Patent Publication Number: US-6990268-B2

Title: Optical wavelength cross connect architectures using wavelength routing elements

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 10/093,844, entitled “OPTICAL WAVELENGTH CROSS CONNECT ARCHITECTURES USING WAVELENGTH ROUTING ELEMENTS,” filed Mar. 8, 2002 by Edward J. Bortolini el al., the entire disclosure of which is herein incorporated by reference for all purposes. This application is also related to the following commonly assigned applications, the entire disclosure of each of which is also herein incorporated by reference for all purposes: U.S. patent application Ser. No. 10/093,843 entitled “METHODS FOR PERFORMING IN-SERVICE UPGRADES OF OPTICAL WAVELENGTH CROSS CONNECTS,” filed Mar. 8, 2002 by Edward J. Bortolini; U.S. patent application Ser. No. 10/126,189, entitled “MULTI-CITY DWDM WAVELENGTH LINK ARCHITECTURES AND METHODS FOR UPGRADING,” filed Apr. 19, 2002 by S. Christopher Alaimo et al.; and U.S. patent application Ser. No. 10/150,810, entitled “BIDIRECTIONAL WAVELENGTH CROSS-CONNECT ARCHITECTURES USING WAVELENGTH ROUTING ELEMENTS,” filed May 17, 2002 by Edward J. Bortolini et al. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates generally to fiber-optic communications. This application relates more specifically to optical wavelength cross-connect architectures used in fiber-optics applications. 
     The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. 
     The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future. 
     In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronics—typically an electronic SONET/SDH system. However, multi-wavelength systems generally require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology. The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called “wavelength routing networks” or “optical transport networks” (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. 
     Optical wavelength cross connects are configured generally to redirect the individual optical channels on a plurality of input optical fibers to a plurality of output optical fibers. Each incoming channel may be directed to any of the output optical fibers depending on a state of the cross connect. Thus, where there are P input fibers and Q output fibers, the optical wavelength cross connect between them may be considered to be a “PN×QN optical switch.” Sometimes herein, the terminology “P×Q optical wavelength cross connect” is used to refer to such a cross connect by referring to the numbers of input and output optical fibers, each of which is understood to have the capacity for carrying N channels. As such the “P×Q optical wavelength cross connect” terminology may be considered to be a shorthand for describing a arbitrarily configurable PN×QN optical device. 
       FIG. 1  provides an example of a prior-art 4×4 optical wavelength cross connect  100  for a DWDM system carrying N individual wavelength channels. Each of the N channels on the four input signals  104  may be redistributed in accordance with a state of the cross connect  100  among the four output signals  116 . The cross connect  100  functions by splitting each of the input signals  104 ( i ) with an optical demultiplexer  108 ( i ) into N signals  120 (1 . . . N, i) that carry only a single wavelength channel λ 1 . . . N . From each of the optical demultiplexers  108 , the signal corresponding to a particular one of the  120 ( j , 1 . . . 4) is directed to a respective one of N 4×4 optical space switches  110 ( j ). Each optical space switch  110  may be configured as desired to redirect the four received signals  120  to four transmitted signals  124 . The transmitted signals  124  are transmitted to optical multiplexers  112  that recombine the reordered individual-wavelength signals onto the four output signals  116 . 
     The efficiency of an arrangement such as shown in  FIG. 1  is limited because it adopts a brute-force-type approach of demultiplexing the four incoming signals into their individual 4N components in order to reroute them. There is a general need in the art for more efficient optical wavelength cross-connect architectures without compromising complete routing flexibility. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the invention thus provide optical wavelength cross-connect architectures having such routing flexibility. In one set of embodiments, an optical wavelength cross connect is provided that receives a plurality of input optical signals that each have a plurality of spectral bands and transmits a plurality of output optical signals that each have one or more of the spectral bands. The optical wavelength cross connect comprises a first plurality of wavelength routing elements. Each of the wavelength routing elements is an optical component adapted for selectively routing wavelength components between a first optical signal and a plurality of second optical signals according to a configurable state. In general, a wavelength routing element may be configured to receive the first optical signal and route the spectral bands to the plurality of second optical signals or may be configured to receive the plurality of second optical signals and route the spectral bands to the first optical signal. As used within the optical wavelength cross connect, each of the first plurality of wavelength routing elements is disposed to receive at least one optical signal corresponding to one of the input optical signals. A mapping of the spectral bands comprised by the plurality of input optical signals to the plurality of output optical signals is determined by the states of the first plurality of wavelength routing elements. The wavelength routing elements may generally be configured in any fashion, and may include, for example, four-pass or two-pass wavelength routing elements. 
     The optical wavelength cross connect may include both working and protection fabrics, with at least one of the first plurality of wavelength routing elements comprised by the protection fabric and the remainder comprised by the working fabric. A plurality of working optical switches are configured to transmit the output optical signals by selecting either a corresponding working signal from the working fabric or a protection signal from the protection fabric. The protection fabric may also include an optical switch for selecting to which of the working optical switches any protection signal should be directed. Either or both of the working and protection fabrics may also include optical amplifiers disposed to amplify the working and/or protection signal prior to transmission to the working optical switches. 
     The signals received by the wavelength routing elements in the cross connect will generally be either the input signals directly or will be equivalents of the input signals resulting from splitting the input optical signals with optical splitters. Such optical splitters may also be disposed to direct equivalents of the input optical signals to the protection fabric. In some embodiments, the optical splitters provide equivalents of every input signal to every one of the first plurality of wavelength routing elements. The working and protection signals are thus defined directly by the respective states of the wavelength routing elements. In another embodiment, the optical splitters provide an equivalent of only one of the input optical signals to each of the wavelength routing elements on the working fabric but provide equivalents of all the input optical signals to the wavelength routing element(s) on the protection fabric. In such an embodiment, a second plurality of wavelength routing elements is disposed to receive each of the outputs from the first plurality of working-fabric wavelength routing elements. The outputs from the second plurality of wavelength routing elements thus correspond to the output optical signals. 
     The cross connect may also be equipped with additional optical equipment in alternative embodiments. For example, in one embodiment, a plurality of optical performance monitors are disposed to measure optical characteristics of various signals, such as of the input optical signals, the output optical signals, or any of the signals propagated through different points of the working and/or protection fabrics. In another embodiment, optical supervisory channels may be disposed to drop spectral bands from the input optical signals or to add spectral bands to the output optical signals. Such optical supervisory drops and adds may be useful where the wavelength routing elements are configured according to a predetermined wavelength grid and the dropped or added spectral bands to not correspond to position on the predetermined grid. 
     In other embodiments, channel-blocking capabilities of wavelength routing elements may be exploited in forming optical wavelength cross connects and in providing methods for distributing spectral bands from input optical signals onto output optical signals. In any of these embodiments, the input optical signals may be equal in number to the output optical signals, although this is not a requirement. In one embodiment, the spectral bands from each of the input optical signals are selectively distributed onto a plurality of intermediate optical signals. The intermediate optical signals are duplicated, with spectral bands being selected from the duplicated optical signals to provide the output optical signals. In selecting spectral bands from the duplicated optical signals, propagation of certain spectral bands may be blocked, such as by using an optical-channel-blocking embodiment of a wavelength routing element. 
     In another embodiment, spectral bands from each of the input signals are selectively distributed onto a plurality of intermediate optical signals, with propagation of certain spectral bands being blocked in such a distribution. Spectral bands from combinations of the intermediate optical signals are selected to provide the output optical signals. 
     In another embodiment, a first plurality of intermediate optical signals are produced that are equivalent to corresponding input optical signals. A second plurality of intermediate optical signals are also produced by selectively distributing spectral bands from each of the plurality of input optical signals. Combinations of the second plurality of intermediate optical signals are combined to provide a portion of the output optical signals. Spectral bands are selected from each of the first plurality of intermediate optical signals to provide a remainder of the output optical signals. The selection of spectral bands from each of the first plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands from the duplicated signals onto the remainder of the output optical signals. Similarly, producing the second plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands onto the second plurality of intermediate optical signals. 
     In yet another embodiment, a first plurality of intermediate optical signals is produced equivalent to a portion of the plurality of input signals. A second plurality of intermediate optical signals is produced by selectively distributing spectral bands from each of a remainder of the plurality of input optical signals. Combinations of the second plurality of intermediate optical signals are combined with selected spectral bands from the first plurality of intermediate optical signals to produce the output optical signals. In some instances, producing the second plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands onto the second plurality of intermediate optical signals. Similarly, in some instances, selecting spectral bands from the first plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands from the first plurality of intermediate optical signals. 
     In still further embodiments, arrays of smaller cross connects configured using wavelength routing elements may be arranged to provide larger cross-connect functions. For example, a K×K optical wavelength cross connect may be configured to distribute spectral bands comprised by K input optical signals among K output optical signals as an array of interconnected optical wavelength cross connects, at least one of which is smaller than K×K. The mapping of the spectral bands from the input ports to the output ports is determined by states of the interconnected optical wavelength cross connects. Examples of array configurations that may be used include crossbar arrays, Benes arrays, Benes arrays with spatial dilation, and Clos arrays. In one embodiment, a generalized Clos array is used in which the array comprises three layers. The first layer has a plurality m of k×n optical wavelength cross connects, the second layer has a plurality k of m×m optical wavelength cross connects, and the third layer has a plurality m of n×k optical wavelength cross connects. Each optical wavelength cross connect in the second layer receives an intermediate input signal from each of the optical wavelength cross connects included in the first layer and transmits an intermediate optical signal to each of the optical wavelength cross connects included in the third layer. In one embodiment, k=2n−1. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and is enclosed in parentheses to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components. 
