CN108693247B - Laser surface acoustic wave detection system based on double measuring beams and use method thereof - Google Patents
Laser surface acoustic wave detection system based on double measuring beams and use method thereof Download PDFInfo
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Abstract
The invention discloses a laser surface acoustic wave detection system based on double measuring beams and a use method thereof, wherein the laser surface acoustic wave detection system comprises: controller, the integrated box of optical detection system and step motor, the integrated box of optical detection system includes: the laser surface acoustic wave detection system is simple in structure and convenient to operate, the using method of the laser surface acoustic wave detection system can measure the speed of the laser surface acoustic wave, the excitation point source of the laser surface acoustic wave can be accurately positioned, the working efficiency can be obviously improved through computer control, a new speed measuring method is provided for the laser surface acoustic wave in practical application, the laser surface acoustic wave detection system has important significance for accurate measurement and industrial automation of the laser surface acoustic wave detection system, and the requirements of nondestructive and automatic measurement are met.
Description
Technical Field
The invention belongs to the technical field of laser surface acoustic wave measurement, and particularly relates to a laser surface acoustic wave detection system based on double measuring beams and a using method thereof.
Background
The surface acoustic wave technology of laser is a technology for exciting surface acoustic waves by applying the interaction between laser and the surface of a sample, and performing accurate and nondestructive measurement on the propagation characteristics of the surface acoustic waves on the surface of the sample. Because the vibration amplitude of the laser surface acoustic wave downwards permeates about 1-2 wavelengths, and the energy is mainly concentrated on the surface layer, the technology is particularly suitable for measuring the surface and the subsurface of a sample and is widely applied to the fields of film, surface layer flaw detection and the like.
The measurement mode of the laser surface acoustic wave has two main types of contact and non-contact, and the non-contact measurement avoids the pollution and damage of the contact to the sample and meets the requirement of nondestructive measurement, so the laser surface acoustic wave measurement method is widely applied to the laser surface acoustic wave measurement. The propagation speed of the laser surface acoustic wave on the surface of a sample is related to the Young modulus of the sample, when the surface of the sample is damaged, the Young modulus is changed, so that the physical basic properties of the surface layer of the sample are changed, and the service performance of industrial parts is directly influenced, and the service life is shortened. Therefore, the measurement of the propagation speed of the laser surface acoustic wave on the surface of the sample has important significance on monitoring the surface performance of the sample. The measurement of the propagation velocity needs to measure the absolute distance between the excitation source of the laser surface acoustic wave and the detection position, and if the mode of directly and manually measuring the excitation source and the detection position is adopted, the excitation source is not accurately positioned, larger measurement errors exist, the experimental efficiency is greatly reduced, and the development requirement of modern automation is not met.
The distance between the excitation source and the detection source directly relates to the measurement of the speed of the laser surface acoustic wave, and the measurement accuracy of the laser surface acoustic wave in an actual application system is influenced, so that the problem cannot be ignored.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a laser surface acoustic wave detection system based on double measuring beams and a using method thereof.
