Abstract:
The present invention overcomes problems associated with switch isolation, noise and crosstalk suppression, insertion loss, spurious reflections, wavelength tolerance, and compactness that are present in varying degrees in other add/drop systems. The present invention includes devices or components that include, but are not limited to high efficiency switchable gratings.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to wavelength division multiplexing and demultiplexing systems, and, more particularly, to optical add/drop multiplexing and related switching systems utilizing switched gratings. 
     With the advent of substantial new uses for electro/optic systems, there exists a greater need to effectively control the multiplexing and routing of signals that are Wavelength Division Multiplexed (WDM), and Dense Wavelength Division Multiplexed (DWDM), on optical fibers. A central operation required in managing such WDM and DWDM systems is the addition of signals into empty channels (Add) and selection of signals from populated channels (Drop). Each of these channels correspond to specific, predefined wavelength ranges in the wavelength division multiplexed signal stream. A typical signal in a given channel consists of a modulated optical carrier that has a center (carrier) frequency corresponding to a wavelength in that predefined wavelength range. The International Telecommunications Union (ITU) has defined grids of WDM channels each with a center frequency, frequency spacing between channels, etc. 
     For example, when a stream of n discrete wavelength channels are multiplexed on a single optical fiber, there is a need to selectively remove the signals from specific wavelength channels and to selectively add signals into specific wavelength channels to the data stream. The initial multiplexing of many different wavelength channels requires wavelength multiplexers. Similarly, wavelength demultiplexers (also, referred to, in the context of this application, as “drop” multiplexers) are often required to separate the different wavelength channels which are multiplexed in WDM systems. Such devices are needed, for example, in digital telecommunication systems and analog RF photonic systems, although, it should be realized that these are just two of numerous electro-optic systems which require the use of such devices. 
     Past approaches for optically multiplexing, demultiplexing, adding, or dropping optical signals of differing wavelengths have deficiencies associated therewith. These deficiencies include, but are not limited to, excessive insertion loss, expense and complexity, size, lifetime issues, and crosstalk. There is still much room for advancement in these prior approaches, particularly with respect to losses, complexity, crosstalk, switch isolation, compactness and multiple reflection suppression. 
     It is therefore an object of this invention to provide optical add/drop multiplexing systems which has superior switch isolation, multiple reflection and crosstalk suppression; less complexity and lower insertion loss; and less stringent wavelength tolerances than systems of the past. 
     It is another object of this invention to provide optical add/drop multiplexing systems which are extremely compact. 
     It is still another object of this invention to provide optical add/drop multiplexing systems which utilize switchable gratings therein. 
     SUMMARY OF THE INVENTION 
     The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow. 
     The present invention overcomes problems associated with switch isolation, noise and crosstalk suppression, insertion loss, spurious reflections, wavelength tolerance, and compactness that are present in varying degrees in other add/drop systems. The present invention includes devices or components that include, but are not limited to high efficiency switchable gratings. 
     For example, in one embodiment of the invention, each of the “base” gratings have substantially identical spatial frequencies and each of the “vertex” gratings have substantially identical spatial frequencies. However, the “vertex” gratings have higher grating frequencies than the “base” gratings. In another embodiment of the invention, the “base” and “vertex” gratings have substantially the same grating frequencies. Further embodiments of the invention, for example, deal with multiple add/drop grating pairs, variations of fixed and switchable gratings, the use of grating pairs to spatially separate the wavelength division multiplexed components of input beams into individual beams, and the replacement of lenses and waveguides with detectors or emitters. 
     For a better understanding of the present invention, together with other and further objects, reference is made to the following description taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation illustrating the overall concept of the add/drop multiplexing system of the present invention; 
         FIG. 2  is a schematic representation illustrating various modes of operation of the add/drop multiplexing system of the present invention; 
         FIG. 3  is a schematic representation illustrating an alternate embodiment of the add/drop multiplexing system of the present invention; 
         FIG. 4  is a schematic representation illustrating a further embodiment of the add/drop multiplexing system of the present invention; 
         FIG. 5  is a schematic representation illustrating a still further embodiment of the add/drop multiplexing system of the present invention; 
         FIG. 6  is a schematic representation illustrating an additional embodiment of the add/drop multiplexing system of the present invention shown in  FIG. 5 ; 
         FIG. 7  is a schematic representation illustrating a further embodiment of the add/drop multiplexing system of the present invention using multiple cascading of dispersive gratings; and 
         FIG. 8  is a schematic representation illustrating an additional embodiment of the add/drop multiplexing system of the present invention shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to better understand the present invention described below, it should be noted that certain terms used in the description of the invention have interchangeable usage. For example, the term “optical” refers not only to optical components, but also to electro-optical components, while “multiplexer” refers to other forms of switching such as demultiplexing. 
     Furthermore, terms such as “beams” and “ports” may also be interchanged, in certain instances, based upon their upon their usage as recognized in the art. 
     In addition, identical components may be referred to with identical reference numerals within the specification and drawings for simplifying an understanding of the various components of this invention. 
     The present invention provides an optical multiplexing, demultiplexing, and add/drop multiplexing/demultiplexing system or device hereinafter generally referred to as an optical add/drop mutiplexing system that is suitable for adding, dropping, or modifying signals that are wavelength multiplexed onto a common optical path. The present device or system, for example, is suitable for Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) applications. The incorporation within this invention of free space switching further adds several distinct advantages over past techniques. More specifically, these advantages include the potential for lower insertion loss, superior switch isolation, multiple reflection and crosstalk suppression, and less complexity. 
     Reference is now made to  FIG. 1  of the drawings which illustrates the broad concept of the invention in schematic fashion, thereby presenting an overview of the optical add/drop multiplexing system  10  of the present invention in one of numerous embodiments, the other embodiments being set forth below with respect to the remaining figures. 
     The embodiment of the optical add/drop multiplexing system  10  of this invention as illustrated in  FIG. 1  of the drawings includes, but is not limited to, static (non-switchable) diffraction gratings  12 ,  14 ,  16 ,  18 ,  20 , and  22  and switchable grating  30 . In the configuration of  FIG. 1 , each of these gratings is parallel to another, and gratings  12 ,  14 ,  16 , and  18  (also referred to as base gratings) have substantially identical spatial grating frequencies. Similarly, the grating spatial frequencies of gratings  20 ,  22 , and  30  (also referred to as vertex gratings) are also substantially identical, but the vertex gratings have higher grating spatial frequencies than the base gratings, such that the light diffracted by the base gratings is symmetrically diffracted from the vertex gratings and recombined at the subsequent base gratings. For example, in system  10  of  FIG. 1 , the vertex gratings  20 ,  22 , and  30  can have, for example, twice (not to be construed as a limitation to this invention) the spatial frequency of the base gratings  12 ,  14 ,  16 , and  18 . Base gratings  12  and  18  are located symmetrically with respect to vertex grating  20 . Similarly base gratings  16  and  14  are located symmetrically with respect to vertex grating  22 . The switchable grating  30  is located at the intersection of the two base-vertex arrangements described above as illustrated in  FIG. 1 . 
     The switchable grating  30  is further divided into individually switchable segments or pixels  32 . These individually switchable segments can be formed, for example, by pixellating the electrode controlling the grating. Conventional electronic control  34  provides the individual control signals, which switch the individual grating segments “on” or “off”. 
     Operation of the add/drop multiplexer system  10  of  FIG. 1  is clearly described below. Input beam  40 , preferably in free space, is typically a collimated or nearly collimated beam of electromagnetic radiation (optical signal) that may include a multiplicity of optical signals each of which are modulated on wavelength-multiplexed optical carriers  50  that each have differing center wavelengths. Beams guided in waveguides or optical fibers  42  may be converted into or from free space beams through the use of lenses  44 . The lenses  44  may be refractive, diffractive, or gradient index, or a combination thereof. Input beam  40  is incident normally on grating  12  (perpendicularly with respect to the surface of the grating  12 ) at a single spatial location. Base diffraction grating  12  angularly disperses the input beam  40  into separate wavelength channels or beams  51  each of which may contain distinct modulated optical carriers  50 , which may hereinafter be referred to interchangeably as beams, optical carriers or optical carrier beams. 
