Abstract:
A method for detection and measurement of trace species in a gas or liquid sample is provided. The method comprises forming a sensor from an optical fiber by tapering a portion the optical fiber along a length thereof, exposing the tapered portion of the optic fiber to the sample gas or sample liquid, emitting radiation from a coherent source, coupling at least a portion of the radiation emitted from the coherent source into the fiber optic ring, receiving a portion of the radiation traveling in the fiber optic ring, and determining the level of trace species in the gas or liquid sample based on a rate of decay of the radiation within the fiber optic ring.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 10/157,400, filed May 29, 2002 now U.S. Pat. No. 7,318,909, which is a Continuation-in-Part of U.S. patent application Ser. No. 10/017,367 filed on Dec. 12, 2001 now U.S. Pat. No. 7,046,362. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to absorption spectroscopy and, in particular, is directed to fiber optic sensors having enhanced evanescent field regions for use with a fiber-optic resonator for ring-down cavity spectroscopy. 
     BACKGROUND OF THE INVENTION 
     Referring now to the drawing, wherein like reference numerals refer to like elements throughout,  FIG. 1  illustrates the electromagnetic spectrum on a logarithmic scale. The science of spectroscopy studies spectra. In contrast with sciences concerned with other parts of the spectrum, optics particularly involves visible and near-visible light—a very narrow part of the available spectrum which extends in wavelength from about 1 mm to about 1 nm. Near visible light includes colors redder than red (infrared) and colors more violet than violet (ultraviolet). The range extends just far enough to either side of visibility that the light can still be handled by most lenses and mirrors made of the usual materials. The wavelength dependence of optical properties of materials must often be considered. 
     Absorption-type spectroscopy offers high sensitivity, response times on the order of microseconds, immunity from poisoning, and limited interference from molecular species other than the species under study. Various molecular species can be detected or identified by absorption spectroscopy. Thus, absorption spectroscopy provides a general method of detecting important trace species. In the gas phase, the sensitivity and selectivity of this method is optimized because the species have their absorption strength concentrated in a set of sharp spectral lines. The narrow lines in the spectrum can be used to discriminate against most interfering species. 
     In many industrial processes, the concentration of trace species in flowing gas streams and liquids must be measured and analyzed with a high degree of speed and accuracy. Such measurement and analysis is required because the concentration of contaminants is often critical to the quality of the end product. Gases such as N 2 , O 2 , H 2 , Ar, and He are used to manufacture integrated circuits, for example, and the presence in those gases of impurities—even at parts per billion (ppb) levels—is damaging and reduces the yield of operational circuits. Therefore, the relatively high sensitivity with which water can be spectroscopically monitored is important to manufacturers of high-purity gases used in the semiconductor industry. Various impurities must be detected in other industrial applications. Further, the presence of impurities, either inherent or deliberately place, in liquids have become of particular concern of late. 
     Spectroscopy has obtained parts per million (ppm) level detection for gaseous contaminants in high-purity gases. Detection sensitivities at the ppb level are attainable in some cases. Accordingly, several spectroscopic methods have been applied to such applications as quantitative contamination monitoring in gases, including: absorption measurements in traditional long pathlength cells, photoacoustic spectroscopy, frequency modulation spectroscopy, and intracavity laser absorption spectroscopy. These methods have several features, discussed in U.S. Pat. No. 5,528,040 issued to Lehmann, which make them difficult to use and impractical for industrial applications. They have been largely confined, therefore, to laboratory investigations. 
     In contrast, cavity ring-down spectroscopy (CRDS) has become an important spectroscopic technique with applications to science, industrial process control, and atmospheric trace gas detection. CRDS has been demonstrated as a technique for the measurement of optical absorption that excels in the low-absorbance regime where conventional methods have inadequate sensitivity. CRDS utilizes the mean lifetime of photons in a high-finesse optical resonator as the absorption-sensitive observable. 
     Typically, the resonator is formed from a pair of nominally equivalent, narrow band, ultra-high reflectivity dielectric mirrors, configured appropriately to form a stable optical resonator. A laser pulse is injected into the resonator through a mirror to experience a mean lifetime which depends upon the photon round-trip transit time, the length of the resonator, the absorption cross section and number density of the species, and a factor accounting for intrinsic resonator losses (which arise largely from the frequency-dependent mirror reflectivities when diffraction losses are negligible). The determination of optical absorption is transformed, therefore, from the conventional power-ratio measurement to a measurement of decay time. The ultimate sensitivity of CRDS is determined by the magnitude of the intrinsic resonator losses, which can be minimized with techniques such as superpolishing that permit the fabrication of ultra-low-loss optics. 
     At present, CRDS is limited to spectroscopic regions where high reflectivity dielectric mirrors can be used. This has significantly limited the usefulness of the method in much of the ultraviolet and infrared regions, because mirrors with sufficiently high reflectivity are not presently available. Even in regions where suitable dielectric mirrors are available, each set of mirrors only allows for operation over a small range of wavelengths, typically a fractional range of a few percent. Further, construction of many dielectric mirrors requires use of materials that may degrade over time, especially when exposed to chemically corrosive environments. Because these present limitations restrict or prevent the use of CRDS in many potential applications, there is a clearly recognized need to improve upon the current state of the art with respect to resonator construction. 
     The article by A. Pipino et al., “Evanescent wave cavity ring-down spectroscopy with a total-internal reflection minicavity,” Rev. Sci. Instrum. 68 (8) (August 1997), presents one approach to an improved resonator construction. The approach uses a monolithic, total internal reflection (TIR) ring resonator of regular polygonal geometry (e.g., square and octagonal) with at least one convex facet to induce stability. A light pulse is totally reflected by a first prism located outside and in the vicinity of the resonator, creating an evanescent wave which enters the resonator and excites the stable modes of the resonator through photon tunneling. When light impinges on a surface of lower index of refraction that the propagation medium at greater than a critical angle, it reflects completely. J. D. Jackson, “Classical Electrodynamics,” Chapter 7, John Wiley &amp; Sons, Inc.: New York, N.Y. (1962). A field exists, however, beyond the point of reflection that is non-propagating and decays exponentially with distance form the interface. This evanescent field carries no power in a pure dielectric medium, but attenuation of the reflected wave allows observation of the presence of an absorbing species in the region of the evanescent field. F. M. Mirabella (ed.), “Internal Reflection Spectroscopy,” Chapter 2, Marcel Dekker, Inc.: New York, N.Y. (1993). 
     The absorption spectrum of matter located at the totally reflecting surfaces of the resonator is obtained from the mean lifetime of a photon in the monolithic resonator, which is extracted from the time dependence of the signal received at a detector by out coupling with a second prism (also a totally reflecting prism located outside, but in the vicinity of, the resonator). Thus, optical radiation enters and exits the resonator by photon tunneling, which permits precise control of input and output coupling. A miniature-resonator realization of CRDS results and the TIR-ring resonator extends the CRDS concept to condensed matter spectroscopy. The broadband nature of TIR circumvents the narrow bandwidth restriction imposed by dielectric mirrors in conventional gas-phase CRDS. The work of A. Pipino et al. is only applicable to TIR spectroscopy, which is intrinsically limited to short overall absorption pathlengths, and thus powerful absorption strengths. In contrast, the present invention provides long absorption pathlengths and thus allows for detection of weak absorption strengths. 
     Various novel approaches to mirror based CRDS systems are provided in U.S. Pat. Nos. 5,973,864, 6,097,555, 6,172,823 B1, and 6,172,824 B1 issued to Lehmann et al., and incorporated herein by reference. These approaches teach the use of a near-confocal resonator formed by two reflecting elements or prismatic elements. 
       FIG. 2  illustrates a prior art CRDS apparatus  10 . As shown in  FIG. 2 , light is generated from a narrow band, tunable, continuous wave diode laser  20 . Laser  20  is temperature tuned by a temperature controller  30  to put its wavelength on the desired spectral line of the analyte. An isolator  40  is positioned in front of and in line with the radiation emitted from laser  20 . Isolator  40  provides a one-way transmission path, allowing radiation to travel away from laser  20  but preventing radiation from traveling in the opposite direction. Single mode fiber coupler (F.C.)  50  couples the light emitted from laser  20  into the optical fiber  48 . Fiber coupler  50  is positioned in front of and in line with isolator  40 . Fiber coupler  50  receives and holds optical fiber  48  and directs the radiation emitted from laser  20  toward and through a first lens  46 . First lens  46  collects and focuses the radiation. Because the beam pattern emitted by laser  20  does not perfectly match the pattern of light propagating in optical fiber  48 , there is an inevitable mismatch loss. 
