Absorption laser differential interferometer

Abstract

The invention provides an absorption laser differential interferometer. The device comprises a signal generator, a laser controller, a laser, a plane mirror FM, a first variable polarization beam splitter vPBS1, a second variable polarization beam splitter vPBS2, a half-wave plate HWP, a first Wollaston prism Wp, a first optimal shape spherical lens L1p, a second optimal shape spherical lens L2p, a first field lens L1c, a second field lens L2c, a second Wollaston prism WC, a non-polarization beam splitter BS and a quarter-wave plate QWP. A linear polarizer LP; and an oscilloscope. The device is simple and reasonable in structural design, can realize more complete and accurate representation of a high-speed flow field, and realizes multi-parameter synchronous measurement of the flow field; when the two technologies are combined for use, the limitation of a single method can be overcome, the physical integrity of measurement is improved, the applicability in a complex flow environment is enhanced, and the measurement capability in the fields of hypersonic wind tunnel experiments, scramjet engine combustion diagnosis, turbulent flow research and the like is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of interference measurement, and in particular to an absorption laser differential interferometer. Background Art

[0002] Interferometry, with its exceptional sensitivity to minute changes in refractive index (or density), offers unique advantages in measurements in hypersonic wind tunnels and high-speed combustion devices (such as scramjets). It can be used to accurately characterize incoming flows and capture complex flow field structures around models. Laser differential interferometry (LDI), developed in the 1970s, achieves high-precision detection of refractive index variations along the entire path of the separated beams by measuring the optical path difference between two closely spaced parallel beams. In recent years, the further developed focused laser differential interferometry (FLDI), with its superior interference rejection and ability to effectively eliminate noise interference from areas outside the flow field (such as the boundary layer on wind tunnel walls / windows), has become an important tool in fluid diagnostics, widely used for incoming flow characterization, boundary layer transition measurements, and turbulent jet diagnostics. Absorption spectroscopy, a mature measurement technique, has a long history in the gas processing industry, atmospheric remote sensing, wind tunnel measurements, shock tube experiments, and combustion diagnostics. This method can synchronously measure the temperature, pressure, gas concentration and flow rate of the fluid through a single laser beam. Due to its simplicity and efficiency, it has been widely used in the field of hypersonic testing, especially in the characterization of wind tunnel flow conditions, showing excellent measurement capabilities.

[0003] While LDI / FLDI and absorption spectroscopy each offer unique advantages in flow field measurement, they still have limitations when used alone. LDI / FLDI can provide high-temporal-resolution density fluctuation information, but accurately correlating these measurements to the fluid's fundamental thermodynamic parameters (temperature, pressure, density, etc.) presents numerous challenges, hindering a comprehensive understanding of the flow field state. While absorption spectroscopy can accurately measure temperature, pressure, and gas concentration, it is limited by its low temporal resolution, making it difficult to resolve the details of rapidly changing flow fields.

[0004] In summary, it is necessary to make further innovations to the existing technologies. Summary of the Invention

[0005] In response to the technical problems existing in the above-mentioned background technology, the present invention proposes an absorption laser differential interferometer, which aims to use the flow field pulsation information (such as density pulsation) measured by laser differential interferometry technology and the basic fluid properties (temperature, pressure, density, etc.) measured by absorption spectroscopy technology. Linking the two can not only make up for their respective shortcomings, but also achieve a more complete and accurate characterization of the high-speed flow field, and realize the simultaneous measurement of multiple parameters of the flow field; when the two technologies are used in combination, they can overcome the limitations of a single method, improve the physical integrity of the measurement, enhance the applicability in complex flow environments, and improve the measurement capabilities in fields such as hypersonic wind tunnel experiments, scramjet engine combustion diagnosis, and turbulent flow research.

