Abstract:
A system for providing photoacoustic spectroscopy. A light source having a quantum dot filter may provide a band of infrared light which is to be reflected by a lamellar grating to a photoacoustic chamber. The light may be modulated by the grating. The chamber may contain a sample of fluid for which spectral information is sought. A sensor may detect acoustic pressures in the chamber which indicate the spectral information. Signals from the sensor may be processed and displayed. Identification and concentration of certain substances in the fluid may be obtained.

Description:
BACKGROUND 
       [0001]    The invention pertains to spectroscopy and particularly to spectroscopy of fluids. More particularly, the invention pertains to photoacoustic spectroscopy. 
       SUMMARY 
       [0002]    The invention is a photoacoustic spectroscopy system which has a source that provides light to a photoacoustic chamber via a grating. The chamber may contain a sample for which spectral information is sought. A sensor may detect pressures in the chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0003]      FIG. 1  is a diagram of a photoacoustic system having a grating; 
           [0004]      FIG. 2  is a diagram of a light source and its components; 
           [0005]      FIG. 3  is a diagram of an example quantum dot filter and its components; 
           [0006]      FIG. 4  is a diagram of a lamellar grating; 
           [0007]      FIG. 5  is a graph of examples of broad band and narrow band light; and 
           [0008]      FIG. 6  is a graph of example absorption lines which may occur at an output of the photoacoustic system. 
       
    
    
     DESCRIPTION 
       [0009]    The present invention may provide a photoacoustic measurement with a quantum dot light source, a grating, a photoacoustic chamber or cell, and a sensitive pressure sensor situated at the photoacoustic chamber. Photoacoustic measurement is based on the tendency of molecules in a gas, when exposed to certain wavelengths of radiant energy (e.g., infrared light), to absorb the energy and reach higher levels of molecular vibration and rotation, thereby attaining a higher temperature and pressure within a measurement cell. When the radiant energy striking a gas is amplitude modulated at a known frequency, the resulting fluctuations in energy available for absorption produce corresponding temperature and pressure fluctuations in the gas, which may be measured as an acoustic signal. The amplitude of the acoustic signal is proportional to the intensity of the radiation and the concentration value of the absorbing gas. Such device may be well suited for measuring very small concentration values of gases (i.e., in the parts-per-billion or better range). 
         [0010]    Photoacoustic spectroscopy measurements should have a broadband infrared light source in order to measure a broad range of analytes. Typically, this may require the use of a glowbar blackbody light source and a Michaelson interferometer. While such a system may enable the measurement of many different analytes, it is typically complex (i.e., having a large number of components), has a potentially large form factor (i.e., not portable), and can have a large power budget. A fluid may be or contain the analytes. The fluid may be a gas or a liquid. 
         [0011]    A U.S. patent application Ser. No. 12/105,241, filed Apr. 17, 2008, U.S. patent application Ser. No. 11/350,541, filed Feb. 9, 2006, and U.S. Pat. No. 6,393,894, issued May 28, 2002, may relate to the present invention. U.S. patent application Ser. No. 12/105,241, filed Apr. 17, 2008, is hereby incorporated by reference. U.S. patent application Ser. No. 11/350,541, filed Feb. 9, 2006, is hereby incorporated by reference. U.S. Pat. No. 6,393,894, issued May 28, 2002, is hereby incorporated by reference. 
         [0012]    The present invention may be a Fourier transform infrared-photoacoustic spectroscopy (FTIR-PAS) system  10 .  FIG. 1  is a diagram of the FTIR-PAS system  10 . A broadband light source  11  may provide a light  12  to a lamellar grating  13  via a reflective mechanism  14 . Light  12  may be modulated by grating  13  as indicated by symbol  15 . A modulation driver  44  may be coupled to grating  13  for modulating light  12 . Driver  44  may be connected to processor  23 . Modulated light  12  may be reflected to a photoacoustic chamber  16  via a reflective mechanism  17  and a window  18  in the chamber. Window  18  may be transparent to the modulated light  12 . Window  18  may be silicon in the case of infrared light  12 . Photoacoustic chamber  16  may contain a sample  19  of gas such as ambient air or a person&#39;s breath brought in through a porous plate  21 , i.e., a gas permeable wall, situated over an opening in chamber  16 . Porous plate  21  may be replaced with a controllable valve which can seal the sample  19  within the chamber. A pressure sensor  22  may be placed in another opening of chamber  16 . An output from the pressure sensor  22  may go to processor  23 . The processor may process the signals into a format which reveals the results of system  10  relative to the sample  19  in chamber  16 . The results may go to a display  24  for viewing. Display  24  may show the results, for example, in an intensity versus wavelength graph  39 . Graph  39  reveals absorption lines  37  and  38  of sample  19 , as shown in  FIG. 6 . The results from processor  23  may also go to an instrument  25  such as a meter for another manner of displaying the results. 
