CN117347343A - Double-optical comb surface enhanced coherent anti-Stokes Raman spectrum gas detection system - Google Patents
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Abstract
The invention discloses a double-optical comb surface enhanced coherent anti-Stokes Raman spectrum gas detection system which comprises a double-optical comb light source module, a light splitting module, a signal triggering module, a beam combining module, a gas detection module and a signal acquisition module. The Raman spectrum detection is a potential path in the current gas detection field, but the common Raman spectrum detection has the problems of small molecular Raman scattering cross section, slow response and the like. The invention has application prospect in the fields of food industry, medical health, environmental monitoring, security inspection and the like.
Description
Technical Field
The invention relates to the technical field of spectrum detection, in particular to a double-optical comb surface enhanced coherent anti-Stokes Raman spectrum gas detection system.
Background
The gas molecular detection is widely applied in the fields of food industry, medical health, environmental monitoring, security inspection and the like, and various detection methods exist in the current gas detection field, and the detection methods can be divided into detection methods based on electric conductivity and thermal conductivity change, electrochemical reaction, spectrum and the like according to principles. Wherein the detection method based on the change of electric conductivity and thermal conductivity is greatly influenced by the environment and cannot detect the kind of the molecule to be detected; the detection method based on electrochemical reaction has selectivity to the gas to be detected, and only gas molecules with electrochemical activity can be detected. Spectroscopic detection is a potential route, and in the spectroscopic detection method, the use of double optical comb spectroscopy to detect gas molecules is a common method, but usually infrared absorption spectra of the molecules are detected, and raman spectra of the detected gas molecules are less. It is difficult to detect low concentration gas molecules, whether by infrared absorption or raman spectroscopy. When raman spectroscopy is used for detecting gas molecules, the raman signal is very weak because of small molecular light scattering cross section; meanwhile, the signal integration time is long, so that the spectrum acquisition speed is slow; in addition, when detecting gas molecules, the gas molecules stay on the surface of the detection substrate for a short time, so that the detection difficulty is greatly increased. In order to solve the above problems, a highly sensitive and rapid gas detection method is required, and low concentration gas molecules of different kinds can be effectively detected.
Disclosure of Invention
Aiming at the problems, the invention aims to solve the problem of rapid detection of low-concentration gas molecules, provides a double-optical comb surface enhanced coherent anti-Stokes Raman spectrum gas detection system, and is used for solving the problems of low response speed, low sensitivity, incapability of specific detection and the like in the current gas molecule detection field.
The design concept of the invention is based on the characteristics of a double optical comb technology and a surface enhanced coherent anti-Stokes Raman spectrum, and the double optical comb technology has wide application in the fields of precise spectrum measurement, high-precision ranging, navigation and positioning systems and optical communication since the invention. The optical frequency comb is ultra-short pulse laser generated by a mode-locked laser, is represented as a series of equally-spaced pulses in the time domain, and is formed by tens of thousands of equally-spaced frequency comb teeth in the frequency domain. The use of double optical comb spectra to detect gas molecules has high measurement accuracy and speed, but there are limitations to the detection of low concentration gas molecules.
The coherent anti-stokes raman scattering is a three-order nonlinear four-wave mixing process composed of pump light, stokes light, detection light and anti-stokes light, and in practical application, the pump light and the detection light can be the same beam of light. When the frequency difference between the pump light and the stokes light and the oscillation frequency of a certain raman active energy level of the molecule to be detected form resonance, the anti-stokes signal will be emitted in the corresponding phase matching wave vector direction. The stimulated resonance signal generated by the coherent anti-stokes raman scattering process has higher detection sensitivity and detection efficiency than spontaneous raman spectra. Although the sensitivity of coherent anti-stokes raman spectroscopy is improved by several orders of magnitude compared to spontaneous raman spectroscopy, detection of low concentration gas molecules is still difficult, and therefore the introduction of a plasmonic enhancement substrate, known as surface enhanced coherent anti-stokes raman scattering, is required. The invention has the advantages of realizing specific detection, along with high detection speed, high sensitivity and small dependence on environment.
In order to achieve the above purpose, the dual optical comb surface enhanced coherent anti-stokes Raman spectrum gas detection system provided by the invention comprises a dual optical comb light source module, a beam splitting module, a signal triggering module, a beam combining module, a gas detection module and a signal acquisition module.
