Abstract:
An apparatus that includes a gain chip assembly, an external cavity, and a controller is disclosed. The gain chip assembly includes first and second gain chips that are coupled optically such that light travels serially between the first gain chip and the second gain chip, each gain chip is electrically biased. The electrical bias of the first gain chip is independent of the electrical bias of the second gain chip. The external cavity has a tunable wavelength selective filter that is changed in response to a control signal. Light in the external cavity passes through the gain chip assembly. The controller determines the tunable wavelength selective filter, and the electrical bias of each of the gain chips so as to cause the apparatus to lase at a wavelength specified by a control signal to the controller.

Description:
BACKGROUND 
       [0001]    Imaging and spectrographic analysis in the mid- and long-wave infrared provide information about chemicals of interest in biologic and chemical settings. Quantum cascade lasers (QCLs) can be used as light sources for such measurements. QCLs can be tuned in wavelengths over a significant range of wavelengths in the infrared. However, the range of tuning could be productively increased. 
       SUMMARY 
       [0002]    The present invention includes an apparatus that includes a gain chip assembly, an external cavity (EC), and a controller. The gain chip assembly includes first and second gain chips that are coupled optically such that light travels serially between the first gain chip and the second gain chip, each gain chip is electrically biased. The electrical bias of the first gain chip is independent of the electrical bias of the second gain chip. The EC has a tunable wavelength selective filter that is changed in response to a control signal, light in the EC passes through the gain chip assembly. The controller determines the tunable wavelength selective filter, and the electrical bias of each of the gain chips so as to cause the apparatus to lase at a wavelength specified by a control signal to the controller. 
         [0003]    In one aspect of the invention, the first gain chip lases at wavelengths in a first band of wavelengths around a first wavelength, and the second gain chip lases at wavelengths in a second band of wavelengths around a second wavelength. The second gain chip is biased such that the second gain chip is transparent to wavelengths in the first band of wavelengths, but does not lase when the first gain chip is biased to lase at wavelengths in the first band of wavelengths. 
         [0004]    In another aspect of the invention, the second gain chip also lases at wavelengths in a third band of wavelengths around a third wavelength and wherein the second gain chip is current biased such that the second gain chip does not lase when the first gain chip is biased to lase at wavelengths in the first band of wavelengths. 
         [0005]    In another aspect of the invention, the second gain chip is biased to lase at a wavelength in the third band of wavelengths, and the first gain chip is biased such that the first gain chip is transparent to the wavelength in the third band of wavelengths, but the first gain chip does not lase, and the second gain chip does not lase at wavelengths in the second band of wavelengths. 
         [0006]    In another aspect of the invention, the apparatus also includes an optical assembly that couples light leaving the first gain chip into the second gain chip. 
         [0007]    In another aspect of the invention, one surface of the first gain chip is abutted to a corresponding surface in the second gain chip. 
         [0008]    In another aspect of the invention, the apparatus includes a QCL. 
         [0009]    In another aspect of the invention, the second gain chip is biased by a signal that is pulsed between first and second states, the first state causing the second gain chip to be transparent to light in the first band of wavelengths and the second state causing the second gain chip to be opaque to light in the second band of wavelengths. 
         [0010]    In another aspect of the invention, the second gain chip is biased by a signal that varies between first and second states, both of the first and second states causing the second gain chip to be transparent to light in the second band of wavelengths without lasing. The first and second states are characterized by different optical path lengths through the second gain chip. The first and second states are chosen to partially compensate for changes in the first gain chip when the first gain chip is pulsed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a typical QCL with an EC for tuning the laser. 
           [0012]      FIG. 2  is a cross-sectional view of one embodiment of a prior art gain chip that can be used in laser  40 . 
           [0013]      FIG. 3  illustrates a more detailed view of active layer  13 . 
           [0014]      FIG. 4  illustrates a simplified cross-section of another prior art active region. 
           [0015]      FIG. 5  illustrates an EC QCL according to one embodiment of the present invention. 
