Abstract:
A photo-thermal interferometric spectroscopy system is disclosed that provides information about a chemical at a remote location. A first light source assembly is included that emits a first beam. The first beam has one or more wavelengths that interact with the chemical and change a refractive index of the chemical. A second laser produces a second beam. The second beam interacts with the chemical resulting in a third beam with a phase change that corresponds with the change of the refractive index of the chemical. A detector system is positioned remote from the chemical to receive at least a portion of the third beam. An adaptive optics system at least partially compensates the light beam degradation caused by atmospheric turbulence. A focusing system is used to bring together the light passed through the chemical; the focusing system includes a multimode fiber for the light collection, The detector system provides information on a phase change in the third beam relative to the second beam that is indicative of at least one of, absorption spectrum and concentration of the chemical.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Ser. No. 10/947,640 filed Jan. 13, 2005 now U.S. Pat. No. 7,277,178 and Ser. No. 11/561,966 filed Nov. 21, 2006 now U.S. Pat. No. 7,426,035, both of which applications are fully incorporated herein by reference. This application is a continuation-in-part of U.S. Ser. No. 10/669/130 filed Sep. 22, 2003 now U.S. Pat. No. 7,327,913 and Ser. No. 11/672,372 filed Feb. 7, 2007. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to systems and methods for chemical detection such as explosives and others, and more particularly to photothermal interferometric spectroscopy devices, and their methods of use, based on optical signal detection. 
     BACKGROUND OF THE INVENTION 
     The principles of photothermal spectroscopy are generally described in a publication by Stephen E. Bialkowski entitled “Photothermal Spectroscopy Methods for Chemical Analysis”, John Wiley &amp; Sons, Inc., 1996, the entire content of which is incorporated by reference herein. Photothermal spectroscopy method allows carrying out extremely sensitive measurements of optical absorption in homogeneous media. It is possible, using, a laser&#39;s coherent and powerful output, to obtain extremely sensitive measurements of optical absorption that exceed those of mass spectroscopy by two or three times, and produce accurate results from only a few molecules. 
     Co-pending U.S. patent application Pub. No. 20050105099 by the same inventor as the present application discloses implementation of coherent receiving technique for photothermal interferometric sensing. Integrated 90-degrees optical hybrid is a key component of the coherent receiver, This method provides improved sensitivity of the detection. 
     Laser free-space propagation is effected by atmospheric conditions such as turbulence and the like which works to cause aberrations in the spatial phase of the wavefront of the laser beams. The beam spreading and scintillation induced by the atmospheric turbulence cannot be compensated by increasing of optical power because of eye safety and power consumption. A considerable improvement is achieved by implementing adaptive optics systems such as described, for example, in U.S. Patent applications Nos. 20040086282 filed Oct. 16, 2003 by Graves (FIG. 4) and 20060024061 filed Feb. 2, 2006 by Wirth (See FIG. 3) and a number of publications, see for example, “Fiber coupling with adaptive optics for free space optical communications” by Weyrauch et al., Proceedings SPIE. 2002, v. 4489, p. 177-183, all of which incorporated herein by references. 
     In “Fiber coupling with adaptive optics for free space optical communications” by Weyrauch et al., Proceedings SPIE, 2002, v. 4489, p. 177-183 the laser beam coupling in multimode and single mode fiber is demonstrated. Obviously implementation of multimode fiber provides a number of advantages. First of all, larger sensitivity of the light beam detection is achieved because the diameter of multimode fiber is about 10 times larger than the diameter of single mode fiber. Secondly, the tolerance to the optical beam misalignment is attained since the diameter of multimode fiber is much larger than the focal spot size of a beam with uniform intensity distribution, while the focal spot size is compatible with a mode-field diameter of single-mode fiber. 
