Abstract:
A device for non contact detection of low concentration substances outside a laboratory environment is disclosed. A probing laser emission is split into two linear orthogonally polarized emission components and one component is delayed in time relative to the other. Both components are directed to a focal region that is proximate to a target medium thought to contain the substance to be detected. Between the arrival of the first and second emission components, an excitation light pulse at a wavelength corresponding to an absorption line in the spectrum of the substance is directed to the focal region. If vapours of the substance are present, they will be heated by the excitation pulse and will change the index of refraction of the focal region before the second emission component passes through it, thus altering the phase of the back-scattered emission returns. The device delays the first returned component by an equal delay and coherently couples the returned emission components. The amplitudes of the orthogonally polarized returned emission components are compared. If the probing laser is pulsed, the ratio of the polarized pulse components is observed to indicate the presence of the substance. Optionally, reference pulses for which no excitation pulse is generated may be introduced to provide a reference signal to rule out effects due to the intrinsic polarization caused by passage of the emission through optical components. If the probing laser is a continuous wave laser, the presence of transients timely correlated with excitation pulses in the detected continuous wave signal will indicate the presence of the substance.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to remote sensing of low-concentration and trace substances. In particular, the present invention relates to the remote sensing of low-concentration and trace substances such as explosives.  
       BACKGROUND OF THE INVENTION  
       [0002]     Traditionally, the identification of substances has involved the identification and measurement of the substances&#39; characteristic spectra, such as through fluorescent and spectroscopic analysis. More recently various photothermal spectroscopic approaches, such as photoacoustic spectroscopy and photo-thermal deflection spectroscopy have been proposed. Such approaches rely on the so-called “thermal lens” effect, in which a weakly absorbing substance is excited by an energy source, such as a flux of photons having a wavelength with which the substance is resonant, producing a change in the refractive index along the energy path, due to the heating of the substance&#39;s vapours by the energy source. The thermal lens thus created has been suggested for measuring absorption and for application in spectrophotometry and spectroscopy.  
         [0003]     The thermal lens effect was first described in Gordon, J. P. et al. “Long—Transient Effects in Lasers with Inserted Liquid Samples”,  Journal of Applied Physics  36, 3 (1965). Buildup and decay transients of laser oscillation were observed when cells containing liquids were placed inside the resonator of a He—Ne laser operating at 633 nm. Similar but less pronounced effects were also observed with two solids. Transverse motion of the cell by about one beam width caused new transients that were similar to the initial ones. The authors believed that the effects were caused by absorption of the He—Ne laser emission in the tested materials, producing a local heating in the vicinity of the beam, and a lens effect due to the transverse gradient of the refractive index. The authors found that absorption of between 10 −3  and 10 −4  cm −1  was sufficient to produce the effect.  
         [0004]     Subsequent to this publication, it was determined that the thermal lens effect provided a mechanism to measure the weak absorption of light in transparent materials.  
         [0005]     In Solmini, Domenico, “Accuracy and Sensitivity of the Thermal Lens Method for Measuring Absoprtion”,  Applied Optics, Vol.  5, No. 12, 1931 (1966), the accuracy and sensitivity of the thermal lens effect for measuring absorption was studied using a geometry in which two lenses were inserted into an optical resonator. The author concluded that the absorbency of transparent materials could not be measured in a simple manner by photometric methods, but confirmed that the thermal lens effect provided a measurement for measuring absorbencies as low as 10 −5  cm −1 . He concluded that the sensitivity of the effect was related to the configuration of the resonator, nearly confocal resonators being the most sensitive. However, the author pointed out that because near confocal resonators manifest effects inimical to precise measurement, cavities that are far from the confocal configuration may be more practical.  
         [0006]     In Jackson, W. B. et al. “Photothermal deflection spectroscopy and detection”,  Applied Optics , Vol. 20, No. 8 1333 (1981), the theoretical foundation of photothermal deflection spectroscopy (PDF) was developed. Two main PDF configurations were considered, namely collinear photothermal deflection, where the gradient of the index of refraction was both created and probed within the sample, and transverse photothermal deflection where the probing of the gradient of the index of refraction was accomplished in the thin layer adjacent to the sample. The authors found that the latter approach is most suited for opaque samples and for materials with poor optical quality. Earlier experiments by other authors were compared and the theoretical predictions were experimentally verified. In summarizing some photothermally-based spectroscopies, the authors provided sensitivities of different experimental set-ups. The sensitivity (in units of (α1) min ×pump power (Watts)) ranged from 10 −4  for microphone photoacoustic spectroscopy to 10 −8  for collinear PDF. Special features were noted as being pertinent to particular set ups.  
