Abstract:
A common multi-gas ring down detector incorporates a cavity that has a piezoelectric mirror and at least two displaced mirrors to define two different transit paths in the cavity. The two paths intersect at the piezoelectric mirror at different angles. Two different laser beams having first and second different wavelengths, can be coupled to the cavity, at different times, by driving the piezoelectric mirror axially. Beam outputs can be evaluated to establish the presence of selected gases in the cavity.

Description:
FIELD 
     The invention pertains to gas sensors, or detectors. More particularly, the invention pertains to cavity ring-down spectrometer (CRDS)-type sensors usable to detect multiple gases. 
     BACKGROUND 
     Various types of cavity ring down sensors/detectors are known. They have been found to be especially useful were very small amounts of contaminants or undesirable gases are to be detected. These include, for example, HF, HCl, and NH3. 
     Representative sensors/detectors include Cole published application 2007/0242266 A1 entitled “Cavity Ring-down Spectrometer Having Mirror Isolation” published Oct. 18, 2007, Cole published application 2008/0239299 A1 entitled “CRDS Mirror for Normal Incidence Fiber Optic Coupling” published Oct. 2, 2008, and Cole et al. U.S. Pat. No. 7,369,242 B2 entitled “Cavity Ring-down Spectrometer for Semiconductor Processing” issued May 6, 2008. The above are all owned by the Assignee hereof and are all incorporated by reference herein. 
     It is at times desirable to measure as many gases as possible using CRDS ring down technology but the absorption lines of gases of interest may not be very close together. This would typically entail using a number of different cavities to measure each gas(es) of interest with any one wavelength range using a tunable laser detector and mirrors designed to be highly reflective in the range of interest. 
     There is a need for sensors of the CRDS-type which can be used to detect the presence of multiple different gases. Preferably such units would be implementable in a cost-effective fashion so as to be less expensive than multiple separate detectors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a multi-gas detector which embodies the invention; and 
         FIG. 2  is a graph illustrating characteristics of the detector of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention, as well as the best mode of practicing same, and is not intended to limit the invention to the specific embodiment illustrated. 
     Embodiments of the invention are capable of measuring multiple different gases whose spectra are widely separated using one cavity. The cavity includes a pair of mirrors that exhibit high reflectance but only transmit within a fairly narrow band. These mirrors have transmission characteristics that make them desirable for light input and output using a single wavelength laser or a tunable laser. 
     One mirror is an ultra high reflectance piezoelectric mirror. By making this element more reflective by adding additional quarter wave pairs to the stack, it is possible to make this mirror substantially totally reflective over a very wide spectral range. Using a mirror of this type beams incident on the mirror from different angles will exhibit a reflectance that is shifted in wavelength. This is because the optical thickness of the layers is different at different angles. This characteristic can be used to implement two or more rings within a common cavity. 
     For the more normal beams, the reflectance is shifted to longer wavelengths. One embodiment of the invention will be capable of measuring three gases, such as HF, HCl, and NH3, with one cavity. The long wavelength HF spectra will be measured at more normal angles relative to the piezoelectric mirror. The HCl lines can be measured at intermediate angles and the short wavelength NH3 lines can be measured at more grazing incidence relative to the piezoelectric mirror. 
     By keeping the beam lengths approximately the same, it is also possible to use similar curved mirrors. These mirrors can be implemented as one of the side mirrors or also can be integrated into the piezoelectric mirror. 
     This CRDS cavity will preferably have input lasers that are tuned to the gas lines of interest. It will also preferably be made of a low expansion material such as glass. An internal center region will be open to gases flowing through it to allow for absorption of the radiation. The piezoelectric mirror will be movable so that the cavity can be coupled to multiple, such as two or three, laser beams for example. 
     Multiple lasers and detectors can be operated substantially simultaneously using the same piezoelectric mirror. Multiple beams will couple into the cavity at different times responsive to the motion of the piezoelectric mirror. Hence, the beams should not substantially interfere with each other in the cavity. 
     In another aspect of the invention, the curved mirror can be part of the piezoelectric mirror. Alternately, one or both of the two other mirrors can be curved. 
       FIG. 1  illustrates a detector  10  which embodies the invention. Detector  10  includes a CRDS-type sensor with a housing  12  which defines an internal cavity R through which a stream of gas can flow. 
     Housing  12  carries a piezoelectric mirror  20 , which can be curved. It can be driven by unit  22  to exhibit reciprocal axial motion  20   a.    
     Housing  12  carries high reflectance mirrors  26   a,b  and  28   a,b . A fixed or tunable wavelength laser  30  emits a beam B 1  of radiant energy of a wavelength that is absorbed by a first gas of interest in the cavity R. That beam of radiant energy B 1  is coupled into the cavity R where it travels on a closed path P 1 . 
     A second laser  32  emits a beam of radiant energy B 2  of a different wavelength that is absorbed by a second, different gas of interest in the cavity R. The second beam of radiant energy B 2  travels on a different closed path P 2  in the cavity R. 
     As will be understood by those of skill in the art, the circulating radiant energy beams on the paths P 1 , P 2  are absorbed by the respective target gases, such as NH3, HCl or HF, and also emitted in part via respective mirrors  26   b ,  28   b  to impinge on detecting elements D 1 , or D 2 . 
     Outputs from D 1 , D 2  can be coupled to control circuits  40  for evaluation. Control circuits  40  are in-turn coupled to drive unit  22  and piezoelectric mirror  20 . Mirror  20  is driven and moves so that two (or more) laser beams B 1 , B 2  can be coupled to the cavity R at different times. The lasers  30 ,  32  can thus be operated simultaneously without interfering with each other in the cavity. 
       FIG. 2  illustrates additional aspects of the embodiment  10  of  FIG. 1 . The L 1  laser,  30  has a wavelength suitable for detecting gas absorption due to either HCL or NH3 on the path P 1  as the mirror  20  vibrates. In that regard, beam B 1  is coupled to cavity R along path P 1  and intersects the mirror  20  at angle A 1 , on the order of 45 degrees, relative to axis  20   b  where mirror  20  is in one position. Beam B 2  is coupled to cavity R and circulates along path P 2  and intersects mirror  20  with angle A 2 , on the order of 15 degrees relative to axis  20   b  where mirror  20  is at a second position. As a result, the two beams B 1 , B 2  do not interfere with each other. 
     The embodiment  10 , illustrated in  FIG. 1 , can be expanded with a third laser source, mirrors and an associated detector to implement a third ring that intersects mirror  20  at a location substantially common to the location of intersection of paths P 1 , P 2 . It will be understood that neither the laser wavelengths nor the specific mirror parameters are limitations of the invention. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.