WO2022034280A1

WO2022034280A1 - Noise loading in an optical system

Info

Publication number: WO2022034280A1
Application number: PCT/GB2021/051777
Authority: WO
Inventors: David Anthony Frisch
Original assignee: Neptune Subsea Ip Limited
Priority date: 2020-08-10
Filing date: 2021-07-12
Publication date: 2022-02-17
Also published as: GB202012386D0

Abstract

A method (1200) for noise loading an optical channel, comprising: allocating (1201) a 5 plurality of wavelength slots of an available spectrum in an optical fibre as comprised in traffic channels or noise loaded channels, wherein each noise loaded channel comprises a noise loaded wavelength slot and a monitoring wavelength slot for receiving data signals. A system (101) for noise loading is also disclosed.

Description

NOISE LOADING IN AN OPTICAL SYSTEM

Technical Field

The present invention relates to a system and method for noise loading an optical channel.

Background

Fibre optic communication networks are used to send information between network nodes, using the principles of internal reflection to send an optical signal through an optical fibre. This technique can be used to communicate information across hundreds or thousands of kilometres - referred to as ‘long haul optics’ . In long haul optics, optical signals are transmitted on a plurality of separate channels over a single optical fibre, according to Wavelength Division Multiplexing (WDM) or Dense Wavelength Division Multiplexing (DWDM), placing distinct channels of the optical signal on separate wavelengths.

There are technical challenges in communicating from one place to another efficiently and effectively using long haul optics and WDM or DWDM.

Typically, communication networks use a range of signal wavelengths that the system is designed to accommodate. Standardised bands are defined for optical telecommunications (e.g. C band, L band etc.). In WDM communication, a band is divided into channels, each comprising a distinct range of wavelengths/frequencies of the band. The transmission band may comprise a number of frequency (or wavelength) slots that could be 6.25, 12.5, 25, 37.5 GHz wide, for example. A channel may be defined as a spectral region comprising one or more adjacent wavelength slots.

Noise loading is used to ensure satisfactory transmission of the channels over the band when one or more channels is not occupied with data signals. Channels may be empty of data signals due to a fault with, or disconnection of, one or more channels in the communication network, for example. Rather than leaving empty channels without optical power, it is common to transmit optical noise on empty channels. Optical communication systems are generally finely optimised on the assumption that all

SUBSTITUTE SHEET (RULE 26) channels are loaded, so noise loading in this way helps maintain the performance of the system.

EP3306835A1 describes some prior art methods for noise loading in the context of optical communications.

Once noise is injected to fill a particular channel, restored data signals on that channel may be difficult or impossible to detect.

Embodiments of the present invention seek to address problems with the prior art.

Summary

An aspect of the present disclosure provides a system for noise loading an optical channel, comprising: an optical switch arrangement comprising an input port configured as a noise port to receive noise, and an output port, wherein the optical switch arrangement is configured to: allocate each of a plurality of wavelength slots at the output port as comprised in: i) a traffic channel, available for receiving data signals; or ii) a noise loaded channel, receiving at least one noise loaded wavelength slot from the noise port, and a monitoring wavelength slot available for receiving data signals. There is at least one noise loaded channel at the output port

The term “optical signal” may refer to any combination of data signal and/or noise signal. The present disclosure refers to wavelength slots - these are essentially equivalent to frequency slots.

The noise may be broadband noise (e.g. resulting from amplified spontaneous emission). The term “noise” is intended to encompass signals that are suitable for loading an empty channel to maintain system performance, and is not restricted to broadband white noise, for example. Any source that produces a signal that approximates noise over the spectral range of the noise loaded wavelength slots may be used as a noise source.

The (or each) noise loaded channel may receive more than one noise loaded wavelength slot. Some or all traffic channels may comprise more than one wavelength slot. At least some traffic channels may have a different number of wavelength slots.

SUBSTITUTE SHEET (RULE 26) In some embodiments, the wavelength slots may have a spectral width up to 37.5 GHz. The wavelength slots may have a spectral width up to: 6.25 GHz, 12.5 GHz, or 25 GHz.

At least some of the traffic channels may comprise a plurality of adjacent wavelength slots. Each noise loaded channel may comprise a plurality of adjacent wavelength slots.

Each traffic channel may comprise a plurality of adjacent wavelength slots.

There may be a plurality of traffic channels, and at least some of the traffic channels may have different spectral widths. The wavelength slots may be equal in spectral width.

The optical switch arrangement may comprise a Wavelength Selective Switch.

The system may further comprise a suitable noise source connected to the noise port.

The system may further comprise a monitor connected to the output port, configured to detect data signals in the monitoring wavelength slot of the noise loaded channel.

The system may further comprise a controller configured to reconfigure the optical switch arrangement in response to the monitor detecting a data signal in the monitoring wavelength slot.

The controller may be configured to cause the noise loaded channel to be configured as a traffic channel in response to detecting the data signal in the monitoring slot of the noise loaded channel.

The system may further comprise an equaliser for controlling relative optical power in each channel of the output port. The monitor may be configured to monitor the optical power in each noise loaded channel and each traffic channel, and to control the equaliser. The equaliser may match the average signal power spectral density in each

SUBSTITUTE SHEET (RULE 26) noise loaded channel with a power spectral density obtained from at least one traffic channel.

The equaliser may be configured to measure power spectral density for one or more of the traffic channels. The equaliser may be configured to store power spectral density information. The equaliser may be configured to determine appropriate signal loading values - using the measured/stored power spectral density information - for wavelength slots reconfigured as noise loaded channels.

The monitor may be configured to control the equaliser to match the total power in a channel between the cases of the channel being a traffic channel or a noise loaded channel. In some embodiments, the power of different traffic channels may vary - for example for performance reasons. Therefore, the equaliser may be configured to match the power spectral density of a noise loaded channel to a previously measured power spectral density for a traffic channel occupying the same wavelength slots. In other embodiments the power spectral density of each noise loaded channel may be configured to match an average power spectral density of the traffic loaded channel(s).

The equaliser may be arranged before the optical switch arrangement, after it, or integrated with it.

The optical switch arrangement may comprise a plurality of input ports, and the noise port for receiving noise may be provided as one of the input ports and the remaining input ports may be traffic ports, and wherein the optical switch arrangement may be configured to: allocate each of a plurality of wavelength slots at the output port to receive a corresponding wavelength slot from one of the input ports; and allocate each of a plurality of channels at the output port as: i) a traffic channel, receiving a corresponding channel from one of the traffic ports; or ii) a noise loaded channel, receiving at least one noise loaded wavelength slot from the noise port, and a monitoring wavelength slot from one of the traffic ports.

The system may further comprise a data signal source connected to each traffic port.

Another aspect of the present disclosure provides a method for noise loading an optical channel, comprising: allocating a plurality of wavelength slots of an available

SUBSTITUTE SHEET (RULE 26) spectrum in an optical fibre as comprised in traffic channels or noise loaded channels, wherein each noise loaded channel comprises a noise loaded wavelength slot and a monitoring wavelength slot for receiving data signals.

The monitoring wavelength slot is not noise loaded (e.g. a noise power spectral density in the monitoring wavelength slot may be less than 5% of the noise power spectral density in noise loaded wavelength slots).

The method may further comprise monitoring the monitoring wavelength slot for a data signal, and re-allocating a noise loaded channel as a traffic channel in response to detecting a data signal from a data signal source in the monitoring slot.

Re-allocating a noise loaded channel as a traffic channel may comprise uncoupling a noise source from the noise loaded channel.

Re-allocating a noise loaded channel as a traffic channel may further comprise coupling the data signal source to each wavelength slot that previously comprised the noise loaded channel.

The method may comprise using a wavelength selective switch to allocate wavelength slots as either traffic channels or noise loaded channels.

The wavelength selective switch may comprise a plurality of input ports and an output port connected to the optical fibre, and one of the input ports may be a noise port receiving noise (e.g. broadband noise), and the remaining input ports may be traffic ports, wherein the wavelength selective switch is configured to: couple each traffic channel in the optical fibre to wavelength slots of the traffic ports; and couple each noise loaded channel in the optical fibre to at least one wavelength slot of the noise port and a monitoring wavelength slot from one of the traffic ports.

In certain embodiments, the wavelength slots may have a spectral width of up to 37.5 GHz, 25 GHz, 12.5 GHz or 6.25 GHz.

The method may further comprise equalising an average power spectral density in each of the traffic channels and noise loaded channels at the output port.

SUBSTITUTE SHEET (RULE 26) The method may further comprise monitoring the power spectral density in each of the traffic channel and noise loaded channels.

According to another aspect, there is provided a system for noise loading an optical channel, comprising: an optical switch arrangement comprising: a noise port to receive noise; at least one traffic port configured to receive data signals; and an output port, wherein the optical switch arrangement is configured to: allocate each of a plurality of wavelength slots at the output port as comprised in: i) a traffic channel, comprising one or more adjacent wavelength slots connected to at least one traffic port; ii) a noise loaded channel, comprising one or more wavelength slots connected to the noise port, and one or more wavelength slots connected to the at least one traffic port.

Each wavelength slot in the noise loaded channel is connected to both the noise port and the at least one traffic port.

According to another aspect, there is provided a method for noise loading an optical channel, comprising: allocating a plurality of wavelength slots of an available spectrum in an optical fibre as comprised in traffic channels or noise loaded channels, wherein each noise loaded channel is connected to both a traffic source and a noise source.

