JPH01307640A - Gas detecting device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a gas detection device that detects gas by a differential absorption method using laser light, and particularly to improvement of gas detection accuracy.

[Prior art] The differential absorption method, which detects gas by using light absorption, which is a chemical and physical property of gas molecules, is described in, for example, the document "Sensor Technology" (Technical Research Committee, February 1986 issue). Page 29 - 3rd
It is disclosed on page 1. The gas detection system disclosed in this document uses a laser beam in the 3.39 μm band of a He-Ne laser with a wavelength that is strongly absorbed by methane gas, and a laser beam with a wavelength that is slightly different from this laser beam and is not absorbed by methane gas. Obtain two wavelengths of laser light by alternating switching with a mechanical chopper or mechanically changing the oscillator length of one laser oscillator, and direct these two wavelengths of laser light to n1 fixed parts such as road surfaces and walls. The system detects methane in the optical path from the difference in intensity of diffusely reflected light. This differential absorption method uses laser light of two wavelengths to compensate for the attenuation of the amount of laser light due to factors other than the gas to be detected, so that gas can be detected with high accuracy.

[Problem to be solved by the invention] Conventional differential absorption methods using laser beams with two wavelengths emit laser beams with slightly different wavelengths using a mechanical chopper or by mechanically changing the oscillator length. Therefore, there was a limit to the wavelength stability and high-speed switching of the two-wavelength laser beam. For this reason, there was a problem in that the gas detection sensor could not be spatially swept at high speed.

The present invention was made to solve these problems, and an object of the present invention is to provide a gas detection device that can detect gas at high speed and with high precision.

[Means for Solving the Problems] A gas detection device according to the present invention includes a first semiconductor laser device that emits a laser beam having a wavelength that matches the absorption wavelength of a measurement gas, and a laser beam that is emitted from the first semiconductor laser device. a second semiconductor laser device that emits a laser beam with a wavelength that is slightly different from the wavelength of the measurement gas and is not absorbed by the measurement gas; a third semiconductor laser device that emits a reference laser beam with a wavelength shorter than the wavelength of each of the laser beams; and one frequency f. (Hz) and modulates the intensity of laser light emitted from the first semiconductor laser device and the third semiconductor laser device; a phase adjuster that outputs a signal and modulates the intensity of the laser light emitted from the second semiconductor laser device; and a phase adjuster that outputs a signal and modulates the intensity of the laser light emitted from the second semiconductor laser device; a first optical receiver that converts the reference laser beam into a reference electric signal; a second optical receiver that receives the reference laser beam projected from the third semiconductor laser device and converts it into a reference electric signal; A lock-in amplifier that detects the amplitude of electrical signals having different phases sent from one optical receiver, and a ratio calculator that calculates the ratio of the amplitudes detected by the lock-in amplifier [Function] ] In this invention, the intensities of laser beams of different wavelengths emitted from the first semiconductor laser device and the second semiconductor laser device are modulated by modulation signals of different phases, thereby changing the intensities of the laser beams of two wavelengths. It is designed to electrically switch between two wavelengths of laser light.

Further, the phase lag due to the aerial transmission of the laser beams of these two wavelengths is corrected by the reference laser beam.

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention.
In the figure, 1 is a first semiconductor laser oscillator (hereinafter referred to as a laser oscillator) that emits light with a wavelength matching the absorption wavelength of the measurement gas, for example, the 1.311111 band; 2 is a semiconductor laser oscillator that drives the laser oscillator 1; Drive device for controlling output, 3
is a second semiconductor laser oscillator that emits a laser beam with a wavelength that is slightly different from the wavelength of the laser beam emitted from the laser oscillator 1 and is not absorbed by the measurement gas; 4 is a driving device for the laser oscillator 3; 5 is a wavelength that is significantly different from the wavelength of the laser light emitted by the laser oscillators 1 and 3, for example, 0.8
6 is a driving device for the laser oscillator 5, which is a third semiconductor laser oscillator that emits a reference laser beam having a wavelength in the 3-band. 7
is an oscillator that oscillates a signal with a frequency f (Hz),
Frequency f (Hz) sent from oscillator 7 to drive devices 2 and 6
The intensity, that is, the amplitude, of the laser light emitted from the laser oscillators 1 and 5 is modulated by the signal. Reference numeral 8 denotes a phase adjuster that synchronizes with a signal of frequency f (Hz) sent from the oscillator 7 and sends a signal having a phase different from this signal by π/2 (rad) to the drive device 4.

The drive device 4 modulates the intensity of the laser light emitted by the laser oscillator 3 based on the signal sent from the phase adjuster 8 . Therefore, the intensities of the laser beams sent from the laser oscillators 1 and 5 and the intensity of the laser beam sent from the laser oscillator 3 are different in phase by π/2 (rad).

9.10 is a semi-transparent mirror that guides the laser beams emitted from each laser oscillator it3+5 to the same optical path; 11 is a reflecting mirror; and 12 is a condensing lens. Reference numeral 13 denotes an optical demultiplexer made of a dielectric multilayer filter, and the optical demultiplexer 13 transmits laser light having wavelengths in the 1, 3-band, emitted from the laser oscillators 1 and 3, and emits it from the laser oscillator 5. The reference laser beam having a wavelength in the 0 and 83 μ band is reflected and separated into laser beams having different wavelength bands.

14 is a photoreceiver that receives the two wavelengths of laser light in the 1.3-band wavelength band demultiplexed by the optical demultiplexer 13 and converts each into an electric signal;
Reference numeral 15 denotes a reference light receiver that receives the reference laser beam in the 0 and 83μ bands that has been demultiplexed by the optical demultiplexer 13 and converts it into a reference electrical signal.

Reference numeral 16 inputs an electric signal generated by laser beams of two wavelengths having a phase difference of π/2 (rad) that is incident on the light receiver 14 and a reference electric signal generated by the reference laser beam that is incident on the reference light receiver 15, and generates a reference electric signal. a lock-in amplifier that detects the amplitudes of electrical signals having different phases, 17 a ratio calculator that calculates the ratio of the amplitudes sent from the lock-in amplifier 1B;
18 is a scattering part of a part to be measured such as a wall.

When detecting gas with the gas detection device configured as described above, after driving each driving device 2, 4.6 to bring each laser oscillator 1, 3.5 into a stable state, the oscillator 7 Send a signal of frequency f(l(z) to .6,
1 emitted from the laser oscillator 1 by the driving devices 2 and 6;
The intensity of the laser beam in the 31Jfll band and the reference laser beam emitted from the laser oscillator 3 is modulated. At the same time oscillator 7
A signal with a frequency f (Hz) is sent from the phase adjuster 8 to the phase adjuster 8.

The phase adjuster 8 sends a signal having a phase different from this signal by π/2 (rad) to the drive device 4 in synchronization with the sent signal.

The driving device 4 modulates the intensity of the laser beam emitted from the laser oscillator 3 using the signal sent from the phase adjuster 8 .

In this way, the laser oscillators 1 and 3 simultaneously emit two laser beams with slightly different wavelengths and different laser beam intensity distributions, and the laser oscillator 5 simultaneously emits a reference laser beam. , 10 and a reflecting mirror 11 to a scattering section 18. The laser light scattered and reflected by the scattering section 18 is focused by the condensing lens 12 and enters the optical demultiplexer 13. The laser light incident on the optical demultiplexer 13 is transmitted to the laser oscillator 1,
Wavelength 1.3 sent from 3. The laser beam with a wavelength of 0.83-band and the laser beam with a wavelength of 0.83-band sent from the laser oscillator 5 are split, and the two laser beams with a wavelength of 1.3-band enter the light receiver 14, and the laser beam with a wavelength of 0.83-band is sent from the laser oscillator 5. The reference laser beam in the band is incident on the reference light receiver 15 . The laser light incident on the light receiver 14 and the reference light receiver 15 is converted into an electric signal and a reference electric signal, respectively, and sent to the lock-in up 16.

FIG. 2 shows the waveform of the electrical signal sent to the lock-in up 16, where a is an electrical signal generated by a laser beam having the same wavelength as the absorption wavelength of the gas emitted from the laser oscillator 1, and b
is emitted from laser oscillator 3, the wavelength is slightly different from that of laser oscillator 1, and the phase of the modulated signal is π
/2 (rad) electric signals generated by different laser beams; C is a reference electric signal generated by a reference laser beam projected from the laser oscillator 5;

Here, the reason why the laser oscillator 1.3 emits the laser beam and the laser oscillator 5 emits the reference laser beam with a short wavelength at the same time is because the reference laser beam corrects the phase delay caused by the aerial transmission of the laser beam, and This is to improve the detection accuracy of the intensity, that is, the amplitude of the laser beam, which has become weak due to scattering and absorption, by using the intensity of the reference laser beam.

That is, the electrical signals sent from the light receiver 14 shown by waveforms a and b in FIG. 2 are compared to the reference electrical signal shown by waveform C.
The waveform is smooth. Therefore, the amplitude of the electrical signal waveform a at the peak point 20 of the waveform C of the reference electrical signal and the phase from the peak point 20 of the waveform C are π/2 (rad).
By detecting the amplitude of the electrical signal waveform at the shifted zero cross point 21, the amplitude can be accurately measured.

The phase detected by lock-in up 16 is π/2 (
rad) different amplitudes are sent to a ratio calculator 17, and the amplitude ratio is calculated.

When laser beams of two slightly different wavelengths are emitted from the laser oscillators 1 and 3 to the scattering section 18 in this way, if there is no gas in the optical path of the laser beam, the gas emitted from the laser oscillator 1 will be Since laser light having the same wavelength as the absorption wavelength is also not absorbed, the amplitude of the waveform a detected by the lock-in amplifier 16 is approximately the same as the amplitude of the waveform S.

On the other hand, if a gas exists in the optical path of the laser beam, the laser beam having the same wavelength as the absorption wavelength of the gas emitted from the laser oscillator 1 will be absorbed depending on the concentration of the gas and the distance where the gas exists, and the laser beam will be received. When the laser beam enters the device 14, its intensity, that is, its amplitude is small. Therefore, the amplitude of waveform a detected by lock-in amplifier 16 is relatively small compared to the amplitude of waveform S. Therefore, by calculating this amplitude ratio using the ratio calculator 17, the presence of gas in the optical path of the laser beam and its concentration can be measured.

[Effects of the Invention] As explained above, this invention modulates the intensity of laser beams with slightly different wavelengths with different modulation signals.
Since the two wavelengths of laser light are switched by changing the intensity of the two wavelengths of laser light, high-speed switching can be performed and gas can be detected at high speed.

In addition, by emitting a high-energy reference laser at the same time, the amplitude detection of the two wavelengths of laser light is corrected, making it possible to accurately detect a wide space.
The gas detection rate can be greatly improved.

Furthermore, since there is no change in the wavelength of the laser light when the intensity of the two wavelengths is switched, gas can be detected at a stable wavelength, which also has the effect of increasing gas detection accuracy.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a waveform diagram of an electric signal input to lock-in up in the above embodiment. 1.3.5...Semiconductor laser oscillator, 2,4.6...
・Drive device, 7... Oscillator, 8... Phase adjuster, 9
゜lO...Semi-transparent mirror, 11...Reflector, 12...Condensing lens, 13...Optical demultiplexer, 14... Light receiver, 1
5... Reference receiver, 16... Lock-in up,
17... Ratio calculator. Agent Patent Attorney Muneharu Sasaki

Claims (1)

[Claims] a first semiconductor laser device that emits a laser beam with a wavelength that matches the absorption wavelength of the measurement gas; and a first semiconductor laser device that emits a laser beam with a wavelength that is slightly different from the wavelength of the laser beam that is emitted from the first semiconductor laser device and is not absorbed by the measurement gas. a second semiconductor laser device that emits a reference laser beam with a wavelength shorter than the wavelength of each of the laser beams;
) and modulates the intensity of laser light emitted from the first semiconductor laser device and the third semiconductor laser device;
d) a phase adjuster that outputs a shifted signal and modulates the intensity of the laser beam emitted from the second semiconductor laser device;
A first light receiver receives two wavelength laser beams emitted from a semiconductor laser device and a second semiconductor laser device and converts them into electrical signals, and a first photoreceiver receives reference laser beams emitted from a third semiconductor laser device and converts them into a reference electrical signal. a second light receiver for converting into
A lock-in amplifier that detects the amplitude of the electrical signal having a different phase sent from the first photoreceiver based on the reference electrical signal sent from the photoreceiver, and a ratio calculator that calculates the ratio of the amplitudes detected by the lock-in amplifier. A gas detection device characterized by comprising: