Laser cavitation diagnosis device and method based on double-frame delay synchronous phase shift

Abstract

The invention discloses a laser cavitation diagnosis device and method based on double-frame delay synchronous phase shift. The device comprises an excitation module and a detection module. The excitation module comprises a first laser, a first 1 / 2 wave plate, a polarization splitting prism, a spectroscope, an energy meter, a convex lens and a water tank; the detection module comprises a second laser, a beam expanding lens group, a second 1 / 2 wave plate, a second polarization splitting prism, a third 1 / 2 wave plate, a first reflector, a second reflector, a fourth 1 / 2 wave plate, a third polarization splitting prism, a third reflector, a fourth polarization splitting prism, a first 1 / 4 wave plate, a first imaging lens, a first micro-polarization camera, a second 1 / 4 wave plate and a second imaging lens; and a second micro-polarization camera. According to the invention, a double-frame delay method is introduced into traditional synchronous phase-shifting interference, the defects of the existing cavitation detection technology are overcome, and instantaneous phase change observation in a single cavitation pulsation period is realized.

Description

Technical Field

[0001] The invention belongs to the field of laser induced cavitation measurement, and in particular relates to a laser cavitation diagnosis device and method based on double-frame time-delay synchronous phase shifting. Background Art

[0002] As an important topic in the field of fluid mechanics, the study of cavitation dynamics and cavitation mechanism has significant engineering application value in many fields such as ship propulsion systems, hydraulic machinery, biomedical engineering, etc. When the local pressure of the fluid is lower than the saturated vapor pressure, the cavitation generated can produce impact pressures of up to hundreds of MPa and high-speed microjets of hundreds of meters per second at the moment of collapse. This physical effect will not only cause cavitation damage to the mechanical surface, but also cause flow field pressure pulsation and reduce energy conversion efficiency.

[0003] Compared with the traditional cavitation research method that relies on flow field disturbance to generate cavitation, laser-induced cavitation technology has become an ideal means of studying cavitation mechanisms due to its unique physical advantages. By adjusting the pulse energy, pulse width and focusing parameters of the laser, a cavitation structure with good spherical symmetry can be accurately generated in a transparent medium, and its initial radius can be precisely controlled in the micron to millimeter range. This non-contact cavitation generation method effectively avoids the flow field interference introduced by the traditional cavitation nucleus, and provides a controllable physical model for studying the pure mechanical behavior of cavitation.

[0004] The instantaneous changes in the early stage of cavitation expansion and the late stage of collapse are obvious. The existing synchronous phase-shifting interferometry is limited by the inability to capture the instantaneous phase changes within a single cavitation pulsation cycle. The high-speed photography method has sufficient resolution but cannot obtain phase information. Therefore, realizing the phase observation research of nanosecond delay during a single cavitation pulsation has become an urgent problem to be solved in laser cavitation diagnosis technology. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of existing cavitation detection technology and to provide a laser cavitation diagnosis device and method based on dual-frame time-delay synchronous phase shifting. The synchronous phase-shifting interference device can be used for a laser beam to provide cavitation information before and after nanosecond delay. The structure is compact, the components used are commonly used components, no special production is required, and debugging is convenient. The principle of the synchronous phase-shifting interference method is simple, and the phase distribution to be measured can be restored by obtaining four frames of phase-shifting interference images at the same time.

[0006] In order to achieve the above-mentioned object, the present invention adopts the following technical scheme: a laser cavitation diagnosis device based on double-frame time-delay synchronous phase shifting, comprising an excitation module and a detection module;

[0007] The excitation module includes a first laser, a first 1 / 2 wave plate, a first polarization beam splitter prism, a beam splitter, an energy meter, a convex lens and a water tank; after the first laser emits a high-energy laser beam, it passes through an attenuator composed of the first 1 / 2 wave plate and the polarization beam splitter prism in sequence, and the laser energy is measured in real time by using a beam splitter and an energy meter, and is focused in water by using a convex lens to generate cavitation;

[0008] The detection module includes a second laser, a beam expander, a second 1 / 2 wave plate, a second polarization beam splitter, a third 1 / 2 wave plate, a first reflector, a second reflector, a fourth 1 / 2 wave plate, a third polarization beam splitter, a third reflector, a fourth reflector, a fourth polarization beam splitter, a first 1 / 4 wave plate, a first imaging lens, a first micro-polarization camera, a second 1 / 4 wave plate, a second imaging lens and a second micro-polarization camera; the second laser emits a parallel light beam through the beam expander, which is divided into two orthogonal linear polarized lights through the second polarization beam splitter, one of which is used as the first detection light and passes through the third 1 / 2 wave plate, and the other is used as the second detection light and passes through the fourth 1 / 2 wave plate through the reflector to delay the light path, and then passes through the synchronous phase shifting system, so that the two micro-polarization cameras collect four laser cavitation interference patterns with phases differing by π / 2 in sequence, and the two groups of interference patterns have a certain time delay.

[0009] Furthermore, the included angle between the fast axis directions of the third 1 / 2 wave plate and the fourth 1 / 2 wave plate and the polarization direction of the respective incident light is 22.5°.

[0010] Furthermore, the included angle between the fast axis direction of the first 1 / 4 wave plate and the second 1 / 4 wave plate and the fast axis or slow axis of the incident linear polarized light is 45°.

[0011] Furthermore, the third polarization beam splitter prism, the third reflector, the fourth reflector, the fourth polarization beam splitter prism, the second 1 / 4 wave plate, the first imaging lens, the first micro-polarization camera, the second 1 / 4 wave plate, the second imaging lens and the second micro-polarization camera constitute a synchronous phase shifting system; the light intensity formulas after the interference of the reference light and the test light reaching the two micro-polarization cameras are both:

[0012]

[0013] Among them, I(x,y) is the light intensity after interference, I 测试 (x, y) is the intensity of the test light passing through the cavitation area, I 参考 (x,y) is the intensity of the reference light that does not pass through the cavitation area, is the phase to be measured, φ(x,y) is the phase shift;

[0014] Then the light intensity formula of the polarization unit in the four polarization directions is:

[0015]

[0016] Among them, I 1 (x,y),I 2 (x,y),I 3 (x,y) and I 4 (x, y) are the light intensities on the four polarization units with the transmission directions differing by 45°. It can be concluded that the cavitation phase to be measured for

[0017]

[0018] Furthermore, the micro polarization camera includes four polarization units with polarization directions of 0°, 45°, 90° and 135°.

[0019] A laser cavitation diagnosis method based on dual-frame time-delay synchronous phase shifting is implemented based on the above-mentioned diagnostic device. The steps of the method are as follows:

[0020] Step 1: The laser beam emitted by the first laser is focused by a laser-resistant convex lens to form a light spot with high laser energy density, which is stimulated to induce cavitation in water, and a high-pressure shock wave and jet are generated during the cavitation pulsation;

[0021] Step 2, the laser beam emitted by the second laser passes through a beam expander, a second 1 / 2 wave plate, and a second polarization beam splitter prism to be divided into orthogonal linear polarized lights, one of which is the first detection light, and the other is delayed by two reflectors to be the second detection light;

[0022] Step 3, the first detection light is separated into p-light and s-light by a third half-wave plate and a third polarization beam splitter prism, wherein the p-light is the test light passing through the cavitation area, and the s-light is the reference light; after being combined by the fourth polarization beam splitter prism, the first detection light is imaged by the first imaging lens at the first micro-polarization camera;

[0023] Step 4, the second detection light passes through the fourth half-wave plate with an angle of 22.5° between the fast axis direction and the polarization direction of the incident light and is divided into p-light and s-light by the third polarization splitter prism, wherein the s-light is the test light passing through the cavitation area and the p-light is the reference light; after being combined by the fourth polarization splitter prism, it passes through the second imaging lens and forms an image on the second micro-polarization camera;

[0024] Step 5: Four synchronous phase-shifting interference images with a phase shift of π / 2 are obtained on both micro-polarization cameras, and the two sets of interference patterns have a delay of nanoseconds;

[0025] Step 6: Use the four-step phase shifting algorithm and phase unwrapping algorithm to process the four phase shifting interference images and calculate the phase distribution to be measured.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The synchronous phase-shifting interferometer device can be used to provide cavitation information before and after a nanosecond delay with a laser beam. It has a compact structure, uses commonly used components and does not require special production. It is easy to debug. The principle of the synchronous phase-shifting interferometer method is simple. The phase distribution to be measured can be restored by obtaining four frames of phase-shifting interferometer images at the same time.

[0028] (2) The present invention introduces a double-frame delay method into the traditional synchronous phase-shifting interferometer, which overcomes the shortcomings of the existing cavitation detection technology and realizes the observation of instantaneous phase changes within a single cavitation pulsation cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a principle diagram of the optical path of a laser diagnostic cavitation device and method based on double-frame time-delay synchronous phase shifting disclosed in an embodiment of the present invention.

[0030] Figure 2 It is a laser cavitation interference pattern collected by a laser cavitation diagnostic device based on dual-frame time-delay synchronous phase shifting.

[0031] Figure 3 It is the laser cavitation phase obtained based on the laser cavitation diagnosis method of double-frame time-delay synchronous phase shifting.

[0032] Figure numerals: 1-first laser, 2-first 1 / 2 wave plate, 3-first polarization beam splitter prism, 4-beam splitter, 5-energy meter, 6-convex lens, 7-water tank, 8-second laser, 9-beam expander group, 10-second 1 / 2 wave plate, 11-second polarization beam splitter prism, 12-third 1 / 2 wave plate, 13-first reflector, 14-second reflector, 15-fourth 1 / 2 wave plate, 16-third polarization beam splitter prism, 17-third reflector, 18-fourth reflector, 19-fourth polarization beam splitter prism, 20-first 1 / 4 wave plate, 21-first imaging lens, 22-first micro polarization camera, 23-second 1 / 4 wave plate, 24-second imaging lens, 22-second micro polarization camera. DETAILED DESCRIPTION

[0033] The present invention proposes a laser cavitation diagnosis device and method based on double-frame time-delay synchronous phase shifting. The device includes an excitation module and a detection module. The experimental optical path system is as follows: Figure 1 shown.

[0034] The excitation module includes: a first laser 1, a first 1 / 2 wave plate 2, a first polarization beam splitter prism 3, a beam splitter 4, an energy meter 5, a convex lens 6, and a water tank 7. The detection module includes: a second laser 8, a beam expander 9, a second 1 / 2 wave plate 10, a second polarization beam splitter prism 11, a third 1 / 2 wave plate 12, a first reflector 13, a second reflector 14, a fourth 1 / 2 wave plate 15, a third polarization beam splitter prism 16, a third reflector 17, a fourth reflector 18, a fourth polarization beam splitter prism 19, a first 1 / 4 wave plate 20, a first imaging lens 21, a first micro-polarization camera 22, a second 1 / 4 wave plate 23, a second imaging lens 24, and a second micro-polarization camera 25. The attenuator and energy meter composed of the 1 / 2 wave plate and the polarization beam splitter prism in the excitation module can achieve real-time monitoring of the laser energy size that generates cavitation. The second polarization beam splitter in the detection module is used to split the light into two linear polarized lights with orthogonal polarization states. The first detection light passes through the second 1 / 2 wave plate, the third polarization beam splitter, the fourth polarization beam splitter, the first 1 / 4 wave plate, and the first imaging lens in sequence, and then forms an image on the first micro-polarization camera. The first detection light that has been delayed by two reflectors passes through the third 1 / 2 wave plate, the third polarization beam splitter, the fourth polarization beam splitter, the second 1 / 4 wave plate, and the second imaging lens in sequence, and then forms an image on the second micro-polarization camera. The two micro-polarization array cameras collect laser cavitation interference images with phase differences of 0, π / 2, π, and 3π / 2, and realize high-resolution observation of nanosecond transient phase evolution.

[0035] Among them, the angle between the fast axis direction of the third 1 / 2 wave plate and the fourth 1 / 2 wave plate and the polarization direction of the respective incident light is 22.5°; the angle between the fast axis direction of the first 1 / 4 wave plate and the second 1 / 4 wave plate and the fast (or slow) axis of the incident linear polarization light is 45°; the micro-polarization camera contains polarization units with four polarization directions (0°, 45°, 90° and 135°).

[0036] The principle of using the device is as follows: the laser beam emitted by the first laser is focused by a laser-resistant convex lens to form a light spot with high laser energy density, which is excited and induced to produce cavitation in water, wherein the attenuator composed of the first 1 / 2 wave plate and the polarization beam splitter prism, as well as the beam splitter and the energy meter are used to measure the laser energy in real time. The laser beam emitted by the second laser is converted into a parallel beam by the beam expander group, and then a part of it is transmitted and a part of it is reflected after passing through the second 1 / 2 wave plate and the second polarization beam splitter prism, forming the first detection light and the second detection light with orthogonal polarization states. The first detection light is divided into p light and s light with orthogonal overall plates by the third 1 / 2 wave plate and the third polarization beam splitter prism, wherein the p light is the test light passing through the cavitation area, and the s light is the reference light. After the two beams of light are combined by the fourth polarization beam splitter prism at the same time, they pass through the first 1 / 4 wave plate to form left-handed and right-handed circularly polarized light, and finally pass through the first imaging lens to generate four interference images with phase shifts that differ by π / 2 on the first micro-polarization camera. The delayed second detection light generated by the first reflector and the second reflector is finally used in the same way to generate four interference images with phase shifts that differ by π / 2 on the second micro-polarization camera.

