WO2013004310A1

WO2013004310A1 - Wdm - pon using passive adm

Info

Publication number: WO2013004310A1
Application number: PCT/EP2011/061497
Authority: WO
Inventors: Filippo Ponzini; Alberto Bianchi
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2011-07-07
Filing date: 2011-07-07
Publication date: 2013-01-10

Abstract

A passive optical network has an optical line terminal (10), passive add drop wavelength multiplexers (20), at different locations along a first optical path, and an optical network termination part (30) coupled to the optical add path and optical drop path of a respective one of the passive add drop multiplexers. The passive add drop wavelength multiplexers are stand-alone parts separate from their respective optical network termination parts, being optically passive, unpowered and having a static configuration of wavelengths. By using such passive ADMs spread along the first optical path acting as a feeder, the PON is no longer limited to a tree structure. Also, the expense of an AWG device can be avoided. By making the ADMs stand-alone passive devices they can be located anywhere, and are non-specific to the types of terminal equipment, so later upgrade or standardisation of the terminal equipment is easier.

Description

WDM -PON USING PASSIVE ADM

Field

The present invention relates to passive optical networks, and to methods of using passive optical networks, and to methods of installing such passive optical networks.

Background

In transport networks, DWDM technology offers many benefits in terms of bandwidth capabilities and scalability. WDM-PON brings this benefit also in access networks, offering high capacity (upgradeable to 10Gb/s), long distances, no bandwidth contention (virtual point-to-point) and service transparency, together with the possibility of smooth upgrades (per channel) in the protocol and in the bit-rate. WDM-PON is an emerging technology also for mobile backhaul, since broadband services and bandwidth demands are quickly increasing. A conventional WDM-PON is realized with a tree topology with a passive AWG at the remote node (RN) acting as a distribution node for mux demux of the channels. This topology supports a high number of ONTs with the same kind of ONT for any AWG port (colorless)

In mobile backhaul WDM-PON permits ultra-broad dedicated bandwidth for each radio base station with high aggregation capacity (up to 96I x 10Gb/s), very low latency, possibility to serve high density and rural areas with the same infrastructure and again the possibility to share the same infrastructure for mobile, residential and business access (Multi-service network).

Conventional WDM-PON networks based on tree topology represent an open and scalable network solution not only for conventional access, but also in metro transport and backhauling scenarios. Despite advantages in terms of bandwidth, scalability and transparency, they suffer from being limited to a tree topology. Nevertheless they are low cost, easy to deploy and able to reach many users with reduced costs for fiber digging, compared to point to point links. Summary

Embodiments of the invention provide improved methods and apparatus. According to a first aspect of the invention, there is provided a passive optical network having an optical line terminal, for sending and receiving wavelength multiplexed signals on an optical first optical path, and two or more passive add drop wavelength multiplexers. These multiplexers are at different locations along the first optical path, and each have a single optical add path and a single optical drop path. They are arranged to add one or more of the wavelengths into the wavelength multiplexed signals, from the optical add path, and arranged for dropping one or more wavelengths from the wavelength multiplexed signals to the optical drop path. One or more optical network termination parts are coupled to the optical add path and optical drop path of a respective one of the passive add drop multiplexers. The passive add drop wavelength multiplexers are stand-alone parts in that they are separate from, not integrated with, their respective optical network termination parts, and are passive in being optically passive, not having optically active components, and unpowered, not needing a power source, and have a static, not dynamic configuration of wavelengths added or dropped.

By using such passive ADMs spread along the first optical path acting as a feeder, the PON is no longer limited to a tree structure as is the case with existing PONs having an AWG for mux and demux of many wavelengths. Hence the PON can now be run over existing fiber runs such as rings more easily, thus reducing installation cost. Also, the expense of an AWG device can be avoided, at least in some parts of the network. By making the ADMs stand- alone passive devices they can be located anywhere, without dependence on power supply, again reducing installation costs compared to known WDM PON rings (see "Chatzi" reference) using integrated remote nodes. By making them passive with no optically active components, they have less effect on the optical levels in the network and so are non-specific to the types of terminal equipment, so later upg rade or standard isation of the term inal eq u ipment can be implemented without changing the ADMs and thus at less cost. Since the choice of wavelengths for adding or dropping is fixed, there is lower complexity and lower cost, and compatibil ity with colourless types of optical network termination part.

