JP6035913B2 - Concentration measuring device and concentration measuring method - Google Patents

Description

  The present invention relates to a concentration measuring apparatus and a concentration measuring method for measuring a concentration of a substance by a differential absorption method using laser light.

  As one of methods for measuring the concentration of a measurement object such as carbon dioxide gas or methane gas, a differential absorption method using laser light is known. In this method, the concentration of the measurement object is based on the transmittance of the laser light having a wavelength that is absorbed by the measurement object (that is, the ON wavelength) and the laser light having a wavelength that is not absorbed by the measurement object (that is, the OFF wavelength). Is calculated. In most cases, the wavelength of the laser light applied to the measurement object is in the infrared region due to the use of absorption. For example, laser light having a wavelength of about 2 μm or 4 μm is used for measuring the concentration of carbon dioxide gas.

  In recent years, exhaust gas regulations have become stricter. In particular, it is a very important issue to suppress the discharge amount of carbon dioxide gas or the like from a factory or the like which is a large-scale emission source. Therefore, it is conceivable to regularly observe the concentration of gas discharged from exhaust facilities such as exhaust ducts and chimneys. However, the exhaust port is usually provided at a high place where it is difficult to enter, and the concentration of harmful substances may be high. Therefore, it is not rational to install the measuring device in the vicinity of the exhaust port, so that a high-intensity laser beam is irradiated toward the exhaust port from a distance, and the concentration of the measurement target is calculated from the intensity of the scattered light. A method can be considered. This is a kind of LIDAR.

JP 2001-325069 A

  As described above, since the wavelength of the laser light used for the differential absorption method is in the infrared region, human beings cannot see it. Therefore, in order to specify the measurement region from a distance, it is conceivable to irradiate visible laser light in parallel with the infrared laser and specify the measurement region by visual observation of the scattered light. However, when the measurement region is at a distance of about several hundred m to several km from the laser light source, it is difficult to specify the scattered light itself. Moreover, it is dangerous because the possibility that a high-intensity laser beam will hit a human is increased. Therefore, it is conceivable to separately provide an optical system for observing the irradiation region of the infrared laser with visible light. Although the apparatus of Patent Document 1 is not a concentration measuring apparatus, it has a configuration in which a CCD camera is installed on the extension of the optical path of the infrared laser light and the irradiation area of the infrared laser can be easily confirmed visually.

  In view of the above situation, an object of the present invention is to provide a concentration measurement device and a concentration measurement method having a function of safely and reliably specifying a measurement object when measuring the concentration of a distant measurement object. .

  A first aspect of the present invention is a concentration measurement apparatus capable of obtaining an irradiation region of probe light as a real image, a laser light source that generates laser light as excitation light, and wavelength conversion of the excitation light, A probe light generation unit that generates on-wavelength and off-wavelength probe light with respect to the measurement object, a first mirror that can be adjusted in angle with respect to the probe light, the first mirror, and the probe light generation And a second mirror that reflects one of the probe light and visible light and transmits the other, and the visible light emitted from the second mirror, and An imaging device for imaging an irradiation region of probe light, a photodetector for detecting the probe light transmitted through or reflected from the measurement object, and the measurement pair based on the intensity of the probe light And summarized in that a control unit for calculating the density of the object.

  The concentration measuring apparatus may further include an optical system for condensing the probe light from the measurement object on the photodetector. In this case, the optical axis of the optical system is substantially parallel to the traveling direction of the probe light emitted from the first mirror toward the measurement object.

  According to a second aspect of the present invention, there is provided a concentration measuring method capable of obtaining an irradiation region of probe light as a real image, wherein the traveling directions of probe light having an on wavelength and an off wavelength with respect to the measurement object are directed toward the measurement object. The visible light from the measurement object traveling in the direction opposite to the traveling direction of the probe light is allowed to pass on the optical path of the probe light, and only the visible light is reflected or transmitted on the optical path. The probe light irradiation region is picked up using the visible light, the probe light transmitted through the measurement object or reflected from the measurement object is detected, and the measurement is performed from the intensity of the probe light. The gist is to calculate the concentration of the object.

  ADVANTAGE OF THE INVENTION According to this invention, when measuring the density | concentration of a distant measurement target object, the density | concentration measuring apparatus and density | concentration measuring method which have the function which can specify the said measurement target object safely and reliably can be provided.

It is a block diagram of the density | concentration measuring apparatus which concerns on one Embodiment of this invention. It is a block diagram of the probe light generation part which concerns on one Embodiment of this invention. It is a block diagram of the probe light generation part which concerns on one Embodiment of this invention, and is a modification of FIG. It is a schematic diagram which shows the relationship between each wavelength of the probe light and reference light which concern on one Embodiment of this invention, and the wavelength of the absorption line of a measuring object.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the common part in each figure, and the overlapping description is abbreviate | omitted.

  FIG. 1 is a configuration diagram of a concentration measuring apparatus according to the present embodiment. FIG. 2 is a configuration diagram of the probe light generator according to the present embodiment. FIG. 3 is a modification of the probe light generator shown in FIG. FIG. 4 is a schematic diagram showing the relationship between the wavelengths of the probe light and the reference light according to this embodiment and the wavelength of the absorption line of the measurement object. As shown in FIG. 1, the concentration measuring apparatus according to the present embodiment includes a laser light source 22, a probe light generator 24, an optical filter 27, a mirror (second mirror) 29, and a mirror (first mirror). 31, a photodetector 26, a control unit (density calculation unit) 28, and an imaging device 35.

  The laser light source 22 generates laser light as excitation light (pump light) 23 input to the probe light generation unit 24 at the subsequent stage. The wavelength and oscillation mode (pulse oscillation or continuous oscillation) of the laser light are selected according to the wavelength conversion specifications (conversion method, output wavelength, etc.) in the probe light generator 24. In this embodiment, an Nd: YAG laser that is a pulse laser light source is used. The Nd: YAG laser outputs a fundamental laser beam of 1064 nm with a pulse width of several ns to several tens of ns and a repetition frequency of 10 Hz to several kHz.

  The probe light generator 24 generates the on-wavelength probe light 10 and the off-wavelength probe light 12 (see FIG. 4) for the measurement object by wavelength conversion of the excitation light 23. Hereinafter, for convenience of explanation, the off-wavelength probe light 12 may be simply referred to as reference light 12. From the viewpoint of increasing the sensitivity of absorption, it is preferable that the wavelength λon of the probe light 10 coincides with the wavelength of the absorption line 14 of the measurement object S, as shown in FIG. However, absorption can be confirmed if at least the wavelength of the absorption line 14 is included in the line width of the probe light 10.

  As shown in FIG. 2, the probe light generator 24 includes a terminal mirror 32 and an output mirror 34 that are arranged along the optical axis (optical path) 20 so that the reflecting surfaces thereof face each other. The distance D between the output mirror 34 and the terminal mirror 32 is, for example, 20 mm. Further, on the optical axis 20 between the terminal mirror 32 and the output mirror 34, a nonlinear optical crystal 36 is provided as an optical element for performing wavelength conversion. As will be described later, the nonlinear optical crystal 36 generates the probe light 10 and the reference light 12 by optical parametric oscillation of the excitation light 23.

  The terminal mirror 32 has a wavelength characteristic that transmits the excitation light 23 and reflects the probe light 10 and the reference light 12 generated by the nonlinear optical crystal 36. Since the wavelength of the excitation light 23 is usually shorter than the wavelengths of the probe light 10 and the reference light 12, the terminal mirror 32 is a so-called long pass filter (LPF). On the other hand, the output mirror 34 also has a wavelength characteristic that reflects the probe light 10 and the reference light 12, similarly to the terminal mirror 32. Accordingly, the terminal mirror 32 and the output mirror 34 constitute a so-called optical resonator. The reflectances of the terminal mirror 32 and the output mirror 34 with respect to the probe light 10 and the reference light 12 are 50 to 99.5%.

  The nonlinear optical crystal 36 is, for example, a KTP crystal or a BBO crystal, and generates on-wavelength probe light 10 and off-wavelength reference light 12 by optical parametric oscillation by the excitation light 23. The center wavelength λon of the probe light 10 is, for example, 2004 nm, and the center wavelength λoff of the reference light 12 is, for example, 1993 nm. The wavelength of light generated by the nonlinear optical crystal 36 can be changed as appropriate by adjusting the angle θ of the optical axis 36a of the crystal with respect to the optical axis of the excitation light 23. Therefore, the nonlinear optical crystal 36 of the present embodiment is mounted on the rotary stage 38 so that the angle θ can be adjusted. In other words, the probe beam 10 and the reference beam 12 are alternately emitted from the output mirror 34 and irradiated onto the measurement object S by repeating the rotation and reverse rotation of the rotation stage 38 at a predetermined cycle, for example. The rotation of the rotary stage 38 is controlled by a control unit (not shown).

