JP2744742B2 - Gas concentration measuring method and its measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art It is known that the presence or absence of a gas can be detected by utilizing the fact that laser light of a specific wavelength is easily absorbed by a certain kind of gas. Sensing technology using this principle is used in industrial measurement, pollution monitoring, etc. Widely used in the field.
If this laser beam is used as a transmission line using an optical fiber, remote monitoring becomes possible.

[0003] Therefore, the present inventors have developed a new remote gas detector using an optical fiber as a transmission line. In a method utilizing this principle, a laser beam is modulated at a high frequency around a driving current of a semiconductor laser, and a laser beam having a modulated wavelength and intensity is oscillated. Further, a laser beam emitted behind the semiconductor laser by controlling the current and the temperature so that the center wavelength of the oscillation becomes the center of the gas absorption line is used for monitoring.
The stabilized laser light emitted forward is transmitted through an optical fiber to a gas cell for measurement containing a gas of unknown concentration, and the transmitted light is guided to a light receiving unit by another optical fiber facing the light source. The gas concentration can be detected at a high S / N ratio by using the double detection signal or the basic detection signal of the laser light obtained by performing phase-sensitive detection using lock-in.

However, focusing on one of the isolated absorption lines of gas, 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. Has a value. Therefore, when performing concentration measurement using the present sensor 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 and pressure is corrected.

Therefore, the present inventors (April 26, 1991)
In Japanese Patent Application “Gas Concentration Measuring Method and Measuring Apparatus”), using a laser that oscillates a laser beam having a wavelength and intensity corresponding to the driving current and temperature, and changing the driving current or temperature of the laser to change the wavelength And oscillates the laser light whose intensity is modulated, sweeps the center wavelength of the laser light, detects the intensity of transmitted light after passing the laser light through the gas atmosphere to be measured, and detects the intensity of the transmitted light. A method and apparatus for measuring the specific gas concentration under the above-mentioned atmospheric pressure from the detected signal by phase-sensitive detection of the specific component are proposed.

FIG. 9 is a diagram showing the relationship between the wavelength of the laser light and the second harmonic phase detection sensitive detection signal. In FIG. 9, the horizontal axis indicates the center wavelength of the laser beam, and the vertical axis indicates the second-harmonic phase-sensitive detection signal. This is an output signal when acetylene gas is used as the detection gas and the absorption wavelength of the detection gas is 1.532 μm. In order to obtain a gas signal from this output signal, a peak value obtained when the center wavelength of the laser light is near the gas absorption line is obtained. Further, 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, and from this value, information on the pressure of the gas atmosphere can be obtained.

[0007]

However, when a low concentration gas is detected under the same modulation condition, periodic noise is superimposed on the output signal in addition to the gas signal to be detected (see FIG. 10). . The periodic noise is generated when laser light emitted from the optical surface and laser light reflected by the optical surface and returned as light interfere with each other to fluctuate an output signal. When such a signal is obtained, it is difficult to obtain the peak value and the spectrum width from the output signal, and the detection accuracy is reduced. FIG. 10 is a diagram showing the relationship between the wavelength of the laser beam in the low-concentration gas and the second-harmonic phase-sensitive detection signal.

Accordingly, the present invention solves the above-mentioned problems, and provides a gas concentration measuring method and a gas concentration measuring method capable of accurately measuring a gas concentration even when a signal such as return light noise that degrades a gas signal is superimposed. To provide.

[0009]

In order to achieve the above object, a gas concentration measuring method according to the present invention uses a laser which oscillates a laser beam having a wavelength and intensity corresponding to a driving current and a temperature. By changing the current or temperature,
The laser beam whose wavelength and intensity are modulated is oscillated, the center wavelength of the laser beam is swept, and the intensity of transmitted light obtained by passing the laser beam through a gas atmosphere to be measured is detected. Fundamental wave component and second harmonic wave component
And a phase sensitive detector, and the ratio of the detected signal is determined by an XY recorder.
Input to the Y-axis and the monitor output of the laser
The gas concentration measuring method for measuring a concentration of a specific gas from the peak value of the waveform obtained by inputting the X-axis of the Y recorder <br/> under the atmospheric pressure, the peripheral using the FFT said detection signal
Decomposed into frequency domain, after removal of Jo Tokoro value or more high-frequency components by the low pass filter, and waveform shaped by the inverse FFT,
And removed gas signal noise, determine the pressure and gas concentration in the gas atmosphere from the waveform <br/> of the gas signal, and correct the gas concentration in the resulting pressure.

The gas concentration measuring device of the present invention oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature and accommodates a laser whose central wavelength is swept and a specific gas to be measured. Together with a measuring gas cell for keeping the temperature of the gas constant, a detector for detecting the intensity of transmitted light obtained by passing the laser light through the measuring gas cell, and a basic signal in the signal from the detector. Wave component and 2nd harmonic
And a lock-in amplifier that detects
When the ratio of the detection signal from the quin amplifier is input to the Y axis
In both cases, when the laser monitor output is input to the X-axis,
XY for outputting a waveform for measuring the concentration of the specific gas
In a gas concentration measurement device provided with a recorder,
FFT for decomposing a wave signal into the frequency domain, and this FFT
High frequency components equal to or higher than a predetermined value are
Low-pass filter that removes the minute and this low-pass filter
Waveform a detection signal from which high-frequency components above a specified value have been removed.
Inverse FFT to be reproduced, obtained detection signal and spectrum width
And an FFT analyzer that corrects the gas concentration by extracting

[0011]

First, in spectral measurement, there is a frequency modulation method as a method for improving measurement sensitivity. This is because when the frequency-modulated light is transmitted through the atmosphere containing the gas to be detected, the detection signal of the transmitted light includes a direct current component, a fundamental wave component having the same frequency as the modulation frequency, and its harmonic components. can get. When the fundamental wave component and the second harmonic component are phase-sensitive detected, the fundamental 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. Thus, when the laser light whose drive current is modulated is transmitted through an atmosphere containing a specific gas and a specific component in a detection signal of the transmitted light is subjected to phase-sensitive detection, information on the gas concentration can be obtained from the detection signal.

A phase-sensitive detection signal of a specific component including noise with respect to the center optical frequency of the laser is decomposed into a frequency domain by using FFT (fast Fourier transform), and a specific harmonic component is removed by a low-pass filter. Waveform reproduction by inverse FFT and gas atmosphere from noise-free gas signal
Pressure and gas concentration, and the obtained pressure
This is to correct the degree . Here, the phase sensitive detection is
Extracting only the component having a specific frequency and phase and measuring its amplitude.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the accompanying drawings. Here, an example in which methane gas is measured 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 have been modulated as described above is transmitted through an atmosphere containing methane gas, a detection signal P of the transmitted light is represented by Expression 1.

[0015]

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

[0016]

[Number 2 is a _{C 0 = T + ((ΔΩ} ) 2/4) · T 02. The detection signal P includes a DC component and cos ωt
Component and a cos2ωt 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 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.
T, T ₀₁ , and T ₀₂ are values of transmittance, Ω = Ω _{0 of} the first derivative dT / dΩ, and the second derivative d ² T / dΩ ² (where ω ₀ is the center frequency of the laser). The shape is shown in FIG.

FIG. 4 shows the transmittance T with respect to the frequency.
FIG. 4 shows a first derivative T ₀₁ and a second 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 and phase component φ of cos ωt in equation 1 are phase-sensitive detected,

[0019]

Equation 3] P (2ω) = A [I 0 ((ΔΩ) 2/4) T 02
+ ΔI · ΔΩ cos φ · T ₀₁ ], and it is understood that the detection signal P (2ω) changes based on T ₀₁ and T ₀₂ .

When the absorption of methane gas is detected by the detection signal P (2ω), the center frequency Ω ₀ of the laser beam is
The fact that the maximum sensitivity is obtained when the center coincides with the center ω ₀ of the absorption line of methane gas is used (see FIG. 4). At this time, since _T01 is "0" and _T02 is the maximum, the second term of Equation 3 is deleted, and only the first term remains. That is, Ω ₀ = ω
_{When 0} , T ₀₂ is

[0021]

[Number 4] _{T 02 (Ω 0 = ω 0} ) = 2 · α becomes _{(ω 0) · c · L} / γ 2. Therefore, substituting this into the first term of Equation 3 gives

[0022]

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)
Varies according to the pressure of the gas atmosphere, as shown in FIG.

FIG. 2 is a diagram 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 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 according to the above formula 5, the values of α (ω ₀ ) and γ ² under the 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.

When the center frequency Ω ₀ of the laser beam is changed, the first term of the equation (3) becomes a waveform obtained by integrating the coefficients of T ₀₂ , and the second term of the equation becomes a waveform of a form obtained by integrating the coefficients of T ₀₁ . The coefficient is
I ₀ , ΔΩ, etc. If the oscillation conditions of the semiconductor laser are set, there is no problem even if they are treated as constants. Therefore, the waveform of the detection signal P (2ω) has a shape in which (b) and (c) in FIG. 4 are integrated with a certain coefficient, respectively, and these are added to each other (see FIG. 3). However, in practice, the first term in Equation 3 is superior to the second term,
The width of the center frequency Omega ₀ between the minimum value of the minimum value and the higher wavelength side of the low-wavelength side shown in FIG. 4 (c) corresponds to the full width at half maximum 2γ in a gas atmosphere pressure. When the full width at half maximum 2γ is obtained in this manner, a pressure can be obtained based on FIG. 2, and an absorption coefficient α (ω ₀ ) under the pressure can be obtained. It should be noted that P (2ω) shown in FIG.
₀ ) / γ ^{2 due} to pressure change, total pressure 100 torr
The maximum value is shown near r.

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

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

In FIG. 1, reference numeral 1 denotes a semiconductor laser, which uses a distributed feedback laser because it is necessary to oscillate laser light of a single wavelength. Reference numeral 2 denotes an optical system for coupling the laser light from the semiconductor laser 1 to the quartz optical fiber 3a, and includes a condenser lens and an optical isolator for cutting the return light from the condenser lens. The end face of the optical system 2 is further subjected to an anti-reflection coating process to minimize return light to the semiconductor laser 1. Reference numeral 4 denotes a Peltier device for mounting the semiconductor laser 1 and controlling its temperature by a Peltier device power supply 5, and the laser module 6 is configured as described above.

Reference numeral 7 denotes a measuring gas cell through which light from the optical fiber 3a passes, and contains methane gas of a constant temperature and an unknown concentration.

Reference numeral 3b denotes a return optical fiber for transmitting a laser beam transmitted through the measuring gas cell 7, and reference numeral 8 denotes a photodetector comprising a pin photodiode for detecting the intensity of the laser beam from the optical fiber 3b. is there.

Here, the end faces of the optical fibers 3a and 3b are
The oblique cut anti-reflection coating or the like is processed so as not to generate an interference system inside.

On the other hand, 9 is an oscillator for outputting a sine wave signal of frequency ω, and 10 is a signal of frequency 2ω
Is a constant current power supply for applying a bias current to the semiconductor laser 1, and the laser driving circuit 12 is configured as described above.

The laser drive circuit 12 drives the semiconductor laser 1 by superimposing a sine wave signal of frequency ω from the oscillator 9 on the output from the constant current power supply 11. In addition, an inductance L is connected to the output side of the constant current power supply 11 to prevent the influence of the output of the oscillator 9, and a capacitor C is connected to the output side of the oscillator 9.

Reference numeral 13 denotes a lock-in amplifier that performs phase-sensitive detection of the output of the photodetector 8 in synchronization with the frequency ω of the sine wave signal from the oscillator 9, and 14 synchronizes with the frequency 2ω of the sine wave signal of the frequency multiplier 10. And a lock-in amplifier 15 for performing phase-sensitive detection of the output of the photodetector 8.
14 is a divider for calculating the output ratio.

On the other hand, 16 is a low-pass filter for cutting the modulation frequency ω component, 17 is a reference power supply for generating a predetermined reference voltage, and 18 is a low-pass filter 16 for obtaining a change in the DC component of the forward voltage of the semiconductor laser 1. A subtractor 23 calculates the difference between the value of the output voltage and the value of the reference voltage, and an amplifier 23 amplifies the output from the subtractor 22. In addition, 24
Is an XY recorder that inputs and outputs the output of the amplifier 19 on the X-axis, the output of the amplifier 19 on the X-axis, and the output of the divider 15 on the Y-axis, respectively. . The measuring means 21 is further provided with an FFT analyzer for performing FFT processing, inverse FFT processing, filter processing, and the like, connected to the XY recorder 20.

Next, the operation of the embodiment will be described.

In FIG. 1, the measurement is performed first by using the semiconductor laser 1.
, A current having a magnitude equal to or larger than a predetermined oscillation threshold is supplied from the constant current power supply 11. The oscillator 9 superimposes a sine wave current having a modulation frequency ω on the bias current to modulate the frequency and intensity of the laser light. Then, the applied current of the Peltier element 4 is adjusted by the variable resistor VR to change the center frequency of the semiconductor laser 1.

At this time, the center frequency of the semiconductor laser 1 is monitored by the voltage of the bias reference power supply 17 and the subtractor 18.
The output of the amplifier 19 is obtained by amplifying the output of the amplifier 19. That is, the oscillation frequency of each semiconductor laser 1 and the amount of change in the forward resistance component have a reproducible relationship. Therefore, the output of the amplifier 19 is input to the X-axis of the XY recorder 20 as a monitor of the center frequency.

On the other hand, the laser light emitted from the semiconductor laser 1 passes through the optical system 2, passes through the optical fiber 3 a, passes through the methane gas atmosphere in the gas measuring cell 7, and is detected through the optical fiber 3 b. To the detector 8 where the intensity is detected. The detection signal from the gas measurement cell 7 is phase-sensitive by the lock-ups 13 and 14, and the fundamental signal P
_S (ω) and the second-harmonic detection signal P _S (2ω) are obtained and input to the divider 15 to obtain P _S (2ω) / P
_S (ω) is input to the Y-axis of the XY recorder 20.

The measuring apparatus shown in FIG. 1 can obtain a waveform similar to the conventional one for a high concentration gas (FIG. 9). The gas concentration signal is obtained from the peak value near the gas absorption line. The pressure is determined from the width of the extreme values that appear on both sides of the peak value. The signal processing section stores the relationship between the voltage difference between the two points and the pressure in advance to determine the pressure, and can correct the gas concentration signal with respect to the pressure.

On the other hand, an output signal waveform similar to the conventional one can be obtained for a low concentration gas. However, in this state, the peak value and the extreme value cannot be obtained from the output signal.

First, data is input to the FFT analyzer 22 and the following processing is performed. The phase sensitive detection signal corresponding to the center optical frequency of the laser is decomposed into a frequency domain using FFT (FIG. 5). For example, when a 256-point FFT analyzer is used, for the output signal of FIG. 5, a region wider than the gas signal region is set as the analysis target region. The analysis target area is divided into 256 equal parts. For this region, FFT frequency analysis is performed on the assumption that the X axis is a time region.

FIGS. 5 and 6 are power spectrum density distributions obtained by performing FFT processing on the respective output waveforms shown in FIGS. 9 and 10. Looking at the waveform without noise, the spectral components of the gas signal are seen in the low frequency region. on the other hand,
Looking at the power spectrum density of the waveform on which noise is superimposed, the distribution is seen up to high frequencies, which is due to noise. Therefore, a low-pass transmission filter process having characteristics as shown in FIG. 7 is performed in comparison with the power spectrum density distribution of a waveform on which noise is not superimposed. This low-pass filter multiplies the spectral components “1 to 8” by a coefficient “1”. On the other hand, “0” is assigned to the spectral components “9 to 128”.
To remove. FIG. 8 shows a waveform reproduction of the subsequent power spectrum density distribution by inverse FFT.

As described above, according to the present embodiment, a high-frequency component having a predetermined value or more is removed from the detection signal to obtain a gas signal. From this gas signal, the pressure of the gas atmosphere and the gas concentration signal are measured. since measuring the concentration of pressure compensation to gas, be superimposed by UNA signal gas signal Ru degrades <br/> such as return light noise can be accurately measured gas concentration.

Note that the low-pass filter used in this case removed the spectral components of N = 9 or more.
This value naturally changes if the width of the wavelength region to be subjected to FT and the number of processing points of the FFT process change. As a setting method of this value, the start point and the end point of the wavelength region to be measured are determined in advance, and the value of N is obtained from the spectral density distribution by the FFT processing at a high density. Further, in this embodiment, methane gas is used as the gas, but the gas is not limited to this, and another gas such as acetylene gas may be used.

[0048]

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

(1) Accurate gas concentration can be measured even when noise that deteriorates the gas signal is superimposed.

(2) The SN ratio in low concentration gas measurement is improved.

[Brief description of the drawings]

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

FIG. 2 shows an absorption coefficient α (ω) and a detection signal P (2) with respect to pressure in a cell used in the gas concentration measuring device shown in FIG.
FIG. 6 is a diagram illustrating a relationship between ω) and a half width 2γ.

FIG. 3 shows X used in the gas concentration measuring device shown in FIG.
FIG. 5 is a diagram illustrating a part of an output waveform obtained by a Y recorder.

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

5 is a power spectrum density distribution obtained by performing an FFT process on the output waveform shown in FIG. 10 by the measuring apparatus shown in FIG. 1;

FIG. 6 is a power spectrum density distribution when the waveform shown in FIG. 9 is subjected to FFT processing by the measuring apparatus shown in FIG. 1;

FIG. 7 is a diagram showing characteristics of a filter in an FFT analyzer used in the measuring device shown in FIG.

8 is a diagram illustrating a waveform reproduced by performing an inverse FFT on the power spectrum density distribution illustrated in FIG. 5 by an FFT analyzer used in the measurement apparatus illustrated in FIG. 1;

FIG. 9 is a diagram showing a relationship between the wavelength of a laser beam in a high-concentration gas and a second-harmonic phase-sensitive detection signal.

FIG. 10 is a diagram showing a relationship between a wavelength of a laser beam and a second-harmonic phase-sensitive detection signal in a low-concentration gas.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Optical system 3a, 3b Optical fiber 4 Peltier element 5 Peltier element power supply 6 Laser module 7 Measurement gas cell 8 Photodetector 9 Oscillator 10 Multiplier 11 Constant current power supply 12 Laser drive circuit 13, 14 Lock-in Amplifier 15 Divider 16 Low-pass filter 17 Reference power supply 18 Subtractor 19 Amplifier 20 XY recorder 21 Measurement means 22 FFT analyzer

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masahiko Uchida 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Prefecture Nippon Electric Cable Co., Ltd. Opto-System Laboratories (56) References JP-A-3-277945 ( JP, A) JP-A-3-505782 (JP, A) JP-A-63-78053 (JP, A)

Claims (2)

(57) [Claims] 1. A laser which oscillates a laser beam having a wavelength and an intensity corresponding to a drive current and a temperature, and oscillates a laser beam having a wavelength and an intensity modulated by changing the drive current or the temperature of the laser. 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 fundamental wave component and the second harmonic component in the detection signal are phase-shifted. Sensitive detection
When the ratio of the detected signal is input to the Y-axis of the XY recorder,
Then, input the monitor output of the laser to the X-axis of the XY recorder.
In a gas concentration measuring method for measuring the concentration of a specific gas under the above-mentioned atmospheric pressure from the peak value of the waveform obtained by applying force ,
The detected signal is decomposed into the frequency domain using FFT,
It was removed Jo Tokoro value or more high-frequency components by-pass filter
After, waveform shaping by the inverse FFT, and gas <br/> scan signal removed of noise from this waveform of the gas signal calculated the pressure and gas concentration in the gas atmosphere, complement the gas concentration in the resulting pressure <br/> A gas concentration measuring method characterized by correcting. 2. A laser which oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature and has a center wavelength swept , and a specific gas to be measured is accommodated.
A measuring gas cell for keeping the temperature of the gas constant, a detector for detecting the intensity of transmitted light obtained by passing the laser beam through the measuring gas cell, and a base in a signal from the detector.
Lock-in for phase sensitive detection of main wave component and second harmonic component
The ratio between the amplifier and the detection signal from this lock-in amplifier is
Input to the Y-axis, and monitor output of the laser is X
When input to the axis, it is used to measure the concentration of the specified gas.
The gas concentration measuring apparatus and an XY recorder for outputting a waveform, decomposing the detection wave signal to the frequency domain FF
T and predetermined from the frequency domain decomposed by this FFT
And a low-pass filter that removes high-frequency components
The low-pass filter removes high-frequency components above a certain value.
And inverse FFT to the detection signal waveform reproduced which extracts the detection signal obtained with a spectral width correcting the gas concentration F
A gas concentration measurement device comprising an FT analysis device.