JP6414257B2 - 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 transmittances 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. Patent Documents 1 to 4 disclose a concentration measuring device using the differential absorption method.

Japanese Patent Laid-Open No. 08-122254 JP 2011-33941 A JP 2012-26949 A JP 2010-181193 A

  In the differential absorption method, the intensity of the laser light is measured before and after entering the measurement object in order to calculate the transmittance of the laser light that has passed through the measurement object. Usually, a photo detector such as a semiconductor detector is individually used for these measurements. However, since the quantum efficiency of the photodetectors varies, an error occurs in the obtained intensity ratio.

  An object of the present invention is to provide a concentration measuring apparatus and a concentration measuring method capable of accurately measuring the concentration of a measurement object.

A first aspect of the present invention is a concentration measuring apparatus, a laser light source that generates laser light as excitation light,
A probe light generator that alternately generates on-wavelength and off-wavelength probe light for the object to be measured by wavelength conversion of the excitation light, and a preset branching ratio, and the on-wavelength and off-wavelength probe light. A beam splitter for branching a part of the beam as intensity reference light, an optical system for condensing the probe light having an on wavelength and an off wavelength transmitted through the measurement object or reflected from the measurement object, and the optical system A mirror that merges the optical paths of the intensity reference light of the on wavelength and the off wavelength not passing through and the optical paths of the probe light of the on wavelength and the off wavelength collected by the optical system into a common optical path, and the common one said intensity reference light-on wavelength and off wavelength through the optical path, and the same receiving the probe light on the wavelength and off wavelength through the common first optical path A photodetector for receiving in for each of the on wavelength and off wavelength, as well as identifying the probe light due to the difference in the time to reach the photodetector of the intensity reference light and the probe light, the probe light and a concentration calculator that calculates the concentration of the measurement object from the intensity, the optical system is a gist that you have placed between the measurement object and the mirror.

A second aspect of the present invention is a concentration measurement method, which generates laser light as excitation light, and alternately generates on-wavelength and off-wavelength probe light for a measurement object by wavelength conversion of the excitation light. Using a beam splitter having a preset branching ratio, branching a part of the probe light having an on wavelength and an off wavelength as intensity reference light , and transmitting the measurement object using an optical system, or The probe light of the on wavelength and the off wavelength reflected from the measurement object is collected, and the optical path of the intensity reference light of the on wavelength and the off wavelength not passing through the optical system using a mirror, and the optical system the optical path of the probe light on the wavelength and off wavelength is condensed are merged into a common first optical path, the intensity reference light-on wavelength and off wavelength via the common first optical path, and Receiving the probe light on the wavelength and off wavelength through the serial common first optical path in the same light-receiving surface for each of the on wavelength and off wavelength, each of the intensity reference light to the light receiving surface of the probe light The gist is to specify the probe light from the difference in arrival time and to calculate the concentration of the measurement object from the intensity of the probe light.

  ADVANTAGE OF THE INVENTION According to this invention, the density | concentration measuring apparatus and density | concentration measuring method which can measure the density | concentration of a measuring object with a sufficient precision 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. It is a block diagram of the optical path branching part and optical path confluence | merging part which concern on one Embodiment of this invention. It is a block diagram of the optical path branching part which concerns on one Embodiment of this invention. It is an example of the measurement result obtained by the density | concentration measuring method which concerns on one Embodiment of this invention.

  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. 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 path branching unit 27, an optical path merging unit 29, a photodetector 26, and a concentration calculating unit 28. With.

  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 is 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, 1998 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 generation unit 24 (25) enter the optical path branching unit 27. The optical path branching unit 27 branches a part of the probe light 10 and the reference light 12 as the intensity reference light 15. The remaining probe light 10 and reference light 12 are applied to the measuring object S. The branching ratio of the optical path branching unit 27 is set in advance. Therefore, the intensity of the probe light 10 and the reference light 12 immediately after exiting the output mirror 34 can be calculated backward by measuring the intensity reference light 15 with a photodetector 26 described later. Furthermore, the calculated intensities of the probe light 10 and the reference light 12 are used as reference values when calculating the transmittances of the probe light 10 and the reference light 12.

  As shown in FIG. 5, the optical path branching unit 27 of the present embodiment is, for example, a beam splitter with a defined reflectance (transmittance). In this case, the intensity reference light 15 extracted by the beam splitter is incident on the optical path merging portion 29 disposed in the front stage of the photodetector 26.

  The optical path merging unit 29 merges the optical path of the intensity reference light 15 extracted by the optical path branching unit 27 with the optical paths of the probe light 10 and the reference light 12 that have arrived from the measurement object, and these lights are passed to the photodetector 26. It is a guide. The optical path merging unit 29 is, for example, a half mirror, and guides all of the merged probe light 10, reference light 12, and intensity reference light 15 to the light receiving surface (not shown) of the photodetector 26.

  Further, as shown in FIG. 6, the optical path branching unit 27 of the present embodiment may be a scatterer that scatters the probe light 10 and the reference light 12. The scatterer is a member having a reflecting surface, such as a line, a rod, or a plate. The scattered light of the probe light 10 and the reference light 12 reaches the light receiving surface of the photodetector 26, and the probe light 10 and the reference light 12 are scattered. Is inserted into a part of

  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. Further, the photodetector 26 also detects the intensity reference light 15 of the probe light 10 and the reference light 12 extracted by the optical path branching unit 27. That is, the photodetector 26 receives these lights on the same light receiving surface. 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 photodetector 26 detects the three types of light described above. Therefore, the detection signal obtained therefrom is generated under the same quantum efficiency. That is, the error in the detection signal due to the individual difference of the photodetectors can be eliminated, so that the accuracy in calculating the concentration of the measuring object S is improved. In addition, the number of detectors used can be reduced as compared with the conventional concentration measuring apparatus. Therefore, the configuration is simplified and the manufacturing cost can be reduced.

  The concentration calculation unit 28 is configured as a part of a control unit (not shown) that controls the entire concentration measurement apparatus, and transmits (absorbance) each of the probe light 10 and the reference light 12 detected by the photodetector 26. From the above, the concentration of the measuring object S is calculated. Specifically, the concentration calculation unit 28 calculates the transmittance (first transmittance) of the reference light 12 from the intensity standard light 15 of the reference light 12 and each intensity of the reference light 12 that has passed through the measurement object. To do. The concentration calculation unit 28 further calculates the transmittance (second transmittance) of the probe light 10 from the intensity reference light 15 of the probe light 10 and each intensity of the probe light 10 that has passed through the measurement object. Considering that the second transmittance is obtained by multiplying the first transmittance by the transmittance due to the absorption of the measurement object S (third transmittance), the concentration calculating unit 28 Using the transmittance of 1, 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 above-described concentration calculation, the concentration calculation unit 28 receives the detection signal of the photodetector 26 while time-resolving, so that the light that has generated the detection signal is the probe light 10, the reference light 12, and these The intensity reference light 15 is specified. As described above, the probe light 10, the reference light 12, and the intensity reference light 15 are all detected by the same photodetector 26. However, either the probe light 10 or the reference light 12 is irradiated onto the measuring object S at a time. Further, the optical path lengths of the probe light 10 and the reference light 12 that reach the light detector 26 via the measurement object S are the optical paths of the probe light 10 and the reference light 12 that reach the light detector 26 as the intensity reference light 15. Overwhelmingly longer than the length. Therefore, the concentration calculation unit 28 receives the detection signal of the photodetector 26 while performing time resolution, thereby obtaining a measurement result as shown in FIG. In this figure, a peak P 1 that appears at time t 1 is a detection signal of the intensity reference light 15. Further, P2 that appears at time t2 (> t1) is a probe signal 10 (or reference beam 12) that is partially extracted as the intensity reference beam 15 of P1, and is a detection signal that has passed through the measuring object S. It is. Since there is such a time difference, the probe light can be identified from the difference in arrival time to the photodetector, and the concentration can be calculated.

  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, 15 ... Intensity reference light, 20 ... Optical axis, 22 ... Laser light source, 23 ... Excitation light, 24, 25 ... Probe light generation part, 26 DESCRIPTION OF SYMBOLS ... Photodetector, 27 ... Optical path branching part, 28 ... Concentration calculation part, 29 ... Optical path confluence part, 30 ... Optical system, 32 ... Termination mirror, 34 ... Output mirror, 36 ... Nonlinear optical crystal, 36a ... Optical axis, 38 Rotating stage 42 Wavelength adjusting mechanism 44 Rotating stage 46 Laser crystal 48 Optical system

Claims (2)

A laser light source that generates laser light as excitation light;
A probe light generator that alternately generates on-wavelength and off-wavelength probe light for the measurement object by wavelength conversion of the excitation light; and
A beam splitter having a preset branching ratio and branching a part of the probe light having an on wavelength and an off wavelength as intensity reference light;
An optical system for condensing the probe light having an on wavelength and an off wavelength transmitted through the measurement object or reflected from the measurement object;
A mirror that joins the optical path of the intensity reference light having the on-wavelength and the off-wavelength not passing through the optical system and the optical path of the probe light having the on-wavelength and the off-wavelength collected by the optical system into a common optical path. When,
A photodetector for receiving said intensity reference beam common on wavelength and off wavelength through an optical path, and the probe light on the wavelength and off wavelength through the common first optical path in the same light-receiving surface,
For each of the on wavelength and off wavelength, the concentration of the intensity reference beam and the difference in the time to reach the photodetector of the probe light with identifying the probe light, the measurement object from the intensity of said probe light A concentration calculation unit for calculating
The density measuring apparatus, wherein the optical system is disposed between the mirror and the measurement object. Generates laser light as excitation light,
By the wavelength conversion of the excitation light, on-wavelength and off-wavelength probe lights for the measurement object are alternately generated,
Using a beam splitter having a preset branching ratio, a part of the probe light having an on wavelength and an off wavelength is branched as intensity reference light,
Using an optical system, the probe light having an on wavelength and an off wavelength transmitted through the measurement object or reflected from the measurement object is collected,
Using a mirror, the optical path of the intensity reference light having the on wavelength and the off wavelength that does not pass through the optical system, and the optical path of the probe light having the on wavelength and the off wavelength collected by the optical system are shared. Join the light path ,
Receiving said intensity reference beam common on wavelength and off wavelength through an optical path, and the probe light on the wavelength and off wavelength through the common first optical path in the same light-receiving surface,
For each of the on wavelength and the off wavelength, the probe light is specified from the difference in arrival time of the intensity reference light and the probe light to the light receiving surface, and the concentration of the measurement object is determined from the intensity of the probe light. A concentration measuring method characterized by calculating.

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