Abstract:
A lens having a reflective surface, and systems that use such a lens. The lens includes a transmissive part for passing a portion of an incident light beam, and a reflective part for reflecting a portion of the incident light beam, and the reflective part is preferably substantially non-transmissive. Such a lens may be particularly suitable in systems that include a back monitor photo detector that is used for sampling and controlling the output power of a light source.

Description:
BACKGROUND 
     Lenses are commonly used in optical systems to direct and/or reconfigure light. In data communication systems, for example, lenses are used to direct and/or reconfigure light provided by a light source to a detector, optical fiber or some other destination. In many cases, the light source is provided in a light source package, and the lens is provided outside of the light source package. 
     To help stabilize the light beam provided by a light source, many light source packages include both a light source and a back monitor photo detector. The back monitor photo detector typically samples a portion of the light beam and provides a signal indicative of the amount of light detected by the signal path photo detector. A controller receives a signal from the back monitor photo detector, and adjusts the power of the light source to a desired, often constant, level. This can be beneficial as some electrical/optical parameters of some light sources, such as lasers, can vary due to effects such as manufacturing tolerance, temperature and aging. As such, control of the power output of light sources can enhance the performance of systems that use the light sources. It is advantageous to have a constant ratio between the response of the back monitor photo detector and the signal path photo detector. 
     In many cases, the back monitor photo detector and the light source are provided adjacent to one another in a common light source package. A flat tilted window, which typically includes a partially reflective coating, is often provided above the back monitor photo detector and the light source and reflects a portion of the light beam from the light source to the back monitor photo detector. A controller receives a signal from the back monitor photo detector, and provides a control signal to the light source to stabilize the output power of the light source over a range of operating conditions. 
     In many systems, a lens is provided outside of the light source package to help direct the light beam to a desired destination such as an optical detector, optical fiber, or some other destination. The lens can, for example, focus the light beam onto a detector, an input facet of an optical fiber, or some other desired destination. 
     Thus, in many optical systems, both a partially reflective window and a separate lens are provided in the path of the light beam. Having to manufacture and mount both of these separate components can increase the cost of the system. In addition, and in some application, there is insufficient room between the light source and the desired destination to accommodate both a partially reflective window and a separate lens. 
     What would be desirable, therefore, is a lens that includes a reflective surface. In some applications, such a lens could replace both the partially reflective window and the lens, thereby providing significant cost savings. In addition, the complexity associated with mounting both the partially reflective window and the lens could be reduced, and the size and/or spacing requirements between the light source and the desired destination may be reduced. All of these may result in more desirable optical systems. 
     SUMMARY 
     Generally, the present invention relates to a lens having a reflective surface, as well as systems that use such lenses. More specifically, the present invention relates to a lens that includes a transmissive part for passing a portion of an incident light beam, and a reflective part for reflecting a portion of the incident light beam. The reflective part is preferably substantially non-transmissive. 
     In one illustrative embodiment, the lens includes a first lens surface and an opposing second lens surface, and the reflective part covers less than half of the surface area of the first lens surface. The reflective part may cover significantly less than half of the surface area of the first lens surface, such as less than 25% or 10% of the surface area of the first lens surface. Rather than specifying a surface area, it is contemplated that the reflective part may reflect less than 50%, and sometimes less than 25%, and sometimes less than 10% of the optical power of the light beam that is incident on the first lens surface. 
     In some embodiments, the reflective part is adapted to focus the reflected light on a predetermined area or location, such as the sensitive area of a back monitor photo detector. By focusing the reflected light, the size of the back monitor photo detector may be reduced. In some cases, the first lens surface is convex, and the reflective part is a concave surface inset into the convex surface, but this is not required in all embodiments. The perimeter of the reflective part may have any desired shape, such as rectangular, circular, elliptical, annular, etc. In some embodiments, the lens is molded using a substantially transparent material, with the reflective part having a reflective coating applied thereto. 
     In some cases, the numerical aperture of the light source may change under different operating conditions. As such, it is contemplated that that the reflective part may be configured to reflect a relatively constant percentage of the output power of the light beam over a range of numerical apertures. In some illustrative embodiments, this may be accomplished by having the reflective part of the lens extend from at or near the center of the illumination beam pattern on the lens to at or near an expected outer perimeter of the illumination pattern. Thus, regardless of the numerical aperture of the light source, a relatively constant percentage of the output power of the light beam may be reflected. 
