Abstract:
An optical coupler having two refractive lenses for coupling an optoelectronic element and an optical medium to each other. One lens may be in contact with the optical medium. The refractive index of the one lens may be similar to the index of the optical medium. The optoelectronic element may be a light source or a detector. The light source may be a laser. The lenses may be glass ball lenses. One of the ball lenses may be a half ball lens. If the optical medium is an optical fiber, one of the lenses may a fiber stop for the fiber when inserted in a receptacle of the coupler.

Description:
BACKGROUND  
         [0001]    The invention relates to optical couplers and particularly to such devices that couple optoelectronic elements and optical fiber. More particularly, the invention relates to couplers having lenses.  
           [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. No. 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]    In the context of the invention, the optoelectronic element may be understood as being a transmitter or a receiver. When electrically driven, the optoelectronic element in the form of a transmitter or light source converts the electrical signals into optical signals that are transmitted as light signals. On receiving optical signals, the optoelectronic element in the form of a receiver or detector converts these signals into corresponding electrical signals that can be tapped off at the output. In addition, an optical fiber may be understood to be any apparatus for forwarding an optical signal with spatial limitation, in particular preformed optical fibers and so-called waveguides.  
           [0004]    A problem with couplers may involve light reflected back to the light source. This may be an issue because, for instance, some fiber optic transmitters suffer from undesirable and performance degrading reflections from the face end of the optical fiber back into the coupled optoelectronic element device (e.g., a semiconductor laser). Here, the fiber&#39;s surface and facing surface of the optoelectronic element device form a Fabry-Perot cavity which may modulate the light from the laser transmitter or semiconductor laser and consequently produce unwanted fluctuations in the power coupled to the optical fiber. Further, the optical energy reflected directly into the laser cavity may cause additional noise in the laser&#39;s output. For these reasons, it would be desirable to reduce and minimize the return reflections from the fiber face in the coupler.  
         SUMMARY  
         [0005]    The invention is an optical coupler which may couple a light source or detector and optical fiber to each other. The coupler may have a fiber stop which is a lens. That is, the optical fiber may have an end that is in contact with a lens of the coupler. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]    [0006]FIG. 1 is an optical diagram of a two ball lens coupler having an air gap between the optical fiber end and the nearest lens.  
         [0007]    [0007]FIG. 2 is a diagram of a two ball lens coupler having a fiber stop lens.  
         [0008]    [0008]FIG. 3 is a diagram of a one and a half lens couple having a flat lens surface as a fiber stop.  
         [0009]    [0009]FIG. 4 is a side sectional view of a two-ball lens coupler apparatus implementing the coupler of FIG. 1;  
         [0010]    [0010]FIG. 5 is a side sectional view of a two-ball lens coupler apparatus implementing the coupler of FIG. 2;  
         [0011]    [0011]FIG. 6 is a side sectional view of a one and a half ball lens coupler implementing the coupler of FIG. 3; and  
         [0012]    [0012]FIGS. 7 a ,  7   b  and  7   c  are a side view and perspective views, respectively, of the coupler housing hardware. 
     
    
     DESCRIPTION  
       [0013]    [0013]FIG. 1 shows an optical layout of a two-ball lens optical coupler  30 . This two-ball lens system may be arranged to focus the light at a point outside the second ball lens  29 , possibly often with light  14  nearly perfectly collimated between ball lenses  28  and  29 . Light  14  may be emitted by a light source  11 . Source  11  may be a laser such as a vertical cavity surface emitting laser. Light  14  may propagate through ball lens  28  and  29 . Light  14  may be focused by lens  28  and  29  on end face  22  of core  23  of optical fiber  33 . Light  14  may propagate from ball lens  29  through air onto the end of core  23 . The Fresnel coefficient of back reflectance  31  of light  14  for coupler  30  may be determined with the following formula, where “n” is an index of refraction of light of the subject material,  
         ((n lens glass −n air )/(n lens glass +n air )) 2 .  
         [0014]    Calculation of reflected light  31  may amount to about 4 percent of the originally emitted light  14  for an n lens glass =1.5 and n air =1.0. This amount of back reflectance light  31  is significant enough to cause unwanted fluctuations in power of light  14  from source  11  coupled to core  23  of optical fiber  33  at end face  22  and additional noise in light  14  at the output of light source  11 .  
         [0015]    [0015]FIG. 2 shows an optical layout of a two-ball optical coupler  10 . Light  14  may be emanated by source  11 . Light  14  may propagate through ball lens  15  and into ball lens  16 , respectively. Ball lens  16  may focus light  14  down to a spot at or near the surface of lens  16  where light  14  may exit lens  16 . Core  23  of fiber  33  may have end face  22  that is situated against the surface of ball lens  16  at that spot where the rays of light  14  converge together. This arrangement may minimize reflectance of light  14  into light  32  that moves towards the direction of light source  11 . One cause of reflected light  32  may be at end face  22  of fiber core  23  being coupled. A reduction of reflectance light  32  may result from having end face  22  of core  23  of fiber  33  of coupler  10  physically in contact with a lens, such as ball lens  16 . The reduction of reflected light  32  may occur because an air interface between lens  16  and fiber end  22  is eliminated at the point of contact. The Fresnel coefficient of back reflectance  32  of light  14 , in view of coupler  10 , may be determined with the following formula,  
         ((n lens glass —n glass fiber )/(n lens glass +n glass fiber )) 2 .  
         [0016]    Calculation of reflected light  32  may amount to about 0.01 percent of light  14  for an n glass fiber =1.47 and n lens glass =1.5. This calculated amount of reflected light  32  in coupler  10  is about 0.25 percent of the calculated reflected light  31  in coupler  30 .  
         [0017]    [0017]FIG. 3 shows an optical layout of a one and a half-ball optical coupler  20 . Light  14  may be emitted by light source  11 . Light  14  may propagate through ball lens  25  and into a half-ball lens  26 . Half-ball lens  26  may focus light  14  down to a spot at or near the flat surface of lens  26  where light  14  may exit lens  26 . Core  23  of fiber  33  may have an end  22  that is situated against the flat surface of ball lens  26  at that spot where the rays of light  14  converge together. This arrangement may minimize reflectance of light  14  as light  34  propagating towards the direction of light source  11 . One cause of reflection may be at end face  22  of fiber core  23  being coupled. A reduction of reflectance light  34  may result from having end face  22  of core  23  of fiber  33  of coupler  10  physically in contact with half-ball lens  26 . The reduction of reflected light  34  may occur because the air interface between lens  26  and fiber end  22  is eliminated with the point of contact. The Fresnel coefficient of back reflectance  34  of light  14 , in view of coupler  20 , may be determined with the following applicable formula,  
         ((n lens glass −n glass fiber )/(n lens glass +n glass fiber )) 2 .  
         [0018]    Calculation of reflected light  34  may amount to about 0.01 percent of light  14  for an n glass fiber =1.47 and n lens glass =1.5. This calculated amount of reflected light  34  in coupler  20  is about 0.25 percent of the calculated reflected light  31  in coupler  30 . The closeness of the indices of refraction of the glass fiber and lens glass appears to result in a minimizing of light reflected from the fiber core end face. The composition of ball lenses  15 ,  16 ,  25 ,  26 ,  28  and  29  may include BK7™ glass or like material.  
         [0019]    The following table indicates the amount of reflected light which is indicated in terms of a percentage of light to the end of the optical medium such as a fiber end face relative to the indices of refraction of the lens proximate to the optical medium and of the optical medium. The formula used for the table is  
         ((n lens glass −n medium )/(n lens glass +n medium )) 2 .  
         [0020]    [0020]                                                     n lens glass     n medium     % of Reflected Light                                1.50   1.00   4.00       1.50   1.10   2.37       1.50   1.20   1.23       1.50   1.30   0.510       1.50   1.35   0.277       1.50   1.40   0.119       1.50   1.41   0.0957       1.50   1.42   0.0751       1.50   1.425   0.0657       1.50   1.43   0.0571       1.50   1.44   0.0416       1.50   1.45   0.0287       1.50   1.46   0.0183       1.50   1.47   0.0120       1.50   1.48   0.00450       1.50   1.49   0.00112                    
         [0021]    If the medium has an index of refraction 10 percent lower than that of the lens, the light reflected is about 0.277 percent of the light going to the medium, which is about 7 percent of light reflected with air as an intervening medium between the lens and the optical medium. If the medium has an index of refraction 5 percent lower than that of the lens, the light reflected is about 0.0657 percent of the light going to the medium, which is about 1.6 percent of light reflected with air as an intervening medium between the lens and the optical medium. In the table, the medium may be the intervening medium. However, if there is contact between the lens and the optical medium the calculation may apply to the index of refraction of the optical medium. Hence, while this discussion has shown that the optimum implementation of this invention includes matching the fiber stop optical element&#39;s index of refraction to that of the fiber, significant practical performance gain (i.e., reduction of reflectance feedback) is accomplished even in imperfectly index matched implementations.  
         [0022]    [0022]FIG. 4 shows an example of coupler  30 . This coupler may be a two ball lens system having an optical fiber  33  interface with a space  35  between the nearest ball lens  29  and fiber face  22 . Space  35  may be a vacuum or filled with air or other optical medium material. Coupler  30  may have a laser light source  11 , such as a vertical cavity surface emitting laser (VCSEL). Source  11  may be contained in a hermetically sealed package  12  having a window  13 . Source  11  may emit light  14  through window  13 , ball lenses  28  and  29 . Lenses  28  and  29  may be structurally supported by an optical subassembly housing  17 . Housing  17  may be structurally supported by fiber optic coupler barrel  18 . Package  12  may be situated in a z-alignment sleeve  19 . Package  12 , for example, may be a TO-56 can. Barrel  18  and sleeve  19  may be fabricated from a stainless metal alloy. The materials of these components, including housing  17 , may be thermally matched. Housing  17  may be of a ceramic such as zirconia or of a metal. Sleeve  19  may be fit into barrel  18  and slide back and forth in order to adjust the distance of source  11  from ball lens  28 .  
         [0023]    After the accomplishment of distance adjustment between source  11  and lens  28 , then sleeve  19  may be fixed or secured to barrel  18  with a weld spot, pressed fit, glue, or the like. A ferrule  24  having an optical fiber  33  in it may be inserted into opening  21 . An end face  22  of fiber  33  may be at a certain distance from ball lens  29 , with air or another medium between end face  22  and ball lens  29 . The other medium between end face  22  and ball lens  29  may be a light transmitting optical medium having a preferred index of refraction. The index of refraction may match the index of fiber core  23  or lens  29 , or both of the latter. The distances of the ball lens  29  from fiber end face  22  and ball lens  28  and of ball lens  28  from light source  11  may be adjusted for another optical medium between ball lens  29  and end face  22 .  
         [0024]    Couplers  10 ,  20  and  30  may be designed to operate at 850 nm, 1310 nm or 1550 nm. They may instead be designed for some other wavelength. These couplers may be designed in various configurations such as with one lens, molded lens or lenses, or more than two lenses.  
         [0025]    [0025]FIG. 5 shows an illustrative implementation of coupler  10 . The structure of coupler  10  may be similar to that of coupler  30  except that ball lens  16 , which is the lens closest to fiber end face  22 , may be a fiber stop for fiber  33  and its core  23 . Coupler  10  may have a lens arrangement, which includes a ball lens  15  near source  11 , has the light focused at or slightly inside the second ball lens  16  surface so that the source  11  to fiber  33  ray path may be much different than that of coupler  30 . Fiber  33  being coupled to may be arranged in such a manner that it is in physical contact with the surface of the second ball lens  16 . The components of coupler  10  may be held in place by an external housing fabricated in such a manner that the laser diode, optical elements and receiving optical fiber cable are held in the correct positions to effect the above-noted focusing.  
         [0026]    [0026]FIG. 6 shows an illustrative implementation of coupler  20 . System  20  may involve a use of a half-ball lens  26  in the system. In this two element full ball-half ball design, the fiber-to-lens contact is thus planar instead of a single point of contact as in the two-ball lens approach of system  10 . The half-ball lens configuration may have the advantages of loosened radial alignment tolerances and reduced contact pressure which may make fiber end face  22  and fiber stop lens  26  less prone to wear or potential surface damage upon repeated insertions of ferrule  24 . Ferrule  24  may be fabricated from a ceramic such as zirconia or form another material. End face  22  of core  23  may be a polished round surfaced tip having a relatively large radius or be flat. There may be a ball lens  25  between lens  26  and source  11 . Fiber  33  may be single mode but could be multi-mode as desired. Likewise, light source  11  may be single mode but could be multi-mode.  
         [0027]    [0027]FIG. 7 a  shows an external side view of couplers  10 ,  20  and  30 , without ferrule  24  inserted, shown in FIGS. 4-6. FIGS. 7 b  and  7   c  are perspective views of these couplers.  
         [0028]    A multitude of the optical couplers may be incorporated in an array-arrangement. Such arrangement may be of a one or two dimensional layout.  
         [0029]    Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications, including aspheric lens variations, modifications and substitutions, 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.