JP2008026190A - Gas detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas detector capable of enhancing detection sensitivity of infrared light and also being downsized. <P>SOLUTION: The gas detector 1 comprises: an infrared light emitting unit 10, a sum-frequency converter 16 and a light detector 17. The infrared light emitting unit 10 is provided with: a seed laser light source 11; an excitation laser light source 12; an optical parametric converter 13; a transmitting/receiving mirror 14; and a scanner 15. An infrared light emitted from the optical parametric converter 13 is transmitted to a target W through the transmitting/receiving mirror 14, and a reflected light is received by the transmitting/receiving mirror 14. The received infrared light is converted to a near infrared light or a visible light by the sum-frequency converter 16, and transmitted to the light detector 17. The light detector 17 can use a light detector for near infrared light or visible light with high detection sensitivity, and can measure with high detection sensitivity, not being affected by thermal noise of the detector itself. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a gas detection device that detects fuel gas such as city gas or harmful gas such as air pollution gas and exhaust gas by infrared light.

Conventionally, a detection method using infrared rays has been proposed in order to detect the leakage of fuel gas such as city gas. For example, in Patent Document 1, a region where gas is leaking from a gas pipe or the like is irradiated with visualization infrared rays including infrared rays of two wavelengths, an absorption wavelength absorbed by the gas and a non-absorption wavelength, and the reflected light is irradiated. The point that the gas concentration distribution is visualized from the comparison of the received light intensity by the image processing circuit after receiving the light by the light receiving element is described. Also in Patent Document 2, laser light including two wavelengths, an absorption wavelength absorbed by gas and a non-absorption wavelength, is irradiated, the irradiation area is photographed with a video camera, and an image of leaked gas is displayed. Points are listed.
JP 2003-294567 A US Pat. No. 6,690,472

  When detecting methane gas contained in city gas, a laser beam having an infrared wavelength with a large absorption intensity is generally selected. However, a detector that detects this wavelength band has a low detection sensitivity and a detection target. Since it becomes infrared light, there is a drawback that it is easily affected by heat noise generated from the detector itself or heat noise generated from a preamplifier incorporated in the apparatus. Therefore, since the minimum detection power is limited and measurement at the shot noise limit is difficult, a high-power laser light source is required to detect the methane gas concentration with high accuracy, and the equipment becomes large and expensive. I must.

  City gas leak detection work needs to be detected along the gas piping, but considering the fact that the installation location of the gas piping is a narrow space such as the basement or the back of the ceiling, it is used for such work in larger equipment. This is currently limited.

  In view of the above, an object of the present invention is to provide a gas detection device capable of increasing the detection sensitivity of infrared light and making the device compact.

  The gas detection device according to the present invention includes an irradiation unit that irradiates a detection target region with infrared light for detecting a detection target gas, a light receiving unit that receives infrared light scattered or reflected from the detection target region, Conversion means for converting received infrared light into near infrared light or visible light by sum frequency conversion, and detection means for detecting a detection target gas based on near infrared light or visible light converted by the conversion means; It is characterized by having. Furthermore, the excitation light source used for the conversion means is used as an excitation light source for generating infrared light in the irradiation means. Further, the conversion means includes a quasi phase matching nonlinear optical crystal that performs sum frequency conversion. Furthermore, the irradiating means includes a quasi phase matching nonlinear optical crystal for generating predetermined infrared light. Further, the irradiating means includes a quasi-phase matching nonlinear optical crystal having a plurality of light propagation portions having different domain periodic structures for generating infrared light having different wavelengths, and the converting means includes infrared light having different wavelengths. A quasi-phase-matching nonlinear optical crystal having a plurality of light propagation portions having different domain periodic structures for performing the sum frequency conversion.

  Since the infrared light to be detected is converted into near-infrared light or visible light and detected by providing the above-described configuration, it can be detected with high sensitivity for near-infrared light or visible light. Detection means can be used and detection is performed with near-infrared light or visible light, so that it is not affected by thermal noise generated from a preamplifier or the like of the apparatus itself, and detection sensitivity can be further increased.

  That is, as a detection means for near-infrared light or visible light, for example, a photomultiplier tube, the detection sensitivity of which is improved by about 100 times compared to an In-As detector that is a conventional infrared detector. Has been put to practical use, and the minimum detection power capable of receiving light is improved, so that even a slight gas concentration can be detected with high accuracy. Therefore, it is not necessary to generate and irradiate high-output infrared light, and the apparatus can be made compact, and the manufacturing cost of the apparatus can be reduced. If the apparatus can be made compact, the apparatus can be carried in a narrow place such as the basement or the ceiling, and is particularly suitable as a leakage gas detection apparatus.

  In addition, since the excitation light source used for the conversion means is used as the excitation light source for generating infrared light in the irradiation means, it becomes possible to share the excitation light source, thereby reducing the size and weight of the apparatus. You can go further.

  Further, by providing the conversion means with a quasi phase matching nonlinear optical crystal that performs sum frequency conversion, the minimum detection capability can be further improved and the detection sensitivity can be increased.

  In addition, by providing the irradiating means with a quasi phase matching nonlinear optical crystal for generating predetermined infrared light, it is possible to use a small light source with low output, further reducing the overall size of the apparatus and reducing the cost. Can be planned.

  In addition, a quasi-phase matched nonlinear optical crystal having a plurality of light propagation parts with different domain periodic structures that generate infrared light of different wavelengths is used as the irradiation means, and sum frequency conversion of infrared light of different wavelengths is used as the conversion means. By using a quasi-phase-matching nonlinear optical crystal having a plurality of light propagation parts with different domain periodic structures, the infrared light corresponding to the absorption wavelength of the detection target gas is switched and output, and the received infrared light is It becomes possible to perform sum frequency conversion to near-infrared light or visible light, and even if it contains multiple types of harmful gases, such as air pollutant gases and automobile exhaust gases, it can handle each harmful gas. Infrared light can be generated and detected.

  Hereinafter, embodiments according to the present invention will be described in detail. The embodiments described below are preferable specific examples for carrying out the present invention, and thus various technical limitations are made. However, the present invention is particularly limited in the following description. Unless otherwise specified, the present invention is not limited to these forms.

  FIG. 1 is a schematic configuration diagram relating to an embodiment of the present invention. The gas detection apparatus 1 includes an infrared light irradiation unit 10, a sum frequency converter 16, and a light detector 17. The infrared light irradiation unit 10 includes a seed laser light source 11, an excitation laser light source 12, and an optical parametric converter. 13, a transmission / reception mirror 14 and a scanning mechanism 15 are provided.

  Laser light from the seed laser light source 11 and the excitation laser light source 12 is introduced into the optical parametric converter 13 to emit infrared light having a predetermined frequency. In the optical parametric converter 13, the laser light of the introduced seed laser light source 11 is simultaneously incident on the nonlinear optical crystal together with the laser light from the excitation laser light source 12, thereby achieving a high output that has been converted to a single frequency by the optical parametric interaction. Infrared light can be obtained. Therefore, infrared light matched with the absorption wavelength of the detection target gas can be emitted, and even a very small amount of gas can be detected with high accuracy.

  The above-described optical parametric converter 13 is a difference frequency converter and is suitable for generating a single frequency. In addition to such a converter, an excitation laser light source is incident on a nonlinear optical crystal to generate a plurality of different wavelengths. Alternatively, an optical parametric generator that generates the laser beam may be used. In the case of the optical parametric generator, laser light is generated in addition to the desired infrared light, but only the infrared light can be output by passing the emitted light through an interference filter. An optical parametric oscillator may be used in the same manner as the optical parametric generator. The optical parametric oscillator uses a resonant mirror to resonate the laser light from the pumping laser light source that is incident on the nonlinear optical crystal to generate a plurality of laser beams with different wavelengths. Only external light can be output.

