Abstract:
An edge-emitting laser having an edge-emitting output and a surface-emitting output. The edge-emitting laser has a resonator having a high order grating, preferably a second order grating capable of producing an edge-emitting component and a surface-emitting component. A waveguide is also located in the resonator and a cap layer is located above the waveguide and grating. The cavity is tested by taking measurements from the surface-emitting component eliminating the need for taking measurements from the edge-emitting component which is time consuming and expensive.

Description:
FIELD OF THE INVENTION 
     This invention relates to multiple edge-emitting lasers manufactured on a single wafer and also relates to testing of the lasers while still located on the wafer. This is called “on-wafer” testing. 
     BACKGROUND OF THE INVENTION 
     Edge-emitting lasers are lasers that emit outputs in a horizontal direction through the edges of a laser. When manufacturing edge-emitting lasers, multiple laser cavities (resonators) are usually formed in parallel strips on the surface of a single wafer using a first order grating. Each strip is divided into multiple segments to form multiple coaxial lasers on the wafer so that each edge of the lasers can be exposed horizontally for testing. 
     On-wafer testing is critical for achieving low cost production of edge-emitting lasers or Distributed Feedback (DFB) lasers by avoiding the testing of each individual die. Assuming the cost of labor is $20 per hour, a worker can only spend minutes testing each individual device to achieve a goal of $20 per die. Any time spent on testing an individual device will add significant cost to the final product. However, if an entire wafer containing 25,000 dies could be automatically tested in even an hour, the cost per die will be dramatically reduced. 
     For the traditional edge-emitting laser fabrication process, laser testing has to be done after bar cleaving when the length of the laser cavity is determined. Bar cleaving is a process which breaks the wafer into multiple individual pieces defining the length of each laser cavity. 
     For telecommunications lasers such as a Distributed Feedback laser (DFB), the testing of the laser cannot be done until after the bar cleaving step. Therefore, a large portion of the manufacturing costs occurs during laser testing. 
     On-wafer testing will reduce the cost of production of edge-emitting lasers and more particularly DFB lasers due to the reduction of testing cost. 
     Conventional testing of each edge-emitting laser requires emitting and measuring the outputs emitted through the edges of the laser. However, this is not possible to do on a single wafer because the edges from which the outputs are emitted are too close to one another. Therefore, the wafer must be subjected to a cleaving process which breaks the wafer into multiple individual pieces, each individual piece being a single edge emitting laser. The output of each individual laser which is emitted through the edges of the laser is then tested for quality assurance. 
     SUMMARY OF THE INVENTION 
     The present invention improves upon the present testing of edge-emitting lasers by allowing for on-wafer testing of edge-emitting lasers located on a single wafer. On-wafer testing is an improvement over the current method of testing because it eliminates the cleaving process which breaks the wafer into multiple individual pieces. The cleaving process is eliminated by substituting a high order grating for the first order grating typically used in edge-emitting lasers. By substituting a high order grating for the typical first order grating, an output is not only emitted through the edges of the laser but outputs are also emitted through the surface of the laser. Measurements can therefore be easily taken from the surface of the wafer without cleaving or breaking the wafer into multiple pieces. 
     An edge-emitting laser of the present invention comprises a substrate; an insulating layer located on said substrate defining a first channel having insulating layers located on either side of said first channel; a high order grating layer located in said first channel wherein said high order grating layer has an order greater than a first order grating layer; a waveguide located on top of said grating layer; and a cap layer located on top of said waveguide and said insulating layers. 
     In an alternate embodiment, a component of an edge-emitting laser comprises a first resonator; a grating located in said first resonator having a grating order greater than one; a waveguide located in said first resonator; a cap layer located above said waveguide and said grating wherein said first resonator has an edge-emitting component being transmitted through said first resonator and a surface-emitting component being transmitted in a different direction than said edge-emitting component. 
     An edge-emitting laser of the present invention may further comprise a high order grating layer that is a second order grating layer located in a first channel and also comprises a second channel that is co-axial to said first channel. A cap layer may have an opening located above said waveguide and also comprises metal slabs located on either side of said opening. 
     A method for fabricating an edge-emitting laser cavity of the present invention comprises the steps of providing a substrate; forming an insulating layer on said substrate defining a first channel having insulating layers located on either side of said first channel; forming a high order grating layer in said first channel wherein said high order grating layer is greater than a first order grating layer; forming a waveguide over said grating layer; and forming a cap layer over said waveguide and said insulating layers. 
     In an alternate embodiment, a method for the fabrication of a component of an edge-emitting laser comprises the steps of forming a first resonator; forming a high order grating layer in said first resonator wherein said high order grating layer is of an order greater than one; forming a waveguide in said first resonator; and forming a cap layer over said waveguide and said first resonator. 
     The method of for the fabrication of an edge-emitting laser cavity in accordance with the present invention may further comprise forming said high order grating layer as a second order high order grating layer and also comprises forming a second channel being co-axial to said first channel. The method further comprises etching across said insulating layer and said first channel forming two or more channels and also comprises forming an opening in said cap layer above said waveguide. In addition, the method further comprises forming metal slabs on either side of said opening and also etching an opening above said waveguide in said cap layer. 
     A method for testing edge emitting-lasers located on a single substrate comprises the step of taking measurements from each surface-emitting component of said lasers. 
     In an alternate embodiment, the method for testing edge-emitting lasers located on a single substrate further comprises the step of taking said measurements using a lensed fiber and an electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described below with reference to the accompanying figures wherein: 
     FIG. 1 is a perspective view of a single edge-emitting laser located on a single wafer; and 
     FIG. 2 is a perspective view of multiple edge-emitting lasers located on a single wafer. 
    
    
     DETAILED DESCRIPTION 
     The present invention is drawn to an edge-emitting laser and a method of manufacturing and testing the laser. Referring to FIG. 1 the laser  100  comprises a substrate layer  105 . Substrate layer  105  is preferably an N-Substrate although it can also be other types of substrates such as a P-Substrate. 
     Located above substrate layer  105  is an insulating layer  110 . At least one channel  120  is located in insulating layer  110  so that portions of insulating layer  110  are located on either side of channel  120 . Insulating layer  110  blocks current from flowing outside of channel  120  and is usually a regrown semi-insulating material such as a Semi Insulating Indium Phosphate (SI InP) preferably iron doped. 
     Located inside channel  120  is a grating layer  130  and a waveguide stripe  140  is located above the grating layer  130 . Grating layer  130  is a second order grating which during operation permits feedback and coupling of outputs in a horizontal, lengthwise axial direction through channel  120  depicted by an arrow  150  representing an edge-emitting component and also in a normal direction with respect to channel  120  depicted by an arrow  160  representing a surface-emitting component. Although second order gratings are commonly present in surface-emitting lasers, second order gratings are not used in edge-emitting communication lasers. It is noted that grating layer  130  could be any grating order greater than a first order grating such as a second order grating, third order grating or a fourth order grating. The advantage of using high order gratings is that light is reflected back to the laser cavity to help the laser reach a lasing condition. However, as the grating layer order increases the light will also scatter toward the surface of the laser instead of back to the laser cavity making the laser less efficient. Therefore, a second order grating layer is an ideal candidate for edge-emitting lasers. Similarly etched facet lasers with a 45-degree-angle etched facet could be substituted for the second order grating. 
     Located above the waveguide stripe  140  and insulating layer  110  is a cap layer  170 . Cap layer  170  depicted in FIG. 1 is a P-Cap layer which is a P-type semiconductor material. However, cap layer  170  is not limited to being a P-type semiconductor material and could be other types of semiconductor material such as an N-type semiconductor material. 
     Located above waveguide stripe  140  in cap layer  170  is an opening  180  allowing surface-emitting component  160  to be emitted. Located on either side of opening  180  are metal stripes  190 . 
     In conventional edge-emitting lasers, a first order grating layer is present. A first order grating only provides an edge-emitting component indicated by arrow  150 . A laser with a grating that is an order higher than one emits a surface-emitting component indicated by arrow  160  in addition to the edge-emitting component indicated by arrow  150 . With an additional surface-emitting component represented by arrow  160 , testing of the laser can be performed from the surface of the laser instead from the edge of the laser as in the past. The surface-emitting component indicated by arrow  160  is used for on-wafer testing to obtain all the laser characteristics such as the current/voltage (I/V) relationship used to determine the terminal voltage, and the light/current (L/I) testing used for determining the power to current relations of each laser in one on-wafer measurement. Therefore, the cleaving process is eliminated and measurements can be obtained from the surface-emitting component (represented by arrow  160 ) present when using high order gratings. 
     Referring to FIG. 2, multiple coaxial channels  210  and  220  are located on a single substrate  105 . Trenches  200  separate each coaxial channel  210  and  220 . Electrode  240  is located on metal stripe  190  and lensed fiber  230  is located above opening  180  to receive the surface-emitting output indicated by arrow  160 . 
     A method for fabricating an edge-emitting laser of the present invention comprises the steps of providing a substrate layer  105  and forming an insulating layer  110  on substrate layer  105  defining a channel  120  having insulating layers  110  located on either side of channel  120 . The substrate layer depicted is an N-Substrate. However, substrate layer  105  is not limited to being an N-Substrate. For example, substrate layer  105  may be a P-Substrate. Insulating layer  110  is usually a regrown semi-insulating material such as a SI InP layer. 
     The method further comprises the steps of forming a grating layer  130  in channel  120  and forming a waveguide  140  over grating layer  130 . Grating layer  130  is a high order grating such as a second order grating. However, any order grating greater than one or any grating that provides outputs in multiple directions is appropriate. 
     The next step comprises forming a cap layer  170  over waveguide  140  and insulating layer  110 , forming an opening  180  in cap layer  170  above waveguide  140 , and forming metal strips  190  on either side of opening  180 . Forming opening  180  is typically accomplished by etching cap layer  170 . 
     The final step comprises forming two channels  210  and  220  by forming a trench  200  across insulating layer  110  and channel  120 . 
     Trench  200  is a deep trench and is typically etched below waveguide stripes  140 . The length of laser cavity is defined by the length of channel  210  or  220  and is determined by a deep trench etching below waveguide stripe  140  forming a facet. The wafer is then subjected to a dicing procedure which will not affect the laser mode characteristics and eliminates the need for cleaving. The good lasers can then be directly picked up for packaging. 
     However, in some cases unacceptable lasers are reprocessed through a series of trimming steps that change the effective position of the etched facet. First the facet is coated with a variable amount of dielectric coating and the surface-emitting output  160  is monitored in situ with a fiber probe. In some cases, for instance, when the coating is too thick the facet may be etched. These coating and etching procedures are used as needed until the laser is acceptable. In principal, yields of close to 100% can be achieved. 
     A method for testing edge-emitting lasers located on a single substrate comprises the step of taking measurements from each surface emitting component represented by arrow  160  of each cavity  210  and  220 . 
     A lensed fiber  230  is located above waveguide  140  to receive the surface-emitting component represented by arrow  160  and an electrode  240  is located on metal stripe  190 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention and all such modifications are intended to be included within the scope of the following claims.