Patent Document

BACKGROUND OF THE INVENTION 
     The present inventions relate generally to packaging for optical switches. 
     In recent years there have been extensive efforts to develop commercially viable optical switches. Presently there are a few relatively small optical switches on the market (e.g. eight port switches). There are also on-going efforts to develop larger optical switches (e.g., switches having 64 to thousands of ports). One proposed optical switch architecture contemplates the use of arrays of Micro Electro-Mechanical Systems (MEMS) mirrors to accomplish the switching. A perceived advantage of this approach is that it is potentially scalable to many channels. One such MEMS mirror based optical switching system is diagranmatically represented in FIG.  1 ( a ). 
     In the embodiment shown, the optical switch  5  includes an input fiber array  10 , an input lens array  11 , input and output mirror arrays  12 ,  13 , an output lens array  14  and an output fiber array  15 . The input and output fiber blocks  10 ,  15  each consists of a two dimensional array of fibers with a polished end face. The input fiber block  10  is positioned adjacent an input lens array  11  to provide collimated input beams, while the output lens array  14  is positioned adjacent output fiber block  15  to provide collimated output beams. Each mirror in the input and output mirror arrays is rotatable about two orthogonal axes so that an input beam received on any one of the input fibers can be directed towards any one of the output fibers by appropriately adjusting the orientation of their associated mirrors. 
     In theory, the mirror arrays can be formed using a wide variety of techniques and different companies have adopted different approaches in their attempts to provide suitable mirror arrays. By way of example, one approach is to create movable mirrors by forming MEMS structures on a monolithic silicon substrate. Devices such as these are commercially available from a variety of sources including MCNC of Research Triangle Park, N.C. and Analog Devices of Cambridge, Mass. 
     In some implementations, the mirrors are actuated electrostatically. In the configuration illustrated in FIGS.  1 ( a ) and  1 ( b ), each mirror is rotatable about two orthogonal axes. The mirrors have an equilibrium position, from which they rotate bidirectionally about the respective axes. Since electrostatic actuators ordinarily can move a mirror in only one direction from equilibrium, four electrodes are typically needed to actuate each mirror in each array and eight electrodes are needed per switch channel. Consequently, a very large number of electrical interconnections are needed to drive a large optical switch. 
     Another common optical switch configuration is illustrated in FIG.  1 ( b ). In this configuration, a “folded” optical path is provided by using a fixed mirror  25  that cooperates with a moveable mirror array  27  (which may be implemented as a single mirror array or multiple mirror arrays) so that an input beam is reflected first off of an associated input mirror in moveable mirror array  22 . The first mirror directs the beam to reflect off of the fixed mirror  25  to an output mirror associated with the desired output channel. With this arrangement, all of the moveable mirrors can be placed on the same “side” of the optical switch while the input and output fiber arrays can be placed together on the other side of the optical switch. 
     Regardless of which of the described (or other) approaches is used, the mirror arrays must be packaged in a manner that provides the required number of electrical interconnections. A rigid mechanical structure is also required to accurately align the optical components. In one proposed implementation, the mirror arrays are mounted on a rigid metal structure, while the electrical interconnections are provide by other means such as a flexible electrical cable. A difficulty with this approach is that the electrical cables that are large enough to provide the number of interconnections needed in a large optical switch are not very flexible. This problem is accentuated by the requirement that the traces must be relatively widely spaced due to the high voltages needed to drive current MEMS based electrostatic actuators (typically 100V or more today, although these requirements are expected to decrease as the technology develops). The silicon substrates are quite fragile and there is a danger that the stiffness of the required cables might damage the MEMS mirror arrays through mechanical stress if the interconnecting cables were attached directly to the mirror arrays. Accordingly, there is a need for improved packaging arrangements for optical switches. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects of the invention, a variety of improved optical switch packaging techniques and optical switch components are described. In one aspect of the invention, an optical switch component that includes a die mounted on an interposer is described. The die has an exposed array of mirrors that can be used as part of an optical switch. Typically, the interposer will also have a fiber array mount arranged to receive a fiber array and to position the fiber array appropriately over the array of mirrors. The interposer may also carry electrical connectors suitable for electrical connection to external devices. In this arrangement, bond pads on the die are electrically connected to the electrical connectors through appropriate conductive features on the interposer. 
