Abstract:
The present invention includes: an excitation light source; a probe light source; a filter that mutually multiplexes a probe light emitted from the probe light source and an excitation light emitted from the excitation light source to a same optical axis; a condenser lens that focuses the excitation light and the probe light; a sample cell that stores a sample; a reflection member that is disposed on an inner wall of the sample cell and reflects the probe light; and a detector that detects the probe light reflected at the reflection member.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese Patent application serial No. 2014-254662, filed on Dec. 17, 2014, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a photothermal conversion spectroscopic analyzer, and particularly relates to the photothermal conversion spectroscopic analyzer using a thermal lens effect. 
         [0004]    2. Description of the Related Art 
         [0005]    When a sample is irradiated with a focused light, the sample absorbs the light and temperature rises locally. With this temperature rise, a refraction index of the sample is changed. In many substances, there is an optical effect such as a concave lens generated near a light focusing portion of the sample because the refraction index is decreased due to temperature rise. This effect is generally called a thermal lens effect, and sample measurement utilizing this effect is used as a high-sensitivity measuring method for a non-fluorescent substance. 
         [0006]      FIG. 6A  is a schematic configuration diagram illustrating an exemplary related art in which a photothermal conversion spectroscopic analyzer  302  uses a transmission optical system using the above-described thermal lens effect (JP-2005-164614-A). In  FIG. 6A , an excitation light L 1  emitted from an excitation light source  101  and a probe light L 2  emitted from a probe light source  102  are multiplexed on a same optical axis at a first filter  103 , and the multiplexed light enters a sample  109  contained in a sample cell  107  after having passed through a first condenser lens  106 , such as an objective lens. The excitation light L 1  has a characteristic to be partly absorbed in the sample  109 . Therefore, the excitation light L 1  focused in the sample  109  by the first condenser lens  106  locally heats the sample  109  in a light focusing area. Then, the excitation light L 1  is absorbed at a third filter  110 . On the other hand, a probe light L 2  passing through the optical axis same as the excitation light L 1  has a spreading angle of a light beam irradiated to a detector  111  expanded by a thermal lens generated at the light focusing portion of the excitation light L 1  in the sample  109 . A change amount of the spreading angle of the probe light L 2  is proportional to concentration of the sample  109 , and an incident light amount with respect to the detector  111  is changed. Therefore, the sample concentration can be measured from a detection signal of the detector  111 . 
         [0007]      FIG. 6B  is a schematic configuration diagram illustrating an exemplary related art in which a photothermal conversion spectroscopic analyzer  303  uses a reflection optical system (JP-04-369467-A). 
         [0008]    Proposed is the photothermal conversion spectroscopic analyzer  303  using the reflection optical system in which a laser beam having passed through a sample is reflected at a reflector and made to enter a light focusing optical system, and then analysis is performed by utilizing the reflected light. According to JP-04-369467-A, a single beam method in which a probe light L 2  is also used as an excitation light is adopted, and the probe light L 2  is focused by a first condenser lens  106  to a reflector  113  disposed outside the sample  109  and the sample cell  107 . The probe light L 2  reflected at the reflector  113  is guided by a filter  121  toward a detector  124  formed of a second condenser lens  118 , a cylindrical lens  122 , and a light receiving element  123 , and then a thermal lens signal is measured. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the photothermal conversion spectroscopic analyzing method disclosed in JP-2005-164614-A, there is a problem in which an entire optical system is large-sized. 
         [0010]    On the other hand, according to the reflection optical system in JP-04-369467-A, the optical system can be downsized by integrating the optical system on one side of the sample  109 . Further, since the probe light L 2  reciprocates in the sample  109 , higher sensitivity can be achieved compared to the transmission optical system. 