Another aspect provides a method of using a passive optical network involving using an optical line terminal, to send and receive wavelength multiplexed signals on an optical first optical path, and adding or dropping wavelengths at two or more passive add drop wavelength multiplexers. The multiplexers are at different locations along the first optical path, and each has a single optical add path and a single optical drop path, and are arranged to add one or more of the wavelengths into the wavelength multiplexed signals, from the optical add path, and to drop one or more wavelengths from the wavelength multiplexed signals to the optical drop path. One or more optical network termination parts are coupled to the optical add path and optical drop path of a respective one of the add drop multiplexers, to receive data from the optical line terminal on the dropped wavelength on the optical drop path and to send data to the optical line terminal on the added wavelength on the optical add path. The passive add drop wavelength multiplexers are stand-alone parts as set out above.

Another aspect provides a method of installing a passive optical network having an optical line terminal, for sending and receiving wavelength multiplexed signals on a first optical first optical path, and at least two passive add drop wavelength multiplexers, at different locations along the first optical path. Each of these has a single optical add path and a single optical drop path, and is arranged to add at least one of the wavelengths into the wavelength multiplexed signals on the first optical path, from the optical add path, and arranged for dropping at least one of the wavelengths from the wavelength multiplexed signals on the first optical path to the optical drop path. The installation method involves installing at least one optical network termination part by coupling it to the optical add path and optical drop path of a respective one of the passive add drop multiplexers. The passive add drop wavelength multiplexers are stand-alone parts separate from their respective optical network termination parts, and are passive in being optically passive, unpowered and having a static configuration of which wavelengths are added and dropped.

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

Brief Description of the Drawings:

How the present invention may be put into effect will now be described by way of example with reference to the appended drawings, in which:

Fig 1 shows a schematic view of a WDM PON network according to an embodiment,

Fig 2 shows a passive ADM for use in an embodiment,

Fig 3 shows a schematic view of a WDM PON network for mobile backhaul according to an embodiment,

Fig 4 shows a passive ADM using 1 skipO filters for use in an embodiment,

Fig 5 shows a passive ADM having an add module and a drop module for use in an embodiment,

Fig 6 shows a schematic view of a WDM PON network in a linear structure according to an embodiment,

Fig 7 shows a schematic view of a WDM PON network in a linear structure using a single fiber for a bid irectional first optical path according to an embodiment, and

Fig 8 shows a schematic view of a WDM PON network having a ring structure with a tree structure of nodes on the add and drop paths according to an embodiment.

Detailed Description:

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

Abbreviations

AWG Arrayed Waveguide Grating

RBS Radio Base Station

BBU Base Band Unit

CO Central Office

CPRI Common Public Radio Interface

DWDM: Dense Wavelength Division Multiplexing

O/E Opto-Electric

OLT Optical Line Terminal

ONT Optical Network Termination

PON Passive Optical Network

RRU Radio Remote Unit

Definitions:

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

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

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

References to base stations are intended to encompass any kind of base station, not limited to those of any particular wireless protocol or frequency, and not l imited to being located at a single location and so can encompass distributed base stations having functions at different locations or shared functions in a computing cloud shared between multiple base stations.

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

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

References to stand alone ADMs are intended to encompass parts which are sufficiently separate to enable the ONTs coupled to the respective ADM to be u pg raded or replaced without need ing to d isrupt the ADM , wh ich can encompass having an optical coupling between the parts which is disconnectable using a plug and socket arrangement, or a fiber splice coupling for example.

Introduction

By way of introduction to the embodiments, how they address some issues with conventional designs will be explained . Current techniques involve tree structures, but these don't fit conveniently with existing fiber infrastructure which is often in rings. If an operator wants to use such infrastructure, for example rings, then one possibility is to choose not to use PON equipment, but go towards metro rings. They offer high flexibility, channel add & drop, and resilience. The drawbacks are higher costs, dedicated and power consuming hardware, and complicated management layers.

To address this, some embodiments of the invention involve ways of using WDM-PON equipment in ring topologies and other topologies, keeping low costs, low power consumption and agile transport layers, typical of WDM-PON. One feature which helps enable this is the concept of "distributed AWG". It can in some cases enable re-use of the same HW equipments for OLT and ONTs on a ring topology. In some cases the distribution node of the PON, conventionally implemented by an AWG, will be replaced by passive elements distributed around a first optical path which may be a ring structure. A cheap and easy way to adapt WDM-PON equipment to a ring topology is shown. Rings are typically easier to deploy than tree topologies and they offer low dig costs and often use less fiber for a g iven number of nodes (suitable for applications such as mobile backhaul applications where there are clusters of non-colocated RRUs far from the Main/Baseband Unit).

Additional features of some embodiments

Any features can be added to any of the aspects of the invention, some will be shown in the figures and described below. The passive ADMs can have a path to add the wavelengths to the first optical path in one direction only. This can avoid optical splitting of the optical add path. By avoiding splitting, lower optical power loss is achieved and so there is less need for amplification, or more passive ADMs can be added for a given optical power level, and more simplicity and lower costs for each of the passive ADMs.

The first optical path can be arranged in a ring . This can enable different network topologies to be used and can make it easier to use existing installed fiber.

The first optical path can comprise a single optical fiber. Using a single fiber for first optical path can give lower costs for fiber deployment, and greater simplicity of the passive ADMs, and thus lower costs of this part also. If in a ring then it can be arranged to be unidirectional, if not in a ring then it can be bidirectional. The passive ADMs can each have a first optical filter for adding and a second optical filter for dropping. This can provide simplicity of construction. Any or all of the passive ADMs can be arranged to add and drop adjacent wavelengths. This can help minimize the walk-off between uplink and downlink, with benefits if used for protocols with tight synchronization requirements for example.

The passive ADMs can have a path to provide a single undivided output for the drop to the optical drop path. This can also help avoid splitting. This can reduce optical power losses, just as adding without splitting can.

The passive optical network can have two or more of the optical network terminals coupled to the add and drop paths of the same one of the passive add drop multiplexers, that one of the passive add drop multiplexers being arranged to add and drop two or more of the wavelengths. This can enable a wider range of network topologies to be used.

The passive optical network can be part of a radio access network, for use as a backhaul network. This can help reduce costs for such backhaul functions compared to traditional point to point links used for backhaul. The radio access network can have a base band unit for a radio base station and a number of rad io remote un its, coupled to the base band un it by the passive optical network. The easy and agile approach offered by WDM-PON can be of benefit here where there are typically tight requ irements for synchronization for example to meet CPRI requirements.

The method of using the network can involve adding of the respective one or more wavelengths from the optical add path to the first optical path without optical splitting of the optical add path.

The method can involve sending the wavelength multiplexed signals around the first optical path set up as a ring, and the adding involving adding to the respective wavelength to the ring in the same direction as the multiplexed signals.

The method of using can involve sending backhaul data between nodes of a radio access network using the passive optical network.

The installing can involve instal l ing the at least two passive add drop wavelength multiplexers at different locations along the first optical path. These can be embedded with the fiber as they don't need power supply or control signals, or can be co-located with ONT equipment for example. The installing can involve coupling the optical line terminal to a fiber ring to form the first optical path to have a ring topology. The installing can involve coupling colourless optical network terminals to the passive ADMs. This can be an upgrade step, making use of the stand alone nature of the passive ADMs.

Installing at least two of the optical network terminals to the add and drop paths of the same one of the passive add drop multiplexers, to add and drop at least two of the wavelengths can enable hybrid topologies to be built up.

The installing can involve coupling nodes of a radio access network to the optical line terminal and to the optical network terminals for use as a backhaul network.

Fig 1 , schematic view of a ring network according to a first embodiment

The WDM-PON in figure 1 is shown as a ring 5 though other topologies are feasible. In Figure 1 , the network has an OLT 10 and several ONTs 30 placed along the ring, each of the ONTs being coupled to the ring by a passive ADM in place of the conventional AWG. Three passive ADMs are shown but there could be many more. A passive ADM is provided for each ONT in this case, though several ONTs could be coupled to each passive ADM in an alternative embodiment. Each ONT along the ring is connected to its respective passive ADM with dedicated wavelengths (one for uplink (λ_κ, or λ_Μ), and one for downlink (λ_κ, or λ_Μ)). Optionally more than one wavelength is added or dropped by the passive ADM, different wavelength plans are possible, but for most applications, different wavelengths should be added and dropped by different ones of the passive ADMs. Only if a broadcast capability is needed would the ADMs be arranged to split a wavelength so that part of its optical signal power could be shared by more than one of the ONTs. The positions of the ONTs in the network can be in the same place as the passive ADM or some kilometers away, as implemented in conventional WDM-PON networks.

It is notable that if the ONTs are colorless (a highly desirable condition for each WDM-PON) they can be placed next to any passive ADM and they will able to operate on the proper wavelengths. Exactly the same behavior is provided by the passive ADM as would be expected when ONTs are attached to different ports of a conventional AWG type distribution node in a conventional tree topology.

The proposed ring as shown does not support protection, to keep lower cost and a full hardware and software compatibility with conventional PONs. If resilience is a strong requirement, it can be reached through the deployment of additional fiber links (multipath) and through dedicated hardware such as splitters and additional ADMs for the protection paths.

A method of installation of such a WDM PON may involve identifying an existing fiber run, or laying a new one, then installing the fixed passive ADMs along the first optical path formed by the fiber according to a wavelength add and drop plan. Then ONTs can be coupled to the ADMs. As the ADMs are universal, not specific to a type of ONT, the network can be upgraded by changing some or all of the ONTs without changing the ADMs. Different wavelengths can be used for different types of ONT to create a hybrid multi-standard network, if the OLT is compatible with multiple standards. The ONTs can be colourless types, which is usually more cost effective, or could be wavelength specific in principle.

Fig 2, Passive ADM

Figure 2 shows an example of a passive ADM for use in embodiments such as that of figure 1 . Each ADM is fully passive, having no optically active parts, no electrical components, and in this case, a fixed wavelength selection scheme with no active control of wavelength selection. It can be called a tap, as there many of them tapping into the feeder path at different locations. It has an optical filter 1 00 for dropping a wavelength to an optical drop path, so that other wavelengths are passed on to another optical filter 1 10 for adding a wavelength from an optical add path. Although shown as having a single direction for the first optical path, adding the wavelength in the same direction as the dropped wavelength, the added wavelength could be added in the opposite direction. Notably the passive ADM is simpler and thus typically cheaper to manufacture and install than an AWG. It is not tunable and realizes a static add/drop. This is never a limitation, because it is "embedded" in the fiber infrastructure. Each passive ADM can cover one uplink and one downlink wavelength to serve one ONT, or multiple wavelengths to serve one ADM or to serve multiple ADMs. Fig 3, network for mobile backhaul

Fig 3 shows a schematic view of a WDM PON network for mobile backhaul for a radio access network according to an embodiment similar to that of figure 1 . One useful application of the proposed WDM-PON network is in a radio access network, in particular where there is a centralized radio base station, where the RBS processing equipment is centralized in a BBU (Base Band Unit 40) and the antennas are remotized RRU (Radio Remote Unit 50).

The BBU and RRU can be connected over the WDM-PON network as described above, built using passive ADMs in place of a conventional WDM- PON distribution node. The BBU is coupled to and located next to the OLT and each ONT serves each RRU. Each passive ADM in this case covers one uplink and downl in k wavelength to con nect a Remote Rad io U n it (RRU). The reference transport protocol in this network scenario is CPRI, with strict requirements in terms of bit/rate (2.5G and higher for LTE applications) and synchronization. The easy and agile approach offered by WDM-PON is particularly useful here to fulfill CPRI requirements.

Fig 4, passive ADM using I skipO filters

Fig 4 shows a passive ADM using I skipO filters for use in an embodiment. As shown, two adjacent wavelengths in the filter band can be extracted and/or inserted into the comb of multiplexed wavelengths λι to λ_Ν on the first optical path using a cascade of two I skipO filters (101 , 1 1 1 ) for downlink and uplink operations: the first of these filters (101 ) is able to drop wavelength K, and the second of these filters (1 1 1 ) adds the wavelength K+1 . I skipO filters add or drop one of the wavelengths and allow all others through, in other words no adjacent wavelengths are needed for the slopes or guard bands of the filter characteristic. In principle 1 skip1 filters could be used if the loss of neighbouring wavelengths as guard bands could be tolerated.

This typical wavelength plan of adding and dropping adjacent wavelengths is useful to minimize the walk-off between uplink and downlink, with benefits if used for protocols with tight synchronization requirements: this is a notable constraint in some backhaul scenarios. Alternative wavelength plans can be proposed with non adjacent wavelengths and with more than one wavelength being added or dropped, by choosing different filters. The filters can be any kind of optical filter able to extract or add a particular wavelength on for example a dense WDM comb with spacing of 100Ghz or similar. Technologies to realize the filters can be thin films or gratings for example.

The number of ONTs depends on the available wavelengths, but also on the number of ADMs along the ring. Each ADM loss is typically around half of a dB: similar or lower losses compared to a conventional AWG in a tree configuration if no more than 10 ADMs are present (typical of MBH scenarios). In case of high number of ADMs, additional loss can be easily managed, due to the short haul of the ring compared to a trunk fiber or if necessary slightly increasing the optical power.

Fig 5, passive ADM having modules

Fig 5 shows a passive ADM similar to that of figure 2 or figure 4, but having a drop module 22 containing the drop filter 100 and an add module 24 containing the add filter 1 10, for use in an embodiment of the PON. The modules show that th e add a nd d rop fi lters need not be i nteg rated tog ether, but ca n be manufactured separately and coupled by a conventional optical coupling such as a detachable plug-in optical connector or a fiber spl ice for example. The order of the add and drop filters can be varied so that the add comes before the drop for example, unless the same wavelength is used for add and drop, which is possible in a unidirectional ring network. As before, the direction of the drop or the add can be different, if the first optical path is bidirectional.

Fig 6, linear type network

Fig 6 shows a schematic view of a WDM PON network similar to that of figure 1 but having a linear topology for the fi rst optica l path according to an embodiment. In this case the OLT is coupled to a chain of passive ADMs by a first optical path in the form of a pair of unidirectional fibers. Three passive ADMs are shown but there could be many more. A passive ADM is provided for each ONT in this case, though several ONTs could be coupled to each passive ADM in an alternative embodiment. Each ONT along the ring is connected to its respective passive ADM with dedicated wavelengths (one for uplink (λ_κ, or AM), and one for downl in k (λ_κ, or λ_Μ)). Clearly in this case unlike the ring topology case, the ADMs must be arranged so that the direction of adding the added wavelength is the opposite direction of the drop direction, so that the adding direction is back towards the OLT. Optionally more than one wavelength is added or dropped by the passive ADM, different wavelength plans are possible, but for most applications, different wavelengths should be added and dropped by different ones of the passive ADMs.

Fig 7, linear type network using single fiber

Fig 7 shows a schematic view of a WDM PON network having a linear topology similar to that of figure 6, but using a single fiber 12 for a bidirectional first optical path according to an embodiment. Compared to conventional point to point links (the reference scenario in this particular application) here exactly the same performance can be obtained using just one fiber. If applied to the mobile backhaul application, only one fiber is needed between the BBU and the remote node and therefore lower costs for fiber deployment are involved.

Fig 8, ring with trees topology

Fig 8 shows a schematic view of a WDM PON network having a ring topology for the first optical path similar to figure 1 but with a tree structure of nodes on the add and drop paths according to an embodiment. One example of a tree is shown, but there can of course be many trees from different points on the first optical path. In the example shown, the tree carries three wavelengths which are added to or dropped from the first optical path at a fixed λ passive ADM 26. The tree has a first ADM 21 for dropping a wavelength A_L and adding a wavelength λ_κ from a first ONT. A second ADM 21 of the tree separates the second and third wavelengths to couple them to the second and third ONTs. The tree part is not limited to being a tree structure and can have any topology, including a ring topology. The ADMs can be fixed passive ADMs or any other type, even conventional AWG devices. There can be splitters in some cases to broadcast signals to multiple nodes. Multiple wavelength add/drop in each ADM enables hybrid topologies (such as rings coupled by a ring, or trees coupled by a ring, or rings coupled by a tree, or trees coupled by a tree for example), but the topology should be decided during the deployment of the first optical path, otherwise changes in the ADMs would be needed (which is possible and would still result in lower costs compared to installing a reconfigurable metro ring type network).

Concluding remarks

As has been described, a passive optical network has an optical line terminal 10, passive add drop wavelength multiplexers 20, at different locations along a first optical path, and an optical network termination part 30 coupled to the optical add path and optical drop path of a respective one of the passive add drop multiplexers. The passive add drop wavelength multiplexers are standalone parts separate from their respective optical network termination parts, being optically passive, unpowered and having a static configuration of wavelengths. By using such passive ADMs spread along the first optical path acting as a feeder, the PON is no longer limited to a tree structure. Also, the expense of an AWG device can be avoided. By making the ADMs stand-alone passive devices they can be located anywhere, and are non-specific to the types of terminal equipment, so later upgrade or standardisation of the terminal equipment is easier.

Some embod iments make use of the concept of a WDM-PON in a ring- topology together with passive ADMs spread out to enable this migration. They can be inserted along the ring to replicate the distribution node functions accord ing to a distributed AWG concept. It is notable that there is "full equivalence" in at least some of the embodiments of the proposed solution compared with conventional WDM-PONs (tree topology), so that conventional ONTs and LOTs can be used. Operators will be free to combine the two topologies (tree and ring) according with their requirements, thanks to the possibility to reuse exactly the same relatively inexpensive OLT and ONT hardware built for conventional WDM-PONs.

The "distributed AWG concept" replaces the AWG with a fully passive ADM (which can be regarded as a passive tap) able to extract and/or insert two wavelengths into the comb of wavelength multiplexed signals.

Some effects of particular features of WDM-PON ring equivalent embodiments are:

Reuse of WDM-PON equ ipment for OLT and ONT: operator chooses, according to available infrastructure and without additional costs.

Failure resil ience, in case of multi-path rings, without further digging.

Full Colorless of ONTs: just one kind of ONT for any node. Applicability to Mobile Backhaul applications and CPRI compatibility. To summarize, WDM-PON ring equivalent embodiments discussed above offers operators the possibility to implement WDM-PON not only on tree topologies, but also in rings and hybrid topologies. They can reuse existing infrastructure and available systems, with a full WDM-PON compatibility. Only additional low cost and fully passive ADMs need to be inserted along the first optical path. Possible applications include conventional residential access, and mobile backhauling applications.

References

Ponzini, F.; Cavaliere, F.; Berrettini, G.; Presi, M.; Ciaramella, E.; Calabretta, N.; Bogoni, A. "Evolution Scenario toward WDM-PON", Optical Communications and Networking, IEEE/OSA Journal (Vol: 1 , Issue: 4, 2009 , pages: C25 - C34) Cavaliere, F.; Ponzini, F.; Presi, M.; Ciaramella, E.; "Migration towards high speed optical access enabled by WDM techniques", Communications and Photonics Conference and Exhibition (ACP), 2009 Asia (2009 , Pages: 1 - 2) Ch atzi , S . et a l , " L-band In-line Remote Amplification for an extended WDM/PON Ring Architecture, ICTON 2009.

Claims

Claims:

1 . A passive optical network having:

an optical l ine terminal, for sending and receiving wavelength multiplexed signals on a first optical path,

at least two passive add drop wavelength multiplexers, at different locations along the first optical path, and each having a single optical add path and a single optical drop path, and being arranged to add at least one of the wavelengths into the wavelength multiplexed signals on the first optical path, from the optical add path, and for dropping at least one of the wavelengths from the wavelength multiplexed signals on the first optical path to the optical drop path, and

at least one optical network termination part coupled to the optical add path and optical drop path of a respective one of the passive add drop multiplexers, wherein the passive add drop wavelength multiplexers are stand-alone parts separate from their respective optical network termination parts, and are passive in being optically passive, unpowered and having a static configuration of which wavelengths are added and dropped.

2. The network of claim 1 , the passive ADM having a path to add the respective one or more wavelengths from the optical add path to the first optical path in one direction only.

3. The network of any preceding claim, the first optical path being arranged in a ring.

4. The network of any preceding claim, the first optical path comprising a single optical fiber.

5. The passive optical network of any of the preceding claims, the passive ADMs each comprising a first optical filter for adding and a second optical filter for dropping.

6. The passive optical network of any of the preceding claims, at least one of the passive ADMs being arranged to add and drop adjacent wavelengths.

7. The passive optical network of any of the preceding claims, the passive ADM having a path to provide a single undivided output for the drop to the optical drop path.

8. The passive optical network of any of the preceding claims, and having at least two of the optical network terminals coupled to the add and drop paths of the same one of the passive add drop multiplexers, that one of the passive add drop mu ltiplexers being arranged to add and drop at least two of the wavelengths.

9. A radio access network comprising a backhaul network in the form of a passive optical network as set out in any of the preceding claims.

10. The radio access network of claim 9, having a base band unit for a radio base station and a number of radio remote units, coupled to the base band unit by the passive optical network.

1 1 . A method of using a passive optical network having the steps of:

using an optical line terminal, to send and receive wavelength multiplexed signals on a first optical path,

adding or dropping wavelengths at at least two passive add drop wavelength multiplexers, at different locations along the first optical path, each of the passive wavelength multiplexers having a single optical add path and a single optical drop path, and being arranged to add at least one of the wavelengths into the wavelength multiplexed signals, from the optical add path, and for dropping at least one of the wavelengths from the wavelength multiplexed signals to the optical drop path, and

using at least one optical network termination part coupled to the optical add path and optical drop path of a respective one of the add drop multiplexers, to receive data from the optical line terminal on the dropped wavelength on the optical drop path and to send data to the optical line terminal on the added wavelength on the optical add path,

wherein the passive add drop wavelength multiplexers are stand-alone parts separate from their respective optical network termination parts, and are passive in being optically passive, unpowered and having a static configuration of which wavelengths are added and dropped.

1 2. The method of cla im 1 1 , the add ing of the respective one or more wavelengths from the optical add path to the first optical path being carried out without optical splitting of the optical add path.

13. The method of claim 1 1 or 12, the first optical path comprising a ring and the method having the step of sending the wavelength multiplexed signals around the ring, and the adding involving adding to the respective wavelength to the ring in the same direction as the multiplexed signals.

14. The method of any of claims 1 1 to 13, having the step of sending backhaul data between nodes of a rad io access network using the passive optical network of any of claims 1 to 8, or the radio access network of claim 9 or 10.

1 5. A method of installing a passive optical network having an optical line terminal, for sending and receiving wavelength multiplexed signals on a first optical first optical path, at least two passive add drop wavelength multiplexers, at different locations along the first optical path, and each having a single optical add path and a single optical drop path, and being arranged to add at least one of the wavelengths into the wavelength multiplexed signals on the first optical path, from the optical add path, and for dropping at least one of the wavelengths from the wavelength multiplexed signals on the first optical path to the optical drop path, and the method having the step of:

installing at least one optical network termination part by coupling it to the optical add path and optical drop path of a respective one of the passive add drop multiplexers, wherein the passive add drop wavelength multiplexers are stand-alone parts separate from their respective optical network termination parts, and are passive in being optically passive, unpowered and having a static configuration of which wavelengths are added and dropped.

16. The method of installing as set out in claim 15, having the step of installing the at least two passive add drop wavelength multiplexers at different locations along the first optical path.

17. The method of installing of claim 15 or 16, having the step of installing the optical line terminal coupled to a fiber ring to form the first optical path to have a ring topology.

18. The method of install ing of any of claims 1 5 to 1 7, having the step of coupling colourless optical network terminals to the passive ADMs.

1 9. The method of install ing of any of claims 1 5 to 1 8, having the step of coupling at least two of the optical network terminals to the add and drop paths of the same one of the passive add drop multiplexers, that one of the passive add drop multiplexers being arranged to add and drop at least two of the wavelengths.

20. The method of installing of any of claims 1 5 to 1 8, having the step of coupling nodes of a radio access network to the optical line terminal and to the optical network terminals for use as a backhaul network.