An optical system for period-adjustable coaxial laser ultrasonic testing

Abstract

The present invention is to solve the technical problems that the excitation and detection in the existing laser ultrasonic detection mode are relatively independent, it is difficult to realize the excitation detection or coaxial excitation detection, and at the same time, the detection method needs to realize the adjustable period of different array line sources by replacing the mask, lens array or optical fiber array, and it is difficult to realize the continuous adjustable period of the array line source, and provides an optical system for period-adjustable coaxial laser ultrasonic detection. The system is based on the principles of acousto-optic diffraction and 4f imaging. The excitation light and the detection light are coaxially incident on the acousto-optic crystal. The excitation light diffracted by the acousto-optic interference induces a transient grating on the surface of the sample to be tested. The coaxially incident detection light is diffracted by the surface transient grating. The diffracted detection light carries the ultrasonic response information of the sample, and is separated and detected after returning to the 4f system. The ultrasonic excitation and detection are completed through the same 4f system to realize coaxial excitation and detection.

Description

Technical Field

[0001] The invention relates to the field of nondestructive testing, and in particular to a periodically adjustable coaxial laser ultrasonic testing optical system based on acousto-optic diffraction. Background Art

[0002] Laser ultrasound is a non-contact, non-destructive testing technology that uses laser excitation and laser reception of ultrasonic waves. Its basic working principle is to use a pulsed laser to irradiate the surface of the material. Part of the laser energy is absorbed and converted into heat energy, and the laser irradiation area produces a local rapid temperature rise, resulting in local rapid thermal expansion and ultrasound. At the same time, a continuous or pulsed laser is used to irradiate the sample surface to detect ultrasound, and then the optical receiving device collects and demodulates the ultrasonic signal. Laser ultrasonic testing does not require a coupling agent and can achieve non-contact, long-distance detection. Moreover, due to all-optical excitation and detection, laser ultrasound can achieve real-time online detection in extreme environments such as high temperature, high pressure, corrosion, and strong radiation.

[0003] The traditional laser ultrasound proposed by Lu Minghui et al. (Lu Minghui, Ding Lei, Yan Xuejun, et al. Application and Prospect of Laser Ultrasonic Technology in Industrial Inspection [J]. Vibration. Test and Diagnosis, 2021, 41(4): 14.) is mainly a "point excitation, point detection" mode, that is, the pulsed laser is focused into a point and incident on the sample surface to generate ultrasound, and another laser beam is used to detect the propagated ultrasound after a certain distance. This "point excitation, point detection" mode is widely used, and the functions of the ultrasonic excitation and detection modules are relatively independent. However, this mode is difficult to achieve excitation and detection or coaxial excitation detection, and in laser ultrasonic scanning detection, in order to facilitate ultrasonic solution, it is necessary to accurately control the positional relationship between excitation and detection during the scanning process, which increases the complexity of detection system integration and control.

[0004] In addition, the ultrasonic signal excited by the laser point source is weak, and the detection signal-to-noise ratio is poor. The laser array line source can inject more energy without damaging the sample surface, increase the intensity of the excited ultrasound, and help improve the sensitivity and signal-to-noise ratio of the laser ultrasonic detection system. At the same time, the characteristics of ultrasound (such as the wavelength of the ultrasonic surface wave) can be adjusted by adjusting the line source width, line source period and other parameters of the array line source.

[0005] Traditional laser array line sources are mainly generated by surface masks, lens arrays, and fiber arrays. It is difficult to achieve continuous adjustment of the array line source period. Moreover, a mask or a lens (fiber) array can only generate an array line source with a fixed period. If you want to adjust the laser array line source period, you need to replace different masks or lens (fiber) arrays. The system is complex, expensive, and inflexible. Summary of the invention

[0006] The purpose of the present invention is to solve the technical problems that in the existing laser ultrasonic detection mode, excitation and detection are relatively independent, it is difficult to realize excitation and detection or coaxial excitation and detection, and at the same time, the detection method needs to realize the adjustable period of different array line sources by replacing the mask, lens array or optical fiber array, and it is difficult to realize the continuous adjustment of the array line source period. An optical system for period-adjustable coaxial laser ultrasonic detection is provided.

[0007] The reaction mechanism of the present invention is as follows: based on the principle of acousto-optic diffraction, the ultrasonic excitation light and the detection light are coaxially incident, so that coaxial ultrasonic excitation and detection can be realized, and the usability of the laser ultrasonic detection system is improved; the excitation light interferes on the sample surface to form a laser array line source, and the laser array line source period can be continuously adjustable without replacing the optical element, and more laser energy can be injected without damaging the material surface, thereby improving the intensity of the excited ultrasound; the laser array line source induces a transient grating on the sample surface, and based on the detection light detection of the transient grating diffraction, it is beneficial to improve the sensitivity and signal-to-noise ratio of the laser ultrasonic detection.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] An optical system for period-adjustable coaxial laser ultrasonic detection, which is special in that it includes a beam combiner, an ultrasonic drive source, an acousto-optic crystal, at least one aperture, a 4f system, and a second reflector, a third reflector and a detector arranged between the 4f systems;

[0010] The beam combiner is arranged on the optical path of the detection light, and is used to coaxially combine the excitation light and the detection light, and the combined excitation light and the detection light are incident on the acousto-optic crystal modulated by the ultrasonic driving source along the optical axis direction of the optical system for diffraction;

[0011] The ultrasound generated by the ultrasonic driving source forms a periodic refractive index modulation in the acousto-optic crystal, and the acousto-optic crystal with a modulated refractive index diffracts the incident excitation light and the detection light; the diffracted excitation light and the detection light are incident to the 4f system after being selected by the aperture, and then converge on the surface of the sample to be tested after passing through the 4f system;

[0012] The acousto-optic crystal is located at the front focus of the 4f system; the sample to be tested is located at the back focus of the 4f system;

[0013] The aperture is used to spatially select the excitation light and the detection light of the same diffraction order;

[0014] The diffracted excitation light interferes on the surface of the sample to be tested, forming an array line source and inducing a transient grating and exciting ultrasound; the diffracted detection light diffracts the transient grating on the surface of the sample to be tested, and the diffracted detection light carries the ultrasonic response information of the sample to be tested;

[0015] The detection light carrying the ultrasonic response information of the sample to be tested is reflected by the third reflector and then incident on the detector;

[0016] The second reflector is used to reflect a beam of detection light incident on the 4f system to the detector.

[0017] Furthermore, it also includes a first reflector arranged on the optical path of the excitation light, and the excitation light is incident on the beam combining mirror after being reflected by the first reflector.

[0018] Further, the device comprises a pinhole aperture and a blocking aperture;

[0019] The pinhole aperture is used to block high-order diffraction light of ±2 orders and above;

[0020] The blocking aperture is used to block the 0th order diffraction light.

[0021] Furthermore, it also includes a neutral density filter arranged before the second reflector, which is used to attenuate and adjust the intensity of the detection light.

[0022] Furthermore, the 4f system is a combination of lenses with different focal lengths.

[0023] Further, the 4f system includes a first lens and a second lens;

[0024] The diffracted excitation light interferes on the surface of the sample to be tested to form an array line source, whose period is proportional to the ultrasonic period in the acousto-optic crystal;

[0025]

[0026] Among them, T laser source is the period of the laser array line source formed by the interference of the excitation light, T ultrasonic is the ultrasonic period in the acousto-optic crystal 4, f1 is the focal length of the first lens of the 4f system, f2 is the focal length of the second lens of the 4f system, M 4f This is the 4f system magnification.

[0027] Furthermore, the laser array line source period formed by the interference of the excitation light is the period of ultrasound injected into the acousto-optic crystal 4 regulated by the ultrasonic driving source 3, so that the laser array line source period can be continuously adjustable.

[0028] Furthermore, the acousto-optic diffraction in the acousto-optic crystal is Ramanus diffraction or Bragg diffraction depending on the conditions of the acousto-optic interaction.

[0029] Furthermore, the aperture is of a "two-hole" type or a "two-slit" type.

[0030] Further, the detector is an intensity detector, a balanced detector or an interferometric detection system.

[0031] Compared with the prior art, the present invention has the following beneficial technical effects:

[0032] 1. The optical system for period-adjustable coaxial laser ultrasonic detection provided by the present invention is based on the principles of acousto-optic diffraction and 4f imaging. The excitation light diffracted by acousto-optic interference forms an array line source at the position to be measured after passing through the 4f system. The period of the array line source is proportional to the ultrasonic period in the acousto-optic crystal (the proportional factor is the magnification of the 4f system). By adjusting the ultrasonic wave length in the acousto-optic crystal, the laser array line source period can be continuously adjusted.

[0033] 2. The optical system for period-adjustable coaxial laser ultrasonic detection provided by the present invention can realize coaxial ultrasonic excitation and detection based on the principles of acousto-optic diffraction and 4f imaging. The excitation light and the detection light are coaxially incident on the acousto-optic crystal, and the excitation light diffracted by the acousto-optic interference induces a transient grating on the surface of the sample to be tested. The coaxially incident detection light is diffracted by the surface transient grating, and the diffracted detection light carries the ultrasonic response information of the sample, which is separated and detected after returning to the 4f system. The ultrasonic excitation and detection are completed through the same 4f system, realizing coaxial excitation and detection.

[0034] 3. The optical system for periodically adjustable coaxial laser ultrasonic detection provided by the present invention has low cost and compact structure, and is conducive to realizing automated scanning detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the periodically adjustable coaxial laser ultrasonic detection optical system based on acousto-optic diffraction of the present invention;

[0036] Reference numerals:

[0037] 1-first reflector, 2-beam combiner, 3-ultrasonic drive source, 4-acoustic-optic crystal, 5-pinhole aperture, 6-blocking aperture, 7-first lens, 8-second lens, 9-neutral density filter, 10-second reflector, 11-third reflector, 12-sample to be tested, 13-detector. DETAILED DESCRIPTION

[0038] In order to make the purpose, advantages and features of the present invention clearer, the optical system for period-adjustable coaxial laser ultrasonic detection proposed by the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0039] like Figure 1As shown, the optical system for periodically adjustable coaxial laser ultrasonic detection provided in this embodiment includes a first reflector 1, a beam combiner 2, an ultrasonic drive source 3, an acousto-optic crystal 4, a pinhole diaphragm 5, a blocking diaphragm 6, a first lens 7, a second lens 8, a neutral density filter 9, a second reflector 10, a third reflector 11, a sample to be tested 12 and a detector 13.

[0040] The first reflector 1 is an optical reflector for the excitation light, and the first reflector 1 is arranged on the optical path of the excitation light. After being reflected by the first reflector 1, the excitation light is incident on the beam combining mirror 2; the beam combining mirror 2 is arranged on the optical path of the detection light, and is used to coaxially combine the excitation light and the detection light; the combined excitation light and the detection light are incident on the acousto-optic crystal 4 modulated by the ultrasonic driving source 3 along the optical axis direction of the optical system, and the ultrasound generated by the ultrasonic driving source 3 forms a periodic refractive index modulation in the acousto-optic crystal 4, and the acousto-optic crystal 4 with a modulated refractive index causes the incident excitation light and the detection light to diffract.

[0041] A pinhole aperture 5 and a blocking aperture 6 are provided on the diffraction optical path of the acousto-optic crystal 4, which are used to block the high-order diffracted light of ±2nd order and above and the 0th order diffracted light, respectively, and only retain the ±1st order light of the excitation light and the detection light in the diffracted light. The ±1st order excitation light and the detection light after spatial gating are incident on the 4f system composed of the first lens 7 and the second lens 8 arranged in sequence.

