JP2008241269A - Absorbance measuring instrument and absorbance measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an absorbance measuring instrument, capable of enhancing detection sensitivity and an S/N ratio in the measurement of the absorbance of a laser beam for the purpose of the measurement of gas concentration, and an absorbance measuring method. <P>SOLUTION: The diffused laser beam P is emitted to a measuring target from laser beam sources 11 and 13a, a light detection element array DA comprising a plurality of light detection elements is arranged in the irradiation surface of the laser beam P and the absorbance of the laser beam P in the measuring target is measured, on the basis of the intensity signals outputted from the respective light detection elements. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an absorbance measurement device and an absorbance measurement method mainly using a gas (gas) as a measurement object, and more particularly to an absorbance measurement device and an absorbance measurement method that can be suitably used for measuring a gas concentration.

  With the development of industries using petroleum resources, the problem of global warming has become serious. This global warming occurs when a greenhouse gas in the atmosphere absorbs part of infrared rays, re-radiates part of it to the ground surface, and warms the ground surface. Greenhouse gases include water vapor, carbon dioxide generated when oil resources are burned, methane, nitrogen monoxide, and chlorofluorocarbons generated when mining natural gas and garbage. In recent years, due to an increase in the consumption of petroleum resources, etc., these greenhouse gases have been discharged in large quantities, and in order to suppress this, emission limits on these gases have been set. Therefore, it is necessary to measure the concentrations of these gases in the atmosphere and monitor the concentrations of these gases.

  In addition to global warming, air pollution has become a serious environmental problem. The polluted gases include carbon monoxide, nitrogen oxide (NOx), sulfur oxide (SOx), hydrogen sulfide, ammonia, acetylene, dioxin, halogen, etc., especially carbon monoxide, nitrogen monoxide, hydrogen sulfide, Dioxins, halogens, etc. are dangerous if inhaled. Therefore, it is necessary to measure the concentration of these gases in an environment where these harmful gases may be generated.

  Each of the above gases has an absorption band that absorbs light of a specific wavelength due to rotation of gas molecules, vibration between atoms, and the like. Therefore, many gas concentration measurement methods using this absorption band have been proposed.

  As the above measurement method, an infrared absorption measurement method using infrared rays (Patent Document 1) is known. However, for the reason that measurement with high resolution is possible, the absorbance of gas with respect to laser light is measured. Many gas concentration measurement methods by measuring have been studied (Patent Documents 2 to 5).

  Since many of the above gas molecules have a plurality of vibration / rotation modes, they have a plurality of absorption bands (fundamental bands) at the fundamental frequency, and further, a higher frequency side (shorter wavelength side) than the respective fundamental bands. ) Has a large number of absorption bands related to overtones and combined sounds of the basic band. Among these absorption bands, the absorption intensity in the basic band is generally much larger than the absorption band related to overtones and combined sounds, and when measuring the gas concentration, the basic band is the target. It is advantageous to measure the absorbance.

  However, depending on the type of gas to be measured, the pressure of the sample gas (for example, the atmosphere), or the concentration (partial pressure) of the gas to be measured in the sample gas, an accurate gas can be obtained even by absorbance measurement for the basic band. In some cases, sufficient detection sensitivity or S / N ratio cannot be obtained to measure the concentration.

  As a technique for increasing detection sensitivity and S / N ratio in absorbance measurement, a multiple optical path measurement method is known (for example, Patent Document 4).

FIG. 1A is an explanatory diagram showing the measurement principle of the multiple optical path measurement method. As shown in FIG. 1A, this method uses two (white type cells) or three (Hiriot type cells) mirrors. By reflecting the laser beam P many times between M, the detection sensitivity is enhanced by extending the optical path length of the laser beam P in the sample gas accommodated in the cell (container) C. That is, if k (ν) is the absorption coefficient, L is the cell length, and n−1 is the number of reflections by the mirror M, the absorbance AI in the sample gas can be expressed by the following equation (1). Is reflected, the optical path length is extended, and the absorbance AI increases.
AI = k (ν) × [n × L] (1)

  However, the multiple optical path measurement method is inconvenient in that the angle of the mirror M needs to be adjusted with high accuracy in order to realize a number of reflections. Further, the laser beam P is reflected for each reflection at the mirror M. Due to reflection loss, there is a limit to the improvement in detection sensitivity or S / N ratio by this method.

In addition, the basic sound bands of various gases described above are often located in the infrared band, but wavelength tunable lasers that oscillate in the infrared band are generally expensive and inconvenient such that they need to be produced at a liquid nitrogen temperature. There is. In contrast, in the near-infrared band, a tunable laser that is inexpensive and high-performance for optical communication applications and can operate at room temperature can be easily obtained. However, the absorption band existing in the near-infrared band is an absorption band related to weak overtones or coupled sounds that are weakly absorbed, and obtaining sufficient detection sensitivity or S / N ratio is a measure of the absorbance for the fundamental sound band. There is a problem that it becomes much more difficult than in the case of measurement.
JP 2004-226104 A JP 7-270308 A Japanese Unexamined Patent Publication No. 7-27701 JP-A-5-79976 JP-A-2005-106521

  The present invention has been made in view of the above problems, and achieves one or more of the following objects.

  That is, an object of the present invention is to provide an absorbance measuring apparatus or an absorbance measuring method capable of increasing absorbance detection sensitivity or S / N ratio as compared to conventional absorbance measurement.

  Another object of the present invention is an absorbance measuring apparatus or an absorbance measuring method capable of measuring a gas concentration by measuring absorbance when a laser beam is irradiated to a gas, and compared with a conventional measuring method, An object of the present invention is to provide an absorbance measuring apparatus or an absorbance measuring method capable of measuring a gas concentration with high detection sensitivity or higher S / N ratio.

  Another object of the present invention is an absorbance measuring device or an absorbance measuring method capable of measuring a gas concentration by measuring absorbance when a laser beam is irradiated onto a gas, and is an absorbance measuring device that is smaller or less expensive than conventional ones. Or to provide an absorbance measurement method capable of measuring the gas concentration with a smaller or less expensive device as compared with the conventional measurement method.

  Still another object of the present invention is to provide an absorbance measuring apparatus or an absorbance measuring method capable of measuring a gas concentration by measuring absorbance using a laser beam in the near infrared wavelength region.

The present invention provides a laser source that emits a laser beam diffused toward a measurement object, and an intensity signal of the laser light that is disposed within the irradiation surface of the laser light and that has passed through the measurement object. An absorbance measuring device comprising a photodetecting element array comprising a plurality of photodetecting elements (claim 1), or
The laser beam that has passed through the measurement object by a step of emitting laser light diffused toward the measurement object and a light detection element array including a plurality of light detection elements arranged in the irradiation surface of the laser light The above-described object is achieved by an absorbance measurement method (claim 5) comprising a step of measuring a light intensity signal.

  The laser source of the present invention emits diffused laser light, but the diffusion is performed to such an extent that it is possible to irradiate a plurality of light detection elements constituting the light detection element array with laser light. Say. In addition, when measuring the intensity signal of the laser light in all the light detection elements constituting the light detection element array, it is necessary to irradiate the laser light to all the light detection elements. In the case where the intensity signal of the laser beam is measured by a part of the light detection elements constituting the light beam, it is sufficient that the part of the light detection elements is irradiated with the laser light.

  The plurality of photodetecting elements constituting the photodetecting element array in the present invention are arranged in the laser light irradiation surface, but the arrangement may be in a matrix or a linear form. In view of reducing the size of the apparatus, it is preferable to use a matrix arrangement.

  FIG. 1B is a conceptual explanatory view schematically showing the mechanism of the present invention.

As shown in the drawing, in the present invention, the intensity of the laser light that is irradiated to the container C that accommodates the measurement object and is transmitted through the measurement object is irradiated with the laser light P diffused so that the beam cross section has a predetermined area S. Are measured in parallel by a plurality of photodetectors constituting the photodetector array DA. Therefore, the absorbance AI measured according to the present invention is expressed by the following formula (2), where n is the number of light detection elements, but the following formula (2) is equivalent to the above formula (1). Therefore, as in the case of the multiple optical path measurement method, an increase in absorbance detection sensitivity or S / N ratio is achieved.
AI = n × [k (ν) × L] (2)

  However, in the present invention, unlike the multiple optical path measurement method, there is an advantage that the adjustment of the mirror M is not difficult, and further, since there is no problem of reflection loss due to the mirror M, the number of light detection elements is increased. Thus, it can be said that it is superior to the multiple optical path measurement method in that a higher detection sensitivity or S / N ratio can be achieved than in the multiple optical path measurement method.

  The present invention does not deny the multiple optical path measurement method. For example, the absorbance is measured by arranging the photodetector array DA of the present invention instead of the photodetector D in FIG. It is possible to apply the present invention to a multiple optical path measurement method.

  In the present invention, it is preferable that the measurement object is a gas accommodated in a predetermined container (claim 2).

  In this invention, it becomes possible to increase the detection sensitivity or S / N ratio in the concentration measurement of various gases required to cope with global warming and environmental problems, or the gas concentration measurement is an inexpensive and small device. Can be realized.

  In the present invention, the frequency control means for changing the frequency of the laser beam and the total value or the average value of the intensity signals output from each of the light detection elements with the frequency of the laser beam as the horizontal axis are plotted. It is preferable to further comprise calculation means for calculating the area of a region surrounded by the spectral curve to be measured and a straight line indicating that there is no absorption by the measurement object (Claim 3).

  That is, the inventor of the present application disclosed in Japanese Patent Application Laid-Open No. 2007-004693, irradiates a measurement target gas with laser light while changing the frequency, measures the transmitted light intensity with a light detection element, and sets the frequency of the laser light on the horizontal axis. By calculating the area of the region surrounded by the spectrum curve obtained by plotting the transmitted light intensity measured by the photodetecting element and the straight line representing the absence of absorption of the measurement object, the reference gas An apparatus and a method capable of measuring a gas concentration with high sensitivity without referring to the above and without measuring peak intensity are disclosed. By applying it, it is possible to further improve the measurement accuracy of the gas concentration and to reduce the size and cost of the measuring device.

  FIG. 2 is an explanatory view showing a method for deriving a gas concentration in the invention according to claim 3.

The horizontal axis and the vertical axis in the figure are the oscillation frequency and absorbance of the laser beam P, respectively, and 20a and 20b in the figure are measurement target gases such as CO ₂ in the container C in the apparatus shown in FIG. It is an absorption curve which shows the light absorbency measured by enclosing the sample gas containing and each under different conditions. The absorbance AI = K (ν) × L is in the relationship of the transmitted light intensity I (ν) and the following equation (3), where the incident light intensity is I ₀ .
AI = (I ₀ −I (ν)) / I ₀ (3)

  The inventor determined that the area of the regions 22a and 22b surrounded by the absorption curves 20a and 20 of the measurement target gas measured as described above and the straight line 21 indicating no absorption is other than the measurement target gas. It has been confirmed that it depends only on the pressure of the gas to be measured, regardless of whether or not it contains substances. Therefore, the concentration of the measurement target gas can be easily derived by obtaining the pressure of the measurement target gas from the area of the regions 22a and 22b and dividing the pressure by the total pressure of the sample gas.

  The curves 23a and 23b in FIG. 2 are absorption curves by a Fabry-Perot resonator that resonates every 1.5 GHz or 300 MHz, respectively, and are shown as data for frequency calibration of the laser beam P at the time of the measurement. The convex peak 24 is shown as a guideline indicating a height of, for example, 90% transmittance in the absorption curve.

  In the present invention, it is preferable that the wavelength of the laser beam emitted from the laser source is in the near infrared region.

  That is, most of the gases that are necessary or beneficial to measure from the response to global warming and environmental problems have an absorption band related to the combined sound or overtone in the near infrared region. Although the absorption intensity in these absorption bands is weak compared to the absorption band in the basic sound, in the absorbance measuring device of the present invention, the absorbance is increased by increasing the number of the photodetector elements constituting the photodetector element array. The detection sensitivity or the S / N ratio of the gas concentration can be increased by using an inexpensive and small absorbance measuring device using a laser source that oscillates in the near infrared region, which is cheap and easily available in the market. Measurement becomes possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 3 is an explanatory diagram showing the configuration of the absorbance measuring apparatus 10 according to the present invention.

  In the figure, reference numeral 11 denotes a wavelength tunable laser element. The laser element 11 emits a laser beam P having a transmission wavelength designated by an analog signal from the D / A converter 12 based on a signal from the computer 14, and the laser. The light P is reflected by the total reflection mirrors M1 and M2 and then diffused in the diffusion lens 13a until the beam diameter reaches a predetermined area. The light P is converted into parallel light by the collimator lens 13b. A plurality of light detection elements constituting the light detection element array DA are irradiated with the laser light P guided to the sample container C that accommodates the measurement object and passing through the measurement object. Intensity signals output from the respective light detection elements of the light detection element array DA by receiving the laser light P are input to the signal channels Ch1 to Ch16 and analyzed by the computer 14.

  Further, four half mirrors M3 to M6 are arranged on the optical path of the laser beam P between the total reflection mirrors M1 and M2, and the laser beams P1 to P4 are branched from each of them.

  Among these, the laser beam P1 is guided to the frequency monitor 15 and the transmission wavelength of the laser element 11 is monitored.

  In addition, Fabry-Perot resonators 16a and 16b that resonate every 300 MHz and 1500 MHz are arranged on the optical paths of the laser beams P2 and P3, respectively, and the output light from the Fabry-Perot resonators 16a and 16b is detected by light. The laser light P4 from the half mirror M6 is guided to the light detection element D3 as it is, guided to the elements D1 and D2, and the light detection elements D1 to D3 respectively receive the intensity signals of the received laser lights P2 to P4 on the computer 14. Are output to the signal channels Ch17 to Ch19. Therefore, it is possible to know the exact transmission frequency of the laser element 11 from the peaks at 300 MHz and 1500 MHz in the output signals from the light detection elements D1 and D2, and the output signal from the light detection element D3 is used as a laser power monitor signal. used.

Hereinafter, the result of the absorbance measurement performed using the absorbance measurement apparatus 10 with the measurement object as CO ₂ gas will be described.

In this measurement, an indium-aluminum-gallium-arsenic optical feedback type tunable semiconductor laser (TSL-210 / output 2 mW manufactured by Suntech Co., Ltd.) is used as a laser element, and CO ₂ molecules in a 1.5 μm band are used. The absorption at the R (18) line of the 2ν ₁ + 2ν ₂ + 2ν ₃ band of was measured. The CO ₂ pressure in the measurement was 0.5 to 10.8 kPa, the detection method was the video method (frequency sweep) of 7 MHz / digit, and the oscillation frequency of the laser element 11 was swept at a speed of 15 GHz / 150 mS.

4A is an explanatory diagram showing vibration modes of ν ₁ , ν _2, and ν ₃ which are basic bands of the coupling band 2ν ₁ + 2ν ₂ + ν ₃ , and FIG. Shows the transmittance of the laser beam P in the 2ν ₁ + 2ν ₂ + ν ₃ band. In this measurement, the R (18) line of the 2ν ₁ + 2ν ₂ + ν ₃ band was measured in the near-infrared band where the laser element is easily available, as shown in FIG. 4 (B). As can be seen, in the 2ν ₁ + 2ν ₂ + ν ₃ band, the absorption at the R (18) line is relatively large.

  FIG. 5 shows the result of the measurement performed as described above. In the figure, the vertical axis and the horizontal axis represent the light intensity (voltage value / V) detected by the light detection element array DA, the horizontal axis represents the frequency of the laser beam P, and (A) is a single value. When the light detection elements are used, (B) is an average value of the light intensity detected by each light detection element when four light detection elements are used in the light detection element array DA, and (C) is: It is a total average value when the measurement of (B) is repeated 20 times.

  As shown in the figure, when (A) and (B) are compared, it is clear that the absorbance detection sensitivity or S / N ratio is improved by using a photodetection element array DA comprising a plurality of photodetection elements. is there. Further, in (C), which is the average value of the measurement in (B), a higher detection sensitivity or S / N ratio is obtained than in the case of (B), so that the light detection that constitutes the light detection element array DA is performed. It is considered that further improvement in detection sensitivity or S / N ratio can be achieved by increasing the number of elements.

  The absorbance measuring apparatus and the absorbance measuring method of the present invention are carbon dioxide, methane, nitric oxide, chlorofluorocarbon, carbon monoxide, nitrogen oxidation performed for the purpose of monitoring and regulating greenhouse gases and harmful environmental gases in the atmosphere. It can be used in the field of gas concentration measurement such as substances (NOx), sulfur oxides (SOx), hydrogen sulfide, ammonia, acetylene, dioxin, halogen, etc. In addition to the above, the absorbance of laser light in gases, liquids, dusts, etc. It can be used in various industrial fields including identification or concentration measurement of elements and molecules contained in a sample by measurement.

(A) is explanatory drawing which shows the measurement principle of the multiple optical path measurement method. (B) is a conceptual explanatory view schematically showing the mechanism of the present invention. Explanatory drawing which shows the derivation | leading-out method of the gas concentration in the especially preferable aspect of this invention. Explanatory drawing which shows the structure of the light absorbency measuring apparatus which concerns on one Embodiment of this invention. (A) is an explanatory view showing vibration modes of ν ₁ , ν _2, and ν ₃ , which are fundamental bands of the carbon dioxide binding band 2ν ₁ + 2ν ₂ + ν ₃ , and (B) is 2ν ₁ + 2ν ₂ + ν of carbon dioxide. Explanatory drawing which shows the transmittance | permeability of the laser beam in ₃ bands. Explanatory drawing which shows the measurement result of the light absorbency by the light absorbency measuring apparatus which concerns on one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Absorbance measuring device 11 Laser element 12 D / A converter 13a Diffuse lens 13b Collimator lens 14 Computer 15 Frequency monitor 16a, 16b Fabry-Perot resonator C Container DA Photodetector array D Photodetector M Mirror P Laser light

Claims (5)

A laser source that emits laser light diffused toward the measurement object;
An absorbance measurement comprising: a photodetection element array comprising a plurality of photodetection elements that are arranged in an irradiation surface of the laser light and each output an intensity signal of the laser light transmitted through the measurement object. apparatus.   The absorbance measurement apparatus according to claim 1, wherein the measurement object is a gas contained in a predetermined container. Frequency control means for changing the frequency of the laser beam;
A spectral curve obtained by plotting a total value or an average value of intensity signals output from each of the light detection elements with the frequency of the laser light as a horizontal axis, and a straight line representing no absorption by the measurement object The absorbance measuring apparatus according to claim 2, further comprising: a calculating unit that calculates an area of a region surrounded by.   The absorbance measuring apparatus according to claim 1, wherein the wavelength of the laser light is in a near infrared region. Emitting a laser beam diffused toward the measurement object;
Measuring the intensity signal of the laser beam that has passed through the object to be measured by a photodetector array comprising a plurality of photodetectors arranged within the laser light irradiation surface. Measurement method.

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