimise the received signal in accordance with the comparison.

Furthermore, the present invention provides a method for optimising signal strength in a wavelength division multiplexed (WDM) signal, in which signals for transmission are multiplexed onto a transmission path, the method comprising the steps of; feeding back a value of the signal strength of a channel of the multiplexed signal on the communication path; comparing the value with a threshold value; and adjusting the initial signal transmission power for that channel to optimise the received signal in accordance with the comparison.

Embodiments of the present invention will now be described, by way of example, and with reference to the drawings, in which;

Figure 1 is a schematic view of an optical WDM transmission system embodying the present invention;

Figure 2 is a graphical representation of calculated optical wavelength channel power against amplifier position in optical path; Figure 3 corresponds to Figure 2 and further includes a plot of measured wavelength channel power versus amplifier position; and

Figures 4 and 5 are graphical representations of Figure 3 shown with increasing transmission power of the measured channel power.

Referring to Figure 1, a WDM optical communication system 10 comprises an optical transmitter 12 (typically a laser and optical modulator) and optical receiver 14. A signal 16 for transmission along the system 10 is supplied to the optical transmitter 12 where it is converted to a modulated optical signal of wavelength λ_\. An optical power control unit 18 controls transmission power of the optical signal. In a preferred embodiment, the power control unit 18 is an electro-optic attenuator, for example a Pockels or Kerr cell. Alternatively, a variable density filter operated by a stepper motor can be used.

In an alternative embodiment, the optical power control unit comprises an optoelectronic detector that measures the optical power of the transmitted signal and controls the optical signal output by a power control feedback circuit 20; this power control can be achieved by controlling a bias voltage to the modulator or laser.

The optical signal λι is multiplexed with other wavelength channels λ₂ to λ„ by an optical multiplexer 22 to form a WDM optical signal which is injected into a communication path 23 (typically an optical fibre). Optical amplifiers 24 (only two amplifiers are shown) optically amplify the WDM signal as it is propagates along the optical path 23 to an optical de-multiplexer 26. The WDM signal is de-multiplexed and the part of the signal of wavelength λι is transmitted to the optical receiver 14. Here it is converted from a modulated optical signal back to an electrical signal which is transmitted on to the signal's destination via conventional means 28. The other demultiplexed signals λ'₂ to λ'_n are transmitted to their respective receiver (not shown).

Optical channel power monitors 30 (two are shown in Figure 1) are positioned along the optical path 23 and operable to monitor the optical power of each wavelength channel λj to λ_n at various positions in the optical path. The power monitors 30 feed back information regarding the monitored optical power of the wavelength channel, and the monitor's position in the optical path 23, to a respective power control unit 18 associated with the wavelength channel. Within Figure 1 only the power control unit 18 and optical transmitter 12 for the wavelength channels λ] are illustrated. A comparator which forms a part of the power control unit (not shown) compares the monitored power to a threshold criteria. The monitored power levels are preferably encoded as digital values before being passed back to the control unit. The threshold criteria can also be stored as digital value thereby enabling the monitored value to be compared with the threshold criteria as a numerical comparison.

The power control unit 18 can, if necessary, adjust the optical transmitter's 12 transmission power to exceed the threshold criteria, and optimise the signal to noise ratio of the optical signal. The adjustment of the transmission power is by any of the methods described earlier.

In the preferred embodiment, the power monitors 30 are positioned after each optical amplifier 24 to measure the optical power at the output of each amplifier. However, systems embodying the present invention may have power monitors placed in strategic positions along the optical path, and not necessarily after each amplifier. For example, to avoid redundancy, in alternative embodiments, the power monitors may be placed after every second amplifier or at each node on a network. This will be dependent on the network configuration and design, and can be different from network to network.

Each optical amplifier in the optical path can also include an electronically controlled variable optical attenuator at its input (not shown). This is used to control the total input power into the amplifier so that the total amplifier output power can be set to the correct level. Description of this control is provided below.

The network can have knowledge of its configuration, including the routes of all optical paths through the network. This knowledge may be manually configured information, or in the preferred embodiment, automatically discovered by the network. Automatic network configuration discovery is not part of the present invention.

The number of optical amplifiers in the optical path, and their performance, has an effect on the build up of optical noise.

Referring to Figure 2, the optical channel power 40 (in dBm) is plotted against the amplifiers positions in the optical path 42 for various optical channels 44. The wavelength dependent nature of the gain in the amplifier medium results in each optical channel's power following a slope as it propagates along the optical path. A channel that sees 0.2dB less than nominal gain will reduce in power by 0.2dB for each amplifier that it passes through, resulting in a negative gradient slope 46. Similarly, a channel that sees 0.2dB more than nominal gain will increase in power by 0.2dB for each amplifier that it passes through, resulting in a positive gradient slope 48. A WDM system will have a range of gain experienced by different wavelength channels, resulting in a range of increasing and decreasing power slopes that is dependent on the wavelength of the operating channels.

