JP5443701B2 - Gas concentration measuring device - Google Patents

Description

  The present invention relates to a gas concentration measuring apparatus that measures the concentration of a gas having a characteristic of absorbing infrared rays, such as carbon monoxide and carbon dioxide.

  A gas (gas) such as carbon monoxide or carbon dioxide has a property of absorbing light (mainly infrared rays) having a specific wavelength (absorption wavelength) as shown in FIG. A gas concentration measuring device that measures the concentration of gas is provided (see, for example, Patent Document 1). A conventional gas concentration measuring apparatus is shown in FIG.

  This conventional apparatus includes a container (gas cell) 20 into which a measurement gas is introduced, an infrared light source (for example, a semiconductor laser) 21 that emits infrared light, and one end of the container 20 that collects infrared light emitted from the infrared light source 21. A lens 22 that is incident from the side, an optical filter 23 that selectively passes only infrared light having a wavelength that matches the absorption wavelength of the measurement gas from infrared light that has passed through the container 20, and infrared light having an absorption wavelength that has passed through the optical filter 23. A lens 24 that emits light, and an infrared detector 25 (such as a thermopile or a pyroelectric element) that detects the intensity of infrared light having an absorption wavelength collected by the lens 24 are provided. For example, when the measurement gas is carbon monoxide, when infrared light is incident on the container 20 into which a gas containing carbon monoxide has been introduced, infrared light having an absorption wavelength (= 4.7 μm) of carbon monoxide is measured in the container 20. Since it is absorbed by the gas (carbon monoxide), the concentration of the measurement gas (carbon monoxide) in the container 20 can be measured by detecting the intensity of the infrared light that has passed through the optical filter 23 with the infrared detector 25. . As shown in FIG. 11, the optical filter 23 is a band-pass filter that passes only the absorption wavelength of the measurement gas (4.7 μm, which is the absorption wavelength of carbon monoxide in the illustrated example).

  Further, in the conventional apparatus shown in FIG. 9, measurement is performed when the emission intensity of the infrared light source 21 changes or when miscellaneous gas in which the measurement gas and the absorption band (absorption wavelength region) partially overlap is mixed in the container 20. There is a possibility that the gas concentration cannot be measured accurately. Therefore, infrared light (reference light) having a wavelength different from the absorption wavelength of the measurement gas is incident on the container 20, and the difference between the intensity of the reference light that has passed through the container 20 and the intensity of the infrared light at the absorption wavelength of the measurement gas is obtained. A device that corrects a measurement error due to a change in emission intensity of the infrared light source 21 or an influence of various gases has also been proposed.

For example, the conventional apparatus shown in FIG. 12 includes two infrared light sources 21, lenses 22 and 24, optical filters 23, and infrared detectors 25, and the absorption wavelength of the measurement gas that has passed through one optical filter 23 </ b> A (for example, The infrared intensity of 4.7 μm) is detected by the infrared detector 25A, and the infrared intensity of an absorption wavelength different from the measurement gas that has passed through the other optical filter 23B (for example, 4.0 μm) is detected by the infrared detector 25B. The difference between the infrared intensities detected by the two infrared detectors 25A and 25B is obtained to correct the measurement error.
JP 7-218432 A

By the way, in the infrared concentration gas concentration measuring apparatus as described above, the filter characteristics of the optical filter 23 that selectively allows only the absorption wavelength of the measurement gas to pass through greatly affect the measurement accuracy. That is, the narrower the pass band (bandwidth) of the optical filter 23 (the narrower the band), the less the influence of miscellaneous gases other than the measurement gas, and thus high-precision measurement becomes possible. However, since this type of optical filter 23 is usually formed by coating a large number (about 50 to 200 layers) of thin film on the surface of the base material by vapor deposition or sputtering, the base material is deformed by the tension of the thin film. Or there existed a problem that it was destroyed, it was difficult to ensure the adhesiveness of a thin film, or manufacturing cost became high. In addition, since it is very difficult to control the film thickness by coating by vapor deposition, it is difficult to match the center wavelength of the pass band to the desired wavelength (absorption wavelength of the measurement gas), the yield of the optical filter is poor, and the manufacturing cost Becomes higher. In addition, thin film materials must be made of materials that transmit infrared rays, but the types of thin film materials that can be coated are limited (for example, SiO ₂ , ZnO, Ge, etc.), and the use of them makes the production cost low. Becomes higher.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a gas concentration measuring apparatus capable of improving measurement accuracy while suppressing manufacturing cost.

