JPS6140335B2 - - 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 an improvement measure for the infrared spectroscopic analysis method using this wavelength tunable laser, the present inventors previously filed a patent application
37717, 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. It takes advantage of what is established. That is, when the value of the above-mentioned current is I and the frequency of the laser beam is 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 large pollutant gases outdoors. Become. More specifically, this method calculates the derivative of the spectral absorption curve and normalizes the derivative with its primitive function to eliminate fluctuations in measured values due to irregular fluctuations due to 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, the output optical power of the wavelength tunable laser used P ₀ , and the light receiving surface of the light receiving element after passing through the atmosphere. If the collected received light power is Pr, then the relationship Pr=K·P ₀ exp{−α(γ)CL}·f(t) (2) holds true. However, α(γ) is the absorption coefficient of carbon monoxide expressed as a function of the light frequency γ, and f(t) is the irregular fluctuation due to atmospheric fluctuations expressed as a function of time t. It is a term.
Further, K is a proportionality constant. Differentiating both sides of the above equation (2) with respect to γ, we obtain P′r=dPr/dγ =−KP ₀・CLexp {−α(γ)CL}dα/dγ・f(t)……(3) . However, for simplicity, P ₀ is assumed to be constant regardless of the range of minute current fluctuations. Above (2)
Dividing Equation and Equation (3) side by side, we get P'r/Pr=-CLα'(γ)...(4), and the term f(t) indicating irregular fluctuations due to atmospheric fluctuations disappears. , and the absolute value of the right-hand 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 the laser beam based on the intermittent current is The differential coefficient of the optical power emitted from the laser with respect to the laser beam frequency is determined from the output signal of the amount of the component corresponding to the minute amplitude current in the output 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, due to factors such as long-term changes in the laser element and changes in the amount of liquid nitrogen used to cool the laser element, the laser beam length shifts, and the laser beam wavelength set to the absorption value of the laser beam by the contaminated gas changes. Change. As a result, even if the laser drive current is kept constant, it becomes impossible to accurately quantify the contaminant gas concentration.

Therefore, in order to keep the laser's oscillation light wavelength constant and to maintain correct and stable detection accuracy, the intermittent current that excites the laser must be changed at predetermined intervals to emit light at the wavelength at the absorption peak of the contaminant gas. It is necessary to control the correction to the value.

The invention of Japanese Patent Application No. 1987-87717 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 superimposed laser drive current to a wavelength-tunable infrared laser element from a programmable constant current power source, and the infrared laser is used as a light source to pass through a gas space containing a specific gas to be detected. The subsequent laser light is made incident on a photoelectric conversion element and first exchanged into an electrical signal. In the gas concentration detection method of detecting the gas concentration based on the amount of absorption of laser light in the gas space, a first calibration path in which the specific gas concentration is known separately from the gas space to be detected, and a first calibration path in which the specific gas is a second calibration route filled with a different known concentration.
A calibration path is provided, and the laser is sequentially switched to pass through the two calibration paths at a predetermined time, and the intermittent current value supplied to the laser element is scanned in a predetermined range, and each of the laser beams corresponding to the intermittent current is A storage device is provided for storing the difference in absorption values of the calibration paths, and the programmable constant current power source is controlled to supply the laser element with an intermittent current corresponding to the maximum value of the difference in the absorption values stored in the storage device. It was characterized by the fact that

However, the conventional method described above requires not only the first and second calibration paths, but also a plurality of shutter mechanisms that sequentially switch the laser beam to both calibration paths, and a control device for the shutter mechanisms, and each of these parts is required. A delicate adjustment was necessary. Furthermore, the conventional methods described above were aimed at correcting wavelength fluctuations in laser radiation due to relatively slow phenomena such as a decrease in the amount of liquid nitrogen as a cryogen or aging of a laser element. The drawback is that it is not possible to correct wavelength fluctuations in the synchrotron radiation caused by high-speed changing phenomena such as the formation of bubbles or the vibration of the refrigerant in a refrigerator.

The present invention has been made in view of these drawbacks, and instead of the first and second calibration paths, an optical path dedicated to automatic frequency control (hereinafter referred to as AFC) is provided, and laser emitted light detected by this optical path is provided. This system directly transmits wavelength fluctuations to a built-in microcomputer (hereinafter referred to as microcomputer) and uses a programmable constant current power supply to change the laser current according to commands from the microcomputer to perform high-speed response AFC. The details will be explained below using the drawings.

