JPH11108836A - Apparatus and method for detecting very small amount of gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a very small amt. of gas inexpensively by a small-sized lightweight apparatus. SOLUTION: A wavelength variable semiconductor laser device 2 oscillates laser beam of frequency designated by a wavelength sweeping part 11. A receiving and transmitting optical system 3 irradiates a space 100 to be measured where a very small amt. of gas is considered to be present with laser beam emitted from the wavelength variable semiconductor laser device 2. The reflected beam incident on the receiving and transmitting optical system 3 is photoelectrically converted into an electric signal by a photodetector 4 to be inputted to an amplifier 5. The electric signal amplified by the amplifier 5 is converted by an A/D converter 6 to be inputted to a signal processor 1. A concn. detection part 13 obtains the concn. of a very small amt. of the gas in the space 10 to be measured on the basis of the input signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for detecting N
The present invention relates to a trace gas detection device and a trace gas detection method for detecting trace gases such as O ₂ , SO ₂ , O ₃ , and water vapor.

[0002]

2. Description of the Related Art Generally, various gas molecules have an absorption spectrum specific to the wavelength of light. Therefore, by irradiating a light having a wavelength at which a trace gas molecule to be detected shows high absorption from a light source such as a laser, and detecting a decrease in the intensity of the light having the wavelength after passing through a measurement space such as the atmosphere, The presence of trace gas molecules to be detected in the space to be measured can be detected.

[0003] A differential absorption method is a specific method for detecting the concentration of a trace amount of gas in a space to be measured by using an intrinsic absorption spectrum of gas molecules. In the differential absorption method, light having a wavelength at which a gas molecule to be detected exhibits high absorption and light having a wavelength near the wavelength and having low absorption are alternately applied to a measurement space from a laser or the like. Then, the light propagating in the space to be measured is received, and the concentration of the gas is detected from the difference between the received light intensities of the two wavelengths of light. In the differential absorption method, a differential absorption lidar (DIAL: Differential Absorber) that detects scattered light in a measured space is used.
ption lidar), a long optical path difference absorption method for receiving light scattered on a solid target, and the like.

When detecting a trace gas in the space to be measured using the intrinsic absorption spectrum of a gas molecule, the absorption wavelength width is extremely narrow. Must be controlled to match exactly. For this purpose, use a precise wavemeter or enclose the same gas as the gas to be detected as a reference gas in the gas cell, transmit the gas cell while changing the wavelength of laser light, etc., and minimize the amount of transmitted light. That is, the measured space is irradiated with laser light or the like while controlling the wavelength so that the absorption is maximized. Also, when the differential absorption method is used using a tunable semiconductor laser, the oscillation is performed by changing the temperature or changing the state of the external resonator in order to oscillate light of two wavelengths alternately. The wavelength is controlled. In order to match the oscillation wavelength to the absorption spectrum of the gas molecule, the temperature and the state of the external resonator must be rapidly changed, and strictly controlled so that the changed wavelength becomes stable. Therefore, the detection device must be complicated. When an external resonator is used, it is necessary to remove a detection value when the wavelength is switched and to use a detection value after the wavelength is stabilized. This also leads to a complicated device.

[0005] As an apparatus for detecting a trace amount of gas in a space to be measured by utilizing an intrinsic absorption spectrum of gas molecules,
For example, JP-A-56-87845 and JP-A-7-1
Although there are apparatuses described in Japanese Patent Publication No. 28232 and Japanese Patent Publication No. 2-47694, each apparatus has a complicated configuration such that a gas cell for reference or calibration is required.

[0006]

As described above, according to the conventional trace gas detecting device, it is necessary to control the wavelength of light such as laser light so as to strictly match the wavelength corresponding to the absorption spectrum. The control part becomes complicated.
In addition, a high-precision wavelength meter must be used, and a gas cell for reference or calibration must be used. High precision wavemeters are expensive. Also, if the gas cell is enlarged, the device becomes huge, so it cannot be so large. As a result,
It is necessary to fill the gas cell with a gas having a concentration much higher than the concentration of the gas actually measured. Then, dew condensation may occur beyond the saturated vapor pressure of the gas, and the incidental facilities may become large. As described above, the conventional trace gas detection device has a problem that the device becomes complicated, large, or expensive.

Accordingly, an object of the present invention is to provide a trace gas detecting device and a trace gas detecting method which can be realized in a small, lightweight, and inexpensive manner with a simplified mechanism.

[0008]

According to the present invention, there is provided a trace gas detecting apparatus for irradiating light to an area to be measured while continuously changing the wavelength within a predetermined range including a wavelength corresponding to an absorption spectrum of a gas to be detected. Variable light irradiating means, light detecting means for receiving light of each wavelength propagated through the measured area and detecting its intensity, and detecting objects in the measured area based on the degree of each intensity detected by the light detecting means. Detecting means for detecting the presence of gas. The detection means includes a concentration detection unit that obtains the concentration of the gas to be detected from the difference between the received light intensity of light having a wavelength corresponding to a spectrum outside the absorption spectrum of the gas to be detected and the intensity of the lowered level (for example, the lowest level intensity). A configuration may be included. Further, the detection means is configured to include a determination means for determining the amount of the gas to be detected in the measurement target region from a low-level reception intensity (for example, the lowest level) of the light reception intensities of light of a predetermined range of wavelengths. Is also good. Further, the light detection means includes a light detector that converts the received light into an electric signal and an amplifier that outputs a fundamental wave component of the output of the light detector, and the determination means compares the amplitude of the output of the amplifier with a threshold value. A configuration including comparison means for determining the amount of the gas to be detected may be employed.

Further, the method for detecting a trace gas according to the present invention comprises:
Irradiating light to the measurement area while continuously changing the wavelength in a predetermined range including the wavelength corresponding to the absorption spectrum of the gas to be detected, receiving light of each wavelength propagated through the measurement area,
The presence of the gas to be detected in the measurement region or the concentration of the gas to be detected in the measurement region is detected based on the difference in the intensity of the received light of each wavelength.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing one embodiment of a trace gas detection device according to the present invention. As shown in FIG. 1, the wavelength tunable semiconductor laser device 2 oscillates a laser beam having a frequency specified by the wavelength sweeping unit 11 of the signal processing device 1. The transmission / reception optical system 3 irradiates the laser beam emitted from the wavelength tunable semiconductor laser device 2 and having passed through the collimator lens 30 to a measured space 100 where a trace gas is considered to exist. The light that has propagated through the measured space 100 is
Then, the light enters the transmission / reception optical system 3. The light incident on the transmission / reception optical system 3 is photoelectrically converted by the photodetector 4 into an electric signal, and is input to the amplifier 5. The electric signal amplified by the amplifier 5 is subjected to A / D conversion by the A / D converter 6 and then input to the signal processing device 1. In the signal processing device 1, the correction unit 1
After the 2 performs distance correction and the like, the concentration detecting unit 13 obtains the concentration of the trace gas in the measured space 100 based on the input signal, and the display unit 14 displays the obtained concentration.

In this embodiment, in the wavelength tunable semiconductor laser device 2, the oscillation frequency of the semiconductor laser 21 is
Output mirror 23, diffraction grating 24 and total reflection mirror 25
Is controlled by the external resonator 22 and the piezoelectric element 26. The wavelength sweeping unit 11 of the signal processing device 1 controls a voltage applied to the piezoelectric element 26. In this embodiment, the transmission / reception optical system 3 includes a transmission optical system 31 including a beam expander including a concave lens 33 and a convex lens 34, and includes a reception optical system 32 including a condenser lens 35.

Next, the operation will be described with reference to FIG. 1 and FIGS. In the signal processing device 1, the wavelength sweeping unit 11 changes the inclination of the total reflection mirror 24 so that light whose frequency gradually changes is oscillated. Within the frequency change range, the unique absorption spectrum of the trace gas to be detected is included. The total reflection mirror 24 is configured to change its inclination by a piezoelectric element 26 acting as a piezoelectric actuator. Further, the wavelength of the laser light from the tunable semiconductor laser device 2 changes according to the inclination of the total reflection mirror 24. Therefore, the wavelength sweeping unit 11 applies a voltage corresponding to the amount of inclination of the total reflection mirror 24 to the piezoelectric element 26. Then, light whose wavelength gradually changes is emitted from the wavelength variable semiconductor laser device 2.

The light from the wavelength tunable semiconductor laser device 2 is
After being converted into parallel light by the collimating lens 30, the divergence angle is adjusted by a beam expander in the transmission optical system 31 of the transmission / reception optical system 3, and the beam is irradiated onto the detection space 100 in which a trace gas to be detected is considered to be present. Detected space 1
The light that has propagated through 00 is reflected by the wall surface 200, propagates through the detected space 100 again, and is received by the receiving optical system 32 of the transmitting and receiving optical system 3.
Incident on. The light incident on the receiving optical system 32 is condensed by the condenser lens 35 and is incident on the photodetector 4. Photodetector 4
Converts photoelectrically the incident light and outputs an electric signal having an amplitude corresponding to the intensity of the incident light. The electric signal is input to the amplifier 5.

The amplifier 5 amplifies the electric signal in the band corresponding to the wavelength sweep speed, and outputs the amplified electric signal to the A / D converter 6. The A / D converter 6 A / D converts the input electric signal and outputs the signal to the signal processing device 1. In the signal processing device 1, the correction unit 12 multiplies the digitized signal by a coefficient proportional to, for example, the square of the light propagation distance. The intensity of the received signal decreases in inverse proportion to the square of the light propagation distance, but the correction unit 12 corrects that point.

When a trace amount of gas is present in the space to be measured 100, as shown in FIG. 2, light of a specific wavelength is absorbed. The light intensity is weak. Then, the photodetector 4
, The strength (amplitude) of the electrical signal, that is, the reception input also decreases. As shown in FIG. 2, when the concentration of the trace gas is low, the degree of decrease in the reception input is small, but when the concentration is high, the decrease in the reception input becomes remarkable. Further, the difference between the value of the reception input when the reception input does not decrease and the value at the minimum point of the reception input corresponds to the concentration of the trace gas.

Therefore, the density detecting section 13 in the signal processing device 1 obtains the minimum value among the signal values inputted every moment. Then, from the difference between the value and the signal value whose value has not decreased, the concentration of the trace gas present in the measured space 100 is calculated and displayed on the display unit 14.
When the trace gas detection device is used as an alarm device for detecting leakage of water vapor or other gas, if each input signal value becomes lower than a predetermined threshold value (see FIG. 2), the leakage is detected. The concentration detector 13 can issue an alarm when the concentration of the gas has become higher than the predetermined concentration. Since the reception input changes in accordance with the intensity of the laser beam and the like, these factors are considered when setting the threshold.

As described above, according to this embodiment,
Light of a predetermined range of frequencies including a unique absorption spectrum of a trace gas considered to exist in the measured space 100,
Irradiation is performed on the space to be measured 100 while continuously changing the frequency.
Further, a reception input obtained by photoelectrically converting the light reflected by the wall surface 200 is obtained from time to time. Then, the measured space 100
The concentration of the trace gas is obtained from the received input value when the received input from is minimized, or an alarm is issued based on the result of comparison between the received input value and the threshold value. Since the wavelength variable range includes a wavelength corresponding to the absorption spectrum of the gas to be detected, if the gas to be detected exists in the measured space 100, some of the reception inputs obtained every moment have a reduced level. . That is, if there is a low level among the received input values, it can be detected that the detection target gas exists.
In this embodiment, since it is only necessary to emit light whose wavelength continuously changes within a predetermined range, it is not necessary to make the oscillation wavelength of the semiconductor laser 21 strictly coincide with the intrinsic absorption spectrum of the trace gas. Therefore, the device configuration can be simplified.

Further, as shown in FIG. 3, the variable range of the wavelength of the laser light from the wavelength tunable semiconductor laser device 2 is changed from the bottom of the wavelength not absorbed by the gas to be detected to the peak in the width of the absorption spectrum. And the width up to the next hem (on the order of 10 to 100 pm). Then, the wavelength of the laser light is changed so that the oscillation wavelength reciprocates within the range. Further, the reciprocation is repeated at a frequency of, for example, kHz. The wavelength sweep unit 11 sets
The voltage applied to the piezoelectric element 26 in the wavelength tunable semiconductor laser device 2 is changed so that the oscillation wavelength changes. With this configuration, the temporal change of the received input is
When the concentration of the gas to be detected is high, the result is as shown in FIG. 4A, and when the concentration of the gas is low, the result is as shown in FIG. 4B.

