Abstract:
Accordingly, a beam-splitting ball lens is provided. The beam-splitting ball lens has: a ball lens; and a beam-splitter filter disposed within the ball lens. The ball lens preferably has first and second portions wherein the beam-splitter filter is disposed at a junction between the first and second portions. The beam-splitting ball lens can further have a mid-plane optical element disposed at the junction such as, a wavelength selective filter, a polarization component, an amplitude modulation mask, a phase modulation mask, a hologram and/or a grating. Also provided is a method for fabricating the beam-splitting ball lens of the present invention. The method includes the steps of: providing the ball lens; and disposing the beam-splitter filter within the ball lens. Preferably the disposing step includes: dividing the ball lens into first and second portions; and disposing the beam-splitter filter at the junction between the first and second portions. Also provided is a mount for the beam-splitting ball lens of the present invention. The mount has a body, the body having screws to retain the beam-splitting ball lens therein. The body further having access holes for two inputs and two outputs corresponding to the two inputs. The access holes being aligned with the beam-splitter filter such that light inputted to the inputs are directed to corresponding outputs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to beam splitting devices and, more particularly, to a beam splitting ball lens, a method for its manufacture, and an apparatus for its packaging. 
     2. Prior Art 
     Recent advances of technologies have lead to successful deployments of optics to cost-sensitive local area networking environments. To meet rapidly increasing bandwidth requirements for future multimedia computing and communications, planning for 10 Gb/s Ethernet is already underway. As technologies become gradually matured, optics will be used for even shorter data links, from inter-computer distances to intra-computer distances. Innovative, compact, and cost-effective packaging methods of optical components are actively being researched. 
     FIG.  1 ( a ) illustrates a conventional beam splitter  106  and method to perform image relay and split. The beam-splitter  106  is shown surrounded by two pairs of imaging lenses  102 ,  104 . Out of the four possible ports  108 ,  110 ,  112 ,  114  are two-input  108 ,  114  and two-output  110 ,  112  ports where input and split/combined output images are located. Such a system has been used to facilitate optical branching functions for parallel array of FIG.  1 ( a ) is bulky, not to mention potential alignment and packaging complexities resulting therefrom. Thus, conventional beam-splitters  106  do not lend themselves to packaging methods of optical components that will be needed for future multimedia computing and communications. 
     SUMMARY OF THE INVENTION 
     Therefore it is an object of the present invention to provide a beam-splitting ball lens which is more compact than conventional beam-splitters. 
     It is a further object of the present invention to provide a beam-splitting ball lens which is more easily aligned with other optical components than conventional beam-splitters. 
     It is still yet a further object of the present invention to provide a beam-splitting ball lens which is packaged more easily than conventional beam-splitters. 
     Accordingly, a beam-splitting ball lens is provided. The beam-splitting ball lens comprises: a ball lens; and a beam-splitter filter disposed within the ball lens. The ball lens preferably comprises first and second portions wherein the beam-splitter filter is disposed at a junction between the first and second portions. The beam-splitting ball lens can further comprise a mid-plane optical element disposed at the junction, such as, a wavelength selective filter, a polarization component, an amplitude modulation mask, a phase modulation mask, a hologram and/or a grating. 
     Also provided is a method for fabricating the beam-splitting ball lens of the present invention. The method comprises the steps of: providing a ball lens; and disposing a beam-splitter filter within the ball lens. Preferably the disposing step comprises: dividing the ball lens into first and second portions; and disposing the beam-splitter filter at a junction between the first and second portions. 
     Also provided is a mount for the beam-splitting ball lens of the present invention. The mount comprises a body, the body having a means to retain the beam-splitting ball lens therein. The body further having access holes for two inputs and two outputs corresponding to the two inputs. The access holes being aligned with the beam-splitter filter such that light inputted to the inputs are directed to corresponding outputs. 
     Also provided is an add/drop multiplexer for downloading information of a predetermined wavelength from a plurality of wavelengths. The add/drop multiplexer comprises: a ball lens; a wavelength filter disposed within the ball lens for transmitting the predetermined wavelength, and reflecting the plurality of wavelengths except for the predetermined wavelength; an input port for transmitting the plurality of wavelengths to the ball lens; an output port for transmitting the plurality of wavelengths from the ball lens; a drop port for transmitting the transmitted predetermined wavelength from the ball lens; and an add port for adding the predetermined wavelength to the reflected wavelengths. Wherein the input, output, drop, and add ports are arranged about the ball lens such that the reflected wavelengths are transmitted to the output port, the transmitted wavelength is transmitted to the drop port, and the added predetermined wavelength is transmitted to the output port 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG.  1 ( a ) illustrates an image relay/split unit using four lenses and a conventional beam-splitter. 
     FIG.  1 ( b ) illustrates a beam-splitting ball lens of the present invention delivering the same functionality as the image relay/split unit of FIG.  1 ( a ). 
     FIG. 2 illustrates a ray tracing geometry of a conventional ball lens. 
     FIG.  3 ( a ) is a graph illustrating a normalized lateral image error Δ/f vs. normalized output distance X/f for the beam-splitting ball lens of the present invention wherein dotted lines illustrate different input angles a and the solid line illustrates a caustic function indicating an optimum Δ/f. 
     FIG.  3 ( b ) is a graph illustrating trade-off relations between excess power loss (in dB) and the obtainable resolution (in lp/nm) with given input NA. 
     FIG. 4 is a graph illustrating typical power splitting data of transmitted and reflective beams vs. incident beam angle for a λ=650 mm beam splitting ball lens. 
     FIGS.  5 ( a ) and  5 ( b ) illustrate two alternative versions of the beam-splitting ball lens of the present invention wherein one of the halves of the ball lens is shaped to accommodate a mid-plane optical element. 
     FIG.  6 ( a ) is an isometric view illustrating a mount for packaging fiber image guides and a beam-splitting ball lens of the present invention wherein the mount is configured as a 4-way image splitter/combiner. 
     FIG.  6 ( b ) is a sectional view illustrating the mount of FIG.  6 ( a ) taken about line  6   b — 6   b.    
     FIG.  6 ( c ) is a photograph of the mount of FIG.  6 ( a ). 
     FIG.  6 ( d ) illustrates a typical output image of group  3  elements of a target. 
     FIG. 7 illustrates a WDM add/drop multiplexer using a variation of the beam-splitting ball lens of the present invention. 
     FIG. 8 illustrates the beam-splitting ball lens of the present invention used with image fibers having curved ends to minimize off-axis aberrations. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention introduces a new integrated optical component, a beam-splitting ball lens, which integrates the functionality of five discrete optical components, namely four imaging lenses and a beam-splitter, into a single compact component. The integrated beam-splitting ball lens is a useful functional component for parallel channel optical circuitry to handle future short-distance optical interconnection needs. 
     Referring now to FIG.  1 ( b ) there is illustrated a beam-splitting ball system  200  having a beam-splitting ball lens  202  which replaces the functionality of the setup shown in FIG.  1 ( a ). The beam-splitting ball lens  202  can be formed by cutting a conventional ball lens into two portions, preferably halves  204 ,  206 , forming a beam-splitter filter  208  at a mid-plane junction  210  between the halves  204 ,  206 . However, the beam-splitting ball lens  202  may also be integrally formed with the beam-splitter filter  208 , such as by insert injection molding of the ball lens with the beam-splitter filter  208  inserted in the mold. The beam-splitter filter  208  may be a membrane or coating or other types of beam-splitters known in the art. The ball lens may be glass, crystal, semiconductor, or a polymer. The halves  204 ,  206  are preferably the same size and shape such that the mid-plane junction  210  in which the beam-splitter filter  208  is formed or disposed in the middle of the ball lens. However, the beam-splitter filter  208  could be offset from the middle of the ball lens without departing from the scope or spirit of the present invention. Preferably, the two halves  204 ,  206  with the beam-splitter filter  208  are cemented or adhered together back into a ball shape with a suitable optical epoxy. However, the components do not have to be adhered to one another, they can also be retained in a ball shape by other means, such as the packaging mount discussed below. 
     The division of the ball lens does not have to be by time-consuming cutting or polishing, but may be done by directly forming half balls using a polymer injection molding process or a casting process. The ball lens halves may also be formed by conventional grinding. Once the beam-splitting ball lens  202  is formed, it can be used in the FIG.  1 ( a ) geometry to deliver 2×2 image splitting/combining operations. In addition, the beam-splitting ball lens  202  has a larger angular usage range than the setup of FIG.  1 ( a ) which is limited by the placements of the four discrete lenses. Although the beam splitting ball lens  202  is shown with the beam-splitter filter  208  oriented 45° relative to the path of input light, its orientation can be at any angle. Alternatively, the beam-splitting ball lens  202  can also include a midplane optical element (see FIGS. 5 a  and  5   b ) in addition to the beam-splitter filter  208 . Examples of mid-plane optical elements include a wavelength filter, a polarization filter, or an amplitude/phase mask or grating, to name but a few. Thus, a new range of functionalities can be incorporated while the basic and essential imaging function is performed. 
     For imaging, the beam-splitting ball lens  202  has the same feature as a single ball lens of identical size and material except that it offers two output imaging planes. FIG. 2 shows a general ray tracing geometry of a ball lens  300  of radius R and refractive index of n′. For an on-axis ray originated at point A towards point D, the ray enters and exits the ball at B and C before crossing the optic axis at D. For a given input distance L, A lateral error Δ occurs when the output distance is selected at X. An optimum Δ to meet a particular resolution requirement can be computed for the ball lens  300 . First, from the triangle ABO and using the sine-law, we have 
     
       
         sin (α)=L/R sin (σ)  (1) 
       
     
     Using Snell&#39;s law at the boundary point B and Eq. (1), we have 
     
       
         sin (α′)=nL/n′R sin (α)  (2) 
       
     
     From trigonometry and basic properties of a ball, it can be shown that 
     
       
         σ′=2α−2α′−σ  (3) 
       
     
     Now, the application of the sine-law to triangle OCD yields 
     
       
         L′=R sine (α)/sine (α′)=L sine (θ)/sine (θ′)  (4) 
       
     
     Since the lateral error Δ is defined as 
     
       
         Δ=(X−L′) tan (σ′)  (5) 
       
     
     by substituting Eq. (4) into Eq. (5), we have 
     
       
         Δ X,θ =X [sin(θ′)−sin (θ)]/cos (θ′)  (6) 
       
     
     Using Eq. (1) through (3) and Eq. (6), a complicated expression linking θ and Δ can be derived. For unity image magnification which is the primary application of the present invention, θ is maximized for a given Δ while setting 
     
       
         X=L=2 f   (7) 
       
     
     where f is the focal length of the ball lens, using an ideal lens equation derived from paraxial approximations. Since the optimization process does not have an analytic solution, we plot, using dotted or dashed curves, the relations between Δ/f vs. X/f with θ as a changing parameter in FIG.  3 ( a ). The dotted straight line at the bottom corresponds to θ=0, or paraxial approximation. It can be seen that for a non-zero aperture angle σ, a non-zero lateral error Δ is inevitably generated. The envelope function as a solid curve at the top is the caustic function for the optimum relation between Δ/f and X/f. The conclusion is that the smaller the input angle to the ball lens  300 , the better the resolution. However, it is also true that the smaller the input angle or limiting aperture, the larger the power loss of the system. FIG.  3 ( b ) shows some relations between the forced power loss in unit of dB vs. the output resolution in unit of lp/mm with the launching angle as a parameter. 
     Based on BK7 ball lenses of different diameters, several beam-splitting ball lenses  202  were fabricated by polishing, coating and re-cementing. The beam-splitter filter  208  was a coating designed to have a 50/50 power splitting ratio for X=650 nm at a 45° incident angle. As noted before, for large volume productions, molding of the ball lens halves is preferred because the fabrication procedure of the beam splitting ball lens  202  can be simplified. The beam-splitting ball lenses  202  fabricated had diameters ranging from 2.5 mm to 6 mm. These beam-splitting ball lenses  202  were tested using a specially designed test system which measures both the power splitting ratios at different angles and image resolution using an U.S. Air Force (USAF) resolution target, a CCD camera for image acquisition, and a PC for data analysis. For applications to optical interconnections, the target field was confined to within a circular area with a radius of 2 mm which is a standard diameter for a 3,500 pixel polymer fiber-image-guide. For a unity magnification case, all tested beam-splitting ball lenses  202  can resolve&gt;30 lp/mm with no apparent geometric distortion when their input apertures were set to be R/2. It was noticed that this resolution is much greater than that the PFIG can supply (around 20/lp). For power measurement, it was noticed that most beam-splitting ball lenses  202  can deliver a splitting ratio with a variation of about ±6% from the designed 50/50 splitting ratio. FIG. 4 shows measured power splitting ratio of the two beams vs. incident angle. There was a 1.5 dB forced or excess power loss of the system. 
     Referring now to FIGS.  5 ( a ) and  5 ( b ) there are shown alternative versions of the beam-splitting lens of the present invention wherein one of the halves  204  is substantially half the size of the ball lens and the other half  206   a ,  206   b  is smaller by the size of an added mid-plane optical element  208   a ,  208   b . FIG.  5 ( a ) illustrates the beam-splitting ball lens  202  of the present invention having both a beam-splitter filter coating  208  and a filter substrate  208   a . The filter substrate  208   a  is rectangular in shape, thus, to maintain the ball shape of the ball lens, one of the halves  206   a  is smaller than the other half  204  by the size of the rectangular filter substrate  208   a . FIG.  5 ( b ) illustrates a similar embodiment, however, the filter substrate  208   b  is wedge shaped and the other half  206   b  is shaped to accommodate the wedge shaped filter substrate  208   b . Of course, other shaped mid-plane optical elements are possible. 
     Referring now to FIGS.  6 ( a ) and  6 ( b ), to facilitate packaging of the beam-splitting ball lens  202 , a mount  600  is provided which helps to couple light between the four input/output optical fiber image guides  602 ,  604 ,  606 ,  608  and a 4 mm diameter beam-splitting ball lens  202 . The mount has a body  610 , preferably fabricated from aluminum, having a means to retain the beam-splitting ball lens  202  therein. The body further having access holes  602   a ,  604   a ,  606   a ,  608   a  for the optical fiber image guides  602 ,  604 ,  606 ,  608 . The access holes  602   a ,  604   a ,  606   a ,  608   a  are aligned with the beam-splitter filter  208  such that light inputted to the inputs  602 ,  608  are directed to corresponding outputs  604 ,  606 . 
     The body  610  preferably comprises a unitary block having a threaded through hole  611  for housing the beam-splitting ball lens  202  within the mount  600 . The beam-splitting ball lens  202  is retained in the threaded through hole  611  by means of a screw plug  612  threadingly engaged on both sides of the threaded through hole  611 . Each screw plug  612  has a threaded portion  614  which mates with a corresponding threaded portion  616  of the threaded through hole  611 . Each screw plug  612  preferably has a concavity  618  corresponding to the outer surface of the beam-splitting ball lens  202 . The vertical positioning of the beam-splitting ball lens  202  along arrow A is accomplished by advancing one of the screw plugs  612  while withdrawing the other an equal amount. 
     The mount  600  further has means to fix and adjust the optical fiber image guides  602 ,  604 ,  606 ,  608  in the access holes  602   a ,  604   a ,  606   a ,  608   a  of the body  610 . This means preferably comprises a threaded bushing  620  threadingly fixed in each of the access holes  602   a ,  604   a ,  606   a ,  608   a  and having a bore  620  for passage of a corresponding optical fiber image guide  602 ,  604 ,  606 ,  608 . A cap  622  having an internal threaded portion  622   a  which threadingly mates with a portion of the threaded bushing protruding from the body  610 . The cap  622  further has a bore  622   b  axially aligned with the bore  620   a  of the threaded bushing  620 . Each optical fiber image guide  602 ,  604 ,  606 ,  608  is passed through the bores  622 b,  620   a  of the cap  622  and threaded bushing  620 , respectively, and is retained therein by an o-ring  624  which is squeezed around the outer periphery of the optical fiber image guide when the cap  622  is advanced over the threaded bushing  620 . 
     FIG.  6 ( c ) shows a photograph of a packaged four-way optical image splitter/combiner (mount)  600  which uses a combination of both guided-wave components, i.e. optical fiber image guides, and a free-space component, i.e. a beam-splitting ball lens  202 . The assembly has a 1.5 dB excess and a power splitting ratio of T/R=0.3 dB. 
     FIG.  6 ( d ) shows a typical image (group 3) of the USAF target at an output port of the packaged four-way image combiner/splitter. Thus, only about 11-12 lp/mm can be resolved, primarily due to a resolution limit of the optical fiber image guides (about 20 lp/mm) and an effect of cascading two optical fiber image guides and a lens. Nevertheless, the resolution is sufficient to resolve a  2 D laser pattern with a laser pitched at 125 μm. 
     Referring now to FIG. 7, there is illustrated a WDM add/drop multiplexer (ADM), referred to generally by reference numeral  700  utilizing a variation of the beam-splitting ball lens  202  of the present invention, designated by reference numeral  701 . The ADM ball lens  701  has a wavelength filter  702  disposed at the junction  210  between its two halves  204 ,  206  instead of the beam-splitting filter  208 . The WDM add/drop multiplexer  700  is a 4-port device with a main input port  704 , a main output port  706 , a drop channel output port  708 , and an add channel input port  710 . The ports are typically optical fibers. ADM&#39;s are useful to download information associated with a particular wavelength channel. The main input port  704  carries all WDM channels of λ 1 , λ 2 , . . . . λ n , where data for λ i  is to be downloaded. The main output port  706  transmits all channels to somewhere else. The wavelength filter  702  at the junction  210  of the ball lens halves  204 ,  206  has the functionality that only passes through a designated wavelength band, for instance λ i . The wavelength filter  702  reflects all other wavelength channels just like a mirror. Thus, when all wavelengths are present at the wavelength filter  702  from the input channel  704 , only the λ i  wavelength passes through the ball lens and is focused into the drop channel  708 . All remaining channels are reflected and are focused into the output channel  706 . The missing channel is replaced by a new beam of wavelength λ i  sent by the add channel  710 . Since the wavelength filter  702  is designed to pass through wavelength λ i , this wavelength rejoins the remaining wavelengths at the output port  706  which transmits all wavelengths λ 1 , λ 2 , . . . .λ n  to a trunk line. Thus, with the ADM ball lens  701  of the present invention, a single integrated optical component serves as both a filter and a fiber-filter interface device. 
     Referring now to FIG. 8 there is illustrated the beam-splitting ball lens  202  of the present invention packaged with four fiber image guides  802 ,  804 ,  804 ,  808 . To minimize off-axis aberrations due to the curved surface of the beam-splitting ball lens  202 , the image fibers  802 ,  804 ,  804 ,  808  have curved ends  802   a ,  804   a ,  806   a ,  808   a . Preferably, the curved ends  802   a ,  804   a ,  806   a ,  808   a  each have a radius corresponding to the radius (R) from the center of the beam-splitting ball lens  202 . 
     To summarize, the present invention provides a new integrated optical component, a beam-splitting ball lens which can serve the need for imaging and splitting 2D data patterns for various data communication and sensing applications. Also provided is a compact and flexible packaging system (mount) to allow the use of a beam-splitting ball lens with optical fiber image guides which are cost-effective flexible 2D optical wave-guiding channels. Thus, the beam-splitting ball lens of the present invention and its packaging mount will help ease design concerns of future 2D array based large-bandwidth board- and back-plane-level optical interconnections. 
     While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.