Abstract:
A multicast optical switch uses a diffractive bulk optical element, which splits at least one input optical beam into sub-beams, which freely propagate in a medium towards an array of directors, such as MEMS switches, for directing the sub-beams to output ports. Freely propagating optical beams can cross each other without introducing mutual optical loss. The amount of crosstalk is limited by scattering in the optical medium, which can be made virtually non-existent. Therefore, the number of the crossover connections, and consequently the number of inputs and outputs of a multicast optical switch, can be increased substantially without a loss or a crosstalk penalty.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is a divisional of U.S. patent application Ser. No. 13/558,802 filed Jul. 26, 2012 which claims priority from U.S. Provisional Patent Application No. 61/512,459 filed Jul. 28, 2011 which is incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates to optical switching, and in particular to multicast optical switching in fiberoptic networks. 
     BACKGROUND OF THE INVENTION 
     In an optical communications network, an optical signal is modulated with digital information and transmitted over a length of optical fiber from a source location to a destination location. An optical cross-connect module allows switching of optical signals from one optical fiber to another. A multicasting optical switch allows one to switch optical signals from one optical fiber to not one but, simultaneously, to a plurality of optical fibers, or to switch optical signals from a plurality of input optical fibers to a plurality of output optical fibers, such that optical signals in any of the input optical fibers can be multicast into non-overlapping subsets of the plurality of the output optical fibers. 
     By way of example, referring to  FIG. 1A , a 8×8 multicast optical switch  100  of the prior art includes eight input fibers  102 , eight optical amplifiers  104 , eight 1:8 optical splitters  106 , eight 8×1 optical switches  108 , and eight channel filters  110  coupled to output fibers  112 . Each 1:8 splitter  106  connects an output of each amplifier  104  to an input of each 8×1 switch  108 . A crossover region  107  includes a plurality of optical fibers connecting each of the eight outputs of each 1:8 splitter  106  to an input of each 8×1 switch  108 . In operation, the amplifiers  104  boost multi-wavelength optical signals in the input fibers  102  to compensate for subsequent optical power splitting by the 1:8 splitters  106 . Multi-wavelength signals from each of the eight input fibers  102  are present at the eight inputs of each 8×1 switch  108 , which function to select multi-wavelength signals from only one of the input fibers  102 . Since the 8×1 switches  108  operate independently on each other, the optical signals from any of the input fibers  102  can be multicast into any subset of the channel filters  110 . The channel filters  110  select a channel of interest, that is, an optical signal at a particular center wavelength, to be outputted at the output fibers  112 . The amplifiers  104  and the channel filters  110  are optional, and are included in  FIG. 1A  by way of an example. If the channel filters  110  are not included, the entire multi-wavelength signals will be present in the output fibers  112 . Without the amplifiers  104 , the signal will appear attenuated at the output fibers  112 . 
     One drawback of the multicast optical switch  100  is complexity. The 1:8 splitters  106  and the 8×1 switches  108  are separate devices connected with a multitude of optical fibers in the crossover region  107 , which complicates assembly, increases outer dimensions, and heightens optical losses. Since the 1:8 splitters  106  and the 8×1 switches  108  can both be fabricated using planar lightwave circuit (PLC) technology, one can integrate these components together onto a single substrate to obtain a relatively compact and inexpensive device, as compared with separate splitter and switch components of the multicast optical switch  100 . At least two vendors—Enablence of Toronto, Canada; and Neophotonics of San Jose, USA—are offering such products. However, one challenge with the PLC implementation is the large number of waveguide crossovers involved. Referring now to  FIG. 1B , a waveguide crossover region  117  of a 8×16 PLC multicast optical switch is shown. The example waveguide layout in  FIG. 1B  illustrates the large number of the waveguide crossovers. Even in a simpler case of an 8×12 multicast switch, a single waveguide has to cross up to 83 other waveguides before exiting the PLC. Each waveguide crossing adds loss and creates a possibility for unwanted crosstalk. 
     Accordingly, it is an object of the invention to provide a less complex multicast optical switch, which would not require waveguide or fiber crossovers. 
     SUMMARY OF THE INVENTION 
     The present invention uses optical beams freely propagating in a bulk optical medium, such as vacuum, air, glass, etc., to make optical crossovers required in a multicast optical switch. Freely propagating optical beams can cross each other without introducing mutual optical loss. The amount of crosstalk is limited by scattering in the optical medium, which can be made very low. Therefore, the number of the crossover connections, and consequently the number of inputs and outputs of a multicast optical switch, can be greatly increased. A diffractive bulk optical element can be used to provide optical beam splitting. Diffractive bulk optic beam splitters for splitting a multi-wavelength optical beam into two, four, eight, and sixteen beams of similar optical power are presently commercially available. Another important advantage of a bulk optic diffractive beamsplitter is that a single bulk beamsplitter can be used to split multiple input optical beams. 
     In accordance with the invention, there is provided a multicast optical switch comprising: 
     a first input port for receiving a first optical beam; 
     a diffractive bulk optical element coupled to the first input port, for splitting the first optical beam impinging on the diffractive bulk optical element into first and second beam portions propagating in a bulk optical medium; 
     first and second directors for receiving the first and second portions, respectively, of the first optical beam; and 
     first and second output ports coupled to the first and the second directors, respectively, for outputting the first and the second portions of the first optical beam; 
     wherein the first and the second directors are configured for independently coupling the first and the second portions of the first optical beam, respectively, into the first and the second output ports, respectively. 
     The number of input and output optical ports can be easily scaled. According to the invention, there is provided a multicast optical switch further comprising M input ports for receiving M optical beams, N directors, and N output ports, wherein the first input port belongs to the M input ports, the first optical beam belongs to the M optical beams, the first and second directors belong to the N directors, and the first and second output ports belong to the N output ports, wherein M and N are integer numbers greater than 1; 
     wherein the diffractive bulk optical element is coupled to each of the M input ports, for splitting each of the M optical beams impinging on the diffractive bulk optical element into N beam portions propagating in the bulk optical medium, wherein the first and second portions of the first optical beam belong to the N beam portions; 
     wherein each of the N directors is configured for receiving a corresponding one of the N portions of each of the M optical beams; 
     wherein each of the N output ports is coupled to a corresponding one of the N directors and is configured for outputting a corresponding one of the N portions, respectively, of a selected one of the M optical beams; and 
     wherein each of the N directors is configured for switching the corresponding portion of the selected one of the M optical beams into the corresponding one of the N output ports. 
     In a preferred embodiment, the diffractive bulk optical element is configured to angularly disperse each of the M optical beams into the N beam portions. The N directors are laterally offset from each other. The multicast optical switch of this embodiment further includes an angle-to-offset optical element having a focal length, for coupling each of the angularly dispersed N beam portions to one of the laterally offset N directors. The angle-to-offset optical element, preferably a concave mirror, is disposed one focal length away from the diffractive bulk optical element, and one focal length away from the N directors. The directors can include micro-electro-mechanical system (MEMS) tiltable mirrors. 
     In accordance with another aspect of the invention, there is further provided a dual multicast optical switch comprising first and second optical switches described above, 
     wherein the concave mirrors of the angle-to-offset optical elements of the first and second optical switches comprise a same first concave mirror; 
     wherein the diffractive bulk optical elements of the first and second optical switches comprise a same first diffractive bulk optical element; and 
     wherein the MEMS mirror arrays of the first and second switches optical comprise a same first MEMS mirror array. 
     In accordance with yet another aspect of the invention, there is further provided a method for multi-casting an optical signal, comprising: 
