WO2013170909A1

WO2013170909A1 - Dummy optical signals for optical networks

Info

Publication number: WO2013170909A1
Application number: PCT/EP2012/061689
Authority: WO
Inventors: Gianmarco Bruno; Riccardo Martinotti
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2012-05-15
Filing date: 2012-06-19
Publication date: 2013-11-21

Abstract

A node of an optical communications network sends (300) working optical signals to other nodes of the network, and generates (310) dummy optical signals for multiplexing with the working signals to alter a spectrum of optical power on any of the optical paths. The dummy optical signals are switched to each of the optical paths, and for each of the optical paths, the selected dummy optical signals multiplexed (330) with the working optical signal. By directing the dummy optical signals to the different optical paths, rather than generating sets of dummy optical signals for each optical path, there can be more flexibility in which wavelengths or how many dummy optical signals can be provided. This can increase a proportion of dummy optical signals in use, or reduce the number of dummy optical signals generated. Wavelength selective switches can be used.

Description

DUMMY OPTICAL SIGNALS FOR OPTICAL NETWORKS

TECHNICAL FIELD

This invention relates to methods of operating nodes of optical communications networks, and to corresponding nodes, and to corresponding programs for such nodes.

BACKGROUND

Wavelength division multiplexing is the transmission of several different signals via a single optical fibre, by sending each signal (also called "channel") at a slight different optical frequency or wavelength. Each signal can be a single wavelength or a band in a flex grid arrangement. A multiplexer is used to combine the different channels together into an optical signal for transmission, and a demultiplexer is used to separate the channels.

WDM optical transmission systems or networks are typically composed of a number of spans, and include a variety of network elements such as terminals, line amplifiers, and add/drop nodes. It is known that it is desirable to control the power levels across all of the channels in a WDM system. The channels lose optical power ("span loss") as they are transmitted over each span of optical fibre. If the transmitted power is too low in any channel then bit errors can result from noise at the receiver. If the transmitted power is too high, then bit errors can result due to spectral distortions e.g. caused by non-linear propagation impairments. To prevent such effects, it is thus typically desirable to equalise power levels across all channels in a WDM system.

Correct implementation of power equalisation allows the system reach to be increased, as more nodes can be cascaded. In addition, nonlinear effects can be better predicted, exploited and controlled. In re-configurable optical systems, spectral distortion of the WDM signal is particularly significant, as the pattern of activated channels in the WDM signal can change over time in response to the traffic requests.

The WDM signal is degraded by optical noise accumulation and impairments due to the propagation through the optical fiber and the optical components. Noise accumulation is unavoidably related to the amplification process, performed via erbium-doped fiber amplifiers (EDFA) or Raman amplifiers.

In order to maximize the transmission reach, the channel launch power is usually set to maximize the optical signal-to-noise ratio (OSNR) whilst keeping nonlinearities under a certain tolerable threshold. Nonlinear effects in optical fiber may be broadly classified as follows:

Stimulated Raman Scattering (SRS) effect: which is a transfer of energy from short-wavelength channels to long-wavelength ones. Unless properly counteracted, this produces a spectral tilt and a related OSNR penalty.

Kerr-based effects: which consist of a phase-modulation induced by strong local optical field.

Nonlinear effects are particularly detrimental for advanced techniques (coherent detection and DSP) employed for 40/100Gbit/s transmission and beyond. In these systems, linear impairments like chromatic dispersion (CD) and polarization mode dispersion (PMD) can be effectively mitigated by coherent detection and subsequent digital signal processing (DSP).

In a well-designed system, the transmission performance should be limited by the optical noise only, i.e. the OSNR. In many systems it is important to keep under control the channel power level and the OSNR in all conditions, so as to:

1) minimize nonlinear penalties from signal propagation in fibre

2) maximize tolerance to fiber linear impairments and match back-to-back performance, and

3) guarantee the same performance with changing traffic conditions, i.e. when the number and spectral distribution of channels change in time

Other sources of OSNR degradation are undesired wavelength-dependent effects like fiber attenuation tilt, imperfections in the amplifier transfer function and non- homogeneous behavior of EDFAs. The first two are static phenomena for which some wavelengths experience higher attenuation or higher amplification, respectively, than others. The last phenomenon occurs when the gain profile of the optical amplifiers changes with the channel load at its input.

Another source of power/OSNR penalty is the channel power depletion from noise accumulation. This is particularly detrimental when the total input power at the EDFAs input is low, usually when only few channels are equipped.

For the reasons above, an OSNR margin is allocated by system designers to tolerate imprecise channel power setting, to counteract imperfect fiber tilt and non-ideal amplifier transfer function and to take into account changes in traffic pattern. In a noise-limited system, this OSNR margin translates into a reduction of maximum reach: for example 1 dB margin corresponds to 20% reduction, 2 dB to 37% and so on. Regarding SRS (Stimulated Raman Scattering), this causes a spectral power gradient or spectral tilt i.e. a variation in the power of the optical signal (which typically comprises a plurality of channels) as a function of wavelength. Such a spectral tilt results in different channels having different optical powers. In optical transmission systems utilising WDM, short wavelength channels interact with long wavelength channels via SRS. The net effect is to increase the apparent span loss for short wavelength channels, and to decrease the apparent span loss for longer wavelength channels. The degree of tilt (i.e. the gradient of the variation of power with wavelength) varies with the total optical power of all of the wavelengths.

