JP2698314B2 - Optical gas analyzer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical gas analyzer for irradiating a sample cell into which a gas to be measured has been introduced with a laser beam and detecting the concentration of the gas to be measured from the light absorption intensity thereof.

[0002]

2. Description of the Related Art Conventionally, there has been known an apparatus which measures a light absorption spectrum of a gas introduced into a sample cell and obtains a gas concentration in the sample cell from the light absorption intensity of the spectrum. This optical gas analyzer has an advantage that the gas concentration can be easily measured without contact.

FIG. 4 shows a configuration of a conventional optical gas analyzer. This apparatus includes a semiconductor laser 1 and a sample gas introduction /
The sample cell 2 to which the gas to be measured (sample gas) is introduced from the outlet 3 and irradiated with the laser light output from the semiconductor laser 1, and the photodetector 4 for detecting the intensity of the laser light transmitted through the sample cell 2 And The semiconductor laser 1 can sweep the frequency near the absorption spectrum of the gas to be measured by the temperature sweep by the temperature controller 6.
The output of the laser beam can be controlled by controlling the driving current. Further, the semiconductor laser 1 can be frequency-modulated by being current-modulated by a signal from the oscillator 8 via the current control unit 7.

Therefore, the laser beam from the semiconductor laser 1 whose frequency has been swept and modulated is incident on the sample cell 2,
After a part is absorbed by the gas to be measured in the sample cell 2, the intensity is detected by the photodetector 4. Then
In the lock-in amplifier 5, only the signal synchronized with the oscillator 8 is detected from the intensity signal detected by the photodetector 4, and as a result, the drift of the light intensity of the semiconductor laser 1 can be removed, and the S / N A signal with a good ratio can be detected.

[0005] The signal detected in this manner has a first derivative shape of the light absorption spectrum. Therefore, the concentration of the gas to be measured can be obtained by obtaining the light absorption intensity from the peak value of the detection signal near the frequency of the light absorption spectrum. If the lock-in amplifier 5 detects only the frequency component twice as high as the oscillation signal from the oscillator 8, the second derivative shape of the light absorption intensity can be measured. According to this method, a primary change and a secondary change in the light intensity of the laser light output from the semiconductor laser 1 can be canceled, and the gas concentration can be measured with higher sensitivity.

Meanwhile, in the above-mentioned optical gas analyzer, there is an apparatus using a multi-reflection type long optical path sample cell as an apparatus capable of further improving the detection sensitivity of the gas concentration. As a multiple reflection type long optical path sample cell, a white (White) type sample cell using three concave mirrors,
A Herriot type sample cell using two mirrors is known.

For example, as shown in FIG. 5, the white-type sample cell 9 radiates laser light to concave mirrors 12, 13, and 1.
The laser beam is reciprocated a plurality of times between the laser beams 4 and emitted.
That is, the laser light incident from the entrance window 10 passes by the concave mirror 12, is reflected by the concave mirror 13, and forms an image on the point a of the concave mirror 12. Next, the reflected laser light from point a of the concave mirror 12 is
At the point b of the concave mirror 12. The reflected laser light from the point b is reflected by the concave mirror 13 and forms an image at the point c of the concave mirror 12. Next, the reflected laser light reflected at the point c of the concave mirror 12 is reflected by the concave mirror 14, passes by the concave mirror 12, and is emitted from the emission window 11.

In this case, the adjusting screws 15 and 16 are operated,
A laser beam enters from an entrance window 10 and a
The angles of the concave mirrors 13 and 14 are adjusted so that images are formed at points, b and c, respectively, and the light is emitted from the emission window 11. By adjusting the angles of the concave mirrors 13 and 14, it is possible to increase or decrease the number of reciprocations from when the laser beam enters to when the laser beam exits. The optical path length L at this time is L = 2 (N + 1) d, where d is the distance between the concave mirrors 12 and 13 (14) and N is the number of reciprocations of light, and the optical path length is increased by 2 (N + 1) times. can do. As a result, the light absorption intensity increases in proportion to the optical path length, and the light absorption spectrum can be measured with higher sensitivity.

