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. 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  
       [0001]     This application claims the benefit of U.S. Ser. No. 60/582,889, filed Jun. 25, 2004, and is a continuation-in-part of U.S. Ser. No. 10/669,130, filed Sep. 22, 2003, both of which applications are fully incorporated herein by reference. 
     
    
     FIELD OF INVENTION  
       [0002]     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  
       [0003]     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.  
         [0004]     McLean et al. (E. A. McLean et al. American Journal Applied Physics Letters, 13, p. 369 (1968)) recognized that the optical absorption resulting in sample heating and subsequent changes in refractive index would cause a phase shift in light passing through the heated region. This phase shift can be detected by interferometric means.  
         [0005]     Grabiner et al. (F. R. Grabiner et al. Chemical Physics Letters, 17, p. 189 (1972)) proposed to use two lasers for photothermal interferometric spectroscopy: pulsed infrared laser for the medium excitation and visible probe laser for the refractive index change measurement.  
         [0006]     In the U.S. Pat. No. 5,408,327 a process and arrangements for photothermal spectroscopy by the single-beam method with double modulation technique is disclosed. A single-beam method is developed making use of the advantages of double modulation technique in detecting the photothermally generated difference frequency without requiring partial beams and while achieving extensive absence of intermodulation, the intensity of the laser beam is modulated before striking the object in such a way that the modulation spectrum substantially contains a carrier frequency (f 1 ) and two sideband frequencies (f 1 +−f 2 ), wherein f 2  is the base clock frequency of the modulation, a regulating detector and a control loop intervening in the modulation process suppress that component of the base clock frequency (f 2 ) in the same phase with the mixed frequency of the carrier frequency and sideband frequencies. After interaction with the object the optical response of the object is measured by means of a measurement detector and frequency-selective and phase-selective device as the amplitude of that component of the base clock frequency (f 2 ) which, as the photothermal mixed product, has the same phase as the mixed frequency of the carrier frequency (f 1 ) and sideband frequency (f 1 +−f 2 ). Use for nondestructive and noncontact analysis of the material parameters of areas of solid bodies close to the surface is described.  
         [0007]     In the U.S. Pat. No. 6,709,857 a system and method for monitoring the concentration of a medium using photothermal spectroscopy is disclosed. The system and method each employs an energy emitting device, such as a laser or any other suitable type of light emitting device, which is adapted to emit a first energy signal toward a location in the container. The first energy signal has a wavelength that is substantially equal to a wavelength at which the medium absorbs the first energy signal so that absorption of the first energy signal changes a refractive index of a portion of the medium. The system and method each also employs a second energy emitting device, adapted to emit a second energy signal toward the portion of the medium while the refractive index of the portion is changed by the first energy signal, and a detector, adapted to detect a portion of the second energy signal that passes through the portion of the medium. The system and method each further employs a signal analyzer, adapted to analyze the detected portion of the second energy signal to determine an amount of a sample in the container based on a concentration of the medium in the container.  
         [0008]     There is a need for remote methods and systems for detecting for the presence of chemicals in the field.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, an object of the present invention is to provide improved methods and systems directed to chemical detection, such as explosives and the like.  
         [0010]     Another object of the present invention is to provide methods and systems directed to remote detection of chemicals, such as explosives and the like.  
         [0011]     Yet 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.  
         [0012]     Still a further 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, based on optical signal detection.  
         [0013]     These and other objects of the present invention are achieved in, a photo-thermal, 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. 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.  
         [0014]     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 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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      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.  
         [0016]      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.  
         [0017]      FIG. 3  is a schematic diagram of a balanced detector of the present invention with 90-degrees optical hybrid.  
         [0018]      FIG. 4  illustrates interference of the pulse with the delayed pulse on the detector from  FIG. 3 .  
         [0019]      FIG. 5  illustrates a polarization multiplexed pair of pulses in one embodiment of the present invention.  
         [0020]      FIG. 6  illustrates polarization multiplexed signal generation and detection setup in one embodiment of the present invention.  
         [0021]      FIG. 7  illustrates the change of the refractive index of interrogated chemical and time location of the strobe and probe pulses in the case of a multiple pulses probe beam in one embodiment of the present invention.  
         [0022]      FIG. 8  is a schematic diagram of a tuneable light strobe source that is used in one embodiment of the present invention.  
         [0023]      FIG. 9  is a block diagram of a photothermal interferometric spectroscopy system with a spatial referenced beam in one embodiment of the present invention.  
         [0024]      FIG. 10  illustrates an experimental setup for gas detection by a photothermal interferometric spectroscopy system in one embodiment of the present invention.  
         [0025]      FIG. 11  illustrates experimental results on acetylene gas detection by photothermal interferometric spectroscopy from  FIG. 10 .  
         [0026]      FIG. 12  illustrates an experimental setup for gas detection by photothermal spectroscopy with the probe laser divided into two paths for one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]     In one embodiment of the present invention, an optical device is provided, the block diagram of which is 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 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 centered over in the wavelength range of 0.2-20 μm. 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. The electrical output signal  27  from the coherent detector is processed in DSP unit  28 . Digital synthesizer and control unit  29  controls DSP unit  27 , optical parametric oscillator  17 , laser  14  and optical comb generator  15 .  
         [0028]     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.  
         [0029]      FIG. 1 ( b ) shows another 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  13  on the opposite sides of the interrogated chemical volume  27 . This allows the chemical detecting without background reflection surface.  
         [0030]     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.  
         [0031]     In the preferred embodiment the light of two orthogonal polarizations is used for the chemical illumination to provide complete information for data recovery.  
         [0032]     Probing of the interrogated chemical is performed by one of two methods: 
        (1) Temporal referenced method,     (2) Spatial referenced method. 
 
 Temporal Referenced Method 
       
 
         [0035]     The probe pulse ( FIG. 2 ) is split into 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 detectible concentration is 10 −10  cm −1  that is better than 1 ppb.  
         [0036]     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° 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°. 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  58 . The resulting four output signals A, B, C, D coming, respectively, from the outputs  59 ,  60 ,  61  and  62 , all having 90° relative phase difference of the form: A=S+L, B=S−L, C═S+jL and D=S−jL.  
         [0037]     In the preferred embodiment the balanced detector is used as described in the U.S. patent application Ser. No. 10/669,130 “Optical coherent detector and optical communications system and method” by I. Shpantzer et al. incorporated herein by reference.  
         [0038]      FIG. 4  shows the overlapping of the time delayed signal 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.  
         [0039]     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.  
         [0040]     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. 6 .  
         [0041]     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. 6 .  
         [0042]     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 polarization maintaining combiners  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.  
         [0043]     The sensitivity of the coherent detection is the following:  
                 SNR   INT     ~         η   2     ⁢     γ   2         hv   2         ⁢   f   ⁢           ⁢   Δ   ⁢           ⁢     tA   2     ⁢     Q   1   2     ⁢         Q   2     ⁡     [       n   -   1         λ   2     ⁢     w   2     ⁢   κ   ⁢           ⁢   T       ]       2             (   1   )             
 
         [0044]     Here index 1 refers to the strobe and index 2 to the probe, γ is the collection efficiency, η is the detector&#39;s quantum efficiency, w is the strobe beam radius, κ is the specific heat, Q is the pulse energy, Δt is the time of measurement. The time delay t d  is short that eliminates the atmospheric and vibration noises. For the same system the DIAL SNR is the following:  
                 SNR   DIAL     ~         η   1     ⁢     γ   1         4   ⁢     hv   1           ⁢   f   ⁢           ⁢   Δ   ⁢           ⁢     tA   2     ⁢     Q   1             (   2   )             
 
         [0045]     Thus using the coherent detection we obtain the enhancement factor of  
             E   =           [     2   ⁢       n   -   1         λ   2     ⁢     w   2     ⁢   κ   ⁢           ⁢   T       ⁢         Q   1     ⁢     Q   2           ]     2     ⁢         η   2     ⁢     λ   1           η   1     ⁢     λ   2           =         [       20       w   2     ⁢     λ   2         ⁢         Q   1     ⁢     Q   2           ]     2     ⁢         η   2     ⁢     λ   1           η   1     ⁢     λ   2                     (   3   )             
 
 where all the energies are in Joules, the beam radius is in cm and wavelengths are in micrometers. Assuming that both strobe and probe energies are about 10 mJ in say ins, the strobe wavelength is 10 μm and the probe wavelength is 1 μm. Assume furthermore distance of 100 m and the lens diameter 15 cm (w˜0.4 cm). Note that 10 mJ is just about equal to the saturation energy. The enhancement factor is then of the order of E˜15. 
 
