Abstract:
The multi-unit wavelength switch enables multiple independent wavelength switching of a plurality of incoming multiplexed optical beams simultaneously on the same optical platform. The different units can have similar functionality or provide disparate functionality, e.g. any one or more of switching, dynamic gain equalization, wavelength blocking, and power monitoring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent Application No. 60/789,564 filed Apr. 6, 2006, which is incorporated herein by reference for all purposes. 
     
     TECHNICAL FIELD 
       [0002]    The present invention relates to a multi-unit wavelength dispersive optical device, and in particular to the integration of a plurality of independent wavelength dispersive optical devices onto a single platform. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventional optical wavelength dispersive devices, such as those disclosed in U.S. Pat. No. 6,097,859 issued Aug. 1, 2000 to Solgaard et al; U.S. Pat. No. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al; U.S. Pat. No. 6,707,959 issued Mar. 16, 2004 to Ducellier et al; U.S. Pat. No. 6,810,169 issued Oct. 26, 2004 to Bouevitch; and U.S. Pat. No. 7,014,326 issued Mar. 21, 2006 to Danagher et al, separate a multiplexed optical beam into constituent wavelengths, and then direct individual wavelengths or groups of wavelengths, which may or may not have been modified, back through the device to a desired output port. Typically the back end of the device includes individually controllable devices, such as a micro-mirror array, which are used to redirect selected wavelengths back to one of several output ports, or an array of liquid crystal cells, which are used to block or attenuate selected wavelengths. 
         [0004]      FIG. 1  illustrates a top view of a typical platform  102 A for a wavelength dispersive device in which a light redirecting element having optical power in the form of a spherical reflector  120  receives a beam of light from a front-end unit  122 . The spherical reflector  120  reflects the beam of light to a diffraction grating  124 , which disperses the beam of light into its constituent wavelength channels. The wavelength channels are again redirected by the spherical mirror  120  to a backend unit  126 . 
         [0005]    In the case of a wavelength blocker (WB) or a dynamic gain equalizer (DGE) the front end unit  122  can include a single input/output port with a circulator, which separates incoming from outgoing signals, or one input port with one output port. Typically the front end unit  122  will include a polarization diversity unit for ensuring the beam (or sub-beams) of light has a single state of polarization. The backend unit  126  for a WB or a DGE is an array of liquid crystal cells, which independently rotate the state of polarization of the wavelength channels to either partially attenuate or completely block selected channels from passing back through the polarization diversity unit in the front end  122 . 
         [0006]    In the case of a wavelength selective switch (WSS) the front end unit  122  includes (See  FIG. 2 ) an array  132  of input/output fibers  132 A to  132 D, each of which may have a corresponding lens  134 A to  134 D, respectively, forming a lens array  134 . An angle to offset (or switching) lens  136  converts the lateral offset of the input fibers  132 A to  132 D into an angular offset at a point  138 , which is imaged by the spherical lens  120  onto the backend unit  126 . The lens array  134  can be removed depending on the relative positions of the switching lens  136 . The backend unit  126  in an WSS is typically a micro-electro-mechanical (MEMS) array of tilting mirrors which can be used to steer each of the demultiplexed beams to one of several positions corresponding to a desired output port. The angle introduced at the back end unit  126  is then transformed by the angle to offset lens  136  to a lateral offset corresponding to the desired input/output fiber  132 A to  132 D. Alternatively, a liquid crystal phased array (LC or LCoS, if incorporated on a silicon driver substrate) can be used to redirect the light. 
         [0007]    In operation as an WSS, a multiplexed beam of light is launched into the front-end unit  122  and optionally passes through a polarization beam splitter  138  and a waveplate  140 A or  140 B (See  FIG. 3 ) to provide two sub-beams of light having the same state of polarization. The two sub-beams of light are transmitted to the spherical reflector  120  and are reflected therefrom towards the diffraction grating  124 . The diffraction grating  124  separates each of the two sub-beams into a plurality of channel sub-beams of light having different central wavelengths. The plurality of channel sub-beams are transmitted to the spherical reflector  120 , which redirects them to the MEMS or LC phased array  126 , where they are incident thereon as spatially separated spots corresponding to individual spectral channels. 
         [0008]    Each channel sub-beam can be reflected backwards along the same path or a different path, which extends into or out of the page in  FIG. 1  to the array of fibers  132 , which would extend into the page. Alternatively, each channel sub-beam can be reflected backwards along the same path or a different path, which extends in the plane of the page of  FIG. 1 . The sub-beams of light are transmitted, from the MEMS or LC phased array  126 , back to the spherical reflector  120  and are redirected to the diffraction grating  124 , where they are recombined and transmitted back to the spherical reflector  120  to be transmitted to a predetermined input/output port shown in  FIG. 2 . 
         [0009]      FIG. 4  illustrates a conventional in-plane or horizontally switching platform, in which an input beam with optical wavelength channels λ 1  and λ 2  is launched via input/output port  31  through switching lens  35  to concave mirror  40 . The input beam is redirected and collimated onto a diffraction grating  50 , which laterally disperses the optical wavelength channels, and directs them at the concave mirror  40 . Each optical wavelength channel is directed at and focused onto a different independently controllable micro-mirror, e.g.  61  and  62 , which make up a MEMs array  60 . The first optical wavelength channel λ 1  is reflected straight back and therefore exits the input/output port  31 , while the second optical wavelength channel λ 2  is reflected at a predetermined angle corresponding to the lateral position of a second input/output port  32 . 
         [0010]    A transmission path correction element, i.e. wedge,  100 , with front and rear non-parallel faces  101  and  102 , respectively, is installed between the concave mirror  40  and the MEMS array  60 . The purpose of this correction element  100  is to modify the paths of the optical signals focused by the concave mirror  40 , so as to effectively rotate the best fit planar surface approximation FP into coplanar coincidence with the optical signal-receiving surface MP  65  of the MEMS array  60 . Non-limiting examples of a suitable (field-flattening) transmission path correction element that may be used for this purpose include a portion or segment of a cylindrical lens and an optical transmission wedge. With the curvilinear focal surface LP of the spherical mirror  40  being transformed into a focal plane FP, and with that plane FP being coincident with the MEMS array plane MP  65 , variation in loss, as minimized by the best fit linear approximation of the focal plane, will be effectively eliminated. 
         [0011]    Unfortunately, each time a customer wishes to purchase a WB, a DGE, an MWS or any form of monitor therefor, they must purchase a separate dispersion platform, i.e. spherical lens and diffraction grating along with associated opto-mechanics and packaging. An object of the present invention is to overcome the shortcomings of the prior art by providing a multi-unit wavelength dispersive device, in which a plurality of independent front and backend units can utilize the same dispersion platform and share the same opto-mechanics and packaging. 
       SUMMARY OF THE INVENTION 
       [0012]    Accordingly, the present invention relates to a multi-unit wavelength dispersing device comprising: 
         [0013]    a first input port for launching a first multiplexed optical input beam including a plurality of wavelength channels; 
         [0014]    one or more first output ports for outputting one or more of the plurality of wavelength channels from the first optical input beam; 
         [0015]    a first switching lens having a first optical axis for converting a lateral displacement corresponding to a position of the first input port relative to the first optical axis into an angular displacement relative to the first optical axis, and for converting an angular displacement of an outgoing optical beam into a lateral displacement corresponding to a position of a selected one of the one or more first output ports; 
         [0016]    a second input port for launching a second multiplexed optical input beam including a plurality of wavelength channels; 
         [0017]    one or more second output ports for outputting one or more of the plurality of wavelength channels from the second optical input beam; 
         [0018]    a second switching lens having a second optical axis for converting a lateral displacement corresponding to a position of the second input port relative to the second optical axis into an angular displacement relative to the second optical axis, and for converting an angular displacement of an outgoing optical beam into a lateral displacement corresponding to a position of a selected one of the one or more second output ports; 
         [0019]    a main lensing element with optical power, having a central axis, for directing and focusing the first and second input optical beams; 
         [0020]    a wavelength dispersing element for dispersing the first and second multiplexed optical input beams into constituent wavelength channels; 