         [0013]    System  10  may have several aspects. One is that the system may use a MEMS (Micro-electro-mechanical systems) based lamellar grating  13  to frequency modulate different wavelengths of light from the broadband infrared (IR) source  11 . The lamellar grating  13  may reduce the number of components in system  10  compared to other Fourier transform (FT) IR light source systems. Also, the system may use a quantum dot conversion filter to obtain a certain band of light from source  11 . The filter may effect a down conversion of a high energy photon to a low energy photon, or possibly multiple low energy photons. Typically, broadband IR light sources may use a blackbody, such as a glowbar or an incandescent light bulb. For instance, quantum dot conversion filters may be used for attaining broadband IR light, result in lower costs and provide higher efficiencies compared to blackbody sources. The conversion filters may also be used for attaining narrow band IR light. 
         [0014]    System  10  may use the lamellar grating  13  to frequency modulate different wavelengths of light  12  from source  11 . Light  12  from the source  11  may be directed toward a surface of the lamellar grating  13  interferometer. 
         [0015]    The light  12  from light source  11  may be produced by a quantum dot conversion filter. In brief, a collection of nanocrystalline quantum dots (i.e., nanocrystals) may be disposed on an optically transparent surface. The surface may be at the output of the light source  11 . An excitation light, such as that of a low-cost LED, may optically pump the quantum dot collection. The quantum dots may be judiciously chosen such that the fluorescence response to the optical stimulus results in, for example, a broadband infrared light source  11 . 
         [0016]    System  10  may provide a rapid method of gas sensing and identification over a continuum spectral region without a need of multiple discrete sources to cover such spectral region and provide virtually all of the benefits of photoacoustic gas detection. 
         [0017]    Photoacoustic spectroscopy may provide a highly sensitive approach for detecting very small concentrations of gases with a microphone at ppb (part-per-billion) levels, or with cantilever-interferometric pressure sensing at ppt (part-per-trillion) levels. 
         [0018]    System  10  may be based primarily on the coupling of a Fourier transform infrared (FTIR) spectrometer illumination source  11  with a photoacoustic (PA) gas sensor or chamber  16 . The Fourier transform (FT) spectrometer portion may generate a modulated infrared beam  12  which is coupled into the PA sensor measurement chamber  16 . Each spectral wavelength of the output  12  of source  11  may be modulated at a frequency proportional to its wavelength and be dependent on how fast the FT spectrometer is scanned or modulated. If a gas of sample  19  absorbs this wavelength, then this absorption may generate a unique frequency of sound wave in the photoacoustic chamber  16  which is detected by the pressure sensing mechanism  22  (e.g., microphone, cantilever-interferometer, or the like) coupled to the chamber. There may be different detected sounds for different absorption peaks. For example, for a 3.3 micron peak, there may be a 10 Hz sound wave, and for a 4.3 micron peak, there may be a 7.7 Hz sound wave. 
         [0019]    Processing the spectral content of the pressure sensing mechanism&#39;s  22  output signal may allow one to obtain the absorption spectral signatures or fingerprints of the gases present in the sample  19  within the photoacoustic measurement chamber  16 . 
         [0020]    System  10  may use source  11  which can be tailored for whatever spectral waveband one would like to cover. In  FIG. 2 , source  11  may be based on an excitation light portion  26 , for example, a simple LED pumping a quantum dot (QD) filter  27  with light  28 . By selection of the appropriate characteristic QD&#39;s, the desired spectral range of light  12  may be obtained with high efficiency generation with the pumping from an LED. This may allow for the obtaining of a highly efficient low powered IR source  11  covering the spectral range of interest. 
