Abstract:
Optical feedback assisted cavity enhanced absorption spectroscopy systems and methods for measuring trace gases with improved long-term stability and reproducibility include a laser coupled with a resonant optical cavity containing a gaseous medium and having at least two cavity mirrors and a plurality of optical resonance cavity modes. The laser emits continuous wave laser light with a mean optical frequency of the laser being adjustable over a range of frequencies, and the laser is responsive to optical feedback light emerging from the cavity. The transmissivity of at least one of the cavity mirrors is selected such that the intensity of the optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to generally to trace gas detection and more specifically to cavity enhanced absorption spectroscopy (CEAS) systems and methods for measuring the trace gases. 
         [0002]    In cavity enhanced optical absorption spectroscopy systems and methods, radiation of a laser is directed into a resonance cavity, and the optical intensity inside the cavity is observed. The optical frequency of the laser can be periodically scanned. If it is assumed for clarity that the laser linewidth is much smaller than the cavity resonance width, at the moment when the laser light frequency coincides with a cavity mode transmission peak the optical intensity inside the resonance cavity reflects total cavity loss, and the total cavity loss can be quantitatively determined provided that the incident intensity and cavity parameters are known. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption of a gas mixture present in the cavity. The lower the cavity mirror losses, or equivalently, the higher each mirror&#39;s reflectivity—the smaller the absorption of the intra-cavity gas mixture that can be detected. With very high reflectivity mirrors, the laser linewidth will become too large compared to the cavity resonance width, thus limiting achievable enhancement of the gas mixture absorption by the cavity. This can be helped by narrowing the laser linewidth using optical feedback from the cavity and a laser that is sensitive or responsive to optical feedback from the cavity. With such a laser during the scan, as the frequency of the laser light approaches the frequency of one of the cavity modes, the laser locks to that mode. By saying that the laser is locked to the mode it is meant that the laser linewidth becomes much smaller than the resonance mode width, and that regardless whether the frequency scan range of the unlocked laser may be large, in a locked condition the optical frequency of the laser will change only within the resonance peak. As the laser frequency scan continues, the laser will lose the lock to the current cavity mode and relock to the next cavity mode that it approaches. Due to the optical feedback effect, the laser optical frequency during the scan will essentially take the number of discrete values corresponding to the peaks of the cavity mode resonances that are equidistant in optical frequency. A discrete absorption spectrum of the analyzed gas can thus be obtained by sequential coupling to the entire set of the cavity modes within the scan range, and the trace gas concentration can be derived from the absorption spectrum. This sub-family of cavity enhanced optical absorption spectroscopy systems and methods that uses optical feedback will be referred to as optical feedback cavity enhanced absorption spectroscopy (OF CEAS). 
         [0003]    In OF CEAS, the strength of the optical feedback from the resonance cavity to the laser has to be in certain limits, otherwise it is impossible to provide reproducible scan-to-scan mode coupling as the laser scans. In OF CEAS systems and methods that are known so far, complex optical components are used for this purpose, such as Faraday isolators, variable optical attenuators, or polarization rotators. Adverse interference effects, temperature drifts and aging drifts may result from these components in the system. Achieving high stability and high reproducibility of the optical absorption measurements becomes a major problem. 
         [0004]    Therefore it is desirable to provide OF CEAS systems and methods that overcome the above and other problems. In particular, very high stability and reproducibility of the absorption measurements should be achieved while retaining reproducible scan to scan sequential coupling to every mode in the cavity with no modes missing. 
       BRIEF SUMMARY 
       [0005]    The present invention provides optical feedback assisted cavity enhanced absorption spectroscopy systems and methods for measuring trace gases with improved long-term stability and reproducibility. 
