Abstract:
Agile OADM structures having a range of tradeoffs between costs and flexibility are disclosed. In certain implementations, cyclic AWGs (arrayed waveguide gratings) are employed. Excellent optical performance is achieved along with relatively low initial and upgrade costs. An economically optimal level of network flexibility may thus be achieved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/630,582 filed Jul. 29, 2003, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to optical networking and more particularly to systems and methods for Wavelength Division Multiplexing (WDM) communications. 
   Impressive strides have been made in the development of WDM communication links. Modern WDM communication links can carry a large number of wavelengths each modulated by a very high data rate signal. Also, the distance over which WDM signals can be transmitted without regeneration by way of optical-electrical-optical conversion has been increased. Furthermore, the distance between purely optical amplification sites along such links has increased. 
   Telecommunication service providers are, however, also very interested in the economic performance of WDM communication links. Revenue garnered by such links may be low initially and then grow over time as traffic increases. To allow profitable operation even before the maturation of traffic growth, it is desirable to install capacity in stages to the extent that technology allows. Rather than initially installing all of the optical components and systems necessary for a full capacity link immediately it is preferable to set up a modularized architecture where lower cost partial initial deployments are possible. 
   To support this type of modular installation and upgrade path, it is important to provide an agile optical add-drop multiplexer (OADM) architecture. An OADM adds and/or drops wavelengths of a WDM signal. In the typical traffic growth scenario, the number of added/dropped wavelengths will grow over time. A conventional WDM that fulfills the maximum expected add/drop capacity requirement will be very costly relative to initial revenues. 
   There are known WDM structures that provide the needed flexibility. Although OADM flexibility postpones certain costs into the future, flexibility itself may also carry a cost due to the types of components that are used. It is thus necessary to find the right trade-off between required flexibility in installation plus upgrade costs. 
   Flexible OADM structures are known. One type of known flexible OADM structure provides automatic reconfigurability using, e.g., optical switches. This may be referred to as Reconfigurable OADM (R-OADMs). Another type of flexible OADM is manually reconfigurable using e.g., fiber patch-cords and wavelength selective devices. These manually reconfigurable OADMs can be referred to as Flexible OADMs (F-OADMs). Technologies are available currently for implementing both R-OADMs and F-OADMs. For example, it is known to implement an F-OADM using a multiplexer arrayed waveguide grating (AWG) having a number of input ports corresponding to the maximum number of wavelengths to be added and a demultiplexer AWG having a number of output ports corresponding to the maximum number of wavelengths to be dropped. 
   The flexibility of the known OADM architectures comes at high initial cost and thus does not support the desired business model. Furthermore, many of the current agile architectures suffer from poor optical performance, e.g., high insertion loss on added/dropped wavelengths and/or injection of additional noise on added wavelengths. Existing fixed OADM structures are cost and performance effective only for low counts of channels to be added or dropped and only where traffic reconfiguration or future growth is not an issue. What is needed are OADM structures that provide reconfigurability to accommodate future growth and changes in traffic, that have good optical performance, and that have relatively low initial and upgrade costs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a top-level representation of OADM functionality. 
       FIG. 2  depicts a split-and-select architecture for an OADM. 
       FIG. 3  depicts an add module according to a first embodiment of the present invention. 
       FIG. 4  depicts an add module according to a second embodiment of the present invention. 
       FIG. 5  depicts an add module according to a third embodiment of the present invention. 
       FIG. 6  depicts an add module according to a fourth embodiment of the present invention. 
       FIG. 7  depicts an add module according to a fifth embodiment of the present invention. 
       FIG. 8  depicts an add module according to a sixth embodiment of the present invention. 
       FIG. 9  depicts a drop module according to a first embodiment of the present invention. 
       FIG. 10  depicts a drop module according to a second embodiment of the present invention. 
       FIG. 11  depicts a drop module according to a third embodiment of the present invention. 
       FIG. 12  depicts a drop module according to a fourth embodiment of the present invention. 
       FIG. 13  depicts how an add module may be integrated to provide a low cost, high performance upgrade path according to one embodiment of the present invention. 
