Abstract:
A free-space optical switch for switching light beams between waveguides of planar lightwave circuits (PLCs). Switching is accomplished using a combination of lenses and micromirrors. The lenses and the controlled tilt of the micromirrors can establish a one-to-one interconnection path between waveguides of the PLCs.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to free space optical switching, and more particularly to apparatus for switching optical signals between waveguides of planar lightwave circuits.  
         [0003]     2. Description of the Related Art  
         [0004]     Free-space micro electromechanical systems (MEMS) optical switches have been found to offer a number of advantages including high port count, low insertion loss, low crosstalk, optical transparency, and polarization insensitivity. However, practical application of prior art switch designs is often limited because of alignment issues, assembly issues, and size issues due to the bulk optical elements and long free-space propagation distances.  
         [0005]     Planar lightwave circuits (PLCs) have proven to be a convenient platform for fabricating components of lightwave communication systems. PLCs offer the advantages of precise manufacturing, small form factor, simple packaging and thermal stabilization.  
         [0006]     Compact optical switches can be realized by combining MEMS devices and PLCs. Prior art MEMS-PLC based optical switch designs, however, typically only offer small port counts (1×2 or 2×2) without cascading multiple switches (which raises crosstalk and loss issues), or employ complex designs requiring the use of multiple mirrors in the optical signal path, which gives rise to manufacturing and alignment issues.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention provides free-space switching solutions between waveguides of multiple PLCs using tilting micromirrors. Each PLC has one or more waveguides that terminate at an edge facet of the PLC. Switching between the waveguides of the PLCs is accomplished using a combination of lenses and a single micro electromechanical systems (MEMS) micromirror or linear array of MEMS micromirrors. The tilt of the micromirrors can establish a one-to-one interconnection path between waveguides of the PLCs.  
         [0008]     The disclosed embodiments provide relatively low cost, effective MEMS-PLC based optical switches, which resolve many of the size, packaging, alignment and stability issues of the prior art. The various embodiments of the invention include different lens/micromirror combinations that provide vertical direction interconnection, localized region interconnection, or a single interconnection from one waveguide to any other waveguide of the PLCs. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0009]     The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments that are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
         [0010]     In the drawings:  
         [0011]      FIG. 1  is an illustration showing a stacked arrangement of planar lightwave circuits;  
         [0012]      FIGS. 2   a - b  are top and side views of an optical switch according to one embodiment of the invention;  
         [0013]      FIGS. 3   a - b  are top and side views of an optical switch according to another embodiment of the invention;  
         [0014]      FIGS. 4   a - b  are top and side views of an optical switch according to another embodiment of the invention;  
         [0015]      FIGS. 5   a - b  are top and side views of an optical switch according to another embodiment of the invention;  
         [0016]      FIGS. 6   a - b  are top and side views of an optical switch according to another embodiment of the invention;  
         [0017]      FIGS. 7   a - b  are top and side views of an optical switch according to another embodiment of the invention;  
         [0018]      FIGS. 8   a - b  are top and side views of an optical switch according to another embodiment of the invention;  
         [0019]      FIGS. 9   a - b  are top and side views of an optical switch according to another embodiment of the invention; and  
         [0020]      FIGS. 10   a - b  are top and side views of an optical switch according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention provides an apparatus for free-space switching between waveguides of a plurality of PLCs. As can be understood from  FIG. 1 , which depicts three PLCs  100 ,  110  and  120  in a stacked arrangement including a plurality of waveguides,  102 ,  104 ,  106 ,  112 ,  114 ,  116 ,  122 ,  124 ,  126 , light emitted from one of the waveguides (referred to hereinafter as “light beams”) can be redirected back into a desired waveguide. For example, light beams emitted from any waveguide (e.g.  112 ,  114 , or  116 ) on any PLC can be redirected to any other waveguide (e.g.  122 ,  124 , or  106 , respectively) on any PLC.  
         [0022]     In one preferred embodiment, an optical switch apparatus  200 , shown in  FIGS. 2   a - b , is provided, which comprises a plurality of PLCs  210 ,  220 ,  230 , a lens  275 , and an array of tilting micromirrors  280 .  
         [0023]     The plurality of PLCs  210 ,  220 ,  230  shown in the illustrated embodiment are arranged in a stacked configuration. It will be appreciated by those skilled in the art that, alternatively, any number of PLCs can be arranged in a variety of configurations in accordance with the present invention for switching light beams between waveguides of the PLCs.  
         [0024]     Each PLC  210 ,  220 ,  230  has one or more waveguides (e.g.  212 ,  214 ,  216 , hereinafter referred to collectively as “the waveguides”) that terminate at an edge facet  218 ,  228 ,  238  of the PLC. The edge facets  218 ,  228 ,  238  are preferably anti-reflection coated to reduce loss and prevent back reflection.  
         [0025]     The PLCs  210 ,  220 ,  230  are preferably arranged in a stack such that the waveguides are aligned for vertical switching. Specifically, in preferred embodiments at least some of the waveguides of the stacked PLCs  210 ,  220 ,  230  are substantially aligned relative to a vertical axis (e.g. axis  240 ) of the stacked PLCs  210 ,  220 ,  230  to facilitate switching between vertically aligned waveguides.  
         [0026]     It is to be understood that the terms “horizontal” and “vertical” are used herein in discussing the preferred embodiments for reference purposes only and are not intended to limit the scope of the claimed invention.  
         [0027]     As can be understood from  FIGS. 2   a - b , a light beam (e.g. light beam  250 ) emitted from a waveguide on PLC  220  (e.g. the waveguide below waveguide  212 ), radiates normally to the edge facet  228  into free space and through a lens  275  in the path of the light beam. The lens  275  collimates the light beam in a vertical direction and images the light beam in a horizontal direction (e.g. as light beam  252 ).  
         [0028]     The lens  275  is preferably an anamorphic lens constructed from two crossed cylindrical lenses, having different focal lengths; F 1  in a horizontal direction and F 2  in a vertical direction, where 2F 1 =F 2 . The anamorphic lens  275  is preferably positioned between the PLCs  210 ,  220 ,  230  and the micromirror array  280 , at a distance of 2F 1  (or F 2 ) from each of the PLCs  210 ,  220 ,  230  and the micromirror array  280 . It can be understood by those skilled in the art from  FIG. 2   a  that the anamorphic lens  275  can alternatively be constructed from a spherical lens and a cylindrical lens having focal lengths as discussed above.  
         [0029]     The light beam ( 252 ) is imaged onto a micromirror (e.g. micromirror  282 ) of the micromirror array  280 . The micromirror ( 282 ), like all mirrors in the micromirror array  280 , can be tilted or rotated about an axis to direct beams at a desired angle and in a preferred direction. The light beam  252  is reflected and directed by the micromirror (e,g, as light beam  254 ) through the lens  275 , which images the beam (e.g. as light beam  256 ) onto the edge facet  218  of a waveguide  212  of PLC  210 , and couples the light beam ( 256 ) into the waveguide (e.g. waveguide  212 ). As  FIG. 2   a  illustrates (in a somewhat exaggerated manner), the light beam  256  coupling into the waveguide  212  is not normal to the edge facet  218 , which results in some coupling loss. This issue is addressed below with reference to other embodiments of the invention.  
         [0030]      FIGS. 3   a - b  illustrate an alternative embodiment of an optical switch  300  according to the present invention. As can be understood from  FIGS. 3   a - b , micromirrors (e.g. micromirror  382 ) of the micromirror array  380  are capable of tilting about two axes such that, for example, a light beam (e.g. light beam  350 ) emitted from a waveguide on a PLC (e.g. the waveguide below waveguide  312  on PLC  320 ), can be imaged by a lens  375  (e.g. as light beam  352 ) onto micromirror (e.g. micromirror  382 ) and reflected back through the lens  375  (e.g. as light beam  354 ) to be imaged onto a waveguide (e.g. waveguide  312  on PLC  310 ) as a normally incident light beam (e.g. light beam  356 ), for effective coupling into the waveguide ( 312 ). Preferably, the lens  375  is an anamorphic lens constructed from two crossed cylindrical lenses (or alternatively, a spherical lens and a cylindrical lens), having different focal lengths; F 1  in a horizontal direction and F 2  in a vertical direction, where 2F 1 =F 2 .  
         [0031]     In another alternative embodiment, shown in  FIGS. 4   a - b , an optical switch  400  is provided wherein the edge facets  418 ,  428 ,  438  of PLCs  410 ,  420 ,  430  are formed (e.g. polished) to have a curved profile. The curved profile of the edge facets  418 ,  428 ,  438  intersects the waveguides of the PLCs  410 ,  420 ,  430  causing light beams radiating from the waveguides to radiate at an angle (due to refraction at the boundary between the waveguide and free space). For example, a light beam  450  emitted from a waveguide on PLC  420  (e.g. the waveguide below waveguide  412 ) is imaged by a lens  475  to strike a micromirror  482  normally, reflect back as light beam  454  through the lens  475  and efficiently couple into waveguide  412  of PLC  410 , as light beam  456 . This arrangement only requires the micromirrors of the micromirror array  480  to tilt about one axis to vertically switch light between waveguides of the PLCs  410 ,  420 ,  430 . As discussed above with reference to other embodiments, the lens  475  is preferably an anamorphic lens constructed from two crossed cylindrical lenses (or alternatively, a spherical lens and a cylindrical lens), having different focal lengths; F 1  in a horizontal direction and F 2  in a vertical direction, where 2F 1 =F 2 .  
         [0032]     In another alternative embodiment of an optical switch  500  according to the invention (shown in  FIGS. 5   a - b ), an imaging system  570  is used so that light beams each strike a micromirror of a micromirror array  580  and an edge facet  518 ,  528 ,  538  of a PLC  510 ,  520 ,  530  normally. The imaging system  570  preferably comprises three lenses  575 ,  576 ,  577 . In the horizontal direction, the optical system  570  effectively comprises a telecentric imaging system (two lenses  575  and  576 , each of focal length F 1 ), such that a light beam (e.g. light beam  550 ) emitted from a waveguide on a PLC (e.g. the waveguide below waveguide  512  on PLC  520 ), is collimated (e.g. as light beam  551 ) between the lenses  575 ,  576 , and is imaged normally (e.g. as light beam  552 ) on a micromirror (e.g. micromirror  582 ). In the vertical direction, the optical system  570  collimates the light beam ( 550 ) onto a micromirror ( 582 ) using a cylindrical lens  577  of focal length F 2 . Light beams are reflected off of a micromirror (e.g. as light beam  554 ) and through the imaging system  570  (e.g. as collimated light beam  555  between the lenses  575 ,  576 ), and efficiently couple into a waveguide (e.g. waveguide  512  of PLC  510 , as light beam  556 ). The micromirrors (e.g. micromirror  582 ) only need to pivot about a single axis to switch light beams to another vertically aligned waveguide (e.g. between the waveguide below waveguide  512  on PLC  520  and waveguide  512  on PLC  510 ).  
         [0033]     In another alternative embodiment of an optical switch  600  according to the invention (shown in  FIGS. 6   a - b ), each of the waveguides on the PLCs  610 ,  620 ,  630  include a mode conversion region  601 , preferably positioned adjacent to the edge facets  618 ,  628 ,  638  of the PLCs  610 ,  620 ,  630 , where each of the waveguides broadens adiabatically. These mode conversion regions  601  of the waveguides cause light beams to emerge from the waveguides in an elliptic mode such that the numerical aperture (NA) in the vertical direction is high relative to the NA in the horizontal direction (i.e. a light beam emitted from a waveguide rapidly diverges in the vertical direction and slowly diverges in the horizontal direction, relatively). Since the beam is slowly diverging in the horizontal direction, no imaging operation is needed for the horizontal component. Due to the slow divergence in the horizontal direction, the beam propagates in free space, strikes the mirror, reflects back to the PLC and efficiently couples into the waveguide.  
         [0034]     In operation, for example, a light beam  650  emitted from a waveguide on PLC  620  (i.e., the waveguide below waveguide  612 ), radiates from the PLC  620  and diverges in the vertical direction. The light beam  650  passes through a lens  675 , which collimates the light beam  650  in the vertical direction (as light beam  652 ). The light beam  652  strikes a micromirror  682  of the micromirror array  680 . The micromirrors (e.g. 682 ) of the micromirror array  680  can be tilted or rotated about an axis to direct beams at a desired angle and in a preferred direction. The light beam  652  is reflected and directed by the micromirror (as light beam  654 ) through the lens  675 , which images the light beam  656  in a vertical direction onto the edge facet  618  of waveguide  612  of PLC  610 , and couples the light beam  656  into the waveguide  612 .  
         [0035]     In another alternative embodiment of an optical switch  700  according to the invention (shown in  FIGS. 7   a - b ), each of the waveguides of the PLCs  710 ,  720 ,  730  include a mode conversion region  701 , preferably positioned adjacent to the edge facets  718 ,  728 ,  738  of the PLCs  710 ,  720 ,  730 , where each of the waveguides broadens adiabatically. The edge facets  718 ,  728 ,  738  are preferably formed with curved, cylindrical lens-like profiles in a horizontal direction. The mode conversion regions  701  and the curved profiles of the edge facets  718 ,  728 ,  738  cause a beam emitted from the waveguides to have a converging mode in a horizontal direction, focusing the beam on the micromirrors.  
         [0036]     In operation, for example, a light beam  750  emitted from a waveguide on a PLC  720  (i.e., the waveguide below waveguide  712 ), radiates from the PLC  720  and diverges in a vertical direction, while converging in the horizontal direction. The light beam  750  passes through a cylindrical lens  775 , which collimates the light beam  750  in the vertical direction (as light beam  752 ). The light beam  752  strikes a micromirror  782  of micromirror array  780 . The micromirrors of micromirror array  780  can be tilted or rotated about an axis such that light beams (e.g light beams  752 ) are reflected and directed by the micromirror (e.g. as light beam  754 ) through the lens  775 . The lens  775  images the light beam  754  (as light beam  756 ) in the vertical direction onto the edge facet  718  of another PLC  710 , and couples the light beam  756  into a waveguide  712 . Alternatively, the edge facets  718 ,  728 ,  738  can be formed with curved profiles such that a beam emitted from the waveguides is collimated in a horizontal direction.  
         [0037]     In another alternative embodiment of an optical switch  800  according to the invention (shown in  FIGS. 8   a - b ), a cylindrical lens array  871  is positioned adjacent the plurality of PLCs  810 ,  820 ,  830  to cause light beams (e.g. light beam  802  from PLC  820 ) emitted from waveguides on the PLCs  810 ,  820 ,  830  to converge in a horizontal direction (e.g. as light beams  850 ,  852 ), as similarly discussed with regard to the embodiments of  FIGS. 7   a - b . The light beams (e.g. light beam  802 ) emitted from the PLCs  810 ,  820 ,  830  are collimated in a vertical direction by a cylindrical lens  875  such that the light beams strike a micromirror (e.g. micromirror  882 ) of the micromirror array  880 . The micromirrors of the micromirror array  880  can be tilted or rotated about an axis such that light beams (e.g.  854 ,  856 ) are reflected and directed by the micromirrors through the lens  875  and the cylindrical lens array  871 . The lens  875  and the cylindrical lens array  871  image the light beams in the vertical and horizontal directions, respectively (e.g. as light beam  808 ), onto an edge facet of a PLC  810 ,  820 ,  830 , coupling the light beams into waveguides (e.g. waveguide  812 ) on a PLC  810 ,  820 ,  830 .  
         [0038]     Those skilled in the art will appreciate that the switching functionality of the present invention is not limited to the vertical direction. Various modifications can be made which allow for switching between waveguides on a single PLC, between groups of waveguides on one or more PLCs, etc. For example, in an optical switch  900  according to another embodiment of the invention shown in  FIGS. 9   a - b , an array of spherical lenses  975  is placed near the edge facets  918 ,  928 ,  938  of the plurality of PLCs  910 ,  920 ,  930 . Preferably, the aperture of each lens in the array of spherical lenses  975  covers a subset of the waveguides (e.g.  912 ,  914 , etc. or  916 ,  917 , etc) on the PLCs  910 ,  920 ,  930 .  
         [0039]     In operation, for example, a light beam  950  emitted from a waveguide on a PLC (e.g. PLC  920 ), radiates from the PLC and diverges in the vertical and horizontal directions. The light beam  950  passes through a lens  975   a  of the array of spherical lenses  975 , which collimates the light beam  950  in the vertical and horizontal directions (as light beam  952 ). The light beam ( 952 ) strikes a micromirror  982  of the micromirror array  980 . The micromirrors  982  of the micromirror array  982  can be tilted or rotated about two axes to direct beams at a desired angle and in a preferred direction. The light beam  952  is reflected and directed by the micromirror (as light beam  954 ) through the lens  975   a , which images the light beam  956  in the vertical and horizontal directions onto the edge facet  918  of another PLC  910 , and couples the light beam  956  into a waveguide  914 .  
         [0040]     In another alternative embodiment of an optical switch  1000  according to the invention shown in  FIGS. 10   a - b , the array of spherical lenses  975  of the embodiment shown in  FIGS. 9   a - b  can be replaced by a single spherical lens  1075  having an aperture covering each waveguide of the plurality of PLCs  1010 ,  1020 ,  1030 , and the micromirror array  980  can be replaced by a single micromirror  1082 .  
         [0041]     In operation, for example, a light beam  1050  emitted from a waveguide on a PLC (e.g.  1020 ), radiates from the PLC  1020  and diverges in the vertical and horizontal directions. The light beam  1050  passes through the spherical lens  1075 , which collimates the light beam  1050  in the vertical and horizontal directions (as light beam  1052 ). The light beam  1052  strikes the micromirror  1082 , which can be tilted or rotated about two axes to direct the beam (as light beam  1054 ) through the lens  1075 , imaging the light beam (light beam  1056 ) in the vertical and horizontal directions onto an edge facet  1018  of another PLC  1010  and coupling the light beam  1056  into a waveguide  1016 .  
         [0042]     It will be appreciated by those skilled in the art that an optical switch according to the present invention can function as a wavelength-selective switch by using demultiplexer/multiplexer devices on the PLCs which generate optical signals at unique wavelengths on the waveguides of the PLCs. By then controlling the coupling of light beams into selected waveguides wavelength-selective switching can be achieved.  
         [0043]     It will also be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.