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Title:
DETECTING OPTICAL FIBRE STRESS IN AN OPTICAL COMMUNICATIONS NETWORK
Document Type and Number:
WIPO Patent Application WO/2014/040619
Kind Code:
A1
Abstract:
A method (10) of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes. The method comprises: a. at at least one node at which a respective lightpath across the network terminates, determining an indication of a polarisation mode dispersion value of the lightpath (12); b. comparing the polarisation mode dispersion value of each said lightpath to a respective threshold polarisation mode dispersion value (14); and c. identifying each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value as a stressed lightpath comprising a respective stressed fibre span (16), and if a stressed lightpath is identified, generating an alarm signal comprising an identification of said stressed lightpath (18).

Inventors:
D ERRICO ANTONIO (IT)
FONDELLI FRANCESCO (IT)
Application Number:
PCT/EP2012/067821
Publication Date:
March 20, 2014
Filing Date:
September 12, 2012
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ERICSSON TELEFON AB L M (SE)
BOTTARI GIULIO (IT)
D ERRICO ANTONIO (IT)
FONDELLI FRANCESCO (IT)
International Classes:
H04B10/079; G01M11/08; H04B10/2569
Domestic Patent References:
WO2010075893A12010-07-08
Foreign References:
EP0863626A21998-09-09
US20110142449A12011-06-16
US20040105682A12004-06-03
Other References:
N. SAMBO ET AL.: "Lightpath establishment in distributed transparent optical networks using network kriging", EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION, 20 September 2009 (2009-09-20)
Attorney, Agent or Firm:
PARKINSON, Neil (Midleton GateGuildford Business Park,Guildford, Surrey GU2 8SG, GB)
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Claims:
CLAIMS

1. A method of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes, the method comprising:

a. at at least one node at which a respective lightpath across the network terminates, determining an indication of a polarisation mode dispersion value of the lightpath;

b. comparing the polarisation mode dispersion value of each said lightpath to a respective threshold polarisation mode dispersion value; and

c. identifying each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value as a stressed lightpath comprising a respective stressed fibre span, and if a stressed lightpath is identified, generating an alarm signal comprising an identification of said stressed lightpath.

2. A method as claimed in claim 1 , wherein there are a plurality of lightpaths each terminating at a respective one of the nodes and at least one said lightpath crosses a plurality of fibre spans and at least a further one of the nodes of the network, and wherein if the polarisation mode dispersion value of at least one said lightpath is greater than the respective threshold polarisation mode dispersion value the method comprises, after step b, cross-correlating the said lightpath with at least one said further lightpath and cross-correlating the polarisation mode dispersion value of the said lightpath with the polarisation mode dispersion value of said further lightpath, the further lightpath sharing at least one fibre span with the said lightpath and terminating at one of the nodes of the said lightpath, for identification of a respective stressed fibre span of the said lightpath.

3. A method as claimed in claim 2, wherein, if the cross-correlation identifies a plurality of the fibre spans of the said lightpath as potentially being stressed fibre spans, the method further comprises generating and transmitting a probe optical signal across at least one of the fibre spans identified as potentially being stressed fibre spans and determining the polarisation mode dispersion of the at least one fibre span by measuring the probe optical signal following transmission across said fibre span.

A method as claimed in any preceding claim, wherein each node comprises a coherent receiver and the indication of the polarisation mode dispersion value of each said lightpath terminating at the node is determined by analysis of a filter impulse response of an adaptive equaliser of the coherent receiver.

A method as claimed in any preceding claim, wherein in a. the indication of the polarisation mode dispersion value is determined over a time window of a preset duration and b. further comprises:

for each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value, determining whether a bit error rate of the lightpath within the time window is above a respective threshold bit error rate, and wherein c. comprises:

identifying each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value and the bit error rate is above the respective threshold bit error rate as a stressed lightpath comprising a respective stressed fibre span.

A method as claimed in any preceding claim, wherein the method comprises identifying the nodes within the network and arranging the nodes in a list with each node that is known to have previously terminated a stressed lightpath being provided at the beginning of the list, and wherein a. comprises obtaining the list of the nodes and for each node in order in the list, identifying each lightpath terminated at the node and determining an indication of the polarisation mode dispersion value of each said terminated lightpath.

7. A fibre stress detection element for an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes, the fibre stress detection element comprising a processor configured to:

a. receive an indication (of a polarisation mode dispersion value of each lightpath across the network which terminates at one of the nodes;

b. compare the polarisation mode dispersion value of each said lightpath to a respective threshold polarisation mode dispersion value; and

c. identify each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value as a stressed lightpath comprising a respective stressed fibre span and if a stressed lightpath is identified, generate an alarm signal comprising an identification of said stressed lightpath.

8. A fibre stress detection element as claimed in claim 7, wherein there are a

plurality of lightpaths each terminating at a respective one of the nodes and at least one said lightpath crosses a plurality of fibre spans and at least a further one of the nodes of the network, and wherein the processor is configured to, if the polarisation mode dispersion value of at least one said lightpath is greater than the respective threshold polarisation mode dispersion value, after b., cross- correlate the said lightpath with at least one said further lightpath and cross- correlate the polarisation mode dispersion value of the said lightpath with the polarisation mode dispersion value of said further lightpath, the further lightpath sharing at least one fibre span with the said lightpath and terminating at one of the nodes of the said lightpath, for identification of a respective stressed fibre span of the said lightpath.

9. A fibre stress detection element as claimed in claim 8, wherein, if the cross- correlation identifies a plurality of the fibre spans of the stressed lightpath as potentially being stressed fibre spans, the processor is configured to, after performing the cross-correlation, generate and transmit a probe control signal arranged to cause a probe optical signal to be generated and transmitted across at least one of the fibre spans identified as potentially being stressed fibre spans and the processor is configured to receive an indication of the polarisation mode dispersion of the at least one fibre span measured using the probe optical signal.

A fibre stress detection element as claimed in any of claims 7 to 9, wherein each node comprises a coherent receiver and the indication of the polarisation mode dispersion value of each lightpath terminating at the node is a filter impulse response of an adaptive equaliser of the coherent receiver.

