Abstract:
An optical switch actuator moving an optical element into or out of an optical pathway. The optical element is coupled to a movable shuttle and driven by a motor between two rest positions. The motor includes two stationary coils and a magnet attached to the shuttle. The shuttle is magnetically latched in the rest positions. The optical element&#39;s position at the extended rest position is controlled with a stop that contacts the shuttle to provide accuracy and precision about multiple axes. The material used to construct the actuator&#39;s components aids in repeatedly positioning the optical element with precision.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation-In-Part of Ser. No. 09/473,455, filed on Dec. 28, 1999, now U.S. Pat. No. 6,606,429, and Ser. No. 10/347,067, filed on Jan. 17, 2003. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   This invention pertains to an actuator for a fiber optical device. More particularly, this invention pertains to an assembly that linearly moves an optical element into an optical path thereby altering the optical path. 
   Description of the Related Art 
   In fiber optic networks, light signals are transmitted along optical fibers to transfer information from one location to another. Optical switches are used to selectively couple light from an input fiber to an output fiber. Optical fibers typically have very small cross-sections and narrow acceptance angles within which light entering the fiber must fall to promote efficient propagation of the light along the fiber. As such, optical switches must transfer light with precise alignment. 
   One type of electromechanical optical switch operates by moving a mirror while maintaining the optic fibers and optical pathway stationary. In response to electrical signals, a relay arm moves a mirror into and out of an optical pathway. The relay arm moves the mirror substantially parallel to its reflective surfaces. The travel of the relay arm along that axis is limited by stops that determine the position of the mirror. The relay arm is constrained at the stops by only a single contact point, thereby allowing inaccuracies in the radial position due to rotation of the arm. Examples of such switches include U.S. Pat. No. 5,133,030, issued to Lee on Jul. 21, 1992, entitled “Fiber Optic Switch Having a Curved Reflector,” and U.S. Pat. No. 4,057,719, issued to Lewis on Nov. 8, 1977, entitled “Fiber Optics Electro-Mechanical Light Switch.” 
   One problem with such a switch is that the relay mechanism may not be able to provide the accuracy and precision in positioning the mirror that may be required by some optical switching networks. Accuracy is the ability to achieve a desired position with any given movement. Precision is the ability to repeatedly achieve the same position over a number of movements, regardless of where that position is located. Because the movement of the relay arm is constrained by only a single point of contact with the stopper, the switch may only be able to provide accurate alignment along a single axis (in the direction of the arm&#39;s movement). The use of a single contact point may result in position inaccuracies due to the freedom of the relay arm to rotate about additional axes. Furthermore, relay mechanisms are typically constructed of materials that may be susceptible to significant wear from component contact through repeated use. Such material wear may lead to problems with precise placement of the mirror over time, in addition to position inaccuracies. 
   Another problem with electromechanical switches is that they use a large electromechanical actuator that may not permit the placement of mirrors in the packing density that may be required for multiple switch arrays. 
   Other types of systems use electromagnetic actuators, for example, disk drive systems. These systems typically use actuators to position drive components over different regions of a disk. One problem with such electromagnetic actuators is that they require a control servo loop in order to operate. With a servo loop, the position component must be actively adjusted to maintain proper positioning. As such, actuators of this type are unable to repeatedly return components to the same position when actuated, without the use of an active control loop. This adds complexity to a system&#39;s design and, thereby, may undesirably increase its cost. 
   BRIEF SUMMARY OF THE INVENTION 
   According to one embodiment of the present invention, an optical switch actuator is provided. The actuator includes an optical element, a shuttle, and a motor. The optical element, in one embodiment, includes a mirror. The shuttle moves longitudinally within a cylinder, which has a cylindrical stopper that contacts a flat on the shuttle, thereby precisely locating the optical element in the extended position. The motor includes two stationary coils located outside a housing. The shuttle has a permanent magnet that interacts with the coils. The magnet is acted upon by to the coils, thereby moving the shuttle between two positions. 
   In one embodiment the shuttle, the cylinder sleeve it moves within, and the stopper are made of a close grained ceramic material that has low coefficient of thermal expansion, is not susceptible to cold metal bonding or welding, and exhibits little wear with repeated use. 
   The stopper permits the optical element to have high repeatability by stopping the shuttle at a fixed point and inhibiting the shuttle from moving longitudinally and from moving about its longitudinal axis. The optical element is attached to the shuttle with an adhesive with a low coefficient of thermal expansion and contains micro-spheres, which allow the optical element to maintain alignment once positioned. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which: 
       FIG. 1  is a perspective view of a of an optical switch actuator; 
       FIG. 2  is a cross-sectional view of the actuator; 
       FIG. 3  is a schematic diagram of the actuator; 
       FIG. 4  is an exploded view of the stopper and shuttle cylinder; 
       FIG. 5  is cross-sectional view of the stopper and shuttle cylinder; and 
       FIG. 6  is an exploded view of the shuttle and permanent magnet. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An apparatus for moving an optical element  132  between an extended position and a retracted position is disclosed. The apparatus is an optical switch actuator  10  suitable for use in optical switches. 
     FIG. 1  illustrates the actuator  10 , which has a cylindrical body  114  with electrical leads  142   a ,  142   b ,  142   c  at one end and an optical element  132  at the opposite end. The optical element  132  is illustrated in the extended position with the optical element  132  positioned at its furthest from the cylindrical body  102 . In the retracted position, the optical element  132  is positioned closer to the end of the cylindrical body  102 . A motor inside the cylindrical body  102  drives a shuttle  112 , and the optical element  132 , between the two positions. In one embodiment, the optical element  132  is a mirror. In another embodiment, the optical element  132  is a filter. In one embodiment, a wavelength division multiplexed (WDM) switch made with an optical element  132  being a partially reflective filter. The use of optical elements, such as mirrors and filters, to propagate light between fiber collimators is well known in the art; accordingly, a more detailed discussion of their operation is not provided. As shown in  FIG. 1 , the optical element  132  is attached to the shuttle  112 , which is constrained by a cover  104 . 
   The electrical leads  142  are attached to a flange  232  located at one end of the cylindrical body  102 . The electrical leads  142  are electrically connected to a motor, or driver. The motor, or driver, moves the shuttle  132  linearly between the extended position and the retracted position. In the extended position, the optical element  132  on the shuttle  132  interacts with an optical pathway and in the retracted position the optical element  132  is out of the optical pathway. 
     FIG. 2  shows the actuator  10  in cross-section. The motor, or driver, performs the driving function and includes a first coil  214  wrapped around the cylindrical body  102 , an adjacent toroidal core  212 , a second coil  224  wrapped around the cylindrical body  102 , an adjacent toroidal core  222 , and a magnet  218  attached to the shuttle  112 . In the illustrated embodiment, the shuttle  112  is integrated with the driver such that motions of the magnet  218  are directly translated to motions of the shuttle  112 . In one embodiment, the magnet  218  is a permanent magnet. In another embodiment, the magnet  218  is a rare earth magnet. 
   The shuttle  112  moves within the cylindrical body  102  between the extended and the retracted position. At the extended position, the shuttle  112  contacts a stopper  204 , which prevents the shuttle  112  from moving further in a longitudinal direction. The stopper  204  determines the accuracy with which the shuttle  112 , and thereby the optical element  132 , can be positioned. However, the wear of the shuttle  112  may affect the precision with which the optical element  132  can be repeatedly positioned. 
   The shuttle  112 , the cylindrical body  102 , and the stopper  204  are constructed from a hard material having a small grain size, for example, ceramic. Ceramic may be polished to higher degree than softer materials such as plastics. When a material is polished, the grain size of the material determines its surface roughness and, thus, its surface area of contact. As a result, when materials come into contact with each other, the area of contact is determined by the grain size of the contacting materials. Materials having a small grain size will have a greater number of grain particles in contact with each other over a given surface area. As such, a smaller grain size results in more contact between the surface of the shuttle  112 , the cylindrical body  102 , and the stopper  204 . 
   In one embodiment, for example, the grain size is approximately in the range of 0.3 to 0.5 microns and the distance of travel of shuttle  112  is approximately 2 millimeters. When materials having this grain size come into contact with each other, the contact accuracy may be approximately 0.2 microns. Such a contact accuracy over a distance of approximately 2 mm results in an angular accuracy of approximately 0.0001 radians. 
   The wear of the material results from the dislodging of surface grains, of which the size of the grains is one factor. The more grains that are dislodged, the greater the wear of the material. However, a large force is required to dislodge a grain of any given size. A surface material having a greater number of small gains will tend to have fewer gains dislodged than a material having a fewer number of larger grains. As such, due to the larger number of grain contacts with small grained surfaces, less discernable wear may result than with a material having a larger grain size. 
   In another embodiment, other fine grained materials that reduce wear on shuttle  112 , the cylindrical body  102 , and the stopper  204  are used, for example, zirconia, silicon carbide, silicon nitride, and aluminum oxide. In yet another embodiment, shuttle  112  and cylindrical body  102  are constructed from a metal or plastic material. If a larger grained material, such as a metal, is desired to be used, the speed at which shuttle  112  is moved is slowed to prevent the generation of forces that may increase the wear on shuttle  112 , the cylindrical body  102 , and the stopper  204 . However, the use of ceramics provides greater precision and switching speed than is attainable with larger grained materials. As such, the proper selection of the material for shuttle  112 , the cylindrical body  102 , and the stopper  204  may aid in achieving a high precision and repeatability in the positioning of optical element  132 . Grain size, however, is only one of several factors that may contribute to the wear resistance of a material. Other factors that may contribute to the wear resistance of a material include, for example, coefficients of friction, modulus of rapture, tensile strength, compressive strength, and fracture toughness. The operation of such factors is well known in the art; accordingly, a more detailed discussion is not provided. 
   Actuator  10  is not limited to only having components constructed from the materials described above. In an alternative embodiment, shuttle  112 , the cylindrical body  102 , and the stopper  204  are coated with the materials described above. For example, shuttle  112 , the cylindrical body  102 , and the stopper  204  are constructed of any rigid material and coated with a wear resistant ceramic such as titanium nitride or aluminum oxide. The coating is applied using techniques that are well known in the art, for example, chemical vapor deposition. 
   An advantage, other than wear resistance, to using a ceramic material for the interface between the shuttle  112 , the cylindrical body  102 , and the stopper  204  is that ceramic is not susceptible to cold-metal bonding or welding. Cold-metal bonding occurs when two components in contact are placed under pressure, the more extreme the pressure, the greater the chance of cold-metal bonding occurring. Without cold-metal bonding, less motor power is required to break loose the shuttle  112  and to overcome inertia and start the shuttle  112  moving. 
     FIG. 3  illustrates the electrical schematic of the actuator  10 . In one embodiment, a direct current voltage is momentarily applied to electrical leads  142   a ,  142   b , which energizes the first coil  214  and causes the permanent magnet  218  to be attracted to the first core  212 . The movement of the permanent magnet is shown by the arrow  312  on FIG.  3 . The voltage pulse causes the permanent magnet  218 , and the shuttle  112 , to move toward the first core  214  and when the permanent magnet  218  is near the first core  214 , magnetic attraction to the first core  214  latches the shuttle  112  in the extended position. The shuttle  112  is constrained from extending by the stopper  204 , which ensures that the shuttle  112  stops at the same point each time it is in the extended position. 
   To move the shuttle  112  to the retracted position, a direct current voltage pulse applied to electrical leads  142   b ,  142   c  momentarily energizes the second coil  224  and causes the permanent magnet  218  to be attracted to the second core  222 . The magnetic force induced in the second core  222  overcomes the magnetic attraction of the permanent magnet  218  to the first core  212  and causes the shuttle  112  to move toward the second core  222 . When the permanent magnet  218  approaches the second core  222 , the magnetic attraction to the second core  222  pulls the permanent magnet  218  toward the second core  222 . With the optical element  132  in the retracted position, the shuttle  112  is latched by the magnetic attraction of the permanent magnet  318  to the second core  222 . 
   In another embodiment, a direct current pulse is applied to one coil  214  or  224  to attract the permanent magnet  218  and another pulse is applied simultaneously to the other coil  224  or  214  to repulse the permanent magnet  218 . The polarity of the voltage applied to the coils  214 ,  224  is reversed to move the permanent magnet  218  in the opposite direction. This has the effect of having one coil  214  or  224  attracting the permanent magnet  218  and the other coil  224  or  214  pushing, or repelling, the permanent magnet  218 , thereby requiring less power to move the shuttle  112  between its two positions. 
   Position indication of the shuttle  112 , and the optical element  132 , is achieved by measuring the inductance of each of the coils  214 ,  224 . The shuttle  112  has two possible positions corresponding to the optical element  132  being extended and retracted: with the optical element  132  extended, the permanent magnet  218  is adjacent the first core  212  and with the optical element  132  retracted, the permanent magnet  218  is adjacent the second core  222 . The coil that the permanent magnet  218  is closest will have a different inductance than the other coil. In one embodiment, the relative inductance is measured by connecting the coils to a circuit that responds to changes in inductance and that responds to a measured inductance above or below a predetermined threshold value. 
     FIG. 4  illustrates the cylindrical body  102 , the stopper  204 , and the cover  104 .  FIG. 5  is a cross-sectional view of the same three components. The stopper  204  is a rod-shaped member that rests in a notch cut in the cylindrical body  102  and is held in position by the cover  104 . In one embodiment, the stopper  204  is fixedly held in position when the cover  104  is attached to the cylindrical body  102 . In another embodiment, the stopper  204  is loosely held in position and when the shuttle  112  is in the extended position, the incline surface  632  forces the stopper  204  upwards and away from the longitudinal axis of the cylindrical body  102  until the stopper  204  is wedged between the cover  104 , the notch cut in the cylindrical body  102 , and the incline surface  632 . The shuttle  112 , after being wedged by the stopper  204 , is rigidly held in a fixed position. The stopper  204 , after contacting the incline surface  6326 , secures the shuttle  112  in a fixed position and prevents the shuttle  112  from rotating about its longitudinal axis, that is, the shuttle  112  cannot rotate about its axis in any manner, including rotating around its axis in the cylindrical body  102  or rotating obliquely around any point on the axis. 
   The repeatability of the shuttle  112  in the extended position is assured by the stopper  204  contacting the cover  104  and the cylindrical body  102  and by the incline surface  632  of the shuttle  112  contacting the stopper  204  and having the shuttle  112  rotate within the cylindrical body  102  such that the incline surface  632  aligns with the stopper  204 . Further, the stopper  204  causes the shuttle  112  to be wedged between the stopper  204  and the upper inside surface of the cylindrical body  102  opposite the stopper  204 . The shuttle  112 , after being wedged, is constrained by the cylindrical body  102  and the stopper  204  from rotating about the axis of the cylindrical body  102 , both radially and obliquely. The interaction between the shuttle  112 , the cylindrical body  102 , and the stopper  204  eliminates the need for tight tolerance between the shuttle  112  and the cylindrical body  102  and results in the shuttle  112  returning to the same extended position during repeated operations with little spatial deviation. 
   The ends of the stopper  204  do not extend past the outside cylindrical surface of the cylindrical body  102 . The cover  104  fits over the end of the cylindrical body  102 . In one embodiment, the cover  104  is sized for an interference fit with the cylindrical body  102  and the cover  104  is pressed onto the end of the cylindrical body  102 , thereby securing the stopper  204  in a fixed position. In another embodiment, the cover  104  is secured in place with an adhesive. 
     FIG. 6  illustrates the shuttle  112  and permanent magnet  218 . The shuttle  112 , in one embodiment, is an MU ferrule that is machined to have a first surface  636  for mounting the optical element  132  and a second surface  634  and an incline surface  632  for interfacing with the cylindrical stopper  204 . The second surface  634  allows the shuttle  112  to slide past the stopper  204  and the incline surface  632  contacts the stopper  204  when the shuttle  112  is in the extended position. The permanent magnet  218  is a disk with an outside diameter equal to or less than that of the shuttle  112 , and the magnet  218  is attached to the inside end of the shuttle  112 . The magnet  218 , in one embodiment is attached to the shuttle  112  with an adhesive. 
   The optical element  132  is secured to the first surface  636  with an adhesive. The optical element  132  attachment to the shuttle  112 , along with the repeatability of the shuttle  112  location in the extended position, is critical. The precise alignment of the optical element  132  relative to the cylindrical body  102  is critical. Any misalignment can result in an attenuation of the optical signal or the loss of the signal. By matching the coefficient of thermal expansion of the individual components and adhesives, the components of the actuator  10  remain in alignment over a wide temperature range such that the optical path does not suffer degradation as the temperature varies. In one embodiment, the temperature range is from −40° to +85° Centigrade. In another embodiment, the transition point of the adhesive is outside the operating temperature range, which enhances the dimensional stability of the connection of the optical element  132  to the first surface  636 . In one embodiment, keeping the transition point outside the operating range is accomplished by using fillers. In still another embodiment, the adhesive has limited shrinkage, which can be accomplished with a filler. Further, the adhesive can be cured in place. In one embodiment the adhesive is cured by ultraviolet light. 
   In one embodiment the adhesive is a quick curing adhesive blended with amorphous silica spheres of a selected diameter. The adhesive is compressed between the optical element  132  and the first surface  636 , with the spheres forming a monolayer, which results in dimensional stability when the adhesive is cured. In another embodiment the adhesive is Dymax OP66LS, which has a coefficient of thermal expansion similar to that of the shuttle  112  such that the optical element  132  remains in alignment as the temperature varies within the operating range of the actuator  10 . 
   In addition to the adhesive, the repeatability of the optical element  132  location relative to the cylindrical body  102  is achieved by the stopper  204  contacting the incline surface  632 , which makes the shuttle  112  self-aligning. The stopper  204  is fixed in position by the cover  104  and the cylindrical body  102 . The stopper  204  contacts the incline surface  632  of the shuttle  112  which stops the shuttle  112  from extending further. Also, the stopper  204 , by its length contacting the width of the incline surface  632 , prevents the shuttle  112  from rotating within the cylindrical body  102  in the fully extended position. Therefore, the interaction of the stopper  204  and the incline surface  632  serve to ensure that the optical element  132  position is highly repeatable when in the extended position. 
   From the foregoing description, it will be recognized by those skilled in the art that an actuator  10  for linearly moving an optical element  132  has been provided. The actuator  10  has opposed coils  214 ,  224  that force a magnet  218  to move between two positions. The magnet  218  movement is translated to a shuttle  112  with the optical element  132 . The shuttle  112  engages a stopper  204  at the extended position, which causes the shuttle  112  to stop at a fixed point with high repeatability. The stopper  204  prevents the shuttle  112  from moving longitudinally along the longitudinal axis. The stopper  204  also prevents the shuttle  112  from rotating about the longitudinal axis. Polishing the inside of the sleeve  202  and the shuttle  112  allows for low friction motion of the shuttle  112 . The low thermal expansion of the ceramic materials, along with the low friction surfaces and the self-aligning shuttle  112 , results in high repeatability and easier alignment of the optical element  132  within an optical pathway. 
   While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.