阅读说明：本技术 基于光学频率梳干涉测量法进行三维成像的装置及方法 (Device and method for three-dimensional imaging based on optical frequency comb interferometry ) 是由 元晋鹏 汪丽蓉 王三丹 于 2020-12-22 设计创作，主要内容包括：本发明属于材料形貌表征的技术领域,具体涉及一种可以对固体材料进行三维成像的装置和方法,装置包括光源系统、分光元件、二维平移台、第一反射镜、光谱仪和信号处理系统,光源系统包括飞秒光纤激光器,飞秒光纤激光器的重复频率和偏置频率被锁定至原子钟上形成光学频率梳；光源系统发出的光束经分光元件分为两束后,一束作为参考光束垂直入射至第一反射镜；一束作为探测光束入射至放置在二维平台上的待测样品表面；参考光束和探测光束分别被反射后返回分光元件,在分光元件处重合并发生干涉,干涉信号进入光谱仪后,经信号处理系统计算得到待测样品的三维形貌。本发明实现待测样品的三维形貌成像,其测量精度高,可以广泛应用于材料领域。(The invention belongs to the technical field of material morphology characterization, and particularly relates to a device and a method capable of carrying out three-dimensional imaging on a solid material, wherein the device comprises a light source system, a light splitting element, a two-dimensional translation table, a first reflector, a spectrometer and a signal processing system, the light source system comprises a femtosecond fiber laser, and the repetition frequency and the bias frequency of the femtosecond fiber laser are locked on an atomic clock to form an optical frequency comb; a light beam emitted by the light source system is divided into two beams by the light splitting element, and one beam is used as a reference beam and vertically enters the first reflector; one beam is used as a detection beam to be incident on the surface of a sample to be detected which is placed on a two-dimensional platform; and the reference beam and the detection beam are respectively reflected and then return to the light splitting element, the light splitting element is overlapped and interfered, and after an interference signal enters the spectrometer, the three-dimensional appearance of the sample to be detected is obtained through calculation of the signal processing system. The invention realizes the three-dimensional shape imaging of the sample to be measured, has high measurement precision and can be widely applied to the field of materials.)

1. An apparatus for three-dimensional imaging based on an optical frequency comb interferometry is characterized by comprising a light source system, a light splitting element (8), a two-dimensional translation stage (10), a first reflector (11), a spectrometer (14) and a signal processing system, wherein the light source system comprises a femtosecond fiber laser (1), and the repetition frequency and the bias frequency of the femtosecond fiber laser (1) are locked on an atomic clock to form an optical frequency comb; a light beam emitted by the light source system is divided into two beams by the light splitting element (8), and one beam is used as a reference beam and vertically enters the first reflector (11); one beam is used as a detection beam to be incident on the surface of a sample (9) to be detected which is placed on a two-dimensional platform (10); the reference light beam and the detection light beam are respectively reflected and then return to the light splitting element (8), the light splitting element (8) is overlapped and generates interference, and after an interference signal enters the spectrometer (14), the signal processing system calculates to obtain the three-dimensional appearance of the sample to be detected.

2. The device for three-dimensional imaging based on the optical frequency comb interferometry according to claim 1, wherein the signal processing system (iii) comprises signal processing software (15) and a display screen (16), the signal processing software (15) is configured to perform fast fourier transform on the interference signal obtained from the spectrometer (14) to obtain a fourier transform spectrogram, calculate a slope of an unwrapped phase of the interference signal, calculate an optical path difference between the reference beam and the measurement beam according to the slope of the unwrapped phase, and generate a three-dimensional shape of the sample to be measured according to the optical path difference at different coordinates and transmit the three-dimensional shape to the display screen (16) for display.