       FIG. 14  depicts how an add module may be integrated to provide a low cost, high performance upgrade path according to an alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts a representative OADM architecture to which embodiments of the present invention may be applied. An OADM  100  adds and drops wavelengths of a bi-directional WDM link. The bidirectional link consists of two unidirectional links flowing in opposite directions. One of the unidirectional links  102  is said to flow from west to east. The other unidirectional link  104  can be said to flow from east to west. A west side add/drop module  106  adds wavelengths to the signal flowing to the west and drops wavelengths from the signal flowing from the west. Similarly, an east side add/drop module  108  adds wavelengths to the signal flowing to the east and drops wavelengths from the signal flowing from the east. 
   Thus it can be seen that an OADM is an optical network element having two bi-directional line interfaces and two bidirectional client (or tributary) interfaces. The OADM is able to extract/insert a subset of wavelengths from/to the incoming/outgoing WDM wavelength set and route these wavelengths to/from the client drop/add interfaces. 
   The number of individual wavelength client interfaces define the add/drop capacity which can be described as a ratio between the number of wavelengths that can be added or dropped and the total number of wavelengths in the WDM grid. For example, an OADM able to drop up to 16 wavelengths out of a total of 32 has an add/drop capacity of 50%. 
   To quantify OADM flexibility, it is useful to introduce a parameter that measures the number of supported possible configurations. Assuming an add/drop capacity of 100%, an ideal agile OADM would be able to add/drop any combination of the N wavelengths of the WDM grid. For this ideal case, any particular wavelength may be added/dropped or simply passed through. Each wavelength thus may be understood to have an associated bit (“ 1 ” or “ 0 ”) to describe the add/drop or pass-through state. The number of possible states for this ideal agile OADM is therefore 2N. However, in order to optimize parameters such as cost and optical performance, it may be desirable to provide an agile OADM architecture with less flexibility where certain combinations of states for all the wavelengths are not achievable. The ratio between the number of supported states in a particular agile OADM architecture and the ideal case of 2N states is a useful measure of flexibility. 
     FIG. 2  depicts further details of add/drop east side module  108  that employs a so-called “split and select” architecture. For the WDM signal flowing from east to west, a splitter  202  taps off a portion of that WDM signal and forwards it to a drop module  204 . Drop module  204  then separates out the various wavelengths to be dropped. Wavelengths to be added to the optical signal flowing from west to east are combined in an add module  206 . The combined add signal is then mixed into the west to east WDM signal by a coupler  208 . Embodiments of the present invention provide improved implementations of drop module  204  and add module  206 . 
   Add Module 
     FIG. 3  shows an add section structure that provides 100% capacity (and 100% flexibility). In the examples described herein, the number of wavelengths in the grid is N=32. In  FIG. 3 , within add module  206  there is a cyclic arrayed waveguide grating (AWG)  302  that combines wavelengths at P input ports into one output port. Each input port of cyclic AWG  302  accepts N/P wavelengths spaced apart at P times the system&#39;s WDM grid spacing. (It will be appreciated that many of the “accepted” wavelengths may not actually be present.) In the example of  FIG. 3 , a 1:8 cyclic AWG is used for a 32 wavelength grid. In this case each of the P input ports carries four wavelengths. If the overall grid is 100 GHz, each input port grid has a spacing of 800 GHz. Cyclic AWG  302  is wavelength-selective in that at any particular input port the wavelengths other than the accepted ones are rejected. This provides a highly beneficial noise filtering effect not found in previous add structure implementations that combine numerous wavelengths without filtering. 
   A description of the details of cyclic AWG technology may be found in Kaneko, et al., “Design and Applications of Silica-based Planar Lightwave Circuits,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 5, September/October 1999, the contents of which are herein incorporated by reference in their entirety for all purposes. Although the present invention is described with reference to the use of cyclic AWGs for multiplexing and demultiplexing, one may also substitute, e.g., optical interleavers and deinterleavers or other suitable devices. 
   Although cyclic AWG  302  accepts four wavelengths at each of 8 inputs, all of the wavelengths may not be present at all of the inputs. In fact, it would be very typical that upon initial installation only a few wavelengths are utilized while others are added later. The design of  FIG. 3  provides 100% capacity and flexibility by providing separate inputs for each of the 32 wavelengths. 
   Thus, four wavelengths are combined to produce one input to cyclic AWG  302 . Each wavelength is passed through an optional variable optical attenuator (VOA)  304  that maintains polarization. As shown by the horizontal and vertical arrows, each pair of wavelengths is preferably input with linear orthogonal polarization states. This allows combination of the wavelength pairs to be performed by polarization beam combiners  306 . The polarization beam combiners may employ, e.g., InP technology, fused fiber technology, etc. The polarization beam combiners  306  introduce relatively low insertion loss, e.g., approximately 0.3 dB, but reduce noise level by causing the noise from each channel to add incoherently. In an alternative embodiment, polarization beam combiner  306  incorporates a quarter waveplate  310  that rotates the linear polarization state of one input by 90 degrees. In this way all of the input wavelengths may have the same linear polarization state, simplifying configuration. 