         [0046]     In the preferred embodiment 10 mJ pulses at required decent repetition rate are obtained using regenerative amplifiers produced by Positive Light, Santa Clara, Calif.  
         [0047]     The estimated minimal detectable concentration is calculated below. 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  
               A   min     ⁢         hv   2         η   2     ⁢       γ   2     ⁡     (     f   ⁢           ⁢   Δ   ⁢           ⁢   t     )       ⁢     Q   2           ⁢         λ   2     ⁢     w   2     ⁢   κ   ⁢           ⁢   T         (     n   -   1     )     ⁢     Q   1                 (   3   )             
 
         [0048]     Assuming that the frequency is 1 kHz and acquisition time is 0.1 s, and the collection efficiency is 1%. For the pulses of A min ˜10 −7  or for the 10 cm path we obtain α min =10 −8 . Assuming that the cross-section of the absorbent is σ=10 −18  cm 2  the minimum detectable concentration is 10 −10  cm −1 . This is better than 1 ppb.  
         [0049]     Even better accuracy in concentration detection can be achieved if the pulse sequence  110 - 115 , shown in  FIG. 7 , is used for probing.  
         [0050]     A schematic diagram to obtain a high power tuneable light strobe source covering whole mid-IR range is shown in  FIG. 8 . It is a combination of optical parametric oscillator (OPO)  120  and DFG  121  fed by two semiconductor lasers  122  and  123  operating in the 880 nm and in 980 nm range correspondingly. Periodically poled ZnSe (PPZS) serves at both OPO  120  and DFG  121  (with different pitches). The tuning can be accomplished by either temperature change or having period of PPZS graded laterally. Then moving PPZS in the lateral direction will allow the tuning.  
         [0051]     Mode locked Yb doped fiber laser  124  consists of a gain element (Yb doped fiber) and an electro-optic modulator. The role of electro-optic modulator is to provide timing for when the mode locked pulse is generated. The pulse length of the mode-locked laser is of the order of a few picoseconds and the wavelength is 1060 nm. An Yb-doped fiber amplifier-125 boosts the power of mode locked pulses to 10 W average power.  
         [0052]     The OPO  120  converts the 1060 nm radiation into the tunable radiation in the 1700-2800 nm ranges. It consists of the PPZS crystal placed into optical cavity.  
         [0053]     The mode locked Er doped fiber laser  126  consists of gain element (Yb doped fiber) and electro-optic modulator. The role of electro-optic modulator is to provide timing for when the mode locked pulse is generated. The pulse length of the mode-locked laser is of the order of a few picoseconds and the wavelength is 1550 nm. An Er-doped fiber amplifier  127  that boosts the power of mode locked pulses to 10 W average power.  
         [0054]     The clock  128  synchronizes the pulses of both Er and Yb lasers.  
         [0055]     Following the amplifier the radiation is split into two parts: one part  129  becomes the probe radiation that measures the phase changes induced by the strobe. The other part  130 , as well as the light from the Yb fiber amplifier  125  impinges upon the second PPZS crystal  121  that is not placed into optical cavity and serves as a difference frequency generator that produces pulses of tunable frequency (3.5-20 mcm)  131 .  
         [0056]     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.  9 ( 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  140  is focused in the location of the strobe laser focus  141 , and the reference probe beam  142  is focused out of the area of the strobe laser influence. After reflection from the reflective surface  24  ( FIG. 9 , 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 Ser. No. 10/669,130 “Optical coherent detector and optical communications system and method” by I. Shpantzer et al. incorporated herein by reference. The information on the interrogated chemical concentration is recovered. Since the coherent detection is used the sensitivity 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.  
       EXAMPLE 1  
     Results on Remote Gas Detection Using Photothermal Spectroscopy  
       [0057]     In this example, preliminary test results are provided for a proof-of-concept experiment of a strobe-probe photothermal spectroscopy system of the present invention, using acetylene gas cell. In one embodiment of the present invention, systems and methods are provided to demonstrate the feasibility of using a laser beam to probe the photothermal effect in gas induced by an intense strobe laser beam, and the detection of the photothermal signal transcribed onto the probe laser beam.  
         [0058]     Acetylene gas ( 12 C 2 H 2 ) has a rich absorption lines in the range of 1510 to 1540 nm. Its absorption spectrum is well-documented and readily available. It is also commercially available in gas cell form with AR-coated end faces and optical fiber couplings.  
         [0059]     The setup for the strobe-probe photothermal measurement using direct detection is shown in  FIG. 10 . A tunable laser  150  with wavelength range within the acetylene absorption was used as the excitation or strobe laser. The strobe laser was amplified using an erbium-doped fiber amplifier (EDFA)  151 . The amplified output was connected to a fiber U-bench  152  to direct the laser beam to free-space so that the beam can be modulated by a mechanical chopper wheel with 50% duty cycle located inside the U-bench  152 . The laser beam was chopped at a frequency of f strobe  of about 1 kHz which is lower than the lowest resonance frequency of the gas cell. The strobe is then directed to the gas cell  153  through a circulator  154 . The acetylene gas pressure in the cell was 400 Torr. The strobe power to the input fiber of the gas cell was about +16 dBm (peak power). This power varies slightly with the strobe wavelength due to the gain variation of the EDFA with wavelength. The gas cell  153  has an off-resonance insertion loss of about 1 dB or 0.5 dB per interface. Therefore, the strobe power into the gas cell was about +15.5 dBm. The laser beam size within the gas cell was about 0.35 mm. A CW probe laser  159  operating at 1547.5 nm was directed (after passing the circulator  155 ) to the other end of the gas cell so that the strobe and the probe beams are counter-propagating. The transmitted probe beam was directed to optical bandpass filters  156  through the circulator  154 . The bandpass filters reject any residual unabsorbed or reflected strobe power. A photodetector  161  was used to convert the probe laser into electrical signal, which was fed to the input of a lock-in amplifier  157  synchronized with the chopper controller  158 . The probe power at the photodetector was about −5 dBm. The magnitude output  162  of the lock-in amplifier  130  (V out ) represents the photothermal signal which reflects the absorption strength of the gas. A computer  160  was used to record V out  as the strobe laser&#39;s wavelength is tuned which was computer-controlled.  FIG. 11  shows typical result of V out  versus the strobe wavelength. The absorption spectrum of 50 Torr of acetylene gas published by NIST is also plotted for comparison.  
         [0060]     In this setup, direct detection converts the amplitude modulated probe signal to electrical signal. The probe laser was amplitude modulated due to thermal blooming of the probe beam as a result of the gas heating induced by the strobe laser thus reducing the refractive index. The reduction of the refractive index near the center of the beam causes a negative lens effect or divergence of the probe beam thus its intensity at the other end of the gas cell was decreased. This effect, however, is quite small and the lock-in signal at peak absorption is only about 20 mV which indicates the amplitude modulation of the probe is quite inefficiency. However, the probe is also phase modulated which can be converted to amplitude modulation using interferometric measurement. The setup is similar to that of  FIG. 9  except that the probe laser is divided into two paths where one path was directed to the gas cell. The two paths were combined and detected. The lock-in amplifier output signal in this case increases up to 500 mV at peak absorption. The powers of the modulated and un-modulated probe laser at the detector were −17 and −11 dBm, respectively. The detector output, however, fluctuates due to random phase variation as a result of environmental perturbation of the fibers as well as laser frequency drifts. Feedback control loop can be used to compensate for such random phase variation which will be implemented in future experiments.  
         [0061]     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.