     In general, the optical carriers with longer wavelengths are diffracted by grating  12  through larger angles, such that the longest wavelength optical carrier  52  is incident on grating  20  at a higher location than mid-wavelength optical carrier  54 , which is in turn incident on grating  20  at a higher location than shortest wavelength optical carrier  56 . In such fashion each of the optical carriers  50  or optical carrier beams are incident at distinct spatial locations on vertex gratings  20 ,  22  and  30  in a manner described in greater detail below. 
     Similarly beam  62  is input to base grating  16  which angularly disperses input beam  62  into separate channels or beams  76 . These beams  76  are dispersed from input beam  62  analogously to the beams  51  dispersed from input beam  40  as described above. 
     Switchable grating  30  is located symmetrically in the region where the optical carrier beams  70 ,  72 ,  76 , and  74  intersect as shown in  FIG. 1 . Further, the size scale of the optical add/drop multiplexing system  10  and width of beams  40 ,  60 ,  62 , and  64  are chosen such that the individual optical carriers of differing center wavelengths are spatially separated on gratings  20 ,  22 , and  30 . The present invention provides a system for a switchable grating segment or pixel  32  along each of optical carrier beams  70  (or subsets thereof) such that for each of optical carrier beams  70 , if the intersecting segment of grating  30  is switched “off”, the beam is transmitted through grating  30  becoming a respective optical carrier beam  72 . If the segment or pixel  32  is switched “on” the beam associated therewith is diffracted and becomes respective optical carrier beam  74 . Each of the optical carrier beams or channels can contain modulated optical carriers with different center wavelengths For typical WDM telecom applications, there may be locations for 2, 4, 8, 20, 40, 80, 160, or even more optical carrier beams on an ITU wavelength grid in a given spectral band or carrier group. Thus the number of optical carriers varies considerably from application to application. 
     For illustrative purposes, but not for limitation, consider the case of all segments or pixels  32  of the switched grating  30  being in the “off” state. In such a case all light incident on grating  30  is transmitted, as if grating  30  did not exist. Initially, all optical carriers  50  are spatially overlapping on a single spot and are normally incident on base grating  12  from beam  40 . Because of the symmetric location of vertex grating  20  with respect to base gratings  12  and  18  and the grating frequency relationship described earlier, all optical carriers  50  are angularly diverging toward vertex grating  20  and are spatially separated on vertex grating  20  where they are diffracted symmetrically into optical carrier beams  70  which are converging toward a single spot or location on base grating  18 . As these optical carrier beams  70  are transmitted through “off” grating  30 , they are transmitted into optical carrier beams  72 . When these optical carrier beams  72  are incident on grating  18 , they are all diffracted symmetrically back into a single beam  64  in which all differing wavelength optical carriers propagate in a single beam with identical propagation directions. This beam  64  may then readily be coupled into an optical fiber or waveguide  42  using lens  44 . Thus, with all switched grating pixels  32  switched off, all the wavelength multiplexed signals in input beam  40  are spatially separated and then recombined in a wavelength multiplexed beam  64 . 
