Abstract:
Devices and methods are provided for a reconfigurable optical add/drop multiplexer (ROADM) having a static circulator, a selectable grating and a reversible circulator. The use of a reversible circulator in a known optical drop multiplexer configuration allows the selectable grating to be used for selecting both the add-wavelength and the drop-wavelength while maintaining an East/West architectural split to allow for SONET compliant maintenance. This invention provides a cost-effective enhancement to a duplex reflective wavelength selective ROADM.

Description:
RELATED INVENTION  
       [0001]    This application claims priority from U.S. Provisional Patent Application Serial No. 60/394,926 filed on Jul. 11, 2002 (Kelly). 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to optical network communications. More particularly, the invention relates to re-configurable optical add/drop multiplexers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Recent advances in optical communications technology have provided an optical building block incorporating selectable optical gratings and a circulator. This building block is well suited to building efficient reconfigurable optical add/drop multiplexers. FIG. 1 shows such a re-configurable optical add/drop multiplexer (ROADM) building block, generally indicated by  20 . The building block provides optical drop capability and comprises an optical circulator  22  connected to a selectable fiber Bragg grating  24 . In operation, a wavelength division multiplexed (WDM) optical signal is introduced at input port  26  of the optical circulator  22 , which directs the optical signal to a first port  28  of the selectable fiber Bragg grating  24 . The selectable fiber Bragg grating  24  is controlled at  30  to reflect a selected wavelength of the optical signal back through the first port  28  to the circulator  22 , which directs this reflected wavelength to a “drop” port  32  of the circulator  22 . The portion of the WDM optical signal, which is not reflected at  30 , is passed through the selectable fiber Bragg grating  24  to a second port  34 .  
           [0004]    [0004]FIG. 2 shows a ROADM module, generally indicated by  40 . A second circulator  36  is added to the building block of FIG. 1 to provide optical add capability. This configuration allows the selectable fiber Bragg grating  24  to be reused to add an optical signal having the selected wavelength. In operation, the configuration of FIG. 2 behaves similarly to that of FIG. 1. The WDM optical signal minus the dropped portion continues from the second port  34  to the second circulator  36  to output port  38 . An optical “add” signal having the same wavelength as the selected wavelength is presented at “add” port  42  of the second circulator  36  which directs it to the second port  34  of the selectable fiber Bragg grating  24 . The “add” signal is reflected by the selectable fiber Bragg grating  24  at  30 . The “add” signal passes back through the second port  34  and through the second circulator  36  to the output port  38 . The configuration of FIG. 2 thus provides increased functionality to that of FIG. 1 with only a small incremental increase in cost. It also has the advantage of little additional increase in insertion loss on the through path from input port  26  to output port  38 .  
           [0005]    Two-fiber optical ring networks typically use fiber pairs to communicate between nodes, one fiber for transmitting and one for receiving. FIG. 3 illustrates two of the ROADM blocks of FIG. 2, shown as  40 A and  40 B, used to form a bi-directional ROADM node, generally indicated by  43 . ROADM block  40 A receives WDM signals at  26 A from a “West” facing node, drops and adds signals of a desired wavelength at  32 A and  42 A respectively and sends the WDM signal at  38 A to an “East” facing node. Likewise, ROADM block  40 B receives WDM signals at  26 B from the “East” facing node, drops and adds signals of a desired wavelength at  32 B and  42 B respectively and sends the WDM signal at  38 B to the “West” facing node.  
           [0006]    Standard two-fiber ( 2 F) SONET bi-directional line switched rings (BLSRs) require that a failure of node equipment can be handled by normal protection switching. A disadvantage of the configuration of FIG. 3 is that a failure of any of the components of a ROADM block takes the associated fiber path out of service and thus results in a traffic outage. For example, if there is a failure of the fiber Bragg grating  24 A or  24 B, the optical drop capability ceases to function because the selected wavelength will not be reflected correctly to circulator  22 A or  22 B. Similarly, the optical add capability stops as well because the optical signal to be added will not be reflected by the fiber Bragg grating  24 A or  24 B, back to the circulator  36 A or  36 B and out to the output port  38 A or  38 B, instead it will continue through the fiber Bragg grating  24 A or  24 B to the first circulator and will be directed to the “drop” port  32 A or  32 B. Worse still, performing maintenance on the node  43  by replacing components will result in a traffic outage.  
           [0007]    [0007]FIG. 4 illustrates a bi-directional ROADM configuration having an “East/West” architectural split. The bi-directional ROADM node is split into a “West” module  44 A and an “East” module  44 B. Instead of the selectable fiber Bragg grating  24 A of module  44 A handling wavelength selection for both the “drop” port  32 A and the “add” port  42 A, the selectable fiber Bragg grating  24 A in “West” module  44 A handles only the wavelength selection of the “drop” port  32 A. A second selectable fiber Bragg grating  24 A′ is added in “East” module  44 B to select the wavelength to be added at the “add” port  42 A. A failure in “West” module  44 A would appear as a fiber failure, which can easily be handled by the SONET layer through normal protection switching and not affect the entire node. The failure would not affect the function of “add” port  42 A in “East” module  44 B. Similarly, selectable fiber Bragg grating  24 B handles wavelength selection for only the “drop” port  32 B and another selectable fiber Bragg grating  24 B′ handles the wavelength selection for the “add” port  42 B.  
           [0008]    A disadvantage of the configuration of FIG. 4 is that the cost savings advantage of reusing a selectable fiber Bragg grating to provide the wavelength selection for both drop and add functions of a ROADM, as described in relation to FIG. 2 and FIG. 3, is lost.  
         SUMMARY OF THE INVENTION  
         [0009]    It is therefore an object of the present invention to overcome the aforementioned disadvantages in the prior art. Accordingly, devices and methods are provided for improved optical add/drop multiplexing.  
           [0010]    One broad aspect of the invention provides an optical add/drop multiplexer having an optical wavelength selective device, a first optical circulator and a second optical circulator having a first operating mode and a second operating mode. The optical wavelength selective device has a first port and a second port and is adapted to reflect optical signals having a selected wavelength and to pass optical signals having wavelengths other than said selected wavelength. The first optical circulator has an input port and a drop port and is adapted to direct optical signals from the input port to the first port of the wavelength selective device and to direct optical signals from the first port of the wavelength selective device to the drop port. The second optical circulator has an output port and an add port. The second optical circulator, in the first operating mode, is adapted to direct optical signals from the second port of the wavelength selective device to the output port, and for directing optical signals from the add port to the second port of the wavelength selective device. In the second operating mode, the second optical circulator is adapted to direct optical signals from the add port to the output port.  
           [0011]    In some embodiments, the optical wavelength selective device is adapted to select the selected wavelength from a plurality of wavelengths.  
           [0012]    In some embodiments, the optical wavelength selective device is a selectable optical grating.  
           [0013]    In some embodiments, the optical wavelength selective device is a selectable Bragg grating.  
           [0014]    In some embodiments, the second optical circulator is a reversible optical circulator.  
           [0015]    Another broad aspect of the invention provides a building block for a bi-directional optical add/drop multiplexer. The building block has an optical wavelength selective device, a first optical circulator and a second optical circulator having two operating modes. The optical wavelength selective device has a first port and a second port and is adapted to reflect optical signals having a selected wavelength and to pass optical signals having wavelengths other than the selected wavelength. The first optical circulator has an input port and a drop port and is adapted to direct optical signals from the input port to the first port of the wavelength selective device and to direct optical signals from the first port of the wavelength selective device to the drop port. The second optical circulator has an output port and an add port. The second optical circulator, in the first operating mode, is adapted to direct optical signals from an external wavelength selective device to the output port, and for directing optical signals from the add port to the external wavelength selective device. In the second operating mode, the second optical circulator is adapted to direct optical signals from the add port to the output port.  
           [0016]    In preferred embodiments the building block is integrated on a single substrate.  
           [0017]    In some embodiments the external wavelength selective device is a wavelength selective device of a corresponding building block.  
           [0018]    Some embodiments of the invention provide an optical network node having at least two of the building blocks.  
           [0019]    Some embodiments of the invention provide an optical network having an interconnected plurality of the optical network nodes.  
