Trace gas detection method and detection device based on photothermal spectroscopy

Abstract

The invention discloses a trace gas detection method and a detection device based on photothermal spectroscopy; the detection method comprises: inputting a trace gas to be measured into an annular reflection pool; passing a first laser beam into the annular reflection pool, wherein the central wavelength of the first laser beam is the same as the absorption peak of the trace gas to be measured, the first laser beam is reflected multiple times in the annular reflection pool, and the trace gas to be measured absorbs the energy of the first laser beam and the temperature changes periodically; passing a second laser beam into the annular reflection pool, and using a Mach-Zehnder interferometer to measure the phase difference information generated by the second laser beam due to the periodic change of the temperature of the trace gas; wherein the central wavelength of the second laser beam is far from the absorption peak of the trace gas to be measured; processing the phase difference information of the second laser beam and converting it 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.

Description

Technical Field

[0001] The present application belongs to the field of gas detection technology, and specifically relates to a trace gas detection method and detection device based on photothermal spectroscopy. Background Art

[0002] Sensitive and selective detection of trace gases is crucial in fields such as atmospheric environment monitoring and biomedicine. As an important sensing technology, laser spectroscopy has the advantages of high sensitivity, strong specificity, and fast response speed. Photothermal spectroscopy has been widely used in the measurement of trace gases such as carbon monoxide, hydrogen sulfide, and methane.

[0003] Photothermal spectroscopy is a highly sensitive laser spectroscopy method that measures optical absorption and thermal properties of samples. When the gas to be measured interacts with a periodically modulated characteristic wavelength beam, part of the light energy is absorbed by the gas molecules, and the molecules transition from the ground state to the excited state. The molecules in the excited state release the absorbed energy in a non-radiative relaxation manner and convert all or part of it into heat energy, resulting in periodic changes in the medium temperature. This temperature change will cause changes in the properties of the sample cell fiber, thereby causing a change in the phase. By detecting the phase change, the concentration information of the trace gas can be inverted. Summary of the invention

[0004] In view of this, on the one hand, the technical solution disclosed in some embodiments is a trace gas detection method based on photothermal spectroscopy, and the detection method includes:

[0005] Inputting the trace gas to be measured into the annular reflecting pool;

[0006] A first laser beam is passed into the annular reflection pool, wherein the central wavelength of the first laser beam is the same as the absorption peak of the trace gas to be measured, the first laser beam is reflected multiple times in the annular reflection pool, and the trace gas to be measured absorbs the energy of the first laser beam and the temperature changes periodically;

[0007] Passing the second laser beam into the annular reflection pool, using a Mach-Zehnder interferometer to measure the phase difference information generated by the second laser beam due to the periodic change in the temperature of the trace gas; wherein the central wavelength of the second laser beam is far away from the absorption peak of the trace gas to be measured;

[0008] Processing the phase difference information of the second laser beam and converting it into light intensity change information of the second laser beam;

[0009] The light intensity variation information of the second laser beam is processed to obtain the concentration of the trace gas to be measured.

[0010] Further, in some embodiments of the trace gas detection method based on photothermal spectroscopy, the annular reflecting pool has a closed cavity structure, the inner surface of the closed cavity structure includes N identical spherical mirrors connected in sequence, one of which is an incident surface, and the remaining N-1 spherical mirrors are reflection surfaces. The first laser beam enters the annular reflecting pool from the incident surface at a set incident angle, and is emitted from the incident surface after N-1 reflections on the N-1 reflection surfaces. The total optical path L of the first laser beam in the annular reflecting pool is expressed as:

[0011]

[0012] in,

[0013]

[0014]

[0015] r is the inner diameter of the annular reflecting pool; d is the distance from the center point of the annular reflecting pool to the center point of the spherical mirror; R is the radius of curvature of the 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; N is the total number of spherical mirrors, which is an odd number; α is half of the central angle of the spherical mirror.

[0016] In some embodiments of the present invention, the maximum incident angle θ of the first laser beam is max Expressed as:

[0017]

[0018] The incident angle θ of the first laser beam ranges from -θ max <θ<θ max ; Among them, the incident angle θ is the angle between the center line of the annular reflection pool and the incident direction of the first laser beam, r is the inner diameter of the reflection pool; d is the distance from the center point of the reflection pool to the center point of the spherical mirror.

[0019] Some embodiments disclose a method for detecting trace gases based on photothermal spectroscopy, which uses a Mach-Zehnder interferometer to measure the phase difference information generated by a second laser beam due to periodic changes in the temperature of the trace gas to be measured, including:

[0020] A second laser beam is input into the Mach-Zehnder interferometer;

[0021] The second laser beam is divided into two laser beams with equal intensity in the Mach-Zehnder interferometer and enters the detection arm and the reference arm respectively; wherein the detection arm is connected to the annular reflection pool, and the reference arm is arranged outside the annular reflection pool, and the detection arm and the reference arm are of equal length;

[0022] The temperature of the trace gas in the annular reflecting pool changes periodically, causing the phase of the second laser beam in the detection arm to change, resulting in a phase difference with the second laser beam connected to the reference arm;

[0023] 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 with each other, so that the phase difference information of the second laser beam is converted into light intensity change information.

[0024] Some embodiments disclose a method for detecting trace gases based on photothermal spectroscopy, which processes the light intensity change information of the second laser beam to obtain the concentration of the trace gas, specifically including:

[0025] Converting the light intensity variation information of the second laser beam into two electrical signals;

[0026] Eliminate the common mode noise of two electrical signals and obtain one electrical signal;

[0027] After the common mode noise is eliminated, the electrical signal is demodulated and an analog signal is output;

[0028] Convert the analog signal into a digital signal and extract the second harmonic signal data;

[0029] The obtained second harmonic signal data is used to obtain the concentration information of the trace gas to be measured.

[0030] On the other hand, some embodiments disclose a trace gas detection device based on photothermal spectroscopy, comprising:

[0031] A laser providing assembly, for providing a first laser beam and a second laser beam;

[0032] The annular reflecting pool is used to place the trace gas to be measured and realize multiple reflection processes of the first laser beam therein. The trace gas to be measured in the annular reflecting pool absorbs the energy of the first laser beam and the temperature changes periodically.

[0033] A Mach-Zehnder interferometer, used to measure the phase difference information of the second laser beam caused by the periodic change of the temperature of the trace gas after the second laser beam passes through the annular reflecting pool;

[0034] Information processing component; used for processing the phase difference information of the second laser beam to obtain the trace gas concentration.

[0035] In some embodiments of the present invention, the laser providing module of the trace gas detection device based on photothermal spectroscopy includes:

[0036] A first laser, used to output a first laser beam;

[0037] A second laser, used for outputting a second laser beam;

[0038] The laser control module is used to control the first laser to output a first laser beam with a set wavelength and intensity, and to control the second laser to output a second laser beam with a set wavelength and intensity.

[0039] Some embodiments disclose a trace gas detection device based on photothermal spectroscopy, wherein an annular reflecting pool has a closed cavity structure, the inner surface of the closed cavity structure includes an odd number of identical spherical mirrors, the spherical mirrors are arranged in such a way that the spherical mirrors are connected in sequence, one of the spherical mirrors is the incident surface of the first laser beam, and the remaining spherical mirrors are reflection surfaces, the first laser beam is incident from the incident surface at a set angle, and is emitted from the incident surface after continuous reflection at multiple reflection surfaces.

[0040] Some embodiments disclose a trace gas detection device based on photothermal spectroscopy, wherein the Mach-Zehnder interferometer comprises:

[0041] a beam splitter configured to split the input second laser beam into two identical beams;

[0042] A detection arm is arranged to be connected with the beam splitter and pass through the annular reflection pool;

[0043] A reference arm is arranged to be connected to the beam splitter and is outside the annular reflecting pool; wherein the detection arm and the reference arm have the same length;

[0044] The coupler is connected to the detection arm and the reference arm, and is used to couple the second laser beam output from the detection arm and the second laser beam output from the reference arm into one laser beam.

[0045] In some embodiments of the present invention, the information processing component of the photothermal spectroscopy-based trace gas detection device includes:

[0046] A first detector is arranged to be electrically connected to the Mach-Zehnder interferometer and is used to detect and convert a light intensity signal output by the Mach-Zehnder interferometer;

[0047] A second detector is arranged to be electrically connected to the Mach-Zehnder interferometer and is used to detect and convert a light intensity signal output by the Mach-Zehnder interferometer;

[0048] A differential module, arranged to be electrically connected to the first detector and the second detector respectively, and used to eliminate common mode noise of the light intensity signals output by the first detector and the second detector;

[0049] The data processing module is arranged to be connected with the differential module, and the data processing module comprises a phase-locked amplifier, a data acquisition card and a computer which are electrically connected in sequence, wherein the phase-locked amplifier is arranged to demodulate and process the electrical signal output by the differential module, the data acquisition card is used to convert the analog signal output by the phase-locked amplifier into a digital signal, extract the second harmonic signal data, and the computer is used to draw a graph of the second harmonic signal to obtain a second harmonic graph, and obtain the concentration information of the trace gas to be measured according to the peak value of the second harmonic signal.

[0050] The trace gas detection device and detection method based on photothermal spectroscopy provided in the embodiments of the present invention utilize an annular reflection pool to totally reflect the laser multiple times inside the pool and then emit it from the incident window, thereby increasing the absorption optical path, increasing the gas's absorption rate of the laser, and improving the system's detection sensitivity. The detection device is an all-optical structure without electronic devices, and can be used in harsh conditions such as strong electromagnetic environments and gas environments. It has the beneficial technical effects of high sensitivity, fast response speed, and strong anti-interference ability, and has good application prospects in this field. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 Example 1 Schematic diagram of the composition of a trace gas detection device based on photothermal spectroscopy;

[0052] Figure 2 Example 2 Schematic diagram of the composition of a trace gas detection device based on photothermal spectroscopy;

[0053] Figure 3 Example 2 Schematic diagram of Mach-Zehnder interferometer setup;

[0054] Figure 4 Example 4 Schematic diagram of the structure of an annular reflecting pool;

[0055] Figure 5 Example 4: Schematic diagram of incident angle of annular reflecting pool;

[0056] Figure 6 Schematic diagram of the reflection trajectory of the first laser beam in the annular reflecting pool of Example 4.

