Abstract:
A system and method of testing multiple edge-emitting lasers on a common wafer is provided. This is accomplished by providing, for each edge-emitting laser on a common fabrication wafer, a structure that re-directs a portion of the edge-emitted light from each edge-emitting laser in a direction such that the re-directed portion from each edge-emitting laser can be measured while the edge-emitting lasers are still on the fabrication wafer. Each edge-emitting laser on the fabrication wafer can therefore be easily tested before cleaving or breaking the wafer into multiple pieces.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 09/353,792 filed Jul. 15, 1999, entitled “Multiple Edge-Emitting Laser Components Located on a Single Wafer and the On-Wafer Testing of the Same”, now U.S. Pat. No. 6,337,871, which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to edge-emitting semiconductor lasers that are manufactured on a single fabrication wafer and, more specifically, to an edge-emitting semiconductor laser that is capable of being optically tested while it is still located on the fabrication wafer (prior to cleaving or sawing). 
     2. Background of the Related Art 
     Edge-emitting lasers are semiconductor lasers with horizontal laser cavities (resonant cavities that are parallel to the plane of the wafer on which they are fabricated) that emit outputs in a horizontal direction through the edges of the laser. When manufacturing edge-emitting lasers, multiple laser cavities are usually formed in parallel strips on the surface of a single fabrication wafer. Each strip is divided into multiple segments to form multiple coaxial lasers on the wafer. The fabrication wafer is then cleaved or sawed to separate the individual lasers. 
     As many as 25,000 or more individual lasers can generally be fabricated on a single fabrication wafer. Each individual laser is typically tested for quality control purposes. This generally involves measuring the laser emission as a function of applied current. “On-wafer” testing, in which each laser is tested while they are still on the fabrication wafer, is the most efficient and cost effective way of testing the lasers. 
     However, because edge-emitting lasers emit light from their respective edges, the light emitted from most of the lasers (except the lasers at the very edge of the fabrication wafer) get blocked by other adjacent lasers, and thus is not detectable. Thus, edge-emitting lasers must be tested after they have been separated by cleaving or sawing the fabrication wafer. This is very time consuming, and thus increases manufacturing costs significantly. 
     Vertical cavity surface-emitting lasers were developed, in part, because on-wafer testing is so desirable in terms of efficiency and cost reduction. Vertical cavity surface-emitting lasers have resonant cavities that are perpendicular to the plane of the fabrication wafer, and thus emit light vertically with respect to the fabrication wafer. The vertically-emitted light from each laser can be measured prior to cleaving or sawing of the fabrication wafer, because the light is not blocked by adjacent lasers. 
     However, vertical resonant cavities are difficult to fabricate and require complicated device processing techniques. Further, vertical cavity surface-emitting lasers have lower output powers and quantum efficiencies than edge-emitting lasers. 
     The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve at least the above problems and/or disadvantages discussed above and to provide at least the advantages described hereinafter. 
     The present invention provides a system and method of testing multiple edge-emitting lasers on a common fabrication wafer. This is accomplished by providing, for each edge-emitting laser on a common fabrication wafer, a structure that re-directs a portion of the edge-emitted light from each edge-emitting laser in a direction such that the re-directed portion from each edge-emitting laser can be measured while the edge-emitting lasers are still on the fabrication wafer. Each edge-emitting laser on the wafer can therefore be easily tested without cleaving or breaking the wafer into multiple pieces. 
     The present invention may be practiced in whole or in part by an edge-emitting laser, comprising a resonant cavity and a second order or higher grating formed in the resonator, wherein the second order or higher grating transmits a first portion of light in the resonator along a first direction as edge-emitted light, and directs a second portion of light in the resonator along a different direction. 
     The present invention may also be practiced in whole or in part by a horizontal cavity edge emitting laser comprising a resonant cavity and a trench positioned proximate to the resonant cavity that is configured to scatter a portion of light in the resonant cavity such that the scattered portion can be measured. 
     The present invention may also be practiced in whole or in part by a method of testing at least two horizontal cavity edge-emitting lasers on a common wafer, comprising re-directing a portion of the light generated by each edge-emitting laser such that the re-directed portion can be detected while the edge-emitting lasers reside on the common wafer. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     The invention is described below with reference to the accompanying figures wherein: 
     FIG. 1 is a schematic view of a horizontal cavity edge-emitting laser, in accordance with one embodiment of the present invention; 
     FIG. 2 is a perspective view of a horizontal cavity edge-emitting laser, in accordance with another embodiment of the present invention; 
     FIG. 3 is a perspective view showing how multiple edge-emitting lasers on a single fabrication wafer can be tested while still on the fabrication wafer, in accordance with the present invention; 
     FIG. 4 is a schematic view of a multi-mode edge-emitting laser, in accordance with another embodiment of the present invention; and 
     FIG. 5 is a schematic perspective view showing how multiple multi-mode edge-emitting lasers on a common fabrication wafer can be tested while still on the fabrication wafer, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic view of a horizontal cavity edge-emitting laser  10  that incorporates a structure that re-directs a portion of the edge-emitted light from the edge-emitting laser  10  in a direction such that the re-directed portion from the edge-emitting laser  10  can be measured while the edge-emitting laser  10  is still on the fabrication wafer, in accordance with one embodiment of the present invention. 
     The edge-emitting laser  10  includes an active region  20 , a grating structure  30  and front and rear facets  40  and  50  The facets  40  and  50  act as reflectors to provide optical feedback. The grating structure  30  is a second order or higher grating that, in addition to providing optical feedback, also redirects a portion  55  of a guided edge-emitted component  60  in a direction that allows the portion  55  to be measured while the edge-emitting laser  10  is still on the fabrication wafer. 
     Although the grating structure  30  is shown above the active layer  20 , it should be appreciated that the grating structure  30  could also be positioned below or to the side of the active layer  20  while still falling within the scope of the present invention. In general, the grating structure  30  can be positioned at any position that will allow it to provide optical feedback and re-direct the portion  55  of the guided edge-emitted component  60  in a direction that will allow the portion  55  to be measured while the edge-emitting laser  10  is still on the fabrication wafer. 
     FIG. 2 is a perspective view of a horizontal cavity edge-emitting laser  100  that incorporates a second order or higher grating structure, in accordance with another embodiment of the present invention. 
     The edge-emitting 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. However any material with a higher bandgap than the channel  120  may be used. 
     Located inside channel  120  is a grating layer  130  and a waveguide stripe  140  is located above the grating layer  130 . Grating layer  130  permits feedback and coupling of outputs in a horizontal, lengthwise axial direction through channel  120 , depicted by an arrow  150  which represents an edge-emitted component. Prior art edge-emitting laser with grating layers, also referred to as distributed feedback (DFB) lasers, utilize first order gratings that provide optical feedback. However, the grating layer  130  utilized in laser  100  is a second order or higher grating, in addition to providing optical feedback, also redirects a portion of the edge-emitted component as a surface-emitted component  160 , in a direction that allows the surface-emitted component to be measured while the edge-emitting laser  100  is still on the fabrication wafer. 
     In a preferred embodiment, a second order grating is used for the grating layer  130 . Gratings with orders greater than two may be used, but there is a trade-off with laser efficiency due to the increased light scatter as the grating order increases. An aspect of the present invention is the recognition that the use of a second order grating provides sufficient scattering of the light towards the surface of the laser for monitoring purposes, while not significantly reducing the laser&#39;s efficiency. 
     Located above the waveguide stripe  140  and insulating layer  110  is a cap layer  170 . Cap layer  170 , shown in FIG. 2, is a P-Cap layer formed from a P-type semiconductor material. However, it should be appreciated that cap layer  170  may be formed of other types of semiconductor material, such as an N-type semiconductor material, while still falling within the scope of the present invention. 
     Located above waveguide stripe  140  in cap layer  170  is an opening  180  allowing surface-emitted component  160  to be emitted. Located on either side of opening  180  are metal stripes  190 , that are used to provide power to the laser  100 . 
     The edge-emitting laser  100  of the present invention emits a surface-emitted component  160 , in addition to the edge-emitted component  150 . The surface-emitting component  160  is used for on-wafer testing to obtain certain laser parameters, such as the current/voltage (I/V) relationship used to determine the terminal voltage, the light/current (L/I) relationship used for determining the power to current characteristics of each laser, and the mode behavior of the laser (also referred to as “wavelength testing”). Thus, expensive and time consuming testing of each individual laser after cleaving, scribing, dicing or sawing is avoided. 
     On-wafer testing identifies the lasers whose performance characteristics are unacceptable. In some cases, lasers with unacceptable mode behavior may be reprocessed, prior to separation from the fabrication wafer, by changing the coating on the facets and/or changing the effective position of the facets. In principal, yields of close to 100 % can be achieved. 
     FIG. 3 is a perspective view showing how multiple edge-emitting lasers on a single fabrication wafer  105  can be tested while still on the fabrication wafer  105 , in accordance with one embodiment of the present invention. The edge-emitting laser  100  of FIG. 1 are fabricated in multiple numbers on a common fabrication wafer  105 , using well-known semiconductor laser manufacturing techniques. However, a second order or higher grating  130  is used in place of the first order grating used in prior art DFB lasers. 
     Multiple coaxial channels  210  and  220  are located on the fabrication wafer  105 . Trenches  200  separate each coaxial channel  210  and  220 . Electrode  240  is used to provide power to the laser being tested, by contacting the metal stripe  190  of the laser being tested. A lensed fiber  230  is preferably used to receive the surface-emitted output  160  from the laser being tested. However, any means of receiving the surface-emitted output  160 , such as a lens or a fiber, may be used. 
     As discussed above, the edge-emitting laser  100  of FIG. 2 is a DFB laser, which are generally single-mode. However, the present invention can also be applied to multi-mode edge-emitting lasers. FIG. 4 is a schematic cross-sectional view of a multi-mode edge-emitting laser  300  that incorporates a structure that re-directs a portion of the edge-emitted light from the edge-emitting laser  300  in a direction such that the re-directed portion from the edge-emitting laser  300  can be measured while the edge-emitting laser  300  is still on the fabrication wafer, in accordance with another embodiment of the present invention. 
     The laser  300  includes an active region  310 , and front and rear facets  320  and  330 . The facets  320  and  330  act as reflectors to provide optical feedback. This type of laser is also know as a Fabry-Perot laser, and receives all of its optical feedback from the facets  320  and  330 . 
     The laser  300  also includes a scattering trench  340 , that scatters a portion of the guided edge-emitted component  350  towards the surface of the laser  300 . The scattered portion exits the laser as a surface-emitted component  360 . The scattering trench is preferably designed so that the surface-emitted component is strong enough to be detected, without significantly affecting the edge-emitted component  350 . In a preferred embodiment, the scattering trench is 3-5 μm wide, and makes an angle of approximately 45 degrees with respect to the plane of the active region  310 . The bottom  370  of the scattering trench  340  is preferably positioned 100-200 nm from the active region. 
     FIG. 5 is a schematic perspective view of multiple multi-mode edge-emitting lasers  300  on a common fabrication wafer  380 , in accordance with the present invention. The lasers  300  are fabricated in multiple numbers on a common fabrication wafer  380  using semiconductor laser fabrication techniques well-known in the art. However, as part of the fabrication process, trenches  340  are formed, preferably using vertical etching techniques such as reactive ion etching (RIE) or chemical assisted ion beam etching (CAIBE). 
     After fabrication, each individual laser can be tested by applying a drive current to their respective electrodes  390 , and measuring the surface-emitted component  360 , preferably with a lensed fiber  230 . 
     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. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.