Each wavelength slot in the noise loaded channel may be connected to both the noise port and the at least one traffic port.

Features of each aspect may be combined with those of any other aspect, including optional features.

Detailed Description

Examples are shown in the figures, in which:

SUBSTITUTE SHEET (RULE 26) Figure 1 shows a portion of a communication band comprising plurality of fixed width traffic channels, corresponding with a fixed-grid.

Figure 2 shows a portion of a communication band configured with traffic channels having different spectral width, corresponding with a flexible-grid, and with vacant regions of spectrum that are not occupied by any signals.

Figure 3 shows the portion of a communication band of Figure 2, with noise loading in the previously vacant regions of spectrum.

Figure 4 shows a portion of a communication band comprising traffic channels, and noise loaded channels in accordance with an embodiment, with a monitoring wavelength slot in each noise loaded channel.

Figure 5 shows the example of Figure 4, with a data signal in the monitoring wavelength slot.

Figure 6 shows an optical switch arrangement for noise loading according to an embodiment.

Figure 7 shows the optical switch arrangement of Figure 6, in combination with a further optical switch arrangement for combining traffic signals with the noise loading.

Figure 8 shows an optical switch arrangement according to examples of the present disclosure.

Figure 9 shows MUX units and switches according to examples of the present disclosure.

Figure 10 shows gratings and adjustable reflectors according to examples of the present disclosure.

Figure 1 1 shows a WSS according to examples of the present disclosure.

Figure 12 shows a system according to examples of the present disclosure.

Figure 13 shows a method according to examples of the present disclosure.

Embodiments of the present disclosure are described by way of example in more detail below.

Current opto-electronics uses coherent modulation at rates of 30-90 billon symbols per second (i.e. 30-90 Gbaud) to transport up to 800 Gbits per second per channel. An optical fibre can currently carry approximately 88 channels, each of 50GHz spectral range - in fixed grid technology. Current ITU-T standards (ITU-T G694.1 02/2012) defines fixed grid channel spacings of 12.5 GHz, 25 GHz, 50GHz or 100 GHz. The

SUBSTITUTE SHEET (RULE 26) available spectral band is divided into equal channels that are separated by the fixed grid spacing.

In flex grid technology, the spectral band allocated to each channel is flexible, with different channels allowed to have different spectral width. Current ITU-T standards (as referenced above) define a slot width granularity of 12.5 GHz so that each channel has a wavelength/frequency slot width of m - 12.5 GHz, where m is an integer. The granularity of the flexible grid is defined as 6.25 GHz, so that each channel can be positioned in steps of 6.25 GHz. It is possible, for example, to provision a channel with 6.25GHz guard bands to either side (not allocated to any other channels). Flex allows channels with different bandwidths to be accommodated in a single fibre with greater spectral efficiency, since each channel uses only the amount of spectrum necessary. Channels with high modulation rates (capable of supporting high bandwidth) typically require channel widths of 37.5 to 100 GHz (or, in wavelength terms 0.3-0.8nm).

It is helpful in some cases for the spectral power density across the band of an optical fibre to be even. Figure 1 shows a WDM spectrum with fixed channel width, in which all channels are present.

Figure 2 shows a spectrum that may be more typical with flexible grid channel allocation. Because of evolution towards higher symbol rates and wider traffic signals in telecommunications, the optical signal includes a mixture of traffic types (with some channels having more spectral width than others) and some parts of the spectrum are vacant of optical signal.

The missing parts of the spectrum may be a result of a fault in the optical network, or certain wavelength slots may be empty for future use. As stated above, repeatered optical links operate better with an even, or substantially even, distribution of power over the spectrum. Additionally, repeatered amplifiers often operate at a fixed optical power designed to accommodate a full spectrum of channels. Therefore, missing parts of the spectrum present a potential problem.

Missing parts of the spectrum may therefore be filled with noise power, in order to create an even (or substantially even) distribution of optical power. This may be

SUBSTITUTE SHEET (RULE 26) referred to as noise loading. Figure 3 illustrates a spectrum with traffic channels 127 and noise loaded channels 125 in the vacant portions of the spectrum. It is possible to automatically allocate noise loading to vacant portions of spectrum, by monitoring the spectrum, and then coupling a noise source to any portions of spectrum that are identified as vacant.

It may be challenging to subsequently detect if a vacant portion of spectrum has become occupied with a data signal. Firstly, the noise on the vacant channel may make it difficult to detect a data signal. Secondly, connecting a noise source to a channel may disconnect the channel from any data signal source.