The double-optical comb light source module consists of two mode-locked lasers which are respectively used as pump light and Stokes light, the repetition frequencies of the pump light and the Stokes light are different, and the difference value of the repetition frequencies is a gravity frequency difference delta f r Measurement time of spectrumResolution of spectrum ΔΩ ≡δf r The heavy frequency difference needs to balance the spectral resolution and the measurement time to proper values.
The double-light comb is used as a light source, and the output end of the double-light comb is connected with the input end of the light splitting module. One path of pulse laser emitted by the light splitting module enters the signal triggering module.
The signal triggering module comprises a frequency multiplication crystal and a photoelectric detector. In the time domain scanning process, the effective coincidence time of the pump light and the Stokes light is the effective measurement time of the molecular vibration energy level, and a trigger signal is introduced to enable the data acquisition card to acquire data only in the effective time, so that the effective sampling rate of the data acquisition card is improved. The two paths of pulse laser beams are combined and then pass through a frequency doubling crystal to generate a second harmonic signal, the second harmonic signal is received by the photoelectric detector and used as a trigger signal, when the trigger signal received by the photoelectric detector reaches a preset intensity threshold value, a trigger instruction is sent out, and the signal collection module starts to collect data.
The other two paths of pulse laser emitted by the beam splitting module enter the beam combining module. Two paths of pulse laser are stretched in the time domain after passing through the nonlinear crystal, wherein one path of pulse laser is stretched in wavelength after passing through the photonic crystal fiber, and wide-spectrum Raman signal identification is provided. The two paths of pulsed laser beams enter a gas detection module through a first objective lens above a gas detection pool after being aligned, a Raman signal of a gas molecule to be detected on a plasmon enhanced substrate is excited, a signal generated by forward scattering is collected by a second objective lens below the gas detection pool, and the signal passes through an optical filter and then enters a photoelectric detector of a signal acquisition module. The nonlinear crystal material is selected from materials having excellent nonlinear response to the wavelength of the optical fiber comb light source.
The signal acquisition module consists of a photoelectric detector and a data acquisition card, and when the data acquisition card receives a trigger instruction sent by the signal trigger module, the data acquisition module starts to acquire data. The acquired original signal is subjected to low-pass filtering, non-resonance background subtraction, intensity and baseline correction to obtain a final spectrum.
The gas detection cell integrated with the plasmon enhancement substrate is sealed outside the gas flow channel inlet and the gas flow channel outlet. The plasmon enhancement substrate consists of a substrate and a plasmon enhancement structure on the substrate, wherein the material of the plasmon enhancement structure is a material for supporting plasmon excitation in a corresponding wave band, and preferably, the plasmon enhancement substrate can be any one of gold, silver, aluminum or copper; the substrate material is a material with light transmittance exceeding 90% in the wavelength range of anti-Stokes, pumping and Stokes; preferably, the substrate is transparent in the visible and near infrared bands and may be of other materials such as quartz or ITO glass.
The plasmon enhancement substrate provided by the invention can be prepared according to a top-down or bottom-up method, has a sub-nanometer gap, can provide hot spots for plasmon enhancement, and can be made of gold, silver, copper or the like, and the plasmon resonance needs to be satisfied. The surface of the plasmon enhancement substrate can be assembled with a molecular layer to detect adsorption type gas, and can also detect non-adsorption type gas, the plasmon enhancement substrate has high scattering in anti-Stokes light and Stokes light wave bands, has strong near field enhancement at pumping light, stokes light and anti-Stokes light, and meets the Fano resonance condition.
The technical principle of the invention is as follows:
the spontaneous raman signal intensity of the molecules is usually low, and it is required to enhance the molecules by noble metal nano structures to realize high-sensitivity molecular detection. Two noble metal nano structures with a distance of several nanometers can generate strong collective resonance of surface plasmons under the action of excitation light, the resonance can break through the diffraction limit of the light, the excitation light is converted into a local electromagnetic field which is bound to a plurality of nanometer spaces on the surface of the noble metal nano structures, the local electromagnetic field is called a hot spot, and the electromagnetic field enhancement factor at the hot spot can reach 10 10 The electromagnetic field enhancement effect can greatly enhance the signal of the molecule. The Fano resonance is realized by designing a proper plasmon enhancement substrate, and meanwhile, a sub-nanometer gap is utilized to construct a hot spot, so that the enhancement of a local electromagnetic field can be realized, and the sensitivity of signals is further improved.