A fibre stress detection element as claimed in any of claims 7 to 10, wherein the indication of the polarisation mode dispersion is obtained over a time window of a pre-set duration and the processor is configured additionally to, at b.:

for each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value, obtain a bit error rate of the lightpath within the time window and determine whether the bit error rate is above a respective threshold bit error rate, and wherein the processor is configured to, at c:

identify each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value and the bit error rate is above the respective threshold bit error rate as a stressed lightpath comprising a respective stressed fibre span.

A fibre stress detection element as claimed in any of claims 7 to 11, wherein the processor is configured to generate a list of the nodes within the network with each node that is known to have previously terminated a stressed lightpath being provided at the beginning of the list, and wherein the processor is configured to implement each of a. to c. for each node in order in the list.

An optical communications network comprising:

a plurality of optical fibre spans;

a plurality of nodes; and

a fibre stress detection element as claimed in any of claims 7

14. An optical communications network as claimed in claim 13, wherein the optical communications network is a dense wavelength division multiplexed mesh network comprising a control plane.

15. A data carrier having computer readable instructions embodied therein for providing access to resources available on a processor, the computer readable instructions comprising instructions to cause the processor to perform the method of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes as claimed in any of claims 1 to 6.

Description:
DETECTING OPTICAL FIBRE STRESS IN AN OPTICAL COMMUNICATIONS

NETWORK

Technical Field

The invention relates to a method of detecting optical fibre stress in an optical communications network. The invention further relates to a fibre stress detection element for an optical communications network and an optical communications network

comprising the fibre stress detection element.

Background

Optical communications networks are affected by physical layer impairments, including fibre stress, the accumulation of which can result in the quality of transmission, QoT, of lightpaths across the network falling below an acceptable QoT threshold. The QoT of a lightpath is typically measured in terms of its bit error rate, BER. For example by a forward error correction, FEC, element of the transponder at the terminating node of the lightpath, or by measuring a number of link-additive metrics (optical signal to noise ratio, OSNR, polarization mode dispersion, PMD, chromatic dispersion, CD, and self phase modulation, SPM) and calculating the BER from these values. N. Sambo et al, "Lightpath establishment in distributed transparent optical networks using network kriging", European Conference on Optical Communication, 20-24 September 2009, Paper 1.5.3, reports the use of BER determined from these link-additive parameters, together with network kriging, to obtain QoT parameter information for a new lightpath to be established across a network. However BER is a summarized parameter in that it takes account of all the physical layer impairments experienced by an optical signal, and therefore does not enable fibre stress within a lightpath to be separately identified.

Summary

It is an object to provide an improved method of detecting optical fibre stress in an optical communications network. It is a further object to provide an improved a fibre stress detection element for an optical communications network. It is a further object to provide an improved optical communications network comprising the fibre stress detection element. A first aspect of the invention provides a method of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes. The method comprises step a. of, at at least one node at which a respective lightpath across the network terminates, determining an indication of a polarisation mode dispersion value of the lightpath. The method comprises step b. of, comparing the polarisation mode dispersion value of each said lightpath to a respective threshold polarisation mode dispersion value. The method comprises step c. of, identifying each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value as a stressed lightpath comprising a respective stressed fibre span. If a stressed lightpath is identified, an alarm signal comprising an identification of said stressed lightpath is generated.

The method may enable stressed lightpaths to be identified using polarisation mode dispersion, PMD, which is directly caused by fibre stress induced by fibre manufacture and packaging, and by environmental factors including thermal variations, mechanical modifications and fibre ageing. In contrast, BER monitoring only reveals more general signal degradation, and does not allow fibre stress to be identified as the specific cause. The method may enable remedial action to be taken before it is too late by recovering traffic carried by a stressed lightpath at an upper layer of the network or scheduling in- field maintenance. The method may be used without interrupting or disrupting communications traffic across a network.

In an embodiment, there are a plurality of lightpaths each terminating at a respective one of the nodes and at least one said lightpath crosses a plurality of fibre spans and at least a further one of the nodes of the network. If the polarisation mode dispersion value of at least one said lightpath is greater than the respective threshold polarisation mode dispersion value the method comprises, after step b, cross-correlating the said lightpath with at least one said further lightpath and cross-correlating the polarisation mode dispersion value of the said lightpath with the polarisation mode dispersion value of said further lightpath, for identification of a respective stressed fibre span of the said lightpath.. The further lightpath shares at least one fibre span with the said lightpath and terminates at one of the nodes of the said lightpath.

The method may enable a stressed fibre span to be identified using PMD, which is directly caused by stress within the fibre. Where multiple lightpaths travel from several ingress nodes to several, different, egress nodes the cross-correlation of the PMD values of lightpaths for which the PMD value is above the respective PMD threshold the method may enable the number of fibre spans suspected as being stressed to be restricted. As the number of lightpaths in the network increases, the accuracy of the method may increase accordingly because the cross correlation of lightpaths spanning different fibre spans and nodes reduces the number of ambiguities in the identity of a stressed fibre span. The method may enable remedial action to be taken before it is too late, for example before a fibre breaks, by recovering traffic carried by a stressed lightpath at an upper layer of the network or scheduling in- field maintenance to remove the cause of the stress or replace the fibre.

In an embodiment, if the cross-correlation identifies a plurality of the fibre spans of the said lightpath as potentially being stressed fibre spans, the method further comprises generating and transmitting a probe optical signal across at least one of the fibre spans identified as potentially being stressed fibre spans. The method further comprises determining the polarisation mode dispersion of the at least one fibre span by measuring the probe optical signal following transmission across said fibre span. Using a probe signal may enable an ambiguity in the identity of the stressed fibre span to be resolved.

In an embodiment, the probe optical signal has a wavelength outside a bandwidth of the optical communications network. In an embodiment, the probe optical signal is transmitted on an optical supervisory channel of the optical communications network. In an embodiment, the probe optical signal carries test traffic only. Use of a probe signal wavelength outside the bandwidth of the network and carrying only test traffic may reduce any cross-talk between the probe signal and communications traffic across the network.

In an embodiment, the indication of the polarisation mode dispersion value is a mean differential group delay of the said lightpath. The mean differential group delay is proportional to the polarisation mode dispersion by a constant dependent on fibre characteristics of each fibre span crossed by the lightpath.

In an embodiment, step a. comprises determining the mean differential group delay of the said lightpath and calculating the polarisation mode dispersion value of the said light path.

