SYSTEM USING VIRTUALLY IMAGED PHASED ARRAY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/230,173, filed on August 6, 2021 , entitled “ONE-DIMENSIONAL INTERFEROMETRIC RAYLEIGH SCATTERING SYSTEM USING VIRTUALLY IMAGED PHASED ARRAY,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to a system and method for measuring velocity of a gas-phase flow, and more particularly, to a system that uses a one-dimensional interferometric Rayleigh scattering with a virtually imaged phase array for measuring the velocity and/or temperature of the gas flow.

DISCUSSION OF THE BACKGROUND

[0003] Gas-phase velocity measurement techniques are widely applicable in the aerospace and combustion industries. Such techniques are specifically designed for measuring the velocity of a gas flow, e.g., through a reactor or a jet engine. However, they are more challenging compared to particle-based velocity measurement techniques in which the individual particles are tracked with the help of high-speed cameras because the gas is not visible and thus, not trackable using high-speed cameras. Several diagnostic techniques such as interferometric Rayleigh scattering (IRS) [1 , 2], laser Doppler velocimetry (LDV) [3], molecular tagging velocimetry, and optical-flow-based techniques have been used to obtain both time- averaged and time-resolved velocity measurements in various applications such as turbulent jet flames and high-speed flow facilities.

[0004] Interferometric Rayleigh scattering is a classic example of a gas-phase velocimetry technique. The Rayleigh scattering signal from a gas medium is collected, collimated, and transmitted through a Fabry-Perot (FP) etalon to obtain optical fringes on a camera. When the scattering medium has a velocity component relative to the laser beam or/and the collection optics, a Doppler shift occurs in the scattered signal frequency, and this results in a shift in the position of the fringes. The velocity of the gas flow is then obtained based on this shift in the fringe location, by calibrating the pixel position of the camera against the frequency shift using the known free spectral range of the etalon. The relation between the velocity and Doppler shift is given by the following equation: where Av _D is the Doppler shift in the scattered signal, k _s is the wave vector of the scattered signal, k _t is the wave vector corresponding to the incident light, and u is the flow velocity vector. The technique has also been extended to 100 kHz repetition rate using a pulse burst laser [4, 5], Multipoint versions of the technique have also been implemented using a FP-etalon with the help of multiple passes of laser beams. [0005] However, these interferometric Rayleigh scattering systems used a FP- etalon, which allows only multipoint velocity measurements. Thus, there is a need for a new interferometric Rayleigh scattering system that is capable of conducting onedimensional velocity/temperature measurements.

BRIEF SUMMARY OF THE INVENTION

[0006] According to an embodiment, there is a one-dimensional, 1 D, interferometric Rayleigh scattering, IRS, system for measuring a velocity of a gas flow, and the system includes a housing having an input optical port and an output optical port, the input optical port being configured to receive a Rayleigh scattered signal corresponding to Rayleigh scattering due to an interaction between the gas flow and a laser beam, a virtually imaged phased array, VI PA, located inside the housing and configured to generate two-dimensional, 2D, light fringes from the Rayleigh scattered signal; and first to fourth lenses (L1 to L4) configured to focus the 2D light fringes. The VI PA has a body made of a transparent material, and the body is sandwiched between a totally reflective mirror and a partially reflective mirror.

[0007] According to another embodiment, there is a virtually imaged phased array, VIPA, based system for measuring a velocity of a gas flow. The VIPA based system includes a light source configured to generate a light beam, a target zone configured to host an interaction between the gas flow and the light beam to generate a Rayleigh scattered signal, a one-dimensional, 1 D, interferometric Rayleigh scattering, IRS, system configured to generate 2D light fringes from the Rayleigh scattered signal based on a virtually imaged phased array, VIPA, a camera configured to record positions of the 2D light fringes, and a processor configured to calculate a speed of the gas flow based on positions of the 2D light fringes from the camera.

[0008] According to yet another embodiment, there is a method for measuring a velocity of a gas flow, and the method includes providing a light source, which is configured to generate a light beam, generating a gas flow at a target zone to obtain an interaction between the gas flow and the light beam to generate a Rayleigh scattered signal, receiving the Rayleigh scattered signal at a one-dimensional, 1 D, interferometric Rayleigh scattering, IRS, system, generating 2D light fringes from the Rayleigh scattered signal with the 1 D IRS system, recording the 2D light fringes with a camera provided downstream of the 1 D IRS system, and calculating with a processor a speed of the gas flow at a single point in the target zone based on positions of the 2D light fringes from the camera.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0010] Figure 1 is a schematic diagram of a virtually imaged phased array based-system for determining the speed and/or temperature of a gas flow that includes particles not visible with a camera;

[0011] Figure 2 schematically illustrates the implementation of the virtually imaged phased array;

[0012] Figure 3 illustrates two-dimensional (2D) light fringes obtained with the virtually imaged phased array from the Rayleigh scattered light due to the gas flow;

[0013] Figure 4 schematically illustrates a correspondence between actual points in a target area in the gas flow and the 2D light fringes recorded on a camera;

[0014] Figure 5 illustrates the shift between no-flow and flow data acquired with the virtually imaged phased array based-system;

[0015] Figure 6 illustrates the relationship between the temperature of the gas flow and the location of measurement in the flow;

[0016] Figure 7 illustrates the 2D light fringes obtained with the virtually imaged phased array from the surface scattered light from a reference scattering plate;

[0017] Figure 8 shows the axial velocities of the gas flow measured at the center of the flow as a function of flow rates; [0018] Figure 9 illustrates the velocity profile obtained for the gas flow at a given target location downstream from a nozzle’s exit; and

[0019] Figure 10 is a flow chart of the method for measuring the velocity of a gas flow with the virtually imaged phased array based-system shown in Figure 1.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a velocimetry system that uses interferometric Rayleigh scattering and a virtually imaged phased array for gas-phase applications. However, the embodiments to be discussed next are not limited to a gas or to measuring only the velocity, but may be applied to a particulate flow and to measuring the temperature or pressure of such a flow.

[0021] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0022] According to an embodiment, a one-dimensional interferometric Rayleigh scattering velocimetry system is implemented using a virtually imaged phased array (VI PA). A continuous-wave laser, for example, emitting at 532 nm, is used as the source and a low-noise CCD camera is used as the detector. The VI PA has a free spectral range of 15 GHz and was coated for the wavelength range 500- 600 nm. Velocity measurements at different flow rates showed an accuracy of

10 m/s. The instrument precision was between 5-10 m/s. The same system may also be used to measure the temperature of the gas flow, or even its pressure, as discussed later.

[0023] According to an embodiment, as illustrated in Figure 1 , a onedimensional (1 D) interferometric Rayleigh scattering (IRS) system 100 uses a virtually imaged phased array (VI PA) 102 [6] and plural lenses L1 to L4 for optically processing an incoming light beam. The 1 D IRS system 100 is configured to receive Rayleigh scattered light 104-1 for processing as the incoming light beam. The VI PA 102 can be considered as acting as a one-dimensional version of the traditional Fabry-Perot (FP) etalon and the functioning of a VI PA is illustrated in Figure 2.

Unlike in an FP-etalon where the light travels perpendicular to its parallel reflecting surfaces, the Rayleigh scattered light 104 enters VI PA 102 at an angle slightly off from 90 degrees relative to the two parallel mirrors 102A and 102B and then undergoes multiple reflections R between them, as shown in the figure. For each reflection on the partially reflective mirror 102B, one light beam 204-I exits the partially reflective mirror 102B, after being totally reflected at the totally reflective mirror 102A, where I is an integer. The plural light beams 204-I create 2D light fringes when recorded, as discussed later with regard to Figure 3. Note that the use of the VI PA instead of the FP etalon makes the present system to be able to achieve a 1 D velocity and/or temperature measurements. The FP etalon is capable only to achieve multipoint velocity measurements.

