JP2866230B2 - Gas concentration measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration measuring device for measuring a gas concentration using a semiconductor laser device.

[0002]

2. Description of the Related Art It is known that the presence or absence of gas can be detected by utilizing the fact that laser light of a specific wavelength is easily absorbed by a certain kind of gas. Sensing technology using this principle is used in industrial measurement and pollution monitoring. Widely used in such as. As an example, it is possible to detect the presence or absence of methane with high sensitivity by utilizing the fact that light having a wavelength of 3.392 μm generated from a He-Ne laser is strongly absorbed by methane. Since methane is the main component of city gas, leakage of city gas can be detected by detecting methane. Although methane is contained in the atmosphere at a concentration of about 2 ppm, it has been increasing in recent years and is said to be the leading cause of global warming after carbon dioxide gas. It is important to measure well. Here, the concentration is defined as the ratio between the partial pressure of the gas of interest and the total pressure of the atmosphere.

FIG. 8 is a schematic configuration diagram of a conventional gas concentration measuring device.

[0004] A gas concentration measuring device is a device for measuring the concentration of a specific gas contained in the atmosphere. It has reflecting mirrors 1 and 2 at both ends, and has a cylindrical multiplex shape through which the air can freely flow in and out. A reflection long optical path cell 3, a laser oscillator 4 for irradiating the atmosphere from the outside to the inside of the cell 3, and an oscillation wavelength of the laser oscillator 4 modulated at a predetermined frequency and stabilized at the center of an absorption line of a gas of interest. A laser modulation / controller 5 for controlling, a photodetector 6 for receiving the laser light passing through the cell 3, and a phase sensitive detection signal _{If for} detecting the output of the photodetector 6 by phase sensitive detection.
A phase sensitive detector 7 for outputting a phase-sensitive detector 8 for outputting a second harmonic wave phase-sensitive detection signal I _2f, dividing the second harmonic wave phase-sensitive detection signal I _2f the fundamental wave phase-sensitive detection signal I _f And a divider 9.

When such a device is used to continuously measure the concentration of methane contained in the atmosphere, the measurement is performed while the air is always sucked in from the inlet 10 of the cell 3 and discharged from the outlet 11. The laser light emitted from the laser oscillator 4 is 2
Since the light is received by the light receiver 6 after reciprocating between the two reflecting mirrors 1 and 2 a plurality of times, the distance passing through the atmosphere in the cell 3 is extended, and the measurement sensitivity is improved. The laser light received by the light receiver 6 is converted into an electric signal and input to the phase sensitive detectors 7 and 8. The divider 9 converts the second-harmonic phase-sensitive detection signal I _2f (proportional to the intensity of the laser beam and the methane concentration) output from the phase-sensitive detector 8 into a fundamental wave phase output from the phase-sensitive detector 7. The methane concentration is calculated by dividing by the sensitive detection signal _If (which is proportional to the intensity of the laser beam).

[0006]

The above-mentioned gas concentration measurement utilizes the fact that the second-harmonic phase-sensitive detection signal I _2f is proportional to the gas concentration. However, as will be described later in the description of the embodiments, the second-harmonic phase-sensitive detection signal I _2f depends not only on the gas concentration but also on the temperature and pressure of the atmosphere. The prior art does not take this point into account and cannot accurately determine the gas concentration.

The present invention has been made in view of the above points, and an object of the present invention is to accurately measure the concentration of a gas regardless of the temperature and pressure of the atmosphere.

[0008]

According to the present invention, there is provided, in accordance with the present invention, a modulating means for modulating a laser beam with a modulating current having a predetermined frequency and amplitude, and an atmosphere containing a gas whose concentration is to be measured. A measuring cell formed so that the laser beam passes through the air that has flowed in, a temperature sensor that detects the temperature in the measuring cell, and a laser that has passed through the air in the measuring cell. A photodetector for receiving light, a first phase-sensitive detector for outputting a fundamental phase-sensitive detection signal from the output of the photodetector, and a second phase-sensitive detector for outputting a second-harmonic phase-sensitive detection signal from the output of the photodetector A phase-sensitive detector, a divider that calculates the gas concentration by dividing the value of the second-harmonic phase-sensitive detection signal by the value of the fundamental-wave phase-sensitive detection signal, and a temperature measured by the sensor. To correct the density by And a No. processor, the modulation means
The amplitude of the modulation current is determined by the amount of absorption relative to the frequency of the laser light
Is achieved by a gas concentration measuring device having a value corresponding to approximately 2.2 times the half width at half maximum of

