Abstract:
A multipass system for sampling by Raman spectroscopy enhances the collected signal and provides a system having improved sensitivity. The system incorporates an injection element for inserting collimated excitation radiation into an optical path, an objective lens for focusing the excitation radiation into the sample and for collecting radiation, a blocking filter that is substantially perpendicular to the optical path and that transmits Raman shifted radiation and reflects the excitation radiation, and a mirror for causing the excitation radiation to reflect excitation radiation back and forth between the mirror and the blocking filter multiple times while Raman shifted radiation is passed through the blocking filter for collection and analysis.

Description:
This application claims the benefit of provisional application serial No. 60/335,473, filed Nov. 1, 2001, the specification of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The instant invention lies generally in the field of spectroscopy, and specifically in the field of Raman scattering spectroscopy. 
     BACKGROUND 
     Molecular spectroscopy is a family of analytical techniques that provide information about molecular structure by studying the interaction of electromagnetic radiation with the materials of interest. In most of these techniques, the information is generally obtained by studying the absorption of radiation as a function of optical frequency. 
     Raman spectroscopy is unique in that it analyzes the radiation that is emitted (or scattered) when the sample is irradiated by an intense optical signal consisting of a single frequency, or a narrow range of frequencies. FIG. 1 is a simplified view of a typical application of Raman spectroscopy. As shown, a laser source  10  outputs an excitation beam  11  that intensely irradiates a sample that is contained within a sample cell or a flow cell  12 . Note that the laser&#39;s excitation radiation  11  is shown as a solid line. As further shown by dashed lines  13 , some Raman scattering of variously shifted wavelengths occurs due to the laser light&#39;s Raman interaction with the sample&#39;s molecular bonds. As shown by more solid lines  14 , however, a great deal more Rayleigh scattering occurs at the original laser frequency due to the laser&#39;s interaction with the atoms as opposed to with the bonds. In the typical system, a long pass laser rejection filter, or simply blocking filter  20 , blocks the Rayleigh scattering (solid lines  14 ) while passing the Raman scattering (dashed lines  13 ,  15 ) on to a spectrometer  31  for detection and analysis (typically with the assistance of a separate general purpose computer  32  as shown). 
     As described above, the “Raman scattering” signal is essentially an emission spectrum with frequency dependent intensities. The individual bands in this spectrum are shifted from the frequency of the excitation signal by amounts that are related to the structure of the molecules present in the sample. 
     Modern Raman spectrometers typically use powerful single wavelength lasers as the source of excitation radiation. Nonetheless, Raman scattering is an extremely rare event, so very little of the laser radiation is actually converted to Raman shifted energy. Most of it simply travels on through the sample without interaction, or is Rayleigh scattered by the sample without having its frequency altered. The weakness of the Raman signal is in part offset by the very high sensitivity of the visible and infrared detectors used. However, it is important to minimize the amount of reflected or Rayleigh scattered laser radiation that gets into the receiving optics since this can swamp the detector either on its own or by generating fluorescence and/or Raman radiation in the receiving optics. 
     Most sample interfacing systems currently being used in Raman spectroscopy employ optical filters to separate the excitation signal from the Raman shifted signal being studied. Some examples are given in I. R. Lewis &amp; P. R. Griffiths, “Raman Spectroscopy with Fiber-Optic Sampling”,  Applied Spectroscopy . Vol. 50, pg. 12A, 1996, FIGS. 5 through 11. (Also see U.S. Pat. Nos. 5,112,127 and 5,377,004.) In most of these designs, a dichroic filter inclined to the axis of the optical path combines the transmitted and received paths. In at least one case (FIG. 5, b  of Ref. 1), the transmitted and received paths are inclined at an angle to each other. 
     Mirrors have been proposed for enhancing Raman radiation signal levels in Raman Spectroscopy, but the enhancement provided by the embodiments known to the inventor do not provide as much gain as is possible with the present invention. In “Raman Spectroscopy for Chemical Analysis”, Wiley Interscience, New York, N.Y. 2000, pg. 121, for example, Richard L. McCreery reported an enhancement in the collected signal by providing two passes of excitation radiation by reflecting the excitation radiation back through the sample. The enhancement of signal levels is due to two factors: (1) the increased laser radiation intensity caused by folding the beam back through the sample; and (2) the fact that the mirror facilitates the collection of forward scattered Raman radiation as well as back scattered radiation. While signal levels have been improved by this method, the inventor has found that it is less than double. The enhancement is reduced by the fact that only half of the focal region is available for interaction of the beam with the sample. 
     There remains a need, therefore, for a Raman spectroscopy system that provides significantly increased Raman radiation signal levels beyond those currently available. 
