Abstract:
An optical coupler for coupling light along an axis between a light port on one side of the coupler and a plurality of light ports on another side of the coupler. The coupler comprises a one-side stage with a light port and a many-side stage with a plurality of arms situated about the axis, each having a light port. A midput region separates the one-side and many-side stages and is situated along the axis where the plurality of arms at least initially starts to split from each other in a direction towards the many-side light ports along the axis. Cross sections of each of the respective initial portions of the arms along the direction are arranged about the same distance from the axis. The cross sectional areas of the arms along the axis are larger at a break point region at which the arms fully separate from each other along the axis than at the many-side light ports. At least a pair of the arms each start to split along the direction in a substantially symmetrical manner about the axis.

Description:
FIELD OF THE INVENTION 
     The present invention relates to optical couplers for coupling light between one and a plurality of light ports, and more particularly to such couplers in which the etendue, or brightness, of the light is preserved to a high extent. 
     BACKGROUND OF THE INVENTION 
     A common problem in designing fiber-optic lighting systems is to minimize the size (i.e., diameter) and number of light guides required to deliver sufficient light. The smaller the fibers required, the lower the cost and the easier to install. Smaller fibers are more flexible and more easily concealed. If multiple outputs are required, the least expensive and neatest method is to run one large fiber from a light source to an intermediate position and then split the light into a number of smaller fibers. If the large fiber is transporting light at angles up to its acceptance angle, then the coupler should make the split without substantially increasing the angular distribution of light, to reduce light loss. This is possible in principle, if the total areas at input and output are the same, which is often not practical due to design issues, owing to the law of conservation of etendue (or brightness or sparkle of light). The law only states that it is possible, but does not require it. 
     A primary goal in designing a coupler is to avoid light loss in the coupler. In an ideal case, the aggregate area of the outputs can be the same as the area of the input where the numerical apertures of the input and outputs are the same. Since this is not always practical due to fabrication issues, it is often necessary to allow the aggregate area of the outputs to increase to approximately a factor of two over the input area. 
     A further goal is to avoid light loss in coupling light to, for instance, output light guides. For this it is necessary that the angular distribution of light at the output not exceed the acceptance angle of the output light guides. Where the numerical aperture of the input and output fibers are the same, this is accomplished by not substantially increasing the angular distribution of light during splitting. 
     A yet further goal of some embodiments is to split the light between the output ports in a controlled manner. The most typical example is splitting the light evenly between the different ports. It is most often desired, if not a requirement of a fiber system, that the fraction of light and the color of the light be the same for all the ports so that post installation testing can be minimized. 
     Another goal is to provide output ports which are spatially separate from each other. In the case of typical large core plastic optical fibers, this eliminates the need to strip the cladding and jacket off of the fiber. In the case where the coupler is used as a combiner, this allows space for mechanical packaging of the input sources. 
     A yet further goal is to minimize the area of the output, since smaller-sized fibers are more economical, etc. It is also desirable to preserve the etendue of the coupled light to a high degree. Preserving the etendue is especially important since it enables the size of the optics that are attached to the fibers, such as automotive headlamps, to be minimized. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment of the invention provides an optical coupler for coupling light along an axis between a light port on one side of the coupler and a plurality of light ports on another side of the coupler. The coupler comprises a one-side stage with a light port and a many-side stage with a plurality of arms situated about the axis, each having a light port. A midput region separates the one-side and many-side stages and is situated along the axis where the plurality of arms at least initially starts to split from each other in a direction towards the many-side light ports along the axis. Cross sections of each of the respective initial portions of the arms along the direction are arranged about the same distance from the axis. The cross sectional areas of the arms along the axis are larger (and preferably substantially larger) at a break point region at which the arms fully separate from each other along the axis than at the many-side light ports. At least a pair of the arms each start to split along the direction in a substantially symmetrical manner about the axis. 
     The coupler according to the foregoing embodiment beneficially has low light loss, while minimizing the area of the output. It beneficially can be made compact in length and diameter, and may have at least a pair of output arms substantially parallel to each other to facilitate coupling to output light guides. Alternatively, the coupler can be designed to combine light from a plurality of light sources. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a coupler according to one embodiment of the invention. 
     FIG. 2 is a perspective view of a coupler that is alternative to that of FIG.  1 . 