[0024] A body 202 of the VI PA 102 is made of a material that is transparent to the incoming light 104, for example, glass or plastic. Note that although VI PA 102 stands for a “virtually imaged” array, it is in fact a real device having the body 202 and the two mirrors 102A and 102B. The totally reflective mirror 102A also has an aperture 206 for receiving the incoming beam 104. The term “virtual” in the VIPA’s name indicates that an image of the light source created by the beams 204-2 to 204- N is virtual, where N is any natural number larger than 2.

[0025] VI PA 102 can be implemented in other ways than the one shown in

Figure 2. VI PA 102 has a few features that distinguish it from the traditional FP etalon. Out of the two mirrors 102A and 102B that cause multiple reflections R resulting in the fringes (not shown), only one is partially reflecting (102B) and the other one (102A) is totally reflecting. As a result, the overall loss of the VI PA 102 is lower compared to that in an etalon, in which both mirrors are totally reflecting. In addition, the VIPA 102 can achieve a spectral resolution in the order of MHz.

However, the advantage of their 1 D nature has not been explored for quantitative measurements. To the inventors’ knowledge, this is the first time when a VIPA is used for 1 D quantitative measurements of flow properties.

[0026] Returning to Figure 1 , the 1 D IRS system 100 further includes a spherical 200-mm focal length lens L1 , for example, with a diameter of 50 mm. Other dimensions may be used for this lens. The lens L1 is configured to directly receive the Rayleigh scattered signal 104-1 from an interaction of the gas flow 110 with a laser beam 122 generated by a laser 120. In one embodiment, the beam 122 may be generated with other light sources such as a high-intensity monochromatic light source, in which case it is not a laser beam, but just a light beam. The lens L1 collimates the Rayleigh scattered signal 104-1 to generate a collimated signal 104-2, which is then focused to the entrance of the VIPA 102, using a 150-mm focal length cylindrical lens L2. Other dimensions may be used for this lens. The focused signal 104 is thus provided as input to the VI PA 102. The VI PA 102 may be coated for the spectral range of 500-600 nm that covers the laser wavelength and has a free spectral range (FSR) of 15 GHz (0.5 cm ^-1 ). The light 104-3 transmitted through the VI PA 102 is then focused on a camera 130 using another cylindrical lens L3 of focal length 750 mm. Other dimensions may be used for this lens. The cylindrical lens L3 generates a focus light 104-4. Lenses L2 and L3 are configured and operated to focus the light in the horizontal plane XY (the focal line is vertical). The focal length of the lens L3 determines the number of fringes that will appear on the camera, and hence the spectral resolution. Another cylindrical lens L4 of focal length 300 mm is focusing the incoming light 104-4 in the vertical plane YZ, and is used to spatially image the laser beam on the camera 130. This lens L4, along with the collection lens L1 , determines the magnification in the spatial (vertical in this case) direction. The lens L4 generates the light beam 104-5, which enters the camera 130 and is recorded by its sensor 132.

[0027] The entire 1 D IRS system 100 may be placed in a housing 106, which has two ports, an input optical port 106A for receiving the Rayleigh scattered light beam 104-1 and an output optical port 106B for allowing the focused light beam 104- 5 to exit the housing 106 and enter the camera 130. An optical port is made of a material that allows a laser or light beam to pass through the port, but it prevents particles or a gas flow to pass.

