Abstract:
An optical switching device including an optical switching engine may be packaged by omitting an optical bench and disposing optical elements directly on a base of a housing of the optical switching device. The optical switching engine may be disposed on a ceramic portion of the base, and thermally matched to the ceramic base. The base may be reinforced by the housing walls and optional internal rigidity ribs. The optical elements may be thermally matched to the base, and the lid may be strain relieved by thinning lid edges. The housing may be mounted to an external chassis using soft grummets.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Provisional Patent Application No. 61/953,977 filed Mar. 17, 2014, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to optical devices, and in particular to optomechanical packaging of optical switching devices. 
       BACKGROUND 
       [0003]    Optical devices typically include optical elements held in a pre-determined spatial relationship by mechanical supports and holders. A mechanical package or housing may be provided to protect sensitive optical elements of an optical device from dust, shock, vibration, and other unwanted influences of the outside environment. To prevent external mechanical stresses from shifting optical components out of alignment, a dedicated rigid plate, termed “optical bench”, may be suspended inside the housing, and the optical elements may be mounted directly to the optical bench. When the optical bench is rigid enough, the mechanical stresses may be decoupled from the optical elements supported by the optical bench, reducing chances of an optical misalignment. 
         [0004]    Telecommunications equipment is frequently held in racks, or crates, having an array of parallel vertical slots for receiving individual telecommunication modules. The slots usually have a fixed width. Accordingly, optical modules used in telecommunication equipment need to be narrower than a multiple of the slot width, and preferably narrower than a single slot width, to fit into their dedicated slots in the racks or crates. The requirement of maximal allowed width, or height if placed horizontally, may provide a restriction of the maximal height of optical devices used in the telecommunication modules. A mere increase of the optical device height by one millimeter may render the optical device unusable, or it may require a stepwise increase of the telecommunication module width by an entire slot width. Many similar modules may be required for a multi-channel optical telecommunications system, further multiplying the required slot space. For this reason, it may be highly desirable to reduce height of individual optical modules. 
         [0005]    In view of the foregoing, it may be understood that there may be significant problems and shortcomings associated with current solutions for mounting optical components of optical devices into housings. 
       SUMMARY 
       [0006]    Optical benches for holding optical components may add a significant height to optical devices. It may be preferable to omit optical benches by placing individual optical components directly onto a base of a housing, and to attach a cover enclosing all the optics directly to the base. To reduce the stresses and resulting deformation, one may thermally match the optical components to the base, and strengthen the base by rigidly attaching walls of the cover, and/or dedicated rigidity ribs, to the base. The housing may be then flexibly suspended in an external frame or chassis, thereby reducing mechanical stresses exerted on the housing. By way of a non-limiting example, the housing may be suspended using a set of soft grummets or flexures coupled to the external chassis. 
         [0007]    In accordance with one aspect of the disclosure, there is provided an optical switching device comprising at least one input port for inputting an optical signal comprising a plurality of wavelength channels, at least one output port for outputting at least one of the wavelength channels, optics for dispersing and redirecting the wavelength channels between the input and output ports, a switching engine optically coupled with the optics for redirecting at least one of the wavelength channels towards a selected output port, and a housing for enclosing the optics and the switching engine. 
         [0008]    The housing may include a base comprising a first portion comprised of ceramic having a coefficient of thermal expansion (CTE) matched to that of the switching engine to within 2 ppm/° C. The switching engine may be mounted directly to the ceramic first portion. The housing may further include a cover enclosing the optics and the switching engine, wherein the cover is mounted directly to the base for stiffening the base. The optics may be CTE matched to the base to within 2 ppm/° C. and mounted directly to the base. The first ceramic portion of the base may include an extension extending beyond the cover and including electrical leads electrically connected to the switching engine and extending beyond the cover. The ceramic may be made of high-temperature co-fired ceramic (HTCC) assembly, which allows such leads to be integrally formed within the ceramic. 
         [0009]    The base may include a second portion extending from the first portion, for supporting at least some of the optics. By way of a non-limiting example, the second portion may be comprised of ceramic or metal. The material of the second portion may be CTE matched to the ceramic of the first portion to within approximately 4 ppm/° C. or less. In one embodiment, the housing may include a CTE compensator mounted directly to the second portion and having a CTE different from a CTE of the second portion by at least 1 ppm/° C., for lessening a wavelength drift of the optical switching device with temperature. The cover may include a frame and a lid mounted, e.g. soldered, to the frame along the frame perimeter. In one embodiment, the cover may further include a seal ring mounted directly to the frame and the lid outside of the frame along the perimeter of the lid. The frame may be mounted directly to the base, to stiffen the base, and at the same time to protect the optics and the switching engine. 
         [0010]    The housing may be mounted to an external chassis via mounts having elastic modulus of less than 20 MPa, and preferably less than 2 MPa. Depending on the type of the optical switching device, the optics may include a concave mirror, a lens, polarizing and directing optics, and a wavelength dispersing device such as a diffraction grating. The switching engine may include e.g. a liquid crystal in silicon (LCoS) array, a micro-electro-mechanical system (MEMS) array, a diffractive beamsplitter, or another optical element capable of redirecting multiple optical beams impinging thereon. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Exemplary embodiments will now be described in conjunction with the drawings, in which: 
           [0012]      FIG. 1A  illustrates a side cross-sectional view of an optical switching device of the present disclosure; 
           [0013]      FIG. 1B  illustrates a top view of the optical switching device of  FIG. 1A ; 
           [0014]      FIG. 2  illustrates a three-dimensional view of a temperature-stabilized, non-hermetic variant of the optical switching device of  FIGS. 1A and 1B ; 
           [0015]      FIG. 3  illustrates a side cross-sectional view of an optical switching device of the present disclosure, including a stiffener rib and a wavelength drift compensator; 
           [0016]      FIG. 4  illustrates a magnified side cross-sectional view of a lid mounted to a frame of a housing of the optical switching device of  FIGS. 1A-1B ,  2 , or  FIG. 3  using a seal ring; 
           [0017]      FIG. 5  illustrates a side cross-sectional view of a lid variant, which is thinned at the edges for reduction of pressure-induced base deformation of the optical switching device of  FIGS. 1A-1B  and  FIG. 3 ; 
           [0018]      FIGS. 6A to 6C  illustrate three-dimensional views of an optical switching device of the present disclosure at progressive stages of assembly; and 
           [0019]      FIG. 6D  illustrates a three-dimensional view of an assembled optical switching device. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. In  FIGS. 1A ,  1 B,  FIG. 2 ,  FIG. 3 , and  FIGS. 6A-6D , similar reference numerals refer to similar elements. 
         [0021]    Referring to  FIGS. 1A and 1B , an optical switching device  100  may include at least one input port  102  for inputting an optical signal  101  ( FIG. 1A ) comprising a plurality of wavelength channels  103 . At least one output port  104  may be provided for outputting at least one of the wavelength channels  103 . Optics  106  e.g. lenses, mirrors, polarizers, waveplates, etc., may be disposed and configured for dispersing and redirecting the wavelength channels  103  between the input  102  and output  104  ports. A switching engine  108  may be optically coupled with the optics  106  for redirecting at least one of the wavelength channels  103  towards a selected output port e.g. the output port  104  shown in  FIG. 1A . By way of an illustrative example and without limitation, the switching engine  108  may include a liquid crystal array, a LCoS array, a MEMS array, a diffractive beamsplitter, etc. The optical switching device  100  may include a multiport optical switch, a wavelength selective optical switch, a multicasting optical switch, etc. Dimensions of the optical switching device  100  may vary. A low height is preferred for reasons stated above, even for a larger footprint devices. For instance, the optical switching device  100  may have a footprint of at least 90 mm×50 mm and a height less than 27 mm, more preferably less than 20 mm or even less than 14 mm. 
         [0022]    A housing  110  may be provided for supporting and protecting the optics  106  and the switching engine  108  from dust, humidity, etc. The housing  110  may include a base  112 , e.g. a base plate, which may have a first portion  112 A and a second portion  112 B extending from the first portion  112 A. In one embodiment, the first portion  112 A is comprised of ceramic. The second portion  112 B may be comprised of ceramic or metal, which is CTE matched to the ceramic within 4 ppm/° C., more preferably to within 2 ppm/° C., and most preferably to within 0.5 ppm/° C. The second portion  112 B may support at least some of the optics  106 . The switching engine  108  may be mounted directly to the ceramic first portion  112 A, which may be CTE matched to the switching engine 108, e.g. to within 4 ppm/° C., more preferably to within 2ppm/° C., and most preferably to within 0.5 ppm/° C. In one embodiment, the first portion  112 A is comprised of a low-expansion ceramic e.g. aluminum nitride ceramic, and the second portion  112 B is comprised of a low-expansion alloy e.g. Kovar. 
         [0023]    The housing  110  may further include a cover  114  enclosing the optics  106  and the switching engine  108 . The cover  114  may be mounted directly to the base  112  for stiffening the base  112  and protecting the optics  106  and the switching engine  108 . Herein and throughout the rest of the specification, the term “mounted directly” means that the two parts are mounted one to another without any intermediate mechanical parts or components. Suitable attaching or bonding agents, such as solder, epoxy, etc., may be used to mount the two parts directly to each other. Thus, the cover  114 , when soldered to the base  112 , is still considered to be “mounted directly”, even though the cover  114  and the base  112  may be held together by a layer of solder spread in between the cover  114  and the base  112 . The optics  106  may be CTE matched to the base  112 , e.g. to within 4 ppm/° C., or more preferably to within 2 ppm/° C., and mounted either directly to the base  112 , or by using CTE-matched sub-mounts. 
         [0024]    Depending on the attachment method employed, the cover  114  may function as a stiffener of the base  112 . The stiffening provided by the cover  114  may improve optomechanical stability of the optical switching device  100  upon mounting of the optical device  100  to an external chassis  122  ( FIG. 1B ). Without the stiffening of the base  112 , small misalignments may even occur under the weight of the base  112  and the optics  106 . Resistance of the optical switching device  100  to shock and vibration may also be improved by stiffening the base  112 . The housing  110 , when hermetically sealed, may become susceptible to variations in ambient atmospheric or internal gas pressure, causing a deformation of the base  112  and a resulting misalignment of the optics  106 . The stiffening of the base  112  by the cover  114  may also lessen this pressure caused deformation. Hardness of solder used to attach the cover  110  to the base  112  may impact the stiffening effect of the cover  114  on the base  112 . 
         [0025]    In the embodiment shown in  FIG. 1A , the ceramic portion  112 A includes an extension  116  extending beyond the cover  114 . The extension  116  may include electrical leads  118  electrically connected to the switching engine  108  and extending beyond the cover  114 , as shown. The electrical leads  118  may be conveniently provided by using high temperature co-fired ceramic (HTCC), which may include integrated wiring. HTCC is a multilayered, sealed and highly reliable ceramic comprising multiple co-fired layers of ceramic tape including e.g. 92% aluminum oxide ceramic. The co-fired tape layers may incorporate metalized circuit patterns on ceramic players, which form the electrical leads  118 . The ceramic tape layers may be laminated under pressure, and the ceramic and metallization may be co-sintered at 1600° C., creating a monolithic structure with the electrical leads  118  locked within the ceramic. The electrical leads  118  may be coupled to an optional connector  120  disposed on the extension  116  of the ceramic portion  112 A, as shown in  FIG. 1A . 
         [0026]    Turning to  FIG. 1B  with further reference to  FIG. 1A , the cover  114  typically includes a frame  126  ( FIG. 1A ) for mounting directly to the base  112 , and a lid  128  having a perimeter  129  ( FIG. 1B ) and mounted to the frame  126  along the lid perimeter  129  forming a unified structure for stiffening the base  112 . The housing  110  of the optical switching device  100  may be mounted to the external chassis  122  ( FIG. 1B ) via optional mounts  124 , e.g. soft grummets or flexures. The mounts  124  may have modulus of elasticity of no greater than 20 MPa, and more preferably no greater than 2 MPa, to relieve the mechanical stresses caused by mounting the optical switching device  100  to the external chassis  122 . 
         [0027]    In one embodiment, the frame  126  includes an outer section, e.g. outer vertical solid wall,  126 A surrounding the optics  106  and the switching engine  108 , and an additional inner section, e.g. inner vertical solid wall, rib or pedestal,  126 B disposed between the optics  106  and the switching engine  108  and mounted directly to the base  112  for further stiffening the base  112 . The inner section  126 B is surrounded by the outer section  126 A, and may be implemented as a part of the second portion  112 B of the base  112 . For cases where the second portion  112 B is made of metal, the inner portion  126 B may be made out of the same metal as the second portion  112 B, and may extend from the second portion  112 B. The inner section  126 B may include at least one opening  132  for propagating at least one of the wavelength channels  103  through the at least one opening  132 . The inner section  126 B may function as a rigidity rib for additionally strengthening the base  112 . 
         [0028]    Referring to  FIG. 2  with further reference to  FIGS. 1A and 1B , an optical switching device  200  of  FIG. 2  is similar to the optical switch device  100  of  FIGS. 1A and 1B . The optical switch device  200  of  FIG. 2  may include a housing  210  having the base  112 , which includes the first ceramic portion  112 A, to which a switching engine  208  is mounted, the second metal portion  112 B, and a cover  214 . The cover  214  may include a solid, e.g. vertical wall, frame  226 , and a tube  217  for feeding through input and output optical fibers, not shown. Similar to the optical switch device  100  of  FIGS. 1A and 1B , the first ceramic portion  112 A of the optical switch device  200  of  FIG. 2  may include the extension  116  and the connector  120  disposed on the extension  116 . Flanges  212  may extend from the frame  214  for mounting to an external frame or chassis, not shown. Optics of the optical switch device  200  may include a diffraction grating  202 , a lens  204 , a compensating prism  206 , a folding prism  209 , and polarizing optics  211 . 
         [0029]    Heaters  232  may be used to maintain the optical switching device  100  at a constant temperature. To lessen pressure induced deformation of the base  112 , the cover  214  of the optical switch device  200  may include a vent, or an opening  216  for equalizing outside and inside air pressure. The effect of varying atmospheric pressure on the optical switching device  200  may be lessened by providing a pressure sensor, not shown, and by controlling the optical switching device  200  to compensate for the varying atmospheric pressure. By way of a non-limiting example, when the optical switching engine  208  comprises a LCoS array, effect of varying atmospheric pressure on a wavelength shift of the diffraction grating  202  may be lessened by shifting the addresses of individual pixels of the LCoS array in accordance with the known wavelength-pressure coefficient. Additionally, a moisture sensor may be provided to compensate for a change of refractive index of air with change in ambient humidity. 
         [0030]    Referring now to  FIG. 3  with further reference to  FIGS. 1A and 1B , an optical switching device  300  of  FIG. 3  is similar to the optical switch device  100  of  FIGS. 1A and 1B , and includes similar elements. A base  312  of the optical switch device  300  of  FIG. 3  may include a first portion  312 A and a second portion  312 B extending from the first portion  312 A. One distinctive feature of the optical switching device  300  of  FIG. 3  is that a CTE compensator, e.g. a compensating plate  304 , may be mounted directly to the second portion  312 B. The compensating plate 304 may have a CTE different from a CTE of the second portion 312B by at least 1 ppm/° C., and more preferably by at least 3 ppm/° C. In operation, the compensating plate  304  introduces a controllable amount of mechanical stress on the second portion  312 B dependent on ambient temperature, to compensate for thermal stresses, thereby lessening center wavelength drift of the optical switching device  300 . The size, shape and CTE of the compensating plate  304  may be selected accordingly, whereby the second portion  312 B can be selected from a wider variety of materials and shapes, e.g. less expensive and/or better suited for other purposes. 
         [0031]    The housing  110  may be hermetically sealed to ensure durability of the optical switching device  100 . However, a standard lid sealing processes, e.g. resistance seam welding process, may cause a deformation of the base  112  due to residual sealing stresses. For this reason, a following low temperature sealing process may be preferable. 
         [0032]    Turning to  FIG. 4  with further reference to  FIGS. 1A ,  1 B, and  FIG. 3 , the cover  128  of the optical switch devices  100  and  300  may include a seal ring  400  ( FIG. 4 ) mounted directly to the frame  126  and the lid  128  outside of the frame  126  along the perimeter  129  of the lid  128 . The seal ring  400  allows the process of packaging the optical switch devices  100  and  300  to be conveniently performed in three stages. At the first stage, the frame  126  may be soldered to the bases  112  ( FIG. 1A) and 312  ( FIG. 3 ) using a solder having a melting temperature of no greater than 300° C., or alternatively brazed at approximately 800° C. At the second stage, the lid  128  may be affixed to the frame  126  e.g. by an epoxy, to protect the optics  106  from flux fumes during subsequent soldering. Using flux-assisted soldering broadens the possible selection of metallization materials for the lid  128 ; for instance, a simple Ni coating may be used for metallization of the lid  128 . At the third stage, the seal ring  400  may be soldered to the frame  126  and the lid  128  e.g. using a low temperature solder such as 48 InSn melting at 118° C. The solder may have a melting temperature of less than 140° C. The frame  126  may have a recess  132  to accommodate the lid  128 , or alternatively the seal ring  130  may be stepped, in which case the frame  126  may remain flat-edged. 
         [0033]    For embodiments where the housing  110  is hermetically sealable (e.g.  FIG. 1A and 3 ) , the optics  106  may become misaligned by deformation of the housing  110  caused by a pressure differential between the inside and outside of the housing  110 . Although the deformation may be lessened by increasing the thickness of the base  112  ( FIG. 1A ), it may be preferable to control the pressure caused deformation by a proper construction of the lid  128 . 
         [0034]    Typically, the lid  128  is much more flexible than the frame  126  and the base  112 A. As a result, deflection of the lid  128  due to atmospheric pressure differential is much greater (e.g.  100  times greater, or even more) than that of the base  112 . The deflection may even become comparable to the lid  128  thickness, for the lid  128  thickness of about 1 mm or less. This may create substantial membrane forces in the deformed lid  128 , like a string under tension. These membrane forces on the lid  128  may apply reaction forces and moments on the frame  126  of the housing  110 , driving walls of the frame  126  inward, thereby contributing to deformation of the base  112 . 
         [0035]    Referring to  FIG. 5 , a lid  528  may be provided to lessen pressure-induced deformation of a housing  510 . The lid  528  may include a tapered portion, or thinner constant-thickness edge portion  502  around a perimeter  529  of the lid  528 , for lessening a deformation of the base, not shown, caused by atmospheric pressure differential between inside and outside of the housing  510 . In one embodiment, a thickness t of the tapered or thinner constant-thickness edge portion  502  is no greater than 0.5 mm. Also in one embodiment, a geometrical area of the tapered or thinner constant-thickness edge portion  502  is no greater than 25% of a total geometrical area of the lid  528 . It has been verified by a direct experiment that the base deformation is indeed lessened by using the lid  528  instead of the lid  128  ( FIGS. 1A ,  1 B, and  FIG. 3 ). One possible mechanism of the deformation reduction is that the membrane forces from the tapered or thinner constant-thickness edge portion  502  apply a compensating moment of force on the frame  126  walls, thereby reducing the cave-in of the frame  126  walls, which in turn results in lessening of the base deflection (the base is not shown in  FIG. 4 ). The lid  528  may result in less than one micrometer deformation at the base thickness of approximately 3 mm. 
         [0036]    Referring to  FIGS. 6A ,  6 B,  6 C, and  6 D, an assembly procedure of an optical switching device  600  ( FIG. 6D ) according to the present disclosure is illustrated. Referring specifically to  FIG. 6A , a frame  626  may be mounted on an alumina ceramic base  612 , which may include a sunk portion  612 A and a connector mounting area  651 . The frame  626  may include mounting flanges  613 , a pressure-equalizing opening  616 , and a tube  617  for feeding through input and output optical fibers, not shown. In  FIG. 6B , an optical switching engine  608  may be mounted on the sunk portion  612 A, and a folding mirror  609  may be mounted to the optical switching engine  608  for directing light onto the optical switching engine  608 . 
         [0037]    Turning to  FIG. 6C , four heat spreader plates  630  may be attached to four inner walls of the Kovar frame  626 . The number of the heat spreader plates 630 may of course vary. The heat spreader plates may have a thermal conductivity of at least 120 W/m·K. A diffraction grating  602 , a lens  604 , a compensating prism  606 , and polarizing/redirecting/collimating optics  611  are aligned and mounted to the ceramic base  612 . The base  612  and frame  626  materials may vary. A connector  620  may be mounted on the connector mounting area  651 . 
         [0038]    Referring specifically to  FIG. 6D , a three-dimensional view of the assembled optical switching device  600  illustrates a propagating input optical beam  601  split into individual wavelength sub-beams  603  by the diffraction grating  602  or other suitable means. A lid  628  is shown semi-transparent for convenience of viewing. The lid  628  may be mounted to the frame  626  using one of the lid mounting methods described above. Alternative pressure-equalizing openings  616 A are provided in the lid  628 . A pair of external heaters  632  may be thermally coupled to the frame  626 , for maintaining the housing  610  at a substantially constant temperature. At least one heater  632  may be provided, but two heaters  632  may allow a better heat distribution. Elastomer grummets  624  may be used for flexible mounting the optical switching device  600  to an external chassis, not shown. The materials of the base  612 , the frame  626 , the lid  628 , etc., may be varied. Thermally conductive materials having thermal conductivity of at least 30 W/m·K, which can be CTE matched to the optical switching engine  608 , the diffraction grating  602 , the lens  604 , the compensating prism  606 , the polarizing/redirecting/collimating optics  611 , etc., are preferred. 
         [0039]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.