           [0020]    Another broad aspect of the invention provides a method of wavelength management in an optical network. The method involves providing at least one network node with at least a first and a second building block for a bi-directional optical add/drop multiplexer. Each building block has an optical wavelength selective device, a first optical circulator and a second optical circulator having at least two operating modes. The optical wavelength selective device has a first port and a second port and is adapted to reflect optical signals having a selected wavelength and to pass optical signals having wavelengths other than the selected wavelength. The first optical circulator has an input port and a drop port and is adapted to direct optical signals from the input port to the first port of the wavelength selective device and to direct optical signals from the first port of the wavelength selective device to the drop port. The second optical circulator has an output port and an add port. The second optical circulator, in the first operating mode, is adapted to direct optical signals from the second port of the wavelength selective device of the other building block, to the output port, and for directing optical signals from the add port to the second port of the wavelength selective device of the other building block. In the second operating mode, the second optical circulator is adapted to direct optical signals from the add port to the output port.  
           [0021]    The method further involves operating each second optical circulator in the first operating mode and when a failure is detected in one of the building blocks, operating the second circulator of the other building block in the second operating mode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings in which:  
         [0023]    [0023]FIG. 1 is a block diagram of a conventional re-configurable optical add/drop multiplexer (ROADM) building block;  
         [0024]    [0024]FIG. 2 is a block diagram of a conventional re-configurable optical add/drop multiplexer (ROADM) module;  
         [0025]    [0025]FIG. 3 is a block diagram of a conventional bi-directional re-configurable optical add/drop multiplexer (ROADM) node;  
         [0026]    [0026]FIG. 4 is a block diagram of a conventional bi-directional re-configurable optical add/drop multiplexer (ROADM) node having an “East/West” architectural split;  
         [0027]    [0027]FIG. 5 is a block diagram of a bi-directional re-configurable optical add/drop multiplexer (ROADM) node using a reversible circulator in accordance with an embodiment of the invention;  
         [0028]    [0028]FIG. 6 is a block diagram of a bi-directional re-configurable optical add/drop multiplexer (ROADM) node using a reversible circulator and having an “East/West” architectural split in accordance with an embodiment of the invention; and  
         [0029]    [0029]FIG. 7 is a flowchart of a method of wavelength management in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Referring now to FIG. 5, shown is a block diagram of a bi-directional ROADM node  45  according to an embodiment of the present invention. The ROADM node  45 , comprises two ROADM blocks  46 A and  46 B. The ROADM node  45  has a configuration similar to that of the bi-directional ROADM node  43  of FIG. 3, with the important difference that the second circulator  47 A and  47 B, of each ROADM block  46 A and  46 B is a reversible circulator.  
         [0031]    Note that for ease of description, the following discussion will be restricted to only one half of FIG. 5, namely ROADM block  46 A. Because the ROADM node  45  is symmetrical, the description applies equally to the other half of FIG. 5 and thus the reference numerals of corresponding elements of ROADM block  46 B will be shown in parentheses. The reversible circulators  47 A ( 47 B) can operate in one of two modes, a forward mode or a reverse mode (a first mode or a second mode respectively). Typically, when there is no failure in the ROADM block  46 A ( 46 B), the reversible circulator  47 A ( 47 B) is operated in the forward mode (first mode), in which case the ROADM block  44 A ( 44 B) behaves identically to the ROADM block  40 A ( 40 B) of the ROADM node  43  of FIG. 3 as described above. The ROADM block  46 A ( 46 B) benefits from the costs savings of reusing the selectable fiber Bragg grating  24 A ( 24 B) to select both the wavelength to drop at  32 A ( 32 B) and to select the wavelength to add at  42 A ( 42 B). Another important benefit is reduced optical path losses through the ROADM block  46 A ( 46 B), that is, between input port  26 A ( 26 B) and output port  38 A ( 38 B).  
         [0032]    The important difference between the embodiment of the present invention and the prior art as shown in FIG. 3, becomes apparent if there is a failure in the ROADM block  46 A ( 46 B), in which case the reversible circulator  47 A ( 47 B) is operated in the reverse mode (second mode) so that all optical signals appearing at the “add” port  42 A ( 42 B) are directed to the output port  38 A ( 38 B). ROADM block  46 A ( 46 B) thus becomes a broadband add device.  