In order to achieve the above object, the invention according to claim 1 is an infrared ray that emits infrared rays including a container into which a measurement gas is introduced, a first wavelength absorbed by the measurement gas, and a wavelength other than the first wavelength. A light source, a first wavelength filter that selectively reflects only infrared light having a first wavelength from infrared light emitted from the infrared light source and enters the container, and detects the intensity of infrared light having the first wavelength that has passed through the container. A first infrared filter, and the first wavelength filter includes a sub-wavelength optical element in which a portion having a different refractive index is formed in a lattice shape on the incident surface side of the substrate. The present invention is characterized in that a lattice-shaped portion formed of a silicon oxide film and a lattice-shaped portion formed of a silicon nitride film are alternately arranged at a constant pitch .

  According to the first aspect of the present invention, the first wavelength filter includes the sub-wavelength optical element in which the portions having different refractive indexes are formed in a lattice pattern on the incident surface side of the substrate. Compared to the wavelength filter, a narrow band filter characteristic can be realized while suppressing the manufacturing cost. As a result, the measurement accuracy can be improved while suppressing the manufacturing cost.

In order to achieve the above object, the invention according to claim 2 radiates infrared rays including a container into which a measurement gas is introduced, a first wavelength absorbed by the measurement gas, and a wavelength other than the first wavelength. A light source, a first wavelength filter that selectively reflects only infrared light of a first wavelength from infrared light emitted from an infrared light source and passed through a container; and infrared light of a first wavelength reflected by the first wavelength filter A first infrared ray detector for detecting the intensity of the first wavelength filter, the first wavelength filter comprising a sub-wavelength optical element in which portions having different refractive indexes are formed in a lattice shape on the incident surface side of the substrate, The shaped portion is characterized in that a lattice-like portion formed of a silicon oxide film and a lattice-like portion formed of a silicon nitride film are alternately arranged at a constant pitch .

  According to the invention of claim 2, since the first wavelength filter is composed of the sub-wavelength optical element in which the portions having different refractive indexes are formed in a lattice pattern on the incident surface side of the substrate, the thin film is coated in multiple layers. Compared to the wavelength filter, a narrow band filter characteristic can be realized while suppressing the manufacturing cost. As a result, the measurement accuracy can be improved while suppressing the manufacturing cost.

  According to a third aspect of the present invention, in the first aspect of the present invention, only infrared light having a second wavelength different from the first wavelength and not absorbed by the measurement gas is selectively reflected from the infrared light emitted from the infrared light source and incident on the container. And a second infrared detector that detects the intensity of the infrared light having the second wavelength that has passed through the container. The second wavelength filter has a different refractive index on the incident surface side of the substrate. It is characterized by comprising sub-wavelength optical elements whose parts are formed in a lattice shape.

  According to invention of Claim 3, the intensity | strength of the infrared rays radiated | emitted from an infrared light source by taking the difference of the infrared ray intensity detected with a 1st infrared detector, and the infrared ray intensity detected with a 2nd infrared detector, etc. The measurement accuracy can be improved by suppressing the influence of fluctuations and other gases other than the measurement gas.

  According to a fourth aspect of the present invention, in the second aspect of the invention, only the second wavelength infrared light that is different from the first wavelength and is not absorbed by the measurement gas is selectively reflected from the infrared light emitted from the infrared light source and passed through the container. And a second infrared detector for detecting the intensity of the infrared of the second wavelength reflected by the second wavelength filter, wherein the second wavelength filter is on the incident surface side of the substrate And a sub-wavelength optical element in which portions having different refractive indexes are formed in a lattice shape.

  According to invention of Claim 4, the intensity | strength of the infrared rays radiated | emitted from an infrared light source by taking the difference of the infrared ray intensity detected with a 1st infrared detector, and the infrared ray intensity detected with a 2nd infrared detector, etc. The measurement accuracy can be improved by suppressing the influence of fluctuations and other gases other than the measurement gas.

  The invention of claim 5 is the invention of any one of claims 1 to 4, wherein the first and second wavelength filters form a sub-wavelength grating in which the grating-like portions having different refractive indexes have a one-dimensional period. It is characterized by comprising a resonant mode grating filter.

  The invention of claim 6 is the invention according to any one of claims 1 to 4, wherein the first and second wavelength filters form a sub-wavelength grating in which the grating-like portions having different refractive indexes have a two-dimensional period. It is characterized by comprising a resonant mode grating filter.

In order to achieve the above object, a seventh aspect of the present invention provides a container into which a measurement gas is introduced, a first wavelength that is absorbed by the measurement gas, and a second wavelength that is different from the first wavelength and is not absorbed by the measurement gas. An infrared light source that emits infrared light including a wavelength, and a first light that selectively emits infrared light having a first wavelength and a second wavelength from the infrared light emitted from the infrared light source to linearly polarized light whose directions are orthogonal to each other. 3 wavelength filter, a first polarizing filter that passes only the first wavelength infrared light reflected by the third wavelength filter and then passed through the container, and passes through the container after being reflected by the third wavelength filter A second polarizing filter that passes only the infrared light having the second wavelength, a first infrared detector that detects the intensity of the infrared light having the first wavelength that has passed through the first polarizing filter, and a second polarizing filter Through And a second wavelength detector that detects the intensity of the infrared light having the second wavelength, and the third wavelength filter has a sub-wavelength in which portions having different refractive indexes are formed in a lattice pattern on the incident surface side of the substrate The lattice portion is formed of an optical element, and the lattice portion formed of a silicon oxide film and the lattice portion formed of a silicon nitride film are alternately arranged at a constant pitch . It is characterized by that.

  According to the invention of claim 7, since the third wavelength filter is composed of the sub-wavelength optical element in which the portions having different refractive indexes are formed in a lattice pattern on the incident surface side of the substrate, the thin film is coated in multiple layers. Compared to the wavelength filter, a narrow band filter characteristic can be realized while suppressing the manufacturing cost. As a result, the measurement accuracy can be improved while suppressing the manufacturing cost. In addition, by taking the difference between the infrared intensity detected by the first infrared detector and the infrared intensity detected by the second infrared detector, the intensity fluctuation of infrared rays emitted from the infrared light source and other than the measurement gas are measured. The measurement accuracy can be improved by suppressing the influence of gas, and the number of wavelength filters can be reduced as compared with the invention of claim 3, and the infrared rays of the first and second wavelengths are further reduced. The measurement accuracy can be improved because it passes through the same optical path.

In order to achieve the above object, the invention according to claim 8 is a container into which the measurement gas is introduced, a first wavelength absorbed by the measurement gas, and a second wavelength different from the first wavelength and not absorbed by the measurement gas. An infrared light source that emits infrared light including a wavelength, and a first light that selectively emits infrared light having a first wavelength and a second wavelength from the infrared light emitted from the infrared light source to linearly polarized light whose directions are orthogonal to each other. 3 wavelength filter, a polarization beam splitter that separates the first wavelength infrared ray and the second wavelength infrared ray that have passed through the container after being reflected by the third wavelength filter, and the second wavelength filter separated by the polarization beam splitter A first infrared detector that detects the intensity of the infrared light having the first wavelength; and a second infrared detector that detects the intensity of the infrared light having the second wavelength separated by the polarization beam splitter; Filter consists subwavelength optical element different sites refractive index on the incident surface side of the substrate is formed in a lattice shape, the lattice sites, the lattice-shaped portion and the silicon nitride which is formed of a silicon oxide film It is characterized in that it is formed such that lattice-shaped portions formed of a film are alternately arranged at a constant pitch .

  According to the invention of claim 8, since the third wavelength filter is composed of the sub-wavelength optical element in which the portions having different refractive indexes are formed in a lattice pattern on the incident surface side of the substrate, the thin film is coated in multiple layers. Compared to the wavelength filter, a narrow band filter characteristic can be realized while suppressing the manufacturing cost. As a result, the measurement accuracy can be improved while suppressing the manufacturing cost. In addition, by taking the difference between the infrared intensity detected by the first infrared detector and the infrared intensity detected by the second infrared detector, the intensity fluctuation of infrared rays emitted from the infrared light source and other than the measurement gas are measured. The measurement accuracy can be improved by suppressing the influence of the gas. Furthermore, compared with the invention of claim 7, since one polarization beam splitter is used instead of the two polarization filters, the first and Measurement accuracy can be improved because infrared rays of the second wavelength pass through the same optical path.

  According to the present invention, it is possible to improve measurement accuracy while suppressing manufacturing costs.

  Hereinafter, an embodiment in which the technical idea of the present invention is applied to a gas concentration measurement apparatus using carbon dioxide as a measurement gas will be described. However, the measurement gas is not limited to carbon dioxide, and may be a gas whose absorption band overlaps with the infrared wavelength region, for example, carbon monoxide, nitrogen oxide, or nitrogen sulfide as shown in FIG. .

(Embodiment 1)
In the present embodiment, as shown in FIG. 1, a container (gas cell) 1 into which a measurement gas (carbon dioxide) is introduced, a first wavelength λ1 (= 4.3 to 4.4 μm) absorbed by the measurement gas, and An infrared light source 2 that emits infrared light including a wavelength other than the first wavelength λ 1, a lens 3 that collects infrared light emitted from the infrared light source 2, and a first wavelength λ 1 from the infrared light collected by the lens 3. A first wavelength filter 4 that selectively reflects only infrared rays to enter the container 1, a lens 5 that collects infrared rays having the first wavelength λ 1 that has passed through the container 1, and a first light that is collected by the lens 5. A first infrared detector 6 that detects the intensity of infrared light having a wavelength λ1 of 1 and an infrared absorber 7 that absorbs infrared light other than the first wavelength that has passed through the first wavelength filter 4 are provided. The infrared light source 2 includes a light bulb having a filament, a light source having a minute heating element formed by so-called MEMS technology, a semiconductor laser, or the like. The first infrared detector 6 includes a thermal detection element such as a thermopile or a pyroelectric element, a quantum detection element such as InSb or PbSe, or a thermoacoustic detection element. The infrared absorber 7 is for preventing infrared rays not reflected by the first wavelength filter 4 from becoming stray light and causing noise, and is constituted by a cylindrical light trap, an infrared absorption film, or the like. Is done.

As shown in FIG. 2A, the first wavelength filter 4 has a SiO ₂ thin film (film thickness: 2 to 3 mm) 4a formed on a silicon substrate (not shown), and the SiO ₂ film 4a. The sub-wavelength optical element (diffraction grating) is formed so that two types of gratings 4b and 4c having different refractive indexes are arranged at a constant pitch. One lattice 4b is formed by etching a silicon nitride film formed by CVD (chemical vapor deposition), and the other lattice 4c is formed by a silicon oxide film between the lattices 4b by a sol-gel method or CVD. It is formed by forming a film. However, the surfaces (incident surfaces) of the gratings 4b and 4c are made to have a desired thickness (for example, 1.072 μm) and smoothed by etch back or polishing. Here, since the refractive index of the SiO ₂ film 4a is 1.52, the refractive index of the grating 4b is 2.0, and the refractive index of the grating 4c is 2.1, the pitch of the gratings 4b and 4c is set to 2.512 μm. Then, as shown in FIG. 2B, it is possible to form a sub-wavelength optical element that converts infrared light having a wavelength of 4.4 μm, which is an absorption wavelength of carbon dioxide, into S-polarized light and reflects it. However, in this sub-wavelength optical element, as shown in FIG. 2 (b), 4.0 μm infrared light is also converted to P-polarized light and reflected, so that P-polarized light is shielded (absorbed) using a polarizing filter. Is desirable.

  Thus, when the infrared light emitted from the infrared light source 2 is collected by the lens 2 and enters the incident surface of the first wavelength filter 4, the first wavelength λ1 included in the absorption band of the measurement gas (carbon dioxide). Of infrared light (S-polarized light) is reflected and passes through the container 1, and the infrared light having the first wavelength λ 1 that has passed through the container 1 without being absorbed by the measurement gas is incident on the first infrared detector 6. Based on the output of the first infrared detector 6, the concentration of the measurement gas (carbon dioxide) in the container 1 can be measured.

  Here, since the first wavelength filter 4 is formed of a sub-wavelength optical element, there are the following advantages. That is, as compared with the optical filter 23 formed by coating a large number (about 50 to 200 layers) of thin film on the surface of the base material by vapor deposition or sputtering as in the conventional example, the reflection area (bandwidth) is narrowed. It is possible. Further, since a silicon substrate that transmits infrared rays can be used as a material, the material cost is lower than that of germanium or the like. Furthermore, since the gratings 4b and 4c can be formed with high accuracy by using a normal semiconductor process, it is easy to make the center wavelength of the reflection region coincide with the desired wavelength (absorption wavelength of the measurement gas), and the yield is good. Manufacturing costs can be reduced.

  Instead of causing the infrared ray having the first wavelength λ1 reflected by the first wavelength filter 4 to enter the container 1, as shown in FIG. 3, only the infrared ray having the first wavelength λ1 is transmitted from the infrared ray that has passed through the container 1. A configuration may be adopted in which the light is reflected by the first wavelength filter 4 and is incident on the first infrared detector 6. In the configuration of FIG. 1, unnecessary infrared rays do not enter the container 1 by arranging the first wavelength filter 4 in the previous stage of the container 1, so that, for example, a gas other than the measurement gas mixed in the container 1 The infrared rays of wavelengths other than the first wavelength λ1 are absorbed and the temperature in the container 1 does not rise. Further, since the first wavelength filter 4 composed of the sub-wavelength optical element has a large wavelength dependency with respect to the incident angle of the infrared rays with respect to the incident surface, the reflected wavelength greatly changes depending on a slight angle difference. In the configuration, since the optical path length from the first wavelength filter 4 to the first infrared detector 6 is shorter than that in the configuration of FIG. 1, the first wavelength filter 4 is changed when the angle of the first wavelength filter 4 is changed. There is an advantage that the amount of movement of infrared rays incident on the infrared detector 6 is reduced, and as a result, the angle adjustment is facilitated.

  By the way, the sub-wavelength optical elements constituting the first wavelength filter 4 are the one-dimensional resonance mode grating 30 shown in FIG. 4A and the two-dimensional resonance mode grating 40 shown in FIG. It doesn't matter. In the one-dimensional resonance mode grating 30, a waveguide layer 32 is formed on a substrate (substrate) 31, and a sub-wavelength diffraction grating 33 is formed on the waveguide layer 32. The period of the diffraction grating 33 is set so long that a diffracted wave is generated in the waveguide layer 32 having a high refractive index, and short enough that no diffracted wave is generated in the substrate 31 or the incident space. When the light (infrared rays) diffracted by the diffraction grating 33 satisfies the propagation conditions of the waveguide layer 32, the light wave once enters the waveguide layer 32, propagates a short distance, and then is influenced by the diffraction grating 33. It is radiated to the outside again. At this time, the light emitted to the substrate 30 side is inverted in phase with respect to the directly transmitted light wave, cancels each other, and the intensity of the transmitted light becomes almost zero. On the other hand, the light waves radiated to the incident space side above the diffraction grating 33 are intensified because they have the same phase as the directly reflected light wave (this is referred to as “resonance reflection”). In addition, when the diffracted wave does not satisfy the propagation condition of the waveguide layer 32, light does not enter the waveguide layer 32, so that it is the same as the transmitted light of the thin film. The conditions under which resonant reflection occurs are narrow, and a narrow-band wavelength filter can be easily realized.

  A two-dimensional resonance mode grating 40 shown in FIG. 4B is described in Japanese Patent Application Laid-Open No. 2000-275415, and includes a substrate 41, a waveguide layer 42 formed on the substrate 41, a conductive layer. And a lattice layer 43 formed on the wave layer 42. In the lattice layer 43, rectangular recesses 44 are arranged in a lattice pattern at an equal pitch, and the lattice layer 43 is configured by the recesses 44, for example, air, and a medium having a low refractive index other than the recesses 44, The high refractive index waveguide layer 42 becomes a waveguide. The light incident on the grating layer 43 resonates with the grating layer 43, and only the reflected light with a specific wavelength is reflected. Further, in this resonance mode grating 40, since the polarization dependency when incident light is incident obliquely with respect to the normal direction of the surface of the grating layer 43 can be eliminated, the sub-wavelength optical element shown in FIG. Thus, two types of reflected light of P-polarized light and S-polarized light are not generated, and it is not necessary to remove unnecessary reflected light (polarized light) using a polarizing filter in order to remove unnecessary reflected light.

(Embodiment 2)
In the present embodiment, as shown in FIG. 5, only the infrared ray having the second wavelength λ2 which is different from the first wavelength λ1 and is not absorbed by the measurement gas from the infrared ray emitted from the infrared light source 2 is selectively reflected to the container 1. A second wavelength filter 8 to be incident, a lens 9 for condensing infrared light of the second wavelength λ 2 that has passed through the container 1, and a second detector for detecting the intensity of infrared light of the second wavelength λ 2 collected by the lens 9. There is a feature in that two infrared detectors 10 are provided. However, since the basic configuration of the present embodiment excluding such feature points is the same as that of the first embodiment, the same components are denoted by the same reference numerals and description thereof is omitted.

  As described in the prior art, when the emission intensity of the infrared light source 2 changes, or when miscellaneous gas that partially overlaps the measurement gas and the absorption band (absorption wavelength region) is mixed in the container 1, In view of the possibility that the concentration cannot be accurately measured, in this embodiment, infrared light (reference light) having a wavelength λ2 different from the absorption wavelength λ1 of the measurement gas is incident on the container 1 and the reference light (passed through the container 1) ( Measurement based on the change in emission intensity of the infrared light source 2 and the influence of miscellaneous gas by obtaining the difference between the intensity of the infrared light of the second wavelength λ2 and the intensity of the absorption wavelength of the measurement gas (first wavelength λ1). The error can be corrected.

  That is, if the infrared light transmitted through the first wavelength filter 4 is incident on the reflection surface of the second wavelength filter 8, the second wavelength λ2 (for example, λ2 = 4.0 μm) is formed on the reflection surface of the second wavelength filter 8. ) Only the infrared rays (reference light) can be selectively reflected and incident on the container 1. For example, if the measurement gas is carbon dioxide, the first wavelength λ1 = 4.4 μm and the second wavelength λ2 = 4.0 μm. If the measurement gas is carbon monoxide, the first wavelength λ1 = 4. What is necessary is just to set it as 7 micrometers and 2nd wavelength (lambda) 2 = 4.0 micrometers. The second wavelength filter 8 is a sub-wavelength optical element having the same configuration as that of the first wavelength filter 4.

  Thus, according to the present embodiment, infrared light (reference light) having a wavelength (second wavelength λ2) different from the absorption wavelength (first wavelength λ1) of the measurement gas is incident on the container 1 and passes through the container 1. By calculating the difference between the intensity of the reference light and the infrared intensity of the absorption wavelength (first wavelength λ1) of the measurement gas to correct the measurement error due to the change in emission intensity of the infrared light source 2 and the influence of various gases Measurement accuracy can be improved. In addition, since the second wavelength filter 8 that selectively reflects only the second wavelength λ2 is composed of the sub-wavelength optical element, a large number (50 layers) are formed on the surface of the base material by vapor deposition or sputtering as in the conventional example. Compared with the optical filter 23 formed by coating a thin film of about ~ 200 layers), it is possible to narrow the reflection area (bandwidth). Further, since a silicon substrate that transmits infrared rays can be used as a material, the material cost is lower than that of germanium and the like, and the lattices 4b and 4c can be formed with high accuracy by using a normal semiconductor process. It is easy to make the center wavelength of the region coincide with a desired wavelength (absorption wavelength of the measurement gas), and there is an advantage that the yield is good and the manufacturing cost can be reduced.

  Instead of causing the infrared rays having the first and second wavelengths λ1 and λ2 reflected by the first and second wavelength filters 4 and 8 to enter the vessel 1, the infrared rays that have passed through the vessel 1 as shown in FIG. From the infrared rays having the first wavelength λ1 reflected by the first wavelength filter 4 to be incident on the first infrared detector 6, and the infrared rays having passed through the first wavelength filter 4 to the second wavelength λ2 Only the light reflected by the second wavelength filter 8 may be incident on the second infrared detector 10. In the configuration of FIG. 5, unnecessary infrared rays other than infrared rays having the first and second wavelengths λ <b> 1 and λ <b> 2 are incident on the container 1 by arranging the first and second wavelength filters 4 and 8 in the front stage of the container 1. Therefore, for example, a gas other than the measurement gas mixed in the container 1 does not increase the temperature in the container 1 by absorbing infrared light having a wavelength other than the first and second wavelengths λ1 and λ2. In addition, the first and second wavelength filters 4 and 8 made of sub-wavelength optical elements have a large wavelength dependency with respect to the incident angle of infrared rays with respect to the incident surface, so that the reflected wavelength greatly varies depending on a slight angle difference. However, in the configuration of FIG. 6, the optical path lengths from the first and second wavelength filters 4, 8 to the first and second infrared detectors 6, 10 are shorter than the configuration of FIG. When the angles of the first and second wavelength filters 4 and 8 are changed, the amount of infrared rays incident on the first and second infrared detectors 6 and 10 is reduced, and as a result, the angle adjustment is facilitated. There is an advantage. Further, in the configuration of FIG. 5, the infrared of the first wavelength λ1 and the infrared of the second wavelength λ2 are incident and passed in parallel to the container 1, whereas in the configuration of FIG. Since infrared rays including the first and second wavelengths λ1 and λ2 are incident and transmitted, there is an advantage that the size of the container 1 can be relatively reduced.

(Embodiment 3)
Since the basic configuration of the present embodiment is the same as that of the second embodiment, the same components are denoted by the same reference numerals and description thereof is omitted. In the present embodiment, as shown in FIG. 7, only infrared rays having the first wavelength λ1 and the second wavelength λ2 are converted from the infrared rays emitted from the infrared light source 2 into linearly polarized light whose directions are orthogonal to each other and selectively reflected. The first wavelength filter 13 that passes through the container 1 after being reflected by the third wavelength filter 13 and passes only the infrared light having the first wavelength λ 1, and the third wavelength filter 13. And a second polarizing filter 12 that passes only the infrared light having the second wavelength λ2 that has passed through the container 1 after being reflected by the light.

  The third wavelength filter 13 includes the sub-wavelength optical element (see FIG. 2A) described in the first embodiment, and converts the infrared light having the first wavelength λ1 (= 4.4 μm) into S-polarized light and reflects it. At the same time, infrared light having the second wavelength λ2 (= 4.0 μm) is also converted to P-polarized light and reflected (see FIG. 2B). The first and second polarizing filters 11 and 12 are configured by a conventionally known wire grid polarizing plate, sub-wavelength optical element (diffraction grating), or the like.

  That is, infrared rays of the first and second wavelengths λ 1 and λ 2 reflected by the third wavelength filter 13 are incident on the container 1 and passed through the container 1. (S-polarized light) and infrared light of the second wavelength λ2 (P-polarized light) are discriminated by the first and second polarizing filters 11 and 12, and the intensity of the infrared rays of the discriminated first and second wavelengths λ1 and λ2 is determined. The first infrared detector 6 and the second infrared detector 10 can detect each separately.

  Thus, according to the present embodiment, as in the second embodiment, an infrared ray (reference light) having a wavelength (second wavelength λ2) different from the absorption wavelength (first wavelength λ1) of the measurement gas is incident on the container 1. The difference between the intensity of the reference light that has passed through the container 1 and the intensity of the infrared light of the absorption wavelength of the measurement gas (first wavelength λ1) is obtained, thereby changing the emission intensity of the infrared light source 2 and the influence of various gases. The measurement accuracy can be improved by correcting the measurement error. In addition, since the third wavelength filter 13 that selectively reflects only the first and second wavelengths λ1 and λ2 is composed of the sub-wavelength optical element, vapor deposition or sputtering is performed on the surface of the base material as in the conventional example. Therefore, it is possible to narrow the reflection area (bandwidth) as compared with the optical filter 23 formed by coating a large number (about 50 to 200 layers) of thin films. Further, since a silicon substrate that transmits infrared rays can be used as a material, the material cost is lower than that of germanium and the like, and the gratings 41 and 42 can be formed with high accuracy by using a normal semiconductor process. It is easy to make the center wavelength of the region coincide with a desired wavelength (absorption wavelength of the measurement gas), and there is an advantage that the yield is good and the manufacturing cost can be reduced. Moreover, in the second embodiment, the first and second wavelength filters 4 and 8 are used to selectively reflect the infrared rays having the first and second wavelengths λ1 and λ2, whereas in the present embodiment, the first and second wavelengths are filtered. Since the infrared rays of the first and second wavelengths λ1 and λ2 are reflected only by the three wavelength filters 13, the configuration can be simplified and the cost can be reduced, and the infrared rays of the first and second wavelengths λ1 and λ2 are the same. Therefore, there is also an advantage that the accuracy of correction using the difference in infrared intensity, that is, the measurement accuracy can be improved.

(Embodiment 4)
Since the basic configuration of this embodiment is the same as that of the third embodiment, the same components are denoted by the same reference numerals and description thereof is omitted. In this embodiment, instead of the first and second polarizing filters 11 and 12, as shown in FIG. 8, the infrared ray having the first wavelength λ1 that has passed through the container 1 after being reflected by the third wavelength filter 13 and It is characterized in that a polarizing beam splitter 14 that separates infrared rays of the second wavelength λ2 is provided.

  The polarization beam splitter 14 is composed of a sub-wavelength optical element in which a polarization grating 14b is formed on one surface of a light-transmitting substrate 14a so that S-polarized infrared light travels straight and P-polarized infrared light is refracted (reflected) ( For example, refer to JP 2002-90534 A). The polarization beam splitter 14 having such a configuration can be manufactured by resin molding using a mold or a semiconductor process for the substrate 14a and the polarization grating 14b.

  Thus, according to the present embodiment, as compared with the third embodiment, one polarizing beam splitter 14 is sufficient instead of the two polarizing filters 11 and 12, and further, the polarizing filters 11 and 12 are inexpensive. The light that does not pass through (infrared rays) is lost, whereas the polarization beam splitter 14 can completely separate S-polarized light and P-polarized light, so that there is an advantage that there is no loss of light.

It is a schematic block diagram which shows Embodiment 1 of this invention. The 1st wavelength filter in the same as above is shown, (a) is a perspective view and (b) is a filter characteristic figure. It is a schematic block diagram which shows the other structure same as the above. (A), (b) is a perspective view which shows the other structure of the 1st wavelength filter in the same as the above. It is a schematic block diagram which shows Embodiment 2 of this invention. It is a schematic block diagram which shows the other structure same as the above. It is a schematic block diagram which shows Embodiment 3 of this invention. It is a schematic block diagram which shows Embodiment 4 of this invention. It is a schematic block diagram which shows a prior art example. It is explanatory drawing for demonstrating the absorption wavelength of various gas. It is a filter characteristic figure which shows the characteristic of an optical filter. It is a schematic block diagram which shows another prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Container 2 Infrared light source 4 1st wavelength filter 6 1st infrared detector

Claims (8)

