Abstract:
An optical coupling system having a high efficiency in coupling light from a light source to an optical fiber. The system may have a ball lens and an aspherical lens situated along the same optical path. The ball lens may be glass and the aspherical lens may be plastic. The ball lens may have optical aberrations common to spherical lenses. The aspherical lens may compensate for such aberrations. The glass ball lens may carry more power and have better thermal properties than the plastic lens, and thus compensate for latter&#39;s possibly weaker thermal properties. Together, the system may have sufficient power, low distortion, good thermal characteristics and high coupling efficiency.

Description:
BACKGROUND  
       [0001]     The invention pertains to optical couplers and particularly to couplers used for conveying laser light from a source into an optical fiber.  
         [0002]     Several patent documents may be related to optical coupling between optoelectronic elements and optical media. They include U.S. Pat. No. 6,086,263 by Selli et al., issued Jul. 11, 2000, entitled “Active Device Receptacle” and owned by the assignee of the present application; U.S. Pat. No. 6,302,596 B1 by Cohen et al., issued Oct. 16, 2001, and entitled “Small Form Factor Optoelectronic Receivers”; U.S. Pat. No. 5,692,083 by Bennet, issued Nov. 25, 1997, and entitled “In-Line Unitary Optical Device Mount and Package therefore”; and U.S. Pat. 6,536,959 B2, by Kuhn et al., issued Mar. 25, 2003, and entitled “Coupling Configuration for Connecting an Optical Fiber to an Optoelectronic Component”; which are herein incorporated by reference.  
         [0003]     Coupling efficiency between light sources and optical media is an important factor in various communications and other applications. Coupling efficiency, for instance, from a laser source to a single mode fiber not only is affected by a mismatch between the laser field/fiber-mode but also by aberrations in the coupling optics. A single ball lens may be used for single mode fiber coupling, but because of the spherical aberration from the ball lens, the coupling efficiency may be only about fifty percent. However, many communications applications need higher coupling efficiencies because of distance, weak light sources and high data rates. An aspherical glass lens is able to achieve high fiber coupling but its cost may be too high for practical use.  
       SUMMARY  
       [0004]     The present invention is a low cost, highly efficient system for coupling light from a light source into optical fiber. Among other features, it may have a spherical lens and an aspherical lens situated on the same optical path. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0005]      FIG. 1  is a diagram of an optical system with only a ball lens;  
         [0006]      FIG. 2  shows an optical system with a convex aspherical design;  
         [0007]      FIG. 3  shows an optical system with an alternative convex aspherical design;  
         [0008]      FIG. 4  shows an optical coupling system with a concave aspherical design;  
         [0009]      FIG. 5  is a graph of coupling efficiency versus deviation of the output relative to the optical fiber of the system of  FIG. 4 ;  
         [0010]      FIG. 6  is a graph of the coupling efficiency of the system versus it temperature; and  
         [0011]      FIG. 7  is a graph of the ray aberrations of the system of  FIG. 4 . 
     
    
     DESCRIPTION  
       [0012]      FIG. 1  shows a system  30  for coupling a light source  32  to an optical fiber  33 . System  30  may have only one lens  35  which is a glass ball lens. Light  31  may propagate from source  32  through window  34  and lens  35 . From lens  35  is spherically focused light  37  of which may match the fiber mode and couple into the end of fiber  33 . The other spherically focused light  38  from lens  35  may mismatch the fiber mode and miss the end of fiber  33  and therefore will not be coupled into the fiber  33 .  
         [0013]     Source  32  of  FIG. 1  may be about  381  microns (15 mils) from the closer surface of window  34 . One may note that if a flip chip is used, window  34  may be spaced such that it is in intimate contact or nearly so with source  32 . Window  34  may be about 203 microns (8 mils) thick. Window  34  may be about 264 microns (10.4 mils) from lens  35 . Lens  35  may have a diameter of about 1.5 millimeters (59 mils). The distance from lens  35  may be about 1.212 millimeters (47.7 mils) from the end of fiber  33 . The above-noted length measurements are along an optical axis  18 .  
         [0014]      FIG. 2  reveals an illustrative example of the invention. Coupler system  10  may be a two-lens device used for coupling light  11  from a light source such as, for example, a vertical cavity surface emitting laser (VCSEL)  12 , into a single mode (SM) optical fiber  13 . Light  11  may propagate through a spherical ball lens  15 . Light  17  may exit lens  15  and be focused on an end of a fiber  13  like that of light  37  of  FIG. 1 . However, because of the spherical aberration from the ball lens, light  19  might not be focused on the end of fiber  13  along with light  17  in the same manner as light  38  is not focused along with light  37  on the end of fiber  33  in  FIG. 1 . Light  17  and  19  in  FIG. 2  may enter an aspherical lens  16 . Lens  16  may be shaped in a non-spherical way to focus light  17  and  19  on to the end of fiber  13  at the same time. A similar arrangement and principle of focusing may appear in coupling systems  20  and  40  of  FIGS. 4 and 3 , respectively.  
         [0015]     VCSEL  12  may be a single mode source. Light  11  may propagate through a protective window  14  of a (hermetically) sealed package containing the VCSEL onto a ball lens  15 . The distance between VCSEL  12  and the surface of window  14  closer to VCSEL  12  may be about 380 microns (15 mils). Window  14  may be about 203 microns (8 mils) thick and consist of BK7™, Corning #7052, or any suitable transmissive material. The distance between the surface of the window  14  (closer to lens  15 ) and lens  15  may be about 280 microns (11 mils) along the optical axis. Spherical lens  15  may be about 1.5 millimeters (59 mils) in diameter. Lens  25  may be a glass ball lens. It may be composed of BK7™, LaSFN9, or any suitable material. Light  11  may move through lens  15  and out of it into an aspherical lens  16 . The distance between lens  15  and lens  16  may be about 561 microns (22.1 mils). Light  11  may propagate through lens  16  into optical fiber  13 . The end of fiber  13  may be in physical contact with lens  16  but not required to be so. The length of lens  16  may be about 209 microns (82.3 mils). The above-noted length measurements are along the optical axis. Lens  16  may be a convex lens made from Zeonex™ E48R available from Zeon Chemicals L.P., 4111 Bells Lane, Louisville, Ky. 40211. The lens may also be made from GE ULTEM. A 1.5 mm ball lens  15  of BK7™ material may be available from Edmund Industrial Optics, 101 East Gloucester Pike, Barrington, N.J. 08007-1380. Optical fiber  13  may be an SMF-28™ single mode optical fiber available from Corning Incorporated, One Riverfront Plaza, Corning, N.Y. 14831. One may note that the dimensions illustrated above are typical and other geometries may be functional as well.  
         [0016]     The present optical coupler may have both high coupling efficiency and low cost. The coupling optics may use a glass ball lens and a molded aspherical lens. The aberration of the ball lens may degrade the efficiency of the coupling system. However, the ball lens&#39; spherical aberration may be compensated by the light ray directing properties of the aspherical plastic lens. Since the ball lens may have significantly more optical power than the plastic lens in the coupling system, the plastic lens&#39; poor thermal properties may be compensated for and minimized. Therefore, an appropriately designed combination of a glass ball lens and plastic molded aspherical lens may provide a thermally stable and highly efficient optical coupling system.  
         [0017]     Lens  16  may be composed of glass or be a single aspherical glass lens. Glass aspherical lenses may have good thermal properties and less aberration than a ball lens. They may be somewhat expensive and difficult to produce. Plastic aspherical lenses may be easily and inexpensively producible; however, they do not have thermal properties as good as the glass lenses. Yet the plastic aspherical lenses have much less aberration than the ball lenses. For instance, light rays coming from a spherical lens periphery may form an image before the ideal focal point. For this reason, the spherical aberration (a blurred image) may occur at the center portion of the image formed. Or if the focus is readjusted for the center portion of the image, then the spherical aberration (again, a blurred image) may occur at the periphery of the image. In other words, it may not be possible for all of the parallel rays going through a spherical lens to converge at one point. An awkward and cumbersome multitude of spherical lenses might be designed to partially correct this aberration problem. However, one aspherical lens may be designed to gather or converge all of the parallel rays of light to one focal point. The aspherical lens may have surface with a specially designed curvature to achieve this convergence of the light rays. The aspherical lens surface does not completely conform to the shape of a sphere like that of a spherical lens. Mass production technologies including plastic mold technology may be used to mold aspherical lenses by pouring or injecting plastic material into a rather precise aspherical mold. Further, the aspherical lens may achieve a coupling efficiency into a single mode fiber above ninety percent for coupling systems  10 ,  20  and  40 . This is a desired performance feature for VCSEL communication applications since VCSEL optical power is relatively low compared to other laser sources. Significant power is better conveyed with a glass aspherical lens; however, the cost of a glass aspherical lens is high (i.e., greater than eight dollars per lens in year 2000 with high volume pricing). The inexpensive (i.e., less than a dollar with high volume pricing) aspherical lens may be the poured or injection molded plastic lens. The aspherical lens may be made of another material similar to plastic. The plastic lens may have poor thermal characteristics but a glass ball lens may compensate for those characteristics in a coupling system with the plastic lens. The ball lens may be made of another material similar to glass.  
         [0018]     A design for the aspherical convex lens  16  may be indicated by the following equation and parameter values. 
 
 z={cr   2 /[1+(1−(1 +k ) c   2   r   2 ) 1/2   ]}+A   6   r   6   +A   8   r   8  
 
         [0019]     Surface 1 
        c=1/R; R=1.457374 (Unit: mm)     k=−18.455693     A 6 =−24.768767     A 8 =−20.028863        
 
         [0024]     Illustrative examples of the invention have an optical design which may possess both high coupling efficiency and low cost. The spherical aberration of ball lens  15  may be compensated for by aspherical plastic lens  16 . Because ball lens  15  may convey the most optical power in system  10 , the combination of a glass ball lens and plastic molded optics may provide thermal stability and high coupling efficiency for optoelectronic element and single mode optical fiber coupling applications.  
         [0025]      FIG. 3  is a layout of an optical coupler system  40  having a convex aspherical lens  46  that may have a different design than that of convex aspherical lens  16  of system  10 . Similarly, light source  42  may emit a light  41  that goes through a hermetically sealed window of the package wherein light source  42  is situated. Light  41  may go through a ball lens  45  and convex aspherical lens  46 . The light from lens  46  may enter fiber  43 . Components  42 ,  45 ,  46  and  43  may be situated on an optical axis  18 . Aspherical lens  46  may be a plastic lens. The materials of the components and the dimensions may be similar those of system  10 .  
         [0026]      FIG. 4  reveals another illustrative example of the invention. Coupler system  20  may be a two-lens device for coupling light from a single mode (SM) VCSEL  22  into an SM optical fiber  23 . The wavelength of the laser light from VCSEL  22  may be 1310 nm. Light  21  may propagate through a protective window  24  into a ball lens  25 . VCSEL  22  may be about  381  microns ( 15  mils) from the closer surface of window  24 . Window  24  may be about 203 microns (8 mils) thick. The surface of window  24  closer to lens  25  may be about 305 microns (12 mils) from lens  25 . Lens  25  may have a diameter of about 1.5 millimeters (59 mils). It may be a glass ball lens. Light  21  may propagate through lens  25  and out of it into an aspherical lens  26 . The distance between lens  25  and lens  26  may be about 76 microns (3 mils). Light  21  may propagate through lens  26  into optical fiber  23 . Optical fiber  23  has an end that may be in contact with lens  26 . The length of lens  26  may be about 205.7 microns (81 mils). The above-noted length measurements may be along the optical axis. The dimensions may be illustrative examples and may be of other appropriate magnitudes. Lens  26  may be a concave Zeonex™ E48R (or any other suitable plastic material) lens. However, lens  26  could be composed of glass, but because of the high cost (as noted above) of glass aspherical lenses, lens  26  may be a poured or an injected molded plastic lens.  
         [0027]     Lens  25  may be a 1.5 mm ball lens made of LaSFN9™ material available from Edmund Industrial Optics. Lens  26  may be made of Zeonex™ E48R material available from Zeon Chemicals L.P. Fiber  23  may be an SMF-28™ single mode optical fiber available from Corning Incorporated. Window  24  may be made from BK7™ material available from various vendors. Window  24  may be a hermetically sealed window of a TO-56 can or other package incorporating light source  22  such as a VCSEL.  
         [0028]     Like system  10 , coupler system  20  may have thermal stability and high coupling efficiency for coupling light into SM (single mode) optical fiber  23 . In the above-described systems  10 ,  20  and  40 , end faces of optical fibers  13 ,  23  and  43 , respectively, may be situated so as to be in contact with aspherical lenses  16 ,  26  and  46 , as shown in the respective  FIGS. 2-4 , or the end faces of fibers  13 ,  23  and  43  may be situated at distance from lenses  16 ,  26  and  46 , respectively (not shown). Also, the order of ball lenses  15 ,  25  and  45  and of aspherical lenses  16 ,  26  and  46  along optical axis  18  may be different than that as shown. The systems disclosed here may be operated with a light source having a wavelength of about 1310 nm but may be at another wavelength, such as 850 nm or 1550 nm as well as other wavelengths. The light source may be replaced with a detector and the source of light may be from the optical medium or fiber.  
         [0029]     In systems  10 ,  20  and  40 , light sources  12 ,  22  and  42  may be single mode VCSELs or other sources of that mode. However, they may be multimode VCSELs or other sources of that mode. The optical fibers  13 ,  23  and  43  of these systems may be single mode or multimode, as applicable.  
         [0030]     A design for aspherical concave lens  26  may be indicated by the following equation and parameter values. 
 
 z={cr   2 /[1+(1−(1 +k ) c   2   r   2 ) 1/2   ]}+A   2   r   2   +A   4   r   4  
 
         [0031]     Surface 1 
        c=1/R; R=−1.576039 (Unit: mm)     k=33.774232     A 2 =0.018687     A 4 =−2.347015        
 
         [0036]     The following chart shows the coupling efficiency of system  20  versus deviation of the alignment of the output of the system with optical fiber  23 . This chart appears to reveal system  20  as having a good tolerance to some misalignment of its output with optical fiber  23  to which system  20  is coupling light from light source  22 .  
                                                                                                   Coupling Efficiency            YDE:       .00000   .00100   .00200   .00300   .00400   .00500                    XDE                                   .00000   |   .96565   .90779   .77890   .61595   .41840   .26323       .00100   |   .90779   .85319   .73169   .57822   .39225   .24636       .00200   |   .77890   .73169   .62675   .49452   .33446   .20926       .00300   |   .61595   .57822   .49452   .38938   .26231   .16329       00400   |   .41840   .39225   .33446   .26231   .17535   .10808       .00500   |   .26323   .24636   .20926   .16329   .10808   .06577                  
 
         [0037]      FIG. 5  is a graph that charts coupling efficiency of the present coupling system  20  versus deviation of the alignment of the output of the system with the optical fiber  23  with the use of ray-based tracing. The y-axis or ordinate axis indicates the coupling efficiency from 0 to 1.0 or 100 percent. An x-axis or abscissa axis indicates the horizontal or x-direction deviation of the core center of fiber  23  from 0 to 5 microns relative to optical axis  18 . Each graph line represents a vertical or y-direction deviation of the core center of fiber  23  from optical axis  18 . Lines  50 ,  51 ,  52 ,  53 ,  54  and  55  represent a y-direction or vertical deviation of 0, 1, 2, 3, 4 and 5 microns, respectively, of the core center of fiber  23  relative to optical axis  18 .  
         [0038]      FIG. 6  is a graph of the coupling efficiency of system  20  versus it package soak temperature from −45 to 100 degrees Centigrade (−49 to 212 degrees F.) as shown by line  60 . This graph may demonstrate the thermal stability of system  20 . System  10  may be regarded to have similar coupling efficiencies under conditions like those of system  20 .  
         [0039]      FIG. 7  graphs the aberrations of the across the output face of system  20  in an x-axis direction and a y-axis direction. This graph appears to reveal system  20  to have a rather distortion free output.  
         [0040]     Coupler systems  10 ,  20 ,  30  and  40  may be a part of an array of light sources such as VCSELs and an array of fibers to which that the light is coupled. On the other hand, components  12 ,  22 ,  32  and  42  may be detectors receiving light from their respective coupling systems that are receiving light from an optical fiber or fibers. The coupled light may include light signals such as communications signals.  
         [0041]     Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.