A variety of solutions have been proposed to address the issue of spectral tilt. In many instances, such solutions require additional equipment and/or communication links (communication channels) between nodes.

For example, US 6,275,313 describes how the tilt due to SRS is approximately linear on a dB/nm scale, and depends solely on total input power and not on the input power distribution. Thus it is suggested that the total input power to the fibre should be maintained at a constant level, such that the resulting gradient can be compensated for or cancelled by using a fixed optical filter. In particular, it is suggested that an optical control signal is provided at a power level over the fibre, in addition to the plurality of optical communications signals, such that the total power is maintained at a pre- determined value irrespective of the number of the optical communication signals.

It is known from US 7,738,791 to provide filling channels in a WDM signal, and to change the optical power of the filling channels to compensate for changes in optical power of the information carrying signals. The compensation is distributed across filling channels of different wavelengths so as to provide a minimum change in centre of gravity of the common spectrum of all the channels.

SUMMARY

According to a first aspect of the invention there is provided a method of operating a node of an optical communications network, having the steps of sending working optical signals on two or more optical paths from the node to other nodes of the network, and generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. The method also involves switching each of the dummy optical signals onto a selected one or more of the optical paths, and for each of the optical paths, multiplexing the one or more dummy optical signals with the working optical signal.

By selecting which of the dummy optical signals to use on the different optical paths, rather than generating sets of dummy optical signals for each optical path, there can be more flexibility in which wavelengths or how many dummy optical signals can be provided for each optical path. This can increase a proportion of dummy optical signals in use, or reduce the number of dummy optical signals which need to be available for use. This can lead to simpler lower cost hardware, and reduced power consumption, particularly of power used by dummy optical signal generators in a standby mode. See figs 1 and 2 for example.

Any additional features can be added to these features, and some such additional features are set out below and set out in dependent claims and described in more detail. One such additional feature is the step of selecting which wavelengths the dummy optical signals have for each of the optical paths. This can enable more flexibility and better spectral control. The selecting can encompass for example determining which ones to select, or responding to an indication of which ones to select and outputting the selected wavelengths. See figs 4 and 8 and 9 for example.

Another such additional feature is the step of generating the dummy optical signals comprising generating a common dummy optical signal and branching the dummy optical signals from the common dummy optical signal. This can also reduce the number of optical sources and thus reduce costs and power consumption, and reduce a footprint of the hardware. Also it can help reduce a number of different spare parts that need to be made available, thus simplifying maintenance. The branching can encompass for example splitting or demultiplexing by wavelength or polarization. See fig 4, fig 8, and fig 11 for example.

Another such additional feature is the common dummy optical signal comprising a white noise signal and having the step of selecting at least one subset of wavelengths. This can enable more flexibility in terms of which wavelengths can be used, and can enable better spectral control. Compared to tuning of tunable components, selecting subsets of wavelength is typically quicker. It can also reduce a footprint of the hardware and help reduce a number of different spare parts that need to be made available, thus simplifying maintenance. Another such additional feature is the step of selecting the one or more subsets being dynamically controllable. This can also enable more flexibility and better spectral control. See figs 4 and 8 and 9 for example.

Another such additional feature is the step of using an optical selector to switch one or more additional working optical signals to replace one or more dummy optical signals before the multiplexing. This can enable more rapid set up of new wavelengths without causing transients and without delays caused by for example needing to reset optical amplifiers along the optical path to cope with changes in optical power spectrum. There need not be a one to one correspondence in the replacement, so each working signal may replace several dummy signals or several working signals may replace a single dummy signal for example. See fig 6 for example.

Another such additional feature is where there are two or more dummy optical signals on the same optical path, there is a step of selecting which of the two or more dummy optical signals to replace, according to which of the dummy optical signals has wavelengths closer to that of the additional working optical signal. This can enable better spectral control and thus less fluctuation in optical power spectrum. See fig 6 for example. In an alternative example, the selection may not be the closest in wavelength, if this can produce better performance for any reason, for example to counteract spectral hole burning, or non-homogenous behaviour of amplifiers, or to reduce bunching of wavelengths at one side of a band.

Another such additional feature is where multiple working signals are multiplexed onto a respective one of the optical paths, there is a step of detecting a loss of one of the multiplexed working optical signals and in response, selecting a dummy optical signal and multiplexing it with the remaining ones of the working optical signals. This can reduce transient fluctuations in optical power spectrum and can give better spectral control. See fig 8 for example.

Another such additional feature is the step of controlling an attenuation level of the dummy optical signals before they are multiplexed, to alter a spectrum of optical power on a respective one of the optical paths. This can enable better spectral control and thus less fluctuation in optical power spectrum.

Another such additional feature is where the dummy optical signals are coupled to inputs of a wavelength selective switch configured to switch the working optical signals, and the step of switching the selected dummy optical signals is carried out by controlling the wavelength selective switch to switch selected wavelengths of the dummy optical signals to the selected one of the optical paths. This can make use of existing equipment and thus improve resource usage and reduce hardware costs.

Another aspect of the invention provides a node for an optical communications network, configured to send working optical signals on two or more optical paths from the node to other nodes of the network, and having one or more optical sources for generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. A switch is provided configured to switch each of the dummy optical signals to a selected one or more of the optical paths, and for each of the optical paths, a multiplexer configured to multiplex the one or more dummy optical signals with the working optical signal.