         [0007]     In U.S. Pat. No. 4,544,274 issued to Cremers et al., there is disclosed a variant of the thermal lens method, in which a cell containing the sample is inserted into a laser resonator for measurement of weak optical absorptions. In the Cremers et al. method, the output coupler of the resonator is deliberately tilted relative to the CW laser beam circulating in the resonator to produce a pulsed laser output, whose pulse width could be related to the sample absorptivity by a simple algorithm or calibration curve, thus demonstrating a measured absorption of 10 −5  cm −1 .  
         [0008]     In Kawasaki et al., “Thermal Lens Spectrophotometry Using a Tunable Infrared Laser Generated by a Stimulated Raman Effect”,  Anal. Chem.  59, 523 (1987), thermal lens spectrophotometry utilizing a tunable infrared laser source was applied to record the spectrum of ammonia in gaseous phase to a spectral resolution of 0.1 cm −1 . The detection limit was 6% for the line at 1025.69 nm when available 0.13 mJ, 10 ns pulses at 1015 nm-1040 nm were focused into a flow cell. The authors felt that once more powerful infrared lasers were created, the sensitivity of the method could be improved by several orders of magnitude.  
         [0009]     In U.S. Pat. No. 4,310,762 issued to Harris et al, there is disclosed a technique based on laser induced thermolens. In that technique a laser beam travels through two cells, a reference cell and a sample cell. The cells are located at points in the beam path such that any modification in the beam caused by a change in the index of refraction of the medium in the reference cell is cancelled by the use of the same medium in the sample cell. Therefore, any detectable modification in the beam, such as beam expansion or change of its divergence as it escapes the sample cell, must be caused by the change in the thermal lens in the material under identification.  
         [0010]     In the foregoing exemplary references, as well as others, the thermo-optical effect was exploited for determining weak light absorption in different transparent media for finding trace substances and for other spectroscopic purposes. However, each disclosed high sensitivity methods and apparatus that were suitable for the laboratory environment only.  
         [0011]     There have been developed a number of optical techniques, based mainly on lidars, which are capable of the remote detection of trace substances in air, on water and on ground surface. None of these methods use the thermo-optical effect. However, if such a method could be developed, it would provide an effective tool for the remote detection of ultra-low concentration substances, such as vapour/gas leaks, side products of the hazardous waste industry as well as trace explosive materials, with high spatial resolution.  
         [0012]     In Bubis, E. L., et al., “Research of low-absorptive media for SBS in near infrared spectral band”,  Optica e Spektroskopiya , Vol. 65, No. 6, 1281 (1988), the thermal lens method was combined with the dark-field method to determine weak absorption of liquids used in phase conjugate mirrors. This approach has demonstrated the possibility of using the thermo-optical effect for the remote detection of low concentration admixture in different transparent media. The authors focused 0.2 ms pulses of between 0.1-5 J of a neobdynium laser having a beam waist of about 0.2 mm into a cell with liquid. A collimated probing beam of a He—Ne laser traversed through the waist along the axis of the pumping beam and was blocked by a copper foil 1 mm in diameter. A portion of the probing beam was scattered due to phase distortions caused by heat deposition in the focal region. The scattered component of the probing beam was registered by a photodetector. It was shown that the so-called critical energy, which is a feature of the tested liquid, particularly its absorbance, determined the weakest distortions detectable. In fact, it was possible to detect heat-induced distortion at 1/100 of the critical energy. With this method the authors measured absorbance as low as 10 −6  cm −1 .  
         [0013]     In Andreyev, N. F., et al., “Locked Phase Conjugation for Two Beam Coupling of Pulse Repetition Rate Solid-State Lasers”,  IEEE J. of Quant. Electr., Vol.  27, No. 9, 1024 (1991), the authors taught a method of coherent beam coupling.  
       SUMMARY OF THE INVENTION  
       [0014]     It is therefore an object of the present invention to detect low concentration and trace substances in an industrial environment.  
         [0015]     It is a further object of the present invention to detect trace substances in air.  
         [0016]     It is another object of the invention to detect trace substances in a thin layer near targets.  
         [0017]     It is yet another object to detect trace substances with high spatial resolution.  
         [0018]     The present invention extends the thermooptically-based method of detecting low concentration substances beyond a laboratory environment. It makes use of the thermal lens effect in conjunction with a method of coherent beam coupling to provide, in an industrial environment, a method and apparatus for detecting low concentration substances in air. The inventive method and apparatus may detect such substances, whether in the form of a gas, vapour or a cloud of dust particles. Typical applications of the inventive method and apparatus include the detection of vapour or gas leaks, side products of hazardous industries and trace explosive materials. Furthermore, the thermal lens effect may now applied to the remote sensing of trace substances with high spatial resolution.  
