Abstract:
In accordance with the present invention, an ultra-sensitive Raman chemical sensor is provided that is based on an enhanced spontaneous emission as a result of cavity quantum electrodynamic effects. More specifically, the sensor in accordance with the present invention makes use of a double resonance of a microcavity with both the excitation laser frequency and the Raman frequency. As such, the Raman shift corresponds to an integer times the free spectral range of the microcavity. Because the Raman frequency directly depends on the excitation laser&#39;s frequency, the fulfillment of the resonance condition for the excitation laser frequency guarantees that resonance with the Raman frequency is also satisfied.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority to U.S. Provisional Patent Application No. 61/505,270, filed on Jul. 7, 2011 and entitled “Microcavity Raman Sensor and Method of Use”. 
    
    
     BACKGROUND OF THE INVENTION 
     When a Raman active medium is placed in an optical cavity, a large enhancement in the scattering efficiency can be realized. The enhancement can be observed when the cavity is made resonant with either the wavelength of the excitation light or the wavelength of the Raman scattered light. The effect is maximized when the cavity is made simultaneously resonant with both, i.e., when a double resonance is obtained. The exact magnitude of the enhancement depends on, among other factors, the finesse of the cavity at the two wavelengths. 
     In the prior art the double resonance condition could be achieved only for a very thin solid sample with a thickness of less than one wavelength. The sample was made to have plane parallel surfaces, effectively creating a planar resonator with two flat partial reflectors forming the cavity. Because of the fixed spacing of the cavity, resonance with the wavelength of the excitation light could be established only for an off-axis laser beam. As a result, the resonance was accompanied by a high walk-off loss. Furthermore, planar resonators are inherently only quasi-stable, and are therefore precluded from having a high finesse even for axial beams because of diffraction. Finally, the extension of this approach to fluid samples is clearly not possible. 
     Accordingly, what is needed in the art is a microcavity arrangement capable of sustaining the double resonance condition with high finesse for Raman scattering in fluids. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention is provided a novel Raman sensor based on quantum electrodynamic effects capable of detecting trace amounts of chemicals in gases and liquids with a high degree of accuracy. 
     In accordance with an embodiment of the invention, an ultra-sensitive chemical sensor is provided that is based on an enhanced spontaneous Raman emission as a result of cavity quantum electrodynamic effects. More specifically, the sensor in accordance with the present invention makes use of a double resonance of a stable Fabry-Perot microcavity with both the excitation laser frequency and the Raman frequency. As such, the Raman shift corresponds to an integer times the free spectral range of the microcavity. Because the Raman frequency directly depends on the excitation laser&#39;s frequency, the fulfillment of the resonance condition for the excitation laser frequency guarantees that resonance with the Raman frequency is also satisfied. 
     In a particular embodiment, a Raman sensor for detecting the concentration of a species of interest in a sample may include a first substantially flat reflector and an adjustable optical fiber, wherein the adjustable optical fiber is translatable in a direction parallel to the fiber optic axis of the optical fiber, and a second substantially concave reflector positioned at a proximate end of the optical fiber and aligned substantially normally to the first reflector, the second reflector separated from the first reflector by a distance to form a microcavity having a microcavity length equal to the distance between the first reflector and the second reflector, wherein the microcavity length is dependent upon the Raman transition of a species of interest to be detected. The sensor may further include an excitation laser positioned to emit an excitation wavelength incident upon the first reflector and the second reflector, wherein the wavelength of the excitation laser is dependent upon the microcavity length between the first reflector and the second reflector and a Raman emission signal detector positioned to receive the Raman emission coupled out of the first reflector and to detect the concentration of the species of interest within a sample positioned between the first reflector and the second reflector. 
     To couple the laser beam into the microcavity, the present invention may include a dichroic filter positioned between the excitation laser and the first reflector, the dichroic filter having high reflectance at the wavelength of the excitation laser and high transmittance at the Raman emission wavelength. 
     To mode match the laser beam into the microcavity and to collimate the Raman emission coupled out of the first reflector onto the Raman emission signal detector, the present invention may include a lens positioned between the first reflector and the dichroic filter. 
     In a particular embodiment, the optical fiber may further be positioned within a close fitting ferrule to constrain the optical fiber and to allow translation of the optical fiber in the direction parallel to the fiber&#39;s optic axis. 
     The present invention may further include, an excitation light detector positioned at a distance from the distal end of the optical fiber to detect the transmitted laser power and a piezoelectric transducer coupled to a distal end of the optical fiber to adjust the length of the microcavity by translating the optical fiber in the direction parallel to the fiber&#39;s optic axis dependent upon the detected excitation laser power. 
     In operation, a sample containing a species of interest is introduced into a microcavity having a microcavity length dependent upon the Raman emission of the species of interest. An excitation laser signal having a wavelength determined by the microcavity length is then introduced into the microcavity. The Raman emission of the sample is then measured to determine the concentration of the species of interest within the sample. 
     The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the disclosure set forth hereinafter and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed disclosure, taken in connection with the accompanying drawings, in which: 
         FIG. 1  shows a preferred embodiment of the Fabry-Perot microcavity; 
         FIG. 2  is a schematic for the doubly resonant microcavity Raman sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The ability of a cavity-resonant electric field to influence the spontaneous emission rate of an atomic transition is well known. In the case of Raman scattering, the rate of Raman photons spontaneously emitted into the cavity mode per molecule can be written as: 
                     A   C     =           c   3     ⁢     v   L     ⁢   ϕ       3   ⁢     v   R   3     ⁢     V   ⁡     (       Δ   ⁢           ⁢     v   C       +     Δ   ⁢           ⁢     v   R         )           ⁢       ⅆ   σ       ⅆ   Ω                 (   1   )               
where ν L  and ν R  are the frequencies of the laser and Raman photons, φ is the laser photon flux density, V the cavity mode volume at the Raman wavelength, Δν c  the cavity linewidth at the Raman wavelength, Δν R  the Raman transition linewidth, and dσ/dΩ the differential Raman scattering cross section. If there are ηv molecules responsible for the Raman scattering in the mode volume, then the total rate of emission into the cavity mode can be found from:
 
                       η   V     ⁢     A   C       =           c   3     ⁢     v   L     ⁢   N   ⁢           ⁢   ϕ       3   ⁢           ⁢       v   R   3     ⁡     (       Δ   ⁢           ⁢     v   C       +     Δ   ⁢           ⁢     v   R         )           ⁢       ⅆ   σ       ⅆ   Ω                 (   2   )               
where N is the density of molecules of interest. The differential scattering cross section itself varies as the fourth power of the Raman shifted frequency, making the net emission rate into the cavity mode vary approximately as the square of ν R  for a fixed pump photon flux density.
 
     Since the cavity mode in a stable Fabry-Perot resonator has an axially varying beam diameter, the photon flux density φ in Eqs. (1) and (2) is to be interpreted as an average. Thus, the result given in Eq. (2), along with the laws governing the scaling of Gaussian modes with changes in the resonator&#39;s dimensions, shows that for a given excitation laser power, Raman emission into the cavity mode is increased as the resonator length is decreased. 
     For fundamental Gaussian modes, the double resonance condition requires that: 
                       mc     2   ⁢   nL       -         m   ′     ⁢   c       2   ⁢   nL         =     Δ   ⁢           ⁢     v   V               (   3   )               
where Δν v  is the Raman frequency, m and m′ are integers and n is the refractive index of the sample (the same refractive index is assumed for both the laser and Raman shifted wavelengths, because they are generally very close to each other).
 
     When the frequencies are expressed in terms of wavenumbers, Equation (3) may be written as 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       m 
                     
                     
                       2 
                       ⁢ 
                       nL 
                     
                   
                   = 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         v 
                         _ 
                       
                       V 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Thus it is seen that only a discrete set of cavity lengths are permitted for any given Raman transition. These cavity lengths in turn dictate a discrete set of permissible excitation laser wavelengths. 
     With reference to  FIG. 1 , in one embodiment the microcavity is comprised of a concave reflector  2  fabricated at the end of an optical fiber  1  and a flat (or slightly concave) reflector  3  deposited on a thin substrate  4 . The concave reflector  2  conforms to a spherically shaped crater formed in a polished end of the optical fiber  1 . This micro-indentation feature can be created, for example, by the process of ablation with the use of one or more pulses of a focused beam from a CO 2  laser. The reflectors  2  and  3  can be produced, for example, by ion beam sputtering, which is known to be capable of yielding coatings with extremely high reflectivities. The sample from which Raman emission is sought is made to fill the space between mirrors  2  and  3 . For maximum enhancement of the Raman emission from the sample, the reflectivities of the mirrors should be made as large as possible at both the wavelength of the excitation laser and the wavelength of the Raman emission. 
     In a preferred embodiment, the reflector  3  is kept in a fixed position while the reflector  2  is constrained to be translatable only in the direction parallel to the fiber axis. The excitation laser beam is made incident into the microcavity from the right in  FIG. 1 . Raman emission from the sample is coupled out of the microcavity through the reflector  3 . 
     A schematic illustrating the overall Raman sensor is shown in  FIG. 2 . Output from a stabilized laser  11 , tuned to the appropriate excitation wavelength, is made incident on a dichroic reflector  12 , which is highly reflective at the excitation wavelength and highly transmitting at the Raman emission wavelength. The laser light from the stabilized laser  11  is then mode matched into the microcavity  5  with the use of lens  10 . The optical fiber  1  is inserted into a close fitting ferrule  6  which permits free translation in the direction parallel to the fiber&#39;s optic axis. At the other end of the ferrule  6  the fiber is attached to a piezoelectric transducer  7  with the use of adhesive  8 . The laser light exiting the fiber is made incident on detector  9 . The output from the detector  9  is used to keep the microcavity always resonant with the excitation laser wavelength through the use of feedback electronics  14 . The piezoelectric transducer  7  may adjust the cavity length (i.e. separation between the reflectors  2 ,  3 ) as necessary for initial set-up as well as during subsequent operation of the sensor to compensate for any dimensional changes due to temperature variations. The Raman emission coupled out of the flat reflector positioned on the substrate  4  of the microcavity is collimated by lens  10 , passed through the dichroic filter  12 , and made incident on detector  13 . Once the system has been calibrated, the Raman signal measured by detector  13  will give directly the concentration of the species of interest which produces the Raman emission. 
     The capability of the doubly resonant microcavity Raman sensor will now be illustrated in the context of its application to the measurement of atmospheric CO 2  concentration. Suppose the 1,388 cm −1  Raman transition in CO 2  is to be employed and the microcavity is semi-confocal. Then one possible cavity length is 25 μm (for Δm=7), and one possible excitation laser wavelength is 640 nm (for m=78), with a corresponding Raman emission wavelength of 702 nm. The differential Raman cross section for the 1,388 cm −1  transition for 640 nm excitation is approximately 2(10) −3 ′ cm 2 /sr. 
     Suppose further that a laser power of 100 mW is available and that the microcavity mirrors have a reflectivity of 99.99% at both 640 nm and 702 nm. At a typical atmospheric CO 2  concentration of 400 ppm and for an expected Raman linewidth of around 2 GHz, one calculates a Raman emission rate through mirror  3  of 5(10) 5  photons/s with the use of Eq. (2). If the detector  13  is a single photon counter and a measurement time of 10 s is employed, then a shot noise of approximately 2(10) 3  counts can be expected. This implies that changes in the CO 2  concentration of about 0.2 ppm (i.e. 0.05% relative change) should be detectable with a response time of 10 s. 
     It will thus be seen that the objects set forth above, and those made apparent from the foregoing disclosure, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing disclosure or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.