Air laser enhancement and auxiliary single-beam coherent Raman spectrum detection device and method based on cascade amplification

Abstract

The invention discloses an air laser enhancement and auxiliary single-beam coherent Raman spectrum detection device and method based on cascade amplification. In the device, enhancement of air laser is realized in low-pressure nitrogen through a multi-stage focusing system, and femtosecond pump light is broadened by utilizing a nonlinear effect. The broadened pump light can establish vibration and rotation coherence of various molecules (such as N2, O2, CO2, SF6, O3 and the like), natural delay exists between the enhanced air laser and the pump light, and the enhanced air laser serves as probe light in a coherent Raman scattering process and acts with coherently excited target molecules to generate Raman signals. The optical filter combination and the diaphragm are used for frequency spectrum and space filtering, and high-sensitivity detection or temperature measurement of gas molecules is achieved. The invention has the characteristics of simple device and suitability for complex environment measurement, and can be used for component detection or temperature measurement of atmosphere or combustion environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of nonlinear ultrafast spectroscopy, and in particular relates to an air laser enhancement based on cascade amplification and its auxiliary single-beam coherent Raman spectroscopy detection device and method. Background Art

[0002] Ultra-intense, ultrashort lasers can excite atmospheric molecules to generate optical gain, creating "air lasers" with high intensity, excellent directivity, and long-distance transmission (see Air Lasing. Cham: Springer, 2018). Due to their unique advantages, such as cavity-free amplification, bidirectional transmission, and narrow linewidth, air lasers have great application potential in fields such as atmospheric remote sensing and combustion diagnostics. However, the low energy of air lasers currently generated by commercial femtosecond laser systems in the millijoule range limits the detection sensitivity and practical application of related spectroscopy techniques.

[0003] Coherent Raman spectroscopy plays a huge role in combustion diagnosis and other fields due to its advantages such as non-contact measurement, accurate temperature measurement, and simultaneous detection of temperature components (see Optics Letters, 2021, 46(7), 1688-1691; Combustion and Flame, 2022, 237, 111738; Optics Express, 2022, 30(20), 35232-35245). However, traditional coherent Raman spectroscopy detection schemes are usually implemented by multiple laser beams of different wavelengths, which requires spatial overlap of multiple beams and delay control of multiple optical paths (see Progress in Energy and Combustion Science, 2010, 36(2), 280-306; Plasma Sources Science and Technology, 2017, 26(10), 103001). This multi-beam design is difficult to operate stably in extreme environments (such as high temperature, high pressure or turbulent combustion fields), which restricts the widespread application of this technology. In addition, some existing technologies fail to fully utilize the nonlinear effects of femtosecond lasers, resulting in limited spectral broadening and difficulty in simultaneously exciting the vibrational coherence of multiple molecules. The generation and spatiotemporal control of picosecond detection light also face technical bottlenecks, affecting the sensitivity and accuracy of detection. Summary of the Invention

[0004] To address the problems of low energy in existing air lasers, complex optical paths in traditional coherent Raman spectroscopy, and limited detection sensitivity, the present invention proposes a device and method for air laser enhancement based on cascade amplification and its auxiliary single-beam coherent Raman spectroscopy detection. The femtosecond laser is repeatedly cascade-focused in low-pressure nitrogen. The air laser generated by the previous focusing can be used as seed light and further amplified in the next focusing process. This fully utilizes the gain generated by the ionization of a single femtosecond laser beam in nitrogen, achieving saturation amplification of the air laser. Simultaneously, the spectrum of the femtosecond pump light is broadened during the multiple focusing and ionization processes, enabling the establishment of vibrational and rotational coherence for a variety of molecules (such as N2, O2, CO2, SF6, O3, etc.). Using the spectrally broadened pump light and the multi-focused air laser as the light source, combined with hybrid femtosecond / picosecond coherent Raman spectroscopy technology, a device and method for air laser enhancement based on cascade multi-pass amplification and its auxiliary single-beam coherent Raman spectroscopy detection is proposed, achieving highly sensitive detection of gas molecules or temperature measurement. The present invention utilizes the air laser enhancement and femtosecond laser nonlinear spectrum broadening effect in the cascade focusing process, has the characteristics of simple device and suitability for complex environment measurement, and can be used for component detection or temperature measurement in the atmosphere or combustion environment.

