Abstract:
Systems and methods are disclosed that provide a direct indication of the presence and concentration of an analyte within the external cavity of a laser device that employ the compliance voltage across the laser device. The systems can provide stabilization of the laser wavelength. The systems and methods can obviate the need for an external optical detector, an external gas cell, or other sensing region and reduce the complexity and size of the sensing configuration.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application No. 61/592,297 filed 30 Jan. 2012 entitled “Gas Detection via Compliance Voltage Measurement in an External Cavity Semiconductor Laser”, and U.S. Provisional Patent Application No. 61/653,991 filed 31 May 2012 entitled “Methods for Normalizing Power of Spectroscopic Signatures from Chemical Sensors Employing Lasers and for Stabilizing Power of Lasers via Compliance Voltage Sensing”, which references are incorporated herein in their entirety. 
    
    
     STATEMENT REGARDING RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with Government support under Contract DE-AC05-76RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to laser-based chemical sensors. More particularly, the invention relates to use of an external cavity quantum cascade laser both as a light source and as a chemical detection mechanism. Finally, the invention relates to wavelength stabilization of an external cavity quantum cascade laser. 
     BACKGROUND OF THE INVENTION 
     Traditional gas-phase spectroscopic absorption techniques use a laser, a gas cell or interaction region of some kind, and a photodetector to detect the intensity of light passing through the interaction region. Absorbed light at specific wavelengths indicates spectroscopic absorption features of a gaseous species. Photothermal spectroscopy is one conventional technique that employs a pulsed or modulated laser. The emission (beam) from the laser is absorbed by a gaseous analyte in a cell or interaction region that causes local heating within the gas. The heating of the gas changes the refractive index of the gas. The change in the refractive index is then detected (e.g., interferometrically) with a second laser. The second laser need not be the same laser as the first pulsed laser in either power or wavelength. Photoacoustic spectroscopy is another conventional technique in which the emission of a pulsed laser is absorbed by a gaseous analyte in a cell or interaction region that causes acoustic excitation of the surrounding gas. The acoustic excitation of the gas is detected with a microphone. In one variation, the laser emission is passed between the tines of a small turning fork and absorbed by gaseous analytes surrounding the tuning fork. Resulting pulsating pressure changes excite the fundamental mode of the turning fork, causing an electrical signal to be emitted through the electrical connections of the device which are subsequently detected. 
     Quantum Cascade Lasers (QCLs) are an important light source for chemical detection in the mid infrared (MIR) range (3 to 20 microns) because the emission wavelengths coincide with the fundamental absorption bands of many chemical species of interest. Tunable QCLs, in particular, external cavity QCLs (ECQCLs), are of particular appeal because they represent a single device with a typical tuning range that is 10%, and often up to 20%, of the center wavelength. Thus, only a few ECQCLs are needed to cover large swaths of the MIR. In conventional ECQCL configurations, the ECQCL is used to develop tunable wavelengths as optical outputs. The emission outputs interact with gaseous samples and allow their detection by various detectors (e.g., photodetectors) located external to the ECQCL. Both QCLs and ECQCLs (and other semiconductor lasers) can thus be used in concert with various traditional detection approaches. 
     However, in all of these approaches, despite the increasing capabilities of the lasers and their various laser configurations, the techniques are limited by the detection limits of the photodetectors and acoustic detectors in the MIR. The present invention addresses this problem by providing improved QCL and ECQCL laser configurations and methods for precise and sensitive detection of analytes without the need of a separate detector. 
     SUMMARY OF THE PRESENT INVENTION 
     A method is described for identification and quantitative determination of an analyte or analytes present in an external cavity of an external cavity laser (ECL) by measuring changes in the compliance voltage appearing across a laser device within the ECL. The method may include supplying a drive current to a laser device located within the ECL; setting an operating wavelength of the ECL to a value corresponding to an absorption feature of the analyte or analytes; and measuring the compliance voltage across the laser device to identify the analyte or analytes. The method may also include modulating the drive current to the ECL or modulating the operating wavelength of the ECL, and demodulating the compliance voltage. 
     A method is also described for identification and quantitative determination of an analytes that may include sweeping an operating wavelength of the ECL. The method may include displaying and/or processing the compliance voltage as a function of operating wavelength, which may provide a compliance voltage spectrum. The method may also include modulating the drive current to the ECL or modulating the operating wavelength of the ECL and demodulating the compliance voltage to obtain a demodulated compliance voltage spectrum. 