[0028] A laser 120 or any other light source, placed outside the housing 106, is configured to emit the laser beam 122 at 532 nm. Other wavelengths may be used depending on the scattering efficiency of the gas flow or its speed. In this embodiment, an optical power of 2 W is used for the laser 120. Because the

Rayleigh scattering at the gas flow 110 is highly polarized, a half-wave plate 124 may be used to optimize the polarization of the laser beam 122 for maximum scattering efficiency. The laser beam 122 is then focused vertically down to a target region 125, using a convex lens 126 of 300 mm and then redirected to a beam dump 128. Plural mirrors M1 , M2 may be used to direct the laser beam 122 at the desired locations. The target region 125 is a region located next to the input optical port 106A of the 1 D IRS system 100, where the gas flow 110 interacts with the laser beam 122. Note that target region 125 may be an open space that allows the gas flow 110 to flow unimpeded.

[0029] The velocity measurements for the gas flow 110 are conducted in this embodiment for a jet of air produced using a contoured converging nozzle 112 with an exit diameter of 1 cm. Any other diameter may be used for the nozzle. Air is supplied to the nozzle 112 at a pressure of 7 bar. Other pressures may be used. Measurements are conducted in this embodiment at a location 2.8 cm downstream from the nozzle’s exit. Any other distance may be used for the measurements. For scanning the flow 110 spatially, the nozzle 112 may be mounted on a translation stage 114 and thus it may be moved in steps of 2mm along the laser beam axis. In one application, a processor 160 is configured to control the translation stage 114 to achieve any desired location or to achieve any desired angle between the laser beam direction/axis and the gas flow direction. The nozzle 112 is oriented in this embodiment at an angle of 45° relative to the vertical direction Z and in the same plane containing the incident beam 122 and the axis of the scattered light 104-1 . The scattered light 104-1 is collected at 90° with respect to the incident laser beam 122 because in this orientation, the system provides the highest sensitivity to the flow’s velocity and the Doppler shift Av _D becomes equal to where u is the flow velocity A and A is the laser wavelength.

[0030] The output light beam 104-5 is received by the camera 130, which is an electron-multiplied CCD camera in this embodiment. Other type of cameras may be used. Because the lenses L3 and L4 can be focused separately (i.e. , independent from each other), the magnification in the horizontal and vertical axes of the camera can be controlled independently, thus giving flexibility over the spectral resolution and the spatial magnification of the system, respectively.

[0031] When the 1 D IRS system 100 is used with the laser 120 and camera 130 to measure the gas flow 110’s velocity, a 2D image of the fringes 310 obtained for the Rayleigh scattered signal 104-1 are recorded by the camera 130, as shown in Figure 3. The processor 160, which is in communication with the light source 120, camera 130, and, in some embodiments with the nozzle 112 and/or the translation stage 114 (to control the orientation of the nozzle) is configured to calculate a speed of the gas flow 110 based on positions of the 2D light fringes 310 from the camera 130. A fringe 310 is the thinnest at the center and then broadens on either side. Also, a strong curvature is present for the fringes. It is observed that the shorter the focal length of lens L1 , the stronger is this bowing effect. Figure 3 plots a position where the light beam 104-5 strikes the sensor 132 of the camera 130, along the Y direction, versus a pixel number of the sensor 132. For example, line 320 in Figure 3 corresponds to a vertical position -1 mm relative to the sensor 132 and to multiple pixel numbers on the pixel axis PA. This line corresponds to a single point measurement, i.e., this line characterizes a single point (the point at location -1 mm) in the target region 125. This 1 D measurement is not possible with the existing FT etalons. Figure 4 schematically illustrates the correspondence between various points (only one point 410 is labeled for simplicity) at the intersection of the laser beam 122 and the gas flow 110, that generate the scattered signal 104-1 , and the signals 104-5 recorded by the sensor 132 of the camera 130. In other words, an actually measured target point 410 (1 D measurement) has its associated data present in the light beam 104-5, and this information is spread along the line 320 in the sensor 132. Thus, the line 320 corresponds to a single point measurement 410 in the target zone 125 in Figure 1 , and for this reason this method is called a 1 D measurement method.