3. The device for three-dimensional imaging based on optical frequency comb interferometry according to claim 1, wherein the light source system further comprises an erbium-doped fiber amplifier (2), a frequency doubling module (3),f-2fan interferometer (4), a circuit locking unit (5) and a GPS module (6); the femtosecond laser emitted by the femtosecond fiber laser (1) is amplified by the erbium-doped fiber amplifier (2), a part of the femtosecond laser is subjected to laser frequency multiplication by the frequency multiplication module (3) and then is output, and a part of the femtosecond laser enters the frequency multiplication modulef-2fAn interferometer (4) for detecting an offset frequency, and a circuit lock unit (5) for locking the repetition frequency and the offset frequency of the optical frequency comb, the GPS moduleThe block (6) is for connecting an atomic clock to provide a lock reference frequency to the circuit lock unit.

4. The device for three-dimensional imaging based on the optical frequency comb interferometry according to claim 3, wherein the femtosecond laser (1) emits the femtosecond laser wavelength at 1560nm, the pulse repetition frequency is 250MHz, and the precision is 10^-14The circuit locking unit changes the cavity length by changing the piezoelectric ceramic voltage on the femtosecond fiber laser (1) so as to lock the repetition frequency of laser pulses;f-2fthe interferometer (3) is used to detect the offset frequency of the optical frequency comb.

5. The device for three-dimensional imaging based on optical frequency comb interferometry according to claim 1, further comprising a second mirror (7), a third mirror (12) and a lens (13), wherein the second mirror (7) is arranged between the output end of the optical frequency comb light source and the beam splitting element (8), and the third mirror (12) and the lens (13) are arranged between the beam splitting element (8) and the spectrometer (14).

6. The apparatus of claim 1, wherein the spectrometer has a wavelength range of 600-1700nm, a wavelength precision of ± 0.01nm, and a wavelength resolution of 0.02 nm.

7. A method for three-dimensional imaging based on optical frequency comb interferometry, which is characterized in that the device for three-dimensional imaging based on optical frequency comb interferometry according to any one of claims 1-6 is adopted, and comprises the following steps:

s1, obtaining interference signals: interference fringes formed by the reference beam and the detection beam at the light splitting element (8) are collected through a spectrometer;

s2, Fourier transform analysis: carrying out fast Fourier transform analysis on the obtained interference signal to obtain a Fourier transform spectrumThen calculating the slope of the unwrapped phase of the resulting interference signalk；

S3, calculating the optical path difference: the calculation formula is as follows:

；

wherein, c =3 × 10⁸m/s, representing the speed of light, n representing the refractive index of air, d representing the optical path difference between the probe beam and the reference beam;

s4, morphology characterization: moving the sample to be measured at fixed step pitch in two-axis plane for scanning, repeating the steps of S1-S3, and obtaining optical path difference between the reference beam and the measuring beam at different positions of the sample to be measured in two-dimensional direction (x-y)dAnd then drawing the change of the optical path difference in a three-dimensional coordinate system (x-y-d) to obtain the surface distribution conditions of different positions of the sample to be detected, thereby realizing the three-dimensional imaging of the sample to be detected.

8. The method of claim 7, wherein the step size is 0.5 μm.

Technical Field

The invention belongs to the technical field of material morphology characterization, and particularly relates to a device and a method capable of carrying out three-dimensional imaging on a solid material.

Background

The optical frequency comb is represented by an optical frequency sequence with equal frequency intervals in a frequency domain, and is represented by an electromagnetic field oscillation envelope with femtosecond time width in a time domain, the frequency spectrum width of the optical frequency sequence and the time width of the electromagnetic field oscillation envelope meet the Fourier transform relation, and the distribution characteristics of the ultrashort pulses in the time domain and the frequency domain are similar to the comb used in daily life, and the frequency comb is called an optical band frequency comb in an visualized mode, and is called an optical comb for short. The optical comb is equivalent to an optical frequency comprehensive generator, is the most effective tool for absolute optical frequency measurement so far, can accurately and simply link an atomic microwave frequency standard and an optical frequency standard, provides a carrier for developing a frequency standard with high resolution, high precision and high accuracy, also provides an ideal research tool for scientific research directions such as precise spectrum, astronomical physics, quantum control and the like, and is gradually applied to the fields of optical frequency precise measurement, atomic ion transition level measurement, remote signal clock synchronization, satellite navigation, absolute distance precise measurement and the like by people.

In order to be able to characterize the morphology of a material with high accuracy and high sensitivity, scientists have proposed a variety of three-dimensional imaging methods, which are critical in various fields of scientific research and practical industry. Photogrammetry is the earliest proposed material characterization method, which can capture a sample to be measured in a wide range and perform appropriate processing on the sample photograph, however, in order to realize three-dimensional imaging of the sample, two or more camera cameras are required, which not only complicates the measurement system, but also has relatively low measurement accuracy. The cmm is also useful for high precision imaging of a target sample surface after continuous scanning of the sample, however, the probe of the cmm needs to be moved over the sample surface, which may damage the sample.

In order to overcome the technical problem of the current three-dimensional imaging, a new optical imaging method needs to be provided to realize the accurate three-dimensional imaging of the surface of the solid material and meet the actual requirements of industrial application.

Disclosure of Invention

The invention overcomes the defects of the prior art, and solves the technical problems that: the device for three-dimensional imaging based on the optical frequency comb interferometry has high sensitivity and simple operation.

In order to solve the technical problems, the invention adopts the technical scheme that: the device for three-dimensional imaging based on the optical frequency comb interferometry is characterized by comprising a light source system, a light splitting element, a two-dimensional translation stage, a first reflecting mirror, a spectrometer and a signal processing system, wherein the light source system comprises a femtosecond fiber laser, and the repetition frequency and the bias frequency of the femtosecond fiber laser are locked on an atomic clock to form an optical frequency comb; a light beam emitted by the light source system is divided into two beams by the light splitting element, and one beam is used as a reference beam and vertically enters the first reflector; one beam is used as a detection beam to be incident on the surface of a sample to be detected which is placed on a two-dimensional platform; and the reference beam and the detection beam are respectively reflected and then return to the light splitting element, the light splitting element is overlapped and interfered, and after an interference signal enters the spectrometer, the three-dimensional appearance of the sample to be detected is obtained through calculation of the signal processing system.

The signal processing system comprises signal processing software and a display screen, wherein the signal processing software is used for performing fast Fourier transform on interference signals obtained from the spectrometer to obtain a Fourier transform spectrogram, calculating the slope of the unfolded phase of the interference signals, calculating the optical path difference between the reference beam and the measuring beam according to the slope of the unfolded phase, and generating the three-dimensional shape of the sample to be measured according to the optical path difference at different coordinates and transmitting the three-dimensional shape to the display screen for display.

The light source system also comprises an erbium-doped fiber amplifier and a frequency doubling module,f-2fthe system comprises an interferometer, a circuit locking unit and a GPS module; the femtosecond laser emitted by the femtosecond fiber laser is amplified by the erbium-doped fiber amplifier, part of the femtosecond laser is output after the frequency doubling of the laser by the frequency doubling module, and part of the femtosecond laser enters the frequency doubling modulef-2fThe interferometer is used for detecting the offset frequency, the circuit locking unit is used for locking the repetition frequency and the offset frequency of the optical frequency comb, and the GPS module is used for connecting an atomic clock to provide a locking reference frequency for the circuit locking unit.

The femtosecond laser wavelength emitted by the femtosecond fiber laser is 1560nm, the pulse repetition frequency is 250MHz, and the precision is 10^-14The circuit locking unit changes the cavity length by changing the piezoelectric ceramic voltage on the femtosecond fiber laser, so as to lock the repetition frequency of the laser pulse;f-2fthe interferometer is used to detect the offset frequency of the optical frequency comb.

The device for carrying out three-dimensional imaging based on the optical frequency comb interferometry further comprises a second reflecting mirror, a third reflecting mirror and a lens, wherein the second reflecting mirror is arranged between the output end of the optical frequency comb light source and the light splitting element, and the third reflecting mirror and the lens are arranged between the light splitting element and the spectrometer.