The slopes are calculated using known formulae and characteristics of the system optics, including the amplifier type and gain for each channel's wavelength. Using these appropriate formulae, it is possible to calculate an initial optical power at the output of the first amplifier on the optical path for each wavelength channel. The theoretical power levels of the signal as it propagates along the optical path can be calculated to provide an overall gain or power profile. This calculated data is stored in network elements, or stores, as a look-up table.

The initial optical power at transmission of each wavelength channel can be determined so that the power of the channel closely follows a slope shown in Figure 2. The slope is dependent on the initial power, the wavelength of the optical channel being amplified, and the gain experienced by that wavelength channel in the amplifier. The combination of slope and initial power level define a minimum power profile for that slope. If an optical channel is at or above this power profile along its complete path, then it is certain to achieve acceptable OSNR at the receiver; the preferred threshold criteria has been met. If the wavelength channel experiences positive gain, that is a positive slope on the minimum power profile, then it may be necessary to set the amplifier input attenuator to attenuate the initial optical power to such a level that the overall optical gain in the system does not exceed OdB. This is particularly important in a ring network to avoid laser oscillation build up.

Preferably, the amplifier input attenuator adds optical attenuation to the system before the signal is amplified so that the total loss of the preceding section equals the optical gain of the amplifier for that channel. In the preferred embodiment it is necessary to automatically control the input attenuator settings. This can be achieved by including an extension of the optical channel power control system.

The optical channel's powers are controlled on a channel by channel basis. The process described below is repeated for each optical channel in the WDM system.

A system embodying the present invention knows which combinations of amplifiers are present along a given optical path, given the network and channel configurations. A set of minimum power profiles that corresponds to this combination are selected.

Referring to Figure 3, the transmission power of the channel transmitter is adjusted until the optical power in that channel measured at the first amplifier output 50 on the channel path is equal to the minimum power profile 52 that has the lowest initial power. The theoretical amplifier output power is compared with the actual output power and the difference between the two values is used to set the input attenuator's value. Some hysteresis is required in the system to prevent large fluctuations in attenuator values and hence power levels. This is the chosen power profile for this iteration.

The optical power 54 in the channel is monitored at each monitoring point along its path and is compared with the power required 52 for that chosen profile at the final amplification stage. If the difference 56 between the channel's power and the chosen profile is positive (that is the channel's power is greater than, or equal to, the chosen profile's power) then no further action is required; the received signal has sufficient power and an acceptable OSNR is achieved, and a threshold criteria has been met.

If the difference 56 is negative, as shown in Figure 3, then the system increases the input optical power for the channel so that the power after the first amplification stage is equal to the channel power for the next power profile.

Referring to Figure 4, the optical signal's power after the first amplification stage 58 is increased to match the next power profile 60. The process is repeated until the difference 56 is positive, as shown in Figure 5.

Referring to Figure 5, in the preferred embodiment of the present invention, the iterative process described above is repeated until all of the measured optical power 54 is greater than or equal to the chosen power profile 62 along the whole optical path (at every amplification stage). This is the preferred threshold criteria. In this embodiment of the present invention, the system can control the optical transmission power of the signal so that the received optical signal has optimum OSNR. The system regularly compares the power level along the path for each optical channel with the power profile. If the optical signal falls below the chosen power profile at any time, the transmission power is re-adjusted to bring the measured profile onto another suitable profile. Likewise, if the optical power along the path becomes substantially greater than its chosen power profile at all points along the path, then the system reduces that channel's transmission power to bring it closer to a different profile. The preferred embodiment has some built in hysteresis to avoid constant changes to the transmission power. In this way, the system automatically compensates for changes in the network.

The present invention also provides a means for the network to have knowledge of its topology. The system is able to detect which channels are in use from measured channel power values. Knowledge of the profiles chosen for each optical channel allow the system to discover which profiles are in use for each optical channel that passes through a given amplifier, and knowledge of the network and channel configuration allow it to determine at which point along the profile each optical channel is at that amplifier. This information allows the system to calculate a theoretical total output power of the amplifier if each channel is on its chosen power profile.

In a preferred embodiment, the system can detect which channels are currently active by measuring their power levels. A channel with zero power is not current active. For example, a failure condition may have closed down one of the configured channels.

Using knowledge of the active channels and their target, or predicted, power levels at an amplifier, the system can calculate the theoretical total output power levels at that amplifier, assuming that all channels are at their target power levels at that point. The theoretical output power is compared to the measured output power at the amplifier to generate an error signal. The error signal is used to drive the amplifier's input attenuator so as to match the amplified output power of the amplifier with the theoretical output power.

In some circumstances, it may be necessary to modify the minimum power profiles. These modifications are made when the profiles are calculated and are included in the look-up tables.

Normally, OSNR builds up in WDM optical communication systems in such a way that an optical channel with a positive gradient slope starts with a lower transmission power in comparison to an optical channel with a negative gradient slope. This assumption can be built into the system control of the preferred embodiment.

With knowledge of the channel wavelength and expected gain in the system experienced by that wavelength, the system determines whether the transmission power should be higher or lower. For example, if the channel is likely to experience high levels of gain according to the calculation look-up tables, then the initial transmission power is low, as shown in figure 3. The opposite applies if the channel is likely to experience low gain, or loss in the system. In this way, some channels achieve excellent OSNRs.

On optical paths that only include a small number of amplifiers, the OSNR build up is reduced, and power levels to achieve the desired OSNR are less. This may result in some, or all, of the optical power of the channels at the receiver being less than the optical power required to meet the power budget of the receiver. In this circumstance, the power of such channels is raised so that it reaches the necessary optical power at the output of the final amplification stage on the path and the desired OSNR is achieved.

Tolerances in the optical power measurements of the power monitors 30 can be compensated by raising the power profile levels such that the received power level is increased by that tolerance level. For example, if the monitors tolerances are +/-ldB, then the power profile values are increased by ldB from their calculated values.

A network may support many types of amplifiers. In which case, the wavelength dependent nature of the gain in each amplifier type will be different. Each type of amplifier contributes to the build-up of OSNR differently. Different types of amplifiers can appear along the path in any order, which can result in a very large number of lookup tables for each type of amplifier combination.

The number of look-up tables can be minimised by assuming the worst case placement of amplifiers, and then having a look-up table for each combination of numbers of amplifiers. For example, one set of power profiles is required for all paths that have two type R amplifiers and three type S amplifiers, rather than ten profile sets required for every combination of the two R and three S amplifiers (RRSSS, RSRSS, RSSRS, etc.).

The worst case situation depends on the power profile slope. For an optical channel that reduces in power along its path, the worst case is when those amplifiers that contribute most of the OSNR are at the end of the path. The opposite is true for a channel that increases in power along its path. Profiles that are calculated for the worst case scenario achieve the desired OSNR for all amplifier combinations.

In an alternative embodiment where optical power monitors are only placed at strategic positions along the path, and not after every amplifier, the channel powers for amplifiers without monitors can be determined by interpolation.

It is not necessary to store complete minimum power profiles in the network. If the power profiles follow a constant gradient (straight line) then it is necessary to store the initial power and gradient value for each profile.

There a several ways to monitor the optic channel power, all of which are well known in the art. A common monitor works by de-multiplexing a small proportion of the WDM signal and measuring the power in each component of the signal, typically by using an array of PIN diodes. Power monitors that scan the across the multiplexed signal bandwidth are also suitable; such monitors use a tuneable filter and a single detector, typically.

Many modifications to the embodiments described are possible and will occur to those skilled in the art without departing from the scope of the invention. For example, the signal optical power may be measured at the amplifier input, rather than the output. Also, the control unit may control more than one wavelength channel at a time.

Claims

1. A wavelength division multiplex (WDM) power control system for use in an optical transmission system, the transmission system comprising: a transmission power controller providing an input to a multiplexer for controlling the power of each wavelength channel in a multiplexed signal, and a signal amplifier on a communication path between the multiplexer and a de-multiplexer for amplifying the multiplexed signal, the WDM power control system comprising: feedback means for feeding back an optical power value for a given transmission wavelength channel to the transmission power controller for that channel; a comparator for comparing the optical power value to a threshold value; and a power optimiser for adjusting the transmission power to optimise the received signal in accordance with the result of the comparison.

2. A WDM power control system according to claim 1, further comprising a monitor for monitoring the optical power of each channel in the multiplexed signal.

3. A WDM power control system according to claim 1 or 2, wherein the threshold value is determined from a plurality of pre-determined values.

4. A WDM power control system according to claims 1, 2 or 3, wherein the predetermined values are stored in a store.

5. A WDM power control system according to any preceding claim, wherein the signal amplification means comprises a plurality of optical amplifiers.