         [0019]     This is achieved by focusing an excitation energy pulse having a wavelength for which the substance or substances to be detected is resonant, at the targeted area to provide a noticeable absorption over a short distance corresponding to the focal waist. A sensing probing pulse will be modified by the change in the refractive index in the focal area if even a low concentration of the substance is present to resonantly absorb the heating pulse. The modification is detected by comparison of the ratios of the orthogonal linear polarization components of the modified probing pulse and of a reference pulse, transmitted through the same focal region, but unperturbed by the excitation pulse, recreated through coherent beam coupling.  
         [0020]     Alternatively, a CW stream of probing photons may be used. In this case, detection of the modification to the probing stream is shown by transients in the amplitude of the returned stream that correspond temporally to the introduction of these excitation pulses.  
         [0021]     The inventive method takes advantage of a long focal distance objective (typically in the range of tens of meters) and high spatial resolution due to a narrow beam waist (typically in the range of hundreds of microns).  
         [0022]     According to a broad aspect of the invention, there is disclosed an apparatus for non-contact detection of a substance in a target region, comprising; a laser source for generation of a probing light emission; an optical subsystem adapted to split a light emission into first and second emission components and to introduce a first delay to the second emission component relative to the corresponding first emission component; a lens subsystem adapted to accept all of the components in sequence and direct them to a focal region proximate to the target region along an optical axis; an excitation source adapted to direct energy at a wavelength corresponding to an absorption line in the spectrum of the substance, through the lens subsystem to the focal region, at a time between the first and second components so as to change the refractive index in the focal region if the substance is present in the target region before the passage of the second component through the focal region; an emission coupler adapted to: recover back-scattered returns of the emission components, introduce a second delay to the first returned emission component relative to the second returned emission component in an amount equal to the first delay, and coherently couple the emission components into a returned light emission; and a detection subsystem adapted to measure components of the returned emission to determine if there has been a change in the phase of the second returned emission component as a result of the presence of the substance in the target region.  
         [0023]     According to a second broad aspect of the invention, there is disclosed a method for non-contact detection of a substance at a target region, comprising the steps of, radiating a probing light emission; splitting the emission into a first and second emission component; delaying in time the second emission component relative to the first emission component; directing all of the emission components in sequence to a focal region proximate to the target region; directing energy at a wavelength corresponding to an absorption line in the spectrum of the substance to the focal region at a time between the first and second emission components so as to change the refractive index in the focal region if the substance is present in the target region; recovering back-scattered returns of the emission components; delaying in time the first returned emission component relative to the second returned emission component by a value equal to the initial delay; coherent coupling the emission components into a returned emission; measuring components of the returned emission to determine if there has been a change in the phase of the second returned emission component as a result of the presence of the substance in the target region. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0024]      FIG. 1  is an optical diagram of an apparatus according to a first embodiment of the present invention;  
         [0025]      FIG. 2  is a timing diagram showing an exemplary sequence of pulses that can be generated by the apparatus of  FIG. 1 ;  
         [0026]      FIG. 3  is a schematic representation of the focusing, by the objective of  FIG. 1 , of light beams into a target region; and  
         [0027]      FIG. 4  is a timing diagram showing the temporal relationship between the pumping pulse and the responses received at the photodiodes of  FIG. 1 , according to a second, continuous wave (CW) embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Referring now to  FIG. 1 , there is shown a simplified schematic diagram of a possible optical apparatus in accordance with a first embodiment of the present invention shown generally at  10  for remote detection of low concentration gas admixture in air in a target region (not shown) proximate to a focal region  29 .  
         [0029]     The apparatus  10  comprises a plurality of optical components, including a plurality of polarizers  11 ,  14 ,  17 ,  18 , mirrors  15 ,  19 ,  20 ,  23 , half wave plates  13 ,  16 , Faraday rotators  12 ,  22 , photodiodes  27 ,  28 , lenses  24 ,  25 ,  26  and an aperture  21 .  
         [0030]     The apparatus  10  accepts laser light emissions as inputs  40  and  50 . In this first embodiment, such laser emission  40 ,  50  are in pulsed form. Pulses  40  input along optical path t are incident on a polarizer  11 , while pulses  50  input along optical path u are incident on a dichroic mirror  23 . For ease of explanation, the direction along which photons proceed from path t through apparatus  10  until they pass through the focal region  29  along path q is denoted the forward direction, while the opposite direction is denoted the reverse direction.  
