JP2540670B2 - Multi-type gas detector using optical fiber - 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-type gas detector which employs a laser beam as a light source and an optical fiber as a transmission line.

[0002]

2. Description of the Related Art It is known that the presence or absence of a gas can be detected by utilizing the fact that light of a specific wavelength is easily absorbed by a certain gas. Sensing technology applying this principle is used in industrial measurement, pollution monitoring, etc. Widely used in. In addition, if this light is transmitted using an optical fiber, it is possible to remotely monitor gas.

As an example, a system capable of remotely detecting methane gas is disclosed in Japanese Patent Laid-Open No. 59-212738. In this system, since there is an oscillation line in the 1.6 μm band that is strongly absorbed by methane gas, the light from the LED light source containing this wavelength is guided to the measurement gas by the optical fiber, and the transmitted light from that gas is converted into another light. Received by the fiber and guided to the light receiving part,
The light receiving section uses a band-pass filter or the like to separate light of a wavelength absorbed by methane gas into light of a wavelength not absorbed, and obtains the concentration of methane gas from the difference in the attenuation amount of each of these two lights.

[0004]

However, in the methane detection system having such a configuration, a large number of optical components such as a half mirror and a band-pass filter are used, so that the optical system becomes complicated and the SN ratio (signal pair It is also disadvantageous in terms of noise ratio).

Therefore, a new remote gas (methane gas) detecting device using the frequency modulation method and using an optical fiber as a transmission line has been proposed (Japanese Patent Application No. 2-78498). In this detection device, the drive current of the semiconductor laser is modulated at a high frequency centering on a predetermined current value, and laser light having a modulated wavelength and intensity is oscillated. Further, a part of the laser light emitted from the semiconductor laser is used for monitoring, and the current and temperature of the laser are controlled so that the center wavelength of oscillation coincides with the predetermined position of the methane absorption line. Then, the laser beam thus stabilized and emitted is guided through an optical fiber to a gas cell containing methane gas of unknown concentration, and the light transmitted through the gas cell is received by another optical fiber and guided to a light receiving unit, where it is received. The gas concentration is obtained from the second harmonic detection signal of the laser light or the fundamental wave detection signal. According to this, extremely high SN
The ratio can detect methane gas.

However, this detector has a problem that only one kind of gas can be quantitatively measured. Usually, in order to prevent a gas explosion or the like in an environment where a special gas is used, there are several kinds of gases to be monitored. In this case, in the conventional method, it is necessary to separately sense these plural kinds of gases, so that another detection device must be newly configured and the plural devices must be used in parallel.

The present invention has been made in view of the above circumstances, and an object thereof is to detect various gases using a novel optical fiber capable of measuring many kinds of gases using one optical transmission line. To provide a device.

[0008]

In order to achieve the above object, the present invention provides a plurality of semiconductor lasers each having a single gas absorption line as an oscillation line for a specific gas to be measured, and each of the semiconductor lasers is prepared. The drive current of the semiconductor laser is modulated at a high frequency by the laser driving means, the modulated laser light from each semiconductor laser is combined into one light by the branch coupler, and then the test cell is transmitted through the outgoing optical fiber. Lead to. Then, the light transmitted through the test cell is received by the optical fiber for the return path and guided to the photodetector, and the specific component in the detection output is phase-sensitively detected by the measuring means, and each specific gas existing in the test cell is detected. It is configured to simultaneously measure the concentration. Here, it is preferable to change the modulation frequency of the laser drive current. Further, it is preferable that the end face of each optical fiber be obliquely polished.

[0009]

By preparing a plurality of semiconductor lasers for various kinds of specific gases and combining the lights from these semiconductor lasers into one light by the branch coupler, one set of the optical fiber for the outward path and the test cell Also, the concentrations of various specific gases can be simultaneously measured by using the return optical fiber. Moreover, when the modulation frequencies of the semiconductor lasers are changed, a plurality of kinds of gases can be easily detected only by obtaining the phase sensitive detection signal for each modulation frequency from the detection output of the transmitted light. If the end face of the optical fiber is obliquely polished, it is possible to suppress internal interference or noise due to the returning light and stabilize the signal.

[0010]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

The present invention is directed to the remote sensing of many types of gases. When a silica-based optical fiber is used as an optical transmission line to perform measurement at a remote location, the optical fiber
It exhibits low loss characteristics around 0.8 μm to 1.7 μm. Therefore, a gas having an absorption line in this band is suitable as a gas to be detected, and in this example, methane gas and acetylene gas were targeted. Methane is 2 as an absorption line
ν ₃ band Q (6) line (1.6659 μm)
_{A 1} + ν ₃ band P (13) wire (1.5328 μm) was used. FIG. 1 shows an embodiment of a gas detection device for the purpose of simultaneous detection of methane gas and acetylene gas.

In the figure, 1a is a methane gas absorption line (1.
A semiconductor laser that oscillates light in the 6659 μm band) and 1b is a semiconductor laser that oscillates light in the acetylene gas absorption line (1.5328 μm band). Also, 2a and 2b are semiconductor lasers 1.
Collimating lenses 3a and 3b for collimating laser beams emitted forward from a and 1b, respectively, are isolators 4a and 4b for blocking return light from the subsequent optical system.
Is a condenser lens for coupling the laser light from the isolators 3a and 3b to the optical fibers 5a and 5b.

Reference numeral 11 denotes a branching coupler for coupling the respective laser beams transmitted through the optical fibers 5a and 5b, and here, a fiber type in which two optical fibers are heated and drawn in close proximity to each other. Used. 6 is a silica-based outward optical fiber that transmits the light combined by the branch coupler 11, and 9 is a test cell containing methane gas and acetylene gas of unknown concentration, inside which the laser light emitted from the fiber 6 is parallel. A collimating lens 10a for making light,
A collimator lens 10b for condensing the laser light transmitted through the cell 4 is provided. Reference numeral 7 is a silica-based return optical fiber that transmits the transmitted light condensed by the collimator lens 10b, 12 is a photodetector such as a pin photodiode for detecting the intensity of the laser light transmitted through the optical fiber 7, 20 Is an amplifier for amplifying the electric signal from the detector 12. Here, the above-mentioned optical fibers 5a, 5
The end faces of b, 6 and 7 are obliquely polished so that the returning light is reduced and an interference system does not occur inside thereof.

Reference numerals 13a and 13b are reference cells containing known concentrations of methane gas and acetylene gas, and 14a and 14b.
b is a lens for condensing the laser light emitted backward from the semiconductor lasers 1a and 1b, and 15a and 15b are reference cells 13.
p for detecting the intensity of the backward light passing through a and 13b
Photodetectors such as in-photodiodes, 19a, 19b
Is a wavelength control device that monitors the wavelength of the light source from the output components of the photodetectors 15a and 15b and adjusts the temperature of the Peltier elements 16a and 16b. The wavelength stabilizing means is configured as described above. 17a, 17b are semiconductor lasers 1a, 1
A laser drive device (laser drive means) that supplies a drive current modulated to b with a predetermined high frequency changes the modulation frequency of the drive current for each of the lasers 1a and 1b.
Further, 18a and 18b are signal processing devices (measuring means) for calculating the methane gas and acetylene gas concentrations in the test cell 9 from the specific component (that is, the second harmonic component of the above-mentioned modulation frequency) in the output of the photodetector 12. is there.

Next, the operation of the above apparatus will be described.

For the modulation frequencies ω ₁ and ω ₂ for the semiconductor lasers 1a and 1b, it can be expected that the measurement accuracy is improved as the least common multiple thereof is increased. Therefore, the modulation frequency ω ₁ of the laser 1a for methane gas is ω ₁ = 51 kHz,
The frequency ω ₂ of the laser 1b for acetylene gas is ω ₂ = 4
I chose 9 kHz.

In the measurement, first, a methane gas mixed gas containing nitrogen as a balance gas is introduced into the reference cell 13a, and an acetylene gas mixed gas containing nitrogen as a balance gas is introduced into the other reference cell 13b. Then, the laser driving device 17
The semiconductor lasers 1a and 1b are driven by a and 17b.
The driving devices 17a and 17b are a constant current source that supplies a stable current to the semiconductor lasers 1a and 1b, an oscillator that oscillates modulation signals of frequencies ω ₁ and ω ₂ , respectively, and 2s of frequencies 2ω ₁ and 2ω ₂ from the modulation signals. It is composed of a frequency divider for producing a harmonic signal, and a bias current from a constant current source is superposed with a modulation current of frequencies ω ₁ and ω ₂ and supplied to the semiconductor lasers 1a and 1b. That is, each laser 1a, 1
The drive current of b is modulated with a predetermined amplitude and a predetermined frequency with a predetermined bias current as the center, whereby the laser light whose wavelength and intensity are modulated is oscillated.

Of the laser light thus oscillated, the light emitted to the rear side is passed through the reference cells 13a and 13b and detected by the photodetectors 15a and 15b. Detector 15
The output signals obtained from a and 15b are input to wavelength control devices 19a and 19b, respectively, and the Peltier devices 16a and 16b are temperature-controlled by signal processing there, and the temperatures of the semiconductor lasers 1a and 1b are controlled. Wavelength control device 19
a, 19b may lock-in amplifier, an integrator is constituted by a Peltier power source or the like, the detector 15a, respectively modulation frequencies omega ₁ from the output signal obtained from 15b, the fundamental wave phase-sensitive detection signal for omega ₂ Is calculated, and the temperatures of the semiconductor lasers 1a and 1b are adjusted so that the value becomes zero. As a result, the oscillation spectra of the lasers 1a and 1b are
It is stabilized at the center of the absorption line of each detection gas.

When the frequencies are stabilized at the centers of the absorption lines of the methane gas and the acetylene gas in this way, the laser beams emitted forward from the semiconductor lasers 1a and 1b are collimated by the collimator lenses 2a and 2b to form an isolator. After passing through 3a and 3b, a condenser lens 4a,
It is put into the optical fibers 5a and 5b by 4b. The respective laser lights transmitted through these optical fibers 5a and 5b are combined by a branch coupler 11 and transmitted through one outward optical fiber 6 and sent to the test cell 9.

In the test cell 9, the combined laser light is propagated in the air, and the light from the laser 1a is absorbed by the methane gas and the light from the laser 1b is absorbed by the acetylene gas. Fiber 7
Can be received by When the air propagation distance is small, as shown in the figure, a self-focus microlens or the like can be used as the collimator lenses 10a and 10b. It is necessary to take measures such as using a single sheet and taking a large light receiving surface.

The laser light transmitted through the return optical fiber 7 is converted into an electric signal by the photodetector 12, and the amplifier 20
The signal is amplified by and then sent to the signal processing devices 18a and 18b. The signal processing devices 18a and 18b are two lock-in amplifiers that perform fundamental wave phase sensitive detection and second harmonic phase sensitive detection for the corresponding modulation frequencies ω ₁ and ω ₂ , respectively, and a divider that obtains the output ratio of these amplifiers. , A recorder for recording the output of the divider. And 2
Obtain the concentration of methane gas or acetylene gas in the measurement atmosphere from the value obtained by dividing the overtone detection signal by the fundamental wave detection signal.

Now, if the center wavelengths of the light oscillated from the semiconductor lasers 1a and 1b are made to coincide with the centers ω ₀ of the absorption lines of methane gas and acetylene gas respectively, as described above, the second harmonic detection signal and the fundamental wave detection signal Ratio of P (2ω) / P (ω) max
Indicates the peak value R,

[0023]

[Equation 1]

It is expressed as follows. Here, I ₀ is the center intensity of each laser output, ΔI is the intensity modulation amplitude, ΔΩ is the frequency modulation amplitude, α (ω ₀ ) is the absorption coefficient at the absorption line center ω ₀ of the specific gas, and γ is its α ( half-value width 2γ of ω ₀ ),
c · l is the product of the concentration c of the specific gas and the light propagation distance l in the cell 9. As described above, the value R of P (2ω) / P (ω) max is proportional to the product of the concentration c of the specific gas and the propagation distance l of the light in the measurement atmosphere. The concentration c of the specific gas can be calculated by the equation 1.

As described above, according to this embodiment, the driving currents of the semiconductor lasers 1a and 1b are modulated at a predetermined high frequency, and the laser light whose wavelength and output are modulated is supplied to the test cell 9 containing methane gas and acetylene gas. Since the fundamental wave detection signal or the second-harmonic detection signal is obtained from the detection signal of the transmitted light, the concentrations of methane gas and acetylene gas in the cell 9 can be detected at a very high SN ratio.

Moreover, for transmission of laser light from the lasers 1a and 1b to the cell 9 and transmission from the cell 9 to the photodetector 12,
By using the optical fibers 6 and 7, respectively, the specific gas can be quantitatively measured or monitored at a remote place. Further, by providing two semiconductor lasers 1a and 1b having different oscillation wavelengths corresponding to methane and acetylene gas to be measured, and combining the lights from these lasers 1a and 1b into one light by the branch coupler 11. , Outbound optical fiber 6,
The concentration of these gases can be detected individually and simultaneously by using a set of optical transmission system consisting of the test cell 9 and the return optical fiber 7. Therefore, the manufacturing cost can be significantly reduced and the work of laying the optical transmission line can be omitted as compared with the conventional case where a plurality of optical transmission systems are provided according to the type of gas.

In the above embodiment, two types of signal processing devices 18a and 18b for methane gas and acetylene gas are used.
However, it is also possible to monitor the concentration of both gases by obtaining detection signals for the respective modulation frequencies ω ₁ and ω ₂ by the one signal processing device 18a, 18b. For example, if you use a general-purpose device as a lock-in amplifier,
Since automatic measurement by a computer becomes possible, if a computer is added to one of the signal processing devices 18a and 18b to take an automatic measurement form and the processing devices 18a and 18b are assigned to each gas measurement in a time division manner, the other The processing devices 18b and 18a can be omitted.

In the above embodiment, the laser light used for wavelength stabilization is the rear light of the semiconductor lasers 1a and 1b. However, a part of the front light is reflected by a half mirror, an optical fiber type branch coupler or the like. It is also possible to use.

Further, in the above embodiment, only one of the output ports of the branch coupler 11 is used as the light wave for detection, but the light wave from the other port is also used to detect the specific gas at multiple points. May be. Also, as the branch coupler 11,
Although the fiber type is used, a waveguide type, a half mirror type and the like are also applicable. The point is that the output from the two fibers may be input to one fiber, or conversely, the output from the one fiber may be input to the two fibers.

FIG. 2 shows another embodiment of the present invention. In this embodiment, visible light and far infrared light are used as wavelengths of light used for gas detection, and a hydrocarbon-based gas and a nitrogen compound-based gas are simultaneously detected. The configuration of the light receiving part of is shown.

As shown in FIG. 3, the currently used pin photodiode has Si for visible and near-infrared light, and Ge or InGaAs at a wavelength of 1 μm or more.
Are used respectively, and it is necessary to use the pin photodiode properly for the light wavelength.

Therefore, in this embodiment, one pin
It is not possible to deal with this with only the photodiode, and the end portion on the light receiving side of the return optical fiber 7 is connected to the fiber 2 via the branch coupler 21.
3a and 23b, and a Si pin photodiode 22a is provided at the output end of one fiber 23a, and an InGaAs pin photodiode 22b is provided at the output end of the other fiber 23b. Note that these pi
The signal processing devices 18a and 18b similar to those in the above-described embodiment are connected to the outputs of the n photodiodes 22a and 22b, respectively.

[0033]

As described above, according to the present invention,
The semiconductor laser drive current is frequency-modulated, the laser light whose wavelength and output are modulated is passed through the test cell containing the specific gas, and the specific component in the detection signal of the transmitted light is phase-sensitive detected. The specific gas can be detected from the signal with a very high SN ratio. In addition, multiple semiconductor lasers with different oscillation wavelengths were prepared according to the type of gas to be measured, and the light from these lasers was combined into one light by a branch coupler, so one set of multiple types of gas was used. It can be detected at the same time only by using the optical transmission line. Furthermore, since an optical fiber is used for this optical transmission line, it is possible to remotely monitor a specific gas.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a multi-gas detector using the optical fiber of the present invention.

FIG. 2 is a block diagram showing a light receiving portion of another embodiment of the present invention.

FIG. 3 is a diagram showing a spectral sensitivity characteristic of a pin photodiode.

[Explanation of symbols]

 1a, 1b Semiconductor laser 6 Optical fiber for forward path 7 Optical fiber for return path 9 Test cell 11 Branch coupler 12 Photodetector 17a, 17b Laser drive device (laser drive means) 18a, 18b Signal processing device (measurement means) 19a, 19b Wavelength control device (wavelength stabilizing means)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shin Osawa 5-1-1, Hidaka-cho, Hitachi-shi, Ibaraki Inside the Electric Wire Research Laboratory, Nitrate Electric Cable Co., Ltd. (56) Reference JP-A-3-15742 (JP, A) ) JP-A-63-171328 (JP, A) JP-A-1-295135 (JP, A) Actually developed JP-A-62-34353 (JP, U) Special Table 4-501007 (JP, A)

Claims (3)

(57) [Claims] 1. Two or more kinds of semiconductor lasers having different oscillation wavelengths, a laser driving means for modulating a driving current of each of these semiconductor lasers with a predetermined amplitude and a predetermined frequency centered on a predetermined current value, and each of the above laser driving means. Wavelength stabilizing means for stabilizing the central wavelength of the laser light oscillated from the laser at a predetermined position of the absorption line of the corresponding specific gas, a branch coupler for joining the laser light emitted from each laser, and this coupler The optical fiber for the outward path for transmitting the light combined by the test cell, the test cell containing two or more kinds of specific gases to be measured, and the light emitted from the optical fiber for the outward path and transmitted through the test cell are transmitted. The optical path for the return path, the photodetector that converts the light intensity from the optical path for the return path into an electrical signal, and the phase sensitive detection of the specific component in the output of this detector A multi-gas detector using an optical fiber, comprising a measuring means for quantitatively measuring each specific gas. 2. A multi-type gas detection apparatus using an optical fiber according to claim 1, wherein frequencies for modulating the drive current of the semiconductor laser are changed. 3. The multi-gas detection device using an optical fiber according to claim 1, wherein the end face of each optical fiber that transmits the laser light is obliquely polished.