6. A WDM power control system according to any of claims 2, 3 or 4, wherein the pre-determined values comprise predictions of received signal power for each channel of the multiplexed signal at a plurality of points along the communication path.

7. A WDM power control system according to any preceding claim, wherein the monitor comprises detectors for detecting the power of a signal in each channel of the multiplexed signal, disposed at a plurality of points along the communication path.

8. A WDM power control system according to claim 7, wherein the detection means are disposed after each amplifier.

9. A WDM power control system according to any preceding claim, further comprising an adjustable signal attenuator disposed on the communication path before the multiplexer for attenuating the signal.

10. A WDM power control system according to any preceding claim, wherein the signal attenuator is controlled by the controller.

11. A transmission system for transmitting a wavelength division multiplexed optical signal, the system comprising: a multiplexer for multiplexing a plurality of signals onto a communication path disposed between a transmitter and a receiver: the transmitter comprising, a plurality of transmitters each for transmitting a signal into a channel of the multiplexed signal; and a controller for controlling the transmission power of the signal; the communication path comprising, an amplifier on the path for amplifying the multiplexed signal, a feedback loop for feeding back an optical power value to a comparator for comparing the optical power value to a threshold value; and a power optimiser for adjusting the transmission power to optimise the received signal in accordance with the comparison.

12. A transmission system according to claim 11, further comprising a monitor for monitoring the optical power of each channel in the multiplexed signal.

13. A transmission system according to claim 11 or 12, wherein the threshold value is determined from a plurality of pre-determined values.

14. A transmission system according to claims 11, 12 or 13, wherein the predetermined values are stored in a store.

15. A transmission system according to any preceding claim, wherein the signal amplifier comprises a plurality of optical amplifiers.

16. A transmission system according to any preceding claim, wherein the pre- determined values comprise predictions of received signal power for each channel of the multiplexed signal at a plurality of points along the communication path.

17. A transmission system according to any preceding claim, wherein the monitor comprises a detector for detecting the power of a signal in each channel of the multiplexed signal, disposed at a plurality of points along the communication path.

18. A transmission system according to claim 18, wherein the detectors are disposed after each amplifier.

19. A transmission system according to any preceding claim, further comprising an adjustable signal attenuator disposed on the communication path before the multiplexer for attenuating the signal.

20. A transmission system according to any preceding claim, wherein the signal attenuator is controlled by the controller.

21. A method for optimising signal strength in a wavelength division multiplexed (WDM) signal, in which signals for transmission are multiplexed onto a transmission path, the method comprising the steps of; feeding back a value of the signal strength of a channel of the multiplexed signal on the communication path; comparing the value with a threshold value; and adjusting the initial signal transmission power for that channel to optimise the received signal in accordance with the comparison.

22. A method for optimising signal strength according to claim 21, further comprising monitoring the signal strength of a channel of the multiplexed signal on the communication path.

23. A method for optimising a signal strength according to claim 21 or 22, further comprising amplifying the signal as it propagates along the communication path, by a plurality of amplifiers.

24. A method for optimising a signal strength according to claims 22 or 23, wherein the signal is monitored by a plurality of monitors disposed along the path.

25. A method for optimising a signal strength according to any preceding claim, wherein the monitors are disposed after each amplifier on the path.

26. A method for optimising a signal strength according to any preceding claim, wherein the threshold value is determined from a plurality of pre-determined values.

27. A method for optimising a signal strength according to claim 26, wherein the pre-determined values are expected optical powers at points on the path.

28. A method for optimising a signal strength according to claim 27, wherein the position of the monitors corresponds to the points on the path.

29. A method for optimising a signal strength according to any preceding claim, wherein the transmission power is adjusted so that the signal is greater than the pre-determined signal for all monitors.

30. A method for optimising a signal strength according to any preceding claim, wherein the signal is monitored continuously.

31. A method for optimising a signal strength according to any preceding claim, wherein the signal is monitored at pre-determined time intervals.

32. A method for optimising a signal strength according to any of claims 26 to 31, wherein the pre-determined values account for different amplifier types.

33. A method for optimising a signal strength according to any of claims 26 to 32, wherein the pre-determined values are arranged to account for a maximum build up of optical signal to noise ratio.

34. A method for optimising a signal strength according to any preceding claim, wherein the initial signal transmission power is high when that channel is expected to experience low gain in the amplifiers.

35. A method for optimising a signal strength according to any preceding claim, wherein the initial signal transmission power is low when that channel is expected to experience high gain in the amplifiers.

36. A WDM power control system as substantially described herein with reference to the figures.

37. A transmission system as substantially described herein with reference to the figures.

38. A method for optimising a signal strength as substantially described herein with reference to the figures.