     Still further, consider the case where all segments or pixels  32  of the switched grating  30  are in the “off” state (i.e., are transparent), in which case all light, once again, incident on grating  30  is transmitted, as if grating  30  did not exist. Base gratings  16  and  14  are located symmetrically with respect to vertex grating  22 . All optical carriers  76  are initially spatially overlapping on a single spot on base grating  16  and are normally incident on base grating  16  from beam  62 . Because of the symmetric location of vertex grating  22  with respect to base gratings  16  and  14  and the grating frequency relationship described earlier, all optical carriers  76  after diffraction by base grating  16  are angularly diverging toward vertex grating  22 . As these optical carrier beams  76  are transmitted through “off” grating  30 , they become optical carrier beams  74 , and are spatially separated on vertex grating  22  where they are diffracted symmetrically into optical carrier beams  78  which converge toward a single spot or location on base grating  14 . When these optical carrier beams  78  are incident on grating  14 , they are all diffracted symmetrically back into a single beam  60  in which all differing wavelength optical carriers propagate in a singe beam with identical propagation directions. This beam  60  may then readily be coupled into an optical fiber or waveguide  42  using lens  44 . Thus, with all switched grating pixels  32  switched “off”, all the wavelength multiplexed signals in input beam  62  are spatially separated and then recombined in a wavelength multiplexed beam  60 . 
     Due to the symmetric nature of switchable diffraction gratings, a dual mapping occurs simultaneously with optical carrier beams  76  incident on the switched grating  30  with respect to the optical carrier beams  70  incident on the switched grating  30  as described above. Therefore, if all the switchable grating segments  32  are “off”, optical carrier beams  70  are transmitted into optical carrier beams  72 , and optical carrier beams  76  are transmitted into optical carrier beams  74 . In such a case, all the wavelength multiplexed carriers (and the signals they carry) that are input in beam  40  are output in beam  64 ; and all the wavelength multiplexed carriers (and the signals they carry) that are input in beam  62  are output in beam  60 . 
     Alternately, if all the switchable grating segments or pixels  32  are “on”, optical carrier beams  70  are diffracted into optical carrier beams  74 , and optical carrier beams  76  are diffracted into optical carrier beams  72 . In this latter case, all the wavelength multiplexed carriers (and the signals they carry) that are input in beam  40  are output in beam  60 ; and all the wavelength multiplexed carriers (and the signals they carry) that are input in beam  62  are output in beam  64 . 
     Since the individual optical carriers (each with a unique center wavelength) are spatially resolved on switched grating  30  and are each incident on a unique switched grating pixel of group of pixels  32 , the route of each wavelength multiplexed signal in beams  40  and  62  may be individually controlled by setting the state of the corresponding grating pixel  32  such that it is output in either of beams  64  or  60 . Each of the optical carrier beams represent modulated optical carriers with different center wavelengths, and corresponding optical carrier beams  70 ,  72 ,  76 , and  74  (defined by intersections at a common grating segment of grating  30 ) represent modulated optical carriers of a common particular center wavelength. Accordingly, for example, for each optical carrier beam  76 , if the intersecting segment of grating  30  is switched “off”, the beam in optical carrier group  76  is transmitted through grating  30  becoming a optical carrier beam  74  and is ultimately included in output beam  60 . If the input beam  40  contains an optical carrier of the same center wavelength as the optical carrier beam  76  described above, it will be transmitted through the same “off” pixel and output in beam  64 . If this same segment of switched grating  30  described above is switched “on”, the corresponding optical carriers (if present) from inputs  40  and  62  will be diffracted and output in beams  60  and  64 , respectively. 
     Reference is now made to  FIG. 2  for further operation of the wavelength add/drop multiplexing system  10 . For convenience the beams  40 ,  60 ,  62 , and  64  may also be described as input, output, add, and drop ports, respectively. 
     Consider the wavelength division multiplexed (WDM) scenario where the input, output, add, and drop ports or beams  40 ,  60 ,  62 , and  64 , respectively, may each may contain many wavelength multiplexed optical carriers propagating as single multiplexed beams which are incident at respective single port locations. Each of these wavelength multiplexed optical carriers may be modulated with one or more signals. It is conventional in the wavelength division multiplexed (WDM) scenario to universally name each of the many possible WDM channels, each of which are defined by a particular center wavelength. These WDM channels have two states: either the WDM channels are “populated” and contain an optical carrier of the center wavelength defined for the WDM channel, or they are “empty” WDM channels which do not contain an optical carrier. The optical carrier in a given WDM channel may be “dropped” or removed, thus leaving the WDM channel empty. Alternatively, an empty WDM channel can have an optical carrier “added” in which case it is then populated. 
     For the purpose of illustrating the operation of the add/drop multiplexing system  10  shown in  FIG. 2 , three of the many possible named WDM channels  80 ,  90 , and  100  are illustrated. WDM channel  80  is defined by the wavelength used in the longest wavelength optical carrier  52 . WDM channel  90  is defined by the wavelength used in a mid-wavelength optical carrier  54 . WDM channel  100  is defined by the wavelength used in the shortest wavelength optical carrier  56 . There may typically be tens or hundreds of named WDM channels incident on optical add/drop multiplexing system  10 , such as defined by the ITU grid discussed earlier. Pixellated switched grating  30  contains a separately controllable switched grating pixel at the locations of each of the channels or subsets of channels it is desirable to add or drop among ports  40 ,  60 ,  62 , and  64 . Input beam  40  as shown in  FIG. 2  contains carrier  52  in WDM channel  80 , carrier  54  in WDM channel  90 , and no optical carrier in WDM channel  100  (as represented by the dotted line). In  FIG. 2 , empty WDM channels are represented by dotted lines, and populated WDM channels are represented by solid lines. 
     Still referring to  FIG. 2 , add/drop multiplexing system  10  is represented therein with switched grating pixel  110  of grating  30 , corresponding to WDM channel  80 , being switched “on” (i.e., diffracting). The switched grating pixel  115  corresponds to WDM channel  90  and is switched “off” (i.e., non-diffracting). Switched grating pixel  120  corresponding to empty WDM channel  100  is also switched “off”. With these settings it is shown below that optical carrier  52  is transmitted from input port  40  to output port  60 ; carrier  54  is dropped from input port  40  to drop port  64 ; and carrier  57  (along with any information that is modulated upon it) is added from add port  62  to output port  60 . 
     These three cases illustrate the basic functionality of the optical add/drop multiplexing system  10 . When the carriers are added or dropped from any ports, all the information that may be modulated upon the carriers is also added or dropped from the ports. For example, if there are eighty defined or named WDM channels, optical carriers and their information in any of the channels may be input into optical add/drop multiplexing system  10  at port (or multiplexed beam)  40 ; any or all of these carriers can be dropped from output port  60  to drop port  64  by switching off (clearing) the corresponding pixel on grating  30 ; and any or all of these carriers can be added to output port  60  through add port  62  by switching off the corresponding pixel on gating  30 . In typical operation for each named WDM channel, optical carriers may 1) be added to an empty input channel, 2) dropped from a populated input channel, 3) both (i.e., dropped from a populated input channel and a new carrier added to the same channel); and/or 4) neither (in which case any input carrier in the named channel is transmitted from the input port to the output port. In such a fashion, each of the segments or pixels  32  on switched grating  30  are used to control the passage, addition, or dropping of particular modulated optical carriers. 
     The switchable gratings used in the configurations described above may be fabricated using many technologies. In a preferred embodiment, the switchable gratings may be formed using Polymer Dispersed Liquid Crystal (PDLC) volume holographic gratings which can be fabricated with very low insertion loss (e.g., 0.1-0.3 dB/grating) and fast switching times. In another aspect of the present invention, the same PDLC switchable gratings preferred for use in these devices can be used for the static non-switchable gratings that are also used in these devices. Accordingly, the electrodes used to apply electric fields to switch the switchable gratings may be omitted from the gratings for the non-switchable gratings. In such an instance all the advantages of low insertion loss, high diffraction efficiency, low scatter, etc. of the switchable gratings can be provided for the non-switchable gratings, and the performance of the present invention is further enhanced since the absorption and surface reflection losses induced by the transparent electrodes are eliminated in the non-switchable gratings. This principle is also applicable to other forms of switchable holographic elements including lenses, mirrors, and corrector plates. Other preferred recording materials for the non-switchable diffraction gratings include dichromated gelatin and the DMP-128 photopolymer for volume holographic gratings; and holographically exposed photoresists for producing blazed holographic surface relief diffraction gratings. 
     Another embodiment of the optical add/drop multiplexer of this invention is illustrated by optical add/drop multiplexing system  110  shown in  FIG. 3 . As shown in  FIG. 3 , base gratings  12  and  16  disperse the input and add beams from beams/ports  40  and  60 , respectively, as in the optical add/drop system  10 . The entering and exiting beams  40 ,  60 ,  62 , and  64 , respectively, are also referred to as ports of the optical add/drop multiplexing systems. Therefore, the connotation “beam(s)/port(s)” may also be used with respect to the description of this invention for purposes of clarity. As in system  10 , wavelength-division-multiplexed (WDM) beams  40  and  62  contain multiple multiplexed beams or optical channels each with differing carrier wavelengths, and are dispersed by gratings  12  and  16  so that these channels angularly and spatially separate. Similarly base gratings  14  and  18  combine converging optical beams or channels of differing wavelengths into single wavelength-multiplexed output and drop beams  60  and  64 , respectively, as in system  10 . 
     However, in the optical add/drop multiplexing system  110 , the “vertex” gratings  120 ,  122 ,  130  and  140  have the same grating frequency as the base gratings  12 ,  14 ,  16 , and  18 . Gratings  120 ,  122 ,  130  and  140  are typically longer than the base gratings so to intersect all the dispersed carriers  50 , but in other respects can be identical to the base gratings. As a result of the gratings all sharing a common grating frequency, the dispersed carriers  50  exiting base grating  12  are all diffracted by grating  120  into beams  150  that are parallel to each other and to the-input beam  40 . Due to the dispersion of the gratings and the separation between gratings  12  and  120 , the beams  150  are spatially separated. Each of the beams  150  corresponds to a separate WDM channel that may be populated or empty as described earlier. 
     Gratings  140  and  130  in system  110  are parallel to gratings  120  and  122 , respectively, and also share the same grating frequency. However, gratings  130  and  140  are switchable and pixellated into individually controllable grating pixels which may be independently switched “on” or “off”. Each grating pixel on switchable gratings  130  and  140  is chosen so that it covers the area of intersection between one (or a subset) of the channels  150  and the respective grating. 
     The four base gratings  12 ,  14 ,  16 , and  18  are arranged symmetrically in pairs with upper gratings  120 ,  122 ,  140 , and  130 , respectively, as shown in  FIG. 3 . Operation of the optical add/drop multiplexer system  110  can now be described. Input beam  40  is dispersed into its separate channels with grating pair  12 - 120 . This grating pair  12 - 120  forms “input pair”  180 . If the pixels of switchable gratings  130  and  140  are all “off” (transparent), then all the channels input in beam  40  are combined (multiplexed) into single output beam  60  by grating pair  14 - 122 . Any channels incident on grating  122  are combined into a single output beam  60  by grating pair  14 - 122 . This grating pair  14 - 122  forms “output pair”  190 . Channels corresponding to any pixels of grating  130  that are switched “on” (diffracting) are multiplexed into drop beam  64  by grating pair  18 - 130 . This grating pair  18 - 130  forms “drop pair”  160 . Finally, any channels populated in add beam  62  may be added to multiplexed output  60  by switching “on” the grating pixels corresponding to those channels on grating  140 . These channels are added by the grating pair  16 - 140 . This grating pair  16 - 140  forms “add pair”  170 . In typical operation, a signal would be added to a channel that was either unpopulated at input  40  or was populated at input  40  but whose signal was dropped into drop port  64 . 