Note that the probe light generator of this embodiment can be modified as follows. A probe light generator 25 shown in FIG. 3 is a modification of the probe light generator 24 shown in FIG. In the probe light generator 24 of FIG. 2, a nonlinear optical crystal 36 is used as an optical element for performing wavelength conversion. On the other hand, the probe light generator 25 in FIG. 3 uses a laser crystal 46 as an optical element for wavelength conversion. Examples of the laser crystal 46 include Tm: YAG, Tm: YLF, Tm: YVO ₄ , Tm, Ho: YAG, Tm, Ho: YLF, Tm, Ho: YVO ₄ and the like. When any of these is used for the laser crystal 46, a semiconductor laser (LD) is used as a laser light source for generating the excitation light 23. The semiconductor laser generates light having a central wavelength of, for example, 785 nm as the excitation light 23. The light emitted from the semiconductor laser is condensed on the laser crystal 46 by an optical system 48 such as a lens in order to increase the conversion efficiency in the laser crystal 46.

  As shown in FIG. 3, a wavelength adjusting mechanism 42 that selects the wavelength of light emitted from the laser crystal 46 is installed between the emission side of the laser crystal 46 and the output mirror 34. The wavelength adjustment mechanism 42 is, for example, an etalon or a prism, and can select the wavelength of light traveling to the output mirror 34 by the rotation of the rotary stage 44 on which the wavelength adjustment mechanism 42 is mounted. That is, the probe light 10 and the reference light 12 can be alternately emitted by repeating the rotation / reverse rotation of the rotary stage 44 at a predetermined cycle, for example.

  The probe light 10 and the reference light 12 emitted from the probe light generator 24 (25) pass through the optical filter 27 and enter the mirror 29. The optical filter 27 is an infrared transmission / visible light absorption filter that transmits the probe light 10 and the reference light 12 and that the visible laser beam generated by the probe light generator 24 (25) travels downstream. To prevent. Note that this may be omitted when no visible laser beam is generated.

  The mirror 29 is a dichroic mirror that reflects any one of the probe light 10 and the reference light 12 and visible light and transmits the other. The mirror 29 is installed between the rear-stage mirror 31 and the probe light generator 24 (25). As shown in FIG. 1, the mirror 29 reflects the probe light 10 and the reference light 12 toward the total reflection mirror 31, transmits the visible light reflected from the total reflection mirror 31 as it is, and transmits the visible light to the imaging device. Lead to 35. In other words, only visible light is extracted by reflection or transmission on the optical path of the probe light 10 or the like, and the visible light is guided to the imaging device 35.

  The mirror 31 is a total reflection mirror such as an aluminum coating mirror, and reflects all of the probe light 10, the reference light 12, and the visible light. That is, the mirror 31 deflects the traveling directions of the on-wavelength and off-wavelength probe lights 10 and 12 toward the measuring object S, and travels the probe lights 10 and 12 on the optical path of the probe lights 10 and 12. Visible light from the measuring object S traveling in a direction opposite to the direction is allowed to pass. The mirror 31 is installed in a biaxial angle adjustment mechanism 33 that can adjust the angle (reflection angle) with respect to these lights, and the irradiation position can be manipulated two-dimensionally toward the measuring object S. The angle adjustment mechanism 33 is controlled by the control unit 28.

  An imaging device 35 is installed on the side opposite to the side where the mirror 31 is provided across the mirror 29. The imaging device 35 is a CCD camera, for example, and images the irradiation areas of the probe lights 10 and 12 using visible light emitted from the mirror 29. Between the measuring object S and the mirror 29, the probe lights 10 and 12 and the visible light are located on the same optical path. Therefore, when the optical axis of the imaging device 35 is aligned with the optical axis of visible light, the irradiation positions of the probe lights 10 and 12 are located at the center of the visual field. That is, the irradiation position can be easily selected and specified. Further, since irradiation with high-intensity light such as visible laser light is not necessary, the irradiation position can be selected and specified safely.

  The light detector 26 detects the probe light 10 and the reference light 12 transmitted through the measurement object S or reflected from the measurement object S. In the present embodiment, a known semiconductor detector is used as the photodetector 26. The semiconductor detector outputs a voltage proportional to the light intensity as a detection signal. In addition, an optical system 30 such as a lens for condensing the probe light 10 and the reference light 12 is provided in the previous stage of the photodetector 26, and the light condensing rate is improved.

  The optical axis 30a of the optical system 30 of the present embodiment is substantially parallel to the traveling direction of the probe lights 10 and 12 emitted from the mirror 31 toward the measurement object S. The concentration measuring apparatus of the present embodiment assumes a distant object to be measured, and sets the focal length of the optical system 30 as large as possible. In this case, the deviation of the probe lights 10 and 12 from the visual field of the optical system 30 due to the change in the irradiation position of the probe lights 10 and 12 can be suppressed as much as possible.

  The control unit 28 controls the entire concentration measuring device. Further, the control unit 28 calculates the concentration of the measurement object S from each transmittance (absorbance) of the probe light 10 and the reference light 12 detected by the photodetector 26. Specifically, the control unit 28 calculates the transmittance (first transmittance) of the reference light 12 from the intensity of the reference light 12 that has passed through the measurement object S. The control unit 28 further calculates the transmittance (second transmittance) of the probe light 10 from the intensity of the probe light 10 that has passed through the measurement object S. In consideration of the fact that the second transmittance is obtained by multiplying the first transmittance by the transmittance due to the absorption of the measuring object S (third transmittance), the control unit 28 determines that the first transmittance is the first transmittance. Then, the third transmittance is calculated backward from the second transmittance. As a result, the concentration of the measuring object S is calculated from the third transmittance.

  In the present embodiment, carbon dioxide gas is used as the measurement object. However, the measurement object to which the present invention is applied is not limited to this, and can be applied to other types of gases. Moreover, it is applicable also to phases (namely, liquid and solid) other than gas.

  Further, the present invention is not limited to the above-described embodiment, is shown by the description of the scope of claims, and further includes all modifications within the meaning and scope equivalent to the description of the scope of claims.

  DESCRIPTION OF SYMBOLS 10 ... Probe light, 12 ... Reference light (probe light), 14 ... Absorption line, 20 ... Optical axis, 22 ... Laser light source, 23 ... Excitation light, 24, 25 ... Probe light generation part, 26 ... Photo detector, 27 DESCRIPTION OF SYMBOLS ... Optical filter, 28 ... Control part, 29 ... Mirror (2nd mirror), 30 ... Optical system, 31 ... Mirror (1st mirror), 32 ... Terminal mirror, 34 ... Output mirror, 35 ... Imaging device, 36 ... Non-linear optical crystal 36a ... Optical axis 38 ... Rotary stage 42 ... Wavelength adjustment mechanism 44 ... Rotary stage 46 ... Laser crystal 48 ... Optical system

Claims (3)

A concentration measuring device capable of obtaining an irradiation region of probe light as a real image,
A laser light source that generates laser light as excitation light;
A probe light generator for generating on-wavelength and off-wavelength probe light for the measurement object by wavelength conversion of the excitation light;
A first mirror installed to be adjustable in angle with respect to the probe light;
A second mirror that is installed between the first mirror and the probe light generator, reflects one of the probe light and visible light, and transmits the other;
An imaging device that images the irradiation region of the probe light using the visible light emitted from the second mirror, and a photodetector that detects the probe light transmitted through the measurement object or reflected from the measurement object When,
And a control unit that calculates the concentration of the measurement object from the intensity of the probe light. An optical system for condensing the probe light from the measurement object on the photodetector;
The concentration measuring apparatus according to claim 1, wherein an optical axis of the optical system is parallel to a traveling direction of the probe light emitted from the first mirror toward the measurement object. A concentration measurement method capable of obtaining an irradiation region of probe light as a real image,
Deflecting the traveling direction of on-wavelength and off-wavelength probe light toward the measurement object toward the measurement object;
Visible light from the measurement object traveling in the direction opposite to the traveling direction of the probe light is passed on the optical path of the probe light,
Taking out only the visible light by reflection or transmission on the optical path and imaging the irradiation area of the probe light using the visible light,
Detecting the probe light transmitted through the measurement object or reflected from the measurement object;
A concentration measurement method, wherein the concentration of the measurement object is calculated from the intensity of the probe light.

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