Patent Publication Number: US-2013243018-A1

Title: Gain medium with improved thermal characteristics

Description:
RELATED INVENTION 
     This application claims priority on U.S. Provisional Application Ser. No. 61/610,865, filed Mar. 14, 2012 and entitled “GAIN MEDIUM WITH IMPROVED THERMAL CHARACTERISTICS”. As far as permitted, the contents of U.S. Provisional Application Ser. No. 61/610,865 are incorporated herein by reference. 
    
    
     BACKGROUND 
     Lasers can be used for many things, including but not limited to testing, measuring, diagnostics, pollution monitoring, leak detection, security, pointer tracking, jamming a guidance system, analytical instruments, homeland security and industrial process control, and/or a free space communication system. 
     Recently, Quantum Cascade (“QC”) as well as Interband Cascade (IC) gain media have been used in applications that require a mid-infrared (“MIR”) output beam. Unfortunately, a significant amount of heat is generated by the operation of a QC or IC gain medium. Further, the core layers of a QC or IC gain medium typically have very low thermal conductivity. As a result thereof, the QC or IC gain medium can become very hot during operation. This can greatly reduce the operational life of the QC or IC gain medium. 
     SUMMARY 
     A laser assembly that generates a beam when power is directed to the laser assembly includes a QC or IC gain medium having (i) a first facet region that includes a first facet, (ii) a second facet region that includes a second facet, and (iii) an intermediate region that separates the facet regions and connects the facet regions. Additionally, the gain medium includes a substrate layer and a core layer that extend between the facets. In one embodiment, the gain medium is designed so that when power is directed to the gain medium, (i) current flows through the core layer in the intermediate region to generate the beam, and (ii) current does not flow through the core layer in one or both facet regions. Current blocking can be achieved by making the core non-conductive or semi-insulating, and/or by making the upper and/or lower cladding layer non-conducting or semi-insulating. As a result of this design, the temperature of gain medium near one or both facets is reduced. This will reduce the likelihood of failure of the gain medium near one or both of the facets. In addition, higher operating temperature may be achieved for the intermediate region of the core, effectively leading to higher device temperature without exceeding the thermally induced temperature limit of approximately two hundred and twenty degrees Celsius (220 C). 
     In one embodiment, the gain medium includes a cladding layer that is adjacent to the core layer and that extends between the facets. Further, the cladding layer has a higher electrical conductivity in the intermediate region than in one or both of the facet regions. 
     In another embodiment, the gain medium includes an electrically conductive cladding layer that extends between the facets. In this embodiment, the cladding layer is electrically connected to the core layer in the intermediate region, and the cladding layer is electrically insulated from the core layer in one or both of the facet regions. 
     In alternative embodiment, the gain medium is designed so that when power is directed to the gain medium, (i) current flows through the core layer in the intermediate region at a first rate to generate the beam, and (ii) current flows through the core layer in the first facet region at a second rate that is less than the first rate. Further, the gain medium can be designed so that when power is directed to the gain medium, current flows through the core layer in the second facet region at a third rate that is less than the first rate. 
     The present invention is also directed to a method for generating a beam, and an assembly that includes the laser assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1A  is a simplified perspective view of a first embodiment of a laser assembly having features of the present invention; 
         FIG. 1B  is a simplified cut-away view taken on line  1 B- 1 B in  FIG. 1A ; 
         FIG. 1C  is a simplified cut-away view taken on line  1 C- 1 C in  FIG. 1A ; 
         FIG. 1D  is a simplified cut-away view taken on line  1 D- 1 D in  FIG. 1A ; 
         FIG. 2A  is a simplified perspective view of another embodiment of a laser assembly having features of the present invention; 
         FIG. 2B  is a simplified cut-away view taken on line  2 B- 2 B in  FIG. 2A ; 
         FIG. 2C  is a simplified cut-away view taken on line  2 C- 2 C in  FIG. 2A ; 
         FIG. 2D  is a simplified cut-away view taken on line  2 D- 2 D in  FIG. 2A ; 
         FIG. 3A  is a simplified perspective view of another embodiment of a laser assembly having features of the present invention; 
         FIG. 3B  is a simplified cut-away view taken on line  3 B- 3 B in  FIG. 3A ; 
         FIG. 3C  is a simplified cut-away view taken on line  3 C- 3 C in  FIG. 3A ; 
         FIG. 3D  is a simplified cut-away view taken on line  3 D- 3 D in  FIG. 3A ; 
         FIG. 4A  is a simplified perspective view of another embodiment of a laser assembly having features of the present invention; 
         FIG. 4B  is a simplified cut-away view taken on line  4 B- 4 B in  FIG. 4A ; 
         FIG. 4C  is a simplified cut-away view taken on line  4 C- 4 C in  FIG. 4A ; 
         FIG. 4D  is a simplified cut-away view taken on line  4 D- 4 D in  FIG. 4A ; 
         FIG. 5  is a simplified top illustration of an assembly having features of the present invention; 
         FIG. 6  is a simplified side illustration of a gain medium and a heat sink having features of the present invention; and 
         FIG. 7  is a simplified end view of another embodiment of a laser assembly having features of the present invention. 
