Abstract:
A tunable (reconfigurable) OADM provides multiple drop ports and multiple add ports by which desired channels can be removed from, or added to, a composite optical signal. In one embodiment, a programmable demultiplexer is arranged to receive an input signal containing components at x different wavelengths from an optical input port, and distribute the input signal components among K output ports. K−1 of the output ports are the “drop” ports of the OADM, and cumulatively contain w different wavelengths. The remaining port, which is the “through port” that carries the z wavelengths not dropped from the original input signal, is connected to the first port of an M port programmable multiplexer having M−1 other input ports. The remaining M−1 ports are the “add” ports of the OADM, which cumulatively receive v different wavelengths to be added by the OADM.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority of Provisional Application Ser. No. 60/300,272 which was filed on Jun. 22, 2001. 

   TECHNICAL FIELD 
   The present invention relates to optical communications, and more particularly to an arrangement for a tunable, multi-port optical add-drop multiplexer (OADM) that can add optical channels to, and extract optical channels from, an optical signal in a wavelength division multiplexing (WDM) system. 
   BACKGROUND OF THE INVENTION 
   The transmission capacity of fiber-optic communication systems has increased significantly by use of the wavelength division multiplexing (WDM) technique. In a WDM system, multiple channels, where each channel is differentiated by using a different wavelength of light, each carry modulated optical signals in a single optical fiber between transmitter and receiver nodes. In a typical optical communication system, it is desirable to have a few access nodes along the fiber path between the transmitter and receiver end terminals that have the ability to add and/or drop one or more optical channels. A node having this capability is often referred to as an optical add/drop multiplexer (OADM). 
     FIG. 1  illustrates a conventional OADM  110  arranged to drop and add only a single optical channel. OADM  110  has two input ports  120  and  130 , and two output ports  140  and  150 . Input port  120  carries multiplexed optical channels λ 1  through λ N  from the communication line and input port  130  carries a local optical channel λ i-add  that is to be added to the fiber link. Output port  140  contains all the optical channels λ 1  through λ N  from the input port  120 , except the optical channel λ i-drop  that has been extracted and essentially replaced by λ i-add . The dropped optical channel λ i-drop  emerges from output port  150 . 
   Some simple OADM&#39;s of the type shown in  FIG. 1  are fixed, in that only a preassigned optical channel can be added/dropped; in more sophisticated arrangements, a reconfigurable system architecture may be used to implement a tunable optical channel OADM that is able to change the wavelength that is added and/or dropped. 
   A different architecture is conventionally required when an access node in an optical communication system has to add/drop more than one channel.  FIG. 2  illustrates a solution based on a cascade of single channel OADMs at the access node. The multiplexed optical channels are introduced at input port  220  of a first OADM  210 - 1 . The output port  240  of OADM  210 - 1  is connected to the input port of a second OADM  210 - 2 . The output port  260  of OADM  210 - 2  carries all the multiplexed optical channels to be transmitted on the communication channel. OADMs  210 - 1  and  210 - 2  have channel add ports  230 - 1  and  230 - 2  and channel drop ports  250 - 1  and  250 - 2 , respectively. Each OADM may be of the fixed channel type or tunable channel type. 
   While  FIG. 2  shows, for illustrative purposes, a solution with two OADMs that can add/drop one channel each, for a total of two channels, more than two OADMs can be inserted at the access node using the serial cascade approach. The cascading solution, however, suffers from a high through loss for the channels that have to pass all the OADMs in the cascade from the communication system input  220  to the output  260 . 
     FIG. 3  illustrates another conventional solution based on an OADM  310  that can add and drop multiple channels within a single device. An input port  320  carries the multiplexed optical channels from the communication line while input port  330  carries the multiplexed local optical channels that are to be added to the fiber link. The local channels to be added, which are available from transmitters  380 - 1  through  380 -N, are combined in a multiplexer  360  and applied to input  330 . Output port  340  carries multiplexed optical channels consisting of all the added optical channels from input port  330  and the through channels from the system input port  320 . The dropped optical channels emerge from output port  350  as a group of channels, and must be separated in a demultiplexer  370  before being available to receivers  390 - 1  through  390 -N. 