     There is a need for remote methods and systems for detecting for the presence of chemicals in the field which provide improved sensitivity due to elimination of atmospheric turbulence effects and improved coupling efficiency at the receiver. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide improved remote methods and systems directed to chemical detection, such as explosives and the like, where the detector system is positioned at a remote location. The preset invention addresses methods and systems directed to chemical sensing that include adaptive optics system to compensate atmospheric turbulence in a light beam passing through the chemical. 
     Yet another object of the present invention is to provide improved remote methods and systems directed to chemical sensing that implement a multimode fiber for collection of the light passed through the chemical. 
     Another object of the present invention is to provide photothermal interferometric spectroscopy devices, and their methods of use, for the remote detection of chemical, and the like. 
     These and other objects of the present invention are achieved in, a photothermal, interferometric spectroscopy system that provides information about a chemical at a remote location. A first light source assembly is included that emits a first beam. The first beam has one or more wavelengths that interact with the chemical and change a refractive index of the chemical. A second laser produces a second beam. The second beam interacts with the chemical resulting in a third beam with a phase change that corresponds with the change of the refractive index of the chemical. A detector system is positioned remote from the chemical to receive at least a portion of the third beam. An adaptive optics system is positioned to compensate the atmospheric turbulence effects in the light beam. The detector system provides information on a phase change in the third beam relative to the second beam that is indicative of at least one of, absorption spectrum and concentration of the chemical. A multimode fiber is used to collect light that passed through the chemical. A single mode fiber is used to insert this light in a waveguide of the integrated part of the detector system. 
     In another embodiment of the present invention, a method is provided for determining information about a chemical at a remote location. A first beam is directed to a remote location where a chemical is present. The first beam has one or more wavelengths that interact with the chemical and changes a refractive index of the chemical. A second beam is directed to the chemical and interacts with the chemical to form a third beam. The third beam has a phase change relative to the second beam that corresponds with a change of a refractive index of the chemical. At least a portion of the third beam is passed through adaptive optics system and received at a detection system positioned remote from the chemical. A phase shift of the third beam is measured that is induced by the first beam and is indicative of at least, one of, absorption spectrum and concentration of the chemical. The third beam is collected by a multimode fiber, then coupled to a single mode fiber and inserted in a coherent receiver. A phase shift of the third beam is measured that is induced by the first beam and is indicative of at least, one of, absorption spectrum and concentration of the chemical. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a photothermal interferometric spectroscopy system of the present invention that has a temporal referenced beam: (a) with reflected probe beam, (b) with transmitted probe beam. 
         FIG. 2  illustrates a change of the refractive index of interrogated chemical and time location of the strobe and probe pulses in the case of a pair pulses probe beam in one embodiment of the present invention. 
         FIG. 3  is a schematic diagram of a balanced detector of the present invention with 90-degrees optical hybrid. 
         FIG. 4  illustrates interference of the pulse with the delayed pulse on the detector from  FIG. 3 . 