     
    
    
     DESCRIPTION 
       FIG. 1A  is a simplified perspective view of a first embodiment of a laser assembly  10  that includes a gain medium  12  and a power source  14  that directs power to the gain medium  12 . In one embodiment, the gain medium  12  includes a first end having a first facet  16 , and a second end having a second facet  18  that is opposite to the first facet  16 . Further, current from the power source  14  flowing through the gain medium  12  generates light that is emitted as an output beam  20  (illustrated as a dashed line) from one or both facets  16 ,  18 . 
     As an overview, in certain embodiments, the gain medium  12  is a QC or IC gain medium and is uniquely designed so that when current is directed to the gain medium  12  from the power source  14 , current flows through the gain medium  12  intermediate the facets  16 ,  18 , but the current does not flow (or the current level (or the current density) is reduced relative to the center) through the gain medium  12  near one or both facets  16 ,  18 . As a result thereof, the temperature of gain medium  12  near the one or both facets  16 ,  18  is reduced. This will reduce the likelihood of failure of the gain medium  12  near one or both of the facets  16 ,  18 . 
     Some of the Figures provided herein include an orientation system that designates an X axis, a Y axis, and a Z axis. It should be understood that the orientation system is merely for reference and can be varied. Moreover, these axes can alternatively be referred to as a first, second, or third axis. 
     The laser assemblies  10  provided herein can be used in a variety of applications, such as testing, measuring, diagnostics, pollution monitoring, leak detection, security, pointer tracking, jamming a guidance system, analytical instruments, infrared microscopes, imaging systems, homeland security and industrial process control, and/or a free space communication system. It should be noted that this is a non-exclusive list of possible applications. 
     The power source  14  is electrically connected to and directs power to the gain medium  12 . In one embodiment, the power source  14  directs current to the gain medium  12  in a pulsed fashion. Alternatively, the power source  14  can direct current to the gain medium  12  in a continuous fashion. The power source  414  can receive power from a generator (not shown), a battery (not shown), or another power source (not shown). The power source  14  can include one or more processors that are used to control the current flow to the gain medium  12 . 
     In one embodiment, the gain medium  12  is a broadband emitter. Alternatively, both facets of the gain medium  12  can be coated with a reflective coating to form a Fabry-Perot laser. Still alternatively, the gain medium  12  can be tuned to adjust the primary wavelength of the output beam  20 . For example, a wavelength selective element (not shown in  FIG. 1A ) can be incorporated into the gain medium  12 . Alternatively, for example, the laser assembly  10  can include an external wavelength selective element (not shown in  FIG. 1A ) that allows the wavelength of the output beam  20  to be tuned. 
     In one embodiment, the gain medium  12  is a Quantum Cascade (“QC”) gain medium that generates an output beam  20  that is in the mid-infrared (“MIR”) range. In an alternative embodiment, the gain medium  12  can be an Interband Cascade (“IC”) gain media. An Interband Cascade gain medium is a hybrid between laser diodes having a direct bandgap and Quantum Cascade gain media. 
     In  FIG. 1A , the output beam  20  only emits from the first facet  16 . In this embodiment, the first facet  16  is coated with a reflective coating and the second facet  18  is coated with a highly reflective coating. With the reflective coating on the first facet  16 , a portion of the light directed at the first facet  16  is reflected back into the gain medium  12  and a portion of the light directed at the first facet  16  is transmitted through the first facet  16 . Further, with the highly reflective coating on the second facet  18 , a bulk of the light directed at the second facet  18  is reflected back into the gain medium  12 . Alternatively, the facets  16 ,  18  can be coated so that light emits from both facets  16 ,  18 . 
     As a non-exclusive example, a suitable QC gain medium  12  can have (i) a length  22 A (from the first facet  16  to the second facet  18 ) of between approximately 0.5 and six millimeters, (ii) a width  22 B (along the slow axis) of approximately five hundred microns, and (iii) a height  22 C (along the fast axis) of approximately one hundred and fifty microns. However, the gain medium  12  can have a size and shape different than this. 
     Further, as provided herein, the gain medium  12  can be separated into three continuous regions along the length direction, namely (i) a first facet region  24 , (ii) a second facet region  26 , and (ii) an intermediate region  28  that separates and connects the first facet region  24  from the second facet region  26 . It should be noted that the gain medium  12  is physically not separated, however, (i) a first dashed line  29 A is used to illustrate the transition of the first facet region  24  and the intermediate region  28 ; and (ii) a second dashed line  29 B is used to illustrate the transition of the intermediate region  28  and the second facet region. 
     The first facet region  24  includes the first facet  16  and the area immediately near the first facet  16 . Similarly, the second facet region  26  includes the second facet  18  and the area immediately near the second facet  18 . Finally, the intermediate region  28  extends between and separates the first facet region  24  from the second facet region  26 . 
     In one non-exclusive embodiment, (i) the first facet region  24  has a first length  24 A of between approximately ten to fifty microns in length, (ii) the second region  26  has a second length  26 A of between approximately ten to fifty microns in length; and (iii) the intermediate region  28  has an intermediate length  28 A that makes up the remaining length of the gain medium  12 . In alternative, non-exclusive embodiments, the intermediate length  28 A is between approximately 0.5 to ten millimeters, and the first length  24 A and the second length  26 A is between approximately two to twenty-five microns. It is recognized that most embodiments of the current blocking layers will result in additional optical losses caused by absorption and scattering, and that these losses can be compensated by having sufficient gain path length in the intermediate region. 
       FIG. 1B  is a simplified cut-away of the intermediate region  28  of the gain medium  12  taken on line  1 B- 1 B in  FIG. 1A  near the center of the gain medium  12 .  FIG. 1C  is a simplified cut-away view of the first facet region  24  of the gain medium  12  taken on line  1 C- 1 C in  FIG. 1A  near the first facet  16 . Further,  FIG. 1D  is a simplified cut-away view of the second facet region  26  of the gain medium  12  taken on line  1 D- 1 D in  FIG. 1A  taken near the second facet  18 . 
     In this embodiment, the gain medium  12  includes a number of layers, namely (i) a substrate layer  30 , (ii) a first cladding layer  32 , (iii) a core layer  34  (as referred to as the “active region”), (iv) a second cladding layer  36 , (v) a dielectric layer  38 , and (vi) a conductive layer  40 . The thickness and shape of these layers can be varied to suit the design of the gain medium  12 . Further, one or more of the layers can be optional. 
     It should be noted that in certain embodiments, each layer  30 ,  32 ,  34 ,  36 ,  38 ,  40  extends the length of the gain medium  12  between the ends. With this design, each layer  30 ,  32 ,  34 ,  36 ,  38 ,  40  can be separated into the first facet region  24 , the second facet region  26  and the third facet region  28 . Stated in another fashion, with this design, each facet region  24 ,  26 ,  28  includes a portion of each of the layers  30 ,  32 ,  34 ,  36 ,  38 ,  40 . 
     In certain embodiments, the power source  14  (illustrated in  FIG. 1A ) is electrically connected to the conductive layer  40  and the substrate layer  30 . With this design, current from the power source  14  directed to intermediate region  28  of the gain medium  12  causes current to flow through the core layer  34  and the generation of the output beam  20  (illustrated in  FIG. 1A ). 
     As an overview, as provided herein, during operation, the core layer  34  of a unipolar device, such as a QC gain medium  12  has a much higher temperature than the core layer of a diode laser. The higher temperature of the QC gain medium  12  is caused by the relatively lower power efficiency and poor thermal conductivity (Kcore˜1-5 W/m-K) of the core layer  34  of the QC gain medium  12 . Thus, the current in the QC gain medium  12  generates quite a bit of heat. 