[0006] In order to solve the above technical problems, the present invention provides an absorption laser differential interferometer, which includes a signal generator, a laser controller, a laser Laser, a plane mirror FM, a first variable polarization beam splitter vPBS1, a second variable polarization beam splitter vPBS2, a half-wave plate HWP, a first Wollaston prism W p , the first best shape spherical lens L 1p , the second best shape spherical lens L 2p 、First shot L 1c , second scene lens L 2c 、Second Wollaston prism W C , non-polarizing beam splitter BS, quarter wave plate QWP, linear polarizer LP and oscilloscope;

[0007] One end of the laser controller is connected to the signal generator, and the other end is connected to the laser. The signal generator is used to modulate the current input to the laser.

[0008] The plane reflector FM is matched and arranged on the laser output optical path of the laser Laser, and is used to fold the laser output optical path of the laser Laser so that the laser beam is aligned with the main optical axis;

[0009] The first variable polarization beam splitter vPBS1 is matched and arranged on the laser beam output optical path of the plane reflector FM, and is used to receive the laser beam reflected by the plane reflector FM and output it after transmission and reflection;

[0010] The second variable polarization beam splitter vPBS2 is matched and arranged on the reflected light path of the first variable polarization beam splitter vPBS1, and is used to receive the laser beam reflected by the first variable polarization beam splitter vPBS1 and output it after transmission and reflection;

[0011] The half-wave plate HWP is matched and arranged on the transmission light path of the first variable polarization beam splitter vPBS1, and is used to rotate the polarization direction of the laser beam by an angle twice the angle between its optical axis and the polarization direction;

[0012] The first best-shaped spherical lens L 1p The laser beam output path matched with the half-wave plate HWP is used to focus the light passing through the half-wave plate HWP onto the first Wollaston prism W p internal;

[0013] The first Wollaston prism W p Matching is set in the first best shape spherical lens L 1p In the laser beam output optical path, it is used to split the incident polarized light into two laser beams with mutually orthogonal polarization states and separated directions, and then output them;

[0014] The second best shape spherical lens L 2p Matching is set on the first Wollaston prism W p The laser beam output optical path is used to connect the first Wollaston prism W p The two separated light beams are collimated so that they remain parallel when passing through the flow field and then output;

[0015] The second field mirror L 2c Matching is set in the second best shape spherical lens L 2p The laser beam output optical path is used to refocus the two parallel laser beams that were originally separated after passing through the flow field onto the second Wollaston prism W c superior;

[0016] The second Wollaston prism W c Matching is set in the second field lens L 2c In the laser beam output optical path, it is used to bend the two parallel laser beams back to the same straight line direction so that they can be combined and output;

[0017] The first field lens L 1c Matching is set on the second Wollaston prism W c The laser beam is output on the optical path and is used to be reflected by the second Wollaston prism W c The combined laser beam is re-collimated and then output;

[0018] The non-polarization beam splitter BS is matched with the first field lens L 1c The laser beam output optical path is used to receive the laser beam from the first field lens L. 1c The output laser beam is transmitted and reflected and then output, one path is used for absorption signal and the other path is used for interference signal;

[0019] The quarter-wave plate QWP is matched and arranged on the laser beam output path of the non-polarizing beam splitter BS. It is used to eliminate the polarization ellipticity caused by the phase delay of the two combined beams during the propagation process, restore the linear polarization state and output it, creating conditions for interference;

[0020] The linear polarizer LP is matched with the laser beam output path of the quarter wave plate QWP, and is used to force the two combined beams of linear polarization states to have consistent polarization and interfere to produce an intensity signal;

[0021] The signal input end of the oscilloscope is connected to a photodetector and is used to detect and collect the laser beam output after being transmitted and reflected by the second variable polarization beam splitter vPBS2, the laser beam output after being transmitted and reflected by the non-polarization beam splitter BS, and the laser beam output by the linear polarizer LP.

[0022] The absorption laser differential interferometer, wherein: the transmission light path of the second variable polarization beam splitter vPBS2 is also matched with an etalon Etalon, and the laser beam coming out of the transmission light path of the second variable polarization beam splitter vPBS2 passes through the etalon Etalon and is detected and collected by the photodetector.