   The multiple channel OADM of  FIG. 3  eliminates the high through loss associated with the cascading solution of  FIG. 2 ; however, it requires additional hardware for multiplexing (with multiplexer  360 ) and demultiplexing (with demultiplexer  370 ) the added and dropped channels. If the added and dropped channels are a fixed subset, then only the required subset of optical channel transmitters in transmitters  380 - 1  through  380 -N and subset of optical receivers in receivers  390  through  390 -N are populated. This is an efficient solution. However, in a dynamic optical communication system, the added and dropped channels can change over time, according to demand. Complete network flexibility necessitates full population of all the optical channel transmitters  380 - 1  through  380 -N and receivers  390 - 1  through  390 -N. This is a very expensive solution, as only a subset of channels will typically be used at any given time, while the others remain idle. Tunable transmitters and receivers cannot be used with the multiplexers and demultiplexers, due to the fixed channel assignment between the input and output ports of such devices. Passive combining and splitting can be used, but the power budget for that solution is impracticable. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, architectures for implementing an OADM are based upon and use the programmable optical multiplexer/demultiplexer as described in co-pending application Ser. No. 09/944,800 filed concurrently herewith and assigned to the same assignee as the present application. As described in the aforementioned co-pending application, a programmable optical demultiplexer is arranged to receive a multiplexed optical signal containing a plurality of separate channels, each with an associated wavelength, and independently assign each input optical channel to a desired output port. Likewise, a programmable optical multiplexer is arranged to receive a plurality of separate optical channels, each with an associated wavelength, and combine the different wavelengths into a single multiplexed optical signal that is made available at the multiplexer output port. 
   The present invention realizes a tunable (reconfigurable) OADM that provides multiple drop ports and multiple add ports by which desired channels can be removed from, or added to, a composite optical signal. The channels added to and dropped from the optical signal can be individual channels (with a single wavelength per channel) and therefore enabled for direct connection to fixed (or tunable) optical transmitters and optical receivers, respectively. Alternatively, the channels added to and dropped from the optical signal can themselves be multiplexed, enabling more advanced features. The OADM of the present invention provides a low loss architecture for all the optical signals that traverse through the device, as required for transparent optical networks. 
   In one embodiment of the present invention, a programmable demultiplexer is arranged to receive an input signal containing components at x different wavelengths from an optical input port, and distribute the input signal components among K output ports. K−1 of the output ports are the “drop” ports of the OADM, and cumulatively contain w different wavelengths. The remaining port, which is the “through port” that carries the z wavelengths not dropped from the original input signal, is connected to the first port of an M port programmable multiplexer having M−1 other input ports. The remaining M−1 ports are the “add” ports of the OADM, which cumulatively receive v different wavelengths to be added by the OADM. By appropriately controlling the demultiplexer and multiplexer, the OADM can independently both drop and add channels to the optical signal, resulting in an output signal containing y wavelengths. In the foregoing description, v, w, x, y and z are integers, where x+v−w=y and z=x−w=y−v. 
   In another embodiment of the present invention, the OADM includes additional multiplexers and/or demultiplexers, so that (a) the channels to be added are first themselves multiplexed before being added to the optical signal at the OADM, or (b) the channels to be dropped are initially grouped so that multiple channels are dropped at once, and the group of dropped channels is then demultiplexed to recover individual dropped channels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully appreciated by consideration of the following detailed description, which should be read in light of the drawing in which: 
       FIG. 1  is an illustration of a single channel OADM; 
       FIG. 2  is an illustration of a cascade of single channel OADMs for accessing multiple channels; 
       FIG. 3  is an illustration of a multiple channel OADM; 
     FIGS.  4 ( a ) and ( b ) are illustrations of a programmable multiplexer and demultiplexer (respectively) that are the building blocks of the present invention; 
       FIG. 5  is an illustration of an embodiment of an OADM arranged in accordance with the principles of the present invention and that includes a programmable demultiplexer followed by a programmable multiplexer; 
       FIG. 6  is an illustration of an alternative embodiment of the OADM using a cascade of programmable multiplexers and demultiplexers for greater channel count; 
       FIG. 7  is an illustration of another embodiment of the OADM using a single programmable demultiplexer with optical circulators; and 
       FIG. 8  is an illustration of an alternative embodiment of an OADM that uses a wavelength distribution switch with several input ports and several output ports, where optical channels do not occur on more than one input port. 