In an embodiment, each node comprises a coherent receiver. The indication of the polarisation mode dispersion value of each said lightpath terminating at the node is determined by analysis of a filter impulse response of an adaptive equaliser of the coherent receiver. The method may therefore exploit existing hardware resource within a communications network and does not require additional hardware to be added to the network. Use of the coherent receiver at a node to determine the indication of the PMD of each lightpath terminating at the node enables the method to obtain the PMD without using a test signal and expensive measurement equipment, such as a polarimeter. The method is aligned with the current evolution of optical communications network technology because coherent detection is used for current, lOOGbit/s, and future, 400Gbit/s and ITbit/s, optical networks.

In an embodiment, in step a. the indication of the polarisation mode dispersion value is determined over a time window of a pre-set duration. Step b. further comprises, for each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value, determining whether a bit error rate of the lightpath within the time window is above a respective threshold bit error rate. Ste c.

comprises identifying each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value and the bit error rate is above the respective threshold bit error rate as a stressed lightpath comprising a respective stressed fibre span.

Additionally obtaining a bit error rate, BER, of the lightpath within the same time window may provide confirmation that PMD degradation is occurring; if the PMD of a lightpath is really degrading there will be a strict correlation between PMD degradation and BER degradation. This may reduce the occurrence of false alarms.

In an embodiment, the time window is 1 second in duration. The method may therefore be compatible with existing network standards.

In an embodiment, the method comprises identifying the nodes within the network and arranging the nodes in a list with each node that is known to have previously terminated a stressed lightpath being provided at the beginning of the list. Step a.

comprises obtaining the list of the nodes and, for each node in order in the list, identifying each lightpath terminated at the node and determining an indication of the polarisation mode dispersion value of each said terminated lightpath. This may enable the most critical fibre spans to be analysed first.

In an embodiment, the polarisation mode dispersion value of each stressed lightpath is stored. The stored polarisation mode dispersion values may be used to define a stress ranking among stressed fibre spans in order to prioritise in- field repairs. In an embodiment, an optical signal to noise ratio is additionally obtained and stored for each stressed lightpath. This may enable a more accurate stress ranking to be provided.

In an embodiment, the optical communications network comprises a control plane and a fibre stress detection element, and step a. further comprises transmitting the indication of the polarisation mode dispersion value of each said lightpath across the control plane to the fibre stress detection element and steps b. and c. are carried out at the fibre stress detection element. A limited amount of information is transmitted across the control plane therefore the method may be implemented without affecting control plane scalability.

In an embodiment, the control plane is a Generalized Multiprotocol Label

Switching, GMPLS, control plane and the indication of the polarisation mode dispersion value of each said lightpath is transmitted across the control plane using a GMPLS control plane protocol, such as Simple Network Management Protocol, SNMP. In an embodiment, an additional field is provided in the GMPLS control plane protocol to carry the indication of the polarisation mode dispersion value of each said lightpath.

In an embodiment, the optical communications network is one of a wavelength switched optical network, WSON, and a spectrum switched optical network, SSON.

In an embodiment, the respective threshold polarisation mode dispersion value of each said lightpath is set in accordance with ITU-T recommendation G.692. In an embodiment, the respective threshold polarisation mode dispersion value may be set to a value higher than the threshold according to ITU-T recommendation G.692. This may allow local fibre conditions which may raise local levels of PMD to be taken into account within the threshold value.

In an embodiment, the method is run in response to a trigger control signal received from a network management system of the optical communications network. The method may be run periodically to verify the evolution of PMD over time.

In an embodiment, the method is run continuously. This may enable PMD evolution over time to be tracked.

In an embodiment, steps a. to c. are carried out consecutively for each lightpath of each node.

In an embodiment, steps a. to c. are carried out concurrently for each lightpath of each node. A second aspect of the invention provides a fibre stress detection element for an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes. The fibre stress detection element comprises a processor configured to a. receive an indication of a polarisation mode dispersion value of each lightpath across the network which terminates at one of the nodes. The processor is additionally configured to b. compare the polarisation mode dispersion value of each said lightpath to a respective threshold polarisation mode dispersion value. The processor is additionally configured to c. identify each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value as a stressed lightpath comprising a respective stressed fibre span. If a stressed lightpath is identified, the processor is configured to generate an alarm signal comprising an identification of said stressed lightpath.

The fibre stress detection element may enable stressed lightpaths to be identified using polarisation mode dispersion, PMD, which is directly caused by fibre stress induced by fibre manufacture and packaging, and by environmental factors including thermal variations, mechanical modifications and fibre ageing. In contrast, BER monitoring only reveals more general signal degradation, and does not allow fibre stress to be identified as the specific cause. The fibre stress detection element may enable an alarm signal to be generated to cause remedial action to be taken before it is too late by recovering traffic carried by a stressed lightpath at an upper layer of the network or scheduling in- field maintenance. The fibre stress detection element may be operated without interrupting or disrupting communications traffic across a network. In an embodiment, there are a plurality of lightpaths each terminating at a respective one of the nodes and at least one said lightpath crosses a plurality of fibre spans and at least a further one of the nodes of the network. The processor is configured to, if the polarisation mode dispersion value of at least one said lightpath is greater than the respective threshold polarisation mode dispersion value, after b., cross-correlate the said lightpath with at least one said further lightpath and cross-correlate the polarisation mode dispersion value of the said lightpath with the polarisation mode dispersion value of said further lightpath, for identification of a respective stressed fibre span of the said lightpath. The further lightpath shares at least one fibre span with the said lightpath and terminates at one of the nodes of the said lightpath.

The fibre stress detection element may enable a stressed fibre span to be identified using PMD, which is directly caused by stress within the fibre. Where multiple lightpaths travel from several ingress nodes to several, different, egress nodes the cross-correlation of the PMD values of lightpaths for which the PMD value is above the respective PMD threshold the fibre stress detection element may enable the number of fibre spans suspected as being stressed to be restricted. As the number of lightpaths in the network increases, the accuracy of the fibre stress detection element may increase accordingly because the cross correlation of lightpaths spanning different fibre spans and nodes reduces the number of ambiguities in the identity of a stressed fibre span. The fibre stress detection element may enable an alarm signal to be generated to cause remedial action to be taken before it is too late, for example before a fibre breaks, by recovering traffic carried by a stressed lightpath at an upper layer of the network or scheduling in- fie Id maintenance to remove the cause of the stress or replace the fibre.

