Abstract:
Raman spectroscopic analyzer including: a beam-casting unit  3  for receiving a light beam generated by a light source and for converging the light beam on a predetermined position in a perpendicular direction to the longitudinal direction of a measurement chamber through which a liquid sample is passed; and a light-receiving unit placed at a distance in the longitudinal direction from the predetermined position, for receiving scattered light emitted from the fluid sample. Among the scattered light which enters the light-receiving unit, the portion which enters this unit after being reflected by the inner wall surface opposite to this unit is eliminated, so that the amount of noise in the Raman spectroscopic measurement is considerably reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2014/066019, filed Jun. 17, 2014, claiming priority based on Japanese Patent Application No. 2013-145360, filed Jul. 11, 2013, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a Raman spectroscopic analyzer for analyzing a component contained in a fluid sample (gas or liquid sample) using Raman-scattered light emitted from the sample. 
     BACKGROUND ART 
     An apparatus for analyzing a component in a sample by performing a Raman spectroscopic measurement normally includes a light source for generating the light to be cast into the sample (excitation light), an entrance optical system for converging the excitation light and casting it into the sample, a dispersing optical system for collecting the Raman-scattered light resulting from an interaction with a substance in the sample and dispersing the light into a spectrum, as well as a detector for detecting the component wavelengths of the light dispersed by the dispersing optical system. 
     If the intensity of the light from the sample is plotted on a graph with the wavelength as the abscissa and the intensity as the coordinate, a Raman scattering spectrum is obtained on both sides of the wavelength of the excitation light. The lines at the longer wavelengths are called the Stokes lines, and those at the shorter wavelengths are called the anti-Stokes lines. 
     The amount of energy corresponding to the difference between the wavelength of the excitation light and that of a Stokes or anti-Stokes line reflects the amount of energy of the natural vibration of a molecule. Accordingly, by calculating this amount of energy, it is possible to identify a substance in the sample. Furthermore, based on the intensity of each Stokes or anti-Stokes line appearing in the Raman scattering spectrum, the quantity of the substance corresponding to that Stokes or anti-Stokes line can be determined. 
     Patent Literature 1 discloses a gas component analyzer which identifies the components in a gas generated within a gasifying furnace and measures the concentration of each component by performing a Raman spectroscopic measurement. 
       FIG. 1  shows the configuration of the main components of this apparatus. In this gas component analyzer  100 , laser light is cast into a stream of gas flowing in the direction perpendicular to the drawing within a tubular sample-passing unit  110  provided within the area surrounded by the broken line in a measurement chamber  115 , and the Raman-scattered light emitted from the gas is measured. The laser light generated by a laser-casting device  114  controlled by a controller  137  is guided through a first optical fiber  120  into a light-casting means  116  and is converged on a predetermined position within the measurement chamber  115  by a lens  125  provided in the light-casting means  116 . After passing through the gas, the laser light is disposed of at a damper  128 . 
     Additionally, a detection optical system is provided on the wall of the measurement chamber  115  located in a direction perpendicular to the path of the laser light with respect to the aforementioned predetermined position. Among the Raman-scattered light emitted from the gas illuminated with the excitation laser light, the portion of light emitted in the perpendicular direction to the path of the laser light passes through alight-passing window  129  and is converged on a light-receiving unit  132  by a condensing lens  130 . The light incident on the light-receiving unit  132  is guided through a second optical fiber  121  and is separated into component wavelengths by a light-dispersing device  135 , to be eventually detected by a CCD camera  136 . 
     CITATION LIST 
     Patent Literature 
     Patent Literature JP 2011-80768 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the gas component analyzer described in Patent Literature 1, the laser light which has passed through the gas is absorbed by the damper  128 . However, until it arrives at this damper, the light is scattered by the substances present on the optical path (e.g. the lens  125  of the light-casting means  116  or the gas present on the light path), and a portion of this scattered light is directed toward the light-passing window  129 . Furthermore, the light scattered on the optical path is additionally reflected or scattered by the inner wall surface of the measurement chamber  115  and eventually falls onto the light-passing window  129 . These forms of scattered light are also received by the light-receiving unit  132  together with the Raman-scattered light emitted from the gas. 