[0057] Reference numerals

[0058] 1 Laser supply assembly 2 Annular reflecting pool

[0059] 3 Mach-Zehnder interferometer 4 Information processing components

[0060] 5 Collimator 11 Laser control module

[0061] 12 First laser 13 Second laser

[0062] 31 Beam splitter 32 Detection arm

[0063] 33 Reference arm 34 Coupler

[0064] 41 First detector 42 Second detector

[0065] 43 Differential module 44 Data processing module

[0066] 441 Lock-in amplifier 442 Data acquisition card

[0067] 443 Computer DETAILED DESCRIPTION

[0068] The word "embodiment" used here as an "exemplary" does not necessarily mean that any embodiment described is superior to or better than other embodiments. Unless otherwise specified, the performance index tests in the embodiments of this application are performed using conventional test methods in the art. It should be understood that the terms described in this application are only used to describe specific implementation methods and are not used to limit the content disclosed in this application.

[0069] Unless otherwise specified, the technical and scientific terms used in this document have the same meanings as commonly understood by ordinary technicians in the technical field to which this application belongs; other experimental methods and technical means not specifically specified in this application refer to experimental methods and technical means commonly used by ordinary technicians in this field.

[0070] The terms "substantially" and "approximately" used herein are used to describe small fluctuations. For example, they can 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%. The numerical data represented or presented in the range format herein are used only for convenience and brevity, and should therefore be flexibly interpreted as including not only the values ​​clearly listed as the limits of the range, but also all independent values ​​or sub-ranges contained in the range. For example, the numerical range of "1-5%" should be interpreted as including not only the clearly listed values ​​of 1% to 5%, but also the independent values ​​and sub-ranges within the range shown. Therefore, independent values ​​such as 2%, 3.5% and 4% and sub-ranges such as 1%-3%, 2%-4% and 3%-5% are included in this numerical range. This principle also applies to the range of only one numerical value. In addition, such an interpretation applies regardless of the width of the range or the characteristics described.

[0071] In this document, including in the claims, transitional words such as "comprises," "includes," "with," "having," "containing," "involving," "accommodating," etc. are understood to be open-ended, i.e., meaning "including but not limited to." Only the transitional words "consisting of" and "composed of" are closed transitional words.

[0072] In order to better illustrate the content of the present application, numerous specific details are provided in the specific examples below. It should be understood by those skilled in the art that the present application can also be implemented without certain specific details. In the embodiments, some methods, means, instruments, equipment, etc. well known to those skilled in the art are not described in detail in order to highlight the main purpose of the present application.

[0073] Under the premise of no conflict, the technical features disclosed in the embodiments of the present application can be combined arbitrarily, and the resulting technical solutions belong to the contents disclosed in the embodiments of the present application.

[0074] The following contents are combined with the attached Figure 1 , 2 , 3, 4, 5, 6 and specific embodiments are further illustrated.

[0075] In some embodiments, the method for detecting trace gases based on photothermal spectroscopy includes:

[0076] Inputting the trace gas to be measured into the annular reflecting pool;

[0077] The first laser beam is passed into the annular reflection pool, the central wavelength of the first laser beam is the same as the absorption peak of the trace gas to be measured, the first laser beam is reflected multiple times in the annular reflection pool, and the trace gas to be measured absorbs the energy of the first laser beam and the temperature changes periodically; the first laser beam is reflected multiple times in the annular reflection pool, which prolongs the optical path of the first laser beam in the annular reflection pool, effectively prolongs the absorption rate of the gas in the annular reflection pool to the energy of the first laser beam, improves the sensitivity of gas detection, and can realize the detection of trace gas; in order to ensure the energy absorption efficiency of the first laser beam by the gas molecules and produce the expected photothermal effect, the central wavelength of the first laser beam is usually set to be consistent with the strongest absorption peak of the gas to be measured;

[0078] A second laser beam is passed into an annular reflecting pool, and a Mach-Zehnder interferometer is used to measure the phase difference information generated by the second laser beam due to the periodic change in the temperature of the trace gas; wherein the central wavelength of the second laser beam is far away from the absorption peak of the trace gas to be measured; generally, gas molecules in the annular reflecting pool absorb part of the energy of the first laser beam, causing the internal temperature to change, and the change in the temperature in the annular reflecting pool will cause the change in the properties of the optical fiber serving as the detection arm in the annular reflecting pool, while the ambient temperature of the reference arm remains constant, and the properties of the optical fiber serving as the reference arm remain unchanged, thereby causing a phase difference between the detection arm and the reference arm of the Mach-Zehnder interferometer;

[0079] The phase difference information of the second laser beam is processed and converted into the light intensity change information of the second laser beam; the light intensity change information of the second laser beam is processed to obtain the concentration of the trace gas. As an optional embodiment, after the detection arm extends out from the annular reflection pool, it is combined with the reference arm at the 2×2 coupler and interferes. This process converts the phase change of the second laser beam into a light intensity change, which is then converted into an electrical signal by the first photodetector and the second photodetector. The electrical signals output by the two detectors are respectively used as the two input ends of the differential module and are differentially processed by the differential module to eliminate common mode noise; the electrical signal output by the differential module is connected to the signal processing module, first demodulated by the phase-locked amplifier, and then the analog signal output by the phase-locked amplifier is converted into a digital signal by a data acquisition card, and the second harmonic signal data is extracted. The second harmonic graph is obtained by drawing through computer software, and the concentration information of the target gas to be measured can be inverted according to the peak value of the second harmonic signal.

[0080] As an optional embodiment, in the trace gas detection method based on photothermal spectroscopy, the annular reflecting pool 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, and 1 spherical mirror is an incident surface. The first laser beam enters the annular reflecting pool from the incident surface at a set incident angle, and is emitted from the incident surface after N-1 reflections by the N-1 spherical mirrors. The total optical path L of the first laser beam in the annular reflecting pool is expressed as:

[0081]

[0082] in,

[0083]

[0084]

[0085] r is the inner diameter of the annular reflecting pool; d is the distance from the center point of the annular reflecting pool to the center point of the spherical mirror; R is the radius of curvature of the 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; N is the total number of spherical mirrors, which is an odd number; α is half of the central angle of the spherical mirror.

[0086] As an optional embodiment, the maximum incident angle θ of the first laser beam max Expressed as:

[0087]

[0088] The incident angle θ of the first laser beam ranges from -θ max <θ<θ max ; Among them, the incident angle θ is the angle between the center line of the annular reflection pool and the incident direction of the first laser beam.

[0089] As an optional embodiment, using a Mach-Zehnder interferometer to measure the phase difference information of the second laser beam caused by the periodic change of the temperature of the trace gas to be measured, includes:

[0090] A second laser beam is input into the Mach-Zehnder interferometer;

[0091] The second laser beam is divided into two beams of equal intensity in the Mach-Zehnder interferometer and enters the detection arm and the reference arm respectively; wherein the detection arm is connected to the annular reflection pool, and the reference arm is arranged outside the annular reflection pool, and the detection arm and the reference arm are of equal length;

[0092] The temperature of the trace gas in the annular reflecting pool changes periodically, causing the phase of the second laser beam in the detection arm to change, resulting in a phase difference with the second laser beam connected to the reference arm;

[0093] 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 with each other, so that the phase difference information of the second laser beam is converted into light intensity change information.

[0094] As an optional embodiment, processing the light intensity variation information of the second laser beam to obtain the concentration of the trace gas specifically includes:

[0095] Converting the light intensity variation information of the second laser beam into two electrical signals;

[0096] Eliminate the common mode noise of two electrical signals and obtain one electrical signal;

[0097] After the common mode noise is eliminated, the electrical signal is demodulated and an analog signal is output;

[0098] Convert the analog signal into a digital signal and extract the second harmonic signal data;

[0099] The obtained second harmonic signal data is used to obtain the concentration information of the trace gas to be measured.

[0100] Example 1

[0101] The trace gas detection device based on photothermal spectroscopy disclosed in Example 1 includes:

[0102] A laser providing component 1, used for providing a first laser beam and a second laser beam;

[0103] The annular reflecting pool 2 is used to place the trace gas to be measured and realize multiple reflection processes of the first laser beam therein. The trace gas to be measured in the annular reflecting pool absorbs the energy of the first laser beam and the temperature changes periodically.

[0104] The Mach-Zehnder interferometer 3 is used to measure the phase difference information of the second laser beam caused by the periodic change of the temperature of the trace gas after the second laser beam passes through the annular reflection pool 2;

[0105] The information processing component 4 is used to process the phase difference information of the second laser beam to obtain the trace gas concentration.

[0106] Example 2

[0107] The trace gas detection device based on photothermal spectroscopy disclosed in Example 2 includes:

[0108] A laser providing component 1 is used to provide a first laser beam and a second laser beam; the laser providing component 1 includes a first laser 12 for outputting the first laser beam, and a second laser for outputting the second laser beam, and a laser control module 11 is provided to be electrically connected to the first laser 12 and the second laser 13, respectively, and is used to control the first laser 12 to output a first laser beam of a set wavelength and intensity, and to control the second laser 13 to output a second laser beam of a set wavelength and intensity;

[0109] As an optional embodiment, the first laser 12 is a distributed feedback laser, the second laser 13 is a distributed feedback laser, and the laser control module 11 includes a current driver and a temperature controller, which are used to drive and modulate the first laser 12. The wavelength of the first laser beam output by the distributed feedback laser can be modulated by changing the temperature and the current, so that the wavelength scanning range of the first laser 12 sweeps across the strongest absorption peak of the gas, so that the target gas to be measured can absorb the energy of the first laser beam emitted by the first laser 12, and a photothermal effect occurs;

[0110] As an optional embodiment, the first laser 12 is wavelength modulated by a superposition signal of a high-frequency sine wave and a low-frequency sawtooth wave to generate a first laser beam with a wavelength corresponding to a specific absorption peak of the gas to be measured;

[0111] As an optional 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 central 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;

[0112] As an optional embodiment, the laser control module 11 is used to drive the second laser 13 to keep its wavelength stable;

[0113] As an optional embodiment, the detection device includes a collimator 5, which is arranged between the first laser 12 and the annular reflection pool 2, and is used to convert the divergent light of the first laser beam output by the first laser 12 into parallel light, so as to ensure that the first laser beam is incident on the annular reflection pool 2;

[0114] The annular reflecting pool 2 is used to place the trace gas to be measured and realize multiple reflection processes of the first laser beam therein. The trace gas to be measured in the annular reflecting pool absorbs the energy of the first laser beam, and the temperature changes periodically. The annular reflecting pool has a closed cavity structure, and the inner surface of the closed cavity structure includes an odd number of identical spherical mirrors. The spherical mirrors are arranged in such a way that the spherical mirrors are connected in sequence, one of the spherical mirrors is the incident surface of the first laser beam, and the remaining spherical mirrors are used as the reflection surfaces of the first laser beam. After the first laser beam enters the annular reflecting pool from the incident surface, it is emitted from the incident surface after continuous reflection on the reflection surface. Generally, the number of spherical mirrors in the annular reflecting pool is an odd number, for example, an odd number of spherical mirrors greater than 3 constitutes an annular reflecting pool. Reflecting surface in the reflecting pool; generally, the surface of the spherical mirror has a high-reflectivity metal aluminum film, which can effectively reflect the laser beam. A quartz window is arranged on the spherical mirror as the incident surface, which serves as the incident window of the first laser beam; the incident first laser beam enters the annular reflecting pool from the quartz window at a certain angle, first incident to the first reflecting surface for one reflection, and then reflects once on each reflecting mirror surface in turn. After the first laser beam is reflected N-1 times on N-1 reflecting surfaces, it is finally emitted from the incident surface. The first laser beam forms a beam trajectory distribution of a regular N-angle star in the cavity. The reflecting surface reflects the incident first laser beam N-1 times, which effectively increases the total optical path of the first laser beam in the annular reflecting pool;

[0115] The Mach-Zehnder interferometer 3 is used to measure the phase difference information of the second laser beam caused by the periodic change of the temperature of the trace gas after the second laser beam passes through the annular reflection pool 2; the Mach-Zehnder interferometer 3 includes: a beam splitter 31 is configured to split the input second laser beam into two identical beams; a detection arm 32 is configured to be connected to the beam splitter 31 and pass through the annular reflection pool 2; a reference arm 33 is configured to be connected to the beam splitter 31 and is arranged outside the annular reflection pool 2 and maintain its ambient temperature constant to prevent the periodic change of the gas temperature inside the annular reflection pool 2 from affecting the phase change of the second laser beam in the reference arm 33; wherein the detection arm 32 and the reference arm 33 are equal in length; a coupler 34 is configured to be connected to the detection arm 32 and the reference arm 33 to couple the second laser beam output from the detection arm 32 and the second laser beam output from the reference arm 33 into one laser beam; generally, the central wavelength of the second laser beam output by the second laser should be far away from the absorption peak of the gas to be measured to prevent it from being absorbed by the gas to be measured and generating a photothermal effect, thereby affecting the detection sensitivity and accuracy of the gas to be measured;

[0116] As an optional embodiment, the detection arm 32 passes through the annular reflecting pool 2 and is embedded in the annular reflecting pool through the groove at the bottom of the annular reflecting pool 2, while the reference arm 33 is directly placed in the air outside the annular reflecting pool without passing through the annular reflecting pool. The detection arm 32 after passing through the annular reflecting pool 2 and the reference arm 33 pass through the 2×2 coupler 34 and are received by the photoelectric detection component in the information processing component.

[0117] The information processing component 4 is used to process the phase difference information of the second laser beam to obtain the trace gas concentration. The information processing component 4 includes: a first detector 41, which is electrically connected to the Mach-Zehnder interferometer and is used to detect and convert the light intensity signal output by the Mach-Zehnder interferometer; a second detector 42 is electrically connected to the Mach-Zehnder interferometer and is used to detect and convert the light intensity signal output by the Mach-Zehnder interferometer; a differential module 43 is electrically connected to the first detector 41 and the second detector 42, respectively, and is used to eliminate the difference between 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, wherein the phase-locked amplifier 441 is configured to demodulate and process the electrical signal output by the differential module, the data acquisition card 442 is used to convert the analog signal output by the phase-locked amplifier 441 into a digital signal, extract the second harmonic signal data, and the computer 443 is used to draw the second harmonic signal to obtain a second harmonic graph, and obtain the concentration information of the trace gas to be measured according to the peak value of the second harmonic signal.

[0118] Example 3

[0119] Gas concentration calculation method

[0120] In Example 3, the first laser beam emitted by the first laser is modulated as the excitation light and irradiated into the annular reflection pool, and is reflected multiple times in the annular reflection pool. The trace gas to be measured in the annular reflection pool absorbs part of the energy of the first laser beam, and the gas molecules to be measured transition from the ground state to the excited state. The gas molecules in the excited state de-excite back to the ground state through radiationless relaxation, and release energy at the same time. The released energy causes the gas molecules to be measured and their spatial medium to be heated periodically according to the frequency of laser modulation, causing the temperature of the trace gas to be measured in the pool to change, thereby generating a photothermal signal. 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:

[0121] T(t)=c1I0α(v)CL (5)

[0122] Where c1 is a constant, I0 is the input light intensity of the first laser, α(v)=α(v c +δ v cosω0t) is the absorption coefficient of the gas to be measured, v c is the center frequency of the first laser output, δ vis the modulation amplitude, ω0 is the high-frequency sinusoidal modulation frequency, and L is the effective optical path. Temperature changes can cause changes in the properties of the optical fiber. When the temperature of the optical fiber in the detection arm changes relative to the temperature of the optical fiber in the reference arm, the length and refractive index of the optical fiber in the detection arm change, thereby changing the phase of the second laser beam transmitted by the detection arm, and interference occurs. Assume that the length of the two optical fibers of the detection arm and the reference arm is equal to L0, and the phase of the second laser beam propagating in the optical fiber is Φ:

[0123] Φ=Φ0+k0nL0 (6)

[0124] Among them, k0 = 2π / λ0; Φ0 is the initial phase before the second laser beam enters the optical fiber, and the initial phases of the detection arm and the reference arm are the same; k0 is the propagation constant, λ0 is the wavelength of light in a vacuum; n is the refractive index of the optical fiber; L0 is the length of the optical fiber. The temperature of the reference arm optical fiber remains unchanged, the temperature change of the detection arm optical fiber is ΔT, the change of the refractive index n is Δn, and the change of the detection arm optical fiber length is ΔL. Then the phase Φ1 of the detection arm and the phase Φ2 of the reference arm are:

[0125]

[0126]

[0127] When the temperature of the detection arm changes, the phase difference ΔΦ of the two optical fibers at the intersection of the coupler is:

[0128]

[0129] Dividing both sides of the equation by L0 and ΔT, we get:

[0130]

[0131] This formula means the amount of change in the phase of the second laser beam in the optical fiber when the temperature changes by 1°C. The left side of the equation represents the effect of temperature on a unit length of optical fiber, and the Δn and ΔL on the right side of the equation represent the rates of change of the optical fiber refractive index and length with temperature, respectively.

[0132] For silica optical fiber,

[0133]

[0134] We can get:

[0135]

[0136] make is a constant:

[0137] ΔΦ=c2ΔT (13)

[0138] The light intensity at the input end of the Mach-Zehnder interferometer, i.e., the light intensity of the second laser beam, is I i , which is divided into two beams of equal intensity by a 2x2 coupler. After the coupling interference between the detection arm and the reference arm, the output end is divided into two paths with equal intensity, which are connected to the first detector and the second detector respectively. The light intensity at the output end is:

[0139]

[0140] Among them, ΔT=T(t)-T0=c1I0α(v)CL-T0, T0 is the original temperature, we can get:

[0141]

[0142] The first detector and the second detector convert the detected light intensity I into an electrical signal V:

[0143]

[0144] Where s is the sensitivity of the first detector and the second detector.

[0145] Since the formula is relatively complex, the components of the data processing module, such as the phase-locked amplifier, can be used to extract the second harmonic signal data. When the concentration is low, the peak value of the second harmonic signal is linearly related to the gas concentration, that is, V 2f =a*C+b, where a and b are calibrated by the instrument, and the concentration of the trace gas to be measured can be inverted based on the peak value of the second harmonic signal.

[0146] Example 4

[0147] In Example 4, the annular reflecting pool is cylindrical as a whole, and the interior is a closed cavity. The inner surface of the cavity is composed of N reflecting spherical mirrors of the same size, and the value of N is set to 9. The emitting spherical mirrors are connected end to end and arranged in a ring along the side of the cylinder in sequence, so that the reflecting spherical mirrors are distributed on the same cross section of the cylinder to ensure that each reflecting spherical mirror can be reflected once and the outgoing light beam and the incident light beam enter and exit the same reflecting spherical mirror. The reflecting spherical mirrors are distributed on the same cross section of the cylinder, and the line located in the cross section and passing through the center of the incident surface can be called the center line of the annular reflecting pool, such as Figure 4 As shown; the angle between the center line of the annular reflection pool and the incident direction of the laser is the incident angle of the first laser beam, denoted as θ, as Figure 5 As shown in FIG. 1 , when the incident first laser beam directly hits the edge of the first reflecting spherical mirror along the center line of the annular reflecting pool, the incident angle is the smallest, and when the incident first laser beam hits the other edge of the first spherical mirror, the incident angle is the largest, θ max for:

[0148]

[0149] Where r is the inner diameter of the reflecting pool; d is the distance from the center of the reflecting pool to the center of the spherical mirror;

[0150] The two spherical mirrors on both sides of the center line of the annular reflection pool can be used as the first reflection surface, so the range of the incident angle of the first laser beam is -θ max <θ<θ max For example, when N=9, r=50 mm, and R=90 mm, the value range of θ is: -20.29°<θ<20.29°.

[0151] After the first laser beam enters from the incident hole, it is reflected for the first time at the first reflection surface, and then 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 in sequence, and is reflected eight times before finally being emitted from the incident surface; the total optical path L of the first laser beam is expressed as:

[0152]

[0153] in,

[0154]

[0155]

[0156] 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 curvature radius 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.

[0157] Example 5

[0158] In this embodiment 5, the trace gas detection method based on photothermal spectroscopy includes:

[0159] The detection device is started, and the current and temperature parameters of the current driver and the temperature controller in the laser control module are set to drive the first distributed feedback laser and the second distributed feedback laser; and the wavelength of the first laser beam output by the first laser is changed by adjusting the two parameters of temperature and current so that the wavelength corresponds to the wavelength of the gas absorption peak;

[0160] The first laser beam is converted into a parallel beam through a collimator and irradiated into the annular reflecting pool. The incident angle of the first laser beam is set so that the light path is incident into the annular reflecting pool in a straight horizontal direction, ensuring that the first laser beam is incident on the first reflecting surface of the annular reflecting pool and is reflected once on each reflecting surface. The incident angle of the first laser beam is further fine-tuned to ensure that the shape of the reflected light spot on each reflecting surface is as close to a circle as possible to achieve the best experimental effect.

[0161] The second distributed feedback laser splits the second laser beam into two beams of equal light intensity via a beam splitter. The two beams are transmitted through optical fibers, one of which is connected to the annular reflection pool as a detection arm of the Mach-Zehnder interferometer, and the other is connected to the annular reflection pool as a reference arm of the Mach-Zehnder interferometer. The detection arm and the reference arm are received by the photoelectric detection module after passing through a coupler.

[0162] Open the air inlet and outlet of the annular reflecting pool, and fill nitrogen into the chamber from the air inlet to reduce the impact of air on the experimental process;

[0163] A certain concentration of the trace gas to be measured is introduced into the annular reflecting pool from the air inlet;

[0164] By applying a modulation signal to the current controller, the wavelength scanning range of the first distributed feedback laser is controlled to be 2.2 times the half-maximum full width of the absorption peak corresponding to the measured gas, and the central wavelength of the first distributed feedback laser should correspond to the peak wavelength of the absorption peak of the measured gas, and a complete second harmonic signal is obtained through the data processing module;

[0165] The gas to be measured in the annular reflection pool absorbs part of the laser energy, causing the temperature in the reflection pool to change. Due to the photothermal effect, the temperature change in the reflection pool will cause the property of the optical fiber in the sample pool to change, thereby causing the phase of the detection arm of the Mach-Zehnder interferometer to change, while the phase of the reference arm remains unchanged, so that a phase difference is generated between the detection arm and the reference arm. After the detection arm comes out of the annular reflection pool, it is combined with the reference arm at the coupler and interferes. This process converts the phase change into a light intensity change, which is then converted into an electrical signal by the first photodetector and the second photodetector. The electrical signals output by the two photodetectors are respectively used as two input ends of the differential module for differential processing through the differential module to eliminate common mode noise;

[0166] The electrical signal output by the differential module is connected to the data processing module, and is first demodulated by a phase-locked amplifier. The analog signal output by the phase-locked amplifier is then converted into a digital signal using a data acquisition card. The second harmonic signal data is extracted and plotted using computer software to obtain a second harmonic graph. The concentration information of the trace gas to be measured can be inverted based on the peak value of the second harmonic signal.

[0167] The trace gas detection device and detection method based on photothermal spectroscopy provided in the embodiments of the present invention utilize an annular reflection pool to totally reflect the laser multiple times inside the pool and then emit it from the incident window, thereby increasing the absorption optical path, increasing the gas's absorption rate of the laser, and improving the system's detection sensitivity. The core component of the detection device is an all-optical structure without electronic devices, and can be used in harsh conditions such as strong electromagnetic environments and gas environments. It has the beneficial technical effects of high sensitivity, fast response speed, and strong anti-interference ability, and has good application prospects in this field.

[0168] The technical solutions disclosed in this application and the technical details disclosed in the embodiments are merely illustrative of the inventive concept of this application and do not constitute a limitation on the technical solutions of this application. Any conventional changes, replacements or combinations of the technical details disclosed in this application have the same inventive concept as this application and are within the protection scope of the claims of this application.

Claims

1. A trace gas detection method based on photothermal spectroscopy, characterized in that: include: Inputting the trace gas to be measured into the annular reflecting pool; Passing a first laser beam into the annular reflection pool, wherein the central wavelength of the first laser beam is the same as the absorption peak of the trace gas to be measured, the first laser beam is reflected multiple times in the annular reflection pool, and the trace gas to be measured absorbs the energy of the first laser beam and the temperature changes periodically; Passing a second laser beam into the annular reflection pool, and using a Mach-Zehnder interferometer to measure the phase difference information generated by the second laser beam due to the periodic change in the temperature of the trace gas; wherein the central wavelength of the second laser beam is far away from the absorption peak of the trace gas to be measured; Processing the phase difference information of the second laser beam and converting it into light intensity change information of the second laser beam; Processing the light intensity variation information of the second laser beam to obtain the concentration of the trace gas to be measured; The annular reflecting pool has a closed cavity structure, the inner surface of the closed cavity structure includes N identical spherical mirrors connected in sequence, one of which is an incident surface, and the remaining N-1 spherical mirrors are reflecting surfaces. The first laser beam enters the annular reflecting pool from the incident surface at a set incident angle, and is emitted from the incident surface after N-1 reflections by the N-1 spherical mirrors. The total optical path L of the first laser beam in the annular reflecting pool is expressed as: in, r is the inner diameter of the annular reflecting pool; d is the distance from the center point of the annular reflecting pool to the center point of the spherical mirror; R is the radius of curvature of the 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; N is the total number of spherical mirrors, which is an odd number; α is half of the central angle of the spherical mirror.