   A bandpass thin film filter (BP-TFF)  308  combines the outputs of PBCs  306 . BP-TFF  308  is a type of interferential filter that separates or combines two adjacent multi-wavelength subbands. The output of each BP-TFF  308  is fed to one of the P input ports of cyclic AWG  302 . Each of the P input ports to cyclic AWG  302  may have a similar structure for combining four wavelengths. Also, it will be seen that the use of the cyclic AWG  302 , the polarization beam combiners  306 , and the BP-TFF  308  greatly reduce the introduction of optical noise while adding minimal insertion loss. 
     FIG. 4  depicts an alternative add module having 50% add capacity and 5% add flexibility according to one embodiment of the present invention. In  FIG. 4 , there are two available inputs for each of the P ports of cyclic AWG  302 . Thus one may optionally introduce two of the four possible wavelengths for each of the P input ports. A 50/50 optical coupler  402  combines the two input wavelengths. Both wavelengths each may pass through an optional VOA  404 . There is 50% capacity since 16 of 32 channels may be added. The use of a coupler introduces some degree of insertion loss, typically around 3.5 dB. Also since there is no pre-AWG filtering or use of a polarization beam combiner, there is some introduction of out-of-channel noise in the process of combining wavelengths prior to input and a 3 dB optical signal to noise ratio (OSNR) impairment (compared to what would be achieved using a PBC or not input coupling to the cyclic AWG). The cost of this approach is however lower than in the 100% capacity example of  FIG. 3 . 
     FIG. 5  depicts a variation of the add module shown in  FIG. 4  according to one embodiment of the present invention. TFFs  406  are single-channel bandpass filters introduced on each wavelength input. This increases insertion loss but filters the noise introduced on each input so that there is 0 dB OSNR impairment. Each TFF  406  is centered at the particular input wavelength for that input. 
     FIG. 6  depicts another variation on the add module of  FIG. 4  according to one embodiment of the present invention. In  FIG. 6 , a polarization beam combiner  602  combines the two input wavelengths for each input port of cyclic AWG  302 . The arrangement of  FIG. 6  assumes that the two input wavelengths are polarized orthogonally to one another. This can be accomplished by specifying the corresponding transmitters to output orthogonally polarized signals or, alternatively by specifying that they share the same polarization and that a quarter waveplate  604  be introduced as discussed with reference to  FIG. 3 . The arrangement of  FIG. 6  provides very low insertion loss and 0 dB OSNR impairment due to the operation of PBC  602 . For the arrangement of  FIG. 6 , capacity is 50% and flexibility is 5%. 
     FIG. 7  depicts a further alternative add structure that can provide a variable degree of flexibility according to one embodiment of the present invention. Here each input port of cyclic AWG  302  can have two, three, or four associated wavelength inputs. Thin film filters (TFFs)  702 ,  704 , and  706  are cascaded together with TFFs (single-channel bandpass filters)  704  and  706  being optionally installed. Each TFF has two inputs and a single output. If only TFF  702  is present at each input port then two wavelengths may be input to a single AWG port and there is thus a capacity of 50% and a flexibility of 5%. If TFF  704  is added at each input port, then three wavelengths may be introduced and there is a capacity of 75% and a flexibility of 60%. The additional inclusion of TFF  706  then provides 100% capacity and flexibility. It will be appreciated that other levels of flexibility and capacity may be achieved by using disparate numbers of TFFs at each input port. A VOA  708  is optionally installed at the wavelength input of TFF  702 . Insertion loss for the added wavelengths is relatively low and there is no OSNR impairment. 
     FIG. 8  depicts an alternative add structure having 25% capacity and 0.0091% flexibility according to one embodiment of the present invention. One of four wavelengths may be input to each of the input ports of cyclic AWG  302 . An optional VOA  802  is included for each input port. The added wavelengths experience very low insertion loss and there is no impairment of OSNR. Although this implementation provides less flexibility, component cost is relatively low. 
