JPH01304345A - Gas sensor - Google Patents

Abstract

PURPOSE:To switch a gas sensor at a high speed so as to improve the gas sensing speed of the sensor by changing the frequencies of modulating signals given to the 1st and 2nd semiconductor laser devices which emit laser light having slightly different wavelengths. CONSTITUTION:The 1st and 2nd oscillators 5 and 6 are oscillated and signals having an oscillating frequency f1 (Hz) are sent to a drive device 2 from the oscillator 5 so as to modulate the intensity of the laser light emitted from a laser oscillator 1. Similarly, signals having an oscillating frequency f2 (Hz) are sent to another drive device 4 from the oscillator 6 so as to modulate the laser light emitted from a laser oscillator 3 through the device 4. The two kinds of laser light having different frequencies emitted from the oscillators 1 and 3 are scattered at a scattering section 14 through a half mirror 7 and reflecting mirror 8 and condensed by a condenser lens 9. After condensation, the two kinds of laser light are made incident to a photoreceptor 10 and converted into electric signals. The output signals of the photoreceptor 10 are sent to an amplitude measuring instrument 13 respectively through BPFs 11 and 12 and subjected to amplitude measurement after the output signals are synthesized and amplified.

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.

[Conventional technology]

The differential absorption method, which detects gases using light absorption, which is a chemical and physical property of gas molecules, is described in the literature "Sensor Technology" (Technical Research Committee, February 1986 issue), pages 29 to 3, for example.
It is disclosed on page 1. The gas detection system disclosed in this document uses a 3.39 μm band laser beam from a 1ie-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. Switching alternately with a mechanical chopper,
Alternatively, you can mechanically change the oscillation quality of one laser oscillator to obtain two wavelengths of laser light, and then irradiate the two wavelengths of laser light toward a part to be measured such as a road surface or wall, and detect the diffused reflection from there. Methane in the optical path is detected from the difference in light intensity. 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 mechanically change the oscillation quality. There were limitations in wavelength stability and high-speed switching. 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 in order to solve this problem, and its object is to obtain a gas detection device that can sweep spatially at high speed.

[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 with a wavelength that matches the absorption wavelength of a gas to be measured; A second semiconductor laser device that emits laser light with a wavelength that is not absorbed by the measurement gas is different, and a first semiconductor laser device that oscillates a signal with a frequency f1 (Hz) changes the intensity of the laser light emitted from the first semiconductor laser device. The intensity of the laser light modulated by the signal of the oscillator and emitted from the second semiconductor laser device is the signal of the second oscillator that oscillates a signal with a frequency f 2 (Hz) different from the oscillation frequency L (llz) of the first oscillator. Modulate. Then, this different frequency f1. The gas is detected by simultaneously projecting laser light whose intensity is modulated at f2 (Hz) toward the part to be measured, and measuring the amplitude of the laser light received from the part to be measured with an amplitude measuring device. do.

[Effect]

In this invention, the modulation signals given to the first semiconductor laser device and the second semiconductor laser device are modulation signals of different frequencies, thereby changing the intensity of the laser beams of two wavelengths. It is designed to be switched electrically.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention,
In the figure, 1 is a first semiconductor laser oscillator (hereinafter referred to as a laser oscillator) that emits a laser beam with a wavelength matching the absorption wavelength of the measurement gas, and 2 is a drive device that drives the laser oscillator 1 and controls its 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, and 4 is a driving device for the laser oscillator 3.

5 is a first oscillator connected to the drive device 2, and the oscillator 5 oscillates a signal with a frequency f + (Hz).

6 is a second oscillator connected to the drive bag N4, and the oscillator 6 oscillates a signal at a frequency f 2 ()12) different from the oscillation frequency f + (Hz) of the first oscillator 5.

7 is a semi-transparent mirror that combines the laser light emitted from the laser oscillator 1 and the laser light emitted from the laser oscillator 3 into one optical path; 8 is a reflecting mirror; 9 is a condenser lens; It is a light receiver that converts into a signal, and includes a semi-transparent mirror 7,
The reflecting mirror 8, the condensing lens 9, and the light receiver 10 constitute an optical system that emits and receives laser light.

Reference numeral 11 denotes a bandpass filter installed to remove noise in the optical path from the electric signal of the laser beam received by the photoreceiver 10, whose intensity is modulated at frequency f, and 12, whose intensity is modulated at frequency f2. A band pass filter provided to remove noise etc. from the electric signal of the laser beam, 1
3 is an amplitude measuring device that synthesizes the electric signals sent from the bandpass filters 11 and 12 and measures the amplitude of the synthesized signal;
4 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, first, the drive device 2 and the drive device 4 are driven to bring each laser oscillator 1 and 3 into a stable state. Next, the first oscillator 5 and the second oscillator 6 are caused to oscillate, and a signal with an oscillation frequency f I (Hz) is sent from the first oscillator 5 to the drive device 2.
The driving device 2 modulates the intensity of the laser beam emitted by the laser oscillator 1, as shown in FIG. 2A, for example. Similarly, the oscillation frequency f z is transmitted from the second oscillator 6 to the drive device 4.
(Hz) signal is sent, and the drive device 4 causes b in Fig.
As shown in the figure, the intensity of the laser beam emitted by the laser oscillator 3 is modulated.

Laser oscillators 1 and 3 transmit laser beams with two wavelengths that are slightly different in wavelength and have different frequencies in the intensity distribution of the laser beam, as shown in Figure 2.
The light is simultaneously emitted from the mirror 7 and projected onto the scattering section 14 via the semi-transparent mirror 7 and the reflecting mirror 8. The laser beam projected onto the scattering section 1a is scattered and reflected by the scattering section 14, and then sent to the light receiver 1 through condensing.
Enter 0 and add 1.1. The incident laser beam is converted into an electrical signal by the photoreceiver 10, sent to the amplitude measuring device 13 via the bandpass filter IL 12, and synthesized by the amplitude measuring device 13.
jfQ width and its intensity distribution, ie, amplitude, is measured.

When a laser beam is emitted from the laser oscillator 1.3 to the scattering section 14, if there is no gas in the optical path of the laser beam, the laser beam emitted from the laser oscillator 1 will also be the same as the laser emitted from the laser oscillator 3. Like light, it is not absorbed and enters the light receiver 10 with the same scattering intensity. Therefore, the amplitude measuring device 1
3, an electrical signal having the same frequency as the second intensity distribution waveform indicated by alb is sent. These two frequencies are different,
When electrical signals with the same amplitude are combined, the amplitude becomes large as shown in C in FIG.

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 by the gas depending on the concentration of the gas and the distance at which the gas exists. When the laser beam is incident on the photoreceiver, it becomes a laser beam with a small intensity distribution, as shown in FIG. 3d. Therefore, when this laser beam with a small intensity distribution and the electrical signal from the laser beam emitted by the laser oscillator 3 (shown in b of FIG. 3) are combined by the vibration Ifllf measuring device 13, this combined signal is shown in e of FIG. 3. The amplitude becomes smaller by the intensity of the laser light absorbed by the gas.

By measuring this decrease in amplitude, the presence and concentration of gas in the optical path of the laser beam can be determined.

〔Effect of the invention〕

As explained above, the present invention is capable of increasing the intensity of the laser beams of two wavelengths by giving different frequencies to the modulation signals given to the first semiconductor laser device and the second semiconductor laser device, which emit laser beams with slightly different wavelengths. Since the laser beams of two wavelengths are switched by changing the wavelength, high-speed switching can be performed.

Moreover, this high-speed switching allows the space to be swept at high speed, and the gas detection speed can be greatly improved.

Furthermore, since there is no fluctuation in the wavelength of the laser light when switching the intensity of the laser light of two wavelengths, 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, FIG.
FIG. 3 is an explanatory diagram showing the intensity distribution of each laser beam. 1.3... Semiconductor laser oscillator, 2.4... Drive device, 5.6... Oscillator, ■... Semi-transparent mirror, 8... Reflecting mirror, 9... Condensing lens, 10 ...light receiver, 11.
12...Band pass filter, 13...Amplitude measuring device.

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 first oscillator that oscillates a signal with a frequency f_1 (Hz) and inputs it to the first semiconductor laser device to modulate the intensity of the laser light emitted from the first semiconductor laser device; A frequency f_ that is different from the oscillation frequency f_1 (Hz) of the 1 oscillator
a second oscillator that oscillates a signal of 2 (Hz) and inputs it to the second semiconductor laser device to modulate the intensity of laser light emitted from the second semiconductor laser device; a first semiconductor laser device; and a second semiconductor laser device. What is claimed is: 1. A gas detection device comprising: an optical system that simultaneously emits and receives laser light emitted from a gas detector; and an amplitude measuring device that measures the amplitude of the laser light received by the optical system.