Patent Publication Number: US-2023163571-A1

Title: Quantum cascade laser with high efficiency operation and related systems and methods

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 17/002,129 filed Aug. 25, 2020, which is a divisional of U.S. application Ser. No. 16/136,713 filed Sep. 20, 2018, now U.S. Pat. No. 10,811,847, which is based upon prior filed copending Application No. 62/608,761 filed Dec. 21, 2017, which are hereby incorporated herein in their entireties by reference. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under award No. N00014-16-1-2424, which was awarded by the Office of Naval Research&#39;s funding. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to lasers and, in particular, to quantum cascade laser and related methods. 
     BACKGROUND 
     Quantum cascade lasers (QCLs) differ from traditional semiconductor diode lasers in that they do not use a p-n junction for light emission. Multiple active regions are “cascaded” so that each injected electron can emit multiple photons and therefore enhance laser gain. Each active region includes a multi-layered semiconductor material structure. This multi-layered semiconductor material structure is designed to have an electronic band structure that gives the desired emission wavelength, and is manufactured with nanometer-level thickness control. 
     Most commercially available QCLs are of the “edge-emitting” variety. In these approaches, linear ridges are created by etching vertically through the layered structure, which has been grown on a wafer substrate. These are cleaved so that the active region comprises a rectangular ridge several millimeters in length, which forms a waveguide. The laser radiation is amplified by passing back and forth along the axis of the ridge. Laser emission occurs on the end facets of these ridges. 
     In  FIG.  1 A , a typical QCL device  20  is shown. The QCL device  20  includes a substrate  21 , and an active region  22  formed thereon. Here, the active region  22  comprises a narrow configuration, and has a ridge width of 10 μm, a height of 2 μm, and a length of 5 mm. 
     SUMMARY 
     Generally, a QCL may comprise a substrate, and a sequence of semiconductor epitaxial layers adjacent the substrate and defining an active region, an injector region adjacent the active region, and a waveguide optically coupled to the active region. The active region may include a plurality of stages. Each stage may have an upper laser level and a lower laser level defining respective first and second wave functions. The upper laser level may have an upper laser level average coordinate, and the lower laser level may have a lower laser level average coordinate. The upper laser level average coordinate and the lower laser level average coordinate may have spacing therebetween of less than 10 nm. Wave functions for all active region energy levels located below the lower laser level may have greater than 10% overlap with the injector region. Energy spacing between uppermost laser levels may exceed 60 meV. A depth of the active region may be greater than 100 meV. 
     Additionally, the upper laser level average coordinate and the lower laser level average coordinate may be derived based upon ∫x ψ 2 dx. The active region may have a width exceeding 15 μm. 
     In some embodiments, the waveguide and the active region may define a double-channel shallow-ridge configuration with channel width exceeding 50 μm and a ridge width exceeding 15 μm. In other embodiments, the waveguide and the active region may define a two-level ridge configuration, and a first level ridge may extend into the active region. Also, a second level ridge may extend to a depth less than that of the first level ridge. Moreover, some embodiments may have the waveguide and the active region defining a multi-channel configuration with greater than 2 channels. 
     Moreover, the QCL may include an insulator layer over the sequence of semiconductor epitaxial layers, and an electrically conductive layer over the insulator layer. The waveguide may define first and second opposing facets, each facet configured to emit a laser output. 
     Another aspect is directed to a QCL system. The QCL system may include a QCL comprising a substrate, and a sequence of semiconductor epitaxial layers adjacent the substrate and defining an active region, an injector region adjacent the active region, and a waveguide optically coupled to the active region. The waveguide may define first and second opposing facets configured to respectively emit first and second laser outputs. The active region may include a plurality of stages. Each stage may have an upper laser level and a lower laser level defining respective first and second wave functions. The upper laser level may have an upper laser level average coordinate, and the lower laser level may have a lower laser level average coordinate. The upper laser level average coordinate and the lower laser level average coordinate may have spacing therebetween of less than 10 nm. Wave functions for all active region energy levels located below the lower laser level may have greater than 10% overlap with the injector region. Energy spacing between uppermost laser levels may exceed 60 meV. A depth of the active region may be greater than 100 meV. The QCL system may further comprise a plurality of mirrors, and a half-wave plate configured to combine the first and second laser outputs. 
     Yet another aspect is directed to a method of making a QCL. The method may include forming a sequence of semiconductor epitaxial layers adjacent a substrate and defining an active region, an injector region adjacent the active region, and a waveguide optically coupled to the active region. The active region may include a plurality of stages, each stage having an upper laser level and a lower laser level defining respective first and second wave functions. The upper laser level may have an upper laser level average coordinate, and the lower laser level may have a lower laser level average coordinate. The upper laser level average coordinate and the lower laser level average coordinate may have spacing therebetween of less than 10 nm. Wave functions for all active region energy levels located below the lower laser level may have greater than 10% overlap with the injector region. Energy spacing between uppermost laser levels may exceed 60 meV. A depth of the active region may be greater than 100 meV. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a schematic diagram of a prior art narrow-ridge configuration with typical ridge width of less than 10 μm. 
         FIG.  1 B  is a schematic diagram of a broad-area configuration with ridge width of greater than 15 μm and reduced laser core thickness, according to the present disclosure. 
         FIG.  2    is a diagram of experimental pulsed optical power versus current and voltage versus current characteristics for two exemplary structures with J tr =0.4 kA/cm 2  and J tr =1.3 kA/cm 2 , according to the present disclosure. 
         FIGS.  3 A and  3 B  are band diagrams of the structure with J tr =0.4 kA/cm 2  and J tr =1.3 kA/cm 2 , respectively, according to the present disclosure. 
         FIG.  4    is a band diagram of an optimized structure: large active region/injector coupling, vertical transition, E 54 ˜80 meV, and E 32 &gt;100 meV, according to the present disclosure. 
         FIG.  5    is a schematic diagram of a double-channel shallow ridge-waveguide configuration, according to the present disclosure. 
         FIG.  6    is a schematic diagram of a double channel shallow ridge waveguide, according to the present disclosure. 
         FIG.  7    is a diagram of a current density ratio with rollover current density, according to the present disclosure. 
         FIG.  8    is a diagram of temperature spread near laser core, according to the present disclosure. 
         FIG.  9    is a diagram of isothermal curves showing data identical to  FIG.  7   . 
         FIG.  10    is a diagram of a first order transverse mode electric field magnitude, according to the present disclosure. 
         FIG.  11    is a diagram of a second order transverse mode electric field magnitude, according to the present disclosure. 
         FIG.  12    is a schematic diagram of a two-level etch ridge-waveguide configuration, according to the present disclosure. 
         FIG.  13    is a schematic diagram of a multi-channel shallow ridge-waveguide configuration, according to the present disclosure. 
         FIG.  14    is a schematic diagram of a QCL system, according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which several embodiments of the invention are shown. This present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Like numbers refer to like elements throughout, and base  100  reference numerals are used to indicate similar elements in alternative embodiments. 
     INTRODUCTION 
     The 10 W continuous wave (CW) optical power level is presently considered to be potentially difficult with single QCL emitters, which undermines QCLs positioning as a technology of the future in some applications (where high power is needed in a small package). The present disclosure describes a design for a compact laser module projected to deliver over 30 W of optical power into a high brightness Midwave Infrared (MWIR) beam. The unprecedented combination of size, weight, and power for the module may be helpful in these applications and other high power MWIR laser applications. 
     Broad-Area Configuration 
     Optical power from traditional narrow-ridge QCLs will likely stay close to 5 W [1, 2] (which results into 3.5 W to 4.0 W on system level) due to two main reasons: limited wall plug efficiency and optical damage of the front facet. CW MWIR QCL efficiency has been clamped at approximately 20% for the last five years and device modeling results indicate that there is not much room for further improvement. At the same time, even if higher efficiency devices were available, it would be very difficult to substantially increase power level as QCLs tend to fail at optical power densities of 10 MW/cm 2 , which roughly corresponds to total power level of 5 W for narrow (10 μm-wide) devices. 
     Increase in power level from single QCL emitters, therefore, reduces to (a) increase in laser core dimensions (power scaling) and (b) increase in the emission aperture size (lower optical power density at the output facet). 
     The broad-area configuration QCL  23  (substrate  24 ,  FIG.  1 B ) had never been actively pursued to increase CW or average optical power of QCLs before 2016 due to a firm belief that laser core self-heating would prevent CW lasing. The QCL self-heating effect is largely caused by a low thermal conductivity of the laser core  25  in the epitaxial growth direction. To reduce temperature, a traditional approach is to employ narrow ridge devices (7-10 μm), utilize semi-insulating InP lateral overgrowth, and mount the devices epi-side down. 
     Laser core thermal resistance can be drastically reduced and laser self-heating can be suppressed by reducing laser core thickness. Device modeling results demonstrate that this will lead to a lower temperature for the same total electrical power dissipated in the thinner core, or, which is of primary interest here, to the same average temperature for much wider devices [3,4]. This makes power scaling with lateral device dimensions possible. In the following, a laser design and laser packaging tailored to the broad-area configuration is disclosed. The design ideas presented here can also be used for optimizing performance of traditional (narrow ridge) QCLs (See  FIG.  1 A ). 
     Laser Core Design 
     CW broad-area QCL performance depends on laser thermal resistance as well as on its pulsed laser characteristics, such as injection efficiency for the upper laser level, η i , differential modal gain, Γg, waveguide losses, α w , and transparency current, J tr . The present disclosure provides analysis of several QCL designs in the broad-area configuration and observed a general trend: structures that have a low transparency current tend to outperform structures with a higher transparency current even when pulsed laser efficiency is the same in both cases. This is illustrated below. 
       FIG.  2    shows a diagram  60  of experimental pulsed optical power versus current and voltage versus current characteristics for two exemplary structures with essentially identical measured maximum efficiency (J tr =0.4 kA/cm 2  (cross-hatched) and J tr =1.3 kA/cm 2  (normal line)). However, the basic parameters characterizing their performance (η i , Γg, α w , and J tr ) were different. Specifically, ratio of J tr  for the two structure exceeded factor of three. The structure with J tr =0.4 kA/cm 2  had a lower η i  (57% versus 72%), which explains why the two structures had similar overall pulsed efficiency. 
     Measured η i , Γg, α w , and J tr  for the two structures were used for CW optical power projections in the broad-area configuration (ridge width&gt;10 μm) using the model developed and validated in Ref. 4. It was determined that the structure with J tr =0.4 kA/cm 2  had a much more efficient power scaling with increase in ridge width, i.e. a higher projected total CW optical power. Qualitatively, the root cause of the different projected behavior is the following. In addition to increase in ridge width, power scaling assumes employment of long cavity HR-coated devices that have low mirror losses, α m . A combination of a low J tr  and low mirror losses results into a low threshold current density J th  as the latter is defined by (Γg and α w  were approximately the same for the two designs): 
     
       
         
           
             
               J 
               th 
             
             = 
             
               
                 J 
                 tr 
               
               + 
               
                 
                   
                     α 
                     m 
                   
                   + 
                   
                     α 
                     w 
                   
                 
                 
                   Γ 
                   ⁢ 
                   g 
                 
               
             
           
         
       
     
     The low J th , in turn, leads to a lower laser core temperature at threshold and, therefore, a higher slope efficiency in CW mode. As a consequence, one of the goals in broad-area QCL design should be a reduction in transparency current density. 
     Transparency current density is a function of carrier backscattering to the lower laser level 3 and injection efficiency into the upper laser level 4, both very difficult to predict from first principles.  FIGS.  3 A- 3 B  compares band-diagrams for the two structure. The increased carrier backscattering for the structure with J tr =1.3 kA/cm 2  can be explained with carrier build up on the active region level 2 located below the lower laser level 3. The active region is defined here as a sequence of layers where the upper laser level 4 and lower laser level 3 are mostly localized. The buildup is caused by a low active region/injector coupling (tight level 2 confinement within the active region). Injector is defined here as the remaining sequence of layers in the laser core. For the structure with J tr =0.4 kA/cm 2 , on the other hand, carriers more easily escape from level 2 into the injector due to a larger active region/injector coupling, preventing carrier build up on level 2 and, therefore, reducing carrier backscattering to level 3. The band diagram analysis presented above suggests that the backscattering problem can be further suppressed by further increasing active region/injector coupling. 
     However, the active region/injector coupling cannot be made arbitrarily large for the following reasons:
         1. Large coupling can delocalize the lower laser level (so-called bound-to-continuum or continuum-to-continuum active region designs), broadening gain spectrum and lowering peak gain. Lower peak gain entails increase in threshold current density, which is very undesirable for broad-area devices.   2. Large coupling can also reduce energy spacing E 54 , which increases parasitic carrier injection into the lower laser level, leading to a higher transparency current density and lower slope efficiency.       

     The new QCL design principles that were developed in this disclosure and described below may provide an approach to all the potential problems listed above. Specifically, laser core layers thicknesses and compositions are carefully adjusted to achieve one or more the following design characteristics in a single structure:
         1. Increased active region/injector coupling: squared wave functions for all active region energy levels located below the lower laser level have &gt;10% overlap with the injector region (reduced carrier buildup in the active region)   2. Vertical laser transition: distance between average coordinates for the upper and lower laser level &lt;10 nm   3. E 54 &gt;60 meV (suppressed carrier injections into states other than the upper laser level)   4. Active region depth E 32 &gt;100 meV (further reduces carrier backscattering)       

     The disclosed design provides a larger active region depth and/or active region/injector coupling than either single-phonon, double-phonon, or non-resonant extraction designs and, in contrast to the bound-to-continuum design, it is based on a vertical laser transition. Therefore, it is substantially different from all the main QCL designs reported in literature. 
     All these design characteristics are combined in an exemplary laser design presented in  FIG.  4   . For that design, the above mentioned parameters had the following values:
         1. Active region/injector coupling: squared wave functions for all active region energy levels located below the lower laser level have &gt;15% overlap with the injector region   2. Vertical laser transition: distance between average coordinates for the upper and lower laser level &lt;50 nm   3. E 54 ˜80 meV   4. Active region depth E 32 &gt;110 meV in a broad bias range (80 to 100 kV/cm)       

     The design is projected to have a high performance in the broad-area configuration. The design will also likely outperform other active region designs such single-phonon, double-phonon, bound-to-continuum, and non-resonant-extraction designs in the traditional narrow ridge configuration. 
     Beam Quality Control 
     Discrimination against high-order modes is one of the technical challenges in broad-area QCL design. Possible approaches include tilted-cavity waveguide geometry [5], employment of FIB-defined high order mode discriminating features into the waveguide [6], and other methods. The simplest and most practical solution addressed here is the shallow-ridge waveguide (SRW) configuration [7, 8], where etching of the ridge is stopped before reaching the laser core ( FIG.  5   ). Different sequence of layers in regions #1 and #2 above the laser core creates a lateral contrast in effective refractive index and a contrast in waveguide losses. Both can be used for mode control by carefully choosing etching depth and values for d1 and d2. In addition, current spreading, and therefore, gain-guiding, can also be controlled by carefully choosing etching depth. This approach is very cost effective and compatible with a high throughput production. 
     The SRW configurations was explored in Refs.  7  and  8  only in pulsed mode and no SRW design principles for CW operation were outlined in those reports. In addition, only gain-guided configuration was explored in Ref. 8 that is unlikely to be sufficient to ensure an efficient high-order mode suppression in CW mode: laser core self-heating in CW mode reaches its highest value at the center of the laser core, which favors higher order modes. Therefore, a large mode discrimination in terms of losses/gain and mode overlap factor has to be applied to achieve a high quality beam. 
     The present disclosure provides a numerical analysis for the optimal design of the SRW described below. The shallow ridge configuration uses etching features that leaves the slab laser core intact, as shown in  FIG.  6   . A COMSOL model incorporating current flow, joule heating, and waveguide modes was built to study the effects of channel configuration on a modified waveguide of an exemplary structure S2 that emitted at 5.7 μm [9]. The waveguide used S2 as a base design, reducing the epi-side InP:Si 1E17 cm −3  layer to 3 μm, and adding a 300 nm substrate side and 250 nm epi-side InGaAs:Si 4E17 cm −3  layer around the laser core in a separate confinement heterostructure configuration. 
     To simulate the effects of current spreading on gain, and thus gain guided modes, assumptions were made about the temperature behavior of the device. Threshold is treated with a 140 K characteristic temperature [9]: 
     
       
         
           
             
               J 
               th 
             
             = 
             
               
                 ( 
                 
                   
                     J 
                     tr 
                   
                   + 
                   
                     
                       
                         α 
                         
                           w 
                           ⁢ 
                           g 
                         
                       
                       + 
                       
                         α 
                         m 
                       
                     
                     
                       Γ 
                       ⁢ 
                       g 
                     
                   
                 
                 ) 
               
               ⁢ 
               
                 e 
                 
                   
                     T 
                     - 
                     
                       T 
                       R 
                     
                   
                   
                     T 
                     0 
                   
                 
               
             
           
         
       
     
     Transparency current is taken to have the same behavior with characteristic temperature T 0 , forcing differential gain having an inverse relationship: 
     
       
         
           
             
               
                 
                   
                     J 
                     tr 
                   
                   ( 
                   T 
                   ) 
                 
                 = 
                 
                   
                     
                       J 
                       tr 
                     
                     ( 
                     
                       T 
                       R 
                     
                     ) 
                   
                   ⁢ 
                   
                     e 
                     
                       
                         T 
                         - 
                         
                           T 
                           R 
                         
                       
                       
                         T 
                         0 
                       
                     
                   
                 
               
                   
             
             ⁢ 
             
 
             
               
                 g 
                 ⁡ 
                 ( 
                 T 
                 ) 
               
               = 
               
                 
                   g 
                   ⁡ 
                   ( 
                   
                     T 
                     R 
                   
                   ) 
                 
                 ⁢ 
                 
                   e 
                   
                     
                       T 
                       - 
                       
                         T 
                         R 
                       
                     
                     
                       - 
                       
                         T 
                         0 
                       
                     
                   
                 
               
             
           
         
       
     
     This gives form for gain (and imaginary index) in the laser core: 
     
       
         
           
             gain 
             = 
             
               
                 
                   g 
                   ⁡ 
                   ( 
                   
                     T 
                     R 
                   
                   ) 
                 
                 ⁢ 
                 
                   ( 
                   
                     
                       
                         J 
                         y 
                       
                       ⁢ 
                       
                         e 
                         
                           
                             T 
                             - 
                             
                               T 
                               R 
                             
                           
                           
                             - 
                             
                               T 
                               0 
                             
                           
                         
                       
                     
                     - 
                     
                       
                         J 
                         tr 
                       
                       ( 
                       
                         T 
                         R 
                       
                       ) 
                     
                   
                   ) 
                 
               
               = 
               
                 
                   
                     4 
                     ⁢ 
                     π 
                   
                   
                     λ 
                     0 
                   
                 
                 ⁢ 
                 
                   n 
                   ″ 
                 
               
             
           
         
       
     
     Taking this as the temperature and (growth direction) current dependent form of gain in the laser core, the simulation is run in two steps. The first finds steady state current and temperature distribution assuming that the epi-side is kept at 293 K and 0 V, with a positive voltage applied to the substrate side. 
     Current spreads beyond the width of the shallow ridge as it reaches the laser core. Temperature buildup occurs nearly entirely in the laser core. Temperature spreads through the substrate around the area of high current density (high heat generation) and reaches the heat sink. 
     The current distribution on the epitaxial side of the laser core features spikes in density just below the walls of the central ridge, creating “ears”. Overall gain begins to degrade before the “ears” exceed rollover due to device heating, so local spikes in current density should not be a concern. The substrate side shows a more Gaussian behavior, with current spreading causing a wider and lower distribution. Gain is then stronger in the epi-side direction. Full width at half maximum (FWHM) of the current spreading for the 30 μm ridge is &gt;50 μm. 
       FIG.  7    shows a diagram  80  of a current density ratio with rollover current density at a 7 V bias for a 30 μm ridge and 60 μm channels etched 2.7 μm deep. Curve  81  indicates epi-side (top) current density along the edge of the laser core, and curve  82  indicates substrate side (bottom) current density. 
     Temperature build up in the device (diagrams  83 ,  84  of  FIGS.  8    {7 V bias, 30 μm ridge, 60 μm wide by 2.7 μm deep channels} and  9 ) causes performance degradation past 7 V bias in the S2 laser core. The temperature distribution at increasing bias gives a gain distribution that allows the secondary transverse mode to become more competitive in terms of modal gain. The placement of the channels allows for greater waveguide absorption in the higher order modes, as the increased proximity to SiN and Au gives enhanced losses. 
       FIGS.  10  and  11    include diagrams  85 ,  86  respectively showing first order and second order transverse mode electric field magnitude. The central ridge width can be used to tailor maximum gain for the fundamental mode. Wider central ridges are more accommodating to higher order modes, and may allow lasing of the second order mode at high bias. Too narrow a ridge will concentrate the current density into too small a region relative to fundamental mode size, and will also lead to greater local heating, reducing performance. 
     The depth of the channel provides vertical confinement. In general, deeper channels lead to greater mode confinement in the laser core, and thus, more modal gain. There is a “squeezing” effect for higher order modes, allowing them to spread laterally with deep enough channels, pushing them toward the high gain central region. Channel depth should be chosen (after central ridge width) to allow just enough gain to not exceed mirror losses for higher order modes to allow for maximum fundamental mode net gain. 
     The overall width of the channels was constructed to allow for an overall width of 150 μm for central ridge and channels combined. Wide channels are necessary to provide as much loss as possible for higher order modes. This width left 100 μm of un-etched material on each side for structural support during epi-down mounting. The optimal design for maximum gain with no net gain for higher order modes was a 30 μm ridge with 60 μm wide by 2.7 μm deep etched channels in a 10 mm long device. 
     The structure reported in Ref. 7 was the only shallow-ridge RWG QCL structure explored so-far. It had 50 μm-wide channels. Channel width&lt;50 μm is typically used for QCLs. The present disclosure provides that it is advantageous to use &gt;50 μm wide channels to ensure higher order mode suppression in high duty cycle operation and only narrow ridges without metal contact openings at the edges of the chip are sufficient for structural support in the epi-down configuration required for CW operation. 
     Current spreading and mode losses are controlled with choosing proper etching depth in the proposed configuration. Mode losses can be independently controlled in the modified “two-level etch” configuration presented in  FIG.  12   . When width of the lower ridge is less than FWHM of current spreading, current uniformly pumps the active region. At the same time, depth of the first etch controls higher mode losses. This configuration has not been explored so far for CW QCLs. 
     The traditional double-channel SRW can also be extended to a multiple-channel (&gt;2 channels) design shown in  FIG.  13   . In the multi-channel SRW configuration, exact position for each ridge and channel, as well as etching depth are chosen to suppress high order mode operation while insuring that overall efficiency is not sacrificed. 
     Facet Damage Control and Packaging 
     As mentioned above, high power CW operation leads to QCL output facet damage. The problem is exacerbated (damage threshold reduces) when HR-coating is applied to the back facet and AR-coating is applied to the front facet of the laser due to a residual absorption in the coating materials. To maximize optical power from a single emitter (especially in the broad-area configuration with a very high output power level), the present disclosure provides a concept of a new packaging for a single QCL emitter that can be used both for traditional QCLs and for the broad-area devices. 
     A QCL chip in this configuration is used without any coating material applied to either facet or with coating material that has a high damage threshold. The basic idea of the new design is to independently collect optical power emitted from the two laser facets and then recombine the two beams using one of many possible beam combining approaches, such as the spatial beam combining (placing the two beam close to each other), polarization beam combining, etc. 
       FIG.  14    illustrates a high power module utilizing polarization beam combining. After the propagation direction of the optical beam emitted from the right facet of the laser is changed by 180° (two mirror reflections), polarization for the beam is rotated by 90° using a half-wave plate, followed by polarization beam combining with the beam emitted from the front laser facet using a polarizer. Beam combining for this approach has the advantage of low losses: 1-3% for each of the optical elements in the setup. An estimated module size for this configuration is 2″×2″×1.5″, resulting into an unprecedented combination of size, weight, and power for a MWIR emitters. This configuration can be extended to multiple emitters. 
     Specifically, beam from several uncoated chips lasing at different wavelengths can be combined into a single high brightness beam using polarization beam combining. 
     Referring now additionally to  FIGS.  3 A- 5   , another embodiment of the QCL  123  is now described. In this embodiment of the QCL  123 , those elements already discussed above with respect to  FIG.  1 B  are incremented by 100 and most require no further discussion herein. This embodiment differs from the previous embodiment in that this QCL  123  illustratively includes a substrate  124 , and a sequence of semiconductor epitaxial layers adjacent the substrate and defining an active region  125 , an injector region  127  adjacent the active region, and a waveguide  126  optically coupled to the active region. The active region  125  illustratively includes a plurality of stages, each stage having an upper laser level and a lower laser level defining respective first and second wave functions  63 - 66 ,  68 ,  70 - 72 ,  74 - 77  (diagrams  62 ,  67  &amp;  73 ). The upper laser level has an upper laser level average coordinate, and the lower laser level has a lower laser level average coordinate. The upper laser level average coordinate and the lower laser level average coordinate have spacing therebetween of less than 10 nm. Wave functions for all active region energy levels located below the lower laser level have greater than 10% overlap with the injector region. Energy spacing between uppermost laser levels exceeds 60 meV. A depth of the active region is greater than 100 meV. 
     Moreover, the QCL  123  illustratively includes an insulator layer  128  over the sequence of semiconductor epitaxial layers, and an electrically conductive layer  130  over the insulator layer. Additionally, the upper laser level average coordinate and the lower laser level average coordinate are derived based upon ∫xψ 2 dx. 
     In illustrated embodiment, the waveguide  126  and the active region  125  define a double-channel shallow-ridge configuration with channel width exceeding 50 μm and a ridge width exceeding 20 μm. The active region  125  has a width exceeding 20 μm. Also, the QCL  123  defines two channels  131 ,  132  abutting opposing longitudinal sides of the ridge. 
     Referring now additionally to  FIG.  12   , another embodiment of the QCL  223  is now described. In this embodiment of the QCL  223 , those elements already discussed above with respect to  FIGS.  3 A- 5    are incremented by 200 and most require no further discussion herein. This embodiment differs from the previous embodiment in that this QCL  223  illustratively includes the waveguide  226  and the active region  225  defining a two-level ridge configuration. The QCL  223  illustratively includes a first level ridge  231 ,  232  extending into the active region  225 . Also, the QCL  223  illustratively includes a second level ridge  233 ,  234  extending to a depth less than that of the first level ridge  231 ,  232 . 
     Referring now additionally to  FIG.  13   , another embodiment of the QCL  323  is now described. In this embodiment of the QCL  323 , those elements already discussed above with respect to  FIGS.  3 A- 5 ,  12    are incremented by 300 and most require no further discussion herein. This embodiment differs from the previous embodiment in that this QCL  323  illustratively includes the waveguide  326  and the active region  325  defining a multi-channel configuration with four channels  335   a - 335   d.    
     Referring now additionally to  FIG.  14   , a QCL system  440  including another embodiment of the QCL  423  is now described. In this embodiment of the QCL  423 , those elements already discussed above with respect to  FIGS.  3 A- 5 ,  12 - 13    are incremented by 400 and most require no further discussion herein. The QCL system  440  illustratively includes a QCL  423  as described in the embodiments (QCLs  123 ,  223 ,  323 ,  523 ) disclosed herein. The waveguide defines first and second opposing facets  441 ,  442 , each facet configured to emit a laser output. 
     The QCL  423  system further comprise a plurality of optical elements  443 - 448   b  configured to combine the first and second laser outputs into a combine laser output  450 . The plurality of optical elements  443 - 448   b  comprises first and second collimating lenses  448 - 448   b  configured to collimate laser outputs respectively from the first and second opposing facets  441 ,  442 . The plurality of optical elements  443 - 448   b  comprises first, second, and third mirrors  443 - 445  configured to direct the first and second laser outputs, a half-wave plate  446  receiving the second laser output, and a polarizer  447  configured to reflect the first laser output and pass the second laser output. 
     Yet another aspect is directed to a method of making a QCL  123 . The method includes forming a sequence of semiconductor epitaxial layers adjacent a substrate  124  and defining an active region  125 , an injector region  127  adjacent the active region, and a waveguide  126  optically coupled to the active region. The active region  125  includes a plurality of stages, each stage having an upper laser level and a lower laser level defining respective first and second wave functions  63 - 66 ,  68 ,  70 - 72 ,  74 - 77  (diagrams  62 ,  67  &amp;  73 ). The upper laser level has an upper laser level average coordinate, and the lower laser level has a lower laser level average coordinate. The upper laser level average coordinate and the lower laser level average coordinate have spacing therebetween of less than 10 nm. Wave functions for all active region energy levels located below the lower laser level have greater than 10% overlap with the injector region. Energy spacing between uppermost laser levels exceeds 60 meV. A depth of the active region is greater than 100 meV. 
     Referring now additionally to  FIG.  6   , another embodiment of the QCL  523  is now described. In this embodiment of the QCL  523 , those elements already discussed above with respect to  FIGS.  3 A- 5 ,  12 - 13    are incremented by 500 and most require no further discussion herein. This embodiment differs from the previous embodiment in that this QCL  523  illustratively includes the electrically conductive layer  530  fills the first and second channels  531 ,  532 . Also, in this illustration of a double-channel shallow-ridge configuration, the sequence of semiconductor epitaxial layers is indicated. 
     REFERENCES 
     Contents of Each Reference are Hereby Incorporated by Reference in their Entirety 
     
         
         [1] A. Lyakh, R. Maulini, A. Tsekoun, R. Go, and C. K. N. Patel, Optics Express 20, 4382 (2012) 
         [2] Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, Applied Physics Letters 98, 181102 (2011) 
         [3] M. Semtsiv and W. Masselink, Applied Physics Letters 109. 203502 (2016) 
         [4] M. Suttinger, R. Go, P. Figueiredo, A. Todi, H. Shu, J. Leshin, and A. Lyakh, Optical Engineering 57, 011011 (2017) 
         [5] Y. Bai, S. Slivken, Q. Lu, N. Bandyopadhay, and Razeghi, Applied Physcis Letters 101, 081106 (2012) 
         [6] R. Kaspi, S. Luong, C. Yang, C. Lu, T. Newell, and T. Bate, Applied Physics Letters 109, 211102 (2016) 
         [7] A. Lyakh, P. Zory, D. Wasserman, G. Shu, C. Gmachl, M. D&#39;Souza, D. Botez, and D. Bour, Applied Physics Letters 90, 141107 (2007) 
         [8] I. Sergachev, R. Maulini, A. Bismuto, S. Blaser, T. Gresch, and A. Muller, Optics Express 24, 19063 (2016) 
         [9] Matthew M Suttinger, Rowel Go, Pedro Figueiredo, Ankesh Todi, Hong Shu, Jason Leshin, and Arkadiy Lyakh, Optical Engineering 57, 011011 (2017) 
       
    
     Many modifications and other embodiments of the present disclosure will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the present disclosure is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.