Patent Abstract:
Semiconductor lasers, in particular Quantum Cascade Lasers (QCLs) are tuable especially in the mid-IR spectral range, e.g. in wavelengths of about 3-14 μm, by precisely controlling the laser&#39;s temperature in the vicinity of the active region. The present invention introduces a novel design for locally heating the active region, thereby allowing fast heating and thus tuning a laser. It is generally applicable for lasers across the field, e.g. to QCLs with multi-color emitters or to Vertical-Cavity Single-Emitter Lasers (VCSELs) or to Distributed Feedback (DFB) lasers. Essentially, the invention consists of structurally integrating a heating resistor as part of the laser, placed close to the component to be temperature-controlled, i.e. the active region or the grating, etc., and feeding this resistor with a variable electrical current in order to locally control the thermal dissipation. In multi-emitter lasers, a resistor can be associated with each emitter section to tune the temperature of each section and thus its emitted wavelength. Similarly, in multisection DBR lasers, with a resistor associated with each grating, the latter can be tuned and thus the associated wavelength of the optical cavity. The invention also includes a novel manufacturing process, especially for buried heterostructure lasers.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 USC §119 to European Patent Application No. 13 405 109.3, filed Sep. 13, 2013, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to semiconductor lasers, in particular to Quantum Cascade Lasers (QCLs) that are tunable especially in the mid-IR spectral range, i.e. in wavelengths of about 3-14 μm. 
     DESCRIPTION OF THE RELATED ART 
     Tuning is effected by precisely controlling the laser&#39;s temperature in the vicinity of the active region. The mid-IR spectral range plays an unique role in sensing applications due to the large number of molecules showing fundamental resonances in this region. In the recent decade, Quantum Cascade Lasers (QCLs) have become one of the most frequently used, efficient laser sources in various spectral ranges. Examples are Maulini U.S. Pat. No. 7,944,959 and Faist US patent application 2003/0174751. A QCL laser generating a mid-IR spectrum is also shown in Vurgaftman U.S. Pat. No. 8,290,011. 
     A typical design is a Distributed Feedback (DFB) laser, here a QCL, with a Bragg grating on top of it, producing a specific emission wavelength, i.e. emitting in a single-mode. By changing the operating temperature of the laser, limited spectral tuning is realized. 
     However, the availability of multiple spectral lines is still highly desirable for many applications whereby a stable control of the laser wavelength is often necessary. Thus, it is the objective of the present invention to devise such a QCL which generates precisely controllable, multiple spectral lines in the mid-IR range. 
     However, it should be understood that the present invention is not limited to QCLs of any wavelength, but is generally applicable for lasers across the field, e.g. to QCLs with multi-color emitters or to Vertical-Cavity Single-Emitter Lasers (VCSELs). 
     SUMMARY OF THE INVENTION 
     To achieve the above main objective, the present invention introduces a novel and advantageous technique for tuning a laser, especially a QCL operating in a mid-IR spectral range. 
     First, to allow fast modulation of the laser&#39;s emission wavelength(s) through thermal tuning, a novel method of local heating is disclosed. 
     A buried heterostructure is preferable for implementing a particular design of the novel tunable laser according to the invention. For this implementation, a novel manufacturing process for such a laser is presented. 
     Further, as indicated above, to obtain multiple single, but tunable spectral modes, multiple emitters may be placed in the same optical waveguide or on separate waveguides, using the novel method of local heating. Here again, the novel manufacturing process, mentioned above, may be implemented when using a buried heterostructure design. Thus, tunable wavelength-independent, single beam emissions are guaranteed. By introducing an additional etching step, each emitter can be addressed singularly. 
     Also, a novel technique to obtain widely tunable QCLs using Distributed Bragg Reflectors (DBRs) is disclosed. The reflectivity of each mirror (DBR1, DBR2, explained further below) presents a comb shape with mode spacing of Δλ1 and Δλ2. Lasing will therefore occur for the modes where both mirrors present high reflectivity. An integrated resistor allows to tune thermally the reflectivity of each mirror, thus a quasi-continuous laser tuning can be obtained using the Vernier effect over wide ranges. As in the case of the multi-color emitters, mentioned above, a resistor associated with one or several emitter sections can be used to tune the temperature of the section and thus its optical properties. Similarly, in multi-color lasers with several emitters, a resistor associated with each DFB can be used to tune the latter and thus the associated wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the appended drawings show: 
         FIG. 1  a prior art Buried Heterostructure (BH) fabrication process, 
         FIG. 2  the BH fabrication process according to the invention, 
         FIG. 3  an edge-emitting laser with a single integrated resistor according to the invention, 
         FIG. 4  a top view of the laser shown in  FIG. 3 , 
         FIG. 5  an edge-emitting laser with two integrated resistors, 
         FIG. 6  a top view of the laser shown in  FIG. 5 , 
         FIG. 7  a laser with two asymmetrically arranged integrated resistors, 
         FIG. 8  a top view of the laser shown in  FIG. 7 , 
         FIG. 9  an edge-emitting multi-emitter laser according to the invention, 
         FIG. 10  a top view of the laser shown in  FIG. 9 , 
         FIG. 11  multi-section laser including one or multiple heating electrods, 
         FIG. 12  a top view of the laser shown in  FIG. 11 , 
       The following figures are graphs of 
         FIG. 13  resistance vs. resistor channel thickness, 
         FIG. 14  dissipated power density vs. resistor channel thickness, 
         FIG. 15  active region average temperature increase vs. resistor channel thickness, 
         FIGS. 16 ,  17  2-D temperature patterns for two resistive channel thicknesses, 
         FIG. 18  time dependent active region temperature increase. 
         FIG. 19  a table showing exemplarily the wavelength variation/tuning as function of the injected “heating” current. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Integrated Resistor 
     A frequently used technique for emission-tuning of a semiconductor laser is temperature tuning. This is usually done by controlling or modifying the temperature of the laser submount, resulting in a change of the temperature of the active medium or region which in turn affects the laser&#39;s emission spectrum. 
     This technique has some intrinsic limitations or disadvantages. The first disadvantage is the long time constant, i.e. the time necessary to modify the temperature of the active medium in consequence to the temperature of the laser submount. Hence the maximum tuning speed available is limited to the Hz range, which is too slow for many applications. Secondly, for fabricating single spectral mode lasers, generally narrow active regions are necessary to guarantee single mode emission. As a consequence, due to the elevated thermal conductance of narrow ridge lasers, the tuning amplitude is limited. 
     The present invention introduces a way to locally increase the active region temperature independently of the laser submount or housing temperature by introducing a dissipative or heating region near the laser&#39;s active region, thus allowing a fast and efficient modulation of the active region medium temperature. 
     To obtain this result for a Buried Heterostructure (BH) laser, a modified version of the fabrication process was implemented, described subsequently. Please note that the following description just defines the principle of the modified manufacturing process—it does not go into the details. These are known to the person skilled in the art. 
       FIG. 1  shows the prior art BH fabrication process. On top of a semiconductor substrate  1 , e.g. n-doped InP, an active region  2  is built, onto which an n-doped cladding  3  is deposited which later provides electrical injection into the active region  2  and, simultaneously, confines the optical mode inside the active region. The laser ridge is covered by a mask  4 , then etched and, through selective regrowth of insulating semiconductor material  5 , e.g. InP:Fe, electric insulation is provided. Subsequent manufacturing steps are omitted here. 
       FIG. 2  presents the novel and inventive variation of this manufacturing process according to one aspect of the present invention. After growing the active region  2 ′, a mask  4 ′ is applied and the laser ridge, i.e. the active region, is immediately etched. Then the insulating semiconductor layer  5 ′, e.g. InP:Fe, is grown. After that, the n-doped cladding  3 ′ is deposited on the whole device in a planar regrowth. Again, subsequent manufacturing steps are omitted here. 
     The important difference to the prior art structure of  FIG. 1  is that the epitaxial regrowth sequence is inverted and that the cladding  3 ′ now overlays the whole structure, i.e. extends also over the insulating layer  5 ′. 
     This results in some important advantages. First, it provides a wide contact region where a dissipative “heater region” according to the invention can be inserted. Also, the reduced thickness of the selective regrowth results in an increased quality of the layers and a reduced defect density. 
     Using the above process, a way of generating a dissipative section is the insertion of a resistive channel inside the cladding  3 ′, which channel allows a controlled heating of the active region. 
     However, the present invention is not limited to the described fabrication method, but covers any case in which an additional electrode and/or a specific resistor is added to the semiconductor laser functioning to independently modify the local active region temperature. 
     Please note that, in the subsequent description and the associated drawings, not all components are shown which are necessary for the operation of the lasers (e.g. the obligatory mirrors, any antireflection coatings, etc.) The person skilled in the art will know where to place and how to design the omitted parts and components. For the sake of clarity, the following descriptions focus on those parts and components that distinguish the present invention over the prior art. 
     A First Embodiment 
     In principle, the waveguide cladding layer of a QCL laser is generally a highly n+doped region into which electrons are injected directly from a superposed metal contact. The doping of the cladding layer is usually decreased close to the active region in order to lower the optical losses of the laser&#39;s n-doped region. By etching an appropriate trench into the cladding layer, two different, independent contacts can be generated. The electric resistance between these two contacts depends on the doping, the thickness, and the width/length of the linking region of the cladding. 
       FIG. 3  shows an implementation of this concept in detail. An active region  12  on a substrate  11 , usually n-doped InP, is enclosed by insulating layers  15 , preferably of InP:Fe. Extending over the active region  12  and the insulating layers  15  is a cladding layer  13 , preferably consisting of lowly doped InP. Shown on top of the cladding layer  13  are two separate, highly doped contact areas  17   a  and  17   b , preferably of n++doped InP, divided by a trench  16 . Each of these contact areas  17   a  and  17   b  is covered by a metallic, usually Au, electrode  18   a  and  18   b , resp. It should be noted that the contact areas  17   a  and  17   b  are not necessary for the functioning of the invention so that they may be omitted. In this case, the electrodes  18   a  and  18   b  would be placed directly on the two islands of the cladding  13 , separated by the trench  16 . A counter electrode or cathode  10  is placed on the bottom of the substrate  11 . 
     The current injected into the active region  12  through the electrode  18   b , the “heating” or “secondary” electrode, is partially dissipated in the lowly doped section of the waveguide cladding  13 , while the current from the electrode  18   a , the “primary” electrode, is practically directly injected into the active region  12 . Because of the intrinsic resistance of the cladding layer  13 , the current injected by the secondary electrode  18   b  generates heat in the current path  14 , which in turn warms up the active region  12  and, to some extent, also the insulating layer  15 . This heating modifies the optical properties of the laser&#39;s cavity and consequently the emission wavelength. 
     By modifying the current injected into the electrode  18   b , and/or by modifying the ratio between the currents injected through the two electrodes  18   a  and  18   b , resp., or by adapting the voltages applied to the two electrodes  18   a  and  18   b , resp., the current through the path  14  is altered and thus the temperature of the active region  12 . This in turn controls the laser&#39;s emission spectrum. An example of this functioning method is provided in the table depicted in  FIG. 19 , showing exemplary values for the variation or tuning of the emitted wavelength as function of the injected “heating” current. 
     The inherent resistance of the current path  14 , i.e. the “heating section”, can be adapted during the fabrication process. For example, increasing the etching depth of the trench  16 , or placing the trench  16  further away from the active region  12 , and/or increasing the width of the trench  16 , or reducing the doping level of the cladding  13  increases the resistance of this current path  14 , i.e. the “heating section” and vice versa. 
       FIG. 4  shows a top view of the first embodiment. To avoid overheating and thus damaging the active region  12  in the vicinity of the facets  19   a  and  19   b , i.e. to avoid so-called COD, it may be preferable to keep the secondary electrode  18   b  shorter than the length of the associated active region  12 ; this is not shown in  FIG. 4  however. If there is a contact area  17   b —which, as explained above, is not necessarily so—this is preferably shorter, too. The same goal can also be achieved by modifying the form of the trench  16  and/or modifying the doping of the cladding  13 . 
     A Second Embodiment 
       FIG. 5  displays a second embodiment. The main difference with regard to the first embodiment is that two “heating” or “secondary” electrodes  28   b  and  28   c  and the associated structures are provided on both sides of the active region  22  in a practically symmetric design. There are two trenches  26   a  and  26   b  and resistive paths  24   a  and  24   b , both of nearly identical shape. The substrate  21  with its rear contact  20 , the insulating layer  25 , and the cladding  23  have the same function as described above in connection with  FIGS. 3 and 4 . Note that there are no contact areas as shown in  FIGS. 3 and 4 . The function of this structure is as described above with the difference that there are two heating regions. This allows for a more uniform heating of the active region  22 , a further advantage. Further, the two distinct heating sections that can be separately controlled, allowing for any variety of desired heating of the so constructed laser. 
       FIG. 6  presents a top view of  FIG. 5 , showing the arrangement of the second embodiment with the two symmetrically arranged heating electrodes  28   b  and  28   c , the primary electrode  28   a  in the middle, and the two trenches  26   a  and  26   b  forming the two practically symmetrical resistive paths. The whole structure can be constructed identical or slightly different. 
     A Third Embodiment 
       FIGS. 7 and 8  show a third embodiment. Here again, several “heating” or “secondary” electrodes are provided around an active region, but the whole is of significantly different, asymmetric design. First, the primary electrode  38   a  is not centered on the active region  32 , but displaced in relation to it. This e.g. allows to design an emission window, providing a vertical exit window of a so-called vertical-cavity laser. 
     One of the “heating” or “secondary” electrodes, namely  38   b  and the associated structure is much closer to the primary electrode  38   a  than the other secondary electrode  38   c . Consequently, the trench  36   a  between the primary electrode  38   a  and the secondary electrode  38   b  is narrower than the trench  36   b  between the primary electrode  38   a  and the secondary electrode  38   c . Also, the two trenches  36   a  and  36   b  are shown with different depths to indicate the possible variations of the design. Further, the two distinct and different heating sections can be controlled separately, allowing for any desired heating of the so constructed laser. 
     The substrate  31  with its rear contact  30 , the insulating layer  35 , and the cladding  33  have the same function as described above in connection with  FIGS. 3 and 4 . However, note again that there are no contact areas as shown in  FIGS. 3 and 4 . As mentioned, the advantage of the structure shown in  FIGS. 7 and 8  is that it allocates space for an emission window of a vertical-cavity surface-emitting laser (VCSEL), providing the advantages of the present invention for this type of lasers, e.g. allowing to fine-tune it. 
       FIG. 8  presents a top view of  FIG. 7 , showing the arrangement of the third embodiment with the asymmetrically arranged secondary or heating electrodes  38   b  and  38   c . The other components displayed correspond to the parts depicted in  FIG. 7 . Usually, this design would be an edge-emitting laser of the same kind as described hereinbefore. However, if desired and as mentioned above, the laser shown in  FIGS. 7 and 8  may be designed as vertical-cavity surface-emitting laser (VCSEL), mentioned above, and thus incorporate a (vertical) exit window  39 , shown here as example. 
     A Multi-emitter Embodiment 
       FIG. 9  displays a multi-color/wavelength, edge-emitting laser structure implementing the invention. In this design, several lasers  49   a  . . .  49   c  emitting different wavelengths through the same active region  42  are placed on a common substrate  41 . Each single laser  49   a  . . .  49   c  features a primary electrode, each indicated here by its primary connection  47   a  . . .  47   c , and at least one secondary or heating electrode, each here indicated by its secondary connection  48   a  . . .  48   c . An additional laser  49   n  is shown at the right end, indicating that the number of lasers on the substrate  41  is not limited to three. A counter electrode or cathode  40  is placed on the bottom of the substrate  41 . 
     The insulation layer and the cladding  43   a  . . .  43   c  of each single laser  49   a  . . .  49   c  are only adumbrated in  FIGS. 9 and 10  since their positions and functions should be clear from the foregoing description. 
     Each of the lasers can be controlled separately, allowing to fine-tune the laser structure&#39;s output one by one. One possibility is to affect the active region&#39;s temperature of each laser and thus the laser&#39;s wavelength through the described, implemented heating system. 
     If the described multi-color/wavelength laser is of the DFB type, the temperature control affects the grating on each laser and thus the laser&#39;s wavelength through the described heating system. 
       FIG. 10  presents a top view of the design depicted in  FIG. 9 , showing the arrangement of the multi-color, multi-emitter laser structure implementing the invention. The depicted embodiment shows, on each single laser, the primary electrode with its connection  47   a  . . .  47   c , the insulation layer/cladding  43   a  . . .  43   c  underlying that electrode, the single secondary or heating electrode with its respective connection  48   a  . . .  48   c , and the common substrate  41 . 
     Multisection Laser Laser 
       FIG. 11  shows a multisection, edge-emitting laser with distributed Bragg reflectors. The Bragg gratings serving as distributed feedback reflectors (DBR sections)  54   b  and  54   c  are embedded in the structure, e.g. on the top of the active region  52 . A counter electrode or cathode  50  is placed on the bottom of the substrate  51 . 
     Looking somewhat similar to the previously described multi-emitter laser, there are several arrangements of primary and secondary electrodes/connections on top of this DFB laser. However, there is a significant difference: the laser is in this case constituted by multiple sections, e.g. front mirror, back mirror, phase section, and the temperature and the drive current in each of these sections is used to tune the emission wavelength of the laser. 
     In detail: whereas the primary electrode and its associated connector  57   a  and the secondary electrode/connection  58   a  control the temperature of the active region  52  and thus its optical properties, the electrode/connection pairs  57   b / 58   b  and  57   c / 58   c  control the temperature of the DBR gratings  54   b  and  54   c , resp., i.e. the mirror properties. Thus, tuning of the laser is available through altering/controlling both the temperature of the active region  52  and the temperature of the DBR gratings  54   b  and  54   c.    
       FIG. 12  is a top view of the design depicted in  FIG. 11 , showing the arrangement of the edge-emitting DBR laser. The primary electrode/connection  57   a  and the secondary or heating electrode/connection  58   a  are located in the center of the laser and control the active region. The two pairs of primary and secondary electrodes/connections  57   b / 58   b  and  57   c / 58   c  are located towards both ends of the active region; they control the grating serving as front and back mirrors. 
     Also shown in  FIG. 12  are the insulation layers/claddings  53   a  . . .  53   c  underlying the various electrodes, the various electrodes with their respective connections  57   a  . . .  57   c  and  58   a  . . .  58   c , and the common substrate  51 . 
     Generally speaking, the present invention has a further advantage also when applied to standard distributed feedback (DFB) lasers. Since such a laser usually includes a grating, the heating provided by the additional “heating” electrode may be used to quickly fine-tune the optical mode phase, i.e to adjust the phase of the longitudinal optical mode in accordance with the cavity mode, thus improving the single-mode yield and reducing the risk of mode-hopping. 
     In the following, the  FIGS. 13 to 18  will be discussed; these figures show diagrams or graphs of various functions and dependancies. 
     The measurements were made on an edge-emitting QCL laser with a ridge 7.5 μm wide and a dissipative region 5 μm wide. The dissipative region is doped 10 16  cm −3 . In this structure, the thickness of the resistive channel was varied and the stationary temperature observed. 
     In  FIG. 13 , the resistance of the dissipative or heating region is plotted as function of the thickness of the resistor channel, calculated using the Drude model. 
       FIG. 14  shows the calculated dissipated power for an injected current of 1A as function of the resistor channel thickness which in turn is a function of the etching depth. 
       FIG. 15  displays the stationary increase of the temperature as function of the resistor channel thickness. As shown, a temperature increase as high as 200K can be observed if the thickness of the resistive channel is changed from 500 nm to 2 μm. 
       FIGS. 16 and 17  compare the temperature patterns for two resistive channel thicknesses of 500 nm ( FIGS. 14 ) and 2 μm ( FIG. 15 ) in the case of an injection current of 1A inside the resistive paths. 
     As mentioned above, one of the main advantages of the present invention is the possibility to obtain a high modulation speed of the active region temperature and therefore of the emission wavelength. 
       FIG. 18  depicts this advantage. The temperature transient for a resistive channel of 500 nm depth is analyzed—left curve—and compared to the temperature transient—right curve—in the case of an instantaneous submount temperature change. 
     The right curve shows time constant (or sluggishness) of temperature changes for a prior art submount heating design. The left curve shows the time constant for a system according to the present invention. In this latter case, the temperature can be increased of up to 20K in 1 μs, while in the case of a change of the submount temperature, the same increase can only be obtained in 300 μs or more, even supposing an instantaneous temperature change of the laser&#39;s back contact. Thus, in a “heating contact design” according to the present invention, the temperature can be modulated in the MHz range, guaranteeing a wide tuning of the laser emission not obtainable with standard prior art designs. 
     Finally, the table shown in  FIG. 19  presents the emission tuning for a resistive path, 500 nm thick, as function of the current dissipated in the resistive path for a laser emitting at 2200 cm −1 . 
     The above detailed description of the function and of various embodiments of the present invention permit a person skilled in the art to devise further implementations without departing from spirit and scope of the present invention. 
     Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.

Technology Classification (CPC): 7