         [0031]     Polarizer  11  accepts input photons  40  along optical path t and is optically connected to a Faraday rotator  12  by optical path a and to a photodiode  28  by optical path s. Polarizer  11  transmits p-polarized components of photons in both the forward and reverse directions, and reflects s-polarized components. Thus, in the forward direction, p-polarized components incident upon it along path t are transmitted through it along path a and impinge upon Faraday rotator  12 , while s-polarized components are effectively reflected off. In the reverse direction, p-polarized components incident upon it along path a are transmitted through it along path t, while s-polarized components incident on it are reflected by it along path s to impinge upon photodiode  28 . As will be discussed below, the optical configuration of the apparatus  10  ensures that there will be effectively no s-polarized components incident upon polarizer  11  along path a.  
         [0032]     Photodiode  28  captures optical pulses reflected from polarizer  11  in the reverse direction, along path s and converts them into electrical pulses in proportion to the density of photons incident upon it. The electrical amplitude response of photodiode  28  is measured (not shown) for processing as discussed below.  
         [0033]     Faraday rotator  12  is optically connected to polarizer  11  by optical path a and to a half wave plate  13  by optical path b. The Faraday rotator  12  is an irreversible optical element that rotates the polarization of the photons incident upon it by a certain angle, in this embodiment, +45°. Thus, photons travelling in the forward direction along path a from polarizer  11  to Faraday rotator  12 , exit from it along path b with their linear polarization rotated by +45°, to impinge upon half wave plate  13 . Photons travelling in the reverse direction along path b from half wave plate  13  to Faraday rotator  12 , exit from it along path a with their linear polarization rotated by +45°, to impinge upon polarizer  11 .  
         [0034]     Half wave plate  13  is optically connected to Faraday rotator  12  by optical path b and to a polarizer  14  by optical path c. Half wave plate  13  is a reversible optical element that rotates the linear polarization of the photons incident upon it by +45°. Thus, photons travelling in the forward direction along path b from Faraday rotator  12  to half wave plate  13 , exit from it along path c with their linear polarization rotated by an additional +45°, to impinge upon polarizer  14 . Thus, in the forward direction, Faraday rotator  12  and half wave plate  13  act so as to change the linear polarization of photons incident upon Faraday rotator  12  along path a in the forward direction by +90°, that is to change p-polarized components to s-polarized components. However, photons travelling in the reverse direction along path c from polarizer  14  to half wave plate  13 , exit from it along path b with their linear polarization rotated by −45°, to impinge upon Faraday rotator  12 , so that the combined effect on the linear polarization of photons travelling in the reverse direction, of the half wave plate  13  and the Faraday rotator  12 , is zero rotation angle.  
         [0035]     Polarizer  14  is optically connected to half wave plate  13  by optical path c, to a photodiode  27  by optical path r and to a mirror  15  by optical path d. Polarizer  14  transmits p-polarized components of photons in both the forward and reverse directions, and reflects s-polarized components. Thus, in the forward direction, p-polarized components incident upon it along path c are transmitted through it and are effectively discarded, while s-polarized components are reflected by it along path d to impinge upon mirror  15 . As will be discussed below, the optical configuration of the apparatus  10  ensures that there will be effectively no p-polarized components incident upon polarizer  14  along path c in the forward direction. In the reverse direction, p-polarized components incident upon it along path d are transmitted through it along path r to impinge upon photodiode  27 , while s-polarized components incident on it along path d are reflected by it along path c to impinge upon half wave plate  13 .  
         [0036]     Photodiode  27  captures optical pulses reflected from polarizer  14  in the reverse direction along path r and converts them into electrical pulses in proportion to the density of photons incident upon it. The electrical amplitude response of photodiode  27  is measured (not shown) for processing as discussed below.  
         [0037]     Mirror  15  is optically connected to polarizer  14  by optical path d and to a half wave plate  16  by optical path e. Mirror  15  reflects photons incident upon it from optical path d to optical path e and vice versa. Thus, photons incident upon it in the forward direction along path d are reflected along path e to half wave plate  16 , while photons incident upon it in the reverse direction along path e are reflected along path d to polarizer  14 .  
         [0038]     Half wave plate  16  is optically connected to mirror  15  by optical path e and to a polarizer  17  by optical path f Half wave plate  16  is a reversible optical element that rotates the linear polarization of the photons incident upon it by +45°. Thus, photons travelling in the forward direction along path e from mirror  15  to half wave plate  16 , exit from it along path f with their linear polarization rotated by +45°, to impinge upon polarizer  17 . However, photons travelling in the reverse direction along path f from polarizer  17  to half wave plate  16 , exit from it along path e with their linear polarization rotated by −45°, to impinge upon mirror  15 .  
