JPH03215730A - Gas detector using optical fiber - Google Patents

Abstract

PURPOSE:To increase the mechanical strength, to improve the accuracy and to expand the gas detection range by forming the sensor and of an optical fiber member which is exposed to inspection gas by using only a core and forming other light propagation parts by providing a core and a clad. CONSTITUTION:The sensor part of the optical fiber member 20 which is exposed to the inspection gas along the longitudinal direction is formed of only the core and other light propagation parts are formed while equipped with the core and clad. A gas cell 30 is arranged in a state incorporating the sensor part 22 of the member 20. The inspection gas is admitted to the gas cell 30 through an entrance pipe 31 and guided out through an exist pipe 32. Laser beam which is propagated in the member 20 is emitted from the right end of the member 20 to reach a germanium photodetector 42 through a V-groove connection part 40 and a quartz dummy optical fiber 41. The photodetector 42 outputs a detection signal corresponding to the intensity of the photodetected laser beam. The detection signal is amplified by an amplifier 50 and inputted to a computer 51.

Description

[Detailed Description of the Invention] Industrial Applications The present invention relates to gas detection using an optical fiber, which detects gas concentration by utilizing the fact that light propagating along the optical fiber is absorbed by a test gas. It is related to the device.

Conventional technology> Conventional technology for detecting gas concentration using an optical fiber. 1
2. & 6 P. 437-439 (1987)
There is one shown in J. The outline of the experimental apparatus shown in this paper will be explained with reference to FIG. In the figure, the optical fiber 1 has a core diameter of 50 μm, a core diameter of 125 μm, and a total length of 40 cm or shorter, excluding the sensor portion 1a. The sensor portion 1a is made thinner by heating and stretching, and has a diameter of 1.8 to 7 μm and a length of 5 μm.
~10m. Note that the sensor portion 1a also includes a core and a cladding.

Light output from laser oscillation Wi2 (wavelength 3.392μ
m) is guided by mirrors and lenses, enters the -#4 (right end in the figure) of the optical fiber 1, propagates through the optical fiber 1, exits from the other end (left end in the figure), and passes through the lens. Scattered into the optical sensor 3. When light propagates in the optical fiber 1, most of the light waves travel through the core and some through the cladding.Furthermore, some of the light waves propagate through the external region other than the core and cladding. In some situations, this light wave is called an evanescent wave.Tail. Proceed with that.

The optical sensor 3 generates a photocurrent according to the intensity of the received light, this photocurrent is amplified by the amplifier 4, and the photocurrent value (the intensity of the received light) is recorded by the recorder 5.

The gas mixer 6 combines methane gas and nitrogen gas in a required ratio, and passes the mixed gas through the duct 7 to the cenosa portion 1a.
burst into speech. For this reason, the Senosa part 1al should be surrounded by a mixed gas atmosphere.

When the sensor portion 1& is exposed to the mixed gas, the optical fiber 1
As the light propagates along, it is absorbed by the methane gas. Therefore, the intensity of light propagating along the optical fiber 1 decreases.

In this device, by changing the concentration of methane gas, the intensity of the received light changed according to the rate of change. Therefore, conversely, by detecting the intensity of the received light, the methane gas concentration of the atmosphere to which the Ceresa portion 1a is exposed can be detected. In addition, it is possible to detect at least 1
Frequency: 1%.

11211>By the way, in the above-mentioned prior art, the outer diameter of the sensor portion 1a is 1.8 to 7.
Since it has an extremely small diameter of μm, there is a drawback that the sensor portion 1a is easily broken by external force.

Also, since a large optical transmission loss occurs in the sensor portion 1a, the sensor portion 11! If a number of sliding doors were formed, the transmission loss would become extremely large, making concentration detection impossible. ? Only one sensor portion can be formed, and the range in which gas concentration can be detected is limited to the area around the sensor portion 1a, making it difficult to detect gas concentration in a wide area.

SUMMARY OF THE INVENTION In view of the above-mentioned prior art, it is an object of the present invention to provide a gas detection device using an optical fiber that has low mechanical strength, high accuracy, and can widen the detection range. .

<s! Means to decide I> Above! ! The structure of the present invention that solves the problem of the weir is that among the longitudinal parts, the sensor part exposed to the test gas is formed by only the core, and the other light propagation parts are formed by an optical fiber member having a core and a cladding. A light incidence part that inputs a test light into one end of the optical fiber member; The present invention preferably includes a light receiving section that detects light intensity, and a determining section that determines the concentration of the test gas based on the light intensity that increases at the light receiving section.

Effect> In the present view, a portion of the test light propagating along the optical fiber member is absorbed by the test gas in the sensor portion. Since the degree of absorption of the test light corresponds to the concentration of the test gas, the concentration of the test gas can be determined based on the luminous intensity of the test light that has propagated through the optical fiber member.

<Example> Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 1 shows a filter gas detection device according to a fifth embodiment of the present invention.

This gas detection device W1 detects the concentration of methane gas. In the same figure, the semiconductor laser oscillation error 10 has a wavelength of 1.66
A μm laser beam (inspection light) is output, and this laser beam is input to the left end of the optical fiber member 20 via the quartz-based dummy optical fiber 11 and the V-groove connection portion 12. The wavelength (1.66 μm) of this laser beam is set to double the infrared vibrational absorption wavelength of methane gas. With this setting, when the laser beam travels through methane gas, a portion of the laser beam is absorbed by the methane gas.

The optical fiber member 20 is composed of light propagation parts 21 and 23 and a sensor part 22.

As shown in FIG. 2, which is an enlarged view, the light propagation portion 21,
23 is a normal silica-based optical fiber, each of which is made of SiO
Cores 21a and 23a made of two glasses and fluorine-doped Si
It is formed of claddings 2lb and 23b made of 02 glass and coatings 21c and 23c. Ko121a
, 23m and the refractive index difference C between the claddings 21b and 23b
0.7%, core 2 1 m? The diameter of 2 3 m is 100 Atrn, the diameter of cladding 2lb, 23b is 140 μm, the diameter of coating 21c, 23c is 2
It is 50 μm. On the other hand, the sensor portion 22 should consist only of a core made of Sin2 glass. Sensor part 22
has a diameter of i 0 0, qm and a core of 21 m,
It is equal to the diameter of 23a, and its length is 20 mm. When the length of the sensor portion 22 is 20 cITI, when a laser beam (wavelength 1.66 μm) propagates through it, there is an optical loss;
It has a relatively low loss of 30 KB/km. Note that when the length of the sensor portion 22 is increased, the loss increases, but the sensitivity of gas concentration detection improves. Therefore, the optimum length of the sensor portion 22 is determined depending on the inspection purpose. This sensor portion 22 is fabricated by drawing an SIO2 glass rod as a bare fiber. Both ends of the sensor portion 22 are connected to the cores 21m and 21m of the light propagation portions 21 and 23, respectively.
23a by fusion splicing.

The gas cell 30 is connected to the sensor portion 22 of the optical fiber member 20.
It is arranged so that it contains inside. This gas cell 30
Methane gas (test gas) is introduced into the gas cell 30 through an inlet pipe 31, and the gas inside the gas cell 30 is led out through an outlet pipe 32.

The laser beam that has propagated through the optical fiber member 20 is emitted from the right end of the optical fiber member 20, and then reaches the groove connecting portion 40.
The light then reaches the germanium photodetection plate 42 via a quartz-based dummy optical fiber 41. The germanium light detection v#42 outputs a detection signal according to the intensity of the received laser light. This detection signal is amplified by an amplifier 50 and input to a computer 51.

The mode and results of a gas-rich guest detection experiment conducted using the gas detection device of this example having the above-described configuration will now be described. During this experiment, the semiconductor laser oscillator 10
Laser light (wavelength: 1.66 μm) is output from the optical fiber member 20 and transmitted through the optical fiber member 20. Laser light is sensor part 2
2, a portion of the laser beam propagates in a state where it seeps out around the sensor portion 22, that is, in the state of an evanescent wave. The gas cell 30 was initially filled with air, and the intensity of the laser light received by the germanium photodetector 42 was examined. After that, 5vo# is placed in the gas cell 30.
% concentration of methane gas. As a result, a part of the evanescent wave generated in the sensor portion 22 was absorbed by methane gas, and the received light intensity V (level) was attenuated by 4 dB.

