JP2007212212A - Two-wavelength simultaneous outside resonance type semiconductor laser device and gas detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a two-wavelength simultaneous outside resonance type semiconductor laser device suitable for measuring the light absorption of a gas having a wide band absorption spectrum like a nitrogen dioxide gas and having a small-sized simple constitution, and a gas detector. <P>SOLUTION: The two-wavelength simultaneous outside resonance type semiconductor laser device is composed of a first semiconductor laser constituted so that coating for reducing the reflectivity of light is applied to its one end surface, a second semiconductor laser different in wavelength from the first semiconductor laser and constituted so that the coating for reducing the reflectivity of light is applied to its one end surface, and an outside resonator for resonating the emitted light from the first semiconductor laser and the emitted light from the second semiconductor laser. Further, the gas detector is composed of the two-wavelength simultaneous outside resonance type semiconductor laser device and a photodetector. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a highly sensitive gas measurement light source by absorption spectroscopy and an external resonant semiconductor laser device suitable as a long optical path cell.

  Many studies have been made on an apparatus for simultaneously resonating one laser beam and light obtained by converting the wavelength of the laser beam with a nonlinear optical crystal (see, for example, Patent Document 1). Since it is very difficult to resonate the laser beam, it is not often performed. In general, in order to stably resonate two independent wavelengths of light simultaneously in a single resonator having a high finesse, a laser light source and a stabilized power source with extremely stabilized wavelengths are required. Furthermore, in order to control the resonator length to an integral multiple of the wavelength of the laser beam, control with a piezoelectric element or the like is necessary. Also, feedback control of temperature and power supply current is necessary. Therefore, the apparatus becomes expensive, and the configuration is large and complicated.

There is a cavity ring down spectroscopy (CRDS) which is a kind of absorption spectroscopy. CRD spectroscopy is a spectroscopic method that makes an effective optical path very long by utilizing the fact that when a laser beam is confined in a resonator used as a gas cell, the light reciprocates many times within the resonator mirror. This makes it possible to perform very high sensitivity measurement. In general, a substance having a very narrow band absorption spectrum is observed by scanning the wavelength using a narrow band laser beam. Thus, the concentration is quantified.

In general spectroscopy, in order to measure a substance such as carbon dioxide (CO ₂ ) having a very narrow absorption spectrum, a wavelength-tunable single-wavelength laser light source and a resonator length are precisely set. Gas spectroscopy is performed using the mechanism to be controlled. Also, the current applied to the laser light and the temperature of the laser light source are adjusted very precisely.
JP-A-8-194240 JP 2005-140558 A Wolfgang Schneider, 4 others (WOLFGANG SCHNEIDER et al.), "Absorption section of NO2 in the ultraviolet and visible region (200-700nm) at 298K"("ABSORPTION CROSS-SECTIONS OF NO2 IN THE UV AND VISIBLE REGION (200 -700 nm) AT 298 K "), Journal of Photochemistry and Photobiology; A: Journal of Photochemistry and Photobiology, A: Chemistry, 1987, 40, pp.195-217

In recent years, nitrogen dioxide (NO ₂ ) gas has been regarded as extremely important as an environmental pollutant.
FIG. 5 is a diagram showing a light absorption spectrum of nitrogen dioxide (NO ₂ ) gas (Non-patent Document 1).
. The horizontal axis indicates the wavelength of light, and the vertical axis indicates the absorption cross section of nitrogen dioxide. There is no wavelength exhibiting sharp absorption, and the absorption spectrum can be regarded as flat if it is in the range of about 1 nm in this region. The light absorption spectrum of nitrogen dioxide (NO ₂ ) gas is very gentle compared to the very sharp light absorption spectrum of carbon dioxide (CO ₂ ) gas. Measurement of a gas having a gentle absorption spectrum, such as nitrogen dioxide gas, does not require a measuring device in which the wavelength of light is controlled very precisely in order to measure a narrow-band absorption spectrum. In addition, in order to identify the gas species, it is necessary to be able to simultaneously measure the absorption of light having a plurality of wavelengths.

It is an object of the present invention to provide a two-wavelength simultaneous external resonance semiconductor laser device and a gas detector having a small and simple configuration suitable for measurement of light absorption of a gas having a broad absorption spectrum such as nitrogen dioxide gas. And

  The present invention employs the following means in order to solve the above problems.

That is, the present invention
A first semiconductor laser having a coating with reduced light reflectance on one end face;
A second semiconductor laser having a wavelength different from that of the first semiconductor laser and having a coating with reduced light reflectance on one end face;
An external resonator in which the light emitted from the first semiconductor laser and the light emitted from the second semiconductor laser resonate;
This is an external resonance type semiconductor laser device.

  According to the present invention, laser beams from two independent semiconductor lasers can be resonated inside one external resonator.

The present invention also provides:
A first semiconductor laser having a coating with reduced light reflectance on one end face;
A second semiconductor laser having a wavelength different from that of the first semiconductor laser and having one end surface coated with a reduced reflectance;
An external resonator in which the emitted light from the first semiconductor laser and the emitted light from the second semiconductor laser resonate and can enclose gas;
A photodetector for detecting light emitted from the external resonator;
It is a gas detection apparatus provided with.

  According to the present invention, it is possible to measure light absorption of a gas having a broad light absorption spectrum such as nitrogen dioxide gas.

  ADVANTAGE OF THE INVENTION According to this invention, the external resonance type semiconductor laser apparatus and gas detector by a small and simple structure suitable for the measurement of the optical absorption of the gas which has a broad absorption spectrum like nitrogen dioxide gas can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The configuration of the embodiment is an exemplification, and the present invention is not limited to the configuration of the embodiment.

Embodiment
<Configuration of gas detector>
FIG. 1 is a diagram illustrating a configuration example of a gas detection device including a two-wavelength simultaneous external resonance semiconductor laser device 10.

The two-wavelength simultaneous external resonance type semiconductor laser device 10 includes a power source 5A for supplying power to the semiconductor laser 4A, a semiconductor laser 4A connected to the power source 5A, and a condensing lens 3A for condensing light emitted from the semiconductor laser 4A. A power source 5B for supplying power to the semiconductor laser 4B, a semiconductor laser 4B connected to the power source 5B, a condensing lens 3B for condensing the light emitted from the semiconductor laser 4B, and the light from the condensing lens 3A A beam splitter 2 that partially reflects and transmits part of the light from the condensing lens 3B and enters light from the two directions into the external resonator 1, and an external resonator that receives light from the beam splitter 2 1.

  The gas detector includes a two-wavelength simultaneous external resonance semiconductor laser device 10, an inspection system 6 that detects light emitted from the external resonator 1 of the two-wavelength simultaneous external resonance semiconductor laser device 10, and a detector control device 20. .

  The detector control device 20 executes a program to realize an information processing function, a communication interface 22 having a communication function with the outside such as a detector, and a memory 27 in which a program executed by the CPU 26 is expanded. , A storage device 25 for storing data such as wavelength dependency of gas absorption cross section and measurement data, an input device 23 for detecting an input operation by a user, an output device 24 for outputting data, and a removable external device And a storage device 21.

  With the above configuration, the light absorption of the gas enclosed in the external resonator 1 of the gas detection device can be measured.

(Power supply)
The power source 5 supplies power to the semiconductor laser 4. The power source 5 can continuously supply power to the semiconductor laser 4. The power source 5 can also apply rectangular wave modulation to the power supply to the semiconductor laser 4. Furthermore, the power supply 5A and the power supply 5B can be synchronized.

(Laser light source)
The semiconductor laser 4 emits laser light. One end face of the semiconductor laser 4 has an anti-reflection coating (AR coating). The reflectivity of the AR-coated end face is approximately 0.01%. As a result, there is almost no reflection. This is because laser oscillation does not occur inside the semiconductor laser 4.

  The first semiconductor laser 4A is a light source that emits laser light having a first wavelength. From the semiconductor laser 4 </ b> A, S-polarized laser light is emitted to the beam splitter 2. The second semiconductor laser 4B is a light source that emits laser light having a second wavelength. A different wavelength from the first wavelength is selected as the second wavelength. From the semiconductor laser 4 </ b> B, P-polarized laser light is emitted to the beam splitter 2.

  As the oscillation wavelength of the first semiconductor laser 4A, for example, one having a wavelength of 638 nm is used. In addition, as the oscillation wavelength of the second semiconductor laser 4B, for example, a wavelength of 673 nm is used. The oscillation wavelength is not limited to these oscillation wavelengths. The oscillation wavelengths of the first semiconductor laser 4A and the second semiconductor laser 4B can be freely selected independently.

  Both or one of the first semiconductor laser 4A and the second semiconductor laser 4B is installed on a mount that can be controlled in the horizontal direction and the vertical direction, and that can control the direction of the end face on the emission side of the semiconductor laser 4. Can do. By installing in the mount, the emission direction of the laser light can be adjusted.

(Condenser lens)
The condensing lens 3 is disposed on the end face side of the semiconductor laser 4 which is coated with no reflection. The condensing lens 3 condenses the laser light emitted from the semiconductor laser 4. An aspherical lens or the like is used as the condenser lens.

(Beam splitter)
The laser beam condensed by the condenser lens 3 is reflected by the beam splitter 2 or transmitted through the beam splitter 2.

  The beam splitter 2 has polarization direction dependency. The reflectance when S-polarized light is incident on the beam splitter 2 is higher than the reflectance when P-polarized light is incident. Similarly, the transmittance when P-polarized light is incident on the beam splitter 2 is higher than the transmittance when S-polarized light is incident.

  Here, S-polarized light refers to polarized light in a direction perpendicular to the plane in which the vibration direction of light incident on the plane includes the normal line of the plane and the normal line of the wavefront that is the traveling direction of the light. P-polarized light refers to polarized light in a direction in which the vibration direction of light incident on a surface is included in a surface including the normal of the surface and the traveling direction of the light. The direction of P-polarized light is a direction perpendicular to the direction of S-polarized light.

In the embodiment of the present invention, the beam splitter 2 is a non-polarized light source,
Reflectivity: transmittance = 50%: 50%
I used the following. Due to the properties of the beam splitter 2 described above, about 60% to 70% of the S-polarized laser light emitted from the first semiconductor laser 4A is reflected by the beam splitter 2. Similarly, about 60% to 70% of the P-polarized laser light emitted from the second semiconductor laser 4B is transmitted through the beam splitter 2. By using the laser beam polarized in this way, the laser beam can be efficiently incident on the external resonator 1.

  The beam splitter 2 can be installed on a linear motion stage that can be controlled in the horizontal direction and a kinematic mount that can adjust the direction of the surface of the beam splitter 2. With the mount on which the semiconductor laser 4 is installed and the mount on which the beam splitter 2 is installed, the optical axes of the two laser beams emitted from the first semiconductor laser 1 and the second semiconductor laser 2 can be matched. . As a result, the two laser beams can be incident on the external resonator 1 with their optical axes aligned.

(External resonator)
The external resonator 1 includes a first reflecting surface 7A and a second reflecting surface 7B. The first reflecting surface 7A and the second reflecting surface 7B face each other. The light from the beam splitter 2 enters from the second reflecting surface 7B side. The external resonator 1 can be a gas cell that can be sealed by introducing gas.

  The reflectivity of the two reflecting surfaces of the external resonator 1 needs to be very high. This is because the laser light incident on the external resonator 1 is confined in the external resonator 2 for a long time. The reflectance of the first reflecting surface 7A is preferably 99.99% or more, more preferably 99.999% or more. The reflectance of the second reflecting surface 7B is preferably 99.94% or more, more preferably 99.97% or more. The reflectance of the second reflecting surface 7B is set lower than the reflectance of the first reflecting surface 7A. Thereby, the incidence of the laser beam from the beam splitter 2 can be facilitated from the second reflecting surface 7B side.

  In the embodiment of the present invention, the resonator length of the external resonator 1 is approximately 8 cm. The resonator length of the external resonator 1 is not limited to this and can be freely selected. The cavity length is very large compared to the wavelength of incident laser light (1 μm or less).

(Inspection system)
In the inspection system 6, the laser light emitted from the external resonator 1 is converted into a photomultiplier tube, a semiconductor photodetecting element (for example, a charge coupled device (CCD), a photodiode,
Detection is performed using a photodetector such as a phototransistor, an avalanche diode, a photoconductive element, or a photovoltaic element. A prism, a diffraction grating, or the like may be disposed in front of the photodetector so that the light emitted from the external resonator 1 is dispersed and simultaneously detected.

(Detector control device)
The detector control device 20 controls the inspection system 6 and performs detection gas detection and the like. The CPU 26 executes a program developed on the memory 27 to realize functions such as detector control and detection gas determination. The input device 23 receives information input by an input operation by the user and notifies the CPU 26 of the information. The input device 23 includes a pointing device such as a keyboard and a mouse. The output device 24 implements output such as display of measurement data. The output device 24 includes a display, a speaker, and the like. The storage device 25 stores measurement data and data such as the wavelength dependence of the absorption cross section of the sample gas. The storage device 25 can be replaced with a recording medium. That is, the data recorded in the storage device 25 can be recorded on a recording medium attached to the removable external storage device 21 and used. It is also possible to use both the storage device 25 and the removable external storage device 21 at the same time.

The detector control device 20 may be a personal computer (PC), a digital storage oscilloscope (DSO), or a combination thereof.

<Operation example>
(Laser oscillation)
Power is supplied to the semiconductor laser 4 whose one end face is coated with no reflection. A laser beam having a broad and discrete frequency over several hundred GHz (wavelength: several nm) generated inside the semiconductor laser 4 is emitted from the side of the end surface of the semiconductor laser 4 which is coated with no reflection. The laser light is condensed by the condenser lens 3. The condensed laser light is reflected or transmitted by the laser splitter 2 and enters the external resonator 1. When the laser light is incident on the external resonator 1, laser oscillation occurs at a frequency (resonance wavelength) that matches the resonator length of the external resonator 1. A part of the laser light oscillated returns to the semiconductor laser 4 along a path opposite to that described above. This returned laser light locks the frequency of the semiconductor laser 4. Thereby, the resonance wavelength of the semiconductor laser 4 is determined following the resonance wavelength of the external resonator 1. When laser beams from the first semiconductor laser and the second semiconductor laser are simultaneously incident on the external resonator 1, they resonate inside the external resonator 1 at the same time.

  FIG. 2 is a diagram illustrating a state in which the light emitted from the first semiconductor laser 4A and the second semiconductor laser 4B oscillates in the external resonator 1. Here, the oscillation frequencies of 638 nm and 673 nm were used. The oscillation frequency is not limited to these and can be freely selected.

(Measurement of attenuation of external resonator)
When rectangular wave modulation is applied to the semiconductor laser 4 and the incident light to the external resonator 1 is blocked, the light intensity inside the external resonator 1 is exponentially attenuated as in the following equation (Equation 1).

ここで、
ｔ：時間
I：外部共振器の内部の光強度、
I₀：ｔ＝０での光強度
τ_p：外部共振器の減衰の時定数
である。τ_pは、次式（数２）のように表される。

here,
t: time
I: Light intensity inside the external resonator,
I ₀ : Light intensity at t = 0 τ _p : Time constant of attenuation of the external resonator. τ _p is expressed as the following equation (Equation 2).

ここで、
Ｒ₁、Ｒ₂：外部共振器を構成する２つの反射面の反射率
ｃ：光速
ｄ：外部共振器の共振器長
α：外部共振器の共振器内損失
である。

here,
R ₁ , R ₂ : reflectance of two reflecting surfaces constituting the external resonator c: speed of light d: resonator length of the external resonator α: loss inside the resonator of the external resonator.

  In the case where there is a gas medium that absorbs the loss in the resonator of the external resonator 1, that is, the wavelength of the light source, the time constant of attenuation of the external resonator 1 is shortened according to the loss. By observing the leakage light intensity from behind the external resonator 1, such an attenuation curve can be obtained. FIG. 3 is a diagram illustrating changes in the time constant of attenuation of the external resonator 1. From FIG. 3, it can be seen that the attenuation time constant of the attenuation curve 102 in the presence of the absorbing medium is shorter than that of the attenuation curve 101 in the absence of the absorbing medium.

  The concentration of the absorbing medium can be quantified using the following equation (Equation 3).

ここで、
Ｎ：分子数密度
σ：吸収媒質の吸収断面積
τ：吸収媒質が存在する場合（すなわち、α≠０）の減衰の時定数
τ₀：吸収媒質が存在しない場合（すなわち、α＝０）の減衰の時定数
である。吸収媒質の吸収量の異なる２つの波長（Ａ，Ｂ）で、本測定を行うことにより、選択的に、対象ガスの濃度定量を行うことができる。

here,
N: Molecular number density σ: Absorption cross section of absorption medium τ: Time constant of attenuation when absorption medium exists (ie α ≠ 0) τ ₀ : Absorption medium does not exist (ie α = 0) This is the decay time constant. By performing this measurement at two wavelengths (A, B) having different absorption amounts of the absorption medium, the concentration of the target gas can be selectively quantified.

ここで、
τ_0A、τ_0B：それぞれの波長（Ａ、Ｂ）での吸収媒質が存在しない場合の減衰の時定数
τ_A、τ_B：それぞれの波長（Ａ、Ｂ）での吸収媒質が存在する場合の減衰の時定数
σ_A、σ_B：それぞれの波長（Ａ、Ｂ）での吸収媒質の吸収断面積
である。それぞれの波長（Ａ、Ｂ）での吸収媒質の吸収断面積（σ_A、σ_B）は、既知の量である。あらかじめ、吸収媒質が存在しない状態、つまり外部共振器１の内部が真空の状態で、レーザー光を外部共振器１に入射して検査系６で減衰の時定数を測定することにより、τ_0A及びτ_0Bを求めることができる。したがって、サンプルガスによる減数定数τ_A
、τ_Bを求めることにより、外部共振器１の内のガスの密度を求めることが可能となる。

here,
τ _0A , τ _0B : decay time constant when there is no absorbing medium at each wavelength (A, B) τ _A , τ _B : when there is an absorbing medium at each wavelength (A, B) Attenuation time constants σ _A , σ _B : Absorption cross sections of the absorbing medium at the respective wavelengths (A, B). The absorption cross sections (σ _A , σ _B ) of the absorbing medium at the respective wavelengths (A, _B ) are known quantities. In advance, when the absorption medium is not present, that is, the inside of the external resonator 1 is in a vacuum state, laser light is incident on the external resonator 1 and the time constant of attenuation is measured by the inspection system 6, so that τ _0A and τ _0B can be obtained. Therefore, the reduction constant τ _A due to the sample gas
, Τ _B can be obtained to obtain the density of the gas in the external resonator 1.

Further, if τ _0A , τ _0B , τ _A and τ _B can be measured, the ratio of σ _A and σ _B can be obtained from the equation (Equation 3). Using this value, the gas species can be identified.

  When the gas to be measured contains impurities, gases having different absorption cross sections exist at the same wavelength. At this time, the time constant of attenuation of the external resonator 1 has two time constants. Therefore, when it is found by measurement that two time constants are obtained at the same wavelength, it is understood that impurities are contained.

  The resonance wavelength of the laser beam in the external resonator 1 of the embodiment of the present invention is not precisely controlled, and a plurality of modes are set over a range of about 1 nm from the center wavelength of the semiconductor laser 4. However, if the absorption spectrum is flat in the range of about 1 nm from the center wavelength as in the light absorption spectrum of nitrogen dioxide gas (FIG. 5), there is no problem in measuring attenuation due to light absorption. This is because the absorption cross-sectional area can be considered to be constant when the wavelength difference is in the range of about 1 nm. That is, a multimode oscillation laser can be used for such a gas species having a flat spectral region, and a trace gas can be measured with an inexpensive and compact configuration of both the light source and the power source.

Differential absorption spectroscopic measurement of nitrogen dioxide (NO ₂ ) gas was performed using the external resonator 1 of the two-wavelength simultaneous external resonance semiconductor laser device 10 as a gas cell and using light sources of two wavelengths. FIG.
It is a figure which shows the measurement test result of nitrogen dioxide by the gas detector of embodiment of this invention. It can be seen from FIG. 4 that the equation (4) is well suited. The apparatus according to the embodiment of the present invention can measure a concentration of about 5 ppm to about 50 ppm with respect to nitrogen dioxide gas.

FIG. 6 is a diagram illustrating an operation flow of determination of the measurement target gas. The light intensity data detected by the inspection system 6 is sent to the detector control device 20 (S102). The detector control device 20 obtains the time constant of attenuation of the external resonator 1 at each measurement wavelength (the wavelength of the semiconductor laser 4) from the measured data. Further, when the measurement gas and the measurement wavelength are designated (S104), the detector control device 20 extracts the absorption cross section data for the gas recorded in the storage device 6 (S106). The detector control device 20 determines whether or not the measured attenuation time constant is 1 for each measurement wavelength (S108). When two or more attenuation time constants exist for one measurement wavelength, it means that gases having different absorption cross sections exist inside the external resonator 1 (gas cell). Therefore, when there are two or more decay time constants (S108; N
O) displays “possibility of impurity contamination” and ends (S110). At each measurement wavelength, when the attenuation time constant is one, it is determined that there is one kind of gas inside the external resonator 1 (gas cell), and the process proceeds to the next step (S108; YES). The detector control device 20 compares the ratio of the absorption cross sections of the two measurement wavelengths obtained from the data of the measurement target gas extracted from the storage device 25 and the ratio of the absorption cross sections of the two measurement wavelengths obtained from the measured data. Are compared (S112). As a result of the comparison, if the ratios are different, it is determined that the gas is not a measurement target gas (S112; NO), and “Possible gas type is different” is displayed and the process ends (S114). If the ratios are the same, it is determined that the measurement target gas has been measured (S112; YES), “Successful measurement” is displayed, and the process ends (S116).

<Effect of embodiment>
According to the embodiment of the present invention, only two inexpensive semiconductor lasers 4 and a power source 5, a beam splitter 2 that guides two laser beams to the external resonator 1, and only the external resonator 1 having a pair of reflecting surfaces. It consists of Therefore, it is possible to resonate two wavelength laser beams inside a high finesse external resonator without using a piezoelectric element and a feedback control device as in the past and at a small size and at low cost.

  In addition, by resonating laser light of two wavelengths inside the external resonator 1 with high finesse, selective and highly sensitive gas by cavity ring-down spectroscopy using the external resonator 1 with high finesse. Measurement can be realized.

It is a figure which shows the structural example of the 2 wavelength simultaneous external resonance type | mold semiconductor laser apparatus and gas detection apparatus of embodiment of this invention. It is a figure which shows the mode of resonance in the external resonator of the laser beam of 2 wavelengths of embodiment of this invention. It is a figure which shows the change of the time constant of attenuation | damping of the external resonator of embodiment of this invention. It is a figure which shows the measurement test result of the nitrogen dioxide by the gas detector of embodiment of this invention. Is a diagram showing the wavelength dependence of absorption cross-section of nitrogen dioxide (NO _2). (Wolfgang Schneider et al .: Journal of Photochemistry and Photobiology, A: Chemistry, 40 (1987) 195-217) It is a figure which shows the operation | movement flow of determination of measurement object gas.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 External resonator 2 Beam splitter 3 Condensing lens 3A 1st condensing lens 3B 2nd condensing lens 4 Semiconductor laser 4A 1st semiconductor laser 4B 2nd semiconductor laser 5 Power supply 5A 1st power supply 5B 2nd Power supply 6 Inspection system 10 Two-wavelength simultaneous external resonance semiconductor laser device 20 Measuring device control device 21 Removable external storage device 22 Communication interface 23 Input device 24 Output device 25 Storage device 26 CPU
27 Memory 101 Attenuation curve when no absorbing medium is present 102 Decaying curve when an absorbing medium is present

Claims (6)

A first semiconductor laser having a coating with reduced light reflectance on one end face;
A second semiconductor laser having a wavelength different from that of the first semiconductor laser and having a coating with reduced light reflectance on one end face;
An external resonator in which the light emitted from the first semiconductor laser and the light emitted from the second semiconductor laser resonate;
An external resonant semiconductor laser device comprising: The external resonator has a first reflecting surface and a second reflecting surface facing each other,
2. The external resonant semiconductor laser device according to claim 1, wherein at least one of the first reflecting surface and the second reflecting surface has a reflectivity of 99.99% or more. A first semiconductor laser having a coating with reduced light reflectance on one end face;
A second semiconductor laser having a wavelength different from that of the first semiconductor laser and having one end surface coated with a reduced reflectance;
An external resonator in which the emitted light from the first semiconductor laser and the emitted light from the second semiconductor laser resonate and can enclose gas;
A photodetector for detecting light emitted from the external resonator;
A gas detection device comprising: Whether or not the gas is a measurement target from the time constant of attenuation measured for the wavelength of light from the first semiconductor laser and the time constant of attenuation measured for the wavelength of light from the second semiconductor laser. Means for determining whether or not
The gas detection device according to claim 3, further comprising: The external resonator has a first reflecting surface and a second reflecting surface facing each other,
At least one of the first reflective surface and the second reflective surface has a reflectance of 99.99% or more.
The gas detection device according to any one of claims 3 and 4. The gas detector according to claim 3, wherein the photodetector is a photomultiplier tube or a semiconductor photodetector element.

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