RU2480737C1 - Method of measuring transparency, concentration of gaseous components of scattering media on two-wave laser - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to monitoring environmental pollution and can be used to measure transparency and component composition (concentration of gaseous components) of scattering media (atmosphere, smokiness of car emissions, chimneys of industrial plants etc). The method involves sending through the analysed section of the gas jet beams of probing radiation towards each other and receiving from two boundary points of the section of the medium at an angle of 90° to the direction of probing scattered radiation, the intensity value of which is used to determine transparency of the section of the medium. The radiation source used is a two-wave laser, wherein a mode for successive optoelectronic recirculation of the first and second boundary points is realised. The length of the inspected path ΔR is determined from the difference in recirculation frequencies, and the unknown gas concentration is determined from the ratio of intensity of scattered radiation at two wavelengths.

EFFECT: high accuracy of measuring attenuation coefficient and broader functional capabilities of the measuring device.

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Description

The invention relates to the field of environmental pollution control and can be used to measure transparency and component composition (concentration of gas components) of scattering media (atmosphere, smoke emissions from automobiles, pipes of industrial enterprises, etc.).

Known meter transparency of the atmosphere [1], containing two lasers, two radiation receivers, a computing unit. However, this system does not allow measuring the smoke emissions of automobiles, the concentration of emissions from pipes of industrial enterprises, etc. due to the fact that it is impossible to register backscatter signals from adjacent points.

Closest to the proposed method is a method for measuring the transparency of a scattering medium [2], which consists in sending through the studied section of a gas jet towards each other beams of probe radiation and receiving at an angle of 90 ° to the direction of sounding of the scattered radiation. The value of the radiation intensity determines the transparency or smokiness of the jet section. The disadvantage of this device is the limited measurement accuracy due to inaccurate alignment of probe radiation beams, fluctuations in the difference frequency of the radiation, and inaccurate knowledge of the length of the monitored portion of the medium.

The objective of the invention is to improve the accuracy of measuring the attenuation coefficient and expand the functionality of the meter. The expansion of functionality consists in providing the ability to simultaneously measure the concentration of gas components in the medium, as well as in the measurement process to determine the length of the controlled section of the medium and take into account its value when calculating transparency.

The problem is solved by the fact that in the method for measuring transparency, the concentration of the gas components of scattering media using a two-wave laser [2], which consists in sending through the studied section of the gas jet towards each other beams of probe radiation and receiving from two boundary points of the medium at an angle of 90 ° to the direction of sounding of scattered radiation, the intensity of which determines the transparency of the medium, using a two-wave laser as a radiation source and m optoelectronic recirculation in series with respect to the first and second boundary points; the length of the controlled path ΔR is determined by the difference in the recirculation frequencies from the expression

where f ₁ , f ₂ are the recirculation frequencies relative to the first and second boundary points, respectively, and the attenuation coefficient of the medium section is determined from the expression

where S _λ2 (R ₀ , R ₁ ), S _λ2 (R ₃ , R ₁ ), S _λ2 (R ₀ , R ₂ ), S _λ2 (R ₃ , R ₂ ) are the intensities of the scattered radiation in the first (R ₁ ) and the second (R ₂ ) boundary points at a wavelength of λ ₂ for the forward and backward paths, respectively; the desired gas concentration is determined from the expression

where ΔK is the differential absorption coefficient at wavelengths λ ₁ and λ ₂ , S _λ1 (R ₀ , R ₁ ), S _λ1 (R ₃ , R ₁ ), S _λ1 (R ₀ , R ₂ ), S _λ1 (R ₃ , R ₂ ) are the intensities of the scattered radiation at the first and second boundary points at a wavelength of λ ₁ for the forward and backward paths, respectively.

раз по сравнению с прототипом. Расширение функциональных возможностей заключается в обеспечении возможности одновременно с измерением коэффициента ослабления измерять концентрацию газовых компонент в контролируемой среде.The properties appearing in the claimed object are an increase in the accuracy of measuring the attenuation coefficient, due to the fact that the length of the controlled section of the medium is measured and its value is taken into account when calculating transparency. Since two probing signals are generated in one active region of the laser, the optical axes of the probing signals are combined and the difference in frequency of the probing radiation is more stable. Since the wavelengths λ ₁ and λ ₂ slightly differ, then in the absence of a controlled gas component, averaging the results provides an increase in the accuracy of measuring the attenuation coefficient in

times compared to the prototype. The expansion of functionality lies in the possibility of simultaneously measuring the attenuation coefficient to measure the concentration of gas components in a controlled environment.

