Open Access
Add to BibSonomy
Abstract
Interferometric Rayleigh scattering technique is commonly employed to measure single-point velocity fluctuation and its standard deviation in a high-speed flow due to many benefits, such as high accuracy, easy data interpretation, and high sampling rate. However, this technique suffers from a severe problem often referred to as the weak Rayleigh scattering signal, especially in the supersonic and hypersonic flow with an extremely low gas molecule density. An asymmetry cavity structure that could cost-effectively improve the Rayleigh scattering (RS) signal of interest is designed and used in the interferemetric Rayleigh scattering technique. The ZEMAX simulations suggest that the parallel beam can be repeatedly reflected in the resonant cavity and can be focused in a measurement region with the order of 0.67 mm×1.31 mm. The number of propagating rays inside the cavity can reach about 50. The fidelity of this proposed cavity is then verified by the Rayleigh scattering imaging experiments. Results show that this cavity allows the laser beam to reflect several times in the resonant cavity, and the RS signal intensity in the major axis can be 10.4 times larger than that of the incident laser. The cavity is finally employed under realistic supersonic flow velocity measurements, where the results conclusively illustrate that the Rayleigh scattering signal of interest in a single direction can be improved by a factor of 4∼5. In addition, the bi-directional (both the axial and radial directions) velocity parameters can also be obtained simultaneously. The axial velocity and its standard deviation are similar to conventional single-line ones.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the advantages of non-intrusive, on-beam, high temporal, and spatial resolution, laser diagnostic technique is usually regarded as a primary tool for gaining a better understanding of fluid dynamics and combustion physics. Laser-based velocity measurement technique is an important branch [1–4]. Current techniques used for non-intrusive velocity measurements in a turbulent flow, such as laser-Doppler velocimetry (LDV) [5–7] and particle image velocimetry (PIV) [8–10], require the use of seed particles. However, the use of seed particles would bring new problems, such as window contamination and following performance in high-speed turbulent flows [11]. Besides, a comprehensive list of some molecular tracing techniques may include Molecular-Tagging Velocimetry (MTV) [12], Raman Excitation plus Laser-Induced Electronic Fluorescence (RELIEF) [13], Air Photolysis and Recombination Tracking (APART) [14], Femtosecond-Laser Electronic-Excitation Tagging (FLEET) [15] and so on. Most of them have shortcomings related to signal levels and molecular species. Therefore, they lack sensitivity at high flow velocities and harsh environments. For example, the MTV technique is only sensitive to the H2O molecule and cannot be applied in the combustion and H2O-free flow [16].
The Interference Rayleigh scattering (IRS) technique is expected to become one of the most effective approaches to solve these problems [17–21]. It depends on the elastic scattering of light from flow molecules without adding any seed particles. Through detecting the Doppler shift of molecular RS with a Fabry–Perot etalon, the velocity of flow can be measured. Meanwhile, a high-power single-frequency continuous-wave (CW) laser beam is used to irradiate the detected flow field, and the sampling rate can reach several kHz [22]. Therefore, this technique can be used in high sampling-rate velocity measurements in the high-speed gas-flow. Meanwhile, it is able to measure several flow parameters (velocity, temperature and species concentration) simultaneously with a single laser beam and applicability over a wide range of working gases (e.g., air, N2, CO2.) [23]. To date, IRS velocimetry has been applied in the study of high-speed jets, wind tunnels [19–22,24] and high-speed combustion flows [25]. Both nanosecond pulsed lasers [21,25] and narrow linewidth continuous-wave (CW) lasers [22,24] have been employed. Estevadeordal J. et al. used IRS technique at a 100-kHz repetition rate to measure the high-speed flow-velocity fluctuation and achieve simultaneous multi-point and multi-parameter results. High temporal resolution was obtained with a quasi-continuous burst-mode laser that is capable of providing bursts of 10 ms duration with a pulse width of 10–100 ns [26,27]. However, Rayleigh scattering is an elastic scattering phenomenon and the signal intensity is proportional to the density of the gas molecule, and so the detected RS signal is relatively weak in the high-speed flow [28,29].