         [0011]    Meanwhile, the excitation light L 1  and the probe light L 2  are slightly absorbed in a component other than the sample  109  where these lights pass through, thereby deteriorating optical properties and causing noise of a signal. Influence therefrom is largely given to the sample cell  107  where the excitation light L 1  and the probe light L 2  that are focused by the first condenser lens  106  and have high light density pass through. To achieve high-sensitivity analysis for the sample  109 , it is important to have a short optical path length that passes through the sample cell  107 . 
         [0012]    However, the probe light L 2  having passed through the sample cell  107  is reflected at the reflector  113  and passes through the sample cell  107  again. Therefore, there may be a problem in which noise of the signal is doubled compared to the invention disclosed in JP-2005-164614-A using the transmission optical system. 
         [0013]    The present invention is made in view of the above-described problems in the related arts, and directed to providing a reflection-type photothermal conversion spectroscopic analyzer capable of performing high-sensitivity sample analysis. 
         [0014]    To achieve the above object, a photothermal conversion spectroscopic analyzer according to the present invention includes: an excitation light source; a probe light source; a first filter configured to mutually multiplex a probe light emitted from the probe light source and an excitation light emitted from the excitation light source to a same optical axis; a first condenser lens configured to focus the excitation light and the probe light; a sample cell configured to store a sample; a reflection member disposed on an inner wall of the sample cell irradiated with the probe light having passed through the sample, and configured to reflect the probe light; and a detector configured to detect the probe light reflected at the reflection member, wherein the sample contacts the reflection member. 
         [0015]    To achieve the above object, a photothermal conversion spectroscopic analyzer according to the present invention includes: an excitation light source; a probe light source; a first filter configured to mutually multiplex a probe light emitted from the probe light source and an excitation light emitted from the excitation light source to a same optical axis; a second filter configured to divide the probe light into a first probe light and a second probe light; a reflector configured to reflect one of the divided probe lights; a first condenser lens configured to focus the other one of the divided probe lights and the excitation light; a sample cell configured to store a sample; a reflection member disposed on an inner wall of the sample cell irradiated with the probe light having passed through the sample, and configured to reflect the probe light; and a detector configured to detect a probe light obtained by multiplexing the first probe light reflected at the reflector and the second probe light reflected at the reflection member, wherein the sample contacts the reflection member. 
         [0016]    To achieve the above object, preferably, an excitation light irradiation adjustment unit to adjust both or one of a position and a diameter of the excitation light irradiated to the sample is provided. 
         [0017]    To achieve the above object, preferably, a probe light irradiation adjustment unit to adjust both or one of a position and a diameter of the probe light irradiated to the sample is provided. 
         [0018]    According to the present invention, size reduction and high sensitivity can be achieved in the photothermal conversion spectroscopic analyzer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a diagram illustrating a photothermal conversion spectroscopic analyzer  300  according to a first embodiment; 
           [0020]      FIG. 2  is an explanatory diagram for a light beam of a probe light when a thermal lens is generated inside a sample; 
           [0021]      FIG. 3  is a diagram illustrating a relation between a position of a reflection member and a light amount of the probe light detected by a detector; 
           [0022]      FIG. 4  is a configuration diagram illustrating a photothermal conversion spectroscopic analyzer  301  according to a second embodiment; 
           [0023]      FIG. 5  is a schematic diagram illustrating wavefronts of a probe light L 2   a  and a probe light L 2   b  entering a filter; and 
           [0024]      FIGS. 6A and 6B  are schematic configuration diagrams illustrating a conventional photothermal conversion spectroscopic analyzer using a thermal lens effect. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Embodiments of the present invention will be described below with reference to the accompanying drawings. 
       First Embodiment 
       [0026]      FIG. 1  is a diagram illustrating a photothermal conversion spectroscopic analyzer  300  according to a first embodiment of the present invention. 
         [0027]    An excitation light L 1  emitted from an excitation light source  101  passes through a first filter  103 , and a probe light L 2  emitted from a probe light source  102  is reflected at the first filter  103 . Then, these lights are multiplexed on a same optical axis and the multiplexed light passes through a second filter  104  and a first quarter wavelength plate  105 . 
         [0028]    The excitation light L 1  having passed through the first quarter wavelength plate  105  is focused by a first condenser lens  106  in a sample  109  inside a sample cell  107 , and then reflected at a reflection member  108 . Inside the sample  109 , a so-called thermal lens is formed based on a photothermal conversion phenomenon in which the excitation light L 1  is partly absorbed and heat is generated. The excitation light L 1  reflected at the reflection member  108  passes through the first condenser lens  106  and the first quarter wavelength plate  105 . After that, the excitation light L 1  is subsequently reflected at a second filter  104  and absorbed by a third filter  110 . In the drawing, a direction corresponding to an optical axis direction of the first condenser lens  106  and also a light traveling direction from the first condenser lens  106  to the sample  109  is set as an X-axis direction. A direction corresponding to a direction vertical to the X-axis and also a traveling direction of the excitation light L 1  after being reflected at the second filter  104  is set as a Y-axis direction. 
         [0029]      FIG. 2  is an explanatory diagram for a light beam of the probe light L 2  when the thermal lens is generated inside the sample  109 . 
         [0030]      FIG. 2  illustrates a trajectory of the light beam of the probe light L 2  in the case where a light focusing position of the probe light L 2  coincides with the reflection member before the thermal lens is generated. Note that the trajectory of the light beam changed by the thermal lens generation is indicated by dotted lines. 
         [0031]    As illustrated in  FIG. 2 , the probe light L 2  having passed through the second filter  104  and the first quarter wavelength plate  105  is focused by the first condenser lens  106 , and then irradiated to the sample  109 . 
         [0032]    In the case where the thermal lens is formed by the excitation light L 1  inside the sample  109 , a light focusing angle of the probe light L 2  having passed through the thermal lens becomes large as illustrated by the light beam of the dotted lines in the drawing. In other words, a light flux of the probe light L 2  having passed through the thermal lens broadens. The probe light L 2  reflected at the reflection member  108  passes through the first condenser lens  106  again, passes through the first quarter wavelength plate  105  as a weak divergent light, and is reflected at the second filter  104 . The probe light L 2  reflected at the second filter  104  passes through the third filter  110  and is focused by a second condenser lens  118 . A pinhole  115  is disposed at a focusing point of the probe light L 2  when the thermal lens is not generated. Therefore, a light flux of the probe light L 2  having passed through the second condenser lens  118  as the weak divergent light is not sufficiently focused at the pinhole  115  position, and the light flux is partly lost at the pinhole  115  and then irradiated onto a light receiving element  116 . In other words, a light amount of the probe light L 2  that passes through the pinhole  115  and is detected by the light receiving element  116  is changed proportional to a light amount of the excitation light L 1  absorbed in the sample  109 . 
         [0033]    A current signal of the probe light L 2  received by the detector  111  is converted to a voltage signal at a current/voltage conversion circuit  201 , and received in a lock-in amplifier  203 , and then measured together with a reference signal from a drive circuit  202  that controls a light amount of the excitation light L 1  output from the excitation light source  101 . A signal indicating a measurement result output from the lock-in amplifier  203  is received in a computer  205 , and the sample  109  is analyzed. 
         [0034]    Meanwhile, in the case where the excitation light source  101  is a light source that cannot perform modulation at a high speed such as gas laser, a device such as a chopper that can modulate, at a high speed, an optical intensity of the excitation light L 1  output from the excitation light source  101  may be disposed between the excitation light source  101  and the first filter  103 . At this point, a reference signal from a chopper control device is used in the lock-in amplifier  203  instead of the reference signal from the drive circuit  202 . 