         [0006]    Embodiments of the present invention advantageously enable achieving a laser frequency scan across cavity modes without missing any of the cavity many modes while providing the measurement of optical absorption of the intra-cavity gas mixture with no additional optical elements in the path between the laser and the cavity dedicated to control the optical feedback strength. In prior systems, additional optical components are used to help ensure that no such modes may be missed. In the present embodiments, the laser can be reproducibly locked to any optical resonance cavity mode within the laser&#39;s adjustable frequency range, without the added complexity present in prior systems, and more importantly without the instability and the perturbations caused by such additional elements. 
         [0007]    In order to reduce complexity and obtain sequential coupling, e.g., when the optical locking range is not larger than the cavity FSR (free spectral range), the optical feedback strength is kept below a threshold value,. In certain embodiments, this is accomplished by one of three different approaches, e.g., by one of approaches 1) or 2) below, or 3) their combination:
       1) For the first approach, the total round-trip loss of the cavity is chosen to provide desirable instrument sensitivity, whereas the transmission or transmissivity of the mirror from which the feedback light emerges from the cavity is selected or set to provide optical feedback strength below a threshold value.   2) For the second approach, in case of using a laser that has intrinsic linearly polarized output emission and a cavity having two sets of linearly polarized modes orthogonal to each other, the laser is oriented in such a way that its output polarization makes a non-zero angle to orthogonally polarized cavity mode sets. The angle value is set to provide optical feedback strength for each set of modes below the threshold value.       
 
         [0010]    In certain embodiments, e.g., if it turns out that the optical feedback strength is above the threshold value in the entire range of mutual orientation angle between the laser and the cavity eigenpolarizations, the second approach is combined with the first approach, namely the transmission of the coupling mirror is set to provide optical feedback strength below the threshold for both mode sets. When there is non-zero angle between the laser light and cavity mode polarizations, the laser can be locked to two sets of the orthogonally polarized cavity modes. The mode losses for two different polarizations are different and fixed by the design of the mirror coatings, e.g., multi-layer mirror coatings. 
         [0011]    This second approach provides several additional advantages, for example:
       the loss difference between orthogonally polarized modes can be used as an internal standard in cavity enhanced absorption measurements;   same loss difference between orthogonally polarized mode sets provides wider dynamic range with weaker spectral lines being measured using a lower loss mode set while strong spectral lines may be saturated, but the strong spectral lines will then be measured using a higher loss mode set;   the existing difference of the mode frequencies between the orthogonally polarized mode sets results in higher spectral resolution.       
 
         [0015]    In certain embodiments, systems and methods are provided for detecting trace gases using a resonance optical cavity, containing a gas mixture, that has two or more mirrors and that is capable of being frequency-scanned by changing the optical length of the cavity. A laser or other light source that is capable of being frequency-scanned is coupled to the cavity though one of the cavity mirror, e.g., a “coupling mirror”. When the frequency of the laser light approaches the frequency of one of the cavity modes, the laser begins to fill the cavity to that mode. Optical intensity inside the resonance cavity reflects total cavity loss at the moment when the laser light frequency coincides with the cavity mode transmission peak. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption of the analyzed gas mixture. Control of the feedback intensity of the light incident from the cavity to the laser enables reproducible locking to any optical resonance cavity mode within the adjustable frequency range of the laser. 
         [0016]    According to one aspect of the present invention, a system is provided for detecting one or more analyte species present in a gaseous or liquid medium. The system typically includes a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, with the cavity having a plurality of optical resonance cavity modes, and a laser that emits continuous wave laser light, wherein the laser is capable of being scanned whereby a mean optical frequency of the laser is adjustable over a range of frequencies, and wherein the laser is responsive to optical feedback light emerging from the cavity. The system also typically includes mode matching optics configured to couple the laser light to the cavity via the cavity coupling mirror, and a detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity. The cavity is typically designed wherein the transmissivities of the at least two cavity mirrors are selected or set such that the intensity of the optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity. For example, to advantageously avoid using a separate optical feedback strength adjustment element, the transmissivity (i.e., a metric indicative of the amount of light passing from the cavity through a mirror) of all mirrors, and in particular of the mirror through which the feedback light comes back to the laser, is selected such that the optical feedback strength to the laser is below the threshold value. This usually means that the transmissivity of the mirror providing feedback to the laser should be much lower than that of the other mirrors. For example, to advantageously bring the system to the same enhancement as a system having three mirrors with the same reflectivities and including a separate optical feedback strength adjustment element (called for this example “system  1 ”), the cavity roundtrip loss can be made equal to that of the cavity of system  1  (but without using such adjustment element); the loss factor of the cavity roundtrip is sum of natural logarithms of the cavity mirrors reflectivities, so one can make the two cavities equal by choosing 4*ln(R)=ln(R 1 )+2*ln(R 2 )+ln(R 3 ) where R is relectivity of the mirrors of system  1 , and for the V-cavity embodiment, R 1  is reflectivity of one end mirror, R 2  is the reflectivity of the coupling mirror (providing feedback to the laser), and R 3  is the reflectivity of the other end mirror. 
         [0017]    According to another aspect of the present invention, a system is provided for detecting one or more analyte species present in a gaseous or liquid medium. The system typically includes a resonant optical cavity containing the medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, with the cavity having a plurality of optical resonance cavity modes, wherein the cavity has two sets of linearly polarized cavity modes orthogonal to each other, and a laser that emits continuous wave laser light, wherein the laser has a linearly polarized output, wherein the laser is capable of being scanned whereby a mean optical frequency of the laser is adjustable over a range of frequencies, and wherein the laser is responsive to optical feedback light emerging from the cavity. The system also typically includes mode matching optics configured to couple the laser light to the cavity via the cavity coupling mirror, and a detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity. In one embodiment, an orientation of the laser relative to the cavity is selected so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes so that the intensity of the optical feedback light impinging on the laser is below a threshold intensity value so as to ensure locking to one or more of the plurality of optical resonance cavity modes that have a frequency within said range of frequencies of the laser. In certain aspects, the orientation of the laser relative to the cavity is adjustable so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes so that the intensity of the optical feedback light impinging on the laser is below the threshold intensity value. In certain aspects, the system includes a means for adjusting the laser position and/or orientation so as to adjust the angle of the output polarization with respect to the polarization of the cavity modes. 
         [0018]    According to yet another aspect of the present invention, a system is provided for detecting one or more analyte species present in a gaseous or liquid medium. The system typically includes a resonant optical cavity containing the medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, with the cavity having a plurality of optical resonance cavity modes, and a laser that emits continuous wave laser light, wherein the laser is capable of being scanned whereby a mean optical frequency of the laser is adjustable over a range of frequencies, and wherein the laser is responsive to optical feedback light emerging from the cavity. The system also typically includes mode matching optics configured to couple the laser light to the cavity via the cavity coupling mirror, and a detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity. In one embodiment, the transmissivity of a laser output coupler is selected such that the intensity of the optical feedback light impinging on the laser or the active laser medium is below a threshold intensity value so as to ensure locking to one or more of the plurality of optical resonance cavity modes that have a frequency within said range of frequencies of the laser. 
         [0019]    According to still a further aspect, a system is provided for detecting one or more analyte species present in a gaseous or liquid medium. The system typically includes a resonant optical cavity containing the medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, with the cavity having a plurality of optical resonance cavity modes, wherein the cavity has two sets of linearly polarized cavity modes orthogonal to each other, and a laser that emits continuous wave laser light, wherein the laser has a linearly polarized output, wherein the laser is capable of being scanned whereby a mean optical frequency of the laser is adjustable over a range of frequencies, and wherein the laser is responsive to optical feedback light emerging from the cavity. The system also typically includes mode matching optics configured to couple the laser light to the cavity via the cavity coupling mirror; and a detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity. In one embodiment, the intensity of the optical feedback light impinging on the laser is controlled to be below a threshold intensity value that ensures locking to one or more of the plurality of optical resonance cavity modes that have a frequency within said range of frequencies of the laser by selection of an orientation of the laser relative to the cavity so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes and/or by selection of the reflectivities of the at least two cavity mirrors. 
         [0020]    In certain aspects, the laser includes a semiconductor diode laser. In certain aspects, the optical feedback light impinges on the laser from the cavity coupling mirror. In certain aspects, the reflectivity of the cavity coupling mirror is greater than the reflectivity of the other cavity mirror(s). 
         [0021]    In the various embodiments, as the mean optical frequency of the laser is sequentially adjusted (scanned) over the range of frequencies, the laser advantageously locks to sequential cavity modes without missing any cavity modes. The laser can also be reproducibly locked to any optical resonance cavity mode within the adjustable frequency range of the laser. 
         [0022]    In certain aspects, the cavities of the various embodiments can have a structure of a ring cavity having three or more cavity mirrors, a linear cavity having two cavity mirrors, or a V-shaped cavity having three cavity mirrors. 
         [0023]    In certain aspects for the various embodiments, a means for adjusting a phase of the optical feedback light is provided. In certain aspects for the various embodiments, a means for adjusting the intensity of the optical feedback light is provided. Such means may include, for example, an optical attenuator element positioned between the laser and the cavity along a path of the optical feedback light. 
         [0024]    Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  illustrates a cavity enhanced absorption spectroscopy (CEAS) system according to one embodiment. 
           [0026]      FIGS. 2 and 3  illustrate a cavity enhanced absorption spectroscopy (CEAS) system  200  wherein selection or adjustment of polarization orientation is used to control the intensity of the optical feedback to the laser source according to another embodiment. 
           [0027]      FIG. 4  illustrates the relationship of the reflectivities of the cavity mirrors for a v-shaped cavity that defines the intensity of the feedback to the laser source. 
           [0028]      FIG. 5  illustrates a cavity enhanced absorption spectroscopy (CEAS) system according to yet another embodiment. 
           [0029]      FIG. 6  illustrates a cavity enhanced absorption spectroscopy (CEAS) system according to yet a further embodiment. 
           [0030]      FIG. 7  illustrates a CEAS system having a v-shaped cavity configuration according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The present invention provides cavity enhanced absorption spectroscopy systems and methods for measuring trace gases with improved scan to scan mode coupling efficiency and improved optical feedback control. 
         [0032]    Embodiments of the present invention provide simple, precise and reliable cavity enhanced absorption spectroscopy systems and methods for detecting trace gases that have improved accuracy and stability as compared to existing systems and methods based upon similar principles. This is achieved, in part, by excluding from the optical path between the laser and the cavity components that may cause optical interference effects, temperature, and/or ageing drifts, etc., and adding to or enhancing features of the remaining components to provide improved device operation in the absence of the removed components. For example, embodiments of CEAS systems as disclosed herein advantageously do not require or need any settable attenuator, e.g., placed in the path between the laser and the cavity, or other element to attenuate or control the laser-cavity coupling rate. 
         [0033]      FIG. 1  illustrates a cavity enhanced absorption spectroscopy (CEAS) system  100  according to one embodiment. As shown, CEAS system  100  includes a light source  1  that emits continuous wave coherent light, such as continuous wave laser light, an optical cavity  4  and two detectors, detector  8  and detector  9 . As shown, cavity  4  is a V-shaped cavity defined by cavity coupling mirror  5  and mirrors  6  and  7 . It should be appreciated that the cavity could be a linear cavity with two or more mirrors, or a ring shaped cavity with three or more mirrors, or any other structure having three or more mirrors. An enclosure or housing (not shown) provides an air tight seal for cavity  4  such as to allow control of the environment within the housing and hence the cavity  4 . One or more optical components  2  are configured and arranged to facilitate directing laser light from source  1  to the optical cavity  4  via cavity coupling mirror  5  and to ensure the mode matching of the laser to the cavity. In the embodiment shown in  FIG. 1 , a beam splitting element  3  is positioned and aligned so as to allow substantially all of the incident light  12  emitted or generated by source  1  to impinge on cavity coupling mirror  5 . A portion of the incident light beam  12  is directed (e.g., reflected or refracted) by element  3  to detector  8 . Cavity coupling mirror  5 , in this embodiment, is arranged at an angle with respect to beam  12 , although it could be perpendicular to beam  12 . A portion of incident light  12  enters cavity  4  via mirror  5 . Depending on the frequency of incident light  12  and the optical length of cavity  4  (e.g., optical length from mirror  7  to mirror  5  to mirror  6 ) light  18  circulating in the cavity may build up and resonate at one or a plurality of cavity modes defined by the optical length of the cavity. A portion of the intra-cavity light  18  circulating in cavity  4  between mirrors  7 ,  5  and  6 , emerges or escapes via mirror  5  and impinges on element  3 . Element  3  allows a portion  20  to pass back to source  1 . 
         [0034]    In certain aspects, source  1  includes a laser or other coherent light source that is sensitive or responsive to optical feedback. One useful laser is a semiconductor diode laser that is sensitive to optical feedback from light  20  impinging on the laser from the cavity, e.g., from coupling mirror  5  in the current configuration. In general, useful laser sources might include diode lasers, quantum cascade lasers and solid state lasers, any external cavity laser, etc. Selection of the reflectivities (or transmissivities) of mirrors  5 ,  6  and  7  defines the intensity of beam  20  and hence the optical feedback intensity (see e.g.,  FIG. 4 ) provided to laser  501 . By setting or controlling the optical feedback intensity, the laser can be coupled to any cavity mode which is in the laser scanning range. 
         [0035]    Source  1  is also preferably capable of being frequency scanned, whereby a mean optical frequency of the laser is adjustable over a range of frequencies. This can be accomplished as is well known, such as, for example, by adjusting the current applied to a diode laser and/or adjusting a temperature of the laser medium. In certain aspects, the cavity  4  is also capable of being frequency scanned, e.g., by changing or adjusting an optical length of the cavity, whereby an optical frequency of a cavity resonance peak is adjustable over a range of frequencies. Adjustment of the optical length of the cavity can include adjusting or modulating a relative position of one or more of the cavity mirrors, adjusting a pressure of the medium within cavity  4  or other ways as are known to one skilled in the art. 
         [0036]    In certain embodiments, CEAS system  10  is useful for detecting trace gases within a gas mixture present in the cavity  4 . When the frequency of the light  12  emitted by source  1  approaches the frequency of one of the cavity modes, the light  12  entering the cavity  4  begins to fill the cavity to that mode. The optical intensity of the light  18  circulating inside the resonance cavity reflects total cavity loss at the moment when the light frequency of light  12  coincides with the cavity mode transmission peak. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption by one or more components of the gas mixture present in the cavity. Analyte absorption, e.g., absorption losses caused by absorption by the one or more gas components, is determined based on the difference of the cavity loss when the absorbing component is present in an analyzed gas and the cavity loss when the absorbing component is absent in a reference gas. 
         [0037]      FIGS. 2 and 3  illustrates a cavity enhanced absorption spectroscopy (CEAS) system  200  wherein selection or adjustment of polarization orientation is used to control the intensity of the optical feedback to the laser source according to another embodiment. The principle of operation of CEAS system  200  is similar to that of CEAS system  100 , including operation of a v-shaped cavity structure  204 , with cavity mirror  205  being a cavity coupling mirror. Here, as shown the source  201  has an intrinsic linearly polarized emission and the cavity has two sets of linearly polarized modes orthogonal to each other. In  FIG. 2 , the polarization of the incident laser beam  212  coincides with the polarization of the cavity mode. In  FIG. 3  the polarization of the incident laser beam  212  does not coincide with the polarization of the cavity mode, and hence the intensity of the optical feedback to laser  201  is reduced. In this embodiment, a means to rotate the polarization of the incident laser light  212  is provided for use in setting, controlling or adjusting the optical feedback intensity to the laser  201 . The means, in certain aspects, includes a mechanical element coupled with the laser, or a platform holding the laser, that physically rotates the laser so that the output polarization is rotated. For example, if the polarization vector of beam  212  is rotated by angle α relative to the polarization of the cavity mode, the intracavity power is proportional to cos(α), and the intensity of the light  220  directed back to the laser  201  is also proportional to cos(α), however, the intensity of the feedback light coupled to the laser mode is proportional to cos 2 (α). It should be noted that no attenuator between the cavity and laser need be used as the mutual orientation of the cavity and the laser affects the cavity coupling rate. 
         [0038]      FIG. 4  shows the relationship of the intensity of the feedback to the laser source as a function of the reflectivities of the cavity mirrors for a v-shaped cavity ( FIG. 4   a ) and for a linear cavity ( FIG. 4   b ). For simplicity, any absorption losses on the mirrors in the cavity are neglected. Generally, it is desirable that the reflectivity (R) of the cavity coupling mirror be greater than, or even significantly greater than, the reflectivities (R 1 ) of the other mirror(s) of the cavity. Or, said another way, it is desirable that the transmissivity (T=1−R) of the cavity coupling mirror be less than, or even significantly less than, the transmissivities (T 1 ) of the other mirror(s) of the cavity.  FIG. 4 . also shows two configurations: a) in which the mirror, from which the feedback light emerges from the cavity, is the same as an input mirror ( FIG. 4   a ); and b) where the input and output mirrors are different. The second configuration where the input and feedback mirrors are different is advantageous for OFCEAS and PAS applications as such configuration typically provides for a greater intracavity power. Useful configurations for controlling and steering the output/feedback beam are discussed below with reference to  FIG. 7 . 
         [0039]      FIG. 5  illustrates a cavity enhanced absorption spectroscopy (CEAS) system  500  according to yet another embodiment. The principle of operation of CEAS system  500  is similar to that of CEAS system  100 , including a v-shaped cavity structure  504 , with cavity mirror  505  being a cavity coupling mirror. Cavity coupling mirror  505 , in this configuration, is positioned and oriented such that incident light beam  512  generated by source  501  impinges upon mirror  505  at an angle relative to the plane defined by mirror  505  at the area of impact. The mirror  505  could be oriented perpendicular to beam  512 . Detector  509  detects light emerging from mirror  507  and generates a signal representing the intracavity optical power of light circulating in the cavity  504 . An intelligence module (not shown) receives and processes the detector output signals. Here, laser output coupler  503  of laser  501 , has its transmissivity set, or adjusted, so as to control the intensity of feedback light interacting with laser  501 , or laser medium  522 , e.g., to provide optical feedback intensity below a threshold value. Laser  501  includes any laser having an output coupler, e.g., an external cavity laser. 
         [0040]      FIG. 6  illustrates a cavity enhanced absorption spectroscopy (CEAS) system  600  according to yet a further embodiment that allows for both photo-acoustic spectroscopy (PAS) and direct absorption spectroscopy measurements using the same cavity. The principle of operation of CEAS system  600  is similar to that of CEAS system  100 , including a v-shaped cavity structure  604 , with cavity mirror  605  being a cavity coupling mirror. Cavity coupling mirror  605  is positioned such that incident light beam  612  generated by source  601  impinges upon mirror  605  at an angle relative to the plane defined by mirror  605  at the area of impact. The mirror  605  could be oriented perpendicular to beam  612 . Detector  609  detects light emerging from mirror  607  and generates a signal representing the intracavity optical power of light circulating in the cavity  604 . Detector  608 , in this embodiment, is positioned internal to the cavity (e.g., within the cavity housing structure) to detect a response of the gas mixture filling the optical cavity  604  to the intracavity optical power in the form of an acoustic signal. U.S. patent application Ser. No. 12/660,614, filed on Mar. 2, 2010, provides details of photo-acoustic spectroscopy, useful photo-acoustic sensors and photo-acoustic measurement techniques and parameters, the contents of which are hereby incorporated by reference in its entirety. An intelligence module (not shown) receives and processes the detector output signals, and provides output signals representative of the PAS and/or CEAS measurements. 
         [0041]    In certain embodiments, a means for organizing an unidirectional beam path around the optical cavity is provided. For example, such an arrangement might include one or more optical isolators, which prevent light propagation from the cavity to the laser in opposite direction. In certain embodiments, a means for adjusting the intensity of the optical feedback light is provided. For example,  FIG. 7  illustrates a cavity enhanced absorption spectroscopy (CEAS) system  700  according to such an embodiment. The principle of operation of CEAS system  700  is similar to that of CEAS system  100 , for example, including a v-shaped cavity structure  704 , with cavity mirror  705  being a cavity coupling mirror. Cavity coupling mirror  705  is positioned such that incident light beam  712  generated by laser diode source  701  impinges upon mirror  705  at an angle relative to the plane defined by mirror  705  at the area of impact so that light is reflected to photodetector  708 . Optional beamsplitting element  703  directs a portion of incident beam  712  to optional detector  710 . Photodetector  709 , in this embodiment, is positioned to receive and detect the portion of the intra-cavity light  718  circulating back and forth within cavity  704  between mirrors  705 ,  706  and  707  that emerges or escapes via mirror  706 . Similar to the operation of CEAS  100 , photodetector  710  detects and generates a signal representing the intensity of the laser light  712  incident on the cavity coupling mirror  705 , detector  708  detects and generates a signal representing the intensity of the laser light reflected by the cavity coupling mirror  705 , and detector  709  detects and generates a signal representing the intracavity optical power of light circulating in the cavity  704 . An intelligence module (not shown) receives the three detector output signals and processes these signals to produce or generate output signal(s). 
         [0042]    Also as shown in  FIG. 7  are additional elements to enhance control of the optical feedback, specifically control of the optical feedback to source  701 . As shown, light emerging from cavity mirror  707  passes through a phasor  720  and returns to source  701 , via beamsplitting element  713 . Optical isolators  718  and  719  are provided to completely block light which travels in the opposite direction. For example, optical isolator element  719  blocks light returning (e.g., reflected light or light escaping from the cavity via mirror  705 ) from mirror  705  toward source  701 , and optical isolator element  718  prevents light returning from phasor  720  (e.g., light reflected by phasor  720  or source light reflected by beamsplitter  713  that is passing through phasor  720  on an opposite path) from impinging on mirror  707 . Selection of the cavity mirror reflectivities (e.g., R 1 , R 2  and R 3 ) defines the optical feedback intensity provided to source  701 . Use of phasor  720  advantageously allows for phase control of the optical feedback provided to source  701  from the cavity  704 . It should be appreciated that similar structures and/or functionality for guiding a beam and controlling feedback intensity can be applied to linear cavities and ring cavities. 
         [0043]    As used herein, the terms “threshold intensity value” or “threshold value” when used with reference to optical feedback of the laser source is intended to mean the intensity of the optical feedback above which the laser will lock to a cavity mode for one FSR (free spectral range) of the cavity. Two examples show the condition when the optical feedback strength is above the threshold value: 1) when a cavity mode is scanned for more than one FSR, but the laser continues to be locked to the same cavity mode; 2) when the laser current or temperature of the laser is adjusted so that the laser frequency is scanned, and when unperturbed by optical feedback the laser frequency would be scanned for more than one cavity FSR, whereas in the presence of the (high) optical feedback the laser continues to be locked to the same cavity mode. As above, it is desirable in cavity enhanced absorption systems and methods to avoid this situation, and indeed the above embodiments advantageously ensure that the laser locks to sequential cavity modes as the laser wavelength is being scanned without missing any cavity modes. 
         [0044]    While the invention has been described by way of example and in terms of the specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.