JPH1123450A - Method and equipment for detecting gas concentration - Google Patents

Abstract

PROBLEM TO BE SOLVED: To separate a resonant signal from an unresonant signal without requiring any precise and intricate wavelength discriminator, e.g. a high resolution spectrometer or a narrow band interference filter, and to measure the gas concentration without being affected by fluctuation in the irradiating atmosphere or the laser intensity. SOLUTION: The method for detecting the gas concentration comprises a step for irradiating a region 2 to be measured with a wide band laser light 1, a step for collecting the backscattering light 4 from the region 2, a step for splitting the backscattering light 4 into light beams 6a, 6b, a step for passing one light beam 6a through a high concentration gas to be measured to a first photodetector 10a and passing the other light beam 6b, as it is, to a second photodetector 10b, and a step for calculating the mean gas concentration in the region 2 to be measured based on the intensity of backscattering light detected by the photodetectors 10a, 10b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration detecting device having a laser backscattered light detecting mechanism.

[0002]

2. Description of the Related Art Gas sensing technology utilizing the fact that monochromatic light of a specific wavelength is easily absorbed by a specific gas is widely used in industrial measurement, pollution monitoring and the like. In particular, the method of irradiating the atmosphere with two types of high-intensity pulsed laser beams having different wavelengths and deriving (detecting) the concentration and distribution of a specific gas type from the difference in the time variation of the backscattered light intensity between the two is as follows. Known as a differential absorption lidar (DIAL), it is used to measure ozone and water distribution in the upper atmosphere and the stratosphere, and to measure carbon dioxide, SO _x , and NO _x in urban areas.

FIG. 4 shows the entire system of a conventional differential absorption lidar. As shown in FIG. 4, the differential absorption lidar irradiates a first laser which irradiates the atmosphere with a narrow band laser beam 30 having a wavelength width about the absorption line width resonated with the absorption line of the target gas in the measurement target region. 31 and a second laser 33 for irradiating the atmosphere with a non-resonant narrow band laser beam 32 to the absorption line of the target gas in the measurement target region. These first and second lasers 31, 33, two types of narrow-band laser light, that is, a resonance laser light 30 and a non-resonance laser light 3
2 are transmitted through the laser light transmitting / combining mirrors 34 and 35 and combined with each other, and the combined laser light 36 is irradiated onto the measurement target area 38 via the laser mirror 37. ing.

The light (backscattered light) 39 scattered backward from the measurement target area 38 is converted into a laser light 36.
The light is condensed by a telescope 40 having the same optical axis as that of the light, and is detected by a light detection system including a photodetector 41 including a photomultiplier tube, a photodiode, and an amplifier 42. If necessary, a wavelength discriminator 43 composed of a spectroscope, an interference filter and the like is included in the light detection system as shown in FIG. 4 to reduce the influence of external light. Thus, this signal is converted to a signal processing unit (control unit) 4.
4 to derive (calculate) the concentration distribution. Note that the resonance laser light 30 and the non-resonance laser light 32 can also be oscillated with a time shift by one device.

Here, the principle of gas concentration detection by the differential absorption lidar will be outlined as follows.

The backscattered light intensity P (R) from a position separated from the light source and the detection unit by a distance R is given by the following laser radar equation.

(Equation 1) Here, P _{0 is} the output (W) of the pulse light. τ: pulse time width. (P ₀ τ is the pulse output (J).) L = cτ / 2; half (m) of the laser pulse space length. c: speed of light (m / s). K: total efficiency of transmitting / receiving optical system (-) T (R): atmospheric transmittance (-). β (R); volume backscattering coefficient of the scatterer (sr ⁻¹ · m ⁻¹ ). _Ar : opening area of the receiving optical system (m ² ). Y (R): a parameter indicating the overlap between the transmission beam and the reception field. (1 when the laser and the receiving light are coaxial.)

In the above equation (1), the terms other than T (R) can be considered to be equal with a very good approximation when the resonance and non-resonance laser light wavelengths are almost the same. Further, T (R) can be expressed by the following equations (2) and (3) for resonance and non-resonance, respectively.

(Equation 2)

[Outside 1]

Further, assuming that the absorption cross sections of the laser at resonance and non-resonance are σ _on and σ _off ,

(Equation 3) It is. Here, n (R) indicates the density (m ⁻³ ) of the target molecule at the R position.

