Abstract:
Optical spectroscopy system and method possessing spectral selectivity sufficient to distinguish isotopic line of the metal of interest. Each of the light beams, counter-propagating through vial with vapor of the sample, has been originated from the same light output of the laser source and modulated at a corresponding judiciously-determined frequency. The light-output, in turn, possesses a carrier frequency and two side-band frequencies defined with respect to a mean value of excitation frequencies of isotopes in the vapor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from and benefit of the U.S. Provisional Patent Application No. 62/275,530 filed on Jan. 6, 2016 and titled “SUB-DOPPLER INTERMODULATED LASER-INDUCED-FLUORESCENCE SPECTROMETER.” The disclosure of this provisional application is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of spectrometry and, in particular, to a system and method for determination of isotopes with the use of sub-Doppler fluorescence spectroscopy and an intermodulated light source. 
       SUMMARY 
       [0003]    Embodiments of the invention provide an optical system and method for implementing high-resolution optical spectroscopy employing isotopic lines of metal(s) of interest with the use of sub-Doppler measurement technique and fluorescence caused by irradiation of the isotopes with light, from a laser light source, the spectrum of which has a carrier frequency and first and second side-band frequencies and that is intermodulated. 
         [0004]    In particular, embodiments provide an optical system for measuring fluorescence produced by isotopes. Such system includes a frequency-tunable light source configured to generate light output that has a carrier frequency and first and second side-band frequencies. The system additionally includes a system of reflectors positioned to direct first and second beams of light, formed from the light output and modulated at respectively-corresponding first and second modulation frequencies, to co-axially counter-propagate in a region of interest (ROI). The system further includes an optical detection unit configured to receive light emitted by a sample (in response to being expose to these counter-propagating beams of light) that is disposed across the first and second beams of light in the ROI. 
         [0005]    A related embodiment of the invention provides a method containing the steps of directing first and second portions of light output, produced by a frequency-tunable light source, towards one another to define first and second co-axially counter-propagating beams; receiving fluorescence, produced by a vapor-phase sample disposed across the first and second co-axially-propagating beams, with an optical detector to form optical data; and determining a spectral characteristic of a component of the sample based on the optical data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention will be more fully understood by referring to the following Detailed Description of Specific Embodiments in conjunction with the generally not-to-scale Drawings, of which: 
           [0007]      FIGS. 1A and 1B  provide schematic diagrams illustrating an embodiment of the system of the invention; and 
           [0008]      FIG. 2  illustrates an embodiment of a method for measuring fluorescence produced by isotopes. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    The present invention is based on the realization that the high-resolution spectroscopy can be effectuated with the use of isotopic lines of chosen materials, in one implementation—metals. Each specific metal isotope absorbs incident radiation at a slightly different, specific to such isotope wavelength(s). Accordingly, the determination of the presence and concentration and/or change in concentration of certain isotope(s) with the use of high spectral resolution laser-based spectroscopy, according to the idea of the invention, turns on the determination of the irradiance of fluorescent light, I, generated by a given isotope 
         [0000]        I=I   0 exp( −εcl )  (1),
 
         [0010]    where I 0  denotes incident light intensity (the intensity of excitation light from the laser source, that has been intermodulated before arriving at the isotope), c is the concentration of the given isotope in the overall isotope mixture, ε is the absorption coefficient of the particular type of isotopes, and I is the path length of the excitation light through the isotope material (for example, the length of the cell containing the isotope material). 
         [0011]    In particular, detection of a relative shift in the abundance of metal isotopes can be effectuated with the use of a spectrometer that employs counter-propagating beams of judiciously-modulated laser light. For the purposes of this disclosure, abundance refers to a relative amount of one isotope to the total amount of isotopes in a mixture of isotopes. 
         [0012]      FIGS. 1A, 1B  schematically illustrate an embodiment  100  of the system of the invention, in side and front plan views, respectively. In this embodiment, configured to precisely measure the ratio(s) of various metal(s) isotopes, the metallic material(s) of interest is placed in a sample vial  110  (for example, made of quartz) that is heated with the heater  120  (in contact with the vial  110 ) to entrain a portion of the metal sample into the vapor phase inside the vial. Both the vial and the heater block are shaped such as to provide reliable physical contact between their corresponding surfaces. For example, the vial  110  has a tubular cylindrical body, while the surface  120 B of the heater in contact with the vial  110  is dimensioned in a complementary fashion; as shown—to define a concave cylindrical surface with a radius substantially equal to the radius of the tubular body of the vial  110 . Optionally, either an outer surface of a vial  110  or a (complementarily-shaped) surface of the heater block  120  is judiciously coated with a thin-film  124  designed to increase reflectance of light at wavelengths at which metal isotopes within the sample vial  110  fluoresce. The heater block can be heated with cartridge heaters, in one embodiment. 
         [0013]    In the proposed scheme, a tunable-frequency laser source is used to generate resonant-frequency light for excitation of the gas-phase metal in the vial. The sub-Doppler technique of inter-modulated fluorescence is employed. The required spectral resolution is achieved by a non-linear response of the gas-phase sample to counter-propagating laser beams. The gas phase-metal is excited with light over the wavelengths of interest to detect laser-induced fluorescence (LIF) with an optical detector (and optionally—via a fiber bundle). 
         [0014]    In particular, the vapor-phase sample in the vial  110  is irradiated with two beams of light  130 A,  130 B that have been formed with a beamsplitter (in one implementation—a 50/50 amplitude beam splitter  132 ) from the same light-output  134 , produced by the laser source  138  (as shown—a laser diode). Each of the components of the original beam  134  is transmitted through a corresponding modulator ( 140 A,  140 B) so that the beams  130 A,  130 B contain light modulated at different rates, f 1  and f 2 , respectively. (In one example—at 500 Hz and 1,000 Hz, respectively) to produce intermodulated beams. So-modulated (with the use of appropriately programmed electronic circuitry) light beams are then delivered, via reflectors  144 A,  144 B, to propagate through (traverse) the vapor-phase sample along the same axis but in different directions. In other words, intermodulated beams  130 A and  130 B are directed to counter-propagate (specifically, towards one another) while overlapping in space and having a common axis. Such propagation of the beams  134 A,  134 B is defined, for the purposes of this disclosure, with the term “co-axial counter-propagation”. 
         [0015]    The light beam  134 , produced by the light source  138 , has the frequency spectrum containing of which has a central, carrier frequency fC and two side-bands, fC+ and fC−. The carrier frequency fC of the beam  134  is chosen such as to be offset from mean value(s) of frequencies of emission of isotope(s) in the vial  110 . Such a choice prevents the situation when two isotopes are excited by the beams  134 A,  134 B at once. (Otherwise, if two isotopes were excited at once, the determination of a fluorescence signal produced by a particular isotope on the background of the aggregated signal produced by these two isotopes would be substantially complicated). At the same time, the side bands of the laser-output  134 , produced at the frequencies of interest fC+ and fC−, do not shift relative to the carrier frequency fC and have equal optical powers. 
         [0016]    Due to the counter-propagating geometry of light beams  134 A,  134 B, chosen in this embodiment, fluorescence of interest only occurs at a frequency f_sum equal to the sum of frequencies (f 1 +f 2 ). Moreover, only those atoms of metal(s) in vapor-phase material, contained in the vial  110 , that have zero velocity (that is, which are stationary) interact simultaneously with the two beams  134 A,  134 B. Accordingly, only such stationary atoms will be caused, by light beams  134 A and  134 B, to fluoresce and, in contradistinction with systems of related art, the Doppler-shift does not occur during the acquisition of the required optical data. 
         [0017]    The fluorescent light emitted by the atoms of metal isotopes is further collected with the use of a lens  150 , which is dimensioned to optimize the collection of fluorescent light from the vial  110 . As shown in  FIGS. 1A, 1B , in one embodiment the lens  150  is configured as a cylindrical lens extended along the length of and parallel to the vial  110 . The collected fluorescent radiation is further delivered to the optical detector unit  154  (which may include a photomultiplier tube, PMT, and lock-in amplifier) via an optical fiber contraption  158  (shown inclusive of the fiber optic feed  158 A in operable communication with fiber cable  158 B). 
         [0018]    A skilled artisan will readily appreciate that, owing to the counter-propagation scheme of  FIGS. 1A, 1B , electronic circuitry  160  (which may include a programmable processor and data-processing electronic circuitry, in operable communication with tangible, non-transitory storage medium containing program codes) may be used to govern the operation of the sub-systems of the embodiment—such as the heater module and/or light source driver, for example; to govern the operation of any of the modulators  140 A,  140 B, and/or to process optical data acquired from the optical detection unit  154  to extract the sought-after spectral information representing the isotope(s) of the sample in the vial  110 . 
         [0019]    In operation, the laser source  138  is scanned over the spectral frequencies representing two known atomic transitions. Relative abundances of each of the corresponding isotope is further determined from the ratio of the area of each spectral feature as given by Equation 1. 
         [0020]    Referring to  FIG. 2 , the embodiment of a method of determining a spectral characteristic of a vapor-phase sample (in one example, a vapor-phase metal isotope) is illustrated. In step  205 , a portion of the metal sample is entrained into the vapor phase inside the vial  110  as described previously. In step  210 , a tunable-frequency laser source is used to generate resonant-frequency light for excitation of the gas-phase metal in the vial. Further, in step  215 , first and second portions of the generated excitation light (i.e., light output that has been produced by a frequency-tunable light source) are directed towards one another to define first and second co-axially counter-propagating beams. Moreover, in step  220 , a component in the vapor-phase sample disposed across said first and second co-axially-propagating beams is excited to produce fluorescence. In step  225 , the fluorescence, produced by the vapor-phase sample that has been disposed across the first and second co-axially-propagating beams, is collected with a use of a lens to form optical data by an optical detector. In addition, in step  230 , a spectral characteristic of a component of said sample based on said optical data is determined. The sought-after spectral characteristics may include at least one of a relative abundance of the sample and spectral distribution of intensity of fluorescence produced by the sample in response to irradiation with excitation light. In generating the excitation light, care is taken to generate light output having a carrier frequency and first and second side-band frequencies, where the carrier frequency is not equal to a mean value defined by frequencies of excitation of first and second isotopes in the sample. The method further includes modulating said first and second portions at respectively-corresponding first and second frequencies that differ from one another. Optionally, the light source can be spectrally-tuned across a spectral region that includes frequencies at which first and second isotopes in the vapor-phase sample absorb light from the intermodulated first and second portions of the excitation light output. 
         [0021]    While the invention is described through the above-described example of embodiment(s), it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the disclosed inventive concepts. Disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment(s).