     In general, the Raman-scattered light emitted from gas has low intensities. Therefore, the scattered light which additionally originates from the excitation laser light enters the light-receiving unit  132  together with the Raman-scattered light constitutes a noise factor in the Raman spectroscopic analysis, which eventually deteriorates the accuracy in the identification of the components in the sample gas as well as the determination of the concentration of each component. 
     Although the previously described example is concerned with the case where the sample is in a gas form, a similar problem also occurs in the case of a measurement of the Raman-scattered light emitted from a liquid sample passing through the measurement chamber. 
     The problem to be solved by the present invention is to reduce the amount of noise resulting from the scattering of the excitation light in a Raman spectroscopic analyzer which detects Raman-scattered light emitted from a sample passing through a measurement chamber. 
     Solution To Problem 
     The Raman spectroscopic analyzer according to the present invention developed for solving the previously described problem includes: 
     a) a sample-passing unit in the form of a tube, for passing a fluid sample in a longitudinal direction of the tube; 
     b) a beam-casting unit for converging a light beam generated by a light source on a predetermined position within the sample-passing unit; and 
     c) a light-receiving unit placed at a distance in the longitudinal direction from the predetermined position, including a light-receiving lens for receiving scattered light emitted from the fluid sample. 
     In a preferable mode of this analyzer, no inner wall surface of the sample-passing unit is present opposite to the light-receiving lens across the predetermined position. “No inner wall surface is present” means that no inner wall surface is present within a predetermined range of distances where the stray light caused by the inner wall surface can enter the light-receiving lens; it does not mean that no inner wall surface should be present over an infinitely long distance. In other words, there is no problem having an inner wall surface at a distance where the stray light caused by the inner wall surface does not affect the light-receiving lens. 
     In the Raman spectroscopic analyzer having the previously described configuration, the light beam generated from the light source converges on the predetermined position within the sample-passing unit and is cast into the fluid sample passing through the sample-passing unit. When the light beam is cast from the light source, scattered light is generated from the sample. Among the light scattered in the longitudinal direction of the sample-passing unit, the light scattered on the side where the light-receiving unit is placed is received by this light-receiving unit, and a Raman scattering spectrum is created by an appropriate analyzing system. Furthermore, the kinds and quantities of the substances contained in the sample are determined by an analysis of the Raman scattering spectrum. 
     In the Raman spectroscopic analyzer according to the present invention, the light-receiving unit is placed within the sample-passing unit, at a distance in the longitudinal direction of the sample-passing unit from the predetermined position where the light beam cast into the sample converges. In other words, the visual field of the light-receiving unit is directed in the longitudinal direction, with the predetermined position at its center. Therefore, if a portion of the light beam generated from the light source is scattered and directed onto the inner wall surface of the sample-passing unit, the thereby reflected light is prevented from directly entering the light-receiving unit. Thus, in the Raman spectroscopic analyzer according to the present invention, among the scattered light which enters the light-receiving unit, the portion which enters this unit after being reflected by the inner wall surface opposite to this unit is eliminated, so that the amount of noise which occurs in the Raman spectroscopic measurement due to the scattering of the excitation light is considerably reduced. 
     If the beam-casting unit and the light-receiving unit are independently constructed, the relative position of the beam-casting unit and the light-receiving unit easily changes, which causes a misalignment of the visual field of the light-receiving unit from the converging position of the incident beam and consequently lowers the detection efficiency of the Raman-scattered light. 
     Accordingly, the beam-casting unit and the light-receiving unit should preferably be integrally constructed. This construction prevents the unwanted change in the relative position of those units. 
     As for the arrangement of the beam-casting unit and the light-receiving unit in the Raman spectroscopic measurement for a fluid sample passing through a pipe, there are two possible forms, as shown in  FIGS. 2A and 2B . 