[0032] The magnification in the spatial direction is estimated using a target with an array of 50 pm pinholes separated by 200 pm. The camera is hardware- binned by 4 pixels in the vertical direction resulting in a super-pixel resolution of 31 pm. The field of view was 2.2 mm wide. No binning is applied in the horizontal direction to achieve maximum spectral resolution. Based on the application and requirements of magnification, spectral resolution, collection efficiency, etc., other combinations of lenses can be used.

[0033] The VI PA 102 may be placed on a rotation stage 109 (which is configured to rotate the VIPA relative to a desired axis, i.e., the direction of the scattered signal 104-1 , which also corresponds to axis X in Figure 1), which may be controlled by the controller 160, and thus, it was rotated about 3° with respect to the direction of the collection axis. The fine-tuning of this angle is conducted by the processor 160 based on the number of fringes appearing on the camera 130 (see Figure 3). This is because the relative wavelength calibration of the camera is conducted based on the fringe position and the free spectral range of the VI PA. A large number of fringes would appear for larger tilt angles of the VI PA, and this would result in lower spectral resolution because there would be fewer pixels between the fringes. On the lower limit, at least three-four fringes are preferred for an accurate polynomial fit to convert the pixel number to frequency. For the specific VI PA 102 and focal length of the lens L3 used for the embodiment of Figure 1 , five fringes were obtained on the camera 130, as shown in Figure 3. The first line is omitted from the analysis since this fringe is partially clipped at the edges, and a 3rd order polynomial fit is used for frequency calibration fit. Thus, in one embodiment, the processor 160 rotates the VIPA 102 until between 3 and 5 fringes are obtained on the sensor 132 of the camera 130.

[0034] Each row 320 of the 2D image in Figure 3 is processed separately as 1 D data to extract the velocity at a given spatial location 410. The curvature of the fringes 310 does not affect the spatial (vertical direction on the camera) coordinates, as verified by the pinhole array target. A wavelet denoising algorithm may be used to reduce noise from the data. Measurements are conducted with and without the air flow at each spatial location. The measurements 300 with the air flow are shown in Figure 3 and the measurements without the air flow (the reference measurement set) are not shown. The Doppler shift between these two data sets is determined by shifting the flow data in the frequency space until the least squared difference between the fringes is achieved. The velocity is then estimated based on this Doppler shift using equation (1) discussed above.

[0035] The 1 D profiles of the Rayleigh signal fringes at the central pixel of the camera 130, with and without the flow 110, are shown in Figure 5 for a flow 110’s velocity of 242 m/s. The fringes are peak-normalized and the centroids C1 and C2 of the peaks are shown with dashed lines. The shift 510 in the peaks is visible in Figure 5. In fact, the fringe profile 520 for the case with the flow is thinner in cross-section and larger in amplitude compared to the no-flow case 522 because of the lower temperature in the expanded flow.

[0036] Various measurements performed with the system 100 showed that the laser frequency drifts in time and this results in a bias in velocity and/or temperature measurements. This phenomenon was also observed from the measurements conducted in static air. To track this frequency drift, the laser frequency needs to be monitored during the gas flow and/or temperature measurements, either by sampling a small portion of the laser beam and sending it directly to the camera with the help of a beam splitter, or using a scattering plate 140 to scatter a portion of the laser beam 122 to the system. As the latter strategy was implemented in the embodiment of Figure 1, a portion 142 of the laser beam 122 is sampled and allowed to fall on the scattering plate 140 placed behind the nozzle 112. A beam splitter 144 is used to split the laser beam 122. The scattered light 142 produces reference fringes 700 as shown in Figure 7, and it can be used for tracking the laser frequency variations.

