WO2006048863A1

WO2006048863A1 - Reconfigurable optical add drop multiplexer (roadm)

Info

Publication number: WO2006048863A1
Application number: PCT/IL2005/001126
Authority: WO
Inventors: Eyal Lichtman
Original assignee: Eci Telecom Ltd.
Priority date: 2004-11-03
Filing date: 2005-10-27
Publication date: 2006-05-11
Also published as: IL165011A0

Abstract

A re-configurable OADM (ROADM) is proposed, suitable for being inserted at an optical multichannel transmission line and comprising a multiplexer and a demultiplexer. The demultiplexer is adapted to receive a multiplexed optical signal comprising a plurality of optical channels and, upon demultiplexing thereof, obtain at egress ports of the demultiplexer separate egress optical channels. The multiplexer is adapted to receive at its ingress ports separate ingress optical channels for further multiplexing thereof. At least one egress port of the demultiplexer and at least one corresponding ingress port of the multiplexer relate to one and the same corresponding optical channel. The ROADM is characterized in that the at least one egress port is connectable with its corresponding ingress port by an external removable optical connector, thereby allowing conversion of the corresponding optical channel from a through state to an add/drop state and vice versa.

Description

Reconfigurable Optical Add Drop Multiplexer (ROADM)

Field of the invention

The invention proposes an architecture of ROADM.

Background of the invention

Reconfigurable optical add/drop multiplexers (ROADMs) are the new hot topic in the industry of optical networks equipment. The buzz is reminiscent of the excitement surrounding tunable lasers a few years ago. Once touted for their ability to deliver dynamic provisioning on the fly, tunable lasers are still used mostly for inventory management and sparing. The industry continues to wait for prices to drop and the technology to mature before widespread deployment can occur. It seems that ROADMs are destined for a similar fate. While it is quite clear that every network provider requires ROADM functionality, it is too early to expect that the providers will be ready to put that functionality into e^~very node. Many experts assume that as the price of an ROADM drops, the ROADM functionality will be pushed into the networks.

Fig. 1 (prior art) shows a simple Demux/Switch/Mux (D SM) architecture 10, typical of first generation ROADM nodes. Note that only one direction of a bi-directional path is shown. The full wavelength division multiplexed (WDM) spectrum is demultiplexed by demultiplexer 12, switched by a multi-port optical switch 14 and then remultiplexed by a multiplexer 16 as a monolithic block of wavelengths. The switch 14 enables dropping some of the incoming wavelengths, as well as adding some wavelength instead (the added and dropped channels are schematically shown as 15). Another module of the switch 14 serves the opposite directio of transmission (not shown). The use of electronically Variable Optical Attenuators (eVOAs) 18 on each wavelength through- path allows for power adjustments in specific channels. The multiplicity of optical taps 20 can be used to monitor per- wavelength power levels.

The architecture 10 suffers from several disadvantages. The first of them is high start-up cost, since the entire ROADM fabric, including demultiplexer, multi-port optical switch and remultiplexer must be deployed and paid for before the first service is even lit. Such architecture is cost-effective only in the densest and high growth network and market environments. The second disadvantage is in that the switching unit represents a single-point-of-failure in the node design. Failure in any of the fiber connections inside the switch, and even at one direction only results in having to replace the entire switching element. Thirdly, the switch has quite a high insertion power loss. Fourthly, the switch technology is complex and yet immature, that usually brings reliability problems.

The DSM architecture may be modified to obtain a banded DSM- based ROADM, by dividing the spectrum into "bands", so that the single optical switch is broken down into smaller switching elements, one per band. The modified architecture has almost the same disadvantages as the parent one.