     Another embodiment of the optical add/drop multiplexer of the present invention is system  200  shown in  FIG. 4 . Here the input pair  180  and output pair  190  of the type described with respect to system  110  are used in conjunction with multiple add pairs  170  and drop pairs  160 , all of which are described above. The use of multiple add and drop grating pairs, however, forms more flexible systems which can have zero or multiple add beams (or add ports) and zero or multiple drop beams (or drop ports). The add and drop pairs  160  and  170  can be interspersed in any order. Also system  200  allows for zero, one, or multiple multiplexed signals to be added or dropped from any of the add or drop pairs, respectively. The flexibility of optical add/drop system  200  is evidenced by its ability to add and drop single channels from each of multiple ports instead of adding and dropping all the desired channels from single add and drop ports. This flexibility (of the former case) may be valuable since it obviates the need to later de-multiplex the signals of the latter case. 
     Another embodiment of the optical add/drop multiplexer of the present invention is system  250  shown in  FIG. 5 . System  250  is a variation of system  10 , where in system  250 , base gratings  12 ,  14 ,  16 , and  18  function the same as in System  10 . There are also three vertex gratings in system  250 , however, as shown in  FIG. 5 , switchable vertex grating  30  (of system  250 ) is replaced by fixed (non-switchable) vertex grating  260 , which is placed at the intersections of beams (channels)  51  and  78 . Similarly, fixed vertex gratings  20  and  22  are replaced by similar but pixellated and switchable vertex gratings  270  and  280 , respectively. 
     Operation of the optical add drop multiplexer system  250  can now be understood by referring to  FIG. 5 . If all the pixels of gratings  270  and  280  are switched off (clear), then the multiplexed signals present in input beam  40  are symmetrically dispersed and recombined into output beam  60 . Any of the pixels of grating  270  that are switched on (diffracting) will drop the signals in the corresponding channels into drop beam  64 , and in so doing, drop the same signals from output beam  60 . Similarly any of the pixels of grating  280  that are switched on (diffracting) will add the signals in the corresponding channels from add beam  62  into output beam  60 . Generally signals will be added into channels that are either unpopulated in input beam  40 , or into channels that have been dropped into drop beam  64 . However, signals can be added into populated channels by switching on the corresponding pixels of grating  280  and introducing the signals in add beam  62 . In this case, the added signals will be diffracted by grating  280  into the output beam  60 . The signals existing in those same channels at grating  260  will be simultaneously diffracted out of the system by grating  280 . These signals diffracted out of the system can be further utilized, as shown in later configurations. 
     In optical add/drop multiplexing system  250  it is seen that in the operation of dropping a signal and adding a new signal to a single channel, there is twice the crosstalk suppression afforded than that of a single switched grating. In other words, consider the drop/add process for signals in the channel k corresponding to “on” pixels  271  and  281 . The diffracting pixel  271  drops the signals incident in input beam  40  on channel k to output beam  64 . Lets assume for example (and not for restriction) that switched gratings  270  and  280  operate at a contrast of 30 dB (meaning that when on, the light is diffracted and the spurious beam that is transmitted is attenuated by 30 dB). The “crosstalk” signal that leaks through the diffracting pixel is also diffracted out of the system by pixel  281  which simultaneously “adds” the new signal into that same channel. Thus with grating contrasts of 30 dB, the remnant of the dropped signal in the output is down by 60 dB. 
     In a similar fashion, additional pixellated switched gratings may be added in a redundant fashion to act as crosstalk suppressors. For example, grating  260  may be substituted with a pixellated switched grating, and can be set clear for pixels that have been dropped at grating  270 . This would add an additional measure of suppression of the remnant dropped signal in the output beam  60 . 
     In the prior description of optical add/drop multiplexing systems  10 ,  110 ,  200 , and  250 , examples were given (without restriction intended) of beams  40  and  62  being input and add beams; and beams  60  and  64  being output and drop beams in the context of a  4 -port add/drop multiplexer. It should be noted that in fact the operation of the systems is symmetric, and not only can beams  60 ,  40 ,  64 , and  62  function as input, output, add, and drop beams, respectively, but that these sets of functionality can occur simultaneously in a bi-directional optical add/drop multiplexing functionality. 
