Patent Publication Number: US-2010129076-A1

Title: Method and apparatus for spectral band management

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to optical communication systems and components and, more particularly, to a method and apparatus for spectral band management. 
     2. Description of the Related Art 
     In a wavelength division multiplexing (WDM) optical communication system, information is carried by multiple channels, each channel having a unique wavelength. WDM allows transmission of data from different sources over the same fiber optic link simultaneously, since each data source is assigned a dedicated channel. The result is an optical communication link with an information-carrying capacity that increases with the number of wavelengths, or channels, incorporated into the WDM signal. In this way, WDM technology maximizes the use of an available fiber optic infrastructure; what would normally require multiple optic links or fibers instead requires only one. 
     As the demand for optical communication networks increases, it is desirable to increase transport efficiency of an optical fiber, i.e., the amount of information carried by the optical fiber. This can be accomplished by increasing the number of channels in a WDM signal carried by a fiber and/or by increasing the data signaling rate, i.e., the bit rate, of the WDM signal. 
     Channel spacing is the amount of bandwidth allotted to each channel in a WDM communications system, and is defined as the spacing between center wavelengths of adjacent optical channels. To increase the number of channels in a WDM signal, the channel spacing is decreased. For example, a fiber may carry a WDM signal with a channel spacing of 100 GHz and consisting of 10 wavelength channels. When the channel spacing of the WDM signal is reduced to 50 GHz, the same fiber may instead carry 20 channels. Thus, when transmitting an optical signal using a modulation format with higher spectral efficiency, a narrower bandwidth is required for each channel, and the channel spacing for a WDM signal can be decreased. 
     Different modulation formats for digital modulation of an optical carrier signal include return to zero (RZ), non-return to zero (NRZ), dual binary (DB), differential phase-shift keying (DPSK), quadrature phase-shift keying (QPSK), and binary phase-shift keying (BPSK), among others. For an optical carrier signal having a given bit rate, each modulation format can produce a different modulation bandwidth, where “modulation bandwidth” is defined as the peak width of a modulated signal at 50% of the peak height, i.e., full-width at half-maximum (FWHM). For example, a 10 Gigabit per second (Gpbs) DB signal occupies approximately one third as much bandwidth as a 10 Gbps signal that is formatted in NRZ, and, consequently, the modulation bandwidth of the 10 Gbps DB signal is approximately one third the bandwidth of the 10 Gbps NRZ signal. 
     Increasing the bit rate of a WDM signal can also improve the transport efficiency of a signal, since more data is transmitted over the same fiber per unit time. However, it is known that the modulation bandwidth of a modulated signal increases with bit rate. Thus, when the bit rate of a WDM signal is increased, the modulation bandwidth of each channel in the WDM signal broadens, which can require a wider channel spacing to ensure adequate isolation between adjacent channels. 
     In sum, the information-carrying capacity of an optical communications network can be improved without replacing or increasing the number of fibers in the optical communications network by decreasing channel spacing, increasing the bit rate, and/or changing the modulation format of in a WDM signal. 
     However, to convert an existing optical communications network to process WDM signals having a narrower channel spacing, a higher bit rate, and/or a different modulation format, a number of network components must be replaced, including lasers, wavelength lockers, and optical switches, among others. To avoid obsoleting existing optical network components that may still have significant useful service life, and to minimize the network downtime associated with such an overhaul, the network can instead be modified to transmit multiple heterogeneous optical signals. Thus, existing network hardware can transmit and receive channels in a WDM signal at one bit rate and modulation format, while newly installed network hardware can be selected to take advantage of higher speeds and/or different modulation formats, as described below. 
       FIG. 1A  illustrates a schematic representation of the available transmission spectrum  104  of an optical fiber used in an optical communication network. A graph is superimposed on available transmission spectrum  104  depicting the light intensity (I) distribution of a demultiplexed optical carrier signal  100  vs. horizontal position (X), where the optical carrier signal  100  includes a plurality of transmission bands  101 . The horizontal position of each band corresponds to a specific segment of available transmission spectrum  104 , and each of transmission bands  101  is populated by a wavelength channel  109 . Wavelength channels  109  each have substantially the same modulation bandwidth  102 , and transmission bands  101  are distributed on a uniform wavelength grid  105 , i.e., each of transmission bands  101  is separated from adjacent ands by channel spacing  103 , e.g., 50 GHz. Channel spacing  103  is selected to be larger than modulation bandwidth  102  to ensure that each of wavelength channels  109  is adequately isolated from each adjacent wavelength channel after demultiplexing. As shown, transmission bands  101  of optical carrier signal  100  do not occupy the entire available transmission spectrum  104  allocated for optical carrier signal  100 , leaving a region of excess capacity  108  of available transmission spectrum  104 . Thus optical carrier signal  100  can be expanded to include additional channels, as illustrated in  FIG. 1B . 
       FIG. 1B  illustrates a schematic representation of the light intensity distribution of an optical carrier signal  110  vs. horizontal position after being demultiplexed. Optical carrier signal  110  includes the plurality of transmission bands  101  from optical carrier signal  100  as well as additional bands  111 A,  111 B. To utilize excess capacity  108  of available transmission spectrum  104 , additional bands  111 A,  111 B are positioned on uniform wavelength grid  105  in the region of excess capacity  108 . As part of optical carrier signal  110 , additional channels  119 A,  119 B populate additional bands  111 A,  111 B as shown, and are transmitted and received over the same optical fiber as wavelength channels  109 , using components that have been added to the original optical network. For example, an optical network can be enhanced with additional nodes that transmit and receive additional channels  111 A,  111 B. Therefore, instead of installing an additional fiber ring for carrying the traffic contained in additional channels  119 A,  119 B, available transmission spectrum  104  of the original fiber is utilized. 
     Additional channels  119 A,  119 B transmit information at a higher bit rate than wavelength channels  109  and, thus, have a modulation bandwidth  112  that is wider than modulation bandwidth  102  of wavelength channels  109 . For example, wavelength channels  109  are 10 GHz DPSK signals and additional channels  119 A,  119 B are 40 GHz DPSK signals, while channel spacing  103  is 50 GHz. As shown in  FIG. 1B , channel spacing  103  is too narrow to accommodate additional channels  119 A,  119 B, thereby resulting in overlap therebetween. Such interference between wavelength channels is highly undesirable in an optical network, and a wider channel spacing is needed for optical carrier signal  110  to function properly. 
       FIG. 1C  illustrates a schematic representation of the light intensity distribution of an optical carrier signal  120  vs. horizontal position after being demultiplexed. Optical carrier signal  120  includes wavelength channels  109  and additional channels  119 A,  119 B from optical carrier signal  110 . In optical carrier signal  120 , wavelength channels  109  and additional channels  119 A,  119 B are each contained in one of widened bands  130 . As shown, widened bands  130  are distributed on a uniform wavelength grid  125 , which has a wider channel spacing  123  than channel spacing  103  of uniform wavelength grid  105  in  FIGS. 1A ,  1 B. Wider channel spacing  123  prevents interference between additional channels  119 A,  119 B. With wider channel spacing  123  of widened bands  130 , wavelength channels having a wider modulation bandwidth than wavelength channels  109  can be carried by optical carrier signal  120 . Therefore, additional channels  119 A,  119 B can be included in optical carrier signal  120  to utilize excess capacity in an optical fiber, such as excess capacity  108  in  FIG. 1A , and additional channels  119 A,  119 B can include wavelength channels having a higher bit rate and/or a different modulation format than wavelength channels  109 . 
     However, in order to uniformly distribute bands  101  and additional bands  111 A,  111 B on uniform wavelength grid  125  so that channels having different modulation bandwidths can be included in a single optical carrier signal, other portions of available transmission spectrum  104  are not efficiently used. Because modulation bandwidth  102  of wavelength channels  109  is substantially narrower than wider channel spacing  123 , widened bands  130  are larger than necessary to accommodate transmission of wavelength channels  109 . Consequently, bandwidth segments  129 , which are disposed between wavelength channels  101 , remain idle and are not utilized for transmitting optical signals. Thus, an optical network as known in the art can be configured with bands accommodating a heterogeneous collection of wavelength channels, i.e., a plurality of wavelength channels having different modulation bandwidths, but only in a manner that does not efficiently utilize all portions of the usable bandwidth of an optical fiber. 