     The laser radiation is approximately mode-matched into a ring down cavity (RDC) cell  60 . A reflective mirror  52  directs the radiation toward a beam splitter  54 . Beam splitter  54  directs about 90%, of the radiation through a second lens  56 . Second lens  56  collects and focuses the radiation into cell  60 . The remaining radiation passes through beam splitter  54  and is directed by a reflective mirror  58  into an analyte reference cell  90 . 
     The radiation which is transmitted through analyte reference cell  90  is directed toward and through a fourth lens  92 . Fourth lens  92  is aligned between analyte reference cell  90  and a second photodetector  94  (PD  2 ). Photodetector  94  provides input to computer and control electronics  100 . 
     Cell  60  is made from two, highly reflective mirrors  62 ,  64 , which are aligned as a near confocal etalon along an axis, a. Mirrors  62 ,  64  constitute the input and output windows of cell  60 . The sample gas under study flows through a narrow tube  66  that is coaxial with the optical axis, a, of cell  60 . Mirrors  62 ,  64  are placed on adjustable flanges or mounts that are sealed with vacuum tight bellows to allow adjustment of the optical alignment of cell  60 . 
     Mirrors  62 ,  64  have a high-reflectivity dielectric coating and are oriented with the coating facing inside the cavity formed by cell  60 . A small fraction of laser light enters cell  60  through front mirror  62  and “rings” back and forth inside the cavity of cell  60 . Light transmitted through rear mirror  64  (the reflector) of cell  60  is directed toward and through a third lens  68  and, in turn, imaged onto a first photodetector  70  (PD  1 ). Each of photodetectors  70 ,  94  converts an incoming optical beam into an electrical current and, therefore, provides an input signal to computer and control electronics  100 . The input signal represents the decay rate of the cavity ring down. 
       FIG. 3  illustrates optical path within a prior art CRDS resonator  100 . As shown in  FIG. 3 , resonator  100  for CRDS is based upon using two Brewster&#39;s angle retroreflector prisms  50 ,  52 . The polarizing or Brewster&#39;s angle, Θ B , is shown relative to prism  50 . Incident light  12  and exiting light  14  are illustrated as input to and output from prism  52 , respectively. The resonant optical beam undergoes two total internal reflections without loss in each prism  50 ,  52  at about 45°, an angle which is greater than the critical angle for fused quartz and most other common optical prism materials. Light travels between prisms  50 ,  52  along optical axis  54 . 
     Although, when compared with the other spectroscopy methods, ring down cavity spectroscopy is a simpler and less expensive to implement, it is still costly in that a ring down cavity spectroscopy system can cost on the order of many thousands of dollars per unit. In addition, conventional CRDS devices are prone to misalignment between the optical elements while being fabricated as well as during use. 
     To overcome the shortcomings of the known approaches to improved resonator construction, a new optic-fiber based optical resonator for CRDS is provided. An object of the present invention is to replace conventional fiber optic sensors with sensors having enhanced evanescent field portion, thereby providing a more sensitive fiber optic sensor. 
     SUMMARY OF THE INVENTION 
     To achieve that and other objects, and in view of its purposes, the present invention provides an improved apparatus for trace species detection and measurement in a sample gas. The apparatus includes a passive fiber optic cable; at least one sensor in line with the fiber optic cable, the at least one sensor having a portion thereof exposed to the sample gas or sample liquid; a coherent source of radiation; coupling means for i) introducing a portion of the radiation emitted by the coherent source to the passive fiber optic ring and ii) receiving a portion of the resonant radiation in the passive fiber optic ring; a detector for detecting a level of the radiation received by the coupling means and generating a signal responsive thereto; and a processor coupled to the detector for determining a level of the trace species in the gas sample or liquid sample based on the signal generated by the detector. 
     According to another aspect of the invention, the sensor has a tapered portion exposed to the sample gas or sample liquid. 
     According to a further aspect of the invention, the sensor has an exposed portion with a “D” shaped cross section. 
     According to yet another aspect of the invention, the level of the trace species is determined based on a rate of decay of the signal generated by the detector means. 
     According to a further aspect of the invention, a filter is placed between the coupling means and the detector to selectively pass the received portion of radiation from the passive fiber optic loop to the detector. 
     According to yet another aspect of the invention, the coupler includes i) a first coupler for introducing the portion of the radiation emitted by the coherent source to a first section of the fiber optic ring and ii) a second coupler for receiving the portion of the radiation in the passive fiber optic ring at a second section thereof. 
     According to still another aspect of the invention, the exposed portion of the fiber is the cladding of the fiber. 
     According to yet a further aspect of the invention, the exposed portion of the fiber is the inner core of the fiber. 
     According to another aspect of the invention, the coherent source is an optical parametric generator, an optical parametric amplifier, or a laser. 
     According to yet another aspect of the invention, an evanescent field of the radiation traveling within the fiber is exposed to the sample gas or sample liquid. 
     According to still another aspect of the invention, the absorption of the radiation from the fiber increases a rate of decay of the radiation. 
     According to yet a further aspect of the invention, the passive resonant fiber has a hollow core. 
     According to yet another aspect of the invention, the apparatus further comprises a sensor formed from a cylindrical body and wrapped with a section of the exposed portion of the resonant fiber such that exposure of the evanescent field to the trace species is enhanced by increasing the penetration depth of the evanescent field. 
     According to a further aspect of the invention, at least a portion of the passive fiber optic ring is coated with a material to selectively increase a concentration of the trace species at the coated portion of the fiber optic ring. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
         FIG. 1  illustrates the electromagnetic spectrum on a logarithmic scale; 
         FIG. 2  illustrates a prior art CRDS system using mirrors; 
         FIG. 3  illustrates a prior art CRDS cell using prisms; 
         FIG. 4  is an illustration of a first exemplary embodiment of the present invention; 
         FIG. 5A  is a end view of a conventional optical fiber; 
         FIG. 5B  is a perspective view of a sensor according to an exemplary embodiment of the present invention; 
         FIG. 6A  is a cross sectional view of fiber optic cable illustrating propagation of radiation within the cable; 
         FIG. 6B  is a cross section of a fiber optic sensor illustrating the evanescent field according to an exemplary embodiment of the present invention 
         FIG. 6C  is a cross section of a fiber optic sensor illustrating the evanescent field according to another exemplary embodiment of the present invention; 
         FIG. 6D  is a cross-section of a fiber optic sensor according to another exemplary embodiment of the present invention; 
         FIG. 7  is an illustration of a second exemplary embodiment of the present invention; 
         FIGS. 8A-8D  are illustrations of a fiber optic sensor according to a third exemplary embodiment of the present invention; 
         FIGS. 9A-9C  are illustrations of a fiber optic sensor according to a fourth exemplary embodiment of the present invention; and 
         FIGS. 10A-10C  are illustrations of a fiber optic sensor according to a fifth exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The entire disclosure of U.S. patent application Ser. No. 10/017,367 filed Dec. 12, 2001 is expressly incorporated herein by reference. 
       FIG. 4  illustrates fiber optic based ring-down apparatus  400  according to a first exemplary embodiment of the present invention through which trace species, or analytes, in gases and liquids may be detected. In  FIG. 4 , apparatus  400  includes resonant fiber optic ring  408  which has fiber optic cable  402  and sensors  500  (described below in detail) distributed along the length of fiber optic cable  402 . The length of resonant fiber optic ring  408  is easily adaptable to a variety of acquisition situations, such as perimeter sensing or passing through various sections of a physical plant, for example. Although as shown, sensors  500  are distributed along the length of fiber optic loop  408 , the invention may be practiced using only one sensor  500 , if desired. The distribution of more than one sensor  500  allows for sampling of a trace species at various points throughout the installation site. The invention may also be practiced using a combination of sensors  500  with straight section of fiber  402  exposed to sample liquids or gases, or with only straight sections of fiber  402  exposed to the sample liquid or gas. It is contemplated that the length of resonant fiber optic ring may be as small as about 1 meter or as large as several kilometers. 
     Coherent source of radiation  404 , such as an optical parametric generator (OPG), optical parametric amplifier (OPA) or a laser, for example, emits radiation at a wavelength consistent with an absorption frequency of the analyte or trace species of interest. Coherent source  404  may be a tunable diode laser having a narrow band based on the trace species of interest. An example of a commercially available optical parametric amplifier is model no. OPA-800C available from Spectra Physics, of Mountain View, Calif. 
     Examples of frequencies of coherent source  404  versus analytes are outlined in Table 1. Table 1 is merely illustrative and not intended as restrictive of the scope of the present invention. Further, it is contemplated that the present invention may be used to detect a variety of chemical and biological agents harmful to humans and/or animals. It is also contemplated that such detection may be enhanced by coating the surface of the passive fiber optic ring with antibodies that specifically bind the desired antigen. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Approximate 
                 Approximate 
               
               
                 Analyte or 
                 Wavelength(s) 
                 Wavelength(s) 
               
               
                 Trace Species 
                 Near Infrared 
                 Mid Infrared 
               
               
                   
               
             
             
               
                 Water (H2O) 
                 1390 nm 
                 5940 nm 
               
               
                 Ammonia (NH3) 
                 1500 nm 
                 10300 nm  
               
               
                 Methane (CH4) 
                 1650 nm 
                 3260 nm 
               
               
                 Carbon Dioxide (CO2) 
                 1960 nm 
                 4230 nm 
               
               
                 Carbon Monoxide (CO) 
                 1570 nm; 2330 nm 
                 4600 nm 
               
               
                 Nitric Oxide (NO) 
                 1800 nm; 2650 nm 
                 5250 nm 
               
               
                 Nitrogen Dioxide (NO2) 
                 2680 nm 
                 6140 nm 
               
               
                 Nitrous Oxide (N2O) 
                 2260 nm 
                 4470 nm 
               
               
                 Sulfur Dioxide (SO2) 
                   
                 7280 nm 
               
               
                 Acetylene 
                 1520 nm 
                 7400 nm 
               
               
                 Hydrogen Fluoride (HF) 
                 1310 nm 
               
               
                 Hydrogen Chloride (HCl) 
                 1790 nm 
                 3400 nm 
               
               
                 Hydrogen Bromide (HBr) 
                 1960 nm 
                 3820 nm 
               
               
                 Hydrogen Iodide (HI) 
                 1540 nm 
               
               
                 Hydrogen Cyanide (HCN) 
                 1540 nm 
                 6910 nm 
               
               
                 Hydrogen Sulfide (H2S) 
                 1570 nm 
               
               
                 Ozone (O3) 
                   
                 9500 nm 
               
               
                 Formaldehyde (H2CO) 
                 1930 nm 
                 3550 nm 
               
               
                 Phosphine (PH3) 
                 2150 nm 
                 10100 nm  
               
               
                 Oxygen (O2) 
                  760 nm 
               
               
                   
               
             
          
         
       
     
     In the first exemplary embodiment, radiation from coherent source  404  is provided to resonant fiber optic ring  408  through optional optical isolator  406 , coupler  410 , and evanescent input coupler  412 . When coherent source  404  is a diode laser, using optical isolator  406  provides the benefit of minimizing noise in the laser by preventing reflections back into the laser. Evanescent input coupler  412  may provide a fixed percentage of radiation from coherent source  404  into resonant fiber optic ring  408 , or may be adjustable based on losses present throughout resonant fiber optic ring  408 . Preferably, the amount of radiation provided by evanescent input coupler  412  to resonant fiber optic ring  408  matches the losses present in fiber optic cable  402  and the connectors (not shown). A commercially available evanescent coupler providing 1% coupling (99%/1% split ratio coupling) of radiation is manufactured by ThorLabs of Newton, N.J., having part number 10202A-99. In a preferred embodiment, evanescent input coupler  412  couples less that 1% of the radiation from coherent source  404  into fiber  402 . 
     In one exemplary embodiment, to detect the trace species or analyte, a portion of the jacket  402   a  covering the fiber optic cable  402  is removed to expose cladding  402   b  that surrounds inner core  402   c  of fiber optic cable  402 . Alternatively, either both jacket  402   a  and cladding  402   b  may be removed to expose inner core  402   c , or the jacketed portion of fiber optic cable  402  may be exposed to the sample liquid or gas. The latter approach may be useful for example, in the case where the evanescent field (discussed below) extends into the jacket for interaction with the trace species (which has been absorbed or dissolved into the jacket). Removing both the jacket and cladding may not be the most preferred, however, because of the brittle nature of inner core  402   c  used in certain types of fiber optic cables. A cross section of a typical fiber optic cable is shown in  FIG. 5A . 
     Bending a total internal reflection (TIR) element changes the angle at which the incident electromagnetic wave contacts the reflection surface. In the case of bending an optical fiber about a cylindrical body, the angle of reflection on the surface of the fiber core opposite the body is closer to normal, and the penetration depth of the evanescent field is increased. By wrapping several turns of optical fiber  402  around cylindrical core element  502  (see  FIG. 5B ), the evanescent field penetration depth is increased and a greater length of fiber can be exposed to the detection fluid in a smaller physical volume. An experimental, verification of the improvement in optical fiber sensing through varying bending radii is discussed by D. Littlejohn et al. in “Bent Silica Fiber Evanescent Absorption Sensors for Near Infrared Spectroscopy,” Applied Spectroscopy 53: 845-849 (1999). 
       FIG. 5B  illustrates an exemplary sensor  500  used to detect trace species in a liquid or gas sample. As shown in  FIG. 5B , sensor  500  includes cylindrical core element  502  (which may be solid, hollow or otherwise permeable), such as a mandrel, with a portion of fiber optic cable  402 , with cladding  402   b  exposed (in this example), wrapped around core element  502  over a predetermined length  506 . It is also possible to fabricate sensor  500  by wrapping core element  502  where core  402   c  of fiber optic cable  402  is exposed. The diameter of core element  502  is such that fiber core  402   c  is formed with less than a critical radius r, at which point excess radiation may be lost through fiber core  402   c  as it circumscribes core element  502 , or fiber integrity is compromised. The critical radius r is dependent on the frequency of the radiation passing through fiber optic cable  402  and/or the composition of the fiber. In a preferred embodiment of the present invention, the radius of core element  502  is between about 1 cm and 10 cm, and most preferably at least about 1 cm. As illustrated, radiation from fiber  402  is provided at input  504  and extracted at output  508 . Cylindrical core element  502  may have a spiral groove on its surface in which fiber  402  is placed as well as a means to secure fiber  402  to cylindrical core element  502 . Such securing means may take may forms, such as a screw tapped into cylindrical core element  502 , an adhesive, such as epoxy or silicon rubber, etc. The invention may be practiced where sensors  500  are integral with fiber  402  or may be coupled to fiber  402  utilizing commercially available fiber-optic connectors. 
       FIG. 6A  illustrates how radiation propagates through a typical fiber optic cable. As shown in  FIG. 6A , radiation  606  exhibits total internal reflection (TIR) at the boundary between inner core  402   c  and cladding  402   b . There is some negligible loss (not shown) by which radiation is not reflected, but is absorbed into cladding  402   b . Although  FIG. 6A  is described as a fiber optic cable,  FIG. 6A  and the exemplary embodiments of the present inventions are equally applicable to a hollow fiber, such as a hollow waveguide, in which cladding  402   b  surrounds a hollow core. 
       FIG. 6B  is a cross sectional view of one exemplary embodiment of sensor  500  which illustrates the effect of wrapping fiber optic cable  402  around core element  502 . As shown in  FIG. 6B , only jacket  402   a  is removed from fiber optic cable  402 . Radiation  606  travels within core  402   c  and exhibits total internal reflection at the boundary between inner core  402   c  and the portion of cladding  402   b - 1  adjacent core element  502  with a negligible loss  609 . On the other hand, in the presence of trace species or analyte  610 , evanescent field  608  passes through the interface between inner core  402   c  and the exposed portion of cladding  402   b - 2 . This essentially attenuates radiation  606  based on the amount of trace species  610  present and is called attenuated total internal reflection (ATR). It should be noted that if there is no a trace species present having an absorption band compatible with the wavelength of the radiation, radiation  606  is not attenuated (other than by inherent loss in the fiber). 
       FIG. 6C  is a cross sectional view of another exemplary embodiment of sensor  500  which illustrates the effect of wrapping fiber optic cable  402  around core element  502  with a portion of jacket  402   a  remaining intact. As shown in  FIG. 6D , only an upper portion of jacket  402   a  is removed from fiber optic cable  402 . Similar to the first exemplary embodiment of sensor  500 , radiation  606  travels within core  402   c  and exhibits total internal reflection at the boundary between inner core  402   c  and the portion of cladding  402   b - 1  adjacent core element  502  with negligible loss  609 . On the other hand, in the presence of trace species or analyte  610  evanescent field  608  passes through the interface between inner core  402   c  and the exposed portion of cladding  402   b - 2 . 