         FIG. 1  is a schematic diagram illustrating a prior-art cross connect used in DWDM applications; 
         FIGS. 2A ,  2 B, and  2 C are schematic top, side, and end views, respectively, of an optical wavelength routing element used in certain embodiments of the invention; 
         FIGS. 3A and 3B  are schematic top and side views, respectively, of an optical wavelength routing element used in certain embodiments of the invention; 
         FIG. 4  is a schematic top view of an optical routing element according to a third embodiment of the invention; 
         FIGS. 5A–5D  are schematic diagrams showing examples of P×1 cross-connect building blocks that include wavelength routing elements in accordance with embodiments of the invention; 
         FIGS. 6A and 6B  are schematic diagrams showing examples of 2×3 and 3×2 cross-connect building blocks that include wavelength routing elements in accordance with embodiments of the invention; 
         FIGS. 6C ,  6 D, and  6 E schematically summarize various categories of P×1 cross-connect building blocks that include wavelength routing elements in accordance with embodiments of the invention; 
         FIGS. 7A and 7B  are schematic diagrams showing optical wavelength cross-connect architectures using a broadcast-and-select arrangement according to embodiments of the invention; 
         FIG. 8A  is a flow diagram showing a method for performing an in-service upgrade of an optical wavelength cross-connect in accordance with an embodiment of the invention; 
         FIGS. 8B–8G  are schematic diagrams illustrating the configuration of an optical wavelength cross-connect at different stages of an in-service upgrade performed in accordance with the flow diagram of  FIG. 8A ; 
         FIGS. 9A and 9B  are schematic diagrams showing optical wavelength cross-connect architectures using a distribute-and-select arrangement according to embodiments of the invention; 
         FIG. 10  is a schematic diagram showing an optical wavelength cross-connect architecture in another embodiment of the invention; 
         FIG. 11  is a schematic diagram showing an optical wavelength cross-connect architecture in a further embodiment of the invention; and 
         FIGS. 12A–12E  are schematic diagrams illustrating alternative cross-connect architectures according to further embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Introduction 
     The following description sets forth embodiments of optical wavelength cross-connect architectures according to the invention. The general operation of such cross-connect architectures is to receive P input signals at respective input ports and output Q output signals at respective output ports. Each of the input and output signals comprises a plurality of spectral bands, with the cross connect capable of achieving a configuration that results in a desired redistribution of input spectral bands corresponding to equivalent channels among the output signals. Although the signals could each have a continuous spectrum, adjacent segments of which could be considered different spectral bands, it is generally contemplated that the spectrum of the incoming light will have a plurality of spaced bands, denoted as corresponding to channels  1 ,  2 ,  3 , . . . N. In some instances, the examples provided herein focus on symmetric cross connects in which P=Q, but this is not a requirement and embodiments of the invention may readily be adapted to nonsymmetric cross connects also. 
     The terms “input port” and “output port” are intended to have broad meanings. At the broadest, a port is defined by a point where light enters or leaves the system. For example, the input (or output) port could be the location of a light source (or detector) or the location of the downstream end of an input fiber (or the upstream end of an output fiber). In specific embodiments, the structure at the port location could include a fiber connector to receive the fiber, or could include the end of a fiber pigtail, the other end of which is connected to outside components. The optical character of the system also permits the input ports and output ports to be interchanged functionally, permitting, for example, a P×Q element to be used as a Q×P element. 
     The International Telecommunications Union (ITU) has defined a standard wavelength grid having a frequency band centered at 194,100 GHz, and another band at every 50 GHz interval around 194,100 GHz. This corresponds to a wavelength spacing of approximately 0.4 nm around a center wavelength of approximately 1550 nm, it being understood that the grid is uniform in frequency and only approximately uniform in wavelength. Embodiments of the invention are preferably designed for the ITU grid, but finer frequency intervals of 25 GHz and 100 GHz (corresponding to wavelength spacings of approximately 0.2 nm and 0.8 nm) are also of interest. 
     2. Wavelength Routing Element 
     Embodiments of the invention for an optical wavelength cross connect include one or more wavelength routing elements (“WRE”). As used herein, a “1×L WRE” refers to an optical device that receives multiplexed light at a WRE input port and redirects subsets of the spectral bands comprised by the multiplexed light to respective ones of a plurality L of WRE output ports. Such a 1×L WRE may be operated as an L×1 WRE by interchanging the functions of the input and output ports. Specifically, a plurality L of optical signals, each multiplexed according to the same wavelength grid are provided at the L output ports (functioning as input ports). A single optical signal is output at the input port (functioning as an output port) and includes spectral bands selected from the L multiplexed optical signals according to the wavelength grid. Thus, the single output optical signal has, at each position on the wavelength grid, no more than one spectral band received at the same position on the wavelength grid from the L multiplexed optical signals. Accordingly, reference herein to a WRE adapted for routing wavelength components “between” a first optical signal and a plurality of second optical signals is intended to include a WRE configured to operate as a 1×L WRE or a WRE configured to operate as an L×1 WRE. 
     Embodiments for the cross connects that use a WRE may generally use any configuration for routing subsets of a plurality of spectral bands that achieve these functions. In some instances, a particular WRE may be provided in a one-pass, two-pass, four-pass, or other configuration. Some examples of suitable WREs are described in detail below, and additional examples of WREs that may be comprised by certain embodiments are described in the copending, commonly assigned U.S. patent application, filed Nov. 16, 1999 and assigned Ser. No. 09/442,061 (“the &#39;061 application”), entitled “Wavelength Router,” by Robert T. Weverka et al., which is herein incorporated by reference in its entirety, including the Appendix, for all purposes. 
     In some embodiments, wavelength routing functions within the WRE may be performed optically with a free-space optical train disposed between the WRE input port and the WRE output ports, and a routing mechanism. The free-space optical train can include air-spaced elements or can be of generally monolithic construction. The optical train includes a dispersive element such as a diffraction grating. The routing mechanism includes one or more routing elements and cooperates with the other elements in the optical train to provide optical paths that couple desired subsets of the spectral bands to desired WRE output ports. The routing elements are disposed to intercept the different spectral bands after they have been spatially separated by their first encounter with the dispersive element. 
       FIGS. 2A ,  2 B, and  2 C are schematic top, side, and end views, respectively, of one embodiment of a 1×L (or, equivalently, L×1) WRE  210 . This embodiment may be considered to be a four-pass WRE. Its general functionality is to accept light having a plurality N of spectral bands at a WRE input port  212 , and to direct subsets of the spectral bands to desired ones of a plurality L of WRE output ports, designated  215 ( 1 ) . . .  215 (L). The output ports are shown in the end view of  FIG. 2C  as disposed along a line  217  that extends generally perpendicular to the top view of  FIG. 2A . Light entering the WRE  10  from WRE input port  212  forms a diverging beam  218 , which includes the different spectral bands. Beam  218  encounters a lens  220  that collimates the light and directs it to a reflective diffraction grating  225 . The grating  225  disperses the light so that collimated beams at different wavelengths are directed at different angles back towards the lens  220 . 
     Two such beams are shown explicitly and denoted  226  and  226 ′, the latter drawn in dashed lines. Since these collimated beams encounter the lens  220  at different angles, they are focused towards different points along a line  227  in a transverse plane extending in the plane of the top view of  FIG. 2A . The focused beams encounter respective ones of a plurality of retroreflectors, designated  230 ( 1 ) . . .  230 (N), located near the transverse plane. Various examples of micromirror configurations that may be used as part of the retroreflectors, among others, are described in the following copending, commonly assigned applications, each of which is herein incorporated by reference in its entirety for all purposes: U.S. patent application Ser. No. 09/898,988, entitled “SYSTEMS AND METHODS FOR OVERCOMING STICTION USING A LEVER,” filed Jul. 3, 2001 by Bevan Staple et al.; U.S. patent application Ser. No. 09/899,000, entitled “FREE-SPACE OPTICAL WAVELENGTH ROUTER BASED ON STEPWISE CONTROLLED TILTING MIRRORS,” filed Jul. 3, 2001 by Victor Buzzetta et al.; U.S. patent application Ser. No. 09/899,001, entitled “TWO-DIMENSIONAL FREE-SPACE OPTICAL WAVELENGTH ROUTER BASED ON STEPWISE CONTROLLED TILTING MIRRORS,” filed Jul. 3, 2001 by Victor Buzzetta; U.S. patent application Ser. No. 09/899,002, entitled “MEMS-BASED, NONCONTACTING, FREE-SPACE OPTICAL SWITCH,” filed Jul. 3, 2001 by Bevan Staple and Richard Roth; U.S. patent application Ser. No. 09/899,004, entitled “BISTABLE MICROMIRROR WITH CONTACTLESS STOPS,” filed Jul. 3, 2001 by Lilac Muller; U.S. patent application Ser. No. 09/899,014, entitled “METHODS AND APPARATUS FOR PROVIDING A MULTI-STOP MICROMIRROR,” filed Jul. 3, 2001 by David Paul Anderson; and U.S. patent application Ser. No. 09/941,998, entitled “MULTIMIRROR STACK FOR VERTICAL INTEGRATION OF MEMS DEVICES IN TWO-POSITION RETROREFLECTORS,” filed Aug. 28, 2001 by Frederick Kent Copeland. 
     The beams are directed back, as diverging beams, to the lens  220  where they are collimated, and directed again to the grating  225 . On the second encounter with the grating  225 , the angular separation between the different beams is removed and they are directed back to the lens  220 , which focuses them. The retroreflectors  230  may be configured to send their intercepted beams along a reverse path displaced along respective lines  235 ( 1 ) . . .  235 (N) that extend generally parallel to line  217  in the plane of the side view of  FIG. 2B  and the end view of  FIG. 2C , thereby directing each beam to one or another of WRE output ports  215 . 
     Another embodiment of a WRE, designated  210 ′, is illustrated with schematic top and side views in  FIGS. 3A and 3B , respectively. This embodiment may be considered an unfolded version of the embodiment of  FIGS. 2A–2C  and operates as a two-pass WRE. Light entering the WRE  10 ′ from WRE input port  212  forms diverging beam  218 , which includes the different spectral bands. Beam  218  encounters a first lens  220   a , which collimates the light and directs it to a transmissive grating  225 ′. The grating  225 ′ disperses the light so that collimated beams at different wavelengths encounter a second lens  220   b , which focuses the beams. The focused beams are reflected by respective ones of plurality of retroreflectors  230 , which may also be configured as described above, as diverging beams, back to lens  220   b , which collimates them and directs them to grating  225 ′. On the second encounter, the grating  225 ′ removes the angular separation between the different beams, which are then focused in the plane of WRE output ports  215  by lens  220   a.    
     A third embodiment of a WRE, designated  210 ″, is illustrated with the schematic top view shown in  FIG. 4 . This embodiment is a further folded version of the embodiment of  FIGS. 2A–2C , shown as a solid glass embodiment that uses a concave reflector  240  in place of lens  220  of  FIGS. 2A–2C  or lenses  220   a  and  220   b  of  FIGS. 3A–3B . Light entering the WRE  210 ″ from input port  212  forms diverging beam  218 , which includes the different spectral bands. Beam  218  encounters concave reflector  240 , which collimates the light and directs it to reflective diffraction grating  225 , where it is dispersed so that collimated beams at different wavelengths are directed at different angles back towards concave reflector  240 . Two such beams are shown explicitly, one in solid lines and one in dashed lines. The beams then encounter retroreflectors  230  and proceed on a return path, encountering concave reflector  240 , reflective grating  225 ′, and concave reflector  240 , the final encounter with which focuses the beams to the desired WRE output ports. Again, the retroreflectors  230  may be configured as described above. 
     3. Cross-Connect Building Blocks 
     Architectures for large cross connects made in accordance with certain embodiments of the invention use L×1 optical elements that include one or more WREs. Such an element is referred to generically herein as an “L×1 WRE,” including arrangements that have more than one WRE, provided at least one WRE is comprised by the element. Thus, one example of an embodiment of an L×1 WRE that may be used in cross-connect architectures according to the invention is a single structure that has one input (output) port and L output (input) ports. Other embodiments of an L×1 WRE comprised of smaller WREs are illustrated in  FIGS. 5A–5D . 
     For example,  FIG. 5A  shows how a 4×1 WRE  510  may be configured with three 2×1 WREs. Each of the 2×1 WREs used in any of these embodiments may be configured as one of the WREs described in the &#39;061 application or may be configured according to another WRE design. The 4×1 WRE  510  accepts four input signals  502  and outputs a single output signal  515 . The four input signals  502  are received in pairs by two of the 2×1 WREs  504 . The outputs from the 2×1 WREs  504  are used as inputs to the third 2×1 WRE, which output the output signal  515 . 
     This arrangement of 2×1 WREs may thus be considered to be a tree arrangement. At each level of the tree, the number of distinct spectral bands across all optical signals at that level is reduced by the action of the 2×1 WREs  504  until, at the final level, only the desired spectral bands remain on the output signal  535 . The resulting 4×1 WRE  510  thus functions according to the definition provided above for the operation of a WRE by mapping selected spectral bands from each of the input signals  502  according to a wavelength grid. 
     The embodiment of  FIG. 5A  may also be used as a 1×4 WRE to perform the reverse mapping according to the wavelength grid by interchanging the functions of the input and output ports. In such an instance, spectral bands originating on the single input signal are progressively directed to the desired ones of the plurality of output signals by separating them with the 2×1 WREs  504  at each level of the tree. It is thus evident for a 1×4 WRE (and more generally for a 1×L WRE) that certain wavelength-grid positions of at least some of the output signals will be inactive by carrying no spectral bands. 
       FIG. 5B  shows an extension of the tree arrangement of 2×1 WREs  504  to an architecture that provides an 8×1 WRE  520 . Spectral bands from eight input signals  522  are routed according to a unique wavelength-grid assignment to a single output signal  525 . The eight input signals  522  are received in pairs by four 2×1 WREs  504 , and the four outputs from those 2×1 WREs are received by the 4×1 WRE  510  shown in  FIG. 5A . The resulting 8×1 WRE  520  functions according to the definition provided above for the operation of a WRE by mapping selected spectral bands from each of the input signals  522  according to a wavelength grid. It may also be used as a 1×8 WRE to perform the reverse mapping according to the wavelength grid by interchanging the functions of the input and output ports. 
     It is evident that larger WREs may be configured by including more layers in the tree. Adding still another layer of 2×1 WREs to the 8×1 WRE of  FIG. 5B  results in a 16×1 WRE. More generally, for a tree having p full layers of 2×1 WREs, the resulting element functions as a 2 P ×1 WRE, mapping spectral bands from  2   P  input signals according to a wavelength grid onto a single output port. Such an element may alternatively be used as a 1×2 P  WRE to perform the reverse mapping according to the wavelength grid by interchanging the functions of the input and output ports. 
     It is not necessary that every level of the tree be completely filled with 2×1 WREs. For example,  FIG. 5C  provides a schematic illustration of an embodiment similar to that of  FIG. 5B  except that two of the 2×1 WREs  504  at the widest level of the tree have been removed. Accordingly, this embodiment functions as a 6×1 WRE  530  that maps selected spectral bands from each of six input signals  532  according to a wavelength grid onto a single output signal  535 . Interchanging the functions of input and ports results in a reverse mapping according to the wavelength grid so that element  530  functions as a 1×6 WRE. It is noted by showing the component 4×1 WRE  510  with the dashed line that this embodiment may alternatively be considered as a configuration having a complete tree, but with different sizes of WREs on a given level. The 6×1 WRE  530  shown comprises a tree having a 4×1 WRE  510  and a 2×1 WRE  504  on its widest level, these WREs feeding into a 2×1 WRE  504  at the top level. 
     Similarly,  FIG. 5D  eliminates some 2×1 WREs  504  from two levels of the tree when compared with  FIG. 5B . The illustrated embodiment functions as a 5×1 WRE  540  by mapping selected spectral bands from each of five input signals  542  according to a wavelength grid onto a single output signal  545 . As for the other embodiments, element  540  may function as a 1×5 WRE by interchanging the functions of the input and output ports. Also, like the embodiment shown in  FIG. 5C , element  540  may be considered as having WREs of different sizes, specifically in this example of comprising a 4×1 WRE  510  and a 2×1 WRE  504 . 
     It is evident that various other combinations may be made according to the principles described with respect to  FIGS. 5A–5D  to produce L×1 and 1×L WREs for any value of L. 
       FIGS. 6A and 6B  provide examples respectively of 2×3 and 3×2 cross-connect building blocks. The illustrated embodiments use combinations of 2×1 WREs, which may be configured as described in the &#39;061 application or otherwise, and optical splitters. The embodiment shown in  FIG. 6A  functions as a 2×3 cross connect that maps spectral bands from two input optical signals  612  according to a wavelength grid onto three output signals  615 . Each of the input optical signals  612  encounters a 1:3 optical splitter  608  connected with three 2×1 WREs  604 . This arrangement thus provides a duplicate of both input signals  612  to each of the 2×1 WREs  604 , each of which is configured to select the desired spectral bands for its corresponding output signal  615 . Notably, this arrangement permits any combination of the spectral bands available from either of the input signals  612  to be included on any of the output signals  615 , subject to the constraint imposed be the wavelength grid. Thus, for example, the specific spectral band at λ 0  on the wavelength grid for, say, the first input signals  612 ( 1 ), may be included on one, two, or even all three of the output signals depending on the configuration of the 2×1 WREs  604 . It is even possible for all of the output signals  615  to include an identical set of selected spectral bands from the two input signals  612 . 
       FIG. 6B  provides an example of a 3×2 cross-connect building block that operates on similar principles. Each of the three input signals  622  encounters a 1:2 optical splitter which directs duplicates of the input signals according to the arrangement illustrated in the figure. The routing of the duplicates with the illustrated set of 2×1 WREs permits each of the output signals  625  to include any desired combination of spectral bands from the input signals  622 , subject to the wavelength-grid constraint and depending on the states of the 2×1 WREs. As for the arrangement shown in  FIG. 6A  for a 2×3 cross-connect building block, a specific spectral band from any of the input signals may be included on any (or all) of the output signals if desired. 
     The cascaded arrangements of WREs described with respect to  FIGS. 5A–5D  are themselves subsets of a more general classification of L×1 WREs that is summarized more comprehensively in  FIGS. 6C–6E . The cascaded arrangements are configured so that the output of one component WRE is in optical communication with the input of another component WRE. In addition to such cascaded arrangements, the tabulation shown in  FIGS. 6C–6E  also provides examples of “flat WRE” embodiments and optical-channel-blocking (“OChB WRE”) embodiments. In addition to the other capabilities of WREs, the OChB WREs have the ability to selectively block a spectral band present on an input from appearing at an output. This may be achieved, for example, by using component WREs in which an “off” state is provided, such as by providing another state to the retroreflectors  230  identified in  FIGS. 2A–4 . For example, where such retroreflectors comprise tiltable mirrors, three states may be provided for a 1×2 OChB WRE by using mirrors that may be fully tilted to the left, fully tilted to the right, or in an intermediate untilted position. Several specific examples of such OChB WRE configurations are enabled by the WREs with off positions described in copending, commonly assigned U.S. patent application Ser. No. 10/099,392, entitled “ONE-TO-M WAVELENGTH ROUTING ELEMENT,” filed Mar. 13, 2002 by Nicholas C. Cizek et al., the entire disclosure of which is herein incorporated by reference for all purposes. For the flat and OChB embodiments shown in  FIGS. 6C–6E  that use a plurality of component WREs, no component WRE has its output in optical communication with the input of another component WRE. Furthermore, in the flat WRE embodiments that use a plurality of component WREs, each of the component WREs has an unused port while in the OChB embodiments that use a plurality of component WREs, at least one of the component WREs has all of its ports used. In particular OChB embodiments, the minimum number of WREs possible of a particular L are used. Such embodiments have advantages of reduced insertion loss, greater reliability, and more modest space requirements than do some other embodiments. 
     The tabulation in  FIGS. 6C–6E  illustrates certain examples of 3×1, 4×1, 6×1, and 8×1 WREs that may be made using component 1×2, 1×3, or 1×4 WREs. Such component WREs in this illustration are configured as single structures having one input (output) port and two, three, or four output (input) ports respectively. The three columns of  FIG. 6C , denoted collectively by reference numeral  640 ( 1 ), use component 1×2 WREs, with each of columns  642 ( 1 ),  642 ( 2 ), and  642 ( 3 ) corresponding to cascaded, flat, and OChB configurations respectively. Similarly, the three columns of  FIG. 6D , denoted collectively by reference numeral  640 ( 2 ), use component 1×3 WREs, with each of columns  644 ( 1 ),  644 ( 2 ), and  644 ( 3 ) corresponding to cascaded, flat, and OChB configurations respectively. Further arrangements using 1×4 WREs are shown in the three columns of  FIG. 6E   640 ( 3 ), also for the cascaded  646 ( 1 ), flat  646 ( 2 ), and OChB  646 ( 3 ) configurations. The tabulation may clearly be extended both in terms of the size of the L×1 WREs and in terms of the component 1×M WREs for arbitrary M and L. Moreover, the tabulation shown is not exhaustive since other configurations that may be grouped according to the classifications are possible, even for those examples of specific L×1 WREs and component 1×M WREs already shown in  FIGS. 6C–6E . 
     4. K×K Optical Wavelength Cross Connects 
     a. Broadcast-and-Select Optical Wavelength Cross Connects 
     The building-block architectures illustrated in  FIGS. 6A and 6B  are examples of a more general class of cross-connect architectures described herein as “broadcast-and-select architectures.” A common feature of such architectures is that the input optical signals are duplicated with optical splitters, with a duplicate of each of the input signals being provided to a WRE, which may then be configured to select any of the desired spectral bands. There is therefore no constraint prohibiting a specific spectral band from any of the input signals from appearing on whatever number of output signals is desired. Such a capacity may be especially suitable for certain applications, including video applications among others. 
     1. High-Reliability Embodiments 
     The broadcast-and-select architectures may be equipped with a protection capability. An example of such an architecture for a 4×4 cross connect  700  is illustrated in  FIG. 7A , although it is evident how the principles may be used for a K×K cross connect of any size. In  FIG. 7A , the cross connect comprises working fabric, denoted  704 , and protection fabric, denoted  708 . The working fabric includes a number K of K×1 WREs  712 , each of which is configured to receive a duplicate of each of the input signals  702 . In a manner similar to that described with respect to  FIG. 6A , each of the K×1 WREs  712  may be configured to select whichever spectral bands are desired for the corresponding output signals  740 . 
     The protection fabric  708  also includes a K× 1  WRE  712 ′ configured to receive a duplicate of each of the input signals  702 . In the event of a failure in the system affecting the output from one of the WREs  712  included on the working fabric  704 , the protection WRE  712 ′ may be configured to substitute for the WRE  712  affected by the failure. Such substitution is accomplished with an arrangement of fiber switches. First, the protection fabric  708  comprises a 1×K fiber switch that receives the output of the protection WRE  712 ′ and directs it to one of K fiber switches provided as 2×1 fiber switches  716 . Each of these 2×1 fiber switches  716  may select between a signal received from an associated WRE  712  on the working fabric and a signal from the 1×K fiber switch on the protection fabric, i.e. corresponding to a signal from the protection WRE. 
     Thus, in normal operation, each of the 2×1 fiber switches  716  is configured to transmit the optical signals received from their respective WREs  712  comprised by the working fabric  704  as output signals  740 . In the event of a failure affecting one of the working WREs  712 , transmission of the otherwise affected output signal  740  is preserved by using the protection fabric: the protection WRE  712 ′ is configured to reproduce the signal otherwise produced by the affected working WRE  712 ; the 1×4 fiber switch comprised by the working fabric  708  is configured to transmit that signal to the 2×1 fiber switch  716  corresponding to the affected working WRE  712 ; and that 2×1 fiber switch  716  is configured to transmit the signal from the protection path, i.e. from the 1×4 fiber switch  717 , rather than the signal from the working path. 
     Duplicates of the input signals  702  are provided to all of the 4×1 WREs by 1:5 optical splitters  720  configured to encounter each of the input signals  702  and directing their outputs to the WREs. More generally, with a broadcast-and-select configuration for a K×K cross connect having a single protection WRE, the optical splitters used will be 1:(K+1) splitters. The strength of the duplicate signals will be reduced as a result of the power splitting performed by the splitters  720 . Accordingly, optical amplifiers  714  and  714 ′ are provided on the working and protection fabrics  704  and  708  to strengthen the output signals  740 . In the illustrated embodiment, the optical amplifiers  714  and  714 ′ are positioned respectively after the working and protection WREs  712  and  712 ′ along the optical paths followed by the optical signals, although other positions within the cross-connect architecture may alternatively be used. In one embodiment, the optical amplifiers  714  and  714 ′ comprise erbium-doped fiber amplifiers (“EDFAs”). 
     Further optional components of the cross connect  700  include a plurality of optical performance monitors (“OPMs”)  732 . The OPMs act as low-power taps into the optical paths that may be used for monitoring the optical channels. Such OPMs  732  are shown in the illustrated embodiment positioned in three distinct positions along the optical paths: before the optical splitters  720 ; after the optical amplifiers  714  on both the working and protection fabrics  704  and  708 ; and after the 2×1 fiber switches  716 . Each of these locations may be used for different monitoring purposes. The OPMs  732  positioned before the optical splitters  720  permit identification of signal faults before the active operation of the cross connect so that corrective action may be taken. Similarly, the OPMs  732  positioned after the 2×1 fiber switches  716  may be used to ensure no signal faults propagate along the output signals  740 . Including OPMs  732  on the working and protection fabrics  704  and  708  provides signal monitoring that permits isolation of each of the WREs  712  and  712 ′ in diagnosing the source of any faults that may be detected. Such flexibility is particularly useful in some instances, including during the performance of an in-service upgrade as described below. 
     Optical supervisory channel (“OSC”) components may also optionally be included on the input and output signals. Such components are useful in instances where the input signal  702  includes out-of-band signals, i.e. includes additional spectral bands distinct from the wavelength grid used by the WREs. An OSC drop  724  may thus be included on each of the input signals  702  to detect and remove any such out-of-band signals. The out-of-band signals may then be added back to the output signals  740  with OSC adds  728  on the output signals. 
     2. Ultrahigh-Reliability Embodiments 
     The reliability of the K×K cross connect may be increased by several orders of magnitude by including an additional WRE on the protection fabric. This is illustrated for an 8×8 cross connect  750  in  FIG. 7B . The basic operation of the cross connect  750  is the same as the cross connect  700  described with respect to  FIG. 7A . Working fabric  754  includes eight 8×1 WREs  762  configured to receive duplicates of input signals  752  from optical splitters  770 . The optical splitters  770  are 1:10 splitters to accommodate the eight WREs  762  comprised be the working fabric and two WREs  762 ′ comprised by the protection fabric  758 . Each of the signals output by the working WREs  762  encounters a 3×1 fiber switch  766  that receives signals from its associated working WRE  762  and may also receive signals from two 1×8 fiber switches  767  included on the protection fabric  758  and optically connected with each of the protection WREs  762 ′. The 3×1 fiber switches  766  transmit the appropriate signal as output signal  790 . Amplifiers  764  and  764 ′ may be included as shown on the working and protection fabrics  754  and  758  to accommodate the power losses from the optical splitters  770 , or may be positioned elsewhere in other embodiments. OPMs  782  may also be provided to monitor the performance of optical signals at different points in the cross-connect architecture, including on the working and protection fabrics  754  and  758  in some embodiments. OSC drops  774  and adds  778  may be provided for removing out-of-band signals from the input signals  752  and including them on the output signals  790 . 
     Operation of the 8×8 cross connect shown in  FIG. 7B  is substantially the same as operation of the 4×4 cross connect shown in  FIG. 7A . The large overall increase in reliability afforded by the two protection WREs  762 ′ may be understood by noting that there is statistically a mean time to failure for any element comprised by the cross connect. In the event of a failure, traffic is rerouted to one of the protection WREs  762 ′ for at a time period corresponding to a mean time to repair the failure. During that repair-time window, there is a small but finite probability of a second failure, which may be protected by routing traffic to the second protection WRE  762 ′. It is even possible to accommodate the very remote possibility of certain types of third failures under some circumstances where the working path suffered only a partial failure. Traffic from the third failure, if consistent with the maintained operation of the partially failed path, may be rerouted to the partially failed path. 
     It is evident from the description of  FIGS. 7A and 7B  how to arrange a K×K cross-connect architecture having F protection paths. Each of K WREs included on the working fabric and F WREs included on the protection fabric comprises a K×1 WRE. A total of K optical splitters are thus provided as 1:(K+F) splitters. The fiber switches provided on the protection fabric are 1×K fiber switches and the fiber switches transmitting the output signals are (F+1)×1 fiber switches. 
     3. In-Service Upgrades 
     The broadcast-and-select architectures described with respect to  FIGS. 7A and 7B  permit an existing K i     1   ×K j     1    architecture to be upgraded to a K i     2   ×K j     2    architecture without disrupting traffic using the cross connect. The illustration below focuses on the special case of symmetric cross connects where K i     1   =K j     1    and K i     2   =K j     2   , but such a limitation is not necessary and the upgrade may be performed more generally on asymmetric cross connects. The method for performing such an upgrade is shown schematically for the general case with the flow diagram of  FIG. 8A  and is shown at sequential stages in  FIGS. 8B–8G  for the specific case of upgrading a 2×2 cross connect (K i     1   =K j     1   =2) having a single protection path to a 4×4 cross connect (K i     2   =K j     2   =4). The following description thus refers simultaneously to the general method of  FIG. 8A  and the specific example in  FIGS. 8B–8G . For convenience of description, the example of  FIGS. 8B–8G  omits OSC components, amplifiers, and OPMs, although it is understood that such components may be included in the architecture in different embodiments. 
     At block  801  of  FIG. 8A , the method begins with a K i     1   ×K j     1    cross connect such as the 2×2 cross connect shown in  FIG. 8B . The 2×2 cross connect includes two 2×1 WREs on the working fabric and a 2×1 WRE  814 ′ optically connected with a 1×2 fiber switch  818  on the protection fabric. Optical splitters  812  encounter the input signals  810  to direct duplicates to the WREs  814  and  814 ′. The output signals  820  are emitted from 2×1 fiber switches  816  configured to transmit working or protection signals depending on the configuration of the cross connect. 
     At block  802  of  FIG. 8A , new working fabric is added to the cross-connect architecture to accommodate the new input and output ports. In upgrading the exemplary 2×2 cross connect to a 4×4 cross connect, the WREs on the working and protection fabrics will be replaced with 4×1 WREs. Thus in  FIG. 8C , the cross connect is shown with two 4×1 WREs  824  added to the working fabric and optically connected with two new 2×1 fiber switches  816  that will be used to transmit new output signals. 
     At block  803  of  FIG. 8A , the protection fabric is upgraded to the larger size. Thus, as shown in  FIG. 8D  for the exemplary upgrade to a 4×4 cross connect, the 2×1 WRE  814 ′ and 1×2 fiber switch  818  are replaced with a 4×1 WRE  824 ′ and a 1×4 fiber switch  828  respectively. Optical connections between the resulting protection fabric and other optical components of the system are completed. It is noted that such a four-channel protection fabric is capable of protecting traffic input from any number of input signals  810  up to four, including in particular the two input signals  810  being maintained during the upgrade. Also, while  FIG. 8D  illustrates simply upgrading the existing protection fabric to a larger size, in other embodiments the protection fabric is further upgraded by adding one or more additional protection paths with additional WREs and fiber switches, producing a protection fabric similar to that described with respect to  FIG. 7B . 
     At this point in the method, each component of the working fabric is upgraded sequentially by transferring the traffic that uses that component of the working fabric to the protection fabric and performing the upgrade of that component. This is illustrated with a loop in the flow diagram of  FIG. 8A , with traffic from the first component being transferred to the protection fabric at the first encounter of block  804 . After upgrading the bypassed component of the working fabric at block  805 , the cross connect appears as in  FIG. 8E , with the first 2×1 WRE  814  substituted with a 4×1 WRE  824 . 
     In addition,  FIG. 8E  illustrates how the operation of the optical splitters may be extended by adding 1:2 optical splitters as necessary at the outputs of the existing optical splitters. Such 1:2 optical splitters are added at points in the process when those outputs are not actively in use. Thus, in  FIG. 8E  1:2 optical splitters  822  are added to the first outputs of the 1:3 splitters  812  while the traffic for the first input signal  810 ( 1 ) is routed to the protection traffic. Optical connections are also made between the upgraded working WRE  824  and the (new) first outputs of the effectively expanded optical splitters. Optical splitters to be configured for encountering the new traffic to be added to the finally upgraded cross connect may be similarly configured as a combination of 1:3 and 1:2 splitters as shown. Alternatively, 1:5 splitters may be substituted. In the event that an additional protection channel is to be added, 1:6 splitters may be used instead to accommodate it. 
     After upgrading the bypassed component of the working fabric, the transmission of the bypassed traffic is switched back to the upgraded working fabric at block  806  of  FIG. 8A . A determination is made at block  807  whether all of the working fabric has been upgraded. If not, the next component of the working fabric is upgraded at block  808  with the cycle repeating until the entire working fabric has been upgraded. Thus,  FIG. 8F  shows a further intermediate illustration of the cross connect when the traffic for the second input signal  810 ( 2 ) is bypassed to the protection fabric. A similar upgrade is performed on the second component of the working fiber from a 2×1 WRE  814  to a 4×1 WRE  824  while the traffic is bypassed. Also, the 1:2 optical splitters  822  are added to the second outputs of the 1:3 optical splitters  812  and optically connected with the new 4×1 WRE  824 . 
     After all the working fabric has been upgraded, additional optical signals  810 ( 3 ) and  810 ( 4 ) may be provided to, and additional output signals  820 ( 3 ) and  820 ( 4 ) emitted from, the cross connect, as shown in  FIG. 8G . The cross connect now functions in every respect as a 4×4 broadcast-and-select cross-connect architecture. As shown in  FIG. 8G , the third output of all the 1:3 optical splitters  812  is connected with a 1:2 optical splitter  822 , resulting in an unused output. As alluded to above, the unused outputs may be optically connected with a second protection WRE on the protection fabric. Furthermore, for an architecture that uses only a single protection WRE, it is unnecessary to include the 1:2 optical splitters  822  on the third outputs of the 1:3 optical splitters  812  to include all optical connections with the working and protection fabrics; they are therefore omitted in an alternative embodiment. It is also noted that if the original K×K cross connect includes two protection paths, one of the protection paths may still function operationally as a protection path during the upgrade while the other protection path is used as a bypass while upgrading the working fabric. 
     It is noted that while FIGS.  8 A and  8 B– 8 G illustrate one embodiment for an in-service upgrade in which the protection fabric is upgraded before the working fabric is upgraded, such an order is not necessary. More generally, the upgrade of the protection fabric may more be performed at any point in the method, i.e. before, after, or even during the upgrade of the working fabric. With respect to the flow diagram of  FIG. 8A , the position of block  803  may thus be freely moved within the method without exceeding the scope of the invention. 
     b. Distribute-and-Select Optical Wavelength Cross Connects 
     In alternative embodiments, the optical wavelength cross connect is configured as a distribute-and-select cross connect. Illustrations of a 4×4 cross connect configured in this way are provided in  FIGS. 9A and 9B  and correspond in functionality to the 4×4 broadcast-and-select cross connect shown schematically in  FIG. 7A . To illustrate the basic operation of the distribute-and-select architecture,  FIG. 9A  provides an example of a configuration without protection. The working fabric in the distribute-and-select cross connect  900  includes a set of 1×4 WREs  910  and a set of 4×1 WREs  912 . In the illustrated embodiment, each of the 1×4 WREs  910  receives one of the input signals  902 . 
     The four outputs of each 1×4 WRE  910  are connected to an input port for each of the four 4×1 WREs  912 . In normal operation, each of the 1×4 WREs  910  acts to distribute the spectral bands from its associated input signal  902  to the particular 4×1 WREs  912  that correspond to the desired output signal  940  for those spectral bands. Each of the 4×1 WREs  912  thus receives the spectral bands to be included on its associated output signal  940  and acts to multiplex those spectral bands accordingly. 
     This distribute-and-select architecture may be supplemented by additional features in a manner similar to that described with respect to the broadcast-and-select architecture. For example, amplifiers  914  may be included to compensate for loss of signal strength as optical signals are propagated through the system. In one embodiment the amplifiers  914  comprise erbium-doped fiber amplifiers. The amplifiers  914  may be positioned as shown in the embodiment of  FIG. 9A , directly after the 4×1 WREs  912  along the optical paths followed in the cross connect, or may be positioned at other points in the system in other embodiments. 
     The cross connect  900  may also comprise OPMs  952  positioned at various points in the cross connect  900  to act as low-power taps for monitoring the optical channels. Such OPMs  952  are shown in the illustrated embodiment positioned in before each of the 1×4 WREs  910  and after the amplifiers  914  that follow the 4×1 WREs  912 , although other positions may be used alternatively. The OPMs  952  permit isolated signal monitoring of the WREs in diagnosing the source of any faults that may be detected. 
     In addition, OSC drops  944  and adds  948  may be provided respectively on the input and output signals  902  and  940  for removing and adding out-of-band signals. 
     In some embodiments, the distribute-and-select architecture may include a protection fabric, such as illustrated in  FIG. 9B . The operation of the protection-fabric embodiment shown in  FIG. 9B  may be explained more simply by labeling optical signals internal to the cross connect. This is done illustratively in  FIG. 9A , in which internal optical signals emanating from the 1×4 WREs  910  are labeled Oαβ and internal optical signals received by the 4×1 WREs  912  are labeled Iβ′α′. Signals that are directed from the kth 1×4 WRE  910 (k) to the kth 4×1 WRE  912 (k) are unlabeled. For each of the other “O” signals emanating from the 1×4 WREs  910 , α defines the originating 1×4 WRE  910 (α) and β defines the receiving 4×1 WRE  912 ; because the labeling convention limits β to 1, 2, or 3, the same value of β may be used in different Oαβ labels to refer to different receiving 4×1 WREs  912 . Similarly, for each of the other “I” signals received at the 4×1 WREs  912 , α′ defines the originating 1×4 WRE  910  and β′ defines the receiving 4×1 WRE  912 (β′); because the labeling convention limits α′ to 1, 2, or 3, the same value of α′ may be used in different Iβ′α′ labels to refer to different originating 1×4 WREs  910 . 
     This labeling convention for internal signals is also used in  FIG. 9B , in which the 4×4 distribute-and-select cross connect  900 ′ includes a protection fabric  908 . The working fabric is denoted  904  and has a general functionality that corresponds to that described with respect to  FIG. 9A . Rather than receive the input signals  902  directly, however, splitters  920  are disposed to intercept each of the input signals  902 , sending an equivalent of each to both the working fabric  904  and the protection fabric  908 . For input signal  902 (k), the equivalent signal directed to the protection fabric is denoted OPk. Similarly, 2×1 fiber switches  916  are provided to select signals from either the working fabric  904  or the protection fabric  908  and to direct the selected signals as output signals  940 . The signal received from the protection fabric  908  and corresponding to output signal  940 (k) is denoted IPk. 
     Associated with each 1×4 WRE  910  on the working fabric  904  are 1×2 fiber switches  932  that may be configured to direct an optical signal received from the 1×4 WRE  910  either to elsewhere on the working fabric  904  or to the protection fabric  908 . Similarly, associated with each 4×1 WRE  912  on the working fabric  904  are 2×1 fiber switches  934  that may be configured to direct an optical signal received either from elsewhere on the working fabric  904  or from the protection fabric  908  to the 4×1 WRE  912 . 
     As shown in  FIG. 9B , the protection fabric  908  includes a 1×4 WRE  910 ′ and a 4×1 WRE  912 ′ configured similarly to the corresponding elements comprised by the working fabric  904 . A selection of the equivalent optical signals OPk directed to the 1×4 WRE  910 ′ is determined by a state of a 4×1 fiber switch  924  disposed to intercept the equivalent optical signals OPk. Similarly, a 1×4 fiber switch  927  is disposed to determine how to direct the signal propagated by the 4×1 WRE  912 ′ onto signals IPk. Associated with the 1&#39;4 WRE  910 ′ are 1×4 fiber switches  926  that may be configured to redirect signals from the 1×4 WRE  910 ′ to different points in the working fabric  904 . Associated with the 4×1 WRE  912 ′ are 4×1 fiber switches  928  that may be configured to direct signals from the working fabric to the 4×1 WRE  912 ′. 
     As in  FIG. 9A , the internal optical signals on both the working and protection fabrics  904  and  908  are conventionally labeled Oαβ for signals emanating from the 1×4 WREs  910  and  910 ′, and Iβ′α′ for signals directed to the 4×1 WREs  912  and  912 ′. The same conventional notation is used as in  FIG. 9A , with the WREs  910 ′ and  912 ′ on the protection fabric  908  being denoted as the fifth WREs so that O 5 β and I 5 α′ refer to optical signals internal to the protection fabric  908 . The propagation of the internal optical signals to effect the operation of the cross connect is determined by the states of the 1×2 fiber switches  932 , the 2×1 fiber switches  934 , the 1×4 fiber switches  926 , and the 4×1 fiber switches  928 . For example, with the configuration of such switches as shown in  FIG. 9B , the cross connect  900 ′ does not use the protection fabric and operates identically to the cross connect  900  as shown in  FIG. 9A . For example, in both  FIGS. 9A and 9B , from the first 1×4 WRE  910 ( 1 ), signal O 11  is directed to signal I 21 , signal O 12  is directed to signal I 31 , signal O 13  is directed to signal I 41 , and the unlabeled signal is directed directly to the 4×1 WRE  912 ( 1 ). Similarly, in both  FIGS. 9A and 9B , to the first 4×1 WRE  912 ( 1 ), signal I 11  is received from signal O 21 , signal I 12  is received from signal O 31 , signal I 13  is received from signal O 41 , and the unlabeled signal is received directly from the 1×4 WRE  910 ( 1 ). It can be readily verified that all of the other internal working signals in the figures correspond. 
     In the event of a failure, traffic may be protected by changing the configuration of certain fiber switches and selecting the appropriate configurations for the protection WREs  910 ′ and  912 ′. For example, in the event of a failure of one of the first WREs  910 ( 1 ) or  912 ( 1 ), traffic that would use those WREs  910 ( 1 ) and  912 ( 1 ) may be rerouted to the protection fabric. This is done by: (1) setting the 1×4 fiber switch  924  to OP 1  so that traffic otherwise directed to the first working 1×4 WRE  910 ( 1 ) is instead directed to the protection 1×4 WRE  910 ′; (2) setting the 4×1 fiber switch  927  to IP 1  and the 2×1 fiber switch  916 ( 1 ) to IP 1  so that traffic otherwise received from the first working 4×1 WRE  912 ( 1 ) is instead received from the protection 4×1 WRE  912 ′; and (3) setting fiber switches  926 ,  928 ,  932 , and  934  so that the O 5 β signals will be directed as would otherwise be the O 1 β signals and that the I 5 α′ signals will be received as would otherwise be the I 1 α′ signals. Specifically, (3) is achieved by: setting fiber switch  932 ( 4 ) to direct O 21  to I 51 ; setting fiber switch  934 ( 4 ) to direct O 51  to I 21 ; setting fiber switch  932 ( 7 ) to direct O 31  to I 52 ; setting fiber switch  934 ( 7 ) to direct O 52  to I 31 ; setting fiber switch  932 ( 10 ) to direct O 41  to I 53 ; setting fiber switch  934 ( 10 ) to direct O 53  to I 41 ; setting fiber switch  926 ( 1 ) to direct O 51  to I 21 ; setting fiber switch  926 ( 2 ) to direct O 52  to I 31 ; setting fiber switch  926 ( 3 ) to direct O 53  to I 41 ; setting fiber switch  928 ( 1 ) to direct O 21  to I 51 ; setting fiber switch  928 ( 2 ) to direct O 31  to I 52 ; setting fiber switch  928 ( 3 ) to direct O 41  to I 53 ; and leaving the settings of the other fiber switches  932  and  934  as they are for working traffic. Similar settings may be used in the event it is necessary to reroute traffic to respond to a failure in a different part of the working fabric  904 . 
     While not shown explicitly in  FIG. 9B , the distribute-and-select architecture with protection may also include amplifiers on the working and/or protection fabrics. Such amplifiers may be disposed as desired to compensate for loss of signal strength as optical signals are propagated through the system. In particular, such losses may be greater with the protection fabric as shown in  FIG. 9B  than those in  FIG. 9A  because of the power splitting introduced by the optical splitters  920 . Also, while not explicitly shown, OPMs may additionally be included at various points in the cross connect to monitor the optical channels. Suitable positions include, among others: before the optical splitters  920  to identify signal faults before active operation of the cross connect  900 ′; after optical amplifiers positioned on the working and/or protection fabrics  904  and  908  to permit isolated signal monitoring of the WREs in diagnosing the source of detected faults; and after the 2×1 fiber switches  916  to permit identification of signal faults after active operation of the cross connect  900 ′. Additionally, OSC drops and adds may be provided respectively on the input and output signals  902  and  940  for removing and adding out-of-band signals. 
     It is evident that the basic architecture shown for a 4×4 cross connect in  FIG. 9B  may be extended to an arbitrary K×K cross connect that receives K input signals and emits K output signals and has F protection arrangements. The working fabric comprises a first set of K WREs each configured to operate as a 1×K WRE and a second set of K WREs each configured to operate as a K×1 WRE. The protection fabric comprises F similar pairs of 1×K WREs and K×1 WREs. A set of K optical splitters are disposed to be encountered by the input signals, each comprising a 1:(F+1) optical splitter and the K fiber switches transmitting the output signals comprise (F+1)×1 fiber switches. Each of the 1×2 fiber switches  932  is substituted with a 1×(F+1) fiber switch to redirect traffic to any of the protection arrangements and each of the 2×1 fiber switches  934  is substituted with an (F+1)×1 fiber switch to receive traffic from any of the protection arrangements. The fiber switches  926  and  928  associated with each protection arrangement are provided respectively as 1×K and K×1 fiber switches so that each protection arrangement may interchange traffic with any of the working components as necessary. 
     One advantage of the distribute-and-select architecture is that component failures are readily detected with a simple power detector placed on the output. This ability results from the fact that a failure can only act to remove a desired spectral band from one of the output signals, but cannot act to substitute the desired spectral band with an undesired spectral band. As an example, consider the failure of a single retroreflector in one of the WREs comprised by the working fabric. That failure will be manifested on the output signals as an absence of a desired spectral band. A further advantage to the distribute-and-select architecture is that the power splitting of the input signals is modest, even for very large cross connects, since they are used only to provide duplicate signals as needed for the protection fabric. 
     c. Cross-Connect Architectures Using Optical-Channel-Blocking Configurations 
     In some embodiments, the cross-connect architectures make use of optical-channel-blocking component WREs, which permit configurations in which spectral bands that appear at an input are blocked from propagation to an output of the component WRE. Specific examples of WRE arrangements that use component OChB WREs to effect higher-order WRE operations are shown in the third columns of  FIGS. 6C ,  6 D, and  6 E respectively for 2×1 OChB WREs, 3×1 OChB WREs, and 4×1 OChB WREs. In addition to the OChB WRE components, such arrangements may comprise one or more optical combiners. Inputs to the arrangement are provided to the plurality of component WREs, with outputs from each of the component WREs being combined by the one or more optical combiners. In some instances, it is convenient for all of the component WREs to accommodate the same number of inputs, i.e. for them all to be M×1 WREs, but this is not a requirement. For example, column  642 ( 3 ) of  FIG. 6C  shows WRE arrangements that use only 2×1 WREs, column  644 ( 3 ) of  FIG. 6D  shows WRE arrangements that use only 3×1 WREs, and column  646 ( 3 ) shows WRE arrangements that use only 4×1 WREs. In other instances, the WRE arrangements may use combinations of WREs having different numbers of input ports. Merely by way of example, a 9×1 optical-channel-blocking WRE arrangement could comprise a 2×1 WRE, a 3×1 WRE, and a 4×1 WRE. The optical combiners are used to combine the outputs from each of the component WREs. 
     One embodiment of a cross-connect architecture that uses an OChB WRE is illustrated in  FIG. 10 . In this example, a 4×4 cross-connect architecture  1000  is illustrated, although it will be evident that the principles used may be extended to architectures of other sizes. As shown, the architecture permits routing of spectral bands from a plurality of input optical signals  1002  onto a plurality of output optical signals  1040 , although it will be appreciated by those of skill in the art that the architecture  1000  may alternatively be used by propagating light in the opposite direction, thereby interchanging the roles of the input and output optical signals. The operation of the architecture  1000  is similar to the distribute-and-select architecture discussed above, but uses some of the channel-blocking features permitted by the use of the OChB WREs. 
     Thus, each of the input signals  1002  encounters a distribution arrangement  1004  configured to distribute their spectral bands among selection elements  1022 . While the distribute-and-select architecture described above uses 1×K WREs for the distribution elements, the architecture  1000  shown in  FIG. 10  uses a combination of smaller WREs and optical splitters to achieve a more limited form of distribution. For the 4×4 cross connect shown, each distribution element  1004  includes a 1×2 WRE  1006  with a pair of 1:2 optical splitters  1008 . Spectral bands comprised by a particular input signal  1002 (k) are thus distributed among two intermediate signals within the respective distribution element  1004 (k), with those intermediate signals being duplicated by the 1:2 optical splitters  1008 . The resulting signals are distributed among the selection elements  1022 . One consequence of the duplication of intermediate signals by the optical splitters  1008  is that substantially identical intermediate signals may be provided to more than one of the selection elements  1022 . 
     In one embodiment, the selection elements comprise optical-channel-blocking WRE arrangements that include at least one WRE with channel-blocking capability. As shown in  FIG. 10 , this may include a plurality of OChB WREs, with outputs of the plurality of such WREs being combined by an optical combiner. In the specific example of the 4×4 architecture  1000  shown in  FIG. 10 , each of the selection elements comprises a pair of 2×1 OChB WREs  1006 ′ with the outputs of the OChB WREs  1006 ′ combined by optical combiner  1008 ′. This structure corresponds to the configuration previously shown in the second row of column  642 ( 3 ) of  FIG. 6C . The inputs to the OChB WREs  1006 ′ are configured to receive signals output from the distribution elements  1004 . For each selection element  1022 , each input receives an intermediate signal from a different distribution element  1004 . In one embodiment, the selection elements  1022  may be grouped with distinct subsets of the selection elements  1022  receiving equivalent sets of intermediate signals. This is the case, for example, in the specific embodiment shown in  FIG. 10  in which selection elements  1022 ( 1 ) and  1022 ( 2 ) receive signals originating from the same set of optical splitters  1008 ; selection elements  1022 ( 3 ) and  1022 ( 4 ) similarly receive signals that originate from the same set of optical splitters. 
     The specific spectral bands comprised by the output signals  1040  are determined by configurations of the 1×2 WREs  1006  comprised by the distribution elements  1004  and the 2×1 WREs comprised by the selection elements  1022 . These elements may be configured so that any spectral band from any of the input signals  1002  may be directed to at least a desired one of the output signals  1040 . Similarly, when the architecture is used in direction reverse to that shown in  FIG. 10 , any spectral band from signals  1022  may be directed to a desired one of signals  1002 . 
     The principles described with respect to the architecture  1000  shown in FIG. may also be used for cross connects of other sizes. For example, an arbitrary K×K cross connect may be configured to function with a duplication level d. In such an embodiment, each of the K input signals encounters a distribution element that outputs K intermediate signals in d groups, i.e. so that sets of K/d intermediate signals are equivalent. This may be achieved in some embodiments with a distribution element that comprises a 1×d WRE that receives the respective input signal and outputs each of d signals to a 1:K/d optical splitter. Each of the K intermediate signals output from the distribution element is provided as an input to a different one of K selection elements that each comprise a WRE with the capability of blocking on optical channel. Each of the selection elements receives an intermediate signal from each of the distribution elements and extracts the desired spectral bands for propagation as one of the output signals depending on a configuration of the selection element. One structure that permits such functionality for each of the selection elements comprises a number d of K/d×1 WREs that receive the K intermediate signals. Outputs of the K/d WREs are combined by a d:1 optical combiner. In some embodiments, the cross connect may be configured so that a group of selection elements receive equivalent signals. This may be accomplished by providing the equivalent intermediate signals from the distribution elements to corresponding selection elements, resulting in a grouping of the K distribution elements into K/d groups that each include d distribution elements. The 4×4 architecture  1000  shown in  FIG. 10  corresponds to the case where K=4 and d=2. 
     In reverse operation, input optical signals are provided to a plurality K of 1×K OChB WREs. The spectral bands for each output optical signal are selected by a d×1 WRE that receives, at each of its d inputs, combinations of spectral bands from K/d of the 1×K OChB WREs. 
     In still other embodiments, these principles may also be used to construct a asymmetric cross connect. For a K 1 &#39;K 2  cross connect, a set of K 1  distribution elements may each output K 2  intermediate signals, such as by using an arrangement comprising a 1×d WRE in communication with a plurality d of 1:K 2 /d optical splitters. The intermediate signals are received by K 2  selection elements, each selection element receiving K 1  intermediate signals and comprising a WRE with channel-blocking capability. The desired operation of the selection elements may be achieved in one embodiment with each selection element having a plurality d of K 1 /d×1 WREs coupled with a d:1 optical combiner. 
     Another embodiment that makes use of WREs having optical-channel-blocking capability is shown in  FIG. 11 . Like the embodiment described with respect to  FIG. 10 , this embodiment uses two distinct sets of elements to achieve the routing functions. A specific illustration is provided for a 4×4 cross connect  1100 , although the principles may readily be applied to cross connects of other sizes, as will be evident to those of skill in the art. The first set of elements  1104  may be considered to be types of distribution elements, although they function differently than the distribution elements described previously. Each of these elements  1104  receives an input optical signal  1102  having a plurality of spectral bands and provides intermediate signals, at least some of which are equivalents to the input signal  1102 . This may be achieved by using elements that comprise an optical splitter  1108  that passes equivalents to the input signal; one of the equivalents is passed to one of the second set of elements, perhaps after further splitting by another optical splitter  1108 , and another of the equivalents is provided to an OChB WRE  1106 . 
     While each of the first set of elements  1104  performs the same type of distribution of spectral bands, two different structures are used for the second set of elements, depending on the origin of the intermediate signals received by those elements. Elements  1122  function as 4:1 optical combiners and combine the spectral bands received from the OChB WREs  1106  components of each of the first set of elements  1104 . The configurations of these OChB WREs  1106  thus determine which spectral bands are included on the output optical signals  1140 ( 1 ) and  1140 ( 2 ) associated with the 4:1 optical combiners  1122 . The 4:1 optical combiners  1122  are shown as comprising a cascaded arrangement of 2:1 optical combiners  1108 ′, but may more generally include any arrangement that provides the 4:1 optical combination. 
     The signals that are propagated from the first set of elements  1104  as equivalents to the input signals  1102  are received by elements configured as 4×1 OChB WREs  1124 . In one embodiment, the functionality of the 4×1 OChB WRE  1124  is achieved by using a pair of 2×1 OChB WREs  1106 ′ whose outputs are combined by a 1:2 optical combiner  1108 ′. This structure is the same as used for the selection elements  1022  in the embodiment of  FIG. 10  and corresponds to the OChB WRE structure shown in the second row of column  642 ( 3 ). These 4×1 OChB WREs  1124  thus receive as inputs equivalents to all of the input signals  1102 . The configurations of the component 2×1 OChB WREs for a particular 4×1 OChB WRE  1124  thus determine which spectral bands from the input signals  1102  are propagated as part of the respective output signals  1140 . 
     While the above description of the operation of the architecture  1100  has  20  looked at one direction for the propagation of light, it will be apparent to those of skill in the art that it may alternatively be used with light propagating in the opposite direction. Regardless of which direction light propagates in the system, it is possible to direct a spectral band from any of the input (output) signals  1102  ( 1140 ) to any of the output (input) signals  1140  ( 1102 ). 
     Also, while the above description has provided an example of this architecture for a 4×4 cross connect, the same principles may be used for differently sized cross connects. In such a configuration, a symmetric K×K cross connect receives K input optical signals at a first set of K elements that each output K intermediate optical signals. A portion, such as half, of the intermediate optical signals output from each of the first set of elements are equivalent to the respective input optical signals and the remainder carry spectral-band subsets of the respective input optical signals, depending on a state of a 1×K/2 OChB WRE comprised by the element. In some instances, the 1×K/2 OChB WRE may itself comprise an arrangement of smaller component OChB WREs. Each of the intermediate optical signals that is equivalent to one of the input optical signals is received by a K×1 OChB WRE, and each such K×1 OChB WRE receives an equivalent to each of the input optical signals. The K×1 OChB WRE may comprise an arrangement of smaller OChB WREs. Each of the intermediate optical signals that carry spectral-band subsets of the input optical signals is received by a K:1 optical combiner, and each such K:1 combiner receives an intermediate optical signal having a spectral band from each of the input optical signals. The K:1 optical combiner may comprise an arrangement of smaller optical combiners or may comprise a larger optical combiner with some unused ports. The number of K×1 OChB WREs and K:1 combiners is thus equal to K/2. 
     In reverse operation, such an architecture receives half of the input optical signals at 1:K optical splitters and the remainder of the input optical signals at 1×K OChB WREs. The components of each output signal are selected by combining outputs from each of the 1×K OChB WREs with spectral bands selected by a 1×K/2 OChB WRE from equivalents to the optical signals provided to the 1:K optical splitters. 
     There is also no requirement that a cross connect using the architecture illustrated in  FIG. 11  be symmetric. In instances where the principles are adopted for an asymmetric K 1 ×K 2  cross connect, each of the first set of elements receives one of K 1  input signals and outputs K 2 /2 equivalents to the respective input signal as well as K 2 /2 intermediate signals that include a subset of the spectral bands from the respective input signal. Each of the K 2 /2 equivalents is received by one of K 2 /2 elements that acts as a K 1 ×1 OChB WRE. Each of the other intermediate signals is received by one of K 2 /2 elements that acts as a K 1 :1 optical combiner. The output signals correspond collectively to the signals output by the K 1 :1 optical combiners and the K 1 ×1 OChB WREs. 
     d. Other Cross-Connect Architectures 
     Other cross-connect architectures that use at least one WRE are also within the scope of the invention. A number of such alternative architectures are provided below as illustrations of specific embodiments, but it will be appreciated that these illustrations are not intended to be limiting and that there are various other alternative embodiments that are also within the scope of the invention. 
     1. Crossbar Cross Connects 
       FIG. 12A  shows an example of a 4×4 crossbar cross connect  1210 . This 4×4 cross connect  1210  comprises a two-dimensional arrangement of 2×2 cross connects  1212 , each of which may be configured according to the broadcast-and-select or distribute-and-select architectures described above. The input signals  1218  are received by respective ones of the 2×2 cross connects  1212  distributed along a first dimension of the array. The output signals  1220  are emitted by respective ones of the cross connects along the second dimension of the array. 
     Within the array, one of the two output ports for each 2×2 cross connect  1212  is optically connected with an input port of the next 2×2 cross connect  1212  in the first of the two dimensions and the second of the two output ports is optically connected with an input port of the next 2×2 cross connect  1212  in the second of the two dimensions. Each of the edge 2×2 cross connects  1212  at the opposite side of the array from where the output signals  1220  are emitted has one unused input port  1214  since there is no 2×2 cross connect  1212  to couple with that input port  1214  in the first dimension. Similarly, each of the edge 2×2 cross connects  1212  at the opposite side of the array from where the input signals  1218  are received has one unused output port  1216  since there is no 2×2 cross connect  1212  to couple with that output port  1216  in the second dimension. 
     It is apparent how a more general K×K crossbar cross connect architecture may be provided with a K×K array of 2×2 cross connects configured in the same fashion as  FIG. 12A  but extended even further in the two dimensions of the array. Except for the edge elements where optical signals are received or transmitted, or have corresponding unused ports, each 2×2 element in the array receives a signal from an element next to it in both directions in the array and transmits a signal to an element next to it in both directions. Such a generalized crossbar cross connect still functions to distribute a plurality of spectral bands on the K input optical signals among the K output optical signals as desired. 
     2. Benes Cross Connect 
       FIG. 12B  provides an illustration of a 4×4 Benes cross connect  1230  configured to distribute spectral bands from multiplexed input optical signals  1238  among output optical signals  1240  as desired according to a configuration of the component 2×2 cross connects  1232 . In this embodiment, six 2×2 cross connects  1232  are provided and configured as illustrated. The Benes cross connect  1230  includes three pairs of 2×2 cross connects  1232 . The first pair receives the four input signals  1238  at its four input ports and the third pair transmits the four output signals  1240  at its four output ports. Each of the intermediate pair of 2×2 cross connects  1232  receives at its input ports signals from each of the first pair of 2×2 cross connects  1232  and transmits at its output ports signals to each of the third pair of 2×2 cross connects  1232 . It will be appreciated that the example of a 4×4 Benes cross connect is used for illustrative purposes only. More generally, the Benes configuration may be used to construct K×K cross connects from a plurality of smaller component cross connects, such as with 2×2 cross connects. 
     3. Benes Cross Connect With Spatial Dilation 
       FIG. 12C  provides an illustration of a 4×4 Benes cross connect with spatial dilation  1250 . As for the other embodiments, the cross connect  1250  distributes spectral bands from multiplexed input optical signals  1262  among output optical signals  1264  as desired according to a configuration of the cross connect  1250 . In this embodiment, two 4×4 cross connects  1256  are used in combination with four 2×1 WREs  1260 . Each of the four input signals  1262  encounters an optical splitter  1252  to direct a duplicate of each input signal to each of the 4×4 cross connects  1256 . Each of the 2×2 WREs  1260  is similarly configured to receive one of the outputs from each of the 4×4 cross connects  1256 . The spatial dilation of the architecture has the effect of overcoming the rearrangably nonblocking character of the Benes architecture. It thus allows for a strictly nonblocking arrangement without requiring rearrangements when switching new connections. It will be appreciated that the example of a 4×4 Benes cross connect with spatial dilation is used for illustrative purposes only. More generally, the Benes configuration with spatial dilation may be used to construct K×K cross connects by increasing the capacity of the intermediate cross connects  1256 . 
     4. Clos Cross Connect 
       FIG. 12D  provides an illustration of a 4×4 Clos cross connect  1270  that distributes spectral bands from multiplexed input optical signals  1278  among output optical signals  1279 . The Clos cross connect  1270  comprises three layers of cross connects. The first layer comprises two 2×3 cross connects  1272  that receive the input signals  1278  at their four input ports. The third layer comprises two 3×2 cross connects  1276  that transmit the output signals  1279  from their four output ports. The 2×3 and 3×2 cross connects  1272  and  1276  may be configured in one embodiment as described above with respect to  FIGS. 6A and 6B . The intermediate (second) layer comprises three 2×2 cross connects  1274 , with each such 2×2 cross connect  1274  being disposed to receive signals at its input ports from both of the first-layer 2×3 cross connects  1272  and to transmit signals from its output ports to both of the third-layer 3×2 cross connects  1279 . In certain configurations, the Clos cross connect  1270  effectively operates by using the first layer of 2×3 cross connects  1272  to distribute spectral bands among the 2×2 cross connects  1274  of the intermediate layer; from here, they are routed as desired to the 3×2 cross connects  1276  of the third layer for integration as desired onto the output signals  1279 . 
     The basic structure of the Clos cross connect may be generalized as shown in  FIG. 12E  to act as a K×K cross connect  1280  for distributing a plurality of spectral bands on K input signals  1288  among K output signals  1289 . As for the 4×4 Clos cross connect discussed with respect to  FIG. 12D , the generalized Clos cross connect  1280  comprises three layers of elemental cross connects. The first layer comprises a plurality m of n×k cross connects  1282  that receive the K input signals  1288 . In an embodiment where the number of input ports provided by the n×k cross connects is exactly equal to the number K of input signals, m≡K/n, although this is not a necessary requirement for an operation cross connect  1280 . Similarly, the third layer comprises a plurality m of k×n elemental cross connects that transmit the K output signals  1289 . The intermediate layer comprises a plurality k of m×m cross connects  1284  disposed so that each m×m cross connect  1284  receives signals from all of the first-layer n×k cross connects  1282  at its input ports and provides signals to all of the third-layer k×n cross connects  1286  from its output ports. For the generalized cross connect  1280  to operate without spectral bands blocking each other during their propagation through the system, k≧2n−1. Thus, in an embodiment where this nonblocking criterion is met exactly, k=2n−1. 
     A number of special cases of the Clos cross-connect configuration are explicitly noted. For example, the 4×4 Benes cross connect described with respect to  FIG. 12B  is equivalent to the configuration  1280  of  FIG. 12E  in the special case where n=m=k=2, and is therefore an example of a configuration in which the nonblocking criterion is not met exactly. The 4×4 Clos cross connect described with respect to  FIG. 12D  corresponds to nearly the same circumstances, but meets the nonblocking criterion exactly by having n=m=2 and k=3. Examples of larger cross connects that meet the nonblocking criterion exactly include an 8×8 cross connect in which n=2, k=3, and m=4, and a 16×16 cross connect in which n=2, k=3, and m=8. Still other examples of cross connects that may be formed are evident from the description above. 
     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. In particular, a number of examples have been provided illustrating discrete principles. Alternative embodiments may include elements in arrangements that use multiple of those principles. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.