A container into which the measurement gas is introduced; an infrared light source that emits infrared light including a first wavelength absorbed by the measurement gas and a wavelength other than the first wavelength; A first wavelength filter comprising: a first wavelength filter that selectively reflects only infrared light and enters the container; and a first infrared detector that detects the intensity of infrared light having the first wavelength that has passed through the container. Comprises a sub-wavelength optical element in which a portion having a different refractive index is formed in a lattice pattern on the incident surface side of the substrate, and the lattice-shaped portion includes a lattice-shaped portion formed of a silicon oxide film and a silicon nitride film The gas concentration measuring device is characterized in that it is formed so as to be alternately arranged at a constant pitch . A container into which a measurement gas is introduced; an infrared light source that emits infrared light including a first wavelength and a wavelength other than the first wavelength absorbed by the measurement gas; and an infrared light that is emitted from the infrared light source and passes through the container A first wavelength filter that selectively reflects only infrared light of the first wavelength; and a first infrared detector that detects the intensity of infrared light of the first wavelength reflected by the first wavelength filter; The first wavelength filter includes a sub-wavelength optical element in which a portion having a different refractive index is formed in a lattice shape on the incident surface side of the substrate, and the lattice portion is a lattice-shaped portion formed of a silicon oxide film. A gas concentration measuring apparatus, wherein the portion and the lattice-like portion formed of the silicon nitride film are alternately arranged at a constant pitch .   A second wavelength filter that selectively reflects only an infrared ray having a second wavelength that is different from the first wavelength and is not absorbed by the measurement gas from the infrared ray emitted from the infrared light source, and a second wavelength filter that passes through the vessel. A second infrared detector for detecting the intensity of infrared rays having a wavelength of a second wavelength filter, wherein the second wavelength filter is a sub-wavelength optical element in which portions having different refractive indexes are formed in a lattice pattern on the incident surface side of the substrate The gas concentration measuring apparatus according to claim 1, wherein   A second wavelength filter that selectively reflects only infrared light having a second wavelength that is different from the first wavelength and that is not absorbed by the measurement gas from infrared light emitted from the infrared light source and passed through the container; and a second wavelength filter. A second infrared detector that detects the intensity of the reflected infrared light having the second wavelength, and the second wavelength filter has a portion having a different refractive index formed in a lattice pattern on the incident surface side of the substrate. 3. The gas concentration measuring apparatus according to claim 2, comprising a sub-wavelength optical element.   5. The first and second wavelength filters each include a resonance mode grating filter in which lattice-like portions having different refractive indexes form a one-dimensional periodic sub-wavelength grating. The gas concentration measuring device according to 1.   5. The first and second wavelength filters each include a resonance mode grating filter in which grating-like portions having different refractive indexes form a sub-wavelength grating having a two-dimensional period. The gas concentration measuring device according to 1. An infrared light source that emits infrared light including a container into which a measurement gas is introduced, a first wavelength that is absorbed by the measurement gas, and a second wavelength that is different from the first wavelength and that is not absorbed by the measurement gas; A third wavelength filter that selectively converts infrared rays of the first and second wavelengths from the emitted infrared rays into linearly polarized light whose directions are orthogonal to each other and reflected by the third wavelength filter. A first polarization filter that passes only infrared light of the first wavelength that has passed through the container later, and a second polarization that passes only infrared light of the second wavelength that has passed through the container after being reflected by the third wavelength filter. A filter, a first infrared detector that detects the intensity of infrared light having a first wavelength that has passed through the first polarizing filter, and a first detector that detects the intensity of infrared light having a second wavelength that has passed through the second polarizing filter. 2 An infrared detector, a third wavelength filter consists subwavelength optical element different sites refractive index on the incident surface side of the substrate is formed in a lattice shape, the lattice sites, the silicon oxide film A gas concentration measuring apparatus, wherein the lattice-shaped portions formed in step 1 and the lattice-shaped portions formed of a silicon nitride film are alternately arranged at a constant pitch . An infrared light source that emits infrared light including a container into which a measurement gas is introduced, a first wavelength that is absorbed by the measurement gas, and a second wavelength that is different from the first wavelength and that is not absorbed by the measurement gas; A third wavelength filter that selectively converts infrared rays of the first and second wavelengths from the emitted infrared rays into linearly polarized light whose directions are orthogonal to each other and reflected by the third wavelength filter. A polarization beam splitter that separates infrared light of the first wavelength and infrared light of the second wavelength that has passed through the container later, and first infrared detection that detects the intensity of the infrared light of the first wavelength separated by the polarization beam splitter And a second infrared detector for detecting the intensity of the infrared light having the second wavelength separated by the polarization beam splitter, and the third wavelength filter has a portion having a different refractive index on the incident surface side of the substrate. There consists subwavelength optical elements formed in a grid pattern, the lattice sites, the lattice-shaped portion and a constant pitch formed by the lattice-shaped portion and a silicon nitride film formed by the silicon oxide film The gas concentration measuring device is formed so as to be alternately arranged .

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