     A method is also described for stabilizing an operating wavelength of an external cavity laser (ECL). The method may include: supplying a drive current to a laser device located within the ECL; modulating the operating wavelength of a laser mode of the ECL at a preselected modulation frequency, waveform, and amplitude when an analyte or analytes is present in the external cavity of the ECL; setting an average operating wavelength of the ECL to a value corresponding to an absorption feature of the analyte(s); measuring a compliance voltage developed across the laser device; demodulating the compliance voltage at a preselected demodulation frequency, waveform, and amplitude to obtain an error signal; passing the error signal to a control member; applying an output of the control member to at least one wavelength-selective element within the external cavity of the ECL to form a feedback loop that acts on the operating wavelength of the ECL; and adjusting the gain, phase, and bandwidth of the control member to stabilize the feedback loop and the operating wavelength of the ECL. 
     Sweeping can include a range of wavelengths that encompasses one or more absorption features of the analytes or obtaining a signal corresponding to the at least one absorption feature of the analyte(s). Sweeping may include continuous variation or piece-wise variation over the selected range of wavelengths. 
     Modulating the operating wavelength and setting the operating wavelength may employ the same wavelength-selective element within the external cavity of the ECL or may employ distinct wavelength-selective elements within the external cavity of the ECL. 
     Modulating may include a modulation frequency in the range from 0 Hz to 100 GHz. 
     Demodulating may include a demodulation frequency that is a multiple of the modulation frequency. The multiple may be an integer. The multiple may also be a rational fraction. 
     Identifying the analyte may include obtaining a corrected compliance voltage by applying a mathematical function to the compliance voltage. 
     The mathematical function may also be a single-valued mathematical function. The mathematical function may also be a binary mathematical function. 
     The method may include comparing the compliance voltage to a known value of the compliance voltage to determine the analytes. The compliance voltage may also be compared to a known value of the compliance voltage that is measured in the absence of an analytes. The compliance voltage may also be compared to a known value of the compliance voltage that is measured with a selected concentration of analytes in the ECL. 
     The demodulation frequency can range from 0 Hz to 100 GHz. 
     The demodulated compliance voltage may be obtained as a function of the wavelength sweep signal of a wavelength-selective element within the external cavity of the ECL. 
     Sweeping the operating wavelength may include simultaneously modulating the operating wavelength at a selected frequency, waveform, and amplitude. 
     Sweeping the operating wavelength can include supplying a sweep signal to a wavelength-selective element located within the external cavity of the ECL. 
     The wavelength-selective element may be a diffraction grating. 
     The wavelength-selective element may also be a mirror that acts in concert with a diffraction grating. 
     The wavelength-selective element may also be a piezo-electric element. 
     Supplying the drive current to the laser device may include modulating the current at a selected frequency, waveform, and amplitude. The modulation frequency can range from 0 Hz to 100 GHz. 
     Identifying analytes may also include comparing one or more features of the corrected compliance voltage spectrum with absorption features appearing in a known spectrum or known spectra obtained from a spectral database of known analytes. 
     Identifying analytes may also include a least squares fit analysis or a weighted least squares fit analysis. 
     Setting the operating wavelength may include modulating the operating wavelength at a preselected frequency, waveform, and amplitude. The modulation frequency can range from 0 Hz to 100 GHz. 
     Stabilizing the operating wavelength of the ECL may include: modulating the operating wavelength of a laser mode of the ECL at a selected modulation frequency, waveform, and amplitude when an analyte or analytes is present in the external cavity of the ECL; demodulating a compliance voltage (or signal) developed across the laser device located within the ECL at a selected demodulation frequency, waveform, and amplitude to obtain an error signal; passing the error signal to a control system, member, or device; applying an output of the control system or device to at least one wavelength-selective element within the external cavity of the external cavity laser to adjust the wavelength of the laser mode of the external cavity laser to correspond to at least one absorption feature of the at least one analyte within the external cavity of the external cavity laser to form a feedback loop through the laser mode; and positioning the feedback loop formed through the laser mode on the at least one absorption feature appearing on the compliance voltage of the laser and locking same thereon. 
     The control system, member, or device may include amplifiers, limiters, servos, servo-controllers, integrators, differentiators, filters, notches, computers, delays, and combinations of these various devices and elements 
     Positioning a feedback loop may include controlling gain and filter parameters of the feedback loop such that the wavelength of the laser mode of the ECL is locked to one or more absorption features of the analytes within the external cavity. 
     Wavelength-selective elements can include, but are not limited to, e.g., gratings, etalons, piezo-electric elements or devices, and the like. 
     The range of wavelengths scanned may encompass one or more absorption features of an analyte or analytes to obtain a signal corresponding to the absorption feature of the analytes. 
     Lasers can include, but are not limited to, e.g., semiconductor lasers, diode lasers, quantum cascade (QC) lasers, inter-band cascade lasers (iCLs), continuous wave (CW) lasers, pulsed lasers, distributed feedback (DFB) quantum cascade (QC) lasers (DFB-QCLs), components thereof, and combinations of these various lasers and laser systems. 
     Modulating the operating wavelength may include modulating the drive current to the laser device located within the ECL. 
     Modulating the operating wavelength may also include modulating with at least one wavelength-selective element located within the external cavity of the ECL. 
     Modulating the operating wavelength and setting the operating wavelength employ the same wavelength-selective element within the external cavity of the ECL. 
     Modulating the operating wavelength and setting the operating wavelength employ distinct wavelength-selective elements within the external cavity of the ECL. 
     Positioning the feedback loop may include controlling gain and filter parameters such that the wavelength of the laser mode of the external cavity laser is locked to the at least one absorption feature of the analyte(s) within the external cavity thereof. 
     Analytes can include gases, liquids, solids, plasmas, and combinations of these various analyte forms. 
     Analytes may be deliberately introduced or passively introduced into the external cavity of the ECL. 
     Analytes may also fill the external cavity of the ECL. 
     Analytes can fill the entire internal volume of the external cavity of the ECL. Analytes may also be contained within a gas cell such as a multipass cell located within the external cavity of the ECL. 
     Identifying analytes may include introducing a gas cell within the external cavity of the ECL such that the optical path of the ECL passes through the gas cell where analytes in the gas cell can interact with the optical beam circulating within the external cavity of the ECL. 
     Absorption features including, e.g., spectral or spectroscopic features of the one or more analytes may be within the wavelength tuning range of the ECL. 
     The purpose of the foregoing abstract is to enable the United States Patent and Trademark Office and the public generally, especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
     Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions, the various embodiments, including the preferred embodiments, have been shown and described. Included herein is a description of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiments set forth hereafter are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a system that includes an external mirror for direct voltage detection and wavelength scanning in a Littrow ECQCL configuration, according to one embodiment of the invention. 
         FIG. 2  shows a system that includes a QCL with a high reflectance coating for direct voltage detection and wavelength scanning in a Littrow ECQCL configuration, according another embodiment of the invention. 
         FIG. 3  shows a system for direct voltage detection and wavelength scanning of an unrestrained gas therein in a Littman-Metcalf ECQCL configuration, according yet another embodiment of the invention. 
         FIG. 4  shows a system that includes a gas cell configured for direct voltage detection and wavelength scanning of a gas contained within the gas cell in a Littman-Metcalf ECQCL configuration, according still yet another embodiment of the invention. 
         FIG. 5  shows a system employing demodulated voltage detection for wavelength scanning and modulation in a Littrow ECQCL configuration, according to yet another embodiment of the invention. 
         FIG. 6  shows a system that includes a separate demodulation device and control device for direct voltage detection and wavelength stabilization in a Littman-Metcalf ECQCL configuration, according still yet another embodiment of the invention. 
         FIG. 7  shows a system that includes a combined demodulation device and control device for direct voltage detection and wavelength stabilization in a Littman-Metcalf ECQCL configuration, according still yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed herein are methods for identification and quantitative determination an analyte or analytes without the need for external detectors or external gas interaction regions. The following description details a best mode of at least one embodiment of the present invention. While the various embodiments described herein involve an external cavity quantum cascade laser (ECQCL), the invention is not intended to be limited thereto. For example, other lasers and laser systems may be employed. Lasers include, but are not limited to, e.g., semiconductor lasers, diode lasers, quantum cascade (QC) lasers, inter-band cascade lasers (iCLs), continuous wave (CW) lasers, distributed feedback (DFB) quantum cascade (QC) lasers (DFB-QCLs), components thereof, and combinations of these various lasers and laser systems. It will also be clear from the description that the invention is susceptible of various modifications and alternative constructions. Thus, it should be understood that there is no intention to limit the invention to the specific forms disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. Therefore the present description should be seen as illustrative and not limiting. 
       FIG. 1  shows an ECQCL  100  employing compliance voltage measurement for chemical detection, or ECQCL voltage-mediated intra-cavity sensor (EVIS)  100  arranged in a Littrow configuration. The ECQCL may be used both as an optical source and as a chemical detector and/or sensor. Detection of analytes may occur internal to (i.e., within the external cavity of) the ECQCL by monitoring changes in the compliance voltage appearing across the quantum cascade laser (QCL). The voltage appearing across the QCL may be distinct from the current used to drive the QCL device within the ECQCL. And, detection of analytes does not need optical outputs that are delivered from the ECQCL, as detailed herein. For example, EVIS  100  may operate as a chemical sensor incorporated within an optical source. EVIS  100  includes an ECQCL  110  that includes a QCL device  120 , an external mirror  130 , and a wavelength-selective element or device  140  such as an optical grating, an etalon, or an interferometer. An optical laser cavity (OLC)  112  in which a laser mode or ECQCL mode  118  exists, is defined by these same items: external mirror  130 , the QCL device  120 , and optical grating  140 . The external cavity (EC)  115  is defined as that portion of the optical laser cavity  112  external to the QCL device  120 . ECQCL mode  118  is common to both QCL  120  and EC  115 . Optical grating  140  may be mounted on an actuator  145  that tunes the wavelength of ECQCL  110 . EVIS  100  may also include a signal-generating device  143  that supplies actuator  145  with a scanning signal  147 . Scanning signal  147  provides rotation of the grating  140 , e.g., in a periodic manner. EVIS  100  may further include a current controller  121 , which supplies QCL  120  with current  122 . Compliance voltage  150  appearing across QCL  120  (and thus simultaneously across current controller  121 ) may be monitored with a measurement system  160  that may include amplifiers, mixers, filters, and computers or CPUs, but is not limited thereto. Compliance voltage  150  is preferably recorded simultaneously with scanning signal  147 , and displayed with respect to, or processed as a function of, scanning signal  147 . Presence of an analyte  170  or multiple analytes  170  in external cavity  115  of ECQCL  110  can be passively introduced by way of their natural presence in ambient air pervading ECQCL  110 , or may by deliberately introduced into optical laser cavity  112 , e.g., in a gas stream, in a gas sample, or by another means. Analytes  170  in external cavity  115  of ECQCL  110  cause changes to ECQCL optical mode  118 , which in turn cause changes in the compliance voltage  150  detected across QCL  120 , which changes are detected by measurement system  160 . Analytes  170  are thus detected by observing or registering changes in compliance voltage  150 . 
       FIG. 2  shows an ECQCL voltage-mediated intra-cavity sensor (EVIS)  200  arranged in a Littrow configuration. EVIS  200  may operate as a chemical sensor incorporated within an optical source. EVIS  200  includes an ECQCL  110  that includes a QCL device  220  with a high-reflectance mirror coating  230  and anti-reflection coating  235 , a focusing lens  237 , and a wavelength-selective device (element)  140  such as an optical grating, an etalon, or an interferometer. Optical laser cavity  112  in which an ECQCL mode  118  (or laser mode) exists may be defined by these same items: QCL device  220 , and optical grating  140 . External cavity (EC)  115  is defined as that part of the laser cavity external to QCL device  220 . ECQCL mode  118  is common to both QCL  220  and EC  115 . Optical grating  140  may be further mounted on an actuator  145  that tunes the wavelength of ECQCL  110 . EVIS  200  may also include a signal-generating device  143  that supplies actuator  145  with a scanning signal  147 . Scanning signal  147  may provide rotation of grating  140 , e.g., in a periodic manner. EVIS  200  may further include a current controller  121  to supply QCL  220  with current  122 . Compliance voltage  150  appearing across QCL  220  (and thus simultaneously across current controller  121 ) may be monitored with a measurement system  160  including one or more of, but not limited to, e.g., amplifiers, mixers, filters, and a computer or CPU. Compliance voltage  150  is preferably recorded simultaneously with scanning signal  147  and displayed with respect to, or processed as a function of, scanning signal  147 . Presence of an analyte  170  or multiple analytes  170  in external cavity  115  of ECQCL  110  can be the result of their natural presence (i.e., passive introduction) in ambient air pervading ECQCL  110 , or may be the result of deliberate introduction into optical laser cavity  112 , e.g., in a gas stream, in a gas sample, or by another means. Analytes  170  in external cavity  115  of ECQCL  110  may cause changes to ECQCL optical mode  118 , which in turn may cause changes in compliance voltage  150  detected across QCL  220  as detected by measurement system  160 . Analytes  170  may thus be detected by observing (or registering) changes in compliance voltage  150 . 