         [0021]    In particular, the energy or light source  11  may produce radiant energy or light  12  which is modulated at a known frequency movement  15  with a lamellar grating  13 . The modulated energy or light  12  may be provided to a cell or chamber  16  containing a gas sample  19  that absorbs the light  12  leading to temperature fluctuations in the gas that track the modulation frequency. Temperature is not sensed directly. Rather, pressure fluctuations that accompany the temperature fluctuations may be detected by pressure sensor  22  such as a sensitive microphone situated in chamber  16 . The microphone output may be detected at a modulation or other frequency for obtaining an electrical signal indicative of gas identification and/or concentration. 
         [0022]    Gas sensors based on the absorption of photons by a gas of interest, such as the photoacoustic sensing approach, generally need a modulatable infrared (IR) radiation source  11  that emits at the absorption band of the gas to be detected. 
         [0023]    Light  12  from source  11 , based on the fluorescence of quantum dots  29  ( FIG. 3 ) of a filter  27  in source  11 , may allow modulation of light  12  to kHz levels and higher and might not require an optical interference filter. Higher light  12  modulation frequencies may yield a better signal to noise ratio and reduced sensitivity to background noise. The modulation of light  12  may be provided by lamellar grating  13 . A modulation frequency of the grating may be at hundreds to thousands of hertz. 
         [0024]    The power required for a quantum dot source  11  is potentially lower than that for an incandescent source producing comparable radiation in the waveband of interest. Additionally, a quantum dot source  11  may produce longer wavelengths of IR radiation at a significantly lower cost than is currently possible with other approaches, thereby allowing a low-cost portable photoacoustic sensor  10  to be produced. 
         [0025]    A quantum dot filter  27 , located proximate to an LED, or other source of excitation  26 , may emit a specific wavelength of light  12  to be received by the chamber  16 . The excitation component  26  and filter  27  may constitute light source  11 , as shown in  FIG. 2 . A LED, for instance, may provide excitation light of about 470 nanometers. The specific wavelength emitted by the present quantum dot source  11  may be between 1 and 4.3 microns, with a possible option of extending further into the infrared. Other designs of source  11  may provide light having a wavelength range from less than one micron out to at least sixteen microns. Such range may be sufficient for obtaining a signature or fingerprint of many different fluids. 
         [0026]    Excitation light portion  26  of light source  11  may generate a light  28  spectrum. Excitation portion  26  may be selected based on several characteristics including cost and power consumption. The excitation portion  26  may be an LED, an array of LEDs, an LED pump, a laser, a laser diode, or other suitable device. 
         [0027]    A light  12  spectrum may be generated by source  11 . The spectrum of light  12  may be selected according to the design and sensitivity of quantum dot filter  27  of source  11 . Quantum dots  29  may generally absorb light at a shorter wavelength than the wavelength at which they emit light via fluorescence. Therefore, a light  28  spectrum may be selected so as to obtain a desired wavelength, such as IR, of light emission from quantum dot filter  27 . The light  28  spectrum may be within the spectrum of visible light, but need not be. For instance, the light  28  spectrum may include white light or ultraviolet (UV) light. Quantum dot filter  27  of  FIG. 3  may consist of at least one layer of quantum dots  29  arranged two-dimensionally on an optically transparent substrate  31 . Alternatively, dots  29  may be embedded and arranged two- or three-dimensionally in substrate  31 . Quantum dots  29  may emit light via fluorescence. A photon from excitation light  28  may be absorbed by the quantum dots  29  and result in an electron-hole pair. The electron may be generated at a relatively high energy state and then relax back to the valance band. When this occurs, the electron and hole may recombine and emit a photon having a specific wavelength as light  12 . The overall process may convert a photon from light  28  of one wavelength into a lower energy photon having another wavelength. The specific wavelength of the emitted photon may be dictated in part by the band gap of the quantum dot  29  material, and be essentially monochromatic for a given quantum dot diameter and material composition. 
         [0028]    The quantum dots  29  may include lead selenide (PbSe), lead sulfide (PbS), mercury telluride (HgTe), or another suitable material, or any combination thereof. Dots  29  may be nano crystals. Quantum dots  29  may be of various shapes, although circular or spherical shapes might be common. Quantum dots  29  may have various sizes, although sizes from single digit to double digit nanometers might be common. The quantum dot substrate  31  may be formed by any suitable manner. Quantum dot filter  27  may be formed by direct printing of quantum dots  29  in a random pattern. The quantum dot filter may be formed by direct printing of quantum dots in an arranged structure. Arrangements of quantum dots  29  may be made in view of size, shape, material, intra-dot one-, two- and/or three-dimensional spatial relationships, and so on. If desired, a protective layer  32  may be added over the quantum dots to protect them from the environment. The present quantum dot filter  27  may have a coating of quantum dots  29  applied to a glass substrate  31  and coated with a protective layer  32 . Quantum dots  29  may be mixed in with a substance designed to be a filter or window. For instance, quantum dots  29  may be mixed in with a plastic (e.g., quantum dot doped plastic) which may be used as a light exit window of an LED or the like. 
         [0029]    The quantum dot filter  27  may fluoresce within a narrow band when subjected to the light  28  spectrum and thus emit light  12  of a specific wavelength. The width of the spectral band of the quantum dot filter  27  may be tuned through careful selection and use of quantum dots  29 . The quantum dot substrate  31  may include quantum dots  29  of a uniform material composition and size to produce a monochromatic IR source, or may include quantum dots of varying size and/or composition to produce a source  11  having a complex IR emission spectrum. For example, if it is desirable for quantum dot filter  27  to fluoresce across a wide band of wavelengths, quantum dots  29  of varying sizes may be used to assemble a quantum dot substrate  31 . Similarly, if it is desirable for quantum dot filter  27  to fluoresce across an extremely narrow band, quantum dots  29  having virtually identical sizes and the same material may be used. An array of interchangeable quantum dot substrates  31  may be used, each emitting a suitable predetermined but different specific wavelength, wavelength band, or spectrum. 
         [0030]    The specific wavelength emitted by quantum dot light source  11  may depend generally on the size and composition of the quantum dots on substrate  31 , and may be selected according to the particular gas  19  that the photoacoustic cell or chamber  16  is to detect. The term specific wavelength may refer to a wavelength of the peak intensity of the energy emitted by a quantum dot source  11 . The specific wavelength may be tuned by controlling the geometry of quantum dots  29 . In general, depending on the material, smaller quantum dots  29  may fluoresce at lower wavelengths (into the visible), whereas larger quantum dots  29  may fluoresce in the red and infrared region. For example, a quantum dot substrate  31  assembled from relatively small quantum dots may emit a specific wavelength that is shorter, has higher energy, and is therefore bluer, than a quantum dot substrate assembled from relatively large quantum dots, which may emit a longer, and therefore redder, specific wavelength. The quantum dot substrate  31  may have quantum dots ranging in size from, for example, approximately two to sixty nanometers. 
         [0031]    The specific wavelength may be chosen to broadly coincide with the strongest absorption band of the gas  19  to be detected by photoacoustic chamber  16 . Typically, the specific wavelength may be in the infrared (IR) band. For instance, if photoacoustic chamber  16  is to be used to detect generic hydrocarbons, the specific wavelength may be chosen to fall within the range of approximately 3.0-3.5 microns. Alternatively, a source  11  design may be such that the specific wavelength of light  12  emitted by quantum dot light source  11  is in the range of one to four microns. As another alternative, a design of source  11  may be such that the specific wavelength of light  12  emitted by quantum dot light source  11  is in the range of three to four microns. Such source  11  of system  10  may be designed to detect, for example, generic hydrocarbons, methane (CH 4 ), or sulfur dioxide (SO 2 ). For example, a specific wavelength of light  12  emitted by the quantum dot source  11 , being approximately 3.3 microns, may be used for detecting methane. Another wavelength of light  12 , being approximately four microns, may be used for calibrating photoacoustic chamber  16 . 
         [0032]    Chamber  16  may serve as a measurement volume for system  10 . Chamber  16  may be generally cube, cylindrical, or like shaped, and may have a volume of approximately one cubic centimeter. 
         [0033]    Pressure sensor  22 , such as a microphone, may be sensitive to acoustic signals, and be positioned to detect pressure changes within chamber  16 . Pressure changes within chamber  16  may be caused by gases absorbing the radiant energy of a specific wavelength and changing temperature as a result. The temperature fluctuations in the gas may track the modulation frequency of specific wavelength. Within chamber  16 , pressure fluctuations that accompany the temperature fluctuations may be detected by pressure sensor  22 . Any suitable acoustic transducer, such as the microphone, may be used as sensor  22 . For example, the microphone may be an electret microphone. As another example, the microphone may be one having piezoelectric material. 
