Abstract:
An optomechanical mounting includes an upper spring assembly and a lower spring assembly that support and secure a sphere containing an optical element. The materials in the mounting have the same or nearly the same CTEs and spring assemblies 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 spring assemblies maintain the orientation of the sphere (and the optical element) during operation, but smooth surfaces of the sphere and springs still permit sensitive, precision rotation of sphere for alignment without post-alignment clamping of the sphere. The spring assemblies can be ring-shaped to permit an opening through the spring assembly to the sphere for light paths or for tools that adjust the alignment of the sphere.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/906,869 of Kenneth J. Wayne et al., filed Jul. 16, 2001, entitled “Optomechanical Mount for Precisely Steering/Positioning a Light Beam.” 
    
    
     BACKGROUND 
     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. A conflict exists between the need to align an optical element with great precision and the need to have the optical element remain aligned for the lifetime of the optical system. 
     To provide adjustability for alignment and still securely hold the optical element in position, optomechanical mounts often use clamping systems. In particular, an optical element in the optomechanical mounting can be adjusted or aligned when the clamp is loose, but the optical element is rigidly held when the clamp is tightened. One concern in optomechanical mounts using clamps is disturbance of the optical element&#39;s alignment when clamping the optomechanical mount. 
     Using the same clamping friction during alignment and use of an optical system avoids disturbances that arise from post-alignment clamping but requires a tradeoff between alignment precision and operational stability. In particular, a multi-axis interferometer typically requires optomechanical mounts that can precisely orient laser beams with sub-microradian sensitivity. An optomechanical mount with a pure kinematic design can provide sub-microradian sensitivity, but typically cannot hold that alignment when subjected to shock, vibrations, and temperature changes. A ruggedly clamped, and therefore stable, optomechanical mount generally is difficult to adjust with sub-microradian precision. Accordingly, tradeoffs are generally required between the precision of adjustments and the stability, and a semikinematic design is often the compromise. 
     FIG. 1 shows an optomechanical mounting  100  that emphasizes minimal constraint and adjustment sensitivity. Such optomechanical mountings are further described in U.S. Pat. No. 6,170,795. Optomechanical mounting  100  includes a support  14 , a three-sphere nest  20 , a sphere  12 , a top plate  14 , and a spring preloaded plunger  26  (spring not shown) operated by a clamp screw  28 . Sphere  12  contains an optical element such as a mirror (not shown) that can be rotated on three-sphere nest  20  for an alignment process that changes the orientation of the optical element without changing the position of its optical center. 
     Alignment of the optical element, which is fixed at the center of sphere  12 , generally requires loosening clamp screw  28  to relieve or reduce the clamping force on sphere  12  and permit rotation of sphere  12 . The spring (not shown) bearing on plunger  26  thus applies an initial stabilizing force directed along a radius of sphere  12 . Once sphere  12  is aligned, tightening clamp screw  28  overcomes the spring and causes plunger  26  to apply the clamping force, which holds sphere  12  in the proper orientation. The final clamping force is set by applying a prescribed torque to clamp screw  28 , and the frictional forces resulting from the clamping force resist rotation of sphere  12  to retain the alignment of the optical element. 
     The need to loosen clamp screw  28  before alignment and the need to tighten clamp screw  28  after alignment increase the total time required for the alignment process. Additionally, clamp screw  28  may not be easily accessible in an optical system, which makes the alignment process more difficult. Additionally, tightening clamp screw  28  causes bending of the assembly and hence disturbs the accuracy of the just-completed alignment. For most applications, an optomechanical mounting is desired that maintains precise angular orientation of the optical element without requiring additional procedures to apply a clamping force after the alignment process. 
     SUMMARY 
     In accordance with an aspect of the invention, an optomechanical mounting includes an upper spring assembly and a lower spring assembly that support and secure a sphere containing an optical element fixed at its center. The spring assemblies can be substantially identical so that thermal expansions affecting one assembly compensates for identical opposing thermal expansion affecting the other spring assembly, giving the optomechanical mounting superior thermal stability. Frictional forces on the sphere from the upper and lower spring assemblies maintain the orientation of the sphere (and the optical element in the sphere) during operation but still permit rotation of the sphere for alignment without removing either spring assembly or releasing the spring tension that the spring assemblies apply to the sphere. 
     Each spring assembly can include springs around a perimeter of a ring so that a central region of each assembly is open for an optical path through the assembly. Alternatively, the central region can be open to provide access for tools that facilitate rotation the sphere for alignment of the optical element. 
     Embodiments of the invention can exhibit excellent long-term alignment stability when subjected to temperature changes, shock, and vibration. The symmetry of the optomechanical mount and the use of similar construction materials in the elements of the mount provide excellent thermal stability. High clamping forces between the springs and the sphere resist alignment changes caused by mechanical shock. In particular, frictional forces at multiple points on the sphere resist rotation of the sphere after alignment is achieved, but fine surface finishes on the sphere and spring make smooth, high resolution rotational adjustment achievable with removable alignment tools. Vibration stability results because the springs, which stiffen due to geometrical deformation as a result of high compressive forces, wrap tightly around the sphere to provide a stiff, highly damped spring/mass system having a high resonant frequency, typically greater than 3 kHz. 
     One specific embodiment of the invention is a system that includes a sphere adapted for mounting an optical element, a first set of springs including multiple springs in contact with the sphere; and a second set of springs including multiple springs in contact with the sphere. Generally, each spring in the first set has a corresponding spring in the second set, and each spring in the first set applies a force to the sphere that is collinear with and opposite to a force that the corresponding spring in the second set applies to the sphere. All spring forces are directed through the center of the sphere. The opposing forces from the springs maintain positional stability of the sphere when the optomechanical mounting is subjected to thermal variations, vibrations, or shock. 
     Typically, either set of springs can be mounted on an inner surface of a support ring. The inner surface of the support ring is typically a conic section with fixtures for mounting the springs, and each spring can be a leaf spring set at an angle according to the normal to the sphere&#39;s surface where the spring contacts the sphere. The support rings can be open in the center for light paths of the optical element or for access to the sphere during the alignment process. A case on which the support rings are mounted can control the separation of the first and second set of springs or their associated support rings to control magnitudes of forces that the first and second sets of springs apply to the sphere. 
     Another embodiment of the invention is a system wherein spring washers are used to support and apply force on the sphere. Fewer springs are required, and no fixtures need to be created on the inner surface of the support ring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a known optomechanical mounting. 
     FIG. 2 is a perspective view of a lower spring assembly for an optomechanical mounting in accordance with an embodiment of the invention. 
     FIG. 3 is a perspective view of a sphere containing an optical element and positioned on the lower spring assembly of FIG.  2 . 
     FIG. 4 is a perspective view of an upper spring assembly, a sphere containing an optical element, a lower spring assembly, and a base for an optomechanical mounting in accordance with an embodiment of the invention. 
     FIG. 5 is a perspective view of a complete optomechanical mounting in accordance with an embodiment of the invention. 
     FIGS. 6A and 6B are perspective views showing the relative orientations of supporting springs in alternative embodiments of the invention. 
     FIG. 7 shows a sphere for containing a refractive translator optic for a mounting in accordance with an embodiment of the invention. 
     FIGS. 8A and 8B show a sphere for containing a reflective beam bender/splitter for a mounting in accordance with another embodiment of the invention. 
     FIGS. 9A and 9B illustrate a sphere positioned between two spring washers in accordance with another embodiment of the invention. FIG. 9B is a cross-sectional view of the objects in FIG. 9A, taken along a vertical plane passing through the line B-B′ shown in FIG.  9 A. Support rings have been added to this view, and the sphere cross-section is represented by a circle to keep the picture clear and easily understood. 
     Use of the same reference symbols in different figures indicates similar or identical items. 
    
    
     DETAILED DESCRIPTION 
     In accordance with an aspect of the invention, upper and lower spring assemblies support a sphere containing an optical element. The spring assemblies are substantially identical and are oriented so that each spring applies a force along a radius of the sphere. Each of theses forces is collinear with an opposing force from a spring in the other spring assembly. The springs accordingly hold the sphere in position with a high degree of thermal stability because thermal expansion that changes a force from one spring assembly is matched by a thermal change in the opposing force from the other assembly. 
     Each spring assembly can include springs on a support ring with a central portion of the assembly opened for an optical path or alignment tool access. 
     FIG. 2 is a perspective view of a lower spring assembly  200  in accordance with an exemplary embodiment of the invention. Spring assembly  200  includes three leaf springs  220  attached to a support ring  210 . 
     Support ring  210  is made of a rigid material such as tool steel and is predominantly circular with a conical inner surface. In the exemplary embodiment, support ring is about 14 mm thick and has an outer diameter of about 44.22 mm. The inner surface has a cylindrical portion with a diameter of about 25.7 mm and height of about 6.82 mm at the bottom of support ring  210 . A conical portion extends upward at a 45° from the cylindrical portion. Accordingly, the conical portion has an opening at the top support ring  210  of about 39.92 mm. 
     Located 120° at-intervals around the inner surface of support ring  210  are fixtures  230  for seating and attaching leaf springs  220 . In the exemplary embodiment of the invention, each leaf spring  220  is a rectangular piece of flat metal such as spring steel about 20.70 mm wide and 12.7 mm high with a thickness of about 0.00762 mm. Leaf springs  220  are flat in the exemplary embodiment of the invention but can be convex or concave in alternative embodiments of the invention. 
     Each fixture  230  is machined into the conical portion of the inner surface of support ring  210  and sized to accommodate a leaf spring  220 . In the exemplary embodiment, fixtures  230  provide flat ledges about 4.15 mm wide, and leaf springs  220  are welded to ledges  232  by stitch welds  234  that are no more than 4.0 mm long to avoid welds extending beyond ledges  232 . A locating pin  240 , which is not a part of spring assembly  200 , can help position springs  220  for attachment (e.g., welding) to support ring  210 , and in the exemplary embodiment, locating pin  240  has a diameter of about 20.08 mm. In other embodiments, other means such as epoxy or spring tension can hold leaf springs  220  to support ring  210 , or leaf springs can be free floating in the fixtures  230  of support ring  210 . 
     FIG. 3 shows a sphere  300  resting on lower spring assembly  200 . Sphere  300  contains an optical element (not shown) such as a mirror, a beam splitter, a translating window, a wedge window, or a lens. In the exemplary embodiment, sphere  300  is a precision bearing about 41.275 mm in diameter that is machined to include an opening  310  for the optical element, openings  320  for light paths to and from the optical element. Sphere  300  can further include openings  330  that fit an alignment tool such as an Allen key or lever that can be used to rotate sphere  300  in the finished optomechanical mounting. 
     To assemble the optomechanical mounting, lower assembly  200  is attached (e.g., bolted) to a base plate  410  as illustrated in FIG.  4 . Mounting feet of base plate  410  can be designed to flex rather than slip when the mount encounters differential thermal expansion, e.g., from a thermal gradient or when base plate  410  is attached to a base of a different material. Thus, when the temperature returns to normal the original alignment of the optical element is reestablished. 
     Sphere  300  is placed on lower spring assembly  200 . An upper spring assembly  420 , which sits on sphere  300  is attached inside a cover  510  shown in FIG. 5, and cover  510  is attached to base plate  410  so that upper spring assembly  420  contacts sphere  300  as shown in FIG.  4 . 
     The height of cover  510  of FIG.  5  and the gap between cover  510  and base plate  410  are selected so that cover  510  and base plate  410  apply pressure to upper and lower spring assemblies  420  and  200 . In the exemplary embodiment of the invention, cover  510  has a cavity of height 42.83 mm that contains lower spring assembly  200 , sphere  300 , and upper spring assembly  420 . As a result, in optomechanical mounting  500 , the springs in upper spring assembly  420  and lower spring assembly  200  contact and apply radial forces to sphere  300 . 
     All of the components in optomechanical mounting  500  can be made of the same material or materials that are substantially the same or at least have the same or similar coefficients of thermal expansion (CTEs). If the CTEs are the same, the entire assembly expands or contracts in unison when subjected to a temperature change. Thus, sphere  300  will not rotate during acclimatization, and the angular alignment of the optical element is preserved when the temperature changes. 
     In the exemplary embodiment, upper spring assembly  420  is substantially identical to lower spring assembly  200 , but the attachments of lower spring assembly  200  to base plate  410  and upper spring assembly  420  to cover  500  orient springs  620 - 1 ,  620 - 2 , and  620 - 3  (shown in FIG. 6A ) of upper spring assembly  420  directly opposite corresponding springs  220 - 1 ,  220 - 2 , and  220 - 3  (shown in FIG. 2) along respective lines through the center of sphere  300  as shown in FIG.  6 A. For example, springs  220 - 1 ,  220 - 2 , and  220 - 3  can be located at 0°, 240°, and 120° around a vertical axis of sphere  300 , while springs  620 - 1 ,  620 - 2 , and  620 - 3  are located at 180°, 60°, and 300° around the vertical axis. With this configuration, spring force vectors for a pair of springs ( 220 - 1 ,  620 - 1 ), ( 220 - 2 ,  620 - 2 ), or ( 220 - 3 ,  620 - 3 ) are collinear and pass through the center of sphere  300 . Each spring  220  or  620  thus has a corresponding spring  620  or  220  that provides an equal, collinear opposing force through the center of sphere  300 . Accordingly, springs  220  and  620  do not apply a torque to sphere  300 , and changes in springs  620 , for example, caused by changes in temperature, counter or cancel corresponding changes in springs  220  to keep sphere  300  from shifting position. 
     FIG. 6B illustrates the spring configuration in an alternative embodiment of the invention. The embodiment of FIG. 6B uses four springs  631 ,  632 ,  641 , and  642  Two springs  631  and  632  are in a lower spring assembly (not shown), and two springs  641  and  642  in an upper spring assembly (not shown). Each spring  631 ,  632 ,  641 , and  642  is a leaf spring having a surface perpendicular to the normal to the surface of sphere  300  at the respective contact points. The upper and lower spring assemblies are identical to each other, but the upper spring assembly is rotated by 90° relative to the lower spring assembly so that the contact points of sphere  300  with springs  631 ,  632 ,  641 , and  642  are at the vertices of a symmetric tetrahedron. The resulting spring forces on sphere  300  are directed radially toward the center of sphere  300  and hold sphere  300  without applying a torque to sphere  300 . Other spring configurations could similarly hold sphere  300  without applying a torque, for example, a single spring on top of sphere  300  could oppose and counter the resultant of the three forces from springs  220 . However, such configurations lack the symmetry of the system of FIG. 6A, where each spring is paired with an opposing spring. Accordingly, thermal expansion of a mount using a minimally constrained arrangement such as illustrated in FIG. 6B may change the position or orientation of sphere  300 . 
     Returning to FIG. 5, cover  510  has openings  520  for light paths to and from the optical element in sphere  300  and openings  530  for tools used to rotate sphere  300  during an alignment process. Additionally, since spring assembly  420  is ring-shaped, an opening  540  in the top of cover  500  or bottom plate  410  can allow a light path through the top of cover  510  or allow access to the top of sphere  300  for a tool used to rotate sphere  300  during an alignment process. Similarly, access can be provided through base plate  410  to the bottom of sphere  300 . 
     Optical elements mounted in a sphere  300  can vary widely, but generally, the center of sphere  300  lies on an optical surface, an axis, and/or a symmetry plane of the optical element in sphere  300 . FIG. 7, for example, illustrates a sphere  700  containing a refractive translator optic  710  for shifting the position of a laser beam perpendicular to direction of propagation of the laser beam. Translator  710  can simply be a thick piece of optical quality glass having parallel optical surfaces. An Allen key fit into an opening  720  can be used to rotate sphere  700  in an optomechanical mounting such as described above to change the incidence angle of an input beam and control the amount of shift refraction causes in translator  700 . Additional access ports for tooling can be provided at almost any position, notably at 45° positions in a vertical plane. 
     FIGS. 8A and 8B show perspective views of a sphere  800  used for a beam bender. Sphere  800  has an opening into which an optical element  810  having a highly reflective surface  812  is inserted and attached. Openings  820  are for an input beam and a reflected beam that enter and exit sphere  800 . Openings  830 , which are accessible through openings in the cover of the optomechanical mounting, allow a lever to rotate sphere  800  as required to align highly reflective surface  812  with the input beam. Optical element  810  is positioned so that highly reflective coating  812  is at the center of sphere  800  so that rotation of sphere  800  changes the angle of incidence of the input beam on surface  812  but does not change the point of incidence on surface  812 . 
     FIG. 9A illustrates another embodiment of the invention using spring washers  900 . The sphere  300  is positioned on a lower spring washer  910 , such that the lower spring washer  910  contacts the sphere  300  along a longitudinal circle on the surface of the sphere  300 . An upper spring washer  920  is placed opposite lower spring washer  910 , such that the upper spring washer  920  also contacts the surface of the sphere  300  along a longitudinal circle. 
     The spring washers  900  are preferably made out of metal such as spring steel, stainless steel, or carbon steel. For some applications, the spring washers  900  should be polished to lower surface friction during alignment of the sphere  300 . The spring washers  900  shown in the exemplary embodiment are Belleville spring washers, a type of washer well known in the art. Other washers, such as modified Belleville spring washers, can also be used. The spring washers  900  in the exemplary embodiment have an outer diameter of about 36 mm, an inner diameter of about 22.4 mm, and a thickness between 0.1 mm and 0.3 mm. 
     FIG. 9B is a cross-sectional view of the sphere  300 , spring washers  900 , and support rings  940  (collectively known as assembly  930 ) as assembled in accordance with this embodiment of the present invention. Lower support ring  950  and upper support ring  960  are similar to the support ring  210  shown in FIG. 2; each of the support rings  940  is predominantly circular, but the inner surface is cylindrical rather than conical, and does not have fixtures  230  for holding leaf springs  220  that are found in support ring  210 . The spring washers  900  are positioned on the edge of the cylindrical inner surface of the support rings  940 . In an alternate embodiment, the inner surface of the support rings  940  is slightly conical to help keep the spring washers  900  in place. The spring washers  900  are fastened in well-known ways (e.g., welded) to the support rings  940 . Alternatively, the spring washers  900  can be left unfastened to the support rings  940 . 
     To assemble the optomechanical mounting, the assembly  930  is attached to a base plate  410  (shown in FIG.  5 ). A cover  510  (shown in FIG. 5) is placed over assembly  930 , such that cover  510  and base plate  410  apply force on support rings  940 . The gap between cover  510  and base plate  410  can be varied to vary the magnitude of force applied. The support rings  940  apply force on the spring washers  900 , such that the upper spring washer  920  exerts an equal, collinear opposing force on sphere  300  against the lower spring washer  910 . 
     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.