[0037] Figure 2 The collected laser cavitation synchronous phase-shifting interferograms are shown. Each group of four interferograms is collected by the first micro-polarization camera, and the right one is collected by the second micro-polarization camera. The expansion time of the laser cavitation is 600ns and 603ns respectively. The polarization directions of the four interferograms in each group are 0°, 45°, 90° and 135° respectively.

[0038] Figure 3 Three sets of laser cavitation phase distributions obtained by calculation are shown, and the expansion times of the laser cavitation are 600ns and 603ns respectively.

[0039] The present invention also provides a laser cavitation diagnosis method based on double-frame time-delay synchronous phase shifting, and the specific steps are as follows:

[0040] Step 1: First, turn on the first laser, adjust the attenuator composed of the first 1 / 2 wave plate and the polarization beam splitter prism, and the beam splitter prism to monitor the energy in real time, and focus the laser through a convex lens to produce cavitation.

[0041] Step 2: Adjust the second 1 / 2 wave plate so that the laser emitted by the second laser generates two beams of linear polarized light with the same light intensity and orthogonal polarization states after passing through the second polarization beam splitter prism.

[0042] Step 3: Adjust the third 1 / 2 wave plate so that the first detection light passes through the third polarization prism and is divided into p and b with the same light intensity. 1 Light and S 1 Light, p 1The light passes through the cavitation area as the test light, and then passes through the fourth polarization splitter prism to combine the light; the distance between the first and second reflectors is adjusted to change the time delay of the second detection light, and the fourth 1 / 2 wave plate is adjusted so that the second detection light passes through the third polarization wind prism and is divided into p 2 Light and S 2 The light is then combined by the fourth polarization beam splitter prism.

[0043] Step 4: The two groups of beamed lasers pass through a quarter wave plate to form left-handed and right-handed circularly polarized light, and finally pass through an imaging lens to produce four interference images with phase shifts that differ by π / 2 on each of the two micro-polarization cameras, as shown in Figure 4. Figure 2 shown.

[0044] In this example, the time delay is determined by the sum of the distances L from the first reflector to the second polarization beam splitter prism and from the second reflector to the third polarization beam splitter prism. The time delay formula is:

[0045]

[0046] Where c is the speed of light, assuming that the Jones matrices of the two linearly polarized detection beams after passing through the second polarization peak splitter are E 0 =(1,0) T and E 0 =(0,1) T , the Jones matrix of the quarter wave plate that is 45° to the transmission axis is:

[0047]

[0048] The micro-polarization array contains a series of polarizers, and the Jones matrix of the polarizers at an angle of θ to the transmission axis is:

[0049]

[0050] The Jones matrices of the reference light and the test light in four polarization directions are calculated. The phase differences between the test light and the reference light after interference are 3π / 2, 0, π / 2, and π. The phase to be measured is:

[0051]

[0052] Finally, the real phase can be restored through phase unwrapping. The above derivation proves that the laser cavitation diagnosis method based on double-frame time-delay synchronous phase shift can obtain nanosecond phase changes during a single cavitation pulsation.

Claims

1. A laser cavitation diagnosis device based on double-frame time-delay synchronous phase shift, characterized in that: It includes an excitation module and a detection module; The excitation module comprises a first laser (1), a first 1 / 2 wave plate (2), a first polarization beam splitter prism (3), a beam splitter (4), an energy meter (5), a convex lens (6) and a water tank (7); after the first laser (1) emits a high-energy laser beam, it passes through an attenuator composed of the first 1 / 2 wave plate (2) and the polarization beam splitter prism (3) in sequence, and the beam splitter (4) and the energy meter (5) are used to measure the laser energy in real time, and the convex lens (6) is used to focus the laser beam in water to generate cavitation; The detection module comprises a second laser (8), a beam expander lens group (9), a second 1 / 2 wave plate (10), a second polarization beam splitter prism (11), a third 1 / 2 wave plate (12), a first reflector (13), a second reflector (14), a fourth 1 / 2 wave plate (15), a third polarization beam splitter prism (16), a third reflector (17), a fourth reflector (18), a fourth polarization beam splitter prism (19), a first 1 / 4 wave plate (20), a first imaging lens (21), a first micro polarization camera (22), a second 1 / 4 wave plate (23), a second An imaging lens (24) and a second micro-polarization camera (25); the second laser (8) emits a parallel light beam through a beam expander lens group (9), which is then split into two orthogonal linear polarized light beams through a second polarization beam splitter prism (11); one beam is used as a first detection light and passes through a third 1 / 2 wave plate (12); the other beam is used as a second detection light and passes through a fourth 1 / 2 wave plate (15) through a reflector to delay the light path, and then both beams pass through a synchronous phase shifting system, so that the two micro-polarization cameras collect four laser cavitation interference patterns with phases that differ by π / 2 in sequence, and the two groups of interference patterns have a certain time delay.

2. The laser cavitation diagnosis device based on double-frame time-delay synchronous phase shifting according to claim 1 is characterized in that: The included angle between the fast axis directions of the third 1 / 2 wave plate (12) and the fourth 1 / 2 wave plate (15) and the polarization direction of the respective incident light is 22.5°.

3. The laser cavitation diagnosis device based on double-frame time-delay synchronous phase shifting according to claim 1 is characterized in that: The angle between the fast axis direction of the first 1 / 4 wave plate (20) and the second 1 / 4 wave plate (23) and the fast axis or slow axis of the incident linear polarized light is 45°.

4. The laser cavitation diagnosis device based on double-frame time-delay synchronous phase shifting according to claim 1 is characterized in that: The third polarization beam splitter prism (16), the third reflector (17), the fourth reflector (18), the fourth polarization beam splitter prism (19), the second quarter wave plate (20), the first imaging lens (21), the first micro polarization camera (22), the second quarter wave plate (23), the second imaging lens (24) and the second micro polarization camera (25) constitute a synchronous phase shifting system; the light intensity formulas after the interference of the reference light and the test light reaching the two micro polarization cameras are both: Among them, I(x,y) is the light intensity after interference, I 测试 (x, y) is the intensity of the test light passing through the cavitation area, I 参考 (x,y) is the intensity of the reference light that does not pass through the cavitation area, is the phase to be measured, φ(x,y) is the phase shift; Then the light intensity formula of the polarization unit in the four polarization directions is: Among them, I1(x,y), I2(x,y), I3(x,y) and I4(x,y) are the light intensities on the four polarization units with a 45° difference in the transmission direction. It can be concluded that the cavitation phase to be measured for 5. The laser cavitation diagnosis device based on double-frame time-delay synchronous phase shifting according to claim 1 is characterized in that: The micro polarization camera contains four polarization units with polarization directions of 0°, 45°, 90° and 135°.

6. A laser cavitation diagnosis method based on double-frame time-delay synchronous phase shifting, characterized in that: Based on the device described in claims 1-5, the steps are as follows: Step 1: The laser beam emitted by the first laser is focused by a laser-resistant convex lens to form a light spot with high laser energy density, which is stimulated to induce cavitation in water, and a high-pressure shock wave and jet are generated during the cavitation pulsation; Step 2, the laser beam emitted by the second laser passes through a beam expander, a second 1 / 2 wave plate, and a second polarization beam splitter prism to be divided into orthogonal linear polarized lights, one of which is the first detection light, and the other is delayed by two reflectors to be the second detection light; Step 3, the first detection light is separated into p-light and s-light by a third half-wave plate and a third polarization beam splitter prism, wherein the p-light is the test light passing through the cavitation area, and the s-light is the reference light; after being combined by the fourth polarization beam splitter prism, the first detection light is imaged by the first imaging lens at the first micro-polarization camera; Step 4, the second detection light passes through the fourth half-wave plate with an angle of 22.5° between the fast axis direction and the polarization direction of the incident light and is divided into p-light and s-light by the third polarization splitter prism, wherein the s-light is the test light passing through the cavitation area and the p-light is the reference light; after being combined by the fourth polarization splitter prism, it passes through the second imaging lens and forms an image on the second micro-polarization camera; Step 5: Four synchronous phase-shifting interference images with a phase shift of π / 2 are obtained on both micro-polarization cameras, and the two sets of interference patterns have a delay of nanoseconds; Step 6: Use the four-step phase shifting algorithm and phase unwrapping algorithm to process the four phase shifting interference images and calculate the phase distribution to be measured.

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