[0037] However, in the VIPA-based system 150 (which includes the 1D IRS system 100, the light source 120, the camera 130 and the target area 125) shown in Figure 1, both the reference fringes 700 and the Rayleigh fringes 300 overlap, making it difficult to process the data. Thus, a different strategy was followed for the embodiment shown in Figure 1. A mechanical shutter 146, controlled by a delay generator 148, is placed in the beam 142’s path to the scattering plate 140. The mechanical shutter 146 is configured to close and open (as instructed by the delay generator 148) after every image, resulting in alternate Rayleigh and Rayleighoverlapped surface scattering fringes on the camera 130. The delay generator 148 is in direct communication with the mechanical shutter 146 and the camera 130 for coordinating the closing and opening of the mechanical shutter. The light beam 142 may be directed to the scattering plate 140 with the help of a mirror 151 and a diverging lens 152. The exposure time for each image was 1 second in this embodiment. Shorter or longer exposure times may be used. 50 images of each case were captured and they were separated during post-processing. The averaged Rayleigh scattering signal was subtracted from the Rayleigh-overlapped surface scattered signal to obtain the pure surface scattering component. By fitting a Gaussian profile and finding the central tendency of each surface scattering fringe, the relative variation in laser frequency with time can be tracked and used to correct for the error in velocity estimate from the Rayleigh signal. Since the laser frequency does not drift significantly in a few seconds duration, this shutter-based strategy worked well. In applications that demand simultaneous monitoring of the laser frequency, a second VI PA system or any other frequency monitoring unit can be used to track the laser frequency shift. Based on the 0-velocity measurements, this laser frequency shift correction reduces the error in measurements from values as high as ~45 m/s to ± 3 m/s.

[0038] It should be noted that the velocity measured using the system 150 represents the axial velocity component of the flow 110. Also, any deviation from the assumed 45° angle of the nozzle 112 would result in an under-estimation of the velocity. To measure the accuracy of the system 150, a calibration test with varying flow rates was conducted by connecting a flow meter just upstream of the nozzle 112. The flow rate was varied from 0 SLPM (standard liter per minute) to 700 SLPM at steps of 100 SLPM in randomized order. The result of this experiment is shown in Figure 8, along with the corresponding numerically predicted values of the axial flow velocity. The dots 810 represent the calculated values and the error bars correspond to a 1 % error representing the flow meter uncertainty. The dots 820 represent the experimental results at the center of the flow with error bars representing the standard deviation over 50 images. The average error between the numerically predicted values and the experimental results was ~ ±10 m/s, indicating a high accuracy of the measurements. The dynamic range of the instrument is estimated to be 5,643 m/s with an assumption that the free spectral range is the maximum Doppler shift allowed.

[0039] The mean and standard deviation of the nozzle flow velocity measurements are shown in Figure 9. The total scanned region is ± 12.5 mm from the flow 110’s axis. The core flow velocity is -250 m/s. The profile is asymmetric because the scanning of the nozzle 112 is done along the laser beam 122’s direction, which is tilted by 45° relative to the nozzle flow’s axis. As expected, the slope in velocity at the interface between the flow and the surrounding air is larger in the lower part (negative values in the x-axis of Figure 9) of the flow which is closer to the nozzle exit. The single-shot precision of measurements, quantified by the standard deviation across measurements from 50 images, is also shown by the line 910. The precision is -5 m/s in the center of the selected field of view (FOV) and increases to -10 m/s at the edges of FOV in the low-velocity region. This worsening of precision is due to the slight broadening of fringes and reduced intensity in this region. In the high-velocity region, the standard deviation in velocity is as high as ± 15 m/s near the edges of FOV due to larger disturbances in the flow, resulting in larger fluctuations in the assumed flow angle.

[0040] In one embodiment, it is also possible to extract the 1 D temperature profile of the flow from such data when comparing the two profiles, especially the full width at half maximum (FWHM). For example, Figure 6 shows the 1 D temperature profile obtained in a heated flow experiment when comparing the FWHMs of the profiles 520 and 522. The measurements correspond to a location 2.8 cm downstream from the exit of the contoured converging nozzle 112 through which the heated flow 112 of air was passed. The experimentally obtained fringe shape in Figure 6 was fitted against simulated Rayleigh Brillouin spectra at atmospheric pressure, with temperature as the free variable. The temperature profile is asymmetric because the scan was done along the vertical (laser beam) direction while the nozzle is tilted by 45° with respect to the laser beam direction. Similar 1 D measurement of pressure can also be conducted if VIPAs of higher resolution become available. Figure 6 shows the temperature versus the location of the measurement for the gas flow 110.

[0041] The VIPA-based 1 D interferometric Rayleigh scattering velocimetry system 100, when used as part of the system 150, demonstrated the capability to measure the flow’s velocity with a precision of 5-10 m/s and an accuracy of 10 m/s. The same system demonstrated the capability to measure the temperature of the flow, as illustrated in Figure 6. The system 150 shows the applicability of VIPA 102 for 1 D quantitative measurements, but this strategy also opens the door for other variations and extensions. For example, the system can be implemented using pulsed lasers for instantaneous velocity measurements. It can also be extended to kHz repetition rate using pulse burst lasers and applied to short-duration applications such as shock tunnels and detonation tubes. By including a molecular filter, the system can be used in applications where surface scattering and Mie scattering are problematic. If simultaneous velocity and temperature measurements are possible, the system can be coupled with a 1 D multi-species Raman system to achieve 1 D measurements of temperature, velocity, pressure, and mole fractions of all major species in open flames as well as in enclosed applications such as optical engines. [0042] A method for measuring velocity of a gas flow 110 is now discussed with regard to Figure 10. The method includes a step 1000 of providing a light source, which is configured to generate a light beam, a step 1002 of generating a gas flow at a target zone to obtain an interaction between the gas flow and the light beam to generate a Rayleigh scattered signal, a step 1004 of receiving the Rayleigh scattered signal at a one-dimensional, 1 D, interferometric Rayleigh scattering, IRS, system, a step 1006 of generating 2D light fringes from the Rayleigh scattered signal with the 1 D IRS system, a step 1008 of recording the 2D light fringes with a camera provided downstream of the 1 D IRS system, and a step 1010 of calculating a speed of the gas flow with a processor at a single point in the target zone, based on positions of the 2D light fringes from the camera. The method may further include a step of calculating a temperature of the gas flow based on the shape of the 2D light fringes. In one embodiment, the gas flow includes particles not visible to the camera. [0043] The method discussed above may have several potential applications in aerospace and combustion applications. Gas-phase flow velocities in high-speed flows and turbulent jets can be measured using the system 150 shown in Figure 1. By including a molecular filter in the collection path to suppress any surface scattering component, it can also be applied to measure flow velocities in enclosed facilities such as optical engines, where surface scattering from windows would be a problem. By using a pulse burst laser, the system can be utilized to study short- duration phenomena such as detonation and shock tunnel flows. As discussed with regard to Figure 6, it is also possible to extract the 1 D temperature profile of the gas flow using this technique.

[0044] The disclosed embodiments provide an IRS velocimetry and/or temperature system that uses a VI PA. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0045] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0046] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

References

The entire content of all the publications listed herein is incorporated by reference in this patent application.

[1] R. G. Seasholtz, A. E. Buggele, and M. F. Reeder, Optics and Lasers in Engineering 27, 543-570 (1997).

[2] R. B. Miles, W. R. Lempert, and J. N. Forkey, Measurement Science and Technology 12, R33-R51 (2001).

[3] J. W. Foreman, E. W. George, and R. D. Lewis, Applied Physics Letters 7, 77-78 (1965).

[4] J. Estevadeordal, N. Jiang, A. D. Cutler, J. J. Felver, M. N. Slipchenko, P. M.

Danehy, J. R. Gord, and S. Roy, Applied Physics B 124, 41 (2018).

[5] A. D. Cutler, K. Rein, S. Roy, P. M. Danehy, and N. Jiang, Optics Express 28, 3025- 3040 (2020).

[6] M. Shirasaki, Optics Letters 21 , 366-368 (1996).