Abstract:
A bulk optic (de)multiplexer for fiber optic communications systems includes a diffraction grating having a diffraction surface, a waveguide array including a plurality of waveguides having an input/output end for emitting and receiving optical signals, and a focusing optic in optical communication between the diffraction grating and the waveguide array along an optical axis. The focusing optic focuses beams from the diffraction surface of the grating for optical coupling with the input/output ends of the waveguides. The (de)multiplexer further includes a frame. A fixed mount is provided between the focusing optic and the frame. A first adjustable mount is provided between the waveguide array and the frame and a second adjustable mount is provided between the diffraction grating and the frame. Preferably, the optical axis corresponds to a Z axis of orthogonal X, Y, Z axes and the waveguides of the waveguide array are aligned with the input/output ends along an input/output axis. The first adjustable mount is configured to provide for linear movement of the waveguide array along the Z axis and movement of the input/output axis within a plane parallel to the X, Y axes. The second adjustable mount is preferably configured to provide only for gimbaled movement of the grating about the point on the diffraction surface of the grating intersecting the optical axis. A method of optically aligning a (de)multiplexer as described above includes affixing the focusing optic to the frame with the focusing optic defining the optical axis, moving the waveguide array relative to the focusing optic only linearly along the optical axis and moving the grating only by rotating the grating about three orthogonal axes at a point on the grating intersected by the optical axis.

Description:
RELATED APPLICATIONS 
     This application claims priority from the commonly-owned utility application filed concurrently with the present application entitled “Thermally Stable Multiplexer/Demultiplexer,” which is incorporated herein in its entirety. 
     TECHNICAL FIELD 
     The present invention is directed towards optical communications, and more particularly toward a structure facilitating alignment of the optical elements of a bulk optical multiplexer/demultiplexer. 
     BACKGROUND ART 
     At the inception of fiber optic communications, typically a fiber was used to carry a single channel of data at a single wavelength. Dense wavelength division multiplexing (DWDM) enables multiple channels at distinct wavelengths within a given wavelength band to be sent over a single mode fiber, thus greatly expanding the volume of data that can be transmitted per optical fiber. The wavelength of each channel is selected so that channels do not interfere with each other and the transmission losses to the fiber are minimized. While typical DWDM allows up to 40 channels to be simultaneously transmitted by a fiber, there is an ongoing effort to further increase the number of channels transmitted for a given wavelength band by an optical fiber. 
     DWDM requires two conceptually symmetric devices: a multiplexer and a demultiplexer. A multiplexer takes multiple beams or channels of light, each at a discreet wavelength and from a discreet source and combines the channels into a single multi-channel or multiplexed beam. The input is typically a linear array of waveguides such as a linear array of optical fibers. The output is typically a single waveguide such as an optical fiber. A demultiplexer spatially separates a multiplexed beam into separate channels according to wavelength. Input is typically a single input waveguide or fiber and the output is typically a linear array of waveguides such as optical fibers. 
     There are a number of different DWDM devices known in the art, including array waveguides (see Li, U.S. Pat. No. 5,706,377), devices using a network of filters and/or fiber Bragg gratings for channel separation (see Pan, U.S. Pat. No. 5,748,350), and a variety of bulk optical DWDM devices. Bulk optical multiplexers and demultiplexers consist of discreet optically aligned optical elements. For example, a wavelength dispersive element such as a reflective diffraction grating, a focusing optic such as a lens, and a waveguide array which may consist of a multi-channel or multiplex waveguide such as a single mode optical fiber and a linear array of single channel waveguides, typically also single mode optical fibers. In a demultiplexing operation, the multi-channel or multiplexed optical signal is emitted from the multi-channel waveguide, directed through and collimated by the focusing optic, and reflected off the diffraction grating. The diffraction grating divides the multi-channel beam into single channel beam components which are reflected through the focusing optic and focused by the focusing optic to optical focal points coupling with the single channel optical waveguides. The multiplexer simply works in reverse, with single channel signals being emitted from the single channel optical fibers, combined into a multiplex signal and coupled to the multiplex optical fiber. Because a single device can perform as a multiplexer or a demultiplexer, it is referred to as a (de)multiplexer herein. Critical to the proper operation of a bulk optic (de)multiplexer is maintaining proper optical alignment of the waveguide array, focusing optic, and diffraction grating to provide efficient coupling of the optical signals to the respective waveguides with minimal or no crosstalk. To date, providing a structure for facilitating proper alignment of the optical elements and for maintaining the optical elements in the desired optical alignment has proven illusive. 