         [0034]    An outer wall of chamber  16  may be constructed of any suitable material. The outer wall may include a metal, such as aluminum. In an alternative, the outer wall may include a plastic, or polymer, such as methacrylate. 
         [0035]    A gas permeable wall  21  of chamber  16  may be a porous membrane formed of paper, a porous metal, or a gas permeable polymer. Thus, after the photoacoustic chamber  16  is located for several minutes within a given environment, the gas mixture within chamber may substantially match the gas mixture of the surrounding environment. 
         [0036]      FIG. 5  is a diagram of a graph shows how the light spectrum  33  generated by light source  11  may cover a broad range of wavelengths (shown on the x-axis, in microns). The graph also shows that the specific wavelength  34  emitted by source  11  may be within a narrow range of wavelengths in the IR band, and have a significantly longer wavelength than the wide light spectrum  33 . 
         [0037]    The lamellar grating based FTIR spectrometer  10  may be a simple form of a FTIR spectrometer requiring no beamsplitter. The lamellar grating  13  of  FIG. 4  may be a binary grating having a variable depth, which operates in the zero order of the diffraction pattern. Grating  13  may be like a beamsplitter having two surfaces, like that of a mirror, but one moveable and one fixed. Grating  13  may have an overall concave shape for a focused reflection of light  12  ultimately from source  13  to chamber  16  via the intermediate reflection devices  14  and  17 , respectively. A lens situated in front of a planar lamellar grating may also be used. The lamellar grating  13  interferometer may divide the wavefront into two wavefronts at a grating where the front facets  35  (a set of fixed mirrors) reflect one half of the beam, and the back facets  36  (a set of mobile mirrors with movement  15 ) reflect one half of the beam. One layout of the sets of mirrors may be like interleaved fingers. The distance between the front mirrors  35  and back mirrors  36  may determine the optical path difference (OPD) between the two wavefronts. Grating  13  may incorporate a MEMS comb drive. The acoustic modulation frequency in the photoacoustic cavity for a given radiation wavelength may be equal to 2V/λ, where V is a velocity of the moving mirror of the lamellar grating  13  and λ is the wavelength of the light  12 . 
         [0038]    The lamellar grating FTIR spectrometer  10  may reduce part count and provide a compact form factor for gas spectroscopy. MEMS may be used to produce the lamellar grating  13  and photoacoustic chamber  16  for system  10 . 
         [0039]    System  10  may have a temperature sensor  41 , another pressure sensor  42 , and a photodiode  43 . Temperature sensor  41  may be coupled to pressure sensor  22 . Temperature sensor  41  may measure the temperature of pressure sensor  22  in order to generate a correction signal to compensate for temperature induced changes in sensitivity of pressure sensor or mechanism  22 . Any suitable temperature measurement device may be used. An example of temperature sensor  41  may include a thermocouple. 
         [0040]    Pressure sensor  42  may be situated at chamber  16 . Pressure sensor  42  may measure the atmospheric pressure about chamber  16  in order to generate a pressure correction signal. Pressure sensor  42  may be used to compensate for variations in the environment surrounding photoacoustic chamber  16 . For example, pressure sensor  42 , may be used to compensate for changes in barometric pressure caused by a change in altitude or weather conditions. Any suitable pressure measurement mechanism may be used. 
         [0041]    Photodiode  43  may be to measure the intensity of the light  12  emitted by light source  11 . Photodiode  43  may be used to monitor the intensity of light  12  for purposes of calibrating photoacoustic gas sensing system  10 . 
         [0042]    Processor  23  may receive signals related to pressure changes in chamber  16 . Processor  23  may be electrically connected to light source  11 . Processor  23  may include circuitry for controlling light source  11 , as well as circuitry for receiving and processing signals from pressure sensor  22 , temperature sensor  41 , pressure sensor  42 , and photodiode  43 . Processor  23  may perform calculations on the signals to identify the one or more gases within chamber  16  and a concentration corresponding to each of those gases. The signals from temperature sensor  41 , pressure sensor  42  and photodiode  43  to processor  23  may be used for calibrating photoacoustic chamber  16  and compensating pressure sensor  22 . Modulation driver  44  signals may also be accounted for by processor  23 . Processor  23  may be any type of computational circuit, such as, but not limited to, a microprocessor, a microcontroller, or a computer. 
         [0043]    In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense. 
         [0044]    Although the invention has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications