Abstract:
A means for optically coupling a semiconductor laser to an optical fiber is disclosed. In one embodiment, a volume phase holographic element is disposed on a light-emitting surface of a semiconductor laser. The volume phase holographic element acts as an aberration-corrector for a lens that is disposed between the semiconductor laser and the optical fiber. In this way, an inexpensive lens that is not aberration free can be used. In some embodiments, the volume phase holographic element converts a Gaussian light beam emitted by the semiconductor laser into an annular beam that is more suitable for long distance transmission in multimode fibers.

Description:
The present application claims priority to, under 35 U.S.C. 119(e), U.S. Provisional Patent Application bearing Ser. No. 60/357,074, filed Feb. 12, 2002, which is incorporated herein by reference. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention relates generally to optoelectronic devices. More particularly, the present invention relates to using volume phase holographic optics in an optoelectronic device. 
     BACKGROUND OF THE INVENTION 
     A transmitter optical subassembly (TOSA) is an important component of an optoelectronic transmitter or transceiver. A typical TOSA includes a semiconductor laser, a lens and a receptacle into which an optical fiber can be inserted easily. The lens, which is for converging light beams to the end of the optical fiber, generally needs to be a small aberration-free lens with high numerical aperture. Small aberration-free lenses with high numerical aperture are difficult to fabricate and costly. Sometimes, they have to be made of sapphire and constitute a significant portion of the manufacturing cost of optoelectronic transmitters and tranceivers. 
     In a typical optoelectronic transmitter or transceiver, the semiconductor laser emits light beams that have a substantially Gaussian intensity distribution, sometimes called a Gaussian profile. Such light beams, however, suffer significant intermodal dispersion. Thus, they cannot be transmitted for very long distances in multimode fibers. 
     Accordingly, there exists a need for an inexpensive means for optically coupling a semiconductor laser to an optical fiber. There also exists a need for a means for optically coupling a semiconductor laser to a multimode optical fiber. 
     SUMMARY OF THE INVENTION 
     The present invention provides a means for optically coupling a semiconductor laser to an optical fiber. According to one embodiment of the invention, a layer of photo-sensitive emulsion is disposed onto a light-emitting surface of the semiconductor laser. Then, hologram developing processes are applied to the emulsion to create a volume phase holographic element. When the semiconductor laser is placed within a transmitter optical subassembly (TOSA), which includes a lens for converging emitted light beams to an end of an optical fiber, the volume phase holographic element compensates for aberration of the lens. Because the volume phase holographic element can reduce the aberration, the lens itself does not have to be aberration-free. Thus, inexpensive non-aberration free lenses can be used in TOSAs. 
     In one embodiment, the volume phase holographic element is formed by exposing the photo-sensitive emulsion (disposed on a light-emitting surface of the semiconductor laser) to two or more interfering light beams. In another embodiment, the semiconductor laser is used as a light source itself, and the light beams that it emits are used to form an interference pattern in the emulsion. In yet another embodiment, the emulsion is illuminated by a laser through a phase mask. After interference patterns are formed in the emulsion, various hologram developing processes including both wet and dry processes can be used to form a volume phase hologram in the emulsion. 
     In one embodiment, volume phase holograms are formed in situ within a TOSA. That is, a TOSA containing a semiconductor laser, a lens and an optical fiber is first assembled. Then, the volume phase holograms are formed within the assembled TOSA. In particular, the semiconductor laser is used as a light source, and light emitted by the semiconductor laser and a reflected version of the emitted light together form the interference patterns in the emulsion. In another embodiment, volume phase holograms are formed in situ within a TOSA that does not have a lens for converging the emitted light onto the optical fiber. 
     In yet another embodiment, the volume phase hologram converts a Gaussian light beam emitted by the semiconductor laser into an annular beam. The annular beam is suitable for multimode fiber because this class of beams suffers less intermodal dispersion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present invention will be more readily apparent from the following description and appended claims when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a cross-sectional view of a Transmitter Optical SubAssembly (TOSA) according to an embodiment of the invention; 
         FIG. 2  depicts a top view of a semiconductor wafer with a layer of photo-sensitive emulsion disposed thereon in accordance with an embodiment of the present invention; 
         FIG. 3  depicts a side cross-sectional view of a portion of the wafer of  FIG. 2  undergoing a step of the hologram forming process according to an embodiment of the present invention; 
         FIG. 4  illustrates a process of forming a volume phase hologram according to another embodiment of the present invention; 
         FIG. 5  depicts a cross sectional view of a TOSA according to another embodiment of the present invention; 
         FIG. 6  illustrates a process of forming volume phase holograms according to yet another embodiment of the present invention; and 
         FIG. 7  depicts a cross-sectional view of a TOSA according to yet another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described. It will be appreciated that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     In furtherance of an embodiment of the present invention,  FIG. 1  depicts a cross-sectional view of a Transmitter Optical SubAssembly (TOSA)  100 , which may be part of a optoelectronic transmitter or transceiver. As shown, the TOSA  100  includes a diode mount structure  110  and an optical fiber receptacle structure  120 . Mounted on the diode mount structure  110  is a transistor outline (TO) laser package  115 , which has a semiconductor laser  116  (e.g., an edge emitter, or a Vertical Cavity Surface Emitting Laser (VCSEL)) and a window aligned between the semiconductor laser  116  and the optical fiber receptacle structure  120 . The optical fiber receptacle structure  120  includes a fiber aligning sleeve  196  and a fiber stop  192  for receiving and aligning an optical fiber  124  with the laser package  115 . In the embodiment illustrated in  FIG. 1 , the optical fiber receptacle structure  120  includes a lens  122  that focuses light emitted from the laser package  115  onto an end of the optical fiber  124 . 
     With reference still to  FIG. 1 , the lens  122  is not an aberration-free lens. Aberration of the lens  122  is compensated or reduced by a volume phase holographic element  140  that is disposed on a light-emitting surface of the laser diode  116 . In particular, the volume phase holographic element  140  is a layer of material with volume phase holograms formed therein that redirects light wavefronts emerging from the laser  116 . By redirecting the light in much the same way as would a correcting lens, aberrations in the lens  122  are at least partially compensated. 
     In another embodiment, the volume phase holographic element has volume phase holograms formed therein that reduce the vertex angle of the largest cone of meridional rays emitted by the semiconductor laser  116 . In  FIG. 5 , which depicts a cross section of a TOSA  500 , the vertex angle of largest cone of meridional rays emitted by the semiconductor laser  116  without the volume phase holographic element  540  is denoted as α, and the vertex angle of the largest cone of meridional rays emitted with the volume phase holographic element  540  present is denoted as β. In other words, the volume phase holographic element  540  acts as a lens that limits the vertex angle of the emitted light. This will also reduce the aberration of the lens. An additional benefit of reducing the vertex angle of the emitted light is that a lens with a lower numerical aperture can be used. 
     Attention now turns to processes for fabricating the volume phase holographic elements. In one embodiment, volume phase holographic elements are formed on a wafer of semiconductor laser diodes. Specifically, photo-sensitive emulsions are disposed on the wafer (e.g., deposited and then distributed by spinning the wafer) of semiconductor lasers. The emulsions are then exposed to a laser interference pattern. The emulsions are subsequently developed using a suitable one of the many well known hologram developing processes. 
     Some photo-sensitive emulsions suitable for purposes the present invention include, but are not limited to, the following materials:
         Dichromated gelatin;   Photopolymers, which are compositions based on polymers/oligomers/monomers that can be selectively polymerized and/or crosslinked upon selective exposure by light radiation;   Lithium Niobate (LiNdO 3 ); and   Glass doped with Germanium.       

       FIGS. 2 and 3  illustrate an example process of forming a volume phase hologram in photo-sensitive emulsions.  FIG. 2  depicts a top view of a wafer  200  of semiconductor laser diodes  212 , and  FIG. 3  depicts a cross-sectional view of a portion of the wafer  200 . As shown in  FIG. 3 , a layer of photo-sensitive emulsion  214  is disposed and patterned on the semiconductor laser die  212 . Also shown in  FIG. 3  are two light beams  301  and  302 . The first light beam  301  and the second light beam  302  form an interference pattern in the emulsion layer  214 . The first light beam  301  is preferably a “data beam” that contains information about the aberations in the system to be compensated. The second light beam  302  is preferably a reference beam that interferes with the data beam to form a holographic image in the emulstion layer  214 . After the interference pattern is formed, hologram developing processes are applied to the emulsion layer  214 . Thereafter, individual semiconductor laser dies can be separated. 
       FIG. 4  illustrates a process of forming a volume phase hologram according to another embodiment of the present invention. In this embodiment, a phase mask  216  is disposed and patterned on the emulsion layer  214 . The phase mask has regions of differing indexes of refraction. The phase mask  216  diffracts an incoming light beam  303  into +1 and −1 orders. The +1 and −1 orders form an interference pattern in the emulsion  214 . The interference pattern thus formed is sometimes known as Bragg Gratings. 
     According to another embodiment of the present invention, volume phase holograms are formed in situ in a TOSA. In this embodiment, which is shown in  FIG. 6 , a volume phase hologram is formed in a TOSA  600  by exposing the emulsion to an emitted light beam and a second light beam that is sent through the optical fiber  124 . The emitted light beam and the second light beam form interference patterns in the emulsion layer  610 . Once the interference patterns are formed, the TOSA  600  may be dissembled and then reassembled such that hologram-developing processes can be applied. The light beam that is sent through the optical fiber  124 , can be a reflected version of the light beam emitted by the laser (e.g., by transmitting light from the emitted light beam down the optical fiber to a reflector and then back through the optical fiber and through the end of the optical fiber facing the laser diode). 
     Note that, in  FIG. 6 , a lens  122  is disposed between the semiconductor laser  116  and the optical fiber  124  when the hologram is formed. The volume phase hologram acts as a corrector for the lens  122  and compensates for or reduces the aberration of the lens  122 . In other embodiments the volume phase holographic element is formed in situ in the TOSA without the lens  122 . 
       FIG. 7  depicts a cross-sectional view of a TOSA  700  according to yet another embodiment of the present invention. The laser diode  116  emits a light beam that has a substantially Gaussian intensity distribution, sometimes called a Gaussian profile. As shown, the volume phase holographic element  710  converts the Gaussian light beam emitted by laser diode  116  into an annular beam  720 . The annular beam  720  is particularly suitable for a multimode fiber  724  because this class of beams suffers less intermodal dispersion than beams that have a Gaussian profile. As a result, longer lengths of multimode fiber can be used. Volume phase holographic elements that can produce annular beams from Gaussian beams are produced with processes similar to those described above. 
     While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art having the benefit of this disclosure without departing from the inventive concepts described herein.