Apparatus for protecting an optical link

According to the present invention, there is provided apparatus for providing protection of an optical link. The apparatus comprises a first port for coupling to the optical link and a second port for coupling to a further optical link. The apparatus further comprises a third port configured to receive an upstream optical signal to be transmitted over the optical link, and to output a downstream optical signal received over the optical link. The apparatus further comprises protection switching apparatus operable to selectively couple the third port to the first port or to the second port. The apparatus further comprises modifying apparatus configured to modify the upstream optical signal, received at the third port, before it is output from the first port, such that the switching upstream optical signal has a distinctive physical characteristic. The apparatus further comprises detecting apparatus, coupled between the first port and the third port, configured to receive a portion of an incoming optical signal, received at the first port, and to selectively detect the presence of the upstream optical signal or the downstream optical signal in the portion of the incoming signal based on the distinctive physical characteristic. The apparatus further comprises control circuitry configured to provide a control signal to the protection switching apparatus based on an output from the detecting apparatus.

TECHNICAL FIELD

The present invention relates to apparatus for protecting an optical link. The present invention also relates to an optical network, and to a radio access network comprising the optical network. The present invention further relates to a method for upgrading an optical network to provide protection for an optical link.

BACKGROUND

Optical networks are considered attractive for use in radio access networks, such as mobile front-haul networks. For example, in a mobile front-haul network, an optical network, such as a wavelength division multiplexed (WDM) optical network, may be used to couple a baseband processing unit to a plurality of remote radio units.

Separate optical fibres may be used to convey upstream and downstream optical signals respectively between the baseband processing unit and the remote radio units. However, using a single optical fibre to transmit both the upstream and downstream optical signals may save costs, which is particularly desirable in radio access networks. This is not only because fewer optical fibres may be required to transmit all of the optical signals, but also because fewer associated components such as splitters, couplers and splice enclosures may be required. Further, capital and labour costs associated with lighting up fibre may be reduced, and capital investment in fibre installations may thus be maximised. Better use may be made of existing dark fibre, and thus the “time to trench” may be extended. For example, instead of laying additional fibres, metro area networks nearing fibre exhaust, which currently use two fibres for transmission of upstream and downstream optical signals respectively, could be upgraded for single fibre operation.

In single fibre operation, in order to mitigate Rayleigh-Backscattering x-talk between the upstream and downstream optical signals, which can severely limit network performance, the upstream and downstream optical signals will typically have different wavelengths.

SUMMARY

The Applicant has appreciated that it would be desirable to provide protection of an optical link, for example an optical fibre, which is arranged to carry optical signals in an upstream direction and a downstream stream direction (i.e. in opposite directions) at the same time.

FIGS. 1aand 1bshow protection apparatus suitable for protecting an optical link10used for transmission of optical signals in a single direction only.FIG. 1ashows a 1+1 protection scheme.

FIG. 1bshows a 1:1 protection scheme. In both Figures, there is a further optical link12, indicated by a dashed line, which can be used to carry the signals travelling over optical link10, in the event that optical link10fails, for example because of an optical fibre cut or other defect. A protection switch14(shown on the left hand side of the Figures) is coupled, at a first port16, to an end of the optical link10and, at a second port18, to an end of the further optical link12. The protection switch14has a further port20configured to receive one or more upstream optical signals for transmission over the optical link10. The protection switch14is however operable to selectively couple the further port20to the first port16or to the second port18, whereby the one or more upstream optical signals, received at the further port20, may be transmitted from the first port16over the optical link10, or from the second port18over the further optical link12.

In the 1+1 protection scheme, shown inFIG. 1a, at the opposite end of the optical links10,12, there is a 2:N splitter22, which has a first port24coupled to optical link10and a second port26coupled to further optical link12. The 2:N splitter22is configured to split the one or more upstream optical signals, received over optical link10or further optical link12, into respective optical signals to be transmitted, over further optical links, to respective optical receivers (not shown).

