Abstract:
An optical system is adapted to produce reduced back reflection from a receiving detector back to a light source for increased system performance. The system may optically condition light signals from the light source for projection onto the detector. The conditioning may result in a light spot on the detector that has an annular intensity distribution or profile. The annular distribution may be attained in any number of ways including providing a slope discontinuity in the lens surface, providing an axicon lens function, and/or providing a defocused light spot on the detector surface.

Description:
[0001]    This application is a continuation-in-part of U.S. Ser. No. 10/610,256, filed Jun. 30, 2003, entitled “A High Speed Optical System”, the entire contents of which are herein incorporated by reference. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates to optical systems for delivering light from a light source to a destination such as a detector, and more particularly, to such systems that are adapted to reduce back reflection from the destination back to the light source.  
           [0003]    Optical technology is used in a wide variety of fields including telecommunications, computers, and medical fields. In many applications, a light guide—such as an optical fiber—is used to deliver a light signal from a light source to a light detector. An important consideration in many of these systems is the optical coupling performance between the light source, the optical fiber, and the light detector. In many cases, the optical fiber is simply “butt-coupled” to the light source and/or light detector. While this may be adequate for some applications, it has been found that in some cases, some of the light that is delivered to the light detector is reflected by the light detector back into the optical fiber, and in some cases, back into the light source. Such back reflection can in some cases create significant noise. For example, it has been found that such back reflections can create optical feedback in some light sources, which can produce increased jitter and increased Reflective Intensity Noise (RIN). Back reflections can also cause interferometric noise in some light sources by converting some light source phase noise into light source intensity noise. For optical communications systems, this can result in increased bit error rates (BER), and reduced performance. For other applications, such as computer and medical applications, this noise can result in reduced system performance and/or reduced reliability.  
         SUMMARY  
         [0004]    The present invention is directed to a method and apparatus for reducing back reflection into an optical system. In one illustrative embodiment, an optical element is provided between a light source and a light detector. The optical element is adapted to direct light delivered by the light source to the light detector. In some cases, the optical element is further adapted to reduce or prevent light that is reflected off the detector from substantially coupling back to the light source. This may be accomplished by, for example, including an axicon type function in the optical element. It is contemplated that the light source may be an optoelectronic device, an optical fiber driven by an optoelectronic device, or any other light source, as desired.  
           [0005]    In one illustrative embodiment, the optical element may include a plano-convex lens that has a flat side and a convex side. In this embodiment, the light source may be positioned adjacent to the flat side, and the detector may be positioned adjacent to but spaced from the convex side. The plano-convex lens may be configured to receive a light beam from the light source and produce an annular shaped light pattern on the detector surface. When the annular shaped light pattern strikes the detector, most of the light reflected by the detector surface will not be directed or focused back to the light source by the plano-convex lens. This may help reduce the optical feedback at the light source, which can reduce jitter and Reflective Intensity Noise (RIN) in the system. This may also reduce the interferometric noise at the light source. With the reduced noise, decreased bit error rates (BER), and increased performance may be achieved.  
           [0006]    While an annular light pattern is described above in one illustrative embodiment, it is contemplated that any light pattern may be used that helps reduce the amount of back reflection that is coupled back into the light source. In many cases, this may correspond to a light pattern that has a reduced light intensity near the center of the light pattern, such as an annular, semi-annular or other like pattern.  
           [0007]    Alternatively, or in addition, it is also contemplated that the optical element may have a focal length, and that the detector may be placed in front of or behind the focal point. This may help defocus the light at the detector, which may help reduce the amount of back reflection that is coupled back into the light source. The detector may also have an anti-reflective coating to further help reduce the amount of back reflection, if desired. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic view of an optical system according to an illustrative embodiment of the present invention;  
         [0009]    [0009]FIG. 2 is a spot diagram of an illustrative light pattern on the detector of FIG. 1;  
         [0010]    [0010]FIG. 3 is a spot diagram of an illustrative return spot on the light source of FIG. 1;  
         [0011]    [0011]FIGS. 4 and 5 show cross-sectional views of lenses having slope discontinuities; and  
         [0012]    [0012]FIG. 6 reveals several focal adjustments of an optical element. 
     
    
     DESCRIPTION  
       [0013]    [0013]FIG. 1 is a schematic view of an optical system  100  according to an illustrative embodiment of the present invention. In the illustrative embodiment shown in FIG. 1, a light source  110  emits light rays  120 , which pass through optical element  130 . Suitable light sources include, for example, an optical fiber that delivers light, a laser such as a Vertical Cavity Surface Emitting Laser (VCSEL), a Light Emitting Diode (LED), or any other suitable device or element, or combination of suitable devices or elements, capable of producing or delivering light. After passing through optical element  130 , light rays  140  impinge on the surface  150  of a detector  160 . The detector  160  may be any suitable light detector, as desired.  
         [0014]    In the illustrative embodiment, the light rays  120  are conditioned by the optical element  130  into light rays or signals  140 , which form an annular light pattern on the detector surface  150 , as better shown in FIG. 2. FIG. 2 shows a spot diagram of an illustrative light pattern produced by optical element  130  on the detector surface  150 .  
         [0015]    When the illustrative annular shaped light pattern (see FIG. 2) strikes the detector surface  150  of detector  160 , most of the reflected light is not directed by the optical element  130  back to the light source  110 , as shown by FIG. 3. FIG. 3 shows the back reflection or return spot on the light source  110 . As can be seen, the back reflection is small (&lt;10%), indicating that the optical feedback at the light source  110  is reduced, which can reduce jitter, Reflective Intensity Noise (RIN), and/or interferometric noise at the light source  110 . As indicated above, this reduced back reflection noise may help provide a decreased bit error rate (BER) and/or an increase in performance of the optical system. More specifically, and in a fiber communications example, the reduced back reflection may help meet the return loss specification of Ethernet and fiber channels and improve the performance of the fiber communications system.  
         [0016]    In the illustrative embodiment shown in FIG. 1, the optical element  130  is a plano-convex lens. The plano (i.e. flat) side  210  of the lens  130  may act as a fiber stop to a light source (e.g. fiber)  110 . In some embodiments, the plano side  210  of the lens  130  may make physical contact with the fiber  110  facet. This physical contact may be maintained using spring loading. To help reduce back reflection caused by the boundary between the piano side  210  of the lens  130  and the fiber  110  facet, the index of refraction of the lens material may be selected to match or substantially match the index of the fiber  110  core. In one illustrative embodiment, the optical element  130  is one piece and made or molded from Ultem R  1010, which is a General Electric Company plastic. In some cases, an optical grease or optical adhesive may be placed between the plano side  210  of the lens  130  and the fiber  110  facet, if desired.  
         [0017]    As shown in FIG. 2, the convex side  220  of the lens  130  may be configured to form an annular or ring spot pattern on the detector surface  150 . When an annular shaped light pattern impinges on the detector surface  150 , most of the reflected light will not be directed or focused by the plano-convex lens  130  back to the light source  110 . This may help reduce the optical feedback to the light source, which can reduce jitter, Reflective Intensity Noise (RIN), and interferometric noise in the system. As such, decreased bit error rates (BER), and increased performance may be achieved.  
         [0018]    While an annular light pattern is shown above in FIG. 2, it is contemplated that any light pattern that helps reduce the amount of back reflection that is coupled back into the light source  110  may be used. In many cases, this may correspond to a light pattern that has a reduced light intensity near the center of the light pattern on the detector surface  150 . That is, in many cases, the optical element  130  may redistributed the power of the light source  110  from the center to the outskirts of the beam that is projected on to detector surface  150 .  
         [0019]    Alternatively, or in addition, it is contemplated that the optical element  130  may have a focal length that images the light from the light source  110  onto a focal point  162  or focal plane, as desired, and the detector  160  may be placed in front of or behind the focal point  162  or focal plane. This may help defocus the light at the detector surface  150 , which may help reduce the amount of back reflection that is coupled back into the light source  110 . The detector surface  150  may also have an anti-reflective (AR) coating to further help reduce the amount of back reflection, if desired. While the optical element  130  is shown as a plano-convex lens in FIG. 1, it is contemplated that the optical element  130  may be any optical element that produces a light pattern on the detector surface  150  that helps reduce the back reflection into the light source  110 .  
         [0020]    Attaining an annular distribution of light on the detector surface  150  may be achieved in any number of ways. For example, and in one illustrative embodiment, an axicon lens, also known as conical lens or rotationally symmetric prism, may be used to convert a parallel laser beam into a ring, a doughnut shaped ablation or an annular intensity profile.  
         [0021]    In some cases, an appropriate slope discontinuity may be provided in the surface  220  of the optical element  130  at or near the optical axis  130 , although this is not required in all embodiments. The slope discontinuity may help provide the axicon function to optical element  130 . An illustrative surface  220  of an axicon optical element having a slope discontinuity at optical axis  230  is shown in FIG. 4. Line  240  shows the slope of the upper part of surface  220  at optical axis  230  (r=0). Line  250  shows the slope of the lower part of surface  220  at optical axis  230 . As one follows surface  220  across axis  230 , there is a disruptive change of slope from slope  240  to slope  250 . Slope discontinuities may be implemented in various ways. FIG. 5 shows a slope or curvature discontinuity  340  as a small notch-like shape, cusp, indentation or protrusion in surface  220  at area  260  about optical axis  230 . Discontinuity  340  may be sharp, abrupt, rough or smooth. Discontinuity  340  may be of any shape or contour that helps enhance the axicon function. Elsewhere, the slope may be continuous, such as a function of the distance from optical axis  230  or of the radius, except at optical axis  230 . In some cases, slope discontinuity  340  of surface  230  may appear imperceptible to the eye. Apart from point or area  260 , surface  220  may be aspherical or spherical, depending on the application.  
         [0022]    Alternatively, or in addition, much or all of the surface  220  of optical element  130  may be configured such that an annular or ring pattern of light  140  is transmitted onto the detector surface  150  of detector  160 . For example, the surface  220  may cone shaped, with the tip of the cone at the vertex of the surface. Surface  220  may also be rotationally symmetric about the optical axis (e.g. z axis), and described by a single parameter θ, where θ is the angle measured between the plane normal to the z axis at the vertex of the cone and the surface  220 . The surface sag of the surface  220  may be defined by, for example:  
           z =tan(θ) r   
         [0023]    where “z” is the surface sag and “r” is the radial coordinate in lens units.  
         [0024]    Alternatively, the lens surface  220  may be defined by the following formulas, constants and variables:  
           z={cr   2 /[1+(1−(1+ k ) c   2   r   2 ) 1/2   ]}+A   1   r   1   +A   2   r   2    
           c =1 /R; R =−0.3715 mm  
           k =−1.171438 E +008  
         A 1 =0.01270  
           A   2 =−0.7737 mm −1   
         [0025]    In some illustrative embodiments, an annular light pattern may be produced on detector surface  150  by defocusing the light spot produced by the optical element  130  relative to the detector surface  150 . In one illustrative embodiment, detector  160  may be positioned either in front of or behind the focus point or focal plane of optical element  130 . This may cause an annular light intensity pattern on detector surface  150 . The area of lower or no intensity in the center of the annular or ring distribution may be referred to as the dark spot of Arago in a well-corrected optic.  
         [0026]    [0026]FIG. 6 reveals three focus positions of an illustrative optical element  130 . Detector position  270  shows an annular intensity profile of light  140  launched on detector surface  150 . The intensity is shown by coordinate I and the distance from optical axis  23  is shown by coordinate R. Detector position  280  shows a profile having the intensity of light  140  concentrated on or near optical axis  230 . Detector position  290  shows an annular intensity profile similar to the profile of detector position  270 . Either detector position  270  or  290  may be used to achieve an annular or ring distribution of light intensity on the detector surface  150 . It is contemplated that optical system  100  may incorporate either or both of the axicon and defocusing components to attaining an annular light pattern on the detector surface  150 .  
         [0027]    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.