Abstract:
A rotatable optical beamsplitter comprises an optically-transparent material and a partially-reflective layer. The optically-transparent material has a unitary spherical exterior surface. The partially-reflective layer is located at least partially within the optically transparent material. The spherically shaped optical beamsplitter demonstrates reduced optical distortion.

Description:
GOVERNMENT INTEREST 
   The invention described herein may be manufactured, used, and licensed by or for the United States Government without payment therefor. 

   CROSS-REFERENCE TO RELATED APPLICATION 
   This application is related to inventor&#39;s application titled Cylindrically Shaped Optical Beamsplitter filed on even date. 
   BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The invention relates to optical systems and elements. More specifically, the invention relates to an optical beamsplitter that can be used to propagate optical signals. 
   2. Description of the Related Art 
   A beamsplitter is an optical device that receives an incident light beam and divides it into two components. The first component is propagated from the beamsplitter as a transmitted beam. The second component is propagated as a reflected beam, angularly displaced with respect to the transmitted beam. 
   A conventional beamsplitter comprises a cube of optically-transparent material that encases a thin layer of semi-reflecting material oriented diagonally across its interior. Approximately one-half of the light entering one face of the beamsplitter is reflected by the semi-reflecting material through an adjacent face as a reflected light beam. The remaining portion of the light from the incident light is transmitted by the layer of semi-reflecting material through the opposite face as a transmitted light beam. 
   Beamsplitters are used in optical devices such as in scanners, interferometers and back-scattering detectors for measuring back-scatter from a particle system. One problem associated with conventional cubic beamsplitters is distortion caused by light striking beamsplitter faces at oblique, i.e. non-perpendicular, angles. The transmission through the beamsplitter is refracted according to Fresnel&#39;s Equations. The resulting light beam is parallel to the incident beam but displaced from it. The amount of displacement depends on the angle of incidence and the index of refraction of the optical materials used in the beamsplitter. This distortion can be compensated for, to some extent, by repositioning system components. 
   U.S. Pat. No. 6,414,797 to Gorden Videen discloses a Beamsplitter Prism with Spherical Faces for Transmitting or Reflecting Spherical Waves Without Magnification. 
   SUMMARY OF THE INVENTION 
   A spherically shaped optical beamsplitter, system and method are provided. An optical beamsplitter comprises an optically-transparent material and a partially-reflective layer. The optically-transparent material has a unitary spherically shaped exterior surface. The exterior surface is the boundary defining an interior. The partially-reflective layer is positioned in the interior. Means is provided for rotating the spherically shaped optical beamsplitter. 
   An embodiment of an optical system comprises an optical beamsplitter incorporating an optically-transparent material and a partially-reflective layer. The optically-transparent material has a unitary spherically shaped exterior surface. The exterior surface defines an interior. The partially-reflective layer is positioned at least partially within the interior. The beamsplitter receives incident light and divides the incident light into a first component and a second component. The first component is transmitted from the beamsplitter. The second component is reflected by the partially-reflective layer at an angle relative to the first component and transmitted from the optical beamsplitter. 
   An embodiment of a method for using an optical beamsplitter comprises the steps of:
         i.) providing an optical beamsplitter comprising an optically-transparent material and a partially-reflective layer, the optically-transparent material having a substantially spherically shaped exterior surface, the exterior surface defining an interior, the partially-reflective layer located at least partially within the interior;   ii.) rotating the optical beamsplitter to a first orientation relative to a sample; and   iii.) acquiring first information corresponding to the first orientation.       

   Other devices, systems, methods, features and/or advantages will be apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional devices, systems, methods, features and/or advantages be included within this description. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Many aspects of the disclosure can be better understood with reference to the following drawings. Components in the drawings are not necessarily to scale. Like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is a schematic diagram of an embodiment of a spherically shaped optical beamsplitter. 
       FIG. 2  is a schematic diagram of the embodiment of the beamsplitter of  FIG. 1 , depicting propagation of an incident ray and a back-scattered ray. 
       FIG. 3  is an embodiment of an optical system incorporating an embodiment of a spherically shaped optical beamsplitter. 
       FIG. 4  is a flowchart depicting functionality of the embodiment of the optical system of  FIG. 3 . 
       FIG. 5  is a schematic diagram depicting another embodiment of an optical system that incorporates an embodiment of a spherically shaped optical beamsplitter. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As will be described in detail here, spherically shaped optical beamsplitters, systems and methods are provided. The exterior surface of the beamsplitter that directs light is unitary and essentially spherically shaped. As a result; various advantages may be achieved. For example, light incident upon such a beamsplitter can be received at an angle that is normal to the exterior surface of the beamsplitter. Light that is propagated from the beamsplitter can be transmitted at an angle that is normal with respect to the exterior surface. Therefore, at least some of the light received and transmitted can be propagated without distortion. 
   Referring now to the drawing,  FIG. 1  is a schematic diagram showing a spherically shaped optical beamsplitter. Beamsplitter  100  is made of a material that is optically transparent with respect to at least one wavelength of light. For example, glass, quartz, plastic and epoxy are optically transparent to visible light. Quartz is transparent to ultraviolet light. Silicon and germanium are transparent to infrared light. The optically-transparent material may be optically transparent with respect to more than one wavelength of light. 
   The exterior surface  102  of the material is defined by a substantially constant radius r extending from center C. One or more portions of the non-transmitting exterior surface can be other than spherically shaped. For instance, opposing poles of the exterior surface could be flattened to facilitate engagement with a mechanism for rotating the beamsplitter. 
   Beamsplitter  100  also includes a partially-reflective layer  104  which is oriented generally within a plane that intersects center C. Two orthogonal axes, longitudinal axis L and transverse axis T extend outwardly from center C. In  FIG. 1 , the L-T plane in which the layer  104  is oriented is parallel to the X-Y plane. 
   The partially-reflective layer is formed of one or more of various materials known in the art. In  FIG. 1 , the partially-reflective layer  104  is continuous and extends across the optically-transparent material. The partially-reflective layer  104  extends across the optically-transparent material so that its outer periphery  106  is adjacent to the exterior surface  102 . Another embodiment, the partially-reflective layer  104  is discontinuous. The partially-reflective layer  104  may not extend to or in the alternative may extend beyond the exterior surface of the beamsplitter. 
     FIG. 2  is a schematic diagram of the beamsplitter of  FIG. 1 , showing propagation of an incident ray and a back-scattered ray. As shown in  FIG. 2 , incident ray  202  is received by the beamsplitter  100  and is incident upon partially-reflective layer  104 . Face  203  of the partially-reflective layer  104  reflects a first component of the incident ray. This first component is propagated from the beamsplitter as a first reflected ray  204 . The remaining portion of the incident ray is propagated through the layer  104  and is transmitted from the beamsplitter as a transmitted ray  206 . 
   In  FIG. 2 , transmitted ray  206  is incident upon an object not shown) that provides a back-scattered ray  208  to the beamsplitter. Both the transmitted ray  206  and back-scattered ray  208  are shown in  FIG. 2  parallel to and laterally offset from each other, for ease of illustration only. In reality, these two rays are co-extensive. 
   Back-scattered ray  208  is received by the beamsplitter and is incident upon an opposing face  209  of the partially-reflective layer  104 . A component of the back-scattered ray  208  is then reflected as a second reflected ray  210  that is propagated from the beamsplitter. Although not shown in  FIG. 2 , the partially-reflective layer  104  may transmit another component of the back-scattered ray  208 . The ray shown in  FIG. 2  is normal to the corresponding portion of the exterior surface of the beamsplitter to which it is incident. 
   Reference is made to  FIG. 3 , which shows an optical system including a spherically shaped beamsplitter. Optical system  300  includes a beamsplitter  302  incorporating a partially-reflective layer  304 . Light source  306  provides light to optional input optics  308 , which focuses light passed to the beamsplitter. The input optics can include a lens. 
   Light incident upon the beamsplitter, shown as an incident ray  310 , is passed through the material of the beamsplitter to the partially-reflective layer  304 . A component of the incident ray is transmitted through the beamsplitter as transmitted ray  312 . The component of the incident ray that is reflected by the partially-reflective layer is not shown. Transmitted ray  312  is directed toward a sample  320  that scatters incident light. A component of the scattered light comes back to the beamsplitter as back-scattered ray  322 . Back-scattered ray  322  is then reflected by the partially-reflective layer  304 . Back-scattered reflected ray  324  passes to optional output optics  328 , which can include one or more lenses for focusing reflected back-scattered ray  324 . The back-scattered reflected ray  324  provided to detector  330  that analyzes the light. For instance, the detector can be an electronic detector such as a photomultiplier tube (PMT), a photodiode (PDA) or a charge-coupled device (CCD), or a non-electronic detector such as photosensitive film or a human eye. The component of the back-scattered ray  322  that is not reflected by partially-reflective layer  304  is not shown in  FIG. 3 . 
   Optical system  300  also includes a controller  340  and a rotation mechanism  342  used to rotate beamsplitter  302  with respect to at least one axis of rotation. The rotation mechanism has a surface that contacts the exterior surface of the beamsplitter so that movement of the rotation mechanism rotates the beamsplitter. As shown in  FIG. 3 , controller  340  provides an input signal to rotation mechanism  342  so that it rotates as indicated by arrow A. In response, beamsplitter  302  rotates as indicated by arrow B. Rotation mechanism can be configured to rotate the beamsplitter with respect to more than one axis. In another embodiment, the beamsplitter can be rotated with the light source. 
   Another embodiment of the optical system is similar to the arrangement shown in  FIG. 3 . The position of light source  306  and input optics is exchanged with the position of detector  330  and output optics  328 . As beamsplitter  302  is rotated, the incident light from the source scans object  320 . The backscattered light from object  312  is transmitted through the beamsplitter to the output optics and detector. 
   The optical system of  FIG. 3  is described with reference to the flowchart of  FIG. 4 . The method, or functionality, begins at block  402 , where a spherically shaped optical beamsplitter is provided. In block  404 , the beamsplitter is rotated with respect to at least one axis of rotation to a first orientation. Then in block  406 , the beamsplitter is used to acquire information corresponding to the first orientation. When the beamsplitter is used in conjunction with an array of photodiodes, the information can correspond to the intensity of the reflected back-scattered light incident upon the photodiodes during an exposure period. 
   Another embodiment of an optical system including a spherically shaped optical beamsplitter is shown in  FIG. 5 . Optical system  500  includes a beamsplitter  502  incorporating a partially-reflective layer  504 . An external light source not shown) illuminates sample  506 . A portion of the light provided by the external source is incident upon the sample, with some of the incident light back scattered to the beamsplitter. Scattered ray  508  is propagated from the sample  506  to the beamsplitter  502 . Scattered ray  508  is then reflected by the partially-reflective layer  504 , with a scattered reflected ray  510  provided to a detector  512  via optional output optics  514 . 
   It should be emphasized that many variations and modifications may be made to the invention. All such modifications and variations are intended to be included within the scope of the following claims.