2. The method for detecting trace gases based on photothermal spectroscopy according to claim 1, characterized in that: The maximum incident angle θ of the first laser beam max Expressed as: Wherein, r is the inner diameter of the reflection pool; d is the distance from the center point of the reflection pool to the center point of the spherical mirror; the incident angle θ is the angle between the center line of the annular reflection pool and the incident direction of the first laser beam; The incident angle θ of the first laser beam ranges from -θ max <θ<θ max .

3. The method for detecting trace gases based on photothermal spectroscopy according to claim 1, characterized in that: The phase difference information generated by the second laser beam caused by the periodic change of the temperature of the trace gas to be measured by using the Mach-Zehnder interferometer includes: A second laser beam is input into the Mach-Zehnder interferometer; The second laser beam is divided into two laser beams with equal intensity in the Mach-Zehnder interferometer and enters the detection arm and the reference arm respectively; wherein the detection arm is connected to the annular reflection pool, the reference arm is arranged outside the annular reflection pool, and the detection arm is equal in length to the reference arm; The temperature of the trace gas in the annular reflecting pool changes periodically, causing the phase of the second laser beam in the detection arm to change, resulting in a phase difference with the second laser beam connected to 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 with each other, so that the phase difference information of the second laser beam is converted into light intensity change information.

4. The method for detecting trace gases based on photothermal spectroscopy according to claim 3, characterized in that: Processing the light intensity variation information of the second laser beam to obtain the concentration of the trace gas specifically includes: Converting the light intensity variation information of the second laser beam into two electrical signals; Eliminate the common mode noise of two electrical signals and obtain one electrical signal; After the common mode noise is eliminated, the electrical signal is demodulated and an analog signal is output; Convert the analog signal into a digital signal and extract the second harmonic signal data; The obtained second harmonic signal data is used to obtain the concentration information of the trace gas to be measured.

5. A trace gas detection device based on photothermal spectroscopy, used to perform the method of claim 1, characterized in that: include: A laser providing assembly, for providing a first laser beam and a second laser beam; An annular reflecting pool is used to place the trace gas to be measured and realize multiple reflection processes of the first laser beam therein, and the trace gas to be measured in the annular reflecting pool absorbs the energy of the first laser beam and the temperature changes periodically; A Mach-Zehnder interferometer, used to measure the phase difference information of the second laser beam caused by the periodic change of the temperature of the trace gas after the second laser beam passes through the annular reflecting pool; The information processing component is used to process the phase difference information of the second laser beam to obtain the trace gas concentration.

6. The trace gas detection device based on photothermal spectroscopy according to claim 5, characterized in that: The laser providing assembly comprises: A first laser, used to output a first laser beam; A second laser, used for outputting a second laser beam; The laser control module is used to control the first laser to output a first laser beam with a set wavelength and intensity, and to control the second laser to output a second laser beam with a set wavelength and intensity.

7. The trace gas detection device based on photothermal spectroscopy according to claim 5, characterized in that: The annular reflecting pool has a closed cavity structure, and the inner surface of the closed cavity structure includes an odd number of identical spherical mirrors. The spherical mirrors are arranged in such a way that the spherical mirrors are connected in sequence, one of the spherical mirrors is the incident surface of the first laser beam, and the remaining spherical mirrors are reflection surfaces. The first laser beam is incident from the incident surface at a set angle, and is emitted from the incident surface after continuous reflection by multiple reflection surfaces.

8. The trace gas detection device based on photothermal spectroscopy according to claim 5, characterized in that: The Mach-Zehnder interferometer comprises: a beam splitter configured to split the input second laser beam into two identical beams; A detection arm, arranged to be connected to the beam splitter and pass through the annular reflection pool; A reference arm is connected to the beam splitter and is outside the annular reflecting pool; the detection arm is equal to the reference arm in length; A coupler is arranged to be connected to the detection arm and the reference arm, and is used to couple 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 trace gas detection device based on photothermal spectroscopy according to claim 5, characterized in that: The information processing component includes: A first detector is arranged to be electrically connected to the Mach-Zehnder interferometer and is used to detect and convert a light intensity signal output by the Mach-Zehnder interferometer; A second detector is arranged to be electrically connected to the Mach-Zehnder interferometer and is used to detect and convert the light intensity signal output by the Mach-Zehnder interferometer; A differential module, arranged to be electrically connected to the first detector and the second detector respectively, and used to eliminate common mode noise of the light intensity signals output by the first detector and the second detector; A data processing module is arranged to be connected to the differential module; the data processing module comprises a phase-locked amplifier, a data acquisition card and a computer which are electrically connected in sequence, wherein the phase-locked amplifier is arranged to demodulate and process the electrical signal output by the differential module, the data acquisition card is used to convert the analog signal output by the phase-locked amplifier into a digital signal, extract the second harmonic signal data, the computer is used to draw a graph of the second harmonic signal, obtain a second harmonic graph, and obtain the concentration information of the trace gas to be measured according to the peak value of the second harmonic signal.

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