JPH10132737A - Remote gas concentration measurement method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a remote gas concentration measuring method and a remote gas concentration measuring method capable of measuring the concentration of a low-concentration gas even using a measuring device having a somewhat large error. SOLUTION: According to a value of a reference gas signal which does not depend on a gas signal component, an alternating current having a frequency twice as high as a modulation frequency is superimposed on a driving current of a laser beam, and an amplitude and a phase of the alternating current are amplified and transferred. By controlling with the phaser 210, the noise component N can be reduced, and an error due to a change in the measurement range can be eliminated. That is, by making the gas signal component G larger than the error components E ₁ and E ₂ , a gas with a low concentration can be measured more accurately. Therefore, even if a measuring device having a somewhat large error is used, a low-concentration gas can be accurately measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a remote gas concentration measuring method and apparatus.

[0002]

2. Description of the Related Art Gas molecules have the property of absorbing laser light of a specific wavelength. It is known that the presence or absence of gas can be detected by utilizing this property, and a sensing technique using this principle is widely used in fields such as industrial measurement and pollution monitoring. In addition, if laser light is transmitted using an optical fiber as a transmission path, gas can be remotely measured.

As a gas concentration measuring device for performing this kind of remote measurement, there is a device described in Japanese Patent Application No. 2-78498. The present inventor has developed a remote gas concentration measuring device using an optical fiber as a transmission line by applying this device.

This remote gas concentration measuring device modulates a driving current of a semiconductor laser at a high frequency around a predetermined value of current, and oscillates a laser beam having a modulated wavelength and intensity. The laser beam emitted backward of the semiconductor laser is monitored and the drive current and temperature are controlled so that the center wavelength of the oscillation wavelength of the laser beam becomes the center of the gas absorption line, thereby stabilizing the oscillation wavelength of the semiconductor laser. . The laser light thus stabilized and emitted forward is transmitted through an optical fiber through a measuring gas cell containing a gas of unknown concentration, and the transmitted light is guided to a light receiving unit by another optical fiber facing the laser beam. By obtaining a double detection signal or a fundamental detection signal of light, the gas concentration can be detected at a high SN ratio.

However, focusing on one isolated absorption line among the gas absorption lines, the shape of the absorption line changes depending on the pressure of the gas atmosphere, and the second-harmonic detection signal used for quantitative measurement of the gas also depends on the pressure. It has a value. Therefore, when measuring the gas concentration in a place where the atmospheric pressure changes rapidly, such as a coal mine or a plant, accurate concentration measurement cannot be performed unless a pressure sensor is separately provided to monitor the pressure change.

Therefore, in order to perform accurate concentration measurement, a laser that oscillates laser light having a wavelength and intensity corresponding to the drive current and temperature is used, and the wavelength and intensity are modulated by changing the drive current and temperature of the laser. While oscillating the laser beam, the central wavelength of the laser beam is swept, the intensity of the transmitted light after passing the laser beam through the gas atmosphere to be measured is detected, and a specific component in this detection signal is detected. A method has been proposed in which phase-sensitive detection is performed and the specific gas concentration under the above-mentioned gas atmosphere pressure is measured from the obtained detection signal (Japanese Patent Application No. 3-96825).

Here, FIG. 7 shows the relationship between the wavelength of the laser beam and the ratio (hereinafter referred to as "gas signal") between the second-order phase-sensitive detection signal obtained by phase-sensitive detection and the fundamental-wave phase-sensitive detection signal. Shown in

In FIG. 1, the abscissa indicates the oscillation wavelength of the laser beam, and the ordinate indicates the gas signal.

The characteristic curve shown in FIG. 1 shows that acetylene gas is used as the detection gas and the absorption wavelength of the gas is 1.532.
The output signal when μm is shown. The gas concentration can be obtained from this output signal by obtaining the peak value obtained when the center wavelength of the laser light is near the gas absorption line. Also, since the wavelength width of the two extreme values appearing on both sides near the gas absorption line indicates the spectrum width of the gas absorption line,
From this value, information about the pressure of the gas atmosphere can be obtained.

When a low concentration gas is detected under the same modulation condition, periodic and aperiodic noises are superimposed on the gas signal to be detected (see FIG. 8). These noises are mainly generated when laser beams reflected on the optical surface in the gas cell section interfere with each other to fluctuate the output signal. When such a fluctuation occurs, it is difficult to obtain the peak value and the spectrum width from the output signal, and the detection accuracy is reduced. FIG. 8 is a diagram showing a state in which noise is superimposed on the gas signal. The horizontal axis represents the wavelength, and the vertical axis represents the signal.

Further, when the semiconductor laser is deteriorated due to its life, there is a possibility that the gas signal to be detected is detected as a low-concentration gas although a high-concentration gas is actually detected.

The inventor of the present invention uses a laser that oscillates a laser beam having a wavelength and an intensity corresponding to the drive current and the temperature, and modulates the drive current around a predetermined current value so that the wavelength and the intensity are modulated. Oscillates the laser light
The center wavelength of the laser light is swept, the transmitted light intensity obtained through the gas atmosphere in which the laser light is measured is detected, and a specific component in the detection signal is phase-sensitively detected. A gas concentration measurement method for determining a concentration, in which a gas to be detected is stored in a gas cell for detection, a gas for reference is stored in a gas cell for reference, and a signal obtained from laser light transmitted through both gas cells is subtracted. A method of measuring the gas atmosphere pressure and the gas concentration from the gas signal obtained by the difference in the processing unit and measuring the gas concentration under an arbitrary pressure was proposed.

In this method, a ratio of a laser beam transmitted through a detection gas cell filled with a gas to be detected, a fundamental phase-sensitive detection signal obtained by phase-sensitive detection of the laser beam, and a second-harmonic phase-sensitive detection signal ( Hereinafter, referred to as a “detection-side gas signal”), a laser beam transmitted through a reference gas cell that does not contain the gas to be detected, a fundamental-wave phase-sensitive detection signal obtained by phase-sensitive detection of the laser beam, and a second-harmonic phase-sensitive detection signal (Hereinafter referred to as “reference-side gas signal”). By taking the difference between the detection-side gas signal (noise component + gas signal component) and the reference-side gas signal (only the noise component), a gas signal containing no noise component (hereinafter referred to as a “difference gas signal”) is obtained (FIG. 9, 10). FIG. 9 is a diagram showing a gas signal before the difference of 50 ppm acetylene gas according to the conventional measuring method, and FIG. 10 is a diagram showing 50 pp according to the conventional measuring method.
It is a figure which shows the gas signal after the difference of m acetylene gas.
9 and 10, the horizontal axis indicates the bias current, and the vertical axis indicates the signal value converted into the gas concentration.

[0014]

However, when a gas having a lower concentration is measured by this method, the baseline of the differential gas signal becomes a curve, and the wavelength width of the peak value and the extreme value can be accurately obtained. This makes it difficult to accurately measure the gas concentration.

For example, consider a case in which 10 ppm acetylene gas is sealed in a detection gas cell and a gas having an acetylene gas concentration of 0 ppm is sealed in a reference gas cell.

FIG. 11 shows 10 ppm by the conventional measuring method.
FIG. 12 is a diagram showing a gas signal before the difference of acetylene gas, and FIG. 12 is a diagram showing a gas signal after a difference of 10 ppm acetylene gas by a conventional measuring method. 11 and 12
In the graph, the horizontal axis indicates a bias current, and the vertical axis indicates a signal value converted into a gas concentration.

It can be seen from FIGS. 2 and 3 that the baseline of the differential gas signal is curved when a low-concentration gas is measured. This is due to a measurement error between the detection-side gas signal and the reference-side gas signal. Things are conceivable. Hereinafter, this will be described.

The measurements of the detection side gas signal consists gas signal component G and a noise component N and the error component E ₁ Tokyo. The measured value of the reference side gas signal includes a noise component N and an error component E ₂
It consists of

Generally, the magnitude of the error ab between the two measured values a and b is evaluated as the sum of the magnitude of the error of the measured value a and the magnitude of the error of the measured value b. Thus, the differential gas signal comprises a gas signal component G and the error component E ₃ Prefecture. However, the magnitude of the error component E ₃ corresponds to the sum of the magnitude of the size and the error component E ₂ error component E _1.

