CN114624192B - Trace gas detection method and detection device based on photothermal spectroscopy - Google Patents
Trace gas detection method and detection device based on photothermal spectroscopy Download PDFInfo
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- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
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Abstract
The invention discloses a trace gas detection method and a detection device based on photo-thermal spectrum, wherein the detection method comprises the steps of inputting trace gas to be detected into an annular reflection tank, introducing a first laser beam into the annular reflection tank, enabling the central wavelength of the first laser beam to be the same as the absorption peak of the trace gas to be detected, enabling the first laser beam to be reflected in the annular reflection tank for multiple times, enabling the trace gas to be detected to absorb the energy of the first laser beam and the temperature to change periodically, introducing a second laser beam into the annular reflection tank, utilizing a Mach-Zehnder interferometer to measure phase difference information generated by the second laser beam due to the fact that the temperature of the trace gas changes periodically, enabling the central wavelength of the second laser beam to be far away from the absorption peak of the trace gas to be detected, processing the phase difference information of the second laser beam to be converted into light intensity change information of the second laser beam, and processing the light intensity change information of the second laser beam to obtain the concentration of the trace gas.
Description
Technical Field
The application belongs to the technical field of gas detection, and particularly relates to a trace gas detection method and device based on a photothermal spectroscopy.
Background
Sensitive and selective detection of trace gases is critical in the field of monitoring the atmospheric environment and biomedical, among others. As an important sensing technology, laser spectroscopy has the advantages of high sensitivity, strong specificity, high response speed and the like. Photothermal spectroscopy techniques have been widely used for the measurement of trace gases such as carbon monoxide, hydrogen sulfide, methane, and the like.
Photothermal spectroscopy is a highly sensitive laser spectroscopic analysis method that measures optical absorption and thermal properties of samples. When the gas to be measured interacts with the periodically modulated characteristic wavelength light beam, part of light energy is absorbed by gas molecules, the molecules transit from a ground state to an excited state, the molecules in the excited state release the absorbed energy in a non-radiative relaxation mode and convert all or part of the absorbed energy into heat energy, so that the temperature of a medium periodically changes, the temperature change can cause the change of the optical fiber property of a sample cell, the phase change is caused, and the concentration information of trace gas can be reversely detected by detecting the phase change.
Disclosure of Invention
In view of this, in one aspect, some embodiments disclose a method for detecting a trace gas based on photothermal spectroscopy, where the method includes:
Inputting the trace gas to be measured into the annular reflection pool;
Introducing a first laser beam into an annular reflecting pool, wherein the central wavelength of the first laser beam is the same as the absorption peak of the trace gas to be detected, the first laser beam is reflected in the annular reflecting pool for multiple times, and the trace gas to be detected absorbs the energy of the first laser beam to periodically change the temperature;
introducing a second laser beam into the annular reflection pool, and measuring phase difference information generated by the second laser beam due to the periodical change of the temperature of the trace gas by using a Mach-Zehnder interferometer, wherein the central wavelength of the second laser beam is far away from an absorption peak of the trace gas to be measured;
processing the phase difference information of the second laser beam, and converting the phase difference information into light intensity change information of the second laser beam;
and processing the light intensity change information of the second laser beam to obtain the concentration of the trace gas to be detected.
Further, in the trace gas detection method based on photo-thermal spectrum disclosed in some embodiments, the annular reflection tank is provided with a closed cavity structure, the inner surface of the closed cavity structure comprises N identical spherical mirrors connected in sequence, wherein 1 spherical mirror is an incident surface, the remaining N-1 spherical mirrors are reflection surfaces, a first laser beam is emitted into the annular reflection tank from the incident surface at a set incident angle, is emitted from the incident surface after being reflected N-1 times by the N-1 reflection surfaces, and the total optical path length L of the first laser beam in the annular reflection tank is expressed as:
Wherein,
R is the inner diameter of the annular reflecting pool, d is the distance from the central point of the annular reflecting pool to the central point of the spherical mirror, R is the curvature radius of the spherical mirror, l is the distance from the central point of the incident surface to the central point of the first reflecting surface, R > l/2;N is the total number of the spherical mirrors and is an odd number, and alpha is half of the central angle of the spherical mirror.
Some embodiments disclose a trace gas detection method based on photothermal spectroscopy, where the maximum angle of incidence θ max of the first laser beam is expressed as:
The range of the incidence angle theta of the first laser beam is-theta max<θ<θmax, wherein the incidence angle theta is an included angle between the central line of the annular reflecting pool and the incidence direction of the first laser beam, r is the inner diameter of the reflecting pool, and d is the distance from the central point of the reflecting pool to the central point of the spherical mirror.
The micro gas detection method based on photo-thermal spectrum disclosed in some embodiments uses a mach-zehnder interferometer to measure phase difference information generated by a second laser beam due to periodic variation of temperature of a micro gas to be detected, including:
inputting a second laser beam into the Mach-Zehnder interferometer;
The second laser beam is divided into two laser beams with equal intensity in the Mach-Zehnder interferometer and respectively enters a detection arm and a reference arm, wherein the detection arm is connected into an annular reflection pool, the reference arm is arranged outside the annular reflection pool, and the detection arm and the reference arm are equal in length;
The temperature of the trace gas in the annular reflecting pool changes periodically to cause the phase of the second laser beam in the detecting arm to change, and the second laser beam in the connecting reference arm generates a phase difference;
The second laser beam output from the detection arm and the second laser beam output from the reference arm are coupled into one path and interfere, and phase difference information of the second laser beam is converted into light intensity change information.
The micro gas detection method based on the photothermal spectrum disclosed in some embodiments processes the light intensity variation information of the second laser beam to obtain the concentration of the micro gas, specifically includes:
converting the light intensity change information of the second laser beam into two paths of electric signals;
Eliminating common mode noise of two paths of electric signals to obtain one path of electric signal;
One path of electric signal after common mode noise is eliminated is demodulated, and analog signals are output;
converting the analog signal into a digital signal, and extracting second harmonic signal data;
and obtaining the concentration information of the trace gas to be detected by using the obtained second harmonic signal data.
In another aspect, some embodiments disclose a micro gas detection device based on photothermal spectroscopy, comprising:
a laser providing assembly for providing a first laser beam and a second laser beam;
The annular reflection pool is used for placing the trace gas to be detected and realizing the multiple reflection process of the first laser beam in the annular reflection pool, the trace gas to be detected in the annular reflection pool absorbs the energy of the first laser beam, and the temperature is periodically changed;
The Mach-Zehnder interferometer is used for measuring phase difference information of the second laser beam, which is caused by periodical change of the temperature of the trace gas after the second laser beam passes through the annular reflecting pool;
and the information processing component is used for processing the phase difference information of the second laser beam to obtain the trace gas concentration.
Some embodiments disclose a trace gas detection device based on photothermal spectroscopy, the laser providing module comprising:
A first laser for outputting a first laser beam;
a second laser for outputting a second laser beam;
The laser control module is used for controlling the first laser to output a first laser beam with set wavelength and intensity and controlling the second laser to output a second laser beam with set wavelength and intensity.
Some embodiments disclose a trace gas detection device based on photothermal spectrum, and the annular reflection tank has airtight cavity structure, and the internal surface of this airtight cavity structure includes odd same sphere mirror, and the setting mode of sphere mirror is, and sphere mirror connects gradually, and one of them sphere mirror is the incident plane of first laser beam, and other sphere mirrors are the reflecting surface, and first laser beam is penetrated with the settlement angle from the incident plane, is penetrated from the incident plane after the continuous reflection of a plurality of reflecting surfaces.
Some embodiments disclose a micro gas detection device based on photothermal spectroscopy, the mach-zehnder interferometer comprising:
a beam splitter arranged to split an input second laser beam into two identical beams;
The detection arm is connected with the beam splitter and penetrates through the annular reflection pool;
The reference arm is connected with the beam splitter and is arranged outside the annular reflection pool, wherein the length of the detection arm is equal to that of the reference arm;
And a coupler connected with the detecting arm and the reference arm, and used for coupling the second laser beam output from the detecting arm and the second laser beam output from the reference arm into a beam of laser beams.
Some embodiments disclose a micro gas detection device based on photothermal spectroscopy, the information processing component includes:
The first detector is electrically connected with the Mach-Zehnder interferometer and is used for detecting and converting light intensity signals output by the Mach-Zehnder interferometer;
The second detector is electrically connected with the Mach-Zehnder interferometer and is used for detecting and converting light intensity signals output by the Mach-Zehnder interferometer;
The differential module is electrically connected with the first detector and the second detector respectively and is used for eliminating common mode noise of light intensity signals output by the first detector and the second detector;
The data processing module is connected with the differential module and comprises a phase-locked amplifier, a data acquisition card and a computer which are electrically connected in sequence, wherein the phase-locked amplifier is used for demodulating and processing an electric signal output by the differential module, the data acquisition card is used for converting an analog signal output by the phase-locked amplifier into a digital signal, extracting second harmonic signal data, the computer is used for drawing the second harmonic signal to obtain a second harmonic graph, and concentration information of trace gas to be detected is obtained according to the peak value of the second harmonic signal.
The trace gas detection device and the trace gas detection method based on the photothermal spectrum provided by the embodiment of the invention utilize the annular reflection pool to emit laser from the incident window after multiple total reflections in the annular reflection pool, so that the absorption optical path is increased, the absorptivity of the gas to the laser is increased, and the detection sensitivity of the system is improved.
Drawings
FIG. 1 is a schematic diagram of the composition of a trace gas detection apparatus based on photothermal spectroscopy in embodiment 1;
FIG. 2 is a schematic diagram of the composition of a trace gas detection apparatus based on photothermal spectroscopy according to embodiment 2;
FIG. 3 is a schematic diagram of a Mach-Zehnder interferometer setup of embodiment 2;
FIG. 4 is a schematic view of the structure of the annular reflecting pool in the embodiment 4;
FIG. 5 example 4 is a schematic view of the angle of incidence of a toroidal reflecting pool;
FIG. 6 is a schematic diagram of the reflection trace of the first laser beam in the annular reflection cell of example 4.
Reference numerals
1. Annular reflecting pool of laser providing assembly 2
3. Mach-Zehnder interferometer 4 information processing component
5. Collimator 11 laser control module
12. First laser 13 second laser
31. Beam splitter 32 detection arm
33. Reference arm 34 coupler
41. First detector 42 second detector
43. Differential module 44 data processing module
441. Phase-locked amplifier 442 data acquisition card
443. Computer with a memory for storing data
Detailed Description
The word "embodiment" as used herein does not necessarily mean that any embodiment described as "exemplary" is preferred or advantageous over other embodiments. Performance index testing in the examples of the present application, unless otherwise specified, was performed using conventional testing methods in the art. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure.
Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs, and other test methods and techniques not specifically mentioned herein are meant to be common to those of ordinary skill in the art.
The terms "substantially" and "about" are used herein to describe small fluctuations. For example, they may refer to less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%, such as less than or equal to ±0.2%, such as less than or equal to ±0.1%, such as less than or equal to ±0.05%. Numerical data presented or represented herein in a range format is used only for convenience and brevity and should therefore be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range. For example, a numerical range of "1-5%" should be interpreted to include not only the explicitly recited values of 1% to 5%, but also include individual values and sub-ranges within the indicated range. Thus, individual values, such as 2%, 3.5% and 4%, and subranges, such as 1% -3%, 2% -4% and 3% -5%, etc., are included in this numerical range. The same principle applies to ranges reciting only one numerical value. Moreover, such an interpretation applies regardless of the breadth of the range or the characteristics being described.
In this document, including the claims, conjunctions such as "comprising," including, "" carrying, "" having, "" containing, "" involving, "" containing, "and the like are to be construed as open-ended, i.e., to mean" including, but not limited to. Only the connective "consisting of" and "consisting of" are closed connective words.
Numerous specific details are set forth in the following examples in order to provide a better understanding of the present application. It will be understood by those skilled in the art that the present application may be practiced without some of these specific details. In the examples, some methods, means, instruments, devices, etc. well known to those skilled in the art are not described in detail in order to highlight the gist of the present application.
On the premise of no conflict, the technical features disclosed by the embodiment of the application can be combined at will, and the obtained technical scheme belongs to the disclosure of the embodiment of the application.
The following is a further exemplary description of the invention with reference to figures 1, 2, 3, 4, 5, 6 and specific examples.
In some embodiments, the method of trace gas detection based on photothermal spectroscopy comprises:
Inputting the trace gas to be measured into the annular reflection pool;
The method comprises the steps of introducing a first laser beam into an annular reflecting pool, wherein the central wavelength of the first laser beam is the same as the absorption peak of the trace gas to be detected, the first laser beam is reflected in the annular reflecting pool for multiple times, the trace gas to be detected absorbs the energy of the first laser beam and the temperature is periodically changed, the first laser beam is reflected in the annular reflecting pool for multiple times, the optical path of the first laser beam in the annular reflecting pool is prolonged, the absorption rate of the gas in the annular reflecting pool to the energy of the first laser beam is effectively prolonged, the sensitivity to gas detection is improved, and the trace gas detection can be realized;
The method comprises the steps of introducing a second laser beam into an annular reflecting pool, and measuring phase difference information generated by the second laser beam due to the periodical change of the temperature of trace gas by using a Mach-Zehnder interferometer, wherein the center wavelength of the second laser beam is far away from an absorption peak of the trace gas to be measured;
Processing the phase difference information of the second laser beam, converting the phase difference information into light intensity change information of the second laser beam, and processing the light intensity change information of the second laser beam to obtain the concentration of trace gas. As an alternative embodiment, the detection arm is extended from the annular reflection Chi Nayan and then is combined with the reference arm at the 2X 2 coupler to interfere, the process converts the phase change of the second laser beam into the light intensity change, then the light intensity change is converted into electric signals through the first photoelectric detector and the second photoelectric detector, the electric signals output by the two paths of detectors are respectively used as two input ends of the differential module to be subjected to differential processing through the differential module so as to eliminate common mode noise, the electric signals output by the differential module are connected into the signal processing module, firstly, the electric signals are subjected to demodulation processing through the phase-locked amplifier, then the data acquisition card is used for converting the analog signals output by the phase-locked amplifier into digital signals, the second harmonic signal data are extracted, the drawing is carried out through software of a computer, the second harmonic graph is obtained, and the concentration information of the target gas to be detected can be inverted according to the peak value of the second harmonic signal.
As an alternative embodiment, in the trace gas detection method based on photothermal spectroscopy, the annular reflection tank has a closed cavity structure, the inner surface of the closed cavity structure includes N identical spherical mirrors connected in sequence, wherein N-1 spherical mirrors are reflection surfaces, 1 spherical mirror is an incidence surface, a first laser beam is injected into the annular reflection tank from the incidence surface at a set incidence angle, after N-1 reflections of the N-1 spherical mirrors, the first laser beam is emitted from the incidence surface, and the total optical path length L of the first laser beam in the annular reflection tank is expressed as:
Wherein,
R is the inner diameter of the annular reflecting pool, d is the distance from the central point of the annular reflecting pool to the central point of the spherical mirror, R is the curvature radius of the spherical mirror, l is the distance from the central point of the incident surface to the central point of the first reflecting surface, R > l/2;N is the total number of the spherical mirrors and is an odd number, and alpha is half of the central angle of the spherical mirror.