   Drop Module 
     FIG. 9  depicts a drop structure architecture according to one embodiment of the present invention. Here, a cyclic AWG  902  is used as a demultiplexer. The input to cyclic AWG  902  is a tapped-off portion of the west-to-east signal. The input to cyclic AWG  902  may optionally pass through a VOA  904 . Each output port of AWG  902  carries N/P wavelengths spaced at P times the overall grid spacing. Here N=32 and P=8. Cyclic AWG  902  both separates and filters the wavelengths from the single input port to the P output ports. In the depicted example, the wavelength grid has 32 wavelengths. Depending on traffic demands anywhere from 1 to 32 of these wavelengths may actually be operational and present at the input to cyclic AWG  902 . Each output port presents from one to four of wavelengths depending on which wavelengths are actually operational. Each output port may have an associated divider  906  which further separates the wavelengths. As will be shown by way of example, there are many possible implementations of divider  906 . 
   In  FIG. 10 , divider  906  is implemented as a 50/50 optical splitter  1002  followed by two parallel TFFs  1004 . Each TFF  1004  selects a single wavelength. Two of the four potentially available wavelengths may be received for each of the P output ports of cyclic AWG  902 . This represents a drop capacity of 50% and a drop flexibility of 5%. Splitter  1002  introduces a certain amount of insertion loss. 
     FIG. 11  depicts an alternative divider structure for 906 that exploits cascaded TFFs. Depending on how many TFFs are installed at each output port, one may implement 50%, 75%, or 100% capacity, corresponding to 5%, 60%, and 100% flexibility, respectively. Other levels of capacity and flexibility may be arrived at by using varying numbers of TFFs on each of the P output ports of cyclic AWG  902 . The TFFs  1006 ,  1008 , and  1010  are connected in a cascade fashion, each device having a single input and two outputs. The capacity and flexibility figures vary as TFFs are added as in the add module embodiment of  FIG. 7 . This arrangement provides very good insertion loss characteristics. 
     FIG. 12  depicts another alternative structure for divider  906 . At each of the P output ports, there is a connected BP-TFF  1202 . Bandpass thin film filter  1202  has two outputs, each carrying half of the grid spectrum. The outputs are fed to TFFs (single-channel bandpass filters)  1204  and  1206  respectively. TFFs  1204  and  1206  further filter and separate into individual wavelengths. This design thus provides 100% capacity and flexibility. This design also provides very low insertion loss. 
   Modularity 
   The add and drop structure designs as discussed above can provide beneficial modularity on various levels.  FIG. 13  shows how modularity can be achieved using the add structure design shown in  FIG. 6 . The design of  FIG. 6  provides 50% capacity and 5% flexibility. Two of four possible wavelengths may be added via a single input port of cyclic AWG  302 .  FIG. 13  thus depicts two optional transmitters  1302  and  1304  for a single input port of AWG  302 . Transmitters  1302  and  1304  have the same output linear polarization state. The optical transmitters  1302  and  1304  are selectably installed depending on current need. Any particular input port may have 0, 1, or 2 transmitters installed. As demand increases, more transmitters are installed on the various input ports. Up to 16 of the available wavelengths may be populated with transmitters. This approach can reduce cost and insertion losses. 
     FIG. 14  depicts how the modularity concepts of  FIG. 13  can be extended to cover components other than the transmitters. Here, on each input port a single module, if present, holds the transmitters  1302  and  1304 , the optional VOAs  404  and the polarization beam combiner  602 . Each such module provides the capability for two wavelengths. Wavelengths can thus be added two at a time, as needed. In this way, the costs of not only the optical transmitters but also those of other components can be postponed until justified by traffic demand. Furthermore, the optical connection to the input of AWG  302  is not polarization state dependent. It will be appreciated that many comparable modularity schemes can be implemented on both the add structure and drop structure sides. 
   Compared to the prior art, embodiments of the present invention provide lower optical insertion loss (and thus highly beneficial avoidance of the need to use per-channel optical amplifiers), better optical noise performance, and a cost-efficient family of architectures that allow installation and upgrade costs to be distributed over time to match traffic demands. Furthermore, many of the components used such as the cyclic AWG, polarization beam combiners, VOAs, and quarter waveplates may be integrated on the same optical chip. Also, compared to add and drop structures that employ conventional non-cyclic AWGs, embodiments of the present invention provide lower cost and lower add structure insertion loss for capacity levels up to 50%. 
   It is understood that the examples and embodiments that are described herein are for illustrative purposes only and that various modifications and changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims and their full scope of equivalents.