     The present invention contemplates that the above lens may be used in a wide variety of applications including telecommunication, computer, control, sensor, manufacturing, and/or any other suitable application. In some applications, a light beam having a controlled output power may be desirable. To produce a controlled light beam, a light source, a photo detector and a lens having a reflective part may be provided. In one illustrative embodiment, the light source and photo detector may be positioned adjacent to one another, and the lens may be spaced from both the light source and photo detector. The lens may be in the light beam path of the light source. In one illustrative embodiment, the lens passes most of the light beam to a desired destination such as a photo detector, optical fiber, or any other desired destination. 
     The reflective part of the lens reflects a portion of the light beam back to a photo detector. A controller, which is coupled to the photo detector and the light source, receives a signal from the photo detector that is indicative of the amount of light detected by the photo detector. The controller provides a control signal to the light source that adjusts the power of the light source such that the signal from the photo detector (and thus the power of the light beam) is relatively constant. 
     The fraction of light that is reflected to the photo detector may be determined by the size, shape and position of the reflective part. The shape may be configured to couple a relatively constant fraction of the light from the light source to the photo detector, even though the numerical aperture of the light source may change due to different operating conditions. This may provide a substantially constant tracking ratio. In addition, and as noted above, the reflective part may be shaped to provide a suitable focal length to focus the reflected light onto the photo detector. The ability to focus the reflected light onto the photo detector may allow for a smaller photo detector, which may reduce the cost of the optical system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an optical system according to an illustrative embodiment of the present invention; 
         FIG. 2  is a schematic view including a ray trace of the reflected light in the illustrative embodiment of  FIG. 1 ; 
         FIGS. 3 and 4  show alternative illustrative embodiments of a reflective surface of a lens in accordance with the present invention; 
         FIGS. 5 and 6  are illustrative off-axis ray tracing diagrams showing the reflected light path for the reflective surfaces of  FIGS. 3 and 4 , respectively; 
         FIGS. 7 and 8  are graphs showing the percent of signal on the photo detector versus angular divergence (NA) for the reflective surfaces of  FIGS. 3 and 4 , respectively; 
         FIGS. 9 and 10  are image spots on a back-monitor photo diode for the reflective surfaces of  FIGS. 3 and 4 , respectively; 
         FIGS. 11 and 12  are graphs showing the single mode fiber coupling efficiency versus angular divergence (NA) for the reflective surfaces of  FIGS. 3 and 4 , respectively; 
         FIGS. 13 and 14  are graphs showing the single mode fiber coupling efficiency versus temperature in degrees Celsius for the reflective surfaces of  FIGS. 3 and 4 , respectively; 
         FIG. 15  is a graph showing the tracking ratios versus angular divergence (NA) for the reflective surfaces of  FIGS. 3 and 4 ; 
         FIG. 16  is a schematic view of an alternative optical system according to an example embodiment of the present invention; 
         FIG. 17  is a schematic view of another alternative optical system according to an example embodiment of the present invention; 
         FIG. 18  is a schematic view of yet another alternative optical system according to an example embodiment of the present invention; 
         FIG. 19  is a schematic view including a ray trace of the reflected light in the example embodiment of  FIG. 18 ; 
         FIGS. 20 and 21  are schematic views of example embodiments of reflective surfaces of a lens according to example embodiments of the present invention; 
         FIG. 22  is a schematic view including a ray trace of the reflected light of one example embodiment of the present invention. 
         FIG. 23  is a schematic view including a ray trace of the reflected light in the example embodiment of  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION 
     Throughout the Description, like components will be identified by like reference numerals and letters.  FIGS. 1 and 2  illustrate an optical system  100  according to one illustrative embodiment of the present invention. The system  100  includes a light source  110 , an optical element  120 , an optical fiber  130 , a back monitor photo detector  140 , and a controller  142 . In the illustrative embodiment, the light source  110  and the photo detector  140  are positioned adjacent to one another (see  FIG. 2 ), and the lens  120  is spaced above both the light source  110  and photo detector  140  to receive the light beam of the light source  110 . In the illustrative embodiment, the back monitor photo detector  140  has a sensitive area of about 80 um, and has a closest edge laterally spaced from the light source  110  by about 300 um. 
     As can be seen, the illustrative optical element  120  includes an integrated reflective surface  125 . Emitted light (EL) from the light source  110  strikes the optical element  120  in a light pattern, and most of the emitted light (EL) passes through the optical element  120  as transmitted light (TL) to optical fiber  130 . In the illustrative embodiment, the optical element  120  focuses the transmitted light (TL) onto the input facet of the optical fiber  130 . While an optical fiber  130  is shown, it is contemplated that the optical element  120  may be used to direct the transmitted light (TL) to any desired destination, depending on the application. 
     The optical element  120  may be an integrated micro lens similar to that described in U.S. patent application Ser. No. 10/622,042, entitled “OPTICAL COUPLING SYSTEM”, filed Jul. 17, 2003, owned by the assignee of the present application, and which is herein incorporated by reference. The integrated micro lens may be made of a suitable plastic such as Ultm®, which is a General Electric Company plastic. In some cases, the lens may be made by injection molding. In the illustrative embodiment, the lens is a plano-convex lens and has a length of about 828 um and is placed about 300 um from the light source  110 , which in some cases may be a Vertical Cavity Surface Emitting Laser (VCSEL). The optical fiber  130  may be, for example, a single mode optical fiber that is butt coupled to the flat surface  122  of the plano-convex optical element  120 . To reduce back reflection caused by the interface between the optical element  120  and the optical fiber  130 , the index of refraction of the material used for the optical element  120  may be substantially matched to the index of refraction of the material used for the core of the optical fiber  130 . 
     In the illustrative embodiment, the optical element  120  includes an input surface  124  and an output surface  122 . The input surface  124  is shown generally as a convex shape and the output surface  122  is shown generally as a flat surface. This is only one illustrative embodiment, however. In other illustrative embodiments, the shape of the input surface  124  and the output surface  122  may be: (1) both concave; (2) both convex; (3) one convex and the other concave; (4) one flat and the other concave; (5) one flat and the other convex; or (5) or any other shape as desired, depending on the application. 
     In the illustrative embodiment, the input surface  124  includes a reflective surface  125 . The optical element  120  may be a molded lens, and the reflective surface  125  may be molded into the surface of the molded lens. To make the reflective surface  125  reflective, a reflective coating may be applied to the reflective surface  125 . In one illustrative embodiment, the reflective surface  125  is coated with a noble metal such as gold. Other coatings may include, for example, aluminum or any other reflective coating. The reflective surface  125  is preferably substantially non-transparent to the emitted light (ET), while the remaining portion of the input surface  124  is substantially transparent to the emitted light (ET). 
     In the illustrative embodiment, the reflective surface  125  covers less than half of the input surface of the optical element  120 , and directs a substantially constant fraction of the output power from the light source  110  to a photo detector  140  (see  FIG. 2 ). In some cases, the reflective surface  125  covers less than 25% of the input surface  124  of the optical element  120 , and in other cases, less than 10%. While the reflective surface is shown on the input surface  124  of the optical element  120 , it is contemplated that the reflective surface  125  may be on the output surface  122  of the optical element  120  (see the alternative optical element  120 ′ of the alternative optical system  100 ′ of  FIG. 16 ), or on both the input surface  124  and output surface  122  (see the alternative optical element  120 ″ of the alternative optical system  100 ″ of  FIG. 17 ), as desired. In addition, it is contemplated that either or both of the input surface  124  or output surface  122  may have more than one reflective surface  125  (see the alternative optical element  120 ′″ of the alternative optical system  100 ′″ of  FIGS. 18-21 ), if desired. When more than one reflective surface  125  is provided on the optical element  120 , selected reflective surfaces may be adapted to reflect the light to either a common location (see the alternative optical element  120 ″″ of the alternative optical system  100 ″″ of  FIG. 22 ) or different locations (see the alternative optical system  100 ″″ of  FIG. 19  as well as the alternative optical element  120 ″″ of the alternative optical system  100 ″″ of  FIG. 23 ), as desired. 
     In some embodiments, the reflective surface  125  is adapted to focus the reflected light onto a predetermined spot or area, such as the sensitive area of a back monitor photo detector  140  (see  FIG. 2 ). By focusing the reflected light, the size of the back monitor photo detector  140  may be reduced. In some cases, the input surface  124  is convex, and the reflective surface  125  is a concave surface inset into the convex input surface  124 , but this is not required in all embodiments. The reflective surface  125  may have a perimeter of any desired shape including, for example, rectangular, circular, elliptical, annular, etc. 
     In some cases, the numerical aperture of the light source  110  may change under different operating conditions. For example, when the light source  110  is a Vertical Cavity Surface Emitting Laser (VCSEL), the numerical aperture may change as a function of output power, temperature, as well as other factors. To help reduce the dependence of the tracking ratio on the numerical aperture of the light source  110 , it is contemplated that that the reflective surface  125  may be configured to reflect a relatively constant percentage of the emitted light (EL) beam, over a predetermined range of numerical apertures. This may be accomplished by, for example, having the reflective surface  125  of the optical element  120  extend from at or near a central axis  128  of the emitted light (EL) pattern on the input surface  124  of the optical element  120  to at or near an expected outer perimeter  129  of the emitted light (EL) pattern. This works especially well if the emitted light (EL) pattern is symmetrical in at least one dimension about the central axis  128 . 
     A controller  142  may also be provided. In the illustrative embodiment, the controller  142  may be coupled to the light source  110  and the back monitor photo detector  140 . The controller  142  receives a signal  144  from the back monitor photo detector  140  that is indicative of the amount of light detected by the back monitor photo detector  140 . The controller  142  then provides a control signal  146  to the light source  110  that adjusts the power of the light source  110  such that the signal  144  from the back monitor photo detector  140 , and thus the power of the emitted light (EL), is relatively constant. 
     The fraction of light that is reflected to the back monitor photo detector  140  may be determined by the size, shape and position of the reflective surface  125 . The shape and position may be configured to couple a relatively constant fraction of the emitted light (EL) to the back monitor photo detector  140 , even when the numerical aperture of the light source  110  changes due to different operating conditions, thereby providing a substantially constant tracking ratio. In addition, and as noted above, the reflective surface  125  may be shaped to provide a suitable focal length to focus the reflected light (RL) onto the back monitor photo detector  140 . The ability of the reflective surface  125  to focus the reflected light (RL) onto the back monitor photo detector  140  may allow for a smaller back monitor photo detector  140 , which may reduce the size and cost of the system. 
       FIGS. 3 and 4  show alternative illustrative embodiments of reflective surfaces for an optical element  120  in accordance with the present invention.  FIG. 3  shows a rectangular shaped reflective surface  225 , and  FIG. 4  shows an annular shaped reflective surface  325 . In the illustrative embodiment, the reflective surfaces  225  and  325  are centered on the X axis and offset on the Y axis, however, this is not required in all embodiments.  FIGS. 5 and 6  illustrate the different patterns of reflected light (RL) created by the rectangular  225  and annular  325  shaped reflective surfaces of  FIGS. 3 and 4 , respectively. 
     Referring to  FIGS. 5 and 6 , the emitted light (EL) in the illustrative embodiment engages the optical element  120  in a pattern that is symmetrical about the central axis of the optical element  120 .  FIGS. 5 and 6  are off-axis views. The optical element  120  passes a desired amount (e.g. a majority) of the emitted light (EL) light to the optical fiber  130  (see  FIG. 1 ), while the corresponding reflective surface  225  or  325 ) is adapted to reflect a desired amount of the emitted light (EL) to back monitor photo detector  140  (see  FIG. 2 ). Possible shapes for the reflective surfaces include, but are in no way limited to, polygonal, circular, elliptical, annular, etc. 
     As can be seen in  FIGS. 5 and 6 , the reflective surfaces  225  and  325  may extend from at or near a central axis (see  128 ′ and  128 ″) of the emitted light (EL) beam pattern to at or near an expected outer perimeter of the emitted light (EL) beam pattern. This may provide a relatively constant percentage of the output power of the emitted light (EL) beam to the back monitor photo detector  140  over a range of numerical apertures of the light source  110 . 
       FIGS. 7 and 8  are graphs showing the percent of signal on the back monitor photo detector  140  versus angular divergence (NA) of the light source for the reflective surfaces of  FIGS. 3 and 4 , respectively. As can be seen, the shape and placement of the reflective surfaces  225  and  325  shown in  FIGS. 5 and 6 , respectively, cause a relatively constant percent of emitted light (EL) to be reflected back to the back monitor photo detector  140  over a range of divergence angles (or numerical apertures) of a VCSEL light source  110 .  FIGS. 9 and 10  show image spots on the back monitor photo detector  140  using the reflective surfaces  225  and  325  of  FIGS. 3 and 4 , respectively. In the illustrative embodiment, the black squares shown in  FIGS. 9 and 10  correspond to an 80 um square sensitive area of an illustrative back monitor photo detector. 
       FIGS. 11 and 12  are graphs showing a single mode fiber coupling efficiency versus angular divergence (NA) of the light source for the reflective surfaces  225  and  325  of  FIGS. 3 and 4 , respectively. As can be seen, the rectangular  225  and annular  325  shaped reflective surfaces both provide a relatively constant coupling efficiency into a single mode fiber  130  over a range of divergence angles (or numerical apertures) of a VCSEL light source  110 . 
       FIGS. 13 and 14  are graphs showing the relatively constant single mode fiber coupling efficiency achieved over a relatively wide temperature range with the reflective surfaces  225  and  325  of  FIGS. 3 and 4 , respectively.  FIG. 15  is a graph showing the relatively constant tracking ratios achieved with the rectangular  225  and annular  325  reflective surfaces of  FIGS. 3 and 4 , respectively, over a range of divergence angles (or numerical apertures) of a VCSEL light source  110 . 
     In one illustrative example of the present invention, the photodetector  140  is a back monitor photodiode, and the reflective surface  125  couples a specified minimum amount of power to the back monitor photodiode  140 . In the particular embodiment shown in  FIGS. 1 and 2 , the laser light source  110  may be a VCSEL positioned 300 um in front of optical element  120 , which is 828 um long and abuts single mode optical fiber  130 . The reflective surface  125  is inset into the input surface of the optical element  120 , and has a height of about 100 um, although other dimensions may be used. The back monitor photodiode  140  is positioned 300 um in a lateral direction from the VCSEL, and the sensitive area of the photodiode  140  is 80 um square. 
     In the illustrative example, the reflective surface  125  focuses sufficient light to induce a photocurrent of at least 50 uA in the photodiode  140  when the VCSEL  110  is coupled to a single mode fiber  130  and the laser  110  is operating at the desired power level. In one embodiment, the reflective surface  125  is adapted to reflect about 23% of the laser power to the photodetector  140 . With a back monitor photodiode responsivity of 0.8 A/W, the back monitor current is about 94 uA for 0.5 mW of laser power coupled to the optical fiber. 
     The tracking ratio is the ratio of laser power that is coupled to the optical fiber  130  versus the laser power reflected to the back monitor photodiode  140 . It is generally desired to have an essentially constant tracking ratio over temperature and process variation. The reflective surface  125  may help provide such a tracking ratio. Different process runs produce VCSELs with slightly different numerical apertures (NA). The numerical aperture (NA) of a given VCSEL is also a function of temperature, output power, and other factors. The shape and location of the reflective surface  125  may be selected to provide a constant tracking ratio over a given range of numerical aperture (NA) values. 
     In one embodiment, a VCSEL  110  and back monitor photodiode  140  require a minimum spacing between the two components. Additionally, in order to reduce the cost of the photodiode, the size of the photodiode chip may be minimized. In such a system, the size and shape of the reflective surface  125  may be configured to focus the reflected light onto the photodiode  140 . In one example, such as is illustrated in  FIG. 2 , the distance between the VCSEL lasing area and the center of the photodiode active area may be about 340 um, while other distances may be used. 
     The surface area of the optical element  120  that is obscured by the reflective surface  125  changes the beam shape at the input surface of the optical fiber  130 . This change in beam shape introduces an additional loss of coupling due to the mismatch between the modified VCSEL mode and the mode of the single mode fiber. For a particular application, the area of the reflective surface  125  should be minimized so that this beam shape distortion and additional coupling loss is minimized. For the design shown in  FIGS. 1 and 2 , about 69% of the VCSEL laser power is coupled to the fiber and about 16.5% is coupled to the back monitor photodiode, thus the additional coupling loss due to beam shape distortion and lens attenuation is about 14.5%. 
     Although the present invention has been described with particular detail and illustrated with significant specificity, it should be understood that alternative embodiments of the present invention are also within its scope.