In the 1:1 line protection scheme, at the opposite end of the optical links10,12, there is instead a second protection switch28. This protection switch28also has a first port30, for coupling to the other end of the optical link and a second port32, for coupling to the other end of the further optical link12. This protection switch28has a further port34configured to output the one or more upstream optical signals. The protection switch28is operable to selectively couple the further port34to the first port30(i.e. to the optical link) or to the second port (i.e. to the further optical link12). The further port34of the protection switch28is coupled to a band filter36, which is configured to split the one or more upstream optical signals output from the protection switch28into respective optical signals for transmission to respective optical receivers (again not shown).

It should be noted that from the perspective of the 2:N splitter22/second protection switch28the “upstream” optical signals are “downstream” optical signals.

Each protection switch14,28, in both the 1+1 and 1:1 line protection system, has a power tap monitor (not shown) which is configured to receive a portion of an incoming optical signal, received at the first port1630of the protection switch14,28. In protection switch14, which is configured to transmit the upstream optical signals over the optical link10, if the power monitor detects the presence of an incoming optical signal, this may indicate that the optical link10is cut or otherwise damaged. This is because this may mean that the one or more upstream optical signals are being reflected back to the first port16, for example from a cut interface along the optical link. On the other hand, in protection switch28, if the power monitor detects loss of an incoming signal, received at port30, this may also indicate that the optical link10is cut or otherwise damaged, since it may mean that the “upstream” optical signals are not passing along the length of optical link10. In both cases, in response to this detection, the protection switch14,28couples the further port2034to the second port1832, such that the upstream optical signals are transmitted over, and received from, the further optical link12, instead of the first port16.

Thus, protection of optical link10may be achieved, in a simple manner, and thus in a cost effective and reliable way.

However, the applicant has appreciated that this system does not work for an optical link which is arranged to carry both upstream and downstream optical signals, i.e. optical signals travelling in opposite directions. This is because, if for example the optical link is cut, not only will the upstream optical signals transmitted from one end of the optical link be reflected back to that end of the optical link, but the downstream optical signals transmitted from the other end of the optical link will not reach that end of the optical link. The net result is that there may be no appreciable change in the optical power detected by the power monitor, in the event of an optical link cut or other defect. Or, if there is a change, it may be tricky to perform a proper threshold calibration, which threshold may need to be adjusted if network parameters such as transmission power/number of upstream and downstream signals change.

This problem is illustrated inFIG. 2. In this example, it is seen that a plurality of upstream optical signals (even lambdas2to48) are transmitted from protection switch14, over optical link10. Light from these signals is reflected back to protection switch14, from a cut along optical link10indicated by a X. Downstream optical signals (from the perspective of protection switch14) (odd lambdas1to47) are also transmitted over optical link10, towards protection switch14. However, these signals are blocked from reaching protection switch14by cut X.

One way of protecting optical link10would be to insert a wavelength filter before the power monitors in the protection switches14,28, wherein the wavelength filter is configured to pass only light from the upstream optical signals or the downstream optical signals. In this way, presence or loss of the upstream optical signals or the downstream optical signals may be selectively detected, and thus an optical link fault may still be detected as described above. However, the applicant has appreciated that a disadvantage of this solution is that the network wavelength plan cannot be changed, without reconfiguring the protection apparatus. This may result in less efficient operation of the network, by preventing or limiting dynamic switching of the wavelength plan. And or increase costs, by requiring manual replacement of the wavelength filter in each protection apparatus, each time it is desired to change the wavelength plan. Or, by requiring inclusion of a tuneable wavelength filter in each protection switch14,28, which may be more expensive than a passive filter, and optionally complex control circuitry to communicate for example with a network management system to adjust the wavelength filter automatically when required.

An alternative solution, for example as described in an article titled “A simple single-fibre CWDM metro/access ring network with unidirectional OADM and automatic protection switching” OFC 2005 is to monitor each downstream optical signal dropped by a respective Optical Add Drop Multiplexer (OADM) in the network. However, the applicant has appreciated that this approach has the further disadvantage that the optical nodes in the network may need to be upgraded in order to provide protection of the optical link. And the solution may not even be feasible in all network architectures. Complex control circuitry may also be required in order to trigger protection of the optical link in the event of a detection of Loss of Signal, LOS. For example, co-ordination with other network elements or a network controller may be required.

According to the present invention, there is provided apparatus for providing protection of an optical link. The apparatus comprises a first port for coupling to the optical link and a second port for coupling to a further optical link. The apparatus further comprises a third port configured to receive an upstream optical signal to be transmitted over the optical link, and to output a downstream optical signal received over the optical link. The apparatus further comprises protection switching apparatus operable to selectively couple the third port to the first port or to the second port. The apparatus further comprises modifying apparatus configured to modify the upstream optical signal, received at the third port, before it is output from the first port, such that the upstream optical signal has a distinctive physical characteristic. The apparatus further comprises detecting apparatus, coupled between the first port and the third port, configured to receive a portion of an incoming optical signal, received at the first port, and to selectively detect the presence of the upstream optical signal or the downstream optical signal in the portion of the incoming signal based on the distinctive physical characteristic. The apparatus further comprises control circuitry configured to provide a control signal to the protection switching apparatus based on the detecting by the detecting apparatus.

Embodiments of the present invention have the advantage that they can provide protection of an optical link arranged to carry both upstream and downstream optical signals, in a simpler, more cost effective and more reliable way. Advantageously, the solution of the present invention is independent of the wavelength plan of the optical network, and thus the wavelength plan may be adjusted dynamically so as to improve network performance, without requiring reconfiguration of the apparatus. Further, no modifications are required to other network elements such as OADMs, HUB. The solution is self-contained, and thus advantageously an optical network may be easily upgraded to provide protection of an optical link using apparatus embodying the present invention.

According to an embodiment of the present invention, the control circuitry may be configured to cause the protection switching apparatus to selectively couple the third port to the second port instead of to the first port if the detecting apparatus detects the presence of the upstream optical signal in the portion of the incoming optical signal. Alternatively, the control circuitry may be configured to cause the protection switching apparatus to selectively couple the third port to the second port instead of to the first port if the detecting apparatus does not detect the presence of the downstream optical signal in the portion of the incoming optical signal.

According to a preferred embodiment of the present invention, the modifying apparatus may comprise a modulating apparatus configured to modulate the upstream optical signal, wherein the distinctive physical characteristic is a modulation. The modulation may be an amplitude modulation, a phase modulation or a frequency modulation. Preferably, the modulation is a tone modulation. This means that the modulation is applied to a narrow portion of the bandwidth of the upstream optical signal, for example at a single frequency of the upstream optical signal. For example, the modulation may be a weak modulation applied at a low frequency, such as less than 5 Hz. The modulation does not carry any digital information.

Thus, applying a modulation to the upstream optical signal may “mark” the upstream optical signal such that it can be distinguished from the downstream optical signal. However, advantageously, the modulation may have no significant impact on the upstream optical signal, such that it is not necessary to remove the modulation after the upstream optical signal has traveled over the optical link. The modulation may merely be perceived by the upstream optical signal receiver as a noise, which can be filtered out using conventional techniques. Thus, this may provide a simple, cost effective and reliable way of adding a distinctive physical characteristic to the upstream optical signal.

According to a preferred embodiment, the modulating apparatus may be configured to provide a modulating signal to the protection switching apparatus, whereby when the upstream optical signal passes through the protection switching apparatus the modulation is applied to the upstream optical signal.

According to a preferred embodiment, the detecting apparatus may comprise a modulation detector configured to detect the presence of the modulation in the portion of the incoming optical signal. For example, the detecting apparatus may also comprise an optical power detector configured to convert the portion of the incoming optical signal into an electrical signal, and the modulation detector may be configured to detect the modulation in the electrical signal. This may provide a simple, cost effective, reliable arrangement.

Alternatively, the modifying apparatus may, for example, be configured to add an optical signal to the upstream optical signal at a distinctive frequency, wherein the distinctive frequency is the distinctive physical characteristic. In this example, the detecting apparatus may comprise a wavelength filter configured to selectively pass optical signals at the distinctive frequency. The detecting apparatus may further comprise an optical power detector configured to detect the optical signal(s) passed by the wavelength filter.

According to the present invention, there is also provided a system for protecting an optical link. The system comprises a first apparatus comprising a first port for coupling to a first end of the optical link, a third port configured to receive an upstream optical signal to be transmitted over the optical link, and to output a downstream optical signal received over the optical link. The first apparatus further comprises modifying apparatus configured to modify the upstream optical signal, received at the third port, before it is output from the first port, such that the upstream optical signal has a distinctive physical characteristic.

The system further comprises a second apparatus comprising a first port for coupling to second end of the optical link, a second port for coupling to a further optical link, and a third port configured to receive an upstream optical signal to be transmitted over the optical link, and to output a downstream optical signal received over the optical link. The second apparatus further comprises protection switching apparatus operable to selectively couple the third port to the first port or to the second port. In addition, the second apparatus further comprises detecting apparatus, coupled between the first port and the third port. The detecting apparatus is configured to receive a portion of an incoming optical signal, received at the first port, and to selectively detect the presence of the upstream optical signal or the downstream optical signal in the portion of the incoming signal based on the distinctive physical characteristic. The second apparatus further comprises control circuitry configured to provide a control signal to the protection switching apparatus based on the detecting by the detecting apparatus.

According to the present invention, there is also provided an optical network comprising an optical link, a further optical link, and an apparatus or system for providing protection of the optical link as described above.

There is also provided a radio access network comprising the optical network.

There is also provided a method of upgrading an optical network to provide protection of an optical link. The method comprises providing an apparatus as described above. The method further comprises coupling the first port of the apparatus to the optical link, coupling the second port of the apparatus to a further optical link, and coupling the third port of the apparatus to a network part arranged to output an upstream optical signal and to receive a downstream optical signal.

There is also provided a further method of upgrading an optical network to provide protection of an optical link. The method comprising providing a system as described above. The method further comprises coupling the first port of the first apparatus to a first end of the optical link, and coupling the third port of the first apparatus to a network part arranged to output an upstream optical signal and to receive a downstream optical signal. The method further comprises coupling the first port of the second apparatus to a second end of the optical link, coupling the second port of the second apparatus to a further optical link. And coupling the third port of the second apparatus to a network part arranged to output an upstream optical signal and to receive a downstream optical signal.

DETAILED DESCRIPTION

FIG. 3shows, by way of example, apparatus400embodying the present invention arranged to provide protection of an optical link10in an optical network within a radio access network300. However, it should be appreciated that apparatus embodying the present invention may be used to protect an optical link, which is arranged to convey upstream and downstream signals, in any optical network. In this example, the optical link10comprises an optical fibre.

In this example, the radio access network300is a front-haul network, which has a hub and spoke configuration. The radio access network300comprises a baseband unit302configured to receive and transmit optical signals to and from a plurality of remote radio units, RRUs304. Each of the optical signals is at a respective optical wavelength. The upstream optical signals, transmitted from the baseband unit, are at respective wavelengths, odd lambdas1to37. The downstream optical signals, received by the baseband unit302from the RRUs304, are also at respective, different wavelengths, even lambdas2to48.

The baseband unit302is coupled to a WDM multiplexer/demultiplexer306, which is configured to receive each of the upstream optical signals transmitted by the baseband unit302and to multiplex the signals into a WDM signal, output from port308. The WDM multiplexer/demultiplexer306is further configured to receive a WDM signal comprising the plurality of downstream optical signals, from the RRUs304at port308, and to de-multiplex and output the respective downstream signals to the baseband unit302.

In this example, protection apparatus400is arranged between the input/output308of the multiplexer/demultiplexer306and optical link10, which may be up to a few tenths of a km long. The protection apparatus400has a first port402coupled to the optical link10, a second port404coupled to a further optical link12(which may be referred to as a protection link) and a further port406, which in this example is coupled to output308of WDM multiplexer/demultiplexer.

In this example, the protection scheme is a 1+1 protection scheme, and the other end of the optical link10, and the protection link12, is coupled to a 2:n splitter/combiner310. However, the protection scheme may instead be a 1:1 protection scheme. In that case, a further protection apparatus400may be coupled at the other end of the optical link10and the protection link12instead.

In this example, 2:n splitter combiner310is configured to split the upstream optical signals received over the optical link10or the protection link12(which are from the perspective of 2:n splitter combiner310, “downstream” optical signals) into a plurality of optical signals, at respective wavelengths. These optical signals are transmitted to respective Optical Add Drop Multiplexers (OADMs)312. In this example, each OADM312is coupled to a respective plurality of RRUs304. Each RRU304is configured to receive a “downstream” optical signal dropped from the OADM312, at a respective wavelength, and to transmit an “upstream” optical signal, at a respective wavelength. These upstream optical signals are passed by the respective OADM312to splitter/combiner310. Splitter/combiner310is configured to combine these upstream optical signals, received from the OADMs312, and output these upstream optical signals for transmission over optical link10, and protection link20.

FIG. 4shows protection apparatus400for protecting an optical link according to an embodiment of the present invention.

The protection apparatus400comprises a first port402, for example an interface, for coupling to an optical link. The protection apparatus400further comprises a second port404, for example an interface, for coupling to a further optical link. Each optical link may comprise an optical fibre. The protection apparatus400further comprises a third port406, which is configured to receive an upstream optical signal to be transmitted over the optical link, and to output a downstream optical signal received of the optical link. The upstream optical signal may comprise one or more optical signals. Similarly the downstream optical signal may comprise one or more downstream optical signals. For example, the third port may be configured to receive a plurality of upstream optical signals to be transmitted over the optical link, for example in the form of a WDM optical signal, and to output a plurality of downstream optical signals, for example also in the form of a WDM optical signal.

The protection apparatus400further comprises a protection switching apparatus408. The protection switching apparatus408is coupled between the third port406, and the first port402and the second port404. The protection switching apparatus408is operable to selectively couple the third port406to the first port402or to the second port404. This means that, when the third port406is coupled to the first port402, the upstream optical signal and the downstream optical signal can pass therebetween. And, when the third port406is coupled to the second port404, the upstream optical signal and the downstream optical signal can pass therebetween.

The protection apparatus400further comprises modifying apparatus410configured to modify the upstream optical signal, received at the third port406, before the upstream optical signal is output from the first port402, such that the upstream optical signal has a distinctive physical characteristic. This means that, advantageously, the upstream optical signal may be distinguished from the downstream optical signal received over the optical link at the first port402.

The apparatus400further comprises detecting apparatus412, coupled between the first port402and the third port406. The detecting apparatus412is configured to receive (at least) a portion of an incoming optical signal, received at the first port (i.e. an optical signal received at the first port from the optical link). The detecting apparatus412is configured to selectively detect the presence of the upstream optical signal or the downstream optical signal in the portion of the incoming optical signal based on the distinctive physical characteristic. For example, the detecting apparatus412may be configured to detect the distinctive physical characteristic. The apparatus400further comprises control circuitry414configured to provide a control signal to the protection switching apparatus based on the detecting by, for example on an output from, the detecting apparatus412.

For example, the control circuitry414may be configured to cause the protection switching apparatus408to selectively couple the third port406to the second port404instead of to the first port402if the detecting apparatus408detects the presence of the upstream optical signal in the portion of the incoming optical signal. This may comprise, so as to distinguish from (low) reflection occurring during normal operation of the optical link, triggering the protection switching apparatus408if the detecting apparatus408detects the presence of the upstream optical signal at an optical power indicative of reflection of the upstream optical signal caused by an optical link defect, such as an optical link cut. For example, this may comprise triggering the protection switching apparatus408if the detecting apparatus412detects the upstream optical signal at a power greater than a predetermined threshold.

Alternatively, the control circuitry414may be configured to cause the protection switching apparatus404to selectively couple the third port406to the second port404instead of to the first port402if the detecting apparatus412does not detect the presence of the downstream optical signal in the portion of the incoming optical signal.

Thus, embodiments of the present invention, advantageously, enable a fault, such as an optical fibre cut, along an optical link to be detected, and thus the optical link to be protected by switching of the traffic (upstream/downstream signals) onto a further (or protection) optical link, in a manner independent of the network wavelength plan.

FIGS. 5aand 5billustrate methods of using the protection apparatus400according to embodiments of the present invention, in order to aid understanding.

At step500, the method comprises receiving an upstream optical signal at the third port406. At step502, the method further comprises modifying the upstream optical signal such that the upstream optical signal has a distinctive physical characteristic. At step504, the method further comprises transmitting the (modified) upstream optical signal from the first port402(i.e. for transmission over the first optical link). The method also comprises at506, simultaneously, receiving a downstream optical signal at the first port402, and at step508outputting the downstream optical signal from the third port406. The method further comprises, at516, monitoring the optical link using the detecting apparatus510.

FIG. 5billustrates steps performed by the detecting apparatus412/control circuitry414of the protection apparatus400. It should be noted that the detecting apparatus412and the control circuitry414may be integrated to any degree.

At step512the detecting apparatus412receives at least a portion of an incoming optical signal received at the first port402. At step514, the detecting apparatus412selectively detects the presence (or absence) of the upstream optical signal or the downstream optical signal in the portion of the incoming signal based on the distinctive physical characteristic. At step414the control circuitry414causes the protection switch408to couple the third port406to the second port404instead of to the first port402. For example the protection switch408may cause the third port406to be coupled to the second port404instead of to the first port402, if the upstream optical signal is detected (at a power indicative of a fault)518or if the downstream optical signal is not detected520.

A preferred embodiment of the protection switching apparatus400will now be described with reference toFIG. 6. In this example, the modifying apparatus410is modulating apparatus600configured to modulate the upstream optical signal, wherein the distinctive physical characteristic is a modulation. In this example, the modulation is an amplitude modulation. However, the modulation could be any type of modulation, such as a phase modulation or a frequency modulation.

Further, in this example, the modulation is advantageously a tone modulation. The modulation is applied at a low frequency of the upstream optical signal, for example less than 5 Hz. It should be noted that the modulation does not carry any digital information and thus is a purely “optical” modulation.

It should be appreciated that the modulating apparatus600may be configured in various ways, depending for example on the type of modulation to be applied, as will be understood by those skilled in the art.

In this example, the modulating apparatus600is configured to provide a modulating signal to the protection switching apparatus408, whereby when the upstream optical signal passes through the protection switching apparatus408the modulation is applied to the upstream optical signal. In particular, as shown inFIG. 7a, the modulation may be superimposed onto a control signal700provided to the protection switching apparatus408. Thus, in this case, the modulating apparatus600includes the protection switching apparatus408itself. This may provide a simple arrangement. However, other arrangements are possible.

For example, as shown inFIG. 7b, in this example where an amplitude modulation is applied to the upstream optical signal, a variable optical attenuator710may be provided along the path of the upstream optical stream. The variable optical attenuator710is configured to, in dependence on a control signal, apply the modulation to the upstream optical signal. In the example ofFIG. 7b, this variable optical attenuator710is integrated into the protection switching apparatus408. Alternatively, the variable optical attenuator may separate from the protection switching apparatus408.

It should be noted that in both of these examples the modulating apparatus600is arranged such that the modulation is applied to the upstream optical signal, whether the upstream optical signal is subsequently output from the first port402or the second port404. This enables a fault on the “protection” optical link12, as well as a fault on the optical link10, to be detected, provided detecting apparatus/control circuitry is arranged accordingly.

However, in alternative arrangements, the modulating apparatus600may for example be arranged between the protection switching apparatus408and the first port402, such that the modulation is only applied to an upstream optical signal output from the first port402. Alternatively, two modulating apparatus600may be provided, one between the protection switching apparatus408and the first port402and one between the protection switching apparatus408and the second port404to apply respective modulations to upstream optical signals to be transmitted from the respective ports402,404.

The protection apparatus400may be “revertive”, whereby once a fault on the optical link is fixed, traffic is switched back to that optical link. However, alternatively, the traffic may remain on the further optical link. In that case, the optical link may become a “protection” optical link, and traffic may be switched back to the optical link, in the event that a failure of the further optical link is detected.

Referring again toFIG. 6, in this preferred embodiment, the detecting apparatus412comprises a modulation detector610configured to detect the presence of the modulation in the portion of the incoming optical signal, which in this example is a tone modulation.

In particular, in this embodiment, an optical power detector612is coupled to the first port402. In this example the optical power detector612is arranged between the protection switching apparatus408and the first port402. However, other arrangements are possible, so long as, in this embodiment, the optical power detector612is coupled to receive a portion of the incoming optical signal before the incoming optical signal passes over the point at which the modulating apparatus600applies the modulation. The optical power detector612is arranged to receive a portion of an incoming optical signal received at the first port402, i.e. a portion of the optical signal travelling in the direction from the first port402to the third port406along an internal path therebetween. A portion of the incoming optical signal may be tapped off for example by a tap (not shown), and passed to the optical power detector612. The optical power detector612is configured to convert the portion of the incoming optical signal into an analog electrical signal. The modulation detector610is configured to detect the modulation in the analog electrical signal. First, the analog electrical signal may be passed through a noise filter, to at least partially remove noise from the analog electrical signal. Thus, in this example, the detecting apparatus412can detect the presence (or absence) of the upstream optical signal by virtue of the presence (or absence) of the modulation in the portion of the incoming optical signal.

In some embodiments, the detecting apparatus412may be configured to compare the presence of the modulation in the analog electrical signal to a threshold. If the modulation is present at an optical power greater than the threshold, or for example if the presence of the modulation increases by a predetermined amount, this may indicate that the optical link10is defective, since the upstream optical signal is being reflected back to the apparatus400at a level greater than that expected during normal operation. The control circuitry414may thus cause the protection switching apparatus408to selectively couple the third port406to the second port404instead of to the first port402, for example by a providing a control signal to the protection switching apparatus408.

Thus, in this preferred embodiment, the distinctive physical characteristic is a modulation. However, the distinctive physical characteristic may be any other type of physical characteristic by which the upstream optical can be distinguished from the downstream optical signal.

For example, the modifying apparatus410may be configured to add an optical signal to the upstream optical signal at a distinctive frequency, wherein the distinctive frequency is the distinctive physical characteristic. In this example, the detecting apparatus412may comprise a wavelength filter configured to selectively pass optical signals at the distinctive frequency. The detecting apparatus412may further comprise an optical power detector arranged to receive the optical signals passed by the wavelength filter. If the detecting apparatus412detects the presence of the optical signal at the distinctive frequency (for example if the optical power detector detects an optical power greater than that associated with reflection of the signal during normal operation), the control circuitry414may trigger the protection switching apparatus408to switch the signals onto the protection link.

FIG. 8shows a system for protecting an optical link according to an embodiment of the present invention.

In this example, a first apparatus900is provided at a first end of the optical link, and a second apparatus910is provided at a second end of the optical link. In this example, the first apparatus900comprises a first port402for coupling to a first end of the optical link, and a third port406configured to receive an upstream optical signal to be transmitted over the optical link, and to output a downstream optical signal received over the optical link. The second apparatus comprises a first port402for coupling to second end of the optical link, a second port404for coupling to a further optical link, and a third port406configured to receive an upstream optical signal to be transmitted over the optical link, and to output a downstream optical signal received over the optical link.

This protection system operates in a similar way to the protection apparatus400described above. However, instead of applying the distinctive physical characteristic at the same apparatus (the second apparatus910) which detects the presence of the upstream optical signal or downstream optical signal in an incoming signal, based on the distinctive physical characteristic, the distinctive physical characteristic is applied to the “upstream optical signal” transmitted by the first apparatus900at the opposite end of the optical link. This “upstream optical signal” is a “downstream optical signal” from the perspective of the second apparatus910. This protection system may also be used in a 1:1 or a 1+1 protection scheme.

Thus, the first apparatus900may comprise modifying apparatus410configured to modify the upstream optical signal, received at its third port406, before it is output from its first port402, such that the upstream optical signal has a distinctive physical characteristic. The modifying apparatus410may be configured as described above.

The second apparatus910may further comprise a protection switching apparatus408operable to selectively couple its third port406to its first port402or to its second port404. The second apparatus910further comprises detecting apparatus412, coupled between its first port and its third port. The detecting apparatus412is configured to receive a portion of an incoming optical signal, received at the first port402, and to selectively detect the presence of the upstream optical signal or the downstream optical signal in the portion of the incoming signal based on the distinctive physical characteristic (added by the first apparatus to the “downstream” optical signal received by the second apparatus). The second apparatus910further comprises control circuitry414configured to provide a control signal to the protection switching apparatus408based on an output from the detecting apparatus412. The detecting apparatus412/control circuitry414may be configured as described above.

In a 1:1 protection scheme, the first apparatus900may further comprise a second port404for coupling to a further optical link, and a protection switching apparatus408operable to selectively couple the third port to the first port or to the second port. The first apparatus900may further comprise detecting apparatus412, coupled between the first port402and the third port404, configured to receive a portion of an incoming optical signal, received at the first port402, and to selectively detect the presence of the upstream optical signal or the downstream optical signal in the portion of the incoming signal based on a distinctive physical characteristic. The first apparatus900may further comprise control circuitry configured to provide a control signal to the protection switching apparatus408based on an output from the detecting apparatus412.

The detecting apparatus412(in the first apparatus900) may be configured to detect the presence of the upstream optical signal or the downstream optical signal based on the distinctive physical characteristic applied to the upstream optical signal by the modifying apparatus410in the first apparatus900.

Alternatively, the second apparatus910may further comprise modifying apparatus410configured to modify the upstream optical signal, received at its third port406, before it is output from its first port402, such that the upstream optical signal transmitted by the second apparatus910(i.e. the “downstream optical signal” received by the first apparatus900) has a distinctive physical characteristic. In this case, the detecting apparatus412in the first apparatus900may detect the presence of the upstream optical signal or the downstream optical signal in an incoming signal to the first apparatus, based on this distinctive physical characteristic instead.

FIG. 8illustrates an example of a protection system according to a preferred embodiment of the present invention in a 1:1 protection scheme. In this example, a protection apparatus (apparatus900) substantially as described with respect toFIG. 6is coupled to a first end of the optical link10and a first end of the further optical link12. A further protection apparatus (apparatus910) substantially as described with respect toFIG. 6is coupled to a second end of the optical link10and to a second end of the further optical link12. The difference is that the first and second protection apparatus900and910are configured to detect the modulation applied by the other apparatus900,910respectively. The modulations are different. Thus, in this example, it is possible for the detecting apparatus412to be arranged to receive a portion of the incoming optical signal, after the incoming optical signal has traversed the protection switch408/point at which the modulating apparatus410applies the modulation.

Thus, embodiments of the present invention have the advantage that they can provide protection of an optical link arranged to carry both upstream and downstream optical signals (i.e. optical signals travelling in opposite directions), in a simpler, more cost effective and more reliable way than other solutions. Advantageously, the method is independent of the wavelength plan of the optical network, and thus the wavelength plan may be adjusted dynamically so as to improve network performance, without requiring reconfiguration of the apparatus. Further, no modifications are required to other network elements such as OADMs, HUB. The solution of the present invention is self-contained, and thus advantageously an optical network may be easily upgraded to provide protection of an optical link using apparatus embodying the present invention.

FIG. 10ashows a method of upgrading an optical network to provide protection of an optical link according to an embodiment of the present invention. The method comprises, at100, providing an apparatus400embodying the present invention. At110, the method comprises coupling the first port402of apparatus400to the optical link. At120, the method comprises coupling the second port404of the apparatus400to the further optical link. At130, the method further comprises coupling the third port406of apparatus400to a network part arranged to output an upstream optical signal and to receive a downstream optical signal.

FIG. 10bshows a method of upgrading an optical network to provide protection of an optical link according to a further embodiment of the present invention. The method comprises, at140, providing a system for protecting an optical link as described above, comprising first apparatus and second apparatus. The method comprises, at150, coupling the first port of the first apparatus to a first end of the optical link. At160, the method comprises coupling the third port of the first apparatus to a network part arranged to output an upstream optical signal and to receive a downstream optical signal. At170, the method comprises coupling the first port of the second apparatus to a second end of the optical link. At180the method comprises coupling the second port of the second apparatus to a further optical link. At190, the method comprises coupling the third port of the second apparatus to a network part arranged to output an upstream optical signal and to receive a downstream optical signal.