JPS6249575B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an infrared spectroscopic analysis method, particularly to an improvement in a spectroscopic analysis method using a wavelength tunable laser as a light source.

It is already well known that spectroscopic analysis methods that utilize infrared absorption are convenient for detecting and quantifying harmful gases that cause air pollution, such as carbon monoxide (CO) and sulfur dioxide gas (SO ₂ ). It is also already known that a wavelength tunable laser that emits infrared rays is suitable as a light source for use in this spectroscopic analysis method.
As one way to improve the infrared spectroscopic analysis method using this wavelength tunable laser, the present inventors previously proposed
No. 29127 (Japanese Patent Application No. 10630/1984), we proposed a method in which the laser current flowing through the laser element is an intermittent current, and a modulation current of minute amplitude is superimposed on this to perform measurements.
In other words, the wavelength tunable laser used in the above analysis method generally consists of an element made of an alloy semiconductor containing lead (Pb), and there is a proportional relationship between the current and the frequency of the laser light, at least within a certain wavelength range. Established. That is, when the above-mentioned current value I and the frequency of the laser beam are γ, the following equation holds true.

γ=KI (K is a constant) ......(1) By using this relationship, it is possible to remove irregular fluctuations due to atmospheric fluctuations and perform stable measurements when analyzing air pollution outdoors. becomes. More specifically, this method calculates the derivative of the spectral absorption curve and normalizes the derivative with its primitive function, thereby determining the fluctuation of the measured value due to the irregular fluctuations caused by the above-mentioned atmospheric fluctuations. Its principle is to erase. The principle will be briefly explained below. The atmospheric concentration C of the harmful gas to be detected, such as carbon monoxide, the optical path length L during measurement, and the output optical power of the wavelength tunable laser to be used.
P ₀ If the optical power collected on the light receiving surface of the light receiving element after passing through the atmosphere is Pr, then Pr=K・P ₀ exp{−α(γ)CL}・(t)
......(2) The following relationship holds true. However, α(γ) is the absorption coefficient of carbon monoxide expressed as a function of the light frequency γ, and (t) is a term expressing irregular fluctuations due to atmospheric fluctuations as a function of time t. It is.
Further, K is a proportional constant. Differentiating both sides of equation (2) above with respect to γ, we see P′r=dPr/dγ=−KP ₀・Clexp{−α(γ)CL}dα/dγ・(t)……(3) . However, for simplicity, it is assumed that P ₀ is constant regardless of γ in the range of minute current fluctuations. Dividing equations (2) and (3) above, P'r/Pr=-CLα'(γ) ......(4) becomes a term indicating irregular fluctuations due to atmospheric changes ( t) disappears, and the absolute value on the right side is proportional to the concentration C of the harmful gas. Therefore, the derivative P′r of Pr is Pr
It can be seen from equation (4) above that by normalizing with However, the above
In equation (4), the dash symbol represents a derivative, that is, a differential coefficient.

In order to actually analyze the concentration of pollutant gases in the atmosphere based on the above theory, it is necessary to find the derivative P'r of the absorption spectrum by the atmosphere to be measured. this
In order to obtain P′r by actual measurement, the current supplied to the wavelength tunable laser element is intermittent, a current with a minute amplitude having a period shorter than the intermittent period is superimposed on the intermittent current, and a laser beam is output based on the intermittent current. The differential coefficient of the laser light power with respect to the laser light frequency is determined from the output signal of the amount of the component corresponding to the current of minute amplitude in the signal.

Since the concentration of harmful gases in the atmosphere is determined based on the principle described above, it is necessary that the oscillation wavelength of the laser beam be constant when the excitation current of the laser element is constant. However, when the laser element changes over a long period of time and when the laser element is cooled, the laser light wavelength shifts due to factors such as changes in the amount of liquid nitrogen, and the current value at which the absorption of the laser light by the contaminant gas peaks changes. As a result, even if the laser current is kept constant, it is no longer possible to accurately quantify the contaminant gas concentration.

