A vibration / rotation temperature synchronous measurement device and method based on pure rotation Raman spectrum

Abstract

A device and method for synchronously measuring vibrational / rotational temperatures based on pure rotational Raman spectroscopy, relating to the field of optical diagnostics for non-equilibrium flow fields, is disclosed. A single-longitudinal-mode laser is used as the light source. Strong Rayleigh scattering signals are suppressed by a cascaded Bragg grating. Ultra-fine spectral dispersion is achieved using the orthogonal cross-dispersion of a virtual image phase array and a holographic grating. High-resolution pure rotational Raman spectra are acquired by an sCMOS camera. The spectra are then fitted using a genetic algorithm to directly obtain the vibrational particle number distribution factor and rotational temperature of each vibrational energy level, thereby obtaining the vibrational temperature or characterizing the non-equilibrium distribution of molecular internal energy. This method offers advantages such as high spectral resolution, good signal-to-noise ratio, and strong applicability, meeting the high-precision temperature measurement requirements of complex non-equilibrium flow fields.

Description

Technical Field

[0001] This invention relates to the field of optical diagnostic technology for non-equilibrium flow fields, specifically to a device and method for synchronously measuring vibration / rotation temperature based on pure rotational Raman spectroscopy. Background Technology

[0002] In various non-equilibrium flow field environments such as electric arc wind tunnel plasma, shock wave wind tunnel, and electric arc heater, the vibrational and rotational energy of gas molecules are often in a non-equilibrium distribution state. Accurately measuring the vibrational and rotational temperatures of gas molecules is crucial for a deeper understanding of the non-equilibrium distribution of energy within the molecules of the flow field and for verifying theoretical models.

[0003] Pure Rotational Raman Spectroscopy (PRRS) is a typical method for flow field temperature measurement. It boasts extremely high measurement sensitivity and is an effective technique for high-precision flow field temperature measurement. However, in non-equilibrium flow fields, the vibrational and rotational temperatures of molecules are not equal, requiring the temperature measurement technique to simultaneously acquire both vibrational and rotational temperatures. This presents a new challenge to the application of PRRS temperature measurement technology.

[0004] According to molecular spectroscopy theory, rotational temperature is related to the relative intensity of Raman transitions at different rotational energy levels within the same vibrational energy level; vibrational temperature is related to the relative intensity of Raman transitions at the same rotational energy level within different vibrational energy levels. However, due to the existence of molecular vibrational-rotational coupling effects, the spectral lines of the same rotational transition at different vibrational energy levels are... Only minute displacements exist, and the displacements between adjacent vibrational energy levels... Approximately equal. Taking the commonly used probe molecule N2 as an example, when the rotational quantum number... hour, ; hour, ; hour, Therefore, to simultaneously extract the vibrational and rotational temperatures of molecules from pure rotational Raman spectra, high-resolution spectroscopic measurements are required, with a system spectral resolution of 0.1 cm⁻¹. -1 ~0.5cm -1 Magnitude.

[0005] Traditional grating spectrometers typically have a resolution greater than 1 cm⁻¹. -1 It is difficult to achieve sub-wavelength resolution, and it cannot distinguish the spectral line splitting caused by vibration-rotation coupling effect, thus losing key vibrational energy level distribution information. It is also impossible to accurately infer the vibrational temperature and internal energy non-equilibrium distribution of molecules in high-temperature non-equilibrium flow fields, making it difficult to meet the measurement requirements of pure rotational Raman spectra with high spectral resolution in complex high-temperature non-equilibrium flow fields. Summary of the Invention

[0006] To address the shortcomings of the prior art, this invention provides a device and method for synchronously measuring vibrational and rotational temperatures based on pure rotational Raman spectroscopy. It achieves ultra-fine spectral dispersion through orthogonal cross-dispersion technology, acquires high-resolution, high-signal-to-noise-ratio pure rotational Raman spectra, and simultaneously realizes accurate measurement of molecular vibrational and rotational temperatures in a non-equilibrium flow field, which can accurately characterize the non-equilibrium distribution state of molecular internal energy.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A vibration / rotation temperature synchronization measurement device based on pure rotational Raman spectroscopy, comprising, in sequence along the optical path propagation direction, a light source, an achromatic lens, a Rayleigh signal suppression unit, a cylindrical lens, a virtual image phase array, a grating, and a detection unit;

[0009] The light source is a single-longitudinal-mode 532nm laser, which is used to emit a laser beam into the flow field test area to excite the probe molecules in the flow field to generate Raman scattering signals.

[0010] The achromatic lens is positioned perpendicular to the scattering direction of the incident laser beam to collect the pure rotational Raman signal generated by the probe molecules in the flow field test area and collimate the scattered light into a parallel beam output.

[0011] The Rayleigh signal suppression unit consists of two cascaded volume Bragg gratings, used to filter out and suppress 532nm Rayleigh scattered light, while transmitting pure rotational Raman signals.

[0012] The cylindrical lens is disposed on the light-emitting side of the Rayleigh signal suppression unit and is used to focus the Raman signal beam after Rayleigh signal suppression and couple it to the virtual image phase array.

[0013] The virtual image phase array is used to perform dispersive beam splitting of the incident pure rotational Raman signal in the vertical direction;

[0014] The grating is a holographic grating, whose dispersion direction is orthogonal to the dispersion direction of the virtual image phase array. It is used to perform secondary dispersion of the beam emitted from the virtual image phase array in the horizontal direction and separate the overlapping interference orders generated by the virtual image phase array.

[0015] The detection unit employs an sCMOS camera and a plano-convex lens. The plano-convex lens is used to focus the Raman signal after two-dimensional dispersion onto the detection surface of the sCMOS camera. The sCMOS camera is used to acquire and record pure rotational Raman spectra.

[0016] Furthermore, the Rayleigh signal suppression unit requires an optical density OD > 8 to suppress 532nm Rayleigh scattering light.

[0017] Furthermore, the free spectral region of the virtual image phase array is 30 GHz, and the theoretical spectral resolution is <0.05 cm⁻¹. -1 .

[0018] Furthermore, the grating has a line count of 2400 lines / mm, an effective width of 10mm, and a spectral resolution smaller than the free spectral region of the virtual image phase array.

[0019] Furthermore, in the detection unit, the sCMOS camera has a pixel size of 6.5μm, a resolution of 2048×2048, and a plano-convex lens with a focal length of 300mm.

[0020] A method for simultaneous measurement of vibration / rotation temperature based on pure rotational Raman spectroscopy includes the following steps:

[0021] Step 1: Build a measurement device according to the optical path structure, and adjust the attitude and position of each optical element so that the sCMOS camera can acquire the pure rotational Raman signal after two-dimensional dispersion.

[0022] Step 2: Gradually increase the emitted laser energy of the light source until the laser beam just barely does not break down in air at room temperature and pressure;

[0023] Step 3: Set the exposure time of the sCMOS camera and acquire the pure rotational Raman spectrum of air at room temperature and pressure, so that the sCMOS camera can clearly record the rotational quantum number. The spectral lines corresponding to the high rotational energy levels;

[0024] Step 4: Increase the emitted laser energy of the light source to the maximum value and collect the pure rotational Raman spectrum of the probe molecules in the non-equilibrium flow field under the preset working conditions;

[0025] Step 5: Based on the pre-calibrated wavelength-pixel mapping relationship, the Raman signal that is dispersed in two spatial dimensions is mapped to the wavelength dimension to obtain the pure rotational Raman spectrum of the final flow field molecules;

[0026] Step Six: Layout with Vibrational Particle Number Factor and rotation temperature To fit the parameters, a genetic algorithm was used to fit the pure rotational Raman spectrum to obtain the rotational temperature under the corresponding operating conditions. Vibrational particle number distribution factor of each vibrational energy level ;

[0027] Step 7: Determine the vibrational particle number distribution factor Does it satisfy the Boltzmann distribution? If so, the vibration temperature can be calculated. If the conditions are not met, then the particle number layout factor of the vibrational dynamics will be used. Characterizes the internal energy distribution of molecules in a flow field.

[0028] Furthermore, in step seven, if With vibrational quantum number If the correlation is linear, it is assumed to satisfy the Boltzmann distribution, and the vibration temperature can be calculated by fitting the Boltzmann distribution expression. Conversely, it is considered a non-Boltzmann distribution, determined by the vibrational particle number distribution factor of each vibrational energy level. Characterizes the internal energy distribution of molecules in a flow field.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] 1. This invention has a wide range of applications and can be used for pure rotational Raman spectroscopy measurements of various probe molecules such as N2 in non-equilibrium flow fields. It can simultaneously measure molecular vibrational temperature and rotational temperature, meeting the diagnostic needs of non-equilibrium flow fields under different working conditions.

[0031] 2. This invention has high spectral resolution and signal-to-noise ratio. It adopts a cascaded Bragg grating to effectively suppress strong Rayleigh scattering. Combined with the orthogonal cross-dispersion scheme of virtual image phase array and holographic grating, it can achieve high spectral resolution of subwavelength level, effectively distinguish spectral line splitting caused by vibration-rotation coupling, and at the same time ensure high signal-to-noise ratio of spectral acquisition, thus solving the problem of insufficient resolution of traditional grating spectrometers.

[0032] 3. This invention has high measurement accuracy and strong adaptability. The spectral fitting method is based on the non-Boltzmann distribution to characterize the particle number layout of the probe molecules. It is applicable to non-equilibrium flow fields with various enthalpy values ​​and can obtain the energy distribution information of molecules more accurately, reducing the measurement error caused by traditional methods. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the optical path structure of the measuring device of the present invention;

[0034] Figure 2 The measurement method of this invention is based on different vibration temperatures. and A schematic diagram of the linear correlation.

[0035] In the diagram: 1. Light source; 2. Achromatic lens; 3. Rayleigh signal suppression unit; 4. Cylindrical lens; 5. Virtual image phase array; 6. Grating; 7. Detection unit. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0037] A device for simultaneous vibrational and rotational temperature measurement based on pure rotational Raman spectroscopy, taking the simultaneous measurement of vibrational and rotational temperatures of N2 molecules in a non-equilibrium flow field as an example, its optical path structure combines... Figure 1 As shown, the device includes, in sequence along the optical path propagation direction, a light source 1, an achromatic lens 2, a Rayleigh signal suppression unit 3, a cylindrical lens 4, a virtual image phase array 5, a grating 6, and a detection unit 7.

[0038] The light source 1 is a single-mode 532nm laser with a rated output power of approximately 60W and a spectral bandwidth of approximately 6kHz, which is much smaller than the 0.1cm required by the temperature measuring device. -1 The spectral resolution will not affect the system's spectral dispersion capability. The laser beam emitted from light source 1 is deflected by a mirror and then incident on the flow field test area, exciting the N2 molecules in the flow field to generate Raman scattering signals, and a laser beam absorption cell (Dump) is set up.

[0039] The achromatic lens 2 is a convex lens with a diameter of 2 inches. It is positioned at a 90° scattering direction perpendicular to the incident laser beam to collect the pure rotational Raman signal generated by N2 molecules in the flow field test area and collimate the scattered light into a parallel beam for output.

[0040] The Rayleigh signal suppression unit 3 is composed of two cascaded volume Bragg gratings (VBGs) fixed by an optical frame. After cascading, it can suppress 532nm Rayleigh scattered light (optical density OD>8), effectively filter out strong Rayleigh scattering background, and at the same time maintain high transmission for pure rotational Raman signals that deviate from 532nm.

