JP2796650B2 - Multi-gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-gas detector using a semiconductor laser device.

[0002]

2. Description of the Related Art It is known that the presence or absence of gas can be detected by utilizing the fact that laser light of a specific wavelength is easily absorbed by a certain kind of gas. Sensing technology using this principle is used for industrial measurement, pollution monitoring, etc. Widely used for As an example, the presence or absence of methane can be detected with high sensitivity by utilizing the fact that the 3.3922 μm oscillation line of the laser light generated by the He—Ne laser is strongly absorbed by methane. Since methane is the main component of city gas, leakage of city gas can be detected by detecting methane gas. However, when gas detection is performed using a semiconductor laser, the oscillation wavelength greatly changes depending on the temperature of the laser element. Therefore, it is necessary to stabilize the oscillation wavelength of the laser light in accordance with the center wavelength of the gas absorption line.

[0003] Japanese Patent Application No. 2-78498 proposes a remote gas (methane gas) detection apparatus to which a frequency modulation method is applied and which uses an optical fiber as a transmission line. In this detection device, the drive current of the semiconductor laser element is modulated by a high-frequency signal around a predetermined current value, and the current and the temperature of the semiconductor laser element are changed by using a part of the laser light emitted from the semiconductor laser element. The oscillation wavelength of the laser beam is stabilized by controlling the center wavelength of the oscillation to coincide with a predetermined position of the absorption line of methane. The stabilized laser light is guided through an optical fiber to a gas cell containing an unknown concentration of methane gas, the light passing through the gas cell is received by another optical fiber, converted into an electric signal by a light receiving unit, and then phase-sensitive. A detector detects a double-wave detection signal and a fundamental-wave detection signal of the laser beam, and obtains a gas concentration from a ratio of these two signals.

However, since this detector can detect only one type of gas concentration, if it is desired to detect the concentration of various types of gases, there is a problem that the number of detection devices must be prepared as many as the number of types of gas. is there.

On the other hand, the applicant of the present application has filed Japanese Patent Application No. 3-96.
No. 826 proposes a multi-gas detecting device for detecting various gases.

FIG. 3 is a schematic configuration diagram of a multi-gas detecting device by the present applicant.

In this device, the absorption line of methane gas is regarded as an oscillation line.
Semiconductor device 1a and absorption of acetylene gas
A semiconductor laser element 1b having a line as an oscillation line,
The semiconductor laser element 1a is rotated by the methane driving device 2a.
Wave number f₁ And the semiconductor laser element 1b is changed to acetylene.
Frequency f₁ Frequency f different from _Two 
Modulate with From the modulated semiconductor laser device 1a
The reference light cell 3a filled with methane gas.
The light is passed through, received by the light receiver 4a, converted into an electric signal,
Installed in the semiconductor laser device 1a by the wavelength control device 5a.
Controlling the temperature of the Peltier element 6a
Laser beam from the laser 1b into the acetylene gas
The electric signal is received by the light receiver 4b after passing through the illumination cell 3b.
And converted to semiconductor by the acetylene wavelength controller 5b.
The temperature of the Peltier element 6b provided in the laser element 1b is
Control.

On the other hand, the laser light from the semiconductor laser elements 1a and 1b is mixed into one light by a branching coupler 8 via an isolator 7a for preventing reflected light, and then a measurement cell is output via an optical fiber 9 for a forward path. Lead to 10 and measure cell 1
The light passing through 0 is guided to the photodetector 12 through the return optical fiber 11 and amplified by the amplifier 13. And further, the component corresponding to the frequency f ₁ in the detection output by the phase-sensitive detection by methane signal processing device 14a, and detects the concentration of methane in the measurement cell 10, the frequency f ₂ in the detection output The corresponding component is subjected to phase sensitive detection by the acetylene signal processor 14b to detect the concentration of the acetylene gas in the measurement cell 10.

[0009]

However, in the multi-type gas detecting device having such a configuration, the reference cell and the light-receiving device that receives the laser beam passing through the cell have the same number as the number of types of gas to be detected. However, it is necessary to stabilize the oscillating wavelength separately, so that there is a problem that the device becomes complicated and the size becomes large.

[0010] The present invention has been made in view of the above points, and an object of the present invention is to provide a gas detection device for simultaneously detecting many kinds of gases using a plurality of semiconductor lasers.
It is to stabilize the oscillation wavelength of each semiconductor laser at the same time.

[0011]

According to the present invention, there is provided a semiconductor device comprising: a plurality of semiconductor lasers having different oscillation wavelengths; and a modulator for modulating a drive current of each semiconductor laser at a different predetermined frequency. A multi-gas detection device for detecting the concentration of gas in a measurement cell using laser light that has been modulated by means and passed through a measurement cell in which a plurality of gases have been sealed, and detects laser light emitted from each semiconductor laser. Mixing means for mixing, a reference cell filled with a plurality of reference gases, a light receiving means for receiving laser light transmitted through the reference cell, and detecting a frequency component equal to a predetermined frequency from an output of the light receiving means. This is achieved by a multi-species gas detection device provided with a plurality of wavelength stabilizing means for stabilizing the wavelength of each semiconductor laser based on the frequency component.

[0012]

The multi-gas detector of the present invention mixes a plurality of laser lights modulated at different predetermined frequencies by a mixing means,
After passing through a reference cell filled with a plurality of reference gases, the light is received by a light receiving unit, and the oscillation wavelength of the laser light corresponding to a predetermined frequency is stabilized by a plurality of wavelength stabilizing units.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of one embodiment of a multi-gas detecting apparatus according to the present invention. Here, the description will be made as an apparatus for detecting two kinds of gases, methane gas and acetylene gas.

The illustrated multi-gas detecting apparatus has two semiconductor laser elements 20a and 20b and a semiconductor laser element 20a.
methane wavelength control device 21a for modulating at a frequency f ₁ with stabilizing the oscillation wavelength of a, the semiconductor laser element 2
0b acetylene wavelength control device 21b for modulating at frequency f ₂ as well as stabilizing the oscillation wavelength of the reference cell 22 which mixed gas is sealed with a known concentration of methane gas and acetylene gas, the semiconductor laser elements 20a , 20b
A beam splitter 23 that mixes the modulated laser light emitted from the light source, a photodetector 24 as a light receiving unit that receives the laser light output from the beam splitter 23 and transmitted through the reference cell 22 and converts the laser light into an electric signal, A measuring cell 25 in which a mixed gas of methane gas and acetylene gas of unknown concentration is sealed, and a photodetector 26 which receives a laser beam output from the beam splitter 23 and transmitted through the measuring cell 25 and converts it into an electric signal. When the methane detector 27a for detecting methane and a signal of frequency f ₁ is outputted from the electric signal and methane wavelength controller 21a output from the photodetector 26, the electrical signal output from the optical detector 26 And the frequency f output from the acetylene wavelength controller 21b
_And an acetylene detector 27b for detecting acetylene from the _two signals.

The semiconductor laser elements 20a and 20b have such characteristics that the oscillation wavelength becomes longer when the temperature of the element itself becomes higher, and the oscillation wavelength becomes shorter when the temperature becomes lower. The semiconductor laser elements 20a and 20b are not shown. A Peltier element is mounted via a heat conducting plate.

The Peltier element is composed of a P-type semiconductor and an N-type semiconductor connected alternately via a metal plate, and the connection is heated or cooled depending on the direction of current flow. The oscillation wavelength of the semiconductor laser elements 20a and 20b can be stabilized by controlling the voltage or the current.

The beam splitter 23 is generally constituted by a half mirror, and splits a light beam incident on the half mirror at a predetermined angle into two parts, a reflected light and a transmitted light.
Laser light passing through the optical path L ₁ emitted from the semiconductor laser device 20a, the then partially transmitted through the beam splitter 23 make incidence to the measurement cell 25, a reference reflecting another part by the beam splitter 23 The light enters the cell 22. Is emitted from the semiconductor laser device 20b laser beam likewise passing through the optical path L _2, a portion thereof is reflected by the beam splitter 23, transmitted along with entering the measurement cell 25, the other part of the beam splitter 23 And enters the reference cell 22.

The semiconductor laser device 20a, and 20b, and the reference cell 22, the measuring cell 25, the optical path L ₃ of the laser beam is emitted from the semiconductor laser element 20a passes through the beam splitter 23, the semiconductor laser device 20b And the laser beam reflected by the beam splitter 23 is disposed so as to coincide with the optical path L ₃ of the laser beam.
Is emitted from 0b to the optical path L ₄ of the laser beam transmitted through the beam splitter 23, the optical path L ₄ of the laser beam reflected by the outgoing beam splitter 23 is arranged to coincide from the semiconductor laser 20a. Therefore, the optical path L ₃ ,
Since L so that the laser beam emitted from the semiconductor laser device the laser light emitted from 20a and the semiconductor laser device 20b are mixed on ₄ the beam splitter 23 functions as a mixing means.

The light receivers 24 and 26 are elements for converting incident laser light into electric signals, and are, for example, photodiodes, but are not limited thereto, and may use phototransistors.

The methane wavelength control device 21a includes the light receiving device 2
The wavelength of the semiconductor laser element 20a is monitored from the output component from the semiconductor laser element 4 and the temperature is adjusted so that the temperature of the Peltier element attached to the semiconductor laser element 20a becomes a predetermined temperature in order to stabilize the oscillation wavelength. modulating the driving current of the element 21a at the frequency f _1. Similarly, the acetylene wavelength control device 21b also
The temperature of the semiconductor laser element 20b is adjusted so that the temperature of the Peltier element attached to the semiconductor laser element 20b becomes a predetermined temperature, and the driving current of the semiconductor laser element 20b is adjusted to the frequency f.
Modulate by ₂ .

The methane detecting device 27a calculates the concentration of methane gas in the measuring cell 25, and calculates the acetylene detecting device 2
7b calculates the concentration of acetylene gas in the measurement cell 25.

Next, the operation of the multi-gas detector will be described.

It is known that the larger the least common multiple of the modulation frequencies f ₁ and f ₂ of the semiconductor laser elements 20 a and 20 b is, the higher the measurement accuracy is. Therefore, for example, the modulation frequency f ₁ of the semiconductor laser element 20 a for methane gas is improved. ₁ to 51
KH _Z , semiconductor laser element 20b for acetylene gas
Chose the modulation frequency f ₂ to 49KH _Z. Modulation frequency 49KH _Z, is a 51KH _Z, may use other frequencies if integer multiples each other.

First, the methane wavelength controller 21a and the acetylene wavelength controller 21b are driven. The semiconductor laser element 20a is modulated at a frequency 51KH _Z with excitation at a wavelength equal to the absorption wavelength of methane, the semiconductor laser device 20b is modulated at a frequency 49KH _Z with excitation at a wavelength equal to the absorption wavelength of acetylene. as a result,
Each of the semiconductor laser elements 20a and 20b oscillates a laser beam modulated at a predetermined frequency with a predetermined bias current as a center.

Of the laser light oscillated in this manner, the laser light emitted from the semiconductor laser element 20a and emitted from the beam splitter 23
Is mixed with the laser light emitted from the semiconductor laser element 20b and reflected by the beam splitter 23, passes through the reference cell 22, and is converted into an electric signal by the light receiver 24. Output signals obtained from the light receiver 24 are input to the wavelength controllers 21a and 21b, respectively.
In the methane wavelength control device 21a, the frequency is 51 KH.
Signal processing is performed only on the modulated wave of _Z to control the voltage of the Peltier device and control the temperature of the semiconductor laser device 20a, so that the oscillation wavelength is stabilized at the absorption wavelength of methane. This signal processing for only the modulation wave frequency 49KH _Z in acetylene control unit 21b is performed by the voltage of the Peltier element is controlled relative to the temperature of the semiconductor laser device 20b is controlled. Wavelength control device 21a,
21b is phase sensitive detector for internal integrator, etc. is constituted by a power supply for the Peltier element, an output signal obtained from the light receiver 24 each modulation frequency 49KH _Z, 51KH
A phase-sensitive detection signal of the fundamental wave for _Z is obtained, and the temperatures of the semiconductor laser elements 20a and 20b are controlled so that the value becomes zero. Thereby, the semiconductor laser element 20a,
The oscillation wavelength of 20b is simultaneously stabilized at the center of the absorption line of each detection gas.

On the other hand, the laser light emitted from the semiconductor laser element 20a and transmitted through the beam splitter 23 and the laser light emitted from the semiconductor laser element 20b and
The mixed light with the laser light reflected by 3 passes through the measuring cell 25, is converted into an electric signal by the light receiver 26, and is input to the methane detecting device 27a and the acetylene detecting device 27b. The detecting device 27a for methane and the detecting device 27b for acetylene are each provided with a corresponding modulation frequency of 49 KH.
_Z, 2 single phase sensitive detector for outputting a phase-sensitive detection signal and the second harmonic wave phase-sensitive detection signal of a fundamental wave for 51KH _Z, divider to obtain an output ratio of the phase-sensitive detector, the output of divider It consists of a recorder to record. Then, the concentration of the methane gas and the acetylene gas in the measurement cell 25 is obtained from a value obtained by dividing the value of the second-harmonic phase-sensitive detection signal by the value of the fundamental-wave phase-sensitive detection signal.

When the center wavelength of the laser light oscillated by the semiconductor laser elements 20a and 20b is made to coincide with the center f ₀ of the absorption lines of methane gas and acetylene gas, respectively, the ratio between the second harmonic detection signal and the fundamental detection signal is obtained. P (2f) / P (f)
_max indicates the peak value R and is expressed as in Equation 1.

[0029]

R = P (2f) / P (f) _max = I ₀ (ΔF) ² α (f ₀ ) · c · L / (2ΔI · γ ² ) where I ₀ is the laser output. The center intensity, ΔI is the intensity modulation amplitude, ΔF is the frequency modulation amplitude, α (f ₀ ) is the absorption coefficient of the specific gas at the absorption line center f ₀ , and γ is the α (f
₀ ), half of the half value width 2γ, c · L, is proportional to the product of the concentration c of the specific gas and the light propagation distance L in the measuring cell 25. The gas concentration c can be calculated.

FIG. 2 is a schematic configuration diagram of another embodiment of the multi-gas detecting device.

The difference between this embodiment and the multi-gas detector shown in FIG. 1 is that lenses 29 and 30 for condensing laser light are used, a fiber coupler 28 is used instead of the beam splitter 23, and a semiconductor Laser elements 20a, 20
b, the fiber coupler 28, the reference cell 24 and the measurement cell 25 are optical fibers F ₁ , F ₂ , F ₃ and F
₄ is connected.

The semiconductor laser element 20a is modulated at the frequency f ₁ by the methane wavelength controller 21a, and the laser light emitted from the semiconductor laser element 20a enters the fiber coupler 28 via the lens 29 via the optical fiber F ₁ . The semiconductor laser device 20b is modulated at a frequency f ₂ by acetylene wavelength control device 21b, a laser beam emitted from the semiconductor laser element 21b is incident on the fiber coupler 28 in the optical fiber F ₂ via the lens 30. Mixed laser beam from the semiconductor laser element 20a and the semiconductor laser device 20b in the fiber coupler 28, the laser beam mixing irradiates an optical fiber F _3, F ₄ reference cell 22 and the measurement cell 25 through the. Reference cell 2
2 is converted into an electric signal by a photodetector 24 and input to a wavelength control device 21a for methane and a wavelength control device 21b for acetylene. The laser beam transmitted through the measuring cell 25 is converted into an electric signal by a photodetector 26. It is converted and input to the methane wavelength control device 27a.

[0033] Similar to the following foregoing various gas detecting device, the voltage of the Peltier element is controlled by the signal processing is performed for only the modulated wave methane wavelength controller 21a the frequency 51KH _Z, the temperature of the semiconductor laser element 20a Is controlled, and the oscillation wavelength is stabilized. Voltage of the Peltier element is controlled by the signal processing is performed for only the modulated wave of acetylene for the wavelength control unit 21b in the frequency 49KH _Z, the temperature of the semiconductor laser device 20b is controlled, the oscillation wavelength is stabilized.

The laser beam and the beam splitter 23 or the fiber coupler and the laser beam of the semiconductor laser device 20b which is modulated at a different frequency f ₂ to the frequency f ₁ of the semiconductor laser element 20a which is modulated in this manner at the frequency f ₁ After mixing in 28 and passing the gas in the reference cell 22, the light is received by the light receiver 24, the methane wavelength controller 21 a stabilizes the oscillation wavelength of the laser light corresponding to the frequency f ₁ , and at the same time, the acetylene wavelength by stabilizing the oscillation wavelength of the laser beam corresponding to the frequency f ₂ by the control unit 21b, prior to the reference cell 22 by the number of types of gases to be measured is required photoreceiver 24, a wavelength stability separate However, according to the present invention, the wavelength can be stabilized by one reference cell and one light receiver at the same time.

In this embodiment, the oscillation wavelength of the gas detector is stabilized. However, the present invention is not limited to this. The oscillation wavelength may be stabilized in a communication device using a laser, for example, a multiplex communication device. Good.

[0036]

As described above, in the present invention,
Mixing means for mixing the modulated laser lights emitted from the respective semiconductor lasers, a reference cell filled with a plurality of reference gases after being mixed by the mixing means, and receiving the laser light transmitted through the reference cells The light receiving means and the plurality of wavelength stabilizing means for detecting a frequency component equal to a predetermined frequency from the output of the light receiving means and stabilizing the wavelength of each semiconductor laser based on the frequency component are provided. The oscillation wavelength of each semiconductor laser of the gas detection device for simultaneously detecting various types of gases using a laser is simultaneously stabilized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of one embodiment of a multi-gas detection device according to the present invention.

FIG. 2 is a schematic configuration diagram of another embodiment of the multi-gas detecting apparatus according to the present invention.

FIG. 3 is a schematic configuration diagram of a multi-gas detection device by the present applicant.

[Explanation of symbols]

 Reference Signs List 20a, 20b Semiconductor laser element 21a Wavelength controller for acetylene 21b Wavelength controller for methane 22 Reference cell 24, 26 Receiver 25 Measurement cell 27a Detector for methane 27b Detector for acetylene 28 Fiber coupler

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-62-98235 (JP, A) JP-A-2-69639 (JP, A) JP-A-1-307640 (JP, A) JP-A-2- 291940 (JP, A) JP-A-3-4147 (JP, A) JP-A-63-165735 (JP, A) JP-A-4-326604 (JP, A) JP-A-2-208531 (JP, A) JP-A-2-122243 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 21/00-21/01 G01N 21/17-21/61

Claims (3)

(57) [Claims] 1. A semiconductor device comprising: a plurality of semiconductor lasers having different oscillation wavelengths; and a modulator for modulating a drive current of each of the semiconductor lasers at a different predetermined frequency. What is claimed is: 1. A multi-gas detector for detecting a concentration of a gas in a measuring cell by using a laser beam passed through a measuring cell, comprising: a mixing unit for mixing laser beams emitted from the respective semiconductor lasers; A reference cell in which the reference gas is sealed, a light receiving unit that receives the laser beam transmitted through the reference cell, and a frequency component equal to the predetermined frequency is detected from an output of the light receiving unit, based on the frequency component. A plurality of wavelength stabilizing means for stabilizing the wavelength of each of the semiconductor lasers. 2. The multi-gas detection apparatus according to claim 1, wherein said mixing means is a beam splitter. 3. The mixing means is a fiber coupler, wherein the fiber coupler and each semiconductor laser are connected by an optical fiber, and the fiber coupler and the reference cell are connected by an optical fiber. The multi-type gas detection device according to claim 1, wherein