[0009]

By using the above-mentioned white type sample cell 9, it is possible to detect the gas concentration of the gas to be measured with high sensitivity. Is a practical limit up to about 100 m. For example, if the optical path length L is set to 1 km and a sample cell for detecting a light absorption spectrum with ultra-high sensitivity is to be realized, the concave mirrors 12 and 13 (14)
If the distance d is 100 cm, the number of reciprocations of the laser light is 499 times, and if the distance between the laser lights imaged on the surface of the concave mirror 12 is 3 mm, the diameter of the concave mirror 12 needs to be about 75 cm or more. Become. Therefore, the white type sample cell 9 having an optical path length of 1 km has a size of about 75φ × 1.
It is not only super large size of 00cm but also capacity is about 4400
The volume becomes as large as liter, and the amount of the gas to be measured to be introduced into the sample cell becomes enormous.

Therefore, in the conventional optical gas analyzer using a white sample cell, the size of the sample cell must be increased in order to measure the concentration of the gas to be measured with ultra-high sensitivity. However, there is a disadvantage that a large amount of gas to be measured is required. Furthermore, since the number of reciprocations of the laser light in the sample cell becomes extremely large, when the angle of the concave mirror is slightly shifted due to the influence of temperature, vibration, or the like, there has been a problem that the stability of the apparatus is reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned inconvenience, and it is an object of the present invention to reduce the size and volume of a sample cell into which a gas to be measured is introduced, and to detect the concentration of the gas to be measured with stable operation and high sensitivity. It is an object of the present invention to provide an optical gas analyzer which can be used.

[0012]

In order to achieve the above object, the present invention provides a method of irradiating a gas to be measured introduced into a sample cell with a laser beam and measuring a light absorption intensity of the laser beam by the gas to be measured. In an optical gas analyzer for detecting the concentration, a reflection mirror capable of transmitting a part of the laser light is disposed at both ends, and a Fabry-Perot resonator type in which a gas to be measured is introduced between the reflection mirrors. A sample cell, an oscillation spectrum width is set to be equal to or less than the fringe width of the sample cell, a frequency variable laser that outputs laser light having a variable oscillation frequency, and a control circuit that sweeps and controls the oscillation frequency of the frequency variable laser, A displacement unit for displacing at least one of the reflection mirrors with respect to the other so as to adjust an interval between the reflection mirrors, and synchronizing with a frequency sweep of the laser light by the control circuit; A driving circuit that drives the displacement unit so that the laser light resonates in the sample cell; and a photodetector that detects the light absorption intensity of the laser light transmitted through the sample cell. And

The present invention also provides an optical gas analyzer for irradiating a gas to be measured introduced into a sample cell with a laser beam and detecting the concentration of the gas from the light absorption intensity of the laser beam by the gas to be measured. A reflection mirror capable of transmitting a part of the laser light is disposed in the portion, and a Fabry-Perot resonator type sample cell in which a gas to be measured is introduced between the reflection mirrors, and a gap between the reflection mirrors are continuously arranged. To sweep
A displacement unit for displacing at least one of the reflection mirrors with respect to the other, a reference cell into which a gas of the same type as the gas to be measured is introduced, and an oscillation spectrum width set to be equal to or less than a free spectrum region of the sample cell; A frequency-variable laser that outputs a laser light with a variable frequency, first light detection means for detecting the light absorption intensity of the laser light transmitted through the reference cell, and the laser light detected by the first light detection means The central frequency f ₀ of the light absorption spectrum of the gas to be measured is obtained from the light absorption intensity of the target gas, and the control circuit for controlling the oscillation frequency of the frequency variable laser to the central frequency f ₀ and the interval between the reflection mirrors are c / ( A driving circuit for driving the displacement means for continuously sweeping in a range of about 2 · f ₀ ), and a driving circuit for detecting the light absorption intensity of the laser light transmitted through the sample cell. And a two-photodetector, wherein the concentration is detected from the light absorption intensity of the laser light detected by the second photodetector.

[0014]

In the optical gas analyzer of the present invention, a Fabry-Perot resonator type sample cell whose optical path length can be adjusted by a displacement means is used, and a laser beam whose oscillation frequency is swept is introduced into this sample cell. By adjusting the optical path length in synchronization with the oscillation frequency and causing resonance, the light absorption intensity of the gas to be measured can be detected with high accuracy, and the concentration of the gas to be measured can be analyzed from this.

Further, in the optical gas analyzer according to the present invention,
The center frequency of the light / absorption spectrum of the laser light transmitted through the reference cell into which the same kind of gas as the gas to be measured has been detected by the gas to be measured is detected. The laser light is introduced into the sample cell, the optical path length of the sample cell is swept within a range determined by the center frequency, and the light absorption intensity of the laser light transmitted through the sample cell is detected with high accuracy.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fabry-Perot resonators are known as optical resonators, spectrum analyzers and optical interferometers. In recent years, a dielectric has been deposited on a glass substrate using ion beam coating technology, and the reflection loss is extremely small, and
A so-called supermirror having a reflectance of 0.9999 or more is realized. The optical gas analyzer according to the present invention realizes a sample cell with a long optical path using a mirror having such excellent characteristics as a Fabry-Perot resonator type cell, which is used for the optical gas analyzer. Things.

FIG. 1 is a block diagram showing a first embodiment of the optical gas analyzer according to the present invention. This optical gas analyzer includes a frequency tunable laser 20 composed of a titanium sapphire laser, a dye laser, a semiconductor laser or the like, a Fabry-Perot resonator type sample cell 22 into which a gas to be measured is introduced, and a photodetector 30.

The frequency variable laser 20 oscillates in a single longitudinal mode, and its oscillation spectrum width is narrowed to be equal to or less than the fringe width of the sample cell 22 by a laser resonator or an external resonator. Further, the oscillation frequency of the frequency variable laser 20 can be changed depending on the distance between the laser resonator or the external resonator, or the ambient temperature and the driving current in the case of a semiconductor device laser.

The sample cell 22 has a cylindrical cavity 26 made of a material having a small coefficient of thermal expansion such as zero-dur glass or super invar, a reflection mirror such as a supermirror having a small reflection loss, and a low surface reflectance. High mirror 24
a, 24b, and a piezo element (displacement means) 28 attached to one of the mirrors 24b to finely adjust the distance d between the mirrors 24a, 24b. Mirror 24a, 2
Reference numeral 4b has a Fabry-Perot resonator structure in which the reflection surfaces are arranged to face each other and resonates laser light of a specific frequency. The mirrors 24a and 24b may be either concave mirrors or flat mirrors.

The control circuit 32 sweeps the oscillation frequency of the frequency variable laser 20, and simultaneously drives the piezo element 28 via the drive circuit 34 to control the distance d between the mirrors 24a and 24b.

Here, the mirrors 24a and 2a of the sample cell 22
Assuming that the interval of 4b is d, the free spectral range (Free Spectral Range, hereinafter simply referred to as FSR) of the sample cell 22 is represented by c / 2nd (where c is the light speed, n
Is the refractive index). When the laser light of the frequency variable laser 20 whose oscillation spectrum width is narrowed to be equal to or less than the fringe width of the sample cell 22 is incident on the sample cell 22, the incident laser light is
After repeating the reciprocating reflection many times between the mirrors 24a and 24b, the light passes through the mirror 24b.

FIG. 2A shows the transmission characteristics of the laser beam transmitted through the sample cell 22. In FIG. 2A, the vertical axis represents the photodetector 3
Light intensity I _t / I _{0 of} the transmitted laser light detected at ₀ (where,
I ₀ is the intensity of the laser beam incident on the sample cell 22, I _t represents the intensity) of the laser beam emitted from the sample cell 22, the horizontal axis represents the oscillation frequency f of the laser beam. The sample cell 2
The center frequency of the light absorption spectrum of the gas to be measured introduced into 2 is f ₀ . As can be understood from FIG. 2A, the light intensity I _t / I ₀ of the transmitted laser light depends on the FSR of the sample cell 22.
Peaks at intervals of.

On the other hand, as the reflectance of the mirrors 24a and 24b increases, the number of times of reflection of the laser beam also increases. For example, if the reflectance of the mirrors 24a and 24b is R, the sample cell 2
The finesse F representing the index of the good resonation of No. ² is represented by F = π × R ^1/2 / (1-R). In this case, the fringe width Δν is Δν = FSR / F. Furthermore, the decay time t _c of the laser light incident on the sample cell 22 can be approximated if the loss is primarily based on the mirror 24a, 24b reflectance, t _c = nd / _C and (1-R).

Here, for example, the distance d between the mirrors 24a and 24b is d = 10 cm, and the reflectance R of the mirrors 24a and 24b is R =
Assuming 0.9999, FSR = 1.5 GHz, F = about 3000,
Δν = 50 kHz and t _c = (3 × 10 ⁵ ) ⁻¹ seconds.
Therefore, the laser light incident on the sample cell 22 reciprocates between the mirrors 24a and 24b having an optical path length of about 1 km and is emitted.

Therefore, the optical gas analyzer of the first embodiment measures the concentration of the gas to be measured in the following manner.

That is, after introducing a gas to be measured, which is a sample, into the sample cell 22 through the sample inlet / outlet 23, a laser beam from the frequency variable laser 20 is incident on the sample cell 22. The laser light incident on the sample cell is, as described above,
The reciprocating reflection is repeated many times in the sample cell 22, and is absorbed by the gas to be measured introduced into the cell and emitted from the sample cell 22. This emitted light is detected by the photodetector 30.

In this case, the oscillation frequency of the variable frequency laser 20 is swept by the control circuit 32, and the optical path length of the sample cell 22 is controlled by the piezo element 28 so as to resonate at the oscillation frequency. . Accordingly, the transmission characteristics of the laser beam (light absorption spectrum of the gas to be measured) detected by the photodetector 30 are as shown in FIG. 2B. Incidentally, in FIG. 2B, the vertical axis represents the light intensity I _t / I ₀ of the transmitted laser beam detected by the photodetector 30, the horizontal axis represents the oscillation frequency f of the frequency-tunable laser 20. The laser light is reflected back and forth many times between the mirrors 24a and 24b in the sample cell 22 and is detected by the photodetector 30 after being absorbed by the gas to be measured. Therefore, the laser light is approximately proportional to the optical path length. As a result, the absorption intensity increases, and peaks at the center frequency f ₀ of the light absorption spectrum of the gas to be measured.

The control of the piezo element 28 is performed in advance with respect to the oscillation frequency of 20 of the frequency-variable laser.
A control signal required to resonate 2 is obtained, and the control circuit 32 outputs a control signal to the piezo element 28 in correspondence with a frequency sweep signal for sweeping the oscillation frequency of the frequency variable laser 20. What should I do? Further, while the control circuit 32 outputs a frequency sweep signal for sweeping the oscillation frequency of the frequency variable laser 20, the light intensity of the emitted laser light detected by the photodetector 30 is maximized, that is, The signal applied to the piezo element 28 may be controlled so that the laser light incident on the sample cell 22 resonates.

In the above embodiment, the mirrors 24a,
The diameter of 4b may be about three times the beam diameter of the incident laser light, and a sample cell 22 having a very small dimension and an optical path length of 1 km or more can be easily realized. Therefore, the optical absorption spectrum of the gas to be measured can be measured with high sensitivity, and the detection and measurement of the gas concentration can be performed with high sensitivity. As an example, if the diameter of the mirrors 24a and 24b used in the above example is 1 cm, the size of the sample cell is 1φ × 10 cm, and the capacity of the sample cell is 0.008 liter. Since the sample cell of the present invention can be realized and can be made compact and robust in terms of structure, it is possible to provide a device which is hardly affected by temperature and vibration and operates stably.

FIG. 3 is a block diagram showing a second embodiment of the optical gas analyzer according to the present invention.

The optical gas analyzer of this embodiment is provided with the sample cell 22 provided with the mirrors 24a and 24b and the piezo element 28 in the embodiment of FIG. 1, and provided in parallel with the sample cell 22 and of the same type as the gas to be measured. Reference cell 3 containing gas
The laser beam from the frequency variable laser 20 is branched by a beam splitter 38 (laser beam splitting means), one of the branched laser beams is incident on the sample cell 22, and the other is transmitted via a mirror 40. Incident on the reference cell 36,
The laser beam emitted from the sample cell 22 is detected by the photodetector 30a.
The laser light emitted from the reference cell 36 is detected by the (second light detecting means) and detected by the light detector 30b (first light detecting means).

The control circuit 32 controls the oscillation frequency of the frequency variable laser 20 so as to lock to the center frequency of the light absorption spectrum of the gas in the reference cell 36. Reference numeral 42 denotes an oscillator for frequency-modulating the frequency-variable laser 20. The signal of the oscillator 42 is input to the lock-in amplifier 44, and the lock-in amplifier 44 detects the signal detected by the photodetector 30b. The oscillator 4
Only signals synchronized with 2 are detected. Reference numeral 46 denotes a sweep circuit, which is a piezo element 28 via a drive circuit 34.
And the distance between the mirrors 24a and 24b is set to c / 2f ₀
(１ ／ wavelength).

The optical gas analyzer of the second embodiment operates as follows. The oscillation spectrum width of the frequency variable laser 20 is narrowed below the FSR of the sample cell 22,
The frequency of the signal from the oscillator 42 is slightly modulated. The laser light of the frequency-variable laser 20 which has been narrowed and frequency-modulated is applied to the beam splitter 3.
At 8, one laser beam is reflected by the mirror 40 and enters the reference cell 36. The laser beam incident on the reference cell 36 is absorbed by a gas of the same type as the gas to be measured sealed in the cell, and then emitted from the reference cell 36 and detected by the photodetector 30b. The laser light detected by the photodetector 30b is converted into an electric signal,
At 4, only the signal synchronized with the signal of the oscillator 42 is detected. The signal detected by the lock-in amplifier 44 is the same as that of the prior art, and has a first-order differential form of the light absorption intensity. The control circuit 32 controls the frequency variable laser 20 so that the oscillation frequency of the frequency variable laser 20 is locked at the zero cross point of the first derivative. By performing such control, the oscillation frequency of the frequency variable laser 20 is set to the center frequency f ₀ of the light absorption spectrum of the gas to be measured.

The other of the laser beams split by the beam splitter 38 is incident on the sample cell 22, and the incident laser beam is applied to the mirror 24 in the same manner as in the first embodiment.
a and 24b are reflected back and forth many times, and are absorbed by the gas to be measured introduced into the sample cell 22;
2 and is detected by the photodetector 30a. The sweep circuit 46 controls the piezo element 28 by the drive circuit 34 and sets the distance between the mirrors 24a and 24b to c / 2f ₀ (1/2).
(2 wavelengths). Then, by obtaining the peak value of the light intensity detected by the photodetector 30a, it is possible to obtain the light absorption intensity of the center frequency f _0.

The piezo element 28 may be controlled so as to maximize the light intensity detected by the photodetector 30a without using the sweep circuit 46. Further, the signal detected by the photodetector 30a is locked. In-amplifier 44a (shown by a broken line in FIG. 3)
To detect only the signal synchronized with twice the frequency of the signal of the oscillator 42, the second derivative shape of the light absorption spectrum can be measured, and the peak of the light absorption spectrum can be detected with higher sensitivity. The light intensity at the position can be obtained, and the gas concentration can be measured with higher sensitivity.

[0036]

As described above, according to the optical gas analyzer of the present invention, a very small sample cell can be easily realized, and the optical absorption spectrum of the gas to be measured can be detected with high sensitivity. Since the measurement can be performed, the gas concentration can be detected and measured with high sensitivity. In addition, it is possible to provide a sample cell with a small structure and a robust structure in terms of structure, to provide a stable operation device that is not easily affected by temperature and vibration, and to minimize the capacity of the gas to be measured. The effect is obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical gas analyzer according to a first embodiment of the present invention.

FIG. 2A and FIG. 2B are diagrams showing resonance characteristics and transmission characteristics of a laser beam in a sample cell.

FIG. 3 is a diagram showing a configuration of an optical gas analyzer according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of a conventional optical gas analyzer.

FIG. 5 is a view showing the structure of a white sample cell used in a conventional optical gas analyzer.

[Explanation of symbols]

Reference Signs List 20 frequency variable laser 22 sample cell 24a, 24b, 40 mirror 28 piezo element 30, 30a, 30b photodetector 32 control circuit 34 drive circuit 36 reference cell 38 beam splitter 42 oscillator 44 44a: Lock-in amplifier 46: Sweep circuit

Claims (2)

(57) [Claims] An optical gas analyzer for irradiating a gas to be measured introduced into a sample cell with a laser beam and detecting its concentration from the light absorption intensity of the laser beam by the gas to be measured. A reflection mirror capable of transmitting a part of light is provided, and a Fabry-Perot resonator-type sample cell in which a gas to be measured is introduced between the reflection mirrors; and an oscillation spectrum width equal to or less than a fringe width of the sample cell. A frequency variable laser that outputs a laser beam having a variable oscillation frequency that is set, a control circuit that sweeps and controls the oscillation frequency of the frequency variable laser, and at least one of the reflection mirrors to adjust an interval between the reflection mirrors. Displacement means for displacing the laser light relative to the other, in order to resonate the laser light in the sample cell in synchronization with the frequency sweep of the laser light by the control circuit. A drive circuit for driving the serial displacement means, optical gas analyzer, characterized in that it comprises a photodetector for detecting a light absorption intensity of the laser light passing through the sample cell. 2. An optical gas analyzer for irradiating a gas to be measured introduced into a sample cell with a laser beam and detecting its concentration from the light absorption intensity of the laser beam by the gas to be measured. A reflection mirror capable of transmitting a part of light is provided, and a Fabry-Perot resonator-type sample cell in which a gas to be measured is introduced between the reflection mirrors; and for continuously sweeping an interval between the reflection mirrors. A displacement unit for displacing at least one of the reflection mirrors with respect to the other, a reference cell into which a gas of the same type as the gas to be measured is introduced, and an oscillation spectrum width set to be equal to or smaller than a free spectrum region of the sample cell; A frequency-variable laser that outputs a laser light having a variable oscillation frequency; a first light detection unit that detects the light absorption intensity of the laser light that passes through the reference cell; A control circuit which obtains a center frequency f ₀ of the optical absorption spectrum of the gas to be measured from the light absorption intensity is detected the laser beam, and controls the oscillation frequency of the variable frequency laser to the center frequency f ₀ by, the reflection A drive circuit for driving the displacement means, and a light absorption intensity of the laser light transmitted through the sample cell, for continuously sweeping the mirror interval in a range of c / (2 · f ₀ ). An optical gas analyzer comprising: a second light detector; and detecting the concentration of the laser light from the light absorption intensity of the laser light detected by the second light detector.