The essence of the method is illustrated using the drawing, which shows a functional diagram of a transparency meter for a scattering medium using a two-wave laser. The system comprises: a two-wave laser 1, a laser power supply 2, a mirror 3, a first radiation receiver 4, a second radiation receiver 5, a processor unit 6, a recirculation unit 7.

As a radiation source, a two-wave semiconductor laser diode with an asymmetric quantum-dimensional heterostructure providing generation at two different optical wavelengths is used [3]. Switching the radiation wavelength in the pulse from λ ₁ to λ ₂ occurs when the amplitude of the pump current in the pulse jumps from I ₁ to I ₂ . The duration of electrical pulses and, accordingly, pulses of emitted light at different wavelengths can be made sufficiently short, less than a few nanoseconds. The difference in the generation wavelengths Δλ = λ ₁ -λ ₂ reaches values of 10-90 nm. If you use a temperature controller and stabilize the amplitude of the injection current, then a high stability of the difference in the generation wavelengths is achieved.

The meter works as follows. A pulsed probe radiation at a wavelength of λ _{1 is} sent through the medium under study by a two-wave laser ₁ . The radiation scattered at the first boundary point R _{1 of the} controlled medium at an angle φ, the value of which S _λ1 (R ₀ , R ₁ ), is recorded by the receiver 4 and goes to the processor 6. The radiation scattered at the second boundary point R ₂ of the medium at an angle φ, the value of which S _λ1 (R ₀ , R ₂ ), is recorded by the receiver 5 and enters the processor 6. Then, the probe radiation is reflected from the mirror 3 and carries out a return passage through the medium under study. At receivers 4 and 5, the values of the signals S _λ1 (R ₃ , R ₁ ) and S _λ1 (R ₃ , R ₂ ) are recorded, which are recorded in the processor memory. Due to different path lengths, scattered radiation will arrive at receivers 4 and 5 at different times, therefore, the digital processing system of the processor allows you to efficiently resolve and identify these signals. For the values of the signals with a direct passage of the medium at a wavelength of λ ₁ scattered at an angle φ to the direction of the sending at points R ₁ and R ₂ , the following expressions can be written:

S _λ1 (R ₀ , R ₁ ) = A ₁ P ₁ σ _φ (R ₁ ) T _λ1 (R ₀ , R ₁ ) T _λ1 (R ₁ , R ₄ ),

S _λ1 (R ₀ , R ₂ ) = A ₂ P ₁ σ _φ (R ₂ ) T _λ1 (R ₀ , R ₁ ) T _λ1 (R ₁ , R ₂ ) T _λ1 (R ₂ , R ₅ ),

where A ₁ , A ₂ are the hardware constants of the receivers 4 and 5, respectively; P ₁ is the radiation power at a wavelength of λ ₁ with a direct pass, σ _φ is the scattering coefficient at an angle φ, R ₄ , R ₅ are the coordinates of the location of the receivers 4 and 5, respectively; R ₀ , R ₃ - the coordinates of the location of the laser 1 and mirror 3, respectively; R ₁ , R ₂ - coordinates of scattering points; T _λ1 (R _i , R _j ) = exp {-ε _λ1 (R _i , R _j ) · (R _j -R _i )} - transparency of the sections [R _i , R _j ), i, j = 0 ... 5 on wavelength λ ₁ .

With the return passage of the medium by radiation at a wavelength of λ ₁ reflected from mirror 3, the values of the signals scattered at an angle (180 ° -φ) at points R ₁ and R ₂ have the form:

S _λ1 (R ₃ , R ₁ ) = A ₁ P ₂ σ _180-φ (R ₁ ) T _λ1 (R ₂ , R ₃ ) T _λ1 (R ₁ , R ₂ ) T _λ1 (R ₁ , R ₄ ),

S _λ1 (R ₃ , R ₂ ) = A ₂ P ₂ σ _180-φ (R ₂ ) T _λ1 (R ₂ , R ₃ ) T _λ1 (R ₂ , R ₅ ),

where P ₂ is the radiation power at a wavelength of λ ₁ with a return pass.

The ratio of the signals scattered at the point R ₁ is

The latter can be written as follows:

where K ₁ = S _λ1 (R ₀ , R ₁ ) / S _λ1 (R ₃ , R ₁ ); B = P ₁ T _λ1 (R ₀ , R ₁ ) / P ₂ T _λ1 (R ₂ , R ₃ );

C ₁ = σ _φ (R ₁ ) / σ _180-φ (R ₁ ); τ _λ1 (R ₁ , R ₂ ) = ε _λ1 (R ₁ , R ₂ ) · (R ₂ -R ₁ ).

For signals scattered at the point R ₂ ,

This expression can be written as follows

where K ₂ = S _λ1 (R ₀ , R ₂ ) / S _λ1 (R ₃ , R ₂ ); C ₂ = σ _φ (R ₂ ) / σ _180-φ (R ₂ ).

The solution of the system of linear equations (1) and (2) with respect to τ _λ1 is equal to:

In inhomogeneous scattering media, in order to exclude the influence of the scattering indicatrix σ _φ , as follows from (3), it is necessary to register scattering at the same angle in each of the scattering points, i.e. at an angle π / 2. In this case, expression (3) takes the form

From expression (4) it is easy to pass to the attenuation coefficient:

Then, pulsed probe radiation at a wavelength of λ _{2 is} sent through the medium under study by a two-wave laser 1. As in the previous measurements, with forward and backward passage of t. R ₁ , scattered emissions of S _λ2 (R ₀ , R ₁ ) and S _λ2 (R ₃ , R ₁ ) are written to the processor, and when passing t. R ₂ radiation S _λ2 (R ₀ , R ₂ ) and S _λ2 (R ₃ , R ₂ ) are also written to the processor.

Similar to the above calculations, we obtain the attenuation coefficient at a wavelength of λ ₂ :

Since the wavelengths λ ₁ and λ ₂ differ slightly, the calculation of the average value of the attenuation coefficient in accordance with the expression

раз по сравнению с прототипом.where ΔP = R ₂ -R ₁ is the length of the controlled path, will provide increased measurement accuracy in

times compared to the prototype.

Simultaneously with measuring the intensity of the scattered radiation in the system, the optoelectronic recirculation mode is carried out sequentially with respect to the first and second boundary points. This is as follows. A pulsed probe radiation at a wavelength of λ _{1 is} sent through the medium under study by laser ₁ . Having registered the signal S _λ1 (R ₀ , R ₁ ), the processor scattered by the first boundary point R ₁ starts the recirculation unit 7, which, through the power supply 2, starts the laser at the wavelength λ ₁ . Thus, as a result of the closure of the optical feedback loop, a recirculation process is established in the system, the period τ of which is determined by the optical delay of radiation at a distance with a constant electrical delay in the recirculation unit. The period (frequency f ₁ ) of recirculation relative to the first boundary point R ₁ will be determined as follows

where, t _opt1 = L ₁ / c is the radiation delay time on the path, L ₁ is the path length consisting of the portion R ₀ ... R ₁ and the portion R ₁ ... R ₄ , c is the speed of light in air, t _e is the electric time delays. The electric delay time t _e the recirculation unit is selected so that it is greater than the total radiation delay over the entire path. Consequently, all pulses of scattered radiation during forward and backward passage of the path will be detected by receivers 4, 5, and only after that the laser sends another optical pulse to the path.

Then the processor, having registered the signal S _λ1 (R ₀ , R ₂ ) scattered by the second boundary point R ₂ , starts the recirculation mode relative to the point R ₂ . The period (frequency f ₂ ) of recirculation relative to the second boundary point R ₂ will be determined as follows

where, t _opt2 = L ₂ / c is the radiation delay time on the path, L ₂ is the path length, consisting of the portion R ₀ ... R ₁ , the portion R ₁ ... R _2, and the portion R ₂ ... R ₅ . From expressions (8, 9), one can find the length of the controlled path ΔR from the difference in recirculation frequencies

where f ₁ , f ₂ are the recirculation frequencies relative to the first and second boundary points, respectively. The obtained value of the length of the controlled path ΔR is taken into account in (5-7) when calculating the optical characteristics. If during the measurement process there is a need to change the boundaries of the controlled area of the medium, the system will automatically take into account the change in the length of the controlled path, which greatly expands the functionality of the meter.

If the difference between the measured coefficients ε _λ1 (R ₁ , R ₂ ) and ε _λ2 (R ₁ , R ₂ ) is of great importance, then the attenuation coefficient is dependent on the wavelength, which can be used to measure gas concentration. The width of the absorption spectral lines of various gases amounts to fractions and units of nanometers; therefore, to control any gas, it is possible to choose the laser generation wavelengths so that one wavelength is in the center of the absorption band of the gas under control and the other outside the absorption band. To measure the gas concentration, the wavelengths of the generation of a two-wave laser are chosen so that the wavelength λ _{1 is} in the center of the absorption band of the gas being monitored, and the wavelength λ ₂ is outside the gas absorption band. We represent the attenuation coefficients ε (λ) at wavelengths λ ₁ and λ ₂ in the form

where K (λ ₁ ) is the absorption coefficient of the investigated gas component at a wavelength of λ ₁ , K (λ ₂ ) is the absorption coefficient of the studied gas component at a wavelength of λ ₂ , C _x is the concentration of the studied gas component, β (λ ₁ ), β (λ ₂ ) - the total absorption and scattering coefficients on λ ₁ , λ _{2 by} all other components present in the medium.

The solution of the system of equations (5, 6), taking into account expressions (11) with respect to C _x, will have the form

, следовательно, искомая концентрация газа будет равнаSince the wavelengths of the generation of a two-wave laser are chosen close in value to λ ₁ ≈λ ₂ with a difference of a few nanometers, then

therefore, the desired gas concentration will be equal to

where ΔK = K (λ ₁ ) -K (λ ₂ ) is the differential absorption coefficient, ΔR is the length of the controlled path.

As can be seen from (5-7, 12), the expressions for the optical characteristics (τ, ε) and gas concentration C _x contain neither instrument constants, nor energies of the transmitted radiation, nor parameters reflecting the influence of the environment. This means that there is no need to calibrate the measuring system and install hardware constants due to their absence in the algorithms that are obtained without using any assumptions or neglect of these constants. The stability of the system to changes in hardware constants also means resistance to pollution of optics.

Since two probing signals are generated in one active region of the laser, the combination of the optical axes of the probing signals and higher stability of the difference frequency of the probing radiation are ensured, which also gives an increase in the accuracy of measuring the attenuation coefficient compared to the prototype. The expansion of functional capabilities consists in providing the possibility, simultaneously with measuring transparency, to measure the concentration of gas components in a controlled environment, as well as to determine the length of a controlled section of the medium and take into account its value when calculating transparency.

Thus, as a result of using a two-wave semiconductor laser as a radiation source in the meter and implementing the optoelectronic recirculation mode, an increase in the measurement accuracy and expansion of the system's functional capabilities are achieved.

Used sources

1. Sergeev N.M. Measurement of atmospheric transparency using two lasers // Proc. doc. VI All-Union Symposium on Laser and Acoustic Sounding of the Atmosphere. Tomsk, part 1, 1980. S.123-125.

2. A.S. USSR No. 1523974, MKI G01N 21/47. The method for determining the transparency of the area of the scattering medium / B.B. Vilenchits and other publ. 1989, Bull. No. 43.

3. Patent RB No. 1385, MKI H01S 3/19, Semiconductor laser / A. A. Afonenko, V. K. Kononenko, I. S. Manak. Publ. 1996.

Claims (1)

A method for measuring transparency, concentration of gas components of scattering media using a two-wave laser, which consists in sending through the studied section of the gas stream towards each other beams of probe radiation and receiving from two boundary points of the medium at an angle of 90 ° to the direction of sounding of the scattered radiation, the intensity value of which is determined transparency of the medium, characterized in that a two-wave laser is used as the radiation source and the optoelectronic recirculation mode is performed rations sequentially with respect to the first and second boundary points; the length of the controlled path ΔR is determined by the difference in the recirculation frequencies from the expression

,
where f ₁ , f ₂ are the recirculation frequencies relative to the first and second boundary points, respectively, and the attenuation coefficient of the medium section is determined from the expression

,
where S _λ2 (R ₀ , R ₁ ), S _λ2 (R ₃ , R ₁ ), S _λ2 (R ₀ , R ₂ ), S _λ2 (R ₃ , R ₂ ) are the intensities of the scattered radiation in the first (R ₁ ) and the second (R ₂ ) boundary points at a wavelength of λ ₂ for the forward and backward paths, respectively; the desired gas concentration is determined from the expression

,
where ΔK is the differential absorption coefficient at wavelengths λ ₁ and λ ₂ , S _λ1 (R ₀ , R ₁ ), S _λ1 (R ₃ , R ₁ ), S _λ1 (R ₀ , R ₂ ), S _λ1 (R ₃ , R ₂ ) are the intensities of the scattered radiation at the first and second boundary points at a wavelength of λ ₁ for the forward and backward paths, respectively.