When the variable range is limited as described above, the operation range of the piezoelectric element 26 is also limited, so that the configuration of the part for controlling the piezoelectric element 26 is simplified. Further, if the sweep of the wavelength of the laser beam is repeated on the order of kHz, the amplification range of the amplifier 5 can be a narrow band (on the order of kHz). Further, the amplifier 5 is configured to amplify only the fundamental wave component. Then, the output of the amplifier 5 is as shown in FIG.
As shown in (B), the sine wave has a frequency twice as high as the reciprocating frequency of the wavelength sweep (one reciprocation is defined as one cycle).

According to the above simplified structure, as shown in FIG. 6, the trace gas detecting device can have a simpler structure. In FIG. 6, the wavelength sweep unit 1
1 is the same as that shown in FIG. 1, but in this case, the output of the amplifier 5 is input to the alarm generator 7. The alarm generator 7 is configured as shown in FIG. 6, for example, and operates a level comparator 72 that inputs the output of the amplifier 5 via a capacitor 71, an alarm 74 such as a buzzer or a lamp, and an alarm 74. Switch 73 is included.

The operation of the trace gas detection device shown in FIG. 6 will be described below with reference to the waveform diagram of FIG. The operation until the wavelength sweeping unit 11 performs control so that the laser light of a predetermined range of wavelength is output and the amplifier 5 amplifies the reception input is the same as the operation in the apparatus shown in FIG. In this case, the output of the amplifier 5 is input to the alarm generator 7. In the alarm generator 7, the sine wave from which the DC component has been removed by the capacitor 71 as shown in FIG. 7 is input to the level comparator 72. The amplitude of the sine wave input to the level comparator 72 corresponds to the concentration of the trace gas present in the measured space 100.

Therefore, the level comparator 72 compares the amplitude of the input sine wave with the alarm threshold. FIG. 7 (A)
If the amplitude of the sine wave exceeds the alarm threshold as shown in FIG. 7, the level comparator 72 turns on the switch 73. Then, the alarm 74 composed of a buzzer and a lamp
Operates and an alarm is issued. If the amplitude of the sine wave does not exceed the alarm threshold as shown in FIG. 7B, the level comparator 72 switches the switch 73 because the concentration of the trace gas present in the measured space 100 is low. Do not conduct. Therefore, no alarm is issued.

As described above, whether the alarm is issued or not is determined according to the magnitude of the amplitude of the reception input. Therefore, the signal processing device 1 as shown in FIG. 1 does not need to be provided. That is,
If only an alarm needs to be issued, the configuration can be simplified and the device can be reduced in size and weight. The wall 2
When the distance to 00 is large or when the laser output is small, the output amplitude of the amplifier 5 does not increase. When it is difficult to distinguish between such a case and a case where the gas concentration is low and the amplitude is small, the DC component of the output of the photodetector 4 is monitored, and the threshold value in the level comparator 72 is set. It should be reflected.

In each of the above embodiments, a continuous wave laser (CW laser) is used. However, when it is difficult to measure the distance, for example, when the distance to the wall 200 is large, the peak output of the laser light is increased. For this purpose, the laser light may be pulsed. Further, the external resonator 22 may have a Q-switch function. In this way, the measurement distance can be increased. In order to further increase the output, a tunable solid-state laser such as a Ti: sapphire laser may be used.

When a gas having an absorption spectrum that cannot be oscillated by the above-described semiconductor laser or solid-state laser is used, the second harmonic (SHG) or the third harmonic (THG) is used.
The light having a wavelength corresponding to the absorption spectrum may be emitted by utilizing the above. Also, light having such a wavelength may be emitted using optical parametric oscillation.

If there are several absorption spectra that are extremely close to each other and it is difficult to identify only a specific spectrum and emit a laser beam in a wavelength range corresponding to the absorption spectrum, as shown in FIG. The variable range may be widened. In this case, if the number of absorption spectra included in the range is large, the output frequency of the amplifier 5 increases, but this can be handled by using the amplifier 5 having a wide operating range or reducing the wavelength variable speed.

[0027]

As described above, according to the present invention, the trace gas detecting device is provided with a wavelength tunable light irradiating means for irradiating light to the measured area while continuously changing the wavelength, and a light propagating through the measured area. Light detecting means for receiving the light of each wavelength and detecting the intensity thereof, and detecting means for detecting the presence of the gas to be detected in the measured area based on the degree of each intensity detected by the light detecting means. With this configuration, it is not necessary to strictly adjust the wavelength of light to a wavelength corresponding to the absorption spectrum of the gas in order to detect a trace gas in the region to be measured. As a result, there is an effect that it is possible to provide a small-sized gas detector having a simplified configuration and reduced size and weight. When the detection means is configured to include a concentration detection unit that obtains the concentration of the detection target gas from the difference between the received light intensity of the light having a wavelength corresponding to the spectrum outside the absorption spectrum of the detection target gas and the reduced intensity of the light. Has the effect of obtaining the concentration of the gas to be detected in the measurement area with a simplified configuration. If the detection means is configured to include a determination means for determining the amount of the gas to be detected in the measurement area from the reception intensity of the received light having a reduced level among the light reception intensities of light in a predetermined range, the detection target This has the effect of simplifying the configuration of the trace gas detection device for detecting the presence of gas. The light detection means includes a photodetector that converts the received light into an electric signal and an amplifier that outputs a fundamental wave component of the output of the photodetector, and compares the amplitude of the output of the amplifier with a threshold to determine the amount of gas to be detected. In the case where the comparing means for judging is provided with the judging means, there is an effect that it is possible to provide a small and light trace gas detecting device.

In the method for detecting a trace amount of gas, a light is irradiated onto an area to be measured while continuously changing the wavelength, light of each wavelength transmitted through the area to be measured is received, and the intensity of the received light of each wavelength is received. If the presence of the gas to be detected in the measurement area or the concentration of the gas to be detected in the measurement area is detected based on the difference between the two, the configuration of the system for detecting the trace gas is simplified and the size and weight are reduced. There is an effect that is made.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a trace gas detection device according to the present invention.

FIG. 2 is an explanatory diagram illustrating a relationship between a reception input and a wavelength of light.

FIG. 3 is an explanatory diagram showing an example of a wavelength variable range.

FIG. 4 is an explanatory diagram showing an example of a reception input obtained every moment.

FIG. 5 is a waveform chart showing an example of an amplifier output.

FIG. 6 is a block diagram showing another embodiment of the trace gas detecting device according to the present invention.

7 is an explanatory diagram illustrating an example of an input of a level comparator in the trace gas detection device illustrated in FIG. 6;

FIG. 8 is an explanatory diagram showing another example of the wavelength variable range.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 signal processing device 2 wavelength tunable semiconductor laser device 3 transmission / reception optical system 4 photodetector 5 amplifier 6 A / D converter 7 alarm generator 11 wavelength sweeper 13 concentration detector 100 measured space

Claims (5)

[Claims] A wavelength tunable light irradiating means for irradiating light to an area to be measured while continuously changing a wavelength in a predetermined range including a wavelength corresponding to an absorption spectrum of a gas to be detected; Light detecting means for receiving light of each wavelength and detecting the intensity thereof, and detecting means for detecting the presence of the gas to be detected in the measured area based on the degree of each intensity detected by the light detecting means. Equipped with a trace gas detector. 2. A concentration detecting unit for obtaining a concentration of the gas to be detected from a difference between a light receiving intensity of light having a wavelength corresponding to a spectrum outside the absorption spectrum of the gas to be detected and a light receiving intensity having a lowered level. The trace gas detection device according to claim 1, comprising: 3. The trace amount according to claim 1, wherein the detection means includes a determination means for determining the amount of the gas to be detected in the measurement area from a low-level reception intensity among the reception intensities of light of a predetermined range of wavelengths. Gas detector. 4. The photodetector includes a photodetector that converts received light into an electric signal, and an amplifier that outputs a fundamental wave component of an output of the photodetector. 4. The trace gas detection device according to claim 3, further comprising a comparison unit that compares the threshold value with the threshold value to determine the amount of the detection target gas. 5. A target area is irradiated with light while continuously changing a wavelength within a predetermined range including a wavelength corresponding to an absorption spectrum of a gas to be detected, and light of each wavelength transmitted through the target area is received. A trace gas detection method for detecting the presence of the gas to be detected in the measurement area or the concentration of the gas to be detected in the measurement area based on a difference in the intensity of the received light of each wavelength.