Abstract:
A multiple spring support for a displaceable mirror in an interferometer maintains the plane in which the flat mirror surface resides perpendicular to the centerline of a wave front at all retardations of the interferometer. In its simplest configuration, two equal length spring sections are connected to a movable rigid beam section at one end of the spring sections and are connected to a fixed rigid base section at the other end of the spring sections. The spacing between spring sections at the movable rigid beam section being the same as the spacing between spring sections at the fixed rigid base section.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to an apparatus for supporting a movable mirror assembly with the apparatus finding advantageous use in an interferometer.  
         BACKGROUND OF THE INVENTION  
         [0002]    In an interferometer, a movable mirror is used to cause constructive and destructive interference between two radiation beams derived from a common source at different movable mirror displacements, or different retardations. The resulting radiation is said to be modulated.  
           [0003]    Various methods have been used to provide bearing support for a movable mirror assembly that attempt to maintain mirror surface perpendicularity to a wave front either while the mirror assembly is moving, or for different displacements of the movable mirror. Air bearings have been widely used for high-resolution mid and near infrared interferometers, but the need for high quality gas is expensive and air bearings are cumbersome. “Porch swing” linkages have been used with success, but are relatively expensive and require great effort and attention to assure proper setup. U.S. Pat. No. 4,991,961 to Strait discloses a moving mirror tilt adjust mechanism in an interferometer to assure such proper alignment. More recently, a glass graphite bearing has been used with success (U.S. Pat. No. 5,896,197). Linear ball bearings are now available that provide acceptable smoothness and linearity, however they are moderately expensive and require great attention to manufacturing tolerances and cleanliness.  
           [0004]    U.S. Pat. No. 4,710,001 to Lacey discloses a moving mirror assembly using a pair of flat springs “created by forming a plurality of open-ended slots in a flat sheet of spring stock, each slot partially enclosing the next innermost slot” (Col. 2, lines 61-64). A frame holds one edge of each spring to position the same within opposed apertures in the frame sidewalls, and a hollow rectangular beam extends between the centers of the springs. While the patent meets the functional criteria required of a moving mirror assembly, it suffers from being overly complex and is subject to environmental influences such as vibrations and external shocks.  
           [0005]    The invention disclosed herein greatly simplifies a movable mirror apparatus and provides a low cost, significantly more robust interferometer, which is less subject to environmental shock and vibrations.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is a device for supporting a mirror in an interferometer or other application so that the plane in which the mirror surface resides can be moved perpendicular to a wave front without tilting or wobbling. The invention meets the requirement for a flat moving mirror used in an interferometer, that is, that the plane which contains the mirror surface remains perpendicular to the wave front for all displacements. This condition is met for our invention even though the actual mirror does not move in a straight-line, but instead in an arc-wise fashion.  
           [0007]    The apparatus can be used in a fast-scan interferometer where measurements are made while the movable mirror is moving at a constant linear velocity, or with all other interferometers, such as a step-scan interferometer, where the movable mirror is moved to a position, stopped while a measurement is made and then moved to another position.  
           [0008]    The present invention discloses the use of springs as part of a movable mirror mechanism for use in an interferometer. When using the term spring, we mean an elastic element that in whole or in part returns substantially to its original form after being forced out of shape. By such definition, the term spring would clearly include metals, plastics, rubbers and other widely accepted elastic materials and would also include such materials as sheet paper or thin sheets of certain other fibrous materials. While paper and certain other fibrous materials are not considered to be highly elastic, they do exhibit the property of substantially returning to their original forms when the sheets are curled or bent but not folded, creased, or otherwise bent beyond their elastic limits.  
           [0009]    While the preferred embodiment shown uses only two springs, it is contemplated that embodiments with more than two springs will exhibit beneficial robustness to external disturbances at low to modest increases in cost. It is also envisioned that flat springs can be replaced with multiple spring steel wires, which are clamped in a fashion similar to that of the flat springs and thereby provide a variation of the preferred embodiment. Furthermore it is contemplated that a significant portion of the movable mirror apparatus can be cast, molded, or extruded out of materials with appropriate elasticity in order to further simplify the apparatus and reduce costs. Two such embodiments are disclosed in FIGS. 4 and 5.  
           [0010]    Because the apparatus is simple and has no parts that exhibit wear characteristics, it is expected that there are additional benefits of low maintenance and durability. Furthermore, since the springs are minimally stressed, it is expected that there will be no deterioration over time and that the movable mirror mechanism will thereby be highly stable over time.  
           [0011]    While the movable mirror mechanism is very simple and low cost, it is highly precise and repeatable even over the greater displacements required for high resolution instruments, which historically have required the use of high cost air bearing systems.  
           [0012]    The two springs supporting the rigid beam and mirror are spaced apart an equal amount at the rigid beam connection and at the frame connection. For the preferred embodiment, during assembly, the spring connections in the at-rest mode are adjusted to cause the springs to be parallel to each other, such that in a side elevation, the lines which can be drawn between adjacent points of the four spring connections define a parallelogram. Meeting these aforementioned conditions causes parallelograms to be defined by lines drawn between adjacent points of the four spring connections at all displacements of the mirror and rigid beam so long as the elastic limits of the springs are not exceeded. This equal spacing of the connections of the springs contributes to the plane of the mirror surface remaining parallel to all other planes in which the mirror resides for all displacements of the rigid beam and mirror assembly permitted by elastic displacement of the springs.  
         BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    In the drawings:  
           [0014]    [0014]FIG. 1 is an isometric view of the preferred embodiment of the support device along with the drive magnet housing assembly for an interferometer;  
           [0015]    [0015]FIG. 2 is a side elevation showing the support device of FIG. 1 in its upright, at rest position, with the drive magnet housing assembly being shown in section;  
           [0016]    [0016]FIG. 3 is a side elevation showing flat spring deflection as it would appear from displacement of the beam and attached mirror during movement or at different retardations, with the at rest position being shown in dashed lines; and with magnet and magnet housing not being shown for clarity of illustration;  
           [0017]    [0017]FIG. 3A is an enlarged, partial side elevation showing a series of different displacements of the flat springs;  
           [0018]    [0018]FIG. 3B is similar to FIG. 2, and shows the beam and flat springs at rest, with a rectangular parallelogram defined by connection points A, B, C, and D of the springs;  
           [0019]    [0019]FIG. 3C is similar to FIG. 3, and shows the beam displaced and the resulting parallelogram defined by connection points A, B, C, and D of the corresponding deflected flat springs;  
           [0020]    [0020]FIG. 4 is a side elevation showing an alternative embodiment that includes an extruded one piece support member that combines a number of components disclosed in the preferred embodiment;  
           [0021]    [0021]FIG. 5 is a side elevation of one variation of an extruded one piece support member; and  
           [0022]    [0022]FIG. 6 is a top plan view of the preferred embodiment of the support device shown as part of an interferometer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    [0023]FIG. 1 discloses an isometric view of the preferred embodiment of the support device in an orientation showing the movable mirror portion above the frame; such orientation is for convenience of description only since the apparatus is capable of functioning in any orientation. As shown in the at-rest condition illustrated in FIG. 1, the movable mirror assembly, indicated generally at  100 , includes two spaced, vertically extending springs  101  and  102 . These springs are preferably made from spring steel and have a thickness in the range of 0.001 to 0.010 inches, with a preferred thickness of 0.003 inches. While rectangular, thin, flat springs  101  and  102  are illustrated and described, it will be appreciated that other spring shapes can be used. For example, when viewed from the left end in FIGS.  1 - 3 , the springs  101  and  102  may have triangular, trapezoidal, and semicircular shapes as well as variations of other multisided shapes. The springs  101  and  102  also could include cut out sections in symmetrical or unsymmetrical patterns. Furthermore certain benefits could be achieved if the thickness of the springs is made different in some sections of the springs to achieve the correct combination of resistance to forces from a variety of directions along with maintaining flexibility in the direction of desired displacement.  
         [0024]    The lower ends of springs  101  and  102  are preferably tightly secured by a clamping assembly, indicated generally at  115 , to a fixed frame  120  of the interferometer. The clamping assembly  115  includes an adjustment block  104 , a spacer block  105  and two spaced clamp plates  107  and  109  positioned at opposite sides of the clamping assembly  115 . The bottom end of spring  101  is sandwiched between the clamp plate  107  and the spacer block  105 , which is attached to frame  120  by fasteners  123 . The lower end of spring  102  is sandwiched between the adjustment block  104 , which is securely attached to frame  120  with fasteners  123 , and clamp plate  109 . The clamp plates  107  and  109  are held in compression against the bottoms of springs  101  and  102 , respectively, by fasteners  121  at one end, and similar fasteners  121  at the other end. Other methods of clamping or securing the bottom ends of the springs to the clamping assembly and frame are also contemplated, such as fasteners  121  extending through the springs and the clamping assembly, or by welding, or otherwise affixing the bottom ends of the springs  101  and  102  directly to the frame. As an alternative to adjustment block  104 , the spacing between the clamped lower ends of the springs  101  and  102  can be made of a single member having the same precise dimensions as the length of beam  103  between the springs  101  and  102 .  
         [0025]    The other or upper ends of springs  101  and  102  are respectively clamped to a rigid, but movable, fixed length beam  103 . At its upper end, spring  101  is clampingly secured or sandwiched, between one end of the fixed length beam  103  and a mirror holder plate  106 . At its upper end, spring  102  is sandwiched between the other end of the fixed length beam  103  and a coil mount plate  108 . The mirror holder plate  106  and the coil mount plate  108  are held in compression against the top ends of springs  101  and  102  by fasteners  122  passing through the entire upper clamping assembly. As with the lower clamping assembly, the upper clamping assembly can be readily modified to have different spacing between the springs, to have different clamping arrangements, and to have alternate means of connecting the upper ends of the springs to the rigid beam  103 .  
         [0026]    The spacing between the upper ends of the springs  101  and  102  at their respective connections to the beam  103  equals the spacing between the springs  101  and  102  at their respective connections to the clamping assembly  115 , which is rigidly mounted to fixed frame  120 . The sections of the springs  101  and  102  between their respective upper and lower clamped ends are unimpeded and are thus free to flex when the rigid beam  103  is displaced or moves. When viewed in side elevation, lines drawn between the four connections of the springs  101  and  102  to the rigid beam and clamp assembly cooperatively define a parallelogram for all displacements of the mirror  110 .  
         [0027]    While two spaced, rectangular springs  101  and  102  are illustrated, it will be appreciated that additional parallel springs could be added as required for the application. For example, four rectangular corner springs of reduced width could also be used to support the rigid beam and mirror (not shown). Also, pre-bent springs could be used instead of the flat rectangular springs shown (not shown).  
         [0028]    Mirror  110 , with reflective surface  111  facing outward, is affixed to mirror holder plate  106 . The mirror  110  is thus affixed to one end of and moves with the rigid beam  103 .  
         [0029]    On the opposite end of the beam  103  is an annular voice coil  112  that is attached to coil mount  108 . The voice coil extends into an aperture  126  in sidewall  127  of magnet housing  113 . As best shown in FIG. 2, the voice coil  112  (with the actual annular electrical coils being illustrated as a blackened rectangle in section) surrounds a permanent magnet  128 , which is fixedly mounted within the housing  113 . The magnet housing  113  is shown attached to magnet housing adapter plate  114 , which is attached to fixed frame  120  by fasteners  129  (FIG. 1).  
         [0030]    To remotely control movement of the mirror  110 , an electrical current is passed through the voice coil  112 . The electrical current can be passed through the electrical coils in either direction to electro-magnetically displace the rigid beam  103  and mirror  110  in either the left or right direction as viewed in FIG. 3. The speed and acceleration of displacement is dependent upon the magnitude of the current and the resistance or assistance of the springs  101  and  102  along with the respective masses of the movable mirror components.  
         [0031]    For rapid scan interferometers, a laser  201  or other optical referencing method is used to observe the position of mirror  110  while a very fast clock is used to provide time for a velocity reference of mirror  110  in a servo loop control circuit. Such methods of velocity or position control are well known to those of ordinary skill in the art, for example, Nichols U.S. Pat. No. 3,488,123 describes such a mechanism. This patent is incorporated herein by reference. There are other schemes, well known in the art, that can be used to sense displacement or velocity of movement of mirror  110  and thereby control the mirror position or the velocity of mirror movement via the amount of current passed through the voice coil  112 .  
         [0032]    The various members of the movable mirror assembly  100  are designed to assure that driving forces are countered with opposing forces substantially along the same axis. The centers of mass of the various components of the movable beam and mirror assembly, with the exception of springs  101  and  102 , lie along an extension of the cylindrical axis of the voice coil  112  and the magnet  128 , which share a substantially common axis  129  (FIG. 2), thereby causing forces due to acceleration to lie along that same common axis. Due to the novel configuration of the springs  101  and  102  relative to the beam  103  and the clamping assembly  115 , the external force resulting from the displacement of the movable portion of the movable mirror assembly  100  is best represented by a resultant force along the longitudinal axis  129  of the voice coil  112 . As best shown in FIGS. 3 and 3A, the bending of spaced springs  101  and  102  allows the fixed beam  103  and mirror  110  to be displaced in an arc, with the beam  103  retaining its horizontal orientation during all displacements (see FIG. 3A and the arcs defined by connection points A and C, of spring  101  and  102 , respectively to beam  103 , at incremental displacements). In longitudinal cross sectional view, the spring connections to the frame and rigid beam cooperatively define a parallelogram at all positions of displacement (see parallelograms having four corners defined by points A, B, C, and D in FIGS. 3B and 3C).  
         [0033]    While the resulting direction of the opposing force from the springs  101  and  102  remains along the longitudinal axis of the voice coil  112  for all displacements, the movable portion of the movable mirror assembly  100  actually moves in an arc-wise path, thereby causing the centerlines of the voice coil  112  and the magnet to separate by a small amount, as represented by the dimension S in FIG. 3. Since the lengths of the springs  101  and  102  can readily be changed by design, the amount of the centerline separations can also be changed. Also, the total displacements required depend upon the optical frequencies of interest and the measurement resolutions desired. For example, in one embodiment for use in a Fourier Transform Mid Infrared Modulator, a four wave number resolution requires a total displacement of around two to three millimeters. With this displacement, which is represented by the dimension D in FIG. 3, the centerline of the voice coil diverges from that of the fixed position magnet by less than one tenth of a millimeter, which has been found to be irrelevant to the measurements being made.  
         [0034]    In order to insure that the necessary dimensional conditions have been met which result in wobble and tilt free movement, a separation adjustment assembly may be provided. During manufacturing, the movable mirror assembly  100  and frame  120  are placed into an alignment fixture to position the mirror surface  111  perpendicular to a collimated beam. While oscillating the movable mirror, the adjustment screw  116  is turned clockwise or counterclockwise to drive a wedge assembly (not shown), which causes the adjustment block  104  to be shifted left or right relative to the spacer block  105 , as viewed in FIG. 2, to control the spacing between the frame connections of the two springs. The spacing of the springs  101  and  102  is adjusted until an acceptable level of wobble and tilt is observed from any misalignment of the images created from the collimated source radiation and the returned reflected radiation from mirror surface  111 . When proper alignment is achieved the images remain aligned and do not move during the oscillation. At which time, adjustable block  104 , along with spring  102  and clamp plate  109 , is rigidly affixed to the frame by securely tightening fasteners  123 .  
         [0035]    [0035]FIG. 4 discloses an embodiment wherein the use of extrusion, molding, or cold rolling techniques to manufacture the apparatus further simplifies the apparatus and reduces costs. Mirror  110  and voice coil  112  are affixed to an extruded one piece support member  130 , which is affixed to frame  120  with fasteners  123 . Extruded support member  130  is integrally comprised of rigid beam section  131 , rigid mount  134  and spring sections  132  and  133  interconnecting the rigid beam and mount. The spring sections  132  and  133  are shown to be thin and of constant and equal thicknesses; however, spring sections  132  and  133  need not be of constant or equal thickneses. The criteria that must be met are that the effective lengths of the springs are the same and the elastic limits of the materials are not exceeded for the required maximum displacements. Extrusion and molding techniques are routinely used to create shapes of polymer, glass, and ceramic materials. Very tight tolerances can be maintained using commercially available technologies. There are currently many highly stable elastic polymers commercially available that would readily provide the properties necessary to extrude or otherwise mold the sections disclosed. In addition, dies for either extruded or molded support members can be designed and manufactured at reasonable costs.  
         [0036]    Cold rolled forming techniques are routinely used to form shapes and to create special metallurgical properties for metals and could be readily adapted for support members made of metals.  
         [0037]    [0037]FIG. 5 discloses a side view of an extruded one piece support member with springs that are not of constant thickness but have increased modulus sections  135  to improve the support member&#39;s resistance to external shocks and vibrations while maintaining sufficiently low resistance to bending along the direction of preferred mirror movement. The integral one piece support members  130  illustrated in FIGS. 4 and 5 have the spacing between spring sections  132  and  133  at the rigid beam section  131  equal to the spacing between the spring sections at the rigid mount section  134 .  
         [0038]    [0038]FIG. 6 discloses the movable mirror apparatus as an integral part of a Fourier Transform Infrared (FTIR) interferometer whose opto-mechanical apparatus is shown generally as  200 . A laser  201  is used as an optical reference. Laser energy  202  is sequentially directed to laser steering mirrors  203 ,  204 , and  205  to be made parallel with optical energy  206  emitted from infrared source  207 . The laser energy  202  from the laser  201  and optical energy  206  from the infrared source  207  together simultaneously pass through, and are reflected off of, beam splitter  208  to fixed mirror  209  and movable mirror  110 . Fixed mirror  209  is attached to mirror support assembly  210 , which in turn is affixed to frame  120 . Movable mirror  110  is an integral part of movable mirror assembly  100  previously described.  
         [0039]    The laser energy  202  passes through and is reflected off of beam splitter  208  to fixed mirror  209  and movable mirror  110 . The split laser energy reflects off of mirrors  209  and  110 , is recombined at beam splitter  208  and then continues on to laser signal detector  211 . The detector  211  converts optical energy to an electrical signal that is used by the electronic control circuitry to send electrical current to voice coil  112 . This current creates an attractive or repulsive force with magnet  128  (which is contained within magnet housing  113 ) to control the displacement and velocity of movable mirror assembly  100 . Infrared energy  206  likewise passes through and is reflected off of beam splitter  208  to fixed mirror  209  and movable mirror  110 . The split infrared energy  206  is reflected off mirrors  209  and  110  and then is recombined at beam splitter  208 . The infrared energy is thereby modulated as the result of the constructive and destructive interferences caused by the change in the movable mirror optical path length for different retardations. Such modulated infrared energy continues past laser detector  211  on to a detecting system (not shown). The design of the detecting system is dependent upon the experiment or experiments of interest. FTIR and FT-NIR detecting systems are well known and widely used commercially.  
         [0040]    Although one embodiment of this invention has been shown and described, various adaptations and modifications can be made without departing from the scope of the invention as defined in the appended claims.