CN117347313A - Detection device for measuring gases with different concentrations by utilizing different polarization directions of laser - Google Patents
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Abstract
The invention belongs to the technical field of laser spectrum, and particularly relates to a detection device for measuring gases with different concentrations by utilizing different polarization directions of laser. The invention adopts the high-precision triangle ring resonator structure to realize the detection of the ring-down spectrum of the optical cavity, avoids the mode beat frequency effect, the multiple ring-down and the etalon effect caused by the direct reflected light of the cavity front mirror fed back to the laser, and improves the accuracy of the ring-down time. The invention uses the phenomenon that the single-pass loss of the light beams with mutually perpendicular polarization directions is different, which is peculiar to the high-precision triangular ring resonator structure, so as to cause different finesses, thereby realizing the separate measurement of gases with different concentration measurement requirements.
Description
Technical Field
The invention belongs to the technical field of laser spectrum, and particularly relates to a detection device for measuring gases with different concentrations by utilizing different polarization directions of laser.
Background
In the twenty-first century, with the development of technology and the continuous improvement of productivity, humans created unprecedented wealth of materials. At the same time, the rapid population growth and the rapid consumption of natural resources, especially the rapid consumption of fossil fuels such as coal, petroleum and the like, lead to serious greenhouse effect and environmental pollution, and bring serious challenges to human survival and sustainable development. In order to cope with the increasingly serious atmospheric environmental problems, corresponding policies and regulations are put out in all countries around the world to control carbon emission and realize emission reduction of greenhouse gases. Therefore, with the importance of human beings on greenhouse gas emission and atmospheric environmental protection, the research of rapid, calibration-free, real-time and online high-sensitivity gas detection technology and the development of corresponding instruments are necessary. The traditional gas detection technology mainly takes contact type and off-line sampling type, and has the problems of low sensitivity, incapability of real-time monitoring, lack of flexibility in detection and the like. In contrast, the laser spectrum gas detection technology does not change the gas property, can realize the on-line measurement of the gas, has the advantages of high sensitivity, high response speed, simple operation and the like, and becomes the primary development technology of the new generation of trace gas detection.
The laser absorption spectrum technology is that when the frequency of the incident laser resonates with the energy level of the target molecule, the laser can be absorbed by the molecule, the quantitative analysis of the substance can be realized by utilizing the size of the light intensity absorption based on the well-known Beer-Lambert law, and the advantages of high sensitivity and resolution are widely applied to the field of trace gas detection. However, due to the limitation of noise, the detection sensitivity of direct absorption is limited by the detection noise of the system, and the sensitivity is low. There are many laser spectroscopy techniques for measuring gases developed in the laser direct absorption spectroscopy technique, and the cavity ring-down spectroscopy method utilizes high-definition optical cavities with effective absorption lengths up to tens of kilometers. Meanwhile, since the CRDS technique determines the intracavity absorption by measuring the intracavity ring-down time, the measurement is not affected by fluctuation of laser power or the like and thus has high detection sensitivity. Since the first proposal of the concept of CRDS by O' Keefa and Deacon in 1988, various cavity ring-down spectroscopy techniques have been proposed and applied. For example, cavity ring-down spectroscopy techniques can be classified into pulsed light source type and continuous light wave type, depending on the light source selected. But the spectral line of the pulsed laser is very broad and therefore determines that it is not possible to have a higher spectral resolution. Compared with pulse CRDS, the CRDS (CW-CRDS) technology based on the continuous light source developed later adopts the continuous light source, and a laser turn-off device and a threshold circuit are added in an experimental device, so that the aspects of spectrum resolution, signal-to-noise ratio, data acquisition rate and the like are greatly improved. Therefore, the continuous wave cavity ring-down spectroscopy technology with stronger cavity output optical power is favored by technological workers, and becomes a preferred scheme of the international commercialized high-precision gas analyzer (such as Picaro and other company products).
The traditional cavity ring-down spectroscopy system comprises a laser source, a light chopping system, a laser matching light path, a high-precision resonant cavity and a data acquisition system. The high-precision resonant cavity in the designed cavity ring-down spectrum experiment system adopts an annular cavity structure, and compared with a linear cavity structure, the annular cavity structure avoids light beams from being directly reflected back to the laser, avoids a mode beat frequency effect, multiple ring-down and an etalon effect, and improves the accuracy of ring-down time. Meanwhile, for the triangular ring resonant cavity structure, the fact that polarized light in two directions is circulated for one circle in the resonant cavity can generate different phase mutation to cause the difference of resonant frequencies is considered. Also, because of the difference in reflectivity of the high reflectivity mirror of the resonator to the two beams (S-polarized component and P-polarized component) having the polarization directions perpendicular to each other, the single pass losses in the two polarization directions are different. This results in a ring cavity having a lower finesse for the P component than for the S component. Therefore, according to the structural characteristic of the ring resonator, the device for measuring the gas with different concentrations by utilizing different polarization directions of the laser beam is provided.
Disclosure of Invention
The invention provides a method for detecting trace gas based on cavity ring-down spectroscopy, which aims at the problems and can be used for detecting trace gas and realizing measurement and detection of gas with different concentration measurement requirements by the same equipment.
The invention adopts the following technical scheme to achieve the aim:
the utility model provides an utilize gaseous detection device of different concentration of different polarization direction measurement of laser, mainly includes laser source, light chopping system, laser cavity matching light path, laser polarization direction adjustment system, triangle ring resonator, data acquisition system, specifically includes: the device comprises a laser, a laser controller, an optical fiber isolator, an optical fiber collimator, a mode matching lens, a first plane reflecting mirror, a second plane reflecting mirror, a lambda/2 wave plate, a polaroid, a triangular ring resonator, a focusing lens, a photoelectric detector, a computer, a digital delay pulse generator, an adder and a function generator;
the invention uses a laser as a laser source, uses a laser controller to control the temperature and current of the laser, and the laser output frequency is tuned by scanning the laser driving current by a function generator. The output end of the laser is connected with the input end of the optical fiber isolator through optical fibers, the output end of the optical fiber isolator is connected with the input end of the optical fiber collimator through optical fibers, and laser output by the output end of the optical fiber collimator enters a free space for transmission.
The laser in free space enters a triangular ring resonator through a mode matching lens, a first plane reflector, a second plane reflector, a lambda/2 wave plate and a polaroid, the laser transmitted by the triangular ring resonator is converged on a photoelectric detector through a focusing lens, and a signal detected by the photoelectric detector is divided into two paths, wherein one path enters a digital delay pulse generator, and the other path enters a computer.
The output end of the function generator is connected with the first input end of the adder, the output end of the adder is connected with the input end of the laser controller, the output end of the laser controller is connected with the input end of the laser, and the output end of the digital delay pulse generator is connected with the second input end of the adder.
Further, the laser adopts a butterfly-packaged DFB laser.
Further, the focusing lens is a short focal length lens of 25mm, and the photodetector is an InGaSa avalanche photodetector.
The laser cavity matching light path is realized by a mode matching lens and a pair of plane reflectors. In the mode matching, the size and the position of the waist spot of the emergent laser are determined by measuring the spatial mode distribution of the emergent laser, and the position of the waist spot of the laser is positioned at the position of the waist spot of the triangular ring resonator by enabling the laser to pass through a mode matching lens, wherein the size of the waist spot is exactly equal to the size of the waist spot of the triangular ring resonator in the self-reproduction mode.
The laser polarization direction adjusting system adopts the lambda/2 wave plate and the polaroid to form the polarization direction rotator, and the laser subjected to mode matching passes through the lambda/2 wave plate and the polaroid, and realizes the conversion of the S polarization direction and the P polarization direction of the laser under the condition that the emergent light intensity is unchanged by rotating the rotation angle of the lambda/2 wave plate and the polaroid.
The triangular ring resonator comprises a cavity, a first plane cavity mirror, a second plane cavity mirror, a plano-concave cavity mirror, an air inlet hole, a pressure sensor, an air outlet hole and a temperature control system, wherein the first plane cavity mirror and the second plane cavity mirror are fixed on the top laser incidence end face and the laser emission end face of the cavity through vacuum glue, the air inlet hole, the pressure sensor, the air outlet hole and the temperature control system are arranged on the side wall of the cavity, and the plano-concave cavity mirror is fixed on the bottom end face of the cavity through the vacuum glue.
Further, the optical path in the triangular ring resonator is of a triangular ring structure, the total length of the optical path in the triangular ring resonator is 476.18mm, the cavity is made of invar steel materials with ultra-low thermal expansion coefficients, and the length, width and height of the cavity are 231.14mm, 30mm and 30mm respectively.
Still further, the first plane cavity mirror and the second plane cavity mirror are plane mirrors with the diameter of 12.7mm, the included angle of the plane mirrors is 88.8 degrees, and the plane concave cavity mirror is a concave mirror with the curvature radius of 1m and the diameter of 12.7 mm.
According to the air channel connection system, the air inlet hole, the air outlet hole and the pressure sensor hole are reserved on the cavity, the vacuum pump and the proportional valve are connected through the air channel connection, the pressure of the cavity is detected through the pressure sensor, and the switch of the proportional valve in the air channel is controlled to accurately control the air pressure in the cavity.
The data acquisition system of the invention is characterized in that an optical signal output by a transmission end of a triangular ring resonator is converged on an InGaSa avalanche photodetector through a short focal length lens of 25mm, the signal detected by the photodetector is divided into two paths, the first path enters a digital delay pulse generator (threshold comparison circuit) to monitor the transmitted light intensity of the cavity, and the second path is connected with a computer (PC) and used for acquiring and processing ring-down signals. The acquisition and processing of the ring-down signal is accomplished using a data acquisition card (24 bit transmission, 10 MHz), the trigger signal for data acquisition is provided by a digital delay pulse generator, the data acquisition card obtains the ring-down event by looking for a transmission peak and performs an e-exponential fit on it to obtain the ring-down time of the cavity. The extraction, processing and storage of the data are realized by Labview program of the computer terminal.
The light chopping system adopts a rapid modulation technology, and the incident laser does not meet the longitudinal mode matching condition by rapidly changing the frequency of the incident light. The invention adopts the current of the external triangular wave scanning laser controller to realize the wavelength modulation function, when the digital delay pulse generator detects that the transmission signal of the resonant cavity reaches the threshold value, the pulse signal output by the digital delay generator is added to the triangular wave scanning signal through the adder, and the scanning current of the laser is suddenly changed in a short time, so that the emergent laser does not meet the longitudinal mode matching condition to realize the laser shutdown in the cavity. When the transmitted light intensity is below the threshold, the digital delay pulse generator output is high, the optical signal is coupled into the resonant cavity and resonates back and forth within the cavity, and the light intensity within the cavity increases.
Compared with the prior art, the invention has the following advantages:
1. the high-precision triangular ring resonator structure is adopted to realize the detection of the ring-down spectrum of the optical cavity, compared with a linear cavity structure, the method has the advantages that the mode beat frequency effect, the multiple ring-down and the etalon effect caused by the direct reflected light of the cavity front mirror fed back to the laser are avoided, and the accuracy of ring-down time is improved.
2. The high-precision triangular ring resonator structure can generate different single-pass loss phenomena for light beams (P polarization direction and S polarization direction) with mutually perpendicular polarization directions, so that different finesses are caused.
3. Simplifying the light path structure and completing the miniaturization and integration design of the whole structure.
Drawings
Fig. 1 is a schematic structural diagram of a detection device according to the present embodiment;
FIG. 2 is a schematic diagram of the triangular ring resonator according to the present embodiment;
fig. 3 is a graph of ring-down signals generated by setting a threshold of a digital delay pulser, (a) is a ring-down signal when the laser is in the P-polarization direction, and (b) is a ring-down signal when the laser is in the S-polarization direction.
Detailed Description
In order to further illustrate the technical scheme of the invention, the invention is further illustrated by the following examples.
As shown in fig. 1, a detection apparatus for measuring gases with different concentrations by using different polarization directions of laser light according to this embodiment includes: the laser comprises a laser 1, a laser controller 2, an optical fiber isolator 3, an optical fiber collimator 4, a mode matching lens 5, a first plane reflecting mirror 6, a second plane reflecting mirror 7, a lambda/2 wave plate 8, a polaroid 9, a triangular ring resonator 23, a focusing lens 17, a photoelectric detector 18, a computer 19, a digital delay pulse generator 20, an adder 21 and a function generator 22;
the laser 1 is used as a laser source, the output end of the laser 1 is in optical fiber connection with the input end of the optical fiber isolator 3, the output end of the optical fiber isolator 3 is in optical fiber connection with the input end of the optical fiber collimator 4, laser output by the output end of the optical fiber collimator 4 enters a free space for transmission, the laser in the free space passes through the pattern matching lens 5, the first plane mirror 6, the second plane mirror 7, the lambda/2 wave plate 8 and the polaroid 9, finally, the laser is coupled into the triangular annular resonant cavity 23 with high fineness through the first plane mirror 10 at an angle of 45 degrees, the laser transmitted by the triangular annular resonant cavity 23 is converged on the photoelectric detector 18 through the focusing lens 17, and signals detected by the photoelectric detector 18 are divided into two paths, wherein one path enters the digital delay pulse generator 20, and the other path enters the computer 19.
The function generator 22 scans the laser by triangular waves with the frequency of 10Hz and the amplitude of 50mv, so that the frequency tuning of the laser is realized, the output end of the function generator 22 is connected with the first input end of the adder 21, the output end of the adder 21 is connected with the input end of the laser controller 2, the output end of the laser controller 2 is connected with the input end of the laser 1, and particularly, a current control port and a temperature control port of the laser controller 2 are respectively connected with a laser base to control the current and the temperature of the laser. The output of the digital delay pulse generator 20 (DG 645) is connected to a second input of the adder 21.
The laser 1 of this embodiment adopts a Eblana Photonics EP1654-DM-B butterfly-shaped packaged DFB laser, the focusing lens 17 is a 25mm short focal length lens, and the photodetector 18 is an InGaSa avalanche photodetector.
The triangular ring resonator 23 of the present embodiment includes a cavity 24, a first planar cavity mirror 10, a second planar cavity mirror 11, a plano-concave cavity mirror 12, an air inlet hole 13, a pressure sensor 14, an air outlet hole 15 and a temperature control system 16, where the first planar cavity mirror 10 and the second planar cavity mirror 11 are fixed on the top laser incident end surface and the laser emergent end surface of the cavity 24, the air inlet hole 13, the pressure sensor 14, the air outlet hole 15 and the temperature control system 16 are arranged on the side wall of the cavity 24, and the plano-concave cavity mirror 12 is fixed on the bottom end surface of the cavity 24. The optical path in the triangular ring resonator 23 is in a triangular ring structure, the total length of the optical path in the cavity is 476.18mm, the cavity 24 is made of invar steel with ultralow thermal expansion coefficient, and the length, width and height of the cavity are 231.14mm, 30mm and 30mm respectively. The first plane cavity mirror 10 and the second plane cavity mirror 11 are plane mirrors with the diameter of 12.7mm, the included angle of the plane mirrors is 88.8 degrees, and the plane concave cavity mirror 12 is a concave mirror with the curvature radius of 1m and the diameter of 12.7 mm.
The laser passes through the first plane cavity mirror 10, the second plane cavity mirror 11 and the plane cavity mirror 12 to form a triangular loop, and the laser resonates in the resonant cavity, so that the laser intensity is enhanced. The laser can be matched with a space mode in the triangular ring resonant cavity with high definition after mode conversion of the mode matching lens, so that the laser can be coupled into the cavity to the greatest extent, the light intensity in the cavity is enhanced, and the generation of a high-order transverse mode in the cavity is inhibited. Wherein the conversion of the S-polarization direction and the P-polarization direction of the laser light is achieved by rotating the rotation angle of the lambda/2 wave plate 8 and the polarizing plate 9. And the fiber isolator 3 functions to return the feedback light in the optical path back into the laser.
The principle of realizing the detection of different concentration gases in the invention is that the fineness of the triangular ring resonator 23 for light beams (S polarization component and P polarization component) with mutually perpendicular polarization directions is different, and further, the requirements of different measurement concentrations can be met by respectively measuring the S polarization component and the P polarization component. In the present invention, the conversion of the S polarization component and the P polarization component is achieved by combining the lambda/2 wave plate 8 and the polarizer 9, and by rotating the lambda/2 wave plate and the polarizer 9. By setting a chopping threshold on the digital pulse generator 20, a ring-down signal is observed at the transmitting end of the resonant cavity, data acquisition of the ring-down signal is performed through a data acquisition card, and e-exponential fitting is performed on the ring-down signal, so that ring-down time is obtained.
The ring-down signal acquired by the photodetector is shown in fig. 3, and a continuous ring-down signal is obtained by scanning the laser frequency. In fig. 3, a is a ring-down signal when a laser P polarization direction beam obtained by rotating the λ/2 plate 8 and the polarizer 9 enters the high-precision triangular ring resonator, and since the high-precision triangular ring resonator has low fineness for the P polarization component of the incident laser, when the laser is in the P polarization direction, the cavity mode signal is high, the coupling efficiency of the resonator is high, but the ring-down time is short. In fig. 3 b is a ring-down signal when a laser beam in S polarization direction obtained by rotating the λ/2 plate 8 and the polarizer 9 enters the high-precision triangular ring-shaped resonator, and a cavity mode signal in S polarization direction is lower in cavity mode amplitude, lower in coupling efficiency of the resonator and longer in ring-down time compared with a cavity mode signal in P polarization direction.
By setting a chopping threshold on the digital delay pulse generator 20, a ring-down signal is observed at the transmission end of the resonant cavity, data acquisition of the ring-down signal is performed through a data acquisition card, and e-exponential fitting is performed on the ring-down signal, so that the ring-down time is obtained. Measuring cavity ring-down time τ by evacuating 0 And ring-down time τ when filled with the medium to be measured, according to the formulaWherein C is the concentration of the medium to be measured, sigma s The effective absorption section of the medium to be measured can be queried through a database, so that the concentration information of the medium to be measured can be obtained.
While the principal features and advantages of the present invention have been shown and described, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing exemplary embodiments, but that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Furthermore, it should be understood that although the present disclosure describes embodiments, not every embodiment is provided with a separate embodiment, and that this description is provided for clarity only, and that the disclosure is not limited to the embodiments described in detail below, and that the embodiments described in the examples may be combined as appropriate to form other embodiments that will be apparent to those skilled in the art.
Claims (6)
1. A detection apparatus for measuring gases of different concentrations using different polarization directions of laser light, comprising: the laser comprises a laser (1), a laser controller (2), an optical fiber isolator (3), an optical fiber collimator (4), a pattern matching lens (5), a first plane reflector (6), a second plane reflector (7), a lambda/2 wave plate (8), a polaroid (9), a triangular ring resonator (23), a focusing lens (17), a photoelectric detector (18), a computer (19), a digital delay pulse generator (20), an adder (21) and a function generator (22);
the laser device is characterized in that the laser device (1) is used as a laser source, the output end of the laser device (1) is connected with the input end of the optical fiber isolator (3) through optical fibers, the output end of the optical fiber isolator (3) is connected with the input end of the optical fiber collimator (4) through optical fibers, laser output by the output end of the optical fiber collimator (4) enters a free space for transmission, the laser in the free space enters a triangular annular resonant cavity (23) through a pattern matching lens (5), a first plane mirror (6), a second plane mirror (7), a lambda/2 wave plate (8) and a polaroid (9), laser transmitted by the triangular annular resonant cavity (23) is converged on a photoelectric detector (18) through a focusing lens (17), and signals detected by the photoelectric detector (18) are divided into two paths, wherein one path enters a digital delay pulse generator (20) and the other path enters a computer (19);
the output end of the function generator (22) is connected with the first input end of the adder (21), the output end of the adder (21) is connected with the input end of the laser controller (2), the output end of the laser controller (2) is connected with the input end of the laser (1), and the output end of the digital delay pulse generator (20) is connected with the second input end of the adder (21).
2. A detection device for measuring gases of different concentrations using different polarization directions of a laser according to claim 1, characterized in that the laser (1) is a butterfly-packaged DFB laser.
3. A detection device for measuring gases of different concentrations using different polarization directions of laser light according to claim 1, characterized in that the focusing lens (17) is a short focal length lens of 25mm and the photodetector (18) is an InGaSa avalanche photodetector.
4. The detection device for measuring gases with different concentrations by using different polarization directions of laser according to claim 1, wherein the triangular ring resonator (23) comprises a cavity (24), a first plane cavity mirror (10), a second plane cavity mirror (11), a flat concave cavity mirror (12), an air inlet hole (13), a pressure sensor (14), an air outlet hole (15) and a temperature control system (16), the first plane cavity mirror (10) and the second plane cavity mirror (11) are fixed on the top laser incidence end face and the laser emission end face of the cavity (24), the air inlet hole (13), the pressure sensor (14), the air outlet hole (15) and the temperature control system (16) are arranged on the side wall of the cavity (24), and the flat concave cavity mirror (12) is fixed on the bottom end face of the cavity (24).
5. The device for measuring gases with different concentrations by using different polarization directions of laser light according to claim 4, wherein the optical path in the triangular ring resonator (23) is in a triangular ring structure, the total length of the optical path in the cavity is 476.18mm, and the cavity (24) is made of invar material with ultra-low thermal expansion coefficient, and the length, width and height of the invar material are 231.14mm, 30mm and 30mm respectively.
6. The device for measuring gases with different concentrations by using different polarization directions of laser light according to claim 4, wherein the first plane cavity mirror (10) and the second plane cavity mirror (11) are plane mirrors with the diameter of 12.7mm, the included angle is 88.8 degrees, and the plane concave cavity mirror (12) is a concave mirror with the curvature radius of 1m and the diameter of 12.7 mm.
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