CN103364371B - The absorption coefficient of atmospheric aerosol Novel differential measurement method of coaxial-type photothermal interference - Google Patents

Abstract

The invention discloses a new method for dual-channel differential measurement of atmospheric aerosol absorption coefficient based on coaxial photothermal interference. The modulated carrier laser is used to form interference through the area of the aerosol sample to be measured, and two-channel interference optical paths are designed. One of the channels Detect the baseline phase shift caused by ambient temperature gradient and platform vibration, another channel detects the total phase change caused by aerosol absorption excitation laser and baseline phase shift, the difference between the two obtains the phase change caused by aerosol absorption, in the interference amplification factor On the basis of calibration, the online detection of aerosol absorption coefficient is realized. The present invention can also be used to detect the concentration of specific aerosol components based on the "fingerprint" absorption characteristics; the present invention overcomes the influence of environmental temperature changes and the vibration of the measurement platform, cancels the adjustment of the phase transfer point, and makes the excitation laser and detection laser The working distance is doubled, and it has the characteristics of convenient operation, fast detection speed, easy miniaturization, and high detection sensitivity.

Description

A New Method for Differential Measurement of Atmospheric Aerosol Absorption Coefficient Using Coaxial Photothermal Interferometry

Technical field:

The invention relates to a new dual-channel differential measurement method using photothermal interference to detect the absorption coefficient of atmospheric aerosol. The interaction between the excitation laser and the interference beam passing through the aerosol area to be measured causes the change of the interference phase, which can be measured in real time according to the phase change The absorption coefficient of the aerosol to be measured, and monitor the evolution of the aerosol absorption coefficient over time. Selecting the excitation laser at a specific "fingerprint" wavelength enables the determination of specific aerosol trace components and their concentrations.

Background technique:

The radiative forcing caused by atmospheric aerosol absorption is much more complicated than that of greenhouse gases, and it is one of the most uncertain factors in the calculation of radiative forcing, which greatly affects the accuracy of atmospheric models. The lack of aerosol absorption information has become one of the main problems in the study of the climate effect of aerosol radiation and atmospheric correction of space-to-Earth remote sensing. When the laser is transmitted in the atmosphere, the laser energy absorbed by the aerosol is transmitted to the atmosphere, causing fluctuations in the refractive index of the atmosphere and complicating the beam distribution. The energy density distribution of the atmosphere transmitted to the target directly affects the design and development of the engineering system. Obtaining the absorption coefficient of atmospheric aerosols through online measurement can not only more accurately evaluate the change law of the complex refractive index of aerosols, especially the imaginary part of the refractive index, deepen the research on the transmission characteristics of lasers in the atmosphere, but also improve the calculation of aerosol radiative forcing. Accuracy, deepen the understanding of climate effects and change laws.

There are also many practical and potential uses for the on-line measurement of the absorption coefficient of aerosols in the atmosphere. For example, the detection of methane content in mines, the detection of harmful gases such as silane, arsine, and phosphine in semiconductor production workshops, and the detection of components of concern in the fields of life sciences and medical diagnosis, etc. In the above application fields, the concentration of each component to be measured is often in the order of ppm (10 ^-6 ) or even below, so the aerosol absorption coefficient detection with high sensitivity, high selectivity, fast response and real-time non-destructive characteristics is designed Technology, using the characteristics that different gas components usually have different "fingerprint" absorption wavelengths, selecting a laser with a specific wavelength can determine the concentration of the component to be measured, so as to make a preventive or emergency treatment plan.

When the light is irradiated on the aerosol particles, the aerosol particles absorb the light radiation energy and cause the thermal state of the air on the light transmission path, such as temperature, pressure and density, to change. When the incident light is modulated, the local thermal state of the air will produce periodicity. Sexual changes. By measuring the degree of this change, the absorption coefficient of the aerosol to be tested can be obtained. The photothermal method is based on the change of a certain property of the sample gas caused by the thermal conversion of the incident laser radiation energy to achieve detection. Using a higher optical power incident laser can improve the detection sensitivity, which is a feature that general absorption spectra do not have. Since the photothermal method measures the thermal diffusion result of light absorption, this result is not sensitive to the scattering and reflection of gas components, so the photothermal method can measure the absorption in a medium where scattering and reflection coexist, which is better than the difference of absorption characteristics obtained by subtracting scattering from extinction The photothermal method is more accurate, and the detector of the photothermal method can recover quickly, will not produce a "poisoning" reaction, and does not require long-term integration to reach the detection limit, so the practical application of using the photothermal effect to detect the aerosol absorption coefficient is very important. attractiveness and market prospects.

