JPH0610636B2 - Gas spectroscope - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gas spectroscopic device that performs spectroscopic analysis of gas molecules using an evanescent wave of an optical fiber.

(Prior Art) Conventionally, among the properties of light waves, changes in intensity, phase, polarization, etc. are used to detect pressure, vibration, acceleration, or electromagnetic quantities such as voltage, current, magnetic field, and There is known an optical fiber sensor that detects a chemical amount such as the type and concentration of a substance by using information about the wavelength of the. The optical fiber sensor is convenient for remote measurement in which a sample is distant or measurement under a situation where the sample cannot be collected, and is used in a wide range of fields such as industrial measurement, pollution monitoring, and medical use.

Taking a spectroscopic sensor using an optical fiber as an example,
As shown in FIG. 11, this type of spectroscopic sensor is placed at a remote observation point and a laser (wavelength λ ₁ , λ ₂ ) 1 as a light emitting source, an optical fiber 2 for transmitting the emitted laser light. The observation cell 3 includes a spectroscope and a photodetector 4 for detecting the light intensity, an amplifier 5 for amplifying the light output, and a signal processing circuit 6 for processing the light output as a signal.

Assuming that the absorption spectrum of the gas to be detected is as shown in FIG. 12, the emission wavelength of the laser 1 is λ ₁
Then, since the light absorption at the wavelength λ ₁ is maximized in the observation cell 3, the presence and concentration of the gas at the location where the observation cell 3 is installed is determined from the attenuation of the light output in the photodetector 4. Can be observed.

However, in such an optical fiber spectroscopic sensor, the observation cell 3 itself is quite large in structure and its installation location and installation condition are limited. In addition, the observation is performed while the laser light is once emitted inside the cell and reflected several times. Since the light is passed through the gas and converged again to be guided to the spectroscope and the photodetector 4, optical loss occurs, and it is difficult to adjust the optical system for converging the diameter of the laser light to be small, and the subsequent maintenance is easy. Not. In addition, if a wide range of spectroscopy including multiple remote observation points is to be performed using such a spectroscopic sensor, an observation cell must be installed at each observation point, which makes the above-mentioned adjustment and maintenance problems even more difficult. Is. Therefore, in order to facilitate the adjustment of the laser beam focusing optical system in the observation cell,
It is conceivable that the optical fibers to the spectroscope and the photodetector are bundled with a large diameter, but it is extremely expensive.

(Object and Structure of the Invention) However, as shown in FIG. 13, the optical fiber 10 comprises a core 11 surrounded by a clad 12 having a refractive index slightly smaller than the refractive index of the core.
Although it propagates while being totally reflected inside the cladding 12, a wave-optical analysis of the state of light propagation reveals that a part of the propagating light leaks into the cladding 12. , This leaching wave is an evanescent wave (Evanesce
nt Wave) Known as W. The appearance of this evanescent wave seepage is the wavelength of the propagating light, the diameter of the core,
It is determined by the refractive index of the core and clad, and the propagation mode of light.

Therefore, focusing on this evanescent wave, a part where the evanescent wave is easily exposed is formed in a part of the optical fiber consisting of the core and the clad, and the specific wavelength component of the light incident on the optical fiber is attenuated by the absorption of the evanescent wave by the substance. A method for spectroscopically measuring the substance by utilizing this is known from JP-A-57-124239 and JP-A-58-501481. None of the ones described in the above documents are applied to gas, and the clad of the optical fiber formed by wrapping the core with the clad is removed to expose the core and form a detection part for a living body or a solution to be detected. However, when laser light is passed through the optical fiber, the scattering of light at this detector becomes large, and satisfactory results are not obtained when applied to spectroscopy of gas, especially gas with low concentration and component ratio. There is.

The present invention has been made in view of the above points, there is no troublesome adjustment of the laser light converging optical system, the light loss is small, it is possible to easily spectral in a wide range and the scattering of light in the detection unit. In order to provide a small gas spectroscope, in order to achieve this object, the gas detection part of the optical fiber is formed by tapering the core of the optical fiber together with the clad surrounding the core. Is.

(Example) Hereinafter, the present invention will be described in detail.

Since the evanescent wave of the optical fiber is used in the gas spectroscopic device according to the present invention, it is necessary to make the optical fiber so that the influence of the evanescent wave greatly affects the atmosphere, and the following method can be considered as a manufacturing method thereof.

(1) Polishing Method As shown in FIG. 2, the groove 1 of curvature in the glass substrate 15
5a is formed, and the optical fiber 16 is formed along the groove 15a.
Is inserted and fixed, and the cladding 16b is polished until the core 16a of the optical fiber 16 is exposed together with the glass substrate 15. Then, the evanescent wave W appears in the atmosphere at the portion A of the optical fiber 16.

(2) Etching method As shown in FIG. 3, the clad 16b of the optical fiber 16 is removed by a hydrofluoric acid-based etching solution to such an extent as to reach the core 16a. Then, the evanescent wave W appears in the atmosphere at the portion A of the optical fiber 16.

(3) Stretching Method As shown in FIG. 4, the optical fiber 16 is locally heated by a micro torch to perform taper stretching. Then, the evanescent wave W appears in the atmosphere at the tapered portion A of the optical fiber 16.

FIG. 5 shows an optical fiber particularly suitable for use in the gas spectroscopic device of the present invention, in which the core and the clad of a conventionally known optical fiber are reversed, and the core is used as the clad 17 to enhance the mechanical strength and to surround it. It is an optical fiber having an annular core structure in which a core 18 is arranged. When light is made incident on the core 18 of the optical fiber having such a structure, the influence of the evanescent wave W strongly appears in the atmosphere as shown in the figure, and detection is possible over the entire length of the optical fiber. It is preferable to set the thickness of the core 18 of the optical fiber having the annular core structure to about the wavelength of the propagating light.

FIG. 1 is a schematic diagram of a gas spectroscopic device according to the present invention.

In the figure, 20 is a white light source that emits white light, 21 is an optical fiber, and a part 21a thereof is a detection unit. The detector 21a removes or extremely thins the clad by the method (1), (2), or (3) described above so that the evanescent wave easily appears in the atmosphere, and the light of a specific wavelength λ ₁ is emitted. It should be located at an observation point B (indicated by a broken line circle) where a gas to be detected that is easily absorbed exists. Reference numeral 22 is a spectroscope that analyzes the spectrum of light that has passed through the detection unit 21a, and 23 is a signal processing circuit that processes the output of the spectroscope 22.

The white light source 20 has a substantially uniform light intensity over a wide wavelength region as shown in the figure. When white light having such a light intensity is sent to the optical fiber 21, the evanescent wave appears in the atmosphere at the detecting portion 21a and is observed. The gas existing at the point B is partially absorbed. As a result, the spectral characteristic of the optical output detected by the spectroscope 22 has a minimum value at the wavelength λ ₁ as shown in the figure. Signal processing circuit 23
Then, the gas concentration can be detected by the difference method between the absorbed light component and the light component of the wavelength λ ₂ which is not absorbed by the gas.

The wavelength of light that is easily absorbed by a gas is unique to each gas, and a typical gas is as follows in a vacuum.

Methane CH ₄ 3.392 μm Carbon monoxide CO 4.666 μm Carbon dioxide CO ₂ 4.257 μm Acetylene C ₂ H ₂ 3.030 μm Therefore, if the wavelength of the light source is determined in advance, the presence or absence of gas that absorbs light of that wavelength and its concentration Can be detected by a decrease in light output.

Further, the detection sensitivity can be increased by making the detection unit 21a of the optical fiber 21 long by making it zigzag or coiled. Further, at an observation point that is difficult to detect, the shape of the detector 21a can be changed to be convenient, so that the spectroscopy can be easily performed. As an example, an experiment for detecting the concentration of methane gas was conducted using the gas spectroscopic device according to the present invention. FIG. 6 shows the optical system used in this experiment.

The wavelength of light that is easily absorbed by methane gas is 3.3922 in vacuum.
Since it is known to be μm, a He-Ne laser 25 that emits laser light of this wavelength, an optical chopper 26 for modulating the laser light at a specific frequency (for example, 275 Hz), and visible light (red The reference He-Ne laser 27 that emits a 0.63 μm laser beam, which is a light beam), the half mirror HM and the mirror M are arranged as shown in the figure, and the optical fiber 30 is provided via the infrared lens 28 for beam focusing. Was placed. A graded multimode fiber having an outer diameter of 125 μm (core diameter of 50 μm) was used as the optical fiber 30, and a part thereof was extended to an average diameter of about 7 μm by a drawing method to form a detection unit 30a. The length of the detection unit 30a was about 10 mm. An infrared lens 29 for condensing a beam is arranged on the output side of the optical fiber 30.
The half mirror HM is arranged on the optical axis of the, and the visible light reflected by the half mirror HM is passed through the visible sensor 31 to the amplifier 32, and the infrared light transmitted through the half mirror HM is In.
It is sent to the preamplifier 34 and further to the lock-in amplifier 35 via the As infrared sensor 33. 36 is a recorder for recording spectroscopic results, 37 is a methane gas (CH ₄ ) tank, 3
Reference numeral 8 is a tank of nitrogen gas (N ₂ ), 39 is a mixer of both gases, and 40 is a pulse generator that generates a pulse of a modulation frequency (for example, 275 Hz). Detection unit 30a of optical fiber 30 from tanks 37 and 38 via mixer 39
The methane gas _{CH 4 100%, 80%,} 60%, 40
%, 20% and a nitrogen gas N ₂ 100% are alternately sprinkled on the recording result of the recorder 36.
As shown in the figure. As can be seen from this figure, there is almost no decrease in output when nitrogen gas N ₂ is sprayed, but the output decreases when methane gas CH ₄ is sprayed. This indicates that the evanescent wave of the light with a wavelength of 3.3922 μm is not absorbed by nitrogen gas, but absorbed by methane gas, and the content ratio of methane gas is 100%.
It can be seen that the output decrease also decreases as the value decreases from 20% to 20% in steps of 20%.

FIG. 8 is a plot of the decrease rate of light output with respect to the content rate of methane gas from the results of FIG. 7. From this figure, it can be seen that the content rate of methane gas and the light absorption rate are in a substantially proportional relationship. In the figure, Po is the light output when there is no methane, and P is the light output for each content rate of methane.

Although the embodiment of the spectroscopic device shown in FIG. 6 is an example of detecting the concentration of a single gas, the same applies to the case of a plurality of gases, and the output is reduced at a wavelength unique to each gas.

Further, as the laser light source, in addition to the one that emits white laser light having a uniform light intensity over a wide wavelength as described above, a laser light source that emits only light of a specific wavelength can be used. Alternatively, the emission wavelength of the light emitting source may be switched to detect a plurality of different gas concentrations.

FIG. 9 shows another embodiment of the gas spectroscopic device according to the present invention. In the figure, the same reference numerals as in FIG. 1 indicate the same components.

In this embodiment, as a laser light source, a pulsed laser light source 40 that emits pulsed light having a wavelength absorbed by any gas molecule is used.
Pulse light is incident on the optical fiber 21. The incident pulsed light is backwardly Rayleigh scattered by each part of the fiber, but if gas molecules are present, it is strongly absorbed and the Rayleigh scattered light is reduced, so that the light intensity is changed to pulsed light by the signal processing circuit 42 via the infrared sensor 41. When observation is performed corresponding to the time delay from the time of incidence, the results shown in FIG. 10 are obtained. In this figure, it is considered that the light was absorbed by the gas molecules at the position D where the intensity of the backward Rayleigh scattered light sharply decreases.

Therefore, when the time from the incidence of pulsed light until the light intensity of the backward Rayleigh scattered light sharply decreases is t, the distance l from the position of the pulsed laser light source 40 to the position where gas molecules are distributed is as follows. It can be calculated from the formula.

l = tC / 2 C: speed of light in fiber The characteristics shown in FIG. 10 are obtained by switching the emission wavelength of the pulse laser light source 40 or by preparing a pulse laser light source emitting pulse laser light of different wavelengths and sequentially switching them. For several different wavelengths,
Since it is possible to determine the existence and concentration of gas molecules that absorb light of that wavelength, it is possible to perform spectroscopy over a wide range.

(Effects of the Invention) As described above, in the present invention, a gas detection part where the evanescent wave is easily exposed is formed in at least a part of the optical fiber, and the gas detection part is provided around the core of the optical fiber. Since it is formed by tapering the diameter with the enclosing clad, there is no need to radiate the spectroscopic light propagating through the optical fiber once and then focus it again, so that the optical loss can be reduced and the radiated light can be converged. The troublesomeness and effort of adjusting the optical system for the operation is unnecessary, and the maintenance becomes easy. Further, since there is no light scattering in the gas detection unit, it is possible to disperse a gas having a small concentration or a small component ratio. Furthermore, it is convenient for observation at a large number of observation points over a wide area because only the exposed part of the evanescent wave needs to be formed for observation. Furthermore, the detection unit itself is significantly smaller than the conventional spectroscopic sensor, and the installation location and installation state do not matter, so that spectroscopy is simplified.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an embodiment of a gas spectroscopic device according to the present invention, FIGS. 2 to 5 are diagrams showing a method for producing an optical fiber used in the gas spectroscopic device according to the present invention, and FIG. FIG. 8 is a diagram showing an optical system for detecting the concentration of methane gas, which was tested by using the gas spectroscopic device according to the invention, FIG. 7 is a graph showing the experimental results obtained by the recorder of the experimental optical system shown in FIG. 6, and FIG. Is a graph showing the relationship between the content rate of methane gas and the reduction rate of light intensity, FIG. 9 is a schematic diagram of another embodiment of the gas spectroscopic device according to the present invention, and FIG. 10 is the embodiment shown in FIG. FIG. 11 is a block diagram showing an example of a conventional spectroscopic sensor, FIG. 12 is a diagram showing an example of a gas absorption spectrum, and FIG. 13 is a structure of a general optical fiber. It is a figure which shows propagation of light. 20 ... White light source, 21 ... Optical fiber, 21a ...
Detector, 22 ... Spectrometer, 23 ... Signal processing circuit,

Claims (3)

[Claims] 1. A laser light source, an optical fiber provided with a gas detector for facilitating the exposure of an evanescent wave of the laser light while introducing a laser light from the laser light source from one end, and an optical fiber at the other end of the optical fiber. In a gas spectroscope having a spectroscope for analyzing the spectrum of the emitted laser light, the gas detecting part of the optical fiber is formed by tapering the core of the optical fiber together with a clad surrounding the core. A gas spectroscopic device characterized by the above. 2. The gas spectroscopic apparatus according to claim 1, wherein the gas detection unit is wound in a coil shape. 3. A laser light source, an optical fiber which surrounds the clad with a core and introduces the laser light from the laser light source from one end, and the spectrum of the laser light guided from the optical fiber at the other end of the optical fiber is analyzed. A gas spectroscope having a spectroscope.