JPH10185804A - Gas concentration measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure gas concentration accurately by obtaining measurement data using laser lights of one absorption wavelength and two nonabsorption wavelengths emitted from a solid laser, e.g. a titanium sapphire laser, and two YAG lasers. SOLUTION: A laser light having an absorption wavelength λ1 of a gas O3 to be measured is emitted from an optical system 1 generating a laser light by operating the tuning mirror of a titanium sapphire laser 12 and the position switch 18 of the optical system 1 generating a laser light. It is detected by a photodiode 20 and inputted to a transient recorder 7. A backscattering light from a forward gas is received by a Cassegrain type telescope 2 and stored in a personal computer 8. Subsequently, a backscattering light data corresponding to laser lights of nonabsorption wavelengths λ2 , λ2 ' is stored in the personal computer 8 through a required operation and a set of backscattering light data is obtained. Thereafter, a concentration value is determined from 100 sets of backscattering light data according to a specified operation. An extremely accurate concentration value can be obtained because the wavelength dependency has been corrected for both backscattering coefficient β and dissipation coefficient α.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a gas concentration measuring device using laser light.

[0002]

2. Description of the Related Art In recent years, the problem of environmental pollution has been widely taken up, and air pollution has become a major theme. NO _X and SO _X is of particular importance among pollution gases, and the laser radar is used to conventionally determine the concentration of these contaminants gases and certain critical attention gases.

In the measurement of gas concentration by a laser radar, a laser is used to generate laser light having an absorption wavelength of the gas to be measured, and the concentration of the gas is measured by detecting the degree of absorption of the laser light by the gas to be measured. The wavelength at which the absorption spectrum of the gas to be measured exists (for example, 450 nm for NO _{2 and} 300 n for O ₃ )
A laser that generates the laser light of m) is used. At this time, in order to obtain the wavelength, two different wavelengths of laser light of a wavelength having a large absorption (hereinafter referred to as “absorption wavelength”) and a wavelength having a small absorption (hereinafter referred to as “non-absorption wavelength”) are used. . Laser radar emits laser light toward the gas to be measured and collects the backscattered light to detect the degree of absorption by the gas.However, the scattered light is weak and noise may be mixed into it. For this reason, laser light having an absorption wavelength and laser light having a non-absorption wavelength are alternately emitted over a certain period of time (for example, several seconds to several minutes), and backscattered light having an absorption wavelength and backscattering light having a non-absorption wavelength therebetween. Are averaged, and the ratio between the two is calculated to obtain the density.

That is, a received signal obtained by condensing backscattered light of a laser beam having a certain wavelength is expressed by the following equation (1).

[0005]

(Equation 1) In the above equation, K: optical system efficiency, P ₀ : laser output, A
_r : Telescope receiving area, β: Backscattering coefficient, c: Speed of light,
τ: laser pulse width, R: distance, α: extinction coefficient, N: measurement target gas density, σ: absorption cross section of the measurement target gas.

The above received signals are obtained for each of the absorption wavelength (λ1) and the non-absorption wavelength (λ2), and the ratio is obtained. The concentration of the target gas is expressed by the following equation (2).

[0007]

(Equation 2) In the above equation, ΔR: distance resolution, Δσ = σ (λ1) −
σ (λ2), the second term is a correction term for the wavelength dependence of the backscattering coefficient, and the third term is a correction term for the wavelength dependence of the atmospheric extinction coefficient.

[0008]

By the way, in the concentration measurement by the above method, in the case of a gas such as SO ₂ , NO, H ₂ O, the wavelength difference between the absorption wavelength (λ1) and the non-absorption wavelength (λ2) is very small. (For example, 0.1n for SO ₂ )
m, NO, H ₂ O, several pm to several tens pm)
Although the second and third terms in the above equation can be ignored, ozone (O
₃ ) In the case of the concentration measurement, absorption lines exist continuously.
In the case of NO ₂ , a wavelength of around 450 nm is often used and the width of the absorption line is relatively wide.
The absorption wavelength and the non-absorption wavelength need to be separated by 1 to several tens of nm. Therefore, if the density is obtained only from the first term of Expression 2, a large error may occur, and the correction by the second and third terms is required.

As a simple method of this correction, the chromatic dispersion that Mie scattering is inversely proportional to the wavelength and Rayleigh scattering (scattering by molecules) is inversely proportional to the fourth power of the wavelength is theoretically known, so it is estimated by calculation. It is possible to do. However, in practice, the accurate concentration of chromatic dispersion varies depending on the atmospheric state at the time of concentration measurement, and thus the actual concentration cannot be easily obtained. Therefore, as a correction method suitable for the actual condition in consideration of the atmospheric state at the time of measurement, the extinction coefficient is obtained from the above equation 2 using data measured using three laser beams of one absorption wavelength and two non-absorption wavelengths. A method is known in which α and the backscattering coefficient β are calculated, and the wavelength dependence is obtained from the values thus obtained. It should be noted that even when NO ₂ is measured at a low concentration, the wavelength difference between the absorption wavelength and the non-absorption wavelength is as small as about 1 nm, which cannot be ignored. The correction method using these three wavelengths is important.

However, in the conventional atmospheric concentration measurement, the above-described correction method using three wavelengths is considered only in the case of ozone concentration measurement. Because (1) ozone has a wide absorption line, two wavelengths, an absorption wavelength and a non-absorption wavelength, must be greatly separated from each other. (2) A tunable wavelength laser that generates laser light near 300 nm, which is the absorption wavelength band of ozone, is used. Absent, discrete wavelengths (eg 308n
The reason is that only three lasers that combine an excimer laser having a wavelength of 248 nm and a third harmonic (355 nm) of a YAG laser or a combination of these lasers and wavelength conversion of hydrogen by a Raman cell can be used. by. Whichever method is used, the apparatus including the optical system becomes complicated, and there is a problem of adjustment of each laser and an installation place. In the former case, there is difficulty in alignment for matching light waves of three wavelengths. . Raman cells use high-pressure hydrogen gas, which is dangerous to handle, needs to be replaced from time to time, and has a problem that the beam characteristics and output intensity of the laser light after wavelength conversion are unstable. Further, in the case of NO ₂ measurement, the wavelength conversion by the Raman cell can only obtain discrete wavelengths, so that it cannot be converted to an arbitrary wavelength, and the converted wavelength often does not match the wavelength required for measurement.
For this reason, although there is an example in which three wavelengths of lasers having different wavelengths are used for NO _2, there has been no report using a Raman cell.

The present invention has been made in view of the above points, and uses a laser beam having three or more different wavelengths, and is a highly accurate gas concentration measuring apparatus in which the adjustment of the optical system is simple and the apparatus configuration is simple. The purpose is to provide.

[0012]

According to one aspect of the present invention, a laser light generating optical system of a gas concentration measuring apparatus includes a first solid-state laser and a first solid-state laser. An oscillation wavelength switching mechanism for sequentially switching the wavelength of laser light output from the solid-state laser, a second solid-state laser, and laser light of different wavelengths of at least three wavelengths switched and output by the oscillation wavelength switching mechanism. And the second
A second nonlinear optical element that converts laser light output from the solid-state laser through the first nonlinear optical element into laser light having a desired wavelength of three or more different wavelengths as a sum frequency and outputs the laser light; And a position switching device for switching the optical position of the nonlinear optical element so that the laser light having the different wavelengths of three or more wavelengths has a phase matching angle in synchronization with the wavelength switching timing by the oscillation wavelength switching mechanism. did.

According to another aspect of the present invention, a laser light generating optical system of a gas concentration measuring device includes a solid laser, a diffraction grating provided on an excitation laser light incident side in the solid laser resonator, and a tuning device. A mirror, an output coupler provided on the oscillation laser light emission side in the solid-state laser resonator,
An oscillation wavelength switching mechanism for sequentially switching and displacing the optical position of the tuning mirror with respect to the diffraction grating,
Each of the three or more different wavelengths of laser light switched and output by the oscillation wavelength switching mechanism and the laser light of a different wavelength from the three or more wavelengths are converted into a desired three or more different wavelengths of laser light as a sum frequency. And the optical position of the nonlinear optical element with respect to the incident laser light and the optical position of the nonlinear optical element in synchronization with the wavelength switching timing by the oscillation wavelength switching mechanism. And a position switching device for switching so that the laser light incident on the solid-state laser is repeatedly amplified between the output coupler and the tuning mirror via the solid-state laser and the diffraction grating. The output coupler outputs laser light having three or more different wavelengths toward the nonlinear optical element.

[0014]

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

FIG. 1 is a block diagram of an embodiment of a gas concentration measuring apparatus according to the present invention. In this embodiment, the concentration of O ₃ is measured as an example. Here, 308 nm was adopted as the absorption wavelength λ1 of O ₃ , and 320 nm and 330 nm were adopted as the non-absorption wavelength λ2.

In the figure, a portion surrounded by a broken line indicates a laser beam of 731.5 nm corresponding to the absorption wavelength of O ₃ which is a gas to be measured and laser beams of 802.8 nm and 869.1 nm corresponding to non-absorption wavelengths. This is a laser light generating optical system 1 that is sequentially and cyclically generated. The laser light generating optical system 1 is configured to change the wavelength of the YAG laser 10 and the wavelength of the input laser light,

[0017]

(Equation 3) A nonlinear optical element (for example, KD ^* P) 11 for generating a second harmonic, which is output by halving according to the relationship, a titanium sapphire laser 12 which generates by switching between two different wavelengths of laser light, and another YAG A laser 13, a non-linear optical element 24 for outputting the wavelength by half,
A mirror 14 for reflecting the laser light from the nonlinear optical element 24; a dichroic mirror 15 for aligning the laser light from the titanium sapphire laser 12 and the laser light from the YAG laser 13 reflected by the mirror 14 on the same optical path; A nonlinear optical element (for example, BBO) 16 which obtains a new wavelength from the wavelength of the input laser light according to the relational expression of Expression 4.
When,

[0018]

(Equation 4) Interference filter 17 for extracting only laser light of a required wavelength
And a position switching device 18 that switches the position of the nonlinear optical element 16 in synchronization with the wavelength switching timing such that the incident angle at which three different wavelengths of laser light enter the nonlinear optical element 16 becomes the phase matching angle. And two Y
A pulse generator 19 for generating a signal for synchronizing the AG lasers 10 and 13 and a switching timing signal of a wavelength switching device 18 for switching the wavelength of the titanium sapphire laser 12 is provided.

On the other hand, laser light reflected by aerosols (suspended particulate matter such as dust and dust) and gas molecules existing in the direction of the object to be measured are reflected by the concave mirror 21 and the convex mirror 2.
2, a Cassegrain telescope 2 that receives light at 2, an interference filter 4 that passes only a wavelength around 330 nm, which is a non-absorption wavelength around 308 nm, which is an absorption wavelength of O ₃ , from a laser beam that has passed through the diaphragm 3, and an extremely weak laser beam A photomultiplier tube 5 for converting the output photoelectrically and outputting it as a light receiving signal; a preamplifier 6 for amplifying a weak light receiving signal with a high amplification factor; and a transient recorder for recording the entire waveform of the amplified light receiving signal by A / D exchange. 7 and Transient Recorder 7
Personal computer 8 that processes the data from the computer and calculates the gas concentration
And a display 9 for displaying the calculation result.

Nonlinear optical element 1 of laser light generating optical system 1
On the output side of 6, a photodiode 20 for detecting a laser beam output from the nonlinear optical element 16 is provided, and its output signal is input to the transient recorder 7 as a trigger signal.

FIG. 2 shows the laser light generating optical system 1 shown in FIG.
1 shows a schematic configuration of an optical system of the titanium sapphire laser 12.

The titanium sapphire laser 12 can alternately generate laser beams of three different wavelengths by an oscillation wavelength switching mechanism, which will be described later. As shown, as shown, the nonlinear optical element 11 shown in FIG. A titanium sapphire crystal 120 on which the laser beam L from the laser enters, a pinhole 121 provided on the emission side, an output coupler 122 made of, for example, quartz crystal, and a beam expander 123 composed of a plurality of prisms. Prism 124
And a beam expander 125 also composed of a plurality of prisms, a diffraction grating 126, and a tuning mirror 127. As other components, an amplifier (not shown) for amplifying the obtained laser light is provided. Contains. The optical position of the tuning mirror 127 with respect to the diffraction grating 126 is rotationally displaced in an arc shape as shown by an arrow A in FIG. 2 according to the oscillation wavelength by an oscillation wavelength switching mechanism 30 described later with reference to FIGS. .

FIG. 3 shows a schematic configuration of the oscillation wavelength switching mechanism 30 and the position switching device 18 and a drive control system thereof.

The oscillation wavelength switching mechanism 30 has a translation stage 32 as a first displacement means which is translated and reciprocated in the direction of arrow B by a lead screw 31, and a sine bar 33 is rotatable on the translation stage 32. Installed. The lead screw 31 is rotationally driven by a step motor 34 via two pulleys 35a and 35b and a belt 36 stretched between the two pulleys. Reference numerals 37a and 37b denote limit switches for regulating the movement position of the translation stage 32. A tuning unit 38 having a built-in drive mechanism such as an ultra-precision linear actuator for vibrating and displacing the tuning mirror 127 at high speed is attached to the lower surface of the translation stage 32. The drive mechanism is mounted on the lower surface of the tuning unit 38. fast (e.g. 10H _Z) tuning stage 39 which is reciprocally driven in the direction of arrow C in is attached by. The tuning unit 38 and the tuning stage 39 constitute a second displacement means.

A spring 40 is stretched between the falling part 33 a on the lower surface of the sign bar 33 and the tuning stage 39, and one end of the tuning stage 39 is attached to the lower surface of the sign bar 33 by the tension action of the spring 40. The ball 41 is always kept in contact with the mounted ball 41.

Reference numeral 50 denotes a driving circuit for driving the step motor 34, and reference numeral 60 denotes a translation stage 32 of the oscillation wavelength switching mechanism 30.
Is a control module for controlling the position of the key based on data input from the keyboard 61, and 62 is a control circuit.

A control module 70 controls the amount of displacement of the tuning stage 39 of the oscillation wavelength switching mechanism 30 based on data input from the keyboard 61.
Is a control circuit. Reference numeral 19 denotes a pulse generator (shown in FIG. 1) for synchronizing wavelength switching and laser oscillation.

On the other hand, since the configuration of the position switching device 18 is basically the same as that of the oscillation wavelength switching mechanism 30, its detailed configuration is omitted in FIG. The drive control system of the position switching device 18 is the same as the drive control system of the oscillation wavelength switching mechanism 30 described above, and the drive circuit 80 for driving the stepper motor of the position switch device 18 includes the control circuit 64 of the control module 63.
, And the displacement of the tuning stage is controlled by the control circuit 74 of the control module 73.

FIG. 4 shows a specific configuration of an example of the oscillation wavelength switching mechanism 30.

One end 33b of the sign bar 33 rises, a head enlargement pin 51 is implanted in the rising portion 33b, and a ball 41 is fixed to the lower surface of the other end. A not-shown spring 40 (shown in FIG. 3) is stretched between the falling portion 33 a near the other end of the sine bar 33 and the lower surface of the tuning stage 39. The side surface 39a of the tuning stage 39 and the ball 41 are always kept in contact with each other by the tensile force. Tuning mirror 1 on head enlargement pin 51
27 mounting bases 52 are mounted with screws or the like so as to be sandwiched by bars 52a from the opposite side of the pins 51.

The tuning unit 38 has a built-in drive mechanism for reciprocating the tuning stage 39 at high speed. The drive mechanism includes a step motor 34 and a pulley 35 for reciprocating the translation stage 32.
a, 35b, a belt 36, a lead screw 31, a drive circuit 50, and the like.
An ultra-precision linear actuator (ministage) described on page 5.6 of the Newport General Catalog can be used. In FIG. 4, 53 indicated by a chain line indicates the diffraction grating 12.
6 is a mounting base for mounting the same.

The mounting table 53 of the diffraction grating 126 is fixed to a fixing table (not shown) for fixing all components of the titanium sapphire laser 12 by screws or the like.
The components of the oscillation wavelength switching mechanism 30 indicated by solid lines in FIG. 4 are attached to the translation stage 32 as shown in FIG.

FIG. 5 shows a specific configuration of an example of the position switching device 18 for switching the position of the nonlinear optical element 16 in synchronization with the wavelength switching timing.

The configuration of the position switching device 18 is basically the same as the oscillation wavelength switching mechanism 30 shown in FIG.

Therefore, only the configuration of the apparatus will be briefly described. The nonlinear optical element 16 whose optical position with respect to the laser light is switched in synchronization with the switching timing of cyclically switching the wavelengths to λ1, λ2, λ2 'is mounted on the mounting base 81.
The mounting base 81 is mounted on the head enlarging pin 82 so as to be sandwiched by the bar 81a from the opposite side. The head enlargement pin 82 is implanted in a rising portion 83b at one end of the sign bar 83, and a ball 84 is fixed to the lower surface of the other end of the sign bar 83. Not shown between the falling portion 83a near the other end of the sine bar 83 and the lower surface of the tuning stage 85 (4 in FIG. 3).
A spring (corresponding to a spring shown as 0) is stretched, and the side surface 85a of the tuning stage 85 and the ball 84 are always kept in contact with each other by the tensile force of the spring.

The tuning unit 86 has a built-in drive mechanism for reciprocating the tuning stage 85 at high speed. The driving mechanism is an oscillation wavelength switching mechanism 30.
Since the driving mechanism is the same as that of FIG.

Next, the O ₃ concentration detection according to this embodiment will be described with reference to FIGS.

(A) 308 nm is used as the absorption wavelength λ1 of O ₃ which is the gas to be measured, 320 nm is used as one non-absorption wavelength λ2, and 3 is used as another absorption wavelength λ2 ′.
When 30 nm is selected, the oscillation wavelengths of the titanium sapphire laser 12 are 731.5 nm, 802.8 nm, 86
9.1 nm. Therefore, titanium sapphire laser 12
In order to generate a laser beam having an oscillation wavelength of 731.5 nm from the keyboard 61, the oscillation wavelength of 731.
When 5 nm is input, the control circuit 6 of the control module 60
2, a control signal is sent to the drive circuit 50, the step motor 34 rotates by a predetermined amount, the lead screw 31 is driven to rotate, and the translation stage 32 moves. Since the tuning stage 39 is attached to the lower surface of the translation stage 32, the sine bar 33 rotates by a predetermined angle with the movement of the translation stage 32, and the optical position of the tuning mirror 127 with respect to the diffraction grating 126 is shifted by a predetermined angle. Displace. This position of the tuning mirror 127 is 7
This is an oscillation position corresponding to an absorption wavelength (λ1) that generates a 31.5 nm laser beam.

Next, in order to generate a laser beam having a wavelength of 802.8 nm corresponding to the non-absorbing wavelength (λ2) from the titanium sapphire laser 12, data of the minute angular displacement of the tuning stage 39 is input from the keyboard 61.
Similarly, data of the minute angular displacement of the tuning stage 39 corresponding to another non-absorption wavelength (λ2 ′) is input. The difference between the absorption wavelength (λ1) and one non-absorption wavelength (λ2) is 802.8 nm−731.5 nm = 71.3 n
m, and the difference between one non-absorbing wavelength (λ2) and another non-absorbing wavelength (λ2 ′) is 869.1 nm-802.
Although 8 nm = 66.3 nm, in this embodiment, the tuning stage 39 is used to change the oscillation wavelength by 1 nm.
Is set to move by 150 μm. To change the oscillation wavelength by the above-mentioned wavelength difference of 71.3 nm and 66.3 nm, the tuning stage 39 is set to 150 μm × 71.3 = 10695 μm and 1595 μm, respectively.
It may be moved by 0 μm × 66.3 = 9945 μm. Therefore, the movement amount of 10695 μm
data “(f)” corresponding to m and 9945 μm,
If "(g)" is input, the tuning stage 39
Is a laser beam oscillation position corresponding to the absorption wavelength (λ1) by the driving mechanism provided in the tuning unit 38 (the position at which the laser beam having the wavelength of 308 nm described above is generated) and 71.3 μm away therefrom. Non-absorption wavelength λ2
Between the laser light oscillation position corresponding to the non-absorbing wavelength λ2 ′ and the laser light oscillation position corresponding to the non-absorption wavelength λ2 ′ further separated by 66.3 μm.
H _Z ).

On the other hand, the conversion efficiency by the sum frequency of the nonlinear optical element 16 for the laser light having the wavelength corresponding to the absorption wavelength (λ1) and the laser light having the wavelength corresponding to the non-absorption wavelengths (λ2) and (λ2 ′) is increased. The nonlinear optical element 16
As a setting operation for switching the optical position of the non-linear optical element 16 with respect to the laser light of 308 nm which is the wavelength corresponding to the absorption wavelength (λ1), the phase matching angle is changed to BBO. At 4
Since it is previously calculated that the angle is 7.3 °, the user inputs this angle 47.3 ° from the keyboard 61. As a result, a control signal is sent from the control circuit 64 of the control module 63 to the drive circuit 80, the step motor of the position switching device 18 rotates by a predetermined amount, the lead screw is driven to rotate, and the translation stage moves. Since the tuning stage is mounted on the lower surface of the translation stage, the sine bar rotates by a predetermined angle as the translation stage moves, and the optical position of the nonlinear optical element 16 fixed to the mounting table 81 is shifted by a predetermined angle. Only displace. This position of the nonlinear optical element 16 is a phase matching angle with respect to a laser beam of 308 nm which is a wavelength corresponding to the absorption wavelength λ1.

Next, the optical position of the nonlinear optical element 16 is set to 320 which is the wavelength corresponding to the non-absorption wavelengths λ2 and λ2 '.
In order to set the phase matching angle with respect to the laser beams of nm and 330 nm, data of the minute angular displacement of the nonlinear optical element 16 is input from the keyboard 61. That is, the phase matching angle for a laser beam having a wavelength corresponding to one non-absorption wavelength λ2 is 48.1 °, and the phase matching angle for a laser beam having a wavelength corresponding to another non-absorption wavelength λ2 ′ is 52.5 °. Is calculated, the difference between the phase matching angle with respect to the wavelength corresponding to the absorption wavelength λ1 and the wavelength corresponding to one non-absorption wavelength λ2 is 0.8 °.
And a wavelength corresponding to one non-absorption wavelength λ2 and another
The difference between the phase matching angles for the wavelengths corresponding to the two non-absorption wavelengths λ2 ′ is 4.4 °, and in this embodiment, the tuning stage 39 is moved by 30 μm to change the angle of the nonlinear optical element 16 by 0.1 °. In order to change the optical position of the nonlinear optical element 16 by the above-mentioned angle differences of 0.8 ° and 4.4 °, the tuning stage 39 must be set to 240 μm and 13 μm.
It may be moved by 20 μm. So keyboard 61
When the data “240” and “1320” corresponding to the movement amounts of 0.8 ° and 4.4 ° are input from the tuning stage 39, the tuning mechanism 39 is driven to the absorption wavelength λ1 by the driving mechanism provided in the tuning unit 38. The optical position corresponding to the phase matching angle with respect to the laser light of the corresponding wavelength, the optical position relative to the laser light of the wavelength corresponding to the non-absorption wavelength λ2 shifted by 0.8 °, and 4.4 ° from that. (in this embodiment example 10H _Z) successively faster and optical position with respect to the laser beam having a wavelength corresponding to the non-absorption wavelength λ2' shifted by switches to cyclically.

After setting the position of the tuning mirror 127 and the minute rotation amount and setting the optical position of the nonlinear optical element 16, the pulse generator 19 of the laser light generating optical system 1 synchronizes the YAG lasers 10 and 13 with each other. (For example, 10 Hz). Also, after the wavelength is switched, the delay of the pulse generator 19 is adjusted so that a pulse for switching the wavelength is generated between pulses for laser oscillation so that the laser oscillates. 1064 from YAG lasers 10 and 13
nm laser light is generated. The laser light output from the YAG laser 10 is converted by a nonlinear optical element 11 (KD ^* P) into a laser light having a half wavelength of 532 nm, and is incident on a titanium sapphire laser 12. In the titanium sapphire laser 12, as described above, the driving mechanism in the tuning unit 38 causes the tuning stage 39 to cause the tuning position of the laser beam having the wavelength corresponding to the absorption wavelength λ1 and the wavelength corresponding to the two non-absorption wavelengths λ2 and λ2 ′. The laser beam oscillation position is sequentially switched at a high speed and cyclically.
3 is transmitted. Since the sign bar 33 is repeatedly rotated and displaced by a small angle with the head enlargement pin 51 fixed to one end thereof as a fulcrum, the mounting base 5 fixed to this pin 51 is rotated.
2 is also pivotally displaced by a small angle.
Is repeatedly rotated by a small angle, and the optical position relative to the diffraction grating 126 changes cyclically. Thereby, three types of laser light whose oscillation wavelengths are slightly shifted, that is, the oscillation wavelength is 731.
5 nm laser light, 802.8 nm laser light, 8
69.1 nm laser light is generated cyclically.

The wavelength 106 output from the YAG laser 13
The laser light of 4 nm has a wavelength of 532 with the nonlinear optical element 24.
After conversion to nm, the light is reflected by the mirror 14 and is reflected by the dichroic mirror 15 on the titanium sapphire laser 1.
The laser light of the three wavelengths output from 2 is aligned with the optical axis and sent to another nonlinear optical element 16 (BBO).

