Abstract:
An optical apparatus and method for coupling output light from a light source to an optical waveguide utilizes an optical turning element with integrated lenses to reduce the difficulty of precisely aligning multiple components of the optical apparatus.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to fiber optics, and more particularly to an optical apparatus for coupling output light from a light source to an optical waveguide. 
     BACKGROUND OF THE INVENTION 
     Semiconductor lasers are commonly used as optical signal generators in fiber optic communications systems. The optical signals generated by a semiconductor laser are transmitted through an optical waveguide, such as an optical fiber, to their destination. An important aspect of an optoelectronic component employing a semiconductor laser is the coupling of the output light from the laser to an optical fiber. Typically, a semiconductor laser is packaged with a lens in an optical subassembly (OSA) to focus the output light from the laser into an exposed end of an optical fiber to ensure good coupling efficiency. Thus, the semiconductor laser and the lens must be properly aligned so that the output light from the laser is effectively transmitted to the optical fiber. 
     There are two distinct types of semiconductor lasers used in fiber optic communications systems. The first type of semiconductor lasers includes edge-emitting lasers, such as Fabry-Perot (FP), Distributed Feedback (DFB) lasers and Distributed Bragg Reflector (DBR) lasers. The other type of semiconductor lasers includes vertical-emitting lasers, such as Vertical Cavity Surface Emitting Lasers (VCSELs). The edge-emitting and vertical-emitting lasers are usually packaged so that the output light from an edge-emitting laser or a vertical-emitting laser propagates along a straight optical path from the laser through a lens and to the fiber (or optical waveguide). Thus, OSAs with vertical-emitting lasers cannot be readily interchanged with OSAs with edge-emitting lasers since the propagating direction of the output light depends on the type of lasers employed in the OSAs. However, OSAs with edge-emitting lasers have been developed that can turn the output light from the original horizontal direction to the vertical direction, which allows these OSAs to be interchanged with OSAs with vertical-emitting lasers. 
     A conventional OSA with an edge-emitting laser that can transmit the output light in the vertical direction includes a 45-degree mirror structure and a lens. The edge-emitting laser and the mirror are mounted on a substrate such that the output light from the laser is reflected off the reflective surface of the mirror structure. Thus, the output light from the laser is redirected from the original horizontal direction to the vertical direction. The lens is attached to the mirror structure to focus the redirected output light propagating in the direction of an optical fiber. 
     A concern with conventional OSAs with edge-emitting lasers that can vertically transmit output light is that precise alignment of various elements of the OSAs, such as the laser, the mirror structure and the lens, is difficult to achieve for proper coupling of the laser to an optical fiber. 
     In view of this concern, there is a need for an OSA with an edge-emitting laser that reduces the difficulty of precisely aligning the various components of the OSA. 
     SUMMARY OF THE INVENTION 
     An optical apparatus and method for coupling output light from a light source to an optical waveguide utilizes an optical turning element with integrated lenses to reduce the difficulty of precisely aligning multiple components of the optical apparatus. The optical turning element includes a reflecting surface to reflect output lights from an array of light sources, such as an array of edge-emitting lasers, so that the output lights can be redirected from an original direction to a modified direction. The optical turning element further includes a receiving surface that may refract the output lights from the array of light sources for redirection. The receiving and reflecting surfaces may be formed using a common silicon fabrication process. 
     An apparatus in accordance with an embodiment of the invention includes a substrate, an edge-emitting light source and an optical element. The edge-emitting light source is positioned on the substrate, along with the optical element. The edge-emitting light source is configured to generate an output light along an original direction. The optical element includes a first surface to receive the output light from the edge-emitting light source such that the output light is transmitted into the optical element. The optical element further includes a second surface to internally reflect the output light to redirect the output light from the original direction to a modified direction. The optical element further includes a third surface with a lens formed on the third surface to optically manipulate the output light as the output light is transmitted through the third surface. 
     A method for coupling output light from a light source to a waveguide includes the steps of transmitting the output light from the light source propagating along an original direction into an optical element, redirecting the output light from the original direction to a modified direction within the optical element, and emitting the output light propagating along the modified direction out of the optical element toward the waveguide. The step of emitting the output light includes optically manipulating the output light into the waveguide. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a first perspective view of an optical subassembly (OSA) in accordance with exemplary embodiment of the present invention. 
         FIG. 2  is a second perspective view of the OSA of FIG.  1 . 
         FIG. 3  is a cross-sectional view of the OSA of FIG.  1 . 
         FIG. 4  is a process flow diagram of a method of coupling output light from an edge-emitting laser to an optical waveguide in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 1 and 2 , an optical subassembly (OSA)  100  in accordance with an exemplary embodiment of the invention is shown. The OSA  100  includes an array  102  of edge-emitting lasers  104 A,  104 B,  104 C and  104 D and an optical turning element  106 , which are attached to a sub-mount substrate  108 . The optical turning element  106  is used to redirect output lights from the edge-emitting lasers from a lateral direction to a vertical direction toward optical waveguides, which may be optical fibers  110 A,  110 B,  110 C and  110 D, as shown in FIG.  1 . The lateral direction is defined herein as the direction parallel to the upper surface of the sub-mount substrate, e.g., the X direction shown in FIG.  1 . The vertical direction is defined herein as the direction perpendicular to the lateral direction, i.e., the Z direction shown in FIG.  1 . Thus, the OSA  100  is compatible to a conventional OSA with vertical-emitting lasers, and consequently, can be interchanged with such conventional OSA. 
     The array  102  of edge-emitting lasers  104 A,  104 B,  104 C and  104 D of the OSA  100  is configured to generate modulated output lights, which are transmitted through the optical fibers  110 A,  110 B,  110 C and  110 D as optical signals. The array of edge-emitting lasers is illustrated in  FIG. 1  as including four edge-emitting lasers. Thus, the illustrated array of edge-emitting lasers is a four-channel laser array. However, the array of edge-emitting lasers may include fewer or more edge-emitting lasers. As an example, the laser array may include twelve edge-emitting lasers. In the exemplary embodiment, the edge-emitting lasers included in the array are Fabry-Perot lasers. However, the edge-emitting lasers may be other type of edge-emitting lasers, such as Distributed Bragg Reflector (DBR) lasers. The edge-emitting lasers are driven by circuitry (not shown), which is connected to the lasers through electrical connections  112 . The array of edge-emitting lasers is attached to the sub-mount  108 , which may be composed of silicon, adjacent to the optical turning element  106 . In other embodiments, the OSA  100  may include a single edge-emitting laser instead of the array of edge-emitting lasers. 
     The optical turning element  106  of the OSA  100  is configured to redirect the output lights of the edge-emitting lasers  104 A,  104 B,  104 C and  104 D from the lateral direction, e.g., the X direction, to the vertical direction, i.e., the Z direction, by reflecting and/or refracting the output lights. Thus, the optical turning element can turn the optical axis of the output lights from the edge-emitting lasers by approximately ninety degrees. In addition, the optical turning element is configured to focus the output light from each of the edge-emitting lasers so that the output lights from the edge-emitting lasers are properly transmitted to the respective optical fibers  110 A,  110 B,  110 C and  110 D. The optical turning element includes a receiving surface  114  (shown in FIG.  1 ), a reflecting surface  116  (shown in  FIG. 2 ) and a focusing surface  118  (shown in both FIGS.  1  and  2 ). The receiving surface is the surface where the output lights from the edge-emitting lasers are transmitted into the optical turning element. As described below, the receiving surface may be angled to refract the output lights as the output lights are transmitted into the optical turning element. The reflecting surface functions as a mirror to reflect the output lights toward the optical fibers. The focusing surface is the surface from where the reflected output lights are emitted from the optical turning element. 
     As illustrated in  FIG. 3 , the receiving and reflecting surfaces  114  and  116  of the optical turning element  106  are angled with respect to the X axis.  FIG. 3  is a cross-sectional view of the OSA  100  across the edge-emitting laser  104 A, the optical turning element  106 , the sub-mount  108  and the optical fiber  110 A. The receiving and reflecting surfaces are angled such that each output light from the edge-emitting lasers  104 A,  104 B,  104 C and  104 D is refracted by the receiving surface and then reflected off the reflecting surface toward the respective optical fiber  110 A,  110 B,  110 C or  110 D, as illustrated in FIG.  3 . The optical turning element  106  is attached to the sub-mount  108  such that the receiving surface of the optical turning element is in close proximity to the light-emitting edges of the edge-emitting lasers  104 A,  104 B,  104 C and  104 D. 
     The focusing surface  118  of the optical turning element  106  is substantially parallel to the X axis. As stated above, the focusing surface is the surface from where the reflected output lights are emitted out of the element toward the optical fibers. The focusing surface includes lenses  120 A,  120 B,  120 C and  120 D, which are formed on the focusing surface. Thus, the lenses are structurally integrated into the optical turning element. These lenses of the optical turning element separately manipulate the output lights from the corresponding edge-emitting lasers  104 A,  104 B,  104 C and  104 D, which have been reflected by the reflecting surface  116 , into exposed ends of the respective optical fibers  110 A,  110 B,  110 C and  110 D. The surfaces of the lenses could be fabricated as etched, or cast refractive or diffractive surfaces. The lenses may be designed so that the is output lights emitted from the optical turning element propagate in a convergent, parallel or divergent manner. The lenses may also be designed so that one or more of the output lights propagate off-axis from the lenses. In the exemplary embodiment, the pitch of the lenses is 0.25 mm. However, the pitch of the lenses may be shorter or longer. As illustrated in  FIGS. 1 and 2 , the optical turning element includes four lenses, which correspond to the four edge-emitting lasers of the array  102 . Thus, the number of lenses included in the optical turning element depends on the number of edge-emitting lasers being used in the OSA  100 . Since the lenses are formed on the optical turning element along with the refractive and reflective surfaces, i.e., the receiving and reflecting surfaces  114  and  116 , the OSA  100  only requires alignment of two components (the laser array  102  and the optical turning element  106 ) rather than three components (a laser, a mirror structure and a lens), as is the case in some conventional OSAs with edge-emitting lasers that can vertically emit output light. 
     In the exemplary embodiment, the optical turning element  106  is made of silicon. However, the optical turning element can be made of other comparable transparent materials, such as injection-molded plastic, pressed glass, sapphire, III-V material, or II-VI material. In addition to being made of silicon, the optical turning element is fabricated such that the receiving and reflecting surfaces  114  and  116  are angled at approximately 49.5 and 59.9 degrees from the X axis, respectively, as illustrated in FIG.  3 . Thus, the receiving surface and the upper surface of the sub-mount  108  create a 49.5 degree angle. Similarly, the reflecting surface and the upper surface of the sub-mount create a 59.9 degree angle. In addition, the receiving surface and the reflecting surface create a 70.6 degree angle. The optical turning element with such receiving and reflecting surfaces can be fabricated by wet-etching a silicon wafer, during which etchant is selectively applied to the surface of the silicon wafer to remove portions of the wafer to form the angled receiving and reflecting surfaces. Due to the crystal alignment of the silicon wafer, the receiving and reflecting surfaces that create a 70.6 degree angle can be formed by anisotropically etching the silicon wafer. Therefore, in the exemplary embodiment, the configuration of the optical turning element allows the use of common silicon fabrication processing, i.e., a wet-etching process, to produce precise crystallographic surfaces, which serve as the receiving and reflecting surfaces of the optical turning element. Although the receiving and reflecting surfaces are angled at approximately 49.5 and 59.9 degrees from the X axis, respectively, in the exemplary embodiment, these surfaces can be orientated at different angles to refract and reflect the received lights toward the lenses  120 A,  120 B,  120 C and  120 D. However, the receiving and reflecting surfaces should form an angle that is approximately 70.6 degrees in order to take advantage of silicon wet-etching process. 
     In operation, electrical control signals are selectively applied to the edge-emitting lasers  104 A,  104 B,  104 C and  104 D of the array  102  via the electrical connections  112  to activate one or more lasers. Each activated edge-emitting laser then emits output light from its light-emitting edge toward the receiving surface  114  of the optical turning element  106  along the lateral direction. The output light travels through the receiving surface into the optical turning element toward the reflecting surface  116  of the element. In the exemplary embodiment, the output light is refracted by the receiving surface due to the angle of the receiving surface. The output light then reflects off the reflecting surface such that the optical axis of the output light is redirected from the original lateral direction to the vertical direction toward the focusing surface  118  of the element. Thus, the output lights is turned by approximately ninety degrees by the receiving and reflecting surfaces of the optical turning element. 
     The reflected output light then travels through one of the lenses  120 A,  120 B,  120 C and  120 D on the focusing surface  118  of the optical turning element  106 , which corresponds to the edge-emitting laser  104 A,  104 B,  104 C or  104 D from where the output light originated. Thus, the output light is emitted out of the optical turning element through the lens toward one of the optical fibers  110 A,  110 B,  110 C and  110 D that is aligned with the lens and the corresponding edge-emitting laser. The lens focuses the output light so that the light is efficiently transmitted into the optical fiber. In this fashion, the output light from each activated edge-emitting laser is transmitted into the optical fiber that is aligned with that edge-emitting laser. 
     The optical turning element  106  of the OSA  100  was analyzed using ray-trace analysis, which indicated that the optical turning element could be very efficient with proper anti-reflection coating. The ray-trace analysis indicated an overall optical efficiency of over ninety-five percent from the edge-emitting lasers  104 A,  104 B,  104 C or  104 D up to the optical fibers  110 A,  110 B,  110 C and  110 D. The optical efficiency does not include losses into the optical fibers or losses due to diffractive inefficiencies. 
     A method for coupling output light from an edge-emitting laser to an optical waveguide in accordance with an exemplary embodiment is described with reference to FIG.  4 . At step  402 , the output light is generated at the edge-emitting laser such that the output light is emitted from an edge of the laser in an original direction. Next, at step  404 , the output light is transmitted into an optical turning element through a receiving surface of the element. At step  404 , the output light is redirected from the original direction to a modified direction, which may be approximately perpendicular to the original direction. In the exemplary embodiment, the output light is redirected by both refracting and reflecting the output light using two surfaces of the optical turning element. Next, at step  406 , the redirected output light is emitted out of the optical turning element through a focusing surface of the optical turning element toward the optical waveguide, which may be an optical fiber. During step  406 , the redirected output light is also optically manipulated into the optical waveguide by a lens formed on the focusing surface. In other embodiments, additional output lights from additional edge-emitting lasers may be coupled to additional optical waveguides in the same manner using the same optical turning element. In these embodiments, the optical turning element includes additional lenses formed on the focusing surface to optically manipulate the additional output lights that are emitted out of the optical turning element. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.