Patent ID: 12222280

In the figures: narrow-linewidth semiconductor fiber laser (1), first fiber polarization controller (2), first fiber polarization beam splitter (3), first fiber circulator (4), second fiber polarization controller (5), interferometer (6), wavelength division multiplexer (7), indication laser (8), fiber collimator (9), biaxial scanning galvanometer (10), sample under test (11), light converging collector (12), collimating lens (13), second fiber circulator (14), third fiber polarization controller (15), second fiber polarization beam splitter (16), first photodetector (17), second photodetector (18), third fiber polarization beam splitter (19), third photodetector (20), fourth photodetector (21), feedback control system (22), data acquisition card (23).

DESCRIPTION OF THE EMBODIMENTS

The present disclosure will be further described below in conjunction with the accompanying drawings.

As shown inFIG.1, the specific instruments for implementation include a narrow-linewidth semiconductor fiber laser1; a first fiber polarization controller2; a first fiber polarization beam splitter3; a first fiber circulator4; a second fiber polarization controller5; an interferometer6; a wavelength division multiplexer7; an indication laser8; a fiber collimator9; a biaxial scanning galvanometer10; a light converging collector12; a collimating lens13; a second fiber circulator14and a third fiber polarization controller15. The output terminal of the narrow-linewidth semiconductor fiber laser1is connected to the input terminal of the first fiber polarization beam splitter3through the first fiber polarization controller2. One output terminal of the first fiber polarization beam splitter3is connected to the first port of the first fiber circulator4, and the second port of the first fiber circulator4is connected to one end of the interferometer6through the second fiber polarization controller5. The other output terminal of the first fiber polarization beam splitter3and the indication laser8are jointly connected to two input terminals of the wavelength division multiplexer7, and the output terminal of the wavelength division multiplexer7is connected to the fiber collimator9. The fiber collimator9outputs the collimated beam, which is irradiated to the sample under test11through the biaxial scanning galvanometer10. A light converging collector12is disposed above the sample under test11. The light converging collector12receives the scattered light from the sample under test11, which is incident into the collimating lens13. The collimating lens13is connected to the first port of the second fiber circulator14. The second port of the second fiber circulator14is connected to the other end of the interferometer6through the third fiber polarization controller15.

The narrow-linewidth semiconductor fiber laser1emits a continuous laser, which is processed by the first fiber polarization controller2to form orthogonally and linearly polarized light with p-polarization state and s-polarization state, and is split by the first fiber polarization beam splitter3to form linearly polarized light with p-polarization state and linearly polarized light with s-polarization state. The linearly polarized light in the s-polarization state serves as the reference light, the reference light is incident into the second fiber polarization controller5through the first fiber circulator4, and is input into the interferometer6after being polarized by the second fiber polarization controller5. The linearly polarized light in the p-polarization state serves as the detection light. The detection light and the visible light output by the indication laser8are multiplexed by the wavelength division multiplexer7and then input to the fiber collimator9. The fiber collimator9starts to output the spatial free light. The spatial free light is irradiated onto the sample under test11through the biaxial scanning galvanometer10. The area illuminated by the sample under test11will modulate the ultrasonic vibration information into the detection laser. The scattered light modulated by the ultrasonic vibration is generated by the surface scattering of the sample under test11. The scattered light is collected and converged by the light converging collector12and then incident on the collimating lens13. After passing through the collimating lens13, the scattered light becomes a signal light. The signal light is incident into the third fiber polarization controller15through the second fiber circulator14. After being subjected to polarization processing by the third fiber polarization controller15, the signal light is input into the interferometer6. Both the reference light and the signal light are reflected and transmitted in the interferometer6, generating the signal light and reference light transmitted by the interferometer6, and the signal light and the reference light reflected by the interferometer6. The signal light transmitted by the interferometer6and the reference light reflected by the interferometer6are combined and then polarized by the second fiber polarization controller5, and is incident into the first fiber circulator4and output from the third port of the first fiber circulator4. The reference light transmitted by the interferometer6and the signal light reflected by the interferometer6are combined and then polarized by the third fiber polarization controller15, and then is incident into the second fiber circulator14and output from the third port of the second fiber circulator14.

The fiber polarization controller is preferably a three-ring mechanical fiber polarization controller. The second fiber polarization controller5and the third fiber polarization controller15have a polarization of one-quarter wavelength. The first fiber polarization controller2changes the polarization state of linearly polarized light into an elliptically polarized state, and the ratio of p-polarized light to s-polarized light is 90:10.