Therefore, in order to keep the laser's oscillation light wavelength constant and to maintain accurate and stable detection accuracy, the intermittent current that excites the laser must be controlled at predetermined intervals to a value that causes emission of light at the absorption peak of the contaminant gas. It is necessary to do so.

The present invention relates to a current control method for correcting the wavelength of laser oscillation light by controlling the current flowing through the laser. An infrared laser is excited by supplying a variable wavelength infrared laser element from a programmable constant current power supply, and the laser light after passing through a gas space containing a specific gas to be detected using the infrared laser as a light source is converted to a photoelectric conversion element. In the gas concentration detection method, the gas concentration is detected by the absorption amount of the laser light in the gas space, in which the specific gas concentration is known separately from the gas space to be detected. A calibration path and a second calibration path filled with a specific gas at a known concentration different from the first calibration path are provided, and the laser beam is sequentially switched to pass through the two calibration paths at a predetermined time and is supplied to the laser element. A storage device is provided for storing the difference in absorption value of each calibration path of the laser beam corresponding to the intermittent current scanned in a predetermined range, and the maximum difference in the absorption value stored in the storage device is provided. The present invention is characterized in that the programmable constant current power supply is controlled to supply an intermittent current corresponding to the value to the laser element.

An embodiment of the gas concentration detection device shown in FIG. 1 will be described below.

Reference numerals 1 and 2 indicate calibration cells filled with the same type of gas as the gas to be measured at a known concentration. 3, 4, and 18 are shutters for switching the laser optical path, 5 is a laser element, 6 is a detector, 7 is a microcomputer, 8 is a programmable constant current power supply, 9
10 is an AC amplifier, 11 is a bandpass filter (B, PF), 12 is a sample hold circuit (SH), 13 is a differential amplifier, 14,1
5 is an A/D converter, 16 is a recorder, and 17 is a display device. Also, 19 to 23 are beam splitters,
24 and 25 are reflecting mirrors, 26 and 27 are concave reflecting mirrors,
28 represents a corner cube, and 29 represents a focusing lens.

When detecting the concentration of gas 30 in the atmosphere with the above configuration, first the laser beam emitted from the laser element 5 is focused by the lens 29, and then the concave reflector 2
6, passes through beam splitters 19, 20, and further 23 to be irradiated into the gas 30 in the optical path to be measured, is reflected by corner cube 28, travels back and forth, and then passes through beam splitters 23, 21, 22.
The electric signal is reflected and focused by the concave reflector 27, detected by the detector 6, and input to the AC amplifier.
7 to detect the gas concentration. This detection method itself is not particularly different from the conventional method, but as mentioned above, the present invention is characterized by a laser current control method for stabilizing the oscillation light wavelength. This will be explained in detail with reference to the following.

First, a laser beam is emitted into two calibration cells 1 and 2 filled with gases to be detected with different concentrations (preferably, the concentration of the gas to be detected in one of the calibration cells is set to zero) by shutters 3 and 4. is applied to the laser element while changing the laser excitation current minutely from the threshold value to the upper limit value at which the laser element 5 can operate as shown in FIG. Calibrate the oscillation light cell 1,
The laser beam power P absorbed by the calibration cells 1 and 2 is detected by the detector 6, and absorption characteristic curves a and b shown in FIG. 3 are obtained. However, curve a in FIG. 3 shows an absorption curve passing through a calibration cell with zero gas concentration, and curve b shows an absorption curve passing through a calibration cell having a predetermined gas concentration.

The respective data of the absorption curves a and b shown in FIG. 3 are taken into the microcomputer 7 and stored in the memory. And the absorbed power P after passing through the two calibration cells 1 and 2
The current value In with the largest difference is detected, and this current value is used as the intermittent current for excitation of the laser element.

The current △I is scanned upward and downward with minute amplitude around this intermittent current In, and the minute change in power P〓 is detected by the differential amplifier 8 while changing the current modulation value △Ix as shown in Fig. 4. do. Next, use the microcomputer to P〓/
The current modulation value that gives the maximum difference in P is detected.

The current is scanned in a current range equivalent to the half width of the P〓/P waveform obtained from the intermittent current value I and the current modulation value △I detected in this way, and the current value In corresponding to the peak position of P〓/P is scanned. , △I at the appropriate time and excite the laser element at the peak position.
By controlling the programmable constant current power source 8, the concentration of the gas 30 in the atmosphere is measured while keeping the oscillation light wavelength constant. Then, the programmable constant current power supply 8 that scans the laser element excitation current is controlled and corrected at predetermined intervals so that the intermittent current value corresponding to the peak position of P/P becomes the center of the scanned current value.

In the present invention, the oscillation wavelength of the laser element is controlled as described above, but when the wavelength gradually shifts in one direction due to long-term operation, the change in only the minute amplitude current as described above is A case may occur in which the peak position of the absorbed power P cannot be detected. In such a case, it is necessary to repeat the above-described method from the beginning to find the laser oscillation wavelength that provides the maximum value of absorbed power.

In other words, the intermittent current I is increased from the threshold value of the laser excitation current to the upper limit value at which the laser element can operate so that the value of the intermittent current corresponding to the peak position of P〓/P becomes the center of the minute change current △I. As shown in the figure, the laser oscillation light corresponding to the laser current is applied to the laser element with slight changes and is passed alternately through the calibration cells 1 and 2, and the laser light power absorbed by the calibration cells 1 and 2 is P is detected by the detector 6, and the absorption curves a and b shown in Figure 3 are obtained. By repeating this process, the oscillation wavelength with the maximum absorption power of the laser beam is known, and the detected gas concentration in the atmosphere is always stably detected. It is necessary to control the laser excitation current so that the

By periodically performing the operations of the present invention as described above and operating the microcomputer and programmable constant current power supply, the oscillation wavelength due to long-term fluctuations of the laser element can be corrected, and the analysis of the gas to be detected can be performed accurately. becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a schematic configuration of an example of a gas concentration detection system to which the present invention is applied, Fig. 2 is an explanatory diagram of laser excitation current adjustment, and Fig. 3 is an absorption diagram of laser light passing through two calibration cells. The diagram showing the curve, FIG. 4, is an explanatory diagram of the ratio P/P of the absorbed power P and the minute change P, and P when the modulation current ΔI is superimposed on the intermittent current In. 1 and 2: Calibration cell, 3, 4 and 18: Shutter, 5: Laser element, 6: Detector, 7: Microcomputer, 8: Programmable constant current power supply, 9: Control for controlling shutter according to commands from microcomputer 7. 10: AC amplifier, 11: BPF, 12: Sample and hold circuit, 13: Differential amplifier, 14,
15: A/D converter, 16: Recorder, 17: Display device, 19, 20, 21, 22, 23: Beam splitter, 24, 25: Reflector, 26, 27:
Concave mirror, 28: corner cube, 29: focusing lens, 30: gas in the atmosphere.

Claims (1)

[Claims] 1. Excite an infrared laser by supplying a laser current in which a minute amplitude current is superimposed on an intermittent current to a wavelength tunable infrared laser element from a programmable constant current power supply, and use the infrared laser as a light source to emit a specific gas to be detected. In the gas concentration detection method, the gas concentration is detected by the amount of absorption of the laser light in the gas space by making the laser light enter a photoelectric conversion element and converting it into an electric signal after passing through the gas space. Separately, a first calibration path in which the concentration of the specific gas is known and a second calibration path in which the specific gas is filled with a known concentration different from the first calibration path are provided, and the two calibration paths are performed at a predetermined time. A memory for sequentially switching and passing a laser beam through a calibration path, scanning an intermittent current value supplied to the laser element in a predetermined range, and storing a difference in absorption value of each calibration path of the laser beam corresponding to the intermittent current. A method for detecting gas concentration, characterized in that the programmable constant current power supply is controlled to supply an intermittent current corresponding to the maximum value of the stored difference between the absorption values to the laser element.