A second widely known and used architecture is a broadcast and select-based ROADM 22 shown in Fig. 2 (prior art) for a unidirectional transmission line. In the broadcast and select architecture, there is just a single through-path 23, and almost the entire spectrum is always available for add/drop at any node. The full wavelength spectrum is split at node ingress by a splitter/coupler 24, and sent to drop filters (or DeMUX) 26 as well as towards the node egress. In this example, a 3dB coupler 24, at the device input, directs the N propagating channels to a DeMUX that further enables manual access to any desired channel/s. A similar 3dB coupler 24 and a MUX 28 at the other end enable the addition of any desired channel/s. A blocker 30 - preferably a combination wavelength blocker/dynamic channel equalizer (DCE) - is necessary to selectively block wavelengths that are used for add/drop, so as to prevent crosstalk and to allow reusing the wavelengths at this or other nodes.

Cost of the second described architecture (22) is much more attractive than of the first one (10). The through-path is deployed at the first install, and requires no upgrading during capacity growth. The channels dropped from DeMUX and the channels added at MUX can be made accessible for manual manipulations. The architecture is therefore flexible. However, the blocker being a programmable high technology element still contributes to a relatively high cost of the architecture and also creates the insertion power loss, together with the two couplers.

Object and summary of the invention

It is therefore the object of the present invention to provide a simple architecture of ROADM capable of satisfying the today's requirements of network providers, namely flexibility, cost-effectiveness and reliability. To achieve the above purpose, the Inventor proposes providing a re- configurable simple OADM (ROADM) suitable for being inserted at an optical multichannel transmission line, the ROADM comprising: a demultiplexer having egress ports and a multiplexer having corresponding ingress ports, wherein at least one egress port of the demultiplexer and at least one corresponding ingress port of the multiplexer relate to one and the same corresponding optical channel handled by the ROADM, the simple ROADM being characterized in that said at least one egress port is connectable with said corresponding ingress port by an external removable optical connector, thereby allowing conversion of the corresponding optical channel from a through state to an add/drop state and vice versa.

The ROADM preferably comprises a set of the external removable optical connectors; the conversion of the optical channels from one of the states to another can be performed as follows: if an external removable optical connector links between an egress port of the demultiplexer and a corresponding ingress port of the multiplexer, a particular optical channel, formed there-between by connecting an egress channel and a corresponding ingress channel, becomes a through channel of the ROADM, and if said external removable optical connector does not link between said egress port of the demultiplexer and said coxresponding ingress port of the multiplexer, a portion of the particular optical channel, constituting said egress channel, is suitable to allow dropping of optical transmissions conveyed along said particular optical channel, while a portion of the particular optical channel, constituting said ingress channel, is suitable to allow adding of optical transmissions to said particular optical channel. In a particular embodiment, the ROADM comprises:

- the demultiplexer adapted to receive a plurality of multiplexed optical channels and, upon demultiplexing thereof, obtain at egress ports of the demultiplexer separate egress optical channels;

- the multiplexer adapted to receive at its ingress ports separate ingress optical channels for further multiplexing thereof; said ingress optical channels having carrier wavelengths respectively equal to the carrier wavelengths of said separate egress channels; - one or more external removable optical connectors; one or more of the egress ports are respectively connectable to the corresponding ingress ports by said one or more external removable optical connectors, so that if an external removable optical connector links between an egress port of the demultiplexer and a corresponding ingress port of the multiplexer, a corresponding optical channel formed there-between becomes a through channel of the ROADM, and if said external removable fiber connector does not linlc between said egress port of the demultiplexer and said corresponding ingress port of the multiplexer, a portion of the corresponding optical channel egressing from said demultiplexer's egress port (and being one of said egress channels) is suitable to become a dropped channel of the ROADM), and a portion of the corresponding optical channel ingressing to the multiplexer's ingress port (and being one of said ingress channels) is suitable to become an added channel of the ROADM. The mentioned one or more external removable connectors can be in the form of optical fiber jumpers each connectable between an egress port of the demultiplexer and a corresponding ingress port of said multiplexer, wherein said egress and said corresponding ingress ports being related to an optical channel of the same carrier wavelength. Access to any desired channel to be dropped and/or added is performed by manually disconnecting the jumper of the desired channel.

Preferably, each of said egress ports of the demultiplexer is bridgeable with the corresponding ingress port of the multiplexer by one of said external removable connectors. In other words, the proposed IROADM can be assembled from the multiplexer and the demultiplexer facing one another by their egress ports and ingress ports, wherein the plurality of separate egress optical channels and the plurality of separate ingress optical channels are respectively interconnected by said removable external optical connectors.

According to one embodiment of the ROADM, at least one of said multiplexer and said demultiplexer is of the type of Variable MUX (VMUX) wherein each of the separate optical channels of the VMUX comprises a variable optical attenuator (VOA). PreferaJbly, the multiplexer is the VMUX, to ensure power equalization of the added channels.

Alternatively, or in addition, the ROADM preferably comprises one or more VOAs switchable into the optical channels to be added to the ROADM.

Advantages of the proposed architecture are:

- simplicity and low cost owing to the absence of any complex internal elements such as a wavelength blocker or an optical switch,

- high flexibility and easy access owing to the simple manual character of selecting and converting the optical channels from one state to another

(from the through state to the add/drop state), and

- almost zero insertion loss at least for the through channels.

Also, no new software is required for supporting the proposed simple ROADM₅ probably except for a patch-program responsible for recognition the ROADM by the system manager. Brief description of the drawings

The invention is described and illustrated using the following non- limiting drawings in which: Fig. 1 (prior art) illustrates one known architecture of ROADM Fig. 2 (prior art) illustrates another known architecture of RO ADM Fig. 3 schematically shows the basic embodiment of the proposed inventive ROADM architecture.

Fig. 4 schematically illustrates another embodiment of the proposed ROADM architecture.

Fig. 5 schematically shows yet a further embodiment of the incentive ROADM architecture.

Figs. 6a and 6b schematically illustrate implementation of the proposed ROADM using VMUX.

Detailed description of the preferred embodiments

Figs. 1 and 2 were referred to in the background portion of the description.

Fig. 3 shows the basic embodiment of the proposed so-called simple ROADM 32.

It should be noted that the ROADM functionality, as implemented by the device 22 of Fig. 2, is achieved by much simpler means in the inventive embodiment 32 of Fig. 3. The device 32 comprises a DeMux 34 and a Mux 36 that are connected "back-to-back". Output ports of the unit 34 are respectively connected to corresponding input ports of the units 36 by so-called jumpers 38 (removable optical fiber connectors). Access to any desired channel for performing drop and/or add fαnction can be obtained just by manually disconnecting the jumper of the necessary channel (for example, position 40 between ports of the channel "k" is the place of one removed jumper).

Fig. 4 illustrates one possible embodiment 42 of the proposed simple ROADM₅ where only some of the optical channels transmitted via an optical line of interest are handled according to the proposed basic scheme of ROADM. The architecture illustrated in Fig. 4 uses a so-called multistage demultiplexing which enables to keep a number of non- demultiplexed through channels in the ROADM. The first-stage demultiplexing is performed in Fig. 4 by using a bandpass filter 41 (called a red/blue filter) separating a band of through channels from a band of channels to be dropped. The second-stage demultiplexing is performed by DeMUX 44. Similar two stages of multiplexing 46, 48 are illustrated at the add portion of the ROADM 42 of Fig. 4. The removable connectors 50 are provided between ports of the DeMUX 44 and MUX 46. However, one or more pairs of the ports of the DeMUX and MUX can be still permanently connected (i.e., without using the removable connectors 50), thereby forming at least one additional through channel such as channel 51. Fig. 5 shows another embodiment 52 of the proposed simple ROADM, using the two stage de-multiplexing performed by demultiplexers 54, 56 at the drop end of the ROADM, and the two-stage multiplexing at the add portion of the ROADM by a pair of multiplexers 58 , 60. Removable connectors 57 serve for configuring add-drop channels at the second stage units 56-58. At least one removable connector 59 can be provided also between the first stage units 54-60, to keep one or more reserved add/drop channels.

Fig. 6a illustrates an embodiment 62 of the simple ROADM, comprising means for power equalization in different optical channels. Usually, the most important task is to ensure that power of the added channels is equalized with power of through channels. To this end, the ROADM 62 comprises spare VOAs (variable optical attenuators) 64 which can be switched in the added channels whenever such are arranged upon removing one or more of the connectors 64. An exemplary add channel "n" is shown, and comprises VOAl. For regulating power of the through and drop channels, the demultiplexer 68 comprises a set of VOAs 67 respectively embedded in all the egress optical channels. The unit 68 therefore forms a so-called variable DeMUX (VDeMUIX). Fig. 6b shows an embodiment 72 alternative to the embodiment 62. The ROADM 72 comprises a DeMUX 74 and a variable MUX (VMUX) 76 with a plurality of VOAs 77. Each of the VOAs 77 is inserted in a specific optical channel and is actually placed at its input port to the multiplexer. The VOAs 77 allow performing power equalization of the added channels with respect to the through channels.

It should be noted that power equalization of optical channels is usually performed together with power monitoring in these channels. In Fig. 6, the power monitoring means can be coupled to the VOAs 77 by taps 78. Alternatively or in addition, a power monitoring unit 79 can be connected to the outgoing line where the optical channels are transmitted in the multiplexed form.

It should be appreciated that other embodiments of the simple ROADM can be proposed, and are to be considered part of the invention, whenever implement the same inventive concept defined by the following claims.

Claims

Claims:

1. A re-configurable OADM (ROADM) suitable for being inserted at an optical multichannel transmission line, the ROADM comprising:

- a demultiplexer adapted to receive a multiplexed optical signal comprising a plurality of optical channels and, upon demultiplexing thereof, obtain at egress ports of the demultiplexer separate egress optical channels;

- a multiplexer adapted to receive at its ingress ports separate ingress optical channels for further multiplexing thereof; wherein at least one egress port of the demultiplexer and at least one corresponding ingress port of the multiplexer relate to one and the same corresponding optical channel, characterized in that said at least one egress port is connectable with said corresponding ingress port by an external removable optical connector, thereby allowing conversion of the corresponding optical channel from a through state to an add/drop state and vice versa.

2. The ROADM according to Claim 1, further comprising said one or more external removable optical connectors, so that one or more of the egress ports are respectively connectable to the corresponding ingress ports by the one or more removable external optical connectors in a manner that: if an external removable optical connector links between an egress port of the demultiplexer and a corresponding ingress port of the multiplexer, a particular optical channel, formed there-between by connecting an egress channel and a corresponding ingress channel, becomes a through channel of the ROADM, and if said external removable optical connector does not link between said egress port of the demultiplexer and said corresponding ingress port of the multiplexer, a portion of the particular optical channel, constituting said egress channel, is suitable to allow dropping of optical transmissions conveyed along said particular optical channel, while a portion of the particular optical channel, constituting said ingress channel, is suitable to allow adding of optical transmissions to said particular optical channel.

3. The ROADM according to Claim 1 or 2, wherein said one or more connectors are in the form of optical fiber jumpers each connectable between an egress port of the demultiplexer and a corresponding ingress port of said multiplexer, said egress and said corresponding ingress ports being related to an optical channel of the same carrier wavelength.

4. The ROADM according to any one of the preceding claims, wherein each of said egress ports of the demultiplexer is bridgeable with the corresponding ingress port of the multiplexer by one of said removable connectors.

5. The ROADM according to any one of the preceding claims, wherein at least one of said multiplexer and said demultiplexer is of the type of Variable MUX (VMUX) wherein each of the separate optical channels of the VMUX comprises a variable optical attenuator (VOA-.).

6. The ROADM according to Claim 5, wherein said multiplexer is the VMUX, thereby ensuring power equalization of the channels to be added.

7. The ROADM according to Claim 4 or 5, additionally comprising one or more VOAs switchable into the optical channels whenever to be added to the ROADM.