WO2013153132A2

WO2013153132A2 - A system for physical layer supervision in optical networks based on tuneable otdr, an hybrid optical switch and a focs

Info

Publication number: WO2013153132A2
Application number: PCT/EP2013/057518
Authority: WO
Inventors: Julio Montalvo
Original assignee: Telefónica, S.A
Priority date: 2012-04-13
Filing date: 2013-04-10
Publication date: 2013-10-17
Also published as: AR090643A1; WO2013153132A3

Abstract

A system for physical layer supervision in optical networks based on tuneable OTDR, an hybrid optical switch and a FOCS. The system comprising: - a light source arranged to inject a monitoring light signal using a plurality of different wavelengths together with at least one data light signal at an input of an optical network or a dark fibre to circulate there through; - a plurality of optical filters provided at different points of said optical network in case of in-line PON monitoring to receive said monitoring light signal and filtering at least part thereof, in the form of respective reflected light signals; - light detecting means arranged to receive said respective reflected light signals from said optical network or said dark fibre; and - analysis means connected to said light detecting means to analyse said received reflected light signals to perform a physical layer monitoring of the optical network; the system further comprises: - a fibre optic cross-connection system, or FOCS, between said light source and said optical network or said dark fibre and comprising at least one hybrid optical switch to deliver said light signal to a desired optical network or to a dark fibre, depending on said plurality of different wavelengths. The hybrid optical switch, for fibre switching a light source injecting a monitoring light signal using a plurality of different wavelengths comprising at least one passive optical wavelength multiplexer-demultiplexer to deliver said light signal to another of said hybrid optical switch or to a desired optical network or to a dark fibre, depending on said plurality of different wavelengths. The fibre optic cross connection system, or FOCS, comprising at least one hybrid optical switch to deliver a light signal to at least one desired optical network or dark fibre, depending on a plurality of different wavelengths.

Description

A System For Physical Layer Supervision In Optical Networks Based On Tuneable OTDR, an Hybrid Optical Switch and a FOCS

Field of the art

The present invention generally relates to optical communications, and more particularly, in a first aspect, to a system to provide physical layer supervision in optical networks based on reflectometry systems.

In a second aspect, the present invention relates to a hybrid optical switch for fibre switching.

In a third aspect, the present invention relates to a FOCS implementation for fibre switching.

By means of FOCS, Fibre Optical Cross Connection System, it will be understood an optical system designed to provide physical layer interconnects between optical fibre interfaces for fibre management and troubleshooting within optical networks.

Prior State of the Art

In today's PON systems, the physical infrastructure is not entirely visible to the Network Management System (NMS). As a direct consequence, a physical failure is not detected before creating service outage in upper layers which in turn may lead to tremendous loss in business for the operators. These arguments have been gaining importance as the warranty on the quality of the infrastructure becomes a deciding factor in the strongly competitive market place [1]. The aim of preventive maintenance is to detect any kind of deterioration in the network that can cause suspended services and to localize these faults in order to avoid specially trained people deployed with dedicated and often expensive equipments, which increases operation-and- maintenance expenses (OPEX).

PON infrastructure does not only suffer from accidental damages and environmental effects (e.g. water penetration in splice closures) but are also subject to a lot of changes after the network is installed and activated. As an example, the optical access network may not be initially fully loaded; subscribers would be turned up, possibly over an extended period of time [2]. Hence, network operators should continuously be aware if a change noticed by its monitoring system is service oriented or indeed a fault. So, the existing maintenance methods in PONs need to be updated.

The most common maintenance tool employed for troubleshooting in long-haul, point-to-point fibre optic links is an Optical Time Domain Reflectometer (OTDR). OTDR measurements can also be applied to PON systems, and there are three main approaches for doing that:

- Using dark fibres accompanying the feeder fibres of PONs, physically bypassing the first level of splitting up to the second level splitting. The efficiency of this approach relies on the gathering of information from active elements [3] and the fact that PON fibres can probabilistically share a high percentage of the same fibre cable infrastructure.

- Performing in-line measurements inside each PON fibre infrastructure by multiplexing an OTDR signal inside each PON feeder fibre at the Central Office, using Wavelength Division Multiplexers. In this approach, the OTDR signal is generated by an external optical source at a different wavelength that data signals.

- Performing in-line measurements inside each PON fibre infrastructure by generating the OTDR signal inside the PON transceiver at the OLT.

The management of the optical layer in PON systems is being standardized in

[4].

