JP2001066250A - Gas detection apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas detection apparatus capable of keeping zero point stability and detecting a gas leak with high accuracy regardless of the presence of various noises such as partial concealing on the way of detection or the like. SOLUTION: An output signal is inputted to a modulation signal adjusting circuit by a signal generating circuit 24, and a modulation signal is outputted to respective laser diodes 16A, 16B so that the laser diodes 16A, 16B become an inverse phase to output modulation lights from the laser diodes. Measuring light is formed from the modulation lights outputted from the laser diodes by a branching filter 18 and three wave synthesizers 20A, 20B, 20C, outputted to the target gas atmosphere of a probe part 14, and received by a light receiver 36. Only a modulation component is extracted by a synchronizing detection circuit 26 to detect gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detection device, and more particularly to a gas detection device for detecting a gas leak or the like using the light absorption characteristics of a target gas.

[0002]

2. Description of the Related Art It is known that a gas has an absorption band for absorbing light of a specific wavelength, and a gas detection device utilizing this has been proposed.

[0003] For example, a method using a differential absorption method as disclosed in Japanese Patent Application Laid-Open No. 5-264446 is known. This modulates the drive current of a single laser diode, alternately emits two types of laser light of a wavelength that is absorbed by the target gas and a wavelength that is not absorbed, splits the light beam with a beam splitter on the way, and splits one. The light is incident on the light receiving element via the target gas, and the other is directly incident on another light receiving element. Then, the output signal of each light receiving element is input to a logarithmic ratio amplifier, the output is converted into a rectangular wave train, and this is passed through a band-pass filter to extract a modulation fundamental wave component, thereby obtaining a gas concentration signal. I have.

However, in the gas detector disclosed in Japanese Patent Application Laid-Open No. 5-264446, the problem that the light output greatly changes as the wavelength of the laser light changes is solved by using a logarithmic ratio amplifier. But
The logarithmic ratio amplifier generally has a large temperature dependency, and is difficult in accuracy and reliability. Further, the gas detection device disclosed in Japanese Patent Application Laid-Open No. 5-264446 is liable to generate noise because it uses two light receivers.

In order to solve the problem of the gas detection device disclosed in Japanese Patent Application Laid-Open No. 5-264446, a gas detection device described in Japanese Patent Application Laid-Open No. 11-142327 has been proposed.

A gas detector disclosed in Japanese Patent Application Laid-Open No. 11-142327 discloses a laser beam having a wavelength that is absorbed by a target gas and a laser beam that has a wavelength that is not absorbed by the target gas. Are combined as modulated light, and used as measurement light. Then, the measurement light passes through the atmosphere of the target gas, is received by a light receiver, and the gas concentration is calculated by an arithmetic circuit from the extracted modulated component.

[0007]

However, in the gas detection device disclosed in Japanese Patent Application Laid-Open No. H11-142327, when there is no gas in the detection range of the target gas, even if the measurement light is partially hidden, there is no output fluctuation. However, if the measurement light is partially concealed using the knife edge method (a method in which the measurement light is shielded from one direction by a knife edge and the output change of the measurement light is observed), the output light Variations occur (FIG. 5). This is because the measurement light itself generated by the modulated light having the opposite phases of the two laser lights has a spatial non-uniformity (that the light intensity cannot be spatially identical). The existence of spatial non-uniformity of the measurement light is based on the difference between the wavelengths of the two lights, absorption and non-absorption. Light emitted from the laser light emitting end or the optical fiber emitting end is diffracted and spreads. If the angle is θ, θ = λ / π
Defined by ω. Here, λ is a wavelength and ω is an outgoing beam diameter, which is a predetermined constant. Accordingly, since the diffraction angle is different due to the different wavelength, a difference occurs in the beam diameter when collimation or the like for measurement is performed.

From the detection principle of the gas detector disclosed in Japanese Patent Application Laid-Open No. H11-142327, when there is no gas in the target gas atmosphere and when the measurement light is partially concealed at that time, there should be no fluctuation in output. Although accuracy measurement is possible,
Actually, when the measurement light is partially concealed, the output light fluctuates for the reason described above.

When the diameters of the two modulated lights are different (FIG. 9)
FIG. 9B shows the variation of the output shown by the solid line and the dotted line in FIG. 9B, which is obtained by using the knife edge method capable of simulating and verifying the partial concealment of the measurement range in the absence of gas. It is shown by the calculation result that the measurement result matches the measurement result.

Therefore, in the gas detector disclosed in Japanese Patent Application Laid-Open No. 11-142327, since the laser beams having two different wavelengths are used, the above problem cannot be avoided in principle. The output fluctuation was clarified by this partial concealment, and the effect of spatial non-uniformity on measurement accuracy was ±
The gas detection device disclosed in Japanese Patent Application Laid-Open No. H11-142327 can be applied to gas leak detection in which the gas leakage is about 1 to 2% and can be ignored. Is a serious problem.

The present invention has been made in order to solve the above-mentioned problem. It is possible to maintain zero-point stability and to achieve ultra-high-precision gas leakage regardless of the presence or absence of various noises such as partial obscuration during detection. It is an object of the present invention to provide a gas detection device capable of performing detection.

[0012]

According to a first aspect of the present invention, there is provided a first laser light generating means for generating a laser light having a wavelength which is absorbed by a target gas; A second laser light generating means for generating a laser light having a wavelength smaller than the wavelength output from the laser light generating means and having a wavelength not absorbed by the target gas; and an output from the first laser light generating means. At a wavelength greater than the wavelength,
The third laser light generating means for generating a laser light having a wavelength not absorbed by the target gas, and the respective laser lights output from the first laser light generating means and the second laser light generating means have the same amplitude. In addition, the laser beams are modulated so that the phases are opposite to each other, and the laser beams output from the first laser light generating means and the third laser light generating means are modulated so as to have the same amplitude and the opposite phases. A modulating means, a measuring light obtained by synthesizing each modulated light modulated by the modulating means as a measuring light, and a measuring light output means for outputting the measuring light into a target gas atmosphere; and It is characterized by comprising a light receiving means for receiving light, and a modulation component extracting means for extracting a modulation component in an output signal of the light receiving means.

According to the first aspect of the invention, when the concentration of the target atmosphere gas is zero, none of the modulation components constituting the measurement light is attenuated, and the modulation components of these modulation lights cancel each other. In addition, the modulation component of the measurement light is reduced to such an extent that no problem occurs. On the other hand, when the concentration of the target atmosphere gas increases, the modulated light constituting the measurement light having a wavelength that is absorbed by the target gas is attenuated according to the gas concentration.
As a result, the magnitude of the modulation component of each modulated light is increased, and as a result, the modulated component of the measurement light increases in accordance with the gas concentration. Therefore, if this is notified when the modulation component of the measurement light exceeds a predetermined value, it can be used as a gas leak detector. Further, if the gas concentration is calculated by substituting the modulation component of the measurement light into a predetermined mathematical formula, it can be used as a gas concentration measuring device.

The first laser light having a wavelength absorbed by the target gas, the second laser light having a wavelength smaller than the first laser light and not being absorbed by the target gas, and a wavelength larger than the first laser light. As described above, the third laser light, which is not absorbed by the target gas, is modulated by the modulating means so that the first laser light and the second laser light have the same amplitude and opposite phases to each other. The laser light and the third laser light are modulated so as to have the same amplitude and opposite phases. Then, by synthesizing and generating the measurement light, it is possible to cancel the output fluctuation caused by partially concealing the measurement light. Therefore, there is no spatial non-uniformity unlike the conventional gas detection device.

In other words, since a logarithmic ratio amplifier is not required and a single photodetector can be provided, high accuracy gas detection can be achieved without causing a change in characteristics due to a change in temperature or a decrease in accuracy due to mixing of noise. It is possible. Further, since only the modulation component of the measurement light is extracted, there is no problem even if there is a certain difference in the intensity of the laser light at the time of non-modulation generated by each laser light generating means.

Accordingly, it is possible to provide a gas detection device which can maintain zero-point stability and perform ultra-high-precision gas leak detection regardless of the presence or absence of various noises such as partial concealment during detection. it can.

According to a second aspect of the present invention, in the first aspect of the present invention, the measuring light output means includes a dividing means for dividing the light output from the first laser light generating means into two parts, A first combining means for combining the light output from the second laser light and one of the two lights divided by the splitting means; and a light output from the first laser light generating means and the splitting means. A second combining unit that combines the other two lights, a third combining unit that further combines the light combined by the first combining unit and the second combining unit into one light, It is characterized by consisting of.

According to the invention of claim 2, according to claim 1,
In the invention described in the above, the measuring light output means is a dividing means for dividing the light output from the first laser light generating means into two,
The light output from the second laser light and the splitting means
First combining means for combining one of the divided lights, second combining means for combining the light output from the first laser light generating means and the other light divided by the dividing means, 1
And the third combining means for further combining the light combined by the second combining means into one measurement light.

According to a third aspect of the present invention, in the first aspect of the present invention, the measurement light output means outputs the light output from the second laser light generation means and the third laser light generation means. And a second combining unit that combines the light combined by the first combining unit and the light output from the first laser light generating unit. And

According to the invention of claim 3, according to claim 1,
In the invention described in (1), the measuring light output means is combined by the first combining means for combining the lights output by the second laser light generating means and the third laser light generating means, and It is possible to comprise a second synthesizing means for synthesizing the emitted light and the light output by the first laser light generating means.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] FIG. The gas detection device 10 includes a controller unit 12 and a probe unit 14. The controller unit 12 includes three laser diodes (LDs) 16A and 16A.
B, 16C, splitter 18, three multiplexers 20A, 20
B, 20C, modulation signal adjustment circuit 22, signal generation circuit 2
4, a synchronous detection circuit 26, an arithmetic circuit 28 and a display 30 are provided. The probe section 14 is provided with a collimator lens 32, a condenser lens 34, and a light receiver 36. The three laser diodes 16A, 16A
B and 16C are the first, second and third laser light generating means of the present invention, respectively.
0B and 20C are modulation signal adjustment circuits 2 for measuring light output means.
Reference numeral 2 denotes a modulating unit, the light receiver 36 corresponds to a light receiving unit, and the synchronous detection circuit 26 corresponds to a modulation component extracting unit. Further, the demultiplexer 18 corresponds to a dividing unit, and the demultiplexers 20A, 20B,
20C corresponds to first, second and third combining means, respectively.

The laser diode 16A oscillates a laser beam having a wavelength λ1 absorbed by the target gas, the laser diode 16B oscillates a laser beam having a wavelength λ2 smaller than the wavelength oscillated by the laser diode 16A. Oscillates a laser beam having a wavelength λ3 larger than the wavelength oscillated by the laser diode 16A. It is desired that the laser beam oscillated by the laser diode 16A has a spectrum width narrower than the absorption line width of the target gas. Further, it is desirable that the laser light emitted from the laser diodes 16B and 16C has a wavelength that is not absorbed by the gas when the gas exists in the gas atmosphere. In the present embodiment, the wavelength λ1 of each laser beam,
λ2 and λ3 use the 1.65 μm band, and these wavelengths λ1, λ2 and λ3 are preferably as close as possible to make optical characteristics such as refractive index similar.

The signal generation circuit 24 outputs a sine wave or a rectangular wave of 5 KHz to the modulation signal adjustment circuit 22, and the modulation signal adjustment circuit 22 outputs the laser diode 1 at a predetermined modulation ratio.
6A and the laser diode 16B, and the laser diode 16A and the laser diode 16C are adjusted so as to be modulation signals of opposite phases, respectively.
22b and 22c are connected to the laser diodes 16A, 16B, 1
Output to 6C.

The splitter 18, the multiplexers 20A, 20B, 20
C comprises directional couplers respectively, and the duplexer 18
The laser light La output from the laser diode La is divided into two parts, a modulated light La1 and a modulated light La2. Multiplexer 20A
Multiplexes the modulated light La1 and the modulated light Lb output from the laser diode 16B to generate a measuring light Ls1, and the multiplexer 20B multiplexes the modulated light La2 and the modulated light Lc output from the laser diode 16C. To generate the measurement light Ls2, and the multiplexer 20C combines the measurement light Ls1 and the measurement light Ls2 to generate the final measurement light Ls. In the intensity modulation ratio for each of the laser diodes 16A, 16B and 16C, the laser diode 16A adjusts the intensity modulation ratio for the modulated light La1 and La2 obtained by dividing the modulated light La into two by the splitter 18. The multiplexer 20A and the multiplexer 20B
Of modulated light La1, Lb and modulated light La2, Lc at
Are defined as Ma1, Mb, Ma2, and Mc, and the sensitivity of the modulated light La, Lb, and Lc in the light receiver 36 is represented by γ.
a, γb, and γc are set so that the modulated light La1 and the modulated light Lb become Mbγb: Ma1γa, and the modulated light L
The measurement light beams Ls1 and Ls in the multiplexer 20C are set such that Mcγc: Ma2γa for a2 and the modulated light Lc.
2 is multiplexed. Laser diodes 16A, 16B, 16
Modulated light La (La1, La2), L output from C
2 (A) and 2 (B) show the changes over time in the light intensity of b and Lc.
As shown in (C), the predetermined intensities I1 and I
The modulation is performed at approximately 5 kHz centering on I2 and I3 with substantially the same amplitude and opposite phases. Further, the demultiplexer 18 and the multiplexer 20
As A, 20B, and 20C, for example, a half mirror or the like can be used instead of using a directional coupler.

The synchronous detection circuit 26 is configured to receive the 5 KHz signal output from the signal generation circuit 24 and to extract only the modulation component IM from the output of the photodetector 36. , Extracted modulation component I
The gas concentration α is calculated from M by the following equation (1), and the display 30 is configured to display the result calculated by the arithmetic circuit 28. α = −1 / KL·log (1−IM / I0) (1) = − 1 / KL·log (1− (Ib + Ic−Ia) / (Ib + Ic)) = − 1 / KL·log (Ia) / (Ib + Ic)) In the above equation (1), I0 (= Ib + Ic) is the measurement light Ls after passing the target gas when the modulated light La is cut.
And Ia, Ib, and Ic are the modulated light La and the modulated light La in the measurement light Ls after passing through the target gas, respectively.
These are the modulation component intensities of Lb and Lc. K is the absorption coefficient of the target gas, and L is the optical path length in the target gas atmosphere.

The controller section 12 and the probe section 14 are connected by an optical fiber, and the collimator lens 32 of the probe section 14 converts the measurement light Ls output from the controller section 12 into parallel light and outputs the parallel light into the target gas atmosphere. .

The condenser lens 34 is configured to condense the measurement light passing through the target gas atmosphere to the light receiver 36. The light receiver 36 uses a photodiode or a photomultiplier to measure the measurement light. It is configured to receive Ls.

Next, the operation of the gas detector 10 configured as described above will be described.

5 KHz output from signal generation circuit 24
The sine wave or rectangular wave output signal is input to the modulation signal adjustment circuit 22, and the modulation signal adjustment circuit 22 sets the modulation ratio to Mbγb: M for the modulated light La1 and the modulated light Lb.
a1γa, and the modulated light La2 and the modulated light Lc
So that Mcγc: Ma2γa, so that the laser diode 16A and the laser diode 16B have opposite phases, and the laser diode 16A and the laser diode 16C have opposite phases.
The modulation signals 22a, 22b, 22D are set so that the laser diodes 16B and 16C have the same phase.
c is output to the laser diodes 16A, 16B, and 16C, whereby the laser diodes 16A, 16B, and 16C are output.
C outputs modulated light La, Lb, and Lc.

The modulated light La output from each laser diode 16A is input to the splitter 18, and the modulated light La is split into two, the modulated light La1 and the modulated light La2. One modulated light La1 of the divided modulated light La and the laser diode 1
The modulated light Lb output from 6B is input to the multiplexer 20A, where it is multiplexed to generate the first measurement light Ls1. Also, the other modulated light La2 of the divided modulated light La
And modulated light Lc output from laser diode 16C
Are input to the multiplexer 20B, where they are multiplexed to generate the second measurement light Ls2.

Then, the first measurement light Ls1 and the second measurement light Ls2 are input to the multiplexer 20C, where they are multiplexed to finally generate the measurement light Ls. Here, the measuring light L
FIG. 2D shows the change over time of the light intensity of s, and the modulated components cancel each other out to have a constant intensity with almost no pulsation.

The measurement light Ls is introduced into the probe section 14 by the optical fiber 38, converted into parallel light by the collimator lens 32, and sent out into the target gas atmosphere. The measurement light Ls that has passed through the target gas atmosphere is input to the light receiver 36 by the condenser lens 34.

If the concentration of the target gas is zero, the measurement light component of wavelength λ1 of the measurement light Ls cannot be attenuated at all if the concentration of the target gas is zero. Is input to the light receiver 36, and the output of the light receiver 36 becomes constant as shown in FIG. On the other hand, when the concentration of the target gas is other than zero, the measurement light component of the wavelength λ1 is attenuated by absorption according to the gas concentration (FIG. 2).
(See the broken line in (A).) Therefore, a modulation component corresponding to the difference between the measurement light component of the wavelength λ1 and the measurement light components of the wavelengths λ2 and λ3 appears in the output of the light receiver 36 (FIG. 4).
(A)).

The output of the photodetector 36 is input to the synchronous detection circuit 26 of the controller 12 and is output to the synchronous detection circuit 2.
In step 6, the signal output from the signal generation circuit 24 is extracted from the modulation signal input from the light receiver 36 to extract only the modulation component IM from the signal output from the light receiver 36 (FIG. 4B). . This modulation component IM becomes zero when the target gas concentration is zero (FIG. 3B). Further, the extracted modulation component IM is output to the arithmetic circuit 28, the gas concentration α is calculated by the arithmetic circuit 28, and the calculation result is displayed on the display 30.

Next, a description will be given of the result of verifying the output fluctuation using the knife edge method capable of simulating and verifying the partial concealment of the measurement range.

In the gas detection device described in Japanese Patent Application Laid-Open No. 11-142327, when the measurement light is partially concealed by using the knife edge method, the output light fluctuates as shown in FIG. This is due to spatial non-uniformity, as described above, and the diffraction angle differs due to the different wavelength. Therefore, a difference occurs in the diameter of the collimated beam for measurement.

Therefore, in the present embodiment, the measurement light Ls1 is generated from the modulated light La1 output from the laser diode 16A and the modulated light Lb having a wavelength smaller than the modulated light La1 output from the laser diode 16B. Also,
The measurement light Ls2 is generated from the modulated light La2 output from the laser diode 16A and the modulated light Lc having a wavelength larger than the modulated light La2 output from the laser diode 16C. Then, the measurement light Ls1 and the measurement light Ls2 are combined to generate the measurement light Ls.

As described above, by combining the two measurement lights Ls1 and Ls2 to generate the measurement light Ls, the output fluctuation using the knife edge method can be changed by the measurement light Ls1 and the measurement light Ls
2, the conventional output fluctuation is compensated and canceled as shown in FIG. Therefore, there is no output fluctuation. Note that the dotted line in FIG. 6 indicates the output fluctuation by the conventional gas detection device or the output fluctuation of the measurement light Ls1 alone in the present embodiment, for example, and the solid line indicates the conventional output fluctuation in the present embodiment.
Alternatively, the output fluctuation of the opposite phase of the measurement light Ls2 alone that compensates for and cancels the output fluctuation of the measurement light Ls1 is shown.

That is, irrespective of the presence or absence of various noises such as partial concealment during detection, zero point stability can be maintained and gas leakage detection with ultra-high accuracy can be performed. [Second Embodiment] The configuration of the controller unit 12 may be as shown in FIG. That is, in FIG. 7, the controller unit 12 includes, as in the first embodiment,
A laser diode 16A, a laser diode 16B, and a laser diode 16C are provided.

In the second embodiment, the laser diode 16
B and modulated light L output from laser diode 16C
b and Lc are combined by the multiplexer 21A and the modulated light Lbc
And the modulated light Lac output from the laser diode 16A and the combined modulated light Lbc output from the laser diodes 16B and 16C are combined by the multiplexer 2
1B, the measurement light Ls is generated. Note that the multiplexers 21A and 21B correspond to the first and second combining means of the present invention.

With such a configuration, the same operation and effect as in the first embodiment can be obtained. Further, the number of the synthesizers provided can be reduced, the controller unit 12 can be downsized, and various adjustments can be easily performed.
The modulated light L output from the laser diode 16A
Since a is not demultiplexed, the output loss of the laser diode 16A is small.

In the above embodiment, the configuration of the probe section 14 may be as shown in FIG. That is, in FIG. 8, in addition to the collimator lens 32, the condenser lens 34, and the light receiver 36,
A half mirror 40 is provided. Other configurations are the same as those of the first embodiment and the second embodiment.

The measuring light Ls is introduced into the collimator lens 32 by the optical fiber 38 from the controller section 12 (see FIG. 1), and is reflected by the half mirror 40 and sent out to the target gas atmosphere. The measurement light Ls that has passed through the target gas atmosphere is reflected or scattered by a reflector or scatterer such as the chamber wall W. Of the measurement light Ls, the light that has been reflected and returned again in the target gas atmosphere passes through the half mirror 40, enters the condenser lens 34, and enters the light receiver 36.

With such a configuration, the same operation and effect as those of the first and second embodiments can be obtained, and
The collimator lens 32 and the condenser lens 34 do not need to be provided at symmetrical positions with respect to the target gas atmosphere as in the first and second embodiments, and can be provided on the same side. At the same time, gas detection can be performed remotely from the target gas atmosphere.

In the above embodiment, the operation circuit 2
8, the gas concentration α is calculated. However, when the gas concentration α is simply used for gas leak detection, the arithmetic circuit 28 is not provided, and the output of the synchronous detection circuit 26, that is, when the modulation component IM exceeds a predetermined value. What is necessary is just to make it the structure which emits a warning signal.

As the modulation component extraction means, a means for measuring the modulation component intensity by Fourier transforming the output of the photodetector, or taking the cross-correlation between the output of the photodetector and the modulation signal, instead of the synchronous detection circuit of the above embodiment. Things can be adopted.

[0047]

As described above, according to the present invention, regardless of the presence or absence of various noises such as partial obscuration during detection, zero point stability is maintained and gas leakage detection is performed with extremely high accuracy. There is an effect that it is possible to provide a gas detection device that can perform the gas detection.

[Brief description of the drawings]

FIG. 1 is a block diagram of a gas detection device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a change over time in light intensity.

FIG. 3 is a diagram showing a temporal change of a photodetector output and a synchronous detection output.

FIG. 4 is a diagram showing a temporal change of a photodetector output and a synchronous detection output.

FIG. 5 is a diagram showing output fluctuation of a conventional gas detection device using a knife edge method.

FIG. 6 is a diagram showing an output fluctuation of the gas detection device according to the embodiment of the present invention using the knife edge method.

FIG. 7 is a block diagram of a gas detector according to a second embodiment of the present invention.

FIG. 8 is a block diagram showing another configuration of the probe unit.

FIG. 9A shows a light intensity distribution when the light beam diameter is different,
FIG. 6B is a diagram illustrating a state where the measurement light beams having different light beam diameters are shielded by a knife edge.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Gas detector 16A Laser diode (first laser light generating means) 16B Laser diode (second laser light generating means) 16C Laser diode (third laser light generating means) 22 Modulation signal adjusting circuit (modulating means) 18 Demultiplexer (dividing means) 20A multiplexer (measuring light output means) 20B multiplexer (measuring light output means) 20C multiplexer (measuring light output means) 21A multiplexer (measuring light output means) 21B multiplexing (Measurement light output means) 26 Synchronous detection circuit (modulation component extraction means) 36 Receiver (light reception means)

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Ito F-term in Toyota Central Research Laboratory Co., Ltd. 1 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi F-term (reference) 2G059 AA05 EE01 EE11 GG01 GG02 GG03 GG09 JJ11 JJ17 KK01 KK02 2G067 BB15 BB17 CC04

Claims (3)

[Claims] A first laser light generating means for generating a laser light having a wavelength absorbed by the target gas; a first laser light generating means having a wavelength smaller than a wavelength outputted from the first laser light generating means; A second laser light generating means for generating a laser light having a wavelength not absorbed, and a laser light having a wavelength larger than the wavelength outputted from the first laser light generating means and having a wavelength not absorbed by the target gas. A third laser light generator, and the laser beams output from the first laser light generator and the second laser light generator are modulated so as to have the same amplitude and opposite phases, and A modulating means for modulating each laser light outputted from the first laser light generating means and the third laser light generating means so as to have the same amplitude and opposite phases to each other; A measuring light output unit that combines the modulated light into a measuring light and outputs the measuring light into the target gas atmosphere; a light receiving unit that receives the measuring light that has passed through the target gas atmosphere; and an output signal of the light receiving unit And a modulation component extracting means for extracting a modulation component in the gas detection device. 2. A splitting means for splitting the light output from the first laser light generating means into two, wherein the measuring light output means comprises:
First combining means for combining the light output from the second laser light and one of the two lights split by the splitting means, and the light output from the first laser light generating means and the splitting means Second to combine the other light divided into two by
2. The combining means according to claim 1, further comprising: third combining means for further combining the light combined by said first combining means and said second combining means into one light. Gas detector. 3. The first light combining means, wherein the measuring light output means combines light outputted from the second laser light generating means and the light outputted from the third laser light generating means. 2. The gas detection device according to claim 1, further comprising: a second combining unit that combines the light combined by the first step and the light output from the first laser light generating unit.