       FIG. 3  shows an ECQCL voltage-mediated intra-cavity sensor (EVIS)  300  arranged in a Littman-Metcalf configuration. EVIS  300  may operate as a chemical sensor incorporated within an optical source. EVIS  300  may include an ECQCL  110  comprising a QCL device  220  with a high-reflectance mirror coating  230  and anti-reflection coating  235 , focusing lens  237 , a wavelength-selective element or device  140  such as an optical grating, an etalon, or an interferometer, and external mirror  381 . Optical laser cavity (OLC)  112  in which an ECQCL mode  118  (or laser mode) exists is defined by these same items: QCL  220 , optical grating  140 , and external mirror  381 . External cavity (EC)  115  is that part of the laser cavity  112  external to QCL device  220 . ECQCL mode  118  is common to both QCL device  220  and EC  115 . External mirror  381  may further be mounted on an actuator  145  that tunes the wavelength of ECQCL  110 . EVIS  300  may also include a signal-generating device  143  that supplies actuator  145  with a scanning signal  147 . Scanning signal  147  may provide rotation of external mirror  381 , e.g., in a periodic manner. EVIS  300  may further include a current controller  121 , which supplies QCL  220  with current  122 . Compliance voltage  150  appearing across QCL  220  (and thus simultaneously across current controller  121 ) may be monitored by a measurement system  160  which may include amplifiers, mixers, filters, and computers or CPUs, but is not limited thereto. Compliance voltage  150  is preferably recorded simultaneously with scanning signal  147  and displayed with respect to, or processed as a function of, scanning signal  147 . Presence of an analyte  170  in external cavity  115  of ECQCL  110  can be the result of their natural presence (i.e., passive introduction) in ambient air pervading ECQCL  110 , or may be the result of deliberate introduction into optical laser cavity  112 , e.g., in a gas stream, in a gas sample, or by another means. Analytes  170  in external cavity  115  of ECQCL  110  cause changes to ECQCL optical mode  118 , which in turn cause changes in compliance voltage  150  detected across QCL  220 , e.g., as detected by measurement system  160 . Analytes  170  are thus detected by observing (or registering) changes in compliance voltage  150 . 
       FIG. 4  shows an ECQCL voltage-mediated intra-cavity sensor (EVIS)  400  arranged in a Littman-Metcalf configuration. EVIS  400  may operate as a chemical sensor incorporated within an optical source. EVIS  400  differs from EVIS  300  in how analytes  170  are introduced into sensor  400 . EVIS  400  may include an optical cell  475  into which analytes  170  are introduced. Optical cell  475  may be a gas cell, a flow cell, an optical cavity, or a multipass cell such as a Herriott cell or a White cell. 
       FIG. 5  shows a modulated ECQCL voltage-mediated intra-cavity sensor (EVIS)  500  arranged in a Littrow configuration. EVIS  500  may be identical to EVIS  100  ( FIG. 1 ) in its optical configuration. EVIS  500  may include an ECQCL  110  and an associated optical laser cavity (OLC)  112  in which ECQCL mode  118  exists. ECQCL  110  and optical laser cavity (OLC)  112  may be defined by: external mirror  130 , optical grating  140 , and QCL device  120 . External cavity (EC)  115  may be that part of optical laser cavity (OLC)  112  external to QCL device  120 . ECQCL mode  118  may be common to both QCL device  120  and EC  115 . EVIS  500  may include a first signal-generating device  143  that supplies actuator  145  with a scanning signal  147  via output signal  598  from summing device  593 . Scanning signal  147  can provide rotation of optical grating  140 , e.g., in a periodic manner. EVIS  500  may further include a current controller  121 , which supplies QCL  120  with current  122 . EVIS  500  may also include a second signal-generating device  590 , which may provide modulation signal  591  to current controller  121 , which may modulate current  122  in amplitude, which may in turn modulate ECQCL optical mode  118  within ECQCL  110  in amplitude, wavelength, or both amplitude and wavelength. Modulation signal  592  from second signal-generating device  590  may be added to scanning signal  147  through a summing device  593 , the output signal  598  from which may modulate the motion of actuator  145  and in turn modulate ECQCL optical mode  118  within ECQCL  110 , e.g., in amplitude, wavelength, or both amplitude and wavelength. 
     Modulation signals  591  and  592  may be used simultaneously to respectively modulate current  122  and motion of actuator  145 . Modulation signals  591  and  592  can independently be of arbitrary waveform and wavelength. Summing device  593  can be an active device such as an amplifier or a summing buffer, or can include passive devices such as resistors, capacitors, and inductors, or can include direct connections between conductors. Further, summing device  593  can be external and separate from signal-generating device  143  and actuator  145 , or can be internal to one of signal-generating device  143  or actuator  145 . Compliance voltage  150  appearing across QCL  120  (and thus simultaneously across current controller  121 ) may be demodulated by demodulator  595  using a reference signal  594  derived from second signal-generating device  590 . Reference signal  594  may be at a frequency corresponding to an integer or rational multiple of either the frequency of modulation signal  591  or modulation signal  592 , or an integer or rational multiple of the product, sum, or difference of the frequencies of modulation signals  591  and  592 . Output  596  of demodulation device  595  is a demodulated compliance voltage  596 . Demodulated compliance voltage  596  may be monitored by a measurement system  160  which may include: amplifiers, mixers, filters, and computers or CPUs, but is not limited thereto. Demodulated compliance voltage  596  may be recorded simultaneously with scanning signal  147 , and displayed with respect to, or processed as a function of, scanning signal  147 . Presence of an analyte  170  or multiple analytes  170  in the external cavity  115  of ECQCL  110  can be a result of their natural presence (i.e., passive introduction) in ambient air pervading ECQCL  110 , or may be a result of deliberate introduction into optical cavity  112 , e.g., in a gas stream, in a gas sample, or by another means. No limitations are intended. Analytes  170  in external cavity  115  of ECQCL  110  may cause changes to ECQCL optical mode  118 , which in turn may cause changes in compliance voltage  150  and thus the demodulated compliance voltage  596  detected by measurement system  160 . Analytes  170  may thus be detected by observing or registering changes in the demodulated compliance voltage  596 . 
     It should be understood that modulation methods and systems described with reference to EVIS  500  are independent of the optical configuration. While EVIS  500  has been described herein based on the optical configuration of EVIS  100 , other embodiments could equally well be based on embodiments  200 ,  300 ,  400 , or indeed other embodiments of the present invention. Thus, no limitations are intended. 
       FIG. 6  shows an ECQCL voltage-mediated intra-cavity sensor (EVIS)  600  arranged in a Littman-Metcalf configuration. Sensor  600  may also be used as a wavelength-stabilized light source. In some embodiments, EVIS  600  may not employ a flow-through cell or interaction region, but may continuously interrogate analyte  170 , e.g., in a gas cell  475  or other fixed gas volume  475 . EVIS  600  may include an ECQCL  110  that includes a QCL device  220  with a high-reflectance mirror coating  230  and anti-reflection coating  235 , focusing lens  237 , a wavelength-selective element or device  140  such as an optical grating  140 , and an external mirror  381 . Optical laser cavity (OLC)  112  in which a ECQCL mode  118  (or laser mode) exists may be defined by the following items: QCL device  220 , optical grating  140  and external mirror  381 . EVIS  600  may generate an output beam  601  that is distinct from ECQCL mode  118 . ECQCL mode  118  may be common to both QCL device  220  and EC  115 . External cavity (EC)  115  may be that part of laser cavity  112  external to QCL device  220 . Fixed gas volume  475  may be considered a part of EC  115 . Fixed gas volume  475  can be the entire volume of EC  115 , of ECQCL  110 , a simple gas cell, a flow cell, an optical cavity, or a multipass cell such as a Herriott cell or a White cell. External mirror  381  may be further mounted on an actuator  145  that tunes the wavelength of ECQCL  110 . EVIS  600  may further include a current controller  121 , which supplies QCL  220  with current  122 . EVIS  600  may also include a signal-generating device  590  which provides modulation signal  591  to current controller  121  to modulate current  122 , e.g., in amplitude, which in turn may modulate ECQCL optical mode  118  within ECQCL  110 , e.g., in wavelength. Modulation signal  591  can be of arbitrary waveform and frequency. Compliance voltage  150  appearing across QCL  220  (and thus simultaneously across current controller  121 ) may be demodulated by demodulator  595  using a reference signal  594  derived from signal-generating device  590 . Reference signal  594  may be at a frequency corresponding to an integer multiple or a rational multiple of the frequency of modulation signal  591 . A demodulated compliance voltage signal (or error signal)  596  may be passed to control system  660 . Control system  660  can include: amplifiers, servos, filters, and computers or CPUs, but is not limited thereto. Control system  660  may drive actuator  145  via control signal  647  to alter the wavelength of ECQCL  110  to keep it coincident with a selected molecular absorption line or feature of the analytes  170 . Control system  660  may move the wavelength of ECQCL  110  rapidly between at least two molecular absorption lines or features of one analyte  170  or of multiple analytes  170 . In this way, EVIS  600  may be wavelength-stabilized to one or more molecular absorption lines of a single analyte  170  or of multiple analytes  170 . Output beam  601  may thus be coincident in wavelength with molecular absorption features of analytes  170 . 
       FIG. 7  shows an ECQCL voltage-mediated intra-cavity sensor (EVIS)  700  arranged in a Littman-Metcalf configuration. Sensor  700  may be used as a wavelength-stabilized light source. EVIS  700  differs from EVIS  600  in how compliance voltage  150  is processed. Compliance voltage  150  appearing across QCL  220  (and thus simultaneously across current controller  121 ) may be measured by a control device or system  760 . Control system  760  may include, e.g., mixers, amplifiers, signal generators, digital controllers, and computers or CPUs, but is not limited thereto. Control system  760  can alter current  122  passed to QCL device  220  from current controller  121  via a control signal  791 . Control system  760  can also alter the wavelength tuning of ECQCL  110  via an actuator  145  and control signal  647 . In this way, control system  760  can vary current  122  and/or wavelength tuning of ECQCL  110  in order to locate, identify, and stabilize ECQCL  110  to one or more molecular absorption lines or features of the at least one analyte  170 . Control system  760  can also move the wavelength of ECQCL  110  rapidly between at least two molecular absorption lines or features of one analyte  170 , or of multiple analytes  170 . In this way, EVIS  700  may be wavelength-stabilized to one or more molecular absorption lines of at the least one analyte  170 . In this way, output beam  601  may be coincident in wavelength with molecular absorption features of analytes  170 . 
     Mathematical Functions 
     Mathematical functions described herein can include, but are not limited to, e.g., single value functions, binary functions, multiplication functions, multiplication by constant value functions, division functions, square-root functions, linear functions, polynomial functions, raising to mathematical (numerical) powers functions, exponential functions, logarithmic functions, trigonometric functions, binomial functions, and combinations of these. 
     Laser Measurements 
     Laser measurements described herein may include, but are not limited to, e.g., absorption measurements, fluorescence measurements, reflection measurements, distance measurements, phase measurements, interferometric measurements, temperature measurements, density measurements, and combinations of these. 
     Laser Spectra 
     Laser spectra described herein may include, but are not limited to, e.g., absorption spectra, fluorescence spectra, reflection spectra, distance spectra, phase spectra, interferometric spectra, temperature spectra, density spectra, and combinations of these. 
     Sweeping of Laser 
     “Sweeping” or “sweeping a laser” as these terms are used herein may refer to continuous tuning of a laser wavelength, or may refer to a piece-wise tuning of a laser wavelength where the wavelength may be set to specific wavelength values for a selected time period or for selected time periods. 
     While a number of embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims, therefore, are intended to cover all such changes and modifications as they fall within the true spirit and scope of the invention.