Different approaches to improve the Rayleigh scattering signal have been investigated in recent years. Ellipsoidal light traps serve to increase the light intensity at the focus [30,31], such as the common ellipsoidal cavity [31], Light-Trapping cells (LTC I and LTC II) [31]. The common ellipsoidal cavity concentrates the intensity on both two focuses. Light-Trapping cell (LTC I) made up of an on-axis ellipsoidal mirror and a flat mirror is also introduced. The flat mirror serves to reflect the rays that approach one focus back through the other focus. Thus, a large number of images of the initial image are focused at a single focus. Then a light-trapping cell (LTC II) is introduced to enhance the gain further. The coaxial spherical-flat mirror is pierced on the ellipsoid focus to provide access for the light entering the cell. Because the redirected light follows an inverted path, the light will make as many passes through the cell. Besides, A retroreflecting multi-pass cell made up of two lenses and both on-axis and off-axis retroreflecting mirror assemblies has been constructed and tested [32,33]. A gain of 20 in Raman scattered signal intensity has been attained in a focal volume.
In this work, an asymmetry cavity that can improve the RS signal of interest is studied. The ZEMAX simulation and experiment results suggest that a remarkable enhancement of the RS signal is expected when the asymmetry cavity is applied. Meanwhile, the bi-directional velocity (axial and radial directions) and the standard deviation parameters in a supersonic free jet from a Laval nozzle with a Mach number of 1.8 can be acquired simultaneously.
2. Theoretical background
2.1 Asymmetry cavity ZEMAX simulation
The asymmetry cavity model is based on a traditional one comprising two identical and confocal spherical reflectors [34,35]. When the light transmits into the traditional resonant cavity, it reflects repeatedly, and the interaction between light and matter can be enhanced immediately. However, this structure can only allow little light to enter the cavity. Hence, we substitute these reflectors for a minor one (small focal length and diameter of the spherical reflector) and a bigger one (large focal length and diameter of the spherical reflector), as shown in Fig. 1(a). These two spherical reflectors are positioned on-axis such that their focuses are coincident, and the distance between these two reflectors df is equal to the sum of two focal lengths fA and fB, as shown in Fig. 1(b). An incident light transmits into this cavity on the edge of the minor reflector, and then the light is propagated many times in the optimized cavity. Therefore, it can be of great benefit for improving the RS signal in the confocal point of the cavity (or the measuring point).
Fig. 1. The 2-dimension graph of ray tracing by ZEMAX simulation (a) and the corresponding location diagram (b)
ZEMAX simulations are conducted to obtain the ray tracing and energy distribution inside the proposed device at different sizes, as shown in Fig. 1. The units of the length, the wavelength and the energy are set separately as centimeter (cm), nanometer (nm) and watt (W), respectively. The incident light power and diameter are set 1 W and 0.2 cm, respectively. The reflectivity of spherical reflectors is set to 95%. The other simulation parameters are detailed in the Table 1. The area detector is defined to map the distribution of the ray tracing and the light field energy so that the light propagation path can be recorded. By analyzing the power of beginning and end rays inside the cavity, the numbers of propagating rays can be calculated directly.
The ray tracing inside the cavity is then simulated by ZEMAX, and the simulation parameters are detailed in the Table 1. For the SM 1, when an incident light transmits into this cavity on the edge of the minor reflector A, the light will be reflected by the bigger reflector B and focused at the measurement point. A light ray that strikes the reflector B at a height hm thus strikes reflector B at a height ηhm, where η= fA: fB. The light is subsequently reflected by reflector A and then focused at the same measurement point. Thus, the light is reflected alternately through the measurement point, and after a number of reflections, the light rays approach parallelism with the major axis. Light rays passing exactly through the focus thus become trapped on the major axis OO′. Meanwhile, the light will propagate outside the cavity after a few times due to the accumulation of the spherical aberration. Therefore, the number of propagating rays N can be calculated by IN=I0(q)N−1, IN and I0 represent the power of the initial and final ray, respectively. q is the reflectivity. As for SM1, IN and I0 can be recorded by the area detector, and IN = 0.341 W, I0 = 1 W, q=0.95, then the number of propagating rays N = logq(IN/I0) + 1 = 22 can be obtained.
Then, the relation between the numbers of propagating rays and focal lengths (fA and fB), reflectors radiuses (RA and RB), the distance d1 is then investigated, as shown in Fig. 2. As for the SM2 (Table 1) and Fig. 2(a), we can conclude that the numbers of propagating rays increase slowly with enlarging the focal length of the minor reflector fA. Due to the decrease of reflection angle θ0∼(d1+RA)/fB∼(d1+RA)η/fA (see Fig. 1(b)), the probability transmitting outside the cavity drops and the number of propagating rays improves immediately. Also, it is worth noticing that there is a critical value at the ratio η0=1:1.3. When η is less than the value η0, the ray will transmit outside the cavity after the first reflection. Meanwhile, by changing η from 1:1.5 to 1:3 (the range of fB is between 75 and 150 cm), the number of propagating rays decreases. The longer focal length fB and the bigger radius can lead to the larger spherical aberration (SA) of the spherical mirror. Therefore, the probability transmitting outside the cavity decreases owing to the larger SA. Moreover, two prominent features are observed in Fig. 2(b) and Fig. 2(c). First, there is an initial leap (R0) with increasing radiuses RA. When RA is below R0, the rays will also transmit outside the cavity after the first reflection. After that, the numbers of propagating rays decrease with enlarging the radius RA, due to the increase of SA. Besides, the rays only reflect on the center region of the mirror B and the SA would be almost irrelevant to the radius RB, so the numbers of propagating rays remain unchanged by enlarging the radius RB, as shown in Fig. 2(c). Finally, different distances d1 (see Fig. 1(b)) induce various numbers of propagating rays, as shown in Fig. 2(d). The numbers of propagating rays enlarge with the distance d1 decreasing. Taking η=1:1.5 as an example, the numbers of propagating rays with d1=0.2 cm, 0.3 cm and 0.4 cm are 50, 47 and 46, respectively. It is also the larger SA and reflection angle θ0 that enables the numbers of propagating rays to decrease. The more transmitting rays would be farther away from the major axis OO′ and the corresponding SA get larger by increasing the distances d1.
Fig. 2. The dependence of the reflection time (or the numbers of propagating rays) inside the cavity on the focal lengths (fA and fB) (a), the radiuses of the spherical reflectors RA (b) and RB (c), the distance between the incident light and the minor spherical reflector d1 (d), simulated by ZEMAX
Table 1. The asymmetric cavity parameters set in the ZEMAX simulation
2.2 IRS measurement principle
The basic principle of the IRS technique is that Rayleigh-scattered light from the flow of interest is imaged through a Fabry–Perot interferometer (FPI, “etalon”), which can distinguish the minimal variation between the wavelengths of two lights accurately. When a parallel beam irradiates the F–P interferometer, a series of broadened circles separated by the airy ring spacing are formed once the condition is fulfilled, as follows [36–38]:
where δ is the phase difference between two transmitted beams; i is the angle between the light beam and the optical axis; n and h are the refractive index and F-P mirror spacing, respectively, and k represents the order of the interferometric rings.The detected images can convert the Doppler shift due to the flow velocity into the spatial–frequency information, because the transmission properties of the etalon depend on the wavelength of the incident light [21,22].
Based on this theory, the frequency shift and flow velocity from the image intensity at the detecting location can be easily derived. Finally, the flow velocity standard deviation I related to the flow turbulence is then given by the following [26,27]. The shorter exposure time and the higher sampling rate in the IRS measurement can lead to the closer error between the velocity standard deviation and the flow turbulence.
The working principle of the bi-directional velocity measurement via the asymmetry cavity is shown in Fig. 3(c). When the focal length is much greater than the reflector radius, the angles between the reflection rays and the horizon, θ1 and θ2, are relatively small and can be ignored. Take SM 1 in Table 1 as an example, fA=50 cm, fB=150 cm, d1=0.4 cm, the radiuses of Reflectors A (RA) and B (RB) are 1 cm and 5 cm, respectively. The maximum shooting angle corresponds to the first reflection and θ0=arcsin((d1+RA)/fB) = 0.53°. Besides, the light rays can be almost parallel with the major axis OO′ after a number of reflections. And the shooting angle θ0 would get smaller to be zero. In addition, the Doppler shifts generated by the rays travelling from the reflector A to B (L1) are different from those passing B to A (L2), because the Doppler shifts depend on the viewing angle and the gas velocity. The bi-directional Doppler shift due to the different velocity directions (v// and v⊥) can be obtained simultaneously by using Eq. (1). According to the corresponding relationship between Doppler shift and RS spectrum, the velocity fluctuation in each direction can be fitted through the obtained interference images.
Fig. 3. (a), signal-enhanced and bi-directional IRS setup by using the asymmetry cavity. (b), the interferogram based on the multi-pass cavity (top) and the local enlarged drawing (bottom) of the top interferogram in the red dashed rectangle. (c), bi-directional velocity measurement method.
3. Experimental setup
A general scheme combining the IRS apparatus and the asymmetry cavity arrangement is shown in Fig. 3(a). A continuous-wave (CW) 532 nm laser with a power of 6.0 W and a linewidth of 5 MHz is used in the system. The output laser beam with the factor M2=1.05 and the divergence angle θh<5°has a good beam quality. It is initially collimated by the lens K1 with a beam diameter of 2 mm, and the polarization of the laser beam is changed by a half-wave-length plate. The laser beam is then split into two beams: 1% of laser light is extended by a diffuser to be the reference laser, and the remaining part transmits into the designed asymmetry cavity. The cavity comprises a minor spherical reflector C1 (diameter is set to 20 mm) and a bigger one C2 (diameter of the spherical reflector is set to 50.8 mm). The reflectivity of these reflectors can reach 96%. Meanwhile, these reflectors are set confocal, and the focus is the same as the measurement point. Subsequently, the transmitted rays reflect repeatedly and focus on the measurement point. A McKenna flat burner is placed under the transporting rays to arrange the dust-free air flow, and the multiple line route can be imaged by the right EMCCD camera, as shown in the image inside the green dashed line (Fig. 3(a)).
The IRS velocity measurement is then performed during the testing of a supersonic free jet produced by a Laval nozzle with a Mach number of 1.8. The high-pressure air source with little steam is produced by the compressor. The measurement is 57 mm far away from the nozzle outlet, and the nozzle inlet pressure is set to 0.7 MPa. The flow direction is oriented at an angle θ of 45° with respect to the primary laser line, whereas the scattered light is collected by the lens K2 (the focal length f2=500 mm) at an angle of 90° relative to the primary laser line. A parallel RS beam is formed by a pair of lenses (K3 and K4). The parallel RS signal and reference beams are then combined via a beam combiner and irradiate parallel to the F-P interferometer. The typical multi-beam interferogram based on the asymmetric cavity is shown in the Fig. 3(b). The circular interference rings are related to the laser wavelength, and each set of elliptical bright spots in the circular rings corresponds to the RS signals on the different measurement locations in the flow. The IRS signal intensity in the major axis OO′ is obviously larger than that in the other locations. Meanwhile, the spatial resolution of the measurement region is on the order of 0.67 mm×1.31 mm. Subsequently, the RS signal for the five rows around the major axis are binned to enhance the sampling rate and the signal to noise ratio (SNR). The IRS spectrum with the Doppler shift can be obtained, as shown in the image inside the blue dashed line (Fig. 3(a)). After that, the reference and the IRS spectrum with the Doppler shift due to the bi-direction flow velocity v// and v⊥ are fitted by the Gaussian solution to obtain the precise locations of all peaks. The peak variation between the IRS signal with the Doppler shift and the reference can be easily obtained. Finally, the corresponding bi-direction velocity (v// and v⊥) can be obtained according to Eqs. (1) and (2). The other detailed parameters, such as focal lengths of reflectors, sampling rate, and camera gain, are listed in Table 2. Case 4 represents the single-line experiments, whereas the other cases represent the asymmetry cavity ones.
Table 2. Experimental parameter: focal lengths of reflectors (fC1 and fC2), distance between the incident ray and the minor reflector d1, as shown in Fig. 1(b).
4. Results and discussion
4.1 Images of multiple rays inside the cavity
The effect of the asymmetry cavity is then studied by the RS imaging experiments. The parameters and results are displayed in Table 2 and Fig. 4, respectively. In the experiments, three schemes were investigated, referred to as Case 1, Case 2, and Case 3. The McKenna flat burner (diameter of the furnace surface is 100 mm) is applied to produce the air gas flow, and the air flow rate is 30 L/min.
Fig. 4. Multiple ray distribution inside the different cavities (the initial incident light is marked by the red arrow, and the measurement point is marked by the white circle)
As shown in Case 1 (fC1 and fC2: 250 and 350 mm, η= fC1 : fC2=1:1.4 and d1=8 mm), the RS signal around the measurement region (marked by the white circle) can be much larger than that of the incident laser (marked by the red arrow). Evidently, the maximum RS signals for the two locations are 3161.2 (incident laser) and 32886.1 (around the measurement point, marked by the white circle), respectively. And the amplification of 10.4 times can be achieved. Considering the cross and opposite rays inside the cavity, the single-direction amplification (SDA) can be approximately 5.2 times, further verifying the improvement of the designed asymmetry cavity. In addition, the SDA in all cases is computed in Fig. 5. As shown in Figs. 4 and 5, by changing the ratio η= fC1 : fC2 to 1:1.6 (350 and 550 mm, Case 3) to 1: 2.2 (250 and 550 mm, Case 2), the SDA declines rapidly to 2.8 and 2.3, respectively. Furthermore, the results are consistent with the ZEMAX simulation. In these solutions, a higher SA can be introduced when decreasing the ratio η (or enlarging the focal length fC2). This condition reduces the reflection time inside the cavity and the corresponding SDA. Meanwhile, the diameter of the laser spot increases gradually (expanding two times after transmitting 6 m) due to the laser Gaussian character. Thus, the laser diverges, and its energy density declines. Finally, the SDA enlarges with the length d1 decreasing. Taking Case 1 as an example, the values of SDA with d1=8 and 15 mm are 5.2 and 4.7, respectively.
4.2 IRS velocity measurement
The designed IRS apparatus (Fig. 3(a)) is performed during the testing of a supersonic free jet produced by a Laval nozzle with a Mach number of 1.8. The sampling rate and sampling length are set to 8810.6 Hz and 2000, respectively. Other parameters are detailed in Table 2. The property of the designed IRS apparatus (Case 5) is experimentally performed, and the velocity results are compared with those of the traditional single-line measurement (Case 4) at the same position of the Laval nozzle, as shown in Fig. 6. The figure shows that the IRS spectrums based on the single-line and the cavity represent some exact Doppler shifts to the reference one (IRS signal without Doppler shift). Meanwhile, the IRS spectrum based on the asymmetry cavity reveals the double-peaking (corresponding to the axial velocity v// and the radial velocity v⊥) character, while the traditional single line result reveals only the single peak (corresponding to the axial velocity v//). In addition, the peak intensity (corresponding to v//) increases from a maximum value of 3423.5 to 16569.7, and it is increased by a factor of 4.8 with the use of the asymmetry cavity. The maximum gain is similar to the SDA of 5.2 in Rayleigh scattering imaging experiments in the stable flow.
Fig. 6. IRS spectrum (accumulated 10 times) difference among the asymmetric cavity, the traditional single-line, and the reference signal.
The results in the measured axial velocity v// and the related power spectrum density (PSD) between the two methods are clearly shown in Fig. 7. Meanwhile, the measured mean velocity and the standard deviation related to the different cases are presented in Fig. 8(d). As shown in Figs. 7 and 8, the distinction between the two methods is small. Take Case 4, Case 5, and Case 6 as examples, the mean velocity results for the three cases are 400.73, 405.33, and 411.52 m/s, respectively. The maximum mean axial velocity bias in the three cases is 10.79 m/s, according to the maximum relative bias of 2.66%. Meanwhile, the amplitudes range and mean amplitudes (marked by the white dashed lines in Fig. 7(d-e)) of lg(PSD) under different frequencies [0:3500 Hz] for these 3 cases are very close. Therefore, the designed IRS system based on the asymmetry cavity model can be used further to measure the flow velocity and the standard deviation. In addition, the radial velocity and the standard deviation can be achieved, as shown in Fig. 8. The figure shows that the average radial velocity results, (19.55 m/s, Case 5), (27.51 m/s, Case 6), and (25.13 m/s, Case 7), are relatively small, whereas the standard deviation results, (23.87%, Case 5), (31.26%, Case 6), and (25.92%, Case 7), are nearly 10 times the axial ones (approximately 3.0%). Moreover, the radial velocity and standard deviation results in the three cases are relatively similar, further verifying that the IRS apparatus based on the asymmetry cavity can be used to measure the bi-directional velocity.
Fig. 7. the axial velocity fluctuation ((a), (b), and (c)) and the power spectrum density (PSD) ((d), (e), and (f)) measured at Case 4, Case 5, and Case 6.
Fig. 8. (a), (b), and (c), the radial velocity fluctuation measured at Case 5, Case 6, and Case 7, respectively; (d) the statistical mean velocity and the standard deviation.
5. Conclusions
In conclusion, an asymmetry cavity for improving the RS signal is designed and used in the IRS apparatus. The ZEMAX simulations suggest that the multi-ray distribution is attributed to the asymmetry cavity. The RS imaging results show that the RS signal with the asymmetry cavity can be improved by a factor of 10.4. The IRS apparatus based on the asymmetry cavity is used to quantitatively measure the velocity and the standard deviation in a supersonic flow. The results are similar to those obtained by the traditional single-line ones. The maximum relative bias is only 2.66% between the proposed methods and the traditional ones. Further experiments verify that the propagational lines in two directions in the cavity can also be used to measure the bi-directional velocity. By further studying the characteristics of the multi-pass lines in the cavity, the designed IRS apparatus based on the asymmetry cavity can be used to measure the flow velocity at different points.
Funding
National Key Research and Development Program of China (2020YFA0405700); National Natural Science Foundation of China (91741205); Foundation of Innovation-oriented Province Construction of Hunan (2019RS2028).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
References
1. U. Sharma, G. Chen, J. U. Kang, I. Ilev, and R. W. Waynant, “Fiber optic confocal laser Doppler velocimeter using an all-fiber laser source for high resolution measurements,” Opt. Express 13(16), 6250–6258 (2005). [CrossRef]
2. R. G. Seasholtz, F. J. Zupanc, and S. J. Schneider, “Spectrally Resolved Rayleigh Scattering Diagnostics for Hydrogen-Oxygen Rocket Plume Studies,” J. Propulsion and Power 8(5), 935–942 (1992). [CrossRef]
3. N. B. Jiang, M. Nishihara, and W. R. Lempert, “Quantitative NO2 molecular tagging velocimetry at 500 kHz frame rate,” Appl. Phys. Lett. 97(22), 221103–21103-3 (2010). [CrossRef]
4. J. P. Drummond, P. M. Danehy, D. Bivolaru, R. L. Gaffney, and A. D. Cutler, “Supersonic Combustion Research at NASA,” Fall Technical Meeting Eastern States Section of the Combustion Institute, University of Virginia, 21-24 Oct. 2007.
5. K. Maru, “Axial scanning laser Doppler velocimeter using wavelength change without moving mechanism in sensor probe,” Opt. Express 19(7), 5960–5969 (2011). [CrossRef]
6. Y. L. Li and R. Baets, “Nonlinear signal errors in homodyne laser Doppler vibrometry induced by strong second-order ghost reflections and their mitigation,” Opt. Express 29(6), 8283–8295 (2021). [CrossRef]
7. C. H. Cheng, L. C. Lin, and F. Y. Lin, “Self-mixing dual-frequency laser Doppler velocimeter,” Opt. Express 22(3), 3600–3610 (2014). [CrossRef]
8. G. E. Elsinga, F. Scarano, B. Wieneke, and B. W. van Oudheusden, “Tomographic particle image velocimetry,” Exp Fluids 41(6), 933–947 (2006). [CrossRef]
9. A. Kaffelabc, J. Moureha, J. L. Harionbc, and S. Russeilbc, “TR-PIV measurements and POD analysis of the plane wall jet subjected to lateral perturbation,” Exp. Therm. Fluid Sci. 77(4), 71–90 (2016). [CrossRef]
10. P. S. Hsu, S. Roy, N. B. Jiang N, and J. R. Gord, “Large-aperture, tapered fiber–coupled, 10-kHz particle-image velocimetry,” Opt. Express 21(3), 3617–3626 (2013). [CrossRef]
11. B. Fond, C. Abram, A. L. Heyes, A. M. Kempf, and F. Beyrau, “Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles,” Opt. Express 20(20), 22118–22133 (2012). [CrossRef]
12. M. J. Schmidt, B. Kslin, and T. Rsgen, “Photoisomeric molecular tagging velocimetry with CCVJ,” Phys. Rev. Fluid 6(4), 044101 (2021). [CrossRef]
13. R. B. Miles, J. Grinstead, R. H. Kohl, and G. Diskin, “The RELIEF flow tagging technique and its application in engine testing facilities and for helium-air mixing studies,” Meas. Sci. Technol. 11(9), 1272–1281 (2000). [CrossRef]
14. N. M. Sijtsema, N. J. Dam, R. H. Klein-Douwel, and J. J. T. Meulen, “Air Photolysis and Recombination Tracking: A New Molecular Tagging Velocimetry Scheme,” AIAA J. 40(6), 1061–1064 (2002). [CrossRef]
15. L. Bo, Y. Tian, G. Qiang, D. Zhang, and Z. Li, “Filamentary anemometry using femtosecond laser-extended electric discharge - FALED,” Opt. Express 26(16), 21132 (2018). [CrossRef]
16. W. R. Lempert, N. B. Jiang, S. Sethuram, and M. Samimy, “Molecular Tagging Velocimetry Measurements in Supersonic Microjets,” AIAA J. 40(6), 1065–1070 (2002). [CrossRef]
17. A. F. Mielke, R. G. Seasholtz, K. A. Elam, and J. Panda, “Time-average measurement of velocity, density, temperature, and turbulence velocity fluctuations using Rayleigh and Mie scattering,” Exp Fluids 39(2), 441–454 (2005). [CrossRef]
18. R. G. Seasholtz, J. Panda, and K. A. Elam, “Rayleigh Scattering Diagnostic for Measurement of Velocity and Density Fluctuation Spectra,” AIAA 40th AIAA Aerospace sciences meeting & exhibit, 14–17 (2002).
19. A. F. Mielke and K. A. Elam, “Multi-property measurements at high sampling rates using Rayleigh scattering,” AIAA Journal 47(4), 850–862 (2009). [CrossRef]
20. A. F. Mielke and K. A. Elam, “Rayleigh scattering diagnostic for measurement of temperature, velocity, and density fluctuation Spectra,” AIAA 44th AIAA Aerospace Sciences Meeting and Exhibit, 837–847 (2006).
21. L. Chen, F. R. Yang, T. Su, W. Y. Bao, X. H. Qi, and S. Chen, “Interferometric Rayleigh Scattering velocimetry using a Fabry-Perot Interferometer,” Guangzi Xuebao 44(1), 112004 (2015). [CrossRef]
22. L. Chen, F. R. Yang, T. Su, W. Y. Bao, B. Yan, and R. B. Li, “High sampling-rate measurement of turbulence velocity fluctuations in Mach 1.8 Laval jet using interferometric Rayleigh scattering,” Chinese Phys. B 26(2), 025205 (2017). [CrossRef]
23. J. A. Lock, R. G. Seasholtz, and W. T. John, “Rayleigh-brillouin scattering to determine one-dimensional temperature and number density profiles of a gas flow jet,” Appl. Opt. 31(15), 2839–2848 (1992). [CrossRef]
24. J. Panda and R. Seasholtz, “Velocity and temperature measurement in supersonic free jets using spectrally resolved Rayleigh scattering,” AIAA-1999-0236, 37th Aerospace Sciences Meeting and Exhibit, Reno, NV, 9–12 Jan. 1999.