By substituting the above equations (2) to (5) into the above equation (1),

(Equation 4) Becomes

In fact, the output by the photons arriving at the detector at times t and t + Δt is: If Δt is sufficiently longer than the laser time width τ,

(Equation 5) Is given by the following equation.

(Equation 6) Here, Ph _on (t) and Ph _on (t + Δt) are outputs at times t and t + Δt at resonance, and Ph _off (t) and Ph _off (t + Δt) are times t and t + Δt at non-resonance.
Is the output at.

Here, if I (t) is defined as in the following equation (11) and is calculated by substituting the above equations (6) and (7),

(Equation 7) Becomes

Therefore, if the absorption cross sections σ _on and σ _off by the laser at the time of resonance and non-resonance are known and the signals at time t and t + Δt can be obtained with high accuracy, the above (1)
The average density n (R) between the distances R to R + ΔR (that is, the density of the target molecule) can be obtained from the expression 2).

[0013] Thus, in the conventional differential absorption lidar, the laser beam having a wavelength resonating with the absorption line of the target gas and a non-resonant laser beam having two wavelengths are radiated into the atmosphere to be backscattered. The time change of the signal (backscattered light) is detected, and the average gas concentration (concentration distribution) is obtained from the above equations (11) and (12).

[0014]

However, in this type of system, laser beams having two different wavelengths, one that resonates with the absorption line of the target gas in the region to be measured and the other that does not resonate, are radiated into the atmosphere. It is necessary to use two laser oscillators or to oscillate laser light of different wavelengths in time with one laser oscillator and irradiate them to the atmosphere. There was a problem of becoming large.

In the case where one laser oscillator oscillates laser beams having different wavelengths with a time lag,
There is also a problem that an error is included in the derived gas average concentration due to fluctuations in the atmosphere, etc., because the two wavelengths of laser light are irradiated with a time shift.

When two laser oscillators are used to simultaneously irradiate a space with narrow-band laser light (resonant laser light 30 and non-resonant laser light 32) of two wavelengths from these laser oscillators, Since the wavelength and the non-resonant wavelength need to be separated from each other by an optical device or an interference filter or more, and the scattering coefficient of the atmosphere may differ between the two wavelengths, the derived gas average concentration (concentration) Distribution) may contain systematic errors.

The present invention has been made in view of the above problems, and an object thereof is to use a precise and complicated wavelength discriminator such as a spectroscope having a high resolution or a narrow band interference filter. Gas concentration detection that can obtain a signal in a resonance state and a signal in a non-resonance state without being affected, and can measure the gas concentration without being affected by fluctuations in the irradiated atmosphere or fluctuations in laser intensity. It is to provide a method and an apparatus.

[0018]

In order to achieve the above object, a gas concentration detecting method according to the present invention comprises: (a) an irradiation step of irradiating a broadband laser beam to a measurement target area;
(B) a light collecting step of collecting backscattered light scattered backward from the measurement target area; (c) a dividing step of dividing the backscattered light into two light rays; and (d) the dividing. One of the light beams transmitted through the high concentration gas to be measured and introduced into the first photodetector,
The other of the split light beams is used as a second light
(E) introducing the light into the light detecting means;
And a calculating step of calculating the average gas concentration in the measurement target area based on the backscattered light intensity detected by the second light detecting means, respectively.

Further, in the gas concentration detecting device according to the present invention, a laser light irradiation mechanism for irradiating the measurement target area with the broadband laser light, and collecting the backscattered light from the measurement target area irradiated with the broadband laser light. A light collecting mechanism, a beam splitter for splitting the backscattered light collected by the light collecting mechanism into two light beams, and a beam splitter filled with a high-concentration gas to be measured and split by the beam splitter. A saturated absorption cell through which one of the divided light beams is transmitted, a light beam transmitted through the saturated absorption cell, and a second light beam into which the other one of the light beams split by the beam splitter is introduced. A first and second photodetector, and a signal processing unit for calculating an average gas concentration in the measurement target area based on the backscattered light intensity obtained from the first and second photodetectors; So that comprise respectively.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 shows an entire system of a gas concentration detecting device used for implementing a gas concentration detecting method according to one embodiment of the present invention. The gas concentration detection device shown in FIG. 1 includes a laser beam irradiation mechanism 3 for irradiating a broadband laser beam 1 having a broader wavelength band than an absorption line width to a measurement target area 2, and measurement performed by irradiating the broadband laser beam 1. A light collecting mechanism 5 for collecting the backscattered light 4 from the target area 2,
A beam splitter 7 for splitting the backscattered light 4 condensed by the light condensing mechanism 5 into two light beams 6a and 6b; Rays 6a and 6b divided at 7
Absorption cell 8 through which one light ray 6a is transmitted
And the first and second photodetectors 10a, 10a into which the light beam 9 transmitted through the saturated absorption cell 8 and the other light beam 6b of the light beams 6a, 6b split by the beam splitter 8 are respectively introduced. 10b, and a signal processing unit 11 including a calculating unit for calculating an average gas concentration in the measurement target area 2 based on the backscattered light intensity obtained from the first and second photodetectors 10a and 10b, respectively. doing.

The above-described laser light irradiation mechanism 2 includes one laser 12 for irradiating the atmosphere with the broadband laser light 1,
A laser mirror 13 for refracting the broadband laser light 1 emitted from the laser 12 toward the measurement target area 2
a and 13b. Laser 12 of this example
Is a device which is tuned to the resonance position of the absorption line of the target gas and oscillates the broadband laser light 2 having a wavelength width wider than the absorption line width and irradiates it to the atmosphere. The condensing mechanism 4 further includes a telescope 14 for condensing the backscattered light 4 from the measurement target area 2, and a light beam condensed by the telescope 14 being converged into a beam having a predetermined diameter to the beam splitter 7. And a converging lens 15 for feeding.

Further, on one optical path 16a of the optical paths 16a and 16b of the light beams 6a and 6b split into two by the beam splitter 7, the saturated absorption cell 8, the condenser lens 17a, and the photomultiplier are provided. A photodetector 10a including a tube, a photodiode, and the like are sequentially arranged. On the other optical path 16b of the optical paths 16a and 16b, a condenser lens 17b and a photodetector 10b including a photomultiplier tube, a photodiode, and the like are sequentially arranged. The detection signals obtained from the photodetectors 10a and 10b are supplied to the calculation unit of the signal processing unit 11 via the amplifiers 18a and 18b, respectively. The signal processing unit 11 includes amplifiers 18a, 18
b) has a signal processing function of calculating the gas average concentration based on the signal supplied from b, has a function of displaying the gas average concentration obtained by the calculation on a screen, and controls the laser 12 described above. It has a function to perform.

Next, the operation and operation of the gas concentration detecting device having such a configuration will be described below. First, a laser beam (broadband laser beam) 1 having a wider bandwidth than the absorption line of the gas to be measured is applied to the laser 1.
2, the broadband laser light 1
Is irradiated through the laser mirrors 13a and 13b into the measurement target area 2. Then, the backscattered light 4 from the measurement target area 2 is collected by the telescope 14, and the collected light is split by the beam splitter 7 into two light beams 6 a,
6b. One of the split light beams 6a and 6b is introduced into the saturated absorption cell 8 containing the gas to be measured through the optical path 16a, and the light beam 6a transmitted through the gas to be measured in the saturated absorption cell 8 is The light is condensed by the condenser lens 17a and is incident on the first photodetector 10a. On the other hand, the other light ray 6b of the split light rays 6a and 6b passes through the optical path 16b and is condensed by the condensing lens 17b and is directly used (without passing through the above-described saturated absorption cell 8).
To the photodetector 10b. Thereafter, detection signals respectively output from the photodetectors 10a and 10b are supplied to the signal processing unit 11, and a predetermined calculation based on these detection signals (calculation for calculating the average gas concentration) is performed.
Is performed in the calculation unit 11a of the signal processing unit 11, and the calculation result is derived as a gas average concentration (concentration distribution). The average gas concentration is displayed on a screen (not shown) of the signal processing unit 11.

Here, the features of the gas concentration detecting apparatus of FIG. 1 according to the embodiment of the present invention and the gas concentration detecting method implemented by using this apparatus will be described as follows. As described above, the conventional differential absorption lidar irradiates the atmosphere with a laser beam having a wavelength that resonates with the absorption line of the target gas and a non-resonant laser beam into the atmosphere to generate a backscattered signal ( The time change of the backscattered light is detected, and (1)
It is necessary to obtain the average gas concentration (concentration distribution) from the expressions (1) and (12). On the other hand, in the present invention, instead of irradiating the laser light of two wavelengths to the atmosphere, the broadband laser light having a wavelength width wider than the absorption line of the target gas is radiated to the atmosphere. The present invention is characterized in that the same effect as the laser light irradiation can be obtained.

The operation of the methane ₃ band will be specifically described below with reference to FIG.

FIG. 2 shows methane 2 near 1.66 μm.
shows the spectrum of the Q- branch of [nu ₃ band. Full width at half maximum 2c tuned to the strongest Q (6) line
A laser beam of about m ^-1 is oscillated by the laser 12 shown in FIG. Of the two rays 6a and 6b collected by the telescope 14 as the backscattered light 4 and split by the beam splitter 7, the ray 6a in the optical path 16a passes through the saturated absorption cell 8 filled with high-concentration methane and The light not absorbed by methane is collected on the first photodetector 10a. The spectrum is shown in FIG.
As shown in (a), the frequency corresponding to the absorption line of methane is absorbed, and the backscattered light intensity becomes zero. On the other hand, the spectrum of the light beam 6b in the other optical path 16b is weakened because the part absorbed by methane in the atmosphere is attenuated according to its concentration (see FIG. 3B). In other words, the signal on the optical path 16a (light ray 6a) corresponds to the backscattered light intensity in the non-resonant state excluding the absorption wavelength of methane, whereas the signal on the optical path 16b (light ray 6b) is in the non-resonant state. This corresponds to the sum of the backscattered light in the resonance state.

The first and second photodetectors 10a when the elapsed time t from the time of laser irradiation, 10b thus each backscattered light intensity sensed I _a ^t, if I _b ^t,

(Equation 8) It is. However, α, in the above equations (13) and (14),
β represents a proportional constant relating to the overall efficiency of light after the beam splitter 7, the gain of the photodetectors 10a and 10b, and the gains of the amplifiers 18a and 18b.

From the above equations (13) and (14), Ph
_{When on} (t) and Ph _off (t) are obtained and substituted into the above equation (11),

(Equation 9) Becomes However, in the above (15), I _a ^{t +} 1, I
_b ^{t + 1} is the detected signal strength at t + Δt.

Therefore, similarly to the above equation (12), the distance R
The average concentration n (R) between ＲR + ΔR is obtained from the following equation (16).

(Equation 10)

Here, in the above equation (15), β / α
Although the value is unknown, this value is stored in the saturated absorption cell 8.
By performing the measurement without filling the tongue,
I_a, I _bIs obtained as a ratio of

In addition, the absorption cross sections σ _on and σ _off by the laser at the time of resonance and non-resonance can be obtained from Rothman et al.
N) "(Appl. Opt. 26 4058 (1986)).

As described above, according to the embodiment of the present invention, the optical paths 16a, 16 divided by the beam splitter 7 into two.
By disposing a saturated absorption cell 8 filled with a high concentration of methane or the like in front of the photodetector 10a in one of the optical paths 16a of the optical path 16b, the complexity of a narrow band spectrometer or a filter can be reduced.
Without using a precise wavelength discriminator, signals in a resonance state and a non-resonance state can be separated, and an average concentration of a gas (for example, methane) can be calculated based on these signals. In addition, since the same (single) laser light (pulse light) is used, there is an advantage that measurement can be performed without being affected by fluctuations in the irradiation atmosphere or fluctuations in laser intensity. Further, since it is not necessary to make the laser light into a narrow bandwidth with an etalon or the like to make it a resonance laser light and a non-resonance laser light, it is not necessary to control resonance-non-resonance.

Although the embodiment of the present invention has been described above, the present invention is not limited to this embodiment.
Various modifications and changes are possible based on the technical concept of the present invention. For example, even when the gas to be measured is other than methane, it goes without saying that the gas concentration detecting method and apparatus according to the present invention can be applied. In that case, the gas for measurement may be filled into the saturated absorption cell 8 at a high concentration.