     In the first form, as shown in  FIG. 2A , the optical axis of the excitation light cast from the beam-casting unit  201  onto the predetermined position  202  of the sample fluid within the pipe  204  (excitation light axis) orthogonally intersects with that of the scattered light directed from the predetermined position  202  toward the light-receiving unit  203  (light-receiving axis). In the second form, as shown in  FIG. 29 , the excitation light axis and the light-receiving axis are coaxially arranged. 
     If the sample-passing unit is at a certain distance from the light source, a light guide (e.g. optical fiber) for propagating the light beam generated from the light source may be used. In the Raman scattering, in addition to the forward or backward scattering along the excitation light axis, the scattering of light also occurs in the direction perpendicular to the direction of polarization of the light cast into the sample. Commonly used optical fibers cannot maintain the direction of polarization of the propagated light, so that a variation occurs in the direction in which the Raman-scattered light is generated. 
     Accordingly, in the case where the excitation light axis orthogonally intersects with the light-receiving axis as shown in  FIG. 2A , if a light guide is used in the beam-casting unit, a polarization-maintaining light guide (e.g. polarization-maintaining optical fiber) should preferably be used. By this system, the Raman-scattered light can be detected with a consistent intensity. 
     In the case where the excitation light axis and the light-receiving axis are coaxially arranged as shown in  FIG. 2B , even if a common type of light guide (e.g. multimode optical fiber, including the one used for energy transmission) is used, the Raman-scattered light can be received with the same degree of consistency as in the case where the polarization-maintaining light guide is used. 
     The polarization-maintaining light guide should preferably maintain the plane of polarization of the light beam in a perpendicular direction to the longitudinal direction. By this configuration, the Raman-scattered light is generated in the direction where the light-receiving unit is placed, so that the detection intensity of the Raman-scattered light is increased. 
     Advantageous Effects of the Invention 
     In the Raman spectroscopic analyzer according to the present invention, if a portion of the light beam generated from the light source is scattered and directed onto the inner wall surface of the sample-passing unit, that light is prevented from directly entering the light-receiving unit, since the light-receiving unit is placed within the sample-passing unit and at a distance in the longitudinal direction of the sample-passing unit from the converging position of the incident beam. Therefore, the amount of noise which occurs due to the scattering of the excitation light is considerably reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating the schematic configuration of a conventional gas component analyzer. 
         FIGS. 2A and 2B  are schematic arrangement diagrams illustrating two examples of the arrangement of the beam-casting unit and the light-receiving unit. 
         FIG. 3  is a schematic configuration diagram of a Raman spectroscopic analyzer as the first embodiment of the present invention. 
         FIG. 4  is a schematic configuration diagram of a Raman spectroscopic analyzer as the second embodiment of the present invention. 
         FIG. 5  is a schematic configuration diagram of a Raman spectroscopic analyzer as the third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A Raman spectroscopic analyzer as the first embodiment of the present invention is described with reference to  FIG. 3 . The Raman spectroscopic analyzer  300  of the first embodiment includes, a laser light source  306  for supplying the excitation light, a pipe  313  (sample-passing unit) for passing a fluid sample, a spectrometer  310  for detecting the scattered light from the fluid sample, a beam-casting unit and a light-receiving unit. The Raman spectroscopic analyzer  300  of the first embodiment is an example of the arrangement in which the excitation light axis orthogonally intersects with the light-receiving axis. 
     As the laser light source  306 , a laser which generates visible light is used. For example, a solid laser (e.g. YAG laser or YVO 4  laser) or gas laser (e.g. Ar laser) can be used. 