The in-line measurements using the integrated OTDR signal inside the data transceiver is a challenging approach whose implementation depends on the transceiver implementation and the physical media dependant layer of a particular PON technology. Even though GPON and EPON standards are completely closed, and XG-PON1 is on its way, no commercial product has appeared up to know for those systems; the technological uncertainty of XG-PON2 and NG-PON2 makes even more difficult the commercial adoption of this approach for PON supervision in a massive way.

On the other hand, external OTDR approaches, either using dark fibres or by multiplexing the OTDR signal inside the PON feeder fibres by using WDMs is an already commercially available tool.

Nevertheless, Central Offices with PON technologies can typically cover between 10 to 50 thousand homes passed (HP). For a typical splitting ratio of 1 :64, this means that there would exist up to 800 PON interfaces from a single CO in case of inline OTDR measurements though WDM filters, and around 200 dark fibres in case that accompanying fibres are used for supervision.

In order to share the cost of OTDR measurement equipment between all these PON interfaces, fibre switches are typically used, thus launching the OTDR pulses on a selected fibre at a certain time either in a periodic way (preventive measurement) or on demand (after a detected alarm), switching to other fibres when required. Fibre Optic Cross-Connected Systems (FOCS) are used to address this need.

The most suitable optical fibre switching technologies for FOCS implementations are Micro-Electro-Mechanical (MEM) switches [5] and opto- mechanical switches [6].

Problems with existing solutions:

The OTDR implementation inside PON transceivers is a very interesting proposal, but suffers from a high uncertainty due to both technological challenges and slow standardization advances, thus being difficult to have a commercial solution and massive deployment able to address the current and mid-term supervision requirements of FTTx network operators.

Regarding the existing external OTDR approaches,

• they lack of efficient scalability as more fibres are deployed requiring to be monitored as FTTx services become to be massively deployed. As new FTTx feeder fibres are deployed smoothly, a large number of active switches are required increasing the power consumption of the system.

• they require a high number of fibres or electrical supply points to be installed in order to increase the number of test ports of a OTDR supervision system. If a new active switch is installed close to fibres under test, a new electrical supply is required. If the same active switch is installed close to an already available electrical supply, longer fibre links are required to deliver the test signal to the fibres to be monitored.

• they have the risk of blocking a fibre connection in case of power supply failure at a certain switching stage of the fibre-optic cross-connect.

Summary of the Invention

It is necessary to offer an alternative to the state of the art which covers the gaps found therein, particularly those related to the lack of proposals which allow the supervision of the physical infrastructure in passive optical networks or other optical networks, either using inline monitoring signals or using dark fibres reserved for supervision purposes.

To that end, the present invention provides in a first aspect, a system for physical layer supervision in optical networks based on tuneable OTDR, comprising: - a light source arranged to inject a monitoring light signal using a plurality of different wavelengths together with at least one data light signal at an input of an optical network or a dark fibre to circulate there through;

- a plurality of optical filters provided at different points of said optical network in case of in-line PON monitoring to receive said monitoring light signal and filtering at least part thereof, in the form of respective reflected light signals;

- light detecting means arranged to receive reflected light signals from said optical network or said dark fibre; and

- analysis means connected to said light detecting means to analyse said received reflected light signals to perform a physical layer monitoring of the optical network;

On contrary to the known proposals, the system of the first aspect of the present invention comprises:

- a fibre optic cross-connection system, or FOCS, between said light source and said optical network or said dark fibre and comprising at least one hybrid optical switch to deliver said light signal to a desired optical network or to a dark fibre, depending on said plurality of different wavelengths.

Other embodiments of the system of the first aspect of the present invention are described according to appended claims 2 and 5, and in a subsequent section related to the detailed description of several embodiments.

In a second aspect, the present invention provides a hybrid optical switch, for fibre switching a light source injecting a monitoring light signal using a plurality of different wavelengths comprising at least one passive optical wavelength multiplexer- demultiplexer to deliver said light signal to another of said hybrid optical switch or to a desired optical network or to a dark fibre, depending on said plurality of different wavelengths.

Other embodiments of the hybrid optical switch of the second aspect of the present invention are described according to appended claims 7 and 8, and in a subsequent section related to the detailed description of several embodiments.

In a third aspect, the present invention provides a fibre optic cross connection system, or FOCS, comprising at least one hybrid optical switch to deliver a light signal to at least one desired optical network or dark fibre, depending on a plurality of different wavelengths.

In a preferred embodiment of the third aspect of the present invention the fibre optic cross connection system, comprises a plurality of said at least one hybrid optical switch constituting a plurality of cascade stages, each stage of said plurality of cascade stages constituted by at least one of said plurality of hybrid optical switches.

