Abstract:
A broadband, integrated quantum cascade laser is disclosed, comprising ridge waveguide quantum cascade lasers formed by applying standard semiconductor process techniques to a monolithic structure of alternating layers of claddings and active region layers. The resulting ridge waveguide quantum cascade lasers may be individually controlled by independent voltage potentials, resulting in control of the overall spectrum of the integrated quantum cascade laser source. Other embodiments are described and claimed.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/902,302, filed 20 Feb. 2007. 
     
    
     GOVERNMENT INTEREST 
       [0002]    The invention claimed herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title. 
     
    
     FIELD 
       [0003]    The present invention relates to quantum cascade lasers. 
       BACKGROUND 
       [0004]    Quantum cascade lasers are semiconductor devices that emit electromagnetic radiation in the mid-to far infrared frequency spectrum, with numerous applications, such as for example chemical monitoring, medical diagnostics, collision avoidance using lidar, and free space communication, to name just a few. Quantum cascade laser are unipolar devices, where a single type of carrier, usually electrons, emit photons when transitioning from an energy band to a lower energy band. Energy bands are engineered with the use of quantum wells. A quantum cascade laser comprises a number of active regions, each active region including an injector region adjacent to a quantum well. Electrons tunnel through an injector region so as to be injected into an adjacent quantum well. The energy bands are structured such that an electron injected into a quantum well emits a photon when transitioning from an energy band to a lower energy band within that quantum well, where the electron then tunnels through the next injector to the next quantum well, where it again may transition from an energy band to a lower energy band within that next quantum well to emit another photon. This cascading process continues, and is one of the reasons why quantum cascade lasers are efficient sources of laser radiation. 
         [0005]    For some applications, it is desirable to have a tunable broadband laser source. For example, a tunable broadband source may be of utility in probing gases for their chemical makeup, where the spectral content of the probing signal gives information about the chemical species, or may be of utility in a communication system, to name a couple of examples. 
         [0006]      FIG. 1  illustrates in a simplified pictorial cross-sectional view a prior art quantum cascade laser for providing broadband radiation. In between cladding layers  102  and  104  are two active regions, each providing radiation at a different wavelength. For ease of illustration, only two active regions are illustrated in  FIG. 1 , active region  106  to provide radiation having a first wavelength (λ 1 ) and active region  108  to provide radiation having a second wavelength (λ 2 ). In practice, however, there may many active regions, each one providing electromagnetic radiation at a different wavelength. The index of refraction of cladding layers  102  and  104  are less than that of the active regions, so that the structure of layers  102 ,  104 ,  106 , and  108  form a ridge waveguide. In the particular example of  FIG. 1 , a voltage potential is provided between metal layer  110  and substrate layer  112 , and the electromagnetic propagation is along the z-axis direction as indicated by the XYZ coordinate system illustrated in  FIG. 1 . 
         [0007]    Each active region in  FIG. 1  includes an injector region with an adjacent quantum well. A quantum well may be referred to as gain region. The injector region usually is a superlattice. The layers making up the superlattice injector regions and the quantum wells are formed along the y-axis direction by various well-known techniques, such as molecular beam epitaxy. By including many active regions, each emitting electromagnetic radiation at a different wavelength, a broadband laser source may be synthesized. However, a problem with quantum cascade lasers of the type depicted in  FIG. 1  is that it may be difficult to control the individual active regions. For example, some active regions may provide more power than other active regions, and it may be difficult to individually tune the active regions so as to provide a desired spectral laser output. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates a cross sectional view of a prior art multi-band quantum cascade laser. 
           [0009]      FIGS. 2 and 3  illustrate cross-sectional views of a quantum cascade laser according to an embodiment. 
           [0010]      FIG. 4  illustrates a perspective view of a quantum cascade laser according to an embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0011]    In the description that follows, the scope of the term “some embodiments” is not to be so limited as to mean more than one embodiment, but rather, the scope may include one embodiment, more than one embodiment, or perhaps all embodiments. 
         [0012]      FIG. 2  illustrates is a simplified pictorial cross-sectional representation of a quantum cascade laser according to an embodiment, where for ease of illustration, only three ridge waveguide lasers are shown. In practice, there may be many individual ridge waveguide lasers, each emitting electromagnetic radiation at a different wavelength so as to provide a broadband source of radiation. A layer with a letter “c” denotes a cladding layer, and a layer with the letter “a” denotes an active layer, where an active layer includes an injector region and an adjacent quantum well (gain region). 
         [0013]    Layers  202 ,  204 , and  206   a  comprise a first quantum cascade laser, layers  206   b ,  208   b , and  210   a  comprise a second quantum cascade laser, and layers  210   b ,  212   b , and  214  comprise a third quantum cascade laser. Current is injected into the first quantum cascade laser by applying a voltage difference to metal contact layers  216  and  218 . Similarly, a voltage difference applied to metal contact layers  220  and  222  provides current to the second quantum cascade laser, and a voltage difference applied to metal contact layers  224  and  226  provides current to the third quantum cascade laser. These three voltage differences may be applied independently of each other. This allows individual control of each quantum cascade laser. 
         [0014]    The three quantum cascade lasers shown in  FIG. 2  are formed from a single monolithic structure comprising various layers of cladding and active regions. This is made clear by referring to  FIG. 3 , where the crosshatched region denotes that portion of the monolithic structure which has been etched away. Note that in  FIG. 3  the metal contact layers are not shown. In  FIG. 3 , layers  302  through  314  are alternating layers of cladding and active regions. The correspondence between the layers in  FIG. 3  and the layers in  FIG. 2  is easily made. Cladding layer  202  in  FIG. 2  is that part of cladding layer  302  remaining after an etching process. Active region layer  204  in  FIG. 2  is that part of active region layer  304  in  FIG. 3  remaining after the etching process. Cladding layers  206   a  and  206   b  in  FIG. 2  are those parts of cladding layer  306  remaining after the etching process. 
         [0015]    Continuing with making the correspondence between  FIG. 2  in  FIG. 3 , active region layers  208   a  and  208   b  are formed from the active region layer  308 , cladding layers  210   a  and  210   b  are formed from cladding layer  310 , active region layers  212   a  and  212   b  are formed from active region layer  312 , and cladding layer  214  is formed from cladding layer  314 . Metal contact layers  216 ,  218 ,  220 ,  222 ,  224 , and  226  are formed by depositing metal on their respective layers. Standard semiconductor processing techniques may be used to form the structure indicated in  FIG. 2  from the monolithic structure indicated in  FIG. 3 . 
         [0016]    It is a matter of semantics whether one may consider layers  206   a  and  206   b  to be two distinct layers or one layer, for they are formed from the same layer ( 306 ) by an etching process. Similar remarks apply to some of the other layers, such as for example layers  208   a  and  208   b  which are formed from the single layer  308 , and so forth. However, note that active region layer  208   a  does not play an active role in the quantum cascade laser formed from layers  202 ,  204 , and  206   a , nor does it play an active role in the quantum cascade laser formed from layers  206   b ,  208   b , and  210   a . Because of the etching process, layer  208   a  is electrically isolated from (i.e., not in electrical contact with) active layer  208   b.    
         [0017]    A simplified perspective view of the embodiments of  FIG. 2  is illustrated in  FIG. 4 . The numerals in  FIG. 2  indicating the various components of the embodiment are also used in  FIG. 4  to denote the same components. Note the orientation of the XYZ coordinate system in  FIG. 4  relative to that of the previous figures. Propagation is along the z-axis direction. For other embodiments, an etching process may be used so that the shapes of cladding layers  206   a ,  210   a , and  214 , and the layers beneath them, are such that contacts  218 ,  212 , and  226  may be placed to the right of their respective quantum cascade lasers, where the “right” direction may be taken along the positive x-axis direction of the XYZ coordinate system. 
         [0018]    For some embodiments, a typical cross-sectional size for a ridge waveguide quantum cascade laser is about 1.5 μm wide by about 14 μm high, where width refers to the x-axis direction and height refers to the y-axis direction. Although not shown in  FIG. 4 , Bragg diffraction gratings may be formed on each of the top cladding layers for each quantum cascade laser so that a single waveguide mode is amplified in each quantum cascade laser. For each quantum cascade laser, a high reflective coating may be formed on a face, where the other face serves as a partial reflector, so that an optical cavity, such as for example a Fabre Perot cavity, may be realized. (The faces are parallel to the x-y plane.) For some embodiments the cavity length for each quantum cascade laser may be on the order of 1.5 mm to 3 mm. For some embodiments the separation between each quantum cascade laser may be about 50 μm. The height of the overall structure depends upon how many quantum cascade lasers are formed, but a typical height for some embodiments may be about 100 μm. 
         [0019]    The ridge waveguide quantum cascade lasers and metal contact pads may be defined by a combination of photo-lithographic patterning, dry and wet etching, oxide and metal evaporation, and MOCVD (metal-organic chemical vapor deposition) epitaxial growth. Various materials may be used for the cladding layers, the injectors and quantum wells within the active region layers, and the substrate. The materials for the cladding layers and active region layers may be lattice strained or lattice matched to their respective substrates. 
         [0020]    For some embodiments, the compounds InP, GaAs, or GaSb may be used for a substrate. Superlattice structures may be used in the cladding layers and active region layers. Particular examples include a GaInAs/AlInAs (gallium indium arsenide/aluminum indium arsenide) heterostructure on an InP substrate; an AlGaAs/GaAs (aluminum gallium arsenide/gallium arsenide) heterostructure on a GaAs substrate; and an AlGaSb/InAs (aluminum gallium antimonide/indium arsenide) heterostructure on a GaSb substrate. Further examples include a superlattice composition of GaInAs/AlInAs for a quantum cascade laser on an InP substrate; a superlattice composition of AlSb/InAs for a quantum cascade laser on a GaSb substrate; and a superlattice composition of AlGaAs/GaAs for a quantum cascade laser on a GaAs substrate. Of course, these are just particular examples for the materials which may be used in an embodiment. Other materials may be used in other embodiments. Typical wavelengths for the laser radiation may be in the range of 5 μm to 20 μm. 
         [0021]    As discussed earlier, each of the quantum cascade lasers making up an embodiment may be individually controlled by way of the applied voltage potentials. Because of this, it is expected that embodiments may find numerous applications in which a mid-to far infrared broadband laser source is desired. For example, an embodiment may be used in a frequency division multiple access communication system, where each of the individual ridge waveguide quantum cascade lasers are turned on and off in some specified fashion.