  The emitted infrared light is guided to the transmission / reception mirror 14 by a reflection mirror or the like, reflected by the transmission / reception mirror 14 and irradiated toward the target W such as a wall surface of the structure. A scanning mechanism 15 is attached to the transmission / reception mirror 14, and the irradiation position on the surface of the target W is scanned and moved in a predetermined direction by rotating the transmission / reception mirror 14 to change the incident angle of infrared light. It has become. Infrared light irradiated toward the target W is scattered or reflected by the surface of the target W, received by the transmission / reception mirror 14, and guided into the detection device 1.

  When the gas G leaked from the gas conduit P stays in the optical path of the infrared light emitted from the transmission / reception mirror 14, the gas G is transmitted if the wavelength of the emitted infrared light is the same as the absorption wavelength of the gas G. In the meantime, infrared light is absorbed. Therefore, the reflected infrared light and scattered light that have been emitted are incident on the transmission / reception mirror 14 while being absorbed and attenuated while passing through the gas G.

  The sum frequency converter 16 introduces laser light from the excitation laser light source 12 and reflected light and scattered light guided from the transmission / reception mirror 14 and performs sum frequency conversion to near infrared light or visible light. In the sum frequency conversion, light having a frequency that is the sum of the frequencies of two incident lights is emitted using a nonlinear optical crystal that satisfies the phase matching condition.

  Since sum frequency conversion is the reverse process of difference frequency conversion, high efficiency conversion can be performed by combining the laser light from the excitation laser light source 12 used in the difference frequency converter in the sum frequency conversion. It is also possible to reduce the size and cost of the apparatus.

Near-infrared light or visible light emitted from the sum frequency converter 16 is incident on the light detector 17 through a bandpass filter, and light having a wavelength converted corresponding to infrared light having an absorption wavelength is detected. The detection output is input to the signal processing circuit 2. Since the light detector 17 detects near infrared light or visible light, its detection performance is significantly improved as compared with a detector that detects infrared light. For example, a conventionally used infrared light detector (In -As detector) has a minimum detection power (W) of 10 ^-8 level, while a near infrared or visible light detector (photomultiplier tube) has a level of 10 ^-10 and the detection sensitivity is It is improved about 100 times. In addition, near-infrared or visible light detectors can detect at the limit of shot noise, which was difficult due to the high noise level of conventional infrared light detectors. The cooling equipment required for this detector is also unnecessary.

  The signal processing circuit 2 performs image processing based on the detection output from the light detector 17 and causes the monitor 3 to display an image of the detection result. The signal processing circuit 2 controls the detection device 1 together with the temperature control circuit 4, the current control circuit 5, and the scan drive circuit 6. The temperature control circuit 4 is a heating means such as a heater so that the temperature of the nonlinear optical crystal used for the seed laser light source 11, the excitation laser light source 12, the optical parametric converter 13 and the sum frequency converter 16 is maintained at a predetermined temperature. The heating control is performed.

  The current control circuit 5 switches and controls the current supplied to the seed laser light source 11 so as to generate laser beams having two different frequencies, and from the optical parametric converter 13 based on the respective laser beams. Two types of infrared light having an absorption wavelength (λon) and a non-absorption wavelength (λoff) of the gas to be detected are emitted alternately at the same level of output. When a semiconductor laser is used as a seed laser light source, the oscillation wavelength can be changed by changing the current value supplied to the semiconductor laser, and the current value is alternately switched between two levels, a high level and a low level. This makes it possible to alternately generate two different wavelengths of infrared light corresponding to the current value level.

  FIG. 2 is a lower graph showing the wavelength λs and output Ps of the laser light emitted from the seed laser light source 11 and an upper graph showing the wavelength λ and output P of the laser light emitted from the optical parametric converter 13. . As can be seen from these graphs, an absorption wavelength λon and an absorption with a strong gas absorption intensity S are obtained from the optical parametric converter 13 corresponding to the two types of laser light having wavelengths λ0 and λ1 emitted from the seed laser light source 11. Infrared light having a weak non-absorption wavelength λoff is output at a high output.

  In general, there are a plurality of absorption wavelengths of the detection target gas. For example, methane gas has absorption wavelengths such as a 1.5 μm band, a 2.5 μm band, and a 3.5 μm band. Then, a wavelength band having low relevance to the absorption wavelength of another gas (for example, water vapor) is selected and used for detection. In methane gas, the 3.5 μm band is generally selected as the absorption wavelength, and a wavelength having a significantly low absorption near the absorption wavelength is selected as the non-absorption wavelength.

In addition to methane gas (CH ₄ ), the infrared light absorption region is also used for substances contained in air pollutants and exhaust gases such as CO ₂ , CO, NO, HF, HCl, SO ₂ , H ₂ S, NH ₃ and HCHO. The absorption wavelength of such a substance and the non-absorption wavelength near the wavelength can be selected and detected.

  The infrared light having the absorption wavelength λon and the non-absorption wavelength λoff is alternately output and irradiated to the target W from the transmission / reception mirror 14, but the scattered light from the target corresponding to the infrared light having the absorption wavelength λon is: As described above, it is absorbed while passing through the gas G, and the received power is attenuated. Scattered light from the target corresponding to the infrared light having the non-absorption wavelength λoff is hardly affected by the gas G.

  The two types of infrared light at each scanning position received in this way are converted into near infrared light or visible light by the sum frequency converter 16 and detected by the light detector 17. Near-infrared light or visible light converted corresponding to infrared light having an absorption wavelength λon and a non-absorption wavelength λoff is converted to light having substantially the same wavelength, but infrared light having an absorption wavelength λon and a non-absorption wavelength λoff is converted. Since the light is emitted alternately, the detection output corresponding to the infrared light having the absorption wavelength λon is detected by detecting the temporal fluctuation of the light having the wavelength, and the detection output corresponding to the infrared light having the non-absorption wavelength λoff. It is possible to detect how much it has been reduced. Then, the concentration of the gas G at each scanning position is calculated based on the ratio at which the detection output decreases, and an image is displayed on the monitor 3 based on the calculated data.

  The scanning drive circuit 6 drives and controls the scanning mechanism 15 in synchronization with the switching control of the current control circuit 5, and each scanning line in which two types of infrared light irradiated from the transmission / reception mirror 14 are set on the surface of the target W It is operated so that it is irradiated along. In this case, the entire region scanned by infrared light in the region from the transmission / reception mirror 14 to the target W is the detection target region, and the concentration of the gas G calculated as described above is an average in the detection target region. Will indicate the concentration.

  FIG. 3 shows an example in which a waveguide-type quasi phase matching nonlinear optical crystal 16 ′ is used as the sum frequency converter 16. Light from the transmission / reception mirror 14 and the excitation laser light source 12 propagates through the optical fiber. The light enters the waveguide-type quasi phase matching nonlinear optical crystal 16 ′ through the fiber coupler 18. The waveguide type quasi phase matching nonlinear optical crystal 16 'can convert the wavelength of incident light to a predetermined wavelength by optical parametric interaction, and changes the wavelength to be converted by changing the domain inversion period. be able to. A typical example is a MgO-doped periodic inversion lithium niobate crystal. By using such a waveguide type quasi phase matching nonlinear optical crystal, it is possible to improve the conversion efficiency for converting the infrared light received by the transmission / reception mirror 14 into near infrared light or visible light. In addition, since such a nonlinear optical crystal may be a small size having a length of 3 cm or less, it is possible to reduce the size of the components of the detection device and improve the portability and convenience of the device.

  FIG. 4 shows an example in which a waveguide-type quasi phase matching nonlinear optical crystal 13 ′ is used as the optical parametric converter 13, and light from the seed laser light source 11 and the pump laser light source 12 passes through the optical fiber. It propagates and enters the waveguide type quasi phase matching nonlinear optical crystal 13 ′ through the fiber coupler 19. Also in this case, it is possible to improve the conversion efficiency of the generated infrared rays, and it is possible to use a low-priced one instead of a high-priced high-output one as an excitation laser light source or a seed laser light source. By using such a waveguide-type nonlinear optical crystal, it is possible to reduce the size of the components of the detection device, further improving the portability and convenience of the device.

  FIG. 5 shows an example in which a quasi phase matching nonlinear optical crystal having a plurality of light propagation portions having different domain periodic structures is used as the optical parametric converter 13 and the sum frequency converter 16. The quasi phase matching nonlinear optical crystal 13 ″ is provided with three light propagation parts having a periodic structure set so that the domain inversion periods Λ1 to Λ3 are different. The domain inversion of each light propagation part is provided. The period is set so that the laser light incident from the excitation laser light source 12 ′ is converted into wavelengths corresponding to the absorption wavelengths of the plurality of types of substances contained in the above-described atmospheric pollutant gas and exhaust gas. A similar function can be realized by using a waveguide type nonlinear optical crystal in addition to such a quasi phase matching nonlinear optical crystal.

  In the case of generating infrared light, the position of the quasi-phase matching nonlinear optical crystal 13 ″ may be shifted so that the laser light from the excitation laser light source 12 ′ is sequentially incident on each light propagation portion. The laser beam from the excitation laser light source 12 ′ may be controlled to be selectively incident on each light propagation unit using an optical means such as a mirror, and supplied to the excitation laser light source 12 ′. And controlling so that infrared light having an absorption wavelength and a non-absorption wavelength of each substance is emitted.

  The infrared light thus emitted is irradiated onto the target W as described above, and the scattered light or reflected light is received and incident on the quasi phase matching nonlinear optical crystal 16 ″, and at the same time, the laser from the excitation laser light source 12 ′. The light is incident. The quasi-phase matching nonlinear optical crystal 16 ″ has three light beams having a periodic structure in which the domain inversion periods Λ1 ′ to Λ3 ′ are set to be different from the quasi-phase matching nonlinear optical crystal 13 ″. Propagation units are provided in parallel, and the domain inversion period of each light propagation unit corresponds to the infrared light emitted from the quasi-phase matching nonlinear optical crystal 13 ″, and each infrared light is converted to near red. The sum frequency is converted to external light or visible light. Therefore, the position of the quasi-phase matching nonlinear optical crystal 16 '' is shifted in synchronization with the change of the light propagation part of the domain periodic structure in which the quasi-phase matching nonlinear optical crystal 13 '' is shifted in position. If the received infrared light and the laser light from the excitation laser light source 12 ′ are incident on the light propagation part of the domain periodic structure, the sum frequency conversion is performed according to the changed infrared light, and the near infrared light Or it can convert into visible light.

  In this way, infrared light corresponding to the absorption wavelengths of a plurality of types of substances is emitted and converted to near-infrared light or visible light by sum frequency conversion according to each infrared light, and the presence or absence of a plurality of types of substances The density can be detected. Therefore, for example, if infrared light having an absorption wavelength of a gas to be detected is selected and irradiated to an exhaust port of a car or an exhaust tower of a factory, exhaust gas from the exhaust port or the exhaust tower is detected in real time. be able to.

  A semiconductor laser (oscillation by injection locking method; oscillation wavelength: 1.55 μm) is used as a seed laser light source constituting the detection device shown in FIG. 1, and an Nd: YAG laser (oscillation wavelength: 1.06 μm) is used as an excitation laser light source. As a parametric converter, an MgO-doped periodic inversion lithium niobate crystal (length 30 mm, thickness 0.5 mm; domain inversion period 30.5 μm) was used. By switching and controlling the current supplied to the seed laser light source, infrared light having an absorption wavelength of 3391.6 nm and a non-absorption wavelength of 3390.2 nm are alternately emitted from the optical parametric converter and stays at a position 5 m away. Irradiated with methane gas. As the sum frequency converter, MgO-doped period-reversed lithium niobate crystal (length 30 mm, thickness 0.5 mm; domain reversal period 22.4 μm) is used to receive the received infrared light together with the laser light from the excitation laser light source. It was. In this case, the received infrared light was converted to near infrared light near 808 nm. As a photodetector, a photomultiplier tube was used to detect the converted near-infrared light near 808 nm. Based on the detection result of near infrared light, the decrease rate of the detection output was calculated from the temporal variation. As a result, it was found that methane gas could be detected to a level of 10 ppm · m.

  If a laser light source having an oscillation wavelength of 980 nm is used as the excitation laser light source, infrared light having an absorption wavelength of 3.4 μm is converted into visible light having a wavelength of 760 nm by the sum frequency converter. It is considered that methane gas can be similarly detected up to a level of 10 ppm · m by using a pipe.

  When carbon dioxide gas (absorption wavelength: 2.1 μm) is detected, when an Nd: YAG laser (oscillation wavelength: 1.06 μm) is used as an excitation laser light source, red light having an absorption wavelength of 2.1 μm is obtained by a sum frequency converter. Since external light is converted into visible light having a wavelength of 704 nm, it is considered that carbon dioxide can be detected to a level of several tens of ppm · m by using a photomultiplier tube as a photodetector. Similarly, if a laser light source having an oscillation wavelength of 980 nm is used as an excitation laser light source, infrared light having an absorption wavelength of 2.1 μm is converted into visible light having a wavelength of 660 nm by the sum frequency converter. Similarly, it is considered that carbon dioxide gas can be detected to a level of several tens of ppm · m by using a double tube.

It is a schematic block diagram regarding embodiment which concerns on this invention. 4 is a graph showing a wavelength λs and an output Ps of laser light emitted from a seed laser light source, and a graph showing a wavelength λ and an output P of laser light emitted from an optical parametric converter. It is a block diagram which shows the case where a quasi phase matching nonlinear optical crystal is used as a sum frequency converter. It is a block diagram which shows the case where a quasi phase matching nonlinear optical crystal is used as an optical parametric converter. It is a block diagram which shows the case where a multi-domain type | mold pseudo phase matching nonlinear optical crystal is used as an optical parametric converter and a sum frequency converter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Gas detection apparatus 2 Signal processing circuit 3 Monitor 4 Temperature control circuit 5 Current control circuit 6 Scanning drive circuit
10 Infrared light irradiation unit
11 Seed laser light source
12 Excitation laser source
13 Optical parametric converter
14 Transceiver mirror
15 Scanning mechanism
16 Sum frequency converter
17 Light detector

Claims (5)

  Irradiation means for irradiating the detection target area with infrared light for detecting the detection target gas, light receiving means for receiving infrared light scattered or reflected from the detection target area, and sum frequency of the received infrared light It is characterized by comprising conversion means for converting to near infrared light or visible light by conversion, and detection means for detecting a detection target gas based on the near infrared light or visible light converted by the conversion means. Gas detector.   The gas detection apparatus according to claim 1, wherein an excitation light source used for the conversion unit is used as an excitation light source for generating infrared light in the irradiation unit.   The gas detection apparatus according to claim 1, wherein the conversion unit includes a quasi phase matching nonlinear optical crystal that performs sum frequency conversion.   The gas detector according to any one of claims 1 to 3, wherein the irradiating means includes a quasi phase matching nonlinear optical crystal for generating predetermined infrared light.   The irradiating means includes a quasi phase matching nonlinear optical crystal having a plurality of light propagation portions having different domain periodic structures for generating infrared light of different wavelengths, and the converting means is a sum of infrared lights of different wavelengths. 3. The gas detection device according to claim 1, further comprising a quasi-phase matching nonlinear optical crystal having a plurality of light propagation portions having different domain periodic structures for performing frequency conversion. 4.

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