Abstract:
The present invention is a mount for optical instruments, and particularly, for a Berek compensator having a birefringent crystal, which can be both tiled and rotated by structure of adjustment rings which are concentric to the optical path of the device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical instruments, and particularly to a Berek compensator having a birefringent crystal, which can be both tilted and rotated by means of adjustment rings which are concentric to the optical path of the device. 
     2. Description of the Prior Art 
     In the field of experimental optics there is a frequent need to introduce a variable phase difference in an optical beam. Various devices exist for this purpose, perhaps the best known of which is the Soleil-Babinet compensator. The Soleil-Babinet compensator utilizes a pair of rectangular-shaped crystal elements having their optical axis perpendicular to each other so that the ordinary ray in one is the extraordinary wave in the other. One of the rectangular elements is further divided into two wedge-shaped sections. One of the wedge sections is movable with respect to the other so that the total length of the optical path through the pair of wedges is variable with respect to the length of the optical path in the undivided rectangular element. Thus, the phase shift of the incident ray is proportional to the relative lengths of the optical paths. Such devices are described in Principles of Optics, 6th Edition, Max Born and Emil Wolf at pages 693-694, and are commercially available for example from Melles Griot, Inc., Irvine, Calif. Another type of variable retarder is described in U.S. Pat. No. 3,924,930, and is commercially available from Cleveland Crystals, Inc., Cleveland, Ohio. 
     While the Soleil-Babinet compensator can provide the requisite optical function of a variable phase shift, it requires the fabrication of two wedge-shaped crystals. It also requires a mechanical mechanism to move one wedge relative to the other while maintaining all elements in precise optical alignment. The device also suffers from the requirement for motion transverse to the optical path, thereby adding to the size of the device. Size is not a trivial aspect since it would be desirable to have a compensator having minimal diameter which would permit mounting in a universal type of optical mount of the type having adjustments facilitating the initial set-up and alignment. Similarly, other commercially available retarders require complex optical and mechanical element fabrication, and are large compared to the usable optical aperture. 
     Another variable phase shift device, known as the Berek compensator, is described in Principles of Optics at page 694. In this device, the active element is a single crystal of birefringent material positioned so that the optical axis is perpendicular to the parallel faces of the crystal. The variable phase shift is created by tilting the crystal relative to the incident beam. While the Berek compensator is well understood and widely known, it has not found wide application in the laboratory. 
     The lack of practical application of the Berek compensator may be due, at least in part, to the requirement for tilting the crystal and the attendant mechanical complexity necessary to provide the requisite precision control. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a Berek compensator, or similar device, having a tilt and rotate adjustment. 
     It is another object of this invention to provide an optical mount of minimal diameter adapted to provide precision tilt and rotational movement to an optical element. 
     Still another object of this invention is to provide an improved Berek optical compensator of minimal diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a side view of the mount of this invention; 
     FIG. 2 is an end view of the mount of this invention taken from the right side of FIG. 1; 
     FIG. 3 is a sectional view of the mount of this invention taken along the line III--III of FIG. 2; 
     FIG. 4 is a sectional view of the optical mount of this invention taken along the line IV--IV of FIG. 2; 
     FIG. 5 is an exploded view of the optical mount of this invention; 
     FIG. 6 is a side view of the base of the mount of this invention; 
     FIG. 7 is an end view of the base of the mount of this invention; 
     FIG. 8 is a sectional view of the base shown in FIGS. 6 and 7 taken along the line VIII--VIII of FIG. 7; 
     FIG. 9 is a side view of the axially rotatable cylinder showing the bearing retention plate; 
     FIG. 9A is a top view of the bearing retention plate illustrated in FIG. 9; 
     FIG. 10 is an end view of the axially rotatable cylinder; 
     FIG. 11 is a sectional view of the axially rotatable cylinder taken along the line XI--XI of FIG. 10; 
     FIG. 12 is a side view of the optical element holder; 
     FIG. 13 is an end view of the optical element holder; 
     FIG. 14 is a sectional view of the optical element holder taken along the line XIV--XIV of FIG. 13; 
     FIG. 15 is a side view of one component of the second axially rotatable cylinder illustrating the axial cam surface; 
     FIG. 16 is a sectional view of the cylinder component shown on FIG. 15; 
     FIG. 17 is an end view of the tilt adjustment ring component of the second axially rotatable cylinder; 
     FIG. 18 is a side view of the tilt adjustment ring component of the second axially rotatable cylinder; and, 
     FIG. 19 is another side view of the tilt adjustment ring component of the second axially rotatable cylinder showing the calibration indicia. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, optical mount of this invention includes a stationary cylindrical base ring 1 having a first, outer cylindrical surface 1a which is slightly greater in diameter than the rest of the mount to facilitate clamping in a conventional clamp. Base ring I is also fitted with a tapped hole 30 to permit it to be affixed to a conventional threaded post. Base 1 has a rotation scale barrel portion 2 with a vernier scale 3 which co-acts with the degree scale 4 on a first, axially rotatable, rotation cylinder 5. A knurled rim 6 affixed to rotation cylinder 5 assists in positioning the cylinder relative to base ring 1. 
     Rotation cylinder 5 has a tilt vernier barrel portion 10 containing the tilt vernier scale 11. The second, axially rotatable tilt cylinder 20 has a tilt scale 21 and a knurled rim 22 affixed thereto for ease in adjusting the tilt mechanism. 
     FIG. 2 is a right end view of the optical mount shown in FIG. 1. The knurled rim 6 is secured to the axially rotatable rotation cylinder 5 by means of screws 7 which pass into tapped holes, not shown, in rotation cylinder 5. 
     FIG. 3 is a sectional view of the optical mount taken along the line III--III of FIG. 2. Parts shown in FIGS. 1 and 2 are identified with the same reference characters. Base ring 1 has a tapped hole 30 which may be used with conventional mounting posts to secure the mount to a bench or optical table. Alternatively, the outer cylindrical surface 32 of base ring 1 can be clamped in devices adapted to hold round optical elements such as the 2-inch multi-function mount Model No. 9850 offered for sale by New Focus, Inc., 340 Pioneer Way, Mountain View, Calif. 
     The inner cylindrical surface 33 of base ring 1 abuts the outer surface 34 of rotation cylinder 5. The end surfaces 35 and 36 of base ring 1 abut the shoulder portions 38 and 39 of the groove in rotation cylinder 5 formed by the surfaces 34 38 and 39. Rotation cylinder 5 is thus held in axial alignment with base ring 1, and is restrained form longitudinal movement along the axis of the system while freedom for rotational movement is retained. 
     Tilt cylinder 20 includes a cam portion 40 described in more detail with reference to FIG. 15. Cam portion 40 has a &#34;V&#34; groove 41 extending about the periphery. This groove is adapted to receive pointed set screws passing through threaded holes in the outer portion 23 of tilt cylinder 20. The pointed set screws provide accurate positioning and locking of the cam portion 40 relative to the outer portion 23 by defining a cylindrical ridge which, in conjunction with the rim 50 on the inner cylindrical surface of rotation cylinder 5, rotatably mounts tilt cylinder 20 within rotation cylinder 5. 
     The cam portion 40 includes an axial cam surface 52 on the interior end of the cylinder. Cam surface 52 engages the cam follower 55 of optical element holder 60, described in more detail with reference to FIGS. 12, 13 and 14. Optical element holder 60 is positioned within the rotation cylinder 5 by means of first and second bearing elements 61 and 62. Bearing elements 61 and 62 include conical seats 61a and 62a, which are adapted to receive the balls 61b and 62b. A corresponding conical seat 62c at the lower end of rotation cylinder 5 receives ball 62b. The upper ball 61b is seated in hole 61c within the bearing spring plate 64, described in more detail with reference to FIGS. 9, 9A, 10 and 11. 
     It can therefore be seen that rotational movement of tilt cylinder 20 relative to rotation cylinder 5 causes the axial cam surface 52 to move cam follower 55 and rotate optical element holder 60 about the axis defined by the bearing elements 61 and 62. A spring 72, described with reference to FIG. 4, holds cam follower 55 in engagement with ball 57 and cam surface 52. 
     FIG. 4, a sectional view taken at 90 degrees to that of FIG. 3, shows optical element holder 60 displaced from the position perpendicular to the optical axis of the system in the broken line view. It will be appreciated that element holder 60 may also be rotated by inserting an object into the interior of rotation cylinder 5 and pressing against the side of holder 60 in a fashion to rotate holder 60 away from axial cam surface 52 against the restraining action of spring 72. FIG. 4 also shows the screw 7 which holds knurled rim 6 to rotation cylinder 5. 
     FIG. 5 is an exploded view of the major component parts of the optical mount of this invention. Base ring 1 accommodates the rotation cylinder 5, which is inserted through the base ring 1. The rotation cylinder 5 has a reduced diameter portion 14 which, after insertion through the base ring 1, provides a support for vernier barrel portion 10, which is fastened to base ring 1 by means of set screws. Cam portion 40 passes through the interior of rotation cylinder 5, and is retained in place by means of the outer portion 10, which is fastened by means of set screws which bear against the &#34;V&#34; groove 41. Optical element holder 60 is positioned within the rotation cylinder 5, and is supported for rotational movement by bearings previously described. The bearings are held tightly against the element holder 60 by bearing spring plate 64, which has a pair of mounting holes 66a and 67a which accommodate screws 66 and 67. Bearing spring plate is slightly flexed when fastened to the rotation ring 5 to load the bearing elements and prevent other than rotational movement. The cam follower 55 on element holder 60 contains a bearing ball 57 which rides on cam surface 52, causing the element holder 60 to tilt about the axis defined by the bearings. Birefringent crystal 65 is mounted on the element carrier 60. The knurled ring 6 is fastened to the right hand end of the rotation cylinder 5. 
     FIG. 6 shows stationary base ring 1 in detail. The outer cylindrical surface 1a has a nominal diameter of 2 inches to make the device compatible with other optical devices and permit the use of the same clamps. The inner cylindrical surface 1b, shown in FIG. 7, provides a bearing surface for the rotation cylinder 5. 
     FIG. 8 is a sectional view along the line VIII--VIII of FIG. 7 showing the tapped mounting holes used with conventional optical mounting posts. 
     FIG. 9 is side view of a portion of the first axial rotation cylinder 5 showing the upper bearing spring plate 64, having a hole 61c with the upper ball 61b positioned therein. Spring plate 64 is shown in detail in FIG. 9a and is secured to rotation cylinder 5 by means of screws 66 and 67. The knurled rim 6 is not shown in this view. 
     The end view of rotation cylinder 5 in FIG. 10 illustrates the tapped holes which receive screws 7 to rotation cylinder 5. 
     The sectional view of FIG. 11, taken along the line XI--XI of FIG. 10 shows the aperture 80 and the slot 81 which accommodate the cam follower 55 of element holder 60 and the bearing spring plate 64. Lower ball 62b is also shown. 
     Optical element holder 60 is shown in top and end views of FIGS. 12 and 13 and the sectional view of FIG. 14. With reference to FIG. 12, element holder 60 has a conical seat 61a adapted to receive the bearing ball, not shown. A cam follower 55 is positioned on the periphery of optical element holder 60 at a point midway between the conical seats 61a and 62a. The end of cam follower 55 has a conical seat 56 to receive a bearing ball 57 which rides on cam surface 52. 
     The sectional view of FIG. 14, taken along the line XIV--XIV of FIG. 13 shows the interior groove 63 which is used to retain the birefringent crystal 65 preferably of Magnesium Fluoride approximately 2 mm in thickness and having parallel, optically flat surfaces. 
     Magnesium Fluoride is a preferred choice for the optical element 65. Alternatively, a Brewster plate may be used as the optical element. 
     The cam portion of the second, tilt, rotatable cylinder is shown in FIGS. 15 and 16. The outer diameter of cylindrical surface 85 matches the interior diameter 90 of outer portion 20. &#34;V&#34; groove 41 is positioned to accommodate the conical point of set screws passing through outer portion 22 and hold the portions 22 in engagement with cam portion 40. Axial cam surface 52 is adapted to displace the cam follower 55 in the axial direction when it is rotated. The sectional view of FIG. 16 is taken along the line XVI--XVI of FIG. 15. The cam surface is a one turn Buttress thread of 8 pitch with a face normal to its axis. 
     FIGS. 17, 18 and 19 show the outer portion 23 of tilt cylinder 20. The interior cylindrical surface 90 fits over the outer cylindrical surface 85 of the cam portion 40. Tapped holes 90a, 90b and 90c accommodate set screws with conical points which engage the &#34;V&#34; groove 41 in cam portion 40. FIG. 18 shows the knurled surface 22 which facilitates adjustment. FIG. 19 shows the indicia which provide an indication of the tilt angle. 
     MODE OF OPERATION 
     In operation, the optical mount of this invention will be set to the zero tilt position and inserted within a conventional optical mount. Once positioned, the radiation source is energized and the mount is moved to a more accurate position by observing the reflected beam at the source. The rotation cylinder 5 is then rotated by grasping knurled rim 6 to provide the correct axial orientation of the crystal 65. The desired phase shift may then be obtained by rotation of the tilt cylinder by grasping the knurled portion of rim 22. 
     Various modifications can be made to the present invention without departing from the apparent scope hereof.