[0005] The technical solutions of the present invention are as follows:

[0006] On the one hand, the present invention provides a device for air laser enhancement and auxiliary single-beam coherent Raman spectroscopy detection based on cascade amplification, which is characterized by including:

[0007] A light source module, a light source module for generating femtosecond pulse laser;

[0008] The air laser generation and cascade amplification module is used to receive the femtosecond pulse laser, shrink (or expand) the beam, and then cascade focus multiple times into filaments in low-pressure nitrogen to ionize nitrogen molecules to generate nitrogen molecular ions. and Population inversion and optical gain are formed between these two energy levels, generating air lasers with a duration of picoseconds, and enhancing the air lasers through the seed amplification effect during multiple cascade focusing processes. represents the vibrational ground state of the second excited state of the nitrogen molecular ion, represents the v=1 vibrational state of the ground state of the nitrogen molecular ion. At the same time, during multiple cascade focusing processes, the femtosecond pump light accumulates nonlinear effects such as self-phase modulation, self-steepening, and strong field ionization, causing spectrum broadening and blue shifting. The enhanced air laser and the spectrally broadened femtosecond pump light naturally overlap in space, with a picosecond time delay, forming a femtosecond-picosecond hybrid beam.

[0009] A coherent Raman scattering signal generating module is used to receive the femtosecond-picosecond mixed light and focus it at a specific position. The spectrally broadened femtosecond pump light can excite the vibration or rotation coherence of molecules in the measurement area; the air laser acts as a probe light and interacts with the coherently excited molecules to generate coherent vibration or rotation Raman scattering signals;

[0010] A spectral signal conversion module is used to receive the coherent Raman scattering signal and convert it into a spectral signal after performing collimation and filtering processing;

[0011] A femtosecond pump light time domain measurement module is used to receive the spectrally broadened femtosecond pump light and measure its electric field time domain waveform based on a frequency resolved optical gating (FROG) method;

[0012] The signal processing module stores the coherent Raman spectra of target molecules (such as N2, O2, CO2, etc.) calculated according to the nonlinear optical model, and determines the target molecules based on the Raman frequency shift. After obtaining the time domain waveform of the femtosecond pump light based on the FROG signal, the received Raman spectrum signal is fitted to determine the temperature.

[0013] Preferably, the light source module includes a femtosecond laser, the air laser generation and cascade amplification module includes a concave mirror and a convex mirror coated with an enhanced silver film, multiple pairs of concave mirrors coated with a protective silver film, and an air cavity filled with low-pressure nitrogen (<100 mbar), the coherent Raman scattering signal generation module includes a filter with high transmittance in the 428nm and 500-900nm bands, a baffle, a plane mirror coated with a protective silver film and an off-axis parabolic mirror, and a gas environment or combustion field with different gases or temperatures, the spectral signal conversion module includes a collimating lens, a dichroic mirror, an adjustable aperture, a short-pass filter, a band-pass filter, a long-pass filter with a steep edge, a plane mirror coated with an ultraviolet-enhanced aluminum film, a collecting lens and a grating spectrometer; the femtosecond pump light time domain measurement module includes a frequency-resolving optical switching device; and the signal processing module is a computer.

[0014] The femtosecond laser outputs a linearly polarized femtosecond laser, which is beam-contracted by the concave mirror and convex mirror coated with an enhanced silver film (or the concave mirror and convex mirror are adjusted to expand the beam in sequence), and then is repeatedly focused into filaments by multiple pairs of concave mirrors coated with a protective silver film in an air cavity filled with low-pressure nitrogen (<100 mbar), generating and enhancing air laser with a wavelength of 427.8 nm. During the multiple focusing processes, the femtosecond pump light accumulates nonlinear effects such as self-phase modulation, self-steepening, and strong field ionization, causing spectrum broadening and blue shifting. The air laser and the femtosecond pump light naturally overlap in space, and after passing through a filter, the supercontinuum extending to below 500 nm is reflected to a baffle. The air laser and most of the femtosecond pump light with a wavelength greater than 500 nm are reflected by a plane reflector coated with a protective silver film and focused by an off-axis parabolic mirror into a gas environment or combustion field with different gases or temperatures. The spectrally broadened femtosecond pump light can excite the vibration or rotation coherence of molecules in the measurement area. The air laser serves as the detection light and interacts with the coherently excited molecules to generate coherent vibration or rotation Raman scattering signals. The signal light is sequentially passed through the collimating lens, the dichroic mirror, the adjustable aperture, the short-pass filter, the band-pass filter, the steep-edge long-pass filter, the plane reflector coated with the ultraviolet-enhanced aluminum film, and the collecting lens, and then focused and incident on the grating spectrometer to be converted into a spectral signal. The femtosecond pump light is reflected by the dichroic mirror to the frequency-resolving optical switch device to be measured to obtain a FROG signal. The coherent Raman scattering signal and the FROG signal are received by a computer, the spectral signal is processed according to a nonlinear optical model, the molecular type is determined according to the Raman frequency shift, the femtosecond pulse time domain waveform is obtained according to the FROG signal, and the ambient temperature is determined according to the Raman spectrum fitting result.

