Abstract:
A mechanism and method for precisely arranging two or more optical elements, such as those incorporated into photoelastic modulators (PEMs), at a specific angular orientation. The method includes supporting one optical element in an annular mounting member that has an optic axis, and supporting other optical elements in other annular mounting members that have optic axes, and concentrically stacking together the two or more mounting members about a central axis in a manner such that one mounting member may be rotated relative to the others about the central axis and such that the optic axes of the mounting members define an optics angle, and rotating one mounting member relative to the others to define the specific angular orientation of the optical elements.

Description:
TECHNICAL FIELD 
       [0001]    This application relates to a mechanism and method for precisely arranging the optical axes of two or more optical elements, such as those incorporated into photoelastic modulators, in a selected angular orientation. 
       BACKGROUND 
       [0002]    A photoelastic modulator (PEM) is an instrument that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term “photoelastic effect” means that an optical element that is mechanically stressed and strained (deformed) exhibits birefringence that is proportional to the amount of deformation induced into the element. Birefringence means that the refractive index of the optical element is different for different components of a beam of polarized light. 
         [0003]    A PEM includes an optical element, such as fused silica, that has attached to it a transducer for vibrating the optical element. The transducer vibrates at a fixed frequency within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz. The mass of the element is compressed and extended along the axis of the optical element as a result of the vibration. The combination of the optical element and the attached transducer may be referred to as an optical assembly. The axis about which the optical element vibrates is referred to as the optical axis of the PEM. 
         [0004]    The optical assembly is mounted within a housing or enclosure that normally includes an aperture through which the light under study is directed through the optical element in a direction generally perpendicular to the optical axis of the PEM. The enclosure supports the optical assembly in a manner that permits the optical element to be driven (vibrated) within it to achieve the above-noted photoelastic effect. 
         [0005]    PEMs are commonly used in measuring polarization properties of either a light beam or a sample. Many instruments use two or more PEMs to provide measurements of certain polarization properties. When two PEMs are used in a single instrument, they are typically arranged so that their optical axes are oriented to be precisely 45 degrees apart (as considered in a direction perpendicular to those two optical axes). 
         [0006]    Examples of typical, two-PEM instruments include complete Stokes polarimeters, Tokomak polarimeters, and a number of other polarimeters and ellipsometers. When four PEMs are used in one instrument, the PEMs are typically grouped in separate pairs. 
         [0007]    The speed and precision with which a pair of PEMs can be oriented so that their optical axes are fixed at a particular, selected angle depends greatly on the precision with which the housing or enclosure to which the PEMs are mounted can be positioned and secured to place the PEMs in that proper orientation. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a mechanism and method for precisely arranging two or more optical elements, such as those incorporated into PEMs, at a specific angular orientation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a top plan view of a mechanism formed in accordance with the present invention, with a cover removed and with an actuator connected thereto for adjusting the mechanism. 
           [0010]      FIG. 2  is a cross sectional view of the mechanism taken along line  2 - 2  of  FIG. 1 . 
           [0011]      FIG. 3  is a perspective view of the mechanism of  FIG. 1  with the cover replaced and the actuator removed. 
           [0012]      FIG. 4  is a top plan view of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    One embodiment of a mechanism  20  formed in accordance with the present invention is depicted in the figures. The mechanism  20  includes two generally annular mounting members, hereafter referred to as a lower mounting member  22  and upper mounting member  24 . The designation of “lower” and “upper” is for reference purposes only. The mounting members  22 ,  24  are nearly identical in construction and are interchangeable. The following description focuses on the upper mounting member  24  with the understanding that the lower mounting member  22  is similarly constructed except where otherwise specified. 
         [0014]    The upper mounting member  24  is metal and is generally annular with a depth (measured vertically in  FIG. 2 ) that is about one-half of its radius (as measured in a plan view). A notch  26  ( FIG. 1 ) is cut through the mounting member  24 . An open end of a somewhat elongated enclosure  28  is attached to the mounting member at the notch  26  to protrude radially outwardly therefrom. (The corresponding enclosure of the lower mounting member  22  is shown at  128 .) It is noteworthy that the mounting member may also be formed of rigid plastic, such as Delrin®. This would be a useful configuration when the mechanism is used in a magnetic field as occurs, for example, in Tokomak polarimeter applications. 
         [0015]    A primary function of each mounting member  22 ,  24  is to support the optical assembly of a photoelastic modulator (PEM)  30 . The primary components of the PEM&#39;s optical assembly include an optical element  32  formed of fused silica. Other material, such as fused quartz, calcium fluoride, zinc selenide, silicon and others may be used to form the optical element. (The corresponding optical element supported in the lower mounting member  22  is shown at  132 .) 
         [0016]    The optical element  32  is a generally square-shaped member but having beveled corners that define flat support surfaces  34 , the function of which is described below. The optical element also has an entry surface  36  against which an incident light beam is directed while the PEM is operating. A quartz piezoelectric transducer  38  ( FIG. 1 ) is bonded to one of the four sides of the optical element  32 . Electrical leads (not shown) from the transducer are connected to a driver circuit for vibrating the optical element  32 . 
         [0017]    The optical element  32  is supported so that its entry surface  36  extends across the central aperture  40  of the upper mounting member  24 . Preferably, the center of the entry surface is aligned with the central axis  41  of that aperture  40  ( FIGS. 2 and 3 ). The optical element  32  is free to vibrate when driven as described above. In this regard, the optical element  32  is mounted to the upper mounting member  24  by somewhat flexible supports  42  ( FIG. 1 ) that secure the optical element  32  at each support surface  34  so that the optical element is substantially suspended within the central aperture  40  of the upper mounting member  24 . 
         [0018]    Each one of the supports  42  includes an elastomeric rod  44  that may be formed, for example, from extruded silicone (polysiloxane) cords that are cut to a specified length to define the rod  44 . One of the two, flat ends of the rod  44  is attached, as by an adhesive, to one of the support surfaces  34  on the optical element  32 . 
         [0019]    The other, free end of the rod  44  fits within a sleeve  46  that is carried inside of a cylindrical slider  48 . The sleeve  46  has a cylindrical axial bore formed through one end to receive the elastomeric rod  44 . The sleeve  46  is a rigid, externally threaded member that is threaded into an internally threaded bore  50  ( FIG. 3 ) of the slider  48 . 
         [0020]    Each on of the four sliders  48  fits inside of a radial hole  52  ( FIG. 3 ) formed through the curved side  56  of the upper mounting member  24 . The slider  48 , with the sleeve  46  threaded into its bore  50 , is slid with the radial hole  52  until the rod  44  is received in the bore of the sleeve  46 . The slider  48  is secured in place via a setscrew  54  that is threaded vertically ( FIG. 1 ) through the upper mounting member  24  to bear against the slider. 
         [0021]    With the slider secured in place, the sleeve  46  is advanced until the free end of the rod  44  (that is, the end not bonded to the optical element support surface  34 ) is completely received within the bore of the sleeve. The sleeve  46  may be advanced by hand or with a tool. In this regard, the outer end of the sleeve  46  may be shaped to define a socket for an Allen-type wrench or the like that can be extended into the bore  50  of the slider to reach the socket in the sleeve  46 . 
         [0022]    The foregoing description of an exemplary support  42  applies to all four supports  42  on both mounting members  22 ,  24 . As depicted in  FIG. 1 , four supports are employed to secure the optical element  32  in place relative to the upper mounting member  24 . The supports are thus arranged in diametrically opposed pairs. Alternative configurations of such supports  42  are contemplated, such as those described in U.S. Pat. No. 7,800,845 owned by the assignee of this application. As another alternative, the rod  44  could be replaced with a glass or plastic conical member with the base of the cone bonded to the support surface  34  and the pointed end seated in the central opening of an annular elastomeric grommet that is mounted on the end of a cylindrical barrel that is secured in the hole  52 . In the figures, the grommet and barrel would appear as the sleeve  46  and slider  48  respectively. The setscrew  54  would hold the barrel and grommet combination in place. 
         [0023]    The transducer  38  is attached to the optical element  32 , and not otherwise supported by the upper mounting member  24 . The transducer  38  is an elongated, bar-like member that extends from the optical element  32  and into the enclosure  28  that protrudes radially outwardly from the outer, curved surface  56  of the upper mounting member  24 . The longitudinal axis  58  of the transducer  38  is aligned with the center of the optical element  32  and, as such, this axis  58  coincides with the optical axis of that optical element. 
         [0024]    For purposes of this description, the projection of the optical axis of the optical element  32  of the PEM  30  onto the structure of the upper mounting member  24  is illustrated by axis line  58 , which will hereafter be referred to as the optics axis  58  of the upper mounting member  24 . The lower mounting member  22  has a similarly defined optic axis  158 , as shown in  FIGS. 1 ,  3  and  4 . 
         [0025]    The angle between these two optics axes  58 ,  158  (as viewed along the central axis  41  (see  FIGS. 1 and 4 ) is referred to as the optics angle  60 . It will be appreciated that the optics angle  60  (and the associated adjustments to that angle discussed below) corresponds directly to the angle between the optical axis of the optical element  32  in the upper mounting member  24  and the optical axis of the optical element  132  in the lower mounting member  22 . Any minor variations between those two axes (which may be attributable to, for example, a slight misalignment of the supports  42  that secure the optical elements  32 ,  132  in place) can be accounted for as will be discussed below. 
         [0026]    As best shown in  FIGS. 1 and 2 , each mounting member  22 ,  24 , includes three, spaced-apart, elongated guide slots  62 ,  162  that, in plan view, are curved about the central axis  41 . The guide slots  62 ,  162  are counterbored into the opposite flat surfaces of the upper and lower mounting members  22 ,  24 . The counterbored portions  64 ,  164  of the guide slots thus provide recesses within which the opposite ends of shoulder bolts  66  are received. 
         [0027]    As shown in the figures, the upper mounting member  24  and lower mounting member  22  are stacked together, concentric with the central axis  41 . The guide slots  62 ,  162  are precisely, concentrically aligned so that the smooth, shoulder portion  68  of each shoulder bolt  66  fits vertically through the stacked mounting members (See  FIG. 2 ) to serve as guide pins so that one mounting member can be precisely rotated relative to the other about the central axis  41 . The head of each shoulder bolt  66  fits inside a counterbored portion  64 . The threaded end of the bolt, to which a flanged nut  70  is fastened, also fits inside of a counterbored portion  164  of the guide slots. The nuts  70  are sized so that they will not rotate with the bolt  66 . When the precise, desired optics angle  60  is established, the shoulder bolts  66  are tightened (as by an Allen wrench applied to the hex socket in the bolt head) to lock the upper mounting member  24  to the lower mounting member  22 , thereby fixing the optics angle. 
         [0028]    In a preferred embodiment, the upper and lower flat surfaces of the stacked upper mounting member  24  and lower mounting member  22  are provided with thin cover plates  72 , the uppermost plate being added after the bolts  66  are all tightened. The underside of the radially protruding portion of the enclosure  28  of the upper mounting member  24  has a cover plate  73 , and the upper side of the radially protruding portion of the enclosure  128  of the lower mounting member  22  has a cover plate  75  ( FIG. 3 ). 
         [0029]    It is contemplated that once the upper and lower mounting members  24 ,  22  are stacked but not rotatably fixed together by bolts  66 , any one of a variety of actuators may be employed for precisely rotating one mounting member relative to the other until the desired optics angle  60  is established. The actuator may be applied to any part of one mounting member to force rotation of that mounting member relative to the other. The actuator can be connected to a work surface adjacent to the rotated mounting member. Alternatively, the actuator can be connected to one mounting member (which member is secured to be stationary) and operable to apply force to the other mounting member. The actuator may be a permanent component of the overall mechanism, or be configured for removal once the precise optics angle is established, and the mounting members locked together. The actuator can be manually operated or mechanically driven under computer control. 
         [0030]    In a preferred embodiment, an actuator  74  ( FIG. 1 ) for providing precise rotation of one mounting member relative to the other comprises a fine adjustment screw assembly  76 . That assembly includes and elongated screw  78 , one end of which  80  is rounded and engages an exterior surface of the enclosure  28  that protrudes radially from the upper mounting member  24  ( FIG. 1 ). The screw  78  is threaded through a bushing  82  that is mounted within a base  84  of the assembly. The base  84  (hence the assembly) is connected to the cover plate  75  of the lower mounting member enclosure  128  via fasteners that are threaded into mounting holes  86  ( FIG. 4 ). Rotation of the ultra fine pitched screw is transferred via the contact of end  80  with the enclosure  28  into rotation of the upper mounting member  24  relative to the lower mounting member  22 , which member  22  may or may not be secured in place while this adjustment is made. 
         [0031]    As noted above, the angle between the two optics axes  58 ,  158  (namely; the optics angle  60 ) that is adjusted as just described corresponds directly to the angle between the optical axes of the optical elements  32 ,  132  in the respective upper mounting member  24  and lower mounting member  22 . Any minor variations between one optic axis  58 ,  158  and the corresponding optical axis of the associated optical elements  32 ,  132  (which variations may be attributable to, for example, a slight misalignment of the supports  42  securing the optical elements  32 ,  132  in place) can be addressed while the mechanism  20  is located in an optical setup with light passing though the optical elements of both PEMs and detected. This approach can be referred to as the PEMs optical angle calibration. 
         [0032]    One approach to this calibration is schematically depicted in  FIG. 2 , where the mechanism  20  is part of a setup that includes a light source  86 , adjustable polarizer  88 , an adjustable analyzer  90 , photodetector  92  and an associated lock-in amplifier  94 . The procedure discussed next is for precisely establishing the angle between the optical axes of the two PEMs to be 45°, although other angles may be selected. 
         [0033]    The polarizer  88  is set at 0° and the analyzer  90  is set at 45°. The upper mounting member  24  is rotated as described above until the optics axis  60  is at 45°. This angle can be measured in any of a number of ways, including the use of angular graduations on the exposed, adjacent surfaces of the mounting members. Next, the PEM  30  in the upper mounting member  24  is operated at a peak retardation of one-half wave while the PEM in the lower mounting member  22  remains off. The 2F signal on the detector  92  is monitored using the lock-in amplifier  94 . The mechanism  20  is then employed to precisely rotate the upper mounting member  24  relative to the lower mounting member  22  until the 2F signal reads “0,” at which point the upper mounting member  24  and lower mounting member  22  are locked together using the shoulder bolts  66  as described above. 
         [0034]    While the foregoing description was made in the context of a preferred embodiment, it is contemplated that modifications to the embodiment may be made without departure from the invention as claimed. For example, it is contemplated that the preferred embodiment of the actuator  74  may include the application of a spring or latch member extending between the adjustment screw and enclosure  28  so that the enclosure will move with both the extension and retraction of the adjustment screw  78 . Further, the actuator may be configured to act on any portion of the mounting members to impart the relative rotation, such portions can be considered protrusions but need not be the radially protruding enclosures discussed above.