JPH07159315A - Optical gas sensor - Google Patents

Abstract

PURPOSE:To provide an optical gas sensor which can detect many kinds of gases with high sensitivity. CONSTITUTION:The optical gas sensor is provided with a plurality of light sources 1a to 1c which oscillate laser beams whose wavelength and intensity coincide with, absorption spectra of a plurality of detection gases according to a driving current and a temperature, ' a gas cell part 6 which is arranged so as,to obtain largest optical coupling at light wavelengths coinciding with the respective absorption spectra of the plurality of detection gases and in which a plurality of optical systems constituted of optical paths which are different from those in conventional cases have been built in and an optical transmission line 7 by which the light sources 1a to 1c are connected to the gas cell part 6 and by which the gas cell part 6 is connected to a photodetector 9. In addition, the optical gas sensor is provided with a plurality of light-uniting means 15 and one light-branching means 16 by which beams of light from different light sources are united to one optical transmission line and by which a beam of light from one optical transmission line is branched into a plurality of optical transmission lines and with the photodetector 9 and a detection device 11 at the end of the optical transmission line which connects the gas cell part 6 to the photodetector 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for optically detecting gas using an optical fiber.

[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 a laser beam of a specific wavelength is easily absorbed by a certain type of gas. Optical sensing technology applying this principle is used in industrial measurement and pollution monitoring. Widely used in. If this laser light is transmitted using an optical fiber, remote gas detection becomes possible.

Prior to the description of the embodiments of the prior art, a method of optically detecting gas will be described. FIG. 6 shows that λ = 1.
The methane infrared absorption spectrum in the vicinity of 65 μm is shown. As shown in this figure, methane gas has a property of absorbing light of a specific wavelength well. This absorption degree depends on the product of the gas concentration and the light absorption length, and is represented by the following equation.

T = exp (-α · c · L) where T: transmittance α: absorption coefficient c: concentration L: spatial optical path length Using the above equation, if the wavelength used and the spatial optical path length are known, the transmission By determining the rate, the gas concentration can be determined.

FIG. 5 shows an embodiment of a conventional optical gas sensor using an optical fiber according to this principle. A semiconductor laser is used as the light source 1. This laser has a single spectrum and the oscillation spectrum width is narrower than the gas absorption spectrum width. When a constant current is supplied to this laser by a constant current source 2 and the temperature control element 3 is controlled by a power source 4 to continuously change the laser temperature, the oscillation wavelength and the oscillation intensity are also changed continuously. Therefore, sweeping at an appropriate temperature allows sweeping on the gas absorption line. This laser light is made incident on the light transmission path 7a by the condenser lens 5 and is transmitted to the gas cell portion 6 in the distance. In the gas cell section 6,
The light incident through the optical transmission line 7a is a space propagation lens 8
The light propagates in space between a and 8b, is condensed on another optical transmission path 7b that faces it, and is transmitted to the light receiver 9. After being converted into an electric signal by the amplifier 10, it is put into the Y-axis component of the XY recorder (detection device) 11. In order to input the temperature of the laser, which is sequentially changed, to the X-axis component of the XY recorder, the output from the temperature measuring element 12 attached to the laser is converted into an electric signal by the amplifier 13 and then input to the XY recorder.

FIG. 4 shows a result obtained by sweeping the wavelength of the light source when the gas cell portion contains methane and detecting by the light receiver. In order to obtain the concentration, the baseline is determined from the change in the amount of light near the absorption line.

The transmittance T = (P ₁ / P ₀ ) is calculated with P _{0 being} the light output value of the baseline at the absorption wavelength center wavelength and P ₁ being the light output value obtained by the actual photodetector.

The density c is obtained from the above equation T = exp (−α · c · L).

The process of will be described.

By the way, when the sensor simultaneously detects gas concentrations of acetylene, carbon dioxide, etc. in addition to methane, it is carried out as follows.

Since each gas has an absorption spectrum peculiar to each gas similarly to methane, a laser that matches the absorption spectrum is prepared and multiplexed by an optical fiber incident multiplexer from each laser. Then, each laser is sequentially driven, the change in transmittance before and after the gas absorption line is measured in the same manner as in FIG. 4, and the gas concentration is measured from the change in the light amount at the peak.

[0012]

However, in order to perform high-sensitivity detection, it is necessary to maximize the coupling of the optical space propagation section of the gas cell section to improve the S / N ratio.

In order to improve the S / N ratio, the spatial optical path length can be lengthened and the efficiency of optical coupling can be increased. From a practical point of view, the spatial optical path length is from a few cm to a dozen or less cm. In order to improve the S / N ratio within this range, high-efficiency coupling of the spatial light transmission line becomes a problem. The incorporation of the high-efficiency optical coupling system not only increases the signal intensity S value, but also reduces the reflected light generated by the optical system, and as a result, it is possible to perform low-noise measurement with small interference noise.

However, the optical coupling using a lens has wavelength dependency due to wavelength aberration, and optimal coupling cannot be performed for a wide wavelength. Therefore, as described in the prior art, if various gases are detected and the wavelength used at that time has a range, all the gases cannot be detected at the maximum S / N, and there are cases where minute gases cannot be detected.

The present inventor has noticed that, when a spatial optical path length of several cm or more is constituted by two sets of opposing lenses and an optical transmission path, the optical arrangement necessary for obtaining the maximum optical coupling efficiency differs depending on the wavelength used. It was

[0016] Therefore, an object of the present invention is to provide an optical gas sensor which was invented in view of such circumstances and which can detect various kinds of gases with high sensitivity.

[0017]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a plurality of light sources that oscillate laser light having wavelengths and intensities that match absorption spectra of a plurality of detection gases according to drive currents and temperatures, and the plurality of detections. A gas cell part containing a plurality of optical systems composed of optical paths arranged so as to obtain maximum optical coupling at a light wavelength that matches each absorption spectrum of gas, a light source and a gas cell part, as well as a path for laser light. , And an optical transmission line connecting the gas cell unit and the light receiver, and light having wavelengths from different light sources are merged into one optical transmission line, or a plurality of optical transmission lines for each wavelength of light from one optical transmission line A plurality of optical multiplexing means and one optical demultiplexing means for splitting the light into a plurality of light sources, and a light receiver and a detector are provided at the end of an optical transmission path connecting the gas cell part and the light receiver. Optical gas detector for multi-type gas detection Ssens.

The multi-type gas described above, characterized in that an optical polarization rotation means for causing 45 ° polarization rotation is provided in the middle of the optical path arranged to obtain the maximum optical coupling of the gas cell section. Optical gas sensor for detection.

The optical transmission line has an optical polarization maintaining mechanism, and optical analyzers are arranged on both sides of the optical polarization rotating means with a shift of 45 ° from each other and emitted from the optical transmission line on the input side. The optical gas sensor for detecting various kinds of gases as described above, characterized in that the polarized light of the light and the direction of the input side analyzer are combined.

The multi-type gas detection device according to claim 1, wherein the optical path arranged so as to obtain the maximum optical coupling of the gas cell section includes a sensor that reacts with gas to change a light absorption coefficient. Optical gas sensor.

Is achieved by

[0022]

EXAMPLES Example 1 FIG. 1 is a diagram showing an example of the optical gas sensor of the present invention. In this example, a case where three kinds of gases of acetylene, methane, and carbon dioxide are detected as the detection gas is shown. The absorption spectrum wavelength of each gas is λ
_C2H2 = 1.53 μm, λ _CH4 = 1.65 μm, λ _CO2 =
It is 1.439 μm. A semiconductor laser (DFB-LD) is used as each of the light sources 1a, 1b, and 1c, and one that oscillates each wavelength laser is provided, and three are provided in total. Each laser includes a condenser lens 5 for optically coupling with an optical fiber, temperature control elements 3a, 3b, 3c for controlling the temperature, temperature measuring elements 12a, 12b, 12c for measuring the temperature, and the like. . Current driving device (not shown) for controlling each light source 1a, 1b, 1c, temperature measuring elements 12a, 12
b, 12c and the temperature control elements 3a, 3b, 3c are connected to the respective light sources 1a, 1b, 1c, but the respective light sources 1a, 1b, 1c perform sequential control and do not perform simultaneous control. One control device is switched by the switching 14. The laser temperature value is XY
It is input to the X axis of the recorder 11.

The laser light oscillated from one of the light sources is combined by the optical combining means 15a and transmitted to the gas cell section 6 through one optical transmission line 7a '. The gas cell unit 6 is a spatial transmission line D that is optically adjusted for each wavelength used.
_C2H2 , D _CH4 , and D _CO2 are arranged. Optical transmission line 7
The laser light from a'is located in front of the gas cell by the optical demultiplexing means 1
The spatial transmission lines D _C2H2 , D _CH4 , D
_It is distributed to either _CO2 . In the middle of each space transmission line, an optical polarization rotation means 17 adjusted so as to cause 45 ° polarization rotation for each transmission wavelength is provided. When this optical polarization circuit means 17 is arranged, the transmitted light is transmitted once. On the other hand, the reflected light which is reflected by the incident light side lens and the output side lens and progresses behind the transmitted light is transmitted three times. As a result, since the optical polarizations of the two have changed by 90 °, the optical interference between the two is minimized, and the interference noise is reduced. This optical polarization rotation means 17 is composed of yttrium-iron-garnet (YI
G) A crystal having a cylindrical structure of a permanent magnet was used.
The light transmitted through the optical polarization rotating means 17 is condensed on the other optical transmission lines 7 _b-1 , 7 _b-2 , 7 _b-3 which face each other. The optical transmission line emitted from the spatial transmission line is 1 by the optical multiplexing means 15b.
It joins two optical transmission lines 7 _{b '} . As a result, the light from the C ₂ H ₂ laser is 7 _a-1 → 7 _{a '} → 7 _{a' -1} → D _C2H2 →
It is transmitted as 7 _b-1 → 7 _{b '} . The light from the laser for CH ₄ is 7 _a-2 → 7 _{a '} → 7 _{a' -2} → D _CH4 → 7 _b-2 → 7 _{b '}
Is transmitted. The light from the CO ₂ laser is 7 _a-3 → 7 _{a '}
→ 7 _{a '-3} → D _CO2 → 7 _b-3 → 7 _b' is transmitted. The laser light that has passed through one optical transmission path is received by the light receiver 9.
The signal from the light receiver 9 is input to the Y-axis of the XY recorder 11 through the amplifier 10.

When a constant current is applied to each gas detecting laser and the laser temperature is changed in a series of operations, the same relationship as in FIG. 4 can be obtained, and the concentration of each gas can be detected.

(Embodiment 2) When an optical transmission line having an optical polarization maintaining mechanism is used, an optical isolator (optical analyzers 45 on both sides of the optical polarization rotating means 17) is provided in the middle of the spatial optical path. It may be arranged such that the polarized light of the light emitted from the optical transmission line on the input side and the direction of the input side analyzer are aligned together).

(Embodiment 3) In addition, as shown in FIG. 3, an optical path different from the conventional one arranged so as to obtain the maximum optical coupling of the gas cell portion of the present invention is a sensor whose absorption coefficient is changed by gas absorption. It is also possible to use. Hydrogen as a detection gas has no absorption wavelength in the long wavelength band. In that case, the WO ₃ thin film 2 which reacts with hydrogen to change the optical absorption coefficient 2
0 is used. Instead of the spatial light transmission path, an optical waveguide 18 and a WO ₃ thin film 20 arranged so as to cover the optical waveguide 18.
And the Pd thin film 21 are provided, and the concentration is obtained from the intensity of the light passing through the optical waveguide 18. The structure of this thin film is simply shown by the following: TiN on the LiNbO ₃ substrate 19
Is thermally diffused to form the optical waveguide 18, and then LiNbO
₃ WO ₃ thin film 20 is vacuum-deposited on the substrate 19 to form Pd.
The thin film 21 is sputtered. The optical transmission line is LiNbO
₃ Adhere to the substrate 19. When hydrogen gas is filled around the thus configured hydrogen sensor, the Pd thin film 21 acts as a catalyst to color the WO ₃ thin film 20. As a result, the ratio of the evanescent wave of the light passing through the optical waveguide 18 in the element increases and the amount of transmitted light decreases.

This hydrogen sensor is used in the gas cell section 6 described above.
It is also possible to detect hydrogen by incorporating it in place of one of the space transmission lines.

[0028]

The optical gas sensor of the present invention enables high-density detection of many kinds of gases.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a basic configuration of an embodiment of the present invention.

FIG. 2 is an explanatory view showing rotation of polarization of transmitted light and reflected light which pass through the optical polarization rotating means of the present invention.

FIG. 3 is a side view showing the structure of a hydrogen gas sensor using another embodiment of the present invention.

FIG. 4 is a graph showing signal waveforms obtained in the related art and the present invention.

FIG. 5 is a circuit diagram of a basic configuration showing an example of a conventional technique.

FIG. 6 is a graph showing an infrared absorption spectrum of methane gas.

[Explanation of symbols]

1, 1a, 1b, 1c Light source 2 Constant current source 3, 3a, 3b, 3c Temperature control element 4 Power supply 5 Condensing lens 6 Gas cell section 7a, 7b, 7a ', 7b'7a _-1 , 7a _-2 , 7 _a-3 ,
7 _{a '-1} , 7 _{a' -2} , 7 _{a '-3} , 7 _b-1 , 7 _b-2 , 7 _b-3
Optical transmission line 8, 8a, 8b Space propagation lens 9 Light receiver 10 Amplifier 11 XY recorder (detection device) 12, 12a, 12b, 12c Temperature measuring element 13 Amplifier 14 Switching 15a, 15b Optical multiplexing means 16 Optical demultiplexing means 17 Optical polarization rotating means 18 Optical waveguide 19 LiNbO ₃ substrate 20 WO ₃ thin film 21 Pd thin film

Claims (4)

[Claims] 1. A plurality of light sources that oscillate laser light having wavelengths and intensities that match the absorption spectra of a plurality of detection gases according to drive current and temperature, and light that matches the absorption spectra of each of the plurality of detection gases. A gas cell part containing a plurality of optical systems composed of optical paths arranged so as to obtain maximum optical coupling at a wavelength, and a light path connecting laser light and connecting a light source and a gas cell part, and a gas cell part and a light receiver. A plurality of optical multiplexing means for merging light having wavelengths from different light sources into one optical transmission path or diverting light from one optical transmission path into a plurality of optical transmission paths for each wavelength An optical gas sensor for detecting multiple types of gas, comprising: a light demultiplexing means; 2. The optical polarization rotating means for causing 45 ° polarization rotation is provided in the middle of the optical path arranged to obtain the maximum optical coupling of the gas cell section. Optical gas sensor for multi-type gas detection. 3. The optical transmission line has an optical polarization maintaining mechanism, wherein optical analyzers are arranged on both sides of the optical polarization rotating means with a shift of 45 ° from each other, and the optical transmission line on the input side is separated from the optical transmission line. The optical gas sensor for detecting various kinds of gases according to claim 2, wherein the polarization of the emitted light and the direction of the input side analyzer are combined. 4. The multi-type gas according to claim 1, wherein the optical path arranged so as to obtain the maximum optical coupling of the gas cell section includes a sensor that reacts with the gas to change the light absorption coefficient. Optical gas sensor for detection.