         FIG. 5  illustrates a polarization multiplexed pair of pulses in one embodiment of the present invention. 
         FIG. 6  illustrates polarization multiplexed signal generation and detection setup in one embodiment of the present invention. 
         FIG. 7  is a schematic diagram of the detector system with an adaptive optics system and a multimode fiber used to collect light that passed through the chemical. 
         FIG. 8  is a block diagram of a photothermal interferometric spectroscopy system with a spatial referenced beam in one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Co-pending U.S. Patent application Publ. No. 20050105099 discloses an optical device which is a part of the block diagram shown in  FIG. 1(   a ), where  10  is a unit that combines strobe generation and targeting,  11  is the unit for optical probe beam generation and targeting,  12  is a signal detection and recovery block, and  13  is electronics control and processing block. The system operates as follows: the strobe laser  14  feeds integrated wideband Li 2 NbO 3  Optical Comb Generator  15 . The comb generator  15  enables the programming and launching of very short pulses (pico-seconds) that are pre-shaped in the frequency domain to match the absorption spectra of the substance under study, such as explosives or another. In the preferred embodiment the pre-shaped strobe is fed to one of the non-linear ZnSe optical mixer  16  while its other input is coupled with the Optical Parametric Oscillator (OPO)  17 . The output of the mixer  16  results in strobe-spectra at the applicable absorption region of the interrogated chemical substance in the wavelength range of 0.2-20 micron. The strobe beam (this beam is called “the first beam” in the present invention) is directed by targeting unit  18  to a specific location inside the examined chemical volume  19  by preferably a MEMs steering mechanism. The chemical under study is also illuminated by a probe beam (this beam is called “the second beam”) or a set of beams  20  coming from the light source  21  and passing the targeting unit  22 . In the preferred embodiment of the present invention, shown in  FIG. 1(   a ), the probe set of beams  23  passed the interrogated chemical is reflected by the reflection surface  24 . Collecting optics  25  collects the part of reflected light (this beam is called “the third beam”) and forwards it to coherent detector  26  that includes 90-degrees optical hybrid. A local oscillator  26   a  provides a reference signal to the coherent detector  26 . The electrical output signal  27  from the coherent detector is processed in DSP unit  28 . Digital synthesizer and control unit  29  controls DSP unit  28 , optical parametric oscillator  17 , laser  14  and optical comb generator  15 . 
     The system of the present invention additionally includes adaptive optics system  12   a  positioned in the receiving unit  12 . 
     Another embodiment of the present invention is a system operating without the background reflection surface. The background surface can be eliminated if there is enough back scattered light in the interrogated chemical volume to carry out the detection. 
       FIG. 1(   b ) shows this embodiment of the present invention. This is the analogous scheme for the chemicals detection but operating in the transmission mode. In certain situations it could be possible to install the light transmitter  11  and detector  12  on the opposite sides of the interrogated chemical volume  19 . This allows the chemical detecting without background reflection surface. The adaptive optics system is a part of the detector  12 . 
     The detected molecules can be brought into the excited state from which it relaxed by the following processes: (i) direct one-photon absorption; (ii) two-photons absorption and (iii) two-photons stimulated Raman process. The stimulated Raman process enables the use of less exotic light sources that simplify and optimize the overall system. 
     In the preferred embodiment the light of two orthogonal polarizations is used for the chemical illumination to provide complete information for data recovery. 
     Probing of the interrogated chemical is performed by one of two methods:
     (1) Temporal referenced method,   (2) Spatial referenced method.   