         [0021]    a first array of wavelength channel redirecting elements for independently directing one or more selected wavelength channels from the plurality of wavelength channels in the first optical input beam to a selected one of the one or more first output ports via the main lensing element and the wavelength dispersing element by providing an angular displacement to the one or more selected wavelength channels for conversion by the first switching lens into a lateral position corresponding to the selected first output port; and 
         [0022]    a second array of wavelength channel redirecting elements for independently directing one or more selected wavelength channels from the plurality of wavelength channels in the second optical input beam to a selected one of the one or more second output ports via the main lensing element and the wavelength dispersing element by providing an angular displacement to the one or more selected wavelength channels for conversion by the second switching lens into a lateral position corresponding to the selected second output port. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0024]      FIG. 1  is a schematic representation of a top view of a conventional wavelength dispersive device; 
           [0025]      FIG. 2  is a schematic representation of a front end of the device of  FIG. 1 ; 
           [0026]      FIG. 3  is a schematic representation of a front end of the device of  FIG. 1 ; 
           [0027]      FIG. 4  is a schematic representation of a top view of another conventional wavelength dispersive switch; 
           [0028]      FIG. 5  is a schematic representation of an isometric view of a wavelength dispersive device according to the present invention; 
           [0029]      FIG. 6  is a schematic representation of a top view of a wavelength dispersive device according to the present invention 
           [0030]      FIG. 7  is a schematic representation of a side view of a wavelength dispersive device according to an embodiment of the present invention; 
           [0031]      FIG. 8  is a schematic representation of a side view of a back-end unit of a wavelength dispersive device according to the device of  FIG. 7 ; 
           [0032]      FIG. 9  is a schematic representation of an isometric view of a back-end unit of a wavelength dispersive device according to the device of  FIG. 7 ; 
           [0033]      FIG. 10  is a schematic representation of a side view of a wavelength dispersive device according to an embodiment of the present invention; and 
           [0034]      FIG. 11  is a schematic representation of a side view of a back-end unit of a wavelength dispersive device according to the device of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    A dual wavelength dispersive device  200 , illustrated in  FIG. 5 , includes a single main lensing element having optical power, preferably in the form of a spherical, e.g. concave, reflector  201 , which receives two independent collimated beams of light from the front-end unit  202 , and which receives and reflects beams of light to and from a wavelength dispersing element, e.g. a diffraction grating  203 , and to and from a backend unit  204 . In this embodiment the front-end unit  202 , the diffraction grating  203 , and the backend unit  204  are each disposed along a single focal plane of the spherical reflector  201 ; however, other arrangements are within the scope of the invention, including using a convex lens (or a series of lenses) and placing the diffraction grating  203  on the opposite side thereof as the front and backend units  202  and  204 , respectively. 
         [0036]    Preferably, the diffraction grating  203 , the spherical reflector  201 , and the backend unit  204  are each constructed of fused silica and mounted together with a beam folding mirror or prism  205  to a supporting plate  215  made of the same or made from a suitable low-expansion material, such as Invar®. The beam folding mirror or prism  205  is provided for space considerations, e.g. a MEMS chip with MEMS mirrors defining the backend unit  204  and their carrier are too large to fit next to the diffraction grating  203 . Accordingly, the beam folding mirror  205  redirects the beams so that the MEMS mirrors can be placed flat under the rest of the optics. Advantageously, the design of  FIG. 5  provides stability with respect to small temperature fluctuations. Moreover, the design of  FIG. 5  is defocus free, since the radius of curvature of the spherical reflector  201  changes in proportion to thermal expansion or contraction of any other linear dimensions. Advantageously, the spherical mirror  201  has substantially no chromatic aberrations. The wavelength dispersing element  203  can be a reflective or a transmissive diffraction grating, with ruled or replicated lines or holographically generated lines 
         [0037]    Preferably, a transmission path correction element  220  is installed between the redirecting element, e.g. the concave mirror  201 , and the backend unit  204 , e.g. a MEMS array  243 , for reasons discussed hereinbefore with reference to  FIG. 4 . 
         [0038]    In the front-end unit  202 , the single switching lens, e.g.  136  or  35 , found in conventional wavelength dispersive devices, is replaced by first and second horizontal cylindrical lenses  231   a  and  231   b  and a single vertical cylindrical lens  232  to create an elliptical beam through the system, for reduced height of the optical system. The first and second horizontal cylindrical lenses  231   a  and  231   b,  are positioned between two fold mirrors  234  and  236 , and act as the switching lens, while creating the desired beam waist size in the vertical direction; the single vertical cylinder lens  232  creates the desired beam waist size in the horizontal direction, i.e. there are separate switching lenses  231   a  and  231   b  (horizontal cylinder lenses) for each beam at the front end unit  202 , while the “conditioning” lens  232  (vertical cylinder lens) is common to all the beams. 
         [0039]    For the sake of simplicity, the fold mirrors  205 ,  234  and  236 , and conditioning lens  232  will be eliminated from any further illustrations. 
         [0040]    With reference to  FIGS. 6 to 9 , the operation of the dual wavelength dispersion device  200  will be described with reference to simultaneously redirecting a pair of wavelength channels λ 1a  and λ 2a  from a first input optical beam including a plurality of wavelength channels λ 1a  to λ 9a , and independently redirecting a pair wavelength channel λ 8b  and λ 9b  from a second input optical beam including a plurality of wavelength channels λ 1b  to λ 9b . Since the number of supported wavelengths usually exceeds the number of output ports, each wavelength channel λ 1a  and λ 2a  can represent one or several wavelength channels. The front end unit  202  includes a first set of input/output ports  241  optically coupled to the first horizontal cylindrical lens  231  a, and a second set of input/output ports  242  optically coupled to the second horizontal cylindrical lens  231   b,  but not optically coupled to the first horizontal cylindrical lens  231   a.  Preferably, the first set of input/output ports  241  are positioned symmetrically on either side of the optical axis of the first horizontal cylindrical lens  231   a,  while the second set of input/output ports  242  are positioned symmetrically on either side of the optical axis of the second horizontal cylindrical lens  231   b  The second set of input/output ports  242  are independent of the first set of input/output ports  241 , i.e. light entering one of the first set of input/output ports  241  will not exit one of the second set of input/output ports  242 . Preferably, the first and second horizontal cylindrical lenses  231   a  and  231   b  are substantially equally spaced on opposite sides of the optical axis OA of the reflector  201 . Typically a multiplexed beam of light is launched into the front-end unit  202  and passes through a polarization beam splitter and a waveplate (See  FIG. 3 ) to provide two sub-beams of light having the same state of polarization; however, for the sake of simplicity only a single input optical beam will be discussed hereinafter. 
         [0041]    The first input optical beam including the plurality of wavelength channels λ 1a  to λ 9a  is launched via one of the input/output ports in the first set of input/output ports  241  and is redirected by the first horizontal cylindrical lens  231   a  through a point  245  in the focal plane of the reflector  201  to become incident on the reflector  201  for a first time at point  1   a.  The first input optical beam is reflected and collimated by the reflector  201  towards the diffraction grating  203 , whereby the first input optical beam is angularly dispersed into constituent wavelength channels, as each wavelength is reflected off of the diffraction grating  203  at a different angle (see  FIG. 6 ). In the preferred embodiment illustrated in  FIGS. 6 and 7 , the wavelengths are dispersed in a dispersion plane, which is in the plane of (or parallel to or at an acute angle to the plane of)  FIG. 6 , but perpendicular to the plane of  FIG. 7 , and perpendicular to the plane including the first and second sets of input/output ports  241  and  242 , respectively, although dispersing the wavelengths in the plane of  FIG. 7  is also possible. 