     The excitation light generated from the laser light source  306  has a specific plane of polarization, such as the linear polarization. This light is introduced into the pipe  313  throb an optical fiber  301  connected to this pipe  313  by a connector  302 . Then, the excitation light is collimated by a first entrance lens  303  and is converged into a central region of a Raman-scattered light measurement area  315  (this region is called the “predetermined position”) by a second entrance lens  311 . The Raman-scattered light measurement area  315  is located near the central axis of the tubular pipe  313 . The excitation light is converged from a direction (y direction) perpendicular to the longitudinal direction (x direction) of the pipe  313  into the central region of the Raman-scattered light measurement area  315 . The optical fiber  301 , first entrance lens  303  and second entrance lens  311  constitute the beam-casting unit. Although the present description deals with the case where the excitation light generated from the laser light source  306  has a specific plane of polarization (e.g. linear polarization), the present embodiment is not limited to this case; it is possible to use a light source that generates light with no specific plane of polarization. 
     After passing through the Raman-scattered light measurement area  315 , the excitation light is trapped by a beam trap  307  and thereby absorbed. This prevents the excitation light from being directed onto the inner wall surface of the pipe  313 . 
     The Raman-scattered light generated within the Raman-scattered light measurement area  315  by the excitation light cast into the fluid sample passing through the pipe  313  in the longitudinal direction of this pipe is collimated parallel to the longitudinal direction (x direction) of the pipe  313  by a light-receiving lens  308  placed next to the predetermined position of the Raman-scattered light measurement area  315  at distance d in the longitudinal direction (x direction) of the pipe  313 . The collimated beam is redirected by a mirror  309  into the perpendicular direction (y direction) to the longitudinal direction (x direction). 
     Since the light-receiving lens  308  is placed in the longitudinal direction (x direction) of the pipe  313 , no inner wall surface of the pipe  313  is present opposite to the light-receiving lens  308  across the predetermined position of the Raman-scattered light measurement area  315 . Therefore, the stray light from the inner wall surface of the pipe  313  is prevented from entering the visual field of the light-receiving lens  308  to be received. 
     Subsequently, the Raman-scattered is introduced through a converging lens  312  into a fiber bundle  305  connected to the pipe  313  by a connector  304 . The fiber bundle  305  comprises a bundle of optical fibers. These optical fibers are arranged in a rectangular form with the longer side extending in the x direction, which form corresponds to a rectangular area whose longer side extends in the y direction in the Raman-scattered light measurement  315 . The light-receiving lens  308 , mirror  309 , converging lens  312  and fiber bundle  305  constitute the light-receiving unit. 
     The fiber bundle  305  can receive light from multiple points within the rectangular area in the Raman-scattered light measurement  315 . The use of the fiber bundle  305  enables an efficient reception of the faint Raman-scattered light. In other words, the light-receiving unit detects the scattered light originating from the Raman-scattered light measurement area  315  (which is a rectangular area whose center lies on the predetermined position, with the longer side extending in the perpendicular direction to the longitudinal direction of the sample-passing unit). 
     The Raman-scattered light is extracted through the fiber bundle  305  to the outside of the pipe  313  and enters the spectrometer  310 . In the spectrometer  310 , the detected Raman-scattered light is separated into component wavelengths, and a wavelength dispersion spectrum of the Raman scattering is obtained. Using this spectrum, the Raman spectroscopic analysis of the fluid sample can be performed. 
     The optical fiber  301 , first entrance lens  303  and second entrance lens  311  constituting the beam-casting unit are connected to the pipe  313  by the connector  302 , while the light-receiving lens  308 , mirror  309 , converging lens  312  and fiber bundle  305  constituting the light-receiving unit are connected to the pipe  313  by the connector  304 , with the beam-casting unit and the light-receiving unit sharing a flat plate  314  in the outer wall surface of the pipe  313 . Since the beam-casting unit and the light-receiving unit are integrally fixed to the same flat plate  314  in the outer wall surface of the pipe  313 , an unwanted change in the relative position of the beam-casting unit and the light-receiving unit does not easily occur. Thus, a Raman spectroscopic analyzer which is highly resistant to vibration and allows for a low frequency of maintenance is obtained. 
     With the Raman spectroscopic analyzer according to the present invention, a polarized Raman spectroscopic analysis can be performed. In general, Raman-scattered light is generated in the perpendicular direction to the polarizing direction. Therefore, in the case of a system configured to detect side-scattered light as shown in  FIG. 3 , if the plane of polarization of the excitation light is not controlled, the detection intensity varies depending on the polarizing direction of the excitation light converging on the Raman-scattered light detection area  315 . To overcome this problem, a polarization-maintaining fiber capable of maintaining the polarizing direction of the light can be used as the optical fiber  301  which propagates the excitation light. In this case, the excitation light having a specific plane of polarization (e.g. linear polarization) generated from the laser light source  306  maintains the same plane of polarization before and after passing through the optical fiber  301  until it is converged on the Raman-scattered light detection area  315 . By aligning this plane of polarization parallel to the light-receiving lens  308 , the Raman-scattered light generated in the perpendicular direction to the direction of polarization can be consistently received. 
     A Raman spectroscopic analyzer as the second embodiment of the present invention is described with reference to  FIG. 4 . The Raman spectroscopic analyzer  400  of the second embodiment includes a laser light source  406  for supplying the excitation light, a pipe  413  (sample-passing unit) for passing a fluid sample, a spectrometer  410  for detecting the scattered light from the fluid sample, a beam-casting unit and a light-receiving unit. The Raman spectroscopic analyzer  400  of the second embodiment is an example of the arrangement in which the excitation light axis orthogonally intersects with the light-receiving axis. 
     As the laser light source  406 , the same type as the laser light source  306  is used. 
     The excitation light generated from the laser light source  406  enters the pipe  413  through a window  402 . Then, this excitation light is converged by a condensing lens  411  into a central region (predetermined position) of a Raman-scattered light measurement area  415 . The Raman-scattered light measurement area  415  is located near the central axis of the tubular pipe  413 . The excitation light is converged from a direction (y direction) perpendicular to the longitudinal direction (x direction) of the pipe  413  into the Raman-scattered light measurement area  415 . The condensing lens  411  constitutes the beam-casting unit. 
     After passing through the Raman-scattered light measurement a  415 , the excitation light is trapped by a beam trap  407  so as to prevent the excitation light from being directed onto the inner wall surface of the pipe  413 . 
     The Raman-scattered light generated from the fluid sample passing through the pipe  413  within the Raman-scattered light measurement area  415  is collimated parallel to the longitudinal direction (x direction) of the pipe  413  by a light-receiving lens  408  placed next to the Raman-scattered light measurement area  415  at distance d in the longitudinal direction (x direction) of the pipe  413 . The collimated beam is redirected by a mirror  409  into the perpendicular direction (y direction) to the longitudinal direction (x direction). The redirected Raman-scatter light is extracted through a window  405  to the outside of the pipe  413  and enters the spectrometer  410  through a converging lens  412 . The light-receiving lens  408 , mirror  409  and converging lens  412  constitute the light-receiving unit. 
     In the spectrometer  410 , the detected Raman-scattered light is separated into component wavelengths, and a wavelength dispersion spectrum of the Raman-scattered light is obtained. Using this spectrum, the Raman spectroscopic analysis of the fluid sample can be performed. 
     The system of the second embodiment is configured so that the Raman-scattered light generated in the Raman-scattered light measurement area  415  is directly introduced into the spectrometer  410 . Unlike the system using a common type of optical fiber through which only a faint Raman-scattered light originating from a single point within the Raman-scattered light measurement area  415  is received, the present system can receive the Raman-scattered light from the entire area and efficiently detect the faint Raman-scattered light. 
     A Raman spectroscopic analyzer as the third embodiment of the present invention is described with reference to  FIG. 5 . The Raman spectroscopic analyzer  500  of the third embodiment includes: a laser light source  506  for supplying the excitation light; an optical fiber  501 ; an illumination beam-converging optical system  507  for converging the excitation light into a fluid sample; a reflecting optical system  505  including a reflection mirror  503  for reflecting the excitation light; a pipe  513  (sample-passing unit) for passing a fluid sample; a detection light-converging optical system  504  for converging the Raman-scattered light, comprising a collimating unit (light-receiving lens)  504 A or  504 C and a converging unit  504 B; and a spectrometer  510  for detecting the scattered light from the fluid sample. The Raman spectroscopic analyzer  500  of the third embodiment is an example in which the excitation light axis and the light-receiving axis are coaxially arranged. 
     The pipe (sample-passing unit)  513  shown in  FIG. 5  is L-shaped, with a window  502  provided in the bend section at such a position where the line of sight through the window is aligned with the longitudinal direction (x direction) of the tubular body of the pipe (sample-passing unit). By using such a pipe, a coaxial arrangement of the excitation light axis and the light-receiving axis can be realized. 
     As the laser light source  506 , the same type as the laser light source  306  is used. 
     The excitation light generated from the laser light source  506  enters the pipe  513  through the window  502 . Then, this excitation light is converged by the illumination beam-converging optical system  507  into a central region (predetermined position) in a Raman-scattered light measurement area  515 . The Raman-scattered light measurement area  515  is located near the central axis of the tubular pipe  513 . The excitation light is converged from the longitudinal direction (x direction) of the pipe  513  into the Raman-scattered light measurement area  515 . 
     Among the Raman-scattered light generated from the fluid sample passing through the pipe  513  within the Raman-scattered light measurement area  515 , the back-scattered component passes through the window  502 , the detection light-converging optical system  504  comprising the collimating unit (light-receiving lens)  504 A or  504 C and the converging unit  504 B, as well as the optical fiber  501 , to eventually enter the spectrometer  510 . 
     In the spectrometer  510 , the detected Raman-scattered light is separated into component wavelengths, and a wavelength dispersion spectrum of the Raman-scattered is obtained. Using this spectrum, the Raman spectroscopic analysis of the fluid sample can be performed. 
     The system of the third embodiment is configured so that the back-scattered component of the Raman-scattered light generated in the Raman-scattered light measurement area  515  is made to enter the spectrometer  510 . The excitation light axis and the light-receiving axis are coaxially arranged. Such an arrangement can also satisfy the condition that no inner wall surface of the pipe  513  is present opposite to the detection light-converging optical system  504  across the predetermined position in the Raman-scattered light measurement area  515 . Therefore, the stray light from the inner wall surface of the pipe  513  is prevented from entering the detection light-converging optical system  504  to be received. 
     REFERENCE SIGNS LIST 
     
         
           100  . . . Gas Component Analyzer 
           110  . . . Sample-Passing Unit 
           114  . . . Laser-Casting Device 
           115  . . . Measurement Chamber 
           116  . . . Light-Casting Means 
           120  . . . First Optical Fiber 
           121  . . . Second Optical Fiber 
           125  . . . Lens 
           128  . . . Damper 
           129  . . . Light-Passing Window 
           130  . . . Condensing Lens 
           132  . . . Light-Receiving Unit 
           135  . . . Spectrometer 
           136  . . . CCD Camera 
           137  . . . Controller 
           201  . . . Beam-Casting Unit 
           202  . . . Predetermined Position 
           203  . . . Light-Receiving Unit 
           300 ,  400 ,  500  . . . Raman Spectroscopic Analyzer 
           301 ,  501  . . . Optical Fiber 
           302 ,  304  . . . Connector 
           303 ,  308 ,  311 ,  312 ,  408 ,  411 ,  412  . . . Lens 
           305  . . . Fiber Bundle 
           306 ,  406 ,  506  . . . Laser Light Source 
           307 ,  407  . . . Beam Trap 
           309 ,  409 ,  503  . . . Mirror 
           310   410 ,  510  . . . Spectrometer 
           313 ,  413 ,  513  . . . Pipe 
           314  . . . Flat Plate 
           315 ,  415 ,  515  . . . Raman-Scattered Light Measurement Area 
           402 ,  405 ,  502  . . . Window 
           504 A,  504 C . . . Collimating Unit 
           504   505 ,  507  . . . Optical System 
           504 B . . . Converging Unit