     FIGS. 2-4 depict three different versions  50 ,  60 ,  70  of the RFP-400 Series Raman probes manufactured by the inventor and disclosed in the provisional application referred to above. As shown, all three probes  50 ,  60 ,  70  are based on a unique design in which the transmitted excitation beam  11  is injected along a path which is parallel to the received collection beam  15  by means of a small reflecting optical element such as a mirror or rhomboid  54 ,  64 ,  74 . As a result, the long pass filter  57 ,  67 ,  77  used to eliminate the laser signal from the receiving optics can be perpendicular to the path  15 . The illustrated probes  50 ,  60 ,  70  are not multi-pass probes. The characteristics of the RFP-400 design shown in FIGS. 2-4, however, pave the way for the unique family of enhancements that are the subject of this application. 
     SUMMARY OF THE INVENTION 
     The preferred embodiments of the present invention take advantage of the facts that: 
     1. The difference between the laser excitation frequency and the frequencies of the Raman shifted radiation; 
     2. The fact that Raman scattering events are rare whereby very little excitation radiation is lost on each pass through the sample; 
     3. The fact that most interference filters are highly reflecting at frequencies there they block the laser signal from being transmitted; and 
     4. The fact that many samples are highly transparent in the frequency range characteristic of the Raman shifted radiation. 
     Each of the embodiments includes the following elements: 
     1. The use of a laser frequency blocking filter approximately perpendicular to the axis of the optical system; 
     2. A lens or other focusing device positioned in the common transmitted and received optical path so as to focus the laser radiation into a small region within the sample and, at the same time, collect Raman scattered radiation from this region; 
     3. A mirror within or behind the sample to be analyzed positioned so as to reflect the transmitted laser signal back through the sample so that at least a portion of it strikes the blocking filter at approximately normal incidence; 
     4. A means for injecting the laser signal into the optical path between the blocking filter and the lens so as to obscure only a minor portion of the receiving optical path. 
     The invention may be regarded as a multi-pass Raman sampling system that illuminates a sample with laser excitation radiation to produce Raman shifted radiation, the system comprising: an injection element located in an optical path, the injection element obscuring only a portion of the optical path while injecting a substantially collimated beam of laser excitation radiation toward the sample in a first direction; an objective lens positioned in the optical path and defining an optical axis and a focal point, the objective lens focusing the collimated beam of laser excitation radiation traveling in the first direction into the sample to produce Raman shifted radiation, the objective lens collecting radiation that is emanating from at or near the focal point and then transmitting the radiation into the optical path in a second direction that is substantially opposite to the first direction; a mirror located within or behind the sample at or near the focal point, the mirror reflecting both laser excitation radiation and Raman shifted radiation back through the sample toward the objective lens and the injection element in the second opposite direction; a blocking filter located optically beyond the injection element such that the injection element is in the optical path between the objective lens and the blocking filter, the blocking filter being substantially reflective to the laser excitation radiation and substantially transparent to the Raman shifted radiation, the blocking filter passing the Raman shifted radiation out of the system for analysis and reflecting the laser excitation radiation back in the first direction toward the objective lens, the sample, and the mirror; and signal enhancement means for causing at least a portion of the laser excitation radiation that is reflected by the mirror to miss the injection element and strike the blocking filter where it is reflected back to the sample through the objective lens to produce more Raman shifted radiation. The just summarized invention is best understood with reference to the following drawings taken together with the accompanying description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified view of conventional Raman spectroscopy; 
     FIG. 2 is a sectional view of a first prior art Raman probe; 
     FIG. 3 is a sectional view of a second prior art Raman probe; 
     FIG. 4 is a sectional view of a third prior art Raman probe; 
     FIG. 5 is a diagrammatic view of a first preferred multi-pass Raman sampling system  100  constructed in accordance with this invention; 
     FIG. 6 is a diagrammatic view of a multi-pass Raman sampling system  200  that includes a slightly tilted mirror  240 ; 
     FIG. 6A is a cross-section of FIG. 6 taken along section lines A—A; FIG. 7 is a diagrammatic view of a multi-pass Raman sampling system  300  that includes an offset injection element  320 ; 
     FIG. 8 is a diagrammatic view of a multi-pass Raman sampling system  400  that includes an offset injection element  420  and a tilted blocking filter  450 ; 
     FIG. 8A shows the approximate intersections of ray paths  1 ,  2 ,  4  and  6  when FIG. 8 is viewed along section lines A—A; 
     FIG. 9 is a diagrammatic view of a multi-pass Raman sampling system  500  that includes an offset and titled injection element  520 ; 
     FIG. 10A is a diagrammatic view of a multi-pass Raman sampling system  600  that uses a long lightguide  631 ; 
     FIG. 10B is a diagrammatic section view taken along lines  10 B— 10 B showing how reflections from the inner walls of the light guide  631  will create a curved arc  632  of illumination; 
     FIG. 10C is a diagrammatic view corresponding to FIG. 10B showing how multiple passes will create an annular ring  633  of illumination, only a portion of which will be blocked by the injection element; 
     FIG. 11 is a diagrammatic view of a multi-pass Raman sampling system  700  that includes an offset injection element  720  in combination with a sample cell  760  in the form of a hollow light guide  761  with a window  762  and a perpendicular end mirror  740 ; 
     FIG. 12 is a diagrammatic view of a multi-pass Raman sampling system  800  that includes a sample cell  860  in the form of a hollow light guide  861  with a window  862  and a tilted end mirror  840 ; 
     FIG. 13 is a diagrammatic view of a multi-pass Raman sampling system  900  that includes a small diameter probe tip of the type illustrated in FIG. 3; 
     FIG. 14 is a diagrammatic view of a multi-pass Raman sampling system  1000  that includes a concave spherical mirror  1040 ; 
     FIG. 15 is a perspective view of a multi-pass Raman sampling system  1100  that use an RFP-420 Raman probe  60  as illustrated in FIG. 3, in combination with a sample cell  80  having a flow aperture  83  and a mirror  1140 ; 
     FIG. 16 is a cross-section of FIG. 15 taken from the side; and 
     FIG. 17 is a cross-section of FIG. 15 taken looking down. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 is a diagrammatic view of a first preferred multi-pass Raman sampling system  100  constructed in accordance with this invention. The illustrated system  100  uses the standard design of Axiom Analytical&#39;s Model RFP-410 probe (FIG.  2 ), and adds a flat mirror  140  located within or behind the sample and positioned approximately at the focal point of the lens  130 . As shown, the laser radiation  11  diverging from the end of an optical fiber is initially collimated by a collimating lens  110  and then passed through a bandpass filter  111 . The laser radiation  11  is then injected into the optical path with an injection element  120  (shown here as a pair of angled mirrors  121 ,  122 ) and finally focused into the sample flow  18  and onto the mirror  140  with an objective lens  130 . Raman shifted radiation and Raleigh scattered radiation will travel along the optical path and ultimately impinge upon the blocking filter  150 . The Raman radiation  15 , as suggested by FIG. 1, will pass through the blocking filter  150  and, thereafter, be focused into a fiber-optic cable with a lens  160 . 
     In the case where the laser radiation  11  is perfectly collimated and the mirror  140  is positioned exactly in the focus and perpendicular to the optical axis  101  (as shown in solid lines), the laser signal  11  reflected by the mirror  140  will be recollimated by the lens  130  and directed back to the optical injection element  120 . In this case, none of the laser radiation  11  reflected by the mirror  140  will reach the blocking filter  150 . 
     In practice, however, the laser beam  11  is not perfectly collimated, but instead has an inherent beam spread. Furthermore, various means for enhancing the signal may be employed, such as moving the mirror  140  slightly out of the focal plane of the lens  130  can increase the beam spread of the reflected laser signal. This is illustrated by the dashed lines in FIG.  5 . Optical beam spread, whether inherent or induced, will enable some portion of the reflected laser signal to be reflected by the blocking filter  150 . Once reflected, it will be refocused by the lens  130  into the sample  18 , where it can generate more Raman shifted radiation. 
     Depending on the dimensions of the various elements, some of this second pass laser radiation (shown with dashed lines for clarity) can again be directed back to the blocking filter where it can again be reflected and again be focused into the sample. With the configuration shown in FIG. 5, it is doubtful that much of the radiation will make more than three round-trip passes through the sample. 
     FIG. 6 provides a variation on the configuration of FIG.  5 . In both case, the injection elements  120 ,  220  are on the optical axis  101 ,  201 . In the embodiment of FIG. 6, however, a different enhancing means is employed. In particular, the mirror  240  is tilted slightly so that the returned signal misses the injection element  220  completely. In this case, nearly all of the laser signal  11  can make two round trip passes through the sample before returning to the injection element  220 . Again, slightly defocusing the position of the mirror will allow a portion of the returning beam to miss the injection element on this pass, allowing it to undergo additional passes through the sample. 
     In studying FIGS. 5 and 6, it should be noted that the presence of a mirror  140  or  240  at the focal point would lead to some enhancement of the Raman signal even if reflection from the blocking filter  150 ,  250  were not employed. In practice, however, it has been found that the enhancement obtained in this case is generally well under a factor of two. In contrast, the reflection from the blocking filter  150 ,  250  used in the preferred embodiments of this invention typically produces the unexpected results of enhancements ranging between factors of four and ten. 
     FIG. 7 is another variation on FIG.  5 . Here, the injection element  320  is offset from the optical axis  301 . In this case, it is not necessary to tilt the return mirror  340 . The first return pass (pass number  2 ) will be displaced on the opposite side of the axis  301  an equal amount and hence will not strike the injection element  320 . The second return pass (pass number 4) will again strike the injection element  320  if the laser signal  11  is perfectly collimated. 
     FIG. 8 illustrates an embodiment of a fourth system  400  that enables the laser signal  11  to make more than four passes through the sample without striking the injection element  450 . This embodiment is similar to the design illustrated by the system  300  of FIG. 7 in that both use a injection element that is offset from the optical axis. The FIG. 8 system  400 , however, further includes the concept of tilting the blocking filter  450 . The effect of tilting the blocking filter  450  can be best understood by referencing FIG. 6A which is a cross-section of FIG. 6 taken along section lines A—A in FIG.  6 . The FIG. 6A section shows the location of the first two passes in the region of this cross section where, as in FIG. 6, the blocking filter  25  is not tilted. With reference to FIGS. 6 and 6A, consider now the effect of tilting the blocking filter  450  ( 250 ) about the axis indicated by  8 — 8 . In such case, the projections of ray paths  1  through  6  in this plane of  8 - 8  appears as shown in FIG. 8 We can see that, for the dimensions illustrated, return path  4  falls closer to the axis than the injection element  420  and thus misses it. Like path  2 , path  6  will fall on the other side of the axis  401  from the injection element  420 . It is contemplated that the dimensions can be chosen so that even more passes will be possible without striking the injection element  420 . FIG. 8A shows the approximate intersections of ray paths  1 ,  2 ,  4  and  6  when FIG. 8 is viewed along section lines A—A. 
     The FIG. 8 design involve two regions of interaction between the radiation and the sample, one on-axis and one slightly displaced. Incident beams  1 ,  5 ,  9 , etc. will focus on-axis. Beams  3 ,  7 ,  11 , etc. will focus at a common point off-axis. In order for both of these regions to contribute to the received or collected signal, the separation between them must be smaller than the radius of the projection of the collection fiber in the sample plane. In order to accomplish this, it will be necessary for the distance between the lens and the injection element to be substantially greater than the focal length of the lens. Different ratios may be employed for varying degrees of benefit. However, locating the lens away from the injection element a distance of a factor of ten or greater relative to the focal length of the lens is believed reasonable for this purpose. 
     FIG. 9 illustrates a system  500  having a configuration in which the reflex mirror  540  and blocking filter  550  remain perpendicular to the central axis  501  of the system, but the injection element  520  is tilted slightly. When the injection element  520  is tilted in the same plane as its offset from the axis (as shown in the illustration), the fourth pass returns to approximately the same region as the insertion element  520 . In this case, tilting the element may not provide any benefit over the non-tilted case shown in FIG.  6 . However, if the injection element  520  is tilted about a different axis (such as one at 45 deg. to the offset plane), path  4  will return out of the offset plane and miss the injection element  520 . It thus can be seen that this configuration will provide similar benefits to that illustrated by FIG.  8 . 
     A lightguide may be desirable in certain circumstances. With any of the configurations discussed so far, as the distance between the injection element  120 - 520  and the objective lens  130 - 530  is increased, the inherent beam spread will result in some of the radiation  11  falling outside of the area of the objective lens  130 - 530 . This tendency can be reduced by placing a lightguide in the region between the injection element  120 - 520  and the objective lens  130 - 530 . This approach is employed in Axiom Analytical&#39;s longer probes, such as the RFP-480 illustrated in FIG.  3 . 
     FIG. 10A shows a system  600  that uses a long lightguide  631 . As suggested above, a lightguide in conjunction with a reflex mirror  640  can provide an additional benefit. In FIG. 10A, the injection element  620  is tilted. If the light guide  640  is long enough, the laser beam  11  emerging from the tilted element  620  will eventually strike the inside of the light guide  631  before striking the objective lens  630 . Since the light emerging from the injection element  620  is not perfectly collimated, but rather a diverging bundle, the image in the plane of the reflex mirror  630  will not be a point, but rather a spot. 
     In the absence of reflection within the light guide  631 , the spot would be a round spot of finite area. However, after reflection from the inner walls of the light guide  631 , the spot will tend to form a curved arc  632  with the center of curvature on the system central axis  601  as shown in FIG.  10 B. Tilting the injection element  420  out of the plane of offset will accentuate this effect by creating a higher percentage of skew rays. The net effect, after several passes, will be an annular ring or circle  633  of illumination centered on the axis  601  as shown in FIG.  10 C. Significantly, only a small percentage of the rays which give rise to this circle  633  will intersect the position of the injection element  620 . 
     FIGS. 11 and 12 show two embodiments  700 ,  800  in which the subject invention is combined with a sample cell  760 ,  860  in the form of a hollow light guide  761 ,  861  with a window  762 ,  862  on one end and a mirror  740 ,  840  on the other. The cell  760 ,  860  would generally be provided with flow fittings (not shown) to allow continuous flow of a liquid or gas. In FIG. 11, the injection element  720  is mounted off of the central axis  701  and is furthermore to be tilted in a plane other than the offset plane. As a result, the excitation radiation will be in the form of a bundle of skew rays. After a few reflections from the walls and reflection from the end mirror  740 ,  840 , the radiation will be in the form of an annular bundle of rays similar to that shown in FIG.  10 C. Most of this bundle arrays will miss the injection element  720 ,  820 . 
     In FIG. 11, the injection element  720  is placed on axis. In this case, we can rely on the inherent spread of the beam to fill the light guide  760 , resulting in a larger bundle of rays on return, some percentage of which will miss the injection element  720 . Alternatively, we can either slightly taper the lightguide  760  to provide a tapered sample cell, or we can tilt the end mirror  740  (as shown). In either case, the divergence of the excitation beam will be increased, resulting in much of the beam missing the injection element  720 . 
     From the two examples of the sample cells  760 ,  860  of FIGS. 11 and 12, it can be seen that most of the techniques discussed above can be combined with a lightguide type sample cell  760 ,  860  to provide multiple passes through the sample and enhanced sensitivity. 
     FIG. 13 shows a system  900  that a light guide  960  for a small diameter probe tip of the type illustrated in FIG. 3 above. In this embodiment, the purpose of the light guide  960  is not to contain the sample, but rather to confine the radiation to a small diameter region. The ultimate purpose is to provide a small diameter probe tip with a high ratio of length to diameter. The simplest embodiment would simply use the standard RFP-420 probe (FIG. 3) with a reflex mirror placed in or behind the sample (see e.g. the embodiment of FIGS. 15-17 discussed below). The position of the mirror  940  can be slightly defocused to increase the divergence of the reflected radiation, insuring that some of it misses the injection element  920 . Alternatively, the embodiment of FIG. 13 can include a converging lens  930  to aid in injecting the excitation beam into the small diameter light guide  960 . Any of the embodiments discussed above can also be combined with the small diameter light guide  960  as long as the physical parameters of the device can be met in manufacturing. 
     FIG. 14 shows one further embodiment of an alterative system  1000 . In the embodiments discussed above, a plane mirror  140 - 940  is generally placed in or near the focal plane of the objective lens  130 - 930 . In many cases, however, the plane mirror can be replaced by a concave spherical mirror  1040  with its center of curvature at or near the focal point of the lens  1030 . This has the advantage of allowing Raman scattered radiation to be collected from regions on both forward and rearward sides of the focal point  1031 . 
     In the plane mirror case of FIG. 7, offsetting the injection element  320  to one side allowed for four passes through the sample before the excitation beam returns to and rejoins the injection element. However, a concave mirror  1040  of FIG. 14, has the disadvantage of directing the first reflected or returned path beam toward the injection element  1020  as shown in FIG.  14 . Of course, the various techniques discussed above can be used in conjunction with this embodiment to insure that at least part of the reflected signal misses the injection element  1020 . 
     FIGS. 15-17 illustrate a particular application of a system  1100  wherein a RFP-420 Raman probe  60 , as illustrated in FIG. 3, is combined with a sample cell  80  having a flow aperture  83 . The flow cell  80  is connected to the flanges  82  of process or process development pipe  84 . As with many of the above-described embodiments, a mirror  1040  is provided for creating a multi-pass Raman sampling system. As can be appreciated from FIG. 15, with the probe  60  and the mirror  1040  positioned correctly (note the arrows suggesting that it may be moved in and out of the focal plane), the excitation beam will make multiple passes through the passing sample and provide an enhanced Raman signal. 
     While the embodiments above have been described with respect to particular details shown in the figures, it is to be explicitly understood that there are many additional variations within the spirit and scope of the instant invention. Therefore, the invention is to be limited only by the following claims.