Figure 4 illustrates an example of a frequency range including traffic channels 127 and noise loaded channels 125 according to an embodiment, showing power with respect to frequency (corresponding with wavelength). In this example, a communication band 1403 is shown, comprising a plurality of 12.5GHz frequency/wavelength slots. The band shown is 300 GHz wide, and may correspond with a portion of a wider communication band (e.g. C-band etc.). The band in this example is divided into wavelength slots of 12.5 GHz. Channels 127, 125 are defined, each comprising three adjacent wavelength slots. Channels 1 to 3 and 8 (numbering from left to right) are traffic channels 127 occupied with data signals. Channels 4 to 7 are noise loaded channels. In this example each noise loaded channel 125 comprises a noise loaded wavelength slot 136 on either side of a central monitoring wavelength slot 135, but the monitoring slot 135 can, in principle, be in any position in the noise loaded channel 125.

Each noise loaded channel 125 may have an average power spectral density that matches the average power spectral density in the traffic channels 127. Since the monitoring slot 135 is vacant, the power spectral density in the noise loaded wavelength slots 136 may be higher than the average power spectral density of the traffic channels 127. The average power spectral density Pioadmg- f°^r the noise loaded wavelength slots 136 in a noise channel 125, may be determined from the following:

SUBSTITUTE SHEET (RULE 26) Where P_Signai is the average power spectral density for a traffic channel, n_clianneL is the total number of wavelength slots in the noise loaded channel and n_noise is the number of noise loaded wavelength slots.

The number of wavelength slots in each channel need not be three, and in other examples, different numbers of wavelength slots may be used per channel (e.g. two wavelength slots per channel). Different channels (e.g. noise loaded channels or traffic channels) may comprise different numbers of wavelength slots. For example, some channels may be relatively broad (facilitating high data rates) and others narrower (supporting lower data rates). It is not essential that every channel include multiple wavelength slots. The allocation of wavelength slots may comprise a mix of unmonitored noise loaded channels (without a monitoring wavelength slot) and monitored noise loaded channels (comprising a mix of at least one noise loaded wavelength slot and at least one monitoring wavelength slot).

The monitoring wavelength slots may be monitored, so as to detect a data signal (for example in the case that traffic appears on a portion of spectrum that has previously been allocated as a noise loaded channel due to it being otherwise vacant). A data signal in the monitoring slot may be detected by simply detecting an increase in total signal power in the noise loaded channel. Alternatively, a monitor may be configured to detect optical signals within the monitoring slot (i.e. with a spectral resolution sufficient to distinguish between signals in the monitoring wavelength slot from any adjacent noise signals. The noise loaded channel can be re-configured as a traffic channel in the event a data signal is detected (for example, with the noise loaded wavelength slot(s) connected to the same port that is connected to the monitoring wavelength slot at which a data signal is detected).

Figure 5 shows the same set of 8 channels, as in Figure 4, but with optical signal power present in the monitoring wavelength slot 135. This may indicate that traffic has been restored to that channel and can be used to prompt reconfiguration of that channel from a noise loaded channel to a traffic channel.

Figure 6 shows a system 101 for noise loading one or more optical channels including an optical switch arrangement 110. The optical switch arrangement 110 has an input

SUBSTITUTE SHEET (RULE 26) port configured as a noise port 105 (i.e. configured to receive noise, or loading signals that approximate noise). A noise source 115 may be connected to the noise port 105.

The optical switch arrangement 110 also has an output port 109, from which the optical switch arrangement 110 may be configured to transmit an optical signal into an optical fibre 113 for onward transmission. A “wavelength blocker” is one example of an optical switch whereby the wavelength blocker attenuates, or blocks, one or more of the wavelength slots from input port 105 to output port 109 in optical switch arrangement 110.

The optical switch arrangement 110 is configured to allocate each of a plurality of wavelength slots at the output port 109. One or more traffic channels (each traffic channel comprising one or more adjacent wavelength slots) may be allocated for receiving data signals - e.g. WDM signals.

The optical switch arrangement 110 is configured to allocate a noise loaded channel for receiving at least one noise loaded wavelength slot from the noise port 105. In each noise loaded channel, a monitoring wavelength slot is allocated which is kept free from noise loading, so that data signals in the noise loaded channel can be detected. The optical switch arrangement 110 is thereby arranged to enable an appearing data signal (which may be a new data signal or a data signal that reappears after a fault, for example) to be identified in the monitoring wavelength slot.

Figure 7 shows an example system for noise loading comprising a first optical switch arrangement 110a, second optical switch arrangement 110b and coupler 114. The first optical switch arrangement 110a is as described with reference to Figure 4, and is configured to allocate noise loaded channels.

The second optical switch arrangement 110b is configured to allocate traffic channels from one or more traffic ports 107 (each connected to a traffic signal source 117). The traffic channels and noise loaded channels may be combined at the coupler 114, so as to produce a spectrum similar to that shown in Figures 4 and 5.

Traffic channels may be combined with optical signals from the output port 109 using an optical coupler (downstream of the optical switch arrangement 110).

SUBSTITUTE SHEET (RULE 26) Allocating a monitoring wavelength slot to a noise loaded channel enables an elegant approach to noise loading. Figure 8 shows an example system 101 for noise loading, in which the optical switch arrangement 110 comprises N input ports and an output port 109. A noise port 105 for receiving noise is provided as one of the N input ports. The remaining N-l input ports are traffic ports 107 - i.e. there are N-l traffic ports 107. For example, in the most basic arrangement N is two and there is one traffic port 107. Each traffic port 107 may be configured to receive a data signal and may be connected to a data signal source 117 configured to deliver a data signal/network traffic.

In some embodiments, the switch arrangement 110 is configured to allocate each wavelength slot at the output port 109 to only one of the N input ports (or for connection to none of the input ports, for example in the case of a guard band). In the simple case where there is a single traffic port 107, each wavelength slot at the output port is connected to either the noise port 105 or the traffic port 107. In the event that a lack of a data signal is detected in a traffic channel (comprising a plurality of wavelength slots allocated to a traffic port) at the output 109, the switch arrangement 110 may configure the traffic channel at output port 109 to be a noise loaded channel by re-allocating all except one of the wavelength slots to be connected to the noise port 105. The single wavelength slot in the noise loaded channel remains connected to the traffic port as a monitoring wavelength slot, by which data signals appearing on that channel from the connected traffic port can be readily detected. In some channels (e.g. a 100G channel, comprising many wavelength slots), it may be possible to configure a noise loaded channel with multiple monitoring slots, which may be connected to different traffic ports 107 of the switching arrangement 110.

In the event that data signals are detected in a monitoring slot, the noise loaded channel can be reconnected to the traffic port from which data signals are detected (e.g. the traffic port that was previously allocated to that channel), for example to automatically restore a traffic channel after a failure is corrected.

The optical switch arrangement 110 may take any suitable form for allocating channels as described above.

SUBSTITUTE SHEET (RULE 26) Figure 9 shows an example optical switch arrangement 1 10 comprising a noise input MUX 605, a data signal input MUX 607, an output MUX 609 and optical switches 61 1. In this simple depiction, there are two input MUX units 605,607 and one output MUX unit 609, but it will be appreciated that an arbitrary number of data signal input MUXs 607 may be provided, depending on the number of traffic ports that are to be accommodated.

Noise input MUX unit 605 comprises a noise port 105 for receiving noise and is configured to de-multiplex the noise into a plurality of wavelength slots. In this embodiment, each wavelength slot corresponds with a channel. Data signal input MUX 607 comprises a traffic port 107 for receiving data signals that occupy various wavelength slots in the band and is configured to de-multiplex these data signals into the same wavelength slots as noise input MUX 105. Each optical switch 61 1 has an output connected to an input port of the output MUX 609, and is configured to select whether each input MUX 605,607 is connected to that input port of the output MUX 609. In this simple example, each switch 61 1 is a 2x1 switch, and is configured to, for each wavelength slot of the output MUX 609, connect the corresponding wavelength slot from the data signal input MUX 107 or connect the corresponding wavelength slot from both the data signal input MUX 107 and the noise input MUX 105.

The output MUX unit 609 multiplexes the data signals and noise onto an output fibre 1 13. The switches 61 1 ensure that each wavelength slot in the output fibre is connected either to a traffic port or to both the noise port and a traffic port. In this way, a traffic signal can be detected re-appearing in wavelength slot that is noise loaded. This approach of connecting both a noise source and a traffic source to a channel may be applicable to any of the other embodiments - it is not essential that a monitoring wavelength slot is always left vacant of noise loading.

In some embodiments, the optical switch arrangement 1 10 may be a WSS, which may provide a more elegant optical switch arrangement 1 10 than the multiple MUX units of Figure 9, for example, as the elements performing multiplexing and switching are housed in a single piece of equipment. Furthermore, WSSs may be compatible with flexgrid technology.

SUBSTITUTE SHEET (RULE 26) In general, a WSS comprises at least a dispersing mechanism (for de-multiplexing the optical signal into constituent wavelengths) and a switching mechanism. Figure 10 shows a WSS comprising a set of gratings 703, 709 and adjustable reflectors 711. This arrangement is illustrative of the basic operation of an example WSS. The dispersing mechanism - for example, the gratings 703 - de-multiplex an incoming optical signal to be sent to the reflectors 711, which redirect the optical signal to an output grating 709 to recombine the wavelengths. The WSS 111 may also include one or more beam focusing elements.

There are a number of different arrangements of dispersing mechanisms and switching mechanisms that the WSS 111 could employ. For example, the WSS 111 may harness Micro-Electrical-Mechanical Systems (MEMS) technology - for example, the WSS 111 may have MEMS-micromirror architecture, for example the adjustable reflectors 711 may be or may comprise a micromirror array. Each wavelength may have a dedicated MEMS-micromirror. Each MEMS-micromirror (or other reflector 711) may be individually adjustable, for example individually tiltable/rotatable. The adjustable reflectors 711 may be arranged to tilt/rotate about an axis perpendicular or substantially perpendicular to the incoming optical signal direction.

Alternatively, the WSS 111 may employ liquid crystal switching as a switching mechanism. The WSS 111 may employ liquid crystal on silicon (LCoS) as a switching mechanism, for example. The WSS 111 may include a liquid crystal cell - applying a voltage to the liquid crystal cell may adjust the reflective angle. The WSS 111 may include a liquid crystal array such that optical phase adjustments of the array pixels may adjust the reflective angle for certain wavelengths or wavelength slots.

The reflective angle may be a vertical reflective angle, and the reflective angle from the liquid crystal cell may be set to direct the optical signal to the output port 109 of the WSS 111. The reflective angle may be different for different wavelength slots.

The WSS 111 may include one or more corrective elements, for example a collimator, a polarizer or the like, configured to process the optical signal for efficient switching by the WSS 111. Different wavelengths may be directed to different parts of the liquid crystal cell to be reflected - for example, the one or more corrective elements may be operable to direct a wavelength to a part of the liquid crystal cell accordingly.

SUBSTITUTE SHEET (RULE 26) The WSS 111 of the present disclosure may be configured to operate with a different dispersive mechanism and/or a different switching mechanism.

Whichever dispersive and/or switching mechanism is employed, the WSS 111 may have an input 103 including two or more ports - shown as incoming arrows in Figure 8. Current WSSs typically have 9-20 ports and offer a wavelength granularity of 6.25 GHz.

Figure 11 shows a WSS 111 comprising inputs 103 and output 109. The WSS 111 may be a 1XN switch, having multiple inputs 103 and one output 109. The WSS 111 may comprise a noise port for receiving noise from a noise source 115 and at least one traffic port 107 for receiving data signals.

As explained above for the optical switch arrangement 110 in general, the WSS 111 may comprise a plurality of traffic ports 107; however, for illustrative purposes Figure 11 shows one traffic port 107. For example, the WSS 111 may include more than 4, or more than 8 traffic ports.

As described with reference to Figure 10, the WSS 111 is configured to allocate each of a plurality of wavelength slots at the output port 109 to be connected to one of the input ports 105, 107. At least some of the wavelength slots are designated as channels that comprise multiple adjacent wavelength slots. Some wavelength slots at the output port may not be connected to any of the input ports (e.g. may be reserved as guard bands).

The WSS 111 may be configured to allocate some channels of the output 109 as noise loaded channels. Each noise loaded channel receives at least one noise loaded wavelength slot from the noise port 105. Leaving aside wavelength slots configured as guard bands, if the optical switch arrangement 110 (e.g. the WSS 111) were to allocate noise channels at the output 109 for each wavelength slot that is not occupied by a data signal from any of the traffic ports, as is typically done in prior art noise loading, a spectrum at the out would be as shown in Figure 3.

SUBSTITUTE SHEET (RULE 26) In certain embodiments, not every wavelength slot in each noise loaded channel is noise loaded. At least one wavelength slot in each noise channel is configured as a monitoring wavelength slot and is not noise loaded. The monitoring wavelength slot of the noise loaded channel may then be used to monitor for data signals in the noise loaded channel, received from one of the traffic ports. A data signal/traffic in the monitoring wavelength slot may indicate that a fault has been resolved, for example. Figure 12 shows a system 101 for optical communication comprising: a WSS 111, noise source 115, traffic sources 117, a monitor 131, controller 151 and equaliser 171.

The WSS 111 may function as described with reference to Figure 11. In other embodiments the WSS 111 may be replaced with any optical switch arrangement 110 that functions similarly (e.g. as described with reference to Figure 10).

The monitor 131 is connected to the output 109. The monitor 131 may be an optical channel monitor (OCM). An optical channel monitor (OCM) is a device that detects the power and/or spectral power profile of an optical signal within a channel. The monitor 131, which may be an OCM, may have a minimum spectral resolution of just one slot or some other minimum value that is smaller than the channel. The monitor 131 may be provisioned with 12.5GHz detection slots, so that power and/or optical signals can be detected in each 12.5 GHz slot of a wavelength/frequency grid. The monitor 131 may be configured to monitor (at least) each monitoring wavelength slot (in each noise loaded channel) for an optical signal and/or power (e.g. data and/or power).

If the monitor 131 - for example, the OCM - detects a significant loss of the traffic channel power (a fault condition), then the controller 151 may switch a noise signal into the channel (reconfigure the traffic channel as a noise loaded channel).

The optical switch arrangement 110 may be configured to allocate one or more slots to the noise source which would contain a similar amount of optical power as the original data channel and may allocate one or more slots to a fraction of the original data signal which is currently at low or zero power due to the fault condition.

The optical switch arrangement 110 may allocate one or more slots to a combination of data signal and noise signal, for example 50% power of each within the one or more

SUBSTITUTE SHEET (RULE 26) slots, such that the total noise power within the channel is similar to the total power of the original data signal within the channel.

The monitor 131 may include an optical signal analyzer to identify when an optical signal within a channel as detected by the monitor 131 - for example OCM - is composed of both a noise signal and a data signal of significant power. This may be due to detecting a data signal of significant power within a slot where there is no or insignificant noise power. This may be due to detecting a significantly higher level of power within one or more of the slots or the entire channel than would be present from just the noise signal itself.

When the optical signal analyzer detects both noise signal and data signal within the channel, the controller 151 may switch the noise signal out of the channel and switch the data signal into the channel, returning to an original (no fault) switch configuration.

The monitor 131 may be configured to provide an output signal to the controller 151 and/or the equaliser 171 indicating the total power in each slot (or the power spectral density).

The equaliser 171 receives the signal from the monitor 131 indicating the relative powers in each wavelength slot of the output 109 of the WSS 1 11, and may be configured to automatically adjust the optical power in each wavelength slot to achieve the same power spectral density per channel (whether the channels are noise loaded or traffic). The equaliser 171 may, for example, comprise a variable optical attenuator for each wavelength slot.

If a data signal/traffic has been detected in a monitoring wavelength slot, the WSS 1 1 1 may be reconfigured, as other wavelength slots filled with noise will block other wavelengths of the appearing data signal.

The controller 151 may be configured to cause the WSS 1 1 1 to reconfigure in response to the monitor 131 detecting a data signal (i.e. traffic) and/or optical power in a monitoring wavelength slot. The controller 151 may, for example provide a control signal to the WSS 1 1 1 to cause the noise channel (where a data signal has been

SUBSTITUTE SHEET (RULE 26) detected in a monitoring wavelength slot) to be reconfigured as a traffic channel. The controller 151 may be integrated with the WSS 111 or monitor 131.

Figure 13 shows method steps 1200 according to the present disclosure. The method 1200 includes an allocating step 1201 - the allocating step 1201 includes allocating a plurality of wavelength slots of an available spectrum in an optical fibre as comprised in traffic channels or noise loaded channels, wherein each noise loaded channel comprises a noise loaded wavelength slot and a monitoring wavelength slot for receiving data signals.

A first monitoring step 1203 may comprise monitoring the monitoring wavelength slot for a data signal, as described above.

A re-allocating step 1205 may comprise re-allocating the noise loaded channel as a traffic channel in response to detecting a data signal from a data signal source in the monitoring slot. The re-allocating step 1205 may also comprise uncoupling a noise source from the noise loaded channel, and may further comprise coupling the data signal source to each wavelength slot that previously comprised the noise loaded channel.

A wavelength selective switch - for example the WSS 111 - may be used to allocate wavelength slots as either traffic channels or noise loaded channels as part of the allocating 1201 or re-allocating 1205 steps.

The method 1200 may include coupling steps 1207, in which the wavelength selective switch may be configured to couple each traffic channel in the optical fibre to wavelength slots of the traffic ports and couple each noise loaded channel in the optical fibre to at least one wavelength slot of the noise port and a monitoring wavelength slot from one of the traffic ports.

The method 1200 may include an equalising step 1209, including equalising an average power spectral density in each of the traffic channels and noise loaded channels.

SUBSTITUTE SHEET (RULE 26) The method 1200 may include a second monitoring step 1211 including monitoring the power spectral density in each of the traffic channel and noise loaded channels.

Although specific examples have been described, these are not intended to limit the scope of the invention, which should be determined with reference to the accompanying claims.

SUBSTITUTE SHEET (RULE 26)

Claims

1. A system for noise loading an optical channel, comprising: an optical switch arrangement comprising an input port configured as a noise port to receive noise, and an output port, wherein the optical switch arrangement is configured to: allocate each of a plurality of wavelength slots at the output port as comprised in: i) a traffic channel, available for receiving data signals; or ii) a noise loaded channel, receiving at least one noise loaded wavelength slot from the noise port, and a monitoring wavelength slot available for receiving data signals; wherein there is at least one noise loaded channel at the output port.

2. The system of claim 1, wherein at least some of the traffic channels comprise a plurality of adjacent wavelength slots.

3. The system of claim 2, wherein each traffic channel comprises a plurality of adjacent wavelength slots.

4. The system of any preceding claim, wherein there are a plurality of traffic channels, and at least some of the traffic channels have different spectral widths.

5. The system of any preceding claim, wherein the optical switch arrangement comprises a Wavelength Selective Switch.

6. The system of any preceding claim, further comprising a noise source connected to the noise port.

7. The system of any preceding claim, further comprising a monitor connected to the output port, configured to detect data signals in the monitoring wavelength slot of the noise loaded channel.

8. The system of any claim 7, further comprising a controller

SUBSTITUTE SHEET (RULE 26) configured to reconfigure the optical switch arrangement in response to the monitor detecting a data signal in the monitoring wavelength slot.

9. The system of claim 8, wherein the controller is configured to cause the noise loaded channel to be configured as a traffic channel in response to detecting the data signal in the monitoring slot of the noise loaded channel.

10. The system of claim 7, further comprising an equaliser for controlling relative optical power in each channel of the output port, wherein the monitor is configured to monitor the optical power in each noise loaded channel and each traffic channel, and to control the equaliser to match a signal power spectral density measured from at least one traffic channel.

11. The system of any preceding claim wherein: the optical switch arrangement comprises a plurality of input ports, and the noise port for receiving noise is provided as one of the input ports and the remaining input ports are traffic ports, and wherein the optical switch arrangement is configured to: allocate each of a plurality of wavelength slots at the output port to receive a corresponding wavelength slot from one of the input ports; and allocate each of a plurality of channels at the output port as: i) a traffic channel, receiving a corresponding channel from one of the input ports; or ii) a noise loaded channel, receiving at least one noise loaded wavelength slot from the noise port, and a monitoring wavelength slot from one of the traffic ports.

12. The system of claim 11, further comprising a data signal source connected to each traffic port.

13. A method for noise loading an optical channel, comprising: allocating a plurality of wavelength slots of an available spectrum in an optical fibre as comprised in a traffic channel, or a noise loaded channel, wherein each noise loaded channel comprises a noise loaded wavelength slot and a monitoring wavelength slot for receiving data signals.

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14. The method of claim 13, further comprising monitoring the monitoring wavelength slot for a data signal, and re-allocating the noise loaded channel as a traffic channel in response to detecting a data signal from a data signal source in the monitoring slot.

15. The method of claim 14, wherein re-allocating the noise loaded channel as a traffic channel comprises uncoupling a noise source from the noise loaded channel.

16. The method of claim 15, wherein re-allocating the noise loaded channel as a traffic channel further comprises coupling the data signal source to each wavelength slot that previously comprised the noise loaded channel.

17. The method of any of claims 13 to 16, comprising using a wavelength selective switch to allocate wavelength slots as either traffic channels or noise loaded channels.

18. The method of claim 17, wherein the wavelength selective switch comprises a plurality of input ports and an output port connected to the optical fibre, and one of the input ports is a noise port receiving noise, and the remaining input ports are traffic ports; wherein the wavelength selective switch is configured to: couple each traffic channel in the optical fibre to wavelength slots of the traffic ports; and couple each noise loaded channel in the optical fibre to at least one wavelength slot of the noise port and a monitoring wavelength slot from one of the traffic ports.

19. The method of any of claims 13 to 18, wherein the wavelength slots have a spectral width up to 37.5 GHz

20. The method of any preceding claim, further comprising equalising an average power spectral density in each of the traffic channels and noise loaded channels.

21. The method of claim 20, further comprising monitoring the power spectral density in each of the traffic channel and noise loaded channels.

22. A system for noise loading an optical channel, comprising:

SUBSTITUTE SHEET (RULE 26) an optical switch arrangement comprising: a noise port to receive noise; at least one traffic port configured to receive data signals; and an output port, wherein the optical switch arrangement is configured to: allocate each of a plurality of wavelength slots at the output port as comprised in: i) a traffic channel, comprising one or more adjacent wavelength slots connected to at least one traffic port; ii) a noise loaded channel, comprising one or more wavelength slots connected to the noise port, and one or more wavelength slots connected to the at least one traffic port.

23. The system of claim 22, wherein each wavelength slot in the noise loaded channel is connected to both the noise port and the at least one traffic port.

24. A method for noise loading an optical channel, comprising: allocating a plurality of wavelength slots of an available spectrum in an optical fibre as comprised in traffic channels or noise loaded channels, wherein each noise loaded channel is connected to both a traffic source and a noise source.

25. The method of claim 24, wherein each wavelength slot in the noise loaded channel is connected to both the noise port and the at least one traffic port.

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