Abstract:
A compact mid-IR laser device utilizes a quantum cascade laser to provide mid-IR frequencies suitable for use in molecular detection by signature absorption spectra. The compact nature of the device is obtained owing to an efficient heat transfer structure, the use of a small diameter aspheric lens and a monolithic assembly structure to hold the optical elements in a fixed position relative to one another. The compact housing size may be approximately 20 cm×20 cm×20 cm or less. Efficient heat transfer is achieved using a thermoelectric cooler TEC combined with a high thermal conductivity heat spreader onto which the quantum cascade laser is thermally coupled. The heat spreader not only serves to dissipate heat and conduct same to the TEC, but also serves as an optical platform to secure the optical elements within the housing in a fixed relationship relative on one another. A small diameter aspheric lens may have a diameter of 10 mm or less and is positioned to provided a collimated beam output from the quantum cascade laser. The housing is hermetically sealed to provide a rugged, light weight portable MIR laser source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/354,237 filed Jan. 15, 2009, and entitled “COMPACT MID-IR LASER,” which is a continuation of U.S. application Ser. No. 11/154,264 filed on Jun. 15, 2005, and entitled “Compact Mid-IR Laser,” now U.S. Pat. No. 7,492,806. The disclosures of each of the above patent applications are hereby incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the invention relate to a compact Mid-Infrared (MIR) laser which finds applications in many fields such as, molecular detection and imaging (e.g., thermal) instruments for use in medical diagnostics, pollution monitoring, leak detection, analytical instruments, homeland security (e.g., weapon guidance, explosive detectors, thermal detection of objects and individuals, etc.) and industrial process control. Embodiments of the invention are also directed more specifically to the detection of molecules found in human breath, since such molecules correlate to existing health problems such as asthma, kidney disorders and renal failure. 
         [0004]    2. Description of the Related Art 
         [0005]    MIR lasers of interest herein may be defined as, lasers having a laser output wavelength in the range of approximately 3-12 μm (3333-833 cm −1 ). More broadly, however, “MIR” may be defined as wavelengths within a range of 3-30 μm. The far-IR is generally considered 30 300 μm, whereas the near IR is generally considered 0.8 to 3.0 μm. Such lasers are particularly advantageous for use in absorption spectroscopy applications since many gases of interest have their fundamental vibrational modes in the mid-infrared (e.g., thermal) and thus present strong, unique absorption signatures within the MIR range. 
         [0006]    Various proposed applications of MIR lasers have been demonstrated in laboratories on bench top apparatuses. Actual application of MIR lasers has been more limited and hampered by bulky size and cost of these devices. 
         [0007]    One laser gain medium particularly useful for MIR lasers is the quantum cascade laser (QCL). Such lasers are commercially available and are advantageous in that they have a relatively high output intensity and may be fabricated to provide wavelength outputs throughout the MIR spectrum. QCL have been shown to operate between 3.44 and 84 μm and commercial QCL are available having wavelengths in the range of 5 to 11 μm. The QCL utilized two different semiconductor materials such as InGaAs and AlInAs (grown on an InP or GaSb substrate for example) to form a series of potential wells and barriers for electron transitions. The thickness of these wells/barriers determines the wavelength characteristic of the laser. Fabricating QCL devices of different thickness enables production of MIR laser having different output frequencies. Fine tuning of the QCL wavelength may be achieved by controlling the temperature of the active layer, such as by changing the DC bias current. Such temperature tuning is relatively narrow and may be used to vary the wavelength by approximately 0.27 nm/Kelvin which is typically less than 1% of the of peak emission wavelength. 
         [0008]    The QCL, sometimes referred to as Type I Cascade Laser or Quantum Cascade Laser, may be defined as a unipolar semiconductor laser based on intersubband transitions in quantum wells. The QCL, invented in 1994, introduced the concept of “recycling” each electron to produce more than one photon per electron. This reduction in drive current and reduction in ohmic heating is accomplished by stacking up multiple “diode” regions in the growth direction. In the case of the QCL, the “diode” has been replaced by a conduction band quantum well. Electrons are injected into the upper quantum well state and collected from the lower state using a superlattice structure. The upper and lower states are both within the conduction band. Replacing the diode with a single-carrier quantum well system means that the generated photon energy is no longer tied to the material bandgap. This removes the requirement for exotic new materials for each wavelength, and also removes Auger recombination as a problem issue in the active region. The superlattice and quantum well can be designed to provide lasing at almost any photon energy that is sufficiently below the conduction band quantum well barrier. 
         [0009]    Another type of Cascade Laser is the Interband Cascade Laser (ICL) invented in 1997. The ICL, sometimes referred to as a Type II QCL (Cascade Laser), uses a conduction-band to valence-band transition as in the traditional diode laser, but takes full advantage of the QCL “recycling” concept. Shorter wavelengths are achievable with the ICL than with QCL since the transition energy is not limited to the depth of a single-band quantum well. Thus, the conduction band to valance band transitions of the Type II QCLs provide higher energy transitions than the intra-conduction band transitions of the Type I QCLs. Typical wavelengths available with the Type II QCL are in the range of 3-4.5 μm, while the wavelengths for the Type I QCLs generally fall within the range of 5-20 μm. While Type II QCLs have demonstrated room temperature CW operation between 3.3 and 4.2 μm, they are still limited by Auger recombination. Clever bandgap engineering has substantially reduced the recombination rates by removing the combinations of initial and final states required for an Auger transition, but dramatic increases are still seen with active region temperature. It is expected that over time improvements will be made to the ICL in order to achieve the desired operating temperature range and level of reliability. 
         [0010]    For purposes of the present invention, QCL and ICL may be referred to under the generic terminology of a “quantum cascade laser” or “quantum cascade laser device”. The laser gain medium referred to herein thus refers to a quantum cascade laser. In the event that it is needed to distinguish between QCL and ICL, these capitalized acronyms will be utilized. 
         [0011]    For the purposes of the present invention, the term “subband” refers to a plurality of quantum-confined states in nano-structures which are characterized by the same main quantum number. In a conventional quantum-well, the subband is formed by each sort of confined carriers by variation of the momentum for motion in an unconfined direction with no change of the quantum number describing the motion in the confined direction. Certainly, all states within the subband belong to one energy band of the solid: conduction band or valence band. 
         [0012]    For the purposes of the present invention, the term “nano-structure” refers to semiconductor (solid-state) electronic structures including objects with characteristic size of the nanometer (10-9) scale. This scale is convenient to deal with quantum wells, wires and dots containing many real atoms or atomic planes inside, but being still in the size range that should be treated in terms of the quantum mechanics. 
         [0013]    For the purposes of the present invention term “unipolar device” refers to devices having layers of the same conductivity type, and, therefore, devices in which no p-n junctions are a necessary component. 
         [0014]    The development of small MIR laser devices has been hampered by the need to cryogenically cool the MIR lasers (utilizing, for example, a large liquid nitrogen supply) and by the relatively large size of such devices hampering their portability and facility of use and thus limiting their applicability. 
       SUMMARY OF THE INVENTION 
       [0015]    In accordance with embodiments of the invention, there is provided a MIR laser device having a monolithic design to permit the component parts thereof to be fixedly secured to a rigid optical platform so as to provide a highly portable rugged device. The MIR laser has a housing; a thermo electric cooling (TEC) device contained within the housing; a heat spreader contained within the housing and positioned either above a top surface of the TEC or above an intermediate plate which is positioned between the top surface of the TEC and the heat spreader. The MIR laser has a quantum cascade laser contained within the housing and fixedly coupled to the heat spreader; and an optical lens (e.g., refractive lenses, diffractive lenses, Fresnel lenses, etc.) contained within the housing and fixedly mounted to the heat spreader for collimating light output from the quantum cascade laser and directing the collimated light to the exterior of the housing. The heat spreader serves to distribute heat to the TEC and also serves as an optical platform to fixedly position said quantum cascade laser and said optical lens relative to one another. 
         [0016]    The TEC device provides cooling by means of the well known Peltier effect in which a change in temperature at the junction of two different metals is produced when an electric current flows through the junction. Of particular importance herein, there is no need for bulky and costly cryogenic equipment since liquid nitrogen is not utilized to effect cooling. The TEC device is used to cool the quantum cascade laser in a manner to permit it to stably operate for useful lifetimes in the application of interest without cryogenic cooling. 
         [0017]    In one embodiment of the invention, the top surface of the TEC device serves as a substrate onto which is mounted the heat spreader. The heat spreader is effective to spread the heat by thermal conduction across the upper surface of the TEC device to efficiently distribute the heat from the quantum cascade laser to the TEC device for cooling. In preferred embodiments of the invention, the heat spreader has a high thermal conductivity such as a thermal conductivity within the range of approximately 150-400 W/mK and more preferably in the range of approximately 220-250 W/mK. The latter range includes high copper content copper-tungstens. An example of a suitable high conductivity material is copper tungsten (CuW), typically a CuW alloy. In accordance with other embodiments of the invention, a high thermal conductivity sub-mount is employed intermediate the quantum cascade laser and the heat spreader. The high thermal conductivity sub-mount may comprise industrial commercial grade diamond throughout its entirety or may be partially composed of such diamond. Diamond is a material of choice due to its extremely high thermal conductivity. In alternative embodiments, the high thermal conductivity sub-mount may be composed of a diamond top section in direct contact and a lower section of a different high thermal conductivity material, such as, for example CuW. 
         [0018]    In other preferred embodiments, the heat spreader serves as an optical platform onto which the quantum cascade laser and the collimating lens are fixedly secured. The optical platform is as a rigid platform to maintain the relative positions of the lens and quantum cascade laser which are secured thereto (either directly or indirectly). The use of the heat spreading function and the optical platform function into a single material structure contributes to the small size and portability of the MIR laser device. 
         [0019]    The quantum cascade laser is the laser gain medium of preference in accordance with embodiments of the invention and provides the desired mid-IR frequencies of interest. The quantum cascade laser may be one of the Type I or Type II lasers described above. Such a laser generates a relatively strong output IR beam but also generates quite a bit of heat, on the order of 10 W. Thus, the TEC device is an important component needed to remove the heat thereby permitting long lived operation of the quantum cascade laser. The optical lens is positioned such as to collimate the laser output of the quantum cascade laser to provide a collimated output beam directed outside of the housing. For this purpose, the quantum cascade laser is positioned a distance away from the optical lens equal to the focal length of the optical lens. In this manner, the source of light from the quantum cascade laser is collected and sent out as an approximately parallel beam of light to the outside of the housing. 
         [0020]    Preferably, in accordance with embodiments of the invention, the overall size of the housing is quite small to permit facile portability of the MIR laser device, and for this purpose, the housing may have dimensions of approximately 20 cm×20 cm×20 cm or less, and more preferably has dimensions of approximately 3 cm×4 cm×6 cm. Further to achieve the desired small size and portability, the optical lens is selected to have a relatively small diameter. In preferred embodiments, the diameter of the lens is 10 mm or less, and in a most preferred embodiment, the diameter of the lens is approximately equal to 5 mm or less. 
         [0021]    Other embodiments of the invention employ additionally an electronic sub-assembly (e.g., including a power source such as a battery) incorporated into the housing. The electronic subassembly has a switch and a summing node, contained within said housing. The MIR laser device also has an input RF port for inputting an RF modulating signal into the electronic sub-assembly through an impedance matching circuit, and a drive current input terminal electrically connected to said quantum cascade laser for inputting drive current to said quantum cascade laser. There is further provided a switching control signal input terminal for inputting a switching control signal into the electrical sub-assembly of the housing for switching said switch between a first and second state. The first state of the switch passes the drive current to the quantum cascade laser permitting it to operate (on position of the quantum cascade laser) and the second state of the switch shunts the drive current to ground thus preventing the drive current from reaching the quantum cascade laser thereby ceasing operation of the quantum cascade laser (turn it off). Controlling the amount of on time to the amount of off time of the laser causes the laser to operate in pulse mode, oscillating between the on and off states at regular intervals according to a duty cycle defined by the time of the on/off states. This duty cycle control of a laser is well known to those skilled in the art and may be used to control the laser to operate in pulsed mode or, in the extreme case, maintaining the laser on all the time results in cw operation of the laser. 
         [0022]    The summing node of the electronic sub-assembly is interposed in an electrical path between the drive current input terminal and the quantum cascade laser to add the RF modulating signal which is input at the RF input port to the laser drive current. RF modulation, also known as frequency modulation, is well known in absorption spectroscopy and is used to increase the sensitivity of a detecting system which detects the laser beam after it has passed through a sample gas of interest. The absorption dip due to absorption of the particular molecules of interest in the sample gas traversed by the laser beam is much easier to detect when the laser beam has been frequency modulated. 
         [0023]    In accordance with other embodiments of the invention, there is provided a MIR laser device having a housing; a quantum cascade laser contained within the housing; and an optical lens contained within the housing and mounted for collimating light output from the quantum cascade laser. In order to achieve the small sizes needed for facile portability and ease of use, the optical lens is chosen to be quite small and has a diameter of approximately 10 mm or less. The optical lens may be movably positioned a variable distance away from the quantum cascade laser, e.g., equal to its focal length so that the optical lens serves to collimate the lens and direct a parallel laser beam toward the exterior of the housing. For example, the collimated laser beam can be directed towards a target located exterior to the housing. The target can include but is not limited to a living being, an inanimate object or chemicals or gases, etc. The laser beam can optically interact with the target (e.g. be absorbed by the target, be scattered by the target, be reflected by the target, be redirected by the target, etc.) and form an infra-red or a thermal image of the target. The housing is preferably hermetically sealed (to keep out moisture) and provided with an output window through which the collimated laser beam is passed to the exterior of the housing. In other preferred embodiments, the diameter of the lens is chosen to be 5 mm or less. 
         [0024]    The electronic sub-assembly described above, with its RF modulation and switch for controlling the duty cycle of operation, may also be used in connection with the small lens diameter embodiment described immediately above. 
         [0025]    In accordance with yet other embodiments of the invention, there is provided a MIR laser device having a housing; a quantum cascade laser contained within the housing; and an optical lens contained within the housing and mounted for collimating light output from the quantum cascade laser. In order to achieve the small sizes needed for facile portability and ease of use, the housing is chosen to be quite small and has a size of approximately 20 cm×20 cm×20 cm or less. The housing is preferably hermetically sealed (to keep out moisture) and provided with an output window through which the collimated laser beam is passed to the exterior of the housing. In other preferred embodiments, the size of the housing is approximately 3 cm×4 cm×6 cm which is compact enough to be a handheld device. 
         [0026]    The MIR laser device, in accordance with principles of embodiments of the invention, is very compact and light weight, and uses a quantum cascade laser as the laser gain medium. The quantum cascade laser may be selected for the particular application of interest within the frequency range of 3-12 μm by appropriate selection of the thickness of quantum wells and barriers. Such a compact, MIR laser enables a number of instruments to be developed in the fields of medical diagnostics (e.g., on humans and other subjects), homeland security (e.g., on humans or devices), and industrial processing, and other applications based on laser absorption spectroscopy for molecular detection. For example, the beam from a compact handheld MIR laser according to several embodiments described herein can be directed (e.g., aimed or pointed) towards a target (e.g. a living being, an internal organ in the human or animal body, inanimate objects, leaking gases, containers containing chemicals, etc.) located exterior to the MIR laser. The directed beam can intersect with the target and form an infra-red or a thermal image of the target which can be viewed with thermal imaging systems. In some embodiments, intersection of the laser beam with the target can result in the beam being absorbed by the target, or reflected by the target, or scattered by the target, or redirected by the target. Important characteristics of the MIR device is the use of a quantum laser as the laser gain media, short focal length aspheric lens, enhanced cooling techniques that do not require liquid nitrogen and the use of high integration and packaging. The resulting structure presents a foot print that is extremely small with a package size (housing size) of approximately 20 cm (height)×20 cm (width)×20 cm (length) or less. The length is taken along the optical axis. The packages size may be any integer or fraction thereof between approximately 1-20 cm for the length dimension combined with any integer or fraction thereof between approximately 1-20 cm in width dimension combined with any integer or fraction thereof between approximately 1-20 cm in the height dimension. A preferred footprint is approximately 3 cm (height)×4 cm (width)×6 cm (length) for the laser package. 
         [0027]    Some advantages of the MIR device according to embodiments of the invention include high brightness with diffraction limited spatial properties and a narrow spectral width (&lt;100 MHz=0.003 cm-1). The quantum laser gain medium enables high output power (50 mW) and allows easy modulation at high frequency with very low chirp. The packaging technology is mechanically and environmentally robust with excellent thermal properties and provides for dramatic miniaturization. 
         [0028]    In most conventional systems, cryogenic cooling has been required for MIR lasers. In contrast, the MIR laser device, in a preferred embodiment, can be temperature controlled close to room temperature without the need for bulky cryogenic cooling but rather employing thermo-electric coolers. Further, the MIR laser device in accordance with embodiments of the invention uses a packaging that specifically accommodates the designs associated with MIR photonics products with specific emphasis on thermal, optical and size requirements. 
         [0029]    Further conventional drawbacks to a compact MIR laser device results from the high heat output of quantum cascade lasers—typically 10 W and even up to 15 W. This heat needs to be removed from the cavity efficiently to maintain cavity temperature and wavelength. This heat load typically requires a large heat sink to effectively remove the heat. In the MIR laser device according to embodiments of the invention, a high conductivity, heat-spreader is used and serves as a small but efficient transfer device to transfer the heat to a thermoelectric cooler. 
         [0030]    An additional impediment to a compact MIR laser design is the conventional use of relatively large size lenses associated with MIR radiation. Typically, these lenses are &gt;10-15 mm in diameter and often 25 mm or more. In contrast, the MIR laser device, in accordance with embodiments of the invention, uses a small aspheric lenses (approximately equal to or less than 5 mm D) that can be used in conjunction with the quantum cascade laser. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIGS. 1A-1C  show perspective views of the MIR laser device; 
           [0032]      FIGS. 2A and 2B  show exploded perspective view of the MIR laser device with  FIG. 2B  being rotated so show a back side of the laser device relative to  FIG. 2A ; 
           [0033]      FIG. 3  shows a plan view of the MIR laser device with the top or lid removed to show the internal structure; 
           [0034]      FIG. 4A  shows a cross sectional view of the MIR laser device taken along lines A-A of  FIG. 3 ; 
           [0035]      FIG. 4B  shows an enlarged view of a portion of  FIG. 4A ; and 
           [0036]      FIG. 5  shows a schematic diagram of the electronics sub-assembly of the first embodiment. 
           [0037]      FIGS. 6A and 6B  illustrate embodiments of tunable quantum cascade laser. 
           [0038]      FIG. 7  is a plot of the power output by an embodiment of a tunable laser over different wavelength ranges. 
           [0039]      FIG. 8  is a plot of the absorbance of ethanol for radiation in the wavelength range of approximately 7-11 μm emitted from an embodiment of a tunable quantum cascade laser as compared to the standard absorption spectrum of ethanol. 
           [0040]      FIG. 9  is a plot of an absorption spectrum of a gas mixture including CO 2 ,  13 CO 2  and  18 OCO as compared to the simulated absorption spectrum for CO 2 ,  13 CO 2  and  18 OCO. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]      FIGS. 1A-1C  show perspective views of a MIR laser device  2  in accordance with a first embodiment of the invention.  FIG. 1A  shows the MIR laser device  2  with the housing  4  including the lid or top cover plate  4   a  and mounting flanges  4   b .  FIGS. 1B and 1C  show the MIR laser device  2  with the lid  4   a  removed, thus exposing the interior components.  FIGS. 2A and 2B  show exploded perspective, views of the various components of the MIR laser.  FIGS. 3 and 4A  show plan and side views respectively of the laser device and  FIG. 4B  shows an enlarged portion of  FIG. 4A . 
         [0042]    As may be seen from these figures, the MIR laser device is seen to include a laser gain medium  6  mounted on a high thermal conductivity sub-mount  8 . There is further provided a temperature sensor  10 , a lens holder  12 , lens mount  13 , output lens  14 , and window  16 . An output aperture  18   a  is provided in the side of the housing  4  with the window positioned therein. The MIR laser device is also comprised a heat spreader  20 , cooler  22  and electronics sub-assembly  24 . The heat spreader  20  also serves as the optics platform to which the key optical elements of the laser device are secured. Thus, more precisely, element  20  may be referred to as the heat spreader/optical platform and this composite term is sometimes used herein. However, for simplicity, element  20  may be referred to as a “heat spreader” when the heat transfer function is of interest and as an “optical platform” when the platform features are of interest. The housing  4  is also provided with an RF input port  26  and a plurality of I/O leads  28  which connect to the electronic sub-assembly  24  and temperature sensor  10 . 
         [0043]    The lens mount  13 , especially as seen in  FIGS. 2A and 2B , is seen to comprise a U-shaped support  13   a , a retention cap  13   b , top screws  13   c  and front screws  13   d . The lens  14  is secured within the lens holder  12 . The lens holder in turn is secured within the lens mount  13  and specifically between the lens U-shaped support  13   a  and the retention cap  13   b . Spring fingers  13   e  secured to the retention cap  13   b  make pressure contact with the top portions of the lens holder  12  when the top screws  13   c  are tightened down to secure the retention cap  13   b  to the U-shaped support  13   a  using the top screws  13   c . The front screws  13   d  secures the U-shaped support  13   a  to the optical platform  20 . In this manner, the lens mount  13 , (and consequently the lens  14  itself) is rigidly and fixedly secured to the optical platform  20 . 
         [0044]    The laser gain medium  6  is preferably a quantum cascade laser (either QCL or ICL) which has the advantages providing tunable MIR wavelengths with a small size and relatively high output intensity. Examples of such a laser include 3.7 μm and 9.0 μm laser manufactured by Maxion. These quantum cascade lasers have reflecting mirrors built into the end facets of the laser gain material. The laser gain medium  6  typically has a size of 2 mm×0.5 mm×90 microns and is mounted directly to the high thermal conductivity submount  8  utilizing an adhesive or weld or other suitable method of securing same. The high thermal conductivity sub-mount  8  is preferably made of industrial grade diamond and may have representative dimensions of 2 mm high×2 mm wide×0.5 mm long (length along the beam path). An alternative dimension may be 8 mm high×4 mm wide by 2 mm long. Other materials may also be used as long as they have a sufficiently high thermal conductivity sufficient to conduct heat from the laser gain medium  6  to the larger heat spreader  20 . The thermal conductivity is preferably in the range of 500-2000 W/mK and preferably in the range of approximately 1500-2000 W/mK. In alternative embodiments, the high thermal conductivity submount  8  may be made of a layer of diamond mounted on top of a substrate of another high thermal conductive material such as CuW. For example, the overall dimensions of the submount may be 8 mm high×4 mm wide×2 mm long (length along the beam path), and it may be composed of a diamond portion of a size 0.5 mm high×2 mm wide×2 mm long with the remaining portion having a size of 7.5 mm high×2 mm wider 2 mm long and composed of CuW. In a most preferred embodiment of the invention, the size of the housing is 3 cm (height).×4 cm (width)×6 cm (length) where the length is taken along the optical axis and includes the two mounting flanges  4   b  on each end of the housing  4 . 
         [0045]    The heat spreader  20  may be fabricated from copper-tungsten or other material having a sufficiently high thermal conductivity to effective spread out the heat received from the high thermal conductivity sub-mount  8 . Moreover heat spreader may be composed of a multilayer structure of high thermal conductivity. The high thermal conductivity sub-mount  8  may be secured to the heat spreader  20  by means of epoxy, solder, or laser welded. 
         [0046]    The heat spreader  20  is placed in direct thermal contact with the cooler  22  which may take the form of a thermo-electric cooler (TEC) which provides cooling based on the Peltier effect. As best seen in  FIG. 4 , the cooler  22  is placed in direct thermal contact with the bottom wall of the housing  4  and transfers heat thereto. The bottom surface of the heat spreader  20  may be secured to the top surface of the cooler  22  by means of epoxy, welding, solder or other suitable means. Alternatively, an intermediate plate may be attached between the top surface of the cooler  22  and the bottom surface of the heat spreader  20  in order to provide further rigidity for the optical platform function of the heat spreader  20 . This intermediate plate may serve as a substrate on which the heat spreader is mounted. If the intermediate plate is not utilized, then the top surface of the TEC heat cooler  22  serves as the substrate for mounting the heat spreader  20 . 
         [0047]    The laser device  2  may have its housing mounted to a heat sink (not shown) inside a larger housing (not shown) which may also contain additional equipment including cooling fans and vents to further remove the heat generated by the operation of the laser. 
         [0048]    The cooler  22  is driven in response to the temperature sensor  10 . The cooler may be driven to effect cooling or heating depending on the polarity of the drive current thereto. Currents up to 10-A may be required to achieve temperature stability in CW operation, with less required in pulsed operation. Temperature variations may be used to effect a relatively small wavelength tuning range on the order 1% or less. 
         [0049]    The lens  14  may comprise an aspherical lens with a diameter approximately equal to or less than 10 mm and preferably approximately equal to or less than 5 mm. Thus, the focal length may be one of approximately 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 mm and any fractional values thereof. The focal length of the lens  14  is fabricated to be approximately ½ the size of the diameter. Thus, 10 mm diameter lens will have a focal length of approximately 5 mm, and a 5 mm diameter lens will have a focal length of approximately 2.5 mm. In practice, the lens focal length is slightly larger than ½ the diameter as discussed below in connection with the numeric aperture. The lens  14  serves as a collimating lens and is thus positioned a distance from the laser gain medium  6  equal to its focal length. The collimating lens serves to capture the divergent light from the laser gain medium and form a collimated beam to pass through the window  16  to outside the housing  4 . The diameter of the lens is selected to achieve a desired small sized and to be able to capture the light from the laser gain medium which has a spot size of approximately 4 μm×8 μm. 
         [0050]    The lens  14  may comprise materials selected from the group of Ge, ZnSe, ZnS Si, CaF, BaF or chalcogenide glass. However, other materials may also be utilized. The lens may be made using a diamond turning or molding technique. The lens is designed to have a relatively large numerical aperture (NA) of approximately of 0.6. Preferably the NA is 0.6 or larger. More preferably, the NA is approximately 0.7. Most preferably, the NA is approximately 0.8 or greater. To first order the NA may be approximated by the lens diameter divided by twice the focal length. Thus, selecting a lens diameter of 5 mm with a NA of 0.8 means that the focal length would be approximately 3.1 mm. The lens  14  has an aspheric design so as to achieve diffraction limited performance within the laser cavity. The diffraction limited performance and ray tracing within the cavity permits selection of lens final parameters dependent on the choice of lens material. 
         [0051]    The small focal length of the lens is important in order to realize a small overall footprint of the laser device  2 . Other factors contributing to the small footprint include the monolithic design of the various elements, particularly as related to the positioning of the optical components and the ability to efficiently remove the large amount of heat from the QCL serving as the laser gain medium  6 . 
         [0052]    The monolithic advantages of the described embodiments result from utilizing the heat spreader/optical platform  20  as an optical platform. The output lens  14  and laser gain medium  6  are held in a secured, fixed and rigid relationship to one another by virtue of being fixed to the optical platform  20 . Moreover, the electronic subassembly is also fixed to the optical platform  20  so that all of the critical components within the housing are rigidly and fixedly held together in a stable manner so as to maintain their relative positions with respect to one another. Even the cooler  22  is fixed to the same optical platform  20 . Since the cooler  22  takes the form of a thermoelectric cooler having a rigid top plate mounted to the underside of the optical platform  20 , the optical platform  20  thereby gains further rigidity and stability. The thermoelectric cooler top plate is moreover of approximately the same size as the bottom surface of the heat spreader/optical platform  20  thus distributing the heat over the entire top surface of the cooler  22  and simultaneously maximizing the support for the optical platform  20 . 
         [0053]    The heat spreader/optical platform  20  is seen to comprise a side  20   a , a top surface  20   b , a front surface  20   c , a step  20   d , a recess  20   e  and bridge portion  20   f  and a heat distributing portion  20   g . The electronic sub-assembly  24  is secured to the top surface  20   b . The laser gain medium  6  may be directly secured to the bridge portion  20   f . If an intermediate high thermal conductivity submount  8  is used between the laser gain medium  6  and the bridge portion  20   f , the submount  8  is directly mounted to the bridge portion  20   f  and the laser gain medium  6  is secured to the submount  8 . The lens mount is secured to the front surface of the optical platform  20  via the front screws  13   d . As best seen in  FIG. 4A , a portion of the lens holder  12  is received within the recess  20   e . It may further be seen that the surface of the lens  14  proximate the laser gain medium  6  is also contained within the recess  20   e . Such an arrangement permits the lens, with its extremely short focal length, to be positioned a distance away from the laser gain medium  6  equal to its focal length so that the lens  14  may serve as a collimating lens. The remaining portions of the lens  14  and the lens holder  12  not received within the recess  20   e  are positioned over the top surface of the step  20   d . The heat distributing surface  20   g  of the heat spreader/optical platform  20  is seen to comprise a flat rigid plate that extends substantially over the entire upper surface of the thermo electric cooler  22 . Other than the screw attachments, the elements such as the temperature sensor  10 , laser gain medium  6 , high thermal conductivity submount  8  and electronics sub-assembly  24  may be mounted to the heat spreader/optical platform  20  by means of solder, welding, epoxy, glue or other suitable means. The heat spreader/optical platform  20  is preferably made from a single, integral piece of high thermal conductivity material such as a CuW alloy. 
         [0054]    The housing  4  is hermetically sealed and for this purpose the lid  4   a  may incorporate an “O” ring or other suitable sealing component and may be secured to the housing side walls in an air tight manner, e.g., weld or solder. Prior to sealing or closure, a nitrogen or an air/nitrogen mixture is placed in the housing to keep out moisture and humidity. The window  16  and RF input port  26  present air tight seals. 
         [0055]    The temperature sensor  10  may comprise an encapsulated integrated circuit with a thermistor as the temperature sensor active component. A suitable such sensor is model AD  590  from Analog Devices. The temperature sensor  10  is positioned on the heat spreader  20  immediately adjacent the laser gain medium  6  and is effective to measure the temperature of the laser gain medium  6 . As best seen in  FIGS. 1C and 2A  the temperature sensor  10  as well as the laser gain medium  6  are in direct thermal contact with the heat spreader  20 . The temperature sensor  10  is in direct physical and thermal contact with the heat spreader  20 . In one embodiment, the laser gain medium  6  is in direct physical and thermal contact with the high thermal conductivity submount  8 . However, in other embodiments, the high thermal conductivity submount  8  may be eliminated and the laser gain medium  6  may be secured in direct physical and thermal contact with the heat spreader  20  with all other elements of the laser device remaining the same. The temperature sensor  10  is connected to the I/O leads  28 . The temperature output is used to control the temperature of the cooler  22  so as to maintain the desired level of heat removal from the laser gain medium  6 . It may also be used to regulate and control the injection current to the laser gain medium  6  which also provides a temperature adjustment mechanism. Varying the temperature of the laser gain medium  6  serves to tune the laser, e.g., vary the output wavelength. 
         [0056]    The electronic sub-assembly  24  is used to control the laser gain medium  6  by controlling the electron injection current. This control is done by using a constant current source. In effect the quantum cascade laser behaves like a diode and exhibits a typical diode I-V response curve. For example, at and above the threshold current, the output voltage is clamped to about 9 volts. 
         [0057]      FIG. 5  shows a schematic diagram of the electronics sub-assembly  24 . The electronics sub-assembly is seen to comprise capacitors C 1  and C 2 , resistor R 1 , inductor L 1 , a summing node  30 , switch  32 , and leads  28   a  and  28   b . A trace or transmission line  34   a ,  34   b  (see also  FIG. 3 ) interconnects components. The polarities of the electronics sub-assembly  24  are selected for a chip arrangement in which the epitaxial layer of the quantum cascade laser is positioned downwardly. Polarities would be reversed if the epitaxial layer side is positioned upwardly. In various embodiments, the electronics sub-assembly  24  can be configured to provide suitable drive currents and drive voltages to the MIR laser. In some embodiments, the electronics sub-assembly can comprise a power source (e.g. a battery). 
         [0058]    The RF input port  26  is seen to be fed along the transmission line  34   a  to one side of the first capacitor C 1 . Resistor R 1 , which may comprise a thin film resistor, is positioned between capacitors C 1  and C 2  and connects the junction of these capacitors to ground. The capacitors and resistor implement an impedance matching circuit to match the low impedance of the quantum cascade laser with the 50 ohm input impedance line of the RF input cable. Transmission line  34   b  interconnects inductor L 1  with the switch  32  and connects to the laser gain medium  6 . The inductor L 1  is fed by a constant current source (not shown) via one of the I/O leads, here identified as lead  28   a . Inductor L 1  serves to block the RF from conducting out of the housing through the current lead  28   a . Similarly, a function of the capacitor C 2  is to prevent the DC constant current form exiting the housing via the RF port  26 . The switch  32  may take the form of a MOSFET and is biased by a switching control signal (TTL logic) fed to I/O lead  28   b . Controlling the duty cycle of this switching control signal controls the relative on/off time of the MOSFET which is operative to pass the drive current either to the laser gain medium  6  (when the MOSFET is off) or to shunt the drive current to ground (when the MOSFET is on). With TTL logic in the illustrated circuit, a 0 volt switching control signal turns MOSFET off and thus the quantum cascade laser on, and a −5 volt switching control signal turns the MOSFET on and thus the quantum cascade laser off. By controlling the switching control signal duty cycle, pulse or cw operation may be realized. 
         [0059]    An RF input signal is fed to the RF input port  26 . This RF signal is used to frequency modulate the drive current signal to the laser gain medium  6  and is summed with the drive current at the summing node  30 . Frequency modulation is commonly used to improve sensitivity in absorption spectroscopy. The center frequency is scanned across the expected resonance (using, for example, temperature tuning achieved by variation of the TEC cooler  22  or variation of the current fed to the quantum cascade laser). Frequency modulation places sidebands about the center frequency, and during the wavelength scanning a strong RF modulation may be observed when off resonance due to an imbalance in the absorption of the frequency sidebands. FM modulation thus effectively produces an AM modulation of the absorption signal. However, at resonance, the effect of the frequency sidebands is of opposite phase and equal magnitude so they cancel out. Sweeping the frequency about the resonance peak (dip) using FM modulation thus permits one to pinpoint more accurately the center of the absorption line which corresponds to a minimum in the AM modulation over the sweep range. Techniques for FM modulation are well known to those skilled in the art and reference is made to the following articles incorporated herein by reference: Transient Laser Frequency Modulation Spectroscopy by Hall and North, Annu Rev. Phys. Chem. 2000 51:243-74. 
         [0060]    The quantum cascade lasers utilized herein have an intrinsically high speed. Thus, to effectively perform FM modulation, the modulated signal must be injected in close proximity to the quantum cascade laser to eliminate any excess inductance or capacitances associated with the laser connections to the RF signal. This is especially important in quantum cascade lasers which present a fairly low impedance and thus the reactance of the connections will critically limit the speed with which the device can be modulated. The circuit design (e.g., drive circuit, which may be integrated within the housing) as disclosed herein presents an extremely small footprint for connections of the RF input to the quantum cascade laser. Thus, for a 1 GHz modulation frequency, a representative range of transmission lengths from the RF input port  26  to the laser gain medium (QCL) (the sum of  34   a  and  34   b ) is 2-4 cm or less generally less than or equal to 4 cm. A preferred value is approximately 3 cm. If one desires to choose a broadband input for the FM modulation restricting the maximum frequency to 1 GHz, then the optimal transmission length is approximately 1 cm or greater. Such a transmission length would permit operating at 100 MHz for example or other values up to the 1 GHz level. Thus, in performing FM modulation of the quantum cascade laser a small transmission path is optimal in order to present a low inductance path to the QCL thereby permitting relatively high modulating frequency to be used. The small transmission paths may be suitably contained with the structures of the disclosed electronic sub-assembly  24 . 
         [0061]    It is noted that the entire electronic sub-assembly  24  is rigidly and fixedly mounted on the heat spreader  20  which serves, as indicated above as an optical platform. The fixing of the transmission lines and other electronic components to the optical platform achieves a rugged design which is largely insensitive to outside vibrational disturbances. 
         [0062]    The input leads  28  are seen to comprise leads  28  and  28   b  and the RF input port  26  described above. Other I/O leads to the housing  4  include the +temp drive signal lead for the TEC to cause the TEC to be heated, a −temp drive signal lead to cause the TEC to be cooled, the temperature sensor input lead to provide a bias voltage to the thermistor temperature sensor, a temperature output lead to provided an output signal for the temperature sensor and a ground return path for the constant current input to the quantum cascade laser. 
         [0063]    External cavity quantum cascade (QC) lasers can be tuned very rapidly over broad spectral ranges compared to other types of laser systems. For example, distributed feedback (DFB) QC lasers must be heated or cooled by tens of degrees (Kelvin) to tune over a relatively small range of wavelength. Heating and cooling over such large temperature ranges can still take several seconds, even for systems that have been optimized to have low thermal mass. 
         [0064]    However, it is advantageous to be able to tune over the available wavelength range in order to “freeze” gaseous samples, such that the effects of turbulence and changing density gradients do not modify the spectrum during acquisition. Studies have shown that it is advantageous for spectral acquisition times to be 10 millisecond or less to mitigate the effects of atmospheric turbulence for open path measurements. 
         [0065]    The external cavity allows for many types of rapid tuning mechanisms. Consider, for example, a grating tuned laser illustrated in  FIG. 6A , and its rapid tuning variant realized by spinning the diffraction grating as illustrated in  FIG. 6B . This system can tune over an available gain bandwidth of the laser in less than 10 millisecond depending on the speed of rotation. U.S. patent application Ser. No. 12/353,223 and U.S. Provisional Patent Application No. 61/313,858, both of which are incorporated herein by reference in their entirety, provide more information about fast tunable laser systems. The grating pitch and QC gain device can be changed to access any spectral region covered by QC devices. 
         [0066]    The embodiment illustrated in  FIG. 6A  is an external Quantum cascade laser including a grating. The uncoated facet  601  of the quantum cascade device (e.g. quantum cascade laser, quantum cascade gain medium, etc.) and the surface  602  of the grating form the external cavity. In the illustrated embodiment, the diffraction grating is in Littrow configuration and is configured to provide feedback (e.g. frequency selective feedback). In various embodiments, the wavelength of the laser can be tuned by changing the grating angle θ. 
         [0067]    In the embodiment illustrated in  FIG. 6B , the grating can be mounted on a rotating turntable (e.g. a spinning spindle of a DC servo motor). The turntable can be rotated continuously. In an example embodiment, the turntable can be configured to rotate the grating at a speed of 600 rpm (or at a frequency of 10 Hz) and have an angular turning range of about 30°. This configuration can provide a sweep of the full spectrum in approximately 8 msec. 
         [0068]      FIG. 7  is a plot of the power output by an embodiment of a tunable laser (e.g. a quantum cascade laser) that is operated in the pulsed mode over different wavelength ranges. For example, curve  701  illustrates that in one embodiment, the tunable laser can be tuned from approximately 7 μm to approximately 9 μm and have a maximum peak pulsed power of approximately 350 mW at a wavelength of approximately 8 μM. From  FIG. 7  it is evident that various embodiments of the tunable laser described in the instant application can be tuned over a broad range of mid infra red wavelengths (e.g. from approximately 3 μm to approximately 12.5 μm). 
         [0069]      FIG. 8  is a plot of the absorbance of ethanol for radiation in the wavelength range of approximately 7-11 μm emitted from an embodiment of a tunable quantum cascade laser as compared to the standard absorption spectrum of ethanol. Curve  801  is the measured absorption spectrum of ethanol acquired in a time less than approximately 10 msec when a rapidly tunable quantum cascade laser is tuned from approximately 7.5 μm to approximately 10.5 μm. Curve  802  is the standard absorption spectrum for wavelengths between approximately 7.5 μm to approximately 10.5 μm provided by PNNL (Pacific Northwest National Laboratory). As seen from  FIG. 8 , the measured absorption spectrum of ethanol corresponds to the standard absorption spectrum of ethanol with high fidelity. 
         [0070]      FIG. 9  is a plot of an absorption spectrum of a gas mixture including CO 2 ,  13 CO 2  and  18 OCO shown by curve  904  as compared to the simulated absorption spectrum for CO 2 ,  13 CO 2  and  18 OCO represented by curve  903 . To obtain the measured spectrum, a tunable quantum cascade laser maintained at a temperature of about 35° C. was tuned in a spectral range from approximately 4.31 mm to approximately 4.37 mm in approximately 1 msec. The output from the tunable quantum cascade laser was allowed to propagate through 1 m of the gas mixture which has ambient concentration levels of CO 2 . The measured spectrum (curve  904 ) has been inverted to allow easy comparison with the simulated spectrum (curve  903 ). As seen from  FIG. 9 , the measured absorption spectrum of the gas mixture corresponds to the simulated absorption spectrum with high fidelity. Curves  901  and  902  are the simulated absorption spectrum for  18 OCO and  13 CO 2  respectively. 
         [0071]    While the invention has been describe in reference to preferred embodiments it will be understood that variations and improvements may be realized by those of skill in the art and the invention is intended to cover all such modifications that fall within the scope of the appended claims.