Abstract:
The invention relates to a semiconductor laser diode structure with increased catastrophic optical damage (COD) power limit, featuring three sections, sometimes called windows, at the output facet of the diode. These include an optically transparent section, a current blocking section and a partially current blocking section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Patent Application No. 60/826,238 filed Sep. 20, 2006, entitled “SEMICONDUCTOR LASER DIODE WITH ADVANCED WINDOW STRUCTURE”, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to semiconductor lasers, in particular high power lasers where catastrophic optical damage to the facets is one of the main factors limiting the maximum optical output power. 
     BACKGROUND OF THE INVENTION 
     One limiting factor for the maximum power of a semiconductor laser diode design is the catastrophic optical damage (COD) limit at the output front facet of the laser diode. Essentially the very high optical power density, current density and carrier density interact with defects, non-radiative recombination centers, optical absorption areas and the semiconductor/coating/air interface to cause excess heating and eventually destructive failure. Various methods have been employed to increase the COD limit which will be discussed along with the new inventions. 
     Note that the COD generally occurs at the output front facet due to the higher optical power density relative to the rear facet. This invention applies primarily to the front facet window, however the same considerations as disclosed in this invention can also be applied to the rear facet. The front facet typically has a low reflectivity coating or coating stack deposited after an optional surface passivation. The rear facet is typically coated with a high reflectivity coating or coating stack. Also note that this window design applies to any high power semiconductor laser at any lasing wavelength including, but not limited to, single-mode lasers, multi-mode lasers, fiber-coupled lasers, distributed Bragg reflector (DBR) lasers and distributed feedback (DFB) lasers. 
     One of the first structures to improve COD is the use of a transparent window area  3  as shown in  FIG. 1 , which shows an output end  10  of a semiconductor laser in longitudinal cross-section. Basically a quantum well active region  2  is isolated from semiconductor/facet coating  6 /air interface at front facet  5 . This reduces the local optical absorption and heating near the front facet  5 . Also typically the transparent window area  3  blocks most of injected current  1 , which is reduced to a leakage current  11  considerably smaller than the injected current  1 . The transparent window area  3  can be formed using a variety of methods including, but not limited to, local etch and regrowth, ion implantation, or diffusion intermixing. 
     Welch et al. (U.S. Pat. No. 4,845,725) disclose a preferred structure, which employs impurity induced disordering to smear the interfaces between active region and cladding layers producing a waveguide layer with increased bandgap and thus a transparent window region at the laser facet and a graded transverse refractive index profile. Window regions having transparent waveguide layers can be produced by impurity induced disordering, i.e. the diffusion of silicon, zinc, tin or other impurity through the semiconductor layers to form the window region. 
     The fabrication of the structure by Welch et al. entails several diffusion or implantation steps as well as crystallographic disordering, all of which are notorious for introducing non-radiative recombination centers. Since the introduced non-radiative recombination centers extend into the active layer of the laser, which is electrically pumped, they represent a serious laser degradation and reliability risk. 
     There are two problems with this design. There can be leakage current through the transparent window area  3  or, even worse, along the facet  5 , which has non-radiative recombination centers, even with facet coating passivation  6 . Also the interface  16  between the quantum well and transparent window typically will have defects and current flowing near those defects will recombine non-radiatively. 
     Another structure for improving COD uses a current blocking area, or unpumped structure  4 , as shown in  FIG. 2 , which is a longitudinal cross-sectional view of an output end  20  of a semiconductor laser. In this case one or more blocking materials or layers  4  prevent current flow near the front facet  5  and therefore reduce non-radiative recombination near the front facet  5 . The current blocking area  4  can be formed using a variety of methods including, but not limited to, patterned contact metal, a blocking insulator (such as silicon nitride, silicon dioxide, aluminum oxide, titanium oxide, etc.), a current blocking implant or diffusion, or current blocking semiconductor layers (such as etching off the contact layer or doing an etch and blocking re-growth). 