A successful application of photothermal method in the detection of aerosol absorption coefficient composition and concentration is photoacoustic spectroscopy. The detection of absorption coefficient, but photoacoustic spectroscopy must be completed in an optical resonant cavity, which is subject to many restrictions.

Due to the change of the air temperature state, the refractive index of the beam path or the optical path has a slight change. This change can be obtained by the method of phase detection. The photothermal interferometry has a very high sensitivity in phase detection. Under the conditions of light propagation in the Earth's atmosphere, phase fluctuations do not appear to be saturated like light intensity fluctuations, so the applicable range of phase fluctuation results based on perturbation theory is much larger than that of light intensity fluctuations. The strength of the photothermal interference signal depends on the absorption coefficient of the aerosol to be measured and the excitation laser power. HBLin et al. measured the absorption characteristics of ammonium sulfate by photothermal interferometry, but the measurement results were easily disturbed by external vibrations, which greatly limited the application of interferometry. MAOwens et al. realized the synchronous control of the reference optical path and the signal optical path by using a combination of a coated Yaman board and a gold-plated metal plate, and adjusted the interference phase through piezoelectric ceramics to always be at the phase inversion point, and carried out fine measurement of ammonia according to the absorption characteristics; To solve the impact of vibration on the results, it is very cumbersome to place the detection system in the cavity of the sound-absorbing material and suspend it. A. Sedlacek et al. used the foldback Yaman structure to simplify the dual optical path design of the interference device, but the foldback structure still needs to use piezoelectric ceramics to adjust the phase inversion point, and the optical surface of the prism needs to be continuously purged with _N2 to keep it clean. Still inconvenient to apply.

Throughout the development of photothermal interferometry, there have always been three bottlenecks that limit its application: 1. The vibration of the measurement platform; 2. The dynamic adjustment of the phase transition point; 3. The action distance of the excitation laser and the detection laser. Since the photothermal interferometry measures the phase change, and the disturbance of the optical path by mechanical vibration is enough to generate phase information that affects the measurement results, the common method to solve the vibration is to shield the measurement equipment with special materials. In order to ensure the linear relationship between the phase change amount and the modulated laser power, the interference beam must be at the point of phase inversion, which is usually realized by a low-frequency piezoelectric ceramic feedback circuit. When the frequency of the feedback circuit is close to the frequency of the excitation laser, it is very troublesome. . In order to increase the working distance of the excitation laser and the detection laser, in the early days, the materials that pass through the detection laser and fully reflect the excitation laser were used to realize the coaxiality of the two, but the heating of the excitation laser to the reflective material has an impact on the optical path, so Nowadays, the method of maintaining a certain angle between the excitation laser and the detection laser is mostly used, and the required interaction distance can be achieved by reducing the angle or increasing the length of the sample cell.

Aiming at the above three factors, the present invention proposes a new method for differential measurement of the atmospheric aerosol absorption coefficient of coaxial photothermal interference, which forms interference by modulating the carrier laser, adopts dual detection interference channels, and measures the interference channel with the modulation excitation laser The results include the phase baseline change caused by the slow change of ambient temperature and platform vibration and the periodic phase change caused by the modulated excitation laser, while the other channel only measures the phase baseline change caused by the slow change of ambient temperature and platform vibration. The phase change caused by aerosol absorption is used to obtain the absorption coefficient of the aerosol to be measured, which fundamentally gets rid of the limitations of environmental temperature changes, platform vibration, and phase inversion points; the coaxial interference method makes the interaction between the excitation laser and the interference optical path The operating distance is expanded to twice that of the non-coaxial method. The invention greatly simplifies the operation of photothermal interference and improves the detection ability of photothermal interference.