         [0039]     Polarizer  17  is optically connected to half wave plate  16  by optical path f, to a polarizer  18  by optical path g and to a mirror  19  by optical path h. Polarizer  17  transmits p-polarized components of photons in both the forward and reverse directions, and reflects s-polarized components. Thus, in the forward direction, p-polarized components incident upon polarizer  17  along path f are transmitted through it along path g to impinge upon polarizer  18 , while s-polarized components are reflected by it along path h to impinge upon mirror  19 . In the reverse direction, p-polarized components incident upon it along path g are transmitted through it along path f to impinge upon half wave plate  16 , while s-polarized components incident upon it along path h are reflected by it along path f to impinge upon half wave plate  16 .  
         [0040]     Mirror  19  is optically connected to polarizer  17  by optical path h and to a mirror  20  by optical path i. Mirror  19  reflects photons incident upon it along optical path h to optical path i and vice versa. Thus, photons incident upon it in the forward direction along path h are reflected along path i to mirror  20 , while photons incident upon it in the reverse direction along path i are reflected along path h to polarizer  17 .  
         [0041]     Mirror  20  is optically connected to mirror  19  by optical path i and to polarizer  18  by optical path j. Mirror  20  reflects photons incident upon it from optical path i to optical path j to polarizer  18 , while photons incident upon it in the reverse direction are reflected along optical path i to mirror  19 . Optical paths h, i and j are designed such that s-polarized components are delayed relative to their corresponding p-polarized components by a time interval chosen to be small enough that there is little likelihood that the index of refraction in a target region (not shown) will be changed during the interval. By way of example only, this time interval may be on the order of 20 ns in the described embodiment.  
         [0042]     Polarizer  18  is optically connected to polarizer  17  by optical path g, to mirror  20  by optical pathj and to an aperture  21  by optical path k. Polarizer  18  transmits p-polarized components of photons in both the forward and reverse directions, and reflects s-polarized components. Thus, in the forward direction, p-polarized components incident upon it along path g are transmitted through it along path k to impinge upon aperture  21 , while s-polarized components incident upon polarizer  18  along path j are reflected by it along path k to impinge upon aperture  21 . In the reverse direction, p-polarized components incident upon it along path k are transmitted through it along path g to impinge upon polarizer  17 , while s-polarized components incident upon it along path k are reflected by it along path j to impinge upon mirror  20 .  
         [0043]     Aperture  21  is optically connected to polarizer  18  by optical path k and to a Faraday rotator  22  by optical path  1 . Aperture  21  selects the so-called TEM oo  mode (transverse excited mode) in the beam of photons passing therethrough in order to ensure the lowest possible divergence of the photon beam.  
         [0044]     Faraday rotator  22  is optically connected to aperture  21  by optical path l and to dichroic mirror  23  by optical path m. The Faraday rotator  22  is an irreversible optical element that rotates the linear polarization of the photons incident upon it by +45°. Thus, photons travelling in the forward direction along path l from aperture  21  to Faraday rotator  22 , exit from it along path m with their linear polarization rotated by +45°, to impinge upon dichroic mirror  23 . Photons travelling in the reverse direction along path m from dichroic mirror  23  to Faraday rotator  22 , exit from it along path l with their linear polarization increased by +45° to impinge upon aperture  21 .  
         [0045]     Dichroic mirror  23  is optically connected to Faraday rotator  22  by optical path m and a concave lens  24  by optical path n. It transmits photons incident upon it along path m to path n and vice versa. The dichroic mirror  23  also accepts incident photons  50  along optical path u and reflects them along path n, along the same path as photons incident upon dichroic mirror  23  along path m. Thus, photons  50  incident upon dichroic mirror  23  along path u as well as photons incident upon it along path m exit from it along path n to impinge upon lens  24 , while photons incident upon dichroic mirror  23  along path n are transmitted therethrough and exit from it along path m to impinge upon Faraday rotator  22 .  
         [0046]     Concave lens  24  is optically connected to dichroic mirror  23  by optical path n and to a convex lens  25  by optical path o. Convex lens  25  is optically connected to concave lens  24  by optical path o and to an objective lens  26  by optical path p. Concave lens  24  has a common optical axis with convex lens  25  and works in conjunction therewith to form a telescope to expand the beam diameter in the forward direction, for example, to 20 cm as shown in  FIG. 3 , to have a narrower beam waist in the focal region  29 , typically in the range of hundreds of microns, as shown in  FIG. 3 , to provide high spatial resolution. Those having ordinary skill in the art will readily recognize that the introduction of such a telescope contributes no new aspects to the inventive principle described herein, but rather assists in obtaining satisfactory results in the practical implementation of the inventive principle. In any event, in the forward direction, photons incident upon the telescope along path n exit from it along path o to impinge on objective lens  26 .  
         [0047]     Objective lens  26  is optically connected to convex lens  25  along optical path p and to a focal region  29  along optical path q. Objective lens  26  focuses the beam into focal region  29 . Preferably, the focal distance from the objective to the focal region  29  is long, typically in the range of tens of metres, as shown in  FIG. 3 . It is presumed that focal region  29  is positioned proximate to some surface  30  that will permit some back-scattering of the beam, which may be solid or liquid, also as shown in  FIG. 3 .  
         [0048]     Having now explained the components of the apparatus  10 , the manner in which an unknown substance located in a target region (not shown), proximate to the focal region  29 , may be analysed thereby to detect trace amounts of a substance or substances under investigation can now be understood.  
         [0049]     The apparatus  10  accepts as input, a sequence of both probing pulses  40  and excitation pulses  50 . The apparatus  10  delivers both pulses  40 ,  50  to the focal region  29  and returns the pulses  40  back to at least polarizer  14 . Focal region  29  is thought to contain a gaseous admixture including trace or higher concentrations of a substance to be detected (not shown) situated proximate to a target region (not shown) corresponding to vapours of a target medium under investigation.  
         [0050]     For reasons that will be described later, probing pulses  40  comprise two separate and alternating pulse trains, each of which is identical and initially linearly polarized. Pulses of the first pulse train will be denoted reference pulses R and pulses corresponding to the second pulse train will be denoted probing pulses P.  
         [0051]     For ease of reference in the following discussion, the linearly polarized components of reference pulse R will be denoted R 1  and R 2  respectively. Likewise, the linearly polarized components of probing pulse P will be denoted P 1  and P 2  respectively. To identify their polarization from time to time, the suffix (s) or (p) will be applied to such components as appropriate. As well, pulses and/or components traveling in the reverse direction will be so indicated by the adoption of a b  superscript.  
         [0052]     Probing pulses P follow their corresponding reference pulses R in time by a delay Δt P-R , which may be of a duration of at least twice that of the delay provided by the passage of p-polarized components along paths h-i-j relative to the passage of the corresponding s-polarized component along path g, an exemplary value of which, in the described embodiment, is 20 ns. Thus, a suitable value for Δt P-R  may be 40 ns, as shown in  FIG. 2A . Probing pulse P and the next reference pulse R are separated by a duration sufficient to permit processing of the p- and s-polarization components monitored at photodiodes  27 ,  28  of the previous pulse pair, which may, by way of example only, be 1 ms in the described embodiment, also as shown in  FIG. 2A .  
         [0053]     The optical beam path followed by each reference pulse R in the probing beam  40  will now be described. The R pulse encounters the optical apparatus  10  at polarizer  11 . The p-polarized component of the R pulse, R(p), passes entirely through polarizer  11 , while any s-polarized component is reflected off. Faraday rotator  12  and half wave plate  13  act so as to change the linear polarization of photons incident upon Faraday rotator  12  along path a in the forward direction by +90° so that R(s) is s-polarized when it encounters polarizer  14  along path c. Because R(s) is s-polarized, polarizer  14  reflects it along optical path d, whereupon it encounters mirror  15  and is reflected along path e through half wave plate  16  along path f Half wave plate  16  is oriented such that the linear polarization of pulses emerging from it along path f is oriented at +45°. Thus, upon exit from half wave plate  16 , reference pulse R has both s-polarized and p-polarized components of equal amplitude, denoted R 1 (p) and R 2 (s) respectively. R 1 (p) is transmitted by polarizer  17  along path g to polarizer  18 . On the other hand, R 2 (s) is reflected by polarizer  17  along path h and is reflected by mirrors  19  and  20  to polarizer  18  along paths i and j respectively. Mirrors  19  and  20  serve to delay in time the arrival at polarizer  18  of the s-polarized pulses R 2 (s) relative to their p-polarized counterparts R 1 (p), by a time interval Δt R2-1 , which in the described embodiment, may be 20 ns, as shown in  FIG. 2B .  
         [0054]     Component R 1 (p) is transmitted by polarizer  18  along path k through aperture  21  to Faraday rotator  22 . After a time interval Δt R2-1 , component R 2 (s) is reflected by polarizer  18  along path k through aperture  21  to Faraday rotator  22 . Faraday rotator  22  rotates the linear polarization of both components by +45°. R 1  and R 2  thereafter pass through dichroic mirror  23  and telescope lenses  24  and  25  along paths l through p respectively, whereupon they are focused into the focal region  29  by objective lens  26  along path q.  
         [0055]     The pulse components R 1  and R 2  are back-scattered by surface  30  proximate to focal region  29 . A small fraction of these back-scattered pulse components, denoted R 1   b  and R 2   b  respectively, is captured by objective lens  26  and sent in the reverse direction back through the telescope lenses  25  and  24  and dichroic mirror  23  to Faraday rotator  22  along paths q through m respectively, further rotating their linear polarization by +45°, or a total of 90° by the double passage through the Faraday rotator  22 . As a result, pulse component R 1   b (p) exits Faraday rotator  22  along path  1  as R 1   b (s). Accordingly, after passing through aperture  21  along path k, pulse component R 1   b (s) is reflected by polarizer  18  along path j and thereafter by mirrors  20  and  19  along paths i and h to polarizer  17 . At this point, R 1   b (s) has phase Φ R1 .  
         [0056]     In the same way, the pulse component R 2   b (s) exits Faraday rotator  22  along path l as R 2   b (p) and is transmitted through polarizer  18  along path g to polarizer  17 , having at that point, phase Φ R2 . Because Δt· R2-1  is sufficiently small, the index of refraction will not be changed during this interval so that Φ R2 =Φ R1 .  
         [0057]     Upon passing through polarizer  17  along path f, the delay introduced into R 2 (s) in the forward direction is compensated by introducing a corresponding delay into R 1   b (s) in the reverse direction, so that the two components are again simultaneous and thus coherently coupled back into a single reference pulse Rb, which is able to return along path f through the half wave plate  16 . This restores the polarization state of R b  along path e to that of R along path e in the forward direction. Accordingly, the return reference pulse R b  exits half wave plate  16  along path e with s-polarization only and is reflected by mirror  15  along path d to encounter polarizer  14 . R b  is only s-polarized, so it is reflected by polarizer  14  along path c to half wave plate  13 . As earlier indicated, in the reverse direction, the passage of a pulse through half wave plate  13  and Faraday rotator  12  does not affect the pulse&#39;s polarization because half wave plate  13  rotates the linear polarization by −45°, while Faraday rotator  12  rotates the linear polarization by +45°. Thus, when R b  emerges along path a from Faraday rotator  12 , it continues to have s-polarization. Accordingly, it is reflected by polarizer  11  along path s to photodiode  28 , which will detect its incidence.  
         [0058]     In the absence of an excitation pulse  50  or if there is no resonant absorption of an excitation pulse  50  in the focal region  29  as discussed below, the apparatus  10  affects the probing pulse P of the input beam  40  in similar fashion. Thus, the pulse component P 2 (s) will arrive at polarizer  18  a time interval Δt P2-1 , as shown in  FIG. 2B , after the pulse component P 1 (p) once it passes along paths h, i and j past mirrors  19  and  20 . As well, pulse component P 1   b (s) will arrive at polarizer  17  with phase Φ P1 , while pulse component P 2   b (P) will arrive at polarizer  17  with phase Φ P2 .  
         [0059]     The only difference between probing pulse P and reference pulse R is in the interposition, between the polarized probing pulse components P 1 (p) and P 2 (s), of an excitation or pumping pulse  50 . The excitation pulses E are powerful laser pulses at a tuned wavelength that corresponds to an absorption line of the substance(s) to be detected in the target region. Each excitation pulse E is directed at dichroic mirror  23  and reflected along path n, through telescope lenses  24 ,  25  along paths o and p respectively whereupon it is focused into focal region  29  by objective lens  26  along path q. As can be seen from  FIG. 2B , which shows the timing of the train of pulses as it passes along beam path n (on the way to the target region), the excitation pulse E is timed to pass between probing pulses P 1  and P 2 .  
         [0060]     Thus, if focal region  29  contains a resonantly absorbing substance, the refractive index of the focal region  29  will be changed due to heat deposited into it by the excitation pulse E. This change in the refractive index causes a change in the phase of the wave back-scattered by surface  30  and transmitted through the heated focal region  29  in the reverse direction. We note that while resonant absorption can induce change in the refractive index through a number of nonlinear optical mechanisms, the present invention exploits the thermooptical effect only.  
         [0061]     If there was no trace of the substance(s) to be detected in the focal region  29 , there would be no change in the refractive index of the focal region  29  as a result of the excitation pulse E, and Φ P2 =Φ P1  because, as shown in  FIG. 2B , Δt P2-1  is the same as Δt R2-1  and sufficiently small that the index of refraction will not be changed. Thus, the linear polarization of the back-scattered probing pulse P b  would be identical to that of the input linear polarized probing pulse P.  
