JP3450938B2 - Gas concentration measuring method and apparatus - Google Patents

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 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 phenomenon, and the sensing technology applying this principle is widely used in industrial measurement, pollution monitoring, and the like. Further, if this laser light is transmitted using an optical fiber as a transmission line, remote monitoring becomes possible.

Therefore, the present inventors have filed Japanese Patent Application No. 2-7849.
By applying No.8, we have developed a new remote gas detector using an optical fiber as a transmission line. In the method utilizing this principle, the driving current of the semiconductor laser is modulated at a high frequency centering on a current having a predetermined value, and laser light having a modulated wavelength and intensity is oscillated. Further, a laser beam emitted to the rear of the semiconductor laser so that the central wavelength of oscillation is at the center of the gas absorption line by controlling the current and temperature is used for monitoring. Then, the laser beam stably emitted in the forward direction is transmitted through the optical fiber to the gas cell for measurement containing the unknown concentration, and the transmitted light is guided to the light receiving portion by another optical fiber facing the optical fiber. The gas concentration can be detected with a higher SN ratio than the overtone detection signal or the fundamental wave detection signal.

However, focusing on one isolated absorption line in the gas absorption line, the shape of the absorption line changes due to the pressure of the gas atmosphere, and is used for quantitative measurement of the gas.
The harmonic detection signal also has a pressure-dependent value. Therefore, when the concentration is measured using this sensor in a place where the atmospheric pressure changes drastically, such as in a coal mine or a plant, the pressure cannot be accurately measured unless a pressure sensor is separately provided for pressure monitoring and pressure correction.

Therefore, the present inventors have used a laser that oscillates a laser beam having a wavelength and intensity according to the drive current and temperature, and changing the drive current or temperature of this laser to modulate the wavelength and intensity of the laser beam. And the central wavelength of the laser light is swept, the intensity of the transmitted light after passing the laser light through the gas atmosphere to be measured is detected, and a specific component in this detection signal is detected by phase sensitive detection. , Proposed a method for measuring the specific gas concentration under the above-mentioned gas atmosphere pressure from this detection signal (Japanese Patent Application No. 3-96825).
issue).

FIG. 26 shows the relationship between the wavelength of the laser light and the ratio of the second harmonic phase sensitive detection signal obtained by phase sensitive detection to the fundamental wave phase sensitive detection signal (hereinafter referred to as gas signal).
Shown in.

In the figure, the horizontal axis represents the central wavelength of the laser beam, and the vertical axis represents the gas signal. Acetylene gas is used as the detection gas, and the absorption wavelength of the gas is 1.53
This is an output signal when it is set to 2 μm. In order to obtain the gas concentration from this signal, the peak value obtained when the center wavelength of the laser light is near the gas absorption line is obtained. Further, the wavelength widths of the two extreme values appearing on both sides in the vicinity of the gas absorption line show the spectrum width of the gas absorption line, and from this value, information on the pressure of the gas atmosphere can be obtained.

[0008]

By the way, when 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. 27). These noises are generated when the laser light is reflected from the optical surface and interferes as return light to change the output signal. When such a signal is obtained, it becomes difficult to obtain the peak value and the spectral width from the output signal,
The detection accuracy will decrease.

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

On the other hand, the present inventors decomposed the gas signal obtained by the phase sensitive detection into the frequency domain by using FFT (Fast Fourier Transform) to obtain the BPF (low pass filter).
A method and an apparatus for obtaining the gas atmosphere pressure and the pressure-corrected gas concentration from the crest value and the spectrum width of the gas signal whose waveform is reproduced (noise reduction processing) by inverse FFT after removing the specific harmonic component (Patent application 4-2
95866).

However, the above-mentioned waveform reproduction (noise reduction processing) method has the following problems. FIG. 2 shows the relationship between the gas signal and the wavelength of the laser light processed by this method.
8, shown in FIG. 28 (a) and 29 (a) show waveforms before signal processing, and FIGS. 28 (b) and 29 (b).
Shows the waveform after processing.

(1) If noise is periodically superimposed, FF
Although noise can be substantially removed by T, there is a problem that the noise component cannot be completely removed unless the appearance of noise is periodic as shown in FIG.

(2) As shown in FIG. 28, the removal processing causes the gas signal waveform to be rounded. This blunting phenomenon appears when measurement is performed with the atmospheric pressure inside the gas sensor cell lowered. This is because the spectrum width (spreading) of the gas absorption line becomes narrower when the gas atmosphere pressure is lowered, and the frequency component of the gas signal becomes higher in the time domain in the FFT processing. Since the processing is performed with the low-pass filter fixed, even gas signal components are removed.

As described above, it is difficult to fix and handle the low-pass filter while the gas atmosphere pressure changes.

Therefore, an object of the present invention is to solve the above problems and to provide a method and apparatus for accurately measuring gas concentration even if a signal that deteriorates a gas signal is superimposed or even if the laser deteriorates due to its life. To provide.

[0016]

According to a first aspect of the present invention, a laser that oscillates a laser beam having a wavelength and intensity according to a driving current and temperature is used, and the driving current is modulated at a frequency ω with a predetermined current value as a center. By oscillating a laser beam whose wavelength and intensity are modulated by sweeping, sweeping the central wavelength of the laser beam, detecting the intensity of the transmitted light obtained by passing the laser beam through the gas atmosphere to be measured, In a gas concentration measurement method that determines the gas concentration from the gas signal obtained by phase-sensitive detection of a specific component in the detection signal, the gas to be detected is housed in the detection gas cell and the gas is detected in the reference gas cell .
Gas in the output gas cell Other than the gas to be detected,
Criteria for removing noise that does not absorb laser light
Gas for use in gas detection, the laser light transmitted through both gas cells is detected by two photodetectors, and the detection gas signal is detected.
No. (gas signal component + noise component) and reference gas signal (no
With phase-sensitive detection by a lock-in amplifier synchronizing the two gases signal noise component only) to the frequency omega, detected gas signals
After phase-sensitive detection in other lock-in amplifier synchronizing the two gases signal No. and the reference gas signal to frequency 2 [omega, phase Satoshi
The output ratio of the detection gas signal frequency ω and 2ω after the detection
The frequency ω and 2 of the reference gas signal after phase sensitive detection
calculated output ratio of omega, these two output ratio calculated the atmosphere pressure and the gas concentration from the gas signal obtained by the difference in the difference processing unit, a method for measuring the gas concentration under any pressure.

The invention of claim 2 comprises: a detection signal obtained at the initial use of the laser, by comparing the detection signal in the current laser, the gas concentration, together with the correction of the atmospheric gas pressure, The gas concentration detecting method according to claim 1 , wherein the life time of the laser is grasped.