[0042] The acousto-optic crystal 4 is located at the front focus of the first lens 7, and the sample 12 to be tested is set at the back focus of the second lens 8. The ±1-order excitation light in the transmitted light interferes on the surface of the sample 12 to be tested. The interference bright fringes formed by the interference are the laser array line source, which induces a transient grating on the surface of the sample 12 to be tested and excites ultrasound; at the same time, a beam of ±1-order detection light coaxially incident is diffracted by the transient grating on the surface of the sample 12, and the diffracted detection light carries the ultrasonic response information of the sample 12 to be tested.

[0043] After being reflected, the detection light carrying the ultrasonic response information of the sample 12 to be tested is collected by the second lens 8 and returned to the 4f system and is reflected by the third reflector 11 to the detector 13 disposed between the first lens 7 and the second lens 8 .

[0044] Another beam of ±1-order detection light as reference light is incident on the neutral density filter 9 and the second reflector 10 arranged in sequence after passing through the first lens 7, and is incident on the detector 13 after being reflected by the second reflector 10. The reference light and the detection light diffracted by the transient grating on the surface of the sample 12 to be tested are incident on the detector 13 at the same time for coherent detection or differential amplification.

[0045] The detector 13 receives the simultaneously incident detection light and reference light carrying the ultrasonic response information of the sample 12 to be tested, and detects the ultrasonic information carried by the signal light through balanced detection or interference detection. In this optical system, the excitation and detection of ultrasound are completed through the same 4f system, and the period is adjustable to realize coaxial laser ultrasonic detection.

[0046] In the embodiment, the acousto-optic diffraction in the acousto-optic crystal 4 may be Ramanus diffraction or Bragg diffraction depending on different acousto-optic interaction conditions, such as different lengths and different frequencies of ultrasonic waves.

[0047] The pinhole diaphragm 5 and the blocking diaphragm 6 only play the role of spatially selecting ±1-order light and blocking other orders of light. They can be other customized diaphragms such as "two-hole" type or "two-slit" type, and the structure is not limited.

[0048] The ±1 order of excitation light and detection light incident on the 4f system can be selected as other orders of diffraction light according to the actual acousto-optic diffraction type and space, for example, other orders of diffraction light are blocked and only the ±2 orders of excitation light and detection light are retained to enter the 4f system.

[0049] The 4f system composed of the first lens 7 and the second lens 8 can be a lens combination with different focal lengths according to actual applications, or can be other equivalent magnification or reduction optical imaging systems. The system magnification can be greater than, less than or equal to 1.

[0050] The neutral density filter 9 is a neutral density filter for detection light, and is used to attenuate and adjust the light intensity of the reference light, thereby improving the sensitivity and signal-to-noise ratio of the balanced detection or interference detection of the detector 13 .

[0051] The detector 13 can be an intensity detector, a balance detector or an interference detection system according to actual detection needs.

[0052] In this embodiment, the ±1 level of the excitation light diffracted by the acousto-optical light interferes at the position to be measured after passing through the 4f system to form an array line source, and the period of the array line source is proportional to the ultrasonic period in the acousto-optic crystal (the proportional factor is the magnification of the 4f system):

[0053]

[0054] Among them, T laser source is the period of the laser array line source formed by the interference of the excitation light, T ultrasonic is the ultrasonic period in the acousto-optic crystal 4, f1 is the focal length of the first lens 7 of the 4f system, f2 is the focal length of the second lens 8 of the 4f system, M 4f It is the 4f system magnification.

[0055] Therefore, without changing the components and structure of the optical system, the ultrasonic period injected into the acousto-optic crystal 4 can be regulated by the ultrasonic driving source 3, so that the laser array line source period can be continuously adjustable.

[0056] The excitation light and the detection light are coaxially incident on the acousto-optic crystal. The excitation light diffracted by the acousto-optic interference induces a transient grating on the surface of the sample to be tested. The coaxially incident detection light is diffracted by the surface transient grating. The diffracted detection light carries the ultrasonic response information of the sample and is separated and detected after returning to the 4f system. The ultrasonic excitation and detection are completed through the same 4f system to achieve coaxial excitation and detection.

[0057] 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned 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 present invention.

Claims

1. An optical system for period-adjustable coaxial laser ultrasonic testing, characterized in that: It comprises a beam combining mirror (2), an ultrasonic driving source (3), an acousto-optic crystal (4), at least one aperture, a 4f system, and a second reflecting mirror (10), a third reflecting mirror (11) and a detector (13) arranged between the 4f systems; The beam combining mirror (2) is arranged on the optical path of the detection light and is used to coaxially combine the excitation light and the detection light. The combined excitation light and the detection light are incident on the acousto-optic crystal (4) modulated by the ultrasonic driving source (3) along the optical axis direction of the optical system for diffraction; The ultrasound generated by the ultrasound driving source (3) forms a periodic refractive index modulation in the acousto-optic crystal (4), and the acousto-optic crystal (4) with a modulated refractive index causes the incident excitation light and the detection light to diffract; the diffracted excitation light and the detection light are incident on the 4f system after being selected by an aperture, and then converge on the surface of the sample to be tested (12) after passing through the 4f system; The acousto-optic crystal (4) is located at the front focus of the 4f system; the sample to be tested is located at the back focus of the 4f system; The aperture is used to spatially select the excitation light and the detection light of the same diffraction order; The diffracted excitation light interferes on the surface of the sample (12) to be tested, forming an array line source and inducing a transient grating and exciting ultrasound; the diffracted detection light diffracts the transient grating on the surface of the sample (12) to be tested, and the diffracted detection light carries the ultrasonic response information of the sample to be tested; The detection light carrying the ultrasonic response information of the sample to be tested is reflected by the third reflector (11) and then incident on the detector (13); The second reflector (10) is used to reflect a beam of detection light incident on the 4f system to the detector (13); The 4f system is a combination of lenses with different focal lengths; the 4f system comprises a first lens (7) and a second lens (8); The diffracted excitation light interferes on the surface of the sample to be tested to form an array line source, whose period is proportional to the ultrasonic period in the acousto-optic crystal; Among them, T lasersource is the period of the laser array line source formed by the interference of the excitation light, T ultrasonic is the ultrasonic period in the acousto-optic crystal (4), f1 is the focal length of the first lens (7) of the 4f system, f2 is the focal length of the second lens (8) of the 4f system, M 4f is the 4f system magnification; The laser array line source period formed by the interference of the excitation light is a period of ultrasound injected into the acousto-optic crystal (4) regulated by an ultrasonic driving source (3), so that the laser array line source period can be continuously adjustable.

2. The optical system for period-adjustable coaxial laser ultrasonic testing according to claim 1, characterized in that: It also comprises a first reflector (1) arranged on the optical path of the excitation light, and the excitation light is incident on the beam combining mirror (2) after being reflected by the first reflector (1).

3. The optical system for period-adjustable coaxial laser ultrasonic testing according to claim 2, characterized in that: The aperture comprises a pinhole aperture (5) and a blocking aperture (6); The pinhole aperture (5) is used to block higher-order diffraction light of ±2 orders or more; The blocking aperture (6) is used for blocking the 0th order diffraction light.

4. The optical system for period-adjustable coaxial laser ultrasonic testing according to claim 3 is characterized in that: It also includes a neutral density filter (9) arranged before the second reflector (10) and used for attenuating and adjusting the intensity of the detection light.

5. The optical system for period-adjustable coaxial laser ultrasonic testing according to any one of claims 1 to 4, characterized in that: The acousto-optic diffraction in the acousto-optic crystal (4) is Ramanus diffraction or Bragg diffraction depending on the conditions of the acousto-optic interaction.

6. The optical system for period-adjustable coaxial laser ultrasonic testing according to claim 5, characterized in that: The aperture is of a "two-hole" type or a "two-slit" type.

7. The optical system for period-adjustable coaxial laser ultrasonic testing according to claim 6, characterized in that: The detector (13) is an intensity detector, a balance detector or an interferometric detection system.

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