         [0035]    Meanwhile, the reflection member  108  disposed on an inner wall of the sample cell  107  contacts the sample  109  in the first embodiment. Therefore, the excitation light L 1  reciprocates only in a surface of the sample cell  107  close to the first condenser lens  106  in a component where the excitation light L 1  focused by the first condenser lens  106  passes through. Therefore, compared to the conventional reflection optical system in which the excitation light L 1  passes through two surfaces of the sample cell  107  in a reciprocating manner, deterioration of optical properties caused by absorption of the excitation light L 1  by the sample cell  107  can be suppressed while keeping a characteristic of being capable of performing high-sensitivity analysis for the sample  109 . 
         [0036]    In the photothermal conversion spectroscopic analyzer  300  using the reflection optical system, when the focusing position of the probe light L 2  and the position of the reflection member  108  are changed in the optical direction (X-axis), a signal detected by the detector  111  is deteriorated. Therefore, in the case of using a detachable type sample cell  107 , for example, readjustment of the sample cell  107  and the focal position of the probe light L 2  is needed when the sample cell  107  is replaced. 
         [0037]    A method of adjusting the position of the sample cell  107  in the optical axis direction in the photothermal conversion spectroscopic analyzer  300  according to the present embodiment will be described. 
         [0038]      FIG. 3  is a diagram illustrating a relation between the position of the reflection member  108  and a light amount of the probe light L 2  detected by a detector  111 . 
         [0039]    When there is the pinhole  115  to focus the probe light L 2  by the second condenser lens  118 , in the case where the position of the reflection member  108  of the sample cell  107  is deviated in either positive or negative optical axis (X-axis) direction from a focusing position X 0  of the probe light L 2 , a focusing angle/divergent angle of the probe light L 2  that has been reflected at the reflection member  108  and passed through the first condenser lens  106  again is changed, and a light flux of the probe light L 2  irradiated onto the pinhole  115  is broadened. As a result, the light amount of the probe light L 2  detected at the light receiving element  116  is reduced. More specifically, when the sample cell  107  is replaced, signal deterioration can be suppressed by moving the sample cell  107  and the reflection member  108  with a stage  117  to positions in the optical axis (X-axis) direction at which the light amount of the probe light L 2  detected at the detector  111  becomes maximum. 
         [0040]    Further, the detector  111  is not limited to a general detector formed of only the pinhole  115  and the light receiving element  116 , and may also be a detector adopting an astigmatic method, a knife edge method, and the like. 
       Second Embodiment 
       [0041]    Next, a second embodiment will be described. 
         [0042]      FIG. 4  is a configuration diagram illustrating a photothermal conversion spectroscopic analyzer  301  according to the second embodiment. The basic configuration is same as a first embodiment. However, a configuration different from the first embodiment is that a probe light L 2  emitted from a probe light source  102  is divided into two at a second filter  104  on the way of being emitted to a sample  109 , and the divided lights are multiplexed again on a same optical axis. In the following, the points different from the first embodiment in a sample analysis method using the photothermal conversion spectroscopic analyzer  301  according to the second embodiment will be described. 
         [0043]    The probe light L 2  emitted from the probe light source  102  is reflected at a first filter  103  and then divided into a probe light L 2   a  and a probe light L 2   b  at the second filter  104 . 
         [0044]    The probe light L 2   a  passes through a second quarter wavelength plate  112  in a reciprocating manner while the probe light L 2   a  reflected at the second filter  104  is reflected at a reflector  113  and returns to the second filter  104  again, thereby rotating a polarization plane thereof by 90 degrees. Therefore, the probe light L 2   a  subsequently passes through the second filter  104 . 
         [0045]    The probe light L 2   b  having passed through the second filter  104  is reflected at a reflection member  108  same as the first embodiment, and after that, the probe light L 2   b  is reflected at the second filter  104  and directed to a detector  111 . The probe light L 2   a  reflected at the reflector  113  and the probe light L 2   b  reflected at the reflection member  108  are multiplexed on the same optical axis at the second filter  104 . 