According to a third aspect of the present invention, a laser that oscillates a laser beam having a wavelength and intensity according to a driving current and temperature, an oscillator that modulates the laser with a frequency ω, and a frequency ω of the oscillator is doubled. And a measuring gas cell for accommodating a specific gas to be measured and keeping the temperature of the gas constant, and detecting the intensity of transmitted light obtained by passing the laser light through the measuring gas cell. In a gas concentration measuring device having a detector and a measuring means for measuring the concentration of the specific gas from a detection signal obtained by phase-sensitively detecting a specific component in the signal from the detector, the laser beam is branched. An optical coupler, and a detection gas cell that accommodates a gas to be detected while transmitting one of the branched laser beams,
The other branched laser beam is transmitted and the detection gas
Gas in the cell Other than the gas to be detected and the laser
The reference gas cell that does not absorb the light and that contains the reference gas for removing noise , the photodetector that receives the laser light that has passed through the detection gas cell, and the laser that has passed through the reference gas cell It is a photodetector that receives light and is synchronized with the frequency ω to detect the gas signal (gas signal component + noise component)
A lock-in amplifier for phase-sensitive detection of both gas signals of the quasi- purpose gas signal (only the noise component) and a frequency 2ω,
Another lock-in amplifier that performs phase-sensitive detection of both the detection gas signal and the reference gas signal, and the frequency of the detection gas signal after phase-sensitive detection extracted by both lock-in amplifiers.
Output ratio of several ω and 2ω and reference gas signal after phase sensitive detection
Of the output ratio of the frequency ω and 2ω recording, those having the analyzing recorder for processing, and the difference signal processing unit for differential processing both output ratio data obtained by the analyzing recorder.

[0019]

[0020]

First, the cause of the interference noise will be described below.

The light interference occurs between the main light and the light in the same direction as the main light and delayed by multiple reflection. Now, assuming that the main light is P ₀ and the light that travels with a delay due to reflection is P ₁ as shown in FIG. 24, the phase difference δ between P ₀ and P ₁ is expressed by Equation 1.

[0022]

[Formula 1] δ = 2π · 2L / λ = 4πL / λ where L is the interference length (μm) and λ is the wavelength of light (μm).

Now, assuming that interference occurs between the light having the wavelength λ _{1 and} the light having the wavelength λ ₂ (see FIG. 25), the phase difference change Δδ and the interference length L are expressed by equations 2 and 3.

[0024]

## EQU2 ## Δδ = 4πL (1 / λ ₁ −1 / λ ₂ ) = 2π

[0025]

[Equation 3]

Here, from the waveform actually obtained, λ = 1.
Assuming that 65 (μm) and Δλ = 0.126 (nm), L
= 10.7 (cm) (approximately equal to the optical path length of the current gas cell).

From this, it is considered that the interference points are generated between the optical fibers of the gas cell optical section or between the lenses.

Therefore, the signals are detected separately in the two gas cells, the gas cell containing the gas to be detected and the reference gas cell, and the obtained signals are differentiated to extract only the gas signal component.

When the gas cell to be detected is installed at the detection point, it becomes difficult to remove it and measure the gas detection signal (only the noise component) having a gas concentration of 0 ppm. Therefore, a cell equivalent to the gas cell to be detected is provided as a reference gas cell so that a gas that does not absorb gas such as nitrogen can be filled at any time, and a gas signal (gas concentration 0 p
Only the gas signal component can be extracted by using (pm, noise component only) as a reference signal and taking the difference from the gas signal (noise component + gas signal component) obtained from the gas to be detected. As a result, the interference noise is removed, and an accurate gas concentration can be obtained from this gas signal regardless of the deterioration of the laser due to the life even under an arbitrary pressure.

Further, the gas signal of a specific component including noise with respect to the center wavelength of the laser at a known gas concentration is F
A gas signal corresponding to the gas concentration obtained by simulation calculation from the component (measured power spectrum) decomposed in the frequency domain using FT, the physical characteristic value of the gas to be detected and the basic parameters of the laser is used in the frequency domain using FFT. By inputting the frequency domain decomposition component of the gas signal to be noise-removed by the analysis unit having the neural network in which the correspondence relationship of the component (teacher power spectrum) decomposed into is previously input, even if the noise is not periodic. Even if the gas atmosphere pressure changes, a correct power spectrum can be obtained, and an accurate gas concentration can be obtained from the noise-removed gas signal that is waveform-reproduced by the inverse FFT.

Further, the detection signals are stabilized and fixed for the temperature of the laser when the absorption line of the detection gas is included and the temperature when the absorption line of the detection gas is not included when the drive current of the laser is changed. By subtracting, the noise component is removed and only the gas signal is obtained. The gas concentration and the pressure of the gas atmosphere are obtained from this gas signal, and the gas concentration is obtained by correcting the pressure of the gas concentration signal.

[0032]

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 thus modulated is transmitted through the atmosphere containing methane gas, the detection signal P of the transmitted light is expressed as in Equation 4.

[0034]

## EQU4 ## P = A [I ₀ + ΔIcos (ωt + φ)] ×
_{[C 0 + ΔΩ · T 01} cosωt + ((ΔΩ) 2/4) T
₀₂ cos2ωt] However, this equation 4 satisfies the equation 5.

[0035]

Equation 5] _{C 0 = T + ((ΔΩ} ) 2/4) · T 02 detection signal P is other DC components, cos .omega.t component and cos2
and the ωt component. Here, A is a constant that depends 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, and φ is the position between the modulation frequency ω and the oscillation frequency Ω. The phase difference, ΔΩ, is the frequency modulation amplitude. Further, T, T ₀₁ , and T ₀₂ are the transmittance, respectively, its first derivative dT / dΩ, and its second derivative d ² T / dΩ ² Ω = Ω
₀ (where ω ₀ is the center frequency of the laser) and its shape is shown in FIG.

Further, FIG. 2 shows the transmittance T with respect to frequency,
It is a figure showing the first derivative T ₀₁ and the second derivative T ₀₂ . In each waveform, the horizontal axis is frequency, and the vertical axis is transmittance T (FIG. 2 (a)), first derivative T ₀₁ (FIG. 2 (b)), second derivative T.
₀₂ (Fig. 2 (c)).

When the frequency of cos ωt and the phase component φ in the equation 1 are phase sensitively detected, the equation 6 is obtained, and the detected signal P
It can be seen that (2ω) changes based on T ₀₁ and T ₀₂ .

[0038]

[6] P (2ω) = A [I 0 ((ΔΩ) 2/4) T 02
+ ΔI ・ ΔΩcosφ ・ T ₀₁ ]

Here, the term "phase-sensitive detection" means that only a component having a specific frequency and a phase-sensitive detection signal is extracted and its amplitude is measured.

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. 2). At this time, the first derivative T ₀₁ is “0” and the second derivative T ₀₂ is maximum, so that the second term of the equation 6 is erased and only the first term remains.
That is, the second-order derivative T ₀₂ when Ω ₀ = ω ₀ is given by Equation 7.

[0041]

Equation 7] _{T 02 (Ω 0 = ω 0} ) = 2 · α (ω 0) · c · L / γ 2 Therefore, the number 7 is the number 8 is substituted into the first term of Equation 6.

[0042]

P (2ω) = A · I ₀ (ΔΩ) ² · α (ω ₀ ) · c · L / 2γ 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 gas when Ω ₀ = ω ₀ , 2γ is the 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 detected signal P (2ω) is proportional to the optical path length product c · 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 8
Changes depending on the pressure of the gas atmosphere as shown in FIG.

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

In order to accurately measure the gas concentration by the above-mentioned equation 8, 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 central wavelength Ω ₀ of the laser light is swept before and after the methane gas absorption line.

Now, the center frequency Ω of the laser beam ₀ Change
Then, the first term of Equation 6 is the second derivative T ₀₂ The second term is the primary
Minute T ₀₁ It becomes the waveform of the form that the coefficient in each is integrated.
The coefficient is I ₀ , ΔΩ, etc., and the oscillation of the semiconductor laser
If you set the conditions, it will not interfere even if you handle it as a constant.
Absent. Therefore, the waveform of the detection signal P (2ω) is as shown in FIG.
The waveform shown in FIG. 2B and the waveform shown in FIG.
They are added together with certain coefficients and these are added together.
It becomes a shape (see FIG. 4). 4 is shown in FIG. 1 which will be described later.
The XY recorder used in the gas concentration measuring device
It is a figure which shows a part of output waveform obtained.

However, in reality, the first term of the equation 6 is the second term.
Since it is superior to the minimum, the minimum on the low wavelength side shown in Fig. 2 (c)
Frequency Ω between the maximum value and the minimum value on the high wavelength side ₀ The width of
This corresponds to a full width at half maximum 2γ at the gas atmosphere pressure. like this
Then, if the full width at half maximum 2γ is obtained, the pressure is obtained based on FIG.
And the absorption coefficient α under that pressure
(Ω ₀ ) Can be obtained. Note that P (2
ω) is α (ω in Equation 8 ₀ ) / Γ²Due to the change in pressure
Yes, the maximum value is shown near the total pressure of 100 torr
It

To obtain the atmospheric pressure from FIG. 3, α
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 view of an embodiment of a gas concentration measuring device to which the gas concentration measuring method of the present invention is applied.

As shown in the figure, the gas concentration measuring device is
LD module unit 1, laser drive circuit 2 for driving LD module unit 1, optical coupler 3 for branching laser light from LD module unit 1, and gas to be detected while one of the branched laser lights is transmitted A detection gas cell 4 for accommodating the reference gas, a reference gas cell 5 for accommodating the reference gas while transmitting the other branched laser beam, and a photodetector 6 for receiving the laser light transmitted through the detection gas cell 4. ,
It comprises a photodetector 7 for receiving the laser beam transmitted through the reference gas cell 5 and a signal processing section 8 for processing the signals from both photodetectors 6 and 7, and includes an LD module section 1 and an optical coupler 3. The detection gas cell 4, the reference gas cell 5, and the photodetectors 6 and 7 are connected by optical fibers 9a to 9e.

The LD module unit 1 includes a distributed feedback semiconductor laser (hereinafter referred to as DFB-LD) 10 as a laser that oscillates a laser beam having a single wavelength, and a DFB-LD 10.
Peltier element 11 for heating 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 heat generation and heat absorption of the Peltier element 11. 1
It is composed of 2 and.

The LD module unit 1 includes a DFB-LD1
A connector for coupling the laser light from 0 to the optical coupler 3, a condenser lens for condensing the laser light, and an optical isolator for cutting the return light from the condenser lens (both not shown). The optical system 13 is provided. The end surface of the optical system 13 is further subjected to antireflection coating so that the return light to the DFB-LD 10 is minimized.

The detection gas cell 4 is a cell through which the laser light emitted from the optical fiber 9b is transmitted, and is formed so as to accommodate various gases of unknown concentration to be detected (for example, methane gas, acetylene gas, etc.), It can be easily attached to the place to be measured (detected). Further, at one end of the detection gas cell 4, there is provided an optical fiber 9d for the return path for propagating the laser light transmitted through the detection gas cell 4.

The optical fiber 9d has a photodetector 6 including a pin photodiode for detecting the intensity of laser light.
Are connected.

On the other hand, the reference gas cell 5 includes the optical fiber 9
a cell which the laser light emitted from c is transmitted, detection
Gas in the gas cell 4 other than the gas to be detected, and
For reference that does not absorb laser light and removes noise
It is formed so as to contain a gas (for example, nitrogen gas), and has a structure such that the gas can be easily sucked and discharged, and the internal pressure can be easily adjusted. Further, at one end of the reference gas cell 5, a return optical fiber 9e for propagating the laser light transmitted through the reference gas cell 5 is provided.

The optical fiber 9e has a photodetector 7 including a pin photodiode or the like for detecting the intensity of laser light.
Are connected. The end faces of the optical fibers 9c and 9e are processed by an oblique cut non-reflective coating or the like so that an interference system does not occur.

On the other hand, the laser drive circuit 2 includes an oscillator 14 for outputting a sine wave signal of frequency ω and a frequency multiplier 15 for multiplying the sine wave signal from the oscillator 14 to generate a double wave signal of frequency 2ω. And a bias current source 16 for adding a bias current to the DFB-LD 10, and a triangular wave sweeper 17 for sweeping the current from the bias current source 16.

The laser drive circuit 2 drives the DFB-LD 10 by the current obtained by superimposing the output from the bias current source 16 on the sine wave signal of the frequency ω from the oscillator 14. Further, an inductance L for preventing the influence of the output of the oscillator 14 is connected to the output side of the bias current source 16, and a capacitor C for removing a DC component is connected to the output side of the oscillator 14. ing.

The signal processing unit 8 synchronizes with the frequency ω of the sine wave signal from the oscillator 14 and performs a phase-sensitive detection of the outputs of the photodetectors 6 and 7, and a sine wave of the frequency doubler 15. A lock-in amplifier 19 that performs phase-sensitive detection of the outputs of the photodetectors 6 and 7 in synchronization with the frequency 2ω of the signal, and after the phase-sensitive detection extracted by both lock-in amplifiers 18 and 19.
Frequency of the gas signal for detection (gas signal component + noise component)
The output ratio of several ω and 2ω and the reference gas signal (of the noise component
(2) frequency ω and 2ω output ratio are recorded and calculated, and an analyzing recorder 20 and a difference processing device 21 that performs difference processing on both output ratio data obtained by the analyzing recorder 20.

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

Heat generation and heat absorption of the Peltier element 11 are controlled by the Peltier element power source 12 to fix the DFB-LD 10 at a constant temperature, and the bias current of the DFB-LD 10 is made to have a triangular waveform and swept in one direction. At this time, at the same time, the oscillator 14 superimposes a sinusoidal alternating current (modulation current) on the bias current. A laser beam is incident on the optical system 13 and branched by the optical coupler 3, one of the branched laser beams is transmitted into the optical fiber 9b for the outward path, is spatially propagated in the gas in the gas cell 4 for detection, and is returned again. Laser light is transmitted by the optical fiber 9d for use, and the signal is detected by the photodetector 6.

On the other hand, the other laser beam split by the optical coupler 3 is transmitted into the forward optical fiber 9c, is spatially propagated in the gas inside the reference gas cell 5, and is transmitted again by the backward optical fiber 9e. The transmitted laser light is received by the detector 7 and a detection signal is output.

Of the signals detected by the photodetectors 6 and 7, for detection in synchronization with the frequency ω of the sine wave signal from the oscillator 14 .
Gas signal (gas signal component + noise component) ω1 and reference gas
Both gas signals (noise component only) ω0 are detected by the lock-in amplifier 18, and a detection gas signal (gas signal component + noise) synchronized with the frequency 2ω of the sine wave signal of the frequency doubler 15 is detected.
Component) (2ω) 1 and the reference gas signal (of the noise component)
(2) Both gas signals of (2ω) 0 are detected by the lock-in amplifier 19.

The detection gas signals ω1 and (2ω) 1 after the phase sensitive detection extracted by the lock-in amplifiers 18 and 19 are detected.
Output ratio (2ω) 1 / ω1 of frequency ω and 2ω, and reference gas
The output ratio of the frequencies ω and 2ω of the signals ω0 and (2ω) 0 (2
ω) 0 / ω 0 is transmitted to the analyzing recorder 20. Both output ratios (2ω) 1 transmitted as two gas signals from the analyzing recorder 20 to the difference processing device 21.
/ Ω1, (2ω) 0 / ω0 is the difference calculated by the difference processing device 21.
Then , a gas signal of only the gas signal component is obtained.

The gas concentration can be obtained from the peak value of the gas signal obtained by the difference processing device 21, and the pressure can be obtained from the extreme value or the half-value width appearing on both sides of the peak value. At this time, the noise component is removed, and the accurate pressure and the pressure-corrected accurate density can be obtained.

Further, since both the reference gas signal and the detection gas signal are detected, even if the DFB-LD 10 is in the state of being deteriorated due to its life, it is possible to misunderstand the actual gas concentration. Instead, an accurate gas concentration can be obtained under an arbitrary atmospheric gas pressure. Further, since a gas having a known concentration can be easily enclosed in the reference gas cell 5, a detection signal at the initial stage of use of the DFB-LD 10 is recorded in advance in a recording medium (memory or the like) not shown, and the gas having a known concentration is used. It is possible to know the life time (laser replacement time) of the DFB-LD 10 by detecting the gas signal of No. 2 and comparing it with this.

In summary, since the reference gas cell 5 is provided in addition to the detection gas cell 4 and the gas concentration is measured by using the signal from the laser beam that has passed through both gas cells 4 and 5, the change in gas concentration and the atmospheric gas Since the reference gas signal can always be grasped in response to pressure changes, etc., the gas concentration can be accurately measured even if a signal that deteriorates the gas signal is superimposed or even if the laser deteriorates due to the life. , Gas concentration and atmospheric gas pressure can be calibrated, and DFB
-It is possible to know the life time of the LD 10.

Here, FIG. 5 is a diagram showing a gas signal of methane gas of 50 ppm and a gas signal of methane gas of 0 ppm, the horizontal axis shows the current corresponding to the wavelength, and the vertical axis shows the gas signal.

As shown in the figure, the gas signal of 50 ppm methane gas and the gas signal of 0 ppm methane gas are substantially superposed, and the gas signal of 50 ppm methane gas is almost unknown.

FIG. 6 is a gas signal of 50 ppm obtained by performing a difference process on the gas signals of both methane gases shown in FIG. 5, the horizontal axis shows the current corresponding to the wavelength, and the vertical axis shows the gas signal. There is.

From the figure, it can be seen that the gas signal of methane gas of 50 ppm is extracted. The gas concentration can be known from the peak value of the extracted gas signal, and the pressure of the atmospheric gas can be obtained from the extreme values on both sides of the peak value.

In such a gas concentration measuring device, for example, a gas cell for detection is attached to an OF (Oil Filled = oil filled) cable connection point, and the gas concentration is measured to measure O.
It is possible to diagnose deterioration of the F cable.

FIG. 7 is a schematic view of another embodiment of the gas concentration measuring apparatus to which the gas concentration measuring method of the present invention is applied. still,
For simplification of description, the same reference numerals are used for the same members as those of the gas concentration measuring device shown in FIG. 1, and the description thereof is omitted.

The difference from the embodiment shown in FIG. 1 is that a gas signal of a specific component containing noise with respect to the center wavelength of the laser light is decomposed into a frequency domain by FFT (fast Fourier transform) (measured power spectrum). And a component (teacher power spectrum) obtained by FFT decomposition of the gas signal obtained by simulation calculation from the physical characteristic value of the gas to be measured and the basic parameters of the laser, the analysis using a neural network in advance. The frequency domain component of the gas signal containing noise is input to this analysis unit to extract the noise-free power spectrum, and the inverse FFT is performed on this power spectrum.
This is a point for obtaining a gas signal with reduced noise by applying

As shown in FIG. 7, this gas concentration measuring apparatus has an LD module section 1, a laser drive circuit 2 for driving the LD module section 1, and a laser beam from the LD module section 1 which is transmitted and detected. It is composed of a detection gas cell 4 containing a gas, a photodetector 6 for receiving the laser light transmitted through the detection gas cell 4, and a signal processing unit 30 for processing a signal from the photodetector 6. The LD module unit 1, the detection gas cell 4, and the photodetector 6 are connected by optical fibers 9b and 9d, respectively.

The signal processing unit 30 synchronizes the frequency ω of the sine wave signal from the oscillator 14 with the lock-in amplifier 18 which performs phase sensitive detection of the output of the photodetector 6, and the sine wave signal of the frequency doubler 15. A lock-in amplifier 19 that performs phase-sensitive detection of the output of the photodetector 6 in synchronization with the frequency 2ω, and an analyzing recorder 20 that records and arithmetically processes the output ratio (gas signal) of both lock-in amplifiers 18, 19. It is composed of an analysis processing device 31 which performs a neural net processing of data from the analyzing recorder 20.

Next, the operation of the gas measuring device shown in FIG. 7 will be described.

The DFB-LD 10 is fixed at a constant temperature, the bias current of the DFB-LD 10 is made into a triangular wave, and the DFB-LD 10 is swept in one direction. At this time, at the same time, the oscillator 14 superimposes a sinusoidal alternating current (modulation current) on the bias current. A laser beam is incident on the optical system 13 to be transmitted through the outward optical fiber 9b, spatially propagated in the gas inside the detection gas cell 4, and the laser beam is transmitted again through the backward optical fiber 9d. The laser light is received by and the detection signal is output.

Of the signals detected by the photodetector 6, a signal synchronized with the frequency ω of the sine wave signal from the oscillator 14 is detected by the lock-in amplifier 18, and the frequency 2ω of the sine wave signal of the frequency doubler 15 is detected. The synchronized signal is detected by the lock-in amplifier 19. The output ratio of the signals extracted by both the lock-in amplifiers 18 and 19 is transmitted to the analyzing recorder 20 and transmitted from the analyzing recorder 20 to the analysis processing device 31 as a gas signal. This analysis processing device 31
In, the noise component is removed from the gas signal on which the noise component is superimposed by the neural network built in the analysis processing device 31, and a pure gas signal is obtained.

The gas concentration can be obtained from the peak value of this gas signal, and the pressure can be obtained from the extreme value or half-value width appearing on both sides of the peak value. At this time, the noise component has been removed, so that it is possible to obtain an accurate pressure and an accurate pressure-corrected gas concentration.

Here, the neural network built in the analysis processing device 31 will be described.

First, a component (actually measured power spectrum) obtained by decomposing a gas signal of a specific component containing noise with respect to the central wavelength of laser light at a known gas concentration into a frequency domain using an FFT and a detection target to be measured. Gas cell 4
The correspondence relationship between the gas signal corresponding to the gas concentration obtained by the simulation calculation from the physical characteristic value of the gas in the inside and the basic parameter of the laser and the component (teacher power spectrum) decomposed into the frequency domain by using FFT is learned. The neural network is built in the analysis processing device 31.

Learning for this neural network is performed, for example, as in the three-layer neural network model shown in FIG.
First, input data, that is, actually measured power spectra P (0) to P (30) with respect to the frequency axis of the gas signal detected at a predetermined concentration, and the amount of information relating to them are given to each input unit of the input layer 32 as analog signals. . This data signal is transmitted to the intermediate layer 33, and finally the output layer 3
It is output from 4. A gas signal corresponding to a predetermined gas concentration obtained by simulation calculation from the output value (analog signal), the physical characteristic value of the gas to be measured (gas in the detection gas cell 4) and the basic parameter of the DFB-LD 10 is calculated. The components decomposed in the frequency domain using the FFT, that is, the teacher power spectra (desired output values) are compared, and the coupling strengths u and v are reduced so as to reduce the difference.
Is changed, and the neural network using u and w determined when the difference becomes minimum is the neural network after the learning is completed, and this is incorporated in the analysis processing device 31 of the signal processing unit 30. In addition, neural network learning,
It should be performed under various gas concentrations and various pressures that are actually detected. By making the input and output analog signals, it is possible to learn both the magnitude of the power spectrum (the magnitude of the gas signal) and the power spectrum pattern as the frequency component.

By doing so, when the power spectrum of the gas signal including interference noise or the like, or the amount of information related to them is input to the learning neural network, each frequency component that does not include a noise component is output. The power spectrum of is output. A pure gas signal waveform can be obtained by subjecting this to inverse FFT.

Moreover, this neural network is capable of learning a vast amount of correspondences, and changes the internal structure of the network for given inputs, and repeats this change until a correct output is output. Can complete learning.

Therefore, it becomes possible to additionally learn the power spectrum of the gas signal including a new noise component,
This improves the versatility of the neural network. In addition, additional learning can be done as often as
It can be learned each time and can be easily incorporated into the analysis processing device as a neural network.

As described above, according to the present embodiment, a component (measured power spectrum) obtained by decomposing a gas signal of a specific component containing noise with respect to the center wavelength of a laser at a known gas concentration in the frequency domain using FFT, Learn the correspondence relationship of the component (teacher power spectrum) that decomposes the gas signal corresponding to the gas concentration obtained by simulation calculation from the physical property value of the gas to be measured and the basic parameter of the laser into the frequency domain using FFT. By incorporating this neural network in an analysis processing device, for example, when it is desired to obtain an accurate pressure and gas concentration from a gas signal waveform containing an interference noise component, even if the noise is not periodic, the gas atmosphere pressure Even if changes, the power spectrum for each frequency component of the gas signal is input, Can be obtained a power spectrum that does not contain, it is possible to obtain a pure gas signal reduction of noise by inverse FFT of this.

Therefore, an accurate gas concentration can be obtained from the peak value of the gas signal, and an accurate pressure can be obtained from the extreme value or half width on both sides of the peak value.

Atmospheric gas pressure 1 atm, gas concentration 5
0, 100, 500, 1000ppm, modulation voltage 0.5
A component (measured power spectrum) obtained by decomposing the gas signal of a specific component containing noise with respect to the central light wavelength (bias current) of the laser at ˜3 V (modulation current 3.1 to 18.6 mA) into a frequency domain using FFT (measured power spectrum). , Neural learning the correspondence of the component (teacher power spectrum) that decomposes the gas signal corresponding to each gas concentration obtained by simulation calculation from the physical property value of methane gas and the basic parameter of the laser into the frequency domain using FFT. An example of a noise-reduced gas signal (gas concentration 50 ppm, modulation current 2 V (current 1
2.4 mA)) is shown in the figure below. Here, the number of inputs to the neural network is 31 frequency components.

In this embodiment, the number of FFT processes is 31.
The number of input neurons of the neural network is 31, but if the width of the wavelength region targeted for FFT, the number of FFT processing points, and the waveform of the gas signal due to changes in the atmospheric gas pressure change, the number of neurons will naturally change. change.

As a method of setting this value, the start point and the end point of the wavelength region to be measured are determined in advance, and the number is obtained from the power spectrum distribution by the FFT process at high concentration.

FIG. 9 shows the relationship of the gas signal with respect to the bias current before being input to the neural network, and FIG. 10 shows the power spectrum distribution obtained by converting FIG. 9 into the frequency axis. Further, FIG. 11 shows the relationship of the gas signal with respect to the bias current obtained by simulation, and FIG. 12 shows the power spectrum distribution obtained by converting FIG. 11 into the frequency axis. FIG. 13 shows the relationship of the gas signal with respect to the bias current after the neural network processing is applied to FIG. 9, and FIG. 14 shows the power spectrum distribution after the neural network processing. By performing the neural network processing from FIGS. 9 to 14, only the gas signal component was completely extracted, and the waveform and the magnitude of the peak value were substantially the same as the simulation results, and good results were obtained.

This gas concentration measuring device can diagnose deterioration of the OF cable, like the gas concentration measuring device shown in FIG. Further, the neural network built in the analysis processing device can be applied not only to reducing the noise component of the gas signal but also to other waveform processing and signal processing techniques.

As described above, according to the present embodiment, when it is desired to obtain an accurate pressure and gas concentration from a gas signal waveform containing an interference noise component, the gas atmosphere pressure changes even if the noise is not periodic. Even by inputting the power spectrum for each frequency component of the gas signal to the neural network after learning, a power spectrum without noise can be obtained, and the noise is reduced by inverse FFT. A pure gas signal can be obtained.

As a result, it is possible to obtain an accurate gas concentration from the peak value of the gas signal in which this noise is reduced, and an accurate pressure from the extreme values or half widths on both sides of the peak value.

FIG. 15 is a schematic view of still another embodiment of the gas concentration measuring apparatus to which the gas concentration measuring method of the present invention is applied. For simplification of explanation, the same members as those of the gas concentration measuring apparatus shown in FIG.

The difference from the embodiment shown in FIG. 1 is that the temperature of the laser when the absorption line of the detection gas is included and the laser temperature when the absorption line of the detection gas is not included when the drive current is changed. Stabilize each with respect to temperature, obtain the difference of each signal from the obtained detection signal and use this as a gas signal, measure the gas atmosphere pressure and gas concentration signal from this gas signal, and measure this gas concentration signal This is the point where the gas concentration is corrected and measured.

As shown in FIG. 15, this gas concentration measuring apparatus has an LD module section 40, a laser drive circuit 41 for driving the LD module section 40, and an LD module section 40.
Based on the signal from the detection gas cell 4 which transmits the laser light from the sensor and stores the gas for detection, the photodetector 6 which receives the laser light transmitted through the detection gas cell 4, and the signal from the photodetector 6. Measuring means 42 for measuring gas concentration
And a signal processing device 43 capable of difference between the two signals. The LD module unit 40, the detection gas cell 4 and the photodetector 6 are connected by optical fibers 9b and 9d, respectively.

Peltier element 11 of LD module section 40
For stabilizing the temperature of the DFB-LD 10 when the absorption line of the detection gas is included and the temperature of the DFB-LD 10 when the absorption line of the detection gas is not included when the drive current is changed. A temperature stabilizing power source 44 is connected.

The measuring means 42 is a lock-in amplifier 18,
19, a divider 45 for obtaining the output ratio of both lock-in amplifiers 18, 19, a low-pass filter 46 for removing the modulation frequency component, and a reference power supply 47 for generating a predetermined reference voltage.
And a subtractor 48 for obtaining the difference between the value of the output voltage of the low-pass filter 46 and the value of the reference voltage in order to obtain a change in the direct current component of the forward voltage of the DFB-LD 10, and the output from the subtractor 48 is amplified. It is composed of an amplifier 49 and an XY recorder 50 for recording the output of the amplifier 49 on the X axis and the output of the divider 45 on the Y axis.

Next, the operation of the gas concentration measuring device shown in FIG. 15 will be described.

The DFB-LD1 is controlled by the temperature stabilizing power source 44.
The temperature of 0 is stably fixed at a certain temperature Ta. Then DFB-
A sweep current that uniformly changes in one direction (increase or decrease) is supplied to the LD 10 as a bias current. To enable repeated measurement, a sawtooth current is applied to the DFB-LD 10 to change the center frequency of laser light. Ta is set so as to coincide with the absorption line at the central portion of the current that changes in a saw shape. An oscillator 14 superimposes a sine wave current having a modulation frequency ω on the bias current to modulate the frequency and intensity of the laser light. At this time, the center wavelength of the laser light is monitored by the output value of the amplifier 49 that amplifies the voltage of the reference power source 47 for bias and the output of the subtractor 48. That is, the oscillation frequency of each DFB-LD 10 and the amount of change in the forward resistance component have a reproducible relationship. Therefore, the output of the amplifier 49 is input to the X axis of the XY recorder 50 as a monitor of the center wavelength.

On the other hand, the laser light oscillated from the DFB-LD 10 passes through the optical system 13 and then the methane gas atmosphere in the detection gas cell 4 through the optical fiber 9b and the optical detection through the optical fiber 9d. It is guided to the vessel 6 and the intensity is detected there. The detection signal from the detection gas cell 4 is phase-sensitive detected by the lock-in amplifiers 18 and 19, and the fundamental wave signal Ps (ω) and the second-harmonic detection signal Ps are detected.
(2ω) is obtained. Both signals Ps (ω), Ps (2ω)
Is input to the divider 45, and Ps (2ω) / Ps (ω)
Is input to the Y-axis of the XY recorder 50.

The gas concentration measuring apparatus shown in FIG. 15 can obtain the same waveform as the conventional one for high concentration gas. The gas concentration signal is obtained from the peak value near the gas absorption line. The pressure is obtained from the width of the extreme value that appears on both sides of the peak value. In the measuring means 42, the pressure can be obtained by previously storing the relationship between the voltage difference between these two points and the pressure in a memory or the like (not shown), and the gas concentration signal can be corrected for the pressure.

On the other hand, the output signal waveform similar to the conventional one can be obtained for the low concentration gas. However, as it is, the peak value and the extreme value cannot be obtained from the output signal.

Therefore, next, the same bias current as that described above is used, the set temperature is set to Tb, and the central wavelength of the laser light is similarly changed. However, at this set temperature Tb,
It is assumed that there is no portion where the center wavelength when the drive current is changed matches the absorption line of gas. 16 and 17 are gas signals when measured at temperatures Ta and Tb, respectively.
FIG. 16 includes two noise waveforms and a gas signal. FIG. 17 contains only the noise waveform. Therefore, when the difference between the two signals is performed, noise is removed and a waveform of only the gas signal as shown in FIG. 18 is obtained.

As described above, according to the present embodiment, the temperature of the laser when the absorption line of the detection gas is included and the DFB when the absorption line of the detection gas is not included when the drive current is changed.
-The temperature of the LD is stabilized and the difference between the respective signals is obtained from the obtained detection signal, and this is used as a gas signal. From this gas signal, the pressure of the gas atmosphere and the gas concentration signal are measured, and this gas concentration is measured. Since the signal is pressure-corrected to measure the gas concentration, the accurate gas concentration can be measured even if noise that deteriorates the gas signal is superimposed.
Further, the SN ratio in the measurement of low concentration gas is improved.

By the way, in the above-mentioned gas concentration measuring apparatus, in order to improve the measurement sensitivity, it is necessary to lengthen the optical path length portion of the detection gas cell 4 which is a portion where the light is absorbed by the gas. In the detection gas cell 4 adopting such a detection method, the shape of the detection gas cell 4 becomes large when the optical path length is increased. Where the space for installation of the detection gas cell is restricted, a small detection gas cell will be provided, making it impossible to construct a system with high detection sensitivity.

Therefore, a gas concentration measuring device having high sensitivity without increasing the size of the detection gas cell will be described below.

FIG. 19 is a schematic view of another embodiment of the gas concentration measuring device of the present invention.

The difference from the embodiment shown in FIG. 1 is that a reflection mechanism is incorporated in the gas cell so that the light passing through the gas cell travels back and forth along the same optical path.

As shown in the figure, the gas concentration measuring device is
A light source 60 including a semiconductor laser, a temperature control circuit, and a drive circuit, an isolator 61 that transmits laser light from the light source 60 and blocks return light, and an isolator 6.
An optical fiber coupler 62 for branching the laser light from 1;
An optical fiber 63 that transmits the light emitted from the optical fiber coupler 62, a collimator lens 64 that is connected to the emission end of the optical fiber 63 and that collimates the laser light, a gas to be measured, and a collimator lens 64. A detection gas cell 65 through which the emitted laser light is transmitted, a reflecting mirror 66 provided in the detection gas cell 65, which reflects the laser light from the optical fiber 63 and returns it to the optical fiber 63.
A photodetector 67 which is connected to the emission side of the optical fiber coupler 62 and receives the laser light transmitted through the detection gas cell 65.
And a control unit 68 that controls the light source 60 and processes the signal from the light detection unit 67.

The semiconductor laser of the light source 60 is D shown in FIG.
Like the FB-LD 10, it oscillates a laser beam having a wavelength and intensity according to a drive current and temperature, and the drive circuit changes the drive current of the semiconductor laser to control the wavelength and intensity of the laser beam. is there.

Here, the restrictions of the gas concentration measuring apparatus of this embodiment will be described.

First, as the light source, a laser that oscillates a laser beam having a wavelength according to the driving current and the temperature is used. It is assumed that one of the following two methods is used as the detection method using this light source.

(1) Method 1 The detection gas is irradiated with laser light having a certain wavelength and light intensity, and the wavelength is gradually swept with respect to the absorption line of the gas. Therefore, the temperature of the semiconductor laser is fixed and the driving current of the semiconductor laser is gradually changed, or the driving current of the semiconductor laser is fixed and the temperature of the semiconductor laser is gradually changed and the intensity of light transmitted through the detection gas is increased. How to monitor changes.

(2) Method 2 A laser beam is modulated at a certain frequency, the modulated laser beam is transmitted through a gas atmosphere, and the fundamental wave component and the second harmonic component of the transmitted light are phase-detected to detect the gas. Is something
By superimposing a modulation current on the driving current of the semiconductor laser and fixing the temperature of the semiconductor laser in order to sweep the central wavelength of the modulated laser light onto the gas absorption line in the same manner as in Method 1, bias the driving current of the semiconductor laser. A method of gradually changing the value or fixing the drive current of the semiconductor laser and gradually changing the temperature of the semiconductor laser. This method 2 can detect gas with higher sensitivity than the method 1.

Next, the operation of this gas concentration measuring device will be described.

Method 2 is used as the detection method.

The driving current of the semiconductor laser is fixed, the temperature of the semiconductor laser is changed, and the wavelength of the oscillating laser light is swept onto the gas absorption line. The laser light controlled in this way passes through the isolator, then the optical fiber coupler, and proceeds to the detection gas cell.

The laser light that has proceeded to the detection gas cell 65 is
Since one round trip is made in the detection gas cell 65 having the optical path length L, the spatial optical path length of 2L is eventually propagated.

The laser light reflected by the reflecting mirror 66 and incident on the optical fiber 63 is branched by the optical fiber coupler 62,
One of the branched laser beams proceeds to the photodetector 67. The other laser light branched by the optical fiber coupler 62 is the light source 6
Although it returns to the direction of 0, it is cut off by the isolator 61 and does not reach the semiconductor laser.

The semiconductor laser used in this embodiment has a large noise with respect to the reflected light, but the isolator 61 can reduce the noise.

Next, a method of calculating the gas concentration from the obtained signal will be described.

FIG. 20 is a diagram showing the results of monitoring the intensity of the laser light transmitted through the gas atmosphere by changing the temperature of the semiconductor laser used for the light source shown in FIG. 19, where the horizontal axis represents temperature and the vertical axis represents temperature. The axis shows the light output. The obtained waveform is sloping down to the right because the light intensity decreases as the temperature of the semiconductor laser rises. If the drive current of the semiconductor laser is constant, the light output and the temperature may be considered to be substantially linear. The baseline obtained when it is assumed that the gas does not depend on the detection gas cell 65 can be obtained by drawing a tangent line where the absorption line of the actually obtained waveform does not match.

Next, the transmittance T is obtained from the obtained waveform. The incident light intensity is P ₁ , and the intensity of the light transmitted through the gas is P ₂
Then, from the Lambert Beer's law for the light absorption of gas, the relation expressed by the equation 9 is obtained.

[0128]

[Equation 9] T = P ₂ / P ₁ = exp (-α · c · L) where T is the transmittance, α is the absorption coefficient of gas, c is the concentration, and L is L.
Is the optical path length.

In the data obtained in the experiment, P ₁ is obtained from the baseline intensity and P ₂ is obtained from the transmitted light intensity.
Therefore, the transmittance at each temperature can be obtained.

Since the temperature and the optical frequency have a constant relationship when the driving current of the semiconductor laser is constant, the horizontal axis shown in FIG. 20 can be converted into the optical frequency. The current of the semiconductor laser was fixed in advance, and the relationship between the temperature of the semiconductor laser and the optical frequency was obtained and then converted. The result is shown in FIG.

FIG. 21 is a diagram showing the relationship between the optical frequency and the transmittance, wherein the horizontal axis represents the optical frequency and the vertical axis represents the transmittance.

The gas concentration in the detection gas cell 65 of unknown concentration can be measured by previously investigating the gas absorption coefficient and the like from the value at the peak shown in FIG.

As described above, according to this embodiment, the detection gas cell is provided with the reflection mechanism so that the light passing through the detection gas cell travels back and forth along the same optical path, so that the optical path length is doubled. The sensitivity is improved.

FIG. 22 shows another embodiment of the embodiment shown in FIG.

The shape of the absorption line of the gas is the detection gas cell 65.
It changes under the influence of the pressure of the gas atmosphere inside. Also, if the semiconductor laser of the light source 60 used is used for a long period of time, the characteristics deteriorate and the position of the absorption line also changes. Therefore, if the optical wavelength monitor unit 69 for monitoring the optical wavelength of the laser light oscillated by the semiconductor laser is incorporated in the present apparatus, it is possible to correct them. The light wavelength monitor unit 69 is composed of a gas cell containing a detection gas and a light receiver that receives light after the light from one side of the light transmission path has propagated for a certain optical path length, and the pressure inside the gas cell is kept constant. It is supposed to be. Signal processing is performed by a method similar to the processing method in the photodetection unit 67 described above, and the two waveforms of the signal of the optical wavelength monitor unit 69 and the signal obtained in the gas cell are compared to obtain the signal of the gas cell. The pressure is corrected by obtaining the size of the absorption line and the line width of the absorption line which are several times as large as those of the wavelength monitor. The change with time of the laser can be estimated from the change in the position of the absorption line obtained by the optical wavelength monitor 69.

FIG. 23 is a diagram showing another embodiment of the embodiment shown in FIG.

The laser light emitted from one end of the optical fiber 63 is collimated by the collimator lens 64, propagated in space, and then received by the other optical fiber 70 which is opposed, and reflected on the opposite end face of the received optical fiber 70. The laser light may be emitted from the fiber end face 70a with a reflecting surface instead of the reflecting mirror.

In general, the light emitted from the optical fiber has a beam diameter that gradually expands when the spatial optical path length is long. This expansion phenomenon is small up to a certain distance, but as the distance increases, Fraunhofer diffraction occurs and the coupling efficiency becomes low.

In the reflection by the reflecting mirror, the spatial optical path length is considered to be 2L. On the other hand, in the reflection by the optical fiber, the spatial optical path length is regarded as L + L. Therefore, if 2L is in the Fraun-Hofer region but L is not in the Fraun-Hofer region, the loss can be reduced by this type of optical coupling system.

[0140]

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

Even if a signal that deteriorates the gas signal is superposed or the laser deteriorates due to the life, the gas concentration can be accurately measured.

[Brief description of drawings]

FIG. 1 is a schematic view of an embodiment of a gas concentration measuring device to which the gas concentration measuring method of the present invention is applied.

FIG. 2 is a diagram showing transmittance T with respect to frequency, and its first derivative T ₀₁ and second derivative T ₀₂ .