Therefore, the technical scheme of the invention is as follows:
a laser surface acoustic wave detection system based on dual measuring beams comprises: controller, the integrated box of optical detection system and step motor, the integrated box of optical detection system includes: a polarized laser, a first non-polarized beam splitter prism, a digital oscilloscope, a first beam splitting structure and a second beam splitting structure,
the first non-polarization beam splitter prism is used for splitting the laser emitted by the polarization type laser into two beams of laser with mutually vertical propagation directions and equal light intensity; two beams of laser are respectively emitted into the first light splitting structure and the second light splitting structure; the first light splitting structure and the second light splitting structure respectively comprise: the laser beams emitted into the first light splitting structure and the second light splitting structure are transmitted and reflected when passing through the second non-polarizing light splitting prism, and the transmission direction and the reflection direction of the laser beams are perpendicular to each other, wherein the reflected laser beams are emitted into the dove prism from the longest surface of the dove prism, and are emitted out of the dove prism to form reference beams through the polarizing light splitting prism, the linear polarizer and the first focusing lens; the frequency of the laser transmitted from the second non-polarization beam splitter prism is modulated by the acousto-optic modulator, the modulated laser is transmitted in the polarization beam splitter prism, and then the laser sequentially passes through a quarter wave plate and a second focusing lens, the laser penetrates through the second focusing lens to form a detection beam and is vertically projected and focused on the surface of a sample to form a detection light spot with the diameter of 3-5 microns;
the first light splitting structure and the second light splitting structure vertically project and focus on the surface of the sample to form 2 detection light spots with the distance of 5-15 mm, and a detection light spot A point and a detection light spot B point are obtained; the pulse laser is used for exciting laser surface acoustic waves, and laser emitted by the pulse laser is projected and focused on the surface of a sample to form an excitation point source O point;
the included angle between the fast axis direction of the quarter-wave plate and the polarization direction of the laser passing through the acousto-optic modulator and transmitted from the polarization beam splitter prism is 45 degrees; the linear polaroid is arranged between the polarization beam splitter prism and the first focusing lens, the detection light beam has the same polarization state with the reference light beam after passing through the linear polaroid after being reflected by the surface of the sample, and is interfered with the reference light beam to form an interference signal, and the interference signal is focused by the first focusing lens 7; the photoelectric detector is used for detecting the optical signal focused by the first focusing lens;
the digital oscilloscope is respectively and electrically connected with the photoelectric detectors of the first light splitting structure and the second light splitting structure, and a circuit between each photoelectric detector and the digital oscilloscope is connected with the phase demodulation circuit and used for recovering a laser surface acoustic wave signal in an interference signal; the controller is electrically connected with the digital oscilloscope and is used for controlling the digital oscilloscope to acquire the laser surface acoustic wave signals demodulated by the phase demodulation circuit;
the digital oscilloscope is connected with a trigger device and is used for determining the zero moment of the laser surface acoustic wave signal acquired by the digital oscilloscope; the stepping motor is used for driving the optical detection system integration box body to rotate by taking a detection light beam forming a detection light spot A as a central axis.
In the above technical solution, the laser light transmitted through the quarter-wave plate of the second light splitting structure passes through a plurality of plane mirrors, so that the laser light is vertically projected on the surface of the sample; the triggering mode of the digital oscilloscope is pulse triggering.
In the above technical solution, the length of the optical path between the second focusing lens and the sample surface is equal to the focal length of the second focusing lens; the distance between the detection light spot A point and the excitation point source O point is larger than the distance between the detection light spot A point and the detection light spot B point.
In the technical scheme, the sample is positioned on a three-dimensional precision displacement table.
The method for measuring the speed of the laser surface acoustic wave by using the laser surface acoustic wave detection system comprises the following steps:
1) initializing a system: adjusting the sampling frequency of the digital oscilloscope to be more than 1 GHz;
in the step 1), the sampling frequency of the digital oscilloscope is 1-2.5 GHz;
in the step 1), the average sampling times of the digital oscilloscope are 2-512 times per acquisition point.
2) Measuring the distance between the two detection light spots to obtain d;
and 2) measuring the distance between the detection light spot A and the detection light spot B by using a micrometer screw, wherein the measurement precision of the micrometer screw is 0.01 mm.
3) Opening the pulse laser to enable laser emitted by the pulse laser to form an excitation point source O point on the surface of a sample, and driving the optical detection system integrated box to rotate at least 180 degrees by the stepping motor at a stepping angle of 0.09-3 degrees; in the rotation process, determining the rotation angle theta of the peak value of the laser surface acoustic wave when the peak value reaches the detection spot B from the excitation point source O point or the shortest time, and adjusting the optical detection system integration box body to be fixed at the angle, wherein the excitation point source O point, the detection spot A point and the detection spot B point are positioned on the same straight line;
in the step 3), the step angle of the stepping motor is 0.09-0.18 degrees.
4) The time that the peak value of the laser surface acoustic wave excited by the pulse laser respectively reaches the detection spot A point and the detection spot B point from the excitation point source O point is obtained through a heterodyne interference method, and t is obtainedAAnd tBThe said tA、tBSubstituting d into formula to obtain laser surface acoustic wave velocity VR；
The method for accurately positioning the excitation point source by using the laser surface acoustic wave speed comprises the following steps: the V is put intoRMultiplying the time of the detection light spot to obtain the distance s between the excitation point source and the detection light spot, wherein the time of the detection light spot is tAOr tB。
The laser surface acoustic wave detection system is applied to nondestructive precise measurement of surface and subsurface defects.