           [0016]      FIG. 6  illustrates a laser that utilizes a pair of collimating lenses to allow the gain chips to be separated by a distance that would not be possible with the arrangement shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The manner in which the present invention provides its advantages can be more easily understood with reference to a QCL that uses an EC to tune the output wavelength. Refer now to  FIG. 1 , which illustrates a typical QCL with an EC for tuning the laser. Laser  40  includes a gain chip  41  that is mounted on a mount  42 . Light emitted from the front facet  43  of gain chip  41  is reflected from a grating  46 . The angle of grating  46  relative to the light beam from gain chip  41  is chosen to lock the laser on a particular mode. The angle is set by an actuator  45  that rotates the grating around an axis  53  that is chosen such that the diffracted wavelength and the length of the cavity are maintained to provide the desired wavelength. Lens  47  expands the output beam to the desired size to provide the output light that is used by the measurement system that utilizes laser  40  as its light source. Lens  52  expands the light leaving front facet  43  of gain chip  41  to a diameter that is set to provide the desired wavelength resolution from grating  46 . Larger beam diameters relative to the spacing of the lines on grating  46  provide narrower wavelength bands in the reflected light that reaches gain chip  41 . 
         [0018]    In laser  40 , an optional light output  51  is used to monitor the light intensity in the output beam using a photodetector  49 . Photodetector  49  can be used to provide the signal indicating the output light intensity. 
         [0019]    Refer now to  FIG. 2 , which is a cross-sectional view of one embodiment of a prior art gain chip that can be used in laser  40 . For the purposes of this discussion, gain chip  20  has been reduced to the layers that are most relevant to the QCL operation. In particular, gain chip  20  includes an active layer  13  that will be discussed in more detail below. The active region is sandwiched between two wave guide regions  12  and  14 . Wave guide region  14  is constructed on a substrate  15  that includes one power contact. A second power contact is provided to layer  11 . It should be noted that the various layers may include a number of sub-layers which are not relevant to the present discussion but necessary for the gain chip to operate properly. 
         [0020]    Refer now to  FIG. 3 , which is a more detailed view of active layer  13 . The active layer is made up of a repeating two-layer unit  21 . The first layer is an injection layer  23  for injecting electrons into the second layer  22 , which is a quantum well layer composed of multiple sub-layers which form a series of repeating quantum wells which electrons may propagate (or “cascade”) down under the influence of an applied electric field (voltage). 
         [0021]    The wavelength of the photons emitted when the electrons cascade down the series of quantum wells created by the layers of the active region is determined by the thickness and distribution of the layers. For any single configuration of layers there is a band of wavelengths over which the chip may lase. The specific wavelength emitted by the laser within this band is determined by the external cavity described above. However, the available range of wavelengths for any single homogenous stack of layers is typically limited to tens of wave numbers in the mid-infrared. This is insufficient to provide the needed tuning range for many applications. 
         [0022]    Refer now to  FIG. 4 . which is a simplified cross-section of another prior art active region. One method for increasing the breadth of the gain profile is to use an active layer  33  that has multiple sub-stacks such as sub-stacks  34  and  35 . Each sub-stack has quantum well layers of the same design; however, the quantum well layers  39  in sub-stack  34  have a different design than the quantum well layers  36  in sub-stack  35 . The injection layers  37  and  38  may or may not have the same design, depending on the particular system. When one sub-stack does not provide sufficient gain at a desired wavelength, the other sub-stack may take over. This technique can be used to expand the useful bandwidth of a single gain chip to hundreds of wave numbers. The design of such heterogeneous QCLs is ultimately constrained by the compatibility of optical, mechanical and electronic characteristics of the constituent sub-stacks. 
         [0023]    Even if these constraints are satisfied, broadband operation with an EC for tuning presents challenges. For example, after lasing begins at the wavelength set by the grating and EC, the sub-stack with the highest initial gain contribution at the chosen wavelength may cease to increase in gain with continued increases in electrical current (due to the reduction of its population inversion from stimulated emission). At the same time, the off resonant sub-stacks continue to increase their gain contribution as the current increases. Eventually one of the other sub-stacks will exceed the gain threshold for solitary Fabry Perot (FP) mode lasing and lase near the peak of its own gain curve, resulting in multimode operation and the associated instabilities. 
         [0024]    This problem is connected to the requirement that the sub-stacks are connected in series electrically. Hence, increasing the current to one sub-stack to increase the output of that sub-stack results in all of the sub-stacks receiving an increased current. There is a limit to the current that can be run through a sub-stack before that sub-stack has sufficient gain to lase on its own using the facets of the gain chip as its cavity. At this point, the wavelength of the QCL is no longer responsive to the EC, and the QCL wavelength cannot be tuned. If the corresponding threshold current is below the current needed to provide the desired gain for another sub-stack in the QCL, setting the gain for the desired sub-stack presents challenges. 
         [0025]    The manner in which the present invention provides its advantages can be more easily understood with reference to  FIG. 5 , which illustrates an EC QCL according to one embodiment of the present invention. EC QCL  60  includes two gain chips shown at  61  and  62 . Each gain chip has its own current supply, the current supplies for gain chips  61  and  62  being shown at  67  and  68 , respectively. The current supplies are operated independently as described below. The laser cavity is defined by grating  63  and the front facet  69  of gain chip  62 . Collimating lens  65  expands the light leaving gain chip  61  to provide a wider area of coverage on grating  63  to increase the resolution of the wavelength that is reflected back into gain chip  61 . Collimating lens  66  sets the diameter of the output laser beam. The light from gain chip  61  is coupled into gain chip  62  and vice versa by placing the ends of the gain chips sufficiently close together. The current supplies and position of grating  63  are controlled by controller  75  which responds to an input signal specifying the desired wavelength to be generated. 
         [0026]    To simplify the following discussion, it will be assumed that gain chip  61  has one homogenous active region and gain chip  62  has one homogenous active region with quantum wells of different design than those in gain chip  61 . The case in which one or both of the gain chips each contain sub-stacks with different quantum well sizes will be discussed in more detail below. For the purpose of this discussion, it will be assumed that gain chip  61  lases in a band of wavelengths around λ 1 , and gain chip  62  lases in a band of wavelengths around  20 . When operating at wavelengths around λ 1 , gain chip  61  must be current biased to provide the necessary gain at those wavelengths, while gain chip  62  must be current biased such that gain chip  62  is transparent to light in the band of wavelengths around λ 1 , but with insufficient gain to cause gain chip  62  to lase at one of its wavelengths around λ 2 . 
         [0027]    The present invention is based on the observation that the transparency of a gain chip for a QCL is a function of the bias current through the chip. At low bias currents, gain chip  62  may be too opaque to pass sufficient light to allow the gain of gain chip  61  to compensate for the losses, and hence, EC QCL  60  will not operate. As the bias current through gain chip  62  is increased, the transparency of gain chip  62  may increase to a point near unity without causing gain chip  62  to lase in its band of wavelengths around λ 2 . If the bias current through gain chip  62  is increased further, gain chip  62  will eventually start to lase at one of its wavelengths. Hence, the bias currents are set such that gain chip  61  has sufficient current to lase at the desired wavelength and gain chip  62  is transparent, but non-lasing. Similarly, if EC QCL  60  is to be operated at one of the wavelengths of gain chip  62 , the bias current through gain chip  61  is set to increase the transparency of gain chip  61  to a value that will allow any losses to be made up by gain chip  62 . The gain of gain chip  62  is set such that gain chip  62  lases at the desired wavelength in the band of wavelengths around λ 2 . 
         [0028]    The above-described embodiments rely on coupling light between gain chips  61  and  62  based on the proximity of the two gain chips to each other. If the losses in this configuration are too large, or the chips must be moved farther apart, a pair of collimating lens can be introduced to provide the coupling. Refer now to  FIG. 6 , which illustrates a laser that utilizes a pair of collimating lenses to allow the gain chips to be separated by a distance that would not be possible with the arrangement shown in  FIG. 5 . To simplify the following discussion, those elements of laser  70  that serve functions analogous to elements in EC QCL  60  have been given the same numerical designations and will not be discussed further here. Laser  70  differs from EC QCL  60  in that gain chips  61  and  62  have been separated in space. A pair of collimating lenses shown at  71  and  72  images the output of each gain chip into the input of the other gain chip. 
         [0029]    In the above-described embodiments, two gain chips were utilized with each gain chip having a single homogenous quantum well stack in the active region. However, lasers with more gain chips could be utilized. The maximum number of gain chips depends on the degree to which the non-lasing gain chips can be set to have transparencies that are close enough to unity that the lasing gain chip can compensate for the remaining losses. 
         [0030]    The individual gain chips could also have a limited number of sub-stacks having different quantum well design wavelengths. The problems with multiple sub-stacks that are electrically driven in series becomes significantly worse as the number of sub-stacks (and the width of the resulting composite gain envelope) increases. In addition, prior art arrangements with one gain chip having multiple sub-stacks require that a sub-stack be present for each band of wavelengths in which the laser is required to operate. If two sub-stacks are incompatible, because the desired wavelength sub-stack requires a current that excites one of the other sub-stacks to lase, the problem sub-stack cannot merely be removed, as this would lead to a region in which the EC laser could not be tuned. 
         [0031]    The present invention overcomes this problem by reducing the number of sub-stacks that must be placed in a single gain chip and by allowing incompatible sub-stacks to be placed in separate gain chips where the incompatibility is less of a problem. Since only the combination of the multiple gain chips needs to have a sub-stack or stack that covers each of the desired wavelength regions, a problematic sub-stack can be placed in its own separate gain chip or in a gain chip with other sub-stacks that do not excite the problem sub-stack when the other sub-stacks are being driven to lase in their desired frequency bands. 
         [0032]    In the above-described embodiments, the inactive gain chip is biased to allow that gain chip to pass the light from the active gain chip without having sufficient gain to lase. As noted above, the inactive gain chip when unbiased, is sufficiently opaque to prevent the lasing based on the active gain chip. In principle, the inactive gain chip can provide a gating function by switching from the unbiased mode to the biased mode. 
         [0033]    The active gain chip can be run in either a continuous mode or a pulsed mode. Similarly, the inactive gain chip can be operated in a continuous or pulsed mode. In the pulsed mode, the output of the active gain chip varies in frequency slightly due to the large bias current that is suddenly applied to the active gain chip. This frequency shift is known as a “chirp” in the laser output. The chirp is the result of a change in the index of refraction due to thermal and charge carrier density effects as well as modifications to the physical size of the active gain chip material (thermal expansion). These changes, in turn, lead to a change in the effective optical path length of the external cavity that contains the active gain chip. 
         [0034]    In one aspect of the invention, the inactive gain chip is used to partially compensate for the change in optical path length. As noted above, the inactive gain chip must be operated at a current above a minimum current that is needed to maintain the transparency of the inactive gain chip to the laser light generated in the active gain chip. In addition, the bias current is preferably less than the bias current that would cause the inactive gain chip to lase. Between these two limits, the bias current can be adjusted. For example, if the inactive gain chip is biased toward the high limit when the active gain chip is pulsed, and then the bias is reduced to the lower limit during the pulse period. The change in optical path length introduced by the switching and subsequent heating of the active gain chip can be partially compensated by the change in optical path length introduced by the switching and subsequent cooling of the inactive gain chip corresponding to the reduction in the current flowing through the inactive gain chip. 
         [0035]    The inactive gain chip can also be used to gate the signal from the active gain chip when the active gain chip is run in either the continuous or pulsed mode by switching the bias current from a value that provides transparency to a value that is below the transparency threshold. Hence, the laser can be operated in pulsed mode by pulsing either the active or inactive gain chips. If both gain chips are operated in pulsed mode with pulses that only partially overlap, the resulting laser pulse can be much shorter than either of the pulses applied to the gain chips. 
         [0036]    In the above-described embodiments, the non-selected gain chip is biased such that the non-selected gain chip does not lase and the non-selected gain chip is transparent. For the purposes of this discussion, a gain chip will be defined to be transparent to light of a particular wavelength if the fraction of the light that is absorbed by that gain chip when light of that wavelength passes through that gain chip is less than some predetermined value that depends on the gain of the gain chip that is lasing at that wavelength. If the lasing gain chip can make up for the loses in the non-lasing gain chip, the non-lasing gain chip is defined to be transparent at the lasing wavelength. It should be noted that the non-lasing gain chip could be less transparent at other wavelengths without interfering with the operation of the EC laser. 
         [0037]    The above-described embodiments use a diffraction grating to form one wall of the external cavity and to also filter the wavelengths that are reflected within the cavity to increase the selectivity of the wavelength of the mode that is lasing. In principle, other structures could be used to selectively filter the wavelengths at which lasing takes place. The present invention does not require any specific external cavity arrangement. 
         [0038]    The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.