         [0046]    Meanwhile, according to the second embodiment, the probe light L 2   a  reflected at the reflector  113  and the probe light L 2   b  reflected at the reflection member  108  are multiplexed at the second filter  104  to interfere with each other. In order to make the probe light L 2   a  and the probe light L 2   b  favorably interfere with each other at the position of the second filter  104 , preferably a wavefront of the probe light L 2   b  is formed same as a wavefront of the probe light L 2   a  in a state that no thermal lens is generated. More specifically, the wavefront of the probe light L 2   b  can be formed same as a plane wave of the probe light L 2   a  at the position of the second filter  104  by matching a focusing position of the probe light L 2   b  to be focused at a first condenser lens  106  with the position of the reflection member  108  in an X-axis direction. 
         [0047]      FIG. 5  illustrates Y-axis cross-sectional wavefronts of the probe light L 2   a  having passed through a polarization filter  119  and the probe light L 2   b.    
         [0048]    The probe light L 2   a  having passed the polarization filter  119  and the probe light L 2   b  have the same polarization plane and have coherency. In an initial state in which no thermal lens is generated inside the sample  109 , adjustment is made such that the wavefronts of the probe light L 2   a  having passed through the polarization filter  119  and the probe light L 2   b  become same by moving the reflector  113  in the optical axis (Y-axis) direction with a stage  120 . In the case where no thermal lens is generated inside the sample  109 , both the probe light L 2   a  and the probe light L 2   b  are parallel lights. Therefore, the wavefronts of the probe light L 2   a  having passed through the polarization filter and the probe light L 2   b  are constant and have high coherency, and coherent light intensities are strong. 
         [0049]    However, as illustrated in  FIG. 2 , when the thermal lens acting as a concave lens is generated inside the sample  109  located in the middle of an optical path, the probe light L 2   b  having a focal point connected to the reflection member  108  inside the sample cell  107  has the focal position moved in a negative X-direction. As a result, the probe light L 2   b  that has been reflected at the reflection member  108  and passed through the first condenser lens  106  (indicated by dotted lines in  FIG. 2 ) is returned to a first quarter wavelength plate  105  as a divergent light, and the wavefront thereof has a phase varied by a distance from a center of the optical axis. Therefore, the coherent light intensities of the probe light L 2   a  having passed through the polarization filter and the probe light L 2   b  are varied by the distance from the center of the optical axis, and a light amount of an entire light flux is reduced. Then, the probe light L 2   a  having passed through the polarization filter  119  and the probe light L 2   b  are detected by the detector  111  in the same manner as the first embodiment. 
         [0050]    According to the present embodiment, change of a receiving light amount due to change of a focusing angle/divergent angle of the probe light L 2  is detected by the detector  111  same as the first embodiment, but in the present embodiment, the light amount change caused by the phase change is further superimposed. Therefore, there is a characteristic in which the light amount change detected by the detector  111  is increased when the thermal lens is generated inside the sample  109 . 
       REFERENCE SIGNS LIST 
       [0000]    
       
           101  Excitation Light Source 
           102  Probe Light Source 
           103  First Filter 
           104  Second Filter 
           105  First Quarter Wavelength Plate 
           106  first Condenser Lens 
           107  Sample Cell 
           108  Reflection Member 
           109  Sample 
           110  Third Filter 
           111  Detector 
           112  Second Quarter Wavelength Plate 
           113  Reflector 
           115  Pinhole 
           116  Light Receiving Element 
           117  Stage 
           118  Second Condenser Lens 
           119  Polarization Filter 
           120  Stage 
           121  Filter 
           122  Cylindrical Lens 
           123  Light Receiving Element 
           124  Detector 
           201  Current/Voltage Conversion Circuit 
           202  Drive Circuit For Excitation Light Source 
           203  Lock-In Amplifier 
           204  Drive Circuit For Probe Light Source 
           205  Computer 
         L 1  Excitation Light 
         L 2  Probe Light 
           300 - 303  Photothermal Conversion Spectroscopic Analyzer