FIG. 1 shows an embodiment of the gas concentration detection device, 1
2 is a calibration cell filled with a gas of the same type as the gas to be measured 30 at a known concentration. 5 is a laser element, 6 is a detector, 7 is a microcomputer, 8 is a programmable constant current power supply, 10a, 10b is an AC amplifier, 11a,
11b is a band pass filter (B・P・F), 1
2a and 12b are sample and hold circuits (S/
H), 13a13b is a differential amplifier, 14a, 14
b is an A/D converter, 16 is a recorder, and 17 is a display device. Also, 19 and 23 are beam splitters,
26, 27, and 32 are parabolic mirrors, and 28 is a corner cube.

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 reflected by the parabolic mirror 26, passes through the beam splitters 19 and 23, and is Gas 3 in measurement optical path
0, is reflected by the corner cube 28, goes back and forth, passes through the beam splitter 23, is reflected by the parabolic mirror 27, is focused, is detected by the detector 6, and is input to the AC amplifier 10b.The electric signal is sent to the microcomputer. 7, recorder 16 or display 1
7 to detect the gas concentration. Although this detection method itself is not particularly different from the conventional method, the present invention is characterized by a laser current control method for stabilizing the oscillation light wavelength as described above.

It is reflected by the corner cube 28 and enters the optical system 31 from the external space and enters the beam splitter 23.
The branched light is detected by the detector 6 via the parabolic mirrors 27 and 32, but there is a reflecting mirror 21
and a gas cell 2 filled with a gas of known concentration.
is designed to be removable. Therefore, in order to obtain a calibration curve for measuring the concentration of gas 30 in the external space, first insert the reflecting mirror 21 into the optical path A of the optical system 31, and also insert the gas cell 2 in front of the detector 6. are introduced as shown by the arrows B, and the light from the laser 5 passes through the total reflection mirror 2.
1, the absorption value A ₁ shown in FIG. 4 corresponding to the gas concentration D ₁ in the gas cell 2 is determined on the recorder 16. After this, if the reflector 21 is inserted as it is and the gas cell 2 is removed in the direction of arrow C, a gas-free state is realized, so the zero concentration value in this case
D ₀ , the point Q ₀ determined by the zero absorption value A ₀ , and the previous concentration
By connecting D ₁ and point Q ₁ determined by the absorption value A ₁ , a calibration curve O as shown in FIG. 4 can be drawn. Thereafter, if the reflecting mirror 21 is removed, absorption A ₂ by the gas 30 in the external space will occur, so the concentration D ₂ of the gas 30 is determined from the point Q ₂ on the curve O corresponding to this value A ₂ . Here, the drive current from the programmable current source 8 is applied to the laser element while being slightly changed from the threshold value to the upper limit value at which the laser 5 can operate, as shown in FIG. Among them, the laser light power P _A that is split into the optical path E direction by the beam splitter, guided into the gas cell 1, and absorbed in the gas cell 1 is detected by the detector 4.
The absorption characteristic curve shown in FIG. Then, the current value I _o having the largest difference in the absorbed power P _A after passing through the gas cell 1
₀ is detected, and this current value I _o0 is used as the current for driving the laser element.

Next, around the current value _Io0 detected here,
As shown in FIG. 5b, from the programmable constant current power supply 8 within the absorption spectrum band range surrounded by the dotted line in FIG. 3 or FIG. 5a. The intermittent current I _B is modulated by the minute modulation current △I _M and the current that increases sequentially in minute steps △I is transmitted to the laser element 5.
is applied to drive. In this way, since the curve N representing the power P _A in FIG. 5a is differentiated by the minute modulation current ΔI _M , a differential power P' _A is obtained. These P _A and P' _A appear at each output end of the A/D converters 15a, 14a in the signal processing means 40 in FIG.
These are divided in the microcomputer 7, and as a result, the data corresponding to each point on the curve P' _A /P _A in Figure 5a is digitized and stored in the memory 9 of the microcomputer 7. be done.

Here, the above P′ _A /P _A curve has two peaks, and the fifth peak corresponds to the position of one of them.
The current value I _o1 shown in Figure a is naturally detected by the microcomputer.

Now, if the data of P′ _A /P _A stored in the memory 9 is shown graphically, the curve W in FIG.
It is assumed that the peak current value of the curve W is given by _o1 . As mentioned above, since the emission wavelength from the laser element 5 fluctuates toward the short wavelength side or the long wavelength side at a speed of several 100 msec, this fluctuation corresponds to a leftward or rightward fluctuation in the P' _A /P _A curve. appears as

Therefore, P' _A /P _A is detected several hundred milliseconds after the data storage of P' A /P A corresponding to the curve W is completed, and the peak current of P _{' A} /P _A exhibited by the curve _W is detected. When the value I _o2 is detected, the corrected current value to be supplied from the programmable constant current power supply 8 in order to correct the laser emission frequency, that is, apply AFC, is I _o2 - I _o1 = x・△I ...(5) It can be expressed as Here, if the scan current change width ΔI shown in FIG. 5b is set to 100 μA in the programmable constant current power supply 8, and the number of steps The drive current value is 5×100 μA=500 μA.

The current value to be corrected has been determined above, but it is unclear whether the above wavelength fluctuation occurred in the short wavelength direction or in the long wavelength direction, so this needs to be clarified. . To do this, the drive current is actually varied by |Ix| in both positive and negative directions using a programmable constant current power supply.
Both values of P' _A / _PA at each current value of I _o1 +I _X and I _o1 - I _X are detected and viewed. If the values of P′ _A /P _A obtained in this way at each current value are _{p 1} and _{p 2 in} FIG. It is immediately clear whether the direction in which the should be corrected is positive or negative. Therefore, in this case, the microcomputer 7 may be programmed in advance to issue a command so that the laser drive current is shifted in the direction in which P' _A /P _A shows a large value.

If this fluctuation of the laser wavelength does not stop even with the above AFC, and it drifts even further as shown in the curve YO of Fig. 6, the laser wavelength will be changed based on the laser drive current that was corrected last time and becomes I _o2 . hand,
After several 100 msec again, the drive current is changed to +I _X and _-I
Both are changed by the programmable constant current power supply 8. In this way, the value of P' _A /P _A indicates the respective values p ₃ and p ₄ with respect to the curve Y in FIG. 6, and p ₃ > p ₄
Since the microcomputer 7 immediately determines that the current change that requires further correction is to be applied in the positive direction, the correction value is determined to be I _o3 − by the same procedure as above. It turns out that I _o2 , and we can apply AFC again here.

Separately, when observing the gas 30 for a long time using the above device, P' _A /P _A may change to a different value due to changes in the laser element 5 over time, misalignment of the optical axis, etc. Therefore, in order to correct this, the current can be rescanned within a new absorption spectrum band as shown in FIG. 5, and the memory can be updated.

In this way, the optical system consisting of the gas cell 1 whose gas concentration is known and the detector 4 and the amplifier 10a are connected to the A/D.
It is in charge of the AFC of the laser light frequency in the signal processing system that reaches the converters 14a and 15a.
While the AFC function is always activated, the gas 30 in the external space is If the laser light power P that reflected the absorption and entered the detector 6 and its differential value P' are taken out to the output terminal of the A/D converter, and these data are digitized and processed by the microcomputer 7, it can be done at high speed. Since the concentration of the gas 30 can be measured and evaluated while constantly performing automatic frequency control on the fluctuating emission wavelength or emission frequency of the laser element, great practical effects can be expected.

[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 a diagram of laser drive current adjustment, and Fig. 3 is an absorption curve of laser light passing through a calibration cell. Figure 4 shows the calibration curve of gas concentration and absorption amount, Figure 5 a and b shows the pattern of current scan across the gas absorption spectral band, and Figure 6 shows P' _A / FIG. 3 is a diagram for explaining the variation of the P _A curve and how the AFC mechanism tracks the variation. 1 and 2: gas cell, 4, 6: detector, 5:
Laser element, 7: Microcomputer, 8: Programmable constant current power supply, 10a, 10b: AC amplifier, 11
a, 11b: B, P...F, 12a, 12b: sample hold circuit, 14a, 14b, 15a,
15b: A/D converter, 16: recorder, 17:
Display device, 19, 23: Beam splitter, 2
1: Reflector, 26, 27, 32: Parabolic mirror, 2
8: corner cube, 30: gas in the atmosphere.

Claims (1)

[Claims] 1. Infrared laser generation is achieved by repeatedly supplying a laser current, in which a current of minute amplitude is superimposed on each step of a current whose magnitude sequentially changes in minute steps over a predetermined range, to a wavelength tunable infrared laser element from a programmable constant current power supply. By changing the emission frequency and using the infrared laser element as a light source, the laser light after passing through a gas space to be measured containing a specific gas to be detected is incident on a photoelectric conversion element to convert it into an electrical signal, and the laser beam in the gas space is In a gas concentration detection method that detects gas concentration from differential absorption characteristics of changes in absorption amount due to changes in the frequency of light, a gas cell filled with a gas of known concentration is inserted in parallel to the measurement optical path that includes the gas space to be measured. A control optical path is provided, and the differential absorption characteristic of the light that follows the control optical path corresponding to a step change in a predetermined range of the current supplied to the laser element in the known concentration gas cell is detected, and the peak of the absorption characteristic is detected. Based on the current value at the point,
The deviation of the peak point of the absorption characteristic obtained in the next repetition period of the laser current is detected, and the current value corresponding to the previous reference point is increased or decreased in steps so that the difference is reduced. A gas concentration detection method characterized in that frequency changes relative to a reference current value are automatically tracked and corrected.