     (a) splitting a first optical signal into first and second signal portions using a diffractive bulk optical element; 
     (b) causing the first and second signal portions to propagate in a bulk optical medium; and 
     (c) directing the first and second signal portions, respectively, propagated in step (b), to first and second output ports, respectively, using first and second directors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1A  is a block diagram of an 8×8 multicast optical switch of the prior art; 
         FIG. 1B  is a plan view of a crossover region of a PLC 8×16 multicast optical switch of the prior art; 
         FIGS. 2A and 2B  are block diagrams of embodiments of 1×2 and M×N multicast optical switches, respectively, according to the invention; 
         FIG. 2C  is a block diagram of an embodiment of the M×N multicast optical switch of  FIG. 2B , including an angle-to-offset (ATO) optical element; 
         FIGS. 3A and 3B  are side elevational and top views, respectively, of an embodiment of the M×N multicast optical switch of  FIG. 2C , including a concave mirror as the ATO optical element, and a MEMS mirror array; 
         FIG. 4  is a beam profile diagram showing longitudinal beam profiles of Gaussian beams propagating through the ATO optical element of  FIG. 2C ; 
         FIG. 5  is a flow chart of a method for calculating required focal lengths of the concave mirror and microlenses of the M×N multicast optical switch of  FIGS. 3A and 3B ; 
         FIG. 6A  is a side elevational view of a dual multicast optical switch with independent operation of the individual multicast optical switches; 
         FIG. 6B  is a side elevational view of a dual multicast optical switch with a ganged operation of the individual multicast optical switches; 
         FIGS. 7A and 7B  are three-dimensional views of diffractive bulk optical elements configured to angularly disperse an optical beam in a single and a double row, respectively, of the optical beam portions; 
         FIG. 8A  is a side elevational view of a multicast optical switch using a double-row diffractive bulk optical element; 
         FIGS. 8B and 8C  are side elevational views of a dual-row optical fiber array usable in the multicast optical switches of  FIGS. 6A ,  6 B, and  8 A; and 
         FIG. 9  is a flow chart of a method for multi-casting an optical signal according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIG. 2A , a multicast optical switch  200  of the invention includes a first input port  201  for receiving a first optical beam  204 , which includes a plurality of wavelength channels, a diffractive bulk optical element  206  coupled to the first input port  201 , for splitting the first optical beam  204  impinging on the diffractive bulk optical element  206  into first and second portions  211  and  212 , respectively, which propagate in a bulk optical medium  208 . Each of the first and second portions  211  and  212  includes a portion of each of the plurality of wavelength channels. First and second directors  221  and  222 , respectively, receive the first and second portions  211  and  212  of the first optical beam  204 . First and second output ports  231  and  232 , respectively, are coupled to the first and the second directors  221  and  222 , for outputting the first and second portions  211  and  212  of the first optical beam  204 . In operation, the first and the second directors independently couple the first and the second beam portions  211  and  212  into the first and the second output ports  231  and  232 . Arrows  227  and  228  indicate that the directors  221  and  222  couple the first and second portions  211  and  212  by angle tuning the portions  211  and  212 , so as to introduce adjustable attenuation into the output ports  231  and  232 , respectively. 
     By way of example, the “bulk optical medium”  208  can include vacuum, air or another gas, or glass or another dense but transparent material. The diffractive bulk optical element  206  divides the first optical beam  204  into the first and second beam portions  211  and  212 , sending a portion of the first optical beam  204  to each output ports  231  and  232 , substantially independently of wavelength(s) of the first optical beam  204 , which can include a multi-wavelength optical beam of a wavelength-division-multiplexed (WDM) signal. Such beam splitting diffractive optical elements are available in standard or custom designs from multiple suppliers including MEMS Optical, Inc. of Huntsville, Ala., USA, and Leister Process Technologies, Axetris Division, Kaegiswil, Switzerland. 