An additional feature is the node having a wavelength selector for selecting wavelengths of the dummy optical signals. See fig 3 for example.

Another additional feature is the one or more optical sources comprising a common source configured to generate a common dummy optical signal and an optical branching part configured to branch the dummy optical signals from the common dummy optical signal. See fig 3 for example.

Another additional feature is the common source being configured so that the common dummy optical signal comprises a white noise signal and the node also having a selector to select a subset of wavelengths from the white noise signal. See fig 3 for example.

Another such additional feature is the wavelength selecting being dynamically tunable in use to control which wavelengths are selected.

Another such additional feature is an optical selector coupled to an input of the corresponding multiplexer so as to replace one or more dummy optical signals coupled to an input of the corresponding multiplexer with one or more additional working optical signals. See fig 5 for example.

Another such additional feature is two or more of the optical selectors coupled for replacing corresponding dummy optical signals, and an optical switch configured to direct the additional working optical signal to a selected one of the optical selectors so as to replace one of the dummy optical signals, according to which of the dummy optical signals has wavelengths closer to that of the additional working optical signal. See fig 5 for example.

Another such additional feature is the node being configured to multiplex two or more working optical signals onto the optical paths, and to select a dummy signal in response to detection of a loss of one of the multiplexed working optical signals and to multiplex the selected dummy optical signal with the remaining ones of the working optical signals. See fig 7 for example.

Another such additional feature is attenuators configured to control an attenuation level of the dummy optical signals before they are multiplexed, to alter a spectrum of optical power on a respective one of the optical paths. See fig 8 for example.

Another such additional feature is a wavelength selective switch configured to switch the working optical signals to corresponding ones of the optical paths, and having inputs for receiving the dummy optical signals from the one or more optical sources, the wavelength selective switch part being configured to select and direct the dummy optical signals to an output part corresponding to the selected one of the optical paths. See figs 9 and 10 and 11 for example.

Another aspect of the invention provides a computer program having machine-readable instructions which when executed by a processor can cause the processor to perform a method of operating the above mentioned node, to receive an indication of which dummy signals are allocated to which of the optical paths and to control the switch to switch each of the dummy optical signals to a selected one or more of the optical paths according to the allocation. In the case of the switch being implemented by wavelength selective switches, a first wavelength selective switch can distribute the dummy optical signals of all wavelengths to others of the wavelength selective switches one for each optical path. These other wavelength selective switches can select which of the wavelengths to use as dummy optical signals on their optical path.

Yet another aspect of the invention provides an optical communications network comprising a node configured to send working optical signals on two or more optical paths from the node to other nodes of the network. The node comprises one or more optical sources for generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths and a switch configured to switch each of the dummy optical signals to a selected one or more of the optical paths. The node further comprises, for each of the optical paths, a multiplexer configured to multiplex the one or more dummy optical signals with the working optical signal.

Any of the additional features can be combined together and combined with any of the aspects, or be disclaimed from any of the aspects. Other effects and consequences will be apparent to those skilled in the art, especially over compared to other prior art. Numerous variations and modifications can be made without departing from the claims of the present invention. Therefore, it should be clearly understood that the form of the present invention is illustrative only and is not intended to limit the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which:

Fig 1 shows a schematic view of a node according to first embodiment,

Fig 2 shows steps in operating a node according to an embodiment,

Fig 3 shows a node having white noise optical source,

Fig 4 shows steps in operating a node using a common optical source,

Fig 5 shows a node having optical selector,

Fig 6 shows steps in operating a node using an optical selector,

Fig 7 shows a node having detection of loss of working optical signal,

Fig 8 shows steps in operating a node to add further dummy channel to compensate for loss,

Fig 9 shows a node having wavelength selectors WSS and attenuators,

Figs 10 and 11 show a node having WSSs,

Fig 12 shows steps in using a WSS based node,

Fig 13 shows a further embodiment of a node using WSSs, and

Fig 14 shows an embodiment of an optical communications network.

DETAILED DESCRIPTION

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes.

Abbreviations:

EDFA Erbium Doped Fiber Amplifier

OSNR Optical Signal-to-Noise Ratio

OSC Optical Supervisory Channel

ROADM Reconfigurable Optical Add Drop Multiplexer

SRS Stimulated Raman Scattering WDM Wavelength Division Multiplexing

WSS Wavelength Selective Switch

WSON Wavelength Switched Optical Network

Definitions:

Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps and should not be interpreted as being restricted to the means listed thereafter. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

Elements or parts of the described nodes or networks may comprise logic encoded in media for performing any kind of information processing. Logic may comprise software encoded in a disk or other computer-readable medium and/or instructions encoded in an application specific integrated circuit (ASIC), field programmable gate array (FPGA), or other processor or hardware.

References to nodes can encompass any kind of switching node, not limited to the types described, not limited to any level of integration, or size or bandwidth or bit rate and so on.

References to switches can encompass switches or switch matrices or cross connects of any type, whether or not the switch is capable of processing or dividing or combining the data being switched.

References to programs or software can encompass any type of programs in any language executable directly or indirectly on processing hardware.

References to processors, hardware, processing hardware or circuitry can encompass any kind of logic or analog circuitry, integrated to any degree, and not limited to general purpose processors, digital signal processors, ASICs, FPGAs, discrete components or logic and so on. References to a processor are intended to encompass implementations using multiple processors which may be integrated together, or co- located in the same node or distributed at different locations for example.

The functionality described herein can be implemented in hardware, software executed by a processing apparatus, or by a combination of hardware and software. The processing apparatus can comprise a computer, a processor, a state machine, a logic array or any other suitable processing apparatus. The processing apparatus can be a general-purpose processor which executes software to cause the general-purpose processor to perform the required tasks, or the processing apparatus can be dedicated to perform the required functions. Embodiments can be programs in the form of machine-readable instructions (software) which, when executed by a processor, perform any of the described methods. The programs may be stored on an electronic memory device, hard disk, optical disk or other machine-readable storage medium or non-transitory medium. The programs can be downloaded to the storage medium via a network connection.

Modifications and other embodiments of the disclosed invention will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

By way of introduction to features of embodiments of the invention, some discussion of known features will be presented.

OSNR considerations

WDM optical communication networks are systems often composed of a large number of fiber spans and a variety of network elements like terminals, line amplifiers and reconfigurable add/drop nodes (ROADMs) sometimes implemented by means of wavelength selective switches (WSSs). Error-free operation must be guaranteed over system life and with changing traffic pattern as expected in wave length- switched optical networks (WSONs).

A traditional solution to minimize the OSNR penalty is to perform OSNR pre- emphasis, as was implemented in commercial DWDM products. The main problem is the need for a backward communication from the receive node back to the transmit node to properly setting the launch power of the transmitted signals. The complexity of such approach in a meshed network becomes unmanageable. In addition, this solution is unpractical in protection schemes for its intrinsic slowness.

Spectral Hole Burning (SHB) effects are addressed by US Patent 2006/0051093,

"System and method for spectral loading an optical transmission system" by Manna.

The problem of OSNR penalty induced by traffic-dependent SRS, EDFA imprecise flatness and channel power depletion is not addressed. Another known solution is based on using a minimum set of dummy channels, i.e. active channels not carrying information, to keep the total power at the amplifiers above a certain value. This solution aims to counteract the channel power depletion by noise accumulation and was implemented in the commercial Marconi Solstis UPLx DWDM product. The problem of OSNR penalty induced by traffic-dependent SRS, EDFA imprecise flatness and channel power depletion is not addressed by this approach. Traffic-dependent changes are addresses by the above mentioned US 7,738,791 which provides dummy channels called filling channels in a WDM signal, and changes the optical power of the filling channels to compensate for changes in optical power of the information carrying signals.

The advantages of the dummy channels can be summarised as enabling reduced OSNR margin and faster set up of new optical channels, as will be explained.

Reducing OSNR margin

By removing uncertainties in spectral management, it is possible to reduce the system OSNR margin of the order of 1.5 dB. In a typical ONSR- limited long haul design, this would correspond to raising the target reach from, e.g. 1000 to 1000* 10^(1.5/10) = 1400 km, indicatively. An example of margins for a prior art system and one with dummy channels is shown in table 1.

OSNR / OSNR margin Prior art One embodiment

[dB] of current

invention

Target OSNR for BER 16 16

10-3

i.e. 1 12 Gbit/s DP- QPSK

Margin for channel 0.5 0

depletion due to

imperfect ASE noise

accumulation (typ)

Margin for wavelength- 1 1

dependant

power/OSNR variation

(typ, assumed no

OSNR pre-emphasis)

Margin for EDFA non- 0.5 0

homogeneous

behavior

Table 1 Example of OSNR budget

Accelerating channel provisioning in WSON

In WSON path provisioning consists in activating new optical channels and implies the reset of optical amplifier (that need to change their output power accordingly) and the switching of ROADM nodes (to setup a continuous lightpath between source and destination). With respect to optical amplifiers, dummy channels may be used to preload the spectrum, anticipating future traffic requests and reducing virtually to zero the amplifier adjustment time. By adding intentionally N dummy channels to an optical link it is possible to pre-set the optical amplifiers to enable faster channel provisioning of M<=N new traffic channels. Just M of the N dummy channels would be swapped with traffic channels and the time for propagating the control information through a very long chain of optical amplifier would be saved.

This is particularly useful in WSON networks having strings of remote optical amplification stages, where it can take minutes when wavelengths are rerouted and amplifiers need to be re-set to carry the new channels. This delay can be cut to seconds if dummy channels are pre-set and the switching done just at terminals, without resetting amplifiers.

Reducing number of optical sources for dummy channels

Embodiments of the present invention can address one disadvantage of dummy channels which has not previously been recognised. This is related to the proliferation of optical sources required to produce dummy channels that grows (non-linearly) with the number of links L and the required number of dummy wavelengths W. This causes increased cost and complexity. For a network of L links, the required number NS of sources is:

This can be addressed by loading the transmission bandwidth of each transmission section (that is the link between any couple of ROADM nodes) with dummy channels that are generated by optical sources that are shared between all ROADM directions. Fig 1 , Node according to first embodiment

A node architecture which implements this is shown in the embodiment of figure 1. Node 60 has an optical switch part 50 which routes working optical signals either to output line B or output line C. An output multiplexer 10 is provided for combining the working optical signals with dummy optical signals for output to line B. An output multiplexer 20 is provided for combining the working optical signals with dummy optical signals for output to line C. The optical switching parts are for sending working optical signals on two or more optical paths from the node to other nodes of the network. One or more optical sources 40 are provided for generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. An optical switch 30 is provided for switching the dummy optical signals to each of the optical paths, via corresponding multiplexers 10 or 20 for multiplexing with the working optical signal for output on line B or line C. Such switching of the dummy nodes can encompass a one to many switch, that is one which distributes the same dummy signal selectively to any one or more of the optical paths. It can also encompass a many to one switch, that is one which for a given optical path, can select which of a number of dummy signals (identical or different ones) is used on that optical path. It can also encompass a many to many switch, that is one which can select destinations for many dummy signals. If a common optical source is provided, this arrangement provides generation of dummy signals where the number of optical sources can increase proportional with the number of ROADM nodes and not with the number of optical links. In principle the optical sources can be internal or external to the node or shared between a number of nodes.

Fig 2, steps in operating a node according to an embodiment

Figure 2 shows steps in operating a node such as the node of figure 1 or other embodiments. Step 300 shows sending working optical signals on two or more optical paths from the node to other nodes of the network. At the same time, step 310 shows generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. At step 320 the dummy optical signals are switched to each of the optical paths, and at step 330 the selected dummy optical signals are multiplexed with their corresponding working optical signal. By selecting which of the dummy optical signals to use on the different optical paths, rather than generating sets of dummy optical signals for each optical path, there can be more flexibility in which wavelengths or how many dummy optical signals can be provided for each optical path. Thus fewer optical sources are needed.

Fig 3, Node having white noise optical source

Figure 3 shows a schematic view of another embodiment similar to the node of figure 1, and similar reference signs are used for corresponding parts. The one or more optical sources 40 are shown as being implemented by an optical white noise generator 100 provided for generating dummy optical signals. This feeds a white noise optical signal to an optical branching part 1 10 for generating multiple optical signals. In principle this can be an optical splitter, for dividing the optical power producing signals having identical optical spectra. The optical branching part could instead be a wavelength demux device, producing signals having different wavelengths or wavelength bands. Tunable optical filters 120 can be provided to select the wavelengths of each of the dummy signals. In some cases the optical filters can be fixed rather than tunable, or there may be no filtering at all if there is sufficient filtering provided by the optical wavelength demuxer. Some advantages arising if the optical source is implemented as a white noise generator are as follows:

1 There is no need to tune or switch on/off the dummy channels (e.g. because a working signal is being sent at their wavelength): the WSS will just pick the noise slice at another frequency;

2 In WSON path provisioning can be speeded-up thanks to the constant operation of the optical amplifiers: there is no need to change the amplifier output channel power because its power is shared between traffic or dummy channels whose sum is kept constant;

3 Due to the nature of white noise, any kind of flexible grid is supported and there is no need to have sources for a given grid (50 GHz or 100GHz etc);

4 Spare part reduction can be achieved because one kind of optical source for the generation of dummy channels is good for the entire network.

5 Implementation can be less expensive and more reliable. The optical white noise generator can be any kind of broadband optical noise source. One possibility is to use Superluminescent LEDs (SLED) which generate optical noncoherent light over a wide bandwidth, that is, noise. An instrument based on this concept is made by Optilab, part number SLD-LC-20-1550, and shown at

http://www.oequest.eom/getproduct/19899/cat/1282/page/3. Another possibility is to use ASE noise from Erbium Doped Fiber Amplifiers. An example is made by Amonics, Part Number: ALS-C-15-R, s h o w n a t hup ://www.oequest. com/ getproduct/ 19968/cat/ 1845/page/ 1.

In both cases, bandwidth and power levels are suitable. The optical noise source does not have to be perfectly flat because the WSS will equalize it before sending to the DWDM transmission line. Many other examples of such optical sources can be found. Fig 4, steps in operating a node using a common optical source

Figure 4 shows steps in operating a node such as the node of figure 3 or other embodiments. Step 300 shows sending working optical signals on two or more optical paths from the node to other nodes of the network. At the same time, step 315 shows generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. This is done by optical branching of a common dummy signal generated by a common source. A selection 325 is made of which of the dummy optical signals is to be used on each of the optical paths, and to select which wavelength is used. This wavelength selection may involve tuning the optical filters, or, if several wavelengths are fed to the controller, it can select which wavelengths are fed to which output. For each of the optical paths, the selected one or more dummy optical signals is switched to the corresponding multiplexer for multiplexing 330 with the working optical signal.

Fig 5, Node having optical selector

Figure 5 shows a schematic view of another embodiment similar to the node of figure 1 , and similar reference signs are used for corresponding parts. Here, an optical selector 150 is provided at the input of output multiplexer 10 for selecting either the dummy optical signal or a further working optical signal. The optical selector can switch the additional working optical signal to replace the dummy optical signal before the multiplexing. This can enable more rapid set up of new wavelengths without causing transients and without delays caused by for example needing to reset optical amplifiers along the optical path to cope with changes in optical power spectrum. Also this can enable cheaper optical amplifiers to be used which have worse specifications for handling transients on the signals being amplified. The function of the optical selector can be implemented by a WSS in another example as described below.

Fig 6, steps in operating a node using an optical selector

Figure 6 shows steps in operating a node such as the node of figure 5 or other embodiments. As in figure 2, step 300 shows sending working optical signals on two or more optical paths from the node to other nodes of the network. At the same time, step 310 shows generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. At step 320 the dummy optical signals are switched to each of the optical paths, and at step 330 the selected dummy optical signals are multiplexed with their corresponding working optical signal. At step 340, the additional working optical signal is added by replacing a dummy optical signal or signals at the input to the output multiplexer, so as to maintain an overall optical power level. If there are two or more dummy signals, then the dummy signal or signals having wavelengths closer to that of the additional working optical signal are replaced. This can help reduce changes in the optical power and the optical spectrum, and help avoid transients and help avoid delays involved in resetting optical amplifiers downstream of the node.

Fig 7, node having detection of loss of working optical signal

Figure 7 shows a schematic view of another embodiment similar to the node of figure 5, and similar reference signs are used for corresponding parts. Here, an indication of loss of working optical signal is fed to optical switch 30. Typically this indication comes from a message passed from a downstream node over an overhead channel, and can be used to control the switch as will be described.

Fig 8, steps in operating a node to add further dummy signals to compensate for loss Figure 8 shows steps in operating a node such as the node of figure 7 or other embodiments. As in figure 2, step 300 shows sending working optical signals on two or more optical paths from the node to other nodes of the network. At the same time, step 310 shows generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. At step 320 the dummy optical signals are switched to each of the optical paths, and at step 330 the selected dummy optical signals are multiplexed with their corresponding working optical signal. At step 360, the loss of working signal on line C is detected. At step 370, an additional dummy optical signal having a wavelength close to that of the lost working optical signal is switched by the switch so as to be selected for multiplexing in place of the lost working optical signal. Again this can help reduce changes in the optical power and the optical spectrum, and help avoid transients and help avoid delays involved in resetting optical amplifiers downstream of the node. Fig 9, node having wavelength selectors and attenuators

Figure 9 shows a schematic view of another embodiment similar to the node of figure 5, and similar reference signs are used for corresponding parts. In this case, one arrangement for a wavelength select part 121 , for selecting the wavelengths of the optical dummy signals, is shown in more detail. Input signals from the optical sources 40 are each fed to optical wavelength demuxers 250, 260, 270, 280. There can be any number of these. Individual wavelengths are output from the demultiplexers and fed to optical selectors 170. These are controlled to select which wavelengths are fed to the optical switch 30, for switching to line B and line C respectively for example. There can of course be many more lines and any wavelength can be sent to any of the lines. Other ways of implementing the wavelength select part can be envisaged. Attenuators 160 can be provided to control the power levels of each of the dummy signals individually. This can be controlled dynamically for each line to provide a constant optical power level, and in principle can be implemented before switching. The optical sources can be based on a common optical source and an optical branching part, or a bank of individual sources producing a band of wavelengths or producing white noise. The function of the wavelength select part can be implemented by WSS parts in another example as will be described.

Figs 10, 11 node having WSS

Figure 10 shows a schematic view of another implementation of a node 60 according to another embodiment. The node includes a ROADM 62 with three bidirectional WDM lines A, B and C to other nodes. A WSS is provided for each of these lines. WSS 950 is provided for line A, associated with a line amplifier 930. WSS 920 is provided for line B, associated with a line amplifier 910. WSS 960 is provided for line C, associated with a line amplifier 940. Each of the WSSs are interlinked so that wavelengths arriving on any of the lines can be routed to either of the other lines. Optical sources 40 for dummy optical signals are provided to inputs of WSS 950. WSS 950 can then be used as a controller to select and direct the dummy optical signals to be passed to any of the lines.

Figure 1 1 shows a schematic view of internal structure of a WSS such as WSS 960. The bottom part is just a splitter 400 that takes a WDM incoming signal from the line amplifier 940 and splits it into N equal parts which are fed to the N other WSSs. The WDM signal carries M wavelengths.

The top part is for signals in the other direction. It receives a WDM signal from each of the other WSSs in the same node. A demux 440 is provided for each of the N ports. In principle the signals could be received from the other WSSs in demultiplexed form if enough separate paths were provided, in which case there would be no need for this demux. Each demux has M outputs, so there are logically "MxN" wires. A set of M switches (420...430) is provided, one for each of the M wavelengths. Each switch is coupled to receive the same wavelength from all of the N demuxes, and select one for output. Each selected wavelength is then attenuated independently as required. An output mux 410 groups the M wavelengths and produces a WDM signal for output to the line amplifier and then on to the neighbouring node.

This can be regarded as a functional diagram. In some cases the switch and attenuator function may be realized optionally by using a tilting mirror, or other arrangement, but this is an implementation detail. The WSS performs a filtering transfer function because each wavelength passes through a demux and then a mux which together will have a bandpass amplitude response.

For WSS 950 of figure 10, this would be similar to the view of figure 1 1 , but the optical sources for the dummy signals would each be coupled to their own demux 440 if the optical sources provide a broadband signal. For cases where the optical sources provide a narrow band signal of a fixed or selectable wavelength, this could be coupled to an input of the corresponding one of the switches.

This is effectively making use of the "add" function of a ROADM, or if the node is a cross connect rather than a ROADM, the WSS of one of the lines could be used for adding the dummy wavelengths instead of coupling the WSS to a line.

Fig 12, steps in using WSS node

Figure 12 shows steps in operating a node such as the node of figure 10 or other embodiments. As in figure 2, step 300 shows sending working optical signals on two or more optical paths from the node to other nodes of the network. At the same time, step 310 shows generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths. Using optical white noise as common optical source is one possible implementation. At step 339 the WSS 950 distributes each of the dummy optical signals of all wavelengths to all the other WSSs in the same node. At step 341, WSS 920 is controlled to select which wavelengths of the dummy signals are to be used on the optical path of line B, by controlling the appropriate ones of switches 420,430 of WSS 920. At the same time, WSS 960 is used to select which of the wavelengths is used for the dummy signals for line C, again by controlling the appropriate ones of switches 420, 430 of WSS 960. The selected one or more of the demultiplexed wavelengths for the dummy optical signals are passed from outputs of the WSSs to the corresponding multiplexer for multiplexing with the working optical signal. At step 342, the attenuation of the selected wavelengths is controlled before they are multiplexed with the working optical signals.

Thus in order to replace an existing dummy signal with a new working signal at the same wavelength, the working signal will be fed to the demultiplexer 440, and then the switch 420 or 430 corresponding to the wavelength of the signals is controlled to select the new working signal instead of the dummy signal.

In order to move a dummy signal from say line B to line C, the dummy signal is broadcast to all WSSs, and the relevant switch 420, 430 at the WSS 920, and corresponding to the wavelength of the dummy signal, will be switched off, then the relevant switch 420, 430 corresponding to the same wavelength at WSS 960 will be switched on to select the dummy signal.

In order to replace a failed working signal with a dummy signal of a nearby wavelength, the switch 420, 430 corresponding to the wavelength of the failed working signal will be switched off, and the switch 420, 430 corresponding to the dummy signal of a nearby wavelength will be switched on to select the dummy signal.

The WSS arrangement with full interconnections between WSS parts, and full switching of all directions makes this a colourless and directionless type of node. Other examples can be envisaged with more restrictions on colours or directions on all or some of the possible paths, if fewer interconnections or fewer switched paths are provided.

Fig 13, further embodiment of a node using WSS

Figure 13 shows a schematic view of a similar node to that of figure 10. One source of dummy signals serves a ROADM node made of an arbitrary number D of directions. Each direction is equipped with a WSS and a booster amplifier BA. For each wavelength, the WSS picks the traffic from the desired direction or from the dummy source or produces a null output. Interconnections between the WSSs are not shown explicitly, for the sake of clarity, but can be assumed. As in figure 10 a WDM dummy source is shared between all outgoing links. A ROADM node supporting D directions will dedicate one port per WSS to the dummy source. This is an acceptable limitation since D=16 WSS are available today and D=23 are foreseen in the short term.

In one example of a relatively simple WSON with 7 ROADMs, there might be 15 links between nodes. This number of links is in between the minimum (7 if they were in a ring) and (7*6/2)=21 if they were fully connected so it can be taken as a reasonably representative example. If the dummy channels are not provided at every node, since they can pass through nodes so as to be used on multiple links, it may be sufficient to provide four nodes equipped with the optical sources, each generating dummy channels of various wavelengths. If two of the four nodes have four directions, and two have three directions, then the saving in numbers of optical sources can be determined as follows. According to the prior art, one optical source per ROADM per direction is required (making 14 in total). With the proposed method, only one optical source per ROADM is sufficient (making 4 in total in this example). This means fewer line interfaces are required.

Fig 14 illustrates an example of an optical communications network 1400 comprising nodes 60 implemented according to embodiments of the present invention described earlier. The network 1400 may also comprise nodes 1402 that do not implement the present invention.

Power consumption

When evaluating the effect on power consumption, the effects of having dummy channels "on" as described are notable. There is an increase of perhaps tenths of mW of power (optical, at module level) that corresponds to some W of electrical power (electrical, at card level). This and a very small increase in the power requested by amplifiers to amplify the dummy channels can be compensated many times over by the reduction in electrical power consumption due to the reduced number of power-hungry line interfaces.

Concluding remarks

The possibility to arbitrarily and dynamically route the dummy channels among the several ROADM ways, as shown, with no practical wavelength assignment constraints is useful because it can enable much faster path reconfiguration and restoration times (for example in the range of seconds instead of for example 30 minutes, as measured in today's networks). Furthermore, this can also enable use of cheaper EDFAs with less demanding transient specifications (an issue with current EDFAs that shortens the list of potential suppliers).

These benefits will typically outweigh the costs of additional equipment (one EDFA + splitter) required at each ROADM node, and increased WSS cost, due to the additional port reserved to the dummy channels.

This is preferable to using an array of lasers for the dummy channels, managed in some way (i.e. shutting them on/off or changing their power), for a number of reasons.

a) There is a waste of bandwidth if you keep them always on or additional hardware with fast controls if you manage them adaptively.

b) Two dummy channel sources are needed per link (both directions) so the number of sources grows as N*D where N is the number of nodes, and D is the meshing degree (typically D=3 to 4). In a completely meshed network the required sources are N*(N- \)I2.

c) This is unfit for upgrading to a flex grid (unless a set of finely tunable fill lasers is used, which is costly and complex).

As has been described, some examples are based on the recognition that ROADMs are based on high-degree Wavelength Selective Switches and white noise is a relatively cheap, reliable source for the dummy signals, in at least some of the embodiments:

1) A dummy optical source is shared amongst two or more links, i.e. arranging a directionless transmit port of a ROADM node so the number of required said sources is just N, compared to N*D or ~N^A2.

2) WSS functionality present in existing ROADMs is exploited "for free" to perform the on/off control of dummy channels, so there is no need for a separate optical switch for switching the dummy channels.

3) Using broadband white noise as a common source for the dummy channels provides complete freedom in wavelength of the dummy channels, so it is ready for upgrade to flexible grid networks.

4) There is no waste of bandwidth: if you need 100% traffic channels on a link, the transmit WSS just does not select dummy channels to be transmitted on that line thanks to WSS functionality.

5) Switching can be done locally to the ROADM node.

Note also that in optical networks having for example 40G/100G DP-QPSK long haul links there is a particular need for careful control of fill channels where the amplifiers' optical power for few-channel operation is too high and it is more important to fill the spectrum to get the correct in/out power levels.

Other variations and embodiments can be envisaged within the claims.

Claims

1. A method of operating a node of an optical communications network, having the steps of:

sending working optical signals on two or more optical paths from the node to other nodes of the network,

generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths,

switching each of the dummy signals, onto a selected one or more of the optical paths, and for each of the optical paths multiplexing the one or more dummy signals with the respective working optical signal.

2. The method of claim 1, having the step of selecting which wavelengths the dummy optical signals have for each of the optical paths.

3. The method of claim 1 or 2 the step of generating the dummy optical signals comprising generating a common dummy optical signal and branching the dummy optical signals from the common dummy optical signal.

4. The method of claim 3, the common dummy optical signal comprising a white noise signal and having the step of selecting one or more subsets of wavelengths.

5. The method of claim 4, the step of selecting the one or more subsets being dynamically controllable.

6. The method of any preceding claim having the step of using an optical selector to switch one or more additional working optical signals to replace one or more dummy optical signals before the multiplexing.

7. The method of claim 6, where there are two or more dummy optical signals on the same optical path, and having the step of selecting which of the two or more dummy optical signals to replace, according to which of the dummy optical signals has wavelengths closer to that of the additional working optical signal.

8. The method of any preceding claim, where multiple working signals are multiplexed onto a respective one of the optical paths, the method having the step of detecting a loss of one of the multiplexed working optical signals and in response, selecting one or more dummy optical signals and multiplexing the selected dummy optical signals with the remaining ones of the working optical signals.

9. The method of any preceding claim having the step of controlling an attenuation level of the dummy optical signals before they are multiplexed, to alter a spectrum of optical power on a respective one of the optical paths.

10. The method of any preceding claim, where the dummy optical signals are coupled to inputs of a wavelength selective switch configured to switch the working optical signals, and the step of switching the selected dummy optical signals is carried out by controlling the wavelength selective switch to switch selected wavelengths of the dummy optical signals to the selected one of the optical paths.

11. A node for an optical communications network configured to send working optical signals on two or more optical paths from the node to other nodes of the network, and having:

one or more optical sources for generating dummy optical signals for multiplexing with the working optical signals to alter a spectrum of optical power on any of the optical paths,

a switch configured to switch each of the dummy optical signals to a selected one or more of the optical paths, and

for each of the optical paths, a multiplexer configured to multiplex the one or more dummy optical signals with the working optical signal.

12. The node of claim 11, having a wavelength selector for selecting wavelengths of the dummy optical signals.

13. The node of claim 11 or 12, the one or more optical sources comprising a common source configured to generate a common dummy optical signal and an optical branching part configured to branch the dummy optical signals from the common dummy optical signal.

14. The node of claim 13, the common source being configured so that the common dummy optical signal comprises a white noise signal and the node also having a wavelength selector to select a subset of wavelengths from the white noise signal.

15. The node of claim 14, the wavelength selector being dynamically tunable in use to control which wavelengths are selected.

16. The node of any of claims 11 to 15 having an optical selector coupled to an input of the corresponding multiplexer so as to replace one or more dummy optical signals coupled to an input of the corresponding multiplexer with one or more additional working optical signals.

17. The node of claim 16, having two or more of the optical selectors coupled for replacing corresponding dummy optical signals, and an optical switch configured to direct the additional working optical signal to a selected one of the optical selectors so as to replace one of the dummy optical signals, according to which of the dummy optical signals has wavelengths closer to that of the additional working optical signal.

18. The node of any of claims 11 to 17 having attenuators configured to control an attenuation level of the dummy optical signals before they are multiplexed, to alter a spectrum of optical power on a respective one of the optical paths.

19. The node of any of claims 11 to 19, the switch comprising a wavelength selective switch configured to switch the working optical signals to corresponding ones of the optical paths, and having inputs for receiving the dummy optical signals from the one or more optical sources, the wavelength selective switch part being configured to switch selected wavelengths of the dummy optical signals to the selected one of the optical paths.

20. A computer program having machine-readable instructions which when executed by a processor can cause the processor to perform a method of operating the node of any of claims 1 1 to 20, the method having the steps of receiving an indication of which dummy signals are allocated to which of the optical paths and controlling the switch to switch each of the dummy optical signals to a selected one or more of the optical paths according to the allocation.

21. An optical communications network comprising a node configured to send working optical signals on two or more optical paths from the node to other nodes of the network, said node comprising: