Abstract:
Methods for directing an optical beam and for making an apparatus for directing an optical beam are described. One such method may include applying a first force to a plate to move the plate from a first angular orientation to a second angular orientation wherein the plate contacts a stop in the second angular orientation. A reflective portion of the plate is stopped from rotating beyond the second angular orientation. A second force can be applied between the plate and the stop to hold the plate against the stop in a plane substantially parallel to a substantially planar surface of the stop. An apparatus for directing an optical beam may be made by coupling an array of plates to a base assembly wherein each plate is movable between a first angular orientation and a second angular orientation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation and claims the benefit of priority under 35 U.S.C. Section 120 of U.S. application Ser. No. 10/122,451, filed Apr. 12, 2002 now U.S. Pat. No. 6,897,539, which is a divisional of U.S. application Ser. No. 09/511,428, filed Feb. 23, 2000, which claims the benefit of prior U.S. Provisional Application Ser. No. 60/123,496, filed Mar. 9, 1999. 

   STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
   This invention was made with Government support under Grant No. DABT 63-95-C-0055, which was awarded by the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in this invention. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to cantilevered microstructure methods and apparatus. Cantilevered microstructures may be used in many different applications. For example, certain cantilevered microstructures may be used to implement large-port-count optical crossbar switches which facilitate the flow of data over a computer network (e.g., the Internet). 
   The explosive growth of internet traffic in the last few years, and its unabated continuation into the foreseeable future, has created an unprecedented demand on the communication infrastructure of both long distance and interchange carriers. The term “fiber exhaust” was coined in the last few years to describe the saturation of traffic in the present installed base of optical fibers. Thus ushered in the era of wavelength division multiplex (WDM), a technique for using multiple colors of light inside a single strand of fiber in order to boost the capacity of the fiber manifold without actually having to install any new fibers. But as internet traffic continues to grow, the fiber-optic network infrastructure is encountering another bottleneck which WDM or similar solutions cannot solve. Interconnection between the growing number of channels supported by WDM systems demands solutions based on optical-cross-connects (OXCs). Large-port-count optical crossbar switches promise to be key components for performing OXC functions. 
   An optical crossbar switch can provide interchange of data paths between different fibers, at multi-gigabit data rates, without having to first convert them into the electronic domain as is being done in existing networks. An N×N optical crossbar switch consists of N input and N output optical fiber ports, with the capability of selectively directing light from any input port to any output port in a “non-blocking” fashion. Currently, switches deployed in the communication infrastructure operate by converting the input optical signals to electronic signals, directing the electronic signals to the proper output channels, and converting them back into optical signals. In an all-optical OXC, the light is directly deflected from an input fiber port into an output fiber port without any electrical conversion. Each of the optical beams can be expanded and collimated by inserting a microlens at the tip of each input and output fiber port. By propagating an array of optical beams in free space and selectively actuating reflectors in an array of movable reflectors, any one of the N input optical beams can be directed to any one of the N output fibers ports. The core of each input and output fiber port is the region in which most of the optical beam travels. Due to the small diameter of the core, the optical crossbar switch requires the reflectors to be maintained at a precise position in order to direct each optical beam from one fiber port to another. 
   The optical crossbar switch has several inherent advantages over its electronic counterpart, including data rate, format, wavelength independence, and lower costs. Furthermore, with advances in microelectromechanical systems (MEMS) technology, batch-processing and assembly methods similar to those used in the IC industry can be employed to produce optical crossbar switches with high port-counts at very low costs. 
   SUMMARY OF THE INVENTION 
   In one aspect, the invention features a movable microstructure apparatus comprising a base, a plate having a bottom portion coupled to the base so that the plate is movable between a first angular orientation and a second angular orientation, and a stop configured to contact the bottom portion of the plate in a contact area when the cantilever is in the second angular orientation. 
   In another aspect, the invention features a cantilevered microstructure apparatus comprising a base, a cantilever coupled to the base and movable between a first angular orientation and a second angular orientation, and a stop configured to contact the cantilever in a contact area sized so that, upon application of an electrostatic bias between the cantilever and the stop, a sufficient force holds the cantilever against the stop. 
   Embodiments may include one or more of the following features. 
   The plate can be coupled to the base at an anchor location. The plate can be coupled to the base through a flexure. The flexure may be formed from a flexible and resilient material accommodating changes in the angular orientation of the plate about the anchor location with respect to the first angular orientation, and a lateral position of the plate with respect to the anchor location. The plate and the stop may each comprise a respective electrically conductive portion. The flexure may be electrically conductive such that the plate can be set to ground or a voltage potential. Upon application of a magnetic field, the cantilever may move to the second angular orientation and contact the stop in a contact area characterized by a height b and a width w that effectively satisfies the following condition: 
                       ɛ   ⁢           ⁢   w   ⁢           ⁢     V   2         2   ⁢           ⁢     g   2         ×         b   2     +     2   ⁢   ab       2       ≥       k   θ     ⁢   θ             (   1   )               
where ε is a constant representing the permittivity of a material separating the electrically conductive portion of the plate and the electrically conductive portion of the stop when the plate is in contact with the stop, V is a voltage applied to create an electrostatic bias between the plate and the stop, g is a distance separating the electrically conductive portion of the cantilever and the electrically conductive portion of the stop when the plate is in contact with the stop, k θ  is a torsional spring constant of the flexure, θ is the angular orientation of the plate about the anchor location with respect to the first angular orientation, and a is a distance separating the stop and the base. If the second angular orientation is an obtuse angle about the anchor location with respect to the first angular orientation, the plate may move to the second angular orientation and contact the stop in a contact area characterized by a height b and a width w provided two conditions are satisfied: (i) condition 1 defined above; and (ii) the following condition:
 
                       ɛ   ⁢           ⁢   w   ⁢           ⁢     V   2         2   ⁢           ⁢     g   2         ×       b   2     2       ≥     kd   ⁡     (     a   +   b     )               (   2   )               
where k is a lateral spring constant of the flexure, and d is a distance separating the anchor location and a plane defined by the contact area of the stop. Alternatively, if the second angular orientation is an acute angle about the anchor location with respect to the first angular orientation, the plate may move to the second angular orientation and contact the stop in a contact area characterized by a height b and a width w provided two conditions are satisfied: (i) condition 1 defined above; and (ii) the following condition:
 
                       ɛ   ⁢           ⁢   w   ⁢           ⁢     V   2         2   ⁢           ⁢     g   2         ×       b   2     2       ≥   kda           (   3   )               
where k is a lateral spring constant of the flexure, and d is a distance separating the anchor location and a plane defined by the contact area of the stop.
 
   The flexure may be formed from polycrystalline-silicon and may include but not be limited to torsional, serpentine, cantilever, and combination of pin-and-staple rotational hinges and/or hinges and flexures having compliance. The flexure may pull the plate from a second angular orientation to a first angular orientation when the electrostatic force coupling the stop and plate is released. The contact area may comprise a substantially planar surface configured to define a lateral position of the plate with respect to an anchor location and the second angular orientation of the plate about the anchor location with respect to the first angular orientation when a force is applied between the plate and the stop. The contact area may also generally comprise the overlap area of the plate and the stop where textured surfaces are used to prevent sticking effects of substantially planar surfaces. The substantially planar contact area surface may be substantially perpendicular to a top surface of the base. The force may be an electrostatic force. The base, the plate and the stop may be formed from a semiconductor material. The plate may have a current-carrying coil, a hard magnetic material, a soft magnetic material, or a combination of the three. The cantilever may have a light-reflecting surface. The plate may be one of an array of plates coupled to the base, each plate having a respective stop configured to contact the plate in a contact area. The respective stops may be formed or be part of a single global mechanical stop array. 
   In another aspect, the invention features a moveable microstructure apparatus comprising a base, a plate coupled to the base and movable between a first position and a second position, and a stop having a substantially planar surface configured to contact the plate in a contact area sized so that, upon application of a force to the plate substantially normal to the substantially planar surface of the stop, a sufficient force holds the plate against the stop such that the plate lies in a plane substantially parallel to the substantially planar surface of the stop. 
   In yet another aspect, the invention features a method for directing an optical beam from a first port to a second port. The method comprises applying a first force to a plate to move the plate from a first angular orientation to a second angular orientation, wherein the plate contacts a stop in the second angular orientation, and applying a second force between the plate and the stop to hold the plate against the stop in a plane substantially parallel to a substantially planar surface of the stop, such that the plate directs an optical beam from a first port to a second port. The first force may be a magnetic field; the second force may be an electrostatic bias. 
   In yet another aspect, the invention features a method for directing an optical beam from a first port to a second port using a light-reflective plate having a first angular orientation in the absence of an applied force and a static equilibrium position in the presence of a steady force. The method comprises applying a first force to the plate to move the plate from the first angular orientation to a second angular orientation other than the static equilibrium position, wherein the plate contacts a stop in the second angular orientation, and applying a second force between the plate and the stop to hold the plate against the stop in a plane substantially parallel to a substantially planar surface of the stop, such that the plate directs an optical beam from a first port to a second port. 
   Embodiments may include one or more of the following features. 
   The first force may be a time-varying force. The first force may have a profile selected from a group consisting of a step profile, a ramp profile, a sinusoidal profile, and a pulse profile. The first force may be a magnetic field. The second force may be an electrostatic bias. 
   In yet another aspect, the invention features a method for manufacturing an apparatus for directing optical beams. The method comprises coupling an array of plate to a base assembly, each plate being movable between a first angular orientation and a second angular orientation, and forming an array of apertures in a stop assembly, the stop assembly being coupled to the base assembly, and each aperture being positioned to contact its respective plate when the plate is in the second angular orientation. 
   Embodiments may include one or more of the following features. 
   Each aperture may be constructed to have at least one substantially planar sidewall constructed to lie in a plane orthogonal to a top surface of the base assembly. Each plate may be coupled to the base assembly through at least one flexure. 
   Advantages that can be seen in implementations of the invention include one or more of the following. The invention can produce an optical crossbar switch having very low insertion loss. The precise positioning of the reflectors enabled by the invention can be used in applications that integrate micro-optical elements, for example, lasers, lenses, movable reflectors and beam splitters, on a silicon chip. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic perspective view of a moveable microstructure apparatus. 
       FIG. 2   a  is a diagrammatic top view of a movable microstructure apparatus having a different lateral position with respect to an anchor location and a different angular orientation about an anchor location with respect to a first angular orientation. 
       FIG. 2   b  is a diagrammatic side view of a movable microstructure apparatus having a different lateral position with respect to an anchor location and a different angular orientation about an anchor location with respect to a first angular orientation. 
       FIGS. 3   a – 3   f  are diagrammatic perspective views of the moveable microstructure apparatus of  FIG. 1  upon application of various combinations of a magnetic field and an electrostatic bias. 
       FIG. 4   a  is a diagrammatic side view of a moveable microstructure apparatus, where a base and a stop have a slight misalignment. 
       FIG. 4   b  is a diagrammatic side view of the moveable microstructure apparatus of  FIG. 4   a  upon application of a magnetic field. 
       FIG. 4   c  is a diagrammatic side view of the moveable microstructure apparatus of  FIG. 4   a  upon application of an electrostatic bias between the plate and the stop. 
       FIG. 4   d  is a diagrammatic front view of the moveable microstructure apparatus of  FIG. 4   c.    
       FIG. 5   a  is a diagrammatic side view of a moveable microstructure apparatus, where a base and a stop have a slight misalignment. 
       FIG. 5   b  is a diagrammatic side view of the moveable microstructure apparatus of  FIG. 5   a  upon application of a magnetic field. 
       FIG. 5   c  is a diagrammatic side view of the moveable microstructure apparatus of  FIG. 5   a  upon application of an electrostatic bias between the plate and the stop. 
       FIG. 6   a  is a diagrammatic perspective view of an N×M system plate microstructure apparatus used as an optical switch. 
       FIG. 6   b  is a diagrammatic top view of the N×M system moveable microstructure apparatus of  FIG. 6   a.    
       FIG. 7   a  is a diagrammatic side view of the moveable microstructure apparatus of  FIG. 1  upon application of a magnetic field, where the magnetic field is not parallel to a sidewall of a stop. 
       FIG. 7   b  is a diagrammatic front view of an alternate embodiment of a moveable microstructure apparatus. 
       FIG. 7   c  is a diagrammatic side view of the moveable microstructure apparatus of  FIG. 7   b  upon application of a magnetic field, where the magnetic field is not parallel to a sidewall of a stop. 
     Like reference numbers and designations in the various drawings indicate like elements. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an apparatus  100  having a stop  102 , a base  104  and a plate  106 . The plate  106  is coupled to the base  104  and is movable between a first angular orientation and a second angular orientation. The stop  102  has at least one substantially planar sidewall  108  that is configured to contact the plate  106  in a contact area when the plate  106  is in the second angular orientation. In one implementation, a substantially planar sidewall  108  is constructed to lie in a plane which is orthogonal to the top surface of the base  104 . 
   The apparatus  100  is fabricated by a MEMS process. The base  104  may be composed of an insulating layer disposed over a semiconductor substrate; for example, silicon nitride, silicon oxide, or a combination of both, may be disposed over a silicon substrate. The plate  106  may be a rectangular beam formed from a conductive material or a semiconductive material such as polycrystalline silicon. A layer of magnetic material may be plated onto the plate  106 . More than one region of the plate  106  may be so plated. The magnetic material may be one of various combinations of nickel, iron, or other elements, and is usually ferromagnetic characterized by a high saturation magnetization. 
   The plate  106  may be coupled through flexures  110  and  112  to the base  104  at anchor locations. In one implementation, insulative anchors  114  and  116  are used to attach the flexures  110  and  112  to the base  104 . The flexures  110  and  112  may be formed from a flexible and resilient conductive or semiconductive material (e.g., polycrystalline silicon). The flexible material provides the flexures  110  and  112  with a degree of elasticity. The flexures  110  and  112  allow the plate  106  to change its angular orientation about the anchors  114  and  116  with respect to the first angular orientation and its lateral position with respect to the anchors  114  and  116 , as shown in  FIGS. 2   a  and  2   b.    
   In one implementation, the stop  102  is coupled to a voltage source  118  and the base  104  is electrically grounded. An electrostatic clamping circuit can be formed from a switch  120 , a contact  122  for forming a connection between the switch  120  and the flexure  110 , the plate  106 , the flexure  112 , and the anchor  116 , and is switchable between a voltage source  124  and electrical ground  126 . The voltage sources  118  and  124  may be external sources such as power supplies or batteries, or internal sources on the apparatus  100 . An electrostatic bias can be created between the plate  106  and one of the clamping surfaces (base  104  and stop  102 ) depending on the position of the switch  120 . 
   Referring to  FIG. 3   a , in the absence of any applied force, the plate  106  lies in the first angular orientation substantially parallel to the base  104 . The voltage source  124  may be coupled to the electrostatic clamping circuit to create an electrostatic bias between the plate  106  and the base  104  upon application of a voltage V 1 . If a sufficient voltage V 1  is applied, the plate  106  is “clamped” to the base  104  and restrains the plate  106  from rotating in the presence of an applied force, for example, a magnetic field  126  as shown in  FIG. 3   b . If the plate  106  is not clamped to the base  104 , application of the magnetic field  126  would cause the plate  106  to be rotated about the anchors  114  and  116  between the first angular orientation and the second angular orientation until there is an equilibrium between the resultant torque from the torsional stretching of the flexures  110  and  112  and the force on the plate  106  caused by the magnetic field  126 . The angular orientation of the plate  106  at the equilibrium point defines a static equilibrium position. In one implementation, the static equilibrium position is the second angular orientation, as shown in  FIG. 3   c . In another implementation, the static equilibrium position is between the first angular orientation and the second angular orientation, as shown in  FIG. 3   d . In this implementation, the force on the plate  106  resulting from the application of the magnetic field  126  can be time-varying, such that the plate  106  is provided with a momentum that rotates the plate  106  beyond the static equilibrium position to the second angular orientation. The time-varying force on the plate  106  may have a step profile, a ramp profile, a sinusoidal profile or a pulse profile. Once the plate  106  is in the second angular orientation, an electrostatic bias may be created between the plate  106  at electrical ground and the stop  102  having a voltage V 2 , as shown in  FIG. 3   e . The plate  106  clamps to the sidewall  108  in a contact area characterized by a height b and a width w provided the following condition is satisfied, that the torque about an axis defined through anchors  114  and  116  resulting from electrostatic bias created between plate  106  and stop  102  is greater than or equal to torque resulting from torsional stretching of flexures  110  and  112 : 
                       ɛ   ⁢           ⁢   w   ⁢           ⁢     V   2         2   ⁢           ⁢     g   2         ×         b   2     +     2   ⁢   ab       2       ≥       k   θ     ⁢   θ             (   1   )               
where ε is a constant representing the permittivity of a material separating the electrically conductive portion of the plate  106  and the electrically conductive portion of the stop  102  when the plate  106  is in contact with the stop  102 , V is a voltage applied to create an electrostatic bias between the plate  106  and the stop  102 , g is a distance separating the electrically conductive portion of the plate  106  and the electrically conductive portion of the stop  102  when the plate  106  is in contact with the stop  102 , k θ  is a torsional spring constant of the flexures  110  and  112 , θ is the angular orientation of the plate  106  about the anchors  114  and  116  with respect to the first angular orientation, and a is a distance separating the stop  102  and the base  104 . Once the plate  106  is clamped to the sidewall  108 , removing the magnetic field  126  has no effect on the angular orientation of the plate  106 , as shown in  FIG. 3   f.    
     FIG. 4   a  shows the stop  102  coupled to the base  104  with a slight misalignment. In this example, the anchors  114  and  116  are offset from a plane  402  defined through the contact area of stop  102 . The plate  106  is movable through an obtuse angle θ about the anchors  114  and  116  with respect to the first angular orientation. In the absence of an applied force, the plate  106  lies in the first angular orientation substantially parallel to the base  102 . If the plate  106  is not clamped to the base  104 , application of the magnetic field  126  may rotate the plate  106  about the anchors  114  and  116  until the plate  106  contacts a top edge  128  of the stop  102  in the second angular orientation, as shown in  FIG. 4   b . The plate  106  clamps to the sidewall  108  in a contact area characterized by a height b and a width w as shown in  FIGS. 4   c  and  4   d  provided two conditions are satisfied: (i) condition 1 defined above; and (ii) that torque about an axis defined through top edge  128  resulting from electrostatic bias created between plate  106  and stop  102  are equal to or greater than torque resulting from the lateral stretching of flexures  110  and  112 , or 
                       ɛ   ⁢           ⁢   w   ⁢           ⁢     V   2         2   ⁢           ⁢     g   2         ×       b   2     2       ≥     kd   ⁡     (     a   +   b     )               (   2   )               
where k is a lateral spring constant of the flexures  110  and  112 , and d is a distance separating the location of the anchors  114  and  116  and a plane defined by the contact or overlap area  404  of the stop  102 .
 
     FIG. 5   a  shows the stop  102  coupled to the base  104  with an alternative misalignment. In this example, the anchors  114  and  116  are offset from a plane  502  defined through the contact area of the stop  102 . The plate  106  is movable through an acute angle θ about the anchors  114  and  116  with respect to the first angular orientation. In the absence of an applied force, the plate  106  lies in the first angular orientation substantially parallel to the base  102 . If the plate  106  is not clamped to the base  104 , application of the magnetic field  126  may rotate the plate  106  about the anchors  114  and  116  until the plate  106  contacts a bottom edge  130  of the stop  102  in the second angular orientation, as shown in  FIG. 5   b . The plate  106  clamps to the sidewall  108  in a contact area characterized by a height b and a width w as shown in  FIG. 5   c  provided two conditions are satisfied: (i) condition 1 defined above; and (ii) the following condition torque about axis defined through bottom edge  130  resulting from electrostatic bias created between plate  106  and stop  102  is equal to or greater than the torque resulting from the lateral stretching of flexures  110  and  112 , or 
   
     
       
         
           
             
               
                 
                   
                     
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   In the three cases described above and shown in  FIGS. 3   f ,  4   c  and  5   c , in the absence of an applied force, the plate  106  returns to the first angular orientation substantially parallel to the base  104  as the torsional and lateral stretching of the flexures  110  and  112  are relaxed. 
     FIGS. 6   a  and  6   b  show an apparatus  600  having a stop assembly  602  coupled to a base assembly  604 . The base assembly  604  has an array of cantilevers  606 . Each plate is coupled to the base assembly  604  by at least one flexure which permits each plate to change its angular orientation and lateral position. The stop assembly  602  may have an array of substantially planar surfaces. Each substantially planar surface may be configured to contact a respective plate in a contact area sized so that, upon application of a force to the plate substantially normal to the substantially planar surface of the stop assembly  602 , a sufficient force holds the plate against the stop assembly  602  in a plane substantially parallel to the substantially planar surface of the stop assembly  602 . In one implementation, the stop assembly  602  defines an array of apertures  608 , as shown in  FIG. 6   a . Each aperture has at least one substantially planar surface  610  that contacts a respective plate in a contact area. Each substantially planar surface  610  is constructed to lie in a plane normal to the base assembly  604 . The array of cantilever  606  may be coupled to an electrostatic clamping circuit such that each plate may be individually selected to be clamped to its respective surface  610  or to the base assembly  604 . In an alternative implementation (not shown), the stop assembly may define an array of cavities, each cavity having at least one substantially planar surface that contacts a respective plate in a contact area. 
   One application of the apparatus  600  is an optical switch. In one implementation, the array of plates  606  act as reflectors. A suitable reflector coating may be deposited on the portion of each plate above the plane of a top surface  612  of the stop assembly  602  to enhance reflectivity if desired. In an alternative implementation, the array of plates  606  act as beam splitters. Each plate may be constructed from a material that transmits and reflects different parts of an optical beam. Each plate is similarly sized and constructed such that each sidewall  610  contacts a bottom portion of its respective plate. 
     FIG. 6   a  shows the apparatus  600  having three optical inputs  614 ,  616 , and  618 , and three optical outputs  620 ,  622 , and  624 . The N inputs ( 614 ,  616 , and  618 ) are along one side of the apparatus  600  and the M outputs ( 620 ,  622 , and  624 ) are along an adjacent side. The switching elements are the array of plate  606 . Each plate is oriented at a similar angle, for example, 45 degrees to an incoming optical beam. If the mth plate along one of the N input beams is clamped to its respective sidewall, that beam is reflected into the mth of the M outputs. All but one plate in a given input line is held down by the electrostatic bias applied between the plate and the base assembly  604 . The plate that is clamped to its respective sidewall selects the output for that input line. 
   The materials from which the apparatus  600  is fabricated, the voltage sources, the applied electrostatic bias, and the applied magnetic fields may be chosen by a user to adjust the sensitivity of the apparatus  600  for any particular purpose or application. The apparatus  600  may be fabricated using techniques including “lithographic, galvanoformung and abformung” (LIGA), traditional machining, deep anisotropic plasma etching and laser machining. The stop assembly  602  may be fabricated by anisotropic etching of (110)-oriented silicon which ensures the angular uniformity of all the sidewalls  610  on the stop assembly  602 . The array of cantilevers  606 , sidewalls  610  and the base assembly  604  may be fabricated to have textured surfaces on one or more surfaces to reduce sticking when each cantilever is clamped to its respective sidewall or to the base assembly  604 . The textured surface may include dimples, bumps, and ridges. The number of cantilevers defining the N-by-M array of cantilevers  606  may be adjusted based on the application of the apparatus  600 . The apparatus  600  may be fabricated in a single batch-process and consist of a single stop-base module. Alternatively, the apparatus  600  may be fabricated in a two-part process, one process for fabricating the stop assembly  602  and the other process for fabricating the base assembly  604 . The stop assembly  602  may be aligned with the base assembly  604  in a separate alignment step. 
   The applied electrostatic bias may be an attractive force applied by the electrostatic clamping circuit described above or by other means, where the attractive force is defined as any force that pushes or pulls a plate towards a stop. 
   The magnetic fields may be applied using coils located internal or external to the apparatus  600 , or a permanent magnet located internal or external to the apparatus  600 . Current-carrying coils, hard magnetic materials, soft magnetic materials, or a combination of the three formed on each of the array of plates  606  may apply a force to the plate in the presence of magnetic fields. An applied magnetic field  702  that is not perfectly parallel to the sidewall  704  may induce a slight torque and resultant bending in the portion of the plate  706  containing a reflective surface when the plate  706  is clamped to the sidewall  704 , as shown in  FIG. 7   a . The resultant bending may cause a misalignment of a reflected beam.  FIG. 7   b  shows an alternative implementation to the plate  706  that reduces the bending effects on optical performance. The magnetic portion  708  of the plate  710  is connected to the rest of the plate  710  by support arms  712 . The support arms  712  isolate the portion of the plate  710  containing a reflective surface  714  from the magnetic field  702  that is applied on the magnetic portion  708  of the plate  710 . In this implementation, when the plate  710  is clamped to the sidewall  716 , application of the magnetic field  702  that is not perfectly parallel to the sidewall  716  results in minimal bending in the portion of the plate  710  containing the reflective surface  714  as shown in  FIG. 7   c.    
   The invention has been described in terms of particular embodiments. Other embodiments are within the scope of the following claims. For example, the steps of the invention can be performed in a different order and still achieve desirable results.