In another preferred embodiment of the third aspect of the present invention, each stage of said plurality of stages includes as many of said at least one active optical switch arranged to said at least one passive optical wavelength multiplexer- demultiplexer as output ports of previous stages.

Other embodiments of the fibre optic cross connection system of the third aspect of the present invention are described according to appended claims 10 to 15, and in a subsequent section related to the detailed description of several embodiments.

Brief Description of the Drawings

The previous and other advantages and features will be more fully understood from the following detailed description of embodiments, with reference to the attached, which must be considered in an illustrative and non-limiting manner, in which:

Figure 1 shows an example of an external OTDR supervision approach.

Figure 2 shows an example of the basic schematic used in the present invention.

Figure 3 shows an example of conventional fibre switch architecture.

Figure 4 shows an example of the generic active optical switch used in the present invention.

Figure 5 shows a proposed switching stage S'ij configuration, according to an embodiment of the present invention.

Figure 6 shows the FOCS implementation considering a tunable OTDR and WDM scalability, according to an embodiment of the present invention.

Figure 7 shows (a) an example of the State of the Art FOCs and (b) an example of the present invention proposal implementation considering (M=M'=64).

Figure 8 shows (a) an example of the State of the Art FOCs and (b) an example of the present invention proposal implementation considering (M=M'=128).

Figure 9 shows (a) an example of the State of the Art FOCs and (b) an example of the present invention proposal implementation considering (M=M'=256).

Figure 10 shows (a) an example of the State of the Art FOCs and (b) an example of the present invention proposal implementation considering (M=M'=512).

Figure 1 1 ilustrates the power savings estimations using 1 x4 WDMs and 4 wavelength OTDR example FOCS design, according to an embodiment of the present invention. Figure 12 ilustrates the CAPEX savings estimations using 1 x4 WDMs and 4 wavelength OTDR example FOCS design, according to an embodiment of the present invention. Detailed Description of Several Embodiments

In the present invention, it is proposed a cost efficient approach for FOCS implementation and scalability by using Tunable OTDR and WDM passives for fibre switching.

The basic schematic of the invention can be seen in the Figure 2.

Instead of having a single wavelength operating OTDR, the present invention proposes to use different wavelengths and WDM passives in the FOCS. By tuning the OTDR wavelength, the WDM passives combined with optical switches will deliver the test signal to the desired optical network or to a dark fibre reserved for supervision purposes, increasing the number of fibres to be tested in a passive way. Even though the figure 2 example shows only in-line supervision of several PONs, it is trivial to use the proposed invention for in-line metro/core system supervision, as well as dark fibre supervision in PON, metro or long-haul transmission infrastructures.

The present invention relies on two points:

• OTDR wavelength tunability. The OTDR pulses can be launched at different wavelengths, all of them within the legacy waveband already established for fibre monitoring. In the case of in-line monitoring in access, this can be the U- band (1625-1675nm); in the case of WDM metro/core systems, this can be any available channel in the employed wavelength grid in the case of using dark fibres, arbitrary wavelengths could be used.

· Wavelength DEMUX at some stage/s of the FOCS. The OTDR pulses are delivered to a selected fibre in a passive and inherent way depending on the wavelength of the OTDR. At certain parts of the optical fibre switching system, wavelength demultiplexing of OTDR signals is used as a fibre selection mechanism, instead of mechanically moving input fibre to a desired fibre output.

In a conventional fibre switch, several stages with different fibre selectors are typically cascaded, as shown in Figure 3.

The Supervision Network Management System (NMS) will communicate with the local management interface in the Central Office for monitoring the M fibres of the system. The local management interface is a local subsystem performing as interface for the NMS to obtain measurements on specific fibres, thus configuring the OTDR as well as all the switches in all the stages to prepare an optical path for delivering the test signal to a specific fibre to test. The local management should link the inventory information to the physical interconnections of the fibre ports of the different switching stages between them, and with the M fibres to test.

Each output port of the switching cascade can be connected to a filter for multiplexing the test signal with the data signals for in-line monitoring or to a dark fibre reserved for testing purposes.

At a certain switching stage in the Fibre Optic Cross-connect System (FOCS), Stagej, there are as many switches as output ports of the previous Stage_(i-1 ). If it is considered one of the switches at Stagej, namely Sij, it can be seen in Figure 4.

The optical switch Sij comprises either a 1xMij MEM switch or an optomechanical switch and requires electrical power supply, Mij being the total number of fibre outputs of Sij.

It is proposed an alternative S'ij configuration (namely hybrid switch) reducing the active switching components of Sij and/or using optical wavelength multiplexer- demultiplexers WDMijk, see in Figure 5.

At each of the ports of the Sij active switch, Mij, we have a 1 xNijk WDM (k= 1... Mij) passive. The output ports of S'ij are now M'ij= Nijk (k=1 ...Mij).

This configuration allows two relevant advantages:

• Increased number of ports (M'ij>Mij). This can be easily achieved in a passive way by increasing each Sij output port with Nijk ports (Nijk≥1 ).

• Reduced active elements in S'ij. If no increase in the number of output ports is desired for a particular FOCS design, it is possible to reduce the value of Mij and use WDM filters to keep the total number of desired outputs.

This invention also covers the case in which S'ij is totally passive (without active switch). In that case an active switch is completely replaced by a passive wavelength multiplexer-demultiplexer.

The total number of outputs of the proposed FOCS is M'≥M without increasing the power consumption of the system. The number of stages of the FOCS can be in most of cases reduced due to the high scalability of the output ports at each stage using WDM (Figure 6).

Each output port at the final stage Q' is identified by a path that depends on the passing wavelength of the WDM passives ports along its optical path from the OTDR through all the stages. The local management system should then consider the passing wavelength for each test fibre for properly preparing the OTDR operating wavelength and the optical path in the FOCS to deliver the optical pulses to the desired fibre.

If no wavelength blocking is desired from one cascade stage to the following, the operating waveband of each input port of Stage'_i must include all the output wavelengths of Stage'_i-1. The total wavelengths of the tunable OTDR should be higher or equal than Max(Nijk).

Each fibre selector Sij can be implemented using the alternative schematic S'ij reducing the cost and power consumption of Sij and/or increasing the number of outputs. The increased cost of T-OTDR with regards to S'ij a standard OTDR will be much lower than the savings obtained with the proposed S'ij fibre selectors. The total power consumption will also be reduced.

By employing Nijk>1 , then the invented system increases the number of total output ports M'>M in a cost effective and flexible way without increasing the power consumption and avoiding the need for additional points of power supply.

In conclusion, fibre selectors S'ij can replace a design with Sij keeping the same number of fibre outputs (M'=M) for system cost and power consumption reduction, or they can increase the number of output fibres in a cost effective way using the WDM scalability approach of the invention without increasing the power consumption.

The WDM passives may slightly increase the insertion loss of the OTDR signal through the FOCS, nevertheless this should not be considered a restriction in most cases, as the loss of dynamic range of OTDR will not be very significant or could be compensated with an increased measurement time (more averaging) or wider test pulses.

Embodiment of the Invention:

A generic FOCS using the implementation of Figure 4 is compared with an alternative example implementation using a 4 wavelength OTDR (N=4) and 1 x4 ports multiplexers/demultiplexers as in Figure 5 and Figure 6 (Nijk=4).

For each number of total ports, the proposed invention uses an active switch with lower number of outputs that in the conventional system (namely State of the Art, SOTA), and adds ports in groups of 4 by cascading 1 x4 WDMs. (Figures 7, 8, 9 and 10)

Commercial prices of mechanical switches, long-haul OTDR modules (with 1 wavelength for SOTA and 4 wavelengths for the present invention) and 4 channels Thin Film Filter-based Coarse WDMs have been considered for a cost comparative analysis. Each of the wavelengths of the tunable OTDR corresponds to each of the output ports of the WDMs. The 4 wavelength OTDR cost is estimated 104% higher than the single wavelength OTDR.

As shown in Figure 1 1 , the proposed invention can save up to 60% of power consumption of the FOCS for the considered example. Other designs may improve this performance.

Advantages of the Invention:

• OPEX reduction of FTTx infrastructure supervision: as the FOCS is partially implemented in a passive way, the overall power consumption of the system is reduced, and so is OPEX. On the other hand, as at the WDM stages of the FOCs the fibre switching is completely passive, the resiliency of the system to power cuts is also enhanced, which indirectly also reduces OPEX.

• CAPEX reduction of FTTx supervision: The concept described in this invention can reduce the cost of FOCS deployment and scalability due to the WDM approach, as passive demultiplexers are a less expensive approach that fibre switches with a high number of output ports.

As shown in Figure 12, for a number of ports higher or equal that 64, the proposed invention can save more than 60% of the total system CAPEX. The penalty of the OTDR dynamic range is just typically reduced in 1 .0 dB, which is a very low value compared with the total dynamic range (-41 dB) of the considered OTDR modules.

ACRONYMS

CAPEX Capital Expenditure

FOCS Fibre Optic Cross-Connect System

FTTB/C/D/H/T Fibre To The Curb/Desktop/Home/Tower GPON Gigabit Passive Optical Network

HP Home Passed

MEMS Micro-Electro-Mechanical System

MWh MegaWatt hour

NG-PON2 Next Generation Passive Optical Network 2 NMS Network Management System

OPEX Operational Expenditure

OLT Optical Line Termination

OTDR Optical Time Domain Reflectometer

PON Passive Optical Network

SOTA State of the Art

XG-PON1 1 st Generation 10-Gigabit PON

XG-PON2 2nd Generation 10-Gigabit PON

REFERENCES

[1] J. Montalvo, et al. Optical Transmission: the FP7 BONE project experience, s.l. :

Springer Verlag, 201 1.

[2] Optical Layer Monitoring in Passive Optical Networks (PONs): a review. K. Yijksel, et al. 2008. International Conference on Transparent Optical Networks.

[3] G.984.2 Ammendmend 2. ITU-T. G-PON Physical Media Dependent (PMD) layer especification.

[4] WT-287 (draft): PON Optical-Layer Management. BroadBand Forum. 201 1.

[5] Applications of large-scale optical 3D-MEMs switches in fibre-based broadband- access networks. N. Madamopoulos, et al. 1 , s.l. : Photonic Network Communications, 2010, Vol. 19.

[6] 1xN fibre bundle scanning switch. J.E. Ford, et al. 1998. Optical Fibre Communication Conference, pags. 143-144.

Claims

1.- A system for physical layer supervision in optical networks based on tuneable OTDR, comprising:

- a light source arranged to inject a monitoring light signal using a plurality of different wavelengths together with at least one data light signal at an input of an optical network or a dark fibre to circulate there through;

- light detecting means arranged to receive said respective reflected light signals from said optical network or said dark fibre; and

wherein the system is characterised in that it further comprises:

2.- The system of claim 1 , wherein said hybrid optical switch comprises at least one passive wavelength multiplexer-demultiplexer, with one input port and a plurality of output ports, to deliver said light signal to a desired optical network or to a dark fibre, depending on said plurality of different wavelengths.

3. - The system of claim 1 , wherein said hybrid optical switch further comprises at least one active optical switch, with one input port and a plurality of output ports, feeding said at least one passive optical wavelength multiplexer-demultiplexer.

4. - The system of claim 1 , wherein said optical network is: a passive optical network, or PON, an optical metro link, an optical core link, or a dark fibre.

5. - The system of any of previous claims, wherein said light source is an Optical Time Domain Reflectometer, or OTDR, with tuneable operating wavelength.

6. - A hybrid optical switch, for fibre switching a light source injecting a monitoring light signal using a plurality of different wavelengths comprising at least one passive optical wavelength multiplexer-demultiplexer to deliver said light signal to another of said hybrid optical switch or to a desired optical network or to a dark fibre, depending on said plurality of different wavelengths.

7.- The hybrid optical switch of claim 6, further comprising at least one active optical switch, with one input port and a plurality of outputs ports, arranged to said at least one passive optical wavelength multiplexer-demultiplexer, to connect said light source to another of said hybrid optical switch or to a plurality of optical networks or dark fibres.

8.- The hybrid optical switch of claims 6 and 7, wherein the operating wavelength of said at least one active switch and said at least one passive wavelength multiplexer- demultiplexer comprises the operating wavelength of said hybrid optical switch of a previous stage or of said OTDR.

9. - A fibre optic cross connection system, or FOCS, comprising at least one hybrid optical switch to deliver a light signal to at least one desired optical network or dark fibre, depending on a plurality of different wavelengths.

10. - The fibre optic cross connection system of claim 9, comprising a plurality of said at least one hybrid optical switch constituting a plurality of cascade stages, each stage of said plurality of cascade stages constituted by at least one of said plurality of hybrid optical switches.

1 1. - The fibre optic cross connection system of claim 10, wherein each stage of said plurality of cascade stages includes as many of at least one active optical switch arranged to said at least one passive optical wavelength multiplexer-demultiplexer as output ports of previous stages.

12.- The fibre optic cross connection system of claim 1 1 , wherein each of said input port of a following stage feeds said plurality of different wavelengths of said output ports of previous stages.

13. - The fibre optic cross connection system of any of previous claims 9 to 12, further comprising a plurality of filters each of them connected to said plurality of output ports of said at least one active optical switch and each of them allowing multiplexing said light signal together with said at least one data light signal.

14. - The fibre optic cross connection system of claim 9, wherein said optical network is: a passive optical network, or PON, an optical metro link, an optical core link, or a dark fibre.

15.- The fibre optic cross connection system of any of previous claims 9 to 14, wherein said light source is an Optical Time Domain Reflectometer, or OTDR, with tuneable operating wavelength.