Thereafter, the received light intensity was detected while decreasing the methane gas concentration, and as shown in FIG. 3, it was detected that the received light intensity decreased as the methane gas concentration decreased. Then, when the received light intensity is 0.02 dB, which is the detection limit, the methane concentration! tO. 03 voj%. Therefore, according to this device, the minimum detectable concentration is 0.0
This means that you have achieved 3vo#%.

As can be seen from the experiment described above, the correspondence between the received light level and the methane gas concentration shown in FIG.
If set in advance, the concentration of methane gas introduced into the gas cell 30 can be detected by detecting the level of received light.

In this device, the sensor part 22 is only a core and has no cladding, so even though the diameter of this part is as large as 100 μm, many light waves, evanescent waves, seep out of the sensor part 22, resulting in high detection accuracy. . In addition, since the sensor portion 22 has a large diameter, its mechanical strength is high.
During the experiment described above and during long-term storage after the experiment, the optical fiber member 20 did not break.

In addition, in this device, the core 21a of the light propagation portions 21 and 23,
Since the diameter of the optical fiber member 23a and the diameter of the sensor portion 22 are made equal, the loss occurring in the optical fiber member 20 itself is small.

Therefore, the concentration can be detected even if a plurality of sensor parts are provided in one optical fiber member 20, and by doing so, the gas concentration can be detected over a wide range.

In addition, the above j! In the example, the light propagation parts 21 and 23 were made of silica-based optical fibers, but fluoride optical fibers (Zr, Ba
, La, Al jNap Hf , pb, etc.) Method of manufacturing the optical fiber member 20: The method for manufacturing the optical fiber member 20 is not limited to the above-mentioned method, but in short, as long as the sensor part is formed only by the core, It may also be produced by Ike's method.

Of course, this device can be used to detect the concentration of not only methane gas but also other gases. When detecting the concentration of another gas, it is recommended to use a laser beam with a wavelength approximately equal to the infrared vibrational absorption wavelength of that gas.

Effects of the Invention> As specifically explained above in conjunction with the embodiments, the present invention provides the following effects.

b) Since the diameter of the sensor part can be made large, it has high mechanical strength and has sufficient strength even when used in practical use.B) Despite the large diameter of the sensor part, this part has high mechanical strength. Since there is no cladding, many evanescent waves are generated, and as a result, the concentration can be detected even if the concentration of the test gas is small, and the detection accuracy is high.

Since the constrained optical fiber member does not have a narrowed core, the optical loss caused by the optical fiber member itself is small. Therefore, a large number of sensor parts can be provided, and concentration can be detected over a wide range.

2) Also, since the optical loss caused by the optical fiber itself is small, concentration can be detected by remote control. For example, it can be used to discover gas leaks in city gas pipes by remote control.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing an embodiment of the present invention, Fig. 2 is an enlarged view showing the main parts of the optical fiber member, Fig. 3 is a characteristic diagram showing the relationship between light reception level and methane gas concentration, and Fig. 4 is This is a configuration diagram showing the conventional technology, where 0 is a semiconductor laser oscillator, O is an optical fiber member, 1 and 23 are light propagation parts, 2C is a sensor part, 21 is a germanium photodetector, O is an amplifier, and 1 is a computer. be.

Claims (1)

[Claims] An optical fiber member in which a sensor portion exposed to the test gas in the longitudinal direction is formed only by a core, and other light propagation portions are formed by having a core and a cladding; a light input section that inputs inspection light into one end; and a light receiving section that receives inspection light that has propagated through the optical fiber member and exits from the other end of the optical fiber member, and detects the light intensity of the received inspection light. A gas detection device using an optical fiber, comprising: a determination unit that determines the concentration of a test gas based on the light intensity detected by the light receiving unit.