         [0033]    In one embodiment of the present invention the elements of the ROADM node  46 A ( 46 B) of FIG. 5 such as circulator  22 A ( 22 B), selectable fiber Bragg grating  24 A ( 24 B), and reversible circulator  47 A ( 47 B), are discrete devices.  
         [0034]    Any suitable optical wavelength selective device can be substituted in place of the selectable fiber Bragg grating  24 A ( 24 B) described above.  
         [0035]    [0035]FIG. 6 illustrates a preferred embodiment of the present invention. Shown is ROADM node  49  whose elements are identical to those of ROADM node  45  of FIG. 5 and they operate in the same manner but here they are grouped into a “West” module  48 A and an “East” module  48 B. Thus, “West” module  48 A comprises circulator  22 A, selectable fiber Bragg grating  24 A and reversible circulator  47 B. Circulator  22 A has input port  26 A, a “drop” port  32 A and port  28  which is connected to selectable fiber Bragg grating  24 A. Selectable fiber Bragg grating  24 A has output port  34 A which is intended to connect to reversible circulator  47 A of “East” module  48 B. Reversible circulator  47 B is physically part of “West” module  48 A but has no optical connection to other elements of “West” module  48 A. Reversible circulator  47 B is intended to connect to selectable fiber Bragg grating  24 B of “East” module  48 B. Similarly, “East” module  48 B comprises circulator  22 B, selectable fiber Bragg grating  24 B and reversible circulator  47 A. The identical modules “West”  48 A and “East”  48 B are intended to interconnect to create a ROADM node  49 . ROADM node  49  thus has an “East”/“West” architectural split. In the case of a failure in one of the modules “West”  48 A (“East”  48 B), the other module “East”  48 B (“West”  48 A) can operate its reversible circulator  47 A ( 47 B) in reverse mode thus becoming a broadband add device. Removal of “West” Module  48 A (“East” module  48 B) can then appear as a simple fiber failure to underlying equipment such as a SONET add/drop multiplexer (ADM). Such a fiber failure can be handled through normal protection switching and thus, “West” Module  48 A (“East” module  48 B) can be removed and replaced without causing a traffic outage except for a brief forced protection switch.  
         [0036]    Another embodiment of the present invention provides a method of wavelength management in an optical network using the ROADM block  46 A ( 46 B) of the present invention as described with reference to FIG. 6. However, the method is not limited to using ROADM block  46 A ( 46 B).  
         [0037]    Referring now to FIG. 7, a flowchart of the method is illustrated. The method involves providing at least one network node with at least a first and a second building block for a bi-directional optical add/drop multiplexer (step  52 ). Each building block has an optical wavelength selective device, a first optical circulator and a second optical circulator having at least two operating modes. The optical wavelength selective device is adapted to reflect optical signals having a selected wavelength and to pass optical signals having wavelengths other than the selected wavelength. In one embodiment, the wavelength selective device is a selective fiber Bragg grating. The first optical circulator is adapted to direct optical signals from an input port to a first port of the wavelength selective device and to direct optical signals from the first port of the wavelength selective device to a “drop” port. The second optical circulator can be a reversible circulator, and in a first operating mode (forward mode), is adapted to direct optical signals from a second port of the wavelength selective device of the other building block, to an output port, and for directing optical signals from an “add” port to the second port of the wavelength selective device of the other building block. In a second operating mode (reverse mode), the second optical circulator is adapted to direct optical signals from the “add” port to the output port.  
         [0038]    Each second optical (reversible) circulator is operated in a first (forward) mode such that OADM service is provided (step  54 ). If a failure is detected in one building block (step  56 ), the other building block operates its second optical (reversible) circulator in a second (reverse) mode (step  58 ) so as to direct optical signals from the add port to the output port. Optionally but preferably, maintenance is then performed on the building block having the failure (step  60 ). Maintenance can involve removal and replacement of the defective building block. The method then returns to step  56  where failures are detected. If no failure is detected, the reversible circulators are operated in the first (forward) mode.  
         [0039]    Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein.