WO2023201928A1 - 一种自适应全光纤激光超声测量仪 - Google Patents

一种自适应全光纤激光超声测量仪 Download PDF

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WO2023201928A1
WO2023201928A1 PCT/CN2022/109944 CN2022109944W WO2023201928A1 WO 2023201928 A1 WO2023201928 A1 WO 2023201928A1 CN 2022109944 W CN2022109944 W CN 2022109944W WO 2023201928 A1 WO2023201928 A1 WO 2023201928A1
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light
interferometer
fiber
optical fiber
signal
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French (fr)
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陈剑
俞振炀
曾逸轲
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Zhejiang University
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Zhejiang University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/24—Probes
    • G01N29/2418—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B17/00—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations
    • G01B17/02—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations for measuring thickness
    • G01B17/025—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations for measuring thickness for measuring thickness of coating
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/048—Marking the faulty objects
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/32—Arrangements for suppressing undesired influences, e.g. temperature or pressure variations, compensating for signal noise
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/28—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising
    • G02B27/283—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising used for beam splitting or combining
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/28—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising
    • G02B27/286—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising for controlling or changing the state of polarisation, e.g. transforming one polarisation state into another
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • G01N2021/1706—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in solids
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/06—Illumination; Optics
    • G01N2201/061—Sources
    • G01N2201/06113—Coherent sources; lasers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/08—Optical fibres; light guides

Definitions

  • the invention relates to an optical fiber measuring instrument in the technical field of laser ultrasonic measurement, and in particular to an adaptive all-fiber laser ultrasonic measuring instrument and a measurement method thereof.
  • Laser ultrasound is a completely non-contact measurement technology that uses laser to generate and detect ultrasonic signals on the sample to be tested without any coupling agent. It is very suitable for extreme environments such as high temperature, high pressure, strong radiation, strong corrosion, or high-speed movement, etc. Online measurement of various components such as complex structures. Compared with conventional piezoelectric ultrasonic testing systems, laser ultrasonic systems have unique advantages, including long-distance non-contact (up to 10 meters away), multiple modes, wide frequency bandwidth, high resolution, and wide application range. Therefore, laser ultrasonic detection technology has received widespread attention from domestic and foreign researchers.
  • a laser pulse is irradiated to the surface of the sample to be tested to generate thermal expansion stress, thereby exciting ultrasonic waves, and then optical methods are used to detect the laser ultrasonic waves.
  • Laser ultrasound is well suited for a variety of industrial applications, such as the measurement of high-temperature steel pipes, coating thickness, and complex-shaped composite materials.
  • the stability of laser ultrasonic detection technology is seriously affected.
  • the amplitude of the laser ultrasonic echo signal fluctuates greatly.
  • the optical path of the laser ultrasonic testing device is complex and bulky, making it difficult to combine scanning technology with the overall detection of the sample to be tested.
  • a typical shortcoming of the existing laser ultrasonic measurement system based on Fabry-Perot interferometer is that it is difficult to control the interferometer to work stably at the optimal sensitivity for a long time. This is because high temperature, vibration and other environments in industrial sites can easily cause changes in the cavity length of the Fabry-Perot interferometer or laser frequency drift. In order to obtain the best detection performance, the Fabry-Perot interferometer cavity length must be able to adaptively adjust according to the environment and its own deformation to compensate for changes in cavity length caused by laser frequency drift and thermal expansion.
  • the present invention provides an all-fiber laser ultrasonic measuring instrument and measurement method with adaptive capabilities, and also proposes an automatic compensation method for laser ultrasonic echo signals based on normalization.
  • the invention has the characteristics of strong environmental adaptability, high measurement sensitivity, compact structure, strong portability, and convenient maintenance and adjustment.
  • An adaptive all-fiber laser ultrasonic measuring instrument :
  • It includes a narrow linewidth semiconductor fiber laser, a first fiber polarization controller, a first fiber polarization beam splitter, a first fiber circulator, a second fiber polarization controller, an interferometer, a wavelength division multiplexer, an indicator laser, and a fiber collimator.
  • the output end of the narrow linewidth semiconductor fiber laser is connected to the input end of the first optical fiber polarization beam splitter through the first optical fiber polarization controller, and an output end of the first optical fiber polarization beam splitter is connected to the first port of the first optical fiber circulator.
  • the second port of the first fiber circulator is connected to one end of the interferometer through the second fiber polarization controller;
  • the other output end of the first optical fiber polarization beam splitter and the indicator laser are jointly connected to the two input ends of the wavelength division multiplexer.
  • the output end of the wavelength division multiplexer is connected to the optical fiber collimator.
  • the output of the optical fiber collimator is The direct beam is irradiated onto the sample to be tested through a biaxial scanning galvanometer.
  • a light collector is placed above the sample to be tested. The light collector receives the scattered light from the sample to be tested and then enters the collimating lens.
  • the lens is connected to the first port of the second fiber optic circulator, and the second port of the second fiber optic circulator is connected through the third fiber polarization controller and the other end of the interferometer.
  • the narrow linewidth semiconductor fiber laser emits continuous laser light, which is polarized by the first fiber polarization controller to form orthogonal linearly polarized light with p-polarization state and s-polarization state, and is split into p-polarized light by the first fiber polarization beam splitter. Linearly polarized light in the state and linearly polarized light in the s-polarized state;
  • the reference light is incident into the second fiber polarization controller through the first fiber circulator, is polarized by the second fiber polarization controller, and then input into the interferometer;
  • the linearly polarized light in the p-polarization state is used as the detection light.
  • the detection light and the visible light output by the pointing laser are multiplexed by the wavelength division multiplexer and then input to the fiber collimator.
  • the fiber collimator outputs spatial free light, and the spatial free light is
  • the dual-axis scanning galvanometer generates ultrasonic vibrations on the sample to be tested.
  • the surface scattering of the sample to be tested produces scattered light modulated by the ultrasonic vibration.
  • the scattered light is collected by the light convergence collector and then incident on the collimating lens.
  • the straight lens turns into signal light, and the signal light is incident into the third fiber polarization controller through the second fiber circulator, and is input into the interferometer after polarization processing by the third fiber polarization controller;
  • Both the reference light and the signal light are reflected and transmitted in the interferometer
  • the signal light transmitted by the interferometer and the reference light reflected by the interferometer are combined and then incident into the first optical fiber circulator through the second optical fiber polarization controller, and are output from the third port of the first optical fiber circulator;
  • the reference light transmitted by the interferometer and the signal light reflected by the interferometer are combined and then incident on the second optical fiber circulator through the third optical fiber polarization controller, and are output from the third port of the second optical fiber circulator.
  • the fiber polarization controller is a three-ring mechanical fiber polarization controller.
  • the polarization of the second fiber polarization controller and the third fiber polarization controller is a quarter wavelength.
  • the first fiber polarization controller polarizes linearly polarized light. The state becomes an elliptically polarized state, and the ratio of p-polarized light to s-polarized light is 90:10.
  • the first optical fiber polarization controller is used for polarization processing, using the optical fiber to generate birefringence under the action of external force, and changing the polarization state of linearly polarized light into any elliptical polarization state including orthogonal components of P-state and S-state polarization.
  • the specific principle is: the three rings are equivalent to three wave plates: ⁇ /4, ⁇ /2, and ⁇ /4 respectively.
  • the light wave enters the ⁇ /4 wave plate and is converted into linearly polarized light, and then is converted into linearly polarized light by the ⁇ /2 wave plate. Adjust the polarization direction, and finally change the polarization state of the linearly polarized light into an arbitrary polarization state through the ⁇ /4 wave plate.
  • It also includes a second optical fiber polarizing beam splitter and a third optical fiber polarizing beam splitter; the input end of the second optical fiber polarizing beam splitter is connected to the third port of the first optical fiber circulator, and the two optical fiber polarizing beam splitters of the second optical fiber polarizing beam splitter are connected to the third port of the first optical fiber circulator.
  • the output ends are respectively connected to a photodetector; the input end of the third optical fiber polarizing beam splitter is connected to the third port of the second optical fiber circulator, and the two output ends of the third optical fiber polarizing beam splitter are also connected to a photodetector respectively.
  • It also includes a feedback control system, which is respectively connected to the reference light output port after the output of the second fiber polarization beam splitter and the third fiber polarization beam splitter has been reflected and transmitted by the interferometer; and is received by the feedback control system.
  • the reference light after reflection/transmission processing by the interferometer is processed and fed back to the interferometer to adjust the detection sensitivity of the interferometer in real time.
  • the data acquisition card is respectively connected to the output port of the second optical fiber polarization beam splitter and the third optical fiber polarization beam splitter.
  • the output signal light after reflection and transmission processing by the interferometer is collected and received by the data acquisition card.
  • the signal light after reflection and transmission processing by the interferometer is used as a laser ultrasonic signal, and then the surface quality of the sample to be tested is detected and identified.
  • the sample to be tested is placed on the platform and generates ultrasonic vibration after being irradiated by the laser of the biaxial scanning galvanometer.
  • the ultrasound is generated by a pulsed laser, that is, a laser pulse is irradiated onto the surface of the sample to be tested to generate thermal expansion stress, thereby stimulating ultrasonic waves.
  • the samples to be tested are specifically high-temperature steel pipes, coatings and complex-shaped composite materials.
  • the interferometer is a Fabry interferometer.
  • the transmission/reflection curve of the interferometer is adjusted, and the amplitude and slope corresponding to the fixed operating frequency of the interferometer are adjusted, thereby adjusting the sensitivity.
  • optical fibers used in the present invention are all polarization-maintaining optical fibers.
  • the sample to be tested is irradiated by a pulsed laser beam and excited to generate ultrasonic waves.
  • the ultrasonic waves generated based on the thermal expansion effect will cause phase changes in the detection light.
  • the scattered light modulated by the ultrasonic vibration signal is input to the interferometer.
  • the interferometer converts the phase modulation of the input light into amplitude modulation through demodulation.
  • Spectroscopy generates reference light and detection light.
  • the detection light is incident on the target object of the test sample to modulate ultrasonic vibration.
  • the scattered light scattered by the test sample is collected and coupled to the optical fiber after photoelectric conversion to obtain ultrasonic vibration modulation.
  • the signal light and the reference light are input into the interferometer through the fiber circulator and the polarization controller, and are output through interference in the inner cavity of the interferometer.
  • the reference light output by the interference is generated by the photodetector to generate a feedback control signal, which is then used to adjust the interferometer.
  • the length of the cavity is then adjusted by feedback to optimize the detection sensitivity performance of the interferometer.
  • the signal light output from the interference output is used to generate an ultrasonic signal through the photodetector to achieve ultrasonic measurement of the sample to be tested.
  • the reference light and detection light are generated by polarized beam splitting of the light source.
  • the detection light is guided by a biaxial galvanometer and incident on the sample to be tested.
  • the signal light is scattered by the object to be tested and collected through an optical lens group.
  • the signal light is phase-modulated by the ultrasonic waves in the target to be measured.
  • the following method is used for signal compensation: the laser ultrasonic signals scanned and collected at different positions of the biaxial scanning galvanometer are synthesized into a combined signal, and the combined signal is subjected to Hilbert transformation and then envelope processing is performed to obtain For the surface wave envelope signal, the signal with the largest amplitude among the surface wave envelope signals is taken 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 are improved, and then the processed combined signal is imaged. Areas of poor quality can also be clearly displayed on the same image, allowing the sample to be accurately analyzed. Highly sensitive measurement of surfaces.
  • the laser ultrasonic signal with the largest surface wave envelope signal amplitude is the reference signal.
  • the laser ultrasonic signal is normalized according to the reference signal and the surface wave envelope signal.
  • the present invention can improve the signal intensity and signal-to-noise ratio of areas with poor surface quality of the sample, realize complete detection of the test sample with poor surface consistency, and greatly improve the subsequent determination of defects or coating thickness. Accuracy of measurement.
  • the output light of the narrow linewidth semiconductor fiber laser is split by the polarization controller and the polarization beam splitter to generate detection light and reference light, and the detection light is irradiated to the sample to be tested through the wavelength division multiplexer and the biaxial scanning galvanometer.
  • the scattered light modulated by ultrasonic vibration is collected and collected as signal light input.
  • the signal light and reference light are input to the interferometer from both ends of the interferometer in opposite propagation directions.
  • the reference light demodulated by the interferometer generates a feedback control signal for adjusting the interference.
  • the cavity length of the instrument works stably at the best detection sensitivity performance.
  • the signal light demodulated by the interferometer generates an ultrasonic signal to achieve ultrasonic measurement of the sample to be tested, including thickness, defects, material properties, etc.
  • the invention has strong environmental adaptability and can adaptively compensate in real time for laser frequency drift caused by changes in the industrial site environment and changes in cavity length caused by thermal expansion and deformation of the interferometer;
  • the measurement sensitivity of the present invention is high and it can adaptively amplify low signal-to-noise ratio signals caused by weak scattering on the surface of the test sample;
  • the invention has a compact structure and strong portability
  • the present invention is easy to maintain and adjust.
  • Figure 1 is an implementation block diagram of the laser ultrasonic measurement system of the present invention
  • Figure 2 is a work flow chart of the laser ultrasonic measurement system of the present invention.
  • Figure 3 is the optimal detection sensitivity control flow chart of the interferometer
  • Figure 4 is a schematic diagram of the automatic compensation method of laser ultrasonic signals based on normalization.
  • 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), indicator laser (8), fiber collimator (9), biaxial scanning galvanometer (10), sample to be tested (11), light Converging collector (12), collimating lens (13), second fiber optic 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 collection card(23).
  • the specifically implemented instruments include a narrow linewidth semiconductor fiber laser 1, a first fiber polarization controller 2, a first fiber polarization beam splitter 3, a first fiber circulator 4, and a second fiber polarization controller 5 , interferometer 6, wavelength division multiplexer 7, indicator laser 8, fiber collimator 9, biaxial scanning galvanometer 10, light convergence collector 12, collimating lens 13, second fiber circulator 14 and third fiber Polarization controller 15; the output end of the narrow linewidth semiconductor fiber laser 1 is connected through the first fiber polarization controller 2 and the input end of the first fiber polarization beam splitter 3.
  • the terminal and the indicator laser 8 are jointly connected to the two input terminals of the wavelength division multiplexer 7.
  • the output terminal of the wavelength division multiplexer 7 is connected to the optical fiber collimator 9.
  • the optical fiber collimator 9 outputs a collimated beam and is scanned biaxially.
  • the galvanometer 10 irradiates the sample 11 to be tested.
  • a light collector 12 is provided above the sample 11 to be tested.
  • the light collector 12 receives scattered light from the sample 11 to be tested and then enters the collimating lens 13.
  • the straight lens 13 is connected to the first port of the second fiber optic circulator 14
  • the second port of the second fiber optic circulator 14 is connected to the other end of the third fiber polarization controller 15 and the interferometer 6 .
  • the narrow linewidth semiconductor fiber laser 1 emits continuous laser light, which is processed by the first fiber polarization controller 2 to form orthogonal linearly polarized light with p-polarization state and s-polarization state, and is split by the first fiber polarization beam splitter 3 Linearly polarized light in the p-polarized state and linearly polarized light in the s-polarized state are formed; the linearly polarized light in the s-polarized state is used as the reference light, and the reference light is incident into the second fiber polarization controller 5 through the first fiber circulator 4.
  • the second optical fiber polarization controller 5 is polarized and input into the interferometer 6; the linearly polarized light in the p-polarization state is used as the detection light, and the detection light and the visible light output by the indicator laser 8 are multiplexed by the wavelength division multiplexer 7 and then input
  • the optical fiber collimator 9 starts to output free space light.
  • the free space light is illuminated on the sample to be tested 11 through the biaxial scanning galvanometer 10.
  • the area illuminated by the sample to be tested 11 will vibrate ultrasonically.
  • the information is modulated into the detection laser, and the scattered light modulated by ultrasonic vibration is generated by the surface scattering of the sample 11 to be tested.
  • the scattered light is collected by the light convergence collector 12 and then incident on the collimating lens 13. After passing through the collimating lens 13, it becomes a signal.
  • Light and signal light are incident into the third fiber polarization controller 15 through the second fiber circulator 14, and are input into the interferometer 6 after polarization processing by the third fiber polarization controller 15; the reference light and the signal light are in the interferometer 6 Reflection and transmission have occurred, resulting in signal light and reference light transmitted by the interferometer 6, signal light and reference light reflected by the interferometer 6; signal light transmitted by the interferometer 6 and reflected by the interferometer 6
  • the final reference light is combined and polarized by the second optical fiber polarization controller 5 before being incident on the first optical fiber circulator 4 and output from the third port of the first optical fiber circulator 4; the reference light transmitted through the interferometer 6 and
  • the signal light reflected by the interferometer 6 is combined and polarized by the third optical fiber polarization controller 15 before being incident into the second optical fiber circulator 14 and output
  • the fiber polarization controller is preferably a three-ring mechanical fiber polarization controller.
  • the polarization of the second fiber polarization controller 5 and the third fiber polarization controller 15 is a quarter wavelength.
  • the first fiber polarization controller 2 converts linearly polarized light into The polarization state becomes an elliptically polarized state, and the ratio of p-polarized light to s-polarized light is 90:10.
  • the original reference light undergoes two quarter-wave plate polarization processes by the second optical fiber polarization controller 5 to form the reference light transmitted and reflected by the interferometer, and changes from s-state polarization to p-state polarization.
  • the original signal light undergoes two quarter-wave plate polarization processes by the third optical fiber polarization controller 15 to form signal light transmitted and reflected by the interferometer, and changes from p-state polarization to s-state polarization.
  • It also includes a second optical fiber polarization beam splitter 16 and a third optical fiber polarization beam splitter 19; the input end of the second optical fiber polarization beam splitter 16 is connected to the third port of the first optical fiber circulator 4, and the second optical fiber polarization splitter 16 is connected to the third port of the first optical fiber circulator 4.
  • the two output ends of the detector 16 are respectively connected to a photodetector; the input end of the third optical fiber polarization beam splitter 19 is connected to the third port of the second optical fiber circulator 14, and the two outputs of the third optical fiber polarization beam splitter 19 Each end is also connected to a photodetector.
  • the photodetectors connected to the two output ends of the second fiber polarizing beam splitter 16 are the first photodetector 17 and the second photodetector 18 respectively, and the photodetectors connected to the two output ends of the third fiber polarizing beam splitter 19 They are the third photodetector 20 and the fourth photodetector 21 respectively; the first photodetector 17 and the third photodetector 20 are connected to the feedback control system 22, and the second photodetector 18 and the fourth photodetector 21 are connected to the data acquisition card 23.
  • the first photodetector 17 receives the signal light transmitted by the interferometer 6 from the second optical fiber polarization beam splitter 16
  • the second photodetector 18 receives the reference signal reflected by the interferometer 6 from the second optical fiber polarization beam splitter 16
  • the third photodetector 20 receives the signal light reflected by the interferometer 6 from the third optical fiber polarization beam splitter 19
  • the fourth photodetector 21 receives the signal light transmitted by the interferometer 6 from the third optical fiber polarization beam splitter 19 reference light.
  • the feedback control system 22 receives the reference light that has been reflected/transmitted by the interferometer 6, processes it and feeds it back to the interferometer 6, and adjusts the detection sensitivity of the interferometer 6 in real time to achieve the optimal sensitivity.
  • the data acquisition card 23 also includes a data acquisition card 23.
  • the data acquisition card 23 is respectively connected to the output port of the second optical fiber polarization beam splitter 16 and the third optical fiber polarization beam splitter 19, which outputs the signal light after reflection and transmission processing by the interferometer 6.
  • the data acquisition card 23 collects and receives the signal light that has been reflected and transmitted by the interferometer 6 as a laser ultrasonic signal, and then performs detection and identification processing on the surface quality of the sample 11 to be tested.
  • the ultrasound in the sample to be tested 11 is generated by a pulse laser, that is, a laser pulse is irradiated onto the surface of the sample to be tested to generate thermal expansion stress, thereby stimulating ultrasonic waves.
  • the transmission/reflection curve of the interferometer 6 is adjusted, the amplitude and slope corresponding to the working fixed frequency of the interferometer 6 are adjusted, and the sensitivity is adjusted.
  • the abscissa of the transmission/reflection curve is the frequency, and the ordinate is the light intensity transmission coefficient, indicating the response to changes in light intensity. The larger the response amplitude, the better the demodulation effect.
  • the transmission/reflection curve of the interferometer 6 moves to the left, the amplitude corresponding to the working fixed frequency of the interferometer 6 increases, and the slope increases, causing the sensitivity to increase.
  • the cavity length of the interferometer 6 is adjusted so that the amplitude corresponding to the working fixed frequency is located at half of the maximum amplitude, that is, the slope is maximized and the sensitivity is maximized.
  • Feedback can be performed based on one of the reference lights that has been processed by the interferometer 6 for reflection/transmission.
  • the interferometer 6 includes a pair of mirrors, at least one of which is movable, and the distance between the two mirrors determines the interferometer cavity length.
  • the narrow linewidth semiconductor fiber laser 1 is used to generate and provide a laser ultrasonic detection light source, with an output wavelength of 1550nm and a continuous laser power of 100mW.
  • the first fiber polarization controller 2 is used to change the polarization state of the continuous laser
  • the first optical fiber polarization beam splitter 3 is used to divide the orthogonal linearly polarized light contained in the optical fiber into reference light and detection light; the specific implementation of the first optical fiber polarization beam splitter 3 has a beam splitting ratio of 1 ⁇ 2 and an operating wavelength of is 1550nm, and the split ratio of detection light and reference light is 90.
  • the indicator laser 8 is used to output 650nm visible light to mark the position of the detection light.
  • Wavelength division multiplexer 7 is used to combine the detection light and the output light of the indicator laser 8 and couple them into the same optical fiber for transmission; the operating wavelength is 650/1550nm.
  • Optical fiber collimator 9 used to convert the transmission light in the optical fiber into collimated light
  • the biaxial scanning galvanometer 10 is used to guide the detection beam to different positions on the surface of the test sample 11 to detect laser ultrasonic signals;
  • the light collector 12 collects and collects the light scattered from the sample 11 to be tested, and then couples it to the optical fiber through the collimating lens 13 to form signal light, and then passes through the second optical fiber circulator 14 and the third optical fiber polarization controller 15 and then from another optical fiber.
  • One end is input to the interferometer 6;
  • the propagation direction of the signal light in the interferometer 6 is opposite to that of the reference light.
  • the second optical fiber polarization beam splitter 16 receives the light beam output from the first circulator, which is respectively the signal light transmitted by the interferometer and the reference light reflected by the interferometer. Its first output port is the signal light transmitted by the interferometer.
  • the first photodetector 17 converts it into a first ultrasonic signal, and the second output port is the reference light reflected by the interferometer, which is converted into a first feedback control signal by the second photodetector 18 .
  • the third optical fiber polarization beam splitter 19 receives the light beam output from the second circulator, which is respectively the signal light reflected by the interferometer and the reference light transmitted by the interferometer. Its first output port is the signal light reflected by the interferometer.
  • the third photodetector 20 converts the second ultrasonic signal, and the second output port is the reference light transmitted by the interferometer, which is converted into a second feedback control signal by the fourth photodetector 21 .
  • the collimator realizes the coupling of the laser in the optical fiber and the free space.
  • the optical fiber collimator 9 couples the detection light and the indicator light from the optical fiber to the free space, and the collimating lens 13 couples the scattered signal light to the optical fiber.
  • the photodetectors 17 and 20 convert the signal light and output an ultrasonic signal
  • the photodetectors 18 and 21 convert the reference light and output a feedback control signal.
  • the feedback control system 22 receives the first and second feedback control signals, realizes real-time adjustment of the interferometer cavity length according to one of them, and modulates the interferometer cavity length in real time based on the feedback control signal to stabilize it at the best detection performance.
  • the data acquisition card 23 stores the first and second ultrasonic signals and performs post-processing for ultrasonic measurement of the sample to be tested, stores and processes the ultrasonic signals, and implements ultrasonic measurement of the sample to be tested.
  • the data acquisition card 23 is used to implement functions such as thickness measurement of the sample to be tested, defect imaging, and material characterization.
  • the biaxial scanning galvanometer 10 outputs detection light, which can be deflected and scanned in the X and Y directions, and guides the detection light to be obliquely incident on any detection position of the sample 11 to be tested.
  • the optical path is split to generate reference light and detection light
  • the detection light is incident on the target object of the test sample 11 to modulate ultrasonic vibration
  • the scattered light scattered by the test sample 11 is collected and coupled to the
  • the optical fiber obtains signal light modulated by ultrasonic vibration.
  • the signal light and reference light are input to the interferometer 11 through different optical fiber circulators and polarization controllers. They are output through interference in the inner cavity of the interferometer 11.
  • the reference light output by the interference generates feedback through the photodetector.
  • the control signal is then used to adjust the cavity length of the interferometer, and then the feedback adjustment optimizes the detection sensitivity performance of the interferometer.
  • the signal light output of the interference output generates an ultrasonic signal through the photodetector to implement ultrasonic measurement of the sample 11 to be tested.
  • Ultrasonic measurement of the sample 11 to be tested such as thickness measurement, defect imaging and material property parameter characterization, etc.
  • the following method is used for signal compensation: the laser ultrasonic signals scanned and collected at different positions of the biaxial scanning galvanometer 10 are synthesized into a combined signal, and the combined signal is subjected to Hilbert transformation and then envelope processing is performed.
  • the surface wave envelope signal is obtained, the signal with the largest amplitude among the surface wave envelope signals is taken as the reference signal, and each surface wave envelope signal is normalized according to the reference signal.
  • the signal intensity and signal-to-noise ratio of areas with poor surface quality of the sample are improved, and then the processed combined signal is imaged. Areas of poor quality can also be clearly displayed on the same image, allowing the sample to be accurately analyzed. Highly sensitive measurement of surfaces.
  • the invention has compact structure, strong portability and convenient maintenance and adjustment.
  • Figure 2 is a work flow chart of the laser ultrasonic measurement system of the present invention.
  • step 1 the output light of the narrow linewidth semiconductor fiber laser is split by a polarization controller and a polarization beam splitter to generate detection light and reference light.
  • step 2 the detection light is irradiated to the sample to be tested through the wavelength division multiplexer and the biaxial scanning galvanometer, and the indicator laser is used to mark the location of the detection light irradiation.
  • step 3 the scattered light modulated by ultrasonic vibration is collected and collected as signal light input.
  • step 4 the signal light and reference light are input to the interferometer in opposite propagation directions from both ends of the interferometer.
  • step 5 the transmitted/reflected reference light demodulated by the interferometer generates a feedback control signal, which is used to adjust the cavity length of the interferometer to work stably at optimal detection sensitivity performance.
  • step 6 the transmitted/reflected signal light demodulated by the interferometer generates an ultrasonic signal.
  • step 7 the received ultrasonic signal is used to measure the sample to be tested.
  • step 8 output the ultrasonic measurement results of the sample to be tested, such as thickness, defects, material properties, etc.
  • Figure 3 is a flow chart for optimal detection sensitivity control of the interferometer of the present invention. Its working process is mainly as follows:
  • the interferometer cavity length corresponding to the maximum slope point is selected as the stable operating point.
  • the cavity length is adjusted based on the feedback control signal generated by the interferometer's transmitted/reflected reference light. If the amplitude is greater than the set value, the PZT driving voltage is reduced, otherwise, the PZT driving voltage is increased until it stabilizes at the optimal operating point.
  • the cavity length of the interferometer is adjusted by adjusting the PZT drive voltage. If the PZT drive voltage is reduced, the cavity length of the interferometer is reduced.
  • the invention has strong environmental adaptability. Based on the reference light, it can adaptively compensate in real time for laser frequency drift and interferometer cavity length changes caused by changes in the industrial site environment and deformation of the interferometer itself, so that the interferometer can continue to be stably maintained in the best working condition, meeting the requirements High temperature, vibration and other industrial field application requirements.
  • the present invention also performs automatic compensation of laser ultrasonic signals based on normalization for signal measurement.
  • the processing process is as follows:
  • the laser ultrasonic signals at different positions are obtained through point-by-point scanning with a biaxial scanning galvanometer and stored in the data acquisition system;
  • the laser ultrasonic signals are subjected to Hilbert transformation and then envelope processing is performed respectively;
  • the laser ultrasonic signal with the largest amplitude surface wave is taken as the reference signal, and normalized by the surface wave respectively.
  • the invention has high detection sensitivity and can adaptively compensate for the problems of weak laser ultrasonic signal and low signal-to-noise ratio caused by poor surface quality of the sample to be tested.
  • one or more devices in the present invention can be used in a more discrete or integrated manner, and even optical devices can be appropriately removed or added in certain specific application scenarios.

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Abstract

一种自适应全光纤激光超声测量仪及其测量方法。其中，窄线宽半导体光纤激光器(1)输出光经偏振控制器(2)和偏振分束器(3)分光产生探测光和参考光，探测光和指示激光器(8)输出的可见光经波分复用器(7)和双轴扫描振镜(10)照射至待测试样(11)产生超声振动，超声振动调制的散射光汇聚收集作为信号光，信号光和参考光分别从干涉仪(6)两端以相反传播方向输入到干涉仪(6)，经干涉仪(6)解调的参考光生成反馈控制信号，调整干涉仪(6)的检测性能，经干涉仪(6)解调的信号光生成超声波信号，实现待测试样(11)超声测量。该测量仪具有环境适应性强、测量灵敏度高、结构紧凑、便携性强、维护调整方便等特点。

Description

一种自适应全光纤激光超声测量仪 技术领域
本发明涉及激光超声测量技术领域的一种光纤测量仪，特别是涉及了一种自适应全光纤激光超声测量仪及其测量方法。
背景技术
激光超声是一种完全非接触的测量技术，分别使用激光在待测试样产生和检测超声波信号，无须任何耦合剂，非常适合高温、高压、强辐射、强腐蚀等极端环境下或高速运动、复杂结构等各种零部件的在线测量。相较于常规压电超声检测系统，激光超声系统具备其特有的优势，包括远距离非接触(最远可达10米以)、模态多、频带宽、分辨率高以及应用范围广等。因此，激光超声检测技术受到了国内外科研工作者的广泛关注。通常，一个激光脉冲照射到待测试样表面产生热膨胀应力，从而激发出超声波，再用光学方法去探测激光超声波。激光超声非常适合各种工业应用，如高温钢管、涂层厚度和复杂形状复合材料等的测量。但由于工业现场环境比较复杂，如高温、振动等，使得激光超声检测技术的稳定性受到严重影响。同时由于待测试样表面质量一致性较差，激光超声回波信号幅值波动很大。此外，激光超声检测装置光路复杂，体积庞大，难以结合扫描技术实现待测试样整体检测。
现有基于法布里-珀罗干涉仪的激光超声测量系统一个典型缺点是难以控制干涉仪长期稳定工作在最佳灵敏度。这是因为工业现场高温、振动等环境容易造成法布里-珀罗干涉仪腔长变化或者激光频率漂移。为了获得最佳检测性能，法布里-珀罗干涉仪腔长必须能够根据环境和本身变形自适应调整以补偿激光频率漂移和热膨胀引起的腔长变化。
发明内容
为了克服上述现有技术的不足，本发明提供了一种具有自适应能力的全光纤激光超声测量仪和测量方法，同时提出一种基于归一化的激光超声回波信号自动补偿方法。
本发明具有环境适应性强、测量灵敏度高、结构紧凑、便携性强、维护调整方便等特点。
本发明所采用的技术方案是：
一、一种自适应全光纤激光超声测量仪：
包括窄线宽半导体光纤激光器、第一光纤偏振控制器、第一光纤偏振分束器、第一光纤环形器、第二光纤偏振控制器、干涉仪、波分复用器、指示激光器、光纤准直器、双轴扫描振镜、光汇聚收集器、准直透镜、第二光纤环形器和第三光纤偏振控制器；
窄线宽半导体光纤激光器输出端经第一光纤偏振控制器和第一光纤偏振分束器的输入端连接，第一光纤偏振分束器的一个输出端和第一光纤环形器的第一端口连接，第一光纤环形器的第二端口经第二光纤偏振控制器后和干涉仪的一端连接；
第一光纤偏振分束器的另一个输出端和指示激光器共同连接到波分复用器的两个输入端，波分复用器的输出端和光纤准直器连接，光纤准直器输出准直光束经双轴扫描振镜照射到待测试样上，待测试样上方设有光汇聚收集器，光汇聚收集器接收来自待测试样的散射光后入射到准直透镜，准直透镜连接到第二光纤环形器的第一端口，第二光纤环形器的第二端口经第三光纤偏振控制器和干涉仪的另一端连接。
窄线宽半导体光纤激光器发出连续激光，经第一光纤偏振控制器偏振处理后形成具有p偏振态和s偏振态的正交线偏振光，经第一光纤偏振分束器分束后形成p偏振态的线偏振光和s偏振态的线偏振光；
以s偏振态的线偏振光作为参考光，参考光经第一光纤环形器入射到第二光纤偏振控制器中，经第二光纤偏振控制器偏振处理后输入到干涉仪中；
以p偏振态的线偏振光作为探测光，探测光和指示激光器输出的可见光一起经波分复用器复用后输入到光纤准直器，光纤准直器输出空间自由光，空间自由光经双轴扫描振镜照射到的待测试样上产生超声振动，由待测试样表面散射产生超声振动调制的散射光，散射光经光汇聚收集器收集汇聚后入射到准直透镜，经准直透镜后变成信号光，信号光经第二光纤环形器入射到第三光纤偏振控制器中，经第三光纤偏振控制器偏振处理后输入到干涉仪中；
参考光和信号光在干涉仪中均发生了反射和透射；
经干涉仪透射后的信号光和经干涉仪反射后的参考光合束后经第二光纤偏振控制器入射到第一光纤环形器中，从第一光纤环形器的第三端口输出；
经干涉仪透射后的参考光和经干涉仪反射后的信号光合束后经第三光纤偏振控制器入射到第二光纤环形器中，从第二光纤环形器的第三端口输出。
所述光纤偏振控制器为三环机械型光纤偏振控制器，第二光纤偏振控制器和第三光纤偏振控制器的偏振为四分之一波长，第一光纤偏振控制器将线偏振 光的偏振状态变成椭圆偏振态，p偏振态光和s偏振态光比例为90：10。
第一光纤偏振控制器用于进行偏振处理，利用光纤在外力作用下产生双折射，将线偏振光的偏振态变成包括P态和S态偏振正交分量的任意椭圆偏振态。其具体原理为：其中三个环分别等效为λ/4、λ/2、λ/4三种波片，光波进过λ/4波片转换为线偏振光，再由λ/2波片调整偏振方向，最后经由λ/4波片将线偏振光的偏振状态变成任意的偏振态。使用偏振控制器通过180°旋转1/4波片和1/2波片，可以获得偏振态的旋转变化。
还包括第二光纤偏振分束器和第三光纤偏振分束器；第二光纤偏振分束器的输入端连接到第一光纤环形器的第三端口，第二光纤偏振分束器的两个输出端分别连接一个光电探测器；第三光纤偏振分束器的输入端连接到第二光纤环形器的第三端口，第三光纤偏振分束器的两个输出端也分别连接一个光电探测器。
还包括反馈控制系统，反馈控制系统分别连接到第二光纤偏振分束器和第三光纤偏振分束器的输出经干涉仪反射和透射处理后的参考的光输出端口；由反馈控制系统接收经干涉仪反射/透射处理后的参考光后进行处理并反馈到干涉仪，实时调整干涉仪的检测灵敏度。
还包括数据采集卡，数据采集卡分别连接到第二光纤偏振分束器和第三光纤偏振分束器的输出经干涉仪反射和透射处理后的信号光的输出端口，由数据采集卡采集接收经干涉仪反射和透射处理后的信号光作为激光超声信号，后进行针对待测试样表面质量的检测识别处理。
所述的待测试样安置在平台上，在受到双轴扫描振镜的激光照射后，产生超声振动。所述超声由脉冲激光产生，亦即一个激光脉冲照射到待测试样表面产生热膨胀应力，从而激发出超声波。
所述的待测试样具体为高温钢管、涂层和复杂形状复合材料等。
所述的干涉仪为法布里干涉仪。
通过调整干涉仪的腔长，进而调整干涉仪的透射/反射曲线，进而调整干涉仪工作固定频率对应的幅值和斜率，进而调整灵敏度。
本发明采用的光纤均为保偏光纤。
在本发明实现中，待测试样受到脉冲激光束照射并激发产生超声波。同时，基于热膨胀效应产生的超声波会引起检测光的相位变化。然后，经超声振动信号调制的散射光输入至干涉仪。干涉仪则通过解调将输入光的相位调制转换成幅值调制。
二、一种自适应全光纤激光超声测量仪的测量方法：
分光产生参考光和检测光，将探测光入射到的待测试样的目标对象上调制超声振动，汇聚收集待测试样散射出的散射光，经光电转换后耦合至光纤获得超声振动调制的信号光，通过光纤环形器和偏振控制器将信号光和参考光输入干涉仪，在干涉仪内腔内经干涉输出，干涉输出的参考光经光电探测器生成反馈控制信号进而用于调整干涉仪的腔长，进而反馈调节优化干涉仪的检测灵敏度性能，干涉输出的信号光输出经光电探测器生成超声信号实现待测试样的超声测量。
所述参考光和探测光由光源偏振分束产生，所述探测光经双轴振镜引导入射至待测试样，所述信号光由待测对象散射经光学透镜组汇聚收集形成，所述信号光被待测目标中的超声波相位调制。
测量过程中，采用以下方式进行信号补偿：将双轴扫描振镜不同位置扫描采集的激光超声信号合成为一个合路信号，将合路信号经希尔伯特变换后，再做包络处理得到表面波包络信号，取表面波包络信号中具有最大幅值的信号作为基准信号，根据基准信号分别对各个表面波包络信号进行归一化处理。归一化后试样表面质量不佳处的信号强度和信噪比得到提高，再将处理后的合路信号进行成像，在同一幅图上质量不佳处也能够清楚显现，实现对试样表面的高灵敏度测量。
所述表面波包络信号幅值最大的激光超声信号为基准信号。
所述激光超声信号根据基准信号和表面波包络信号进行归一化处理。
本发明通过上述信号补偿处理，能够提高试样表面质量不佳处的信号强度和信噪比，实现了对表面一致性差的待测试样的完整检测，大大提高后续缺陷的判定或涂层厚度测量的准确性。
本发明中，窄线宽半导体光纤激光器输出光经偏振控制器和偏振分束器分光产生探测光和参考光，探测光经波分复用器和双轴扫描振镜照射至待测试样，超声振动调制的散射光汇聚收集作为信号光输入，信号光和参考光分别从干涉仪两端以相反传播方向输入到干涉仪，经干涉仪解调的参考光生成反馈控制信号，用于调整干涉仪的腔长稳定工作在最佳检测灵敏度性能，经干涉仪解调的信号光生成超声波信号，实现待测试样超声测量，包括厚度、缺陷、材料特性等。
与现有技术相比，本发明的有益效果是：
1)本发明环境适应性强，可以实时自适应补偿工业现场环境变化造成的激光频率漂移和干涉仪热膨胀、变形等引起的腔长变化；
2)本发明测量灵敏度高，可以自适应放大待测试样表面弱散射导致的低信 噪比信号；
3)本发明结构紧凑，便携性强；
4)本发明维护调整方便。
附图说明
图1为本发明激光超声测量系统的实现框图；
图2为本发明激光超声测量系统的工作流程图；
图3为干涉仪最佳检测灵敏度控制流程图；
图4为基于归一化的激光超声信号自动补偿方法示意图。
图中：窄线宽半导体光纤激光器(1)、第一光纤偏振控制器(2)、第一光纤偏振分束器(3)、第一光纤环形器(4)、第二光纤偏振控制器(5)、干涉仪(6)、波分复用器(7)、指示激光器(8)、光纤准直器(9)、双轴扫描振镜(10)、待测试样(11)、光汇聚收集器(12)、准直透镜(13)、第二光纤环形器(14)、第三光纤偏振控制器(15)、第二光纤偏振分束器(16)、第一光电探测器(17)、第二光电探测器(18)、第三光纤偏振分束器(19)、第三光电探测器(20)、第四光电探测器(21)、反馈控制系统(22)、数据采集卡(23)。
具体实施方式
下面结合附图对本发明进一步说明。
如图1所示，具体实施的仪器包括窄线宽半导体光纤激光器1、第一光纤偏振控制器2、第一光纤偏振分束器3、第一光纤环形器4、第二光纤偏振控制器5、干涉仪6、波分复用器7、指示激光器8、光纤准直器9、双轴扫描振镜10、光汇聚收集器12、准直透镜13、第二光纤环形器14和第三光纤偏振控制器15；窄线宽半导体光纤激光器1输出端经第一光纤偏振控制器2和第一光纤偏振分束器3的输入端连接，第一光纤偏振分束器3的一个输出端和第一光纤环形器4的第一端口连接，第一光纤环形器4的第二端口经第二光纤偏振控制器5后和干涉仪6的一端连接；第一光纤偏振分束器3的另一个输出端和指示激光器8共同连接到波分复用器7的两个输入端，波分复用器7的输出端和光纤准直器9连接，光纤准直器9输出准直光束经双轴扫描振镜10照射到待测试样11上，待测试样11上方设有光汇聚收集器12，光汇聚收集器12接收来自待测试样11的散射光后入射到准直透镜13，准直透镜13连接到第二光纤环形器14的第一端口，第二光纤环形器14的第二端口经第三光纤偏振控制器15和干涉仪6的另一端连接。
窄线宽半导体光纤激光器1发出连续激光，经第一光纤偏振控制器2骗子处理后形成具有p偏振态和s偏振态的正交线偏振光，经第一光纤偏振分束器3分束后形成p偏振态的线偏振光和s偏振态的线偏振光；以s偏振态的线偏振光作为参考光，参考光经第一光纤环形器4入射到第二光纤偏振控制器5中，经第二光纤偏振控制器5偏振处理后输入到干涉仪6中；以p偏振态的线偏振光作为探测光，探测光和指示激光器8输出的可见光一起经波分复用器7复用后输入到光纤准直器9，光纤准直器9开始输出空间自由光，空间自由光经双轴扫描振镜10照射到待测试样11上，待测试样11照射到的区域会把超声振动信息调制到检测激光中，由待测试样11表面散射产生超声振动调制的散射光，散射光经光汇聚收集器12收集汇聚后入射到准直透镜13，经准直透镜13后变成信号光，信号光经第二光纤环形器14入射到第三光纤偏振控制器15中，经第三光纤偏振控制器15偏振处理后输入到干涉仪6中；参考光和信号光在干涉仪6中均发生了反射和透射，产生了经干涉仪6透射后的信号光和参考光、经干涉仪6反射后的信号光和参考光；经干涉仪6透射后的信号光和经干涉仪6反射后的参考光合束后经第二光纤偏振控制器5偏振处理后入射到第一光纤环形器4中，从第一光纤环形器4的第三端口输出；经干涉仪6透射后的参考光和经干涉仪6反射后的信号光合束后经第三光纤偏振控制器15偏振处理后入射到第二光纤环形器14中，从第二光纤环形器14的第三端口输出。
光纤偏振控制器优选为三环机械型光纤偏振控制器，第二光纤偏振控制器5和第三光纤偏振控制器15的偏振为四分之一波长，第一光纤偏振控制器2将线偏振光的偏振状态变成椭圆偏振态，p偏振态光和s偏振态光比例为90：10。
具体地，原始的参考光经过两次第二光纤偏振控制器5的四分之一波片偏振处理形成干涉仪透射和反射的参考光，由s态偏振变成p态偏振。
具体地，原始的信号光经过两次第三光纤偏振控制器15的四分之一波片偏振处理形成干涉仪透射和反射的信号光，由p态偏振变成s态偏振。
还包括第二光纤偏振分束器16和第三光纤偏振分束器19；第二光纤偏振分束器16的输入端连接到第一光纤环形器4的第三端口，第二光纤偏振分束器16的两个输出端分别连接一个光电探测器；第三光纤偏振分束器19的输入端连接到第二光纤环形器14的第三端口，第三光纤偏振分束器19的两个输出端也分别连接一个光电探测器。
第二光纤偏振分束器16两个输出端连接的光电探测器分别为第一光电探测器17和第二光电探测器18，第三光纤偏振分束器19两个输出端连接的光电探测器分别为第三光电探测器20和第四光电探测器21；第一光电探测器17和第 三光电探测器20连接到反馈控制系统22，第二光电探测器18和第四光电探测器21连接到数据采集卡23。
第一光电探测器17从第二光纤偏振分束器16接收经干涉仪6透射后的信号光，第二光电探测器18从第二光纤偏振分束器16接收经干涉仪6反射后的参考光，第三光电探测器20从第三光纤偏振分束器19接收经干涉仪6反射后的信号光，第四光电探测器21从第三光纤偏振分束器19接收经干涉仪6透射后的参考光。
还包括反馈控制系统22，反馈控制系统22分别连接到第二光纤偏振分束器16和第三光纤偏振分束器19的输出经干涉仪6反射和透射处理后的参考的光输出端口；由反馈控制系统22接收经干涉仪6反射/透射处理后的参考光后进行处理并反馈到干涉仪6，实时调整干涉仪6的检测灵敏度，达到最优灵敏度。
还包括数据采集卡23，数据采集卡23分别连接到第二光纤偏振分束器16和第三光纤偏振分束器19的输出经干涉仪6反射和透射处理后的信号光的输出端口，由数据采集卡23采集接收经干涉仪6反射和透射处理后的信号光作为激光超声信号，后进行针对待测试样11表面质量的检测识别处理。
待测试样11中的超声由脉冲激光器产生，亦即一个激光脉冲照射到待测试样表面产生热膨胀应力，从而激发出超声波。
由于环境温度和超声振动会影响光束中的参数，使得光束和干涉仪6中的透射/反射曲线之间的匹配关系存在误差，干涉仪6对光束的干涉作用效果变差。通过调整干涉仪6的腔长，进而调整干涉仪6的透射/反射曲线，进而调整干涉仪6工作固定频率对应的幅值和斜率，进而调整灵敏度。
透射/反射曲线的横坐标是频率，纵坐标是光强透射系数，表明光强变化响应，响应幅值越大则解调效果越好。
在环境温度局部降低或者超声振动频率局部减小的情况下，干涉仪6的透射/反射曲线向左移动，干涉仪6的工作固定频率对应的幅值增大，斜率增加，使得灵敏度升高。优先地通过调整干涉仪6的腔长，使得工作固定频率对应的幅值位于最大幅值的一半，即使得斜率最大，使得灵敏度最高。
根据经干涉仪6反射/透射处理后的参考光其中之一的参考光进行反馈即可。
具体实施中，干涉仪6包括一对反射镜，其中至少一个反射镜是可移动的，两个反射镜之间的距离确定干涉仪腔长。
窄线宽半导体光纤激光器1用于生成并提供激光超声检测光源，输出波长为1550nm，功率为100mW的连续激光。
第一光纤偏振控制器2，用于改变连续激光的偏振态；
第一光纤偏振分束器3，用于将光纤中含有的正交线偏振光分为参考光和探测光；具体实施的第一光纤偏振分束器3为1×2分束比，工作波长为1550nm，探测光和参考光分光比为90。
指示激光器8用于输出650nm可见光，标识探测光位置。
波分复用器7，用于将探测光和指示激光器8的输出光汇合在一起，并耦合到同一根光纤中进行传输；工作波长为650/1550nm。
光纤准直器9，用于将光纤内的传输光变成准直光；
双轴扫描振镜10，用于将探测光束引导至待测试样11表面不同位置检测激光超声信号；
光汇聚收集器12将从待测试样11散射出的光汇聚收集后经准直透镜13耦合至光纤形成信号光，并经第二光纤环形器14和第三光纤偏振控制器15后从另一端输入到干涉仪6；
信号光在干涉仪6内的传播方向与参考光相反。
第二光纤偏振分束器16接收从第一环形器输出的光束，分别由干涉仪透射的信号光和干涉仪反射的参考光而来，其第一输出口为干涉仪透射的信号光，经第一光电探测器17转换为第一超声信号，第二输出口为干涉仪反射的参考光，经第二光电探测器18转换为第一反馈控制信号。
第三光纤偏振分束器19接收从第二环形器输出的光束，分别由干涉仪反射的信号光和干涉仪透射的参考光而来，其第一输出口为干涉仪反射的信号光，经第三光电探测器20转换为第二超声信号，第二输出口为干涉仪透射的参考光，经第四光电探测器21转换为第二反馈控制信号。
准直器实现激光在光纤与自由空间的耦合，光纤准直器9将探测光和指示光从光纤耦合至自由空间，准直透镜13则将散射信号光耦合至光纤。
光电探测器17、20将信号光转换后输出超声信号，光电探测器18、21将参考光转换后输出反馈控制信号。
反馈控制系统22接收第一和第二反馈控制信号，并根据其中一个实现干涉仪腔长实时调节，基于反馈控制信号实时调制干涉仪腔长使其稳定在最佳检测性能。
数据采集卡23存储第一和第二超声信号并进行后处理用于待测试样的超声波测量，实施存储和处理超声信号，实现待测试样的超声测量。数据采集卡23用于实现待测试样厚度测量、缺陷成像与材料定征等功能。
双轴扫描振镜10输出探测光，在X和Y方向上可偏转扫描，引导探测光以斜入射至待测试样11的任意检测位置。
具体实施中，光路分光产生参考光和检测光，将探测光入射到待测试样11的目标对象上调制超声振动，汇聚收集待测试样11散射出的散射光，经光电转换后耦合至光纤获得超声振动调制的信号光，通过不同光纤环形器和偏振控制器将信号光和参考光输入干涉仪11，在干涉仪11内腔内经干涉输出，干涉输出的参考光经光电探测器生成反馈控制信号进而用于调整干涉仪的腔长，进而反馈调节优化干涉仪的检测灵敏度性能，干涉输出的信号光输出经光电探测器生成超声信号实现待测试样11的超声测量。待测试样11的超声波测量，如测厚、缺陷成像与材料特性参数定征等。
测量过程中，采用以下方式进行信号补偿：将双轴扫描振镜10不同位置扫描采集的激光超声信号合成为一个合路信号，将合路信号经希尔伯特变换后，再做包络处理得到表面波包络信号，取表面波包络信号中具有最大幅值的信号作为基准信号，根据基准信号分别对各个表面波包络信号进行归一化处理。归一化后试样表面质量不佳处的信号强度和信噪比得到提高，再将处理后的合路信号进行成像，在同一幅图上质量不佳处也能够清楚显现，实现对试样表面的高灵敏度测量。
本发明结构紧凑，便携性强，维护调整方便。
图2为本发明激光超声测量系统的工作流程图。
在步骤1中，窄线宽半导体光纤激光器输出光经偏振控制器和偏振分束器分光产生探测光和参考光。
在步骤2中，探测光经波分复用器和双轴扫描振镜照射至待测试样，同时指示激光用于标识探测光照射位置。
在步骤3中，超声振动调制的散射光汇聚收集作为信号光输入。
在步骤4中，信号光和参考光分别从干涉仪两端以相反传播方向输入到干涉仪。
在步骤5中，经干涉仪解调的透射/反射的参考光生成反馈控制信号，用于调整干涉仪的腔长稳定工作在最佳检测灵敏度性能。
在步骤6中，经干涉仪解调的透射/反射的信号光生成超声波信号。
在步骤7中，利用接收到超声波信号实现待测试样测量。
在步骤8中，输出待测试样超声波测量结果，譬如厚度、缺陷、材料特性等。
图3为本发明干涉仪最佳检测灵敏度控制流程图，其工作过程主要是：
扫描干涉仪腔长得到干涉仪透射/反射曲线。
根据透射/反射曲线选取斜率最大点对应的干涉仪腔长为稳定工作点。
调整干涉仪腔长至最佳工作点。
基于干涉仪透射/反射参考光生成的反馈控制信号调整腔长，如果幅值大于设定值，则减少PZT驱动电压，反之则增大PZT驱动电压，直至稳定在最佳工作点。
通过调整PZT驱动电压用于调节干涉仪的腔长，若减少PZT驱动电压则减小干涉仪的腔长。
同时在调整过程中判断干涉仪腔长是否超出了调节范围，如果是的话则需初始化重新开始。
本发明环境适应性强，基于参考光可以实时自适应补偿工业现场环境变化和干涉仪本身变形等引起的激光频率漂移和干涉仪腔长变化，使干涉仪持续稳定维持在最佳工作状态，满足高温、振动等工业现场应用需求。
如图4所示，本发明针对信号的测量还进行基于归一化的激光超声信号自动补偿，处理过程如下：
第一步，通过双轴扫描振镜逐点扫描获取不同位置的激光超声信号并保存在数据采集系统中；
第二步，将激光超声信号经希尔伯特变换后分别做包络处理；
第三步，取具有最大幅值表面波的激光超声信号为基准信号，并分别通过表面波实现归一化处理。
本发明检测灵敏度高，可以自适应补偿待测试样表面质量不佳导致的激光超声信号微弱和信噪比低问题。
对于熟悉本领域的研究人员而言，本发明中的一个或多个器件可以用更加分立或者集成的方式，甚至可以在某些具体应用场景下适当移除或者增添光学器件。
以上所述仅为本发明的一个较佳实施例，并不用以限制本发明，凡在本发明的精神和原则之内所做的近似修改、等同替换和改进等，均应包含在本发明的保护范围之内。

Claims (9)

  1. 一种自适应全光纤激光超声测量仪，其特征在于：
    包括窄线宽半导体光纤激光器(1)、第一光纤偏振控制器(2)、第一光纤偏振分束器(3)、第一光纤环形器(4)、第二光纤偏振控制器(5)、干涉仪(6)、波分复用器(7)、指示激光器(8)、光纤准直器(9)、双轴扫描振镜(10)、光汇聚收集器(12)、准直透镜(13)、第二光纤环形器(14)和第三光纤偏振控制器(15)；
    窄线宽半导体光纤激光器(1)输出端经第一光纤偏振控制器(2)和第一光纤偏振分束器(3)的输入端连接，第一光纤偏振分束器(3)的一个输出端和第一光纤环形器(4)的第一端口连接，第一光纤环形器(4)的第二端口经第二光纤偏振控制器(5)后和干涉仪(6)的一端连接；第一光纤偏振分束器(3)的另一个输出端和指示激光器(8)共同连接到波分复用器(7)的两个输入端，波分复用器(7)的输出端和光纤准直器(9)连接，光纤准直器(9)输出准直光束经双轴扫描振镜(10)照射到待测试样(11)上，待测试样(11)上方设有光汇聚收集器(12)，光汇聚收集器(12)接收来自待测试样(11)的散射光后入射到准直透镜(13)，准直透镜(13)连接到第二光纤环形器(14)的第一端口，第二光纤环形器(14)的第二端口经第三光纤偏振控制器(15)和干涉仪(6)的另一端连接。
  2. 根据权利要求1所述的一种自适应全光纤激光超声测量仪，其特征在于：
    窄线宽半导体光纤激光器(1)发出连续激光，经第一光纤偏振控制器(2)偏振处理后形成具有p偏振态和s偏振态的正交线偏振光，经第一光纤偏振分束器(3)分束后形成p偏振态的线偏振光和s偏振态的线偏振光；
    以s偏振态的线偏振光作为参考光，参考光经第一光纤环形器(4)入射到第二光纤偏振控制器(5)中，经第二光纤偏振控制器(5)偏振处理后输入到干涉仪(6)中；
    以p偏振态的线偏振光作为探测光，探测光和指示激光器(8)输出的可见光一起经波分复用器(7)复用后输入到光纤准直器(9)，光纤准直器(9)输出空间自由光，空间自由光经双轴扫描振镜(10)照射到的待测试样(11)上产生超声振动，由待测试样(11)表面散射产生超声振动调制的散射光，散射光经光汇聚收集器(12)收集汇聚后入射到准直透镜(13)，经准直透镜(13)后变成信号光，信号光经第二光纤环形器(14)入射到第三光纤偏振控制器(15)中，经第三光纤偏振控制器(15)偏振处理后输入到干涉仪(6)中；
    参考光和信号光在干涉仪(6)中均发生了反射和透射；
    经干涉仪(6)透射后的信号光和经干涉仪(6)反射后的参考光合束后经第二光纤偏振控制器(5)入射到第一光纤环形器(4)中，从第一光纤环形器(4)的第三端口输出；
    经干涉仪(6)透射后的参考光和经干涉仪(6)反射后的信号光合束后经第三光纤偏振控制器(15)入射到第二光纤环形器(14)中，从第二光纤环形器(14)的第三端口输出。
  3. 根据权利要求1所述的一种自适应全光纤激光超声测量仪，其特征在于：
    所述光纤偏振控制器为三环机械型光纤偏振控制器，第二光纤偏振控制器(5)和第三光纤偏振控制器(15)的偏振为四分之一波长，第一光纤偏振控制器(2)将线偏振光的偏振状态变成椭圆偏振态，p偏振态光和s偏振态光比例为90：10。
  4. 根据权利要求1所述的一种自适应全光纤激光超声测量仪，其特征在于：
    还包括第二光纤偏振分束器(16)和第三光纤偏振分束器(19)；第二光纤偏振分束器(16)的输入端连接到第一光纤环形器(4)的第三端口，第二光纤偏振分束器(16)的两个输出端分别连接一个光电探测器；第三光纤偏振分束器(19)的输入端连接到第二光纤环形器(14)的第三端口，第三光纤偏振分束器(19)的两个输出端也分别连接一个光电探测器。
  5. 根据权利要求1所述的一种自适应全光纤激光超声测量仪，其特征在于：
    还包括反馈控制系统(22)，反馈控制系统(22)分别连接到第二光纤偏振分束器(16)和第三光纤偏振分束器(19)的输出经干涉仪(6)反射和透射处理后的参考的光输出端口；由反馈控制系统(22)接收经干涉仪(6)反射/透射处理后的参考光后进行处理并反馈到干涉仪(6)，实时调整干涉仪(6)的检测灵敏度。
  6. 根据权利要求1所述的一种自适应全光纤激光超声测量仪，其特征在于：
    还包括数据采集卡(23)，数据采集卡(23)分别连接到第二光纤偏振分束器(16)和第三光纤偏振分束器(19)的输出经干涉仪(6)反射和透射处理后的信号光的输出端口，由数据采集卡(23)采集接收经干涉仪(6)反射和透射处理后的信号光作为激光超声信号，后进行针对待测试样(11)表面质量的检测识别处理。
  7. 根据权利要求1所述的一种自适应全光纤激光超声测量仪，其特征在于：
    通过调整干涉仪(6)的腔长，进而调整干涉仪(6)的透射/反射曲线，进而调整干涉仪(6)工作固定频率对应的幅值和斜率，进而调整灵敏度。
  8. 应用于权利要求1-7任一所述的一种自适应全光纤激光超声测量仪的测量方法，其特征在于：
    分光产生参考光和检测光，将探测光入射到的待测试样(11)的目标对象上调制超声振动，汇聚收集待测试样(11)散射出的散射光，经光电转换后耦合至光纤获得超声振动调制的信号光，通过光纤环形器和偏振控制器将信号光和参考光输入干涉仪(11)，在干涉仪(11)内腔内经干涉输出，干涉输出的参考光经光电探测器生成反馈控制信号进而用于调整干涉仪的腔长，进而反馈调节优化干涉仪的检测灵敏度性能，干涉输出的信号光输出经光电探测器生成超声信号实现待测试样(11)的超声测量。
  9. 根据权利要求8所述的一种自适应全光纤激光超声测量仪的测量方法，其特征在于：
    测量过程中，采用以下方式进行信号补偿：将双轴扫描振镜(10)不同位置扫描采集的激光超声信号合成为一个合路信号，将合路信号经希尔伯特变换后，再做包络处理得到表面波包络信号，取表面波包络信号中具有最大幅值的信号作为基准信号，根据基准信号分别对各个表面波包络信号进行归一化处理。
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