     The QC gain medium  12  generates light through sub-band transitions and these bands do not shrink much if at all due to temperature or current flow. In contrast, laser diodes have bandgaps that shrink when heated. This shrinkage results in the absorption of laser light which causes localized heating that further shrinks the bandgap and creates a runaway condition of self absorption and current flow along the facet. To restate, the current blocking layers in laser diodes prevent band shrinkage/bending, while for QC cores, the current blocking layers prevent the generation of excess heat through optical inefficiency of the QC core. 
     Thus, as provided herein, for a QC gain medium  12 , the temperatures of the core layer  34  is much higher at each facet  16 ,  18 , than in the area therebetween (the intermediate region  28 ) due the discontinuity of material beyond the core layer  34  (i.e. air) at the facets  16 ,  18 . 
     Unfortunately, as provided herein, the higher temperature of the core layer  34  can result in the failure of the gain medium  12  near one or both of the facets  16 ,  18 . For example, the high core temperature subjects the coatings of the facets  16 ,  18  (i) to high temperatures (e.g. approximately one hundred to two hundred and twenty (100-220) degrees Celsius) in a small region at the termination of the core layer  34  on the facets  16 ,  18 , and (ii) to very large radial thermal gradients immediately outside of the core layer  34  due to radial heat spreading geometry. These conditions cause high stress on the coating, which can lead to temperature-induced absorption, delamination and degradation. This excess temperature can cause degradation of the facets  16 ,  18 , even for uncoated facets  16 ,  18 . 
     Further, in certain embodiments, the gain medium  12  includes a solder layer (not shown) that electrically connects the core layer  34  to the respective cladding layer  32 ,  36 . As discussed above, with previous designs, as the core layer  34  is heated the facets  16 ,  18  become very hot. This can cause an Indium or other low temperature solder to be very close to the melting point. This can also cause solder pullback due to surface tension and for certain metallization schemes used on the chip. Additional heating by unwanted light impingement on the substrate layer  30 , the core layer  34 , or solder layer can raise the solder temperature above the melting point. The molten solder can then flow out over the respective facet  16 ,  18 , leading to a short. 
     The present invention teaches a way to inhibit the failure of the gain medium  12  near one or both facets  16 ,  18 . This will enhance the lifespan and durability of the gain medium  12 . Removing the facet temperature limit will also allow the gain medium to operate at a higher temperature while still maintaining reliability. 
     The substrate layer  30  is the typically the thickest part of the gain medium  12  and, as non-exclusive examples, can be made of n-doped indium phosphide (“InP”) or gallium antimonide (“GaSb”). In  FIGS. 1B-1D , the substrate layer  30  is at the bottom, is generally rectangular shaped, and can have a thickness of between approximately 100 and 200 microns. 
     The first cladding layer  32  is positioned between the substrate layer  30  and the core layer  34 , and the second cladding layer  36  is positioned on the top of the core layer  34 . With this design, the core layer  34  is positioned between the first cladding layer  32  and the second cladding layer  36 . The cladding layers  32 ,  36  have a lower refractive index than the core layer  34 . As a result thereof, the cladding layers  32 ,  36  refract light back into the core layer  34  and act as a waveguide. In one embodiment, each cladding layer  32 ,  36 , or portions thereof, can be made from n-doped InP. In certain embodiments, the InP material surrounding the core layer  34  has a relatively high coefficient of thermal conductivity (approximately ˜60 W/m/K). This will help to spread heat from the core layer  34 . This also leads to large temperature gradients of micrometer scale radially about the core layer  34 . 
     The core layer  34  is the active region of the gain medium  12 , defines the two facets  16 ,  18 , and includes a periodic series of thin layers of varying material composition forming a superlattice. The superlattice introduces a varying electric potential across the core layer  34  meaning that there is a varying probability of electrons occupying different positions over the length of the device. By suitable design of the layer thicknesses of the core layer  34 , it is possible to engineer a population inversion between two subbands in the system which is required in order to achieve laser emission. Since the position of the energy levels in the system is primarily determined by the layer thicknesses and not the material, it is possible to tune the emission wavelength of core layer  34  over a wide range in the same material system. 
     In one embodiment, for a QC gain medium  12 , the core layer  34  is a QC core layer that can utilize two different semiconductor materials, such as Indium gallium arsenide (“InGaAs”) and Aluminium indium arsenide (“AlInAs”) to form a series of potential wells and barriers for electron transitions. The output wavelength of the gain medium  12  can be changed by changing the thicknesses of the periodic layers. In one non-exclusive embodiment, the core layer  34  has a thickness of between approximately one to two microns. As mentioned above, the core layer  34  of a QC gain medium  12  typically has a very low thermal conductivity, approximately one to five W/m/K. This leads to high operating temperature of the core layer  34 , which leads to reduced lifetime. 