[0015] The concave and convex mirrors are coated with an enhanced silver film. A femtosecond pulse laser beam is contracted when it passes through the concave mirror first and then the convex mirror, while it is expanded when it passes through the convex mirror first and then the concave mirror. The distance between the concave and convex mirrors is approximately equal to the absolute value of the difference in their focal lengths. By contracting or expanding the beam, different focused light intensities can be achieved without changing the focal lengths of subsequent pairs of concave mirrors, optimizing the generation and enhancement of air laser light.

[0016] The surfaces of the multiple pairs of concave mirrors are coated with a protective silver film, which has high reflectivity in the 400-1000nm band. The concave mirror on one side is used to focus the light beam, and the concave mirror on the other side is used to collimate the light beam. The distance between the corresponding focusing and collimating concave mirrors is slightly smaller than the sum of their focal lengths to reduce the collimation deviation caused by the forward focus shift during the femtosecond laser filamentation process. The concave mirrors on each side are arranged in a front-to-back staggered manner to achieve a compact layout and a smaller concave mirror reflection angle, thereby reducing aberrations.

[0017] The filter has high transmittance in the 428nm and 500-900nm bands, can suppress supercontinuum that broadens to a wavelength below 500nm, reduce interference with coherent Raman signals, and achieve higher detection signal-to-noise ratio and sensitivity.

[0018] The dichroic mirror has an incident angle of 45°, which can reflect signals with wavelengths above 490nm to the frequency-resolving optical switch device, while signals with wavelengths below 490nm have high transmittance.

[0019] The adjustable diaphragm can block part of the supercontinuum background that is focused by the off-axis parabolic mirror and then transmitted through the dichroic mirror by reducing the clear aperture; the short-pass filter and the band-pass filter can reduce the transmittance of the supercontinuum and air laser by adjusting the incident angle, thereby achieving higher detection sensitivity when detecting coherent Raman signals; the long-pass filter with steep edges is used to reduce the transmittance of coherent Raman signals such as oxygen, which have larger Raman frequency shifts and stronger signals due to higher environmental concentrations, and is used to improve the detection sensitivity of low-concentration target molecules (such as CO2 and SF6).

[0020] On the other hand, the present invention also provides a method for single-beam coherent Raman spectroscopy detection based on cascade amplification of air laser enhancement and its auxiliary, which is characterized by comprising:

[0021] After the femtosecond pump light is beam-contracted or beam-expanded by a combination of a concave mirror and a convex mirror coated with an enhanced silver film, the femtosecond pump light is repeatedly focused and collimated in low-pressure nitrogen using multiple sets of concave mirrors coated with a protective silver film to ionize nitrogen molecules to produce nitrogen molecular ions. and Population inversion and optical gain are formed between these two energy levels. The air laser generated by the previous focusing can serve as a seed to enhance the air laser. In the process of multiple focused ionizations, the spectrum of the femtosecond pump light will be broadened and blue-shifted due to the accumulation of nonlinear effects such as self-phase modulation, self-steepening, and strong field ionization. The duration of the enhanced air laser is in the order of picoseconds, and it naturally overlaps with the spectrally broadened femtosecond pump light in space, with a time delay of the order of picoseconds in time, forming a beam of femtosecond-picosecond mixed light.

[0022] Spectrally broadened femtosecond pump light can effectively stimulate the Raman coherence of various molecules;

[0023] The enhanced air laser is used as a probe light to interact with the coherently excited molecules to generate coherent vibrational or rotational Raman scattering signals;

[0024] The time domain waveform of the femtosecond pump light is measured based on the frequency-resolved optical switching method, and the coherent Raman scattering signals of different target molecules are calculated in combination with the nonlinear optical model. The molecular species are determined based on the Raman frequency shift; and the Raman spectral signal is fitted to determine the temperature.

[0025] The method for measuring temperature using the above device comprises the following steps:

[0026] 1) Turn on the femtosecond laser to output femtosecond pulse laser;

[0027] 2) The femtosecond pulsed light laser is beam-contracted by a concave mirror and then a convex mirror (or beam-expanded by a convex mirror and then a concave mirror). The distance between the concave mirror and the convex mirror is approximately equal to the absolute value of the difference in their focal lengths. The distance between the two mirrors is adjusted according to the change in the spot size of the femtosecond laser after beam contraction or expansion with the transmission distance, so that the femtosecond laser spot remains unchanged during transmission.