     Schultheiss, U.S. Pat. No. 4,718,056, is directed to a bulk optical (de)multiplexer including a diffraction grating, a lens and an optical fiber harness. In Schultheiss, the diffraction grating, lens, and fiber harness are all mounted to a frame by adjustable mounts. While having each of the optical elements on its own adjustable mount clearly makes it possible to optimize the optical alignment of the (de)multiplexer optical elements, it actually over complicates alignment because none of the optical elements are fixed relative to the frame to provide a reference point, thus necessitating adjustment of each element during optical alignment. 
     Ignatuc, U.S. Pat. No. 5,195,707, is directed to an optic positioning device for holding an optical element which has a center and for adjusting the optical element relative to the center. The positioning device includes a supporting base having a concave spherical surface and a holding body having a convex spherical surface which is slidably mated with the concave spherical surface. Both the concave and convex spherical surfaces have radial centers at the center of the optical element. Ignatuc allows for gimbaled movement of the optical element about its optical center. However, the mating concave and convex spherical surfaces provide a large surface contact area which can make it difficult to make small, precise movements of the optical element due to “sticktion” between the surfaces. Ignatuc also requires that both spherical surfaces be made to precise tolerances in order to insure the center of the optical element remains at a fixed location. This increases manufacturing costs. 
     The present invention is intended for overcoming one or more of the problems discussed above. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention is a bulk optic (de)multiplexer for fiber optic communications systems including a diffraction grating having a diffraction surface, a waveguide array including a plurality of waveguides having an input/output end for emitting and receiving optical signals, and a focusing optic in optical communication between the diffraction grating and the waveguide array along an optical axis. The focusing optic focuses beams from the diffraction surface of the grating for optical coupling with the input/output ends of the waveguides. The (de)multiplexer further includes a frame. A fixed mount is provided between the focusing optic and the frame. A first adjustable mount is provided between the waveguide array and the frame and a second adjustable mount is provided between the diffraction grating and the frame. Preferably, the optical axis corresponds to a Z axis of orthogonal X, Y, Z axes and the waveguides of the waveguide array are aligned with the input/output ends along an input/output axis. The first adjustable mount is configured to provide for linear movement of the waveguide array along the Z axis and independent movement of the input/output axis within a plane parallel to the X, Y axes. The second adjustable mount is preferably configured to provide only for gimbaled movement of the grating about a point on the diffraction surface of the grating intersecting the optical axis. 
     A second aspect of the present invention is an attachment assembly for attaching a diffraction grating having a diffraction surface of an optical (de)multiplexer to a frame of the (de)multiplexer, the (de)multiplexer having optical elements in addition to the grating, the optical elements being attached to the frame and aligned along an optical axis. The attachment assembly includes a grating mount having a leading surface to which the grating is attached with a diffraction surface in a select orientation relative to the grating mount and a spherical surface having a radial center at a point on the diffraction surface of the grating. A receptacle on the frame has a surface which is conical about a central axis receiving the spherical surface of the grating mount with the optical axis intersecting the point on the refractive surface of the grating. A clamp or stay is operatively associated with the grating mount and the frame for fixing the grating mount relative to the frame with the diffraction surface in a select orientation relative to the optical axis. 
     A third aspect of the present invention is a method of optically aligning a (de)multiplexer for fiber optic communications systems. The (de)multiplexer includes a diffraction grating having a diffraction surface for dividing a multi-channel incident beam into single channel beams, a waveguide array including a plurality of waveguides having an input/output end for receiving the single channel beams, and a focusing optic in optical communication between the diffraction grating and the waveguide array along an optical axis, the focusing optic focusing the single channel beams from the diffraction surface of the grating for optical coupling with the input/output ends of the waveguides. The method includes fixing the focusing optic to a frame of the (de)multiplexer to define an optical axis of the (de)multiplexer corresponding to the optical axis of the focusing optic, moving the waveguide array relative to the focusing optic linearly along the optical axis and moving the grating only by rotating the grating about three orthogonal axes at a point on the grating surface intersected by the optical axis. 
     The bulk optic (de)multiplexer of the present invention provides a combination of fixed and adjustable mounts which eases alignment of the (de)multiplexer optical elements. The fixed mount of the focusing lens allows the optical axis of the focusing lens to define a reference about which the other optical elements can be aligned. The adjustable mount between the grating and the frame not only provides for gimbaled movement of the grating about a point on the grating surface intersected by the optical axis to aid in proper alignment of the grating, it also provides a structure which prevents a change of orientation of the grating relative to the optical axis while providing movement of the grating only along the optical axis due to temperature changes or clamping of the adjustable mount. The adjustable mount associated with the waveguide array allows for independent movement of the waveguide array along the optical axis and for independent movement of the waveguide array in a plane normal to the optical axis. This independence of movement further facilitates efficient alignment. The apparatus further facilitates the claimed method of aligning the (de)multiplexer which simplifies and expediates alignment of the optical elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a (de)multiplexer in accordance with the present invention with the top of the (de)multiplexer housing cut away; 
     FIG. 2 is a perspective view of a (de)multiplexer frame in accordance with the present invention; 
     FIG. 3 is a cross-section of the (de)multiplexer frame of FIG. 2 taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a schematic perspective view of an optical fiber waveguide array used in the present invention; 
     FIG. 5 is an enlarged cross-section of a slidable connection between fixed and moving X, Y stages; 
     FIG. 6 is a front elevation view of a moving X, Y stage in accordance with the present invention; 
     FIG. 7 is a cross-section view of the moving X, Y stage of FIG. 6 taken along line  7 — 7  of FIG. 6; 
     FIG. 8 is a front elevation view of a waveguide array attached to the cantilevered platform of the movable X, Y stage in accordance with the present invention; 
     FIG. 9 is a perspective view of a grating holder depicted in FIG. 3; 
     FIG. 10 is a sectional view illustrating pivoting of the grating holder about the optical axis; 
     FIG. 11 is a schematic representation of a properly aligned (de)multiplexer; 
     FIG. 12 illustrates movement of the focal points of single channel beams as a function of an increase in temperature along with the movement of the waveguide array as a function of increase in temperature to maintain coupling with the single channel beams. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a plan view of a (de)multiplexer in accordance with the present invention. A (de)multiplexer frame  12  resides within a housing  14 . The housing  14  consists of a top and a bottom portion that fit together in a sealing relationship as will be described in greater detail below. In FIG. 1 the top portion has been removed to reveal the (de)multiplexer frame. 
     The (de)multiplexer frame  12  is shown in perspective removed from the housing  14  in FIG.  2 . The (de)multiplexer frame maintains the optical elements of the (de)multiplexer in optical alignment. FIG. 2 also includes a depiction of orthogonal X, Y, Z axes which will be used as reference axes throughout this description. The Z axis is collinear with the optical axis  22 . 
     Referring to FIG. 3, the optical elements of the (de)multiplexer include a wavelength dispersive element  16 , a focusing optic  18 , and a waveguide array  20 . The wavelength dispersive element, the focusing optic, and the waveguide array are maintained by connection to the frame in optical communication with one another along an optical axis  22 . Thus, the elements are in what is commonly known as littrow alignment. 
     The wavelength dispersive element  16  is preferably a reflective diffraction grating formed using conventional techniques from a glass substrate having a negligible coefficient of thermal expansion. One preferred substrate material is ZERODUR, manufactured by the Schott Company. A diffraction surface  24  of the grating has a large number of grooves which are formed parallel to a Y axis normal to the cross-sectional plane of FIG.  3 . The grooved surface has a highly reflective coating, such as gold. Representative gratings include echellette and preferably echelle gratings, as are disclosed in commonly assigned, copending U.S. patent application Ser. No. 09/628,774, entitled “Echelle Grating Dense Wavelength Division Multiplexer/Demultiplexer”, the contents of which are incorporated in their entirety herein. 
     The focusing optic  18  is preferably a spherical symmetric doublet lens, although other lens structures could be suitable as well. The spherical symmetric doublet lens has a select focal length within a relatively narrow range of tolerance along an optical axis. The lens also has a refractive index that varies a known amount as a function of temperature. Furthermore, the lens surfaces deform in a predictable manner with changes in temperature. 
     The waveguide array  20  is shown in greater detail in FIG.  4 . The waveguide array  20  consists of a number of single channel waveguides  26  having an input/output end  28  with the input/output ends  28  aligned along an input/output end axis  30  and a multiplex or multi-channel waveguide  32  vertically below and at the center of the single channel waveguides  26 . Other embodiments known in the art, such as having the multiplex waveguide coincident with the input/output end axis or above it, may also be suitable. Also, stacked multiplex waveguides and single channel arrays (as described in U.S. patent application Ser. No. 09/628,774) may be used with the invention described herein. For the sake of simplicity, a limited number of single channel waveguides, here,  20 , have been shown as comprising the waveguide array. The preferred embodiment will have many more single channel waveguides, such as  48  or more. In the preferred embodiment the single channel waveguides  26  and the multiplex waveguide  32  are all single mode optical fibers. The fibers are held in place between two pieces of a silicon substrate or wafer  34  having V-shaped grooves  36  which are precisely etched in the substrate pieces to maintain the single channel fibers at a precise desired spacing from one another. The silicon is preferably high purity prime grade. Likewise, the multiplex optical fiber  32  is held in a precise location relative to the single channel optical fibers by a third piece of silicon substrate with an appropriate V groove. As stated above, the input/output ends of the single channel fibers terminate along an input/output end axis  30 , and this axis is in the same plane as the input/output end of the multiplex waveguide  32 . As shown in FIG. 4, the spacing of the waveguides increases slightly from left to right so as to couple with the corresponding single channel beams which increase in separation in a like manner. In the preferred embodiment the dimensions of the assembled substrate are 15 mm along the X axis, 1.5 mm high and 12 mm deep. 
     In order to simplify alignment and increase alignment tolerances, a microlens array  37  may be incorporated into the substrate. The microlens array also increases band pass. The microlens array consists of a focusing lens  38  along an optical axis of each waveguide, with the lenses coupled by a frame  39  attached to the substrate  34  made of a material having a CTE matching that of the substrate  34  to thermally expand with the substrate to promote optical coupling along the waveguide optical axes. 
     The (de)multiplexer frame  12  of the preferred embodiment includes a telescope portion  40 , a cone housing portion  42  attached to one end of the telescope portion by conventional fasteners such as screws, and a flexure portion  44  similarly attached to the other end of the telescope portion  40 . Alternatively, the frame could be a single integral piece, two pieces or more than three pieces. The multiplexer frame  12  further includes a first adjustable mount  46  associated with the flexure portion  44 , a second adjustable mount  48  associated with the cone housing portion  42  and a fixed mount  50  within the telescope portion  40 . 
     The first adjustable mount  46  consists of a flexure  52  which is movable axially within a limited range (±1 mm) only along a Z axis as indicated by the arrow  54 . In the preferred embodiment the flexure is integrally formed of the flexure portion of the frame, but it could also be a separate structural element affixed to the frame by suitable connectors. As seen in FIG. 2, the preferred embodiment includes identical top and bottom flexure elements  52 A and  52 B to prevent tipping of the waveguide array as the flexure is moved along the Z axis. A single flexure element suitably arranged could perform the same function as well and is within the scope of the invention. Other structures for providing movement only along the Z axis may be suitable as well. A noninfluencing lock piece assembly  56  preferably connects the flexure  52  to the flexure portion of the frame so that the flexure  54  can be secured in a select position along the Z axis. Referring to FIG. 2, the preferred embodiment uses two noninfluencing lock piece assemblies  56 A and  56 B, one on the top and one on the bottom of the flexure with each assembly secured on opposite sides of the frame. Other stay structures or clamps may be used instead of the noninfluencing lock piece, but substitutes would preferably also not influence the Z axis position as they are secured. 
     The first adjustable mount  46  further consists of a fixed X, Y stage  58  which is fixedly attached to the Z axis flexure  52  by screws or the like (not shown) so that the first X, Y stage moves with the Z axis flexure  52 . The fixed X, Y stage  58  has a planar surface  60  substantially normal to the optical axis  22 . A moving X, Y stage  62  has a planar surface  64  which abuts the planar surface  60  of the fixed X, Y stage  62 . The planar surfaces can be moved relative to one another parallel to the X, Y axis by virtue of sliding connectors  66 , and the planar surfaces are preferably anodized aluminum to facilitate sliding. A sliding connector  66  is shown in greater detail in FIG.  5 . The sliding connector  66  consists of a bolt  68  having a shaft body  70  and a threaded tip  72 . The threaded tip  72  is threadably engaged in a threaded hole  74  in the moving X, Y stage  62 . The shaft body  70  resides in a hole  74  in the fixed X, Y stage  58  having an inner diameter greater than the outer diameter of the shaft body  70  to allow a desired degree of freedom of movement of the moving X, Y stage relative to the fixed X, Y stage in an X, Y plane. In the preferred embodiment this is about ±0.5 mm of movement. A washer  76  resides between the head of the bolt  68  and a planar surface of the fixed X, Y stage  58  opposite the planar surface  60 . To facilitate movement of the moving X, Y stage  62  relative to the fixed X, Y stage  58 , the washer  76  has an annular trough abutting the fixed X, Y stage which is filled with tiny glass beads having a diameter slightly greater than the depth of the annular trough encased in suitable grease. Thus, the sliding connector  66  allows limited movement between the moving X, Y stage and the fixed X, Y stage within an X, Y plane with minimal friction induced sticktion between the planar surfaces of the stationary and moving stages. The moving X, Y stage is secured in place by noninterfering lock pieces  77  on opposing sides of the moving X, Y stage, or other suitable stays or clamps. The non-interfering lock piece assemblies  56  and  77  collectively form a stay for the first adjustable mount  46 . By the combination of the Z axis flexure  52  and the sliding connector  66  between the fixed X, Y stage and the moving X, Y stage, the moving X, Y stage and the associated waveguide array  20  may be moved axially of the Z axis, rotated about the Z axis, and moved linearly within an X, Y plane normal to the Z axis. In addition, the waveguide array  20  can be moved independently along the Z axis. In other embodiments, the first adjustable mount might be limited to the Z axis flexure (or some other structure providing movement along the Z axis) without the moving X, Y stage, or vice versa. 
     The moving stage is shown in greater detail in FIG.  6  and FIG.  7 . FIG. 6 is a front elevation view of the moving X, Y stage and FIG. 7 is a sectional view taken along line  7 — 7  of FIG. 6. A cantilevered connector  80  extends from the moving X, Y stage from a side opposite and in a direction normal to the planar surface  64 . The cantilevered connector  80  consists of a platform  82  which extends in a cantilevered manner from a post  84 . The top surface of the platform has a recess  86 . 
     FIG. 8 illustrates how the waveguide array  20  is attached to the platform  82  by a flexible connector  88  which allows the waveguide array to expand when heated relative to the platform  82 . In the preferred embodiment the flexible connector  88 .consists of a rigid connection  90  between a bottom of the substrate  34  near the post  84  and an elastic connection  92  near the distal end of the cantilevered platform  82 . The rigid connection  90  is made by a suitable rigid curing epoxy and the flexible connection  92  is made by a suitable elastic curing epoxy. The waveguide array  20  is epoxied in place bringing the waveguide array into registration with appropriate guides and spacers (not shown) associated with the platform  82 . Both the rigid and elastic curing epoxies have virtually identical thermal coefficiencies of expansion to preventing tipping of the waveguide array  20  about a Z axis by changes in temperature. 
     The second adjustable mount  48  includes the rear wall of the cone housing portion  42  of the frame  12  having a concave and preferably a conical recess or receptacle  98  formed therein. The conical recess  98  is symmetric about a central axis which is preferably collinear of very close to collinear with the optical axis  22 . A circular hole  100  having a central axis along the optical axis  22  extends from the conical wall of the conical recess  98  through the rear wall. The surface  25  of the rear wall opposite the conical recess  102  is spherical and has a radial center which preferably is at about a point  104  where the optical axis  22  intersects the diffraction surface  24  of the grating  16 . The second adjustable mount  48  further includes a grating mount  106  having a leading planar surface  108  and a trailing spherical surface  110 . The spherical surface  110  may include a low friction coating  111  such as PTFE or carbide. The spherical surface  110  has a radial center at the point  104  where the optical axis intersects the diffraction surface  24  of the grating with the trailing spherical surface  110  nested in the conical recess  98 . A clamp assembly or stay  112  secures the grating mount  106  with its spherical surface engaging the conical recess  98  as illustrated in FIG.  3 . The clamp preferably consists of a threaded tail  114  which extends along a central axis of the spherical surface through the hole  100  in the rear wall of the cone housing. A washer  116  having a spherical recess  118  including an annular contact rib  120  resides between the spherical wall  102  and a locking nut  122  with the contact rib  120  contacting the spherical wall  102 . The contact rib  120  has a low friction coating  121  such as PTFE or carbide. A spring washer  124  is preferably provided between the locking nut and the spherical washer  116 . The spring washer  124  allows the locking nut to be loosened somewhat while the clamp assembly  112  is still held in position so that the grating mount  106  can be repositioned. As should be clear from FIG. 3, the cooperation between the spherical surface of the grating mount and the conical recess  98  allows for true gimbaled movement of the diffraction surface of the grating about the point  104 . In other words, the grating can be rotated about the point  104  along orthogonal three axes within a limited range of motion. This is illustrated in FIG. 10. A notch  125  at the distal end of the threaded tail  114  defines a first planar surface  126  in an Z, Y plane and a second planar surface  127  in an X, Y plane as depicted in FIG. 3, parallel to the grooves of the grating  16 , to facilitate course alignment of the grating  16 . 
     The grating mount  106  is shown in a perspective view in FIG.  9 . Extending on either side of the leading planar surface  108  are standing ribs  128 . The standing ribs  128  are included to ensure that as the grating holder is subjected to changes in temperature, it expands and contracts along the optical axis  22  with the diffraction surface of the grating maintaining its orientation relative to the optical axis  22 , as illustrated schematically in FIG.  12 . The grating mount is preferably made of titanium or  416  stainless steel to ensure rigidity and strength, although other low CTE materials such as INVAR, KOVAR or ZERODUR may be used as well. 
     The grating is attached to the leading planar surface  108  by a suitable epoxy and is positioned on the leading surface in a select orientation by registration with an inner wall of the standing ribs  128  and suitable spacers. 
     FIG. 10 illustrates gimbaled movement of the diffraction surface about the point  104 . With the grating mount  106  tilted relative to the optical axis  22  as shown in FIG. 10, the point  104  is still intersected by the optical axis  22 . Also, the washer  100  is self-aligning relative to the spherical surface  102  to maintain complete contact of the annular contacting rib  120  with the spherical surface. This structure also keeps a ring of contact of the washer opposite that of the spherical surface  110  of the holder to minimize distortion of the cone housing. While in the preferred embodiment the spherical surface  100  and conical recess  98  have central axes along the optical axis  22 , their central axes could be non-colinear with the optical axis and still allow for the desired gimbaled movement. 
     The fixed mount  50  simply consists of an enlarged inner diameter portion  130  of the telescope portion  40  of the frame  12 . The focusing optic  18  is axially inserted in this enlarged diameter of portion  130  and secured in place by an appropriate adhesive or held in place by an appropriate mechanical clamp suitable for maintaining the optical alignment over a wide temperature range, e.g., −5° C.-65° C. 
     The optical elements of the (de)multiplexer  10  are aligned as follows. First, the lens  18  is brought into registration with the wall at the end of the larger diameter portion of the fixed mount  130  and the optical axis of the lens defines the optical axis  22  of the (de)multiplexer. The lens is then cemented or clamped in place. The grating is coarsely aligned by having the first surface  126  parallel to the Z, Y reference plane and the second surface  127  parallel to the X, Y reference plane. The moving X, Y stage is coarsely aligned simply by connection of the X, Y moving stage to the X, Y fixed stage with waveguide array attached to the X, Y moving stage as discussed above. Following coarse alignment, the optical elements are close to littrow alignment. In the preferred embodiment, an actuator for moving the Z flexure along the Z axis is operatively associated with the Z flexure, an actuator for moving the X, Y moving plate along the X axis is associated with the X, Y moving plate, and an actuator for moving the X, Y moving plate along the Y axis is associated with the X, Y moving plate. Likewise, an actuator for each rotational degree of movement of the second adjustable mount is operatively associated with the grating mount  106 . A multiplex beam of the operative wave band of light is propagated to the multiplexer through the multiplex optical fiber  32 . Photo detectors are associated with the end single mode fibers to monitor light output at these points. Alternatively, the actuators could combine one or more degree of movement. An automatic alignment device utilizing a six axis feed back loop controlled algorithm controls actuation of each actuator to move the grating and waveguide array relative to the optical axis to optimize optical signal strength at the extreme ends of the single channel array. First the beam strength is optimized at one end of the signal channel array and then the other by coordinated movements of the six actuators by the automatic alignment device until signal strength in these single channel optical fibers is maximized. Then the first and second adjustable mounts  46 ,  48  are clamped into place as described above. 
     The multiplexer  10  is also configured so that “optical coupling” or the coupling integrity, that is efficiency and the level of crosstalk, remain within acceptable specified levels within a relatively wide range of temperatures, more particularly from about −5° C. to 65° C. For example, for efficiency, loss should not increase greater than 0.5 db and cross-talk should not increase more than 5 db. The problem addressed by the unique “athermalized” frame of the present invention is illustrated schematically in FIG.  11  and FIG.  12 . Referring to FIG. 11, with the multiplexer properly aligned and with the (de)multiplexer temperature at room temperature, that is about 20° C., light from the multi-channel or multiplex waveguide  32  is projected as a multi-channel beam to the focusing optic  18  which collimates the multi-channel beam and directs it off the diffraction surface  24  of the grating  16 . The diffraction grating divides the multi-channel beam by wavelength into a number of single channel beams which are diffracted off the diffraction surface  24  to the focusing optic  18  which directs the focused singled channel beams to select focal points for optical coupling with the input/output ends of the single channel waveguides  26 . 
     FIG. 12 illustrates the effect of an increase in temperature on the beams  140 . As temperatures increase the refractive index of the lens  18  decreases and the surfaces of lens are slightly deformed so as to cause the focal length of the lens  18  to increase. Thus, the select focal points extend beyond the plane of the input/output ends of the optical fibers  26 ,  32 . At the same time, there is a change in the refractive index of the air occupying the space between the diffraction grating  16  and the lens  18  and the air between the lens  18  and the waveguide array  20 . This change in the refractive index of the air causes a lateral shift of the select focal points of the single channel beams  140  along the X axis as is also illustrated in FIG.  12 . If the temperature change is great enough, the location of the focal points of the single channel beams can move far enough out of alignment with the input/output ends of the single channel fibers  26  to significantly degrade the efficiency of optical coupling, leading to an unacceptable loss of efficiency and to crosstalk. 
     In the preferred embodiment, temperature changes result in both axial and radial movement of the focal points as a function of temperature. In alternate embodiments, some form of athermalization could eliminate either the radial or axial movement. In such alternate embodiments, it would be necessary only to provide for a corresponding movement of the input/output ends of the waveguides. 
     Although the position of the grating may move along the optical axis as a result of temperature changes expanding or contracting the frame between the lens and the grating and thermal expansion or contraction of the grating holder itself, the light in this region is collimated so these movements have no optical effect. Also, as discussed above, the grating substrate is athermal (negligible CTE) and the grating holder is configured to maintain the orientation of the diffractive surface of the grating as the grating holder is subjected to temperature changes. Thus, the grating itself is believed to be insensitive to temperature changes for alignment purposes. 
     To address the problem of migration of the focal points as a function of changes in (de)multiplexer temperature, the frame  12  of the (de)multiplexer includes a number of novel CM) features. First, the frame is made of a thermally expansive material, for example, an aluminum alloy such as  6061  T 6  having a coefficient of thermal expansion (CTE) that allows the frame to vary in length as a function of temperature. Briefly, the change in length as a function of temperature is determined by the well known equation ΔL=L αΔT, where ΔL is the change of length, L is the length of the thermally expansive material, αis the coefficient of thermal expansion of the thermally expansive material, and ΔT is the change in temperature. Herein the terms “thermally expansive” and “thermal expansion” and the like are used to mean both expansion or contraction, with the ΔL being positive or expansive as temperature increases and ΔL being negative or there being a contraction as the temperature decreases, unless the context clearly indicates it is intended to be limited to an expansion, such as where the term is associated with an increase in (de)multiplexer temperature. The frame  12  is symmetric about the optical axis  22  to provide equal lengths L of material along the optical axis  22 , and therefore uniform expansion along the optical axis with changes in (de)multiplexer temperature. Thus, in the preferred embodiment the telescope portion of the frame  40 , the cone portion  42  of the frame and flexure portion  44  of the frame are all symmetric about the optical axis. As a result, these portions of the frame change length only along the optical axis  22 , simplifying maintaining alignment during temperature induced length changes. Most importantly, the length of frame material connecting the lens  18  and the waveguide array  20  along with the material(s) from which this portion of the frame is made are selected to provide a length L and CTE such that the length along the optical axis of the portion of the frame between the waveguide array and the lens changes with temperature about the same amount that the focal length of the single channel focusing beams changes as a function of temperature to maintain optical coupling of the focusing points and the input/output ends of the single channel fibers. 
     To compensate for the lateral shift along the X axis of the single channel beams  140  caused by a change in temperature of the air, the structure is similar in principle. Referring first to FIG. 8, the substrate  34  of the waveguide array  20  is a thermally expansive material so that increases in temperature will cause it to expand and decreases in temperature will cause it to contract along the X axis. To move the input/output ends of the single channel waveguides the same direction as the lateral shift of the beams with temperature, the flexible connector  88  includes the first end of the substrate  34  having a rigid connection  90  to the cantilevered connector  80  near the post  84  and an elastic connection  92  near the distal end of the cantilevered connector  80 . As a result, as the substrate  34  grows along its X axis as a function of increases in temperature, the substrate will expand toward the elastic connection  92 , causing distortion of the elastic connection  92  as illustrated in phantom lines in FIG.  8 . This will provide some of the lateral shift in a direction along the X axis necessary to move the input/output ends of the single channel fibers into the desired degree of optical coupling with the focal points of the single channel signals  140 . To further move the input/output ends along the X axis, in the preferred embodiment the cantilevered connector  80  is also made of a thermally expansive material such as an aluminum alloy. Referring to FIG. 8, the distance between the midpoint of the post  84  and the rigid connection  90  (L c ) will provide further lateral displacement of the waveguide array along the X axis with changes in temperature. The amount of change, ΔL will be determined as follows: ΔL c =L c  αΔT. Thus, the distance L c  as well as the coefficient of thermal expansion of the material from which the cantilevered connector  80  is made can be adjusted to provide sufficient movement along the X axis to insure the maintenance of optical coupling between the single channel beams  140  and the input/output ends of the single channel fibers  126 . 
     While the preferred embodiment compensates for lateral shift along the X axis by combining the thermal expansion of the waveguide substrate and the cantilevered connector, in other embodiments either one may provide the necessary lateral shift to maintain suitable optical coupling, and the other could be “athermalized.” For example, the flexible connector between the substrate and the cantilevered connector could have two elastic connections (and no rigid connection) so that only thermal expansion of the cantilevered connector  80  moves the input/output ends a significant amount to maintain optical coupling. Or, the cantilevered connector  80  could be modified to not be cantilevered or it could be made of an athermal material so optical coupling is maintained solely by thermal expansion of the waveguide substrate. 
     FIG. 12 also illustrates schematically the effect of temperature on the grating mount  106 . As discussed above and as illustrated in FIG. 12, the standing ribs  128  insure that the grating mount  106  will move the diffraction grating  16  along the optical axis  22  while maintaining the diffraction surface  24  in the same select orientation relative to the optical axis  22 . 
     Referring back to FIG. 1, vibration damping supports  150  support the (de)multiplexer frame  12  within the housing  14 . The vibration damping supports  150  include vertical supports  152  and horizontal supports  154 . Both the vertical and horizontal supports  152 ,  154  are positioned to suspend the frame within the housing. The vertical and horizontal supports  152 ,  154  are made of an elastomeric material of a select formulation chosen to dissipate vibrations most likely to be encountered by the (de)multiplexer  10 . Representative vibrations of concern would be low frequency vibrations such as might be incurred in an earthquake, middle frequency vibrations such as might be incurred during transport of the assembled (de)multiplexers and high frequency vibrations which might be generated by cooling fans and the like deployed in the vicinity of the multiplexer in operation. Representative materials for the vibration damping supports include natural and synthetic rubber, urethane and other polymeric elastomers, with urethanes being preferred. Choice of a particular material would be a function of many factors, including anticipated frequencies, anticipated temperatures, mass of the (de)multiplexer and the like. 
     Although not shown, a top housing piece mates with the bottom housing piece illustrated in FIG. 1. A seal is deployed between the housing segments so that the interior of the housing can be pressurized with nitrogen or another suitable gas to minimize the risk of corrosion of sensitive optical and mechanical parts as well as to provide for a uniform gaseous medium through which the beams are transmitted within the (de)multiplexer.