     In another embodiment, for an IC gain medium, the core layer  34  is an IC core layer that can utilize Gallium Antimonide (GaSb), Aluminum Indium Arsenide (AlInAs), Gallium Indium Arsenide (GaInAs), Indium Arsenide (InAs), and Aluminum Antimonide (AlSb). 
     In the embodiment illustrated in  FIGS. 1B-1D , the dielectric layer  38  is deposited on the sides of the core layer  34  to guide the current injected into the core layer  34  and guide the light generated by the core layer  34  to the facets  16 ,  18 . The dielectric layer  38  can also be referred to as an undoped or semi-insulating cladding layer and can be made of Indium Phosphide (InP). 
     The conductive layer  40  coats the second cladding layer  36  and the dielectric layer  38  and provides an electrical contact for the power source  14 . Further, the conductive layer  40  assists in removing heat created when producing light. 
     The power source  14  directs current to the gain medium  12  to force current through the core layer  34 . As provided herein, in certain embodiments, the gain medium  12  is uniquely designed to inhibit current from reaching the core layer  34  in each facet region  24 ,  26  (at or near the facets  16 ,  18 ). Stated in another fashion, in certain embodiments, the gain medium  12  is uniquely designed so that when the power source  14  directs current to the gain medium  12 , (i) current flows through the gain medium  12  in the intermediate region  28  (between the facets  16 ,  18 ), and (ii) current does not flow through the gain medium  12  in one or both facet regions  24 ,  26  (near one or both facets  16 ,  18 ). This will greatly reduce the temperatures in each facet region  24 ,  26  (at or near the facets  16 ,  18 ) in which current does not flow in, and this will reduce thermal and physical stress on the coatings on the facets  16 ,  18 . This will also reduce the likelihood of failure of one or both of the facets  16 ,  18 . 
     In certain embodiments, the relatively low thermal conductivity (Kcore˜1-5 W/m/K) of the core layer  34  means that current blocking is only required for a short distance from each facet  16 ,  18  to result in a significant reduction in temperature at each facet  16 ,  18 . Further, a QC gain medium  12  has a relatively low cold-cavity optical self-absorption at the lasing wavelength. Thus, the laser light generated in the actively pumped intermediate region  28  of the core layer  34  can pass through unpumped facet regions  24 ,  26  with only small optical absorption and associated heating. Typical waveguide losses are on the order of 1-5 cm-1. 
     The method used to inhibit the flow of current or reduce the level of current through the core layer  34  in the facet regions  24 ,  26  can vary. For example, in the embodiment illustrated in  FIGS. 1A-1D , (i) in the intermediate region  28 , both of the cladding layers  32 ,  34  are electrically conductive; and (ii) in one or both of the facet regions  24 ,  26 , one or both of the cladding layers  32 ,  34  are electrically non-conductive (fully or partly insulating). With this design, current flows through the core layer  34  in the intermediate region  28  to generate the beam  20 , and current does not flow through the core layer  34  in one or both facet regions  16 ,  18 . 
     In one embodiment, for example, (i) in the intermediate region  28 , both of the cladding layers  32 ,  34  can be made of a doped (illustrated with circles) InP; and (ii) in one or both of the facet regions  24 ,  26 , one or both of the cladding layers  32 ,  34  can be made of undoped or semi-insulating InP. With this design, one or both of the cladding layers  32 ,  34  have a higher electrical conductivity in the intermediate region  28  than in one or both of the facet regions  24 ,  26 . In one embodiment, one or both of the cladding layers  32 ,  34  are electrical conductive in the intermediate region  28 , and are non-conductive in one or both of the facet regions  24 ,  26 . 
     In  FIGS. 1C and 1D , current blocking in the facet regions  24 ,  26  is occurring in each of the cladding layers  32 ,  34 . However, it should be noted that in this embodiment, current blocking in only one of the cladding layers  32 ,  34  in the facet regions  24 ,  26  will inhibit the flow of current near the respective facet  16 ,  18 . 
     It should be noted that the core layer  34  can be mounted in an epi up or an epi down mounting configuration. 
       FIG. 2A  is a simplified perspective view of another embodiment of a laser assembly  210  that includes a gain medium  212  and a power source  214  that are somewhat similar to the corresponding components described above. In this embodiment, the gain medium  212  is again uniquely designed so that when current is directed to the gain medium  212  from the power source  214 , current flows through the gain medium  212  intermediate the facets  216 ,  218 , but the current does not flow through the gain medium  212  near one or both facets  216 ,  218 . The gain medium  212  is again divided into (i) the first facet region  224 , (ii) the second facet region  226 , and (iii) the intermediate region  228  which are similar in length to the corresponding regions described above. 
       FIG. 2B  is a simplified cut-away of the intermediate region  228  taken on line  2 B- 2 B in  FIG. 2A  near the center of the gain medium  212 .  FIG. 2C  is a simplified cut-away view of the first facet region  224  taken on line  2 C- 2 C in  FIG. 2A  near the first facet  216 . Further,  FIG. 2D  is a simplified cut-away view of the second facet region  226  taken on line  2 D- 2 D in  FIG. 2A  taken near the second facet  218 . 
     In this embodiment, the gain medium  212  again includes (i) the substrate layer  230 , (ii) the first cladding layer  232 , (iii) the core layer  234 , (iv) the second cladding layer  236 , (v) the dielectric layer  238 , and (vi) the conductive layer  240  that are somewhat similar to the corresponding components described above. However, in this embodiment, the entire first cladding layer  232  and the entire second cladding layer  234  are made of a conductive material such as doped (illustrated with circles) InP. 
     Further, in this embodiment, gain medium  212  includes (i) a lower first current blocker  250 A (insulator) positioned between the core layer  234  and the first cladding layer  232  in the first facet region  224 ; (ii) an upper first current blocker  250 B (insulator) positioned between the core layer  234  and the second cladding layer  236  in the first facet region  224 ; (iii) a lower second current blocker  252 A (insulator) positioned between the core layer  234  and the first cladding layer  232  in the second facet region  226 ; and (iv) an upper second current blocker  252 B (insulator) positioned between the core layer  234  and the second cladding layer  236  in the second facet region  226 . In this embodiment, each blocker  250 A- 252 B can be made of a partly or fully electrically insulating material. With this design, the current blocking layer is deposited directly on the core layer  234 . 
     In this embodiment, each cladding layer  232 ,  236  is electrically connected and adjacent to the core layer  234  in the intermediate region  228 . Further, each cladding layer  232 ,  236  is electrically insulated (or partly insulated) and spaced apart from the core layer  234  in each facet region  224 ,  226 . 
     It should be noted that a single blocker (either upper or lower) can be used to block current near facet  216 ,  218 . Further, each blocker  250 A,  250 B,  252 A,  252 B can be designed to be a partial insulator that allows for a reduced current flow through each facet region  224 ,  226 . 
       FIG. 3A  is a simplified perspective view of another embodiment of a laser assembly  310  that includes a gain medium  312  and a power source  314  that are somewhat similar to the corresponding components described above. In this embodiment, the gain medium  312  is designed so that when current is directed to the gain medium  312  from the power source  314 , the bulk of the current flows through and there is a higher current density in the gain medium  312  intermediate the facets  316 ,  318 , and a reduced amount of current flows through and there is a lower current density in the gain medium  312  near one or both facets  316 ,  318 . The gain medium  312  is again divided into (i) the first facet region  324 , (ii) the second facet region  326 , and (iii) the intermediate region  328  which are similar in length to the corresponding regions described above. 
       FIG. 3B  is a simplified cut-away of the intermediate region  328  taken on line  3 B- 3 B in  FIG. 3A  near the center of the gain medium  312 .  FIG. 3C  is a simplified cut-away view of the first facet region  324  taken on line  3 C- 3 C in  FIG. 3A  near the first facet  316 . Further,  FIG. 3D  is a simplified cut-away view of the second facet region  326  taken on line  3 D- 3 D in  FIG. 3A  taken near the second facet  318 . 
     In this embodiment, the gain medium  312  again includes (i) the substrate layer  330 , (ii) the first cladding layer  332 , (iii) the core layer  334 , (iv) the second cladding layer  336 , (v) the dielectric layer  338 , and (vi) the conductive layer  340  that are somewhat similar to the corresponding components described above. 
     However, in  FIGS. 3A-3D , (i) in the intermediate region  328 , both of the cladding layers  332 ,  334  are electrically conductive; and (ii) in one or both of the facet regions  324 ,  326 , both of the cladding layers  332 ,  334  are semi-insulating or non-conductive. For example, (i) in the intermediate region  328 , both of the cladding layers  332 ,  334  can be made of a doped (illustrated with circles) InP; and (ii) in one or both of the facet regions  324 ,  326 , both of the cladding layers  332 ,  334  are made of partly doped InP (illustrated with fewer circles than in the intermediate region  328 . The amount of current restriction in each facet regions  324 ,  326  can be varied to achieve the design requirements of the gain medium  312 . For example, providing sufficient voltage and current to achieve optical transparency in regions  324 ,  326  can result in reduced optical self-absorption and associated heating while minimizing heating associated with typical operating current density in the intermediate region  328 . 
     With this design, in alternative, non-exclusive embodiments, when current is directed to the gain medium  312 , the current flow in each facet region  324 ,  326  is at least approximately 40, 50, 60, 70, 80, 90, 95 or 100 percent less than the current flow in the intermediate region  328 . Stated in another fashion, with this design, the current flow in each facet regions  324 ,  326  is reduced when compared to the intermediate region  328 . 
     Stated in yet another way, the gain medium  312  is designed so that when current is directed to the gain medium  312 , (i) the current density in the core layer  334  of the intermediate region  328  is at a first current density, (ii) the current density in the core layer  334  of the first facet region  324  at a second rate that is less than the first current density, and (iii) the current density in the core layer  334  of the second facet region  326  at a third current density that is less than the first current density. With this design, in alternative, non-exclusive embodiments, when current is directed to the gain medium  312 , the first current density is at least approximately 40, 50, 60, 70, 80, 90, or 100 percent greater than the second current density and the third current density. 
     In this embodiment, each cladding layers  32 ,  34  has a higher electrical conductivity in the intermediate region  28  than in each of the facet regions  24 ,  26 . As alternative, non-exclusive examples, one or both of the cladding layers  32 ,  34  have an electrical conductivity in the intermediate region  28  that is at least approximately 50, 100, 200, or 300 percent greater than in one or both facet regions  24 ,  26 . 
     It should be noted that the current flow in one or both facet regions can be reduced or completely blocked in number of other, non-exclusive fashions. For example, modifications (e.g. diffusion-induced disordering) can be done to reduce the conductivity of the core layer in the facet regions while maintaining the conductivity in the intermediate region. 
       FIG. 4A  is a simplified perspective view of another embodiment of a laser assembly  410  that includes a gain medium  412  and a power source  414  that are somewhat similar to the corresponding components described above. In this embodiment, the gain medium  412  is designed so that when current is directed to the gain medium  412  from the power source  414 , the bulk of the current flows through (and there is a higher current density in) the gain medium  412  intermediate the facets  416 ,  418 , and a reduced amount of current flows through (and there is a lower current density in) the gain medium  412  near one or both facets  416 ,  418 . The gain medium  412  is again divided into (i) the first facet region  424 , (ii) the second facet region  426 , and (iii) the intermediate region  428  which are similar in length to the corresponding regions described above. 
       FIG. 4B  is a simplified cut-away of the intermediate region  428  taken on line  4 B- 4 B in  FIG. 4A  near the center of the gain medium  412 .  FIG. 4C  is a simplified cut-away view of the first facet region  424  taken on line  4 C- 4 C in  FIG. 4A  near the first facet  416 . Further,  FIG. 4D  is a simplified cut-away view of the second facet region  426  taken on line  4 D- 4 D in  FIG. 4A  taken near the second facet  418 . 
     In this embodiment, the gain medium  412  again includes (i) the substrate layer  430 , (ii) the first cladding layer  432 , (iii) the core layer  434 , (iv) the second cladding layer  436 , (v) the dielectric layer  438 , and (vi) the conductive layer  440  that are somewhat similar to the corresponding components described above. However, in  FIGS. 4A-4D , (i) in the intermediate region  428 , the core  434  is electrically conductive; and (ii) in one or both of the facet regions  424 ,  426 , the core  434  is only partly conductive or is non-conductive (illustrated with x&#39;s). With this design, in alternative, non-exclusive embodiments, when current is directed to the gain medium  412 , the current flow in each facet region  424 ,  426  is at least approximately 40, 50, 60, 70, 80, 90, 95 or 100 percent less than the current flow in the intermediate region  428 . 
       FIG. 5  is a simplified side view of an assembly  600  that includes a laser assembly  510  that generates an output beam  520 , an object  502  that receives the output beam  520 , and a rigid mounting base  504  that fixedly retains the laser assembly  510  and the object  502 . In this embodiment, the laser assembly  510  is an external cavity laser and the major components include a gain medium  512 , a power source  514 , a laser housing  550 , a wavelength dependent (“WD”) feedback assembly  552  that can be used to tune the primary wavelength of the output beam  520 , a cavity lens  554 , an output lens  556 , and a control system  558  that directs the current to the gain medium  512 . The power source  514  can be a generator (not shown), a battery (not shown), or another power source (not shown). 
     The gain medium  512  can be similar to any of the gain mediums  12 ,  212 ,  312 ,  412  described above. In  FIG. 5 , the gain medium  512  emits from both facets  516 ,  518 . In this embodiment, (i) the second facet  518  that faces the cavity lens  554  and the WD feedback assembly  552 , and (ii) the first facet  516  faces the output lens  556 . In this embodiment, the second facet  518  is coated with an anti-reflection (“AR”) coating and the first facet  516  is coated with a reflective coating. 
     The laser housing  550  houses, encloses, and/or retains many of the components of the laser assembly  510 . In  FIG. 5 , the laser housing  550  encloses and retains the gain medium  512 , the output lens  556 , the cavity lens  554 , and the WD feedback assembly  552 . The laser housing  550  can include a removable top that is not shown so that the components within the laser housing  550  are visible. 
     The WD feedback assembly  552  reflects light back to the gain medium  512  along the lasing axis, and is used to precisely adjust the lasing frequency of the external cavity and the wavelength of the output beam  520 . The design of the WD feedback assembly  552  can vary pursuant to the teachings provided herein. Non-exclusive examples of suitable designs include a diffraction grating, a MEMS grating, prism pairs, a thin film filter stack with a redirector, an acoustic optic modulator, or an electro-optic modulator. The WD feedback assembly  452  can be fixed or adjustable (e.g. a motor that moves a grating). 
     The cavity lens  554  is positioned between the gain medium  512  and the WD feedback assembly  552  along the lasing axis (e.g., along the Z axis), and collimates and focuses the light that passes between these components. For example, in one embodiment, the cavity lens  554  can include an aspherical lens having an optical axis that is aligned with the lasing axis. 
     The output lens  556  collimates and focuses the light that exits the first facet  516  of the gain medium  512 . For example, the output lens  556  can be somewhat similar in design to the cavity lens  554 . 
     The gain medium  512  can generate quite a bit of heat. Accordingly, in certain embodiments, the laser assembly  510  can include a temperature controller (not shown) that transfers the heat away from the gain medium  512  to control the temperature of the gain medium  512 . For example, the temperature controller can include one or more thermoelectric coolers (“TEC”) that transfer the heat from the gain medium  512 . 
       FIG. 6  is a simplified side illustration of the gain medium  612  and a heat sink  661 . In certain embodiment, the gain medium  612  fixedly secured to and positioned directly on the heat sink  661 . Further, the heat sink  661  secures the gain medium  612  to the rigid mounting base  604  (illustrated in  FIG. 5 ). In certain embodiments, the heat sink  661  can be made of a material having a high thermal conductivity so that the heat sink  661  readily transfers heat from the gain medium  612  to the mounting base  504 . In certain embodiments, the heat sink  661  has a thermal conductivity of at least approximately 500-2000 W/mK, and preferably in the range of approximately 1500-2000 W/mK. 
     In this embodiment, the gain medium  612  has a gain medium length  622 A (measured from the first facet  616  to the second facet  618 ) that is approximately equal to a heat sink length  661 A of the heat sink  661  (measured on the lasing axis). With this design, the gain medium  612  can be designed to emit from both facets  616 ,  618  without the influence from the heat sink  661 . 
       FIG. 7  is a simplified end view of another embodiment of the laser assembly  710 . In this embodiment, multiple gain media  712  (only two are illustrated in  FIG. 7 ) are positioned on a common substrate layer  730 . The gain media  712  can be similar to any of the gain mediums  12 ,  212 ,  312 ,  412  described above. 
     While a number of exemplary aspects and embodiments of a laser assembly  10  have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that any claims that may be hereafter introduced with regard to the present invention are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.