[0028] 3) After the femtosecond laser is focused by a concave mirror on one side, it is collimated by a concave mirror on the other side. The distance between the focusing and collimating concave mirrors is slightly less than the sum of their focal lengths. The distance between the two is adjusted according to the change in the size of the collimated femtosecond laser spot with the transmission distance, so that the femtosecond laser spot remains unchanged during transmission. For the concave mirror pair in the second focusing and collimation process, as well as the third concave mirror pair, the distance between the focusing and collimating concave mirrors is determined using the same method.

[0029] 4) After the air cavity is evacuated by a mechanical pump, it is filled with low-pressure high-purity nitrogen; the femtosecond laser is focused multiple times in the low-pressure nitrogen to broaden the spectrum, and at the same time, the nitrogen molecules are ionized to produce nitrogen molecular ions. and A population inversion is formed between the two energy levels, generating an air laser that transmits in the same direction as the femtosecond laser. The air laser signal is continuously enhanced during the step-by-step focusing process.

[0030] 5) After passing through the filter, the air laser and the remaining femtosecond pump light are reflected by a plane mirror onto an off-axis parabolic mirror and then focused onto the measurement area, stimulating the molecular Raman coherence and generating vibrational or rotational Raman signals; part of the supercontinuum is reflected by the filter onto the baffle;

[0031] 6) The output femtosecond laser, air laser and the generated molecular coherent Raman signal are transmitted at a certain divergence angle and collimated by a collimating lens;

[0032] 7) Insert a dichroic mirror, an adjustable aperture, a short-pass filter, a band-pass filter, and a long-pass filter with a steep edge after the collimating lens to obtain a coherent Raman signal with a high signal-to-noise ratio of the target molecule;

[0033] 8) The coherent Raman signal is reflected by a plane mirror coated with a UV-enhanced aluminum film, and then focused by a collecting lens into the slit of a grating spectrometer, which collects and converts the spectral signals.

[0034] 9) The output femtosecond pump light is reflected by the dichroic mirror and enters the frequency-resolving optical switch device to collect the different delays.

[0035] The signal is imported into the computer to calculate the time domain waveform of the femtosecond pump light;

[0036] 10) The collected spectral signal is imported into a computer and, combined with the time-domain waveform of the femtosecond pump light, the coherent Raman spectrum of the target molecule is calculated using a nonlinear optical model. The target molecule type is determined by the Raman frequency shift. The temperature of the measurement point is determined based on the temperature parameter obtained by fitting the experimentally measured coherent Raman signal of the target molecule.

[0037] Compared with the prior art, the technical effects of the present invention are as follows:

[0038] 1) The present invention uses multiple sets of concave mirrors to focus the femtosecond pump light multiple times in low-pressure nitrogen. The air laser generated by the previous focusing can be used as seed light and further amplified in the next focusing process. In this way, the gain generated by the ionization of a femtosecond laser in nitrogen can be fully utilized to achieve saturation amplification of the air laser. The device of the present invention is simple. Under the condition of 6mJ femtosecond laser pumping, the output energy of the air laser can be increased by 1 compared with single focusing.

[0039] More than an order of magnitude.

[0040] 2) The spectrum of femtosecond pump light is broadened during the process of multiple focused ionization. Compared with the femtosecond pulse laser directly output by the femtosecond laser, the broadened pump light can establish a variety of molecules (such as N2, O2, CO2, SF6, O3

[0041] Vibrational coherence of .

[0042] 3) During the cascade focusing process, the air laser and the spectrally broadened femtosecond pump light naturally overlap in space, resulting in a temporal delay of picoseconds. Simultaneously, the enhanced air laser light lasts on the order of picoseconds and has a narrow linewidth. This approach addresses the challenges of generating picosecond probe light and precisely controlling its spatial and temporal relationship with the femtosecond pump light during hybrid femtosecond / picosecond coherent Raman temperature measurement, while also improving detection sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a schematic diagram of an air laser enhancement and auxiliary single-beam coherent Raman spectroscopy detection device based on cascade amplification.

[0044] Among them: 1 is a femtosecond laser, 2 is a concave mirror coated with an enhanced silver film, 3 is a convex mirror coated with an enhanced silver film, 4-9 are concave mirrors coated with a protective silver film, 10 is an air cavity filled with low-pressure nitrogen, 11 is a filter with high transmittance in the 428nm and 500-900nm bands, 12 is a baffle, 13 is a plane mirror coated with a protective silver film, 14 is an off-axis parabolic mirror coated with a protective silver film, 15 is a collimating lens, 16 is a dichroic mirror, 17 is an adjustable aperture, 18 is a short-pass filter, 19 is a band-pass filter, 20 is a long-pass filter with steep edges, 21 is a plane mirror coated with a UV-enhanced aluminum film, 22 is a collecting lens, 23 is a grating spectrometer, 24 is a frequency-resolving optical switching device, and 25 is a computer. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments, but this should not limit the scope of protection of the present invention.

[0046] The core components and optical path arrangement of the present invention are as follows: Figure 1 As shown, it specifically includes the following modules:

[0047] 1. Light source module

[0048] Femtosecond laser 1: outputs linearly polarized femtosecond laser with a central wavelength of 800 nm, a pulse width of 40 fs, a single pulse energy of 6 mJ, and a repetition rate of 1 kHz.

[0049] 2. Air laser generation and cascade amplification module

[0050] Beam reduction / expansion system: It consists of a silver-coated concave mirror 2 with a focal length of 1000mm and a silver-coated convex mirror 3 with a focal length of -750mm. By adjusting the order and spacing between the two (250mm, the absolute value of the focal length difference), the beam diameter can be flexibly controlled (beam reduction to 3 / 4 or beam expansion to 4 / 3).

[0051] Multiple pairs of concave mirrors 4-9 are coated with a protective silver film, resulting in high reflectivity in the 400-1000nm wavelength range. Concave mirrors 4, 6, and 8 focus the light beam, while concave mirrors 5, 7, and 9 collimate it. These mirrors are arranged in a staggered arrangement, which not only achieves a compact layout but also reduces the mirror's reflection angle, minimizing aberrations and improving beam transmission quality. The six silver-coated concave mirrors 4-9 form a three-stage focusing and collimation system within an air chamber 10 filled with low-pressure nitrogen (20 mbar). This helps the femtosecond laser fully function within the low-pressure nitrogen, generating and amplifying air laser light.

[0052] The air cavity 10 is filled with low-pressure nitrogen. The spectrum of the femtosecond laser is broadened after multiple focusing by multiple pairs of concave mirrors in the low-pressure nitrogen. At the same time, the nitrogen molecules are ionized to produce nitrogen molecular ions, forming a population inversion and generating air laser that transmits in the same direction as the femtosecond laser. The air laser signal is continuously enhanced during the step-by-step focusing process.

[0053] 3. Coherent Raman scattering signal generation module

[0054] Filter 11: It has high transmittance in the 428nm and 500-900nm bands, which can suppress the supercontinuum that broadens to a wavelength below 500nm, reduce interference with coherent Raman signals, and improve detection signal-to-noise ratio and sensitivity.

[0055] Baffle 12: Part of the supercontinuum is reflected by the filter onto the baffle, which blocks the interference signal.

[0056] Plane mirror 13: reflects the air laser light and the remaining femtosecond pump light that have passed through the filter to the off-axis parabolic mirror.

[0057] The off-axis parabolic mirror 14 focuses the reflected light into the flame or gas, stimulates the Raman coherence of different molecules, and generates coherent Raman signals.

[0058] 4. Spectral signal conversion module

[0059] Collimating lens 15: collimates the output femtosecond laser, air laser and generated molecular coherent Raman signal so that they are transmitted in a more regular beam form.

[0060] The dichroic mirror 16 has an incident angle of 45°, and reflects the signals with wavelengths above 490 nm to the frequency-resolving optical switch device 24 , while the signals with wavelengths below 490 nm have high transmittance, thereby achieving effective signal separation.

[0061] Adjustable aperture 17: By reducing the clear aperture, it blocks part of the supercontinuum that is focused by the off-axis parabolic mirror and then transmitted through the dichroic mirror, further purifying the signal.

[0062] Short-pass filter 18 and band-pass filter 19: by adjusting the incident angle, the transmittance of supercontinuum and air laser is reduced, thereby improving the detection sensitivity when detecting coherent Raman signals.

[0063] The long-pass filter 20 with steep edges is used to reduce the transmittance of coherent Raman signals of oxygen and other substances that have larger Raman frequency shifts and stronger signals due to higher ambient concentrations, thereby improving the detection sensitivity of low-concentration target molecules (such as CO2 and SF6).

[0064] Plane mirror 21 coated with UV-enhanced aluminum film: reflects the coherent Raman signal to the collection lens.

[0065] Collecting lens 22: focuses the reflected coherent Raman signal into the slit of the grating spectrometer.

[0066] Grating spectrometer 23: collects and converts spectral signals.

[0067] 5. Femtosecond pump light time domain measurement module

[0068] The frequency-resolved optical switching device 24 is used to receive the spectrally broadened femtosecond pump light and measure its electric field time-domain waveform based on the frequency-resolved optical gating (FROG) method.

[0069] 6. Signal processing module,

[0070] Computer 25: stores the coherent Raman spectra of target molecules (such as N2, O2, CO2, etc.) calculated according to the nonlinear optical model, identifies the target molecules based on the Raman frequency shift; after obtaining the time domain waveform of the femtosecond pump light based on the FROG signal, fits the received Raman spectrum signal to determine the temperature.

[0071] The femtosecond laser 1 outputs a linearly polarized femtosecond laser, which is beam-contracted by the concave mirror 2 and convex mirror 3 coated with an enhanced silver film (or the concave mirror and convex mirror are adjusted to expand the beam in sequence), and then is focused into filaments multiple times by the concave mirrors 4-9 coated with a protective silver film in an air cavity 10 filled with low-pressure nitrogen (<100mbar), thereby generating an air laser with a wavelength of 427.8nm and enhancing it. During the multiple focusing processes, the femtosecond pump light accumulates due to nonlinear effects such as self-phase modulation, self-steepening, and strong field ionization, and its spectrum is broadened and blue-shifted. The air laser and the femtosecond pump light naturally overlap in space, and after passing through the filter 11, the supercontinuum extended to below 500nm is reflected to the baffle 12. The air laser and most of the femtosecond pump light with a wavelength greater than 500nm are reflected by the plane reflector 13 coated with a protective silver film and focused by the off-axis parabolic mirror 14 into a gas environment or combustion field with different gases or temperatures, and the femtosecond pump light with broadened spectrum is emitted. The pump light can excite the vibrational or rotational coherence of molecules in the measurement area. The air laser serves as the probe light, interacting with the coherently excited molecules to generate coherent vibrational or rotational Raman scattering signals. The signal light passes through the collimating lens 15, the dichroic mirror 16, the adjustable aperture 17, the short-pass filter 18, the band-pass filter 19, the steep-edge long-pass filter 20, the plane reflector 21 coated with a UV-enhanced aluminum film, and the collecting lens 22 in sequence, and is then focused and incident on the grating spectrometer 23 to be converted into a spectral signal. The femtosecond pump light is reflected by the dichroic mirror 16 to the frequency-resolving optical switch device 24, where it is measured to obtain a FROG signal. The coherent Raman scattering signal and the FROG signal are received by a computer 25, which processes the spectral signals according to a nonlinear optical model, determines the molecular species based on the Raman frequency shift, obtains the femtosecond pulse time-domain waveform based on the FROG signal, and then determines the ambient temperature based on the Raman spectrum fitting results.

[0072] Application Example: Procedure for measuring combustion products and temperature of a standard flat combustion flame:

[0073] 1) Place the flat flame burner near the focus of the off-axis parabolic reflector, turn on the water cooling cycle inside the burner, input combustible gas (such as methane or hydrogen) and synthetic air in proportion through the gas flow meter, and ignite the flame through the electric spark igniter;

[0074] 2) Turn on the femtosecond laser to output femtosecond pulse laser;

[0075] 3) The femtosecond pulsed light laser is beam-contracted by a concave mirror and a convex mirror (or beam-expanded by a convex mirror and a concave mirror).

[0076] 4) After the air cavity is evacuated by a mechanical pump, it is filled with low-pressure high-purity nitrogen; the femtosecond laser is focused multiple times by multiple pairs of concave mirrors in the low-pressure nitrogen, and the spectrum is broadened. At the same time, the nitrogen molecules are ionized to produce nitrogen molecular ions. and A population inversion is formed between the two energy levels, generating an air laser that transmits in the same direction as the femtosecond laser. The air laser signal is continuously enhanced during the step-by-step focusing process.

[0077] 5) After passing through the filter, the air laser and the remaining femtosecond pump light are reflected by a plane mirror onto an off-axis parabolic mirror and then focused onto the flame, stimulating the Raman coherence of different molecules in the flame (such as CO2, O2, N2, etc.) and generating vibrational Raman signals; part of the supercontinuum is reflected by the filter onto the baffle;

[0078] 6) The output femtosecond laser, air laser and the generated molecular coherent Raman signal are transmitted at a certain divergence angle and collimated by a collimating lens;

[0079] 7) Insert a dichroic mirror, an adjustable aperture, a short-pass filter, a band-pass filter, and a long-pass filter with a steep edge after the collimating lens to obtain a coherent Raman signal with a high signal-to-noise ratio of the target molecule;

[0080] 8) The coherent Raman signal is reflected by a plane mirror coated with a UV-enhanced aluminum film, and then focused by a collecting lens into the slit of a grating spectrometer, which collects and converts the spectral signals.

[0081] 9) The output femtosecond pump light is reflected by the dichroic mirror and enters the frequency-resolving optical switch device. The signals with different delays are collected and imported into the computer to calculate the time domain waveform of the femtosecond pump light;

[0082] 10) The collected Raman spectral signal is imported into a computer and, combined with the time-domain waveform of the femtosecond pump light, the coherent Raman spectrum of the target molecule is calculated using a nonlinear optical model. The target molecule type is determined by the Raman frequency shift. The temperature of the measurement point is determined based on the temperature parameter obtained by fitting the experimentally measured coherent Raman signal of the target molecule.

Claims

1. A single-beam coherent Raman spectroscopy detection device based on cascade amplification of air laser enhancement and its auxiliary, characterized in that: include: A light source module for generating femtosecond pulse laser; An air laser generation and cascade amplification module is used to receive the femtosecond pulse laser and achieve air laser enhancement and femtosecond pump light spectrum broadening through multi-stage focusing in low-pressure nitrogen; The coherent Raman scattering signal generation module uses spectrally broadened femtosecond pump light to stimulate molecular coherence and uses enhanced air laser as detection light to generate coherent Raman signals. A spectral signal conversion module is used to receive the coherent Raman scattering signal and convert it into a spectral signal after performing collimation and filtering processing; a femtosecond pump light time domain measurement module, configured to receive the spectrally broadened femtosecond pump light and measure its electric field time domain waveform based on frequency resolved optical switching (FROG) technology; The signal processing module is used to identify molecular species based on Raman frequency shift and determine the ambient temperature through spectral fitting.

2. The air laser enhancement and auxiliary single-beam coherent Raman spectroscopy detection device based on cascade amplification according to claim 1 is characterized in that: The air laser generation and cascade amplification module includes: At least two sets of silver-coated concave mirrors for achieving multiple focusing of the femtosecond laser; A gas chamber filled with nitrogen at a pressure lower than 100 mbar; The air laser generated by the previous stage of focusing serves as the seed light for the next stage, realizing cascade amplification.

3. The air laser-enhanced and assisted single-beam coherent Raman spectroscopy detection device based on cascade amplification according to claim 2, characterized in that: The concave mirrors are arranged in a staggered layout to reduce aberrations and achieve a compact optical path structure. At the same time, the distance between the focusing concave mirror and the corresponding collimating concave mirror is slightly smaller than the sum of their focal lengths to reduce the collimation deviation caused by the forward focus during the femtosecond laser filamentation process.

4. The air laser enhancement and auxiliary single-beam coherent Raman spectroscopy detection device based on cascade amplification according to claim 1 is characterized in that: The coherent Raman scattering signal generation module includes a filter with high transmittance in the 428nm and 500-900nm bands, a baffle, a plane reflector coated with a protective silver film, an off-axis parabolic mirror, and a gas environment or combustion field with different gases or temperatures; the spectral signal conversion module includes a collimating lens, a dichroic mirror, an adjustable aperture, a short-pass filter, a band-pass filter, a long-pass filter with a steep edge, a plane reflector coated with an ultraviolet-enhanced aluminum film, a collecting lens, and a grating spectrometer.

5. The air laser-enhanced and assisted single-beam coherent Raman spectroscopy detection device based on cascade amplification according to claim 4 is characterized in that: The short-pass filter has a cutoff wavelength of 450nm and is used to suppress supercontinuum background; the adjustable band-pass filter has a bandwidth of 10-20nm and is used to select the characteristic peaks of target molecules; the long-pass filter with steep edges has a cutoff edge that is adjustable according to the incident angle to suppress interference from short-wavelength signals.

6. The air laser-enhanced and assisted single-beam coherent Raman spectroscopy detection device based on cascade amplification according to claim 1 is characterized in that: The femtosecond pump light time domain measurement module adopts a frequency resolution optical switch device (24) to measure the time domain waveform of the femtosecond laser, and provides a pump light time domain reference for temperature calculation.

7. The air laser enhancement and auxiliary single-beam coherent Raman spectroscopy detection device based on cascade amplification according to any one of claims 1 to 6, characterized in that: The light source module includes a femtosecond laser, and the signal processing module is a computer. The femtosecond laser outputs a linearly polarized femtosecond laser, which is beam-contracted by the concave mirror and convex mirror coated with the enhanced silver film (or beam-expanded by adjusting the concave mirror and convex mirror in sequence), and then focused into filaments multiple times by the concave mirror coated with the protective silver film in an air cavity filled with low-pressure nitrogen (<100 mbar), thereby generating an air laser with a wavelength of 427.8 nm and enhancing the laser. During the multiple focusing processes, the spectrum of the femtosecond pump light will be broadened and blue-shifted due to the accumulation of nonlinear effects such as self-phase modulation, self-steepening, and strong field ionization; the air laser and the femtosecond pump light naturally overlap in space, and after passing through the filter, the supercontinuum extended to below 500nm is reflected to the baffle; the air laser and most of the femtosecond pump light with a wavelength greater than 500nm are reflected by a plane mirror coated with a protective silver film and focused by an off-axis parabolic mirror into a gas environment or combustion field with different gases or temperatures. The spectrally broadened femtosecond pump light can excite the vibration or rotation coherence of the molecules in the measurement area. The air laser acts as a detection light and interacts with the coherently excited molecules to generate coherent vibration or rotation Raman scattering signals; the signal light passes through the collimating lens, dichroic mirror, adjustable aperture, short-pass filter, etc. in sequence. After passing through a filter, a bandpass filter, a long-pass filter with a steep edge, a plane reflector coated with an ultraviolet-enhanced aluminum film, and a collecting lens, the light is focused and incident on a grating spectrometer to be converted into a spectral signal; the femtosecond pump light is reflected by a dichroic mirror to a frequency-resolving optical switch device to obtain a FROG signal; the coherent Raman scattering signal and the FROG signal are received by a computer, the spectral signal is processed according to a nonlinear optical model, the molecular type is determined according to the Raman frequency shift, the femtosecond pulse time domain waveform is obtained according to the FROG signal, and the ambient temperature is determined according to the Raman spectrum fitting result. The dichroic mirror has an incident angle of 45°, which can reflect signals with a wavelength above 490nm to the frequency-resolving optical switch device, while signals with a wavelength below 490nm have high transmittance.

8. An air laser enhanced coherent Raman spectroscopy detection method based on the detection device according to any one of claims 1 to 7, characterized in that: include: Air laser is generated and intensified in low-pressure nitrogen through multi-stage focusing; Broaden the spectrum of femtosecond pump light using nonlinear effects; Using spectrally broadened femtosecond light to excite molecular coherence and detecting it with air lasers to generate Raman signals; Filter and spectrally analyze Raman signals to achieve molecular recognition; Temperature measurement is achieved by combining FROG measurement results and Raman spectrum fitting.

9. A temperature measurement method based on the detection device according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: 1) Turn on the femtosecond laser to output femtosecond pulse laser; 2) The femtosecond pulsed light laser is sequentially beam-contracted by a concave mirror and a convex mirror (or beam-expanded by a convex mirror and a concave mirror). The distance between the concave mirror and the convex mirror is approximately equal to the absolute value of the difference in their focal lengths. The distance between the two mirrors is adjusted according to the change in the spot size of the femtosecond laser after beam contraction or expansion with the transmission distance, so that the femtosecond laser spot remains unchanged during transmission. 3) After the femtosecond laser is focused by a concave mirror, it is collimated by a concave mirror. The distance between the concave mirror and is slightly smaller than the sum of the focal lengths of the two. The distance between the two is adjusted according to the change of the femtosecond laser spot size after collimation with the transmission distance, so that the femtosecond laser spot remains unchanged during transmission; for the concave mirror in the second focusing and collimation process, and For the third concave mirror, the distance between the focusing and collimating concave mirrors is determined by the same method; 4) After the air cavity is evacuated by a mechanical pump, it is filled with low-pressure high-purity nitrogen; the femtosecond laser is focused multiple times in the low-pressure nitrogen to broaden the spectrum, and at the same time, the nitrogen molecules are ionized to produce nitrogen molecular ions. and A population inversion is formed between these two energy levels, generating an air laser that transmits in the same direction as the femtosecond laser. The air laser signal is continuously enhanced during the step-by-step focusing process; 5) After passing through the filter, the air laser and the remaining femtosecond pump light are reflected by a plane mirror onto an off-axis parabolic mirror and then focused onto the measurement area, stimulating the molecular Raman coherence and generating vibrational or rotational Raman signals; part of the supercontinuum is reflected by the filter onto the baffle; 6) The output femtosecond laser, air laser and the generated molecular coherent Raman signal are transmitted at a certain divergence angle and collimated by a collimating lens; 7) Insert a dichroic mirror, an adjustable aperture, a short-pass filter, a band-pass filter, and a long-pass filter with a steep edge after the collimating lens to obtain a coherent Raman signal with a high signal-to-noise ratio of the target molecule; 8) The coherent Raman signal is reflected by a plane mirror coated with a UV-enhanced aluminum film, and then focused by a collecting lens into the slit of a grating spectrometer, which collects and converts the spectral signals. 9) The output femtosecond pump light is reflected by the dichroic mirror and enters the frequency-resolving optical switch device. The signals with different delays are collected and imported into the computer to calculate the time domain waveform of the femtosecond pump light; 10) The collected spectral signal is imported into a computer and, combined with the time-domain waveform of the femtosecond pump light, the coherent Raman spectrum of the target molecule is calculated using a nonlinear optical model. The target molecule type is determined by the Raman frequency shift. The temperature of the measurement point is determined based on the temperature parameter obtained by fitting the experimentally measured coherent Raman signal of the target molecule.

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