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

4 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. 5: Gas signal of methane gas of 50 ppm and 0 ppm
It is a figure which shows the gas signal of the methane gas of FIG.

6 is a gas signal of 50 ppm obtained by subjecting the gas signals of both methane gases shown in FIG. 5 to differential processing.

FIG. 7 is a schematic view of another embodiment of the gas concentration measuring device to which the gas concentration measuring method of the present invention is applied.

8 is a diagram showing a three-layer neural network model built in the analysis processing device shown in FIG.

FIG. 9 is a diagram showing a relationship of a gas signal with respect to a bias current before being input to a neural network.

FIG. 10 is a diagram showing a power spectrum distribution obtained by converting FIG. 9 into a frequency axis.

FIG. 11 is a diagram showing a relationship of a gas signal with respect to a bias current obtained by simulation.

FIG. 12 is a diagram showing a power spectrum distribution obtained by converting FIG. 11 into a frequency axis.

FIG. 13 is a diagram showing a relationship of a gas signal with respect to a bias current after performing a neural net process on FIG. 9.

FIG. 14 is a diagram showing a power spectrum distribution after being subjected to neural network processing.

FIG. 15 is a schematic view of still another embodiment of the gas concentration measuring device to which the gas concentration measuring method of the present invention is applied.

FIG. 16 is a diagram showing a gas signal when measured at a temperature Ta.

FIG. 17 is a diagram showing a gas signal when measured at a temperature Ta.

FIG. 18 is a diagram showing a waveform of a gas signal from which noise is removed.

FIG. 19 is a schematic view of another embodiment of the gas concentration measuring device of the present invention.

20 is a diagram showing the results of monitoring the intensity of laser light transmitted through a gas atmosphere by changing the temperature of the semiconductor laser used in the light source shown in FIG.

FIG. 21 is a diagram showing a relationship between optical frequency and transmittance.

22 is a diagram showing another embodiment of the embodiment shown in FIG.

23 is a diagram showing another embodiment of the embodiment shown in FIG.

FIG. 24 is an explanatory diagram for explaining interference noise.

FIG. 25 is an explanatory diagram for explaining interference noise.

FIG. 26 is a diagram showing the relationship between the wavelength of laser light and the ratio (gas signal) of the second-harmonic phase-sensitive detection signal obtained by phase-sensitive detection and the fundamental wave phase-sensitive detection signal.

FIG. 27 is a diagram showing a waveform in which periodic and aperiodic noises are superimposed in addition to the gas signal to be measured.

FIG. 28 is a diagram showing a relationship between a laser wavelength and a gas signal in a low concentration gas before and after performing a conventional FFT process.

FIG. 29 is a diagram showing a relationship between a laser wavelength and a gas signal in a low-concentration gas before and after performing a conventional FFT process on the low atmospheric pressure side.

[Explanation of symbols]

4 Gas cell for detection 5 Reference gas cell 10 Laser (DFB-LD, semiconductor laser)

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akiyuki Nakamura 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Prefecture, Nitro Ritsudensen Co., Ltd., Optro System Research Center (72) Inventor Masahiko Uchida, Hitachi City, Ibaraki Prefecture 5-1-1 Takamachi, Opto System Research Laboratories, Nititsu Electric Cable Co., Ltd. (56) Reference JP-A-63-9842 (JP, A) JP-A-62-218843 (JP, A) JP-A-6 -300685 (JP, A) JP 5-256768 (JP, A) JP 5-250715 (JP, A) JP 4-326041 (JP, A) (58) Fields investigated (Int.Cl) . ^7, DB name) G01N 21/00 - 21/01 G01N 21/17 - 21/61 G01M 11/00 - 11/02 PATOLIS

Claims (3)

(57) [Claims] 1. A laser whose wavelength and intensity are modulated by using a laser that oscillates a laser beam having a wavelength and intensity according to the drive current and temperature, and modulating the drive current with a frequency ω centered around a predetermined current value. The light is oscillated, the central wavelength of the laser light is swept, and the intensity of the transmitted light obtained by passing the laser light through the gas atmosphere to be measured is detected,
In the gas concentration measuring method for obtaining the gas concentration from the gas signal obtained by phase-sensitive detection of the specific component in the detection signal, the gas to be detected is housed in the gas cell for detection, and the gas for detection is stored in the gas cell for reference . Gas to be detected
Noise that does not absorb laser light with a gas other than
A reference gas for removal is housed, and the laser light that has passed through both gas cells is detected by two photodetectors, and the detection gas signal (gas signal component + noise component) is detected.
Phase-sensitive detection of both gas signals of reference and reference gas signal (only noise component) with frequency ω and phase-sensitive detection of both gas signal of detection gas signal and reference gas signal with frequency 2ω Frequency of the gas signal for detection after phase sensitive detection after phase sensitive detection by the lock-in amplifier of
The output ratio between the numbers ω and 2ω is calculated, and the reference
Output ratio ω and 2ω of the output signal,
A gas concentration measuring method, characterized in that a gas atmosphere pressure and a gas concentration are calculated from a gas signal obtained by subtracting the ratio in a difference processing unit, and the gas concentration is measured under an arbitrary pressure. 2. The gas concentration and the atmospheric gas pressure are corrected by comparing the detection signal obtained at the initial stage of use of the laser with the detection signal of the current laser, and the life of the laser is expired. The gas concentration detecting method according to claim 1, wherein 3. A laser that oscillates a laser beam having a wavelength and intensity according to a driving current and temperature, an oscillator that modulates the laser with a frequency ω, and a frequency doubler that doubles the frequency ω of the oscillator. A measuring gas cell that contains a specific gas to be measured and that keeps the temperature of the gas constant, and a detector that detects the intensity of transmitted light obtained by passing the laser light through the measuring gas cell, In a gas concentration measuring device having a measuring means for measuring the concentration of the specific gas from a detection signal obtained by phase-sensitively detecting a specific component in a signal from a detector, an optical coupler for branching the laser beam, and a branch One of the separated laser light is transmitted and the detection gas cell that contains the gas to be detected and the other branched laser light is transmitted and the gas in the detection gas cell
Do not absorb laser light with a gas other than the gas to be detected.
A reference gas cell that contains a reference gas for removing noise, a photodetector that receives the laser light that has passed through the detection gas cell, and a photodetector that receives the laser light that has passed through the reference gas cell. , Gas for detection, synchronized with frequency ω
Signal (gas signal component + noise component) and reference gas signal
A lock-in amplifier for the phase-sensitive detection of both gas signal (noise component only), synchronized with the frequency 2 [omega, detected gas signals
Signal is extracted by both lock-in amplifiers and other lock-in amplifiers that perform phase sensitive detection of both the signal and the reference gas signal.
Of the detection gas signal frequencies ω and 2ω after phase sensitive detection
Output ratio and frequency ω of the reference gas signal after phase sensitive detection
An analyzing recorder that records and calculates the output ratio of 2ω and both outputs obtained by the analyzing recorder.
A gas concentration measuring device, comprising: a differential signal processing unit that performs differential processing of ratio data.