Abstract:
An optical carrier drop/add transmission system and method for adding a signal to multiplexed input optical signals conveyed by an optical multiplex input line. The multiplexed input optical signals are demultiplexed to provide isolated input optical signals to an optical switch matrix comprising switches in an array of lines and column, the isolated input optical signals being inputted in a direction parallel to a line of switches in the optical switch matrix. The added optical signal is input in a direction parallel to a column in the optical switch matrix. An output line is selected and the switch that is on the column on which the added optical signal is inputted and on the selected output line is switched.

Description:
This is a continuation of U.S. patent application Ser. No. 09/722,955 filed Nov. 27, 2000 now U.S. Pat. No. 6,928,244 which claimed the benefits of U.S. Provisional Application No. 60/172,732 filed on Dec. 20, 1999 and which also claimed the benefits of U.S. Provisional Application No. 60/204,452 filed on May 16, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention relates to optical communication. More particularly, this invention relates to systems and methods using optical switches for adding and dropping channels from an optical transmission medium. 
   2. Description of Related Art 
   In current optical communication systems, multiple channels are multiplexed onto a single optical transmission medium using multiplexing techniques, such as wavelength-division-multiplexing (WDM). WDM can combine a plurality of communication channels, in the form of discrete wavelengths, onto a single optical fiber. As multiplexing techniques improve, an increasing number of channels are being transmitted on a single optical fiber or group of optical fibers. As the number of channels increase, so too does the need for an ability to add and/or drop a portion of the channels to and/or from the transmission medium. 
   Current communication systems can use an opto-electronic regeneration technique to add and drop channels from a transmission system. With such a technique, in order to receive or transmit data on the optical network using WDM, a node of the network can include at least one optical sensor that receives the optical signal at one or more wavelengths. The optical sensor can include an optical-electrical converter that can convert the optical signal to electrical signals corresponding to the received optical signals. Adding and/or dropping of the signals can then be performed electronically by processing the electrical signals in the electrical domain. The resulting electrical signal can then be modulated onto the network using an electro-optical converter. Such Optical-Electrical-Optical (OEO) conversion can be very complex, costly and time consuming. 
   Additionally, optical wavelength add/drop multiplexers (OADM) can be used in WDM transmission systems. Currently, it has been well recognized that OADMs are needed to avoid the complex and costly OEO conversions. However, currently available OADMs are generally fixed. In other words, a given incoming channel (wavelength) is only associated with a fixed add/drop port. Such a device lacks “client-configurability” and therefore severely limits the selection of which channels to add/drop for a client. 
   Therefore, there exists a need for a device to add and drop channels from a transmission medium that can be readily configured according to the needs of a client. 
   SUMMARY OF THE INVENTION 
   The invention provides an optical switch matrix device and methods that selectively add and drop channels from an optical communication medium. The optical switch matrix can receive an input signal from an optical medium, such as an optical fiber cable. The input signal can include numerous input channels, for example a plurality of channels each having a different wavelength. The optical switch matrix can also receive an add signal which can include numerous add channels for different clients; each add channel can replace an input channel of the input signal that is dropped. 
   Depending on the configuration of the optical switch matrix, any channels of the input optical signal can be dropped from the communication medium to any of the clients. The dropped channels can be received and processed by a receiver. Further, any channels from the add signal can be added to the communication medium. The added channels along with the remaining channels of the input signal can then be outputted and transmitted on the communication medium. Different from fixed optical wavelength add/drop multiplexers (OADMs) described in the related art, the invented optical switch matrix can be configured to allow each client to access any of the input channels, therefore offering client-configurability to the network. 
   The optical switch matrix can be a device that operates on the optical channels in the optical domain. For example, the optical switch matrix can be a device, such as a micro electrical mechanical system (MEMs), having an array of micromirrors that are rotatably mounted on a substrate. The micromirrors may be selectively positioned to interact with passing light, so as to redirect light beams between ports of the optical switch matrix. Accordingly, the optical switch matrix can add/drop channels to/from an optical communication medium. 
   Alternatively, or in conjunction with the MEMs, the optical switch matrix can be a device such as a matrix of switches utilizing bubble technology. As an optical channel passes through the optical switch matrix, bubble switches can be selectively activated causing the channel to be redirected between ports of the optical switch matrix. Accordingly, the optical switch matrix can add/drop channels to/from an optical communication medium. 
   These and other features and advantages of this invention are described in or are apparent from the following detailed description of the system and method according to exemplary embodiments of this invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The benefits of the present invention will be readily appreciated and understood from consideration of the following detailed description of exemplary embodiments of this invention, when taken together with the accompanying drawings, in which: 
       FIG. 1  is an exemplary block diagram of a wavelength add-drop device in accordance with the present invention; 
       FIG. 2  is an exemplary block diagram of a wavelength add-drop device using MEMS technology in a unidirectional network in accordance with the present invention; 
       FIGS. 3 and 4  are exemplary block diagrams of the wavelength add-drop device of  FIG. 2  in two different functioning configurations; 
       FIG. 5  is an exemplary functional block diagram of a wavelength add-drop device according to an embodiment of the present invention; 
       FIGS. 6 and 7  are exemplary functional block diagrams of a wavelength add-drop device using MEMS technology in a bi-directional network in accordance with the present invention; and 
       FIGS. 8 and 9  are exemplary functional block diagrams of a wavelength add-drop device using MEMS technology in a bi-directional network in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows an optical switch matrix system  100  for selectively adding and dropping channels from a transmission medium  120 . The system  100  includes an optical switch matrix  102  having four ports: input port  104 , output port  106 , add port  108  and drop port  110 . The input port  104  is optically coupled to a demultiplexer  112  for receiving input channels  104   a–   104   f  from the transmission medium  120 . The output port  106  is optically coupled to a multiplexer  114  for transmitting optical channels  106   a – 106   f  onto the transmission medium  120 . The add port  108  is optically coupled with the optical switch matrix  102  for inputting added channels  108   a – 108   e  that are connected to different clients. The drop port  110  is optically coupled with the optical switch matrix  102  for transmitting drop channels  110   a – 110   e,  possibly for further processing. 
   Both the multiplexer  114  and the demultiplexer  112  are optically coupled with the transmission medium  120 . The transmission medium  120  can include any structure that allows for the transmission of optical communication signals, such as an optical fiber. The optical communication signals can further include a plurality of channels that are simultaneously transmitted along the communication medium  120 . For example, numerous channels having discrete wavelengths can be combined onto a single optical transmission medium using wavelength-division-multiplexing (WDM). 
   The demultiplexer  112  is a device that is capable of optically dividing input signals received on the transmission medium  120  into a plurality of channels  104   a – 104   f.  Once the input channel is divided, the input channels  104   a – 104   f  are transmitted to the optical switch matrix  102 . As described above, the channels can travel along the transmission medium  120  on different wavelengths. Additionally, the channels of the input signal can be combined on the transmission medium  120  according to any well known communication technique, such as TDMA, CDMA and the like. Any technique that allows multiple channels to be transmitted across the transmission medium  120  and separated by the demultiplexer  112  can be used without departing from the spirit and scope of the present invention. 
   The multiplexer  114  is a device that is capable of optically combining the output channels  106   a – 106   f  received from the optical switch matrix  102  into an output signal that is then transmitted on the transmission medium  120 . As described above, the numerous output channels  106   a – 106   f  can travel along the transmission medium as an output signal in accordance with any known or later developed transmission technique without departing from the spirit and scope of the present invention. 
   The add port  108  is a device that is capable of receiving channels  108   a – 108   e  from different clients, and then transmitting the added channels  108   a – 108   e  to the optical switch matrix  102 . Data sources for the added channels  108   a – 108   e  can be generated by a plurality of light sources, such as tunable laser diodes, included in the add port  108 . Each of the light sources can be adjusted to emit a channel having a specific wavelength. The light sources of the add port  108  can further operate in accordance with instructions received from a controller (not shown) in order to selectively output an added channel of a specific wavelength. For example, added channels  108   a – 108   e  can each be transmitted on different wavelengths λ a –λ f , corresponding to the wavelengths of the input channels. 
   The drop port  110  is a device that is capable of receiving drop channels  110   a – 110   e  from the optical switch matrix  102 . Each of the channels can be of various wavelengths. The drop port  110  can then output any of the drop channels to a processor (not shown) for further processing. 
   The optical switch matrix  102  is a device that is capable of redirecting optical signals passing through the optical switch matrix  102 . In this manner, a portion of the input channels  104   a – 104   f  can pass through the optical switch matrix  102  to the output channels  106   a – 106   f  without any substantial interference. In other words, these channels are permitted to pass nearly unabated through the optical switch matrix  102  and continue to travel on the transmission medium  120 . 
   Alternatively, a portion of the input channels can be selectively redirected to a drop channel  110   a – 110   e  of the drop port  110  as the inputted channels  104   a – 104   f  pass through the optical switch matrix  102 . In a similar manner, added channels  108   a – 108   e  can be selectively redirected to output channels  106   a – 106   f  of the output port  106  for which the corresponding input channel has been dropped as the added channels pass through the optical switch matrix  102 . According to this technique, input channels can be removed/dropped and new channels can be added to the transmission medium  120 . 
   The optical switch matrix  102  can include an array of switches that can be in either an active or inactive state. In an active state, the switch is able to redirect a light beam or channel passing in close proximity to the switch. In an inactive state, the switch allows a light beam or channel to pass without incident. 
   As an example of operation, assume that the optical switch matrix  102  includes at least N×M matrix of switches that are initially in the inactive position. Further assume that the transmission medium  120  is transmitting an input signal having 6 channels (A–F). In the initial state, the input signal can be received by the demultiplexer  112 . The demultiplexer  112  operates on the input signal to optically separate the input signal into input channels  104   a – 104   f.  The input channels  104   a – 104   f  are then transmitted to the optical switch network  102 . 
   In the initial state of the switch matrix  102 , where all of the optical switches are in the inactive state, the input channels  104   a – 104   f  are permitted to pass through the optical switch matrix to the output channel  106   a – 106   f  without being acted upon. Accordingly, the output channels  106   a – 106   f,  corresponding to the input channels  104   a – 104   f  are transmitted to the multiplexer  114 . The multiplexer  114  then optically operates on the output channels  106   a – 106   f  in order to combine the output channels  106   a – 106   f  into an output signal, and then transmit the output signal back onto the transmission medium  120 . 
   During the course of operation, assume that it has now become desirable to replace input channel  104   c  with an added channel  108   b.  Accordingly, as the input channels  104   a – 104   f  are transmitted through the optical switch matrix  102 , one or more optical switches in the path of input channel  104   c  could be switched to an active state whereby the optical switch can redirect the light beam corresponding to input channel  104   c  to the specified drop port  110 , such as dropped signal  110   a.  Furthermore, the add port  108  can begin transmitting an added signal  108   b  into the optical switch matrix  102  and an optical switch in the path of added channel  108   b  could be switched to an active state, and thereby redirect the added channel  108   b  to output channel  106   c  of the output port  106 . 
   Accordingly, the multiplexer would then receive the input channel  104   a  on output channel  106   a,  the input channel  104   b  on the output channel  106   b,  the added channel  108   b  on the output channel  106   c,  the input channel  104   d  on the output channel  106   d,  the input channel  104   e  on the output channel  106   e  and the input channel  104   f  on the output channel  106   f.  The output channels  106   a – 106   f  would then be combined by the multiplexer  114  and transmitted as an output signal across the transmission medium  120 . In this manner, a channel of the input signal,  104   c,  has been replaced (dropped) during the addition of the added channel  108   b.    
   As is to be understood, the switches of the optical switch matrix  220  can be changed at any time during operation to add or drop channels to or from the transmission medium  120 . In this manner, a user can easily configure the optical switch matrix  102  to add or remove all or a portion of information from an optical network. 
   As shown in  FIG. 2 , the optical switch matrix  102  can be a single microelectrical mechanical system (MEMs). This single MEMs design of the optical switch matrix can be particularly useful for dropping and adding channels from an unidirectional ring network. The MEMs includes an array of micromirrors  280  that are rotatably mounted to a substrate  282 . The micromirrors  280  are rotatable between a non-activated and activated position. In the non-activated position, the micromirrors  280  are substantially parallel and flush with the substrate  282 . In the active position, the micromirrors  280  are rotated or flipped to be in a substantially perpendicular position relative to the substrate  280 . Furthermore, in the active position the micromirrors  280  are positioned within the light path of channels passing through the optical switch matrix. 
   This type of optical switch matrix is discussed in detail in  Journal of Microelectro - mechanical Systems,  Vol. 5, No. 4, December 1996, entitled “Electrostatic Micro Torsion Mirrors for an Optical Switch Matrix” by Hiroshi Toshiyoshi and Hiroyuki Fujita, incorporated herein by reference in its entirety. The optical switch matrix is also discussed in co-pending and commonly assigned patent application Ser. No. 09/002,240 filed on Dec. 31, 1997, also incorporated herein by reference in its entirety. 
   As shown in  FIG. 2 , the wavelength add-drop device  200  includes an optical N×M matrix switch  220  coupled to an input demultiplexer  210 , an output multiplexer  230 , an add port  240  and a drop port  250 . 
   The optical N×M matrix switch  220  can be a four-port matrix switch. In this embodiment, the optical N×M matrix switch  220  is a N×M free space MEMS crossconnect that comprises N×M micromirrors  280 . In the exemplary embodiment shown in  FIGS. 2–4 , N=6 and M=5. As described above, each of the 30 micromirrors  280  shown in  FIGS. 2–4  may take one of an active or inactive position. 
   The positions of the micromirrors  280  can be controlled by a matrix controller (not shown in  FIGS. 2–4 ). By energizing the switch that is on the i th  row and the j th  column of the matrix switch, e.g., by flipping up the micromirror switch on line i and column j, and concurrently tuning the light source of  241   j  to the wavelength used on the i th  line, one can thus add a wavelength from light source  241   j  and/or drop a wavelength at sensor  251   j.    
   Although the optical switch matrix  102  of  FIG. 1  has been described in the exemplary embodiments of  FIGS. 2–4  as being a MEMs type switch  220 , it is to be understood that various other switches can be used without departing from the spirit and scope of the present invention. For example, the optical switch matrix  102  can be any type of optical switch with or without a micromechanical element, such as optical switches based on total internal reflection of a fluid-containing planar light wave circuit (PLC), otherwise known as bubble technology. Such technology is more fully described in the article entitled “Compact Optical Cross-Connect Switch based on Total Internal Reflection in a Fluid-Containing Planar Light Wave Circuit” by J. E. Fouquet, in 2000  OFC Technical Digest,  pp. 204 to pp. 206, which is incorporated herein by reference in its entirety. 
   Referring again to  FIG. 2 , as an example of operation, assume that in an initial state of operation, all of the micromirrors  280  are in the inactive position. Next, assume that a determination has been made that a signal conveyed by the input line  260  is to be dropped. If so, it is determined on which line of the matrix the light ray that has the wavelength that carries the signal to be dropped is transmitted. Next, the sensor  251 A to  251 M on which the signal is to be received is determined. The micromirror  280  corresponding to that line and the column of that selected sensor in the matrix is next positioned in the active position. Next, a determination is made whether another signal conveyed by the input line  260  has to be dropped. The above operations are repeated until no other signal conveyed by the input line  260  has to be dropped. 
   As mentioned above, since the N×M matrix switch in  FIG. 2  is implemented in a unidirectional network, the add channels are always associated with the drop channels. That is, the add channel and drop channel on the same column are associated with the same client. By using the backside reflection of the activated micromirrors, and concurrently tuning the lasers of the add channels to selected wavelengths, signals can be added into traffic from the selected add channels. 
     FIGS. 3 and 4  are exemplary functional block diagrams of the wavelength add-drop device of  FIG. 2  in two different functioning configurations corresponding to a same set of dropped light rays in a unidirectional network. In  FIGS. 3 and 4 , the dropped light rays are the lights rays emitted by input ports  211 B and  211 D, on the second and fourth lines of switch matrix  220 . However, in the configuration outlined in  FIG. 3 , the light ray transmitted on the second line is dropped to the sensor  251 D and the light ray transmitted on the fourth line is dropped to the sensor  251 B. In the configuration outlined in  FIG. 4 , the light ray transmitted on the second line is dropped to the sensor  251 C and the light ray transmitted on the fourth line is dropped to the sensor  251 E. 
   Consequently, in the configuration outlined in  FIG. 3 , the added signals are added by inputting light rays from the light sources  241 B and  241 D. In the configuration outlined in  FIG. 4 , the added signals are added by inputting light rays from the light source  241 C and  241 E. 
     FIGS. 3 and 4  show that the wavelength add-drop device according to an exemplary embodiment of the invention can be configured to select which sensor  251 A– 251 E receives the dropped signal and to select the light source  241 A– 241 E that inputs the added signal. 
   The present invention describes a device that offers full client-configurability, permitting any subset of the incoming wavelengths ( 1 ,  2 , . . . N) to be added or dropped at any subset of the light sources  241 A to  241 E or sensors  251 A to  251 E. 
   An exemplary wavelength add-drop device  400  is outlined in  FIG. 5 , and includes a signal manager  410 , the optical N×M matrix switch  102 , the input demultiplexer  112 , the output multiplexer  114 , the add port  108 , the drop port  110 , the input line  160  and the output line  170 . 
   The signal manager  410  comprises a dropped signal processor  420 , a matrix controller  430  and an added signal processor  440 . The dropped signal processor receives the signals output by the dropped channels  110   a – 110   e  of the drop port  110  and processes those signals. The matrix controller  430  determines which micromirrors in the optical matrix switch  102  are to be tuned to their active position and commands the positions of the micromirrors. The added signal processor  440  provides the signals to be added through the add port  108  and the light sources  108 A– 108 E. 
   The signal manager  410 , the dropped signal processor  420 , the matrix controller  430  and the added signal processor  440  may be, in the exemplary embodiment of the invention shown in  FIG. 5 , a microprocessor that uses software to implement exemplary embodiments of the methods and devices according to this invention. 
     FIG. 6  shows a wavelength add-drop device  700  wherein the optical switch matrix  102  includes two optical N×M matrix switches  720 A and  720 B. This configuration of the optical switch matrix  102  having two MEMs can be particularly useful for adding and dropping channels from a bi-directional ring network and/or a linear network. The optical N×M matrix switch  720 A is coupled to an input demultiplexer  710 , a drop port  750  and the optical N×M matrix switch  720 B. The optical N×M matrix switch  720 B is also coupled to an output multiplexer  730  and an add port  740 . 
   As described above, the embodiment of the present invention described in  FIG. 6  may be used in combination with a linear network or a bi-directional ring. Dropping and adding optical signals may be carried out independently by the optical N×M switch matrices  720 A and  720 B which provide full client-configurability since the add port and the drop port associated with the same input channel or wavelength are independent of each other. 
     FIG. 7  shows an example of operation of the embodiment described in  FIG. 6 . In this example, assume that it is desired to replace a channel corresponding to input channel  711 A with a new channel corresponding to added channel  741 C. As described above, the input channel is received by the demultiplexer  710 . The demultiplexer divides the channels into respective input channels  711 A– 711 F that are then input into the optical switch matrix  102 . 
   As can be seen in  FIG. 7 , the first MEMs  720 A of the optical switch matrix  102  receives the input channels. Further, micromirror  722  has now been switched to an active state. Accordingly, the input channel  711 A is redirected to a output channel  751 B of the drop port  750 . Additionally, as can be seen, the remaining input channels  711 B– 711 F are permitted to pass across the first MEMs  720 A without interference. 
   Simultaneous to the dropping of input channel  711 A, an input signal  741 C is added to the output channel  731 A by the add port  740 . As can be seen, a micromirror  724  of the second MEMs  720 B is switched into an active position. Accordingly, the input signal  741 C is redirected to the output port  731 A of the output multiplexer  730 . Additionally, the output ports  731 B– 731 F receive the input channels  711 B– 711 F, respectively. 
   The multiplexer  730  then combines the new combination of output channels  731 A– 731 F into an output signal. The output signal is then transmitted across the transmission medium  770 . Accordingly, the channel corresponding to input channel  711 A has been removed from the transmission medium and the channel corresponding to added channel  741 C has been added in the removed channel&#39;s place. 
   It should be noticed that in the embodiment of the present invention outlined in  FIG. 6 , only one side of the switching mirrors is used. Moreover, the structure shown in  FIG. 6  is strictly non-blocking. In other words, a new connection or a connection change can be made without rerouting the existing non-changing connections. 
     FIG. 8  shows a wavelength add-drop device wherein the optical switch matrix  102  includes one optical N×M matrix switch  820 A and one optical M×M switch  820 B. The optical N×M matrix switch  820 A is coupled to an input demultiplexer  810 , a drop port  850 , the optical M×M matrix switch  820 B and an output multiplexer  830 . The optical M×M matrix switch  820 B is coupled to an add port  840 . 
   The embodiment of the present invention described in  FIG. 8  may also be used in combination with a linear network or a bi-directional ring network. Dropping and adding optical signals may be carried out independently by the optical matrix switches  820 A and  820 B which provide full client-configurability sine the add port and the drop port associated with the same input channel or wavelength are independent of each other due to the optical M×M matrix  820 B. 
     FIG. 9  shows an example of operation of the embodiment described in  FIG. 8 . In this example, assume that it is desired to replace a channel corresponding to input channel  811 A with a new channel, corresponding to the added channel  841 D. As described above, the input signal is received by the demultiplexer  810  and separated into input channels  811 A– 811 F. As the input channel  811 A is transmitted across the MEMs  820 A, the input channel  811 A&#39;s path is obstructed by micromirror  822  which is in an activated position. A front side  822   a  of micromirror  822  causes the input channel  811 A to be redirected to drop channel  851 B. 
   Additionally, as the input channel  811 A is being dropped, the added channel  841 D is being added. The added channel originates from the add port  840  and is transmitted across the MEMs  820 B until it is redirected by activated micromirror  824 . The micromirror  824  redirects the added channel  841 D so that it intersects with an opposite side  822   b  of the micromirror  822 . The opposite side  822   b  of the reflecting mirror  822  redirects the channel  841 D to the output channel  831 A of the multiplexer  830 . 
   As can be seen from the example described above, the input channels  811 B– 811 F will be transmitted across the MEMs  820 A and be received by the corresponding output channels  831 B– 831 F. The input channel  811 A will be dropped to the drop port  851 B, while the added channel  841 D will be transmitted to the output port  831 A. Accordingly, the output multiplexer  830  will combine the individual channels into an output signal and transmit it across the transmission medium  870 . 
   It should be appreciated that, in the embodiment of the present invention outlined in  FIGS. 8 and 9 , two sides of the switching mirrors of the optical N×M switch matrix  820 A are used but only one side of the switching mirrors of the optical M×M switch matrix  820 B are used. Moreover, the structure shown in  FIGS. 8 and 9  is strictly non-blocking. A new connection or a connection change can be made without rerouting the existing non-changing connections. 
   While this invention has been described in conjunction with the exemplary embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.