JPH0548857B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a gas distribution measurement method that uses a two-wavelength laser to measure the gas distribution, that is, the product of the gas concentration and the distribution length.

(Prior art) It is known that the presence or absence of gas can be detected by utilizing the fact that a laser beam of a specific wavelength is easily absorbed by a certain type of gas, and sensing technology that applies this principle is used in industrial measurement, pollution monitoring, etc. Widely used. As an example, the vacuum wavelength of the 3.39 μm band of the laser light generated by the H _e -N _e laser is 3.3922 μm.
There are two oscillation lines, m (λ ₁ ) and 3.33912 μm (λ ₂ ), where λ ₁ is strongly absorbed by methane and λ ₂ is only slightly absorbed by methane. Therefore, it is possible to detect the presence or absence of methane with high sensitivity using a laser beam containing these two wavelength components. Since methane is the main component of city gas, leakage of city gas can be detected by detecting methane gas.

FIG. 9 shows a schematic configuration of a conventional methane detection system that detects methane using two lasers, and is composed of a light source section 1, a light receiving section 2, and a signal processing section 3. The light source unit 1 includes a H _e −N _e laser 1 a that emits a laser beam of 3.3922 μm,
H _e −N _e laser 1 that emits 3.3912 μm laser light
b and emits red light (0.6328μm) for monitoring.
It has mechanical choppers _{1d and} 1e that modulate the laser beams from lasers 1a and 1b at different frequencies.
Red light from laser 1c and modulated laser light from lasers 1a and 1b are transmitted through mirrors M ₁ , M ₂ , M ₃ ,
The light is emitted toward the monitoring area D by M ₄ and half mirrors HM ₁ and HM ₂ , is reflected by obstacles 4 such as roads and walls, and is received by the light receiving unit 2 .

In the light receiving section 2, the incident laser light is sent to the light receiving mirror 2a,
The light is reflected and focused by the light receiving sensor 2b and converted into an electrical signal by the light receiving sensor 2c. The output of the light receiving sensor 2c is first amplified by the preamplifier 3a of the signal processing section 3, then separated by the lock-in amplifiers 3b and 3c synchronized with the modulation frequency of the mechanical choppers 1d and 1e of the light source section 1, and then separated by the recorder 3d. recorded. From the difference between the two recorded signals, it can be determined whether methane is present in the monitoring area D.

Since a methane detection system having such a configuration uses a large number of mirrors or half mirrors, the optical system is complicated and requires a large volume, and optical axis adjustment is troublesome, resulting in a large loss of laser light. In addition, there are many other problems, such as complicated signal processing and operational limitations of the mechanical chopper that make it impossible to perform high-frequency modulation, resulting in a disadvantage in terms of signal-to-noise ratio.

On the other hand, U.S. Patent No. 4,059,356 discloses that when an air circulation cell is installed inside a laser resonator and the laser is brought to a place to be monitored, the atmosphere at that place enters the laser resonator. A gas detector is disclosed that is capable of detecting the presence or absence of methane in the atmosphere. Although this detector does not have the problems mentioned above, remote sensing is not possible.

Therefore, in order to solve these problems, we used a laser that oscillates a laser beam containing two wavelength components.
The gains of the wavelength components are made almost equal, and the resonator length L is set to L.
= λ ₁ λ ₂ /2 | λ ₁ - λ ₂ | (integer + 1/2) and performs minute modulation. As a result, the outputs of the two wavelength components are modulated simultaneously, and the modulation component of the sum of the outputs becomes 0. A laser device configured to automatically control the resonator length so that the resonator length is The outputs of wavelength λ ₁ and λ ₂ components of laser light generated from this laser device are modulated with a phase shift of 180° from each other, but the combined total output is not modulated. However, when this laser light passes through a specific gas atmosphere, the wavelength λ ₁ component is absorbed and the total output has a modulation component, making it possible to detect that specific gas. By using this laser device, gases such as methane can be detected with a simple configuration and with less loss of laser light.

However, although the gas detection method using such a laser device can detect the presence of a specific gas based on the presence or absence of a modulation component of the total output, the total output may be affected by scattering of the laser light in the atmosphere, reflectance from objects, and other factors. Since this changes, it is not possible to measure the amount of distribution using the output value. Furthermore, when the laser light is infrared light, it is necessary to combine visible light for monitoring purposes, but the half mirror used for this combination causes a loss of light quantity, making it impossible to effectively utilize the laser output. There is also.

(Objective and Structure of the Invention) The present invention has been made in view of the above points, and an object of the present invention is to enable measurement of gas distribution amount over a wide area and at a distance with a simple structure and without loss of light amount.
To achieve this purpose, the output of the two wavelength components oscillated from the dual wavelength oscillation laser has the same frequency f ₁
Using a two-wavelength laser beam that is modulated by and automatically controlled so that the modulation component of the sum of both outputs becomes 0,
The laser beam is modulated at a frequency f ₂ different from the frequency f ₁ and then passed through a gas atmosphere whose distribution amount is to be measured, and the components of the modulation frequencies f ₁ and f ₂ of the laser beam that have passed are detected, and both components are detected. The system was configured to measure the amount of gas distribution by performing arithmetic processing including logarithmic transformation on the transmittance calculated based on the difference between .

(Example) The present invention will be explained below based on the drawings.

FIG. 1 shows an embodiment of an apparatus for measuring the distribution amount of methane gas by the method of the present invention, but the present invention is of course not limited to this.

In the figure, 10 surrounded by a broken line is a two-wavelength oscillation laser device, which includes a H _e -N _e discharge tube 10 a,
Methane cell 10b containing methane gas and mirror 1
0c and _{10d .} The mirror 10d is vibrated at a predetermined frequency by the electrostrictive element 10e, so that both mirrors 10
The resonator length L between c and 10d is modulated. 10f is a half mirror placed in the optical axis of the laser beam generated from the H _e -N _e laser;
0g is an optical sensor such as I _o A _s , 10h is an oscillator 10
Optical sensor 10g that changes in synchronization with the frequency f ₁ of i
A lock-in amplifier that detects the output component of 10j
is an integrator that integrates the output of the lock-in amplifier 10h, and 10k is the output of the integrator 10j and the oscillator 10i.
The electrostrictive element 10e is driven by this high voltage.

The H _e −N _e laser has a vacuum wavelength of 3.3922 μm (λ ₁ )
There are two closely spaced oscillation lines of 3.3912 μm (λ 2 ) and 3.3912 μm (λ ₂ ), but under normal conditions, the component at wavelength λ ₁ has a larger gain than the component at wavelength λ ₂ , so as a result of competition, the wavelength λ ₁
Only the component oscillates, and the wavelength λ ₂ does not oscillate. However, since the H _e -N _e laser device is provided with a methane cell 10b that absorbs light of λ ₁ in the resonator,
The overall gain of the wavelength λ ₁ component decreases, and if the methane pressure is appropriately selected, it becomes almost equal to the gain of the wavelength λ ₂ component, making simultaneous oscillation of two wavelengths λ ₁ and λ ₂ possible as shown in Figure 6. . In this case, the absorption of the wavelength λ ₁ component by the Menthacell 10b changes depending on the methane pressure as shown in FIG.
1.3Torr).

Next, considering the relationship between the cavity length and the oscillation output, in the case of gas lasers, the gain curve generally has a Doppler spread around the center frequency unique to the laser medium, and the resonance condition ν _r = nC/2L (L is the cavity length, C is the speed of light, n
is an integer) is _oscillated (see Figure 6). In this case, if ν _r is close to the center frequency, the output will be large, and if it is far from the center frequency, the output will be small. When the resonator length L changes, ν _r crosses the center frequency one after another, so the oscillation intensity changes periodically as the resonator length L changes.

In the case of two-wavelength oscillation, each wavelength component changes in this way, but the resonator length L should be selected so as to satisfy L=λ ₁ λ ₂ /2 | λ ₁ − λ ₂ | (1/2 + integer). For example, as the resonator length L changes in the vicinity, the maximum output points B and C of λ ₁ and λ ₂ come to be located at the midpoint between each other as shown in FIG.

Therefore, if the resonator length L is changed (modulated) with an amplitude of Δl by the frequency f with a certain value L ₀ as the center, the outputs of the laser beams with two wavelengths λ ₁ and λ ₂ will be I ₁ and I ₂ ( (Indicated by chain and dashed lines in Figure 8)
and the total output is I (shown by the solid line in Figure 8).
Then, it can be expressed as follows.

I ₁ = I ₁ (L ₀ ) + dI ₁ (L ₀ )/dL ・△l sin2πftft + higher-order component I ₂ = I ₂ (L ₀ ) + dI ₂ (L ₀ )/dL ・△l sin2πftft + higher-order component I= I ₁ + I ₂ = I ₁ (L ₀ ) + I ₂ (L ₀ ) + (dI ₁ (L ₀ )/dL + dI ₂ (L ₀ )/dL) ・△l sin2πft + higher-order components Therefore, the modulation frequency of the total output I The phase of the f component is detected by the lock-in amplifier 10h (see Figure 1), and the output is used as an error signal to be detected by the high voltage amplifier 10.
When feedback is applied to the electrostrictive element 10e through k, the center of modulation of the resonator length L ₀ is set so as to satisfy dI ₁ (L ₀ )/dL+dI ₂ (L ₀ )/dL=0 (1). is automatically controlled. In this case, automatic control must be performed by appropriately selecting the phase of the error signal so that dI ₁ (L ₀ )/dL=−dI ₂ (L ₀ )/dL≠0 is satisfied.

Optical sensor 10g, lock-in amplifier 10h, integrator 10j, and high voltage amplifier 10k shown in Fig. 1
The total output I of the laser beam is detected by an infrared light sensor 10g, and the modulated frequency component is detected by a lock-in amplifier 10h, integrated by an integrator 10j, and then output by a high voltage amplifier 10k. Determine the bias voltage. As a result, the high voltage amplifier 10k has the oscillation frequency of the oscillator 10i centered around the bias voltage.
A high voltage varying with _f1 is outputted to drive the electrostrictive element 10e. When the modulated wave component disappears due to the feedback effect, the bias voltage of the high voltage amplifier 10k is held by the integral value held by the integrator 10j at that time, and the output high voltage is held constant and oscillation continues. Ru. When the resonator length L changes due to temperature changes during operation, it is corrected by the feedback action of the feedback circuit.

As shown in Figure 2, the outputs of the wavelength λ ₁ and λ ₂ components generated from the H _e −N _e laser under automatic control are modulated with a 180° shift from each other, but the total output is modulated. It has not been.

Returning to Figure 1 again, numeral 11 is a mechanical chopper placed on the optical axis of the H _e -N _e laser.
The mechanical chopper 11 consists of an impeller in which an opening 11a is formed, and a mirror 1 is provided on the back surface of the blade.
1b is coated. The mechanical chopper 11 modulates the laser light at the oscillation frequency f ₂ (f ₁ ≠ f ₂ ) of the oscillator 12, and a visible laser 13 such as an H _e -N _e laser that emits red laser light for monitoring. Visible light from the mirror 11b is alternately projected onto the same optical axis as the measurement laser light. 14 is a mirror; 15 is a filter that cuts out visible light for monitoring and scattered light of sunlight, and passes only wavelengths λ ₁ and λ ₂ ; and 16 is a filter that passes through the atmosphere F of methane gas, which is the gas whose distribution amount is to be measured. Optical sensor 17 receives the laser beam that has passed through the optical sensor 1
6, 18 and 19 are lock-in amplifiers that detect the modulation frequency components of frequencies f ₁ and f ₂ of the optical output amplified by the preamplifier 17, and 20 is the output of the lock-in amplifiers 18 and 19.
A difference amplifier 21 amplifies the difference between H ₁ and H ₂ and outputs p, and 21 uses the output p of the difference amplifier 20 to calculate the methane transmittance of wavelength λ ₁ component u = (1-p) / (1 + p)
22 is a logarithmic amplifier that calculates and amplifies the logarithm of transmittance u, and 23 is a display that displays the measured methane distribution amount (product of concentration c and distribution length l) cl.

Next, a method for measuring the amount of methane distribution using the gas distribution amount measuring device having the above configuration will be explained.

Laser beams with wavelengths λ ₁ (3.3922 μm) and λ ₂ (3.3912 μm) are generated from a H _e -N _e laser, which is a two-wavelength oscillation laser device. If the outputs of laser beams with two wavelengths λ ₁ and λ ₂ are I ₁ and I ₂ respectively, I ₁ and I ₂
In addition, the total output I can be expressed by the following equation when 100% modulation is performed, and its waveform is as shown in FIG.

I ₁ = A ₀ (1-sin2πf ₁ t) I ₂ = A ₀ (1-sin2πf ₁ t) I = I ₁ + I ₂ = I ₀ This two-wavelength laser beam is connected to the mechanical chopper 1
1 at frequency f ₂ . This laser beam becomes as shown in FIG. At this time, visible laser 1
The visible light from 3 is reflected by the mirror 11b of the mechanical stopper 11 and projected onto the same optical axis alternately with the measurement infrared laser light.

This modulated laser beam is reflected by a mirror 14 and passed through an atmosphere F of methane gas, and only wavelengths λ ₁ and λ ₂ components are waved by a filter 15 and received by an optical sensor 16. The wavelength λ ₁ component of the laser light is absorbed by methane gas, but the wavelength λ ₂ component is hardly absorbed, so the total output has a modulation component, and its waveform is as shown in FIG.

The lock-in amplifier 18 is synchronized with the oscillation frequency f ₁ of the oscillator 10i, and the lock-in amplifier 19 is synchronized with the oscillation frequency f ₂ of the oscillator 12. The components of modulation frequencies f ₁ and f ₂ are phase detected and output as H ₁ and H ₂ as shown below.

H ₁ = Aa ₁ (1-u) H ₂ = Aa ₂ (1-u) where u = exp (-αcl), α is the absorption coefficient of light with wavelength λ ₁ and absorption of light with wavelength λ ₂ by methane The difference from the coefficient, c, is the methane concentration, and l is the distribution length. Further, A is an amount that constantly changes due to factors other than methane such as scattering loss of laser light, and a ₁ and a ₂ are amounts that do not change after initial setting due to factors such as the shape of the opening of the chopper.

The differential amplifier 20 outputs wavelengths λ ₁ and λ ₂ components.
From H ₁ and H ₂ , p=a ₂ /a ₁・H ₁ /H ₂ is calculated and amplified, and then in the calculation circuit 21, the methane transmittance of the wavelength λ ₁ component is calculated as u=(1-p)/(1+p ) is calculated.

The distribution amount cl of methane is determined by calculating −1/α l _o u in the logarithmic amplifier 22 . Also,
Even if the modulation is 100% or has never been present, cl can be found in the same way by considering the bias in _H2 . This distribution amount cl serves as an index showing how methane at a certain concentration spreads, and is therefore useful for knowing the amount of leaked methane gas and its effects. Furthermore, if we consider only gas leakage, the change in distribution length is within a range of several times at most, whereas the concentration of leaked gas can be thought to change on the order of 2 to 3 powers of 10. Therefore, the amount of distribution can also be seen as an index of concentration.

The distribution amount cl thus obtained is displayed on the display 23.

Figure 5 shows a laser beam generated from a two-wavelength oscillation laser modulated at a frequency (f ₁ ) of 100 Hz, which is further modulated by a mechanical chopper at a frequency (f ₂ ) of 10 Hz, with a duty ratio of 9:1 and an attenuation rate of 25. % of methane gas.

(Effects of the Invention) As explained above, in the present invention, the outputs of two wavelength components oscillated from a two-wavelength oscillation laser are modulated at the same frequency _f1 , and the modulation component of the sum of both outputs becomes 0. Using a two-wavelength laser beam that is automatically controlled as follows, the laser beam is modulated at a frequency f ₂ different from the frequency f ₁ and then passed through the gas atmosphere to be measured, and the modulation frequency of the laser beam that has passed through the gas is determined.
The gas distribution amount is measured by detecting the f ₁ and f ₂ components and performing arithmetic processing including logarithmic transformation on the transmittance calculated based on the difference between the two components, so it is possible to measure the amount of light with a simple configuration. Gas distribution can be measured remotely and over a wide area without loss. In the present invention, when modulation is performed using a mechanical chopper using the frequency f ₂ of the laser beam, visible light for monitoring can be coaxially projected during modulation, so a half mirror is used for multiplexing as in the past. This eliminates the loss of light amount that would otherwise occur.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an apparatus for implementing the gas distribution measurement method according to the present invention, FIG. 2 is a waveform diagram of a laser beam of a two-wavelength oscillation laser device used in the present invention, and FIG.
Figure 4 is a waveform diagram of a laser beam that has been further frequency modulated, Figure 4 is a waveform diagram of a laser beam that has passed through the gas whose distribution amount is to be measured, and Figure 5 is a methane gas diagram in an experiment using the gas distribution measurement method according to the present invention. FIG. 6 is a waveform diagram of the passing laser beam. FIG. 6 is a diagram showing the relationship between the gain and resonance frequency of the two-wavelength laser beam oscillated by the two-wavelength oscillation laser device used in the present invention. _FIG . Figure 8 shows the relationship between the cavity length and oscillation output of the dual wavelength oscillation laser device used in the present invention. Figure 9 is a schematic diagram of the conventional methane detection system _. It is a line diagram. 10...2 wavelength oscillation laser device, 10a...
H _e -N _e discharge tube, 10b...methane cell, 10e
...Electrostrictive element, 10g...Light sensor, 10h, 1
8, 19...Lock-in amplifier, 10i, 12...
...Oscillator, 10k...High voltage amplifier, 11...Mechanical chopper, 13...Visible laser, 15
... Filter, 16 ... Optical sensor, 20 ... Differential amplifier, 21 ... Arithmetic circuit, 22 ... Logarithmic amplifier, 23 ... Display device.

Claims (1)

[Claims] 1 The output of two wavelength components oscillated from a two-wavelength oscillation laser is modulated at the same frequency _f1 , and the modulation component of the sum of both outputs is automatically controlled to be 0.2
After modulating the laser beam at a frequency f ₂ different from the frequency f ₁ , the laser beam is passed through a gas atmosphere whose distribution amount is to be measured, and the modulation frequency f ₁ and f of the laser beam that has passed through the gas is A method for measuring the amount of gas distribution, characterized in that the amount of gas distribution is measured by detecting _{the second} component and performing arithmetic processing including logarithmic transformation on the transmittance calculated based on the difference between the two components.