[0009]

According to the gas concentration measuring apparatus of the present invention, laser light emitted from a semiconductor laser modulated at a predetermined frequency and amplitude is passed through the atmosphere in a measuring cell, and the passed laser light is received by a photodetector. When receiving the signal and inputting the signal from the photodetector to two phase-sensitive detectors, when obtaining the concentration from the fundamental phase-sensitive detection signal and the second harmonic phase-sensitive detection signal,
The modulating means adjusts the amplitude of the modulation current to the frequency of the laser light.
Equivalent to approximately 2.2 times the half width at half maximum of the absorption amount with respect to the wave number
As a result, the signal processor can correct the density only by the temperature measured by the temperature sensor.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an embodiment of a gas concentration measuring apparatus according to the present invention. The same reference numerals are used for the same components as those of the conventional apparatus shown in FIG. Here, an apparatus for measuring the concentration of methane gas will be described, but the principle is the same when measuring the concentration of another gas.

The gas concentration measuring device is a device for measuring the concentration of methane contained in the atmosphere.
2, a multi-reflection long optical path cell 3 into which air can flow in and out, a laser oscillator 4 for irradiating the atmosphere in the multi-reflection long optical path cell 3 with laser light, and an oscillation wavelength of the laser And a laser modulator / controller 5 for stabilizing control at the center of the methane absorption line, and a photodetector 6 for receiving laser light passing through the multiple reflection long optical path cell 3
A phase-sensitive detector 7 for phase-sensitively detecting the output from the photodetector 6 and outputting a fundamental phase-sensitive detection signal _If ;
Phase-sensitive detector 8 that outputs a harmonic phase-sensitive detection signal I _2f
A divider 9 for calculating the gas concentration by dividing the second-harmonic phase-sensitive detection signal I _2f by the fundamental-wave phase-sensitive detection signal _If , and a temperature sensor provided inside the multiple reflection long optical path cell 3 12
And a signal processor 13 for correcting the concentration of methane based on the temperature of the atmosphere measured by the method.

The multi-reflection long optical path cell 3 is a substantially cylindrical container, having two reflecting mirrors 1 and 2 at both ends, an intake port 10 into which air flows in, and an exhaust port 11 outflowing air.
A temperature sensor 12 is attached near the exhaust port 11. The reflecting mirrors 1 and 2 are arranged so that the laser light incident from the laser oscillator is reflected a plurality of times, emitted to the outside again, and received by the light receiver 6. The multi-reflection long optical path cell 3 is a cell for reciprocating the laser light a plurality of times to increase the optical path length in the atmosphere. However, the present invention is not limited to this. May be passed.

As the temperature sensor 12, a thermistor, a thermocouple or the like is used.

The signal processor 13 is an essential part of the present invention, and has a function of correcting the concentration of methane from the temperature of the atmosphere on the basis of theoretical studies described later. For the signal processor 13, a microprocessor is preferably used.

The laser modulation / controller 5 modulates the driving current of the semiconductor laser at a predetermined frequency and amplitude,
The oscillation wavelength of the laser light is controlled to be stabilized at the center of the methane absorption line.

FIG. 2 is a schematic configuration diagram showing the structure of the laser oscillator 4 and the laser modulation / controller 5.

The laser oscillator 4 includes a semiconductor laser element 14 that emits laser light in both front and rear directions, and a Peltier that functions to change the temperature of the laser element 14 to change the oscillation wavelength by generating or absorbing heat according to the magnitude and direction of the current. Element 15;
A reference gas cell 16 filled with methane, a light receiver 17 for receiving a laser beam after passing through the reference gas cell 16 and converting the laser beam into an electric signal, and a collimating lens for collimating a laser beam in front of the semiconductor laser element 14 18. The forward laser light is incident on the multiple reflection long optical path cell 3 of FIG.

The laser modulation / controller 5 includes a laser element 14
A constant current source 23 for supplying a constant DC current to the oscillator, an oscillator 19 for generating an AC current having a predetermined frequency and amplitude to modulate the oscillation frequency of the laser element 14, and a DC current and an AC current. A current mixer 24 for superposition, a phase sensitive detector 20 for outputting a fundamental phase sensitive detection signal based on a signal from the light receiver 17, an integrator 21 for integrating the output voltage of the phase sensitive detector 20, , A current source 22 for adjusting the current supplied to the Peltier element 15 according to the output of the integrator 21, and a frequency of the oscillator 19 which is doubled and output. And a frequency multiplier 26.

The phase-sensitive detector 20 determines whether the center oscillation frequency of the laser is higher or lower than the center frequency of the methane absorption line, and outputs a voltage having a magnitude and a sign corresponding to that. Then, the temperature of the Peltier element 15 changes accordingly, and the center oscillation frequency of the laser is automatically adjusted to the center of the absorption line of methane.

A laser beam having a frequency coincident with the center of the methane absorption line and frequency-modulated with a constant amplitude is also emitted in front of the laser element 14, and this light is subjected to the multiple reflection shown in FIG. The concentration of methane in the atmosphere after being incident on the long optical path cell 3 is measured.

Next, the output signals of the two phase-sensitive detectors 7 and 8 shown in FIG. 1 will be considered.

FIG. 3 is an explanatory diagram for schematically obtaining a phase-sensitive detection signal. Curve A shows the relationship between the laser oscillation frequency ω (horizontal axis) and the output voltage V of the light receiver 6 (vertical axis) near the absorption line of methane centered at ω ₀ . Assuming that the output voltage when there is no absorption is V ₀ , this curve A is expressed by Equation 1.

[0024]

V = V ₀ (1−αL) where α is called an absorption coefficient and is a function of frequency ω.
It is represented by Equation 2. L is the optical path length, which is the total length of the distance that the laser light passes through the atmosphere.

[0025]

The [number 2] ^{_{α = α 0 γ 2 / [}} (ω-ω 0) 2 + γ 2] γ in equation (2) is referred to as a half width at half maximum of the absorption line, V in the curve A ₀
The difference between ω ₀ and the frequency (there are two) at which the amount of decrease from ω is half the amount of decrease at the center ω ₀ is plotted in FIG.
α ₀ is the absorption center, that is, the value of α at ω = ω ₀ ,
Depends on methane concentration.

Now, assuming that the center of the laser oscillation frequency ω is stabilized at the center ω _{0 of the} absorption line and modulated at a predetermined frequency f and amplitude M as described above, ω is a function of time t. As shown in Equation 3.

[0027]

Ω = ω ₀ + Mcosft When ω in Equation 3 is drawn as a function of time t in FIG.
Becomes Here, the vertical axis is time t. Since ω oscillates with a width of M right and left around ω ₀ , the corresponding position on the curve A oscillates along the curve A between the points a and b. As a result, the output voltage of the light receiver changes as shown by the curve C. Here, the horizontal axis is time t. Note that V oscillates twice while ω oscillates once. Therefore, although the output voltage V has a component oscillating at the frequency 2f,
There is no component oscillating at the frequency f. Further, the curve C is not a simple sine wave but a superposition of frequency components such as 4f and 6f in addition to 2f. The phase sensitive detector 8 is
It has the role of extracting the 2f component from this and determining its amplitude I _2f . As can be seen from FIG. 3, this amplitude I _2f depends on the modulation amplitude M. As the modulation amplitude M increases, the amplitude
I _2f initially increases, but gradually decreases after reaching a local maximum at some point. Detailed analysis (H. Wahlquis
t, J. Chem. Phys. 35 (1961) 170
Using the result of 8), the relationship between the modulation amplitude M and the amplitude I _2f is as shown in _Expression 4.

[0028]

## EQU4 ## I _2f ∝V ₀ α ₀ Lx ² / (1 + x ² ) ^1/2 (1
+ (1 + x ² ) ^1/2 ) ² where x is the ratio between the modulation amplitude M and the half width at half maximum γ.
Is defined by

[0029]

X = M / γ In particular, when x << 1, Equation 4 is approximated by Equation 6.