[0041] The cylindrical lens 4 is a plano-convex cylindrical mirror, which is set on the light-emitting side of the Rayleigh signal suppression unit 3. It is used to focus the Raman signal beam after Rayleigh signal suppression in the vertical direction and couple it into the virtual image phase array 5.

[0042] The virtual image phase array 5 (VIPA) has a free spectral range of 30 GHz, a fineness of approximately 33, and a theoretical spectral resolution of <0.05 cm⁻¹. -1The incident pure rotational Raman signal is reflected and interfered multiple times between the two reflecting surfaces of the virtual image phase array 5, achieving high-resolution dispersion in the vertical direction and separating Raman signals of different wavelengths in the vertical direction.

[0043] The grating 6 is a holographic grating with 2400 lines / mm, an effective width of 10mm, and a spectral resolution of approximately 0.78cm. -1 The grating 6 is set on the light-emitting side of the virtual image phase array 5, and its dispersion direction is orthogonal to the dispersion direction of the virtual image phase array 5. It performs secondary dispersion on the pure rotational Raman signal of the first beam emitted from the virtual image phase array 5 in the horizontal direction, and separates the different interference orders generated by the virtual image phase array 5 in the horizontal direction, avoiding spectral confusion caused by order overlap.

[0044] The detection unit 7 employs an sCMOS camera and a plano-convex lens. The sCMOS camera has a pixel size of 6.5 μm and a resolution of 2048 × 2048. Due to the limited height of the VIPA and the width of the diffraction grating, the spatial distribution of the two-dimensional dispersed light is only about tens of millimeters. Therefore, the image-side aperture angle is small, and the optical aberrations are very weak. Only a simple plano-convex lens is needed to meet the aberration requirements. The plano-convex lens has a focal length of 300 mm and is used to focus the Raman signal after two-dimensional dispersion onto the detection surface of the sCMOS camera. The sCMOS camera is synchronized with the light source 1 through external triggering to complete the acquisition and recording of the dispersed Raman spectrum. Calculations show that the spectral resolution of the grating matched to the system is approximately 0.81 cm⁻¹. -1 It is smaller than the free spectral region of VIPA and satisfies the matching condition for orthogonal dispersion.

[0045] Based on the above measuring device, a method for simultaneous measurement of vibration / rotation temperature based on pure rotational Raman spectroscopy is proposed, including the following steps:

[0046] Step 1: Construct the measurement device according to the optical path structure, and adjust the attitude and position of each optical element in the measurement device, including: adjusting the optical path of the light source 1 so that the laser beam emitted from the single longitudinal mode 532nm laser is stably incident on the flow field test area; adjusting the position of the achromatic lens 2 perpendicular to the incident laser beam, and using a convex lens to collect and collimate the Raman scattered light; adjusting the angle of the two-stage volume Bragg grating in the Rayleigh signal suppression unit 3 so that the Rayleigh scattered light is reflected and suppressed, while the pure rotational Raman signal passes through with high transmittance; adjusting the position and angle of the cylindrical lens 4 so that the Rayleigh signal is suppressed. The Raman signal beam is focused and coupled into the virtual image phase array 5. The angle between the virtual image phase array 5 and the grating 6 is adjusted so that the virtual image phase array 5 performs high-precision dispersion of the Raman signal in the vertical direction. However, the different interference orders of the dispersed light overlap in space. Then, a holographic grating with a spectral resolution smaller than that of the virtual image phase array 5 in the free spectral region is used to separate the overlapping orders in the horizontal direction, so that the spectrum after two-dimensional dispersion is uniformly distributed on the detection surface of the sCMOS camera of the detection unit 7. The dispersed Raman signal is recorded by the sCMOS camera, and the construction and initial debugging of the measurement device are completed.

[0047] Step 2: Gradually increase the energy of the laser beam emitted from light source 1 until the laser beam just barely avoids breakdown in air at room temperature and pressure, ensuring the excitation efficiency of the Raman signal while avoiding plasma interference from air breakdown in the spectral measurement.

[0048] Step 3: Set the exposure time of the sCMOS camera according to requirements, and collect pure rotational Raman spectra of air at room temperature and pressure. By fine-tuning the optical components in the measurement device, improve the optical path coupling efficiency so that the sCMOS camera can clearly record the rotational quantum number. The high-resolution, high-signal-to-noise-ratio pure rotational Raman spectra corresponding to high rotational energy levels were obtained to complete the calibration of the measuring device.

[0049] Step 4: Increase the emitted laser energy of light source 1 to the maximum value, set the sCMOS camera to synchronize with light source 1, and collect the pure rotational Raman spectrum of N2 molecules in the non-equilibrium flow field under preset working conditions to complete the spectral acquisition of the flow field test area.

[0050] Step 5: Obtain the wavelength-pixel mapping relationship based on the two-dimensional spectral model method. The dispersed Raman signal in both spatial dimensions will be mapped to the wavelength dimension, yielding the pure rotational Raman spectrum of the final flow field molecules, where... The expression is:

[0051]

[0052] In the formula, and These are the x and y coordinates of the pixels, respectively. The coefficients of the fitted polynomial, This refers to the diffraction order of the grating.

[0053] Step Six: Construct a theoretical model for pure rotational Raman spectroscopy, using the vibrational particle number layout factor. and rotation temperature To fit the parameters, a genetic algorithm was used to fit the obtained pure rotational Raman spectrum to obtain the rotational temperature of N2 molecules in the flow field under the corresponding operating conditions. and the vibrational particle number distribution factor of the corresponding vibrational energy level. .

[0054] Step 7: Adjust the vibrational particle number distribution factor obtained in Step 6. Process and draw With vibrational quantum number Determine whether the corresponding relationship curves satisfy linear correlation:

[0055] like and They are linearly correlated, combined Figure 2 As shown, it is assumed that the vibrational particle number distribution of N2 molecules in the flow field satisfies the Boltzmann distribution, and the vibrational temperature is further calculated through fitting. The Boltzmann distribution is expressed as follows:

[0056]

[0057] In the formula, For vibrational energy, the expression is:

[0058]

[0059] In the formula, It is a spectral constant, and ;

[0060] like and If the linear correlation is not satisfied, the vibrational particle number distribution of N2 molecules in the flow field is considered to be a non-Boltzmann distribution. In this case, it is impossible to determine the distribution based on a single vibrational temperature. Its energy distribution is characterized directly by the vibrational particle number distribution factor of each vibrational energy level. Characterize the internal energy distribution of N2 molecules in the flow field.

[0061] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0062] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A pure rotational Raman spectroscopy based simultaneous measurement of vibration / rotation temperature device, characterized by: Along the direction of optical propagation, the light source (1), achromatic lens (2), Rayleigh signal suppression unit (3), cylindrical lens (4), virtual image phase array (5), grating (6) and detection unit (7) are arranged in sequence. The light source (1) is a single longitudinal mode 532nm laser, which is used to emit a laser beam into the flow field test area to excite the probe molecules in the flow field to generate Raman scattering signals. The achromatic lens (2) is positioned perpendicular to the scattering direction of the incident laser beam to collect the pure rotational Raman signal generated by the probe molecules in the flow field test area and collimate the scattered light into a parallel beam output. The Rayleigh signal suppression unit (3) consists of two cascaded volume Bragg gratings, used to filter out and suppress 532nm Rayleigh scattered light, while transmitting pure rotational Raman signals. The cylindrical lens (4) is disposed on the light-emitting side of the Rayleigh signal suppression unit (3) to focus the Raman signal beam after Rayleigh signal suppression and couple it to the virtual image phase array (5). The virtual image phase array (5) is used to perform dispersion of the incident pure rotational Raman signal in the vertical direction; The grating (6) is a holographic grating, and its dispersion direction is orthogonal to the dispersion direction of the virtual image phase array (5). It is used to perform secondary dispersion of the beam emitted from the virtual image phase array (5) in the horizontal direction and separate the overlapping interference order generated by the virtual image phase array (5). The detection unit (7) uses an sCMOS camera and a plano-convex lens. The plano-convex lens is used to focus the Raman signal after two-dimensional dispersion onto the detection surface of the sCMOS camera. The sCMOS camera is used to collect and record pure rotational Raman spectra.

2. The pure rotational Raman spectrum based simultaneous measurement of translation / rotation temperature device according to claim 1, characterized in that: The Rayleigh signal suppression unit (3) requires an optical density OD>8 to suppress 532nm Rayleigh scattering light.

3. The pure rotational Raman spectrum based simultaneous measurement of translation / rotation temperature device according to claim 1, characterized in that: The free spectral range of the virtual phased array (5) is 30 GHz, the theoretical spectral resolution is <0.05 cm -1 .

4. The apparatus for simultaneous measurement of vibration and rotation temperature based on pure rotational Raman spectrum according to claim 1, characterized in that: The grating (6) has 2400 lines / mm and an effective width of 10mm. Its spectral resolution is smaller than that of the free spectral region of the virtual image phase array (5).

5. The vibration / rotation temperature synchronization measurement device based on pure rotational Raman spectroscopy according to claim 1, characterized in that: In the detection unit (7), the sCMOS camera has a pixel size of 6.5μm, a resolution of 2048×2048, and a focal length of 300mm for the plano-convex lens.

6. A method for simultaneous measurement of vibration and rotation temperature based on pure rotational Raman spectroscopy, characterized in that: The vibration / rotation temperature synchronization measurement device based on pure rotational Raman spectroscopy according to claim 1, the measurement method includes the following steps: Step 1: Build a measurement device according to the optical path structure, and adjust the attitude and position of each optical element so that the sCMOS camera can acquire the pure rotational Raman signal after two-dimensional dispersion. Step 2: Gradually increase the output laser energy of the light source (1) until the laser beam just does not break down in air at normal temperature and pressure; Step 3: Set the exposure time of the sCMOS camera and acquire the pure rotational Raman spectrum of air at room temperature and pressure, so that the sCMOS camera can clearly record the rotational quantum number. The spectral lines corresponding to the high rotational energy levels; Step 4: Increase the emitted laser energy of the light source (1) to the maximum value and collect the pure rotational Raman spectrum of the probe molecules in the non-equilibrium flow field under the preset working conditions; Step 5: Based on the pre-calibrated wavelength-pixel mapping relationship, the Raman signal that is dispersed in two spatial dimensions is mapped to the wavelength dimension to obtain the pure rotational Raman spectrum of the final flow field molecules; Step Six: Layout with Vibrational Particle Number Factor and rotation temperature To fit the parameters, a genetic algorithm was used to fit the pure rotational Raman spectrum to obtain the rotational temperature under the corresponding operating conditions. Vibrational particle number distribution factor of each vibrational energy level ; Step 7: Determine the vibrational particle number distribution factor Does it satisfy the Boltzmann distribution? If so, the vibration temperature can be calculated. If the conditions are not met, then the particle number layout factor of the vibrational dynamics will be used. Characterizes the internal energy distribution of molecules in a flow field.

7. The method of claim 6, wherein the method is a pure rotational Raman spectroscopy based simultaneous measurement of translation / rotation temperature. In step seven, if With vibrational quantum number If the correlation is linear, it is assumed to satisfy the Boltzmann distribution, and the vibration temperature can be calculated by fitting the Boltzmann distribution expression. Conversely, it is considered a non-Boltzmann distribution, determined by the vibrational particle number distribution factor of each vibrational energy level. Characterizes the internal energy distribution of molecules in a flow field.

Images