Abstract:
A reconfigurable optical add/drop module (ROADM) for dynamically adding or dropping various wavelengths of an optical signal without having to physically replace the module with a wavelength-specific add/drop module, and corresponding methods. A multiplexed optical signal in an optical network enters the reconfigurable optical add/drop module. Filters on the module separate various wavelengths of the optical signal along the module&#39;s various waveguides and a reconfigurable switching matrix directs the various wavelengths of the optical signal to be added, dropped and/or combined with other wavelengths.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/426,115, entitled “Optical Add/Drop Module,” filed Nov. 13, 2002, which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The present invention relates generally to optical communication. More particularly, the present invention relates to modules for dynamically adding or dropping portions of an optical signal. 
   2. The Relevant Technology 
   Networks increasingly employ fiber optic technology to transmit information reliably via a communications network. Networks that employ fiber optic technology are known as optical communications networks, and are marked by high bandwidth and reliable, high-speed data transmission. 
   Optical communications networks often employ a technique known as wavelength division multiplexing (WDM) in order to maximize the amount of information that can be transmitted via the network. To employ this technology, a plurality of optical transmitters, located at the transmission nodes of the optical network, transmit optical signals. Each optical transmitter receives an electrical signal from a network device, such as a computer, and modulates the electrical signal via a laser to an optical signal having a distinct wavelength, called a channel. The distinct channels from the optical transmitters are then combined by a multiplexor to form a multiplexed optical signal. The multiplexed optical signal can then be transmitted via a single fiber optic cable to an optical network, such as a LAN backbone. A reception node of the network then receives the multiplexed, optical signal. 
   Once received by the reception node, the multiplexed optical signal is divided back into its constituent channels by a demultiplexor, and each channel is fed to one of a plurality of optical receivers for modulation into electrical signals. The electrical signals are then forwarded to a network device, such as a computer, for processing. 
   Not every channel, however, may be needed at a given time, and components on an optical network may need to communicate with each other through only one channel of a multiplexed optical signal. Alternatively, components simply may require fewer channels than are present in the multiplexed signal. As such, components may use add/drop modules to manage each of the different wavelength channels in an optical signal. As the name implies, add/drop modules are for adding or dropping component channels of an optical signal. 
   Add/drop modules are used frequently at nodes, or connection points, to manage exchanges of data channel signals in an optical network. Particularly, an add/drop module may drop an unused channel of a multiplexed signal while simultaneously adding a different, appropriate channel. Add/drop modules may interface directly with a computer to communicate Ethernet data from a fiber-optic network, in which case the add/drop modules are used to extract the channel of interest from the multiplexed optical signal. Add/drop modules also may be employed in transceivers or some other nodes in a fiber optic network. 
   More specifically, add/drop modules extract from the multiplexed signal those channels that are to be used by a device in the network. This enables components on a network, such as two computers, to communicate over the same wavelength, or channel, of a fiber optic signal, without necessarily interrupting bandwidth dedicated to the other channels in the signal. For similar reasons, the add/drop module also can be used to insert the dropped channel back into the optical signal, or to include different data that is encoded on the same wavelength as the dropped channel. One problem with conventional add/drop modules, however, is that they tend to be fixed, operating only on specific channels or wavelengths, and they generally cannot add or drop other channels without being replaced by a different add/drop module corresponding to the newly desired channels. 
   In addition to add/drop modules, optical networks also may employ various types of optical switches for switching and routing optical signals. One type of optical switch includes a matrix of thermo-optic switching elements interconnected by waveguides formed on a silica substrate. Other types of switches, known as planar optical switches, are available for some applications. Planar switches, such as switched directional couplers, represent an example of LiNbO 3 -based switches. In addition to traditional signal switching and routing, optical switching applications also may require selective optical signal filtering. 
   Although fixed wavelength add/drop devices are known in the art, there exists a need for add/drop devices that are configurable for use with a variety of wavelengths. Accordingly, a need exists for an optical add/drop module capable of wide-band WDM optical signal transmission and filtration. The optical add/drop module should be capable of dropping and adding a variety of channels without having to be physically replaced. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to a reconfigurable optical add/drop module (ROADM). The module includes waveguides that direct specific channels of a WDM optical signal. For example, the waveguides receive optical signals from input ports and guide the optical signal and/or channels of the optical signal through the module, ultimately to output ports. 
   A filter portion of the module includes a set of discrete focusing filters that direct or reflect one or more wavelengths, or channels, of the optical signal. The filters are arrayed on the substrate in such as way as to direct certain channels differentially. For example, this array enables the filters to direct a given channel by a given displacement. Accordingly, the filters direct specific channels to particular waveguides on the substrate. 
   One of the waveguides may be associated with a drop channel, such that the channel corresponding to that waveguide will be dropped from the optical signal. The remaining channels, i.e., wavelengths not dropped, are recombined in one or more of the waveguides, and thereafter guided out of the module through an output port. 
   The module is reconfigurable in that any of the channels can be selected for the add/drop operation. This is done using a switching matrix that switches channels between the waveguides. For example, the switching matrix may allow any of the channels to be switched to the waveguide that is associated with a drop or add port of the module. Accordingly, the add/drop module need not be replaced to drop or add different channels, rather the channels need only be switched to the appropriate waveguide. This ability to automatically reconfigure the add/drop module using switching technology is in contrast to conventional add/drop modules, which tend to be fixed and are used only with specified channels/wavelengths to be dropped or added. 
   The module also can be configured to add a wavelength received from an input port, the add process being essentially the reverse of the drop process. Multiple add/drop modules may be used in succession. For example, an added channel may contain the data that was contained in a dropped channel or an added channel may include different data that is encoded on the same channel as a dropped channel. Of course, added channels and dropped channels may be completely unrelated as well. Those of skill in the art will recognize that many add/drop combinations are possible and none of the foregoing examples should be interpreted as limiting the scope of the invention. 
   The present invention may be used in a network interface associated with a computer or network device that needs data. The invention may be used to deliver, for example, Ethernet data encoded in an optical signal to a computer. The present invention also may be used with transceivers, since the dropped channel could be detected by a photodetector on the receiver module of the transceiver, the channel later converted into an electrical signal that can be used by, for example, a computer. Such a transceiver may be associated with a computer in a network that uses data, as opposed to the original bank of transceivers that generate multiple wavelengths multiplexed together to form the original, multiplexed data. 
   These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by references to specific embodiments thereof, which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1  illustrates a surface view of an example reconfigurable optical add/drop module in the drop configuration; 
       FIG. 2  illustrates an alternative surface view of an example module in the drop configuration, showing greater detail of the switching matrix; 
       FIG. 3  illustrates an example path of a particular wavelength as it travels from an in port to an out port; 
       FIG. 4  illustrates an example path of a particular wavelength as it travels from an in port to a drop port; 
       FIG. 5  illustrates example paths of several wavelengths for a particular optical signal traveling through the module from the in port to the out port; 
       FIG. 6  illustrates example paths of several wavelengths for a particular optical signal traveling through a module from the in port to the out port, with one wavelength being directed to the drop port; 
       FIG. 7  illustrates example paths of several wavelengths for a particular optical signal traveling through an example module from the in port to the out port, with multiple wavelengths being directed to the drop port; and 
       FIG. 8  illustrates example paths of several wavelengths through an example module in the add configuration, with two wavelengths being combined. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1–8  depict various features and embodiments of the present invention, which is directed to a reconfigurable optical add/drop module (ROADM). The module provides the ability to dynamically add or drop various wavelengths, or channels, of an optical signal. 
   It should be understood, however, that the drawings are merely representations of presently preferred embodiments of the invention. Accordingly, drawings should not be construed to limit the present invention, nor should they be construed as necessarily drawn to scale. 
   The term “optical signal(s)” includes the full range of all electromagnetic radiation that can be used satisfactorily to communicate information through a waveguide and/or fiber optic cable. An optical device incorporating teachings of the present invention may operate on digital or analog signals in the infrared, visible and ultraviolet spectrum. 
   The terms “polymer” and “polymers” include any macromolecule combinations formed by the chemical union of multiple, substantially identical combining units or monomers. Certain polymers have satisfactory characteristics for use as a waveguide for optical signals. Combinations of two, three of four monomers often are referred to respectively as dimers, trimers, and tetramers. Polymers may be further classified as inorganic, organic, natural, synthetic or semi-synthetic. 
   The terms “hologram” and “holographic” should be interpreted broadly to encompass a wide range of arrangements, orientations, and geometries, including volume holograms. Holograms are one example of a filter that is capable of guiding, directing, focusing, and/or dispersing an optical signal. As used in this application, the term “filter” also should be interpreted broadly to encompass a wide range of structures. For example, prisms, diffraction gratings, and holograms, whether stationary or moving, may be used as types of filters. A description of one particular holographic arrangement is attached as Appendix A. While the holographic arrangement described in Appendix A points out some specific benefits for a particular holographic arrangement, nothing in Appendix A should be interpreted as limiting the types of filters that may be used in the present invention. In other words, the present invention may make use of any of the filter structures that guide, focus, or disperse light, which are discussed in Appendix A, whether described favorably or not. 
     FIG. 1  presents a general, surface overview of the present invention.  FIG. 1  shows a substrate  100  having holographic filters  110 , waveguides  120 , and switching matrix  140 . An optical signal  160  enters waveguide  122  through an in port, interacts with the set of holographic filters  110 , and eventually passes through switching matrix  140 . The optical signal  160  may be filtered such that certain wavelengths, or channels, of the signal may travel to drop port  124 , or to out port  136 . While  FIGS. 2–7  show the present invention in a drop configuration, it is understood that the embodiment of  FIG. 1  is easily adapted for an add configuration, i.e., adding channels to the optical signal  160 , shown generally in  FIG. 8 . 
     FIG. 2  shows one example embodiment of the module  100 , with an example switching matrix  140 . In  FIG. 2 , optical signal  160  has at least three distinct channels, wavelength λ 1    162 , wavelength λ 2    164 , and wavelength λ 3    166 , when entering the waveguides  120  (individually labeled  122 – 134 ) through an in port optically coupled to waveguide  122 . As used in this application, optically coupled should be interpreted broadly to encompass any type of optical transmission, direct or indirect, from one component or structure, to another. The holographic filters  110  in  FIG. 2  comprise at least 3 distinct holographic filters  112 ,  114 , and  116 , corresponding respectively to channels λ 1    162 , λ 2    164 , and λ 3    166 . 
   The components of the switching matrix  140  may comprise thermo-optical switches that are compatible with the substrate material and consume relatively little power. For example, the switching module may comprise thermo-optical switches that operate as described in U.S. patent application Ser. No. 09/999,054, entitled “N×N Optical Switching Device Based on Thermal Optic Induced Internal Reflection Effect,” filed on Nov. 1, 2001, which is incorporated herein by reference, or other switches. One benefit of thermo-optical switches, such as those described in the U.S. patent application Ser. No. 09/999,054 can be incorporated into a silicon substrate and are compatible with a substantially planar device. It is understood that other types of known, optical switching technologies can operate with this module, and should not be construed as precluded. It is also noted that each of switches  142 – 150  in switching matrix  140  is shown in the present diagrams as a thermo-optic switch, primarily for convenience. 
   In principle, a portion of a switch of appropriate composition, e.g., thermo-optic switch  148 , may be electrically heated, thereby decreasing or otherwise changing its refraction index relative to the remaining switch portion. When a channel encounters the heated switch from the portion having a relatively greater (or sufficiently different) refraction index (i.e., the cooler portion), the switch reflects the channel consistent with the angle of incidence. An example switch technology is described in Appendix B. 
   Turning now to  FIG. 3 , the diagram shows an example path for channel λ 1    162  of optical signal  160 . Channel  162  enters waveguides  120  through in port  122 . Channel  162  is dispersed by a specific displacement channel-specific hologram  112 , causing channel  162  to enter waveguide  126 . In other words, hologram  112  directs channel  162  to waveguide  126 . Channel  162  then passes along waveguide  126  through switch  148 , since switch  148  is in the “off” configuration. By way of contrast,  FIG. 3  also shows switch  150  in the “on” configuration. Because switch  148  is in the off configuration, channel  162  passes around to waveguide  132  past switch  142 , exiting again toward the holographic filters  110 . When channel  162  reaches channel-specific hologram  112 , which reflects only channel  162  in this particular example, channel  162  is again directed by a specific displacement to waveguide  134  and the out port  134 . 
     FIG. 4  shows substantially the same situation as  FIG. 3  except that in this case switch  148  is in the “on” configuration, thereby directing channel  162  through waveguide  124  to the drop port. As previously explained, activating switch  148  causes switch  148  to alter a portion of its refractive index such it becomes substantially optically reflective. Switch  148  then reflects channel  162  into the switching waveguide  136 , heading toward waveguide  124 . Similarly, switch  150 , also in the “on” configuration, reflects channel  162  to exit along waveguide  124  to the drop port. Thus, when switches  148  and  150  are in the “on” configuration, channel  162  is dropped from the remaining channels of the optical signal. 
     FIG. 5  is similar to  FIG. 3  except that it shows multiple channels, channels  162 ,  164  and  166  of optical signal  160 , traveling through the module, none of which are dropped (switches  142 ,  144 ,  146 , and  148  are configured “off”). As previously described, channel  162  is reflected by hologram  112  back to waveguide  126 . Channel  162  then travels past switch  148  around to waveguide  132 . Channel  162  travels past switch  142  and is directed by hologram  112  into waveguide  134  and the out port. 
   Channel  164  travels through waveguide  122 , to reflect off channel-specific hologram  114 . The hologram  114  directs channel  164  to waveguide  128 , past switch  146 , around to waveguide  130 , and past switch  144 . When channel  164  leaves waveguide  130 , hologram  114  directs channel  130  to the out port, through waveguide  134 . Channel  166  follows a similar scenario as channels  162  and  164  except that hologram  116  reflects channel  166  to waveguide  130 , past switch  144  around to waveguide  128 , again to hologram  116 , and finally to the out port, through waveguide  134 . 
     FIG. 6  shows an embodiment of the add/drop module in the drop configuration where channel λ 3    166  is to be dropped from the signal  160 . Here, the channel paths are substantially the same as in  FIG. 5 , but since switch  144  is in the “on” configuration, channel  166  travels along the switching waveguide  136  past each of the switches  146  and  148  (whether each switch is in the on or off configuration). Ultimately, switch  150  (switched on) reflects channel  166  along waveguide  124  to the drop port. 
   It should be understood that the present add/drop module&#39;s configuration may be used to drop (or add) a plurality of channels from an optical signal  160 , as shown in  FIG. 7 .  FIG. 7  shows the paths of channels  162 ,  164 , and  166  when the switches  144 ,  146 , and  150  are configured as on. Channel  162  follows the same paths as previously described, ultimately exiting to the out port, through waveguide  134 . Channel  166  follows the same path to the drop port through waveguide  124 ; note, however, that channel  166  passes through switch  146  from the opposite direction even though switch  146  is in the on configuration. 
   Channel  164  in  FIG. 7  follows the same initial path as in  FIG. 6 , except that since switch  146  is configured as on, switch  146  reflects channel  164  along switching waveguide  136 . Then, since switch  150  is configured as on, switch  150  reflects channel  164  through waveguide  124  to the drop port. Thus, in this embodiment, at least two channels are dropped from the optical signal. 
   The prior figures show the module  100  in a drop configuration. However, the module is also suitable for an add configuration, essentially the reverse of a drop configuration. To illustrate,  FIG. 8  shows one embodiment of the instant invention in an add configuration. Here, wavelength λ 2    164  is added to an optical signal comprising wavelength λ 1    162 . 
   In  FIG. 8 , optical signal  160 , comprising channel λ 1    162 , enters the add/drop module  100  from the in port through waveguide  172 . A channel λ 2    164  to be added with channel λ 1    162  enters through add/in port waveguide  184 . Channel  162  travels along waveguide  172  until it reaches channel-specific hologram  112 . Hologram  112  directs channel  162  into waveguide  176 . Since switch  148  is configured as on, channel  162  travels along switching waveguide  136  to switch  150 , also configured as on. Switch  150  then reflects channel  162  out along the waveguide  174  to the add/out port. 
   Channel  164  enters though waveguide  184  and interfaces with channel-specific hologram  114 . Hologram  114  reflects channel  164  into waveguide  180 , where channel  164  travels to switch  144 . Since switch  144  is configured as on, switch  144  reflects channel  164  along switching waveguide  136  toward switch  150 . Passing switches  146  and  148 , channel  164  joins channel  162  prior to interfacing with switch  150 , also configured as on. As switch  150  reflects both channels  162  and  164  along waveguide  174  to the add/out port, channel  164  is added to the optical signal comprising channel  162 . 
   The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.