The method for measuring the speed of the laser surface acoustic wave is applied to the measurement of the speed of the laser surface acoustic wave.
The laser surface acoustic wave velocity obtained by the method for measuring the laser surface acoustic wave velocity is applied to the accurate positioning of a laser point source.
Compared with the prior art, the laser surface acoustic wave detection system is simple in structure and convenient to operate, the using method of the laser surface acoustic wave detection system can measure the speed of the laser surface acoustic wave, the excitation point source of the laser surface acoustic wave can be accurately positioned, the working efficiency can be obviously improved through computer control, a new speed measuring method is provided for the laser surface acoustic wave in practical application, the laser surface acoustic wave detection system has important significance for accurate measurement and industrial automation of the laser surface acoustic wave detection system, and the requirements of nondestructive and automatic measurement are met.
Drawings
Fig. 1 is a position relationship diagram of an excitation point source O point, a detection light spot a point, and a detection light spot B point, where d: distance between two detection beams (spots), d' distance of AC, s: the distance between the excitation point source and the detection light spot close to the excitation point source;
fig. 2 is a schematic diagram of a position relationship between a circular area formed by rotation of a detection light spot a and a detection light spot B and an excitation point source O, where 2(a) is that the excitation point source O is located in the circular area, 2(B) is that the excitation point source O is located outside the circular area, and 2(c) is that the excitation point source O is located on the periphery of the circular area;
FIG. 3 is a schematic structural diagram of a laser SAW detection system of the present invention;
fig. 4 shows surface acoustic wave signals detected by the detection spot a and the detection spot B.
Wherein, 1: polarization type laser 2: first non-polarizing beam splitter prism
3: second non-polarizing beam splitter prism 4: polarization beam splitter prism
5: dove prism 6: linear polarizer
7: first focusing lens 8: quarter wave plate
9: first plane mirror 10: second plane mirror
11: sample 12: three-dimensional precision displacement platform
13: the photodetector 14: phase demodulation circuit
15: optical detection system integration box 16: stepping motor
17: fixing the rotating shaft 18: digital oscilloscope
19: the controller 20: second focusing lens
21: acousto-optic modulator
Detailed Description
In the technical scheme of the invention, the sampling frequency of the digital oscilloscope 18 is 1-2.5 GHz, and in the embodiment, the sampling frequency is 2.5 GHz. The average sampling frequency of the digital oscilloscope 18 is 2-512 times per acquisition point, and the triggering mode is pulse triggering; the polarization type laser 1 is a 632.8nm HeNe laser. The wavelength of the pulse laser is 532nm, the pulse width is 1.7ns, and the frequency is 10 Hz. The controller 19 is a computer.
The laser surface acoustic wave detection system and the use method thereof of the present invention are described in detail below with reference to the accompanying drawings.
As shown in the attached figures 1-3, the device comprises: a controller 19, an optical detection system integration box 15 and a stepping motor 16, wherein the optical detection system integration box 15 is positioned above the sample 11. The optical detection system integration box 15 includes: a polarized laser 1, a first Non-polarizing Beam splitter 2 (Non-polarizing Beam splitter: NPBS), a digital oscilloscope 18, a first light splitting structure (not shown), and a second light splitting structure (not shown) identical to the first light splitting structure. The polarization type laser 1 is used as a laser source of a laser surface acoustic wave detection system, has stable frequency and emits P polarized light. The first non-polarization beam splitter prism 2 is used for splitting laser emitted by the polarization type laser 1 into two beams of laser with mutually vertical propagation directions and equal light intensity (the light intensity ratio is 1:1, and the frequencies are both f); two beams of laser are respectively emitted into the first light splitting structure and the second light splitting structure; the first light splitting structure and the second light splitting structure respectively comprise: the laser beams emitted into the first light splitting structure and the second light splitting structure are transmitted and reflected when passing through the second non-polarizing light splitting prism 3, the transmission direction and the reflection direction of the laser beams are perpendicular to each other, wherein the reflected laser beams are emitted into the dove prism 5 from the longest surface of the dove prism 5 and are subjected to 2 times of total reflection in the dove prism 5, and the reflected laser beams are emitted out of the dove prism 5 after being subjected to the 2 times of total reflection and form reference beams through the polarizing light splitting prism 4, the linear polarizer 6 and the first focusing lens 7.