[0023] The absorption laser differential interferometer is characterized in that: the plane mirror FM has a pair, the first plane mirror FM is matched and set on the laser output optical path of the laser Laser, and the mirror surface forms a 45° angle with the laser output optical path of the laser Laser; the second plane mirror FM is matched and set on the reflection optical path of the first plane mirror FM, and the mirror surface forms a 90° angle with the mirror surface of the first plane mirror FM. The second plane mirror FM will receive the reflected light beam from the first plane mirror FM and reflect it again and input it into the first variable polarization beam splitter vPBS1.

[0024] The absorption laser differential interferometer is characterized in that: the signal generator is an arbitrary function generator.

[0025] The absorption laser differential interferometer is based on the following: the laser is a tunable diode laser, which is installed in a heat dissipation unit with a collimating optical device and is used to output a laser beam.

[0026] By adopting the above technical solution, the present invention has the following beneficial effects:

[0027] The absorption laser differential interferometer of the present invention has a simple and reasonable structural design. The data measured by laser differential interferometry technology can be converted into the optical path difference between the detection beams, and then converted into density pulsation. These measurement results are linked to the basic properties of the fluid (temperature, pressure, density, etc.) measured by absorption spectroscopy technology, realizing multi-parameter synchronous measurement in complex environments and improving the physical integrity of the measurement.

[0028] This method uses the principle of interference to measure minute changes in optical path differences, enabling detection of extremely small changes in density, refractive index, or pressure with exceptionally high sensitivity. Absorption spectroscopy quantitatively analyzes flow field parameters through the absorption of specific wavelengths of light by a substance, providing precise spectral information. Therefore, combining the two can improve measurement accuracy and enhance experimental measurement capabilities.

[0029] The present invention can simultaneously and coaxially measure the flow field interference signal (for flow field density pulsation measurement) and the absorption signal (for average flow field characteristic measurement), greatly improving the test efficiency and providing more accurate flow field conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 A schematic structural diagram of an absorption laser differential interferometer provided by an embodiment of the present invention.

[0032] Notes:

[0033] Laser-laser; PM-plane mirror; vPBS1-variable polarization beam splitter; vPBS2-variable polarization beam splitter; HWP-half-wave plate; W p -First Wollaston prism; W C -second Wollaston prism; L1-best shape spherical lens; L2-best shape spherical lens; BS-non-polarizing beam splitter; QWP-quarter wave plate; LP-linear polarizer; Etalon-etalon; PD1-photodetector; PD2-photodetector; PD3-photodetector; PD4-photodetector; subscript P-transmitter; subscript C-receiver. DETAILED DESCRIPTION

[0034] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0035] The present invention will be further explained below with reference to specific embodiments.

[0036] like Figure 1 As shown, the embodiment provides an absorption laser differential interferometer, including a signal generator, a laser controller, a laser Laser, a plane mirror FM, a first variable polarization beam splitter vPBS1, a second variable polarization beam splitter vPBS2, a half-wave plate HWP, a first Wollaston prism W p , the first best shape spherical lens L 1p , the second best shape spherical lens L 2p 、First shot L 1c , second scene lens L 2c 、Second Wollaston prism W C , non-polarization beam splitter BS, quarter wave plate QWP, linear polarizer LP, oscilloscope, etalon Etalon, first photodetector PD1, second photodetector PD2, third photodetector PD3 and fourth photodetector PD4.

[0037] One end of the laser controller is connected to the signal generator, and the other end is connected to the laser.

[0038] The signal generator is an arbitrary function generator, which is used to modulate the current input to the laser.

[0039] The laser uses a tunable diode laser, which is mounted in a heat sink unit with collimating optics for outputting a light beam.

[0040] The plane mirrors FM are a pair. The first plane mirror FM is matched with the laser output optical path of the laser Laser, with its mirror surface forming a 45° angle with the laser beam output by the laser Laser, so that the received laser beam is reflected at a 45° angle. The second plane mirror FM is matched with the reflection optical path of the first plane mirror FM, with its mirror surface forming a 90° angle with the mirror surface of the first plane mirror FM, so that the second plane mirror FM reflects the reflected beam received from the first plane mirror FM. The light emitted by the laser Laser is linearly polarized light. The pair of plane mirrors PM folds the optical path so that the beam can be precisely aligned with the main optical axis (dashed line).

[0041] The first variable polarization beam splitter vPBS1 is matched with the reflection light path of the second plane reflector FM and receives the laser beam reflected by it, and transmits and reflects the laser beam. The second variable polarization beam splitter vPBS2 is matched with a side in a perpendicular direction to the first variable polarization beam splitter vPBS1, specifically matched with the reflection light path of the first variable polarization beam splitter vPBS1, and is used to receive the laser beam reflected by the first variable polarization beam splitter vPBS1 and output it after transmission and reflection.

[0042] The half-wave plate HWP is matched and arranged on the transmission light path of the first variable polarization beam splitter vPBS1, and is used to rotate the polarization direction of the laser beam by twice the angle between its optical axis and the polarization direction, so that the subsequent first Wollaston prism W p It is split into two beams of orthogonal polarization at slightly offset angles.

[0043] The first best-shape spherical lens L 1p The matching is set on the laser beam output optical path of the half-wave plate HWP, which is used to focus the light passing through the half-wave plate HWP on the first Wollaston prism W p (The light beam needs to be focused inside it to achieve a smaller angle of separation, which is convenient for subsequent collimation) inside, spatial beam splitting is achieved.

[0044] The first Wollaston prism W p A W-Wollaston prism is used, which is matched with the first best shape spherical lens L 1p In the laser beam output optical path, it is used to split the incident linearly polarized light into two beams with mutually orthogonal polarization states (such as vertical and horizontal) and slightly separated directions, which is used to measure the refractive index change of the flow field.

[0045] The second best shape spherical lens L 2p Matching is set in the first Wollaston prism W p The laser beam output optical path is used to connect the first Wollaston prism W p The two separated light beams are collimated so that they remain parallel when passing through the flow field and then output.

[0046] The second field lens L 2c Matching is set in the second best shape spherical lens L 2p The laser beam output path is used to refocus the two slightly separated parallel laser beams that passed through the flow field onto another second Wollaston prism W. c To facilitate subsequent interference analysis.

[0047] The second Wollaston prism W c Matching is set in the second field mirror L 2cIn the laser beam output optical path, it is used to "fold back" the two parallel beams to the same straight line direction so that they can be combined and output.

[0048] The first field lens L 1c Matching is set in the second Wollaston prism W c The laser beam is output on the optical path and is used to be reflected by the second Wollaston prism W c The combined laser beam is re-collimated and output to facilitate subsequent interference and absorption measurements.

[0049] The non-polarization beam splitter BS is matched with the first field mirror L 1c The laser beam output optical path is used to receive the laser beam from the first field mirror L. 1c The output laser beam is transmitted and reflected and then output, one way is used for absorption signal and the other way is used for interference signal.

[0050] The quarter-wave plate QWP is matched and arranged on the laser beam output path of the non-polarizing beam splitter BS. It is used to eliminate the polarization ellipticity that may be caused by the phase delay of the two combined beams during the propagation process, restore its linear polarization state and output it, creating conditions for interference.

[0051] The linear polarizer LP is matched with the laser beam output light path of the quarter wave plate QWP and is used to force the two combined beams of linear polarization states to have consistent polarization and interfere to produce an intensity signal.

[0052] The signal output end of the first photodetector PD1 is connected to an oscilloscope, and the signal input end is used to detect and collect the reflected laser beam of the second variable polarization beam splitter vPBS2.

[0053] The signal input end of the etalon Etalon is matched and arranged on the laser beam transmission light path of the second variable polarization beam splitter vPBS2.

[0054] The signal output end of the second photodetector PD2 is connected to the oscilloscope, and the signal input end is connected to the signal output end of the standard device Etalon through the light beam signal. The laser beam coming out of the transmission light path of the second variable polarization beam splitter vPBS2 passes through the standard device Etalon and is detected and collected by the second photodetector PD2.

[0055] The signal output terminal of the third photodetector PD3 is connected to an oscilloscope, and the signal input terminal is used to detect and collect the reflected laser beam of the non-polarization beam splitter BS.

[0056] The signal output end of the fourth photodetector PD4 is connected to an oscilloscope, and the signal input end is used to detect and collect the laser beam output by the linear polarizer LP.

[0057] The working principle of the present invention is:

[0058] The laser controller is connected to a tunable diode laser, which is mounted in a heat sink with collimating optics for outputting a light beam. An arbitrary function generator is connected to the laser controller to modulate the current input to the laser. The light emitted by the laser is linearly polarized. First, the optical path is folded by two plane mirrors PM so that the beam can be precisely aligned with the principal optical axis (dashed line). A variable polarization beam splitter vPBS1 is then used to adjust the transmission / reflection of the light. The reflected light then passes through another identical vPBS2 and is again used to adjust the transmission / reflection of the light. The reflected light is irradiated onto the photodetector PD1 for measuring the pre-absorption signal in the absorption spectrum. The light transmitted from vPBS2 passes through an etalon and then irradiates the second photodetector PD2. The light initially transmitted through vPBS1 is linearly vertically polarized, and then the linear polarization is rotated 45° by the half-wave plate HWP before entering the first Wollaston prism W. p The light is split into two mutually perpendicular polarized beams. A best-shape spherical lens L is placed between the HWP and the WP. 1P , to focus the beam at W P and expand the two split beams outward to another best-shaped spherical lens L 2P On. P Placed in L 1P The focus of L 2P Then place it on W P This ensures that the two beams of light propagate parallel to each other and that each beam is parallel when passing through the test gas. After the two collimated and parallel beams pass through the test gas, there is an identical field lens L on the other side. 2C , focusing these two beams into a matching but opposite second Wollaston prism W C When placed on L 2C When the two beams are at the focus of W C Then another best shape spherical lens L 1CThe collinear beams are collimated. A reflective / transmissive non-polarizing beam splitter (BS) is mounted in a cage cube holder. The reflected light is directed to photodetector PD3 for absorption signal measurement. Although both collinear beams are reflected at BS, they do not interfere, so the absorption signal is unaffected by the optical path difference between the two beams. The transmitted light passes through BS. Because the phases of the two coaxial beams may be shifted / delayed (resulting in a slightly elliptical effective polarization state), a quarter-wave plate (QWP) is used to remove this elliptical polarization and restore the light to standard linear polarization. A linear polarizer (LP) causes the two coaxial beams to interfere, which then illuminates a fourth photodetector (PD4) for measurement using laser differential interferometry. LP is adjusted to the center of the infinite interference fringe to achieve maximum linear sensitivity and avoid phase ambiguity. All photodetectors are connected to an oscilloscope for signal digitization.

