Abstract:
A slab-coupled optical waveguide laser (SCOWL) is provided that includes an upper and lower waveguide region for guiding a laser mode. The upper waveguide region is positioned in the interior regions of the SCOWL. The lower waveguide region also guides the laser mode. The lower waveguide region is positioned in an area underneath the upper waveguide region. An active region is positioned between the upper waveguide region and the lower waveguide region. The active region is arranged so etching into the SCOWL is permitted to define one or more ridge structures leaving the active region unetched.

Description:
GOVERNMENT INTEREST STATEMENT 
     This invention was made with government support under Contract No. FA8721-05-C-0002 awarded by the U.S. Air Force. The government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention is related to the field of slab-coupled optical waveguide lasers (SCOWL), and in particular to a high efficiency SCOWL where the active region of the SCOWL is placed within the interior regions of the SCOWL waveguide, instead of at the edges. 
     The development of high power, reliable and efficient semiconductor lasers emitting in a single spatial mode has been a challenge for the past few decades. 
     Earlier demonstrated SCOWL structures required etching through the quantum-well (QW) active region. Etching through the active region eliminates the possibility of lateral current spreading. Current spreading usually causes the laser mode in the SCOWL device to change dramatically as the current spreading becomes more severe under high current injection levels. The problem with etching through the active region, particularly in the AlGaAs/InGaAs/GaAs materials system, is that etching often introduces surface defects at the exposed QW sidewall location. These surface defects propagate into the QW as the device is operated, and the defects can limit the device reliability and operating time since they lead to non-radiative recombination centers. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a slab-coupled optical waveguide laser (SCOWL). The SCOWL includes an upper waveguide region for guiding a laser mode. The upper waveguide region is positioned in the interior regions of the SCOWL. The lower waveguide region also guides the laser mode. The lower waveguide region is positioned in an area underneath the upper waveguide region. An active region is positioned between the upper waveguide region and the lower waveguide region. The active region is arranged so etching into the SCOWL is permitted to define one or more ridge structures leaving the active region unetched. 
     According to another aspect of the invention, there is provided a method for forming a slab-coupled optical waveguide laser (SCOWL). The method includes forming an upper waveguide region for guiding a laser mode. The upper waveguide region is positioned in the interior regions of the SCOWL. Also, the method includes forming a lower waveguide region for guiding the laser mode. The lower waveguide region is positioned in an area underneath the upper waveguide region. Moreover, the method includes positioning an active region between the upper waveguide region and the lower waveguide region. Furthermore, the method includes arranging the active region so etching into the SCOWL is permitted to define one or more ridge structures leaving the active region unetched. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a slab-coupled optical waveguide laser (SCOWL) formed in accordance of the invention; 
         FIGS. 2A-2F  are schematic diagrams illustrating the process flow for the fabrication of the SCOWL of  FIG. 1 ; 
         FIG. 3  is a graph illustrating measured power versus current, voltage versus current, and power conversion efficiency (PCE) versus current of a 1050 nm SCOWL formed in accordance with the invention; and 
         FIG. 4A-4C  are near field beam profiles for a 1050 nm SCOWL formed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides an improved SCOWL design, where the active region of the SCOWL is placed within the interior regions of the SCOWL waveguide, instead of at the edges. This allows for etching into the SCOWL waveguide that is required for defining a ridge waveguide, and is also essential for defining the slab section of the inventive SCOWL device which provides the mode filtering resulting in the single-spatial mode operation. The required etch depth for the SCOWL operation in this improved design is selected such that the active region is unetched. The distance that the active region is located from the final etch depth is critical because it defines the amount of lateral current spreading that occurs. 
       FIG. 1  shows a cross-section view of a SCOWL  2  formed in accordance of the invention. The SCOWL  2  includes a p-type metal layer  4  positioned on a dielectric layer being  6 . The dielectric layer  6  is positioned on a p-type GaAs cap layer  8 . The SCOWL  2  also includes a p-type upper cladding layer  10  where the p-type GaAs cap layer  8  is positioned on. The p-type upper cladding layer  10  is formed on a p-type AlGaAs upper waveguide region  12 . An undoped active region  14  is positioned between the p-type upper waveguide region  12  and an n-type lower waveguide region  16 . The n-type lower waveguide region  16  is positioned on an n-type lower cladding layer  18 . The n-type lower cladding layer is positioned on an n-type GaAs substrate  20 , where the GaAs substrate  20  is positioned on an n-type metal layer  22 . A number of trench structures are formed in regions that include the dielectric layer  6 , p-type GaAs cap layer  8 , p-type upper cladding layer  10 , and p-type upper waveguide region  12 . In addition, the trench structures  26 ,  28  are lined with a dielectric layers  30 ,  32 . The content within the trench structures  26 ,  28  include the same materials that define the p-type metal layer  4  to form ridge waveguide structures. Alternatively, the trench structures  26 ,  28  can be filled with primarily with air and a metal lining. The dielectric layers  30 ,  32  include the same materials that define the dielectric layer  6 . 
     The p-type upper cladding layer  10  and n-type lower cladding layer  16  assists in confinement of a laser mode  24  propagating through the p-type AlGaAs upper waveguide region  12 , active region  14 , and n-type lower waveguide region  16 . The p-type upper waveguide region can include AlGaAs having a concentration of Al between 0% and 50% and a thickness between 0.10 μm and 1.0 μm with a doping level between 1×10 15  and 1×10 18  cm −3 . The n-type lower waveguide region  16  can include AlGaAs having a concentration of Al between 0% and 50% and a thickness between 1.0 μm and 8.0 μm with a doping level 1×10 15  and 1×10 18  cm −3 . The composition of the p-type upper cladding layer  10  and the n-type lower cladding layer  18  must be higher in Al percentage as compared to the p-type upper waveguide region  12  and the n-type lower waveguide region  16 , respectively. 
     The active region  14  can include undoped quantum wells, barrier layers, and bounding layers. The active region having  14  undoped bounding sublayers can include AlGaAs, where the Al concentration is between 0% and 30%, or GaAsP, where the P concentration is between 0% and 30%, with a thickness between 1 and 20 nm. An active region  14  having undoped barrier layers can include GaAsP, where the P concentration is between 0% and 30%, or AlGaAs, where the Al concentration is between 0% and 30% Al, with a thickness between 1 and 20 nm. Moreover, an active region  14  having undoped quantum wells can include InGaAs, where the In concentration is between 0% and 40%, with a thickness between 1 and 20 nm 
     Because the active region  14  is positioned between the waveguides  12 ,  16 , it is necessary to dope the upper waveguide region  12  p-type, while the lower waveguide region  16  (where the largest fraction of the mode  24  is positioned) is doped n-type. By positioning the p-n junction around the active region  14 , proper injection of electrons and holes into the active region  14  is ensured. By using this arrangement, the active region  14  is confined within the interior regions of the waveguide of the SCOWL  2 . This permits etching in the SCOWL  2  to form the ridge waveguides leaving the active region unetched. This allows a finite amount of lateral current spreading to occur. 
     In conventional ridge waveguide lasers, the active region is typically placed near the center of the waveguide. In contrast, in the improved SCOWL device  2 , the active region  14  is placed asymmetrically within the waveguide of the SCOWL  2 , such that the upper waveguide region  12  thickness is less than the lower waveguide region  16  thickness. The active region  14  placement allows for obtaining a low optical confinement factor, which is essential for the SCOWL concept. 
     With respect to the active region  14  being quantum wells (QWs), it is necessary to reduce the number of QWs as compared with the earlier SCOWL structure when moving the active region inside of the SCOWL waveguide. This is because locating the QW within the SCOWL waveguide causes the optical intensity to be relatively higher than that of a similar design in which the QW is located at the edge of the SCOWL waveguide. One way to keep the optical confinement factor relatively constant is to reduce the number of quantum wells in the active region  14 . In this embodiment, one can use two QWs in the improved SCOWL  2  instead of three QWs, which were used in the earlier SCOWL design. 
     Also, it is important that specific lengths s, w, h within the SCOWL  2  be obtained so optimal performance can be adhered to. The length w, which defines the distance between the trench structures  26 ,  28 , can be between 1 μm and 7 μm. The length s, which defines the critical distance between the final etch depth of the ridge waveguides and the active region  14 , can be between 0 and 0.3 μm. The length h, which defines the combined height of the upper waveguide region  12 , active region  14 , and lower waveguide region  16 , can be between 1.3 μm and 9 μm. 
       FIGS. 2A-2F  illustrate a process flow in the fabrication of the inventive SCOWL  2  as shown in  FIG. 1 .  FIG. 2A  illustrates the material layers  8 - 20  being fabricated using standard crystal growth techniques such as MOCVD, OMVPE, and MBE. The material layers  8 - 20  are the same layers described in  FIG. 1  with the exception to the dielectric layer  6 , p-type metal layer  4 , and n-type metal layer  22  which are fabricated later.  FIG. 2B  shows the formation of the trench structures  26 ,  28 , where a mask was used to form the shape of the trench structure  26 ,  28  and wet chemical or dry etching, such as ICP etching, was used to etch portions of the layers  8 - 12  to form the trench structure  26 ,  28 .  FIG. 2C  shows the deposition of dielectric material on the p-type GaAs cap layer  8  and within the trench structures  26 ,  28  to form layers  6 ,  30 , and  32 . The dielectric material can be any kind of reasonable dielectric, including SiO 2 , Al 2 O 3 , etc. The dielectric deposition method can be pyrolytic, PECVD, ICP-assisted, etc.  FIG. 2D  shows the formation of contact openings  40 ,  42  on the ridge defined by the trench structures  26 ,  28 .  FIG. 2E  shows the deposition of p-type metal materials on the top surface of the dielectric layer  6  to form the p-type metal layer  4  and the interior region of the trench structures  26 ,  28  to define ridge waveguides.  FIG. 2F  shows the backside deposition of an n-type metal on the n-type GaAs substrate  20  to form the n-type metal layer  22 . 
       FIG. 3  is a graph illustrating measured power versus current, voltage versus current, and power conversion efficiency (PCE) versus current of a 1050 nm SCOWL formed in accordance with the invention. Note the inventive SCOWL has kink-free operation up to P&gt;1 W, maximum PCE&gt;50%, and maximum power &gt;2 W. This shows a substantial increase in operational performance over known SCOWLs in the prior art. 
       FIG. 4A-4C  are near field beam profiles for a 1050 nm SCOWL formed in accordance with the invention. The inventive 1050 nm SCOWL demonstrates large nearly circular modes over a large range of operation current levels (250 mA, 500 mA, and 1000 mA). This illustrates substantial improvement in mode confinement as compared to other SCOWLs in the prior art. 
     This invention is a substantial improvement over previous SCOWL devices and also many other types of single-mode semiconductor lasers. Electrical-to-optical efficiency of 53% (CW) and 59% (pulsed) for junction-side up mounted SCOWL devices formed in accordance with the invention have been obtained. This is a substantial improvement over the earlier SCOWL devices, which had electrical-to-optical efficiency of 35%. The invention compares very favorably with conventional ridge waveguide lasers, which have efficiencies between 30 to 50% at this wavelength range. In addition, because the ridge width of the inventive SCOWL can be relatively wide, the inventive SCOWL has better performance at 1 W level output powers. The efficiency does not decline as rapidly at high current injection levels due to series resistance. For example, the peak CW electrical-to-optical efficiency of a ridge waveguide (RWG) laser is as high as 50%, but this rapidly drops off to 30% at power levels close to 1 W. The inventive SCOWL still has efficiencies of &gt;40% at 1 W power levels. This makes a substantial difference in the thermal performance of these devices, particularly in array applications. 
     The high efficiency SCOWL described so far is implemented in the InGaAs/AlGaAs/GaAs material system. It is possible to design and implement the high efficiency SCOWL in other material systems and other wavelengths that are commonly used for semiconductor lasers and amplifiers, including, but not limited to, InGaAsP/GaAs, InGaAsP/InP, InGaAsSb/AJGaAsSb/GaSb, and InGaN/AlGaN/GaN. 
     The inventive SCOWL, when used in arrays, is useful for pumping high power ytterbium-doped fiber lasers. With wavelength beam combining (e.g., in an external cavity), dense SCOWL arrays can in principle enable collimated, high brightness beams with scalable output power, useful for a variety of applications. The amplifier version of this device could be used in high power phase-locked arrays in a seeded-injection amplifier approach. 
     Any of the above-discussed embodiments of high efficiency SCOWL devices and arrays may be incorporated into an associated laser system. Such a laser system may include, for example, the high efficiency SCOWL devices, the beam combining cavity, electrical, thermal, mechanical, electro-optical and opto-mechanical laser control equipment, associated software and/or firmware, and an optical power delivery subsystem. Embodiments of the high efficiency SCOWL and associated laser systems, can be used in applications that benefit from the high power and brightness of the embodied laser source produced using the high efficiency SCOWL devices. These applications may include, for example, materials processing, such as welding, drilling, cutting, annealing and brazing; marking; laser pumping; medical applications; and directed energy applications. In many of these applications, the laser source formed by the high efficiency SCOWL devices may be incorporated into a machine tool and/or robot to facilitate performance of the laser application. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.