The coherent anti-stokes raman scattering is a three-order nonlinear four-wave mixing process, and involves the interaction of four beams of light, which are respectively called pump light, stokes light, probe light and anti-stokes light, and in practical application, the pump light and the probe light can be the same beam of light. When the frequency difference between the pump light and the stokes light and the oscillation frequency of a certain raman active energy level of the molecule to be detected form resonance, the anti-stokes signal will be emitted in the corresponding phase matching wave vector direction. In this process, a large number of coherent phonons are generated, which have the same frequency and phase, so that the intensity of the coherent anti-stokes raman scattering signal is far greater than that of the spontaneous raman scattering signal, and the detection sensitivity and the detection efficiency are very high. The combination of coherent anti-stokes raman scattering and plasmon enhanced substrate may further improve the sensitivity of detection, referred to as surface enhanced coherent anti-stokes raman scattering.
The optical frequency comb is formed by a series of equally-spaced pulses in the time domain and tens of thousands of equally-spaced frequency comb teeth in the frequency domain, has an extremely fine and stable frequency structure, and can improve the measurement accuracy and speed of the spectrum by using the double-frequency comb as a light source. The characteristics of optical frequency comb and surface enhanced coherent anti-Stokes Raman are combined, so that the rapid high-sensitivity detection of gas molecules can be realized.
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. the device uses the double optical comb as a system light source, the signals after the double optical comb beat frequency are positioned in a radio frequency band, and the signals of gas molecules to be detected can be converted from spectrum to radio frequency through three processes of beat frequency, filtering and Fourier transformation, so that the down conversion of spectrum information is realized. The double optical comb is pulse laser with stable phase, has extremely fine and stable frequency structure, and is favorable for realizing high-frequency resolution spectrum measurement.
2. The coherent anti-stokes raman scattering has higher signal intensity than spontaneous raman, and can improve the detection sensitivity.
3. The Fano resonance peak position of the plasmon enhancement substrate is matched with the excitation light wavelength, and meanwhile, a sub-nanometer gap is utilized to construct a hot spot, so that local electromagnetic field enhancement can be realized, and the sensitivity of signals is further improved.
4. The wavelength range of one pulse is widened by utilizing the photonic crystal fiber, so that the detection of the Raman signal with a wide spectrum can be realized. The signals after the double optical comb beat frequency are converted from spectrum to radio frequency through Fourier transformation, and broadband detection can be realized by using a single detector. The whole system can realize rapid, real-time and label-free gas molecule detection with high sensitivity, and is stable and compact.
Drawings
FIG. 1 is a graph showing the electromagnetic field distribution of an enhanced substrate obtained by using numerical simulation calculation in example 1 of the present invention;
in fig. 1: (a) (b) and (c) are respectively enhanced substrate local electromagnetic field distribution diagrams obtained by numerical simulation calculation under different excitation wavelengths.
FIG. 2 is a graph showing the scattering spectrum measured in example 1 of the present invention.
FIG. 3 is a scanning electron microscope image of a plasmon enhanced substrate characterizing morphology according to embodiment 1 of the present invention;
in fig. 3: (a) The diagram is a scanning electron microscope diagram of a periodic array of gold tetramers, and the diagram (b) is a scanning electron microscope diagram of a single gold tetramer structure. The period of the periodic array is 6 μm, the period is the distance between any two gold tetramer structures, the diameter of a single gold tetramer structure is 260nm, the height is 50nm, and the gap is 15nm.
FIG. 4 is a schematic diagram of a system architecture of the present invention;
in fig. 4: 1-1: double-light comb light source module, 1-21: spectroscopic element #a,1-22: spectral element #b,1-3: photonic crystal fiber, 1-4: nonlinear crystal, 1-5: optical filters, 1-61: mirror #c,1-62: mirror #D,1-7: frequency doubling crystal, 1-81: photodetectors #E,1-82: photodetectors #F,1-9: data acquisition card, 1-10: first objective lens, 1-11: gas flow channel inlet, 1-12: gas flow channel outlet, 1-13: a plasmon enhanced substrate; 1-14: and a second objective lens.
FIG. 5 is a plot of the original signal of toluene and the final spectrum after low pass filtering, subtraction of non-resonant background, intensity and baseline correction in example 1 of the present invention.
FIG. 6 is a schematic diagram of the detection of gas molecules using a gas detection cell according to embodiment 1 of the present invention.
Detailed Description
For a better understanding of the present invention, the technical solutions of the present invention will be further described in detail with reference to the accompanying drawings and specific examples, but the present invention is not limited to the following examples.
Example 1
For easy understanding, the plasmon enhancement structure of the present invention has a sub-nanometer gap, which is less than 30nm; the material of the plasmon enhancement structure is metal capable of realizing plasmon resonance, which refers to the characteristics of the plasmon enhancement structure, and various structures capable of realizing plasmon enhancement are not necessarily periodic, and only one optimization scheme is listed in the embodiment, but the unique characteristics of the plasmon enhancement structure cannot be represented.
The double-optical comb surface enhanced coherent anti-Stokes Raman spectrum gas detection system provided by the embodiment can be used for rapidly and highly sensitively detecting toluene gas molecules.
As shown in FIG. 4, the system comprises a double-light comb light source module, a light splitting module, a signal triggering module, a beam combining module, a gas detection module and a signal acquisition module.
The double-optical comb light source module generates seed pulses by two mode-locked lasers 1-1, expands short pulse light in a time domain by utilizing the chirp characteristic of light, amplifies the short pulse light after passing through ytterbium-doped optical fibers, emits two optical frequency comb signals according to preset repetition frequency, and respectively serves as pump light and Stokes light, and provides proper optical frequency comb signals for a signal triggering module and a beam combining module which are described below. Two optical frequency comb signals output by two mode-locked lasers 1-1 of the double optical comb light source module enter the light splitting module; two light frequency comb signals output by the double light comb light source module enter the light splitting module.
The light splitting module consists of two light splitting elements #A1-21, wherein the light splitting elements #A1-21 are broadband polarization light splitting cubes, and the working wavelength is 900-1300nm. The two light-splitting elements #A1-21 are arranged on the light path from which the light-frequency comb signals are emitted, the two light-frequency comb signals output by the double-light-comb light source module enter the light-splitting module through the two light-splitting elements #A1-21 respectively, the light-frequency comb signals reflected by the light-splitting elements #A1-21 are converged and then enter the signal triggering module, and the light-frequency comb signals transmitted by the light-splitting elements #A1-21 enter the beam combining module.
The signal triggering module comprises frequency doubling crystals 1-7 and photodetectors #E1-81; the optical frequency comb signals are reflected by the light splitting elements #A1-21 and then enter the frequency doubling crystals 1-7 to obtain second harmonic signals, the second harmonic signals are received by the photoelectric detectors #E1-81 and used as trigger signals, when the trigger signals received by the photoelectric detectors #E1-81 reach a preset intensity threshold, trigger instructions are sent to feed back to the signal acquisition module, and the signal acquisition module starts to acquire data when receiving the trigger instructions. The trigger signal is introduced to enable the data acquisition cards 1-9 to acquire data only in effective time, so that the sampling rate of the data acquisition cards is improved.
The beam combination module comprises a photonic crystal fiber 1-3, two nonlinear crystals 1-4, a light splitting element #B1-22 and a reflecting mirror #C1-61, wherein the light splitting element #B1-22 is a short-wave-pass dichroic mirror, has higher reflectivity when the wavelength is more than 1060nm, and has higher transmissivity when the wavelength is less than 1060 nm; a beam of optical frequency comb signals transmitted by one of the optical splitting elements #A1-21 is stretched in the time domain through one nonlinear crystal 1-4 and then enters the optical splitting element #B1-22 to obtain an optical frequency comb signal #1, and the nonlinear crystal 1-4 introduces chromatic dispersion to enable each frequency component to generate phase change, so that phase matching of pump light and Stokes light is realized. The other beam of optical frequency comb signal transmitted by the other beam-splitting element #A1-21 is stretched in the time domain after passing through the other nonlinear crystal 1-4, and is stretched in the frequency domain after passing through the photonic crystal fiber 1-3 to obtain an optical frequency comb signal #2, and the spectral range is changed into 1100nm-1500nm after passing through the photonic crystal fiber 1-3, so that the optical frequency comb signal #1 (pump light) and the optical frequency comb signal #2 (Stokes light) can realize detection of most Raman fingerprint areas. The optical frequency comb signal #1 and the optical frequency comb signal #2 are combined into an optical frequency comb signal #3 after passing through the light splitting elements #B1-22, and the optical frequency comb signal #3 enters the gas detection module as a proper input signal.
The output end of the beam combination module is connected with the input end of the gas detection module.
A gas detection module including a reflecting mirror #D1-62, a first objective lens 1-10, a second objective lens 1-14 and a gas detection cell integrating the plasmon enhancement substrate 1-13; the first objective lens 1-10 and the second objective lens 1-14 are respectively arranged above and below the gas detection pool; the first objective lens 1-10, the second objective lens 1-14 and the gas detection pool are respectively arranged on an emergent light path of the optical frequency comb signal #3; the optical frequency comb signal #3 finally emitted from the beam splitting element #B1-22 in the beam combining module is reflected by the reflector #D1-62, enters the gas detection tank through the first objective lens 1-10, excites the Raman signal of the gas molecule to be detected, and enters the signal acquisition module through the second objective lens 1-14 after passing through the gas detection tank.
The gas detection pool is a closed pool provided with gas flow passage inlets 1-11 and gas flow passage outlets 1-12; the plasmon enhancement substrates 1-13 are arranged inside the gas detection cell. The plasmon enhancement substrates 1-13 comprise a substrate and a plasmon enhancement structure on the substrate, wherein the plasmon enhancement structure has a sub-nanometer gap, and the gap is smaller than 30nm; the material of the plasmon enhancement structure may be selected from any of gold, silver or copper (gold is preferred in this embodiment); the substrate material (high transmittance in the visible and near infrared bands) is quartz or ITO glass (quartz is preferred in this embodiment); the nonlinear crystal 1-4 is made of flint glass.
This embodiment is a preferred design, and plasmon enhancement substrates 1-13 specifically include: a substrate (high transmittance in visible and near infrared bands) and a periodically arranged tetramer disk structure, a scanning electron microscope image is shown in fig. 3. The substrate material is quartz plate, the tetramer disc structure material is gold, and the tetramer disc structure material is uniformly and periodically arranged in the plane space at intervals of 6 mu m; the diameter of the gold tetramer disc is 260nm, the height is 50nm, and the gap between the discs is 15nm.
The signal acquisition module consists of an optical filter 1-5, a photoelectric detector #F1-82 and a data acquisition card 1-9; after being collected by the second objective lens 1-14 below the gas detection tank, the Raman signal sent by the gas detection tank enters the photoelectric detector #F1-82 through the optical filter 1-5, and the photoelectric detector #F1-82 feeds back the Raman signal to the data acquisition card 1-9.
In the signal acquisition module, when the data acquisition cards 1-9 receive a trigger instruction sent by the signal trigger module, data acquisition is started, and a final Raman spectrum is obtained.
As shown in FIG. 1, in order to obtain the electromagnetic field distribution of the gold tetramer disc structure through numerical simulation and simulation calculation, the parameters of the gold tetramer disc structure are calculated through simulation, the spectral position of Fano resonance can be regulated and controlled by adjusting the size and the interval of the gold tetramer structure, and the gold tetramer disc structure has higher electromagnetic field enhancement when the diameter of the gold tetramer structure is 260nm, the height is 50nm and the middle gap is 15nm.
As shown in FIG. 2, the scattering spectrum of the gold tetramer disk structure corresponds to Stokes light wavelengths 1060-1260nm and anti-Stokes light 860-1060nm, respectively. The plasmon enhancement substrates 1-13 have high scattering in the anti-Stokes optical band 860-1060nm and Stokes optical band 1060-1260 nm; the near field enhancement is very strong at the positions of the pump light 1060nm, the Stokes light 1060-1260nm and the anti-Stokes light 860-1060nm, and the Fano resonance condition is satisfied.
The embodiment also provides a preparation method of the plasmon enhancement substrates 1-13, which comprises the following specific steps: (S1) spin-coating PMMA glue on a quartz plate, wherein the glue thickness is 150nm; (S2) exposing and developing with an electron beam to obtain a predefined structure, depositing 50nm gold using thermal evaporation; and (S3) removing redundant photoresist from the structure through acetone to obtain the gold tetramer disc periodic array structure.
The working process of the system in this embodiment is as follows:
as shown in connection with fig. 6, example 1 of the present invention uses plasmon enhancement substrates 1 to 13 to detect gas molecules:
the double-light comb light source module generates seed pulse by a mode-locked laser 1-1, expands short pulse light in a time domain by utilizing the chirp characteristic of light, amplifies the short pulse light after passing through ytterbium-doped optical fiber, transmits two paths of pulse laser according to preset repetition frequency, the center wavelength is 1030nm, the pulse width is 100fs, the repetition frequency of a pumping light source is 100MHz, the repetition frequency of a Stokes light source is 100MHz minus the heavy frequency difference of 100Hz, and the heavy frequency difference of delta f r Refers to the repetition frequency difference between the pump light and Stokes light, the measurement time of the spectrumResolution of spectrum ΔΩ ≡δf r The high spectral resolution and short measurement time are possible when the heavy frequency difference is taken to be 100 Hz. The optical frequency comb signal emitted by the double optical comb light source is reflected by the light splitting element #A1-21 and enters the frequency doubling crystal 1-7 to generate a second harmonic signal, twoThe subharmonic signals are received by the photoelectric detectors #E1-81 and used as trigger signals, when the trigger signals received by the photoelectric detectors #E1-81 reach a preset intensity threshold, trigger instructions are sent out, and the data acquisition cards 1-9 start to acquire data. The trigger signal is introduced to enable the data acquisition cards 1-9 to acquire data only in effective time, so that the sampling rate of the data acquisition cards is improved. The optical frequency comb signals emitted by the double-optical comb light source are transmitted into the photonic crystal fibers 1-3 and the nonlinear crystals 1-4 of the beam combining module through the light splitting elements #A1-21, and the optical frequency comb signals #1 and #2 are obtained. The photonic crystal fiber 1-3 widens stokes light in a frequency domain to provide a broad spectrum raman signal identification. After passing through the photonic crystal fiber 1-3, the spectrum range is changed into 1100nm-1500nm, and the detection of most Raman fingerprint areas can be realized by the pump light and the Stokes light frequency comb. The nonlinear crystals 1-4 introduce dispersion to enable each frequency component to generate phase change, so that phase matching of pump light and Stokes light is realized. The optical frequency comb signal #1 and the optical frequency comb signal #2 are combined and then used as proper input signals (optical frequency comb signal # 3), and enter the gas detection module through the light splitting elements #B1-22.
In view of the fact that the gas detection tank is a closed tank which transmits light vertically, and small holes are formed in two sides of the closed tank and are used for connecting pipelines of a gas passage, the closed tank is designed to be a closed tank with gas flow passage inlets 1-11 and gas flow passage outlets 1-12 respectively arranged on the side walls of the gas detection tank; the plasmon enhancement substrate 1-13 is arranged in the gas detection tank; and placing the plasmon enhancement substrates 1-13 into a gas detection tank, opening a toluene gas bottle and a diluent gas nitrogen bottle, and regulating the flow of the two paths of gases to ensure that the indication of the gas sensor is stabilized at 1ppm. The optical frequency comb signal #3 enters a gas detection tank through an objective lens 1-10, a Raman signal of toluene gas molecules on the surface of a plasmon enhancement substrate 1-13 is excited, the objective lens 1-14 below the gas detection tank collects Raman signals generated by forward scattering, the Raman signals enter a photoelectric detector #F1-82 after passing through an optical filter 1-5, and data collection is started when a data collection card 1-9 receives a trigger instruction sent by a signal trigger module.
The final raman spectrum obtained after the low-pass filtering, subtraction of non-resonant background, intensity and baseline correction of the collected original signal is shown in fig. 5: measurement time 6. Mu.s, spectral resolution 3.5cm -1 The detection limit of the gas concentration is 1ppm, the detection sensitivity, the spectral resolution and the detection speed are high, and the toluene gas with the concentration as low as 1ppm can be detected.
Example 2
Example 2 is different from example 1 in that example 1 is a detection of toluene gas molecules, example 2 is a detection of benzene gas molecules, and the procedure is the same as example 1.
The flow rates of the nitrogen gas and the benzene gas are regulated by a flowmeter, the concentration of the mixed gas is monitored by a gas sensor, and then the mixed gas enters a gas detection tank through a gas passage for detection. The two paths of pulse lasers enter signals for exciting toluene gas molecules after being combined through objective lenses 1-10, the objective lenses 1-14 below a gas detection pool collect Raman signals generated by forward scattering, the Raman signals enter photodetectors #F1-82 after passing through optical filters, when a triggering instruction sent by a signal triggering module is received by a data acquisition card 1-9, data acquisition is started, carrier signals, non-resonance background are removed from the data, the intensity and a base line are corrected to obtain a final spectrum, the measurement time is 6 mu s, and the spectral resolution is 3.5cm -1 The detection limit of the gas concentration is 1ppm, and the detection sensitivity, the spectral resolution and the detection speed are high, so that the benzene gas with the concentration as low as 1ppm can be detected.
In particular, the structure of the plasmon-enhanced substrate of the present invention may be varied, and the tetramer is only one of them. The mechanism of multimodal resonance is only one of the references, and the farno resonance is not the only way to achieve the object of the invention. The invention mainly uses the wavelength of the optical fiber optical comb light source and the characteristic Raman peak position coverage area of the gas molecules to be detected as examples, namely, all material selection and structural design of the whole system can be determined. In addition, the application object of the present invention is not limited to gas molecules, and other substances can be detected. Although specific embodiments have been described herein to facilitate an understanding of the invention by those skilled in the art. It should be understood that the invention is not limited to the specific embodiments, but is capable of numerous modifications within the spirit and scope of the invention as hereinafter defined and defined by the appended claims as will be apparent to those skilled in the art all falling within the true spirit and scope of the invention as hereinafter claimed.
Claims (10)
1. A double-optical comb surface enhanced coherent anti-Stokes Raman spectrum gas detection system is characterized in that: the device comprises a double-light comb light source module, a light splitting module, a signal triggering module, a beam combining module, a gas detection module and a signal acquisition module;
the double-optical comb light source module is used for generating pump light and stokes light of a coherent anti-stokes Raman signal; the repetition frequencies of the pump light and Stokes light in the double-light comb light source module are different, and the difference value of the repetition frequencies is the difference delta f of the repetition frequencies r Measurement time of spectrumResolution of spectrum ΔΩ ≡δf r The heavy frequency difference needs to balance the spectrum resolution and the measurement time to obtain proper values;
the light splitting module is used for splitting one path of optical frequency comb signals from the pump light and the Stokes light emitted by the double-optical comb light source module into a signal triggering module, and the transmitted pump light and Stokes light enter the beam combining module;
the signal triggering module is used for triggering and sampling signals when the two paths of pulses are completely overlapped and providing triggering signals for the signal acquisition module;
the beam combination module is used for stretching the optical frequency comb signal emitted by the double-optical-comb light source module in the time domain and the frequency domain, and providing an appropriate input signal, namely an optical frequency comb signal #3, for a gas detection module;
the gas detection module is used for introducing the optical frequency comb signal #3 after beam combination to realize excitation of a Raman signal of a gas molecule to be detected;
the signal acquisition module is used for acquiring the gas Raman signal sent by the gas detection module, and acquiring data when receiving the trigger instruction sent by the signal trigger module, so as to obtain a final Raman spectrum.
2. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 1, wherein:
the double-optical-comb light source module consists of two mode-locked lasers (1-1) which are respectively used as pump light and Stokes light, and provides proper optical frequency comb signals for a signal triggering module and a beam combining module which are described below, and two optical frequency comb signals output by the double-optical-comb light source module enter the beam splitting module;
the light splitting module consists of two light splitting elements #A (1-21), wherein the two light splitting elements #A (1-21) are arranged on a light path from which the optical frequency comb signals are emitted, and the two optical frequency comb signals output by the double-optical-comb light source module enter the light splitting module through the light splitting elements #A (1-21) respectively; the optical frequency comb signals reflected by the light-splitting elements #A (1-21) enter a signal triggering module; the optical frequency comb signals transmitted by the light-splitting elements #A (1-21) enter the beam-combining module.
3. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 2, wherein:
the signal triggering module comprises frequency doubling crystals (1-7) and photodetectors #E (1-81); the optical frequency comb signals are reflected by the light splitting element #A (1-21) and then enter the frequency doubling crystal (1-7) to generate second harmonic signals, the second harmonic signals are received by the photoelectric detector #E (1-81) and used as trigger signals, and when the trigger signals received by the photoelectric detector #E (1-81) reach a preset intensity threshold value, trigger instructions are sent out to be fed back to the signal acquisition module.
4. A dual optical comb surface enhanced coherent anti-stokes raman spectroscopy gas detection system according to claim 3, wherein:
the beam combination module comprises a photonic crystal fiber (1-3), two nonlinear crystals (1-4), a beam splitting element #B (1-22) and a reflecting mirror #C (1-61); wherein, a beam of optical frequency comb signals transmitted by the optical splitting element #A (1-21) is stretched in the time domain through the nonlinear crystal (1-4) and then enters the optical splitting element #B (1-22) to obtain an optical frequency comb signal #1, another beam of optical frequency comb signals transmitted by the optical splitting element #A (1-21) is stretched in the time domain through the nonlinear crystal (1-4) and then is stretched in the frequency domain through the photonic crystal fiber (1-3) to obtain an optical frequency comb signal #2, and the optical frequency comb signal enters the optical splitting element #B (1-22) through the reflecting mirror #C (1-61); the optical frequency comb signal #1 and the optical frequency comb signal #2 are combined to form an optical frequency comb signal #3, and the optical frequency comb signal #3 serves as a proper input signal to enter the gas detection module.
5. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 4, wherein:
the gas detection module comprises a reflector #D (1-62), a first objective lens (1-10), a second objective lens (1-14) and a gas detection pool integrating the plasmon enhancement substrate (1-13); the gas detection tank is a closed tank with light-transmitting upper surface and lower surface and a pipeline for connecting a gas passage arranged on the side wall; the first objective lens (1-10) and the second objective lens (1-14) are respectively arranged above and below the gas detection pool; the output end of the beam combination module is connected with the input end of the gas detection module, namely:
the second objective (1-14) and the gas detection pool are respectively arranged on the emergent light paths of the optical frequency comb signals; the optical frequency comb signal #3 finally emitted by the beam splitting element # B (1-22) in the beam combining module is reflected by the reflector # D (1-62) and enters the first objective lens (1-10); the optical frequency comb signal #3 is taken as a proper input signal, enters the gas detection tank through the first objective lens (1-10), excites a Raman signal of a gas molecule to be detected, and enters the signal acquisition module through the second objective lens (1-14) after passing through the gas detection tank.
6. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 5, wherein:
the signal acquisition module consists of an optical filter (1-5), a photoelectric detector #F (1-82) and a data acquisition card (1-9); the Raman signal enters a photoelectric detector #F (1-82) through an optical filter (1-5), and the photoelectric detector #F (1-82) feeds back the Raman signal to a data acquisition card (1-9); the second harmonic signal in the signal triggering module is fed back to the data acquisition card (1-9); when the data acquisition cards (1-9) receive the trigger instruction sent by the signal trigger module, data acquisition is started.
7. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 6, wherein: the side wall of the gas detection tank is respectively provided with a gas flow passage inlet (1-11) and a gas flow passage outlet (1-12); the plasmon enhancement substrates (1-13) are arranged inside the gas detection cell.
8. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 7, wherein:
the plasmon enhancement substrate (1-13) comprises a substrate and a plasmon enhancement structure on the substrate;
the substrate has high light transmittance in visible light and near infrared bands;
the plasmon enhancement structure has a sub-nanometer gap, the gap being less than 30nm; the material of the plasmon enhanced structure is metal capable of realizing plasmon resonance.
9. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 8, wherein:
the material of the plasmon enhancement structure is a material supporting the excitation of the plasmon in the corresponding wave band; the substrate material is a material with light transmittance exceeding 90% in the wavelength range of anti-Stokes, pumping and Stokes; the nonlinear crystal material is selected from materials with excellent nonlinear response to the wavelength of the optical fiber optical comb light source.
10. The dual optical comb surface-enhanced coherent anti-stokes raman spectroscopy gas detection system of claim 9, wherein:
the plasmon enhancement substrates (1-13) have high scattering in the anti-stokes optical band and stokes optical band; there is a strong near field enhancement at the pump light, stokes light and anti-stokes light.
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