[0059] The present invention utilizes laser differential interferometry technology to measure flow field pulsation information (such as density pulsation) and absorption spectroscopy technology to measure basic fluid properties (temperature, pressure, density, etc.). Linking the two can not only compensate for their respective shortcomings, but also achieve a more complete and accurate characterization of high-speed flow fields, and realize multi-parameter synchronous measurement of flow fields. When the two technologies are used in combination, they can overcome the limitations of a single method, improve the physical integrity of the measurement, enhance the applicability in complex flow environments, and improve measurement capabilities in fields such as hypersonic wind tunnel experiments, scramjet engine combustion diagnosis, and turbulent flow research.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A differential interferometer based on absorption laser, characterized by: The interferometer includes a signal generator, a laser controller, a laser Laser, a plane mirror FM, a first variable polarization beam splitter vPBS1, a second variable polarization beam splitter vPBS2, a half-wave plate HWP, a first Wollaston prism W p , the first best shape spherical lens L 1p , the second best shape spherical lens L 2p 、First shot L 1c , second scene lens L 2c 、Second Wollaston prism W C , non-polarizing beam splitter BS, quarter wave plate QWP, linear polarizer LP and oscilloscope; One end of the laser controller is connected to the signal generator, and the other end is connected to the laser. The signal generator is used to modulate the current input to the laser. The plane reflector FM is matched and arranged on the laser output optical path of the laser Laser, and is used to fold the laser output optical path of the laser Laser so that the laser beam is aligned with the main optical axis; The first variable polarization beam splitter vPBS1 is matched and arranged on the laser beam output optical path of the plane reflector FM, and is used to receive the laser beam reflected by the plane reflector FM and output it after transmission and reflection; The second variable polarization beam splitter vPBS2 is matched and arranged on the reflected light path of the first variable polarization beam splitter vPBS1, and is used to receive the laser beam reflected by the first variable polarization beam splitter vPBS1 and output it after transmission and reflection; The half-wave plate HWP is matched and arranged on the transmission light path of the first variable polarization beam splitter vPBS1, and is used to rotate the polarization direction of the laser beam by an angle twice the angle between its optical axis and the polarization direction; The first best-shaped spherical lens L 1p The laser beam output path matched with the half-wave plate HWP is used to focus the light passing through the half-wave plate HWP onto the first Wollaston prism W p internal; The first Wollaston prism W p Matching is set in the first best shape spherical lens L 1p In the laser beam output optical path, it is used to split the incident polarized light into two laser beams with mutually orthogonal polarization states and separated directions, and then output them; The second best shape spherical lens L 2p Matching is set on the first Wollaston prism W p The laser beam output optical path is used to connect the first Wollaston prism W p The two separated light beams are collimated so that they remain parallel when passing through the flow field and then output; The second field mirror L 2c Matching is set in the second best shape spherical lens L 2p The laser beam output optical path is used to refocus the two parallel laser beams that were originally separated after passing through the flow field onto the second Wollaston prism W c superior; The second Wollaston prism W c Matching is set in the second field lens L 2c In the laser beam output optical path, it is used to bend the two parallel laser beams back to the same straight line direction so that they can be combined and output; The first field lens L 1c Matching is set on the second Wollaston prism W c The laser beam is output on the optical path and is used to be reflected by the second Wollaston prism W c The combined laser beam is re-collimated and then output; The non-polarization beam splitter BS is matched with the first field lens L 1c The laser beam output optical path is used to receive the laser beam from the first field lens L. 1c The output laser beam is transmitted and reflected and then output, one path is used for absorption signal and the other path is used for interference signal; The quarter-wave plate QWP is matched and arranged on the laser beam output path of the non-polarizing beam splitter BS. It is used to eliminate the polarization ellipticity caused by the phase delay of the two combined beams during the propagation process, restore the linear polarization state and output it, creating conditions for interference; The linear polarizer LP is matched with the laser beam output path of the quarter wave plate QWP, and is used to force the two combined beams of linear polarization states to have consistent polarization and interfere to produce an intensity signal; The signal input end of the oscilloscope is connected to a photodetector and is used to detect and collect the laser beam output after being transmitted and reflected by the second variable polarization beam splitter vPBS2, the laser beam output after being transmitted and reflected by the non-polarization beam splitter BS, and the laser beam output by the linear polarizer LP.

2. The absorption laser differential interferometer according to claim 1, wherein: An etalon Etalon is also matched and arranged on the transmission light path of the second variable polarization beam splitter vPBS2. The laser beam coming out of the transmission light path of the second variable polarization beam splitter vPBS2 passes through the etalon Etalon and is detected and collected by the photodetector.

3. The absorption laser differential interferometer according to claim 1, wherein: The plane mirror FM has a pair, the first plane mirror FM is matched and set on the laser output optical path of the laser Laser, and the mirror surface is at a 45° angle to the laser output optical path of the laser Laser; the second plane mirror FM is matched and set on the reflection optical path of the first plane mirror FM, and the mirror surface is at a 90° angle to the mirror surface of the first plane mirror FM. The second plane mirror FM will receive the reflected light beam from the first plane mirror FM and reflect it again and input it into the first variable polarization beam splitter vPBS1.

4. The absorption laser differential interferometer according to claim 1, wherein: The signal generator is an arbitrary function generator.

5. The absorption laser differential interferometer according to claim 1, wherein: The laser adopts a tunable diode laser, which is installed in a heat dissipation unit with a collimating optical device and is used to output a laser beam.

Images