CN108426850B - Absolute measurement of atmospheric CO2Content frequency stabilized cavity ring-down spectrometer - Google Patents
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Abstract
The invention relates to an absolute measurement method for CO in atmosphere2A frequency stabilized cavity ring-down spectrometer of contents, comprising: the gas circuit system provides gas to be detected with preset pressure into the ring-down optical cavity; the outer partA semiconductor laser in the optical path emits laser light that is coupled through at least one optical element to the ring down cavity, where the laser light forms a TEM00 interference mode. The invention can realize absolute measurement of CO2 concentration without calibration, and the uncertainty is 0.057% -0.1%.
Description
Technical Field
The invention relates to a spectrometer, in particular to a spectrometer for absolute measurement of CO in atmosphere2A frequency stabilized cavity ring-down spectrometer of content.
Background
CO in the atmosphere2Mainly comes from the combustion of fossil fuels such as coal, petroleum and natural gas, the exhaust emission of motor vehicles and the CO in the atmosphere2Concentration of (2) 280ppm (10) before the industrial revolution-6The number of gas molecules contained in one million gas molecules) to about 400ppm at present, and CO is present in hundreds of years2The concentration of (c) increased by 40%. Thus limiting and reducing CO2The emission of CO in the atmosphere becomes a major issue for human beings2Monitoring of concentration is a problem that humans are first confronted. By monitoring CO in a region2The concentration condition of the fossil fuel is judged and analyzed to perform targeted treatment and regulation.
Existing CO2The concentration detection technology can be divided into a traditional non-spectral detection method and a novel spectral analysis method according to a detection principle, and the traditional non-spectral detection method is divided into the following steps: thermocatalytic methods, gas chromatography, chemiluminescence, and ultrasound measurements. The novel spectroscopic analysis method is mainly used for checking by using the principle of spectroscopy, namely the interaction characteristic of light and detected molecules, and the essence of the novel spectroscopic analysis method is the absorption of electromagnetic waves by substances. The method has the advantages of wide measurement range, capability of simultaneously measuring different gases, continuous on-line monitoring and the like, and becomes an ideal tool and method for monitoring the concentration of the trace gas. The spectrum analysis method mainly comprises a differential absorption spectroscopy (DOAS) technology, a tunable laser absorption spectroscopy (TDLAS) technology, a Direct Absorption Spectroscopy (DAS) technology and a cavity ring-down/cavity enhanced absorption spectroscopy (CRDS/CEAS) technology. The cavity ring-down absorption spectrum technology based on frequency stabilization enables light to be reflected for multiple times in the cavity due to the high-quality optical cavity, and the effective absorption length can reach dozens of kilometers, so that the frequency stabilization is greatly improvedThe detection sensitivity can be measured on site in real time, a series of problems caused by sampling measurement are avoided, and due to various advantages, the frequency-stabilized cavity ring-down-based absorption spectroscopy is a scheme which is recognized internationally and most hopefully to solve accurate measurement of greenhouse gas components (the signal-to-noise ratio and the sensitivity are both 3-4 orders of magnitude higher than those of the traditional method), and meanwhile, the method can be used for calibrating various concentration measuring instruments on site.
However, most of the system is in the laboratory due to the complexity of the cavity ring-down system, and no mature cavity ring-down system capable of realizing accurate measurement exists.
Disclosure of Invention
The invention aims to provide a frequency stabilized cavity ring-down spectrometer, which can realize absolute measurement of CO2 content in the atmosphere.
The invention provides a frequency stabilized cavity ring-down spectrometer, which comprises: the gas circuit system provides gas to be detected with preset pressure into the ring-down optical cavity; the semiconductor laser in the external optical path emits laser light, the laser light is coupled to the ring down cavity through at least one optical element, and the laser light forms a TEM00 interference mode in the ring down cavity.
The ring-down optical cavity comprises an air inlet hole and an air outlet hole, and the air inlet hole and the air outlet hole are located on the same side of the optical cavity body or on different sides of the optical cavity body.
Wherein the at least one optical element comprises a lens, a first mirror and a second mirror.
The first end part of the ring-down cavity body is provided with a first high reflecting mirror, and the second end part of the ring-down cavity body is provided with a second high reflecting mirror.
The frequency stabilization cavity ring-down spectrometer is integrally positioned on the optical platform.
Wherein, further comprising a data acquisition processing unit.
The invention has the advantages that: 1. the optical cavity adopts low thermal expansion coefficient (2X 10)-7℃-1) The invar steel is realized by controlling the external temperature to be (25.000 +/-0.003) DEG CThe existing cavity length is stable, namely the horizontal axis of the absorption spectrogram is stably measured; 2. the frequency of the working laser is locked on the optical cavity, and the locked central frequency is represented by the maximum value of the number of ring-downs in unit time; 3. the light source is switched off by means of an optical amplifier (BOA); 4. the measured special diagnosis spectral line is R (12), and the absorption center wavelength is 6237.421424cm-1(ii) a 5. The structure is simple, and the measurement repeatability is less than 0.05%; 6. the absolute measurement of the concentration of CO2 can be realized, calibration is not needed, and the uncertainty is 0.057% -0.1%; the instrument can also be calibrated by using standard gas to realize relative measurement, the uncertainty level depends on the level of the standard gas, and the uncertainty is about 0.05 percent; 7. has extremely high repeatability, and the repeatability is 0.01 percent.
Drawings
FIG. 1 is a schematic diagram of a ring down cavity of the present invention;
FIG. 2 is a schematic structural diagram of a high reflection mirror according to the present invention;
FIG. 3 is a schematic structural view of a frequency stabilized cavity ring down spectrometer of the present invention;
FIG. 4 is a schematic view of a measurement spectrum according to the present invention.
Detailed Description
To facilitate an understanding of the present invention, embodiments of the present invention will be described below with reference to the accompanying drawings, and it will be understood by those skilled in the art that the following descriptions are provided only for the purpose of illustrating the present invention and are not intended to specifically limit the scope thereof.
FIG. 1 illustrates the structure of a ring down cavity of the present invention. The ring-down optical cavity 1 comprises an optical cavity body, wherein an accommodating space is arranged inside the optical cavity body, the optical cavity body is provided with a first end part and a second end part, an air inlet hole 2 is formed in the side wall close to the first end part, an air outlet hole 3 is formed in the side wall close to the second end part, and the air inlet hole 2 and the air outlet hole 3 are located on the same side of the optical cavity body or on different sides of the optical cavity body. Inside the side wall in the extension direction of the optical cavity body there is a blind hole extending from the first end to the second end, said blind hole having a predetermined size, preferably said size being 20-60mm or other suitable size, preferably a platinum resistance thermometer or temperature sensor being arranged in said blind hole.
A first high reflecting mirror 4 and a second high reflecting mirror 5 are respectively arranged at the first end and the second end of the optical cavity body, and fig. 2 is a schematic structural diagram of the high reflecting mirrors. The first or second high reflecting mirror is provided with a first side and a second side, the first side is of a plane structure, the first side is of a circular structure with the diameter larger than the size of the first end, the second side is of a concave structure with a preset curvature radius, and the preset curvature radius is matched with the length of the cavity. The first high reflecting mirror 4 and the second high reflecting mirror 5 may have the same structure or different structures according to specific designs. Preferably, the concave structure of the first high reflection mirror 4 is opposite to the concave structure of the second high reflection mirror 5. The high-reflection mirror is adhered to the end face of the cavity through epoxy resin, so that the high-reflection mirror can be sealed on one hand and is used for forming intra-cavity interference on the other hand. Preferably, the blind hole is 50mm deep, and the optical cavity body preferably adopts low thermal expansion coefficient (2 x 10)-7℃-1) The invar realizes the stability of the cavity length by external temperature control at 25.000 +/-0.003 ℃.
FIG. 3 is a schematic structural diagram of a frequency stabilized cavity ring down spectrometer of the present invention. As shown in fig. 3, the entire apparatus is divided into three main sections, section i, section ii and section iii, and the division of the three sections is not intended as a definition or distinction between specific structures and components thereof, but is merely for convenience of understanding and description to aid in understanding the present invention. The frequency stabilization cavity ring-down spectrometer comprises a ring-down cavity, a gas circuit system, an external light path and a data acquisition and processing unit, wherein the cavity ring-down spectrometer is integrally positioned on an optical platform so as to keep the high stability of the whole system.
As shown in fig. 3, the first part comprises a gas path system, the gas path system comprises a gas path control unit and a temperature and pressure measurement unit, the gas inlet 2 and the gas outlet 3 of the ring-down cavity 1 are respectively connected with the first part through pipelines, and the gas control unit controls the pressure and the quality of the gas entering the ring-down cavity 1. The temperature and pressure control unit controls the temperature and ambient pressure within the entire ring down cavity. The method comprises the following steps that gas to be detected is controlled through a mass flow controller 6, the mass flow controller 6 is connected with a gas inlet 2 through a pipeline, the mass flow controller 6 is controlled through a PC (personal computer) or other control systems, the gas to be detected enters a ring-down light cavity through the pipeline, the pressure in the cavity in the ring-down light cavity 1 is measured through a pressure measuring unit 7, the pressure in the cavity is maintained at a preset pressure value or is adjustable within a preset pressure range in the working process, the pressure in the cavity is further preferably maintained at 100torr, an external vacuum pump 8 is connected to the outer side of a gas exhaust hole 3 of the ring-down light cavity 1, and the vacuum pump 8 provides negative pressure for the ring-down light cavity 1; the temperature measuring unit 9 is connected with a standard platinum resistance thermometer in the blind hole and is used for measuring the intracavity temperature of the ring-down cavity 1.
The two ends of the ring-down cavity 1 are respectively provided with a first high reflecting mirror 4 and a second high reflecting mirror 5, the outer side of the first high reflecting mirror 4 is provided with a photodetector (del)10, the photodetector 10 is connected to a Digital Delay Generator (DDG)11, the photodetector 10 is further connected to a high-speed Data Acquisition Card (DAC)12, and the digital delay generator 11 is connected to an optical amplifier (BOA) driver 13. When the output voltage of the photodetector 10 reaches the threshold value of 1.8V, the delay generator 11 will send out a cut-off pulse, on one hand, a signal is sent to the optical amplifier driver 13 to cut off the light source, and on the other hand, a signal is sent to the high-speed data acquisition card 12 to start to acquire ring-down data. The semiconductor driver 14 controls the semiconductor laser 15, and the laser light emitted from the semiconductor laser 15 passes through the optical amplifier 16, passes through the lens L, the second reflecting mirror M2 and the first reflecting mirror M1 in sequence, and is irradiated to the second high reflecting mirror 5 by the light emitted from the first reflecting mirror M1, wherein optical elements such as the lens L, the reflecting mirrors M1 and M2 cooperate to couple the laser light of about 1.6 μ M output from the semiconductor laser to the ring-down cavity 1, and a TEM00 interference mode is formed in the ring-down cavity 1.
FIG. 4 shows measurement line information for the ring down cavity spectrometer of the present invention. When the voltage detected by the photodetector 10 reaches the threshold, the signal of the photodetector 10 is transmitted to the high-speed data acquisition card 12 and the digital delay generator 11, and the digital delay generator 1 is utilized1 sends a cut-off pulse to the driver 13 of the optical amplifier to cut off the light source and measure the rate at which photons are absorbed in the ring-down cavity 1 to obtain the absorption coefficient. After the absorption coefficients at a plurality of frequency points are measured, the absorption spectrum shown in FIG. 4 can be obtained, the area A can be obtained by integration, and further CO can be obtained2Partial pressure p in the gasiThe following were used:
in the formula: 1.656 × 10 ═ S-23cm is CO2Has an absorption center wavelength of 6237.421424cm-1C is the speed of light in vacuum, kBBoltzmann constant, T is temperature.
The total system pressure p measured by combining the pressure measuring unit can obtain CO2The molar concentration x is:
the following description is only for understanding the working process of the ring-down spectrometer of the present invention, and is not meant to be a unique limitation on the structure and working mode thereof, and those skilled in the art can make an adaptive adjustment with the structure of the spectrometer according to specific needs, and further improve the working steps according to the adjusted structure, and the specific measurement and operation processes are as follows:
1. and starting the system for preheating, waiting for the temperature of the system to be stable, starting the vacuum pump 8, and setting the internal pressure of the ring-down cavity 1 to be 100 torr.
2. The laser frequency of the operating semiconductor laser 15 is locked to the ring down cavity. A TEM00 interference is constructed in the optical cavity by adjusting the working laser current of the 1.6-micron semiconductor laser, the frequency of the working laser is locked on the ring-down cavity by taking the number of ring-down as a target, then the absorption coefficient is measured, and the average is obtained after 320 times of measurement;
3. after the measurement, the frequency is hopped by changing the control temperature on the DFB laser 15 by the laser controller 14 to approximately 1 free spectral range (about 100 MHz).
4. And repeating the step 2 to measure the absorption coefficient of the next frequency point, and averaging after 320 times of measurement.
5. Repeating the steps 3 and 4 to obtain the whole absorption spectrum, and processing data by a Labview program to obtain the area. The measured temperature T, total pressure p and the formula (1-2) are combined to obtain CO2And (4) concentration.
The instrument is used for measuring the concentration of standard gas in a gas cylinder, wherein the gas is CO2And N2The mixture of (1) at a concentration of (400.25. + -. 0.20) ppm. The absorption spectrum shown in fig. 4 is obtained through measurement, the area a is obtained through fitting, the partial pressure of CO2 is obtained through combining formula (1) and is pi-5.345 Pa, the total pressure is obtained through measurement of an instrument pressure measuring unit and is p-13363.97 Pa, and therefore the concentration in the gas cylinder is obtained as follows:
the difference from the standard value was 0.04%. The uncertainty analysis is shown in table 1 below:
TABLE 1 uncertainty analysis
The invention has the advantages that: 1. the optical cavity adopts low thermal expansion coefficient (2X 10)-7℃-1) The invar realizes the stability of the cavity length by controlling the external temperature at (25.000 +/-0.003) DEG C, namely, the horizontal axis of an absorption spectrogram is stably measured; 2. the frequency of the working laser is locked on the optical cavity, and the locked central frequency is represented by the maximum value of the number of ring-downs in unit time; 3. the light source is switched off by means of an optical amplifier (BOA); 4. the measured special diagnosis spectral line is R (12), and the absorption center wavelength is 6237.421424cm-1(ii) a 5. The structure is simple, and the measurement repeatability is less than 0.05%; 6. the absolute measurement of the concentration of CO2 can be realized, calibration is not needed, and the uncertainty is 0.057% -0.1%; standard gas pairs can also be usedThe instrument is calibrated to realize relative measurement, the uncertainty level depends on the level of standard gas, and the uncertainty is about 0.05 percent; 7. has extremely high repeatability, and the repeatability is 0.01 percent.
It is to be understood that while the present invention has been described in conjunction with the preferred embodiments thereof, it is not intended to limit the invention to those embodiments. It will be apparent to those skilled in the art from this disclosure that many changes and modifications can be made, or equivalents modified, in the embodiments of the invention without departing from the scope of the invention. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention are still within the scope of the protection of the technical solution of the present invention, unless the contents of the technical solution of the present invention are departed.
Claims (5)
1. Absolute measurement of atmospheric CO2A frequency stabilized cavity ring-down spectrometer of contents, comprising: a ring-down cavity, a gas path system and an external light path,
the method is characterized in that: a blind hole extending from the first end part to the second end part is formed in the side wall of the optical cavity body in the extending direction, the size of the blind hole is 20-60mm, and a platinum resistance thermometer or a temperature sensor is arranged in the blind hole; the optical cavity body adopts a low thermal expansion coefficient of 2 multiplied by 10-7℃-1The invar steel realizes the stability of the cavity length by controlling the temperature at 25.000 +/-0.003 ℃ externally; the gas path system provides gas to be measured with preset gas pressure into the ring-down light cavity; the gas circuit system comprises a gas circuit control unit and a temperature and pressure measuring unit, a semiconductor laser in the external light path emits laser of about 1.6 microns, the laser is coupled to the ring-down cavity through at least one optical element, and a TEM00 interference mode is formed in the ring-down cavity by the laser; the two ends of the ring-down optical cavity are respectively provided with a first high reflector and a second high reflector, the outer side of the first high reflector is provided with a photoelectric detector, the photoelectric detector is connected to a digital delay generator and a high-speed data acquisition card, and the digital delay generator is connected to a high-speed data acquisition cardThe generator is connected to the optical amplifier driver; when the output voltage on the photoelectric detector reaches a threshold value of 1.8V, the time delay generator sends out a cutting pulse to send a signal to the driver of the optical amplifier to cut off the light source, and the high-speed data acquisition card starts to acquire ring-down data; measuring the absorption rate of photons in the ring-down cavity to obtain an absorption coefficient, measuring the absorption coefficients at multiple frequency points to obtain an absorption spectrum, integrating to obtain an area A, and further obtaining CO2Partial pressure p in the gasiThe following were used:
in the formula: 1.656 × 10 ═ S-23cm is CO2Has an absorption center wavelength of 6237.421424cm-1C is the speed of light in vacuum, kBBoltzmann constant, T is temperature;
the total system pressure p measured by combining the pressure measuring unit can obtain CO2The molar concentration x is:
2. the frequency stabilized cavity ring down spectrometer of claim 1, wherein: the ring-down optical cavity comprises an air inlet hole and an air outlet hole, wherein the air inlet hole and the air outlet hole are positioned on the same side of the optical cavity body or on different sides of the optical cavity body.
3. The frequency stabilized cavity ring down spectrometer of claim 1, wherein: the at least one optical element includes a lens, a first mirror, and a second mirror.
4. The frequency stabilized cavity ring down spectrometer of claim 1, wherein: the frequency stabilization cavity ring-down spectrometer is integrally positioned on the optical platform.
5. The frequency stabilized cavity ring down spectrometer of claim 1, wherein: further comprises a data acquisition and processing unit.
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