On the other hand, the position switching device 18 is driven by the pulse generator 19 in synchronization with the YAG lasers 10 and 13 (for example, at 10 Hz). That is, in FIG. 5, the driving mechanism in the tuning unit 86 causes the tuning stage 39 to oscillate the laser light of the wavelength corresponding to the absorption wavelength λ1 and the laser light of the two wavelengths corresponding to the two non-absorption wavelengths λ2 and λ2 ′. The oscillating position is cyclically switched at a high speed, and this minute reciprocating motion is transmitted to the sign bar 83 via the ball 84. Since the sign bar 83 is repeatedly rotated and displaced by a small angle with the head enlarging pin 82 fixed to one end thereof as a fulcrum, the mounting base 81 fixed to this pin 82 is also displaced by a small angle and as a result, The nonlinear optical element 16 fixed to the mount 81 is rotated by a small angle to take a phase matching angle with respect to the laser beam at each position. As a result, the nonlinear optical element 16 converts the laser light having a wavelength of 731.5 nm output from the titanium sapphire laser 12 into a laser light having a wavelength of 308 nm with a maximum efficiency at a phase matching angle of 47.3 °. At a phase matching angle of 48.1 °, a laser beam having a wavelength of 320 nm is emitted with a maximum efficiency of 8
The laser light having a wavelength of 869.1 nm is further converted into a laser light having a wavelength of 330 nm with a maximum efficiency at a phase matching angle of 52.5 °, and is sequentially outputted cyclically. The laser light converted and output by the nonlinear optical element 16 has a wavelength before conversion of 731.5 nm,
Since the laser beam of 802.8nm or 869.1nm and 532nm are included, three wavelengths (308 nm required for measuring O ₃ by passing the interference filter 17,
(320 nm, 330 nm) only, and is emitted toward the front measurement target area.

The measuring laser light scattered by the target area is received by a Cassegrain telescope 2, O ₃ in the interference filter 4 through diaphragm 3 is reflected by the concave mirror 21 and convex mirror 22
Only the wavelengths near 308 nm, which is the absorption wavelength of, and the two non-absorption wavelengths, 320 nm and 330 nm, are transmitted. The light that has passed through the interference filter 4 is converted into a photomultiplier 5
, And is amplified by the preamplifier 6 and input to the transient recorder 7. In the transient recorder 7, the incident laser light is A / D-converted using the output signal from the photodiode 20 as a trigger signal, and the waveform of one laser light pulse emitted from the laser light generating optical system 1 with respect to the scattered light is recorded. . The data recorded in the transient recorder 7 is transferred to a personal computer 8 via a GPIB cable, where the following density calculation taking into account the correction is performed. First, an extinction coefficient α is obtained from a reception signal obtained as a result of emitting two laser beams having non-absorption wavelengths λ2 and λ2 ′ toward a measurement target region by the following equation 5 known as Klett's equation. The expression of Klet is described in the second section of “Applied Optics” Vol. 20 issued on January 15, 1981.
James D., published on pages 11-220. Klet
"Stable analytical inver."
sion solution for process
ing lidar returns].

[0046]

(Equation 5) In the above equation, k: constant, r: distance, P (r): received signal, X (r) = r ² · P (r), and α / β = 30. If the extinction coefficient α is obtained, the backscattering coefficient β becomes α / 3
It is obtained as 0. Also, α (λ1) in the third term of Expression 2
The extinction coefficient α (λ2) and the extinction coefficient α (λ2) obtained above for two non-absorbing wavelengths λ2 and λ2 ′ are calculated using Angstrom's law (α (λ) = constant × λ ^−x , x Takes a value of 0.5 to 2), and obtains the value of x. Using these values, the second term (the correction term of the wavelength dependence of the backscattering coefficient β) and the third term (the correction term of the wavelength dependence of the extinction coefficient α) of Equation 2 are obtained.
Can be obtained, and the ratio between the scattered light data for the absorption wavelength (λ1) and the scattered light data for the non-absorption wavelength (λ2) is calculated for a large number of elapsed times,
Using the average data, the concentration of O ₃ is obtained from Equation 2. In this case, the distance R can be easily calculated based on the elapsed time since the laser light propagates in the atmosphere at the speed of light.

The operation of the gas concentration measuring device according to the present invention will be described with reference to FIG.

After the power of the apparatus is turned on (F-1), the YAG
Wait for 30 minutes to elapse until the characteristics of the laser 10, the titanium sapphire laser 12, and other circuits are stabilized (F-2), and correspond to the absorption wavelength (λ1) of O ₃ from the laser light generating optical system 1 first. It is confirmed that a laser beam having a desired wavelength is oscillating (F-3). Next, the tuning mirror 127 is displaced by a predetermined minute angle by the oscillation wavelength switching mechanism 30 of the titanium sapphire laser 12 to obtain a corresponding phase matching angle (F-4), and the position switching device of the laser light generating optical system 1 By operating 18, it is confirmed that laser light having a wavelength corresponding to one non-absorption wavelength (λ2) is oscillating from the laser light generating optical system 1 (F-5). Next, the tuning mirror 12 of the titanium sapphire laser 12
7 is further displaced by a predetermined minute angle to obtain a corresponding phase matching angle (F-6), and the laser light generating optical system 1
By operating the position switching device 18 of (1), it is confirmed that laser light of a wavelength corresponding to another non-absorption wavelength (λ2 ′) is oscillating from the laser light generating optical system 1 (F-7). At this time, each wavelength is stored in a memory (not shown) for later data arrangement (F-8).

Finally, data collection is started (F
-9). First, the tuning mirror 127 of the titanium sapphire laser 12 is returned to the phase matching angle corresponding to the absorption wavelength λ1, and the position switching device 18 of the laser light generating optical system 1 is returned to the position corresponding to the absorption wavelength λ1 to generate the laser light. The optical system 1 emits laser light of 731.5 nm, which is the absorption wavelength (λ1) of O ₃ . This is detected by the photodiode 20 and is input to the transient recorder 7 as a trigger signal (F-1).
0). Backscattered light from the gas in front is received by the Cassegrain telescope 2 and recorded in the memory of the personal computer 8 (F-11). After that, based on the switching timing signal from the pulse generator 19, the tuning mirror 1 of the oscillation wavelength switching mechanism 30 of the titanium sapphire laser 12
27 is displaced by a small angle to obtain a phase matching angle corresponding to the non-absorption wavelength (λ2) (F-12). As a result, the non-absorbed wavelength (λ2) of O ₃ from the laser light generating optical system 1 is 80.
A 2.8 nm laser beam is emitted and the photodiode 2
The trigger signal from 0 is input to the transient recorder 7 (F-13). Here, the data of the backscattered light is recorded in the memory of the personal computer 8 (F-14).

Thereafter, based on the switching timing signal output from the pulse generator 19, the tuning mirror 127 of the oscillation wavelength switching mechanism 30 of the titanium sapphire laser 12 is further displaced by a small angle, and this time another non-absorbing wavelength (λ2 ′) Take the phase matching angle corresponding to ()
-15). As a result, the laser light generation optical system 1 outputs O ₃
869.1n, which is another non-absorption wavelength (λ2 ′) of
m laser light is emitted, and a trigger signal from the photodiode 20 is input to the transient recorder 7 (F-16). Here, the data of the backscattered light is
(F-17). Thus, one set of backscattered light data for the laser light having the absorption wavelength (λ1) and the two non-absorption wavelengths (λ2) and (λ2 ′) is obtained. In order to obtain data as average as possible, it is preferable to repeat the same operation and collect, for example, 100 sets of data. Then, it is determined whether or not the number of data collections has reached a predetermined number (F-18). If the number has not reached the predetermined number, the process returns to step (F-10) to repeat the same operation until the number reaches the predetermined number.

When the number of times reaches a predetermined number, the density calculation is started based on Equation 2 (F-20). For this purpose, first, an average of 100 backscattered light data obtained for each of the absorption wavelength (λ1) and the non-absorption wavelength (λ2) is obtained (F
−21), the extinction coefficients α (λ2) and α (λ2 ′) are obtained from Equation 5 using the average values obtained for the non-absorption wavelengths (λ2) and (λ2 ′) (F-22, F-23). . Next, using the extinction coefficients α (λ2) and (λ2 ′) thus obtained, the absorption wavelength (λ
1) is determined (F-2).
4). Finally, using the value obtained above, the density N
The calculation of (R) is performed (F-25). In the calculation of the concentration, the correction of the wavelength dependence of the backscattering coefficient β and the correction of the wavelength dependence of the extinction coefficient α are performed, so that the obtained concentration value is extremely accurate. The density value is stored in the personal computer 8 and displayed on the display 9 as necessary (F-2).
6).

In the above embodiment, O ₃ is exemplified.
The present invention can also be applied to the measurement of the concentration of a gas having a property similar to that of O ₃ , that is, a gas that needs to separate the absorption wavelength and the non-absorption wavelength among the three wavelengths for concentration measurement, such as NO _2. it can. As an example, N0 ₂ absorption wavelengths 447.9Nm, non-absorbing wavelength is 446.8nm and 4
42.0 nm, and the phase matching angle was 5 KDP in each case.
5.4 °, 55.3 ° and 36 °. To emit a laser beam of this wavelength, the sum of the second harmonic of the titanium sapphire laser and the YAG laser (1064 nm) is used.

Further, in the above-described embodiment, the three-wavelength switching method is adopted. However, the four-wavelength switching method (one absorption wavelength and three non-absorption wavelengths) or more can be achieved only by changing the control program. The accuracy of the measurement can be further increased.

As a solid-state laser that can be used in the present invention, there is an alexandrite laser having a narrow oscillation wavelength width in addition to the titanium sapphire laser used in the embodiment. In addition, it is a matter of course that the oscillation wavelength switching type laser according to the present invention can be used for devices other than the gas concentration measuring device.

[0055]

As described above, according to the present invention,
By using only two lasers, a solid-state laser such as a titanium sapphire laser and a YAG laser, the wavelength dependence of the backscattering coefficient and the extinction coefficient is corrected, and highly accurate concentration measurement can be performed. Therefore, the device including the optical system is simplified, which is advantageous in cost. In addition, there is no problem in adjusting the laser or installing the laser, and there is no difficulty in aligning light waves of three wavelengths when three lasers are used. In addition, since a Raman cell is not used, there is no problem of troublesome replacement from time to time and instability of beam characteristics and output intensity of laser light after wavelength conversion.

[Brief description of the drawings]

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

FIG. 2 shows a schematic configuration of a part of a laser light generating optical system in the gas concentration measuring device according to the present invention.

FIG. 3 shows a schematic configuration of an oscillation wavelength switching mechanism and a position switching device and a drive control system thereof.

FIG. 4 is a perspective view illustrating a configuration of an example of an oscillation wavelength switching mechanism.

FIG. 5 is a perspective view showing a configuration of an example of a position switching device for a nonlinear optical element.

FIG. 6 is a flowchart showing the operation of the gas concentration measuring device according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser light generating optical system 2 Cassegrain telescope 3 aperture 4, 17 interference filter 5 photomultiplier tube 6 preamplifier 7 transient recorder 8 personal computer 9 display 10, 13 YAG laser 11, 16, 24 nonlinear optical element 12 titanium sapphire laser 14 mirror Reference Signs List 15 dichroic mirror 18 position switching device 19 pulse generator 20 photodiode 30 oscillation wavelength switching mechanism 31 lead screw 32 translation stage 33, 83 sine bar 34 step motor 38, 86 tuning unit 39, 85 tuning stage 40 spring 41, 84 ball 50, Reference Signs List 80 driving circuit 51, 82 head enlarged pin 52, 81 mounting base 60, 70 control module 61 keyboard 62, 72 control circuit 120 titanium saff IA crystal 121 pinhole 122 output coupler

Claims (10)

[Claims] 1. A first solid-state laser, an oscillation wavelength switching mechanism for sequentially switching the wavelength of laser light output from the first solid-state laser, a second solid-state laser, and switching by the oscillation wavelength switching mechanism. Each of the output laser beams having different wavelengths of at least three wavelengths or more and the laser light output from the second solid-state laser via the first nonlinear optical element has a desired frequency of three or more different wavelengths as a sum frequency. A second nonlinear optical element that converts the laser light into a laser beam and outputs the laser light, and the optical position of the second nonlinear optical element is synchronized with the wavelength switching timing by the oscillation wavelength switching mechanism, and the wavelengths of the different three or more wavelengths are changed. And a position switching device for switching the laser light so as to have a phase matching angle. 2. A solid-state laser, a diffraction grating and a tuning mirror provided on an excitation laser light incidence side in the solid-state laser resonator, and an output coupling provided on an oscillation laser light emission side in the solid-state laser resonator. An oscillation wavelength switching mechanism for sequentially switching and displacing the optical position of the tuning mirror with respect to the diffraction grating; each of three different laser beams of three or more wavelengths switched and output by the oscillation wavelength switching mechanism; A nonlinear optical element that converts a laser beam having a wavelength different from the wavelength not shorter than the wavelength into a laser beam having a desired three or more different wavelengths as a sum frequency and outputs the laser beam; and an optical position of the nonlinear optical element with respect to the incident laser beam. In synchronization with the wavelength switching timing by the oscillation wavelength switching mechanism, the phase matching angles of the laser beams having the different three or more wavelengths are adjusted. And a position switching device for switching between the output coupler and the tuning mirror after the laser light incident on the solid-state laser is repeatedly amplified between the output coupler and the tuning mirror via the solid-state laser and the diffraction grating. A laser light generating optical system that outputs laser light having three or more different wavelengths from the laser beam toward the nonlinear optical element. 3. The apparatus according to claim 1, wherein said position switching device comprises: first displacement means for significantly displacing the optical position of said nonlinear optical element; and second displacement means for displacing a small amount.
3. A gas concentration measuring device according to claim 1. 4. A gas concentration measuring apparatus according to claim 3, wherein said first displacement means is a translation stage reciprocated by a lead screw. 5. The apparatus according to claim 3, wherein said second displacement means comprises a stage reciprocating at a high speed, a sine bar pivotally displaced by said stage, and a non-linear element mount fixed to said sine bar. Alternatively, the gas concentration measuring device according to claim 4. 6. The gas concentration measuring apparatus according to claim 5, wherein said stage is an ultra-precision linear actuator. 7. The gas concentration measuring apparatus according to claim 5, wherein said stage and said sine bar are always kept in contact with each other by a spring. 8. The gas concentration measuring apparatus according to claim 1, wherein the first solid-state laser is a titanium sapphire laser. 9. The gas concentration measuring device according to claim 1, wherein the first solid-state laser is an alexandrite laser. 10. A first solid-state laser, a first nonlinear optical element, and an oscillation wavelength switching mechanism for sequentially switching the wavelength of laser light output from the first solid-state laser are aligned in this order with their optical axes aligned. After arranging, by dichroic mirrors, the laser light of at least three different wavelengths output by switching the wavelength and the laser light output from the second solid-state laser via the second nonlinear optical element are aligned. A laser light generating optical system that converts laser light into three or more different desired wavelengths by the second nonlinear optical element and sequentially switches and outputs the laser light; and a light receiving unit that receives scattered laser light from the direction of the gas to be measured. Photoelectric conversion means for photoelectrically converting the scattered laser light received by the light receiving means; recording means for recording optical signal data from the photoelectric conversion means; Calculating means for calculating the concentration of the gas to be measured based on the received light signal data; and switching the optical position of the second nonlinear optical element of the laser light generating optical system with respect to the incident laser light by switching the oscillation wavelength. A gas concentration measuring device, comprising: a position switching device that switches the laser beams of three or more different wavelengths so as to have respective phase matching angles in synchronization with a wavelength switching timing by a mechanism.