On the other hand, the laser light simultaneously transmitted from the second unpolarized beam splitter prism 3 is frequency-modulated (frequency is modulated to f + f) by the acousto-optic modulator 21m，fm80MHz), the modulated laser is transmitted in the polarization beam splitter 4, and then sequentially passes through the quarter-wave plate 8 and the second focusing lens 20, the laser penetrates through the second focusing lens 20 to form a detection beam and vertically projects and focuses on the surface of the sample 11 to form a detection light spot with a diameter of 3-5 μm (the sample is located at the focal point of the second focusing lens, that is, the length of the light path between the second focusing lens 20 and the surface of the sample 11 is equal to the focal length of the second focusing lens).
Due to the size limitation of the optical element, the distance between the two detection light spots cannot be too close. In a specific embodiment, the diameter of the flat mirror 9 in FIG. 3 is 12.7mm, and its component parallel to the sample direction (i.e., horizontal direction) is 9mm, so that the distance between the two beams is greater than 4.5 mm; considering the problem of loss in signal propagation, the distance between two detection light spots cannot be too far, and finally the distance between 2 detection light spots is preferably determined to be between 5mm and 15 mm. Therefore, the first light splitting structure and the second light splitting structure vertically project and focus on the surface of the sample 11 to form 2 detection light spots with the distance of 5-15 mm, and a detection light spot A point and a detection light spot B point are obtained; the pulse laser is used for exciting laser surface acoustic waves, and laser emitted by the pulse laser (not shown in the figure) is focused on the surface of the sample 11 to form an excitation point source O point.
The fast axis direction of the quarter-wave plate 8 forms an angle of 45 ° with the polarization direction of the laser light that has passed through the acousto-optic modulator 21 and has been transmitted from the polarization splitting prism 4, and when the light reflected from the sample surface passes through the quarter-wave plate 8 again, the polarization direction of the light changes by exactly 90 °, and reflection occurs at the polarization splitting prism 4. The linear polaroid 6 is arranged between the polarization beam splitter prism 4 and the first focusing lens 7, the included angle between the transmission axis of the linear polaroid and the polarization direction of incident light of the linear polaroid is more than 0 degree and less than 90 degrees, the detection light beam is reflected by the surface of the sample 11, has the same polarization state with the reference light beam after passing through the linear polaroid 6, and is interfered with the reference light beam to form an interference signal, and the interference signal is focused by the first focusing lens 7; the photodetector 13 is used for detecting the light signal focused by the first focusing lens 7 (the photodetector 13 is located at the focal point of the first focusing lens 7).
As shown in fig. 3, the optical path emitted by the first light splitting structure can be directly projected vertically on the surface of the sample 11 to form a detection spot a, the optical path emitted by the second light splitting structure starts to travel horizontally to the right, and then the laser (i.e., the detection beam) emitted by the second light splitting structure can be projected vertically on the surface of the sample 11 through a plurality of plane mirrors to form a detection spot B. The plurality of plane mirrors include: the optical path is changed from horizontal rightward propagation to horizontal leftward propagation by reflection of the 2 first plane mirrors (as shown in fig. 3), the changed optical path is located below the original optical path, and the optical path is reflected into a downward vertical direction by the second plane mirror 10. If the distance between the two detection light spots needs to be changed, the distance can be adjusted by moving the displacement table of the second plane mirror 10 in the horizontal direction; on the other hand, the sample 11 is located on the three-dimensional precision displacement table 12, and the three-dimensional displacement table 12 enables the sample to precisely move in a plane perpendicular to the detection light beam, so that the speed measurement of the laser surface acoustic wave in different areas of the sample can be rapidly realized.