         [0042]    The dispersed wavelengths λ 1a  to λ 9a  are incident on the reflector  201  a second time at a series of points  2   a,  and are then reflected and focused to a first array of channel wavelength redirecting elements  243  e.g. a MEMs array of mirrors or an LC phased array, in the backend  204  in a first dispersion plane. The first array of redirecting elements  243  includes a plurality of tilting mirrors or LC cells, one for each wavelength channel for independently redirecting each wavelength channel λ 1a  to λ 9a  to any one of the first set of input/output ports  241 . Preferably, all of the mirrors in the MEMs array  243  tilt about a single axis A 1 , which lies in the first dispersion plane (or parallel thereto), i.e. in or parallel to or at an acute angle to the plane of  FIG. 6  and perpendicular to the plane of  FIG. 7 , to enable the wavelength channels λ 1a  to λ 9a  to be redirected out at an acute angle to the first dispersion plane, i.e. out of the plane of  FIG. 6  and in the plane of (or a plane parallel to the plane of)  FIG. 7 . In the illustrated alignment, one or more of the wavelength channels, e.g. λ 1a  and λ 2a , is redirected by the first MEMs array  243  relative to the remaining wavelength channels λ 3a  to λ 9a , which travel back along the same path as the incoming signal hitting the reflector  201  at points  2   a,  recombining at the diffraction grating  203  forming a first multiplexed output beam, hitting the reflector  202  at point  1   a,  and exiting the same input/output port through which the input beam was launched.  FIG. 8  illustrates the nine angles of reflection, i.e. nine angular positions, provided by the MEMs array  243  corresponding to the nine input ports in the first set of input/output ports  241 . More or less reflection angles, i.e. angular positions, are possible depending on the number of input/output ports. The redirected wavelength channels λ 1a  and λ 2a  are directed towards and reflector  201  and are incident thereon for a third time at point  3   a,  after which the wavelength channels λ 1a  and λ 2a  are directed to the diffraction grating  203  at a separate location than before for recombination into a second multiplexed output beam. Subsequently, the second multiplexed output beam, comprised of the wavelength channels λ 1a  and λ 2a , is reflected by the reflector  201  to the front end  202 . The second multiplexed output beam, along with all incoming and outgoing beams, passes through point  245  in the focal plane of the reflector  201  at an angle to the optical axis of the first horizontal cylindrical lens  231   a,  corresponding to the reflection angle provided by the MEMs array  243 , which corresponds to the desired input/output port. The first horizontal cylindrical lens  231   a  converts the angle into a lateral displacement corresponding to the lateral position of the desired input/output port in the set of input/output ports  241 . 
         [0043]    Simultaneously, a second input optical beam including a plurality of wavelength channels λ 1b  to λ 9b  is launched via one of the input/output ports in the second set of input/output ports  242  and redirected by the second horizontal cylindrical lens  231   b  through a point  246  in the focal plane of the reflector  201  to become incident on the reflector  201  for a first time at point  1   b.  The second input optical beam is reflected by the reflector  201  towards the diffraction grating  203 , whereby the second input optical beam is angularly dispersed into constituent wavelength channels, as each wavelength is reflected off of the diffraction grating  203  at a different angle (see  FIG. 6 ). In the preferred embodiment illustrated in  FIGS. 6 and 7 , the wavelengths are dispersed in a dispersion plane, which is in the plane of (or parallel to or at an acute to angle the plane of)  FIG. 6 , but perpendicular to the plane of  FIG. 7 , and perpendicular to the plane including the first and second sets of input/output ports  241  and  242 , respectively, although dispersing the wavelengths in the plane of  FIG. 7  (or a plane parallel to the plane of  FIG. 7 ) is also possible. 
