JPS639843A - Gas detection device - Google Patents

Abstract

PURPOSE:To stably measure the concentration of gas by providing a means which compares the absolute value of the oscillation wavelength of a semiconductor laser with actual oscillation wavelength and detects the deviation and a means which corrects the deviation. CONSTITUTION:The wavelength of the semiconductor laser 1 which is controlled by a temperature controller 10 to necessary temperature is scanned on a pressure reduction cell 13 where pure gas of the same kind with the gas to be measured is charged to specific pressure. Consequently, an absorption spectrum obtained from a laser detector 15 is compared by a comparator 17 with an absorption spectrum in a reference wavelength band corresponding to the pressure reduction cell 13 stored in a memory device previously to detect the deviation in the oscillation wavelength of the semiconductor laser 1. The deviation in the oscillation wavelength can be compensated by controlling the temperature controller 10 so that the temperature is raised when the wavelength is made short or lowered when the wavelength is made long.

Description

[Detailed Description of the Invention] [Summary] In order to solve measurement errors and instability caused by fluctuations in the oscillation wavelength of the laser in a semiconductor laser type gas detection device, the present invention provides a method for installing a vacuum cell immediately before gas concentration measurement. A device that measures the absorption spectrum and compares it with a table of absorption lines of a decompression cell stored in a storage device in advance to determine the shift in the laser's oscillation wavelength band, and corrects this shift by controlling the temperature of the semiconductor laser. It is.

[Industrial application field]

The present invention relates to a gas detection device, and particularly to a semiconductor laser type gas detection device (hereinafter abbreviated as gas sensor).

Pollution gas sensors are required to be small, fast, and highly accurate. Semiconductor laser gas sensors are portable and have desirable features, but are therefore also required to have high environmental resistance.

[Conventional technology]

FIG. 4 is a basic configuration diagram of a conventional gas sensor, and FIG. 5 is a diagram for explaining the principle of gas concentration detection. In FIG. be done. 7'' is a gas concentration measuring system, in which the laser beam that has passed through the gas to be measured 3 is focused by a lens 4 onto a laser detector 5.

The semiconductor laser 1 has a characteristic that the oscillation wavelength of the emitted light becomes shorter by increasing the drive current, and becomes longer by decreasing the drive current.Since the wavelength can be continuously scanned, the oscillation wavelength of the emitted light becomes shorter when the drive current is increased. By measuring the light transmittance while scanning the wavelength, an absorption spectrum of the gas to be measured 3 as shown in FIG. 5 can be obtained. FIG. 4 will be explained below with reference to FIG.

FIG. 5 shows an example of the absorption spectrum when, for example, sulfur dioxide gas SO2 is selected as the gas 3 to be measured.

This figure shows the absorption spectrum characteristics when the wavelength is scanned, with the vertical axis representing the transmittance of the laser beam to the gas to be measured 3 and the horizontal axis representing the wavelength of the laser beam. The spectral characteristics of this sulfur dioxide gas SO8 have a minimum point P of transmittance at wavelength and peak points Q and R of transmittance at wavelength and 4, respectively.

At the minimum point P, the laser beam passing through the measured gas S08 is absorbed due to the presence of the sulfur dioxide gas S08, and its transmittance decreases. If there is no absorption effect, the spectral characteristic should be a straight line connecting point Q and point R.

This straight line is called a baseline.

In the signal processing circuit 6 of FIG. 4, the two peak points Q,
The difference in transmittance from the wavelength wP' to the minimum point P on the line connecting R is determined.

Since the difference in transmittance is proportional to the concentration of the gas to be measured 3, the concentration can be calculated by multiplying by a proportionality coefficient, and the calculated value is displayed on the display device 7.

[Problem that the invention seeks to solve]

In a conventional semiconductor laser type gas sensor, the semiconductor laser 1 is used in a cooled state in order to stabilize its oscillation wavelength.For example, the semiconductor laser 1 is installed in the freezing chamber of a helium circulation type refrigerator, and the cooling temperature is set to an absolute temperature of about 80K. Although the oscillation is controlled, the absolute value of the oscillation wavelength is not controlled, and the oscillation wavelength shifts due to drifts in the control temperature or changes in the characteristics of the semiconductor laser itself, which has the disadvantage that gas concentration cannot be measured stably. Auta.