25. D. Bivolaru, P. M. Danehy, J. W. Lee, R. L. Gaffney, and A. D. Cutler, “Direct-View Multi-Point Two-Component Interferometric Rayleigh Scattering System,” AIAA-2008-0236, 46th Aerospace Sciences Meeting, Reno, NV, 7–10 Jan. 2008.
26. J. Estevadeordal, N. B. Jiang, A. D. Cutler, J. J. Felver, M. N. Slipchenko, P. M. Danehy, J. R. Gord, and S. Roy, “High-repetition-rate interferometric rayleigh scattering for flow-velocity measurements,” Appl. Phys. B 124(3), 41–53 (2018). [CrossRef]
27. N. B. Jiang, A. D. Cutler, K. Rein, S. Roy, and P. M. Danehy, “100-khz interferometric rayleigh scattering for multi-parameter flow measurements,” Opt. Express 28(3), 2838–3040 (2020). [CrossRef]
28. R. G. Seasholtz and J. Panda, “Rayleigh Scattering Diagnostic for Simultaneous Measurements of Dynamic Density and Velocity,” AIAA-2000-0642, 38th Aerospace Sciences Meeting & Exhibition, Reno, NV, 10–13 Jan. 2000.
29. A. F. Fagan, M. M. Clem, and K. A. Elam, “Improvement in Rayleigh Scattering Measurement Accuracy,” AIAA-2012-1060, 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Tennessee, 9–12 Jan. 2012.
30. A. C. Eckbreth, “Laser diagnostics for combustion temperature and species”, Vol. 3. (CRC press, 1996).
31. R. A. Hill and D. L. Hartley, “Focused, multiple-pass cell for raman scattering,” Appl. Opt. 13(1), 186–192 (1974). [CrossRef]
32. R. A. Hill, A. J. Mulac, and C. E. Hackett, “Retroreflecting multipass cell for raman scattering,” Appl. Opt. 16(7), 2004–2006 (1977). [CrossRef]
33. D. A. Santavicca, “A High Energy, Long Pulse Nd: YAG Laser Multipass Cell for Raman Scattering Diagnostics,” Opt. Comm. 30(3), 423–425 (1979). [CrossRef]
34. B. Yan, X. X. Yang, J. Y. Fang, Y. D. Huang, H. Qin, and S. Q. Qin, “Tunable terahertz plasmon in grating-gate coupled graphene with a resonant cavity,” Chin. Phys. B 24(1), 015203 (2015). [CrossRef]
35. B. Yan, J. Y. Fang, S. Q. Qin, Y. T. Liu, Y. Q. Zhou, R. B. Li, and X. A. Zhang, “Experimental study of plasmon in a grating coupled graphene device with a resonant cavity,” Appl. Phys. Lett. 107(19), 191905 (2015). [CrossRef]
36. H. Y. Xia, X. K. Dou, D. S. Sun, Z. F. Shu, X. H. Xue, Y. Han, D. D. Hu, Y. L. Han, and T. D. Cheng, “Mid-altitude wind measurements with mobile rayleigh doppler lidar incorporating system-level optical frequency control method,” Opt. Express 20(14), 15286–15300 (2012). [CrossRef]
37. H. Y. Xia, X. K. Dou, M. J. Shangguan, R. C. Zhao, Y. L. Han, Z. F. Shu, X. H. Xue, Y. Han, C. Wang, and J. W. Qiu, “Stratospheric temperature measurement with scanning Fabry-Perot interferometer for wind retrieval from mobile Rayleigh Doppler lidar,” Opt. Express 22(18), 21775–21789 (2014). [CrossRef]
38. H. Y. Xia, M. J. Shangguan, C. Wang, G. L. Shentu, J. W. Qiu, Q. Zhang, X. K. Dou, and J. W. Pan J W, “Micro-pulse upconversion Doppler lidar for wind and visibility detection in the atmospheric boundary layer,” Opt. Lett. 41(22), 5218–5221 (2016). [CrossRef]