         [0044]    The dispersed wavelengths λ 1b  to λ 9b  are incident on the reflector  201   a  second time at a series of points  2   b,  and are then reflected to a second array of channel wavelength redirecting elements e.g. a MEMs array  244 , in the backend  204  in a second dispersion plane preferably parallel to the first dispersion plane. The MEMs array  244  includes a plurality of tilting mirrors, one for each wavelength channel for independently redirecting each wavelength channel λ 1b  to λ 9b  to any one of the second set of input/output ports  242 , i.e. only the second set of input/output ports  242 , none of the first set of input/output ports  241 . Preferably, the mirrors in the second MEMs array  244  tilt about an axis A 2 , which lies in the second dispersion plane, i.e. in or parallel to the plane of  FIG. 6  and perpendicular to the plane of  FIG. 7 , to enable the wavelength channels λ 1b  to λ 9b  to be redirected out at an acute angle to the second dispersion plane, i.e. out of the plane of (or a plane parallel to the plane)  FIG. 6  and in the plane of (or a plane parallel to the plane of)  FIG. 7 . In the illustrated alignment, one or more of the wavelength channels, e.g. λ 1b  and λ 2b , are redirected by the second MEMs array  244  relative to the remaining wavelength channels λ 3b  to λ 9b , which travel back along the same path as the second input beam hitting the reflector  201  at points  2   b,  recombining at the diffraction grating  203  forming a third multiplexed output beam, hitting the reflector  202  at point  1   b,  and exiting the same input/output port through which the second input beam was launched.  FIG. 8  illustrates the nine angles of reflection, i.e. nine angular positions, provided by the MEMs array  244  corresponding to the nine input ports in the second set of input/output ports  242 . More or less reflection angles, i.e. angular positions, are possible depending on the number of input/output ports. The redirected wavelength channels λ 1b  and λ 2b  are directed towards and reflector  201  and is incident thereon for a third time at point  3   b,  after which the wavelength channels λ 1b  and λ 2b  are directed to the diffraction grating  203  at a separate location than before for recombination into a fourth multiplexed output beam. Subsequently, the fourth multiplexed output beam, comprised of the wavelength channels λ 1b  and λ 2b , is reflected by the reflector  201  to the front end  202 . The fourth multiplexed output beam, along with all incoming and outgoing beams, passes through point  246  in the focal plane of the reflector  201  at an angle to the optical axis of the second horizontal cylindrical lens  231   b,  corresponding to the reflection angle provided by the second MEMs array  244 , which corresponds to the desired input/output port. The second horizontal cylindrical lens  231   b  converts the angle into a lateral displacement corresponding to the lateral position of the desired input/output port in the second set of input/output ports  242 . 
         [0045]    In the illustrated example, the first and second MEMs arrays  243  and  244  are separated by the same amount as the first and second horizontal cylindrical lenses are separated, e.g. by about 1.5 mm, and the first and second sets of input/output ports are separated by approximately 1.5 mm. The first and second MEMs arrays  243  and  244  are preferably fabricated parallel to each other on a single substrate  250 , which would enable precision alignment between the two arrays, thus eliminating the need for separate alignment of the two arrays  243  and  244 . A dual row MEMs array is less expensive than two single row MEMs arrays, and only marginally more expensive than a single row MEMs array. Similarly, the first and second horizontal cylindrical lenses  231   a  and  231   b  can be fabricated as a single molded optical element, thereby enabling precision alignment therebetween, and eliminating separate alignment of the individual lenses. 
         [0046]    Alternative arrangements could have any combination of wavelengths λ 1a  to λ 9a  being output any combination of input/output ports in the first set of input/output ports  241 , and any combination of wavelengths λ 1b  to λ 9b  being output any combination of input/output ports in the second set of input/output ports  242 . Moreover, the first and second MEMs arrays  243  and  244  can be designed to switch the individual wavelength channels within the same dispersion plane, while the first and second set of input ports  241  and  242  can also be aligned in the same dispersion plane. 