[0030]

## EQU6 ## I _2f ∝V ₀ α ₀ Lx ² FIG. 4 is a graph showing the relationship between the amplitude I _2f and x represented by the equation (4). From this, it can be seen that I _2f takes a maximum value near x = 2.2.

[0031] Now, in proportion to the amplitude I _2f is alpha ₀ According to Equation 4, since alpha ₀ is dependent on the methane concentration, if measure the signal strength of the methane gas in advance known concentration, the amplitude I _2f methane The concentration can be measured. This is the principle of concentration measurement.

However, when the intensity of the laser beam received by the light receiver 6 fluctuates due to causes other than the absorption of methane, for example, dust in the air, dirt on the reflecting mirrors 1 and 2 and deformation of the optical system. Cannot immediately determine the concentration of methane from the magnitude of the amplitude I _2f . Therefore, it is desired to measure the intensity of the laser beam received by the light receiver 6 by some method. The phase sensitive detector 7 performs that function. In the discussion so far, the output voltage V of the photodetector 6 has no component oscillating at the frequency f, so that the amplitude _If should always be zero.
There is another cause for the I _f. That is, when the drive current of the semiconductor laser is modulated at the frequency f, the oscillation frequency of the laser is modulated as shown in Expression 3, and the laser output is also modulated at the frequency f. As a result, the output voltage of the light receiver 6 has a component of the frequency f represented by the equation (7).

[0033]

V ′ = V ₁ cos (ft + φ) where the amplitude V ₁ depends on the modulation amplitude of the drive current. Φ represents a phase shift. Therefore, the output _If of the phase sensitive detector 7 is given by Expression 8.

[0034]

Equation 8] determining the ratio of I _f alpha] V ₁ where I _2f and I _f becomes several 9.

[0035]

## _EQU9 ## I _2f / I _f ∝ (V ₀ / V ₁ ) α ₀ Lx ² / (1+
x ² ) ^1/2 (1+ (1 + x ² ) ^1/2 ) ² and this ratio does not depend on the light intensity entering the receiver 6.
This is because both V ₀ and V ₁ are proportional to the light intensity. Therefore, even if the light intensity entering the light receiver 6 varies,
Α ₀ is determined from I _2f / _If , and the concentration of methane can be measured.
The divider 9 has a function of calculating this ratio.

However, it is still not possible to accurately determine the methane concentration using only this ratio. Because α ₀ depends not only on the methane concentration but also on the temperature and pressure of the atmosphere. Further, x in Equation 9 is inversely proportional to the half width γ of the absorption line as in Equation 5, and γ also depends on temperature and pressure. Therefore, in order to determine the methane concentration accurately, it is necessary to consider the effects of atmospheric temperature and pressure. Therefore, it is considered how α ₀ and γ depend on the temperature T and the pressure P of the atmosphere.

First, the value obtained by integrating α given by Equation 2 over the entire region of ω, that is, the integrated intensity is proportional to the product α ₀ γ. On the other hand, this is proportional to the number density of the molecule responsible for absorption. I know. Therefore, when the total molecular number density of methane is n and the distribution rate of the lower level of the absorption transition of interest is g, Equation 10 holds.

[0038]

Α ₀ γ∝gn From this, Expression 11 is obtained.

[0039]

Α ₀ ∝gn / γ Then, the T and P dependencies of n, g, and γ are examined.

First, n is immediately obtained from the equation of state of gas as shown in Expression 12.

[0041]

Where ρ is the methane concentration to be determined and represents the ratio of the partial pressure of methane to the total pressure of the atmosphere. k is Boltzmann's constant.

Next, when the main cause is the collision between molecules, the half-width at half maximum γ of the absorption line is proportional to the density of all molecules and the average velocity of molecules when the simplest model is used. 13 is obtained.

[0043]

[Number 13] γαP / T ^1/2 Finally, examine the T dependence of the distribution rate g. In general, when the energy of a certain level i is E _i and the degree of degeneracy is w _i , the distribution number of the level i in the thermal equilibrium state is w _i exp (−E _i / k
T). Therefore, the distribution ratio g _i of the level _i is given by Expression 14.

[0044]

G _i = w _i exp (−E _i / kT) / Σw _j
exp (−E _j / kT) Here, Σ means the sum of all j. When the sum is executed using the known expression of E _i and w _i for the methane-type molecule, Expression 14 becomes Expression 15.

[0045]

G _J ∝exp [−BJ (J + 1) / kT] /
T ^3/2 Here, J is the quantum number of the rotational level of interest of the molecule, and B is the rotational constant of the molecule.

Using the results of equations (11), (12), (13) and (15), it can be _seen from _equation (4) that the dependence of I _2f on T and P is
Get.

[0047]

## EQU16 ## I _2f ∝V ₀ ρLx ² exp [-BJ (J +
1) / kT] / T ² (1 + x ² ) ^1/2 (1+ (1 + x
² ) ^1/2 ) ² where x = M / γ∝MT ^1/2 / P, where M is the modulation amplitude used in equation (3).

According to equation (16), I _2f is proportional to the concentration ρ, but also depends on the temperature T and the total pressure P (x is T,
Note that it is a function of P, M). So, if we know how I _2f changes with T and P,
The gas concentration can be accurately obtained by the correction.

For methane, B = 5.24 cm ^-1 and the Q of the 2ν ₃ absorption band at a wavelength of 1.66 μm.
Looking at the line (6), since J = 6, Equation 16 becomes Equation 17.

[0050]

## EQU17 ## I _2f ∝V ₀ ρLx ² exp (−317 / T)
/ T ² (1 + x ² ) ^1/2 (1+ (1 + x ² ) ^1/2 ) ² Here, the unit of T is K.

FIG. 5 and FIG. 6 are calculated according to equation (17).
It is a graph which shows T and P dependence of _I2f . T = 29
8K, P = 1013 mbar as a reference state,
It shows how I _2f changes depending on T and P in the vicinity. Reference state in both figures is indicated by the point P _0.

[0052] First, FIG. 5 is a view when the modulation amplitude M is equal to 1.5 times the half width at half maximum of the absorption line gamma (under standard conditions), and I _2f and P to the temperature of the four types Is shown by a solid line. In this case, I _2f changes regardless of which of T and P changes. Therefore, both temperature and pressure measurements are needed to correct the concentration. Then, the measured temperature and pressure are supposed to be the point P ₁ (T = 288K, P =
When 1000 mbar), reading point on the vertical axis corresponding to P ₁ is because it is 1.035, divided by 1.035 to the output of the divider 9 gives the correct concentration.

On the other hand, FIG. 6 is a view similar to the case where M = 2.2γ. The feature in this case is that all the solid lines are substantially horizontal, indicating that I _2f hardly depends on the pressure P. This corresponds to the fact that when x = 2.2 in FIG. 4, I _2f reaches a maximum and the slope of the curve becomes zero. Therefore, if the modulation amplitude is selected so that M = 2.2γ, the concentration can be corrected only by measuring the temperature. If the measured temperature is point P ₂ (T = 288 K), the density is corrected by dividing the output of the divider 9 by the value 1.032 on the vertical axis corresponding to the point P ₂ . The signal processor 13 has a role of performing this correction based on the temperature measured by the temperature sensor 12, and can be realized by, for example, a microprocessor.

As described above, it is an important feature of the present invention to have a signal processor which can correct the density only by measuring the temperature.

FIG. 7 shows a measurement example of the time change (a) of the methane concentration and the time change (b) of the gas temperature by the gas concentration measuring apparatus of the present embodiment (January 14 to 15, 1991).
Day measurement). In FIG. 7A, the horizontal axis represents time (hour: minute), and the vertical axis represents methane concentration (ppm). A concentration fluctuation of about 25% is observed in 24 hours. In the figure, the thicker part of the curve indicates that methane is carried from the swamp and other methane sources by the wind, and the concentration changes quickly and greatly. The vertical axis in FIG. 6B indicates the temperature (K), but the temperature change in the measurement cell was within 3K.

As described above, according to the present embodiment, the laser beam emitted from the semiconductor laser device 14 modulated at the predetermined frequency f passes through the atmosphere containing methane gas in the multiple reflection long optical path cell 3 and then receives light. The signal is received by the detector 6 and its intensity is converted into an electric signal. The fundamental wave phase-sensitive detection signal _If obtained by the phase-sensitive detector 7 is used as the second harmonic phase-sensitive detection signal If obtained by the phase-sensitive detector 8. _When calculating the methane concentration by dividing _2f using the divider 9,
By setting the modulation amplitude to about 2.2 times the half width at half maximum γ, I _2f is maximized. As a result, the temperature is independent of the pressure of the atmosphere and the temperature is measured by the signal processor 13 only from the temperature measurement by the sensor 12 The density is corrected.

[0057]

As described above, in the present invention,
After the laser light emitted from the semiconductor laser element modulated at a predetermined frequency passes through the gas in the measuring cell, the intensity is converted into an electric signal by a light receiver, and the fundamental wave obtained by the two phase-sensitive detectors When obtaining the concentration from the phase-sensitive detection signal and the second-harmonic phase-sensitive detection signal, the modulation amplitude is set to the optimum value, so that the signal processor determines the gas temperature only by the gas temperature regardless of the gas pressure. It is possible to correct the density accurately.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of a gas concentration measuring device according to the present invention.

FIG. 2 is a schematic configuration diagram of a laser oscillator and a laser modulation / controller shown in FIG. 1;

FIG. 3 is a diagram illustrating a relationship between an oscillation frequency of a laser beam and an output of a light receiver, and a relationship between modulation of the two.

FIG. 4 is a graph showing a relationship between a modulation amplitude and a second-harmonic phase-sensitive detection signal.

FIG. 5 is a diagram illustrating temperature and pressure dependence of a second-harmonic phase-sensitive detection signal.

FIG. 6 is a diagram illustrating temperature and pressure dependence of a second-harmonic phase-sensitive detection signal.

FIG. 7 is a graph showing a measurement example of the time change (a) of the concentration of methane in the atmosphere and the time change (b) of the temperature by the gas concentration measurement device of the present embodiment.

FIG. 8 is a schematic configuration diagram of a conventional gas concentration measurement device.

[Explanation of symbols]

 REFERENCE NUMERALS 1, 2 Reflecting mirror 3 Measurement cell 4 Laser oscillator 5 Laser modulation and controller 6, 17 Photodetector 7, 8, 20 Phase-sensitive detector 9 Divider 12 Temperature sensor 13 Signal processor 14 Semiconductor laser device 15 Peltier device 16 Gas Cell for Reference 18 Collimator Lens 19 Oscillator 21 Integrator 23 Constant Current Source 24 Current Mixer 25 Bias Voltage Generator 26 Multiplier

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) G01N 21/00-21/01, 21/17-21/6 1 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. A modulating means for modulating a laser beam with a modulation current having a predetermined frequency and amplitude, and an atmosphere containing a gas whose concentration is to be measured flows in, and the laser beam passes through the atmosphere. Measurement cell formed as described above, a temperature sensor for detecting the temperature in the measurement cell, a light receiver for receiving laser light after passing through the atmosphere in the measurement cell, and an output of the light receiver A first phase-sensitive detector that outputs a fundamental phase-sensitive detection signal from a second phase-sensitive detector, a second phase-sensitive detector that outputs a second-order phase-sensitive detection signal from the output of the photodetector, and a second phase-sensitive detector. A divider that calculates the concentration of the gas by dividing the value of the detection signal by the value of the fundamental phase-sensitive detection signal, and a signal processor that corrects the concentration based on the temperature measured by the temperature sensor. Prepared, front
The amplitude of the modulation current of the modulation means to the frequency of the laser light.
A value equivalent to approximately 2.2 times the half width at half maximum of the absorption amount
A gas concentration measuring device. (2)The gas is methane gas;
The density optical path length product ρL, which is the product of the density ρ and the optical path length L, is
formula I_2f∝V_o ρL_exp (-317 / T) / T² I _2f Is the value of the second harmonic phase sensitive detection signal V _o Is the output power of the receiver when there is no absorption of the laser light.
Pressure T is temperature (K) The density optical path is set and corrected to satisfy
Find long product The gas concentration according to claim 1, wherein
measuring device.