     Accordingly, there is a need in the art for a method and apparatus for efficiently utilizing the available transmission bandwidth of an optical fiber when the fiber is used to carry wavelength channels having different modulation bandwidths. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention contemplate a method and apparatus for selectively switching bands in an optical carrier signal. A method for routing an optical signal, according to a first embodiment, comprises receiving an optical signal having a plurality of bands distributed over a transmission spectrum, directing a first band having a first width along a first optical path, and directing a second band having a second width along a second optical path, wherein the first width and the second width are different. A method for routing an optical signal, according to second embodiment, comprises receiving an optical signal having a plurality of transmission bands of different bandwidths distributed over a transmission spectrum and directing a group of the bands along a selected optical path, wherein widths of at least two bands in the group are different. 
     An optical device, according to an embodiment of the invention, comprises an input port for receiving an optical signal having a plurality of bands of different widths distributed over a transmission spectrum and a switch assembly configured to direct a first group of bands along a first optical path and a second group of transmission bands along a second optical path. The number of bands in the two groups may be different and the widths of the bands in the two groups may be different. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIGS. 1A-1C  illustrate schematic representations of the light intensity distribution of a demultiplexed optical carrier signals vs. horizontal position. 
         FIG. 2A  illustrates a schematic representation of the available transmission bandwidth of an optical fiber used in an optical communication network, according to an embodiment of the invention. 
         FIG. 2B  schematically illustrates the available transmission bandwidth of an optical fiber with a graph of the light intensity distribution of an optical carrier signal superimposed thereon, according to an embodiment of the invention. 
         FIG. 2C  schematically illustrates two resultant optical signals that are produced by selectively directing portions of an optical carrier signal along different optical paths, according to an embodiment of the invention. 
         FIG. 2D  schematically illustrates two resultant optical signals that are produced by selectively directing portions of an optical carrier signal along two different optical paths while broadcasting other portions of the optical carrier signal along both optical paths, according to an embodiment of the invention. 
         FIG. 3  schematically illustrates an optical network configured to transmit an optical carrier signal having a non-uniform wavelength grid, according to an embodiment of the invention. 
         FIG. 4  schematically illustrates a cross sectional view of an LC-based optical switch which may be incorporated into an optical switching device, according to an embodiment of the invention. 
         FIGS. 5A and 5B  schematically illustrate top plan and side views, respectively, of an LC-based optical switching device, in accordance with one embodiment of the invention. 
         FIG. 5C  schematically illustrates a cross-sectional view of an LC array taken at section line a-a, as indicated in  FIG. 5A . 
     
    
    
     For clarity, identical reference numbers have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     Embodiments of the invention contemplate a method and apparatus for selectively switching bands in an optical carrier signal. When an optical carrier signal is demultiplexed, the bands that make up the available transmission bandwidth of an optical fiber may be of non-uniform bandwidth and arranged on a non-uniform wavelength grid so that portions of the optical fiber bandwidth are not left unused. An optical switching device, according to an embodiment of the invention, is used to arrange the wavelength grid for the demultiplexed optical carrier signal based on the bandwidth of each band, where each band may be populated by one or more wavelength channels. In one embodiment, the optical switching device includes a plurality of independently controllable pixel elements, or subpixels, that can be combined as necessary to form macropixels of the appropriate geometry to optically switch each band as desired, regardless of the bandwidth of each band or modulation bandwidth of the wavelength channels populating each band. 
       FIG. 2A  illustrates a schematic representation of the available transmission spectrum  204  of an optical fiber used in an optical communication network. A graph is superimposed on available transmission spectrum  204  depicting the light intensity (I) distribution of a demultiplexed optical carrier signal  200  vs. horizontal position (X), where the optical carrier signal  200  includes a plurality of bands  201 A-D,  202 A-B, and  203 A-C, where the horizontal position of each band corresponds to a specific segment of available transmission spectrum  204 . For purposes of illustration, each of bands  201 A-D,  202 A-B, and  203 A-C is depicted containing a wavelength channel. However, embodiments of the invention also contemplate an optical carrier signal with one or more bands being populated with no wavelength channel or multiple wavelength channels. 
     Because optical carrier signal  200  is demultiplexed, the bands contained therein, i.e., bands  201 A-D,  202 A-B, and  203 A-C, are spatially dispersed. As shown, bands  201 A-D are each populated with a wavelength channel having a relatively narrow modulation bandwidth  211 . Bands  201 A-D are positioned in region  1  of available transmission spectrum  204  with a correspondingly narrow channel spacing  251 . Similarly, bands  202 A-B are each populated with wavelength channels having a relatively wide modulation bandwidth  212 . Bands  202 A-B are positioned in region  2  of available transmission spectrum  204  with a correspondingly wide channel spacing  252 . Bands  203 A-C are each populated with a wavelength channel having a modulation bandwidth  213 , and are positioned in region  3  of available transmission spectrum  204  with an appropriately sized channel spacing  253 . 
     The differences between modulation bandwidths  211 ,  212 , and  213  may be due to the different bit rates and/or modulation formats of the wavelength channels populating bands  201 A-D,  202 A-B, and  203 A-C. For example, the wavelength channels contained in bands  202 A-B may be 40 Gbps DPSK signals while the wavelength channels contained in bands  203 A-C may be 10 Gbps DPSK signals, which have a substantially narrower modulation bandwidth. Alternatively, the wavelength channels populating bands  201 A-D may be transmitted in one modulation format, e.g., DB, and the wavelength channels populating bands  202 A-B may be transmitted in another modulation format, e.g., NRZ, while the wavelength channels contained in bands  203 A-C may be transmitted in a third modulation format, e.g., DPSK. One of skill in the art will appreciate that available transmission spectrum  204  is not made up of bands distributed across on a uniform wavelength grid, as is commonly known in the art. Rather, bands  201 A-D,  202 A-B, and  203 A-C, have different bandwidths as required, so that available transmission spectrum  204  is utilized most efficiently. 
     According to one embodiment of the invention, it is contemplated that bands  201 A-D,  202 A-B, and  203 A-C contained in optical carrier signal  200  may be arranged in a more general fashion, as illustrated in  FIG. 2B .  FIG. 2B  schematically illustrates available transmission spectrum  204  with a graph of the light intensity distribution of optical carrier signal  200  superimposed thereon, where the optical carrier signal  200  includes a plurality of bands  201 A-D,  202 A-B, and  203 A-C arranged in an arbitrary fashion. As shown, bands having similar bandwidth, such as bands  202 A-B, are not necessarily grouped together, and the wavelength grid on which bands  201 A-D,  202 A-B, and  203 A-C are arranged may be highly non-uniform, so that available transmission spectrum  204  is efficiently utilized. 
       FIG. 2C  schematically illustrates two resultant optical signals  291 ,  292  that are produced by selectively directing portions of optical carrier signal  200  along different optical paths, according to an embodiment of the invention. Resultant optical signal  291  includes a plurality of bands from optical carrier signal  200 , i.e., bands  201 A-B,  202 A-B, and  203 B. Resultant optical signal  292  includes the remainder of bands from optical carrier signal  200 , i.e., bands  201 C-D,  203 A, and  203 C. Resultant optical signals  291 ,  292  are selectively directed along different optical paths when optical carrier signal  200  is directed to an optical switching device, such as optical switching devices  341 ,  342 , described below in conjunction with  FIGS. 5A-C . As shown, the bands contained in either resultant optical signal  291  or  292  are not limited to a single bandwidth. In addition, said bands are not limited to a specific location in available transmission spectrum  204 , i.e., the bands contained in either resultant optical signal  291  or  292  need not be selected from a single contiguous portion of available transmission spectrum  204 . Further, each band contained in resultant optical signals  291 ,  292  may be populated by one or more wavelength channels. Resultant optical signal  291  may include bands that are populated with one or more wavelength channels to be routed to a different destination node than wavelength channels populating resultant optical signal  292 . Alternatively, resultant optical signal  291  may include bands populated by “dropped” wavelength channels, in which case resultant optical signal  291  is directed to a light dump. 
       FIG. 2D  schematically illustrates two resultant optical signals  293 ,  294  that are produced by selectively directing portions of optical carrier signal  200  along two different optical paths while broadcasting other portions of optical carrier signal  200  along both optical paths, according to an embodiment of the invention. Resultant optical signals  293 ,  294  are similar to resultant optical signals  291 ,  292 , in  FIG. 2C , except that a portion of the optical energy contained in bands  201 C and  203 A is directed along each optical path. Thus, each of resultant optical signals  293 ,  294  includes bands  201 C and  203 A. As depicted in  FIG. 2D , when bands  201 C and  203 A are broadcast along two optical paths, the intensity of wavelength channels populating bands  201 C and  203 A is reduced by approximately half, but can subsequently be amplified by means well known in the art. 
       FIG. 3  schematically illustrates an optical network  300  configured to transmit optical carrier signal  200  having a non-uniform wavelength grid, according to an embodiment of the invention. Optical network  300  includes optical rings  310 ,  320 , and  330 , which are optically linked via optical switching devices  341 ,  342 , as shown. Optical ring  310  includes transmitting node  311  and receiving nodes  312  and  313 . Optical ring  320  includes receiving node  321  and transmitting node  323 . Optical ring  330  includes receiving node  331  and transmitting node  332 . It is understood that optical components of optical communication networks are typically bidirectional in nature, and therefore may distribute optical signals in both directions, i.e., from a transmitting node, e.g., transmitting node  311 , to a receiving node, e.g., receiving node  331 , and vice-versa. For clarity, the operation of optical network  300  is described using unidirectional optical paths from the transmitting nodes to the receiving nodes. 
     Receiving nodes  312 ,  313 ,  321 , and  331  each include an optical demultiplexer  351  and one or more optical receivers  352 , as shown in  FIG. 3 , where each receiving node is configured with one optical receiver  352  for each optical wavelength channel to be received at that node. For example, receiving node  313  is configured to receive three bands and includes an optical demultiplexer  351  and three optical receivers  352 . Similarly, transmitting nodes  311 ,  323 , and  332  each include an optical multiplexer  353  and one or more optical transmitters  354 , one optical transmitter  354  for each bands to be transmitted from each respective node. 
     The transmitting and receiving nodes of optical network  300  are each configured to transmit or receive wavelength channels that each have a fixed optical wavelength and modulation format and are positioned in a band of available transmission spectrum  204 . However, because optical network  300  is configured with optical switching devices  341 ,  342 , the bands containing the wavelength channels that make up the optical carrier signal transmitted over optical network  300  do not have to be arranged along a uniform wavelength grid. Consequently, each transmitting node of optical network  300  may transmit wavelength channels via bands of different bandwidth. Thus, wavelength channels having different modulation formats and/or bit rates can be arranged to efficiently utilize available transmission spectrum  204 . For example, transmitting node  311  may be configured to transmit the wavelength channels populating bands  201 A-D, transmission node  332  may be configured to transmit the wavelength channels populating bands  202 A-B in  FIG. 2B , and transmission node  323  may be configured to transmit the wavelength channels populating bands  203 A-C in  FIG. 2B . As described above in conjunction with  FIGS. 2A and 2B , the bandwidth of bands  201 A-D may be different than the bandwidth of bands  202 A-B and of bands  203 A-C. Thus, each of optical transmitters  354  may be configured to transmit one wavelength channel having a unique center frequency and modulation bandwidth, where each channel is contained in a band of optical carrier signal  200  having the necessary bandwidth. One of skill in the art will appreciate that the configuration of each optical transmitter  354  in optical network  300  may be selected so that optical carrier signal  200  is divided into bands arranged to efficiently utilize the available transmission spectrum  204  of optical carrier signal  200 . As noted above,  FIGS. 2A and 2B  illustrate two such arrangements of bands  201 A-D,  202 A-B, and  203 A-C. 
     Similarly, each receiving node of optical network  300  may be configured to receive wavelength channels positioned in bands of available transmission spectrum  204  having different bandwidth than the bands configured for other receiving nodes in optical network  300 . For example, receiving node  321  may be configured to receive wavelength channels positioned in bands  201 A-B, receiving node  331  may be configured to receive wavelength channels positioned in bands  201 C-D, receiving node  312  may be configured to receive wavelength channels positioned in bands  202 A-B, and receiving node  313  may be configured to receive wavelength channels positioned in bands  203 A-C. 
     In operation, at each transmission node in optical network  300 , e.g., transmitting node  311 , one or more wavelength channels are transmitted and multiplexed into an optical carrier signal that is circulated over a corresponding optical ring, e.g., optical ring  310 . Optical switching devices  341 ,  342  receive circulated optical carrier signals as input signals, demultiplex each input signal into individual wavelength channels, sort the wavelength channels based on destination, and multiplex and transmit the sorted wavelength channels along the appropriate optical ring. 
     Optical switching devices  341 ,  342  are configured to sort bands of available transmission spectrum  204  that are arranged on a non-uniform wavelength grid, the advantages of optical network  300  over prior art optical networks are threefold. First, wavelength channels having different modulation bandwidths may be transmitted over optical network  300  simultaneously without the need for broadening the wavelength grid to accommodate channels with a wide modulation bandwidth. This allows transmitting and receiving nodes to be added to optical network  300  to efficiently take advantage of available transmission bandwidth, where the added nodes can operate at state-of-the-art bit rates and/or modulation formats. Thus existing node components can be left in place and wavelength channels operating at slower bit rates and/or different modulation formats can be used simultaneously with newly added wavelength channels. Second, by efficiently utilizing the available transmission bandwidth of an existing optical ring, the need for additional fiber rings to be installed may be avoided. Third, some embodiments of an optical switching device, such as those described below in conjunction with FIGS.  4  and  5 A- 5 C, can be reconfigured “on-the-fly.” That is, as network architecture is dynamically modified, for example one or more nodes are added, removed, or reconfigured to transmit and receive different wavelength channels, an optical network configured with optical switching devices as described herein may be dynamically reconfigured. In this way, wavelength channels of any desired modulation bandwidth can be managed and routed with no interruption to network operation due to mechanical modification or replacement of components in optical switching devices  341 ,  342 . This is because the optical beam deflector subpixels that make up the macropixels of an optical switching device can be aggregated into a new configuration using software only. Optical beam deflector subpixels and macropixels contained in one embodiment of an optical switching device are described below in conjunction with FIGS.  4  and  5 A-C. 
     In one embodiment, optical switching devices  341 ,  342  are similar in operation and organization to wavelength selective switches known in the art, and, thus, route light populating each band making up an optical carrier signal, i.e., the individual wavelength channels, from one node in an optical network to another node. For example, optical switching device  341  can demultiplex a wavelength channel transmitted from transmitting node  311  over optical ring  310 , and route the wavelength channel to optical ring  320  for receipt by the appropriate receiving node. In addition, optical switching devices  341 ,  342  route the wavelength channels in an optical carrier signal when the wavelength channels populate bands that are arranged along a non-uniform wavelength grid, as illustrated in  FIGS. 2A ,  2 B. To that end, optical switching devices  341 ,  342  are configured with an array of optical beam deflectors having a plurality of independently controllable pixel elements, or subpixels. The subpixels can be combined to form macropixels having the necessary geometry to direct demultiplexed bands of any desired bandwidth. Thus optical switching devices  341 ,  342  have configurable channel spacings that are not defined by a uniform wavelength grid and instead may be defined by the modulation bandwidth of each wavelength channel routed through optical switching devices  341 ,  342 . 
     Optical beam deflectors suitable for use as subpixels in optical switching devices  341 ,  342  include liquid crystals (LCs), microelectromechanical system (MEMS) micromirrors, and any other optical switching devices that can be miniaturized to the extent necessary to allow organization in a subpixel array, such as electro-optic and magneto-optic switches. By way of illustration, an LC-based optical switching device is described herein that can be incorporated into optical network  300  as illustrated in  FIG. 3 . While the LC-based optical switching device described herein uses liquid crystal polarization modulators in conjunction with a beam steering device to serve as optical beam deflectors, one skilled in the art will appreciate that reflective LC devices may also be used as optical beam deflectors. 
       FIG. 4  schematically illustrates a cross sectional view of an LC-based optical switch which may be incorporated into an optical switching device, e.g., optical switching device  341  or  342 , according to an embodiment of the invention. An LC optical switch  400 , as described herein, may serve as an optical beam deflector subpixel, and includes an LC assembly  401  and a beam steering unit  402 . In the example shown, LC assembly  401  includes two transparent plates  403 ,  404 , which are laminated together to form LC cavity  405 . LC cavity  405  contains an LC material that modulates, i.e., rotates, the polarization of an incident beam of linearly polarized light as a function of the potential difference applied across LC cavity  405 . LC assembly  401  also includes two transparent electrodes  406 ,  407 , which are configured to apply the potential difference across LC cavity  405 , thereby aligning the LCs in LC assembly  401  to be oriented in a first direction, a second direction or somewhere between these two directions. In this way, LC assembly  401  may modulate the polarization of incident light as desired between the s- and p-polarized states. Transparent electrodes  406 ,  407  may be patterned from indium-tin oxide (ITO) layers, as well as other transparent conductive materials. Beam steering unit  402  may be a birefringent beam displacer, such as a YV0 4  cube, or a Wollaston prism. Beam steering unit  402  is oriented to separate a linearly polarized beam  411  directed from LC assembly  401  into two polarized beams  409 A,  409 B, wherein each has a polarization state orthogonal to the other, i.e., p- and s-polarized. In the example shown in  FIG. 4 , polarized beam  409 A is p-polarized (denoted by the vertical line through the arrow representing polarized beam  409 A), and polarized beam  409 B is s-polarized (denoted by a dot). 
     In operation, LC optical switch  400  conditions a linearly polarized input beam  408  to form one or two polarized beams  409 A,  409 B, as shown in  FIG. 4 . LC optical switch  400  then directs polarized beam  409 A along optical path  410 A and polarized beam  409 B along optical path  410 B. For a switching operation, in which a beam is routed along one of two optical paths, LC optical switch  400  converts all of the optical energy of input beam  408  to either polarized beam  409 A or  409 B. For an attenuating operation, LC optical switch  400  converts a portion of the optical energy of input beam  408  into polarized beam  409 A and a portion into polarized beam  409 B, as required, where polarized beam  409 B is then directed to a light sink. For a broadcasting operation, LC optical switch  400  converts substantially equal portions of input beam  408  into polarized beam  409 A and polarized beam  409 B. 
     In the example illustrated in  FIG. 4 , input beam  408  is a beam of p-polarized light, denoted by a vertical line through the arrow representing input beam  408 . Input beam  408  passes through LC assembly  401  and is directed through the LC contained in LC cavity  405  to produce linearly polarized beam  411 . When input beam  408  passes through LC cavity  405 , the polarization state of the beam may be rotated 90°, left unchanged, i.e., rotated 0°, or modulated somewhere in between, depending on the molecular orientation of the LC material contained in LC cavity  405 . Therefore, linearly polarized beam  411  may contain an s-polarized component and a p-polarized component. Beam steering unit  402  produces polarized beam  409 A from the p-polarized component of linearly polarized beam  411 , and polarized beam  409 B from the s-polarized component of linearly polarized beam  411 , as shown in  FIG. 4 . Beam steering unit  402  is oriented to direct polarized beam  409 A along optical path  410 A and polarized beam  409 B along optical path  410 B, where optical paths  410 A,  410 B are parallel optical paths separated by a displacement D. The magnitude of displacement D is determined by the geometry and orientation of beam steering unit  402 . 
       FIGS. 5A and 5B  schematically illustrate top plan and side views, respectively, of an LC-based optical switching device, in accordance with one embodiment of the invention. In the example illustrated in  FIGS. 5A and 5B , optical switching device  500  includes an optical input port  501 , a diffraction grating  502 , a lens  503 , an LC array  504 , a beam steering device  505 , and an output/loss port assembly  506 . 
     A WDM input signal, beam  510 , is optically coupled to diffraction grating  502  by optical input port  501 . Diffraction grating  502  demultiplexes beam  510  into a plurality of N wavelength channels λ 1 -λN, wherein each of wavelength channels λ 1 -λN is spatially separated from the other channels along a unique optical path, as shown in  FIG. 5A . In the example shown, the unique optical paths followed by wavelength channels λ 1 -λN are positioned in the same horizontal plane. Wavelength channels λ 1 -λN are optically coupled to LC array  504  by lens  503 , and each may have a unique channel spacing associated therewith. The spatial separation S between each wavelength channel is proportional to the channel spacing between each of wavelength channels λ 1 -λN. For example, the spatial separation S between two demultiplexed wavelength channels with a 100 GHz channel spacing is twice that for a 50 GHz channel spacing. As described above in conjunction with  FIGS. 2A ,  2 B, the channel spacing, and therefore the spatial separation S, between any two wavelength channels may be non-uniform when projected onto LC array  504 . 
     LC array  504  contains a plurality of LC macropixels  504 A- 504 N, each of which is positioned to correspond to one of wavelength channels λ 1 -λN. Each LC macropixel  504 A- 504 N of LC array  504  contains one or more LC subpixels that may be substantially similar in configuration and operation to LC assembly  401  in  FIG. 4 , where each of the subpixels is independently controlled, but can be aggregated with adjacent subpixels to function as a single macropixel. The organization of the LC subpixels and LC macropixels  504 A- 504 N in LC array  504  is described below in conjunction with  FIG. 5C . As wavelength channels λ 1 -λN pass through LC array  504 , the polarity of each wavelength channel is conditioned by the associated macropixel as desired. As described above in conjunction with  FIG. 4 , for a switching operation, the corresponding LC macropixel of LC array  504  converts all of the optical energy of the wavelength channel to either s-polarized or p-polarized. For an attenuating operation, the corresponding LC macropixel converts a portion of a wavelength channel to s-polarized and a portion to p-polarized, as required. Hence each wavelength channel, or a portion thereof, that is to be routed to output port  506 A is conditioned with a first polarization state, and each wavelength channel, or portion thereof, that is to be routed to output port  506 B is conditioned with a second polarization state that is orthogonal to the first. For example, wavelength channels bound for output port  506 A may be p-polarized and wavelength channels bound for output port  506 B may be s-polarized, or vice-versa. 
     After conditioning by LC array  504 , wavelength channels λ 1 -λN pass through beam steering device  505 , which is substantially similar to beam steering unit  402  of  FIG. 4 . Therefore, depending on the polarization state of each wavelength channel, beam steering device  505  steers each wavelength channel along an upper optical path, a lower optical path, or a portion along both, as depicted in  FIG. 5B . In this way, beam steering device  505  directs s-polarized beams to one output port and p-polarized beams to the other output port, i.e., wavelength channels λ 1   A -λN A  are directed to output port  506 A and wavelength channels λ 1   B -λN B  are directed to output port  506 B. It is noted that when optical switching device  500  performs an attenuation operation on wavelength channels λ 1 -λN, one of the output ports  506 A,  506 B may act as a loss port and the other as a conventional output port. 
       FIG. 5C  schematically illustrates a cross-sectional view of LC array  504  taken at section line a-a, as indicated in  FIG. 5A . LC array  504  includes an LC cavity  520  containing an LC material, a common horizontal electrode  521 , and an array  530  of vertical electrodes  530 A- 530 M, where M equals the number of LC subpixels in LC array  504 . Common horizontal electrode  521  is positioned behind LC cavity  520 , and may be substantially similar in make-up to transparent electrode  406 , described above in conjunction with  FIG. 4 . In the example shown in  FIG. 5B , common horizontal electrode  521  serves as an electrode for all LC subpixels  504 A- 504 M (shaded regions) of LC array  504 . Array  530  of vertical electrodes  530 A- 530 M is adjacent LC cavity  520  and opposite common horizontal electrode  521 . Vertical electrodes  530 A- 530 M are electrically isolated from each other by a gap, and each vertical electrode serves as the second electrode for an LC subpixel of LC array  504 , similar to transparent electrode  407  in  FIG. 4 . Thus, each LC subpixel  504 A- 504 M is defined by a region of LC cavity  520  located between common horizontal electrode  521  and one of the vertical electrodes of array  530 , and can be independently controlled based on the voltage applied to the appropriate vertical electrode. For example, LC macropixel  504 A is the shaded region in  FIG. 5B  corresponding to the portion of LC cavity  520  that is between common horizontal electrode  521  and vertical electrode  530 A. 
     As noted above in conjunction with  FIG. 5A , each of LC macropixels  504 A-N of LC array  504  is made up of one or more subpixels, where the number of subpixels aggregated together to operate as a single macropixel is based on the channel spacing of each wavelength channel directed onto LC array  504 , i.e., wavelength channels λ 1 -λN. Further, each of LC macropixels  504 A-N is positioned to spatially correspond to the requisite wavelength channel. Thus, the wavelength channels contained in a WDM input signal, i.e., beam  510 , may be arranged in an arbitrary fashion and are not required to be distributed along a uniform wavelength grid. For example, LC macropixel  504 A may include five LC subpixels while adjacent LC macropixel  504 B may only include a single LC subpixel, etc. 
     One of skill in the art will appreciate that in lieu of the transmissive, polarization-based optical beam deflectors described above, reflective optical beam deflectors may be used as part of an optical switching device, as described herein. For example, because a MEMS micromirror array consists of a large number of individually controllable pixel elements, such an array is also contemplated as a reconfigurable array of optical beam deflectors. It is understood that embodiments of the invention are not limited to configurations of optical switching device that rely on MEMS micromirror arrays or LC arrays. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.