Invention content:

The purpose of the present invention is to address the shortcomings of the existing commonly used aerosol absorption coefficient measurement technology, to provide a way to form two interference detection channels by modulating the carrier laser, and use the differential method to obtain the phase change caused by aerosol absorption, and to measure it online. A method for measuring the aerosol absorption coefficient; this method gets rid of the limitations of environmental temperature changes, platform vibrations, and phase inversion points; and expands the interaction distance between the excitation laser and the interference optical path to twice that of the non-coaxial method, using adjustable wavelength This method can also effectively determine the concentration of specific aerosol components, and has the characteristics of simple principle, high detection sensitivity, and easy miniaturization.

The technical scheme adopted in the present invention is:

A new method for differential measurement of atmospheric aerosol absorption coefficient by coaxial photothermal interference, characterized in that it includes a carrier laser, an optical beam splitter is placed on the front optical path of the carrier laser, and the incident light of the carrier laser is split by light The optical device is divided into two beams of light, and an optical isolator, an optical circulator, and an optical collimator are arranged in sequence on the front optical path of the two beams of light, and the two optical collimators are located at the front end of the detection pool, and the rear end of the detection pool There is a high-reflective mirror, and the two beams of light are reflected and transmitted on the end face of the collimator respectively. The reflected light returns along the original path, and the transmitted light passes through the sample gas area to be detected in the detection cell and returns to the original path by the high-reflective mirror. The sample gas to be detected in the detection cell passes through the end face of the optical collimator again, and the transmitted light and the reflected light on the end face of the optical collimator form interference light. The two interference lights enter the photodetector through the optical circulator, and one of them interferes The beam detection baseline drift is used as the baseline correction optical path, and the other interference beam detects the sum of the phase change caused by the baseline drift and the aerosol absorption excitation laser, the excitation laser is the modulated light generated by the signal generator far away from the wavelength of the interference light, and the interference amplification factor is calibrated On the basis of this method, the online detection of aerosol absorption coefficient is realized.

The new method for differential measurement of the atmospheric aerosol absorption coefficient of coaxial photothermal interference is characterized in that: a spectroscopic sheet is provided on the optical path in front of the excitation laser, an optical power meter is provided on the reflected optical path of the spectroscopic sheet, and the spectroscopic A light reflector 1 is arranged on the transmission light path of the sheet, and a light reflector 2 is arranged on the reflection light path of the light reflector 1, and the light reflector 1 and the light reflector 2 are respectively arranged on one of the interfering light beams in the detection cell. The excitation laser interacts with the detection interference light path for a certain distance in the detection cell.

The new method for differential measurement of atmospheric aerosol absorption coefficient by coaxial photothermal interference is characterized in that: the detection cell and the interference optical path are fixed on a fixed support with as large a counterweight as possible to reduce the vibration of the measurement platform.

The new method for differential measurement of atmospheric aerosol absorption coefficient by coaxial photothermal interference is characterized in that: the sample gas enters the detection cell from the mixing chamber of the gas preparation system, and the carrier gas of the gas preparation system includes pure Nitrogen and pre-treated air can prepare sample gases with different water vapor contents and direct air sampling. The mixing chamber acts as a buffer pool and eliminates the adverse effects of the vibration of the air pump on the measurement results of the interference optical path.

The new method for differential measurement of the atmospheric aerosol absorption coefficient of coaxial photothermal interference is characterized in that: the interference light path passing through the detection cell is coaxially reversed, so the interaction distance between the excitation laser and the detection interference light path It is twice the length of the detection interference optical path covered by the excitation laser.

The incident light is divided into two beams with the same optical quality by the beam splitter, and two paths of interference light are formed by reflection and transmission. The environmental temperature gradient and platform vibration have the same influence on the two paths of interference light. The difference between the two achieves the photothermal phase change. The accurate measurement avoids the adjustment of the image transfer point by traditional photothermal interference, and expands the dynamic range of measurement frequency and concentration.

The invention realizes the direct detection of the atmospheric aerosol absorption coefficient in an open atmospheric environment; selects a specific "fingerprint" absorption wavelength, and can realize the online component determination and concentration detection of trace gases; the energy fluctuation of the excitation laser is obtained by the spectrometer and the energy meter to improve the accuracy of the calculation results.

Optical fiber is an electrically insulating and corrosion-resistant optical transmission medium. It is light in weight, small in size and negligible in light energy loss and polarization state change for short-distance transmission. Coaxial photothermal interference is realized through single-mode optical fiber.

The theoretical basis of the present invention is:

The thermal conductivity of air is very small (0.026W·m ^-1 ·K ^-1 at 298.2K temperature), and the heat conduction process of air is much slower than the excited state relaxation process of aerosol absorption, so it is considered that the heating of the sample air It is plausible that this is done instantaneously and confined within the excitation laser beam.

Assuming that the collisional relaxation of the aerosol in the air after absorbing light energy causes the temperature to rise by ΔT, under the premise that the aerosol absorption is not saturated, the temperature rise can be expressed as:

$\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}\mathrm{<span\; class="notranslate">\; \&alpha;l</span>}}{\mathrm{<span\; class="notranslate">\; C\&pi;</span>}{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; l\&rho;</span>}\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&rho;C</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

where P _e is the power of the excited laser, α is the absorption coefficient of aerosol particles, l is the interaction length between the excited laser beam and the probe laser beam, a _e is the radius of the excited laser beam, ρ and C are the density and Heat capacity, f _e is the modulation frequency of the excitation laser, its duty ratio is 1:1, and this frequency should be insensitive to environmental noise and mechanical vibration.

Changes in atmospheric temperature cause small changes in the refractive index, according to Gladstone-Dale's law:

Δn=((n-1)/T ₀ )ΔT(2)

Among them, n is the average refractive index of the sample to be tested, and T ₀ is the absolute temperature of the sample to be tested.

The optical path of the probe beam passing through this area changes accordingly due to the change of the refractive index of the air, and the amount of change is represented by the change of the interference phase of the probe beam Δφ _e . For the coaxial reflective structure described in this patent, the active length of the excitation laser beam and the detection laser beam (the maximum when the two overlap, but the optical path is easily affected by the thermal expansion and contraction of the optical device) rises to 2l, and λ is the detection The wavelength of the beam, then:

Δφ _e =2π·2l·Δn/λ(3)

The frequency of the variation of the interference phase Δφ _e is the same as the modulation frequency f _e of the excitation laser, and the equations (1), (2) and (3) are combined to get:

$\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; l</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}\frac{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&rho;</span>}{\mathrm{<span\; class="notranslate">\; Cf</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; l</span>}{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}\mathrm{<span\; class="notranslate">\; \&rho;C</span>}{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

The aerosol absorption coefficient α is:

$\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&lambda;T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}\mathrm{<span\; class="notranslate">\; \&rho;</span>}{\mathrm{<span\; class="notranslate">\; Ca</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; l</span>}{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}{\mathrm{<span\; class="notranslate">\; \&Delta;\&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

Using the phase-sensitive lock-in amplifier as a detector can effectively filter out the noise other than the modulation frequency and improve the anti-interference ability of the system. The signal S from the amplifier is proportional to the absorption coefficient α of aerosol particles and has a linear relationship with the phase change Δφ _e , expressed as:

α∝S=AΔφ _e (6)

Where A is the interference amplification factor, which can be obtained by preparing an aerosol sample for calibration.

Assume that the initial interference beam formed by the modulated laser is of cosine form:

x(t)=Bcos(ωt) (7)

After one of the interfering beams passes through the sample area, its expression is:

x ₁ (t)=Bcos(ωt+φ ₁ ) (8)

Among them, _φ1 mainly comes from the change of ambient temperature and platform vibration. In the case of small platform vibration, φ1 is _a slow variable. After another beam of interference beam passes through the sample area and interacts with the excitation laser, its expression is:

x ₂ (t)=Bcos(ωt+φ ₂ ) (9)

Where φ ₂ =φ ₁ +Δφ _e , Δφ _e is the phase change caused by the modulation excitation laser.

Through the Hilbert transform, (8) and (9) are transformed into:

x ₁ ′(t)=Bsin(ωt+φ ₁ ) (10)

x ₂ ′(t)=Bsin(ωt+φ ₂ ) (11)

(11)*(8)-(10)*(9) gets:

X(t)=Bsin(Δφ _e ) (12)

(9)*(8)+(11)*(10) get:

Y(t)=Bcos(Δφ _e ) (13)

The amount of phase change Δφ _e is:

${\mathrm{<span\; class="notranslate">\; \&Delta;\&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arctan</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; Y</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 14</span>}<span\; class="notranslate">\; )</span>$

Substitute into formula (5) to obtain the absorption coefficient of the aerosol to be measured in real time.

The beneficial effects of the present invention are:

The present invention establishes a method for measuring the absorption coefficient of atmospheric aerosol on-line in real time by photothermal interferometry, obtains environmental factors and periodically modulated excitation laser to signal baseline phase change through two interference optical paths, and obtains the absorption coefficient of the aerosol to be measured by using a differential method And the law of evolution therebetween; the measurement of the aerosol absorption coefficient in the present invention is in-situ measurement, the detection sensitivity is high, and the operation is simple and convenient.

The advantages of the present invention are:

1. The real-time online non-contact measurement of the aerosol absorption coefficient in the atmospheric environment is realized, and the original state of the atmosphere to be measured is maintained to the greatest extent;

2. The optical fiber coaxial reflective structure realizes the doubling of the action distance of the excitation laser beam and the interference beam;

3. The measurement principle of aerosol absorption coefficient is simple, easy to operate and fast in detection speed;

4. Different from the traditional absorption spectrum, the photothermal interference method can improve the detection sensitivity of the aerosol absorption coefficient by increasing the excitation laser power;

5. The method of obtaining the phase change of the modulation and demodulation carrier laser signal avoids the adjustment of the transition phase difference point in the traditional interference method. The length of the interference optical path and the initial phase are not required to be equal, which expands the dynamic range of the measurement frequency and concentration;

6. Dual-channel differential measurement eliminates the influence of environmental temperature gradients and platform vibrations. In theory, one calibration point is enough to correct the interference amplification factor;

7. Without absorption, there is no signal. It is a measurement technology with zero background signal. It uses a wavelength-tunable laser to measure the concentration of specific components.

Description of drawings:

Figure 1 is a schematic diagram of the measurement principle of the aerosol absorption coefficient (the incident light (solid arrow) passing through the end face of the collimator and the optical path (hollow arrow) reflected by the total mirror are coaxial in the detection cell, and the excitation laser optical path and the transmitted light, The coaxial detection optical path composed of reflected light forms a small angle to ensure the absorption length while avoiding the perturbation of the optical path caused by the expansion and contraction of the mirror itself; To accurately determine the aerosol absorption coefficient.

Figure 2 is a working principle diagram of an interference optical path in two interference channels (the dotted line and the solid line both propagate coaxially, and are described separately for the convenience of illustration).

Figure 3 is a flow chart for preparing samples to be tested (samples that can be prepared include: ①aerosol samples of different concentrations with pure nitrogen as carrier gas, ②aerosol samples with different concentrations of pure nitrogen with different concentrations of water vapor as carrier gas, ③zero air Aerosol samples with different concentrations of carrier gas, ④ zero air containing different concentrations of water vapor as carrier gas with different concentrations of aerosol samples, ⑤ ambient atmosphere).

detailed description:

As shown in Figure 1, a new method for differential measurement of the atmospheric aerosol absorption coefficient of coaxial photothermal interference includes a carrier laser 1, an optical beam splitter 2 is placed on the front optical path of the carrier laser 1, and the carrier laser 1 The incident light is divided into two beams by the optical beam splitter 2, and the optical isolator 3, the optical circulator 5, and the optical collimator are arranged in sequence on the front optical path of the two beams of light, and the two optical collimators are located in the detection cell 7. The front end and the rear end of the detection cell 7 are provided with a high reflection mirror 8, and the two beams of light are respectively reflected and transmitted on the collimator end face 6, the reflected light returns along the original path, and the transmitted light passes through the sample area to be detected in the detection cell 7 Reflected by the high reflection mirror 8 and return to the original path, after passing through the sample to be detected in the detection cell 7, it passes through the end face 6 of the light collimator again, and the transmitted light and the reflected light of the end face 6 of the light collimator form interference light, and the two One path of interference light passes through the optical circulator 5 to form an interference beam and enters the photodetector, wherein one path of interference beam detects the baseline drift as the baseline correction optical path, and the other path of interference beam detects the sum of the baseline drift and the phase change caused by the aerosol absorption excitation laser 11, said The excitation laser is the parallel light modulated by the signal generator 12 away from the wavelength of the interference light. On the basis of the calibration of the interference amplification factor, the online detection of the aerosol absorption coefficient is realized.

Exciting laser light 11 front optical path is provided with spectroscopic plate 9, is provided with optical power meter 10 on the reflective light path of spectroscopic plate 9, is provided with light reflector-13 on the transmission light path of spectroscopic plate 9, and is provided with optical reflector-13 on the reflective light path of light reflector-13 Light reflecting mirror 2 14 is provided, and light reflecting mirror 13 and light reflecting mirror 2 14 are respectively arranged on the interference optical path of one of the interfering light beams in the detection pool 7, and the excitation laser 11 is in the detection pool 7 and the detection interference optical path The included angle is within 2 ^O , and the two interact for a certain distance.

The detection cell 7 and the interference optical path are fixed on a fixed support with as large a counterweight as possible to reduce the vibration of the measurement platform.

The sample gas enters the detection cell through the mixing chamber of the gas preparation system. The carrier gas of the gas preparation system includes pure nitrogen and pretreated air, which can prepare sample gases with different water vapor contents and air for direct sampling. The mixing chamber acts as a buffer pool The role of the pump eliminates the adverse effects of the vibration of the air pump on the measurement results of the interference optical path.

The interference optical path passing through the detection cell 7 is coaxially reversed, so the interaction distance between the excitation laser 11 and the detection interference optical path is twice the length of the detection interference optical path covered by the excitation laser 11 .

Example

1. Armored single-mode optical fiber is selected as the transmission medium of the carrier laser 1, and visible light such as HeNe laser is used as the carrier to form an interference optical path through the optical fiber, and the interference light and the incident carrier laser are separated through the optical circulator 5. The reflectivity of the anti-reflection coating on the end face 6 of the light collimator is set between 26% and 30%, so that the intensity of the reflected light and the transmitted light forming interference are approximately equal;

2. For the same atmospheric aerosol, the absorption coefficients corresponding to different wavelengths of the excitation laser 11 are different. A laser that cannot interfere with the carrier laser 11 is selected as the excitation laser. Taking into account the diameter of the excitation laser beam and the angle between the excitation laser and the interference beam, the width of the mirror on the excitation laser adjustment bracket is controlled to about 5mm;

3. In order not to interfere with each other in the measurement results of the two interfering optical paths, the distance between the two parallel beams returning from the original path reflected by the high mirror 8 is set at 8cm to 12cm, and the optical collimator and the high mirror 8 are fixed in a position where the deformation coefficient is as small as possible. For materials such as ceramics, the distance between the two is set at 20cm to 24cm, the angle between the interference beam and the excitation beam is controlled at 1 ^o ~ 2 ^o , and the interaction distance is controlled at about 10cm;

4. The detection pool 7 is made of tempered glass, and the detection pool 7 is fixed with a stable support with a counterweight as large as possible, and the optical fiber passing through the detection pool 7 is filled with an elastic material, so that the vibration of the detection pool 7 to the interference optical path is reduced to a minimum;

5. Since ammonia gas has a relatively large absorption cross-section of 56atm ^-1 cm ^-1 at 273K, ammonia gas is selected as the aerosol standard sample for calibration. The standard concentration of ammonia permeates outward at a certain speed (μg/min) through the permeation tube, and the upper part is continuously purged with pure nitrogen, and the ammonia gas sample from low to high is dynamically diluted through the flow controller in different periods of time to excite the laser. Select a _CO2 laser with an energy of 2W and a wavelength of 9.22 μm, and record the dynamic range of excitation laser energy through the cooperation of the beam splitter 9 and the optical power meter 10, so as to improve the accuracy of demodulation signal processing;

6. Substituting the formula to calculate the phase change Δφ _e corresponding to different concentrations of ammonia gas samples, using one interferometric optical path to record the change of the signal baseline with environmental factors φ ₁ , and using another interferometric optical path to measure the periodic modulation signal φ ₂ to obtain the modulation The absorption intensity S caused by the exciting laser beam is used to calibrate the interference amplification factor A;

7. Through the sample preparation system, the interference amplification factor is corrected, and the absorption coefficient of the configured sample is investigated;

8. The mixing cavity of the sample preparation system is regarded as a buffer pool, and the ambient atmosphere is directly injected into the sample without any treatment through the mixing cavity, and the absorption coefficient corresponding to the wavelength of the excitation laser is investigated;

9. When detecting the concentration of a certain component in the atmosphere, select the absorption wavelength of the “fingerprint” of the component as the excitation laser; if you want to detect several aerosol components with relatively close absorption lines, choose a laser with an adjustable output wavelength , Such as the simultaneous detection of methane, ammonia and carbon monoxide can choose 2.3μm DFB semiconductor laser.

Figure 2 is a working principle diagram of an interference optical path in two interference channels, and the labels in the figure represent:

1. Carrier laser 2. Optical isolator 3. Optical circulator 4. Collimator end face 5. Carrier laser transmitted light 6. High mirror 7. Carrier laser reflected light through high mirror 8. Reflected light passed through collimator The transmitted light 9. The reflected light of the carrier laser through the end face of the collimator 10. The interference light composed of 8 and 9.

Claims (5)

1. A new method for differential measurement of the atmospheric aerosol absorption coefficient of coaxial photothermal interference, characterized in that: a carrier laser is included, an optical beam splitter is placed on the front optical path of the carrier laser, and the incident light of the carrier laser passes through the optical beam splitter. The beam splitter is divided into two beams of light, and the front optical path of the two beams is respectively provided with an optical isolator, an optical circulator, and an optical collimator. The two optical collimators are located at the front end of the detection cell, and the rear end of the detection cell is set There is a high-reflective mirror, and the two beams of light are reflected and transmitted on the end face of the collimator respectively. The reflected light returns along the original path, and the transmitted light passes through the sample gas area to be detected in the detection cell and returns to the original path by the high-reflective mirror, passing through the detection The sample gas to be detected in the cell passes through the end face of the optical collimator again, and the transmitted light and the reflected light on the end face of the optical collimator form interference light. The two interference lights enter the photodetector through the optical circulator, and one of the interference beams detects the baseline The drift is used as the baseline correction optical path, and the other interference beam detects the sum of the baseline drift and the phase change caused by the aerosol absorption excitation laser, which is the modulated light generated by the signal generator away from the wavelength of the interference light, based on the calibration of the interference amplification factor , to realize the online detection of aerosol absorption coefficient. 2. The new method for differential measurement of the atmospheric aerosol absorption coefficient of coaxial photothermal interference according to claim 1, characterized in that: a spectroscopic sheet is provided on the optical path ahead of the excitation laser, and a spectroscopic sheet is provided on the reflected optical path of the spectroscopic sheet. There is an optical power meter, a light reflector 1 is set on the transmission light path of the spectroscopic sheet, and a light reflector 2 is set on the reflection light path of the light reflector 1, and the light reflector 1 and the light reflector 2 are respectively set in the detection cell and one of them interferes On the interference optical path of the light beam, the excitation laser interacts with the detection interference optical path for a certain distance in the detection cell. 3. The new method for differential measurement of atmospheric aerosol absorption coefficient by coaxial photothermal interference according to claim 1, characterized in that: the detection cell and the interference optical path are fixed on a fixed bracket with as large a counterweight as possible. 4. The new method for differential measurement of atmospheric aerosol absorption coefficient of coaxial photothermal interference according to claim 1, characterized in that: the sample gas enters the detection cell from the mixing chamber of the gas preparation system, and the gas preparation The carrier gas of the system includes pure nitrogen and pretreated air, which can prepare sample gases with different water vapor contents and direct air sampling. 5. The new method for differential measurement of atmospheric aerosol absorption coefficient of coaxial photothermal interference according to claim 1, characterized in that: the interference light path passing through the detection cell is coaxially reversed, so the excitation laser and the detection The interaction distance of the interference light path is twice the length of the detection interference light path covered by the excitation laser.