     Temporal Referenced Method 
     The probe pulse ( FIG. 2 ) is split in to two and recombined into a two-pulse sequence  30  and  31 , separated by a time t d ≧t p  where t p  is the duration of strobe pulse  32 . The resulting sequence of pulses in shown in  FIG. 2 . The lower part of the figure shows the rapid change of the refractive index  33  in interrogated media followed by relaxation  34 . The phase delay will be measured by interfering the probe signal with its time delay version using the balanced detector. Major advantage lies in the fact that if the time delay t d  is short, the atmospheric noise and vibration noise are not existent. The calculations below show that the minimum detectable concentration is 10 −10  cm −1  that is better than 1 ppb. 
     The interrogated chemical temperature experiences a rapid rise that leads to the rapid change of the refractive the index that causes a phase delay in the probe beam. The phase delay is measured by interfering the probe signal with its time delay version using the balanced detector. The schematic diagram of the balanced detector is shown in  FIG. 3 . It consists of a 90-degrees optical hybrid  40  and four balanced photodetectors  41 - 44 . Two incoming optical signals  45  and  46 , called, respectively the signal S and the local oscillator L, impinge two inputs  47  and  48  of the optical hybrid. Both signal beam S and local oscillator L beam are divided by the first set of 3 dB couplers  49  and  50  as shown in  FIG. 3 . The beam  51  passes through the phase shifter  52  and gains the additional phase shift of 90 degrees. The beams  53  and  54  are combined together at the directional coupler  55 . Respectively, the beams  56  and  57  are combined together at the directional coupler  59 . The resulting four output signals A, B, C, D coming, respectively, from the outputs  59 ,  60 ,  61  and  62 , all having 90-degrees relative phase difference of the form: A=S+L, B=S−L, C=S+jL and D=S−jL. 
     In the preferred embodiment the balanced detector is used as described in the U.S. patent applications Publ. Nos. 20040096143 and Ser. No. 11/672,372 by the same inventor incorporated herein by reference. 
       FIG. 4  shows one embodiment of data recovery. Two signals, one of which is time delayed, are overlapping at the detector. Incoming signal  70  is splitted at splitter  71 , and the beam  72  experiences the delay at the delay line  73 . The delay time is chosen to be the same as a time delay between two pulses in the pair. As the result of this delaying of one of the beams, the pulses  74  and  75  impinge the coherent detector at the same time. Since the pulse  74  corresponds to the heated chemical, and pulse  75  is the reference pulse, the information of the phase change in the laser beam due to the refractive index change can be recovered after detection. 
     Another embodiment uses polarization multiplexed configuration of probe pulses as shown in  FIG. 5  in order to eliminate the delay line at the receiver. Pulse  80  and pulse  81  have orthogonal polarization states (H and V). There are various techniques to implement such polarization multiplexed dual-pulse probe laser. To help elucidate the principle an example of one such implementation is described next. 
     The dual-pulse probe laser can be constructed by polarization multiplexing using a configuration shown in the  FIG. 6 . The input probe pulse train  90  at the far left is divided into four paths using polarization maintaining or PM fiber-optic couplers (PMCs)  91 ,  92  and  93 . The probe pulses in two of the obtained PM optical fibers are combined orthogonally using a polarization beam combiner (PBC)  94 . The two PM fibers have a relative length difference introduced by a delay line  95 . It corresponds to a relative time delay, τ, which is the temporal separation of the two neighboring probe pulses. The output of the PBC is a probe pulse train with two orthogonally polarized neighboring pulses (V and H) with the H-polarized pulse delayed by τ relatively to the V-polarized pulse as shown in the  FIG. 5 . 
     Strobe pulses  96  reflected from the semitransparent mirror  97  heats up the interrogated chemical. The strobe  96  and probe  98  pulse trains are assumed to be synchronized as shown in  FIG. 5 . 
     The returned probe pulse train  99  is directed to the receiver through a circulator  100  as shown in the  FIG. 6 . A polarization controller  101  followed by a polarization beam splitter (PBS)  102  are used to separate the two orthogonal polarized probe pulses (V and H) into two separate optical PM fibers. The two PM fibers have a relative length difference introduced by the delay line  103 . The length difference corresponds to a relative time delay, τ, similar to above but the V-polarized pulse is delayed so that the two pulses are aligned to overlap in time. The two probe pulses are combined with the two local oscillator (LO) pulses at 90-degrees optical hybrids  104  and  105  before impinging balanced detectors  106  and  107  as shown in the figure. The two outputs of the balanced detectors are then subtracted from each other at  108  in order to cancel out the common-path phase noise experienced by both V- and H-polarized probe pulses. The subtraction can also be performed digitally after passing the outputs of the balanced detectors to analog-to-digital converters. With digital signal processing compensation of the relative time delay of the two signals can be performed digitally thereby eliminating, the fiber delay line at the receiver. 
     It was shown that sensitivity of the coherent detection is in order of magnitude higher than the sensitivity of direct detection, see U.S. Patent application 20050105099 by the same inventor as the present invention. 
     The further improvement of the sensitivity can be achieved by increasing the light collection efficiency γ since the sensitivity is proportional to γ. The sensitivity of the coherent detection is the following. 
     
       
         
           
             
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     Here index  1  refers to the strobe and index  2  to the probe, η is the detector&#39;s quantum efficiency, w is the strobe beam radius, k is the specific heat, Q is the pulse energy, Δ is the time of measurement. The time delay t d  is short that eliminates the atmospheric and vibration noises. 
     In the preferred embodiment 10 mJ pulses at required decent repetition rate are obtained using regenerative amplifiers produced by Positive Light, Santa, Clara, Calif. 
     The estimated minimal detectable concentration is the following. We define the minimum change of absorption that we can detect as A min =α min L, where α is absorption coefficient and L is the length of focus of the strobe laser or the size of outgasing cloud whichever is smaller. 
     
       
         
           
             
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     The minimum detectable concentration is about 10 −10  cm −1 ; this is better than 1 ppb. 
     Further reduction of the minimum detectable concentration can be achieved by increasing the light collection efficiency γ, since it is proportional to 
     
       
         
           
             
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       FIG. 7  shows one of the embodiments of the coherent optical receiver with increased light collection efficiency. 
     The light beam  23  with the phase change that corresponds to the heating of the interrogated volume impinges receiving unit  109 . The part of the beam  23  that passes a beam splitter  110  without reflection impinges adaptive optics (AO) element  111 , which can be a deformable mirror. After reflection from the AO element  111  and semi-reflecting plane  112  of the splitter  110 , the light beam is focused by a focusing element  113  into a fiber  114 . Splitter  115  splits the light beam in the fiber into two directions  116  and  117 . The fiber  117  is connected to a sensor  118  which outputs signal  119  being fed in a controller  120  that controls the adaptive optics element  111 . The fiber  116  is connected to the coherent optical receiver  26 . In the preferred embodiment the fibers  114  and  116  are multimode fibers and the receiver  26  is an integrated receiver as disclosed in co-pending U.S. patent application Ser. No. 11/672,372 filed Feb. 7, 2007 by the same inventor. A multimode-to-single-mode coupler  121  is used to insert light from the multimode fiber to a single mode fiber  122  which is connected to the input  123  of the receiver  26 . A local oscillator light source  124  outputs a light beam  125  which enters another input  126  of the receiver  26 . The receiver  26  is connected to the digital processing unit (DSP)  126  where the data about the phase change is recovered. The phase change is indicative of at least one of, absorption spectrum and concentration of the chemical. 
     U.S. Pat. No. 5,699,464 discloses a multimode-to-singe-mode coupler that may be used as the element  121 . 
     The adaptive optics coupling system includes the splitter  115  connected the sensor  118  providing input to the controller  120  that controls the adaptive optics element  111 . Such system was proposed in “Fiber coupling with adaptive optics for free space optical communications” by Weyrauch et al., Proceedings SPIE, 2002, v. 4489, p. 177-183, which incorporated herein by references. 
     A schematic diagram to obtain a high power tuneable light strobe source covering whole mid-IR range was described in details in the parent application U.S. Publ. No. 20050105099. 
     Spatial Referenced Method 
     In one embodiment of the present invention, two or more probe beams are used, and they are focused on certain distance inside and near the chemical volume under study as shown in  FIGS. 8(   a ) and ( b ) for reflection-type and transmission-type sensing (two beams case is shown). The probe beams generation and targeting unit  11  outputs two beams, which are slightly spatially resolved. One probe beam  130  is focused in the location of the strobe laser focus  131 , and the reference probe beam  132  is focused out of the area of the strobe laser influence. After reflection from the reflective surface  24  ( FIG. 8   a ) two probe beams impinge the coherent detector  12 . The change of phase of the first probe beam relatively to another one is recovered followed by DSP processing  13 . In the preferred embodiment the coherent detector is used as described in the U.S. patent application Publ. No. 20040096143 “Optical coherent detector and optical communications system and method” by the same invention incorporated herein by reference. The information on the interrogated chemical concentration is recovered. Since the coherent detection is used the of this system is higher (similar to time reference system) compared to the standard system described in S. E. Bialkowski Photothermal Spectroscopy Methods for Chemical Analysis, John Wiley &amp; Sons, Inc., 1996, incorporated herein by reference. 
       FIG. 8   b  discloses a similar schematics for chemical detection using spatial reference signal, but operating in a transmission mode. The detector  12  collects light passed through the chemical under study  19 . 
     Experimental results on Remote Gas Detection Using Photothermal Interferometric Spectroscopy are disclosed in the parent U.S. Patent application Publ. No. 20050105099 filed Jan. 13, 2005 by the same inventor, incorporated herein by reference. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.