     Turning to  FIG. 2B  with further reference to  FIG. 2A , a multicast optical switch  240  is an M×N multiport version of the multicast optical switch  200  of  FIG. 2A . The multicast optical switch  240  of  FIG. 2B  further includes M input ports  241  for receiving M optical beams  244 , N directors  261 , and N output ports  271 . By comparing  FIGS. 2A and 2B , one can see that the first input port  201  is one of the M input ports  241 , the first optical beam  204  is one of the M optical beams  244 , the first and second directors  221  and  222  are two of the N directors  261 , and the first and second output ports  231  and  232  are two of the N output ports  271 . In the embodiment shown, the multicast optical switch  240  has four input ports  241  and four output ports  271 . Four ports were selected for ease of illustration. Generally, M and N can be integer numbers equal to or greater than two, of course within practical limits known to one of skill in the art. 
     The diffractive bulk optical element  206  is coupled to each of the M input ports  241 , for splitting each of the M optical beams  244 , impinging on the diffractive bulk optical element  206 , into N beam portions  251  propagating in the bulk optical medium  208 . The first and second portions  211  and  212  of the first optical beam of  FIG. 2A  are two of the N beam portions  251  of  FIG. 2B . Each of the N directors  261  is configured for receiving a corresponding one of the N portions  251  of each of the M optical beams  244 . Each of the N output ports  271  is coupled to a corresponding one of the N directors  261  and is configured for outputting a corresponding one of the N portions  251  of a selected one of the M optical beams  244 . Each of the N directors  261  is configured for switching the corresponding portion of the selected one of the M optical beams  244  into the corresponding one of the N output ports  271 . The directors  261  can include tiltable mirrors, for example MEMS mirrors, phased liquid crystal arrays, or any other suitable devices for controllably steering and/or displacing optical beams. 
     The N portions  251  of the M optical beams  244  freely propagate in the bulk optical medium  208 , to make the optical crossovers required in the multicast optical switch  240 . Any suitable optics can be used to direct the N portions  251  of the M optical beams  244 . Referring now to  FIG. 2C , a preferable optical configuration for the optical crossovers is shown. In a multicast optical switch  280  of  FIG. 2C , the diffractive bulk optical element  206  is configured to angularly disperse each of the M optical beams  244  into the N portions  251 . The N portions of only two of the M optical beams  244 , denoted at  251 A with solid lines, and at  251 B with dashed lines, are shown for clarity. Thus, the beam portions  251 A and  251 B are portions of two respective beams: one emitted from the first input port  201 ; and one emitted from a second input port  202 , respectively, of the plurality of input ports  241 . An angle-to-offset (ATO) optical element  281  couples each of the angularly dispersed N portions  251  to one of the N directors  261 , which are offset from each other in a vertical direction in  FIG. 2C . The ATO optical element  281  is disposed one focal length f away from the diffractive bulk optical element  206 , and one focal length f away from the N directors  261 . Although in  FIG. 2C  the ATO optical element  281  is shown as a lens having an optical axis  282 , other elements having magnifying power, such as concave mirrors, can be used as well. 
     As evidenced by its name, the function of the ATO optical element (lens)  281  is to laterally offset the angularly dispersed N portions  251 , so as to couple the N portions  251  to the corresponding N directors  261 . Still referring to  FIG. 2C , the angularly dispersed N portions  251 A of the beam emitted by the first input port  201 , shown with solid lines, are coupled to the N directors  261 . The angularly dispersed N portions  251 B of the beam emitted by the second input port  202 , shown with dashed lines, are coupled to the same N directors  261 , albeit at different angles of incidence. This occurs due to the ATO optical element  281  being disposed one focal length f away from the diffractive bulk optical element  206 , and one focal length f away from the N directors  261 . The function of the N directors  261  is to select the corresponding angularly dispersed portion  251  of one of the input optical beams  244  for coupling into a corresponding output port of the plurality of output ports  271 . An angularly tunable element, such as a MEMS mirror, can be used for this purpose. 
     Referring to  FIGS. 3A and 3B  with further reference to  FIG. 2C , a multicast optical switch  300  has a concave mirror  381  instead of the lens  281 , a reflective bulk diffractive optical element  206 A in place of the transmissive bulk diffractive optical element  206 , and an array of MEMS mirrors  361  in place of the directors  261 . Using the reflective elements  206 A,  361 , and  381  instead of the transmissive ones  206 ,  261 , and  281  generally results in a more compact device, although in principle either transmissive or reflective elements, or both, could be used. Conveniently, in the reflective multicast optical switch  300 , the bulk diffractive optical element  206 A and the MEMS array  361  can be disposed on a common planar carrier  302 . The concave mirror  381  has an optical axis  382 . 
     An input waveguide array  341  is coupled to the plurality of input ports  241 , and an output waveguide array  371  is coupled to the plurality of output ports  271 . The input and output waveguide arrays  341  and  371  include M input and N output waveguides  303  and  304 , respectively. Input and output microlens arrays  311  and  312 , respectively, include M and N microlenses, respectively. Each of the M and N microlenses of the input and output microlens arrays  311  and  312  is optically coupled to one of the M and N optical waveguides of the input and output waveguide arrays  341  and  371 , respectively. The function of the input and output microlens arrays  311  and  312  is to convert between spot sizes of the input and output waveguides  303  and  304 , on one hand, and spot sizes on the bulk diffractive optical element  206 A and the MEMS mirror array  361 , on the other. This will be considered in more detail below. 
     In operation, the input waveguides  303  of the input waveguide array  341  guide the M input beams  244 , which are collimated by the input microlens array  311 , split into the N beam portions  251 , get reflected by the concave mirror  381 , and impinge onto the MEMS mirror array  361 . The MEMS mirrors of the array  361  reflect the corresponding one of the N portions  251  of each of the M optical beams  244  at an adjustable angle, thereby causing the corresponding portion of each of the M optical beams  244  to bounce again off the concave mirror  381 , get focused by a microlens of the output microlens array  312 , and get coupled and into the corresponding one of the waveguides  304  of the output waveguide array  371 , exiting from the corresponding one of the N output ports  271 . In  FIG. 3B , only one of the N portions  251  of only one of the M input beams  244  is shown for clarity. The remaining N portions  251  of the shown input beam are not shown. The remaining N portions are angularly dispersed by the bulk diffractive optical element  206 A in a fan-like fashion, and are directed by the concave mirror  381  towards the other mirrors of the MEMS mirror array  361 . This can be gleaned by comparing  FIGS. 3B and 2C , which are drawn in the same plane of view (YZ plane). In  FIG. 2C , all of the N portions of two of the M input beams  244 , emanating from the to two input ports  201  and  202 , are shown at  251 A and  251 B, respectively. 
     The angles of tilt of the MEMS mirrors of the array  361  are controlled by a controller  399 . In one embodiment, the controller  399  is suitably configured for tilting at least one of the MEMS mirrors of the MEMS mirror array  361  at an adjustable angle to provide a controllable attenuation of the corresponding portion of the one of the M optical beams  244  coupled into the corresponding one of the N output ports  271 . The hardware used to implement the controller  399  can be implemented with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof. 
     Referring now to  FIG. 4  with further reference to  FIGS. 2C and 3A , a first longitudinal beam profile  401  of a beam portion of the N portions  251  is shown in a solid line. The input optical beams  244  are preferably Gaussian or near-Gaussian optical beams. The microlenses of the input microlens array  311  place the Gaussian beam waist on the bulk diffractive optical element  206 A. The first longitudinal beam profile  401  corresponds to the Gaussian waist radius ω 1DOE . Since the ATO element (lens)  281  in  FIG. 2C , corresponding to the concave mirror  381  in  FIGS. 3A and 3B , is disposed one focal length f away from the Gaussian waist, the N portions  251  will have Gaussian waist at the directors  261 , corresponding to the MEMS mirrors  361 , having a Gaussian waist radius ω 1MEMS . Increasing the Gaussian beam waist size at the bulk diffractive optical element  206  or  206 A from ω 1DOE  to ω 2DOE  will result in decreasing of the Gaussian beam waist at the directors  261  or the MEMS mirrors  361  from ω 1MEMS  to ω 2MEMS , as indicated by a second longitudinal beam profile  402  shown with dashed line. For zero-order Gaussian beams, the relationship between f, ω DOE  and ω MEMS  can be expressed as follows: 
     
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ω 
                         
                           
                             DOE 
                             ω 
                           
                           ⁢ 
                           MEMS 
                         
                       
                     
                     λ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     wherein λ is a center wavelength of the M input optical beams  244 . 
     Turning to  FIG. 5  with further reference to  FIGS. 2C ,  3 A, and  3 B, the focal length f of the ATO element  281  and/or concave mirror  381 , and the focal lengths f 1  and f 2  of the microlenses of the arrays  311  and  312 , respectively, can be calculated as follows. In steps  501  and  502 , spot radiae ω DOE  and ω MEMS  are determined by corresponding optical/geometrical parameters  503  and  504  of the bulk diffractive optical element  206 A and the MEMS array  361 , respectively. The spot radii ω DOE  and ω MEMS  do not have to be equal to each other; in fact, these are likely to be different from each other due to different optical/geometrical parameters  503  and  504  of the bulk diffractive optical element  206 A and the MEMS array  361 , respectively. From the spot radii ω DOE  and ω MEMS , the focal length of the ATO optical element can be computed in a step  505  using the Eq. (1) above. A distance s 1  from a proximal tip of the waveguides  303  of the input waveguide array  341  to the microlenses of the input microlens array  311  can be calculated in a step  506  from the equation 
     
       
         
           
             
               
                 
                   
                     s 
                     1 
                   
                   = 
                   
                     
                       f 
                       1 
                     
                     ( 
                     
                       1 
                       + 
                       
                         
                           
                             
                               ω 
                               DOE 
                               2 
                             
                             
                               ω 
                               
                                 wvg 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                               2 
                             
                           
                           - 
                           
                             
                               z 
                               
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 DOE 
                               
                               2 
                             
                             
                               f 
                               1 
                               2 
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     wherein z R DOE  is a Rayleigh length of the M optical beams  244  impinging on the diffractive bulk optical element  206 A, and ω wvg1  is a mode radius of a light mode propagating in the waveguides  303  of the input waveguide array  341 . Similarly, a distance s 2  from a proximal tip the waveguides  304  of the output waveguide array  371  to the microlenses of the output microlens array  312  can be calculated in a step  507  from the equation 
     
       
         
           
             
               
                 
                   
                     s 
                     2 
                   
                   = 
                   
                     
                       f 
                       2 
                     
                     ( 
                     
                       1 
                       + 
                       
                         
                           
                             
                               ω 
                               MEMS 
                               2 
                             
                             
                               ω 
                               
                                 wvg 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                               2 
                             
                           
                           - 
                           
                             
                               z 
                               
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 MEMS 
                               
                               2 
                             
                             
                               f 
                               2 
                               2 
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     wherein z R MEMS  is a Rayleigh length of the N portions  251  of each of the M optical beams  244 , reflected from the MEMS mirror array  361 , and ω wvg2  is a mode radius of a light mode propagating in the waveguides  304  of the output waveguide array  371 . The Rayleigh length z R  is defined as 
                 z   R     =       π   ⁢           ⁢     ω   2       λ       ,         
wherein λ is a central wavelength.
 
     Turning to  FIG. 6A  with further reference to  FIGS. 3A and 3B , a dual multicast optical switch  600  includes first and second optical switches  601  and  602 , respectively, each being equivalent to the multicast optical switch  300  of  FIGS. 3A and 3B . The concave mirrors  381  of the first and second optical switches  601  and  602  are replaced with a same concave mirror  681 . The diffractive bulk optical elements  206 A of the first and second switches  601  and  602  are replaced with a same diffractive bulk optical element  606 . The MEMS mirror arrays  361  of the first and second switches  601  and  602  are replaced with a same dual-row MEMS mirror array  661 . 
     As the name suggests, the dual-row MEMS mirror array  661  includes first and second rows  661 A and  661 B, respectively, of MEMS mirrors. The MEMS mirrors of the first and second rows  661 A and  661 B are the MEMS mirrors of the first and second switches  601  and  602 , respectively. The first and second switches  601  and  602  are operable independently of each other. Thus, two independent multicast optical switches  601  and  602  are provided with the common diffractive bulk optical element  606 , the MEMS array  661 , and the concave mirror  681 , providing considerable savings of cost and size. 
     Referring now to  FIG. 6B  with further reference to  FIG. 6A , a ganged dual multicast optical switch  620  is similar to the dual multicast optical switch  600  of  FIG. 6A , the difference being that the dual MEMS mirror array  661  is replaced with the single-row MEMS mirror array  361 . The two input waveguide arrays  341  are placed parallel to each other in the ganged dual multicast optical switch  620  of  FIG. 6B , ensuring that beam portions  651  and  652  diffracted by the diffractive bulk optical element  606  fall on same MEMS mirrors of the single-row MEMS mirror array  361 . The two multicast optical switches  601  and  602  operate in a ganged fashion, that is, with the same switch states, since they use the same MEMS mirrors of the MEMS array  361 . By way of example, if an k th  input of the first optical switch  601  is routed to its l th  output, an k th  input of the second optical switch  602  will be routed to its corresponding l th  output. By way of another example, if the k th  input of the first optical switch  601  is multicast to l th  to m th  outputs thereof, the k th  input of the second optical switch  602  will be simultaneously multicast to l th  to m th  outputs thereof. This function is useful in an Add/Drop configuration, where the first multicast optical switch  601  carries Add signals and the second multicast optical switch  602  carries Drop signals. It is common that Add and Drop signals will have the same configuration, since most communication links are bi-directional, sending and receiving information to and from the same direction, so the ganged operation is not a drawback in that application. The advantage is the savings in the MEMS chip  361 , requiring only one MEMS mirror array instead of two. Savings in packaging are also significant. 
     An important advantage of the free-space architecture of the multicast optical switches  200 ,  240 ,  280 ,  300 ,  600 , and  620  of  FIGS. 2A ,  2 B,  2 C,  3 A- 3 B,  6 A, and  6 B, respectively, is their scalability to large port counts. For example, the optical switches  240 ,  280 , and  300  of  FIGS. 2B ,  2 C, and  3 A- 3 B, respectively, can be easily scaled to the port counts with M≧4 and N≧8. In the dual switches  600  and  620  of  FIGS. 6A and 6B , respectively, the port counts of the individual multicast optical switches  601  and  602  can also be increased to M≧4 and N≧8. 
     One drawback of the large port counts is that the angular spread of the diffractive bulk optical elements  206 ,  206 A, and  606  can become too broad, so that optical aberrations and/or footprint of the outer diffracted beam portions become unacceptably large. The problem of aberrations and/or large footprint can be mitigated by constructing the diffractive bulk optical elements  206 ,  206 A, and  606  in such a way as to provide angular spread of the diffracted beam portions  251  and  651  not in one but in two dimensions. 
     The latter point is illustrated in  FIGS. 7A and 7B . A diffractive bulk optical element  706  of  FIG. 7B  is configured to angularly disperse each of the M optical beams  244  into two rows  702  and four columns  704  of eight portions  251  of each of the M optical beams  244 , whereas the diffractive bulk optical element  206 , shown in  FIG. 7A  for comparison, angularly disperses each of the M optical beams  244  into a single row  706  of eight portions  251  of the M optical beams  244 . Of course the two rows  702  and the four columns  704  are only examples, and other numbers N 1  and N 2  of the rows  72  and columns  704  are possible, wherein N=N 1 ·N 2 , for N≧4. The angular pattern of the spread portions  251  can be rectangular, hexagonal, etc, with the MEMS mirror arrays  361  or  661  matching that pattern. The diffractive bulk optical element  706  can also be made reflective, not transmissive as shown in  FIG. 7B  for illustration purposes. 
     Turning to  FIG. 8A  with further reference to  FIGS. 3A and 3B , a multicast optical switch  800  uses a reflective, two-dimensional, diffractive bulk optical element  806  similar to the diffractive bulk optical element  706  of  FIG. 7B  but working in reflection. Functionally, the multicast optical switch  800  is similar to the multicast optical switch  300  of  FIGS. 3A and 3B , with the two-row MEMS array  661  replacing the single-row MEMS array  361 . In operation, the N portions  251  of the M input optical beams  244  are spread by the diffractive bulk optical element  706  into two rows of N/2 portions  251  and are redirected by the concave mirror  381  to impinge on the first and second rows  661 A and  661 B of MEMS mirrors, respectively, which then select a portion of a particular input beam of the M input optical beams  244  for coupling into the corresponding output waveguide array(s)  371 . Since each MEMS mirror of the two-row MEMS mirror array  661  is associated with only one particular waveguide of the output waveguide arrays  371 , the functioning of the multicast optical switch  800  is substantially identical to the functioning of the multicast optical switch  300  of  FIGS. 3A and 3B , requiring tilt of the MEMS mirrors only in the YZ plane; although two-plane tilting may be desirable to provide “hitless” operation. As noted above, the configuration of the MEMS mirror array  661  must be coordinated with the configuration of the beam portions  251  diffracted by the diffractive bulk optical element  806 . For example, if the diffractive bulk optical element  806  diffracts the M input optical beams  244  into N 1  rows and N 2  columns of the beam portions  251 , the MEMS mirror array  661  must also have N 1  rows and N 2  columns of MEMS mirrors, wherein N=N 1 ·N 2 . 
     Turning to  FIGS. 8B and 8C  with further reference to  FIGS. 6A ,  6 B, and  8 A, a dual-row fiber array  820  includes parallel top and bottom rows  821  and  822 , respectively, of optical fibers  830 . Microlenses  811  are coupled to the respective optical fibers  830 . The dual-row fiber array  820  can be used in the dual multicast optical switch  600  and/or  620  of  FIGS. 6A and 6B  in place of the two input  341  and/or the two output  371  single-row fiber arrays. In the multicast optical switch  800  of  FIG. 8A , a multi-row optical fiber array including N 1  rows of N 2  optical fibers can be used in place of the two single-row output fiber arrays  371 . To make input/output beams  840  emitted by the top and bottom rows  821  and  822  of the optical fibers  830  non-parallel to each other, the fibers  830  can be laterally displaced with respect to optical axes  882  of the microlenses  811 , as shown in  FIG. 8C . The lateral displacement along the X axis will ensure that the input/output beams  840  exit at an angle with respect to the optical axes  882  of the microlenses  811 . 
     Turning now to  FIG. 9  with further reference to  FIG. 2A , a method  900  for multi-casting an optical signal includes a step  901  of using the diffractive bulk optical element  206  to split a first optical signal, i.e. the optical beam  204 , into first and second signal portions, i.e. the beam portions  211  and  212 . In a step  902 , the first and second signal portions are propagated in the bulk optical medium  208 . In a step  903 , the first and second directors  221  and  222  are used to direct the first and second signal portions propagated in the step  902  to the first and second output ports  231  and  232 , respectively. Also, in the step  903 , the first and the second signal portions  211  and  212  are independently and controllably coupled into the first and the second output ports  231  and  232 , respectively. 
     Still referring to  FIG. 9  with further reference to  FIG. 2B , the method  900  can be extended for multicast, multiport switching of M optical signals between N output ports. For multicast, multiport M×N optical switching, the step  901  includes using the diffractive bulk optical element  206  to split each of M optical signals including the first optical signal, i.e. the input optical beams  244  including the first input optical beam  204 , into N signal portions including the first and second signal portions, i.e. the N optical beam portions  251  including the first and second beam portions  211  and  212 . The step  902  includes propagating the N portions  251  of each of the M optical signals  244  in the bulk optical medium  208 . The step  903  includes using the N directors  261  comprising the first and second directors  211  and  212  to direct the N signal portions  251 , propagated in the step  902 , to the N output ports  271  including the first and second output ports  231  and  232 . Each of the N directors  261  is configured for receiving a corresponding one of the N portions  251  of each of the M optical signals  244 . Each of the N output ports  271  is coupled to a corresponding one of the N directors  261  and is configured for outputting a corresponding one of the N portions  251  of a selected one of the M optical signals  244 . The step  903  also includes using each of the N directors  261  to switch the corresponding portion of the selected one of the M optical signals  244  into the corresponding one of the N output ports  271 . 
     Still referring to  FIG. 9  with further reference to  FIG. 2C , the ATO element  281  can be used in the step  902  of the method  900  when, in the step  901 , each of the M optical signals  244  is angularly dispersed. In the step  902 , the ATO element  281  is used to couple each of the angularly dispersed N signal portions  251  to one of the N directors  261  laterally offset from each other. The step  902  also includes disposing the ATO optical element  281  substantially one focal length f away from the diffractive bulk optical element  206 , and substantially one focal length f away from the N directors  261 . In one embodiment, the step  903  includes using at least one of the N directors  261  to provide a controllable attenuation of the corresponding portion of the one of the M optical signals  244  coupled into the corresponding one of the N output ports  271 . 
     The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.