As an alternative embodiment, the maximum angle of incidence θ max of the first laser beam is expressed as:
The range of the incidence angle theta of the first laser beam is-theta max<θ<θmax, wherein the incidence angle theta is an included angle between the central line of the annular reflecting pool and the incidence direction of the first laser beam.
As an alternative embodiment, measuring phase difference information generated by the second laser beam by using the mach-zehnder interferometer when the temperature of the trace gas to be measured changes periodically, includes:
inputting a second laser beam into the Mach-Zehnder interferometer;
The second laser beam is divided into two beams with equal intensity in the Mach-Zehnder interferometer and respectively enters a detection arm and a reference arm, wherein the detection arm is connected into an annular reflection pool, the reference arm is arranged outside the annular reflection pool, and the detection arm and the reference arm are equal in length;
The temperature of the trace gas in the annular reflecting pool changes periodically to cause the phase of the second laser beam in the detecting arm to change, and the second laser beam in the connecting reference arm generates a phase difference;
The second laser beam output from the detection arm and the second laser beam output from the reference arm are coupled into one path and interfere, and phase difference information of the second laser beam is converted into light intensity change information.
As an alternative embodiment, processing the information of the light intensity variation of the second laser beam to obtain the concentration of the trace gas specifically includes:
converting the light intensity change information of the second laser beam into two paths of electric signals;
Eliminating common mode noise of two paths of electric signals to obtain one path of electric signal;
One path of electric signal after common mode noise is eliminated is demodulated, and analog signals are output;
converting the analog signal into a digital signal, and extracting second harmonic signal data;
and obtaining the concentration information of the trace gas to be detected by using the obtained second harmonic signal data.
Example 1
The micro gas detection device based on photothermal spectroscopy disclosed in embodiment 1 comprises:
A laser providing assembly 1 for providing a first laser beam and a second laser beam;
the annular reflection pool 2 is used for placing trace gas to be detected and realizing the multiple reflection process of the first laser beam in the annular reflection pool, the trace gas to be detected in the annular reflection pool absorbs the energy of the first laser beam, and the temperature is periodically changed;
a Mach-Zehnder interferometer 3 for measuring phase difference information of the second laser beam caused by periodical change of the temperature of the trace gas after the second laser beam passes through the annular reflecting pool 2;
and an information processing component 4 for processing the second laser beam phase difference information to obtain the trace gas concentration.
Example 2
The micro gas detection device based on photothermal spectroscopy disclosed in embodiment 2 comprises:
the laser providing assembly 1 is used for providing a first laser beam and a second laser beam, the laser providing assembly 1 comprises a first laser 12 used for outputting the first laser beam and a second laser used for outputting the second laser beam, and a laser control module 11 is electrically connected with the first laser 12 and the second laser 13 respectively and used for controlling the first laser 12 to output the first laser beam with set wavelength and intensity and controlling the second laser 13 to output the second laser beam with set wavelength and intensity;
As an alternative embodiment, the first laser 12 is a distributed feedback laser, the second laser 13 is a distributed feedback laser, the laser control module 11 includes a current driver and a temperature controller, and is used for driving and modulating the first laser 12, and by changing the temperature and the current, the wavelength of the first laser beam output by the distributed feedback laser can be modulated, so that the wavelength scanning range of the first laser 12 sweeps through the strongest absorption peak of the gas, and the target gas to be measured can absorb the energy of the first laser beam emitted by the first laser 12, and a photo-thermal effect occurs;
as an alternative embodiment, the first laser 12 is wavelength-modulated by a superimposed signal of a high-frequency sine wave and a low-frequency sawtooth wave, generating a first laser beam of a wavelength corresponding to a specific absorption peak of the gas to be measured;
As an alternative embodiment, the wavelength scanning range of the first laser 12 is controlled to be 2.2 times of the full width at half maximum of the corresponding absorption peak, and the center wavelength of the first laser should correspond to the peak wavelength of the absorption peak, so that a complete second harmonic signal can be obtained after passing through the data processing module;
as an alternative embodiment, the laser control module 11 is configured to drive the second laser 13 so that its wavelength remains stable;
As an alternative embodiment, the detecting device includes a collimator 5 disposed between the first laser 12 and the annular reflecting pool 2, for converting the divergent light of the first laser beam output by the first laser 12 into parallel light, ensuring that the first laser beam is incident into the annular reflecting pool 2;
The annular reflection pool 2 is used for placing trace gas to be detected and realizing the multiple reflection process of the first laser beam in the annular reflection pool, the trace gas to be detected in the annular reflection pool absorbs the energy of the first laser beam, and the temperature is periodically changed; the annular reflecting pool is provided with a closed cavity structure, the inner surface of the closed cavity structure comprises an odd number of identical spherical mirrors, the spherical mirrors are arranged in a mode that the spherical mirrors are sequentially connected, one spherical mirror is an incidence surface of a first laser beam, the other spherical mirrors are used as reflection surfaces of the first laser beam, and the first laser beam enters the annular reflecting pool from the incidence surface and is emitted from the incidence surface after being continuously reflected by the reflection surfaces; the number of the spherical mirrors in the annular reflecting pool is an odd number, for example, an odd number of the spherical mirrors larger than 3 form a reflecting surface in the annular reflecting pool, generally, the surface of the spherical mirror is provided with a high-reflectivity metal aluminum film which can effectively reflect laser beams, the spherical mirror serving as an incident surface is provided with a quartz window serving as an incident window of a first laser beam, the incident first laser beam enters the annular reflecting pool from the quartz window at a certain angle, firstly enters the first reflecting surface to be reflected once, then sequentially reflects on each reflecting mirror surface for once, and finally exits from the incident surface after reflecting N-1 times on N-1 reflecting surfaces, the first laser beam forms a positive N-star-shaped beam track distribution in a cavity, and the incident first laser beam is reflected for N-1 times by the reflecting surface, so that the total optical path length of the first laser beam in the annular reflecting pool is effectively improved;
The Mach-Zehnder interferometer 3 is used for measuring phase difference information of a second laser beam caused by periodical change of the temperature of a trace amount of gas after the second laser beam passes through the annular reflection tank 2, and comprises a beam splitter 31, a detection arm 32, a reference arm 33, a coupler 34 and a coupler 33, wherein the beam splitter 31 is used for splitting the input second laser beam into two identical beams, the detection arm 32 is connected with the beam splitter 31 and passes through the annular reflection tank 2, the reference arm 33 is connected with the beam splitter 31 and is arranged outside the annular reflection tank 2 and keeps the ambient temperature constant, the periodical change of the temperature of the gas inside the annular reflection tank 2 is prevented from influencing the phase change of the second laser beam in the reference arm 33, the detection arm 32 and the reference arm 33 are equal in length, and the coupler 34 is connected with the detection arm 32 and the reference arm 33 and is used for coupling the second laser beam output from the detection arm 32 and the second laser beam output from the reference arm 33;
As an alternative embodiment, the detecting arm 32 passes through the annular reflecting pool 2, is embedded into the annular reflecting pool through a groove at the bottom of the annular reflecting pool 2, and the reference arm 33 is directly placed in the air outside the annular reflecting pool without passing through the annular reflecting pool, and the detecting arm 32 after passing through the annular reflecting pool 2 and the reference arm 33 are received by a photoelectric detecting component in the information processing component after passing through the 2×2 coupler 34;
The information processing component 4 is used for processing the phase difference information of the second laser beams to obtain the concentration of trace gas, the information processing component 4 comprises a first detector 41, a second detector 42, a differential module 43 and a data processing module 44, wherein the first detector 41 is electrically connected with the Mach-Zehnder interferometer and is used for detecting and converting the light intensity signal output by the Mach-Zehnder interferometer, the second detector 42 is electrically connected with the Mach-Zehnder interferometer and is used for detecting and converting the light intensity signal output by the Mach-Zehnder interferometer, the differential module 43 is electrically connected with the first detector 41 and the second detector 42 respectively and is used for eliminating common mode noise of the light intensity signals output by the first detector 41 and the second detector 42, the data processing module 44 further comprises a phase-locked amplifier 441, a data acquisition card 442 and a computer 443 which are electrically connected in sequence, the phase-locked amplifier 441 is used for demodulating and processing the electric signal output by the differential module, the data acquisition card 442 is used for converting the analog signal output by the phase-locked amplifier 441 into a digital signal, extracting the second harmonic signal data, and the computer 443 is used for drawing the second harmonic signal to obtain a second harmonic graph, and the concentration information of the gas to be measured according to the peak value of the second harmonic signal.
Example 3
Gas concentration calculation method
In embodiment 3, the first laser beam emitted by the first laser device is irradiated into the annular reflection tank as an excitation light modulation, and is reflected in the annular reflection tank for multiple times, the trace gas to be detected in the annular reflection tank absorbs the energy of the first laser beam, the gas molecules to be detected transition from the ground state to the excited state, the gas molecules in the excited state are back excited to the ground state through non-radiative relaxation, and meanwhile, the energy is released, and the released energy enables the gas molecules to be detected and the space medium thereof to be periodically heated according to the frequency of laser modulation, so that the temperature of the trace gas to be detected in the tank is changed, and a photo-thermal signal is generated. The photothermal signal is proportional to the temperature change, and the temperature change T (T) is proportional to the concentration C of the gas absorbing molecules:
T(t)=c1I0α(v)CL (5)
Wherein c 1 is a constant, I 0 is an input light intensity of the first laser, α (v) =α (v c+δvcosω0 t) is an absorption coefficient of the gas to be measured, v c is a center frequency of the output laser of the first laser, δ v is a modulation amplitude, ω 0 is a high-frequency sine wave modulation frequency, and L is an effective optical path. Temperature changes can cause changes in the properties of the optical fiber. When the temperature of the detection arm optical fiber changes relative to the temperature of the reference arm optical fiber, the length and the refractive index of the detection arm optical fiber change, so that the phase of the second laser beam transmitted by the detection arm changes, and an interference phenomenon occurs. The lengths of two paths of optical fibers serving as the detection arm and the reference arm are equal to L 0, and the phase of light waves of the second laser beam propagating in the optical fibers is set as phi:
Φ=Φ0+k0nL0 (6)
wherein k 0＝2π/λ0;Φ0 is the initial phase of the second laser beam before entering the optical fiber, the initial phases of the detection arm and the reference arm are the same, k 0 is the propagation constant, lambda 0 is the wavelength of light in vacuum, n is the refractive index of the optical fiber, and L 0 is the length of the optical fiber. The temperature of the optical fiber of the reference arm is unchanged, the temperature variation of the optical fiber of the detection arm is delta T, the variation of the refractive index n is delta n, and the variation of the optical fiber length of the detection arm is delta L, so that the phase phi 1 of the detection arm and the phase phi 2 of the reference arm are respectively:
when the temperature of the detection arm changes, the phase difference delta phi of the two optical fibers at the intersection of the coupler is as follows:
The equation is divided by L 0 and DeltaT on both sides to obtain:
The equation means the amount of change in the phase of the second laser beam in the optical fiber every 1 ℃ of temperature change, wherein the left side of the equation indicates that the optical fiber of unit length is affected by temperature, and the right side of the equation indicates the refractive index of the optical fiber and the rate of change of the length with temperature, respectively.
For a silica optical fiber,
The method can obtain:
Order the Is constant:
ΔΦ=c2ΔT (13)
The light intensity of the input end of the Mach-Zehnder interferometer, namely the light intensity of the second laser beam, is I i, the light intensity of the second laser beam is divided into two light beams with equal light intensity through a 2x2 coupler, the output end of the detection arm and the reference arm after coupling interference is divided into two paths, the two paths of light intensity are equal, the light intensity of the output end is respectively connected to a first detector and a second detector, and the light intensity of the output end is:
Where Δt=t (T) -T 0＝c1I0α(v)CL-T0,T0 is the original temperature, the following can be obtained:
the first detector and the second detector convert the detected light intensity I into an electrical signal V:
where s is the sensitivity of the first detector and the second detector.
Because the formula is complex, the second harmonic signal data can be extracted by using components of a data processing module, such as a lock-in amplifier and the like, and the second harmonic signal peak value and the gas concentration are in a linear relation at low concentration, namely V 2f = a, C and b, wherein a and b are calibrated by an instrument, and the concentration of the trace gas to be detected can be inverted according to the second harmonic signal peak value.
Example 4
In embodiment 4, the whole annular reflecting pool is a cylinder, the inside is a closed cavity, the inner surface of the cavity is composed of N reflecting spherical mirrors with the same size, the numerical value of N is set to 9, and the transmitting spherical mirrors are connected end to end and sequentially arranged along the side surface of the cylinder in an annular mode, so that the reflecting spherical mirrors are distributed on the same cross section of the cylinder, primary reflection can be carried out on each reflecting spherical mirror, and outgoing beams and incident beams enter and exit on the same reflecting spherical mirror. The angle between the center line of the annular reflecting pool and the incidence direction of the laser is the incidence angle of the first laser beam, which is marked as theta, as shown in figure 5, the incidence angle is the smallest when the incident first laser beam directly irradiates the edge of the first reflecting spherical mirror along the center line of the annular reflecting pool, and the incidence angle is the largest when the incident first laser beam irradiates the other edge of the first spherical mirror, wherein theta max is as follows:
wherein r is the inner diameter of the reflecting pool, d is the distance from the center point of the reflecting pool to the center point of the spherical mirror;
Two spherical mirrors adjacent to two sides of the center line of the annular reflecting pool can be used as the first reflecting surface, so that the range of the incident angle of the first laser beam is-theta max<θ<θmax. For example, when N= 9,r =50 mm and R=90 mm are taken, the value range of θ is-20.29 ° < θ <20.29 °.
After the first laser beam is injected from the incidence hole, the first reflection occurs on the first reflection surface, then the first laser beam sequentially passes through the second reflection surface, the third reflection surface, the fourth reflection surface, the fifth reflection surface, the sixth reflection surface, the seventh reflection surface and the eighth reflection surface to be reflected eight times and finally is emitted from the incidence surface, and the total optical path L of the first laser beam is expressed as:
Wherein,
R is the inner diameter of the reflecting pool, d is the distance from the center point of the reflecting pool to the center point of the spherical mirror, R is the radius of curvature of the reflecting spherical mirror, l is the distance from the center point of the incident surface to the center point of the first reflecting surface, and R > l/2, in this embodiment 4, n=9.
Example 5
In this embodiment 5, the trace gas detection method based on photothermal spectroscopy includes:
The method comprises the steps of starting a detection device, setting current and temperature parameters of a current driver and a temperature controller in a laser control module to drive a first distributed feedback laser and a second distributed feedback laser, and changing the wavelength of a first laser beam output by the first laser by adjusting the temperature and the current parameters to enable the wavelength to correspond to the wavelength of a gas absorption peak;
The first laser beam is changed into a parallel beam through a collimator and irradiates into the annular reflecting pool, the incidence angle of the first laser beam is set, so that a light path is straightly incident into the annular reflecting pool in the horizontal direction, the first laser beam is ensured to be incident on a first reflecting surface of the annular reflecting pool and is reflected once on each reflecting surface;
The second distributed feedback laser divides a second laser beam into two beams with equal light intensity through a beam splitter, the two beams are transmitted through optical fibers, one beam of light is used as a detection arm of the Mach-Zehnder interferometer to be connected into the annular reflection pool, the other beam of light is used as a reference arm of the Mach-Zehnder interferometer, the two beams do not pass through the annular reflection pool, and the detection arm and the reference arm pass through the coupler and are received by the photoelectric detection module;
the air inlet and the air outlet of the annular reflecting pool are opened, nitrogen is filled into the cavity from the air inlet for washing, and the influence of air on the experimental process is reduced;
Introducing a trace gas to be detected with a certain concentration into the annular reflection tank from the gas inlet;
the method comprises the steps that a modulating signal is applied to a current controller, the wavelength scanning range of a first distributed feedback laser is controlled to be 2.2 times of the full width at half maximum of an absorption peak corresponding to gas to be detected, meanwhile, the central wavelength of the first distributed feedback laser corresponds to the absorption peak-to-peak wavelength of the gas to be detected, and a complete second harmonic signal is obtained through a data processing module;
The method comprises the steps that laser energy is absorbed by a gas to be detected in an annular reflecting pool to cause temperature change in the reflecting pool, the temperature change in the reflecting pool can cause optical fiber property change in a sample pool due to photo-thermal effect, so that the phase of a detection arm of a Mach-Zehnder interferometer is changed, the phase of a reference arm is kept unchanged, a phase difference is generated between the detection arm and the reference arm, the detection arm and the reference arm are combined into one path at a coupler after coming out of the annular reflecting pool and interfere with each other, the phase change is converted into light intensity change, then the light intensity change is converted into electric signals through a first photoelectric detector and a second photoelectric detector, and the electric signals output by the two paths of photoelectric detectors are respectively used as two input ends of a differential module to be subjected to differential processing so as to eliminate common mode noise;
The electric signal output by the difference module is connected to the data processing module, demodulation processing is carried out through the phase-locked amplifier, the analog signal output by the phase-locked amplifier is converted into a digital signal through the data acquisition card, the second harmonic signal data is extracted, the second harmonic graph is obtained through drawing by software of a computer, and the concentration information of the trace gas to be detected can be inverted according to the peak value of the second harmonic signal.
The trace gas detection device and the trace gas detection method based on the photothermal spectrum provided by the embodiment of the invention utilize the annular reflection pool to emit laser from the incident window after multiple total reflections in the annular reflection pool, so that the absorption optical path is increased, the absorptivity of the gas to the laser is increased, and the detection sensitivity of the system is improved.
The technical details disclosed in the technical scheme and the embodiment of the application are only illustrative of the inventive concept of the application and are not limiting to the technical scheme of the application, and all conventional changes, substitutions or combinations of the technical details disclosed in the application have the same inventive concept as the application and are within the scope of the claims of the application.
Claims (9)
1. The trace gas detection method based on the photothermal spectrum is characterized by comprising the following steps of:
Inputting the trace gas to be measured into the annular reflection pool;
Introducing a first laser beam into the annular reflecting pool, wherein the central wavelength of the first laser beam is the same as the absorption peak of the trace gas to be detected, the first laser beam is reflected in the annular reflecting pool for multiple times, and the trace gas to be detected absorbs the energy of the first laser beam to periodically change the temperature;
introducing a second laser beam into the annular reflection pool, and measuring phase difference information generated by the second laser beam due to the periodical change of the temperature of the trace gas by using a Mach-Zehnder interferometer, wherein the center wavelength of the second laser beam is far away from an absorption peak of the trace gas to be measured;
Processing the phase difference information of the second laser beam, and converting the phase difference information into light intensity change information of the second laser beam;
Processing the light intensity change information of the second laser beam to obtain the concentration of the trace gas to be detected;
the annular reflecting pool is provided with an airtight cavity structure, the inner surface of the airtight cavity structure comprises N identical spherical mirrors which are sequentially connected, 1 spherical mirror is an incident surface, the rest N-1 spherical mirrors are reflecting surfaces, a first laser beam is emitted into the annular reflecting pool from the incident surface at a set incident angle, and is emitted from the incident surface after being reflected for N-1 times by the N-1 spherical mirrors, and the total optical path L of the first laser beam in the annular reflecting pool is expressed as:
Wherein,
R is the inner diameter of the annular reflecting pool, d is the distance from the central point of the annular reflecting pool to the central point of the spherical mirror, R is the curvature radius of the spherical mirror, l is the distance from the central point of the incident surface to the central point of the first reflecting surface, R > l/2;N is the total number of the spherical mirrors and is an odd number, and alpha is half of the central angle of the spherical mirror.
