Abstract:
An optomechanical mounting includes an upper set of balls and a lower set of balls that support and secure a sphere containing an optical element. The materials in the mounting have the same or nearly the same coefficient of thermal expansion and the balls provide opposing radial forces so that thermal expansions are compensated, giving the mounting superior thermal stability. Frictional forces on the sphere from the upper and lower set of balls maintain the orientation of the sphere (and the optical element) during operation, but smooth surfaces of the sphere and balls still permit sensitive, precision rotation of sphere for alignment without post-alignment clamping of the sphere.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/906,869 of Kenneth J. Wayne et al, filed on Jul. 16, 2001, entitled “Optomechanical Mount for Precisely Steering/Positioning a Light Beam.” The disclosure of co-pending application Ser. No. 09/906,869 is herein incorporated by reference. 
     
    
     
       BACKGROUND  
         [0002]    Many optical systems require precision optomechanical mountings that hold optical elements in the positions and orientations required for operation of the system. To achieve proper positioning and alignment of an optical element, an optomechanical mounting generally must allow movement or rotation of the optical element relative to other optical elements during an alignment process, but once the optical element is aligned the mounting must securely hold the optical element to maintain the proper alignment during shipping and use of the optical system.  
           [0003]    [0003]FIG. 1 shows a prior art optomechanical mount  101 , as disclosed in application Ser. No. 09/906869. A sphere  105  is sandwiched between a lower spring assembly  107  and an upper spring assembly  109 . The sphere  105  contains an optical element (not shown) and has openings  110  for light paths to pass through. The sphere  105  also has openings  111  that fit an alignment tool used to rotate the sphere  105  during alignment. When a cover (not shown) is clamped down over the sphere  105  and spring assemblies  107  and  109 , springs (not shown) in the spring assemblies  107  and  109  apply force to keep the sphere  105  in place after alignment.  
           [0004]    The optomechanical mount  101  exhibits excellent long-term alignment stability when subjected to temperature changes, shock, and vibration. However, the optomechanical mount  101  is relatively expensive to manufacture. The spring assemblies  107  and  109 , in particular, have many complex parts, making the optomechanical mount  101  difficult and time-consuming to build. Therefore, it is desirable to have an optomechanical mount that maintains precise angular orientation of the optical element without requiring as many parts or as much assembly time as the optomechanical mount  101 .  
         SUMMARY  
         [0005]    In accordance with a preferred aspect of the invention, rigid balls are used to support and apply force to a sphere containing an optical element fixed at its center. A lower set of balls supports the sphere, and an upper set of balls rests on top of the sphere. Generally, each ball in the lower set has a corresponding ball in the upper set; each ball in the lower set applies a force to the sphere that is collinear with and opposite to a force that the corresponding ball in the upper set applies to the sphere. All ball forces are directed through the center of the sphere. As used herein, the “center” of the sphere refers to the true center of the sphere, not the center of mass. The opposing forces from the balls maintain positional stability of the sphere when the optomechanical mounting is subjected to thermal variations, vibrations, or shock. The balls and other components should be made of the same material or materials having substantially similar coefficients of thermal expansion (CTEs), so that thermal expansion of the assembly and component parts does not result in a rotation of the sphere containing the optical element. This gives the optomechanical mounting superior thermal stability.  
           [0006]    The sphere and balls are enclosed within a housing and held in place by a lid. The housing preferably has cavities recessed in its base to hold the lower set of balls. The housing has openings in the center for light paths of the optical element or for access to the sphere during the alignment process. The sphere and upper set of balls are aligned in the housing on top of the lower set of balls before the lid is attached to the housing. The lid aligns the upper set of balls and applies force, such that each ball in the upper set applies a force to the sphere that is collinear and diametrically opposed to a force from a ball in the lower set. These forces are strong enough to hold the sphere in position and to resist alignment changes due to mechanical shocks once the optomechanical mount is completely assembled. At the same time, the fine surface finishes on the sphere and the balls allow the sphere to be rotated smoothly, and with high resolution, once surface friction between the sphere and the balls is overcome.  
           [0007]    In another embodiment, the sphere can be held in place by high-strength magnets. The magnetic force is sufficient to hold the sphere in alignment, yet weak enough to be overcome when the sphere is rotated with an alignment tool.  
           [0008]    These embodiments require fewer parts and less assembly time than the prior art optomechanical mount  101 . Therefore, the cost of manufacturing for the present invention is lower.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a perspective view of a prior art optomechanical mounting, as disclosed in application Ser. No. 09/906,869.  
         [0010]    [0010]FIG. 2A is a perspective view of an optomechanical mounting in accordance with an embodiment of the invention.  
         [0011]    [0011]FIG. 2B is a perspective view of an optomechanical mounting in accordance with an embodiment of the invention, with the housing removed to show inside detail.  
         [0012]    [0012]FIG. 2C is a perspective view of the housing of an optomechanical mounting in accordance with an embodiment of the invention.  
         [0013]    [0013]FIGS. 3A and 3B show alternate structures that can be used in place of balls to constrain the sphere.  
         [0014]    [0014]FIG. 4A shows an alternate embodiment of the present invention.  
         [0015]    [0015]FIG. 4B shows a means for applying downward force upon an alternate embodiment of the present invention.  
         [0016]    Use of the same reference symbols in different figures indicates similar or identical items.  
     
    
     DETAILED DESCRIPTION  
       [0017]    In accordance with an aspect of the invention, a lower and an upper set of rigid balls supports a sphere containing an optical element. The balls are substantially identical and are oriented so that each ball applies a force along a radius of the sphere. Each of these forces is collinear with an opposing force from a ball from the other set of balls. The balls accordingly hold the sphere in position with a high degree of thermal stability because the housing, sphere, and support balls are made of materials having identical or substantially similar CTEs. Therefore, the housing, sphere, and support balls all expand and contract in unison without imparting rotation to the sphere containing the optical element.  
         [0018]    [0018]FIG. 2A is a perspective view of an optomechanical assembly  201  in accordance with an embodiment of the present invention. A sphere  105  rests on support balls  207  in a housing  203 . The sphere  105  is adapted to receive an optical element (not shown). Upper balls  209  are arranged on the sphere  105  and partially constrained by the sphere  105  and the central bore of the housing  203 . A lid  211  is attached to the housing  203 . The lid  211  has openings  212  that fit over the upper balls  209 . Each ball in the support balls  207  has a corresponding ball in the upper balls  209 . Each pair of balls is diametrically opposed from its matching mate, so that the forces exerted by each pair on the sphere  105  are equal and opposite in direction.  
         [0019]    The housing  203  has openings  205  for light paths to and from the sphere  105 , or to allow access to the sphere  105  during the alignment process. The housing  203  is preferably made out of the same rigid material as the balls, such as steel. Since the housing  203  has flat surfaces and is cubic in shape, it is less costly to machine and manufacture than the rounded spring assemblies  107  and  109  shown in the prior art of FIG. 1.  
         [0020]    [0020]FIG. 2B is a perspective view of the optomechanical assembly  201  shown in FIG. 2A, with the housing  203  removed so as to better illustrate the arrangement of the sphere  105 , the support balls  207 , the upper balls  209 , and the lid  211 . In the exemplary embodiment, the support balls  207  are placed so that when the upper balls  209  are in position, each one of the support balls  207  is diametrically opposed to a corresponding ball in the set of upper balls  209 . In this fashion, equal and opposite forces are applied to the sphere  105 . For example, the balls  207 - 1 ,  207 - 2 , and  207 - 3  can be located at 0°, 120°, and 240° in a plane normal to a vertical axis of sphere  105 , while the balls  209 - 1 ,  209 - 2 , and  209 - 3  are located at 180°, 300°, and 60° around another plane normal to the vertical axis. With this configuration, force vectors for a pair of balls ( 207 - 1 ,  209 - 1 ), ( 207 - 2 ,  209 - 2 ), or ( 207 - 3 ,  209 - 3 ) are collinear and pass through the center of the sphere  105 . Each support ball  207  thus has a corresponding upper ball  209  that provides an equal, collinear opposing force through the center of the sphere  105 . Accordingly, balls  207  and  209  do not apply a torque to the sphere  105 , and changes in the upper balls  209 , for example, caused by changes in temperature, counter or cancel corresponding changes in the support balls  207  to keep the sphere  105  from shifting position.  
         [0021]    The illustrated embodiment depicts three balls in the support balls  207  and three balls in the upper balls  209  for a total of six balls. This is a preferred number of balls, since the sphere  105  is minimally constrained. However, more balls can be used. The support balls  207  and the upper balls  209  are identical in size and shape. In a working embodiment of the invention, the balls used were approximately 11.1 mm in diameter. The balls can be varied in size without affecting the functionality of the invention. The balls precisely position the sphere  105  so that the center of the sphere  105  remains in place during and after alignment.  
         [0022]    The sphere  105  contains an optical element (not shown) such as a mirror, a beam splitter, a translating window, a wedge window, or a lens. Optical elements mounted in the sphere  105  can vary widely, but generally, the center of the sphere  105  lies on the optical center, which may be an optical surface, an axis, and/or a symmetry plane of the optical element in the sphere  105 . In the exemplary embodiment, the sphere  105  is a precision bearing about 41.275 mm in diameter that is machined to include openings  110  for light paths to and from the optical element. The sphere  105  can further include openings  112  that fit an alignment tool such as an Allen key or lever that can be used to rotate the sphere  105  in the finished optomechanical assembly  201 . Additional access ports for tooling can be provided at almost any position, notably at 45° positions in a vertical plane. The sphere  105  can be rotated about any axis running through its center. The forces exerted by the support balls  207  and upper balls  209  hold the sphere  105  in place and protect it from shocks or jars that might disturb the alignment of the sphere  105 .  
         [0023]    The lid  211  has openings  212 , one for each of the upper balls  209 . The openings  212  are narrower than the diameter of the upper balls  209 , so that the edges of the openings  212  will contact the surfaces of the upper balls  209  when the lid  211  is placed over the upper balls  209 . When the lid  211  is attached to the housing  203 , the contact points transfer the downward force from the lid  211  to the upper balls  209 , and keep the upper balls  209  in position. The lid  211  also has a central opening  210 , to allow a light path or an alignment tool to access the sphere  105 .  
         [0024]    All of the components in the optomechanical assembly  201  can be made of the same material or materials that are substantially the same or at least have the same or similar CTEs. If the CTEs are the same, the entire assembly expands or contracts in unison when subjected to a temperature change. Thus, the sphere  105  will not rotate during acclimatization, and the angular alignment of the optical element is preserved when the temperature changes. In an actual working embodiment, stainless steel was used to make the housing  203 , support balls  207 , upper balls  209 , sphere  105 , and lid  211 . However, other rigid materials, such as steel, Invar, etc. can also be used.  
         [0025]    The sphere  105 , support balls  207 , and upper balls  209  should have surface finishes that permit rotation of the sphere during alignment. If the sphere  105 , support balls  207 , and upper balls  209  are all made of the same material, then it is possible that galling (microscopic cold welding) will occur between the sphere  105  and the other balls as the sphere  105  is rotated during adjustment. If the system will only be adjusted a few times, this is not a serious problem and indeed may even be an advantage because the long-term stability of the setting is improved. However, if the system will be adjusted frequently, it is preferable to reduce the surface friction between the sphere  105 , support balls  207 , and upper balls  209  so as to prevent galling.  
         [0026]    One method of reducing surface friction, if cleanliness is not a requirement, is to lubricate the components of optomechanical assembly  201 . If cleanliness is a consideration, another option is to make support balls  207 , or upper balls  209 , or both sets of balls out of a material that has less surface frictional force to overcome. A ceramic such as silicon nitride is one possible material. If the primary concern is reduced surface friction and smooth adjustment for the sphere  105 , a copper alloy such as brass or bronze may also be used. However, using a copper alloy may negatively impact thermal stability, durability, and shock stability.  
         [0027]    [0027]FIG. 2C is a perspective view of just the housing  203 . The housing  203  has depressions  213  in its base. The support balls  207  (not shown) fit into the depressions  213 , and are secured with epoxy, welding, press fitting, screws, or any other method of attachment. The housing  203  has a hole  215  in its base to allow access to the sphere  105  for a light path or an alignment tool.  
         [0028]    To assemble the optomechanical assembly  201 , the support balls  207  are set into the depressions  213  of the housing  203  and fixed in place. The sphere  105  is placed onto the support balls  207 , and then the upper balls  209  are arranged around the sphere  105  as previously described. The upper balls  209  stand slightly above the top face of housing  203 . Finally, the lid  211  is screwed onto housing  203  or otherwise secured over the upper balls  209 . The lid  211  applies a downward spring force to each one of the upper balls  209 . The magnitude of this force is fixed by the height of the ball contact points above the top face of the housing  203 , the stiffness of the lid  211 , and fabrication tolerances. The selection of this magnitude will depend on the shock/vibration environment. In an actual working embodiment, the force was approximately 15 to 20 pounds per upper ball  209 .  
         [0029]    The sphere  105  may be constrained using structures other than support balls  207  and upper balls  209 . For instance, hemispheres and smaller portions of spheres can be used in the place of balls. FIG. 3A shows some possible substitute structures for support balls  207  and upper balls  209 . The best structures have surfaces that will contact the sphere  105  at approximately a single point. The examples shown in FIG. 3A have spherical surface portions that will contact the sphere  105  at approximately a single point. The word “approximately” is used because it is almost impossible to manufacture such perfect surfaces that will contact and remain in contact with each other at exactly a single point.  
         [0030]    Other structures may be used that have many points of contact with the sphere  105 . FIG. 3B illustrates one such example. However, these structures generate higher frictional forces and make alignment of the sphere  105  difficult. There are other well-known methods for supporting the sphere  105  that will maintain rotational freedom about any axis. For instance, using a conical surface to support a sphere is a method well known in the art.  
         [0031]    [0031]FIG. 4A shows an alternate embodiment of the present invention, using high-strength magnets  401  to hold the sphere  105  in place. The magnets  401  can be fixed to the base of the housing  203  (not shown) with any well-known means such as epoxy, press fitting, etc. Some possible high-strength magnets are alnico, ceramic, and rare-earth magnets. The magnets  401  are mounted so that they are slightly angled in towards the sphere  105 , to facilitate contact with the surface of the sphere  105 . Due to the perspective of FIG. 4A, only two magnets  401  can be seen; a third magnet  401  is positioned out of sight behind the sphere  105 , such that all three magnets  401  are positioned at equidistant intervals around the sphere  105 . Although only three magnets  401  are shown in the figure, more than three magnets can be used.  
         [0032]    The magnets  401 , as drawn, are rectangular in shape, but can also be circular or any other shape. Obviously, the sphere  105  should be made of a material that will be attracted to the magnets  401 . The sphere  105  is placed upon the magnets  401 , and the magnetic force holds the sphere  105  in place when the sphere  105  is not being aligned. The magnets should be chosen so that their magnetic force is strong enough to keep the sphere  105  in alignment, yet weak enough to be overcome when the sphere  105  is rotated with an alignment tool. No upper magnets are needed in contact with the sphere  105 , since any upper magnets would tend to pull the sphere  105  away from the magnets  401 .  
         [0033]    If the magnets  401  do not have enough magnetic pull on the sphere  105 , it may be necessary to apply a downward force upon the sphere  105  to prevent it from lifting off of the magnets  401  during the alignment process. FIG. 4B shows one possible means for applying a downward force  403  upon the sphere  105 . A cover  405  for the housing  203  (not shown) has a contact  407  mounted on a spring  309 . When the cover  405  is attached to the housing  203 , the contact  407  pushes against the surface of the sphere  105 . The spring  309  provides the downward force  403  to keep the sphere  105  from lifting off of the magnets  401  while it is being aligned.  
         [0034]    Although the invention has been described with reference to particular embodiments, the described embodiments are only examples of the invention&#39;s application and should not be taken as limitations. For example, although specific dimensions and materials were described for an exemplary embodiment of the invention, those dimensions and materials are subject to wide variations and replacements. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.