In an embodiment, if the cross-correlation identifies a plurality of the fibre spans of the stressed lightpath as potentially being stressed fibre spans, the processor is configured to, after performing the cross-correlation, generate and transmit a probe control signal. The probe control signal is arranged to cause a probe optical signal to be generated and transmitted across at least one of the fibre spans identified as potentially being stressed fibre spans. The processor is configured to receive an indication of the polarisation mode dispersion of the at least one fibre span measured using the probe optical signal. Using a probe signal may enable an ambiguity in the identity of the stressed fibre span to be resolved.

In an embodiment, the probe optical signal has a wavelength outside a bandwidth of the optical communications network. In an embodiment, the probe optical signal is transmitted on an optical supervisory channel of the optical communications network. In an embodiment, the probe optical signal generated carrying test traffic only. Use of a probe signal wavelength outside the bandwidth of the network and carrying only test traffic may reduce any cross-talk between the probe signal and communications traffic across the network.

In an embodiment, the indication of the polarisation mode dispersion value is a mean differential group delay of the said lightpath. The mean differential group delay is proportional to the polarisation mode dispersion by a constant dependent on fibre characteristics of each fibre span crossed by the lightpath. In an embodiment, each node comprises a coherent receiver. The indication of the polarisation mode dispersion value of each lightpath terminating at the node is a filter impulse response of an adaptive equaliser of the coherent receiver.

The fibre stress detection element may therefore exploit existing hardware resource within a communications network and does not require additional hardware to be added to the network. The fibre stress detection element is aligned with the current evolution of optical communications network technology because coherent detection is used for current, lOOGbit/s, and future, 400Gbit/s and lTbit/s, optical networks. In an embodiment, the indication of the polarisation mode dispersion is obtained over a time window of a pre-set duration. The processor is configured additionally to, at b., for each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value, obtain a bit error rate of the lightpath within the time window. The processor is configured to determine whether the bit error rate is above a respective threshold bit error rate. The processor is configured to, at c, identify each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value and the bit error rate is above the respective threshold bit error rate as a stressed lightpath comprising a respective stressed fibre span.

Additionally obtaining a bit error rate, BER, of the lightpath within the same time window may provide confirmation that PMD degradation is occurring; if the PMD of a lightpath is really degrading there will be a strict correlation between PMD degradation and BER degradation. This may reduce the occurrence of false alarms.

In an embodiment, the time window is 1 second in duration.

In an embodiment, the processor is configured to generate a list of the nodes within the network with each node that is known to have previously terminated a stressed lightpath being provided at the beginning of the list. The processor is configured to implement each of a. to c. for each node in order in the list.

In an embodiment, the fibre stress detection element comprises one of a network management system and a computer in communication with the network.

In an embodiment, the processor is configured to cause the polarisation mode dispersion value of each stressed lightpath to be stored. The stored polarisation mode dispersion values may be used to define a stress ranking among stressed fibre spans in order to prioritise in- field repairs. In an embodiment, the optical communications network comprises a control plane and a fibre stress detection element. The processor is configured at a. to receive the indication of the polarisation mode dispersion value of each said lightpath from the control plane. A limited amount of information is transmitted across the control plane therefore the fibre stress detection element may be used without affecting control plane scalability.

In an embodiment, the control plane is a Generalized Multiprotocol Label

Switching, GMPLS, control plane and the indication of the polarisation mode dispersion value of each said lightpath is received from the control plane through a GMPLS control plane protocol, such as Simple Network Management Protocol, SNMP. In an embodiment, an additional field is provided in the GMPLS control plane protocol to carry the indication of the polarisation mode dispersion value of each said lightpath.

In an embodiment, the optical communications network is one of a wavelength switched optical network, WSON, and a spectrum switched optical network, SSON.

In an embodiment, the respective threshold polarisation mode dispersion value of each said lightpath is set in accordance with ITU-T recommendation G.692. In an embodiment, the respective threshold polarisation mode dispersion value may be set to a value higher than the threshold according to ITU-T recommendation G.692. This may allow local fibre conditions which may raise local levels of PMD to be taken into account within the threshold value.

In an embodiment, the processor is configured to operate in response to a trigger control signal received from a network management system of the optical communications network. The fibre stress detection element may be operated periodically to verify the evolution of PMD over time.

In an embodiment, the processor is configured to operate continuously. This may enable PMD evolution over time to be tracked.

In an embodiment, the processor is configured to carry out a. to c. consecutively for each lightpath of each node.

In an embodiment, the processor is configured to carry out a. to c. concurrently for each lightpath of each node.

A third aspect of the invention provides an optical communications network comprising a plurality of optical fibre spans, a plurality of nodes, and a fibre stress detection element. The fibre stress detection element comprises a plurality of optical fibre spans and a plurality of nodes. The fibre stress detection element comprises a processor configured to a. receive an indication of a polarisation mode dispersion value of each lightpath across the network which terminates at one of the nodes. The processor is additionally configured to b. compare the polarisation mode dispersion value of each said lightpath to a respective threshold polarisation mode dispersion value. The processor is additionally configured to c. identify each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value as a stressed lightpath comprising a respective stressed fibre span. If a stressed lightpath is identified, the processor is configured to generate an alarm signal comprising an identification of said stressed lightpath.

The fibre stress detection element may enable stressed lightpaths within the network to be identified using polarisation mode dispersion, PMD, which is directly caused by fibre stress induced by fibre manufacture and packaging, and by environmental factors including thermal variations, mechanical modifications and fibre ageing. In contrast, BER monitoring only reveals more general signal degradation, and does not allow fibre stress to be identified as the specific cause. The fibre stress detection element may enable an alarm signal to be generated to cause remedial action to be taken before it is too late by recovering traffic carried by a stressed lightpath at an upper layer of the network or scheduling in-field maintenance. The fibre stress detection element may be operated without interrupting or disrupting communications traffic across the network.

In an embodiment, there are a plurality of lightpaths each terminating at a respective one of the nodes and at least one said lightpath crosses a plurality of fibre spans and at least a further one of the nodes of the network. The processor is configured to, if the polarisation mode dispersion value of at least one said lightpath is greater than the respective threshold polarisation mode dispersion value, after b., cross-correlate the said lightpath with at least one said further lightpath and cross-correlate the polarisation mode dispersion value of the said lightpath with the polarisation mode dispersion value of said further lightpath, for identification of a respective stressed fibre span of the said lightpath. The further lightpath shares at least one fibre span with the said lightpath and terminates at one of the nodes of the said lightpath.

