JPS60253953A - Measurement system for gas concentration - Google Patents

Abstract

PURPOSE:To measure the concentration of gas at a relavtively high speed with high precision by superposing a modulated current having a lower frequency than a fine sine wave current on light from a semiconductor laser in addition to a DC current and the fine sine wave current, and making a wavelength scan with the modulated current of low frequency. CONSTITUTION:The power source 2 of a laser diode 1 is controlled by a DC power source 3, power source 4 for wavelength scanning, and power source 5 for differential modulation. Laser light 6 is absorbed by a specific gas 7 and inputted to a detector 8. The reception signal of the detector 8 is inputted to a phase detector 10 through a filter 9 which passes only the higher harmonic 2f1 of frequency twice as high as a modulation frequency f1, and phase-detected with the synchronizing signal of the power source 5. The phase-detected signal is supplied to a phase detector 12 through a filter 11 which passes the higher harmonic 2f2 of frequency twice as high as a frequency f2 for wavelength scanning and phase- detected with the synchronizing signal of the power source 4. The phase-detected signal is passed through a smoothing filter 13 and outputted as a signal 14 proportional to the concentration gas. Thus, the gas concentration is measured at a relatively high speed with high precision.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a gas concentration measuring meter (in particular, a gas concentration meter that can obtain gas concentration with high accuracy by modulating the oscillation wavelength of a laser in two directions). Regarding measurement method.

[Conventional technology]

Conventional gas concentration measuring meters include two methods: (1) differential measurement around the center wavelength of the absorption line, and (2) method in which wavelengths are measured at two step-like C points in a wider wavelength range than the absorption line. As the method (2), FIG. 4 shows a diagram of the measurement principle in which the second-order differential value of the gas absorption line is included as the fourth factor. As shown in the figure, the second-order differential value of the gas absorption line is maximum at the center of the absorption line (point α), and the minimum value (point c) is observed before and after it. The second derivative output is low even in the absence of gas, but there are various causes for this, such as the effect of etalon fringes, etc.
As a result, off-sera) hs burns, and in addition, C2 produces a signal corresponding to the summer concentration (2).

Therefore, it is necessary to correct the offset B, but since the offset fluctuates due to factors such as changes in the environmental temperature and the baseline drifts, highly accurate correction is not possible, making it difficult to measure with high precision. . In addition, in the method (2) of measuring the wavelength at multiple points in steps, the wavelength is usually changed by increasing the current of the laser diode in steps, but it takes time for the wavelength to stabilize each time the current is changed in steps. There was a drawback that it was necessary.

[Problem that the invention seeks to solve]

The present invention eliminates the influence of baseline fluctuations caused by optical recoil, which is an obstacle in gas concentration measurement, and eliminates non-measuring time such as waiting for wavelength stabilization. It provides a measurement method.

[Means for solving problems]

As shown in Figure 4, the second-order differential waveform of the gas absorption line has a maximum α and minimum J and c, and the line connecting the two minimums J and C is parallel to the baseline, and from the line segment b-C. The height hm up to the maximum α corresponds to the gas concentration. The present invention focuses on this and attempts to read the difference between the maximum and minimum of the second-order differential waveform by scanning the wavelength between the maximum and minimum using a low frequency modulation signal. Therefore, in the present invention, as shown in Fig. 6, the light source laser (2. , a low-frequency modulation current for wavelength scanning is superimposed and applied.Then, this modulated LED light is passed through a specific gas and absorbed, and a detector detects the signal S changed by the absorption.Then, the detected From the signal S, based on the above-mentioned principle, hm, that is, the difference between the maximum and minimum of the second-order differential waveform of the gas absorption line is read.For example, if the frequency of minute sine wave modulation is f t = I KHz
, the low frequency second modulation wave frequency is 7'2 = 10
ffz, the wavelength will be scanned 10 times per second. Baseline fluctuations normally occur on the order of seconds or more, but in the above case, measurements can be made in 1/10 seconds, so baseline fluctuations have no effect.

〔Example〕

FIG. 1 shows the configuration of an embodiment of the present invention, in which 1 is a laser diode, 2 is its power source, and the power source is a DC power source 6.
Power source for wavelength scanning 4. It is controlled by a differential modulation power source 5. The laser beam 6 is absorbed by a specific gas 7 (6') and input to the detector 8.

The received signal of the detector 8 is the second harmonic 2 of the modulation frequency 11.
It passes through a filter 9 that passes only fI. This 2fI signal is phase detected by the synchronizing signal 5' of the differential modulation power source 5. 10
is the phase detector for that purpose. Next, the phase-detected signal 10 is passed through a filter 11 that passes the second harmonic 2b of the wavelength scanning frequency f (where f2<L). This 2f2
The phase of the signal is detected using the synchronizing signal 4' of the wavelength scanning power source 4. 12 is its phase detector. Next, this phase-detected signal is passed through a smoothing filter 13 to obtain a signal 14 proportional to the gas concentration.

FIG. 2 shows the signal waveforms of each part of this embodiment.
The numbers assigned correspond to the numbers in FIG. A doubly modulated current such as 2 is supplied to the laser diode 1,
If the laser output is absorbed by the specific gas 7, the output of the detector 8 will be 8 (2).

Near the center of absorption, a large harmonic of twice the modulation frequency f1 is output. Therefore, the 2f1 signal (2
(equivalent to the second derivative signal) is extracted. Next, the signal sound 11 is passed through the filter 11 that passes only the second harmonic of the wavelength scanning frequency f2, 2f2, to obtain the signal 11 with the peak value A, and this 2f2
The signal is passed through a smoothing filter 16 to obtain the effective value (i3) of the 2f2 signal (gas a degree (signal proportional to two)).

〔Effect of the invention〕

According to the present invention (2), even if there is an optical offset, the offset is the same frequency component as the wavelength scanning frequency f, whereas the signal due to gas absorption is the frequency component of 2f2, so the 2f2 frequency By observing component (i), it is possible to almost eliminate the offset component.Also, even if the offset changes from moment to moment, the wavelength is scanned faster than the change, so it does not affect the time change of the offset. In addition, when the wavelength is changed discretely, it takes time to stabilize the wavelength (due to the thermal response characteristics of the laser), but according to the present invention, Since the wavelength can be scanned continuously, no waiting time is required and relatively high speed (two measurements can be performed).

[Brief explanation of drawings]

Figure 1 is a configuration diagram for explaining the gas concentration measurement method of the present invention, Figure 2 is a waveform diagram of each part thereof, and Figure 6 shows the detector output when the method of the present invention is used in the gas absorption line. The principle diagram and FIG. 4 are diagrams showing the relationship between the second-order differential value of the gas absorption line and the wavelength. 1...Laser diode 2...Power source 6...DC power source 4...Wavelength scanning power source 5...Differential modulation power source 6...Laser light 7...Specific gas 8... Detector 9...Filter 10 that passes only 2fI...Phase detector 11...Filter 12 that passes 2f2...Phase detector 13...Smoothing filter 14...Signal patent proportional to gas concentration Applicant Fujitsu Limited Agent Patent Attorney Gobe Tamamushi (1 other person) Figure 1 Figure 2 Figure 3 Figure 4 Figure ＼ - Center of absorption line

Claims (1)

[Claims] In the gas concentration measurement method, the gas concentration is measured by reading the difference between the maximum and minimum of the second derivative waveform of the gas absorption line.
A semiconductor laser is used as a light source, and in addition to a direct current and a minute sine wave current, a modulation current having a lower frequency than the minute sine wave current is superimposed on the laser, and wavelength scanning is performed using the modulation current at the lower frequency. Gas concentration measurement method.