[0035]

According to the first aspect of the present invention, there is provided a gas concentration detecting method for irradiating a measurement target region with a broadband laser beam, and collecting backscattered light scattered backward from the measurement target region. A light condensing step, a splitting step of splitting the backscattered light into two light rays, and transmitting one of the split light rays into the high-concentration gas to be measured and passing it to the first photodetector. And introducing the other of the split light beams into the second light detection means as it is, based on the backscattered light intensities detected by the first and second light detection means, respectively. And a calculation step of calculating the average gas concentration in the measurement target area. That is, in the present invention, instead of irradiating the laser light of two wavelengths to the atmosphere, a broadband laser light having a wavelength width wider than the absorption line of the target gas is radiated to the atmosphere to thereby provide a two-wavelength laser. According to the present invention, it is possible to separate a signal in a resonance state and a signal in a non-resonance state only by irradiating one broadband laser light, since the same effect as that of light irradiation can be obtained. Can be obtained. In addition, by obtaining a signal in a resonance state and a signal in a non-resonance state by using the same (single) broadband laser beam, the gas concentration can be measured without being affected by atmospheric fluctuations or fluctuations in laser intensity. Can be performed.

According to a second aspect of the present invention, there is provided a gas concentration detecting device for irradiating a broadband laser beam to a measurement target region, and a rear portion from the measurement target region irradiated with the broadband laser beam. A light-collecting mechanism that collects scattered light, a beam splitter that splits backscattered light collected by the light-collecting mechanism into two light beams, and a high-concentration measurement target gas (for example, high-concentration methane) A saturated absorption cell through which one of the light beams split by the beam splitter is transmitted, a light beam transmitted through the saturated absorption cell, and a light beam split by the beam splitter First and second light detectors, into which the other one of the light beams is respectively introduced, and these first and second light detectors
Signal processing unit that calculates the average gas concentration in the measurement target area based on the backscattered light intensity obtained from the photodetector, so that the saturated absorption cell is provided before the photodetector. By installing, a precise and complicated wavelength discriminator such as a spectroscope having a high resolution or a narrow band interference filter can be eliminated. Also, by separating signals in a resonance-non-resonance state using the same (single) broadband laser light, it is possible to provide a system that can ignore the effects of atmospheric fluctuations and laser light fluctuations.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a gas concentration detection device that performs a gas concentration detection method according to the present invention.

FIG. 2 is a spectrum diagram showing a spectrum of a Q-branch of a methane 2ν ₃ band.

FIG. 3A is a spectrum diagram showing a spectrum of backscattered light transmitted through a saturated absorption cell, and FIG. 3B is a spectrum diagram showing a spectrum of backscattered light not transmitted through a saturated absorption cell.

FIG. 4 is a configuration diagram of a conventional differential absorption lidar.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 broadband laser light 2 measurement target area 3 laser light irradiation mechanism 4 backscattered light 5 light focusing mechanism 6a, 6b light beam 7 beam splitter 8 saturated absorption cell 10a, 10b photodetector 11 signal processing unit (calculation unit) 12 laser 14 telescope 16a, 16b Optical path 18a, 18b Amplifier

Claims (2)

[Claims] (A) an irradiation step of irradiating a measurement target area with a broadband laser beam; (b) a light collection step of collecting backscattered light scattered backward from the measurement target area; c) a dividing step of dividing the backscattered light into two light beams; and (d) one of the divided light beams is transmitted through a high-concentration gas to be measured and passed to a first photodetector. An introduction step of introducing the other one of the divided light rays into the second light detection means as it is, and (e) a rear side detected by the first and second light detection means, respectively. A calculating step of calculating an average gas concentration in the measurement target area based on the scattered light intensity. 2. A laser light irradiation mechanism for irradiating a broadband laser beam to a measurement target area, a light collection mechanism for collecting backscattered light from the measurement target area irradiated with the broadband laser light, and a light collection mechanism A beam splitter that splits the backscattered light condensed into two light beams, and one of the light beams that are filled with a high-concentration measurement target gas and are split by the beam splitter, A saturated absorption cell to be transmitted, and a light beam transmitted through the saturation absorption cell,
And first and second photodetectors into which the other of the light beams split by the beam splitter is introduced, respectively, and backscattered light obtained from the first and second photodetectors. A signal processing unit for calculating an average gas concentration in the measurement target area based on the intensity.