2. The photo-thermal spectrum based trace gas detection method according to claim 1, wherein an incidence angle maximum θ max of the first laser beam is expressed as:
wherein, r is the inner diameter of the reflecting pool, d is the distance from the center point of the reflecting pool to the center point of the spherical mirror, and the incident angle theta is the included angle between the central line of the annular reflecting pool and the incident direction of the first laser beam;
The range of incidence angle theta of the first laser beam is-theta max<θ<θmax.
3. The method for detecting trace gas based on photothermal spectroscopy according to claim 1, wherein measuring phase difference information generated by the second laser beam by periodically varying a temperature of the trace gas to be detected using a mach-zehnder interferometer, comprises:
inputting a second laser beam into the Mach-Zehnder interferometer;
The second laser beam is divided into two laser beams with equal intensity in the Mach-Zehnder interferometer and respectively enters a detection arm and a reference arm, wherein the detection arm is connected into the annular reflection pool, the reference arm is arranged outside the annular reflection pool, and the detection arm and the reference arm are equal in length;
The temperature of the trace gas in the annular reflecting pool changes periodically to cause the phase of the second laser beam in the detecting arm to change, and a phase difference is generated between the second laser beam and the second laser beam connected into the reference arm;
The second laser beam output from the detection arm and the second laser beam output from the reference arm are coupled into one path and interfere, and phase difference information of the second laser beam is converted into light intensity change information.
4. The method for detecting trace gas based on photothermal spectroscopy according to claim 3, wherein processing the information of light intensity variation of the second laser beam to obtain the concentration of the trace gas comprises:
converting the light intensity variation information of the second laser beam into two paths of electric signals;
Eliminating common mode noise of two paths of electric signals to obtain one path of electric signal;
One path of electric signal after common mode noise is eliminated is demodulated, and analog signals are output;
Converting the analog signal into a digital signal, and extracting second harmonic signal data;
and obtaining the concentration information of the trace gas to be detected by using the obtained second harmonic signal data.
5. A photo-thermal spectroscopy based micro gas detection apparatus for performing the method of claim 1, comprising:
a laser providing assembly for providing a first laser beam and a second laser beam;
The annular reflection pool is used for placing the trace gas to be detected and realizing the multiple reflection process of the first laser beam in the annular reflection pool, the trace gas to be detected in the annular reflection pool absorbs the energy of the first laser beam, and the temperature is periodically changed;
The Mach-Zehnder interferometer is used for measuring phase difference information of the second laser beam, which is caused by periodical change of the temperature of the trace gas after the second laser beam passes through the annular reflecting pool;
And the information processing component is used for processing the phase difference information of the second laser beam to obtain the trace gas concentration.
6. The photothermal spectroscopy-based micro gas detection apparatus of claim 5, wherein the laser providing assembly comprises:
A first laser for outputting a first laser beam;
a second laser for outputting a second laser beam;
The laser control module is used for controlling the first laser to output a first laser beam with set wavelength and intensity and controlling the second laser to output a second laser beam with set wavelength and intensity.
7. The micro gas detection device based on photothermal spectroscopy according to claim 5, wherein the annular reflecting pool is provided with a closed cavity structure, the inner surface of the closed cavity structure comprises an odd number of identical spherical mirrors, the spherical mirrors are arranged in a mode that the spherical mirrors are sequentially connected, one spherical mirror is an incident surface of a first laser beam, the other spherical mirrors are reflecting surfaces, the first laser beam is incident from the incident surface at a set angle, and is emitted from the incident surface after being continuously reflected by a plurality of reflecting surfaces.
8. The photo-thermal spectrum based trace gas detection apparatus according to claim 5, wherein the mach-zehnder interferometer comprises:
a beam splitter arranged to split an input second laser beam into two identical beams;
the detection arm is connected with the beam splitter and penetrates through the annular reflecting pool;
the reference arm is connected with the beam splitter and is arranged outside the annular reflection pool, and the length of the detection arm is equal to that of the reference arm;
And the coupler is connected with the detection arm and the reference arm and is used for coupling the second laser beam output from the detection arm and the second laser beam output from the reference arm into one laser beam.
9. The photothermal spectroscopy-based micro gas detection apparatus of claim 5, wherein the information processing component comprises:
the first detector is electrically connected with the Mach-Zehnder interferometer and is used for detecting and converting light intensity signals output by the Mach-Zehnder interferometer;
the second detector is electrically connected with the Mach-Zehnder interferometer and is used for detecting and converting light intensity signals output by the Mach-Zehnder interferometer;
The differential module is electrically connected with the first detector and the second detector respectively and is used for eliminating common mode noise of light intensity signals output by the first detector and the second detector;
the data processing module is connected with the differential module, and comprises a phase-locked amplifier, a data acquisition card and a computer which are electrically connected in sequence, wherein the phase-locked amplifier is used for demodulating and processing an electric signal output by the differential module, the data acquisition card is used for converting an analog signal output by the phase-locked amplifier into a digital signal and extracting second harmonic signal data, and the computer is used for drawing the second harmonic signal to obtain a second harmonic graph and obtaining concentration information of trace gas to be detected according to a peak value of the second harmonic signal.
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