Abstract:
A branching unit is provided for directing individual wavelengths of a WDM optical communication signal among a transmitting terminal, a receiving terminal and a branch terminal. The branching unit includes first and second cross bar switches each having at least a first, second and third port such that in a first state the first cross bar directs an optical signal appearing on the first port to the second port and in a second state the first cross bar directs the optical signal from the first port to the third port. A reflective filter couples the third port of the first switch to the third port of the second switch. The reflective filter is configured to reflect a prescribed wavelength and to transmit therethrough all remaining wavelengths forming the WDM signal. An optical fiber couples the second port of the first switch to the second port of the second switch. First and second circulators are also provided, which each have an input, output and an intermediate port. The intermediate port of the first and second circulators are coupled to the first port of the first and second switches, respectively. The input port of the first circulator is adapted to receive the WDM signal from the transmitting terminal and the output port of the first circulator is adapted to receive the prescribed wavelength and transmit it to the branching terminal. The input port of the second circulator is adapted to receive the prescribed wavelength from the branching terminal and the output port of the first circulator is adapted to receive the WDM signal and transmit it to the receiving terminal.

Description:
FIELD OF THE INVENTION 
     The invention relates to optical signal processing in a lightwave communications system. More particularly, the invention relates to a branching unit that can both transmit and drop selected wavelengths of a wavelength division multiplexed signal. 
     BACKGROUND OF THE INVENTION 
     Lightwave communications systems applied in the field of telecommunications can be broadly classified into two categories. These two categories are referred to as long-haul and short-haul systems, depending on whether the optical signal is transmitted over relatively long or short distances compared with typical intercity distances (approximately 50 to 100 kilometers). Long-haul communications systems require high-capacity trunk lines and can transmit information over several thousands of kilometers using optical amplifiers. 
     Long-haul communications systems are used to carry international communications traffic from one continent to another. Since this often requires the laying of fiber trunk lines underwater, these systems are often referred to as submarine systems. 
     In submarine systems, as well as terrestrial systems, it becomes necessary to direct certain wavelengths of wavelength-multiplexed optical signals carried on these high-capacity fiber trunks. This typically occurs to conform to desired traffic routing parameters. 
     The optical component used to redirect these signals is referred to as an optical add-drop multiplexer (ADM). An ADM is known as a key device for use in splitting and inserting wavelength-division multiplexed optical signals. 
     Undersea optical communication systems include transmitter and receiver terminals connected by a fiber transmission medium and repeaters containing optical amplifiers that compensate for attenuation in the fiber. To provide increased flexibility in undersea network architecture beyond simple point to-point interconnects, a branching unit is provided, which allows traffic to be split or switched to multiple landing points, which are referred to as branch terminals. The branching unit contains the ADM that redirects the optical signals from the trunk connecting the transmitting and receiving terminals to the branch terminal. In addition to serving as optical interconnects, branching units also provide and manage electrical power to the repeaters. In wavelength division multiplexed communication systems, the branching unit drops selected wavelengths or channels to the branch terminals while transmitting the remaining wavelengths that compose the WDM signal. 
     U.S. Appl. Ser. No. 08/728,591 discloses a branching unit which drops optical information signals of selected wavelengths received from a transmitting trunk terminal to a branch terminal. This known branching unit transmits all the wavelengths of a WDM signal except for those information signals carried at the selected wavelengths that are to be added or dropped to the branch terminal. The branch terminal replaces the received optical information signal at each selected wavelength with another optical information signal. The new optical information signal, carried at the selected wavelength, is transmitted by the branch terminal to the branching unit, which in turn multiplexes the selected wavelength onto the WDM signal. The end result is that the branching unit drops certain information signals while receiving additional optical information signals to replace the dropped signals. The additional information signal is then carried along with the other WDM signals. 
     However, one limitation of this branching unit is that the selected wavelength to be added/dropped cannot be changed for a given branching unit. Moreover, the same branching unit cannot process different combinations of two of more selected wavelengths in an add/drop mode. 
     It therefore would be desirable to provide a single branching unit in which any combination of selected wavelengths to be added/dropped can be arranged. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a branching unit is provided for directing individual wavelengths of a WDM optical communication signal among a transmitting terminal, a receiving terminal and a branch terminal. The branching unit includes first and second cross bar switches each having at least a first, second and third port such that in a first state the first cross bar directs an optical signal appearing on the first port to the second port and in a second state the first cross bar directs the optical signal from the first port to the third port. A reflective filter couples the third port of the first switch to the third port of the second switch. The reflective filter is configured to reflect a prescribed wavelength and to transmit therethrough all remaining wavelengths forming the WDM signal. An optical fiber couples the second port of the first switch to the second port of the second switch. First and second circulators are also provided, which each have an input, output and an intermediate port. The intermediate port of the first and second circulators are coupled to the first port of the first and second switches, respectively. The input port of the first circulator is adapted to receive the WDM signal from the transmitting terminal and the output port of the first circulator is adapted to receive the prescribed wavelength and transmit it to the branching terminal. The input port of the second circulator is adapted to receive the prescribed wavelength from the branching terminal and the output port of the first circulator is adapted to receive the WDM signal and transmit it to the receiving terminal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a lightwave communications system in which an embodiment of the present invention may be deployed. 
     FIG.  2 ( a ) is a schematic diagram of a known ADM. 
     FIG.  2 ( b ) is a schematic diagram of a second known ADM. 
     FIG.  2 ( c ) is a schematic diagram of a third known ADM. 
     FIG. 3 is a schematic diagram of an ADM in accordance with an embodiment of the present invention. 
     FIG. 4 shows a schematic diagram of an alternative embodiment of the ADM constructed in accordance with the present invention. 
     FIG. 5 is a block diagram in accordance with a BU incorporating the ADMs shown in FIGS. 3 or  4 . 
     FIG. 6 is a block diagram in accordance with a second embodiment of a BU incorporating the ADMs shown in FIGS. 3 or  4 . 
     FIG. 7 is an alternative embodiment of the ADM shown in FIG. 3 in which only a single cross-bar switch is employed. 
    
    
     DETAILED DESCRIPTION 
     This section describes the present invention with reference in detail to the drawings wherein like parts are designated by like reference numerals throughout. 
     FIG. 1 illustrates a block diagram of a trunk and branch lightwave communications system in which an embodiment of the present invention may be deployed. FIG. 1 illustrates a high-capacity wavelength division multiplexing (WDM) lightwave communications system. In its simplest form, WDM is used to transmit two channels in different transmission windows of the optical fiber. For example, an existing lightwave system operating at λN can be upgraded in capacity by adding another channel of wavelength λP. A typical WDM system operates in the 1550 nanometer (nm) window, for example, λ1 to λN in the range from 1530 nm to 1565 nm. 
     As seen in FIG. 1, the network includes transmission trunk terminal  211  and receiver trunk terminal  215  interconnected by optical fiber links  204 ,  236 ,  205  and  235 , which support bi-directional optical communication. The network also includes branching unit  206  and branch terminal  213 . Branch terminal  213  includes transmitters and receivers (not shown) similar to trunk terminals  211  and  215 . Branching unit  206  is disposed in the transmission path between trunk terminals  211  and  215 . Branching unit  206  directs selected wavelengths to branch terminal  213 . 
     As shown, trunk terminal  211  includes optical communication transmitters  200 ,  214  and  216  to transmit optical communications channels at wavelength λ1, λ2 . . . λN, respectively. Multiplexer  210  multiplexes these signals together to form multiplexed signal  202 . Multiplexed signal  202  is launched into optical fiber  204  for transmission to the receiving end. Since optical fiber  204  is a high-capacity trunk, signal  202  is also referred to as “trunk traffic”. During transmission, multiplexed signal  202  passes through branching unit  206 . Branching unit  206  places multiplexed signal  202  back onto optical fiber  236 . At the receiving trunk terminal  215 , demultiplexer  212  demultiplexes and routes λ1, λ2 . . . λN to receivers  208 ,  218  . . .  220 , respectively. 
     Branching unit  206  places wavelength λi on optical fiber  360  and thereby branches λi to branch terminal  213 . The optical information signal of wavelength λi is referred to as “branch traffic,” since branching unit  206  branches it from trunk  204  to optical fiber  360 . Branch terminal  213  in turn transmits a different optical information signal at wavelength λi onto optical fiber  340 . Branching unit  206  replaces λi, which was dropped onto optical fiber  360 , with the λi it receives from branch terminal  213  on optical fiber  340 . The branch unit  206  multiplexes this λi with λ1, λ2, . . . λn, forming multiplexed optical signal  234 , which is launched on optical fiber  236  toward receiving trunk terminal  215 . Optical fibers  362  and  342  are used to add and drop traffic from terminal  215  in a manner similar to that described above for terminal  211 . 
     It is worthy to note that multiplexed signal  234  is different from multiplexed signal  202  since the optical information signal of wavelength λi has been replaced with a different optical information signal of wavelength λi. That is, although multiplexed signal  202  and  234  may include the same signal wavelengths, they do not necessarily carry the same information. 
     FIG.  2 ( a ) is an example of an ADM of the type disclosed in U.S. Appl. Ser. No. 08/728,591. The ADM shown in this and subsequent figures, is typically incorporated in a BU as previously described. ADM  466  passes all wavelengths but the wavelength(s) being added or dropped (e.g., λi). FIG.  2 ( a ) shows trunk in  496 , trunk out  498 , branch in  492 , branch out  494 , and circulators  476  and  474 , all of which are connected through a reflective filter  472 . In this example, reflective filter  472  is a Bragg grating. Other examples of filters  472  include diffraction gratings, interference induced gratings, Fabry-Perot etalon, wavelength router, or any other mechanism for selectively passing wavelengths. 
     As signals of varying wavelength pass from branch in  492 , they are directed by circulator  474  through fiber grating  472 . Fiber grating  472  reflects the bragg wavelength and passes all other wavelengths. In this manner, the desired wavelength can be added to the multiplexed signal placed on trunk out  498 , while those signals with destinations at other ADMs pass onto branch out  494 . 
     FIG.  2 ( b ) illustrates another example of the ADM  206  disclosed in U.S. Appl. Ser. No. 08/728,591. The ADM  468  performs the same function as the ADM shown in FIG.  2 ( a ), except it does so using couplers rather than circulators. An opto-isolator  484  is added to coupler  488  used for branch in  500 , to prevent signals from entering branch in  500 . 
     FIG.  2 ( c ) illustrates yet another example of an ADM disclosed in the previously mentioned patent application. As with ADM  466  and  468 , ADM  470  performs the identical function. ADM  470 , however, uses coupler  488  and circulator  486  to perform this function. Notice that placement of circulator  486  on the branch in side of the ADM removes the need for an additional opto-isolator, thereby reducing the overall number of components. 
     FIG. 3 shows one embodiment of the ADM  406  constructed in accordance with the present invention. ADM  406  is designed to both selectively transmit and drop a predetermined wavelength. FIG. 3 shows trunk in  496 , trunk out  510 , branch in  500 , branch out  520 , cross-bar switches  490  and  495 ,  3 -port circulators  482  and  485 , and fiber grating  478 . Cross bar switches  490  and  495  operate in two states. In a first or normal state, a signal directed to inputs  1  or  3  is transferred to ports  2  and  4 , respectively. In a second or switched state, a signal directed to inputs  1  or  3  is transferred to ports  4  or  2 , respectively. Cross bar switches  490  and  495  function symmetrically. That is, in the normal state, a signal directed to inputs  2  or  4  is transferred to ports  1  or  3 , respectively, and in the switched state a signal directed to inputs  2  or  4  is transferred to ports  3  or  1 , respectively. 
     In FIG. 3, trunk in  496  is connected to input port  10  of circulator  482  and branch out  520  is connected to output port  14  of circulator  482 . Intermediate port  12  of circulator  482  is connected to port  1  of cross bar switch  490 . Port  2  of cross bar switch  490  is connected to fiber grating  478 , which in turn is connected to port  1  of cross bar switch  495 . Port  4  of cross bar switch  490  is connected to port  3  of cross bar switch  495 . Branch in  500  is connected to input port  10  of circulator  485 . Port  2  of cross bar switch  495  is connected to intermediate port  12  of circulator  485 . Trunk out  510  is connected to output port  14  of circulator  485 . The various circulators, switches, and grating shown in FIG. 3 are all interconnected by optical fibers. As detailed below cross bar switches  490  and  495  are each operable in two different states. The particular state in which the switches  490  and  495  are placed is determined by a command signal that is transmitted to the respective switch in a known manner. The command signal may be in optical or electrical form. 
     Since the cross bar switch  490  used in the FIG. 3 embodiment of the invention only employs one input port and two output ports, the switch  490  need not be a 2×2 cross bar switch. Rather, only a 1×2 cross bar switch is required. Similarly, cross bar switch  485  may be a 1×2 or a 2×2 cross bar switch. 
     In operation, ADM  406  drops and adds a predetermined wavelength λ1 when cross bar switches  490  and  495  are in the appropriate states, while transmitting all remaining wavelengths from trunk in  496  to trunk out  510 . Alternatively, ADM  406  may be directed to transmit all wavelengths, including predetermined wavelength λ1, by changing the states of cross bar switches  490  and  495 . More specifically, cross bar switches  490  and  495  always remain in the same state (either normal or switched). If switches  490  and  495  are in their normal state (so that a signal directed to ports  1  and  3  is directed to ports  2  and  4 , respectively) an incoming WDM signal arriving on trunk in  496  will be directed to port  2  of cross bar switch  490  via intermediate port  12  of circulator  482  and port I of cross bar switch  490 . Fiber grating  478  is arranged to reflect wavelength λ1 and transmit all other wavelengths. Accordingly, fiber grating  478  reflects wavelength λ1 back through port  2  of cross bar switch  490 , which in turn directs wavelength λ1 to port  1  of cross bar switch and ultimately, via circulator  482 , to branch out  520 . All remaining wavelengths other than λ1 will be transmitted through fiber grating  478  to port  1  of cross branch switch  495 . Since cross branch switch  495  is in its normal operating state, the remaining wavelengths will be directed to port  2  of cross branch switch  495  and ultimately, via circulator  485 , trunk out  510 . 
     Wavelength λ1 can be added to trunk out  510  as follows. Wavelength λ1 is directed along branch in  500  to port  2  of cross bar switch  495  via circulator  485 . Since cross bar switch  495  is in its normal operating state, wavelength λ1 is transmitted through port  1  of switch  495  and is reflected by fiber grating  478  back through port  1  to port  2  of switch  495 . Finally, wavelength λ1 is directed to intermediate port  12  of circulator  485  so that it appears on trunk out  510  via output port  14  of circulator  485 . 
     If the states of cross bar switches  490  and  495  are changed to their switched states, all wavelengths directed along trunk in  496  will appear on trunk out  510 . No wavelengths will be added or dropped. An incoming WDM signal arriving on trunk in  496  will be directed to port  1  of cross bar switch  490  via intermediate port  12  of circulator  482 . Since cross bar switch  490  is in its switched state, the WDM signal appears on port  4  of cross bar switch  490  so that it is directed to port  3  of cross bar switch  495 . Accordingly, since the WDM signal avoids fiber grating  478  all the individual wavelengths, including λ1, arrive at port  3  of cross bar switch. Since cross bar switch  495  is also in its switched state, the WDM signal is directed from port  3  to  2  and to trunk out  510  via circulator  485 . 
     In summary by sending the appropriate commands to cross bar switches  490  and  495 , the ADM  406  is configured so that predetermined wavelength λ1 reaches its desired destination. Specifically, when the switches  490  and  495  are in their switched state, all the wavelengths are transmitted to the trunk out and none are dropped or added. When the switches  490  and  495  are in their normal state, predetermined wavelength λ1 is dropped and added while the remaining wavelengths are transmitted to trunk out  510 . 
     The ADM shown in FIG. 3 is reconfigurable to extent that the destination of a single wavelength, e.g., λ1, can be changed. In other embodiments of the invention the destination of two or more wavelengths may be directed independently of one another. For example, in the embodiment of the invention shown in FIG. 4 wavelengths λ1 and λ2 can be added/dropped or transmitted. Depending on the state of the cross bar switches, none, one or both wavelengths λ1 and λ2 may be added and dropped. As seen in FIG. 4, this result is achieved by cascading together multiple ones of the ADMs shown in FIG.  3 . 
     The ADM shown in FIG. 4 includes cross bar switches  490 ,  495  and  493 . Circulator  482  switches  490  and  495  and fiber grating  478  are arranged as previously described with respect to FIG.  3 . An additional cross bar switch  493  and an additional fiber grating  477 , however, are inserted between switch  495  and circulator  485 . More specifically, ports  2  and  4  of switch  495  are respectively connected to ports I and  3  of switch  493 . Fiber grating  477  is inserted in the path between port  2  of switch  495  and port  1  of switch  493 . Fiber grating  477  is arranged to reflect wavelength λ2 and transmit all other wavelengths. In operation, wavelength λ1 can be add/dropped or transmitted in the same manner discussed above in connection with FIG.  3 . When λ1 is to be transmitted, for example, switches  490  and  495  are placed in their switched states and switch  493  is placed in its normal state. Wavelength λ1 will be transmitted through fiber grating  477  since grating  477  transmits all wavelengths but λ2. Alternatively, if it is desired to drop both λ1 and λ2, for example, switches  490 ,  495 , and  493  are all placed in their normal states. If only λ2 is to dropped, switches  490  and  495  placed in their switched states, and switch  493  is placed in its normal state. 
     One of ordinary skill in the art will recognize that the present invention as shown in FIG. 4 may be readily extended to selectively add/drop or transmit more than two predefined wavelengths. This is accomplished by adding an additional cross bar switch and fiber grating for each additional wavelength. The fiber grating is selected to transmit all wavelengths but the additional wavelength. 
     Similar to the known arrangements shown in FIGS.  2 ( a ) and  2 ( c ), other embodiments of the invention may incorporate circulators rather than couplers. 
     FIG. 5 is a block diagram of a BU that includes a plurality of the ADMs shown in FIGS. 3 or  4 . FIG. 5 shows system  41  having input trunk  1 , trunk  2  . . . trunk N, referred to as  42 ,  44  and  46 , respectively. System  41  also has output trunk  1 , trunk  2  . . . trunk N, referred to as  48 ,  50  and  52 , respectively. In addition, system  41  uses fiber pair referred to as branch add input  54  and branch drop output  56 . Finally, ADMs  58 ,  60  and  62  are all attached to branch add input  54  and branch drop output  56 , as well as to trunk pairs  42  and  48 ,  44  and  50 , and  46  and  52 , respectively. 
     More particularly, the ADMs are configured such that the branch out line of one ADM becomes the branch in line of an adjacent ADM. Thus, the topology of system  41  is such that optic fiber  47  serves as both the branch out of ADM  62  and the branch in of ADM  60 . Similarly, optic fiber  45  serves as both the branch out of ADM  60  and branch in of ADM  58 . Optic fiber  43  serves as the branch out of ADM  58 . In this embodiment, optic fiber  43  directs the dropped signal to any desired location. It is, however, possible for optic fiber  43  to serve as the branch in for ADM  62 . 
     Thus configured, system  41  has a single fiber pair to add and drop signals from multiple trunk lines using multiple ADMs. Since ADM  406  only permits those signals of wavelengths different from the added signal and dropped signal, there exists only four possibilities for processing signals through ADM  406 , summarized in the following table: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Trunk Out 
                 Branch Out 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Trunk In 
                 All but λi 
                 λI 
               
               
                   
                 Branch In 
                 λi 
                 All but λi 
               
               
                   
                   
               
             
          
         
       
     
     Therefore, since ADM  406  passes all wavelengths except the Bragg wavelength (or branching wavelength), ADM  58 ,  60  and  62  is transparent with respect to these wavelengths. 
     The present embodiment of the invention can be illustrated through the following example. Let an incoming multiplexed signal be defined as containing signals of wavelength λ1 to λ5 carried on input trunk lines  42 ,  44  and  46 . Further, assume that ADM  62  branches out wavelengths λ2 and λ3, ADM  60  branches out  5 , and ADM  58  branches out λ1 and λ4. 
     As described below, λ1 to λ5 are dropped from trunk in  42 ,  44  and  46  and branched to a desired destination using only a single fiber pair. As λ1 to λ5 pass into ADM  62  from trunk in  42 , ADM  62  branches out λ2 and λ3 onto optic fiber  47 , which carries these signals into ADM  60 . Since the passing device (not shown) of ADM  60  reflects only wavelength λ5, wavelengths λ2 and λ3 pass through ADM  60  onto fiber optic  45  to ADM  58 . ADM  60  also branches out λ5 from trunk in  44  onto fiber optic  45  as well. Thus, λ2, λ3 and λ5 are transmitted to ADM  58 . Since the passing device (not shown) of ADM  58  only reflects wavelengths λ1 and λ4, wavelengths λ2, λ3 and λ5 pass through ADM  58  onto fiber optic  43 . At the same time, λ1 and λ4 from trunk in  42  are placed onto fiber optic  43  by ADM  58 . 
     Similarly, λ1 to λ5 can be added to trunk out  48 ,  50  and  52 . If we assume λ1 to λ5 are transmitted into ADM  62  from fiber optic  54 , the passing device of ADM  62  reflects λ2 and λ3 which are multiplexed together with wavelengths λ1, λ4 and λ5 from trunk in  46 , and sent over trunk out  52 . As λ1, λ4 and λ5 pass into ADM  60 , the passing device of ADM  60  reflects λ5 which is multiplexed together with λ1 to λ4 from trunk in  44 , and sent over trunk out  50 . Finally, as λ1 and λ4 pass into ADM  58 , the passing device of ADM  58  reflects λ1 and λ4 which are multiplexed together with λ2, λ3 and λ5 from trunk in  42 , and sent over trunk out  48 . 
     FIG. 6 is a block diagram of another multi-trunk, multi-ADM BU that incorporates a plurality of ADMs of the type shown in FIGS. 3 and 4. In this arrangement an additional switch  491  is employed between the branch out  86  of ADM  104  and the branch in of ADM  88 . The addition of switch  491  provides the BU with additional flexibility. When switch  491  is in its normal state, the BU operates as explained above in connection with FIG.  5 . When switch  491  is in its switched state, the BU operates in a so-called “all-but” state in which all wavelengths are dropped except for a selected wavelength or wavelengths. For example, if wavelength λk is reflected by either grating  478  or grating  477  to branch out  86 , λk will be dropped if switch  491  is in its normal state but will be transmitted to branch out  78  if switch  491  is in its switched state. 
     Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, the functionality of the inventive ADM shown in the FIG. 3 embodiment may be accomplished in other embodiments with only a single cross-bar switch. FIG. 7 shows one such embodiment.