     In some embodiments, the contact pads are positioned around an opening in the interposer and the array of mirrors is exposed through the interposer opening. In one preferred implementation, the die is attached to the interposer by directly metallurgically joining bond pads on the die to associated contact pads on the interposer. In another embodiment, the die is attached to the interposer and wire bonding is used to facilitate the electrical connection between the die and interposer. 
     In another aspect of the invention, the fiber array mount carried by the interposer includes a base, an alignment stage and a bracket. The base has an alignment ridge and the alignment stage has a stage slot therein that fits over the alignment ridge. The stage also includes a ledge within the slot. The bracket is sized to fit into the slot and has a bracket ledge arranged to rest on the stage ledge within the slot. The bracket also includes a bottom surface recess arranged to nest over the alignment ridge within the slot. Fasteners are provided to couple the bracket to the base to hold the alignment stage and base together. 
     In many of the described embodiments, an optical switch can be formed by putting together two of the described interposer based optical switch components. In some embodiments the two components may be mirror images of one another, although this is not a requirement. When placing two interposer based optical switch components together, an alignment frame may be positioned between the interposers to help maintain a desired spacing between the respective arrays of mirrors. In some such embodiments, the alignment frame may be arranged to rest directly on the interposers and to surround the dice and fiber arrays. 
     In other embodiments of an optical switch, an inner housing is provided that encloses the dice, the fiber array mounts the alignment frame and portions of the interposers, but leaves the connectors exposed. In one preferred implementation incorporating a housing, the inner housing is sealed and a seal is formed between the inner housing and the respective interposers by soldering the inner housing directly to the interposers. A heater or cooler may optionally be provided to heat or cool the inner housing. 
     In various embodiments, the fiber bundle is a collimated array of optical fibers. Ribbon cables may be coupled to the interposer connectors, with each ribbon cable having an external connector for connecting to external systems. 
     In another aspect of the invention, an optical switch is provided which has an outer housing in addition to the inner housing. The outer housing encases the optical switch components and the inner housing and a resilient filler material is provided between the inner and outer housings to provide additional protection to the optical switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG.  1 ( a ) is a diagrammatic representation of the optical paths associated with a three-dimensional optical switch using movable mirrors that scan in two axes. 
     FIG.  1 ( b ) is a diagrammatic representation of the optical paths associated with a three-dimensional optical switch having the input and output fiber arrays on the same side of the switch. 
     FIG. 2 is a diagrammatic perspective illustration of an interposer based optical switch component in accordance with one embodiment of the present invention. 
     FIG. 3 is a diagrammatic top view of the interposer of FIG.  2 . 
     FIG. 4 is a diagrammatic perspective illustration of the fiber array mount portion of the optical switch component of FIG.  2 . 
     FIG. 5 is an exploded perspective view of the fiber array mount of FIG.  4 . 
     FIG. 6 is a diagrammatic perspective illustration of an optical switch incorporating a pair of the optical switch components illustrated in FIG.  2 . 
     FIG. 7 is an exploded diagrammatic perspective view illustrating an inner housing arrangement in accordance with one embodiment of the present invention that is suitable for protecting the optical switch of FIG.  6 . 
     FIG. 8 is an exploded diagrammatic perspective view illustrating the attachment of ribbon cables to the optical switch of FIG.  6 . 
     FIG. 9 is an exploded diagrammatic perspective view illustrating an outer housing arrangement in accordance with one embodiment of the present invention. 
     FIG. 10 is a diagrammatic perspective view illustrating the exterior appearance of the optical switch package of FIG.  9 . 
     FIG. 11 ( a ) is a diagrammatic perspective illustration of a substrate based optical switch component having a fixed mirror in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An optical switch package in accordance with one embodiment of the invention will be initially described with reference to FIGS. 2-10. FIG. 2 is a diagrammatic illustration of an interposer based optical switch component  120  in accordance with one embodiment of the present invention. The optical switch component  120  includes an interposer  122 , a die  123  having an array of mirrors  124 , a fiber array mount  128  and a plurality of electrical interposer connectors  129 . A fiber termination  126  is secured to the interposer  122  by the fiber array mount  128  and a bundle of optical fibers  127  exits from the fiber termination  126 . 
     The interposer  122  can take a wide variety of forms. One suitable interposer construction is illustrated in FIG.  3 . In this embodiment, the interposer  122  has a mirror opening  131  that is surrounded on one side by an array of contact pads  134  that make up a contact pad field. The contact pads, in turn, are electrically coupled to the electrical connectors  129  via electrical traces (not shown). The traces may be any type of electrical conductors produced using modern micro-fabrication techniques. With this arrangement, the die  123  can be mounted on the interposer  122  in a mirror array down configuration to expose the mirror array through the mirror opening  131 . The die  123  can be electrically connected to the contact pads  134  on the interposer  122  by any suitable method. By way of example, direct soldering in a flip chip mounting style or wire bonding may be used. In the illustrated embodiment, a flip chip mounting style is contemplated such that bond pads (not shown) on the die  123  are directly soldered to the contact pads  134  on the interposer using solder balls, posts or the like. In other embodiments, wire bonding, TAB, conductive adhesives, as well as other conventional interconnection techniques can be used to electrically couple the die  123  to the interposer  122 . An alignment hole  136  may be provided to provide a reference for handling equipment during assembly of the optical switch component  120 . 
     In the embodiments shown the die is mounted in a mirror array down configuration so that the mirror array  124  is exposed through the mirror opening  131 . This configuration has several advantages, one of which is that it allows the die  123  to be mounted on the opposite side of the interposer from the fiber termination  126  which makes it easier to prevent interference between the fiber array mount  128  and the die. However, in alternative embodiments, the die  123  may be mounted on the same side of the interposer  122  as the fiber termination  126  which eliminates the need for the mirror opening  131 . This arrangement has some advantages as well. For example, same side die mounting generally permits the use of smaller dice, which can have a significant impact on the production costs of the mirror arrays. 
     Interposers in general (as well as suitable interposer fabrication techniques) are well known in the semiconductor packaging area and any of a wide variety of interposer designs may be used. Generally an interposer is a substrate structure that provides both mechanical support and electrical interconnection. By way of example, the interposers may be formed from ceramic materials such as Alumina or Aluminum Nitride, or from a composite laminate (such as printed circuit board laminates), silicon, polymer composites, ceramic or metal matrices or a wide variety of other materials. The interposer may be constructed with traces on one side, both sides, or in a layered manner with multiple conductive layers depending upon the needs of the optical switch. 
     The fiber termination  126  is mounted to the interposer by fiber array mount  128 . It should be appreciated that the fiber array mount  128  must both hold the fiber termination  126  and align the fiber array relative to the mirror array  124 . A wide variety of mount structures may be used and the actual construction of a particular mount  128  will depend in large part on the nature of the fiber termination being held. In the embodiment shown, a collimator is formed as part of the fiber termination by aligning a lens array (not separately shown) at the face of the optical fibers. In other embodiments, the lens array may not be necessary and/or additional components (such as an optical multiplexer/demultiplexer, optical detectors, etc.) may be made part of the fiber termination  126 . In any of these embodiments, an appropriate mount  128  can be made to secure the fiber array to the interposer  122 . 
     Referring next to FIGS. 4 and 5, a particular mount  128  will be described. As best seen in FIG. 5, the mount includes a base  205 , an adjustable alignment stage  210  and a bracket  215 . The base  205  is substantially U-shaped and includes an alignment ridge  207 . The stage  210  is also substantially U-shaped and includes a substantially U-shaped slot  212 , which has a ledge  214  therein. The slot  212  is arranged to fit over the alignment ridge  207 . The bracket  215  is also sized and shaped to fit into the slot  212  in alignment stage  210  and its lower end is stepped down to form a ledge  216  arranged to rest on the ledge  214 . The bracket  215  has a recess in the bottom surface thereof (not shown) that is arranged to nest over alignment ridge  207  on base  205  to position the stage  210 . Fasteners  218  (which may be screws, bolts or a variety of other suitable fastening or locking means) are then used to secure the bracket  215  to the base  205 . The fiber array termination  126  is held in the stage  210 . Appropriate features (not shown) may be added to either the termination  126  or the stage  210  to help the stage hold the termination in place. In some embodiments, an adhesive such as epoxy may be used to secure the stage to the termination. 
     It should be appreciated that in the described fiber array mount  128 , the position of the stage may be adjusted within the tolerances between the relative widths of the slot  212  and alignment ridge  207 . This permits the fiber array to be relatively precisely aligned relative to the mirror array  124  during installation of the fiber array. Guide grooves  220  may be provided to provide a precision gripping point for handling equipment and may be used in combination with interposer alignment hole  136  to facilitate precise alignment of the fiber array relative to the mirror array  124 . When the proper alignment has been made, stage  210  is locked in place by tightening the fasteners (e.g. screws)  218 . It should also be appreciated that with the described independent alignment stage arrangement, the bracket  215  substantially only presses down against the ledge  214  in stage  210 . Thus only vertical forces are transmitted from the bracket  215  to the stage  210  to lock the stage in place. The nesting of the recess in bracket  215  over ridge  207  absorbs any torsional component without passing any of that force to the stage  210 . Notably, when screws are used as the fasteners, rotational forces are not transmitted from the fasteners  218  to the alignment stage  210  during tightening, which could have the effect of throwing off the alignment of the fiber array. 
     As best seen in FIG. 4, the mount  128  holds the fiber array termination  126  over the mirror array  124 , in a manner that covers only a portion (e.g. half) of the mirror array to leave an optical path for reflected light to pass through. It should be appreciated that the U-shaped nature of the various illustrated mount structures provide a good connection with the interposer without interfering with the optical path. However, a variety of other mount structure geometries and configurations may be used as well. 
     In the illustrated embodiments, the only electrical components carried by the interposers are the dice, the connectors and the conductive features that electrically couple the connectors to the dice. However, it should be appreciated that a wide variety of other electrical components can be incorporated onto the interposer. By way of example, this may include other integrated circuits (such as various ASICs or programmable logic devices) as well as various discrete components (e.g., resistors, capacitors, inductors etc.) mounted on, formed on or formed within the interposer. 
     Referring next to FIG. 6, the assembly of an optical switch  300  using a pair of identical optical switch components  120  in accordance with one embodiment of the invention will be described. The optical functioning of the switch  300  requires that the relative position of the input mirror array and the output mirror array be fixed. In the illustrated embodiment, this is accomplished by mounting an input optical switch component  304  and an output optical switch component  308  to an alignment frame  311  which provides the required spacing between the interposers. That is, the alignment frame  311  cooperates with the interposers to provide the physical structure holding the mirror arrays in a fixed relationship relative to one another thereby maintaining the required linear spacing (in a direction parallel to the mirror planes) and normal spacing (in a direction perpendicular to the mirror planes) between the mirror arrays. It will be appreciated that the required linear and normal spacing are determined by the design characteristics of the switch optics. 
     The nature of the alignment frame  311  may be widely varied. In the embodiment shown, it takes the form of a rectangular open frame. As best seen in FIG. 6, the frame  311  rests directly on the input and output interposers  305 ,  309 . The frame is positioned such that it circumscribes the pairs of mounts  128 , dice  123  and fiber array terminators  126 , while leaving the connectors  129  outside of the frame  311  to facilitate external electrical connections. The frame  311  may be formed in any suitable manner. In the embodiment shown, the frame is composed of two pieces. The first piece is a U-shaped element  316  and the second piece is a cross bar element  318  that is secured to the U-shaped element  316  using an appropriate fastening arrangement such as screws  319 . An alignment pin  321  carried by the frame  311  may cooperate with alignment holes in the interposer to facilitate alignment of the frame relative to the interposers and to hold the frame in position. 
     One noteworthy feature of the described optical switch  300  is that the switch is composed of two identical optical component halves. It should be appreciated that forming a switch from identical switch halves may have some significant production cost advantages over switches that are formed from different components due to standardization. On the other hand, the use of identical switch halves is not required by any means and the described interposer and alignment frame based packaging structure works well regardless of whether the optical switch components are identical. By way of example, it may be desirable to provide the optical connectors with all of the fiber connections on one side. One way that this can be accomplished is to utilize a folded switch geometry as discussed above with respect to FIG.  1 ( b ). In this arrangement, the fixed mirror  145  may be carried by a rigid substrate  148  having a geometry similar to an interposer, while the moveable mirror array is carried by the interposer. It should be appreciated that the fiber arrays would only need to be attached to substrate in this arrangement, while the interposer connectors  129  (which have large number of connections) may only be required on the interposer. This arrangement has the benefit of requiring fewer components than the previously described embodiment. However, the described packaging arrangement can readily be used to protect either arrangement, or with a variety of other optical switch configurations. FIG. 11 illustrates a suitable substrate that carries a fixed mirror. 
     The optical switch  300  has all of the components necessary to form a fully functioning switch. However, since the MEMS mirror arrays in particular are somewhat delicate, it is generally desirable (and necessary) to provide environmental protection for the switch to create a commercially viable product. The environmental protection preferably isolates the switch from dirt, moisture and other contaminants. It also protects the switch from mechanical shock and vibration, electrostatic shock, RF interference and temperature extremes. 
     Referring next to FIG. 7, a housing arrangement suitable for protecting the heart of the switch  300  will be described. In the embodiment shown, an inner housing  330  is arranged to slide over the interposers  305 ,  309  between the alignment frame  311  and the electrical connectors  129 . Thus, the housing has interposer slots  332  arranged to fit over the interposers as well as terminator slots  334  arranged to fit over fiber array terminations  126 . A base  336  forms a cap for the housing. In the embodiment shown, the base  336  is secured to the frame  311  by screws, although this is not required. Flashings  339  are then slipped over the fiber bundles  127  and fiber array terminations  126  to enclose the terminator slots  334 . 
     In some (and possibly most) applications, it will be desirable to seal (and potentially hermetically seal) the inner chamber of the switch. This can readily be done by joining the base  336  to the inner housing  330 , joining the housing  330  to the interposers  305 ,  309  and joining the flashings  339  to the housing  330 . In the described embodiment, the various components are joined by soldering. However, such joining can be accomplished by a wide variety of conventional techniques including soldering, welding, adhesive bonding and the like. In some embodiments, metallic seal lines (not shown) may be formed on the interposer surface to serve as a solder base for soldering the housing to the interposers. When assembled with an inner housing, the optical switch  300  has the appearance illustrated in FIG.  8 . 
     It is not uncommon for optical switches to be placed in environments where it can get relatively cold. Accordingly, a resistive heating blanket  342  may be placed over the inner housing to facilitate heating when necessary or desired. If a heater is desired, a heater cord  345  is provided to power the heating blanket  342 . The heater also allows the package to be held at a constant temperature for improved optical performance. In alternative embodiments it may be desirable to provide a cooler and/or a bi-directional heat pump either in addition to, or in place of the heater. 
     Referring next to FIG. 8, ribbon cables  350  having internal connectors  351  and external connectors  353  may be provided to electrically couple the switch to external drivers. The internal connectors  351  plug into the interposer connectors  129  and the external cable connectors  353  plug into connectors external to the package. Of course, when desired, connectorless joining methods may be used to couple the ribbon cable to the interposer. In the illustrated embodiment, each interposer has four connectors  129  and a separate ribbon cable  350  is provided for each connector. Thus the optical switch has a total of eight ribbon connectors. Of course the actual number of control lines and thus, the number and size of the various connectors that are required will vary significantly based on the needs of a particular switch. In current electrostatic MEMS mirrors that are rotatable in two degrees of freedom, four control lines are required for each mirror. In switches having 256 input ports, that requires over 2000 different control lines for the input and output mirrors alone (four lines per mirror, 256 input mirrors, 256 output mirrors). Additionally, relatively high voltages are currently required to rotate electrostatic mirrors (e.g. voltages on the order of 100 volts in available systems). Thus, high voltage drivers are required. In some embodiments, the high voltage drivers can be mounted on the interposers themselves. However, due to space limitations, it may be impractical to place the high voltage drivers on the interposers. Thus, the drivers can be located on external circuit boards. One effect of this approach is that the spacing between conductors must be a bit larger than would be required in lower voltage applications which tends to increase the size of the required connectors. Of course, these technologies are rapidly advancing and as the technology develops, it is likely (indeed expected) that lower drive voltages and higher connector densities will become common. 
     Although four control lines per mirror may be required in some MEMS based mirror arrays, it should be appreciated that fewer or more control lines may require for other mirror arrays. For example, MEMS based mirror arrays that have rotation though a single degree of freedom, may require just two control lines per mirror and further work in the area may be able to reduce the required number of control lines even further. 
     Once the ribbon cables have been attached an external housing assembly  360 , can be placed over the entire switch assembly to further environmentally protect the switch. Of course, the actual design of the outer housing may be widely varied. By way of example, one suitable embodiment is illustrated in FIG.  9 . In the embodiment shown, the outer housing assembly  360  includes an outer housing shell  362 , an outer housing base  365 , flexible cable strain relief clamps  386 , fiber bundle strain relief clamps  390 , and clamp covers  394 . The various components can be coupled together using appropriate fasteners such as screws (not shown). 
     In the embodiment shown, the fiber bundle strain relief clamps  390  are coupled to the housing shell  362  using screws or other suitable fasteners. The clamp covers  394  are then coupled to the strain relief clamps  390  (again using suitable fasteners such as screws). Similarly, the flexible cable strain relief clamps  386  may be attached to the base  365  by screws or other appropriate fasteners. 
     Foam padding (not shown) or another highly damping resilient filler material is placed within the outer housing assembly  360  so that the only mechanical support suspending the inner housing assembly within the outer housing assembly is the resilient foam padding. In the described embodiment, a loose piece of foam is placed within the housing. However, in alternative embodiments a resilient material may be adhered to or molded into the outer housing shell  362 . The foam padding allows substantially independent movement of the inner housing assembly within the outer housing assembly thereby isolating the switch from vibrations and/or shock impulses that may disturb the outer housing. A thermal path can be provided between the inner and outer housings by using a thermally conductive (yet resilient) filler material. 
     Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. For example, although the illustrated embodiment has been described primarily in the context of a device having optical inputs and outputs carried by different interposers (e.g., using a simple 2 mirror array optical switch approach), it should be appreciated that the majority of the described packaging techniques can be readily applied to folded optical path based optical switches as well. These might include optical switches wherein both the input and output fiber bundles are carried by the same interposer and the second interposer carries all of the moveable mirrors and their associated electronics and/or other components. It may also apply to mirror array based optical switches having other optical paths and to optical switches having a single, or more than two fiber bundles. 
     The described packaging techniques can be used with optical switches of any appropriate size. Current efforts are primarily focusing on building 64, 256 and 1000 channel optical switches, however the described packaging can readily be applied to substantially larger and smaller switches as well. The invention has been described primarily in the context of semiconductor based MIMS mirror array structures. However, it should be apparent that most of the described techniques can apply equally well to switches using other mirror array technologies. Also, a number of unique packaging features have been described that combined to provide a particular optical switch packaging arrangement. However, it should also be appreciated that many of the described features are independent and do not need to be used in combination. Therefore, it should be apparent that the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Technology Category: g