The fibre stress detection element may enable a stressed fibre span to be identified using PMD, which is directly caused by stress within the fibre. Where multiple lightpaths travel from several ingress nodes to several, different, egress nodes the cross-correlation of the PMD values of lightpaths for which the PMD value is above the respective PMD threshold the fibre stress detection element may enable the number of fibre spans suspected as being stressed to be restricted. As the number of lightpaths in the network increases, the accuracy of the fibre stress detection element may increase accordingly because the cross correlation of lightpaths spanning different fibre spans and nodes reduces the number of ambiguities in the identity of a stressed fibre span. The fibre stress detection element may enable an alarm signal to be generated to cause remedial action to be taken before it is too late, for example before a fibre breaks, by recovering traffic carried by a stressed lightpath at an upper layer of the network or scheduling in- field maintenance to remove the cause of the stress or replace the fibre.

In an embodiment, if the cross-correlation identifies a plurality of the fibre spans of the stressed lightpath as potentially being stressed fibre spans, the processor is configured to, after performing the cross-correlation, generate and transmit a probe control signal. The probe control signal is arranged to cause a probe optical signal to be generated and transmitted across at least one of the fibre spans identified as potentially being stressed fibre spans. The processor is configured to receive an indication of the polarisation mode dispersion of the at least one fibre span measured using the probe optical signal. Using a probe signal may enable an ambiguity in the identity of the stressed fibre span to be resolved.

In an embodiment, the probe optical signal has a wavelength outside a bandwidth of the optical communications network. In an embodiment, the probe optical signal is transmitted on an optical supervisory channel of the optical communications network. In an embodiment, the probe optical signal generated carrying test traffic only. Use of a probe signal wavelength outside the bandwidth of the network and carrying only test traffic may reduce any cross-talk between the probe signal and communications traffic across the network.

In an embodiment, the indication of the polarisation mode dispersion value is a mean differential group delay of the said lightpath. The mean differential group delay is proportional to the polarisation mode dispersion by a constant dependent on fibre characteristics of each fibre span crossed by the lightpath.

In an embodiment, each node comprises a coherent receiver. The indication of the polarisation mode dispersion value of each lightpath terminating at the node is a filter impulse response of an adaptive equaliser of the coherent receiver. The network is aligned with the current evolution of optical communications network technology because coherent detection is used for current, lOOGbit/s, and future, 400Gbit/s and lTbit/s, optical networks.

In an embodiment, the indication of the polarisation mode dispersion is obtained over a time window of a pre-set duration. The processor is configured additionally to, at b., for each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value, obtain a bit error rate of the lightpath within the time window. The processor is configured to determine whether the bit error rate is above a respective threshold bit error rate. The processor is configured to, at c, identify each lightpath for which the polarisation mode dispersion value is above the respective threshold polarisation mode dispersion value and the bit error rate is above the respective threshold bit error rate as a stressed lightpath comprising a respective stressed fibre span.

Additionally obtaining a bit error rate, BER, of the lightpath within the same time window may provide confirmation that PMD degradation is occurring; if the PMD of a lightpath is really degrading there will be a strict correlation between PMD degradation and BER degradation. This may reduce the occurrence of false alarms.

In an embodiment, the time window is 1 second in duration.

In an embodiment, the processor is configured to generate a list of the nodes within the network with each node that is known to have previously terminated a stressed lightpath being provided at the beginning of the list. The processor is configured to implement each of a. to c. for each node in order in the list.

In an embodiment, the fibre stress detection element comprises one of a network management system and a computer.

In an embodiment, the processor is configured to cause the polarisation mode dispersion value of each stressed lightpath to be stored. The stored polarisation mode dispersion values may be used to define a stress ranking among stressed fibre spans in order to prioritise in- field repairs.

In an embodiment, the optical communications network comprises a control plane and a fibre stress detection element. The processor is configured at a. to receive the indication of the polarisation mode dispersion value of each said lightpath from the control plane. A limited amount of information is transmitted across the control plane therefore the fibre stress detection element may be used without affecting control plane scalability. In an embodiment, the control plane is a Generalized Multiprotocol Label

Switching, GMPLS, control plane and the indication of the polarisation mode dispersion value of each said lightpath is received from the control plane through a GMPLS control plane protocol, such as Simple Network Management Protocol, SNMP. In an embodiment, an additional field is provided in the GMPLS control plane protocol to carry the indication of the polarisation mode dispersion value of each said lightpath.

In an embodiment, the optical communications network is one of a wavelength switched optical network, WSON, and a spectrum switched optical network, SSON.

In an embodiment, the respective threshold polarisation mode dispersion value of each said lightpath is set in accordance with ITU-T recommendation G.692. In an embodiment, the respective threshold polarisation mode dispersion value may be set to a value higher than the threshold according to ITU-T recommendation G.692. This may allow local fibre conditions which may raise local levels of PMD to be taken into account within the threshold value.

In an embodiment, the processor is configured to operate in response to a trigger control signal received from a network management system of the optical communications network. The fibre stress detection element may be operated periodically to verify the evolution of PMD over time.

In an embodiment, the processor is configured to operate continuously. This may enable PMD evolution over time to be tracked.

In an embodiment, the processor is configured to carry out a. to c. consecutively for each lightpath of each node.

In an embodiment, the processor is configured to carry out a. to c. concurrently for each lightpath of each node.

In an embodiment, the optical communications network is a dense wavelength division multiplexed mesh network comprising a control plane.

A fourth aspect of the invention provides a data carrier having computer readable instructions embodied therein for providing access to resources available on a processor. The computer readable instructions comprise instructions to cause the processor to perform any of the steps of the method of detecting optical fibre stress in an optical

communications network comprising a plurality of optical fibre spans and a plurality of nodes described above.

In an embodiment, the data carrier is a non-transitory data carrier. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief description of the drawings

Figure 1 shows the steps of a method according to a first embodiment of the invention of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes;

Figure 2 shows the steps of a method according to a second embodiment of the invention of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes;

Figure 3 shows the steps of a method according to a third embodiment of the invention of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes;

Figure 4 shows the steps of a method according to a fourth embodiment of the invention of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes;

Figure 5 shows the steps of a method according to a fifth embodiment of the invention of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes;

Figure 6 shows the steps of a method according to a sixth embodiment of the invention of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes;

Figure 7 shows the steps of a method according to a seventh embodiment of the invention of detecting optical fibre stress in an optical communications network comprising a plurality of optical fibre spans and a plurality of nodes;

Figure 8 is a schematic representation of an optical communications network illustrating the step of cross-correlation of the lightpaths and PMD values of any of the methods shown in Figures 1 to 7;

Figure 9 is a schematic representation of a fibre stress detection element according to an eighth embodiment of the invention; Figure 10 is a schematic representation of a fibre stress detection element according to a ninth embodiment of the invention; and

Figure 11 is a schematic representation of an optical communications network according to a fifteenth embodiment of the invention.

Detailed description

Referring to Figure 1, a first embodiment of the invention provides a method 10 of detecting optical fibre stress in an optical communications network. The network comprises a plurality of optical fibre spans and a plurality of nodes.

The method comprises:

a. at at least one node at which a respective lightpath across the network terminates, determining an indication of a polarisation mode dispersion, PMD, value of the lightpath 12;

b. comparing the PMD value of each said lightpath to a respective threshold PMD value 14; and

c. identifying each lightpath for which the PMD value is above the respective threshold PMD value as a stressed lightpath comprising a respective stressed fibre span 16. If a stressed lightpath is identified, generating an alarm signal comprising an identification of said stressed lightpath 18.

The term lightpath will be understood by the person skilled in the art as an optical transmission path from a source node to a destination node in an optical communications network and comprises one or more fibre spans. A lightpath is set up by assigning a dedicated wavelength to it for each fibre span in the lightpath.

Polarization Mode Dispersion (PMD) is a form of modal dispersion where two different polarizations of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections and asymmetries, causing random spreading of optical pulses. It is caused by stresses induced by fibre manufacture and packaging but it is significantly influenced by environmental factors such as thermal variations, mechanical modifications, and fibre ageing. There are common phenomena, imposed on an optical fibre in the field, which cause a detectable PMD variation. By monitoring the PMD evolution over time the method 10 enables such stresses to be detected, so that appropriate remedial action can be taken before it is too late, for example recovering the traffic at an upper layer or planning in field maintenance to remove the causes of stress.

A second embodiment of the invention provides a method 20 of detecting optical fibre stress in an optical communications network. The steps of the method 20 of this embodiment are shown in Figure 2. The method 20 of this embodiment is similar to the method 10 shown in Figure 1, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, there are a plurality of lightpaths each terminating at a respective one of the nodes of the network. At least one said lightpath crosses a plurality of fibre spans and at least a further one of the nodes of the network.

If the PMD value of at least one said lightpath is greater than the respective threshold PMD value the method comprises, after step b, cross-correlating the said lightpath with at least one said further lightpath and cross-correlating the polarisation mode dispersion value of the said lightpath with the PMD value of said further lightpath, for identification of a respective stressed fibre span of the said lightpath 22. The further lightpath shares at least one fibre span with the said lightpath and terminates at one of the nodes of the said lightpath.

A third embodiment of the invention provides a method 30 of detecting optical fibre stress in an optical communications network. The steps of the method 30 of this embodiment are shown in Figure 3. The method 30 of this embodiment is similar to the method 20 shown in Figure 2, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, if the cross-correlation identifies a plurality of the fibre spans of the said lightpath as potentially being stressed fibre spans 32, the method further comprises generating and transmitting a probe optical signal across at least one of the fibre spans identified as potentially being stressed fibre spans. The PMD of the at least one fibre span is determined by measuring the probe optical signal following transmission across said fibre span 34.

A fourth embodiment of the invention provides a method 40 of detecting optical fibre stress in an optical communications network. The steps of the method 40 of this embodiment are shown in Figure 4. The method 40 of this embodiment is similar to the method 10 shown in Figure 1, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, each node in the network comprises a coherent receiver. As will be well know to persons skilled in the art, a coherent receiver used in an optical communications network node comprises an adaptive equaliser. The indication of the PMD value of each said lightpath terminating at the node is determined by analysis of a filter impulse response of the adaptive equaliser 42.

In traditional non-coherent receivers, that employ direct detection, the phase of the optical E-field is lost, making exact equalization of the channel by a linear filter impossible. Coherent detection circumvents this problem by combining the received signal with a local oscillator (LO) laser and by using balanced detection to downconvert it into a baseband electrical output that is proportional to the optical E-field. The resulting signal can then be sampled and processed by digital signal processing (DSP) algorithms, providing a software based, platform able to measure and compensate the PMD.

A fifth embodiment of the invention provides a method 50 of detecting optical fibre stress in an optical communications network. The steps of the method 50 of this embodiment are shown in Figure 5. The method 50 of this embodiment is similar to the method 10 shown in Figure 1, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, the indication of the PMD value is determined over a time window of a pre-set duration 52.

For each lightpath for which the PMD value is above the respective threshold PMD value, the method comprises determining whether a bit error rate, BER, of the lightpath within the same time window is above a respective threshold BER 54.

Each lightpath for which both the PMD value and the BER value are above the respective thresholds is identified as a stressed lightpath comprising a respective stressed fibre span 56.

A sixth embodiment of the invention provides a method 60 of detecting optical fibre stress in an optical communications network. The steps of the method 60 of this embodiment are shown in Figure 6. The method 60 of this embodiment is similar to the method 10 shown in Figure 1, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, the network is one of a wavelength switched optical network, WSON, and a spectrum switched optical network, SSON. The nodes of the optical communications network are considered one by one, consecutively in series, and the lightpaths terminating at each node are themselves considered one by one, consecutively in series. This approach is practical because it is not necessary to consider all of the nodes and all of the terminating lightpaths at the same time; the identification of fibre stress within a network is not as time critical as fault monitoring, which requires immediate action to repair it.

The method 60 of this embodiment commences with listing all of the nodes 62, which in this example are reconfigurable optical add-drop multiplexers, ROAMs, each comprising a coherent receiver. The list of nodes can be ordered according to various criteria, including listing the nodes so that each node that is known to have previously terminated a stressed lightpath is provided at the beginning of the list. The first node in the list is selected 64 and all of the lightpaths which are terminated at the node using a coherent receiver are listed 70.

In this embodiment, the PMD value of a lightpath is obtained by determining the mean differential group delay, DGD, of the lightpath over a pre-set time window, T. As will be well known to the person skilled in the art, the mean DGD is proportional to the PMD by a constant dependent on the fibre characteristics of the fibre spans crossed by the lightpath, and a coherent receiver comprises an adaptive equaliser. The DGD is determined by analysis of the filter impulse response of the adaptive equaliser.

The first lightpath in the list is selected 72 and a check is made as to whether the PMD value can be monitored for the lightpath over the time window, T, 78. If it can, the PMD value within the time window is obtained 80 and compared to a threshold PMD value to determine whether it exceeds the threshold 82. The threshold can be set according to ITU-T Recommendation G.692, or can be defined by the network operator. If the threshold PMD value is exceeded, the lightpath is identified as crossing at least one stressed fibre span 84. This process is repeated for each lightpath in the lightpath list 74, 76, and then for each node in the node list 66. The lightpaths which cross at least one stressed fibre span are then cross-correlated to identify one or more fibre spans which are suspected to be stressed fibre spans 86. If all of the stressed fibre spans are identified without any ambiguities 88, and a stressed fibre span has been identified, an alarm signal comprising an indication of the stressed fibre span is generated 92, so that appropriate remedial action can be taken in- field.

If the cross-correlation identifies a plurality of the fibre spans as potentially being stressed fibre spans, the method further comprises generating and transmitting a probe optical signal across at least one of the fibre spans identified as potentially being stressed fibre spans 90. The PMD of the at least one fibre span is determined by measuring the probe optical signal following transmission across said fibre span. In this embodiment, the probe signal is transmitted on the optical supervisory channel, as defined in ITU-T

Recommendation G.692, and carries test traffic.

A seventh embodiment of the invention provides a method 100 of detecting optical fibre stress in an optical communications network. The steps of the method 100 of this embodiment are shown in Figure 7. The method 100 of this embodiment is similar to the method 60 shown in Figure 6, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, all of the lightpaths are then considered concurrently. As in the previous embodiment, all of the nodes in the network are listed 62. Then all of the lightpaths which are terminated at each node using a coherent receiver are listed 102.

A check is made as to whether the PMD value can be monitored for each lightpath over the pre-set time window, T, 108. If it can, the PMD value within the time window is obtained for each of the lightpaths 108 concurrently. The PMD value for each lightpath is concurrently compared to a respective threshold PMD value to determine whether it exceeds the threshold 110.

The lightpaths are then cross-correlated as before.

Figure 8 illustrates a simple mesh network 120 comprising a plurality of nodes in the form of ROADMs 122 and a plurality of fibre spans 124. The network is GMPLS controlled. A plurality of lightpaths 126 have been established across the network. To simplify explanation, the lightpaths are represented as monodirectional (oriented arrow) but, in general, the lightpaths will be bidirectional.

At each node where at least one lightpath is terminated (dropped), nodes B, C, D, F, G, H, I and N, the PMD value is determined and compared to its respective threshold PMD value. Five of the nodes have PMD values below the respective thresholds 128, and three of the nodes, C, D and N, have PMD values which are above the respective thresholds 130.

The excessive PMD value of lightpath AC terminating at node C could be caused by fibre stress in fibre span AB or fibre span BC. A simple cross-correlation of the lightpaths 126 is carried out by comparing lightpath AC with the other lightpaths and considering the PMD values of the other lightpaths. This comparison indicates that lightpath EB (crossing fibre spans EA and AB) shares a fibre span (AB) with lightpath AC and the PMD value at B, terminating lightpath EB is below its respective threshold.

Therefore it can be concluded that fibre span AB is not stressed and the stressed fibre span in lightpath AC is BC.

Comparing lightpath AD with the other lightpaths and considering the PMD values indicates that the excessive PMD value of lightpath AD, terminating at node D, could be due to stress in fibre span BC (identified in relation to lightpath AC above) but a fibre stress also in fibre span CD can not be ruled out.

The excessive PMD value of lightpath LN, terminating at node N, could be caused by stress in fibre span LM and/or MN. Comparing lightpath LN with the other lightpaths and considering PMD values does not identify any other lightpaths terminating at nodes M or N. Therefore an ambiguity in the identity of the stressed fibre span in lightpath LN exists and a probe optical signal, carrying test traffic, can be transmitted for example from L to M to resolve the ambiguity.

As the network load increases, with more lightpaths being established across the network 120, the accuracy of the method will increase correspondingly because the cross correlation of lightpaths spanning different paths across the network will reduce the number of ambiguities. An eighth embodiment of the invention provides a fibre stress detection element 140 for an optical communications network, as shown in Figure 9. The network comprises a plurality of optical fibre spans and a plurality of nodes.

The fibre stress detection element comprises a processor 142 configured to:

a. receive an indication 144 of a PMD value of each lightpath across the network which terminates at one of the nodes;

b. compare the PMD value of each said lightpath to a respective threshold PMD value; and

c. identify each lightpath for which the PMD value is above the respective threshold PMD value as a stressed lightpath comprising a respective stressed fibre span and if a stressed lightpath is identified, generate an alarm signal 146 comprising an identification of said stressed lightpath.

A ninth embodiment of the invention provides a fibre stress detection element 150 for an optical communications network, as shown in Figure 10. The fibre stress detection element of this embodiment is similar to the fibre stress detection element 140 of Figure 9, with the following modifications. The same reference numbers are retained for

corresponding features.

In this embodiment, there are a plurality of lightpaths each terminating at one of the nodes of the network. At least one of the lightpaths across the network crosses a plurality of fibre spans and at least one other one of the nodes of the network.

The processor 152 is configured to, if the PMD value of at least one of the terminating lightpaths is greater than the respective threshold PMD value, after b., cross- correlate the said lightpath with at least one further lightpath across the network and the PMD value of that further lightpath, for identification of a respective stressed fibre span of the said lightpath. The further lightpath must share at least one fibre span with the said lightpath and terminate at one of the nodes crossed by the said lightpath.

A tenth embodiment of the invention provides a fibre stress detection element for an optical communications network having the same structure as the fibre stress detection element 150 shown in Figure 10 and will be described with reference to Figure 10. The fibre stress detection element of this embodiment is similar to the fibre stress detection element 150 of Figure 10, with the following modifications. In this embodiment, the processor 152 is configured to, if the cross-correlation identifies a plurality of the fibre spans of the said lightpath as potentially being stressed fibre spans, generate a probe control signal 154 arranged to cause a probe optical signal to be generated and transmitted across at least one of the fibre spans identified as potentially being stressed fibre spans. The processor 152 is configured to receive an indication 156 of the PMD of the at least one fibre span measured using the probe optical signal.

An eleventh embodiment of the invention provides a fibre stress detection element for an optical communications network having the same structure as the fibre stress detection element 150 shown in Figure 10 and will be described with reference to Figure 10. The fibre stress detection element of this embodiment is similar to the fibre stress detection element 150 of Figure 10, with the following modifications.

In this embodiment, each node in the network comprises a coherent receiver. The indication of the PMD value of each said lightpath terminating at the node is a filter impulse response of the adaptive equaliser.

A twelfth embodiment of the invention provides a fibre stress detection element for an optical communications network having the same structure as the fibre stress detection element 150 shown in Figure 10 and will be described with reference to Figure 10. The fibre stress detection element of this embodiment is similar to the fibre stress detection element 150 of Figure 10, with the following modifications.

In this embodiment, the indication of the PMD value is determined over a time window of a pre-set duration 52. The processor 152 is configured additionally to, at b., for each lightpath for which the PMD value is above the respective threshold PMD value, obtain a bit error rate, BER, of the lightpath within the same time window and determine whether the BER is above a respective threshold BER.

The processor 152 is configured to, at c, identify each lightpath for which both the

PMD value and the BER value are above the respective thresholds as a stressed lightpath comprising a respective stressed fibre span.

A thirteenth embodiment of the invention provides a fibre stress detection element for an optical communications network having the same structure as the fibre stress detection element 150 shown in Figure 10 and will be described with reference to Figure 10. The fibre stress detection element of this embodiment is similar to the fibre stress detection element 150 of Figure 10, with the following modifications. In this embodiment, the network is one of a wavelength switched optical network, WSON, and a spectrum switched optical network, SSON.

The processor 152 is configured to generate a list of all of the nodes, which in this example are reconfigurable optical add-drop multiplexers, ROAMs, each comprising a coherent receiver. The list of nodes can be ordered according to various criteria, in this embodiment the processor 152 is configured to list the nodes so that each node that is known to have previously terminated a stressed lightpath is provided at the beginning of the list. The processor 152 is configured to implement each of a. to c. for each node in order in the list, as follows.

The processor 152 is configured to select the first node in the list and to list all of the lightpaths which are terminated at the node using a coherent receiver.

In this embodiment, the PMD value of a lightpath is obtained by determining the mean differential group delay, DGD, of the lightpath over a pre-set time window, T. The DGD is determined by analysis of the filter impulse response of the adaptive equaliser. The processor 152 is configured to receive the mean DGD values.

The processor is configured to select the first lightpath in the list and to check whether the PMD value can be monitored for the lightpath over the time window, T. The processor is configured to, if the PMD value can be monitored, obtained the PMD value within the time window and compare it to a threshold PMD value to determine whether it exceeds the threshold. The threshold can be set according to ITU-T Recommendation G.692, or can be defined by the network operator. The processor 152 is configured to, if the threshold PMD value is exceeded, identify the lightpath as crossing at least one stressed fibre span. The processor 152 is configured to repeat this process for each lightpath in the lightpath list and then for each node in the node list.

The processor 152 is configured to cross-correlate the lightpaths which cross at least one stressed fibre span to identify one or more fibre spans which are suspected to be stressed fibre spans. If all of the stressed fibre spans are identified without any ambiguities 88, and a stressed fibre span has been identified, the processor is configured to generate an alarm signal comprising an indication of the stressed fibre span, so that appropriate remedial action can be taken in-field. If the cross-correlation identifies a plurality of the fibre spans as potentially being stressed fibre spans, the processor 152 is configured to generate a probe control signal arranged to cause a probe optical signal to be generated and transmitted across at least one of the fibre spans identified as potentially being stressed fibre spans. The processor 152 is configured to receive an indication 156 of the PMD of the at least one fibre span measured using the probe optical signal. In this embodiment, the probe signal is transmitted on the optical supervisory channel, as defined in ITU-T Recommendation G.692, and carries test traffic.

A fourteenth embodiment of the invention provides a fibre stress detection element for an optical communications network having the same structure as the fibre stress detection element 150 shown in Figure 10 and will be described with reference to Figure 10. The fibre stress detection element of this embodiment is similar to the fibre stress detection element of the previous embodiment, with the following modifications.

In this embodiment, the processor 152 is configured to consider all of the lightpaths concurrently. As in the previous embodiment, the processor 152 is configured to list all of the nodes in the network and then list all of the lightpaths which are terminated at each node using a coherent receiver.

The processor 152 is configured to check whether the PMD value can be monitored for each lightpath over the pre-set time window, T. The processor 152 is configured to, if it can, concurrently obtain the PMD value within the time window for each of the lightpaths. The processor 152 is configured to concurrently compare the PMD value for each lightpath to a respective threshold PMD value and to determine whether it exceeds the threshold.

A fifteenth embodiment of the invention provides an optical communications network 160 as shown in Figure 11.

The network 160 comprises a plurality of optical fibre spans 162, a plurality of nodes 164 and a fibre stress detection element 140, as shown in Figure 9. It will be appreciated that any of the fibre stress detection elements described above may be used instead.

A sixteenth embodiment of the invention provides an optical communications network which is similar to the network 160 shown in Figure 11 and which will be described with reference to that Figure. In this embodiment, the network is a dense wavelength division multiplexed mesh network, such as a WSON or SSON, comprising a control plane, such as GMPLS.

A seventeenth embodiment of the invention provides a data carrier having computer readable instructions embodied therein for providing access to resources available on a processor. The computer readable instructions comprise instructions to cause the processor to perform any of the methods of detecting optical fibre stress in an optical communications network described above in the first to the seventh embodiment of the invention.

The data carrier may be a non-transitory data carrier.