     Yu et al. (U.S. Pat. No. 6,373,875) disclose such a semiconductor laser structure incorporating a current blocking area  4 , specifically to prevent current leakage near the facet  5 , as illustrated in  FIG. 2 . 
     There are two problems with this design. There will be optical absorption in the quantum well active region  2  along the section  25  where it is not pumped to transparency. Also the semiconductor/facet coating  6 /air interface at the front facet  5  will typically be a location of defects causing further optical absorption. 
     Kamejima (U.S. Pat. No. 4,759,025) discloses a semiconductor laser structure which attempts to resolve COD problems by using an intermixing technique. The laser structure is grown with thin multiple layers located at the intended active layer. The area to be pumped electrically by an electric current is thermally interdiffused by laser irradiation to form a mixed crystal exciting region having a band gap narrower than that of the surrounding layer, which is thus transparent to emitted light. In particular the non-interdiffused area near the laser facets becomes transparent to the laser emission. Also, the pumping current flows preferentially through the lower bandgap interdiffused area, thereby reducing exposure of the laser facets to the pumping current. 
     Kamejima&#39;s structure is unsuitable for high power lasers for a fundamental reason, however. To achieve high output power levels and high efficiency, a single or multiple quantum well (MQW) active layer consisting of one or more very thin semiconductor layers is generally preferred. The intermixing process on which Kamejima&#39;s structure depends destroys the quantum well structure. 
     An object of the present invention is to improve the laser structure for use at high optical power output levels by mitigating the adverse thermal effects in the vicinity of the laser facets such as recombination diverting some of the pumping current to non-radiative recombination centers and optical absorption. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention relates to a semiconductor laser structure pumped with an injected electrical current for high optical power output. The key features include an optically transparent section, and a current blocking section at an output facet of the diode. 
     The optically transparent section is protected from the injected pumping current by the overlapping current blocking section. 
     Another aspect of the present invention relates to an additional partially current blocking section. To further shield the output facet from the pumping current, the partially current blocking section produces a reduced profile in the injected pumping current density. Sufficient pumping current is provided however to maintain optical transparency this section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in greater detail with reference to the accompanying drawings, which represent preferred embodiments thereof, wherein: 
         FIG. 1  (prior art) shows an example longitudinal cross-section of an output window design for a semiconductor laser with transparent section; 
         FIG. 2  (prior art) illustrates an example longitudinal cross-section of an output window design for a semiconductor laser with unpumped current blocking section; 
         FIG. 3  shows an example longitudinal cross-section of an output window design for a semiconductor laser with combined transparent and unpumped current blocking sections according to current invention; 
         FIG. 4  shows a top view of an output end of an exemplary laser with combined transparent and unpumped current blocking window design according to current invention; 
         FIG. 5  is a longitudinal cross-section of an output end of an exemplary semiconductor laser with combined transparent and unpumped current blocking window design with additional partially blocked/partially pumped section; 
         FIGS. 6   a - d  presents a top view of example lasers with combined transparent and unpumped current blocking window design, illustrating a variety of usable methods for adding partially blocked/partially pumped sections; 
         FIG. 7  is a plot of the total CW COD current level as a function of current in a short patterned contact section near the front facet; 
         FIG. 8  is a plot of percentage failure versus COD power for devices with pumping sections patterned to give nominally 50% current density; and 
         FIG. 9  is a table comparing yield for devices with and without patterned pumping sections. 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment, shown in longitudinal cross-section in  FIG. 3 , comprises an output facet end  30  of a semiconductor laser incorporating a quantum well active region  2 , a contact region for injecting pumping current  1  into a main section  38  of the quantum well active region  2 , an optical reflector generally known as a laser facet  5  coated with a facet coating  6  for passivation and reflectivity control, a current blocking area  34 , and a transparent window  33 . The quantum well active region  2  may comprise a single quantum well, or a multiple quantum well structure. Semiconductor materials suitable for the quantum well active region  2  include, but are not limited to, AlGaAs, GaAs, GaAsP, InP, InGaAs, and InGaP. The output optical field profile is represented by curve  37 . The transparent window  33  limits the optical absorption under the current blocking area  34 , which substantially removes undesired electrical injection current leakage through the transparent window  33 , or along the front facet  5 . It also substantially removes electrical injection current near the interface  36  between the quantum well active region  2  and transparent window  33 . For this reason length, L 2 , of the current blocking area  34  would typically be longer than length, L 1 , of the transparent window  33 . However, there may be material combinations and structures, in which L 2  is shorter than L 1 . Furthermore these lengths need to be optimized for best laser diode performance, high COD power limit and highest reliability under operating conditions. 
       FIG. 4  is a top view of the first embodiment showing the output facet end  40  of a semiconductor laser with a current injection area  48  which extends to within a distance L 2  from the facet  5  coated with facet coating  6 . The transparent window  43  extends from the facet  5  for a distance L 1 . The output optical field profile is represented by curve  47 . 
     A final problem to be addressed is excess optical loss under the current blocking area  34  that is not within the transparent window area  33  as shown in  FIG. 3 . 
     Accordingly a second embodiment of the invention, shown in longitudinal cross-section in  FIG. 5 , comprises an output facet end  50  of a semiconductor laser incorporating a quantum well active region  2 , a contact region  1  for injecting pumping current into a main section  58  of the quantum well active region  2 , an optical reflector generally known as a laser facet  5  coated with a facet coating  6  for passivation and reflectivity control, a total current blocking section  54  of length L 2A , a partially blocking section  55  of length L 2B −L 2A  through which pumping current from a contact region  51  is injected into a partially pumped section  52  of the quantum well active region  2 , and a transparent window section  53  of length L 1 . The output optical field profile is represented by curve  57 . 
     A section the quantum well active region, which does not receive electrical pumping will absorb lasing light emitted by a pumped section  58  of the quantum well active region  2 , thus reducing lasing efficiency and causing localized heating. The transparent window section  53  is introduced to offset such potential problems by reducing optical losses by absorption and consequent heating. 
     The total current blocking section  54  removes undesired electrical injection current leakage through the transparent window section  53  or along the front facet  5  and removes electrical injection current near the interface  56  between the quantum well active region  2  and transparent window section  53 . The partially blocking section  55  provides a means for reducing electrical current density in the partially pumped section  52  of the quantum well active region  2  produced by pumping current from the contact area  51  at an end portion(s) of the quantum well active region  2 . Typically the current density in the partially pumped section  52  is a fraction of the current density resulting from injecting pumping current from the contact region  1  into the main section  58  of the quantum well active region  2 . 
     For the reasons discussed in relation to the first embodiment, a typical design has L 1 &lt;L 2A . For the partially blocking section  55  to have finite length for obtaining a profiled transition in current density between the total current blocking section  54  and the main section  58  pumped by pumping current from contact region  1 , the lengths must also fulfill the relationship, L 2B −L 2A &gt;0. Furthermore these lengths need to be optimized for best laser diode performance, high COD power limit and highest reliability under operating conditions. 
     Beneath the partially blocking section  55  the current density in the partially pumped section  52  will be lower than in the main lasing section  58  but just enough to reduce optical loss and improve the overall COD power limit. 
     As shown in  FIGS. 6   a - d , the contact areas  68   aa ,  68   bb ,  68   cc  and  68   dd  (corresponding to  51  in  FIG. 5  and disposed generally above the partially blocking section  55 ) can be formed using a variety of methods including, but not limited to, patterned contact metal, multiple contacts, a patterned blocking insulator (such as silicon nitride, silicon dioxide, aluminum oxide, titanium oxide, etc.), a patterned current blocking implant or diffusion, or patterned current blocking semiconductor layers (such as etching off the contact layer or etching and blocking re-growth). 
     In  FIG. 6   a  a top view of the third embodiment shows the output facet end of a semiconductor laser  60   a  with current injection area  68   a . An extra contact  68   aa  adjacent to the total current blocking section  64  and separated by a gap  69  from the main contact area  68   a  can be used to inject a reduced current density in the quantum well active region below it. 
     In  FIG. 6   b  a top view of the fourth embodiment shows the output facet end of a semiconductor laser  60   b  with a plurality of contact stripes  68   bb  disposed between the total current blocking section  64  and the main contact area  68   b  for injecting a reduced current density in the quantum well active region below it (e.g. the partially pumped section  52  in  FIG. 5 ). 
     In  FIG. 6   c  a top view of the fifth embodiment is shown comprising the output facet end of a semiconductor laser  60   c  with current injection area  68   c  comprising a plurality of contact fingers  68   cc  extending to the total current blocking section  64  for injecting a reduced current density in the quantum well active region below it (e.g. the partially pumped section  52  in  FIG. 5 ). 
     The sixth embodiment shown in  FIG. 6   d  is similar to the fifth embodiment, the difference being in the tapering of the extended contact fingers  68   dd  of the current injection area  68   d  for controlling the spatial profile of the injected current density. 
     Various experiments were performed to show that adding a partial pumping region improves the COD limit and device reliability.  FIG. 7  shows results for applying various current levels to a small separate contact (such as  68   aa  in  FIG. 6   a ) near the front facet  5 . In the case of no current in the extra contact  68   aa  near the front facet  5  (corresponding to the embodiment of  FIG. 3 ) the COD level ranges from 3.2 W to 4.0 W. In the case of uniform current injection (corresponding to the design in prior art  FIG. 1  with transparent window only) the COD ranges from about 3.2 W to 3.8 W. At approximately one-half baseline current density the COD range narrows and increases to 3.6 W to 4.1 W. This indicates that a patterned pumping scheme designed to inject about one-half the current density can improve the COD. 
       FIG. 8  shows the result of using a patterned partial pumping section (such as in  FIG. 5  and  FIG. 6 ) designed for injecting about one-half the current density. Going from unpumped to 50 μm partially pumped to 100 μm partially pumped improves COD level significantly. This particular set of samples illustrates the greatest improvement but in no set of samples tested case does the COD level become worse. 
       FIG. 9  shows the yield results of an experiment for laser diodes with a baseline combined transparent and unpumped current blocking section compared to laser diodes with an additional partially pumped/partially blocked section. The yield through a standard burn-in increases significantly from 58% to 95%, while the yield to a destructive COD test after burn-in increases even further from 78% to 100%. Again, this particular set of samples shows the most improvement but in no set of samples does the COD level become worse when using a partially pumped section. 
     The above experiments were conducted while injecting one half of the current density; however, alternative amounts of current injection are within the scope of this invention, e.g. preferred current density about 30% to 70%, more preferred current density about 35% to 60%, most preferred current density about 40% to 50%. 
     Preferred lengths for a laser cavity, L C , are greater than about 0.5 mm, more preferred values range approximately from 0.5 mm to 10 mm, the most preferred range being about 0.7 mm to 5 mm. 
     Accordingly, the current blocking area has a preferred length L 2A  from the output facet of up to about 5% of the total laser cavity length L C , more preferably between about 1 μm and 50 μm, and most preferably from about 2 μm to 30 μm. 
     The length L 1  of the transparent window area is equal to or less than the length L 2A  of the current blocking area. Preferably L 1  should be greater than about 0.5 μm and less than about 1% of L C . 
     The partially pumped (or partially current blocking) section has a length L 2B  from the output facet of up to about 10% of L C , more preferably between about 10 μm and about 5% of L C , and most preferably between about 30 μm and about 3% of the L C , wherein L C &gt;L 2B &gt;L 2A &gt;=L 1 . The partial current blocking area extends from the current blocking area to the desired distance L 2B  from the front facet  5 .