     Additionally, a pixellated switched grating can be introduced in system  250  at the intersection of the dispersed add and drop beams arising from multiplexed add and drop beams  62  and  64 , respectively (see  FIG. 6 ). Use of this additional grating, and the utilization of channels dropped by grating  280  (discussed above), are illustrated in the optical add/drop multiplexing system  300  of  FIG. 6 . The optical add/drop multiplexing system  300  of  FIG. 6  includes of a generalized optical add/drop multiplexing system made up of optical input/output base grating assemblies  290 , and optical input/output base grating assemblies  292  as described in the earlier systems. These base gratings are used to form both upward and downward vertex grating cascades  350  and  355 , respectively. Each of the vertex grating cascades  350  and  355  consist of switchable pixellated gratings  330  and fixed (non-switchable) vertex gratings  340 . 
     The upper vertex grating branch  350  together with corresponding base gratings illustrates how the optical add/drop multiplexing system  250  can be extended to additional input/output, add, and drop beams or ports. The optical add/drop multiplexing and switching system  300  combines two such extended systems and overlaps functionality of the base gratings in input/output assemblies  310 . In assemblies  310 , signals may be input and output to the upper branch  350  from the right beam  313 , and to the lower branch  355  from the left beam  314 . By making the base gratings  312  switchable, coupling between the upper and lower branches is readily obtained by switching off (clearing) grating  312 . In so doing, channels in beams  313  and  314  are coupled, and the channels in the corresponding upper and lower branch beams are coupled. A similar functionality with reduced ports can be obtained by simply omitting assemblies  310  altogether. The optical add/drop multiplexing system  300  provides for selective exchange of specific channels among multiple bi-directional ports. 
     The optical add/drop multiplexing systems  10 ,  110 ,  200 ,  250 , and  300  of the present invention can also take advantage of multiple cascading of dispersive gratings in order to increase dispersion and potentially reduce system size. This type of system variation of the present invention is illustrated in system  400  of  FIG. 7 . This high dispersion system  400  includes an input cascade  480 , and output cascade  490 , and drop cascades  460  together with add cascade(s)  470 . System  400  is analogous to system  110  in which cascades  460 ,  470 ,  480 , and  490  replace pairs  160 ,  170 ,  180 , and  190 . In each cascade-for-pair replacement, each of the gratings of the original pair are broken into cascades of two or more gratings. The cascading of gratings in this manner provides for higher angular dispersion of the channels than would be attainable with single gratings. The larger dispersions can shrink the system size for the case of very closely spaced channels, as found in dense wavelength division multiplexed channels. When switched gratings are broken onto two or more cascaded gratings, only one of the cascade, typically the one with most dispersed channels) needs to be switchable. This is illustrated in  FIG. 7 . Multiple add and drop cascades  470  and  460  can be inserted in the system in any order, as discussed with system  200 . 
     Still a further embodiment of the present invention is the optical multiplexing/de-multiplexing system  500  of  FIG. 8 . In system  500  a grating pair assembly  180 , described earlier in  FIG. 4 , is used to spatially separate the wavelength division multiplexed components of input beam  40  into individual beams  510  propagating parallel to each other as illustrated in  FIG. 8 . Each of these beams can then be separately coupled into optical fibers or waveguides  42 ′ using lenses  514 . Optical multiplexing/de-multiplexing system  500  can be used in either the de-multiplexing mode described above; in the reverse direction as a multiplexer; or both simultaneously in a bi-directional multiplexing/de-multiplexing mode. 
     It should be noted that the demultiplexed beams  510  will contain some residual lateral dispersion. However, for many applications, this is not a limiting feature of this invention. 
     Alternatively in system  500  the lenses  514  and waveguides  42 ′ can be replaced by detectors, or by emitters such as lasers. In the latter case, the lasers would each be tuned or selected to emit signals at the wavelength corresponding to the channel in which they are used. Lenses  514  would still be used as required for coupling to the detectors or lasers. 
     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.