   

   DETAILED DESCRIPTION 
   The present invention describes new architectures for implementing an OADM that advantageously makes use of the programmable optical multiplexer/demultiplexer described in applicant&#39;s co-pending application identified above. For the purposes of completeness, the functionality of that element is described in connection with FIGS.  4 ( a ) and  4 ( b ) herein. As illustrated in FIG.  4 ( a ), a programmable optical multiplexer  420  has K input ports  410 - 1  through  410 -K and a single output port  430 . Each of the input ports can receive an optical signal containing one or more optical channels from the set of λ- 1  through λ-N, provided the channels of each input port are different. The optical signals are combined in the multiplexer, and emerge as a composite signal at output port  430  containing all the optical channels λ- 1  through λ-N. Operationally, multiplexer  420  establishes a unique pathway for each optical channel between any one of the input ports  410 - 1  through  410 -K and the output port  430 , as prescribed by a control signal  440 , physically preventing the detrimental possibility of combining two optical channels operating on the same wavelength from two different input ports. 
   The programmable multiplexer of FIG.  4 ( a ) can also be operated in the reverse direction and function as a programmable demultiplexer  400 , as shown in FIG.  4 ( b ). A single input port  450  receives a multiplexed optical signal containing a plurality of wavelengths or channels, and separates the signal so that one or more of the channels appears at each of the output ports  460 - 1  through  460 -M. The assignment of specific channels to output ports is independent, and is determined by a control signal on input  470 . In this demultiplexer, note that, if desired, one or more wavelengths applied at input port  450  can be output from that same port, instead of being output from one of the other output ports  460 - 1  through  460 -M. This capability will be useful in connection with the OADM arrangement illustrated in FIG.  7  and described more fully below. 
   From the foregoing description, it is seen that the programmable multiplexer  420  of FIG.  4 ( a ) and the programmable demultiplexer  400  of FIG.  4 ( b ) can each be implemented in the same hardware device (assuming that K=M). It is to be noted that the device can be operated so that it concurrently acts as a multiplexer and as a demultiplexer. Using the demultiplexer of FIG.  4 ( b ) as an example, in addition to the processing of wavelengths as described previously, wavelengths can be introduced into the device through ports  460 - 1  through  460 -M at the same time that wavelengths are being output from those ports. However, each wavelength being processed in the device must have a unique path between an input port and an output port, which path may be traversed bi-directionally. 
     FIG. 5  illustrates an embodiment of an OADM arranged in accordance with the present invention, using a programmable multiplexer and demultiplexer of FIGS.  4 ( a ) and  4 ( b ). An input port  510  carries the multiplexed optical channels λ- 1  through λ-N of the communication system. A programmable demultiplexer  520  assigns the optical channels to the various output ports  530 - 1  through  530 -K. The optical channels that are transmitted through the OADM (i.e., not dropped) are assigned to a first one of the output ports, namely output port  530 - 1 . The dropped channels are assigned to the remaining ports, namely ports  530 - 2  through  530 -K. Typically, the dropped channels are detected at the drop site, and therefore each drop port  530 - 2  through  530 -K is usually terminated by an optical receiver  531 - 2  through  531 -K. In this operation mode, a single dropped channel is assigned to an available drop port, so that up to K−1 channels can be dropped. (Note that multiple channels can be assigned to a drop port, as described more fully below.) Also note that optical detection may, instead of being performed directly at the drop port, be performed at a remote location, such as at a customer&#39;s premises. In that case, several dropped channels can be assigned to the drop port that leads to the customer for demultiplexing and detection of the multiple optical channels. 
   Still referring to  FIG. 5 , output  530 - 1  is called a “through route”, and contains one or more of the optical channels that were present in the input signal on line  510 , but of course does not include the channels that were dropped. The through route on output  530 - 1  is connected to the input port  540 - 1  of a programmable multiplexer  550 , that has an additional M−1 input ports  540 - 2  through  540 -M to which the “add channels” are introduced. Typically, a tunable optical channel transmitter  541 - 2  through  541 -M is connected to each add input port and arranged to provide a signal containing a single optical wavelength. However, it is possible to add several multiplexed optical channels at each port  540 - 2  through  540 -M, which may, for example, originate from a remote site, such as a customer&#39;s premise. Output port  560  carries the multiplexed optical channels, comprised of the through channels and the added channels. A control signal  570  directs the programmable demultiplexer  520  and multiplexer  550  to carry out the wavelength add and drop to and from the proper ports. In this embodiment, the add and drop channels are processed by two different devices, namely programmable demultiplexer  520  and programmable multiplexer  550 , enabling the add channel wavelengths to either be different from the drop channels wavelengths or alternatively, have some drop channel wavelengths in common with the add channel wavelengths. 
   If the number of added and dropped channels exceeds the number of available add ports M−1 and drop ports K−1 of the embodiment of  FIG. 5 , it is possible to cascade the programmable multiplexers and demultiplexers, as shown in FIG.  6 . The input optical channels at the OADM are introduced at port  610 , and enter the first programmable demultiplexer  620 . The through channels exit at port  630 - 1 , which is connected to input port  640 - 1  of programmable multiplexer  670 , and emerge at the output port  695 . The signal path of the through channels is identical to the signal path of the through channels in the embodiment of FIG.  5 . 
   Drop channels exit programmable demultiplexer  620  at one of the other ports  630 - 2  through  630 -K, and one or more of the outputs can contain multiple channels. For example, as shown in  FIG. 6 , outputs  630 - 3  and  630 - 2  each contain multiple channels, and are accordingly each connected to a second programmable demultiplexer  650 - 3  and  650 - 2 , respectively. The second programmable demultiplexer increases the number of available drop ports. Such an arrangement is possible due to the ability of the programmable demultiplexer  620  to direct more than one channel to one or more of the output ports  630 . In this embodiment, it is assumed that the second programmable demultiplexers  650 - 3  and  650 - 2  each direct a single the channel to a distinct output port for detection. However, it is possible to again iterate (i.e., nest) the process, if yet additional ports are needed. 
   Still referring to  FIG. 6 , an example of the path taken by a dropped channel is as follows: first, the channel exits programmable demultiplexer  620  from port  630 - 3 , as part of a group of dropped channels. Port  630 - 3  is connected to second programmable demultiplexer  650 - 3 , where the group of dropped channels is then demultiplexed, so that the dropped channel may illustratively exit from port  660 - 3 - 1 . By connecting each of K−1 ports  630 - 2  through  630 -K to a second programmable demultiplexer ( 650 - 2  through  650 -K) that has K output ports, the total number of available drop ports can therefore increase up to K(K−1). Note however, that the OADM of  FIG. 6  may also be implemented such that programmable demultiplexers  650 - 2  through  650 -K having different characteristics than the first programmable demultiplexer  620 , e.g., a greater or lesser number of ports. 
   In the arrangement of  FIG. 6 , the same cascading solution is implemented for the add channels as for the drop channels, just described. In particular, a series of multiplexers  690 - 2  through  690 -M are each arranged to receive a plurality of add channels. For example, multiplexer  690 - 2  receives add channels  690 - 2 - 1  through  690 - 2 -M, multiplexer  690 - 3  receives add channels  690 - 3 - 1  through  690 - 3 -M, and so on. An example of the path of an added channel is as follows: the added channel is introduced at port  680 - 2 - 1 , which is connected to programmable multiplexer  690 - 2 , which is subsequently connected to input port  640 - 2  of programmable multiplexer  670 , which leads to the output port  695 . By virtue of the arrangement of  FIG. 6 , the number of available add ports can therefore increase up to M(M−1). As with the embodiment of  FIG. 5 , the OADM of  FIG. 6  may drop and add different channels. Note that while programmable multiplexers  690 - 2  through  690 -M can be the same as programmable multiplexer  670 , they do not have to be. For example, if desired, some or all of the multiplexers  690 - 2  through  690 -M can be fixed rather than programmable, in order to reduce cost. Likewise, programmable demultiplexers  650 - 2  through  650 -K are not required to be the same as programmable demultiplexer  620 . In  FIG. 6 , individual control signals to the programmable demultiplexers and programmable multiplexers are not explicitly shown, in order to reduce complexity of the drawing. 
     FIG. 7  illustrates another embodiment of an OADM in accordance with the present invention, this embodiment utilizing a single programmable multiplexer/demultiplexer  730  operating in a bi-directional mode, as described previously, and a plurality of circulators for separating the add and drop channels. An input port  710  carrying the input multiplexed WDM channels is connected to a first optical circulator  715 , which directs the input channels to input port  720  of the programmable demultiplexer  730 . The control signal  760  applied to programmable demultiplexer  730  is arranged so that each channel to be dropped is directed to any available one of the output ports  740 - 1  through  740 -K of demultiplexer  730 . Each output port  740 - 1  through  740 -K is attached to a corresponding optical circulator  741 - 1  through  741 -K that directs the dropped channel to the corresponding drop port  742 - 1  through  742 -K. An example of a drop path is from the input  710 , via circulator  715  to input port  720  of programmable demultiplexer  730 , to a demultiplexed output port  740 - 2  and via optical circulator  741 - 2  to drop port  743 - 2 . 
   In the arrangement of  FIG. 7 , added channels are introduced from add ports  743 - 1  through  743 -K, and are connected to respective ports  740 - 1  through  740 -K of programmable multiplexer  730  via the corresponding optical circulators  741 - 1  through  741 -K. The added channels emerge from port  720  where they are directed to output multiplexed port  750  via optical circulator  715 . 
   Through channels enter programmable demultiplexer  730  via port  720  and are routed in programmable demultiplexer to emerge back on the input port  720 . Optical circulator  715  directs the through channel traffic returning from the programmable demultiplexer  730  to the output multiplexed port  750 . 
   As previously described, programmable demultiplexer  730 , when operating in a bi-directional mode, must be arranged such that each wavelength being processed in the device has a unique path between an input port and an output port. Thus, the embodiment of  FIG. 7  requires that each added wavelength be introduced at the same port at which the same wavelength is dropped. 
     FIG. 8  illustrates another OADM embodiment operated without wavelength contention, utilizing the wavelength switch shown in  FIG. 4  of the above-mentioned co-pending application. The wavelength switch shown in the co-pending application has r input ports and s output ports, and is arranged so that any particular wavelength can enter the switch at one of the input ports and emerge from any one of the output ports. In  FIG. 8 , OADM  820  configures the wavelength switch to have a plurality of input ports  810  and  850 - 1  through  850 -P (so that P+1=r), and a plurality of output ports  830  and  840 - 1  through  840 -M (so that M+1=s). Input port  810  carries the WDM input from a communication system, and input ports  850 - 1  through  850 -P are the add ports. Output port  830  carries the WDM output to the communication system and the remaining output ports  840 - 1  through  840 -M are the drop ports. Control signal  860  determines the pathway taken within OADM  820  for each wavelength, between an input port and an output port. In this embodiment, any single input optical wavelength channel may appear at only one input port, preventing a particular optical channel from being both dropped and added concurrently by the OADM. This is because any wavelength to be dropped must inherently have been introduced into the OADM via input port  810 , and that same wavelength cannot also be concurrently introduced at one of the add ports  850 - 1  through  850 -P. 
   The path of a dropped channel is from the input port  810  through the programmable demultiplexer  820 , to an available drop port of  840 - 1  through  840 -M. The path of the through channels is from input port  810  through programmable demultiplexer  820  to output port  830 . The path of the added channels is from an available input port  850 - 1  through  850 -P through programmable demultiplexer  820  to output port  830 . 
   Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.