When measuring a low-concentration gas such that the gas signal component G becomes equal to the error component E ₃ , the baseline of the differential gas signal becomes a curve due to the error component E ₃ , and accurate gas concentration measurement cannot be performed. Therefore, in order to accurately measure a low-concentration gas concentration, it is necessary to use a measurement device having a small measurement error in order to accurately measure the detection-side gas signal and the reference-side gas signal. Are difficult to obtain and expensive.

Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a remote gas concentration measuring method and a remote gas concentration measuring method capable of measuring the concentration of a low-concentration gas even with a measuring device having a somewhat large error (about several percent of the measuring range). It is to provide the device.

[0022]

In order to achieve the above object, a remote gas concentration measuring method according to the present invention uses a laser which oscillates a laser beam having a wavelength and an intensity corresponding to a driving current and a temperature, and uses a predetermined current value. The wavelength and intensity of the laser light are modulated by changing the drive current around the center, and the intensity of the transmitted light obtained by passing the modulated laser light through a gas atmosphere as a measurement target is detected and obtained. In a remote gas concentration measurement method that determines the gas concentration from a gas signal obtained by phase-sensitive detection of a specific component in the detection signal, the gas to be detected is stored in the detection gas cell, and the reference gas is stored in the reference gas cell. Then, a laser beam corresponding to a driving current on which an alternating current having a frequency twice as high as the modulation frequency is superimposed is passed through both gas cells to detect the intensity of both transmitted light and to control the amplitude and phase of the alternating current. It is obtained so as to reduce the noise component in the detection signal by.

A remote gas concentration measuring apparatus according to the present invention comprises: a laser which oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature; and a laser beam which varies the drive current of the laser around a predetermined current value. Modulating means for modulating the wavelength and intensity of light, detecting means for detecting the intensity of transmitted light obtained by passing the modulated laser light through a gas atmosphere as a measurement target, and detecting signals obtained by the detecting means. And a signal processing means for obtaining a gas concentration from a gas signal obtained by phase-sensitive detection of a specific component. A reference gas cell for detecting the intensity of transmitted light that has passed through the detection gas cell according to a driving current on which an alternating current having a frequency twice the modulation frequency is superimposed. Reference gas cell side detection means, and a reference gas cell side detection means for detecting the intensity of light transmitted through a reference gas cell through a laser beam corresponding to a driving current on which an alternating current having a frequency twice the modulation frequency is superimposed, And control means for reducing the noise component in the detection signal by controlling the amplitude and phase of the alternating current.

[0024] Here, the measurement value of the detection-side gas signal consists gas signal component G and a noise component N and the error component E ₁ Tokyo. Differential gas signal consists gas signal G and the error component E ₃ Prefecture. However, the magnitude of the error component E ₃ corresponds to the sum of the magnitude of the size and the error component E ₂ error component E _1.

When measuring a low-concentration gas, the gas signal component of the detection-side gas signal decreases, and the gas signal component G becomes smaller than the error components E ₁ and E ₂ . In this case, an amplifier is provided in front of the measuring device for the double frequency component of the detection side gas signal and in front of the measuring device for the double frequency component of the reference side gas signal. With error components E ₁ ,
It can be increased to E _2.

However, since the noise component N is also amplified at the same time, the measurement range of the measuring device for the gas signal on the detection side must be changed. If you change the measurement range, an error component E ₁ increases, eventually, so that the ratio between the gas signal component G and the error component E _1, E ₂ is not improved. The reason why no amplifier is provided before the measuring device for the fundamental frequency component is that the fundamental frequency component is constant regardless of gas absorption.

Therefore, an alternating current having a frequency twice the modulation frequency is superimposed on the driving current of the laser beam in accordance with the value of the reference gas signal which does not depend on the gas signal component, and the amplitude and phase of the alternating current are controlled. Reduces the noise component N, reduces the error in changing the measurement range, and makes the gas signal component G larger than the error components E ₁ and E ₂ , so that the error is somewhat large (about several percent of the measurement range). The gas concentration of the low-concentration gas can be measured using a measuring device.

[0028]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

First, the principle of the remote gas concentration measuring method of the present invention will be described.

When the laser light is modulated by changing the drive current of the semiconductor laser having the oscillation frequency Ω, not only the oscillation frequency but also the oscillation intensity is modulated. When a laser beam whose oscillation frequency and intensity are modulated is transmitted through an atmosphere containing methane gas, the detection signal P of the transmitted light is expressed by the following equation (1).
It is represented as

[0031]

P = A [I ₀ + ΔI · cos (ωt + φ)] ×
[C ₀ + ΔΩ · T ₀₁ cosωt + (1/4) · (ΔΩ)
² · cos2ωt] where Equation 1 satisfies Equation ² .

[0032]

## EQU2 ## C ₀ = T + (1/4) · (ΔΩ) ² · T _{02 The} detection signal P has a DC component, a cos ωt component and a cos
2ωt component.

Here, A is a constant depending on the reflection conditions and the like,
I ₀ is the central intensity of the laser output, ΔI is the amplitude of the intensity modulation,
ω is the modulation angular frequency of the drive current, φ is the phase difference between the intensity modulation and the frequency modulation, and ΔΩ is the frequency shift of the frequency modulation.

T is the transmittance, T ₀₁ is the first derivative d of the transmittance T
T / dΩ, T ₀₂ is the second derivative of the transmittance T d ² T / dΩ ^2, FIG. 2 (a), (b) , shows the respective waveforms (c). 5A to 5C, the horizontal axis indicates frequency, and the vertical axis indicates transmittance T, first derivative T ₀₁ , and second derivative T ₀₂ , respectively.

When the frequency and phase component φ of cos 2ωt in equation 1 are subjected to phase-sensitive detection, equation 3 is obtained, and it can be seen that the detected signal P (2ω) changes based on the first derivative T ₀₁ and the second derivative T _02. .

[0036]

Equation 3] P (2ω) = A [I 0 ((ΔΩ) 2/4) T 02
+ ΔI · ΔΩcosφ · T ₀₁ ] Here, the phase-sensitive detection means to extract only a component having a specific frequency and a phase-sensitive detection signal and measure the amplitude.

When detecting the absorption of methane gas by the detection signal P (2ω), the fact that the maximum sensitivity is obtained when the center frequency Ω ₀ of the laser beam coincides with the center ω ₀ of the absorption line of methane gas is used. (See FIG. 2). Further, at this time, the first derivative T ₀₁ becomes 0 and the second derivative T ₀₂ becomes the maximum, so that the second term of Equation 3 is deleted and only the first term remains. That is, the second derivative T ₀₂ when Ω ₀ = ω ₀ is given by Expression 4. Substituting Equation 4 into the first term of Equation 3 results in Equation 5.

[0038]

## EQU4 ## T ₀₂ (Ω ₀ = ω ₀ ) = 2 · α (ω ₀ ) · c · L
/ Γ ²

[0039]

P (2ω) = A · I ₀ (ΔΩ) ² · α (ω ₀ ) · c · L / γ ² = K ₁ α ((ω ₀ / γ ² )) · c · L where K ₁ is a constant, α (ω ₀₎ is the absorption coefficient of methane when Ω ₀ = _{ω 0,} 2γ full width at half maximum of the gas absorption line,
The optical path length product c · L is the product of the gas concentration c and the optical path length L.

As described above, the detection signal P (2ω) is proportional to the optical path length product c · L, from which the concentration c of the methane gas can be detected with extremely high sensitivity.

By the way, α (ω ₀ ) and γ in Expression 5
² changes depending on the pressure of the gas atmosphere as shown in FIG.

FIG. 3 is a graph showing the relationship between the absorption coefficient α (ω) for the pressure in the gas cell, the detection signal P (2ω), and the full width at half maximum 2γ described later. In the figure, the horizontal axis represents the pressure (torr), the vertical axis represents the absorption coefficient α (ω),
The detection signal P (2ω) and the full width at half maximum 2γ are shown.

In order to accurately measure the gas concentration according to Equation ⁵ , the values of α (ω ₀ ) and γ ² at atmospheric pressure must be obtained. These accurate values can be obtained from the output waveform of the detection signal P (2ω) when the center frequency Ω ₀ of the laser beam is swept before and after the methane gas absorption line.

Now, when the center frequency Ω ₀ of the laser beam is changed, the first term of Equation 3 is a second derivative T ₀₂ , and the second term is a waveform obtained by integrating the coefficients of the first derivative T _01. Become. The coefficients are I ₀ , ΔΩ and the like. If the oscillation conditions of the semiconductor laser are set, there is no problem even if they are handled as constants.

Accordingly, the waveform of the detection signal P (2ω) is shown in FIG.
The waveform shown in FIG. 2 (b) and the waveform shown in FIG. 2 (c) are integrated by a certain coefficient, respectively, and are added to each other (see FIG. 4). FIG. 4 is a diagram showing a part of an output waveform obtained by a recorder used in the gas concentration measuring device shown in FIG. 1 described later.

However, in fact, the first term of Equation 3 is
2 (c), the minimum on the low wavelength side shown in FIG.
Frequency Ω between the value and the minimum value on the high wavelength side₀The width of
This corresponds to a full width at half maximum 2γ at the gas atmosphere pressure. this
When the full width at half maximum 2γ is obtained in this manner, the pressure is determined based on FIG.
Force, and the absorption coefficient α under that pressure
(Ω₀) Can be obtained. Note that P (2) shown in FIG.
ω) is α (ω) in Equation 5. ₀) / Γ^TwoChange due to pressure
The maximum value is shown near the total pressure of 100 torr.

In order to obtain the atmospheric pressure from FIG. 3, if either the absorption coefficient α (ω ₀ ) or the half width at half maximum γ is known, the pressure can be known, and the pressure-corrected concentration can be detected.

FIG. 1 is a block diagram showing an embodiment of a measuring apparatus to which the remote gas concentration measuring method of the present invention is applied. In this embodiment, a semiconductor laser is used as a light source.
The case where methane gas is measured will be described.

As shown in FIG.
LD (laser diode) module unit 2, laser drive circuit 3 for driving LD module unit 2, optical coupler 4 for splitting laser light from LD module unit 2, and one laser beam split by optical coupler 4 Is transmitted,
A detection gas cell 5 containing a gas to be detected, a reference gas cell 6 containing a reference gas while transmitting the other laser beam branched by the optical coupler 4, and a laser passing through the detection gas cell 5. A light detector 7 serving as a detecting gas cell side detecting means for receiving light; a light detector 8 serving as a reference gas cell side detecting means receiving a laser beam transmitted through the reference gas cell 6; And a signal processing unit 9 for processing a signal from the control unit 8.

The LD module section 2 includes a distributed feedback semiconductor laser (hereinafter, referred to as “DFB-LD”) 10 as a laser that oscillates laser light of a single wavelength, and a DFB-LD.
A Peltier element 11 for mounting or cooling the DFB-LD 10 and a Peltier element power supply for controlling the oscillation frequency (oscillation wavelength) of the DFB-LD 10 by controlling the heat generation and absorption of the Peltier element 11 1
And 2.

The LD module section 2 includes a DFB-LD1
0, a connector for coupling the laser light from the optical coupler 4 to the optical coupler 4, a condensing lens for condensing the laser light, and an optical isolator for intercepting the return light from the condensing lens (all shown). Optical system 13 composed of
And is connected to the input side of the optical coupler 4 via the optical fiber 14. Note that the end face of the optical system 13 is subjected to an anti-reflection coating process so that light returning to the DFB-LD 10 is minimized.

One output side of the optical coupler 4 is connected to one end of a gas cell 5 for detection via an optical fiber 15 for a forward path. The gas cell 5 for detection includes an optical fiber 15 for the forward path.
Is a cell through which laser light emitted from is transmitted, and various gases of unknown concentration (for example, methane gas, acetylene gas, etc.)
And can be easily attached to a location to be measured (detected).

The other end of the detection gas cell 5 is connected to a return optical fiber 16 through which the laser light transmitted through the detection gas cell 5 propagates. The return optical fiber 16 is connected to the photodetector 7 composed of a pin photodiode or the like for detecting the intensity of the laser beam.

The other output side of the optical coupler 4 is connected to one end of the reference gas cell 6 via an outgoing optical fiber 17. The reference gas cell 6 includes an outgoing optical fiber 17.
Is a cell through which laser light emitted from is transmitted, and is formed so as to contain a gas that does not absorb light (for example, nitrogen gas), and can easily inhale and discharge the gas and adjust the internal pressure. It is easy to do. The other end of the reference gas cell 6 is connected to a return optical fiber 18 through which the laser light transmitted through the reference gas cell 6 propagates. The return optical fiber 18 is connected to a photodetector 8 composed of a pin photodiode or the like for detecting the intensity of laser light. The return optical fiber 1
The end faces 6 and 18 are coated with an oblique cut anti-reflection coating or the like so that no interference system is generated.

The laser drive circuit 3 includes an oscillator 19 that outputs a sine wave signal having an angular frequency ω, a frequency multiplier 20 that outputs a sine wave signal having an angular frequency 2ω based on the sine wave signal from the oscillator 19, An amplifying phase shifter 210 serving as control means for outputting a sine wave signal having an angular frequency of 2ω with changed amplitude and phase with respect to the sine wave signal from the frequency multiplier 20, and a sine wave signal from the oscillator 19 An adder 21 that outputs a signal obtained by adding the sine wave signal from the phaser 210, and a DFB-LD
A bias current source 22 for supplying a bias current to the bias current source 10 and a bias current control device 23 for controlling an output current of the bias current source 22 are provided.

The laser drive circuit 3 includes a bias current source 22
The DFB-LD 10 is driven by a current in which a sine wave signal having an angular frequency ω from the oscillator 19 and a sine wave signal having an angular frequency 2ω from the amplification phase shifter 210 are superimposed on the output current from . The output side of the bias current source 22 is connected to an inductor L for preventing the influence from the output from the adder 21. The output side of the adder 21 is connected to a capacitor C for removing a DC component. The amplification phase shifter 210 controls an amplification factor and a phase amount according to an output from a divider 24 of the signal processing unit 9 described later. The bias current controller 23 can sweep the output current of the bias current source 22.

The signal processing unit 9 includes a variable amplifier 25 that can amplify the signal from the photodetector 7 and change the amplification factor, a variable amplifier 26 that amplifies the signal from the photodetector 8, and an oscillator 19 And a lock-in amplifier 27 that performs phase-sensitive detection of the output from the photodetector 7 in synchronization with the angular frequency ω of the sine wave signal from the sine wave signal, and is variable in synchronization with the angular frequency 2ω of the sine wave signal from the frequency multiplier 20. A lock-in amplifier 28 for performing phase-sensitive detection of the output of the amplifier 25, a lock-in amplifier 29 for performing phase-sensitive detection of the output from the photodetector 8 in synchronization with the angular frequency ω of the sine wave signal from the oscillator 19, A lock-in amplifier 30 that performs phase-sensitive detection of the output of the variable amplifier 26 in synchronization with the angular frequency 2ω of the sine wave signal from the frequency multiplier 20, and a divider 31 that obtains an output ratio from the lock-in amplifiers 27 and 28. When A divider 24 for obtaining the output ratio from both the lock-in amplifier 29, and a recorder 32 for recording the output of the divider 31.

Next, the operation of the remote gas concentration measuring device shown in FIG. 1 will be described.

By controlling the heat generation and heat absorption of the Peltier element 11 by the Peltier element power supply 12, the DFB-
The LD 10 is fixed at a constant temperature, and the bias current source 22
Oscillator 19 and amplifying phase shifter 210
DFB-L by the drive current on which the alternating current from
D10 is driven.

The laser light incident on the optical system 13 is branched by the optical coupler 4, and one of the branched laser lights propagates in the outward optical fiber 15 and spatially propagates in the gas in the detection gas cell 5. Then, the laser light propagates again in the return optical fiber 16, is received by the photodetector 7, and a detection signal is output.

On the other hand, the other laser beam split by the optical coupler 4 propagates in the outward optical fiber 17, spatially propagates the gas in the reference gas cell 6, and then propagates again in the return optical fiber 18. Then, the light is received by the photodetector 8 and a detection signal is output.

Of the signals detected by the photodetector 7, the signal synchronized with the angular frequency ω of the sine wave signal from the oscillator 19 is
The signal detected by the lock-in amplifier 27 and synchronized with the angular frequency 2ω of the sine wave signal of the frequency multiplier 20 is amplified by the variable amplifier 25 and then detected by the lock-in amplifier 28. The divider 31 calculates the ratio of the signals extracted by the lock-in amplifiers 27 and 28 (the ratio between the noise component and the gas signal component).

Of the signals detected by the photodetector 8, the signal synchronized with the angular frequency ω of the sine wave signal from the oscillator 19 is
The signal detected by the lock-in amplifier 29 and synchronized with the angular frequency 2ω of the sine wave signal of the frequency multiplier 20 is amplified by the variable amplifier 26 and then detected by the lock-in amplifier 30. The divider 24 calculates the ratio (only the noise component) of the signals extracted by the two lock-in amplifiers 29 and 30. The amplifying phase shifter 210 changes the amplification factor and the phase amount based on the ratio of the signals, so that the two lock-in amplifiers 29, 3
Control is performed so that the signal ratio extracted with 0 is 0 (actually, the electrical noise included in this signal ratio or less). After the amplification phase shifter 210 performs the control, the noise ratio included in the signal ratio obtained by the divider 31 before the control becomes 0 and only the gas signal component remains. When the amplification factors of the variable amplifiers 25 and 26 are increased, the amplification phase shifter 210
Finely adjusts the amplification rate and the phase amount, thereby increasing the noise component removal rate.

The bias current control unit 23
When the bias current supplied to the LD 10 is swept, the amount of the noise component changes with the sweep, so that the amplification phase shifter 2
Reference numeral 10 denotes a divider 24 so that the output of the divider 24 keeps 0.
, The amplification factor and the phase amount are continuously controlled based on the output.

The gas concentration is determined from the gas peak value obtained by the recorder 32, and the pressure is determined from the extreme value or full width at half maximum 2γ appearing on both sides of the peak value. At this time, since the noise component has been removed, an accurate pressure of the gas in the detection gas cell 5 and an accurate concentration after the pressure correction can be obtained.

Here, using the remote gas concentration measuring device 1 shown in FIG. 1, a detection gas cell 5 is filled with 10 ppm acetylene gas and a reference gas cell 6 is filled with 0 ppm acetylene gas. The side gas signal is shown in FIG. 5 (measured with a recorder not shown in FIG. 1).

FIG. 5 shows a diagram of the measuring apparatus 1 shown in FIG.
It is a waveform of a gas signal when acetylene gas of ppm is measured. In the figure, the horizontal axis indicates the bias current,
The vertical axis shows the signal value converted into the gas concentration.

FIG. 6 is a diagram showing a state where the reference side gas signal is controlled to be 0. The horizontal axis indicates the bias current, and the vertical axis indicates the signal value converted into the gas concentration.

The detection-side gas signal shown in FIG.
It can be seen that the measurement error caused by the measurement by the conventional method shown in FIG.

According to the present invention described above, according to the value of the reference gas signal which does not depend on the gas signal component, an alternating current having a frequency twice as high as the modulation frequency is superimposed on the driving current of the laser beam. By controlling the amplitude and phase, the noise component N can be reduced and the error due to the change in the measurement range can be eliminated.
By increasing relative _1, E _2, it is possible to measure low concentrations of gas more accurately. In the present embodiment, the case of measuring the concentration of methane gas has been described, but the present invention is not limited to this, and the concentration of other types of gas may be measured.

[0071]

In summary, according to the present invention, the following excellent effects are exhibited.

The gas to be detected is accommodated in the gas cell for detection, the gas for reference is accommodated in the gas cell for reference, and a laser beam corresponding to a driving current in which an alternating current having a frequency twice the modulation frequency is superimposed is applied to both gas cells. In addition to detecting the intensity of both transmitted light and controlling the amplitude and phase of the alternating current, the noise component in the detection signal can be reduced. The concentration of the concentration gas can be measured.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a measuring device to which a remote gas concentration measuring method of the present invention is applied.

2 (a) is a waveform of the transmittance T, (b) the first derivative T ₀₁ of the waveform of the transmittance T, (c) is the transmittance T second derivative T ₀₂
It is a figure which shows the waveform of.

FIG. 3 is a diagram showing a relationship among an absorption coefficient α (ω) with respect to a pressure in a gas cell, a detection signal P (2ω), and a full width at half maximum 2γ.

FIG. 4 is a diagram showing a part of an output waveform obtained by a recorder used in the gas concentration measuring device shown in FIG.

5 is a waveform of a gas signal when acetylene gas of 10 ppm is measured by the measuring device shown in FIG.

FIG. 6 is a diagram illustrating a state in which a reference side gas signal is controlled to be 0.

FIG. 7 is a diagram illustrating a relationship between a wavelength of a laser beam and a gas signal.

FIG. 8 is a diagram showing a state in which noise is superimposed on a gas signal.

FIG. 9 is a view showing a gas signal before a difference of 50 ppm acetylene gas by a conventional measuring method.

FIG. 10 is a diagram showing a gas signal after a difference of 50 ppm acetylene gas by a conventional measurement method.

FIG. 11 is a diagram showing a gas signal before a difference of 10 ppm acetylene gas by a conventional measurement method.

FIG. 12 is a diagram showing a gas signal after a difference of 10 ppm acetylene gas by a conventional measurement method.

[Explanation of symbols]

 2 LD module section 3 Laser drive circuit 4 Optical coupler 5 Gas cell for detection 6 Gas cell for reference 9 Signal processing section 11 Peltier element 13 Optical system 210 Amplification phase shifter

Claims (2)

[Claims] A laser that oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature, modulates the wavelength and intensity of the laser beam by changing the drive current around a predetermined current value, Detects the intensity of transmitted light obtained by passing the modulated laser light through a gas atmosphere as a measurement target, and obtains a gas concentration from a gas signal obtained by phase-sensitive detection of a specific component in the obtained detection signal. In the remote gas concentration measurement method, the gas to be detected is stored in the detection gas cell, and the reference gas is stored in the reference gas cell,
A laser beam corresponding to a driving current in which an alternating current having a frequency twice as high as the modulation frequency is superimposed is passed through both gas cells to detect the intensity of both transmitted lights, and is also detected by controlling the amplitude and phase of the alternating current. A remote gas concentration measurement method, wherein a noise component in a signal is reduced. 2. A laser which oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature, and modulates the wavelength and intensity of the laser beam by changing the drive current of the laser with a predetermined current value as a center. A modulating means, a detecting means for detecting the intensity of transmitted light obtained by passing the modulated laser light through a gas atmosphere as a measuring object, and a phase-sensitive specific component in a detection signal obtained by the detecting means. In a remote gas concentration measurement device provided with signal processing means for obtaining a gas concentration from a gas signal obtained by detection, a detection gas cell containing a gas to be detected, a reference gas cell containing a reference gas, Detecting gas cell side detecting means for detecting the intensity of transmitted light, which passes laser light according to a driving current on which an alternating current having a frequency twice the modulation frequency is superimposed, through the detecting gas cell; A reference gas cell side detecting means for detecting the intensity of transmitted light passing through a reference gas cell according to a driving current on which an alternating current having a frequency twice as high as the tuning current is superimposed, and an amplitude and a phase of the alternating current Control means for reducing the noise component in the detection signal by controlling the control of the remote gas concentration.