     FIG. 3 is a perspective view of another coupler according to the invention. 
     FIG. 4 is a perspective view of a further coupler according to the invention. 
     FIGS. 5A-5H show sequential, equally spaced cross sections of coupler  10  of FIG.  1 . 
     FIGS. 6A-6G show sequential, equally spaced cross sections of coupler  30  of FIG.  3 . 
     FIGS. 7A-7H show sequential, equally spaced cross sections of coupler  50  of FIG.  4 . 
     FIG. 8 is a detail view showing portions of output arms of a coupler and corresponding light guides connected to the output arms. 
     FIG. 9A is a side view of an angle-to-area converter used to produce a preferred embodiment of the inventive coupler in accordance with a design example. 
     FIG. 9B is an end view of the converter of FIG. 9A taken along line  9 B— 9 B in FIG.  9 A. 
     FIG. 9C is an end view of the converter of FIG. 9A taken along line  9 C— 9 C in FIG.  9 A. 
     FIG. 10A is a side view of a pair of angle-to-area sub-converters used to produce a preferred embodiment of the inventive coupler in accordance with the mentioned design example. 
     FIG. 10B is an end view of the sub-converters of FIG. 10A taken along line  10 B— 10 B in FIG.  10 A. 
     FIG. 10C is an end view of the sub-converters of FIG. 10A taken along line  10 C— 10 C in FIG.  10 A. 
     FIG. 10D is a side view of the sub-converters of FIG. 10A after undergoing a union operation. 
     FIG. 11A is a side view of the converter of FIG.  9 A and sub-converters of FIG. 10D interpositioned with respect to each other, in accordance with the mentioned design example. 
     FIG. 11B is an end view of the converters of FIG. 11A taken along line  11 B— 11 B in FIG.  11 A. 
     FIG. 11C is an end view of the converters of FIG. 11A taken along line  11 C— 11 C in FIG.  11 A. 
     FIG. 12A is a side view of a coupler produced according to the mentioned design example. 
     FIG. 12B is an end view of the coupler of FIG. 12A taken along line  12 B— 12 B in FIG.  12 A. 
     FIG. 12C is an end view of the coupler of FIG. 12A taken along line  12 C— 12 C in FIG.  12 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an optical coupler  10  in accordance with one embodiment of the invention. Coupler  10  includes an input stage  12  having an input port  14 , and an output stage  16  having four output arms  18   a ,  18   b ,  18   c  and  18   d , each with a respective output port  20   a ,  20   b ,  20   c  or  20   d . (In an alternative embodiment, the input and output ports are interchanged, so that light entering ports  20   a - 20   d  becomes combined and directed out of the coupler through port  14 .) 
     A midput region  22  is defined along a central axis  24  of the coupler, where the output arms at least first start to separate from each other along the input-to-output direction of light propagation. Midput region  22  coincides with a break point region  25 , at which output arms  18   a - 18   d  fully separate from each other. The arms separate from each other in an “X” shape  26 , with the “X” being located in a plane. Midput region  22  is shown with zero length along axis  24 , although it could be of non-zero length, e.g., a cylinder. 
     In a preferred construction of coupler  10 , the diameters of input port  14  and output ports  20   a - 20   b  are 7 mm and 5 mm, respectively; the axial distance from input port  14  to midput region  22  is 50 mm, and the axial distance from the midput region to the output ports is 40 mm. 
     Coupler  10  is preferably formed as a unitary piece of acrylic, for example, relying on total internal reflection as the primary means of light propagation. Alternatively, coupler  10  (and the other couplers described herein) could comprise a hollow coupler with reflective walls, such as coupler  28  in FIG.  2 . Another option is to form the device using a material with a non-uniform index of refraction, often called a Gradient Index material. 
     Although optical coupler  10  is shown with four output arms, it could alternatively have other numbers of output arms, such as two, three, five or six. Moreover, the cross sections of either the input port or the output ports could be polygonal (e.g., rectangular), rather than circular as shown. The shape of coupler  10  will be more fully described below. 
     FIG. 3 shows a preferred coupler  30  in accordance with the invention. The configuration of coupler  30  is selected to more closely preserve the etendue of light transmission than coupler  10  (FIG.  1 ). Like reference numerals as between different embodiments refer to like parts. As with coupler  10  (FIG.  1 ), coupler  30  includes an input port  14  and output arms  18   a ,  18   b ,  18   c  and  18   d  with respective output ports  20   a - 20   d . Its input stage  32  extends along axis  24  from input port  14  to midput region  34  at which the output arms first start to separate from each other along the input-to-output direction of light propagation. Input stage  32  may be shorter than as shown. This separation is imparted by a void  36  centered about axis  24  and which starts at midput region  34  and continues for the length of output stage  38  of the coupler. Input stage  32  may be essentially of zero length where void  36  starts at input port  14 . 
     Coupler  30  also includes a break point region  40  at which the output arms are fully separated from each other along the direction of light propagation. Peripheral voids  42  extend along the outer periphery of output stage  38  in elongated manner along the direction of light propagation. Voids  42  face outwardly with respect to axis  24  and are smoothly integrated with a void  44  separating output arms  18   a - 18   d  immediately downstream of break point region  40  (i.e., further along the path of light transmission). By “smooth” is meant that the cross section at any point along the axial length transitions to the next point without any substantial discontinuities. 
     FIG. 4 shows a further coupler  50  according to the invention. Coupler  50  is generally similar to coupler  10  (FIG.  1 ), but its output arms split from each other in an “X” shape  26 , as viewed along axis  24 , that is non-planar so as to reduce light loss owing to the splitting between the arms. Non-planar “X” shape  26  will be further described below. 
     FIGS. 5A-5H show sequential cross sections of coupler  10  of FIG. 1, starting with input port  14  in FIG. 5A, including an “X” shape  58  in FIG. 5E where the output arms separate from each other, and ending with output ports  20   a - 20   d  in FIG.  5 H. 
     FIGS. 6A-6G show sequential cross sections of coupler  30  of FIG. 3, starting with input port  14  and ending with the output ports in FIG.  6 G. FIGS. 6B-6E show cross sections of central void  36 , with a periphery defining four arc segments  36   a - 36   d  (FIG. 6D) corresponding in shape to the inner peripheries  60  (FIG. 6E) of the of the output arms downstream of the point where the arms are fully separated from each other. Peripheral voids  42  are shown in FIG.  6 D. Central and peripheral voids  36  and  42  assist in preserving the etendue of the transmitted light. 
     FIGS. 7A-7G show sequential cross sections of coupler  50  of FIG. 4, starting with input port  14  and ending with output ports  20   a - 20   d . Splitting pattern  26 , in the form of a non-planar “X”, is shown through progressive cross sections, indicating that the center of the “X”, in FIG. 7E, is downstream of the peripheral regions of the “X” in FIGS. 7B-7D. 
     With each of the three couplers  10 ,  30  and  50  described so far, the outer peripheries of the output arms are substantially parallel to each other. By “substantially parallel” is meant in the specification and claims less than 30% of the half angle of output light. (In other words, the output arms may be tilted, with respect to the main object of the splitter by 30% of the half angle of the output light). Further, the output arms are spatially separated from each other. As such, as shown in FIG. 8, output arms  18   a  and  18   b  can be beneficially connected to light guides  68   a  and  68   b  without removing cladding and outer jacket  70   a  and  70   b  from the light guides. A 4 mm spacing between arms  18   a  and  18   b  accommodates typical cladding and outer jacket of adjacent output light guides  68   a  and  68   b . Such spacing is preferably at least about 2 mm, and more preferably at least about 3.2 mm. 
     DESIGN EXAMPLE 
     Preferred steps for designing a coupler  70  (FIG. 12A) are illustrated in connection with FIGS. 9A-12C. Coupler  70  has only two output arms, but is otherwise most similar in to coupler  30  of FIG.  3 . Establishing rotational symmetry about central axis  24  (FIG. 12A) helps ensure that the light is split equally into each output arm. Such symmetry can also be stated as rotational symmetry about axis  24 , or as n-fold symmetry about axis  24 , where “n” is the number of output arms. If desired, however, a coupler need not employ rotational or n-fold symmetry, whereby one arm may be larger than an adjacent arm. However, it is usually desired that each arm be spaced about the same distance from axis  24 . 
     Diameters of the input and output ports  14  and  20   a - 20   b  (FIG. 12A) and the spacing between output arms  18   a  and  18   b  are determined. It is preferred that the area of the input port substantially equals the combined area of the output ports. It is also preferred that the numerical aperture of the input port be substantially the same as that at the output ports. For ease of explanation, specific dimensions are described. Assuming the diameter of the input port is 10 mm, the diameter of each of the two output ports is 10/(2 ½ ) mm, or about 7 mm. A center-to-center spacing between output ports of 11 mm provides a 4 mm gap between ports, which accommodates typical cladding and jacket on downstream light guides as described above with respect to FIG.  8 . 
     In FIG. 9A, a forward converter  72  is designed to start with the desired input port size and to end with a shape surrounding all of the output ports. The shape of converter  72  is chosen to provide an appropriate angle-to-area conversion. While a straight taper is shown for converter  72 , other tapers will be obvious to those of ordinary skill in the art, as now discussed. 
     As reported in X. Ning, R. Winston, and J. O&#39; Gallagher, Appl. Optics, vol. 26, no. 2 (January 1987), pp. 300-305, a dielectric totally internally reflecting concentrator can transform a port of area A1 with maximum angle Theta 13  1 to an area A2 with maximum angle Theta 13  2, where A1*sin(Theta—1) 2  A2*sin(Theta—2) 2  Such an angle-area transformer is also called a Theta—1/Theta—2 converter by the foregoing Ning et al. Reference. 
     One particular embodiment of a Theta — 1/Theta — 2 converter is a dielectric compound parabolic concentrator (DCPC) which are described in W. T. Welford and R. Winston, High Collection Nonimaging Optics, New York: Academic Press, Inc. (1989), chapter 4 (pp. 53-76, 82-84). In some cases, a DCPC can be replaced with a tapered cone or two tapered cones attached to one another, except that the DCPC is typically shorter in length. Another method to implement an angle-area-converter is to combine a gradient index (GRIN) rod with a tapered cone, as discussed in A. Cutolo, et. al., Appl. Optics, vol. 29, no. 9 (Mar. 20, 1990), pp. 1353-1363. 
     Typically, angle-to-area converters have the same shape at the input and output ports. If the shapes are different, the skew invariant, such as discussed in the foregoing Welford et al. reference at pp. 228-230, may limit the performance of a Theta—1/Theta—2 converter. When the input and output areas are different, a slow taper from one shape to the other provides reasonable performance for a Theta—1/Theta—2 converter, as suggested by Garwin, R. L. in “The design of Liquid Scintillation Cells,” Rev. Sci. Instruments, vol. 23 (1952), pp. 755-757. Adjusting the cross-section along the taper may provide improved performance in some cases, especially if the aspect ratio of the output ports is much different than the input port, and the length of the coupler is to be minimized. Some investigations of nonrotationally symmetric, non-imaging optic devices have been performed recently. See “Nonrotationally Symmetric Reflectors for Efficient and Uniform Illumination of Rectangular Apertures”, by Shatz, et. al., SPIE vol. 3428, pp. 176-183. An extension of the ideas presented by the foregoing Shatz article is to create an angle-to-area device which transitions from one shape to another where the intermediate cross-sections have star-like cross-sections. 
     The shorter the forward converter  72  , the shorter the overall coupler produced. A longer coupler will ensure more thorough light mixing and make the illuminance distribution more uniform at the break point (not shown). This can make dividing of light a among the output arms more uniform. On the other hand, a shorter coupler is desirable for compactness. 
     FIG. 9B shows input port  14  with a diameter of 10 mm. FIG. 9C superimposes output ports  20   a  and  20   b , shown in dashed lines, on face  74  of converter  72  (FIG.  9 A). With each output port having a 7 mm diameter and with a 4 mm spacing between ports, as assumed above, the minimum diameter of face  74  is 18 mm. 
     According to FIG. 10A, two angle-to-area converters  76   a  and  76   b  are designed, and are referred to herein as sub-backward converters (or sub-converters). The subbackward converters preferably each overlap the other in the vicinity of numeral  79  so that the axial shape of their left side is the same as the axial shape of their right side. (In contrast, coupler  10  of FIG.  1  and FIGS. 5A-5H was designed with the left-hand ends of the sub-converters being pie- or wedge-shaped, like the 90-degree portion of coupler  10  as shown in FIG. 5E, so that adjacent sub-converters abut each other.) Sub-converters  76   a  and  76   b , as shown, are parallel to each other such that their axial center-to-center spacing (not shown) of 11 mm at the left-shown end is the same as the center-to-center spacing (not shown) at the right-hand end. The two sub-converters need not be parallel to each other, but if they are not parallel, then they may not be axisymmetric, that is, rotationally symmetric about the main axis (e.g., axis  24 , FIG.  1 ). 
     FIG. 10C shows output ports  20   a  and  20   b  dimensioned as noted with respect to FIG.  9 C. FIG. 10B shows end faces  78   a  and  78   b  of converters  76   a  and  76   b  dimensioned so that, when such converters overlap, input port  14  is preferably at least covered to prevent light loss. Thus, the triangle at  80  shows that for input port  14  of 10 mm diameter (5 mm radius), and with a center-to-center spacing between faces  78   a  and  78   b  of 11 mm (or half spacing of 5.5 mm), the minimum diameter for faces  78   a  and  78   b  is 16 mm each (radius of 8 mm). The 8 mm radius in triangle  80  is approximately the square root of the sum of 5.5 mm 2  and 5 mm 2 . 
     FIG. 10D shows both converters  76   a  and  76   b  unioned together according to the logic of the Boolean “OR” function, which, incidentally, eliminates any overlap (e.g. at  79  in FIG.  10 A). 
     FIG. 11A shows the unioned sub-backward converters  76   a  and  76   b  overlapping forward converter  72 , shown in dashed lines for clarity. Only a part of converter  72 , at  73 , is directly visible. FIG. 11B shows the left-hand side of the overlapped converters of FIG. 11A, with input port  14  shown in dashed lines for clarity. FIG. 11C shows the right-hand side of the overlapped converters, with converter  72  shown in dashed lines for clarity. 
     As shown in FIG. 12A, the overlapped converters are then intersected according to the Boolean “AND” function, so that only the portions of each which overlap each other remain in the intersected product, shown as coupler  70 . FIG. 12B shows the left-hand side of coupler  82  with input port  14 , and FIG. 12C shows its right-hand side with output ports  20   a  and  20   b.    
     According to the foregoing design example, the sub-backward converters (or sub-converters), each of which may be substantially axisymmetrical, are unioned together and then intersected with the forward converter. Alternatively, each sub-converter can be first intersected with the forward converter, producing a non-axisymmetrical shape, before being unioned to the other sub-converters. Both approaches can result in the same shape, so that they are mathematical equivalents of each other. 
     Coupler  10  (FIGS.  1  and  5 A- 5 H) constructed according to the preferred dimensions mentioned above achieves a light loss of only about 11%, which is the result of 8% Fresnel reflection losses at the input and output ports and 3% from losses at the “X”. This loss can be minimized through the use of index matching or anti-reflection coatings. Analysis of coupler  50  (FIGS.  4  and  7 A- 7 H) also indicates a reduced light loss of only about 8% from Fresnel reflections, where the 3% loss at the “X” is removed because the “X” is no longer perpendicular to the main axis of light propagation. The combined area of output ports for the various couplers described is preferably limited to no more than approximately twice the area of the input port, and preferably is less than the mathematical product of 0.95, the number of output ports, and the area of the input port (e.g., 1.9 times the area of the input ports for the case of two output ports). The maximum cross sectional dimension of the output ports along the described axis is preferably less than the maximum cross sectional dimension of the input port along the axis. 
     The couplers described herein may be formed, for instance, from a clear plastic material such as acrylic, by molding, casting, machining, or related fabrication methods. The surfaces should be smooth to avoid scattering light passing through the coupler by total internal reflection. The couplers preferably each comprise an integrated, single, preformed unit, into which light guides, etc., can be coupled or integrated if desired. In some cases, it may be desirable to make the input to breakpoint portion of the coupler from one piece of material, and make the breakpoint to output portions separately. In the vicinity of each light port, a short cylindrical section can be added to facilitate coupling to light guides, etc. The couplers are preferably axially surrounded by a protective cover (not shown) having a lower index of refraction than the coupler. 
     While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention.