JPH05256768A - Gas concentration measuring method and measuring apparatus therefor - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a gas concentration measuring method and a measuring device therefor capable of performing pressure correction without using a high-gain and high-stability amplifier. [Structure] The modulated laser light is branched by a branching device 6, one of the branched lights is transmitted through methane gas in a cell for measuring gas 7 of unknown concentration, which is kept at a constant temperature, and is locked via a photodetector 9. The gas concentration is calculated by dividing the value of the secondary differential signal T ₀₂ obtained by the phase-sensitive detection by the in-amplifiers 15 and 16 by the value of the primary differential signal T ₀₁ by the divider 17, and at the same time The reference gas cell 8 containing methane gas having a temperature, a constant pressure and a known concentration is transmitted, and phase-sensitive detection is performed by the lock-in amplifier 19 via the photodetector 10, and based on the obtained second derivative signal, The feature is that the gas concentration is corrected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration measuring method and measuring apparatus.

[0002]

2. Description of the Related Art Methane gas is the main component of city gas,
Leakage of city gas can be detected by detecting methane gas.
For this reason, it is necessary to detect the presence or absence of methane gas safely, reliably, and promptly in a specific area such as an underground mall or a high-rise building. However, conventional semiconductor and combustion type gas sensors have poor reliability, and in recent years, optical gas sensors have been developed.

The principle of the optical gas sensor is that laser light of a specific wavelength is easily absorbed by a certain gas. Sensing technology applying this principle is widely used in industrial measurement, pollution monitoring, and the like. If an optical fiber is used as the transmission path for this laser light, remote monitoring is also possible.

Therefore, the present inventors have developed a new remote gas (methane gas) detector using an optical fiber as a transmission line. In the method utilizing this principle, the driving current of the semiconductor laser is centered at a high frequency to oscillate a laser beam whose wavelength and intensity are modulated. Further, laser light emitted to the rear of the semiconductor laser so that the center wavelength of oscillation is at the center of the methane absorption line by controlling the current and temperature is used for monitoring. Then, the stabilized laser beam emitted forward is transmitted through the optical fiber to the cell for measurement gas containing unknown concentration, and the transmitted light is guided to the light receiving section by another optical fiber facing the laser beam. The gas concentration can be detected at a higher S / N ratio than the second-harmonic detection signal of light or the basic detection signal.

However, focusing on one isolated absorption line of methane gas, the absorption coefficient α has a value depending on the total pressure of the atmosphere. Therefore, when the concentration is measured at a place where the atmospheric pressure changes drastically such as a coal mine or a plant, the pressure cannot be accurately measured unless a pressure sensor is separately provided and the pressure is measured and corrected based on the value.

Therefore, the present inventors oscillate a laser beam having a wavelength and intensity according to the driving current and temperature, sweep the central wavelength of the laser beam, and bring the laser beam to a constant temperature to be measured. We propose a method to detect the intensity of transmitted light after passing through a maintained gas atmosphere, perform phase sensitive detection of a specific component in this detection signal, and measure the specific gas concentration under the above atmospheric pressure from this detection signal. did.

[0007]

In the above-mentioned conventional method, the monitoring of the center frequency of the laser utilizes the fact that the relationship between the oscillation frequency of the laser and the change amount of the forward resistance is reproducible.

First, the difference between the terminal voltage of the laser and a predetermined constant voltage is calculated, and the difference voltage is amplified to monitor the amount of change in the voltage between the laser terminals.

FIG. 3 is a diagram in which the signal obtained by the gas concentration detector is recorded by an XY recorder. The horizontal axis represents the differential voltage, and the vertical axis represents the second-harmonic phase-sensitive detection signal obtained by detecting the light transmitted through the methane gas and performing the phase-sensitive detection.

In the figure, the widths of the two minimum values correspond to the full width at half maximum of the gas absorption line, and they change depending on the pressure of the measurement atmosphere. If a signal processing unit (not shown) grasps the relationship between the voltage difference between these two points and the pressure, the pressure can be converted.

However, since the amount of voltage change between the laser terminals is a small value of about 1/1000, an amplifier with high gain and stable operation is required.

Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a gas concentration measuring method and a measuring apparatus therefor capable of performing pressure correction without using a high gain and high stability amplifier.

[0013]

In order to achieve the above object, the gas concentration measuring method of the present invention uses a laser that oscillates a laser beam having a wavelength and intensity according to a driving current and a temperature, and drives the laser. The intensity of the transmitted light obtained by changing the current or temperature to oscillate the laser beam whose wavelength and intensity are modulated and sweeping the central wavelength of the laser beam and passing the laser beam through the gas atmosphere to be measured. In the gas concentration measuring method of detecting a specific component in this detection signal by phase-sensitive detection, and measuring the concentration of the specific gas under the atmospheric pressure from the detection signal, incident on the gas atmosphere to be measured. The laser light before being branched is branched through a branching device, and the branched laser light is passed through a reference gas atmosphere with a constant temperature, a constant pressure and a known concentration, and the intensity of the transmitted light is detected. Together with phase sensitive detection of the specific component from being detected signals, and corrects the concentration of the specific gas to the measuring object gas by the detection signal.

Further, the gas concentration measuring device of the present invention accommodates a laser that oscillates a laser beam having a wavelength and intensity according to a driving current and temperature, and a specific gas to be measured, and keeps the temperature of the gas constant. Measuring gas cell to keep
A measurement gas side photodetector that detects the intensity of transmitted light obtained by passing the laser light through the measurement gas cell, and a phase sensitive detection of a specific component in the signal from this detection device, and from this detection signal In a gas concentration measuring device equipped with a measuring means for measuring the concentration of the specific gas, an optical branching device for branching a laser beam incident on the measurement gas cell, and a laser beam branched by the optical branching device are transmitted. In addition to detecting the intensity of the laser beam that has passed through the reference gas cell that contains a gas of known concentration that is kept at a constant temperature and a constant pressure, the phase of the specific component in the detection signal is detected. It is provided with a reference gas side photodetector for sensitive detection, and a correction means for correcting the concentration of the specific gas measured by the measuring means with a detection value measured by the reference gas side photodetector.

[0015]

First, there is a frequency modulation method as a method for improving measurement sensitivity in spectroscopic measurement. This is because when frequency-modulated light is transmitted through an atmosphere containing the gas to be detected, the detection signal of the transmitted light has a DC component as well as a fundamental component and its higher harmonic components of the same frequency as the modulation frequency. can get. Of these, when the fundamental wave component and the second harmonic component are respectively phase-sensitive detected, the fundamental wave component corresponds to the first derivative with respect to the absorption line, and the second harmonic component corresponds to the second derivative with respect to the absorption line. From this, when the laser light whose drive current is modulated is transmitted through an atmosphere containing a specific gas and the specific component in the detection signal of the transmitted light is subjected to phase sensitive detection, information on the gas concentration can be obtained from the detection signal.

Since this gas concentration is affected by the change in pressure, the laser beam transmitted through the reference gas cell containing a gas having a constant temperature, a constant pressure and a known concentration is detected, and this detection value is used to detect the above. An accurate concentration can be measured by correcting the concentration of the specific gas to be measured. Here, the phase sensitive detection is to extract only a component having a specific frequency and phase and measure its amplitude.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. Here, an example of measuring methane gas using a semiconductor laser as a light source will be described.

When the driving current of the semiconductor laser is modulated to modulate the oscillation frequency Ω of the laser light, not only the oscillation frequency but also the oscillation intensity is modulated. Now, when the laser light whose frequency and intensity are modulated in this way is transmitted through an atmosphere containing methane gas, the detection signal P of the transmitted light is expressed as in Equation 1.

[0019]

[Number 1] _{P = A [I 0 + ΔIcos} (ωt + φ)] × [C 0 + ΔΩ · T 01 cosωt + ((ΔΩ) 2/4) T 02 cos2ωt] However,

[0020]

[Number 2 is a _{C 0 = T + ((ΔΩ} ) 2/4) · T 02. This detection signal P is a direct current component and cosωt
Component and cos2ωt component. Here, A is a constant depending on the reflection condition, I ₀ is the central intensity of the laser output,
ΔI is the intensity amplitude modulation, ω is the modulation frequency of the drive current, φ is the phase difference between ω and Ω, and ΔΩ is the frequency modulation amplitude.
Further, T, T ₀₁ , and T ₀₂ are the values of Ω = Ω ₀ (where ω ₀ is the center frequency of the laser) of the transmittance, its first derivative dT / dΩ, and its second derivative d ² T / dΩ ² . The shape is shown in FIG.

Here, FIG. 4 shows the transmittance T with respect to frequency.
It is a figure which shows the 1st derivative T ₀₁ and its 2nd derivative T ₀₂ . In each waveform, the horizontal axis represents frequency, and the vertical axis represents transmittance T (a), first derivative T ₀₁ (b), and second derivative T ₀₂ (c).

When the frequency of cos ωt and the phase component φ in equation 1 are detected by phase sensitive detection,

[0023]

Equation 3] P (2ω) = A [I 0 ((ΔΩ) 2/4) T 02 + ΔI · ΔΩcosφ · T 01] is obtained, the change detection signal P (2 [omega) is based on the T ₀₁ and T ₀₂ I understand that

When the absorption of methane gas is detected by the detection signal P (2ω), the center frequency Ω ₀ of the laser light is
The fact that the maximum sensitivity is obtained when the center ω ₀ of the absorption line of methane gas is matched is used (see FIG. 4). Further, at this time, T ₀₁ is “0” and T ₀₂ is maximum, so that the second term of Formula 3 is erased and only the first term remains. That is, Ω ₀ = ω
T ₀₂ when ₀ is

[0025]

## EQU4 ## T ₀₂ (Ω ₀ = ω ₀ ) = 2α (ω ₀ ) cL / γ ² Therefore, substituting this into the first term of Equation 3,

[0026]

(5) P (2ω) = A · I ₀ (ΔΩ) ² · α (ω ₀ ) · c · L / 2γ ² = K ₁ α ((ω ₀ ) / γ ² ) c · L. Here, K ₁ is a constant and α (ω ₀ ) is Ω ₀ = ω
The absorption coefficient of methane gas at ₀ , 2γ is the full width at half maximum of the gas absorption line, and 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 product of the gas concentration c and the optical path length L, and the methane gas concentration c can be detected with extremely high sensitivity.

By the way, α (ω ₀ ) and γ ^{2 in the} _equation 5
Changes with the pressure of the gas atmosphere, as shown in FIG.

FIG. 2 is a diagram showing the relationship between the absorption coefficient α (ω) with respect to 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 pressure (torr), the vertical axis represents absorption coefficient α (ω), detection signal P (2ω), and full width at half maximum 2γ.

In order to accurately measure the gas concentration by the above-mentioned equation 5, the values of α (ω ₀ ) and γ ² under 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 light is swept before and after the methane gas absorption line.

Now, when the central frequency Ω ₀ of the laser light is changed, the first term and the second term of the equation 3 have a waveform in which the coefficients are respectively added to T ₀₂ and T ₀₁ . The coefficient is
I ₀ , ΔΩ, and the like, and if the oscillation condition of the semiconductor laser is set, it can be handled as a constant without any problem. Therefore, the waveform of the detection signal P (2ω) has a shape obtained by integrating (b) and (c) in FIG. 4 with certain coefficients and adding them together (see FIG. 3). However, in practice, the first term of Equation 3 is superior to the second term, so
The width of the center frequency Ω ₀ between the minimum value on the low wavelength side and the minimum value on the high wavelength side shown in FIG. 4C corresponds to the full width at half maximum 2γ at the gas atmosphere pressure. If the full width at half maximum 2γ is obtained in this way, the pressure can be obtained based on FIG. 2, and the absorption coefficient α (ω ₀ ) under the pressure can be obtained. Note that P (2ω) shown in FIG.
₀ ) / γ ^{2 due} to pressure change, total pressure 100 torr
The maximum value is shown near r.

To obtain the atmospheric pressure from FIG. 2, α
If either (ω ₀ ) or γ is known, the pressure is known, and the pressure-corrected concentration can be detected.

FIG. 1 is a schematic configuration diagram of a gas concentration measuring apparatus as one embodiment of the present invention.

In the figure, reference numeral 1 denotes a semiconductor laser, which uses a distributed feedback laser because it is necessary to oscillate a laser beam having a single wavelength. Reference numeral 2 denotes an optical system for coupling the laser light from the semiconductor laser 1 to the silica optical fiber 3a, which comprises a condenser lens and an optical isolator for cutting the return light. The end surface of the optical system 2 is further subjected to antireflection coating treatment to minimize the return light to the semiconductor laser 1. Reference numeral 4 denotes a Peltier element for mounting the semiconductor laser 1 and controlling the temperature thereof by an external current source (not shown), and the laser module 5 is constituted by the above.

Reference numeral 6 is an optical branching device for branching the laser light from the optical fiber 3a and is connected to the optical fibers 3a and 3b. Reference numeral 7 is a measurement gas cell through which the light from the optical fiber 3b passes, and contains methane gas of unknown concentration at a constant temperature. Reference numeral 8 is a reference gas cell through which the light from the optical fiber 3c is transmitted, and contains methane gas having a known concentration at a constant temperature and a constant pressure. The reference gas cell 8 needs to be set to a constant temperature and a constant pressure, and therefore needs to have a structure that is not easily affected by disturbance. Therefore, the reference gas cell 8 is covered with a heat insulating material and is temperature-controlled and kept at a constant temperature.

3d is a silica optical fiber for the return path for propagating the laser light transmitted through the measuring gas cell 7, and 9 is a photodetector comprising a pin photodiode or the like for detecting the intensity of the laser light from the optical fiber 3d. is there. Reference numeral 3c is an optical fiber for allowing the laser light branched by the branching device 6 to pass through the reference gas cell 8, and 10 is light detection including a pin photodiode or the like for detecting the intensity of the laser light passing through the reference gas cell 8. It is a vessel.

Here, the end faces of the optical fibers 3a to 3d are
Diagonal cut non-reflective coating is used to prevent internal interference.

On the other hand, 11 is an oscillator for outputting a sine wave signal of frequency ω, and 12 is a frequency 2 based on this signal of frequency ω.
A frequency doubler for producing a second harmonic signal of ω, 13 is a constant current power supply for adding a bias current to the semiconductor laser 1, and the laser drive circuit 14 is constituted by the above.

The laser drive circuit 14 drives the semiconductor laser 1 by superimposing the sine wave signal of the frequency ω from the oscillator 11 on the output from the constant current power supply 13. In this example,
The modulation frequency was ω = 50 KHz. Further, an inductance L is connected to the output side of the constant current power supply 13 to prevent the influence of the output of the oscillator 11, and a capacitor C is connected to the output side of the oscillator 11.

Reference numeral 15 is a lock-in amplifier which performs phase sensitive detection of the output of the photodetector 9 in synchronization with the frequency ω of the sine wave signal from the oscillator 11, and 16 is synchronized with the frequency 2ω of the sine wave signal of the frequency multiplier 12. A lock-in amplifier for performing phase sensitive detection of the output of the photodetector 9, and 17 for both lock-in amplifiers 1.
It is a divider for obtaining the output ratio of 5 and 16. Reference numeral 18 denotes a concentration calculator for correcting the concentration from the output of the divider 17. Reference numeral 19 is a lock-in amplifier that performs phase sensitive detection of the output of the photodetector 10 in synchronization with the frequency 2ω of the sine wave signal of the frequency doubler 12.

On the other hand, 20 is a low-pass filter for cutting the modulation frequency ω component, 21 is a reference power source for generating a predetermined reference voltage, and 22 is a low-pass filter 20 in order to obtain a change in the direct current component of the forward voltage of the semiconductor laser 1. Is a subtractor for obtaining the difference between the output voltage value of the reference voltage and the reference voltage value, and 23 is an amplifier for amplifying the output from the subtractor 22. In addition, 24
Is an XY recorder for inputting and recording the output of the amplifier 23 on the X axis, the output of the lock-in amplifier 19 on the Y ₁ axis, and the output of the lock-in amplifier 16 on the Y ₂ axis. It is configured. The measuring means 25 is further provided with a signal processing device (not shown) connected to the XY recorder 24.

Next, the operation of the embodiment will be described.

In FIG. 1, in the measurement, first, a constant current power source 13 supplies a current having a magnitude equal to or higher than a certain oscillation threshold value to the semiconductor laser 1. A sinusoidal current having a modulation frequency ω is superimposed on this current to modulate the frequency and intensity of the laser light. Then, by adjusting the applied current to the Peltier element 4,
The center frequency of the semiconductor laser 1 is changed.

At this time, the center frequency of the semiconductor laser 1 is
It is monitored by the output value of the amplifier 23 that amplifies the output of the subtractor 22. This is because the relationship between the amount of change in the oscillation frequency of each semiconductor laser and the amount of change in the forward resistance component is reproducible. Therefore, the output of the amplifier 23 is input to the X-axis component of the XY recorder 24 as a monitor of the center frequency.

On the other hand, the laser light oscillated by the semiconductor laser 1 is partially passed through the optical fiber 3a by the optical branching device 6 through the methane gas atmosphere of unknown concentration in the measuring gas cell 7 at a constant temperature, It is guided to the photodetector 9 through the optical fiber 3d, and the intensity thereof is detected there.

On the other hand, after the remaining light passes through the optical branching device 6 and passes through the optical fiber 3c to the reference gas cell 8 containing the gas of the same kind as the measurement gas of known concentration, constant temperature and constant pressure, It is guided to the photodetector 10 and the intensity thereof is detected there.

The detection signal from the measurement gas cell 7 is phase-sensitive detected by the lock-in amplifiers 15 and 16, and the fundamental wave signal P _S (ω) and the second-harmonic detection signal P _S (2ω) are detected.
Is obtained and is input to the divider 17, where P _S (2
The concentration before pressure conversion can be obtained from ω) / P _S (ω).

[0048] then enter the second harmonic detection signal P _R (2ω) obtained from the light receiver 8 and 9 Y ₁ axis XY recorder 24, the second harmonic detection signal P _S a (2 [omega) Y ₂ Enter on the axis.

FIG. 3 shows the XY recorder 24 as described above.
It is a figure which shows a part of output waveform obtained by. In the figure, the horizontal axis represents frequency and the vertical axis represents the product k · α · c · L of the absorption coefficient α, the concentration c, the optical path length L and the constant k.

The pressure in the measuring gas cell 7 can be obtained from the difference (output voltage of the amplifier 23) γ between the frequency giving the maximum value and the frequency giving the minimum value of the waveform shown in FIG. In the signal processing unit, the relationship between the voltage difference between these two points and the pressure may be stored in advance, and the concentration may be corrected from this relationship. However, the amplification factor of the amplifier 23 depends on the ambient temperature, the temperature of the element used, and the like. As it fluctuates, it needs to be corrected.

Therefore, first, the ratio representing the relationship between the voltage of the minimum value of the second harmonic voltage of the lock-in amplifier 16 obtained from the transmitted light of the reference gas cell 7 and the pressure in the reference gas cell 8 is stored in advance. .. Next, the minimum value voltage of the second harmonic voltage of the lock-in amplifier 16 obtained from the transmitted light of the measurement gas cell 7 is corrected by this ratio, and the pressure is calculated from the corrected value and the value in the table in the signal processing unit. Ask. An accurate concentration can be obtained from the obtained pressure and the concentration before correction.

As described above, according to the present embodiment, the modulated laser light is branched by the branching device 6, and one of the branched lights is transmitted through the methane gas in the measurement gas cell 7 of unknown concentration and pressure to obtain the light. Lock-in amplifiers 15 and 16 via the detector 9
The gas concentration is calculated by dividing the secondary differential signal T ₀₂ obtained by the phase sensitive detection with the primary differential signal T ₀₁ by the divider 17, and the methane gas having a constant temperature, a constant pressure and a known concentration is calculated. Since the cell 8 for the reference gas accommodated therein is transmitted, the phase-sensitive detection is performed by the lock-in amplifier 19 via the photodetector 10, and the concentration is corrected based on the obtained second derivative signal, the gain is high and the stability is high. It is possible to realize a gas concentration measuring method and a measuring apparatus therefor that can perform pressure correction without using a frequency amplifier. Therefore, it is possible to accurately measure the methane gas concentration without installing a new pressure sensor even in places or environments where the atmospheric pressure changes drastically, such as in a coal mine or a plant, which would not be possible without a pressure sensor in the conventional method. it can.

Further, in this embodiment, the reference gas cell and the measurement gas cell are used as phase sensitive detection signals for pressure correction.
Although a harmonic signal is used, the present invention is not limited to this, and a fundamental signal may be used. Further, in the present embodiment, the methane gas was stored in the reference gas cell to measure the methane gas, but another gas may be stored in the reference gas cell to measure the concentration of the other gas.

[0054]

In summary, according to the present invention, the laser light before being incident on the gas atmosphere to be measured is branched through a branching device, and the branched laser light is used as a reference for constant temperature, pressure and concentration. The intensity of the transmitted light is detected through a gas atmosphere, and the concentration of the specific gas to be measured is corrected with this detected value, so pressure can be corrected without using a high-gain and high-stability amplifier. A gas concentration measuring method and a measuring device therefor can be realized.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a gas concentration measuring device as an embodiment of the present invention.

FIG. 2 is an absorption coefficient α (ω) and a detection signal P (2) with respect to pressure in a cell used in the gas concentration measuring apparatus shown in FIG.
It is a figure which shows the relationship between (omega)) and half value width 2 (gamma).

FIG. 3 is an X used in the gas concentration measuring device shown in FIG.
It is a figure which shows a part of output waveform obtained by the Y recorder.

FIG. 4 is a diagram showing a transmittance T with respect to frequency, and its first derivative T ₀₁ and second derivative T ₀₂ .

[Explanation of symbols]

 1 semiconductor laser 6 branching device 7 measurement gas cell 8 reference gas cell 9, 10 light receiver 11 oscillator 12 frequency multiplier 13 constant current power supply 15, 16, 19 lock-in amplifier 17 divider 24 XY recorder

Front Page Continuation (72) Inventor Masahiko Uchida 5-1-1 Hidaka-cho, Hitachi-shi, Ibaraki Hitachi Cable Co., Ltd.

Claims (2)

[Claims] 1. A laser that oscillates a laser beam having a wavelength and intensity according to a drive current and temperature is used, and the drive current or temperature of the laser is changed to oscillate a laser beam whose wavelength and intensity are modulated. The central wavelength of the laser light is swept, the intensity of the transmitted light obtained by passing the laser light through the gas atmosphere to be measured is detected, and the specific component in this detection signal is phase-sensitive detected, and this detection signal is detected. From the gas concentration measuring method for measuring the concentration of a specific gas under the atmosphere pressure, the laser beam before entering the gas atmosphere to be measured is branched via a branching device,
The branched laser light is passed through a reference gas atmosphere having a constant temperature, a constant pressure and a known concentration to detect the intensity of the transmitted light and phase-sensitively detect a specific component from the detection signal,
A gas concentration measuring method, characterized in that the concentration of a specific gas to be measured is corrected with this detection signal. 2. A laser for oscillating a laser beam having a wavelength and intensity according to a driving current and a temperature, a measuring gas cell for accommodating a specific gas to be measured and keeping the temperature of the gas constant, A measurement gas side photodetector that detects the intensity of transmitted light obtained by passing a laser beam through this measurement gas cell, and phase-sensitive detection of a specific component in the signal from this detector, and from the detection signal the above In a gas concentration measuring device having a measuring means for measuring the concentration of a specific gas,
An optical branching device for branching the laser light incident on the measurement gas cell, and a gas of a known concentration, which transmits the laser light branched by the optical branching device and is kept at a constant temperature and a constant pressure, are stored. A reference gas cell, a reference gas side photodetector that detects the intensity of the laser light that has passed through this reference gas cell, and performs phase sensitive detection of a specific component in the detection signal,
A gas concentration measuring device comprising: a correcting unit that corrects the concentration of the specific gas measured by the measuring unit with a detection value measured by the reference gas side photodetector.