     It is contemplated that the removal of jacket  402   a  (in either example of sensor  500 ) may be accomplished by mechanical means, such as a conventional fiber optic stripping tool, or by immersing the portion of the fiber cable in a solvent that will attack and dissolve jacket  402   a  without effecting cladding  402   b  and inner core  402   c . In the case of partial removal of jacket  402   a , the solvent approach may be modified by selectively applying the solvent to the portion of the jacket intended for removal. 
     To enhance the attraction of analyte molecules of the trace species in a liquid sample, a jacket-less portion of the passive fiber optic ring may be coated with a material to selectively increase a concentration of the trace species at the coated portion of the fiber optic ring. An example of one such coating material is polyethylene. Additionally, antigen specific binders may be used to coat the fiber to attract a desired biological analyte with high specificity. 
     Referring again to  FIG. 4 , the radiation that remains after passing through sensors  500  continues through fiber loop  402 . A portion of that remaining radiation is coupled out of fiber optic loop  402  by evanescent output coupler  416 . Evanescent output coupler  416  is coupled to processor  420  through detector  418  and signal line  422 . Processor  420  may be a PC, for example, having a means for converting the analog output of detector  418  into a digital signal for processing. Processor  420  also controls coherent source  404  through control line  424 . Once the signals are received from detector  418  by processor  420 , the processor may determine the amount and type of trace species present based the decay rate of the radiation received. 
     Optionally, wavelength selector  430  may be placed between evanescent output coupler  416  and detector  418 . Wavelength selector  430  acts as a filter to prevent radiation that is not within a predetermined range from being input into detector  418 . 
     Detector  414  is coupled to the output of input coupler  412 . The output of detector  414  is provided to processor  420  via signal line  422  for use in determining when resonant fiber optic ring  402  has received sufficient radiation by which to perform trace species analysis. 
     In the case of detection of trace species or analytes in liquids, the index of refraction of the liquid must be lower than the index of refraction of the fiber optic cable. For example, given a fiber optic cable having an index of refraction of n=1.46, the invention may be used to detect trace species dissolved in water (n=1.33) and many organic solvents, including methanol (n=1.326), n-hexane (n=1.372), dichloromethane (n=1.4242), acetone (n=1.3588), diethylether (n=1.3526), and tetrahydrofuran (n=1.404), for example. An extensive list of chemicals and their respective index of refraction may be found in  CRC Handbook of Chemistry and Physics,  52 nd    edition , Weast, Rober C., ed. The Chemical Rubber Company: Cleveland Ohio, 1971, p. E-201, incorporated herein by reference. There are other types of optical fiber available with different indexes of refraction, and the present invention can be tailored to a given liquid matrix assuming the optical fiber has both a higher index of refraction than the liquid and effectively transmits light in the region of an absorption band by the target analyte. 
     There are many different types of optical fiber currently available. One example is Corning&#39;s SMF-28e fused silica fiber which has a standard use in telecommunications applications. Specialty fibers exist that transmit light at a multitude of different wavelengths, such as a 488 nm/514 nm single mode fiber, manufactured by 3M of Austin, Tex. (part no. FS-VS-2614), 630 nm visible wavelength single-mode fiber manufactured by 3M of Austin, Tex. (part no. FS-SN-3224), 820 nm standard single-mode fiber manufactured by 3M of Austin, Tex. (part no. FS-SN-4224), and 0.28-NA fluoride glass fiber with 4-micron transmission, manufactured by KDD Fiberlabs of Japan (part no. GF-F-160). Further, and as mentioned above, fiber optic cable  402  may be a hollow fiber. 
     It is contemplated that fiber  402  may be a mid-infrared transmitting fiber to allow for access to spectral regions having much higher analyte absorption strengths, thereby increasing the sensitivity of the apparatus  400 . Fibers that transmit radiation in this region are typically made from fluoride glasses. 
       FIG. 7  illustrates a second exemplary embodiment of the present invention through which trace species, or analytes, in gases and liquids may be detected. In describing  FIG. 7 , elements performing similar functions to those described with respect to the first exemplary embodiment will use identical reference numerals. In  FIG. 7 , apparatus  700  uses a similar resonant fiber optic ring  408  including fiber optic cable  402  and sensors  500 . Radiation from coherent source  404  is provided to resonant fiber optic ring  408  through optional optical isolator  406 , coupler  410 , and evanescent input/output coupler  434 . Evanescent input/output coupler  434  may provide a fixed percentage of radiation from coherent source  404  into resonant fiber optic ring  408 , or may be adjustable based on losses present throughout resonant fiber optic ring  404 . In the exemplary embodiment evanescent input/output coupler  434  is essentially a reconfiguration of evanescent input coupler  412  discussed above with respect to the first exemplary embodiment. It a preferred embodiment, evanescent input/output coupler  434  couples less that 1% of the radiation from laser  404  into fiber  402 . 
     Detection of trace species is similar to that described in the first exemplary embodiment and is therefore not be repeated here. 
     The radiation that remains after passing through sensors  500  continues through fiber loop  402 . A portion of that remaining radiation is coupled out of fiber optic loop  402  by evanescent input/output coupler  434 . Evanescent input/output coupler  434  is coupled to processor  420  through detector  418  and signal line  422 . As in the first exemplary embodiment, processor  420  also controls coherent source  404  through control line  424 . Once the signals are received from detector  418  by processor  420 , the processor may determine the amount and type of trace species present based the decay rate of the radiation received. 
     Optionally, wavelength selector  430  may be placed between evanescent input/output coupler  434  and detector  418 . Wavelength selector  430  acts as a filter to prevent radiation that is not within a predetermined range from being input into detector  418 . Wavelength selector  430  may also be controlled by processor  420  to prevent radiation from coherent source  404  “blinding” detector  418  during the time period after the radiation from coherent source  404  was coupled into fiber  402 . 
       FIGS. 8A-8D  illustrates another exemplary sensor  800  used to detect trace species in a liquid or gas sample. As shown in  FIGS. 8A and 8D , sensor  800  is formed from fiber  801  by tapering the inner core  804  and cladding  805  to create tapered region  802  having tapered inner core  808  and tapered cladding  809 . The forming of tapered region  802  may be accomplished using either of two techniques. The first technique is heating of a localized section of fiber  801  and simultaneous adiabatic pulling on either side of the region in which it is desired to form sensor  800 . This procedure creates a constant taper in fiber  801 . This tapered fiber can then be for used as a spectroscopic sensor according to the first exemplary embodiment, for example. In the second exemplary technique, tapered region  802  may be formed by using a chemical agent to controllably remove a predetermined thickness of fiber cladding  805  to form tapered cladding  809 . A detailed description of a sensor formed using the second technique is described below with respect to  FIGS. 10A-10C . 
       FIG. 8B  illustrates a cross section of sensor  800  in the pre taper and post taper regions. As shown in  FIG. 8B , inner core  804  and cladding  805  are in an unmodified state. It should be noted, for simplicity, the illustrations and description do not refer to the jacketing of fiber optic cable  801 , though such jacketing is assumed to be in place for at least a portion of fiber optic cable  801 . 
       FIG. 8C , illustrates a cross section of sensor  800  in tapered region  802 . As shown in  FIG. 8C , tapered inner core  808  and tapered cladding  809  each have a significantly reduced diameter as compared to inner core  804  and cladding.  805 . Tapered region  802  may be of any desired length based on the particular application. In the exemplary embodiment, as shown in  FIG. 8D , for example, the length of the tapered region is approximately 4 mm with a waist diameter  814  of about 12 microns. 
     Referring again to  FIG. 8A , evanescent field  806  in the region of inner core  804  is narrow and confined when compared to enhanced evanescent field  810  in taped region  802 . As illustrated, enhanced evanescent field  810  is easily exposed to the trace species (not shown) as discussed above with respect to the earlier exemplary embodiments and, thus, is better able to detect the trace species in region  812 . 
       FIGS. 9A-9C  illustrate yet another exemplary sensor  900  used to detect trace species in a liquid or gas sample. As shown in  FIG. 9A , sensor  900  is formed from fiber  901  by removing a portion of cladding  905  to create a substantially “D” shaped cross section region  902 . The forming of “D” shaped cross section region  902  may be accomplished by polishing one side of optical fiber cladding  905  using an abrasive, for example. The abrasive is used to remove cladding  905  in continuously increasing depths along region  902  to preserve guided mode quality, ultimately reaching a maximum depth at the point of minimum cladding thickness  909 . This area of lowest cladding thickness represents the region of maximum evanescent exposure  910 . 
       FIGS. 10A-10C  illustrate still another exemplary sensor  1000  used to detect trace species in a liquid or gas sample. Sensor  1000  is formed using the second technique described above with respect to the tapered sensor exemplary embodiment. As shown in  FIG. 10A , sensor  1000  is formed from fiber  1001  by removing a portion of cladding  1005  using a chemical agent, known to those of skill in the art, to create tapered region  1002  having tapered cladding  1009 . It is important that the chemical agent not be permitted to disturb or remove any portion of the inner core, as this may introduce significant losses in sensor  1000 . 
       FIG. 10B  illustrates a cross section of sensor  1000  in the pre taper and post taper regions. As shown in  FIG. 10B , inner core  1004  and cladding  1005  are in an unmodified state. It should again be noted, for simplicity, the illustrations and description do not refer to the jacketing of fiber optic cable  1001 , though such jacketing is assumed to be in place for at least a portion of fiber optic cable  1001 . 
       FIG. 10C  illustrates a cross section of sensor  1000  in tapered region  1002 . As shown in  FIG. 10C , inner core  1004  is not affected while tapered cladding  1009  has a significantly reduced diameter as compared to cladding  1005 . Tapered region  1002  may be of any desired length based on the particular application. In the exemplary embodiment, for example, the length of the tapered region is approximately 4 mm with a waist diameter  1014  of about 12 microns. 
     Referring again to  FIG. 10A , evanescent field  1006  in the region of inner core  1004  is narrow and confined when compared to enhanced evanescent field  1010  in taped region  1002 . As illustrated, enhanced evanescent field  1010  is easily exposed to the trace species (not shown) as discussed above with respect to the earlier exemplary embodiments and, thus, is better able to detect the trace species in region  1012 . 
     With respect to the above described sensors  800 ,  900  and  1000 , losses created in the optical fiber by forming the sensors may be balanced with the amount of evanescent field exposure by determining the appropriate taper diameter or polish depth for the desired detection limits prior to fiber alteration. Further, it may be desirable to provide a protective mounting for sensors  800 ,  900  and/or  1000  to compensate for increased fragility due to the respective tapering and polishing operations. 
     It is contemplated that sensors  800 ,  900  and/or  1000  may be used in either as an unrestricted fiber, on a cylindrical core element  502  (which may be solid, hollow or otherwise permeable), such as a mandrel (shown in  FIG. 5B ) or in a loop or bent configuration (not shown). 
     Sensors  800 ,  900  and  1000  may be further enhanced by coating the sensing region with a concentrating substance, such as a biological agent to attract an analyte of interest. Such biological agents are known to those of ordinary skill in the art. It is also contemplated that several detecting regions  800 ,  900  and/or  1000  may be formed along a length of a fiber optic cable to produce a distributed ring down sensor. 
     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.