The present invention was created in view of the above-mentioned conventional drawbacks, and includes means for detecting a deviation by capturing the absolute value of the oscillation wavelength of a semiconductor laser and comparing this with the actual oscillation wavelength, and a method for correcting the amount of deviation. The purpose is to provide means.

[Means for solving problems]

As shown in the principle diagram of FIG. 1, the gas detection device of the present invention has the following features:
The wavelength of the semiconductor laser 1 whose temperature is controlled to a required temperature by the temperature control device 10 is scanned by controlling the current flowing through the semiconductor laser 1, and the absorption spectrum of the gas to be measured 3 in the atmosphere is measured using the gas concentration measurement system 7. In the gas detection device for measuring the concentration of the gas to be measured, the emitted light of the semiconductor laser l is divided into two parts by a half mirror 11.
A reduced pressure cell 13 which is divided and filled with a pure gas of the same type as the gas to be measured 3 at a predetermined pressure, and a storage device 1 that stores in advance absorption spectrum characteristics in a reference wavelength band corresponding to the reduced pressure cell 13 are provided. At the same time, an absorption spectrum of the decompression cell 13 due to one of the two divided beams is obtained by an A/D converter 16 via a laser detector 15, and this absorption spectrum is compared with an absorption spectrum of the storage device 18. According to the semiconductor laser 1
A comparator 17 is provided for detecting a deviation of the oscillation wavelength band from the reference wavelength band, and the output of the comparator 17 drives the temperature control device 10 to correct the deviation of the oscillation wavelength band. do.

[Effect]

The wavelength of the semiconductor laser 1, which is controlled to a required temperature by the temperature control device IO, is scanned by the wavelength of the semiconductor laser 1, which is controlled to a required temperature by the temperature control device 1O, to the decompression cell 13, which is filled with a pure gas of the same type as the gas to be measured 3 at a predetermined pressure. absorption spectrum [
By comparing the absorption spectrum in the reference wavelength band corresponding to the decompression cell 13 stored in the storage device 18 in advance (see FIG. 2(a)) with the comparator 17, the absorption spectrum of the semiconductor It is possible to detect the shift in the oscillation wavelength of laser 1 (in the comparison in this figure, it can be seen that the oscillation wavelength of the semiconductor laser is shifted toward the Mu side, that is, the longer wavelength, because the Mu absorption line is missing), The shift in the oscillation wavelength can be corrected by controlling the temperature control device 10 so that the temperature is increased when the wavelength is shortened (if the wavelength is shortened) and the temperature is decreased when the wavelength is lengthened in response to this shift.

〔Example〕

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

Note that, in order to make the explanation of the configuration and operation easier to understand, the same parts are given the same reference numerals throughout all the figures, and repeated explanation thereof will be omitted.

FIG. 3 is a diagram showing the configuration of an embodiment of the present invention.
is a helium circulation type refrigerator, and inside this refrigerator there are a mounting base 9a made of a material having good thermal conductivity, such as copper, and a mounting base 9a made of copper.
A heat sink 9 composed of a heater 9b wound around a body, a temperature sensor 9c, etc. is housed, and a semiconductor laser 1 is mounted on a mounting base 9a.

The heat sink 9 is controlled by the temperature side m device 10 to a temperature of about 8
It is cooled to the required temperature of 0K (absolute temperature). 11 is a half mirror which divides the parallel light of the laser beam emitted from the semiconductor laser 1 into two, one of which is a plane mirror 21a~
21C, condenser mirrors 22a to 22b, and three spherical mirrors 23
The light passes through a long optical path cell composed of a to 23c and is focused on a laser detector 5, and from the output thereof, an absorption spectrum due to the gas to be measured 3 (not shown) in the long optical path cell is measured. As explained in FIG. 4 and FIG. 5, the difference in transmittance is determined in the signal processing circuit 6, and the concentration of the gas to be measured 3 (not shown) is calculated from the difference in transmittance by conversion, and the concentration is displayed on the display device 7. indicate.

The other side passes through a reduced pressure cell 13 in which a pure gas of the same type as the gas to be measured (hereinafter referred to as the gas to be measured) is sealed at a predetermined pressure (for example, 5 to 1Q mmHg) via a plane mirror 24 and a condenser mirror 25. The light is then focused on the laser detector 15.

As shown in FIG. 2(b), the absorption spectrum of the gas to be measured, which has been depressurized in this way, has a plurality of absorption lines with wavelengths λ1.
It has a characteristic that a pattern of relative absorption intensity corresponding to Js and its respective wavelengths (the absorption intensity of wavelength λ1 is smaller than the absorption intensity of wavelength islands) can be clearly observed. This absorption spectrum is input to a comparator 17 via an A/D converter 16.

Reference numeral 18 denotes a storage device such as a ROM, which stores absorption spectrum characteristics (each wavelength of a plurality of absorption lines and (relative absorption intensity data corresponding to each wavelength) is stored in a table, and the data is input to the comparator 17 in accordance with the type of gas to be measured.

If the data in the storage device 18 input to the comparator 17 becomes the pattern shown in FIG. 2(a), and the data in the A/D converter 16 becomes the pattern shown in FIG. The data from the A/D converter 16 lacks absorption lines corresponding to wavelengths.In other words, it can be determined that the oscillation wavelength band of the semiconductor laser 1 has shifted toward eight longer wavelengths.This can be corrected. In order to do this, the oscillation wavelength band of the semiconductor laser 1 is shifted to the short wavelength side by controlling the temperature control device 10 to the high temperature side.Conversely, when the oscillation wavelength band is shifted to the short wavelength side, the temperature control device 10 is controlled to the high temperature side. Correction is possible by controlling the device 10 to the low temperature side.

In addition, since the wavelength bandwidth that a single conventional semiconductor laser can continuously scan is narrow, the absorption lines that can be used for concentration measurement are limited to those with a small half-width, and the oscillation wavelength band of the laser Since this is discontinuous and unique to each semiconductor laser, the semiconductor lasers that can be used in the device are limited to those that match the absorption line of the gas to be measured.

Therefore, it was impossible to measure absorption lines with large absorption coefficients.

By reversing the above-mentioned method of correcting the oscillation wavelength by controlling the temperature of the semiconductor laser, it is possible to shift the oscillation wavelength band in a desired direction.

In addition, multiple semiconductor lasers with discontinuous natural oscillation wavelength bands are stored in a refrigerator in a switchable manner, and the discontinuous band is measured by the absolute value of the oscillation wavelength while switching the semiconductor lasers. A scalable laser light source is realized.

This makes it possible to scan a wide wavelength band by switching between multiple semiconductor lasers, making it possible to measure absorption lines with a wide half-width. Furthermore, it has the effect of improving the utility value of a semiconductor laser, which could not be used conventionally because it does not match the absorption line of the gas to be measured.

〔Effect of the invention〕

As described above in detail, according to the gas sensor of the present invention, fluctuations in the oscillation wavelength of the semiconductor laser can be suppressed by controlling the temperature of the heat sink, so that the gas concentration of the gas to be measured can be stably measured.

[Brief explanation of the drawing]

Figure 1 is a diagram of the principle of the present invention, Figure 2 is a comparison diagram of absorption spectrum characteristics obtained from each laser detector shown in Figure 1, Figure 3 is a configuration diagram of an embodiment of the present invention, and Figure 4 is a diagram of the conventional A basic configuration diagram of a gas sensor, FIG. 5 shows a diagram of the principle of conventional gas concentration detection. In the figure, 1 is a semiconductor laser, 3 is a gas to be measured, and 10
is a temperature control device, 13 is a pressure reducing cell, 17 is a comparator, 18
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Claims (1)

[Scope of Claims] The concentration of the gas to be measured (3) in the atmosphere is measured by scanning the wavelength of a semiconductor laser (1) whose temperature is controlled to a required temperature by a temperature control device (10), and based on the absorption spectrum of the gas to be measured (3) in the atmosphere. In a gas detection device that performs Storage device for storing information in advance (18
), and by comparing the absorption spectrum of the decompression cell (13) obtained by wavelength scanning of the semiconductor laser (1) with the absorption spectrum of the storage device (18), the oscillation wavelength band of the semiconductor laser field can be determined. A comparator (17) is provided for detecting a deviation from the reference wavelength band of the temperature control device (10), and the output of the comparator (17)
) to correct a shift in the oscillation wavelength band.