         [0047]    Furthermore, the first and second MEMs array  243  and  244  can be replaced by other optical switching elements, e.g. liquid crystal on silicon (LCoS) phased arrays, such as those disclosed in United States Patent Publication No. 2006/0067611 published Mar. 30, 2006 to Frisken et al, or an array of polarization rotators, e.g. liquid crystal cells, for independently rotating the polarization of individual wavelength channels λ 1b  to λ 9b , whereby a portion, i.e. for a DGE, or the entire wavelength channel, i.e. for a WB or WSS, will be blocked or switched by a beam splitting element provided in the backend unit  204  or in the front end unit  202 , e.g. as part of the polarization diversity element. For a DGE or a WB arrangement, all of the wavelength channels λ 1b  to λ 9b  are recombined by the grating  203  into a single multiplexed output beam, and are returned to the same input/output port, whereby a circulator directs the single multiplexed output beam to an output port. Alternatively, all of the wavelength channels λ 1b  to λ 9b  can be redirected by the polarization rotating device at an angle to the incoming beam and recombined by the grating  203  into a single multiplexed output beam, which is output a different input/output port in the front end unit  202 . 
         [0048]    With reference to  FIGS. 10 and 11 , a multi-unit WSS device  300  preferably includes a single main lensing element having optical power in the form of a spherical, i.e. concave, reflector  301 , which receives three independent collimated beams of light from the front-end unit  302 , and which receives and reflects beams of light to and from a diffraction grating  303 , and to and from a backend unit  304 . In this embodiment the front-end unit  302 , the diffraction grating  303 , and the backend unit  304  are each disposed along a single focal plane of the spherical reflector  301 ; however, other arrangements are within the scope of the invention, including using a convex lens and placing the diffraction grating  303  on the opposite side thereof as the front and backend units  302  and  304 , respectively. 
         [0049]    A transmission path correction element can be installed between the redirecting element, e.g. the concave mirror  301 , and the backend unit  304 , e.g. a MEMS array  343 , for reasons discussed hereinbefore with reference to  FIG. 4 . 
         [0050]    In the front-end unit  302 , the single switching lens, e.g.  136  or  35 , found in conventional wavelength dispersive devices, is replaced by first, second and third horizontal cylindrical lenses  331   a,    331   b  and  331   c  and a single vertical cylindrical lens  332  to create an elliptical beam through the system, for reduced height of the optical system. The first, second and third horizontal cylindrical lenses  331   a,    331   b  and  331   c,  can be positioned between two fold mirrors (not shown), and act as the switching lens, while creating the desired beam waist size in the vertical direction; the single vertical cylinder lens  332  creates the desired beam waist size in the horizontal direction, i.e. there are separate switching lenses  331   a,    331   b  and  331   c  (horizontal cylinder lenses) for each beam at the front end unit  302 , while the “conditioning” lens  332  (vertical cylinder lens) is common to all the beams. For the sake of simplicity, the fold mirrors and conditioning lens have been eliminated from the illustrations. 
         [0051]    The front end unit  302  includes a first set of input/output ports  341  optically coupled to the first horizontal cylindrical lens  331   a,  and a second set of input/output ports  342  optically coupled to the second horizontal cylindrical lens  331   b,  but not optically coupled to the first horizontal cylindrical lens  331   a,  and a third set of input/output ports  343  optically coupled to the third horizontal lens  331   c,  but not the first and second horizontal cylindrical lenses  331   a  and  331   b.  Preferably, the first set of input/output ports  341  are positioned symmetrically on either side of the optical axis of the first horizontal cylindrical lens  331   a,  while the second set of input/output ports  342  are positioned symmetrically on either side of the optical axis of the second horizontal cylindrical lens  331   b,  and the third set of input/output ports  343  are positioned symmetrically on either side of the optical axis of the second horizontal cylindrical lens  331   c.  The second and third sets of input/output ports  342  and  343  are independent of each other and of the first set of input/output ports  341 , i.e. light entering one of the first set of input/output ports  341  will not exit one of the second set of input/output ports  342 . Preferably, the optical axis of the first horizontal cylindrical lens  331   a  is aligned with the central axis of the reflector  301 , while the second and third horizontal cylindrical lenses  331   b  and  331   c  are substantially equally spaced on opposite sides of the optical axis OA of the reflector  301 . Typically a multiplexed beam of light is launched into the front-end unit  302  and passes through a polarization beam splitter and a waveplate (See  FIG. 3 ) to provide two sub-beams of light having the same state of polarization; however, for the sake of simplicity only a single input optical beam will be discussed hereinafter. 
         [0052]    A first array of MEMs mirrors  344  in the back end unit  304  is used to independently direct one or more selected wavelength channels, e.g. λ 1c , from the original set of wavelength channels, e.g. λ 1c  to λ 11c , to selected output ports in the first array of output ports  341 , as hereinbefore described with reference to  FIG. 7 . 
         [0053]    When switching lenses are placed above and/or below the optical axis OA of the spherical reflector  301 , the available numerical aperture on the spherical reflector  301  is reduced, whereby fewer ports can be accommodated. In the multi-unit MWS  300  the first horizontal cylindrical lens  331   a  is positioned on-axis with the reflector  301 , whereby eleven ports can be accommodated in the first set of input/output ports  341 ; however, the farther the second and third cylindrical lenses  331   b  and  331   c  are from the optical axis OA of the spherical reflector  301 , the fewer the number of ports that can be accommodated in the second and third sets of input/output ports  342  and  343 . Accordingly, the second and third sets of input/output ports  342  and  343  can be used for alternative functions, e.g. DGE, WB or reduced port-count WSS (as hereinbefore described) and channel monitoring. In the case in which the second or third set of ports is used for a reduced port count WSS capable of functioning in an Nx1 configuration with N input ports for accepting multiplexed inputs and one common output, external passive combiners can be added to the N input ports to further increase the total input port count. 
         [0054]    For channel monitoring, a plurality of wavelength channels, e.g. λ 1m  to λ 11m , are launched via a first input/output port  342 ′, and one wavelength channel, λ nm , at a time is redirected by an array of MEMs mirrors  345  to a second input/output port  342 ″, which is optically coupled to a photodetector PD for measuring the output optical power of the selected wavelength channel as each wavelength channel is selected sequentially. The remaining wavelength channels are redirected to a third input/output port or back to the first input/output port  342 ′, which includes a circulator for separating the incoming signals from the outgoing signals and directing the outgoing signals to a separate output port. 
         [0055]    The third set of input/output ports  343  can also be used as an WSS, but with a limited number of input/output ports, e.g. four. If the number of addressable ports in the third set of input/output ports  343  is fewer than half of the number of addressable ports in the first set of input/output ports  341 , then the third array of MEMs mirrors  346  can be fabricated in the same process and on the same substrate  350  as the first array  344 , but the third array can be processed to tilt with a limited angular range, i.e. only one direction from the horizontal, e.g. one end of each mirror will only have to tilt clockwise between a horizontal position and below horizontal without having to rotate counterclockwise above the horizontal position. Accordingly, the number of electrodes required per mirror can be reduced, e.g. by at least one half, along with the number of electrical connections thereto, since electrodes will not be required under both sides of the mirrors. 
         [0056]    Similar to  FIG. 8 ,  FIG. 11  illustrates the different output angles provided by the first, second and third arrays of MEMs mirrors  344 ,  345  and  346 , respectively. The mirrors in the first MEMs array  344  have eleven different angular positions corresponding to the eleven different input/output ports in the first array  341  of input/output ports. The mirrors in the second MEMs array  345  have only two different positions for either directing the wavelength channel back to the input port  342 ′ or to the output port  342 ″ for sequential power monitoring. The mirrors in the third array  346  have four different positions corresponding to the limited number of ports provided in the third array of input/output ports  343 . 
         [0057]    In use the output ports of one of the arrays of input/output ports may be optically coupled to the input ports of the other arrays of input/output ports to provide cascaded functionality, e.g. one of the signals output the WWS formed by the first array  341  and  344  can be output to the channel monitor formed by the third arrays  343  and  346  and/or the signal output the channel monitor (third arrays  343  and  346 ) can be then output to an attenuator/WB formed second arrays  342  and  345 . Alternatively, all of the channels can be sent to the channel monitor (third arrays  343  and  346 ) initially and then passed to the WSS (first array  341  and  344 ) and/or to the attenuator/WB (second arrays  342  and  345 ).