Specifically, the original reference light undergoes two quarter-wave plate polarization processing through the second fiber polarization controller5, forming the transmitted and reflected reference light of the interferometer, thereby transforming from s-polarization state to p-polarization state.

Specifically, the original signal light undergoes two quarter-wave plate polarization processing through the third fiber polarization controller15, thereby forming the transmitted and reflected signal light of the interferometer, thereby transforming from the p-polarization state to s-polarization state.

A second fiber polarization beam splitter16and a third fiber polarization beam splitter19are further included. The input terminal of the second fiber polarization beam splitter16is connected to the third port of the first fiber circulator4, and the two output terminals of the second fiber polarization beam splitter16are respectively connected to a photodetector. The input terminal of the third fiber polarization beam splitter19is connected to the third port of the second fiber circulator14, and the two output terminals of the third fiber polarization beam splitter19are also connected to a photodetector respectively.

The photodetectors connected to the two output terminals of the second fiber polarization beam splitter16are the first photodetector17and the second photodetector18respectively, and the photodetectors connected to the two output terminals of the third fiber polarization beam splitter19are the third photodetector20and the fourth photodetector21respectively. The first photodetector17and the third photodetector20are connected to the feedback control system22, and the second photodetector18and the fourth photodetector21are connected to the data acquisition card23.

The first photodetector17receives the signal light transmitted by the interferometer6from the second fiber polarization beam splitter16, the second photodetector18receives the reference light reflected by the interferometer6from the second fiber polarization beam splitter16, the third photodetector20receives the signal light reflected by the interferometer6from the third fiber polarization beam splitter19, and the fourth photodetector21receives the reference light transmitted by the interferometer6from the third fiber polarization beam splitter19.

A feedback control system22is further included. The feedback control system22is respectively connected to the output terminal port of the reference light that is output by the second fiber polarization beam splitter16and the third fiber polarization beam splitter19and is reflected and transmitted by the interferometer6. The feedback control system22receives the reference light that has been reflected/transmitted by the interferometer6, processes the reference light and feeds back the reference light to the interferometer6, thus adjusting the detection sensitivity of the interferometer6in real time to achieve the optimal sensitivity.

A data acquisition card23is further included. The data acquisition card23is connected to the output terminal port of the signal light that is output by the second fiber polarization beam splitter16and the third fiber polarization beam splitter19and is reflected and transmitted by the interferometer6. The data acquisition card23collects and receives the signal light that is reflected and transmitted by the interferometer6and uses the same as a laser ultrasonic signal, and then the surface quality of the sample under test11is subjected to detection and identification processing.

The ultrasound in the sample under test11is generated by a pulse laser, that is, a laser pulse is irradiated to the surface of the sample under test to generate thermal expansion stress, thereby exciting ultrasonic waves.

Whereas the ambient temperature and ultrasonic vibrations may affect the parameters within the light beam, resulting in discrepancies in the matching relationship between the beam and the transmission/reflection curves in the interferometer6, thereby diminishing the interference effect of the interferometer6on the light beam. By adjusting the cavity length of the interferometer6, and consequently modifying the transmission/reflection curves of the interferometer6, the amplitude and slope corresponding to the fixed operating frequency of the interferometer6may be altered, thus enabling the adjustment of sensitivity.

The abscissa of the transmission/reflection curves represents frequency, while the ordinate denotes the light intensity transmission coefficient, indicating the response of light intensity variation. A greater response amplitude signifies superior demodulation efficacy.

In the event of a localized decrease in ambient temperature or a localized reduction in ultrasonic vibration frequency, the transmission/reflection curves of the interferometer6shift to the left. Consequently, the amplitude corresponding to the fixed operating frequency of the interferometer6increases, and the slope becomes steeper, thereby enhancing sensitivity. Preferably, the cavity length of the interferometer6is adjusted to position the amplitude corresponding to the fixed operating frequency at half of the maximum amplitude, thus maximizing the slope and optimizing sensitivity.

Feedback may be provided based on one reference light among the reference lights reflected/transmitted by the interferometer6.

In a specific implementation, the interferometer6includes a pair of mirrors, wherein at least one of which is movable, and the distance between the two mirrors determines the cavity length of the interferometer.

A narrow-linewidth semiconductor fiber laser1is configured to generate and provide a laser source for ultrasonic detection. Said laser emits a continuous wave output with a wavelength of 1550 nanometers (nm) and a power output of 100 milliwatts (mW).

The first fiber polarization controller2is configured to change the polarization state of continuous laser.

The first fiber polarization beam splitter3is configured to split the orthogonally and linearly polarized light contained within the optical fiber into reference light and detection light. Specifically, the implemented first fiber polarization beam splitter3has a 1×2 splitting ratio, operates at a wavelength of 1550 nm, and exhibits a light split ratio of 90 between the detection light and reference light.

The indication laser8is configured to emit 650 nm visible light, serving to demarcate the position of the detection light.

The wavelength division multiplexer7is configured to combine the detection light and the output light of the indication laser8and couple them into the same optical fiber for transmission. The operating wavelength is 650/1550 nm.

The fiber collimator9is configured to turn the transmitted light in the optical fiber into collimated light.

The biaxial scanning galvanometer10is configured to guide the detection beam to different positions on the surface of the sample under test11to detect laser ultrasonic signals.

The light converging collector12converges and collects the light scattered from the sample under test11and couples the scattered light to the optical fiber through the collimating lens13to form a signal light, which is then input to the interferometer6from the other end through the second fiber circulator14and the third fiber polarization controller15.

The propagation direction of the signal light in the interferometer6is opposite to that of the reference light.

The second fiber polarization beam splitter16receives the light beam output from the first circulator, which is respectively derived from the signal light transmitted by the interferometer and the reference light reflected by the interferometer. The first output port of the light beam is the signal light transmitted by the interferometer, which is converted into a first ultrasonic signal through the first photodetector17, the second output port thereof is the reference light reflected by the interferometer, which is converted into a first feedback control signal through the second photodetector18.

The third fiber polarization beam splitter19receives the light beam output from the second circulator, which is respectively derived from the signal light reflected by the interferometer and the reference light transmitted by the interferometer. The first output port of the light beam is the signal light reflected by the interferometer, which is converted into a second ultrasonic signal through the third photodetector20. The second output port thereof is the reference light transmitted by the interferometer, which is converted into a second feedback control signal through the fourth photodetector21.

The collimator facilitates the coupling of laser between optical fiber and free space. The fiber collimator9couples the detection light and indicator light from the optical fiber to free space, while the collimating lens13couples the scattered signal light into the optical fiber.

The photodetectors17and20convert the signal light and output an ultrasonic signal, and the photodetectors18and21convert the reference light and output a feedback control signal.

The feedback control system22receives the first and second feedback control signals, adjusts the cavity length of the interferometer in real time according to one of the first and second feedback control signals, and modulates the cavity length of the interferometer in real time based on the feedback control signal to stabilize the cavity length of the interferometer at the optimal detection performance.

The data acquisition card23stores the first and second ultrasonic signals and performs post-processing for ultrasonic measurement of the sample under test, stores and processes the ultrasonic signals, and implements ultrasonic measurement of the sample under test. The data acquisition card23is configured to implement functions such as thickness measurement for the sample under test, defect imaging, and material characterization.

The biaxial scanning galvanometer10outputs the detection light, which may be deflected and scanned in the X and Y directions, guiding the detection light to be incident obliquely to any detection position of the sample under test11.

In the specific implementation, the optical path is split to generate reference light and detection light, the detection light is incident on the target object of the sample under test11to modulate ultrasonic vibration. The scattered light scattered by the sample under test11is converged and collected and coupled to the optical fiber after photoelectric conversion to obtain the signal light modulated by ultrasonic vibration. The signal light and reference light are input into the interferometer11through different optical fiber circulators and polarization controllers.

Within the cavity of the interferometer11, the reference light is output after interference. The reference light output from the interference is detected by a photoelectric detector, which produces a feedback control signal. This signal is subsequently configured to adjust the cavity length of the interferometer, thereby providing feedback regulation and optimization of the detection sensitivity performance of the interferometer. The signal light output from the interference is detected by a photoelectric detector, generating an ultrasonic signal that enables ultrasonic measurement of the sample under test11. The ultrasonic wave measurement of the sample under test11may include, but is not limited to, thickness measurement, defect imaging, and characterization of material property parameters.

During the measurement process, the following method is used for signal compensation. The laser ultrasonic signals scanned and collected by the biaxial scanning galvanometer10at different positions are combined into a combined signal, and the combined signal is subjected to Hilbert transformation and then subjected to envelope processing to obtain the surface wave. For the envelope signal, the signal with the largest amplitude among the surface wave envelope signals serves as the reference signal, and each surface wave envelope signal is normalized according to the reference signal. After normalization, the signal intensity and signal-to-noise ratio of areas with poor surface quality of the sample under test are improved, and then the processed combined signal is formed into an image. Areas with poor quality may also be clearly displayed on the same image, thus achieving high-sensitivity measurement of the surface of the sample under test.

The present disclosure is characterized by its compact structure, high portability, and ease of maintenance and adjustment.

FIG.2is a work flow chart of the laser ultrasonic measurement system of the present disclosure.

In step 1, the light output by the narrow-linewidth fiber semiconductor laser is split by a polarization controller and a polarization beam splitter to generate detection light and reference light.

In step 2, the detection light is irradiated to the sample under test through the wavelength division multiplexer and the biaxial scanning galvanometer, and the indicator laser is configured to mark the irradiation position of the detection light.

In step 3, the scattered light modulated by ultrasonic vibration is converged and collected as signal light for input.

In step 4, the signal light and reference light are input to the interferometer from both ends of the interferometer in opposite propagation directions.

In step 5, the transmitted/reflected reference light demodulated by the interferometer generates a feedback control signal, which is configured to adjust the cavity length of the interferometer to work stably at optimal detection sensitivity performance.

In step 6, the transmitted/reflected signal light demodulated by the interferometer generates an ultrasonic signal.

In step 7, the received ultrasonic signal is configured to implement measurement of the sample under test.

In step 8, the ultrasonic measurement results of the sample under test, such as thickness, defects, material properties, and so on are output.

FIG.3is an optimal detection sensitivity control flow chart of the interferometer, and the operating process is mainly as follows.

The cavity length of the interferometer is scanned to obtain transmission/reflection curves of the interferometer.

The cavity length of the interferometer corresponding to the maximum slope point is selected according to the transmission/reflection curves and used as the stable operating point.

The cavity length of the interferometer is adjusted to the optimal operating point.

The cavity length is adjusted based on the feedback control signal generated by the reference light transmitted/reflected by the interferometer. If the amplitude is greater than the set value, the PZT driving voltage is reduced, otherwise, the PZT driving voltage is increased until the cavity length is stabilized at the optimal operating point.

The cavity length of the interferometer is adjusted by adjusting the PZT driving voltage. If the PZT driving voltage is reduced, the cavity length of the interferometer will be reduced.

In the meantime, during the adjustment process, it is determined whether the cavity length of the interferometer exceeds the adjustment range. If so, initialization should be performed for a restart.

The present disclosure, characterized by high environmental adaptability, employs a reference light to provide real-time adaptive compensation for laser frequency drift and variations of interferometer cavity length caused by industrial environmental changes and deformations of the interferometer itself. This mechanism enables the interferometer to consistently maintain its optimal operational state, thereby meeting the demands of industrial applications in high-temperature, high-vibration, and other challenging environments.

As shown inFIG.4, the present disclosure also performs automatic compensation of laser ultrasonic signals based on normalization for signal measurement. The processing process is as follows.

Step 1: Laser ultrasonic signals are acquired from various positions through point-by-point scanning using a biaxial scanning galvanometer, and are saved in the data acquisition system.

Step 2: Envelope processing is applied to the laser ultrasonic signals respectively after subjecting them to Hilbert transformation.

Step 3: The laser ultrasonic signal with the maximum amplitude surface wave serves as the reference signal, and normalization processing is performed through surface waves respectively.

The present disclosure exhibits high detection sensitivity and is capable of adaptively compensating for weak laser ultrasonic signals and low signal-to-noise ratios caused by poor surface quality of the sample under test.

For practitioners familiar with the related field, one or more devices in the present disclosure may be implemented in a more discrete or integrated manner, or even appropriately removed or added as optical components in certain specific application scenarios.

The aforementioned description constitutes merely one preferred embodiment of the present disclosure and is not intended to limit the scope thereof. Any approximate modifications, equivalent substitutions, and improvements made within the spirit and principles of the present disclosure should be encompassed within the scope to be protected by the present disclosure.