The digital oscilloscope 18 is electrically connected with the photodetectors 13 of the first light splitting structure and the second light splitting structure respectively, and a phase demodulation circuit 14 (research on demodulation technology of ultrasonic signals in laser heterodyne interference [ D ]. university of north and middle, 2016.) is connected on a circuit between each photodetector 13 and the digital oscilloscope 18, for recovering surface acoustic wave signals in interference signals; the controller 19 is electrically connected to the digital oscilloscope 18 and is used for controlling the digital oscilloscope 18 to acquire the surface acoustic wave signal demodulated by the phase demodulation circuit 14. The surface acoustic wave signal is detected, photoelectrically converted and amplified by a photoelectric detector, demodulated by a phase demodulation circuit and acquired by a computer-controlled digital oscilloscope.
Because an external trigger is needed to determine the zero time acquired by the digital oscilloscope when the surface acoustic wave signal is acquired, namely the time when the laser surface acoustic wave is generated and starts to propagate, the digital oscilloscope 18 is connected with a trigger device for determining the zero time of the surface acoustic wave signal acquired by the digital oscilloscope 18; when the pulse laser for exciting the surface acoustic wave is turned on, most energy of laser beams emitted by the pulse laser is focused on a sample to form an excitation point source, and meanwhile, a small amount of stray light is induced by the trigger device to form a trigger signal of the digital oscilloscope. The trigger device has the functions of photoelectric conversion and signal amplification, is particularly sensitive to light induction of the wavelength corresponding to a laser for exciting the laser surface acoustic wave, and has the response time of nanosecond order in order to ensure the triggering synchronism.
The collected surface acoustic wave signals are subjected to heterodyne interference technology, the detection light beam is modulated at the central frequency of the acoustic-optical modulator mainly by adding the acoustic-optical modulator in the detection light beam of a classical Mach-Zehnder interference system, the phase difference between the reference light beam and the detection light beam can be changed when the surface acoustic wave signals pass through the detection light spot, and the relation 1 of the output light current and the phase difference can be obtained through photoelectric conversion of the photoelectric detector on the average interference signals. Wherein I1And I2Is the DC component generated by the reference light and the detection light, f is the frequency of the reference beam, fmIs the center frequency (modulation frequency) of the acousto-optic modulator, u (t) is the displacement waveform of the laser surface acoustic wave, phi0Is the phase without the surface acoustic wave signal, i.e., the initial phase, and λ is the wavelength of the laser. The photoelectric current output by the photoelectric detector is related to the modulation frequency and the surface acoustic wave signal, and the surface acoustic wave signal can be demodulated through the phase demodulation circuit.
The signal transmission process of the trigger device, the pulse laser and the digital oscilloscope is as follows: when the pulse laser is turned on, most of light beams are focused on the surface of a sample to form an excitation point source to excite a surface acoustic wave signal, and a small amount of stray light is directly transmitted to the trigger device. After receiving the optical signal emitted by the pulse laser, the triggering device performs photoelectric conversion, amplification and the like on the optical signal, and then inputs the optical signal into the digital oscilloscope as a triggering signal for acquiring the surface acoustic wave signal of the laser. When the laser surface acoustic wave signal is transmitted to the detection point A, due to the existence of the trigger signal and the real-time control of the oscilloscope by the controller, the laser surface acoustic wave signal has definite zero time during acquisition and is stably displayed in the controller.
The stepping motor 16 is used for driving the optical detection system integration box 15 to rotate by taking the detection beam forming the detection spot a as a central axis. The surface acoustic wave of the laser emitted by the pulse laser is focused on the surface of the sample 11 to form an excitation point source O point, and since the detection spot in fig. 2(a) and 2(c) may be shielded by an excitation device (composed of the pulse laser and related optical devices) during rotation, the optimal preferred technical scheme of the present invention is as shown in fig. 2(B), that is, the distance between the detection spot a point and the excitation point source O point is greater than the distance between the detection spot a point and the detection spot B point.
Since the surface acoustic wave of the laser excited by the excitation point source propagates outward in the form of a spherical wave, the surface acoustic wave signal can be picked up also when the excitation point source O, the detection spot a, and the detection spot B are not on a straight line, but in this case, as shown in fig. 1, the peak timing at which the surface acoustic wave of the laser is detected at point B '(any point where O, A and B are not collinear by rotating AB around point a) will be different from point B, and cannot be directly calculated by the distance d between AB, but should be calculated by d' which is the distance from a to a at the intersection point C (where OB is equal to OB 'on OB) where a circle is drawn with OB' as the radius O as the center. For ease of calculation, we consider O, A collinear with B as a prerequisite for this measurement method, and make collinear adjustments to O, A and B by rotating the point of detection spot B around a line perpendicular to the sample surface and passing through detection spot A as the axis, with the distance AB as the radius. In actual measurement, the circular domain formed by the rotation of the two measurement spots and the excitation point source have three position relations, namely, the excitation point source O is in the circular domain, outside the circular domain and on the circular domain, which are respectively shown in fig. 2(a), (b) and (c). Equations (2) and (3) can be obtained from graph (a), equations (2) and (4) can be obtained from graph (b), and equation (2) can be obtained from graph (c) according to mathematical geometrical relationships. It can be concluded that OB is longest or shortest when O, A and B are collinear, i.e., the laser surface acoustic wave peak arrival time is maximum or minimum. And the collinear positions of the three points are determined according to the peak time by acquiring the surface acoustic wave signals in real time.
OA+AB＝OA+AB1＞OB1(2)
OA+OB3＝AB2＜OA+OB2(3)
OB3+B3A＜OB2+AB2(4)
The laser surface acoustic wave detection system can be well applied to the measurement of the laser surface acoustic wave speed, and the method for measuring the laser surface acoustic wave speed by using the laser surface acoustic wave detection system comprises the following steps:
1) initializing a system: adjusting the sampling frequency of the digital oscilloscope 18 to be more than 1 GHz;
2) measuring the distance between the two detection light spots by adopting a screw micrometer with the measurement precision of 0.01mm to obtain d;
3) the distance of the second focusing lens 20 from the sample surface is adjusted so that the sample is at the focal point of the second focusing lens. And (2) opening the pulse laser to enable laser emitted by the pulse laser to form an excitation point source O point on the surface of the sample 11, electrically connecting the stepping motor with the controller, and controlling upper computer software of the stepping motor to enable the stepping motor 16 to drive the optical detection system integration box body 15 to rotate by at least 180 degrees at a fixed stepping angle of 0.09-3 degrees and at a lower rotation frequency (less than or equal to 1 Hz). Wherein, the preferred 0.09 ~ 0.18 of step angle of step motor.
And picking up surface acoustic wave signals of a detection spot B point at each rotation angle in the rotation process according to a heterodyne interference method, determining the rotation angle theta of the stepping motor when the wave crest of the laser surface acoustic wave reaches the detection spot B point from an excitation point source O point in the longest time or the shortest time (when the wave crest of the laser surface acoustic wave reaches the detection spot B point, the detection spot B point is positioned at the farthest point from the excitation point source O point, and when the wave crest of the laser surface acoustic wave reaches the detection spot B point in the shortest time, the detection spot B point is positioned at the closest point from the excitation point source O point), and controlling the digital oscilloscope to display, collect and store the. The stepping motor adjusts the optical detection system integration box body 15 to be fixed at the angle theta, namely, the point source excitation point O point, the detection light spot A point and the detection light spot B point are located on the same straight line at the moment.
4) After the excitation point source O and the detection light spots A and B are adjusted to be collinear, the laser surface acoustic wave is transmitted to the detection light spot A point and the detection light spot B point from the point O along the OB straight line. The time for the laser surface acoustic wave peak value excited by the excitation point source to respectively reach the detection light spot A point and the detection light spot B point from the point O is obtained through the heterodyne interference method, and t is obtainedAAnd tBWill tA、tBSubstituting d into formula (5) to obtain the velocity V of laser surface acoustic waveR；
Above VRThe precise positioning of the excitation point source can be well applied. The method for calculating the distance between an excitation point source and a detection light spot by using the obtained laser surface acoustic wave velocity is used for realizing the accurate positioning of the excitation point source, and comprises the following steps: will VRMultiplying the time of the detection light spot to obtain the distance s between the excitation point source and the detection light spot, wherein the time of the detection light spot is tAOr tB。
For example, as shown in fig. 1, the distance s of OA can be obtained by equation (6):
s＝VR*tA(6)
next, a description will be given by taking a (100) single crystal silicon sample as an example, and the detection results of the surface acoustic wave of the laser propagated to the detection spot a and the detection spot B are shown in fig. 4. Extracting the time t corresponding to the peak value of the laser surface acoustic waveA2.515 μ s, tB3.799 μ s and d 6.500mm, the surface acoustic wave of the laser was calculated by the formula (5)The velocity was 5062m/s and it was determined from equation (6) that the point of the excitation point source O was 12.731mm from the point of the detection spot a in the direction away from the point of the detection spot B.
In addition, the laser surface acoustic wave detection system of the invention has better application in the nondestructive precision measurement of surface and subsurface defects, because the current nondestructive measurement of the surface and subsurface defects is mainly a single measuring beam surface acoustic wave detection system which realizes the measurement of the laser surface acoustic waves with different detection distances by moving the position of an excitation source, but when two groups of surface acoustic wave signals are analyzed by using the method, larger measurement errors are caused by the difference of environments at different moments and the difference of the measurement of the two groups of surface acoustic wave signals. The laser surface acoustic wave detection system of the invention does not have the error.
The invention has been described in an illustrative manner, and it is to be understood that any simple variations, modifications or other equivalent changes which can be made by one skilled in the art without departing from the spirit of the invention fall within the scope of the invention.
Claims (10)
1. A laser surface acoustic wave detection system based on double measuring beams is characterized by comprising: controller (19), integrated box (15) of optical detection system and step motor (16), integrated box (15) of optical detection system includes: a polarized laser (1), a first non-polarized beam splitter prism (2), a digital oscilloscope (18), a first beam splitting structure and a second beam splitting structure,
the first non-polarization beam splitter prism (2) is used for splitting laser emitted by the polarization type laser (1) into two beams of laser with mutually vertical propagation directions and equal light intensity; two beams of laser are respectively emitted into the first light splitting structure and the second light splitting structure; the first light splitting structure and the second light splitting structure respectively comprise: the laser device comprises a second non-polarization beam splitting prism (3), a dove prism (5), a polarization beam splitting prism (4), an acousto-optic modulator (21), a photoelectric detector (13), a phase demodulation circuit (14) and a quarter wave plate (8), wherein laser light emitted into the first light splitting structure and the second light splitting structure is transmitted and reflected when passing through the second non-polarization beam splitting prism (3), the transmission direction and the reflection direction of the laser light are perpendicular to each other, the reflected laser light is emitted into the dove prism (5) from the longest surface of the dove prism (5), and the reflected laser light is emitted from the dove prism (5) and passes through the polarization beam splitting prism (4), a linear polarizer (6) and a first focusing lens (7) to form a reference beam; the laser transmitted from the second non-polarization beam splitter prism (3) is subjected to frequency modulation by the acousto-optic modulator (21), the modulated laser is transmitted in the polarization beam splitter prism (4), then the laser sequentially passes through the quarter-wave plate (8) and the second focusing lens (20), the laser penetrates through the second focusing lens (20) to form a detection beam, and the detection beam is vertically projected and focused on the surface of the sample (11) to form a detection light spot with the diameter of 3-5 mu m;
the first light splitting structure and the second light splitting structure vertically project and focus on the surface of the sample (11) to form 2 detection light spots with the distance of 5-15 mm, and a detection light spot A point and a detection light spot B point are obtained; the pulse laser is used for exciting laser surface acoustic waves, and laser emitted by the pulse laser is projected and focused on the surface of a sample (11) to form an excitation point source O point;
the included angle between the fast axis direction of the quarter-wave plate (8) and the polarization direction of the laser passing through the acousto-optic modulator (21) and transmitted from the polarization beam splitter prism (4) is 45 degrees; the linear polaroid (6) is arranged between the polarization beam splitting prism (4) and the first focusing lens (7), the detection light beam has the same polarization state with the reference light beam after passing through the linear polaroid (6) after being reflected by the surface of the sample (11) and interferes with the reference light beam so as to form an interference signal, and the interference signal is focused through the first focusing lens (7); the photoelectric detector (13) is used for detecting the optical signal focused by the first focusing lens (7);
the digital oscilloscope (18) is respectively and electrically connected with the photoelectric detectors (13) of the first light splitting structure and the second light splitting structure, and a circuit between each photoelectric detector (13) and the digital oscilloscope (18) is connected with the phase demodulation circuit (14) for recovering laser surface acoustic wave signals in interference signals; the controller (19) is electrically connected with the digital oscilloscope (18) and is used for controlling the digital oscilloscope (18) to acquire the laser surface acoustic wave signals demodulated by the phase demodulation circuit (14);
the digital oscilloscope (18) is connected with a trigger device and is used for determining the zero moment of the laser surface acoustic wave signal acquired by the digital oscilloscope (18); the stepping motor (16) is used for driving the optical detection system integration box body (15) to rotate by taking a detection light beam forming a detection light spot A as a central axis.
2. The laser surface acoustic wave detection system according to claim 1, wherein laser light transmitted from the quarter-wave plate (8) of the second spectroscopic structure passes through a plurality of plane mirrors so that the laser light is projected perpendicularly on the surface of the sample (11); the triggering mode of the digital oscilloscope (18) is pulse triggering.
3. The laser surface acoustic wave detection system according to claim 2, wherein the length of the optical path between said second focusing lens (20) and the surface of said sample (11) is equal to the focal length of said second focusing lens (20); the distance between the detection light spot A point and the excitation point source O point is larger than the distance between the detection light spot A point and the detection light spot B point.
4. A laser surface acoustic wave detection system according to claim 3, characterized in that said sample (11) is located on a three-dimensional precision displacement stage (12).
5. The method for measuring the velocity of a laser surface acoustic wave by using the laser surface acoustic wave detection system according to any one of claims 1 to 4, comprising the steps of:
1) initializing a system: adjusting the sampling frequency of the digital oscilloscope (18) to be more than 1 GHz;
2) measuring the distance between the two detection light spots to obtain d;
3) turning on the pulse laser to enable laser emitted by the pulse laser to form an excitation point source O point on the surface of a sample (11), and driving the optical detection system integration box (15) to rotate for at least 180 degrees by the stepping motor (16) at a stepping angle of 0.09-3 degrees; in the rotation process, determining the rotation angle theta of the peak value of the laser surface acoustic wave when the peak value reaches the detection spot B from the excitation point source O point or the shortest time, and adjusting the optical detection system integration box body (15) to be fixed at the angle, wherein the excitation point source O point, the detection spot A point and the detection spot B point are positioned on the same straight line;
4) the time that the peak value of the laser surface acoustic wave excited by the pulse laser respectively reaches the detection spot A point and the detection spot B point from the excitation point source O point is obtained through a heterodyne interference method, and t is obtainedAAnd tBThe said tA、tBSubstituting d into formula (5) to obtain the velocity V of laser surface acoustic waveR；
6. The method according to claim 5, characterized in that in 1), the sampling frequency of the digital oscilloscope (18) is 1-2.5 GHz; the average sampling times of the digital oscilloscope (18) are 2-512 times per acquisition point; in the step 2), a micrometer screw is adopted for measuring the distance between the detection light spot A and the detection light spot B, and the measurement precision of the micrometer screw is 0.01 mm; in the step 3), the step angle of the stepping motor (16) is 0.09-0.18 degrees.
7. A method for accurately positioning an excitation point source by using the surface acoustic wave velocity of laser obtained by the method of claim 5, which comprises the following steps: the V is put intoRMultiplying the time of the detection light spot to obtain the distance s between the excitation point source and the detection light spot, wherein the time of the detection light spot is tAOr tB。
8. Use of a laser surface acoustic wave detection system as claimed in any one of claims 1 to 4 for non-destructive precision measurement of surface and subsurface defects.
9. Use of the method of claim 5 for measuring the speed of a laser surface acoustic wave.
10. Use of the method of claim 7 for accurate positioning of a laser point source.
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