         [0062]     However, if the gas admixture in the focal region  29  resonantly absorbs the excitation pulse E, signifying the presence of the substance(s) to be detected in the target, the phase Φ P2  for the back scattered probing pulse P 2   b  traveling through the focal waist would be different from that of the back scattered probing pulse P 1   b , namely Φ P1 . In such a situation, the optical paths for probing pulses P 1   b  and P 2   b  differ by 1·Δn, so that the phase shift between these two pulses at the exit of polarizer  17  in the reverse direction is defined by the relation: 
 
ΔΦ=Φ P2 −Φ P1 =(2·π/λ)(Δ n ·1)  (1) 
 
 where λ is the wavelength of the photons focused into the target region, l is the length of the beam waist and Δn is the change of the refractive index due to heating of the medium in the focal waist. 
 
         [0063]     If ΔΦ=0, signifying that there was no change in the refractive index as a result of the excitation pulse E, and further suggesting the absence of the substance(s) to be detected in the target medium, pulses P 1   b  and P 2   b , after being coherently combined by polarizer  17  as described above, would result in an s-polarized output probing pulse P b  along path e, after passing through half wave plate  16 . Thus, it would be eventually detected by photodiode  28  but not at photodiode  27 .  
         [0064]     On the other hand, if ΔΦ≠0, signifying that there was a change in the refractive index as a result of the excitation pulse E, and further suggesting the presence of the substance(s) to be detected in the target region, a p-polarization component would appear in the polarization of P b . Depending upon the absolute value of ΔΦ, a certain portion P prob , of output probing pulse P b  will be transmitted by polarizer  14  to photodiode  27 .  
         [0065]     Those having ordinary skill in this art will readily recognize that there will always be some depolarization of pulses while passing through optical components. Therefore, it is likely that there will be a p-polarization component P ref  of output reference pulse R b  that will be transmitted by polarizer  14  to photodiode  27  and thus give rise to a false positive reading. However, such depolarization should be the same for pulses traversing the same optical paths.  
         [0066]     Moreover, such false readings may be minimized by comparing the ratio between the s-polarization component S ref  and p-polarization component P ref  of the returned reference signal R b , which is known not to have had the imposition of any excitation pulse E, with the ratio between the s-polarization component the probing pulse S prob  and p-polarization component P prob  of the returned probing signal P b , which has.  
         [0067]     This comparison also obviates the necessity to measure the actual phase of the returned probing signal P b , which may not be trivial. Rather, the apparatus  10  is required only to process the amplitude of the linearly orthogonal polarization components of the returned probing signal P b  (and those of the returned reference signal R b  if optical depolarization is to be ruled out). The amplitude response is easily obtained and can be suitably amplified or attenuated by proper circuit design, such as would be known to a person of ordinary skill in this art.  
         [0068]     Those having ordinary skill in this art will readily recognize that the inventive features of such an apparatus are not technically restricted to operation with pulsed lasers. Indeed, the optical diagram of  FIG. 1  would be equally applicable in an embodiment in which laser emission  40  was not a train of pulses but a continuous wave (CW) laser photon stream.  FIG. 4A  shows the amplitude response as a function of time for the introduction of the excitation pulse E. Assuming that the laser emission  40  is a CW photon stream, one would expect a relatively constant amplitude response at both photodiodes  27  and  28 , irrespective of the interposition of any excitation pulses E, as shown in  FIGS. 4B and 4C  respectively, in the absence of the substance(s) under investigation in the target region.  
         [0069]     Where, however, a substance under investigation is present in the focal region  29 , which resonantly absorbs the excitation pulses E, one would expect a series of transient perturbations in the amplitude response over time of one or both of photodiodes  27  and  28 , as shown in  FIGS. 4D and 4E  respectively. Such transients will temporally correspond to the timing of the excitation pulses E and be delayed by a delay Δt CW . The magnitude and sign of such transient would be dependent upon the actual configuration of the electronics to detect, amplify and display the photodiodes&#39; electrical amplitude response, but the mere presence of such transients would provide a qualitative indication of the presence of the substance under investigation.  
         [0070]     Those having ordinary skill in the relevant art will also recognize that there may exist a mathematical or empirical relation that may allow these perturbations to be measured in order to generate a quantitative approximation of the quantity of the substance under investigation, but the development and explanation of such relations is beyond the scope of the present invention.  
         [0071]     It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiments disclosed herein, consistent with the present invention, without departing from the spirit and scope of the present invention.  
         [0072]     Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein.  
         [0073]     Accordingly, the specification and the embodiments are to be considered exemplary only, with a true scope and spirit of the invention being disclosed by the appended claims.