The wavelength range of the spectrometer is 600-1700nm, the wavelength precision is +/-0.01 nm, and the wavelength resolution is 0.02 nm.

The invention also provides a method for three-dimensional imaging based on the optical frequency comb interferometry, which adopts the claimed device for three-dimensional imaging based on the optical frequency comb interferometry, and comprises the following steps:

s1, obtaining interference signals: collecting interference fringes formed by the reference light beam and the detection light beam at the light splitting element through a spectrometer;

s2, Fourier transform analysis: performing fast Fourier transform analysis on the obtained interference signal to obtain a Fourier transform spectrogram, and then calculating the slope of the unwrapped phase of the obtained interference signalk；

S3, calculating the optical path difference: the calculation formula is as follows:

；

wherein, c =3 × 10⁸m/s, representing the speed of light, n representing the refractive index of air, d representing the optical path difference between the probe beam and the reference beam;

s4, morphology characterization: moving the sample to be measured at fixed step pitch in two-axis plane for scanning, repeating the steps of S1-S3, and obtaining optical path difference between the reference beam and the measuring beam at different positions of the sample to be measured in two-dimensional direction (x-y)dThen drawing the change of the optical path difference in a three-dimensional coordinate system (x-y-d) to obtain a table of different positions of the sample to be measuredAnd (5) the surface distribution condition is realized, so that the three-dimensional imaging of the sample to be detected is realized.

The step size is 0.5 μm.

Compared with the prior art, the invention has the following beneficial effects: the invention provides a device and a method for carrying out three-dimensional imaging based on an optical frequency comb interferometry, which can carry out three-dimensional imaging on a sample to be measured by carrying out two-dimensional scanning and optical path difference measurement on the sample, have the advantage of high universality, can realize the appearance characterization on the sample to be measured with any size, any shape and any thickness, have high measurement precision, and simultaneously have simple measurement operation, are beneficial to equipment integration and can carry out real-time online imaging; these results demonstrate that the present invention will be a powerful tool for three-dimensional imaging.

Drawings

FIG. 1 is a schematic diagram of an apparatus for three-dimensional imaging by optical frequency comb interferometry according to an embodiment of the present invention;

FIG. 2 is a graph of the obtained interference spectrum and unwrapped phase versus slope;

FIG. 3 is a diagram of a sample to be tested and a three-dimensional image;

wherein, 1 is a femtosecond fiber laser, 2 is an erbium-doped fiber amplifier, 3 is a frequency doubling module, and 4 isf-2fThe interferometer, 5 is a circuit locking unit, 6 is a GPS reference frequency, 7 is a second reflecting mirror, 8 is a light splitting element, 9 is a sample to be measured, 10 is a two-dimensional translation stage, 11 is a first reflecting mirror, 12 is a third reflecting mirror, 13 is a lens, 14 is a spectrometer, 15 is signal processing software, and 16 is a display screen.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments; all other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in FIG. 1, the present inventionThe embodiment provides a device for three-dimensional imaging based on optical frequency comb interferometry, which comprises a light source system I, an interferometry system II and a signal processing system III. Wherein, the light source system I comprises a femtosecond fiber laser 1, an erbium-doped fiber amplifier 2 and a frequency doubling module 3,f-2finterferometer 4, circuit lock unit 5, GPS reference frequency 6. The interference measurement system II comprises a second reflecting mirror 7, a light splitting element 8, a sample to be measured 9, a two-dimensional translation table 10, a first reflecting mirror 11, a third reflecting mirror 12, a first lens 13 and a spectrometer 14. The signal processing system III comprises signal processing software 15 and a display screen 16.

The light emitted by the femtosecond fiber laser 1 is firstly amplified in power by the erbium-doped fiber amplifier 2, and the amplified light is divided into two parts which respectively enter the frequency doubling module 3 and the frequency doubling modulef-2fAn interferometer 4 for performing laser frequency doubling and detecting the offset frequency, respectivelyf ₀Passing pulsed lightf-2fThe repetition frequency of the optical frequency comb is carried out by a circuit locking unit 5 after the interferometer 4f _rep And bias frequencyf ₀And finally, the optical frequency comb is locked on an atomic clock connected with the GPS module 6 through the GPS module, so that the repetition frequency of the pulse laser output by the femtosecond fiber laser 1f _rep And bias frequencyf ₀Are all locked on the atomic clock. That is, the light source system I of the present embodiment locks the repetition frequency and the bias frequency of the output laser, and realizes the locking of the optical frequency comb, thereby obtaining an optical frequency comb with high frequency stability, the stability of which is 5 × 10^-12Therefore, the invention can improve the measurement accuracy of material characterization.

Specifically, in this embodiment, the femtosecond laser wavelength emitted by the femtosecond fiber laser is 1560nm, the pulse repetition frequency is 250MHz, and the precision can reach 10^-14It can change the cavity length by changing the piezoelectric ceramic, and further change the pulse repetition frequencyf _rep (ii) a In this embodiment, when the repetition frequency is locked, beat frequencies between adjacent comb teeth (that is, the repetition frequency of the optical pulses) are detected by performing optical heterodyne on the laser light) And comparing the frequency with standard frequency (microwave standard frequency from a GPS module) to obtain an error signal, and loading the error signal to piezoelectric ceramic PZT on a laser cavity mirror so as to realize locking of the repetition frequency of the mode-locked laser. The erbium-doped fiber amplifier is a high-power fiber amplifier, the bandwidth and the shape of an output spectrum can be adjusted by a polarization controller at the input end of the amplifier, and the purpose of adjusting the bandwidth and the shape is to ensure that the central wavelength of laser output by the erbium-doped fiber amplifier is close to 1560nm so as to ensure that the central wavelength of laser frequency after frequency doubling is 780nm, thereby being beneficial to the interaction between light and materials.

Specifically, in the present embodiment, the first and second electrodes,f-2finterferometer for detecting offset frequency of optical frequency combf ₀ (ii) a Due to the dielectric dispersion in the femtosecond fiber laser cavity, the group velocity v of photons is caused_gAnd phase velocity v_pResulting in a phase difference between adjacent pulses output from the chamberThis results in the presence of an optical comb carrier envelope offset frequency:

；（1）

the repetition frequency of the optical frequency comb is:

；（2）

repetition frequencyf _repCharacterizing the frequency of a round trip of photons in the femtosecond fiber laser cavity, where T_pIs a repetition period of the light pulses,L the length of the cavity is taken as the length of the cavity,v _gis the group velocity of the pulsed light field. The group velocity v of photons will be caused by intracavity dielectric dispersion_gAnd phase velocityv _p Resulting in a phase difference between adjacent pulses output from the chamberIn the formulaω ₀The center frequency of the laser pulse. If the phase difference between the carriers and envelopes of adjacent optical pulses is taken into accountThe frequency of the optical frequency comb can be expressed as. Wherein the offset frequency. That is, the repetition frequency of the optical frequency comb is determined by the cavity length of the laser, and the bias frequency is caused by the phase difference between the adjacent optical pulse carrier and the envelope, so that the repetition frequency can be locked by feeding back the error signal to the piezoelectric ceramic PZT on the cavity mirror of the femtosecond fiber laser, and the error signal is fed back to the current source of the femtosecond fiber laser to lock the bias frequency.

Specifically, in the present embodiment, nonlinearityf-2fThe interferometer is formed byf-2fNamed by self-reference techniques, which comb low-frequency components of light frequenciesf _n = nf _rep+f ₀Frequency doubled component 2f _n = 2nf _rep+2f ₀High frequency component of comb with optical frequencyf _2n = 2nf _rep+f ₀Heterodyne beat frequency is carried out to obtain offset frequency:f ₀= (2nf _rep+2f ₀)-(2nf _rep+f ₀) Obtained by beating heterodyningf ₀And comparing with the standard frequency to obtain an error signal, and feeding the error signal back to a current source of the femtosecond fiber laser so as to lock the bias frequency.

In this embodiment, the frequency doubling module 3 is configured to convert pulsed light with a center wavelength of 1560nm that has passed through the erbium-doped fiber amplifier into output pulsed light with a wavelength of 780nm, and interact with a sample to be measured using 780nm laser. The circuit locking unit is used for locking the repetition frequency and the bias frequency; the GPS reference frequency provides a reference for all synthesizers and counters in the process of repetition frequency and offset frequency locking, ensuring that the optical frequency comb eventually locks onto the atomic clock to which the GPS module is connected. In the GPS module, a quartz oscillator outputs a 10MHz reference signal, all frequency synthesizers and counters of a circuit locking unit in a frequency comb are referenced to the 10MHz output by the GPS in the process of locking the repetition frequency and the offset frequency, so that the repetition frequency and the offset frequency are locked to an atomic clock connected with the GPS, and therefore, the repetition frequency and the offset frequency are locked to the same reference, but are not dependent on each other. They are locked separately. .

In this embodiment, the spectrometer is a high-precision spectrometer of the Yokogawa company, model AQ6370D, the wavelength range is 600-1700nm, the wavelength precision is ± 0.01nm, and the wavelength resolution is 0.02nm, which is an ideal choice for interferometric measurement. The two-dimensional translation table is used for fixedly mounting a sample to be detected, and the sample to be detected moves in two dimensions by moving the two-dimensional translation table in two dimensions, so that the appearance characterization of the sample to be detected is realized.

In this embodiment, a light beam emitted by the light source system enters the light splitting element 8 after passing through the second reflector 7, and then is split into two beams, one beam serving as a reference beam enters the first reflector 11 vertically; one beam is used as a detection beam to be incident on the surface of a sample 9 to be detected which is placed on a two-dimensional platform 10; the reference beam and the detection beam are respectively reflected by the first reflector 11 and the sample 9 to be detected and then return to the light splitting element 8, the reference beam and the detection beam are overlapped and interfered at the light splitting element 8, interference signals enter the spectrometer 14 for detection after passing through the third reflector 12 and the lens 13, the interference signals obtained by detection are calculated by the signal processing system to obtain the three-dimensional appearance of the sample to be detected, and finally the three-dimensional appearance is transmitted to the display screen 16 for display.

Specifically, in this embodiment, the signal processing system iii includes signal processing software 15 and a display screen 16, where the signal processing software 15 is configured to perform fast fourier transform on an interference signal obtained from the spectrometer 14 to obtain a fourier transform spectrogram, calculate a slope of an expansion phase of the interference signal, calculate an optical path difference between a reference beam and a measurement beam according to the slope of the expansion phase, and generate a three-dimensional shape of a sample to be measured according to the optical path difference at different coordinates and transmit the three-dimensional shape to the display screen (16) for display.

Further, the embodiment of the invention also provides a method for three-dimensional imaging based on an optical frequency comb interferometry, which comprises four steps of interference signal acquisition, Fourier transform analysis, optical path difference calculation and morphology characterization:

s1, obtaining interference signals: pulsed light emitted from the optical frequency comb is split into two beams at the light splitting element 8, and one beam enters a reference path and is reflected at a first reflecting mirror 11; one beam enters a measuring path and is reflected at a sample 9 to be measured; the last two reflected beams are superposed again at the light splitting element 8, and the superposed beams interfere and enter the spectrometer 14 to observe the spectrum interference fringes.

S2, Fourier transform analysis: performing fast Fourier transform analysis on the obtained interference signal to obtain a Fourier transform spectrogram, and then calculating to obtain the slope k of the expansion phase of the interference signal; wherein k =；The unwrapped phase of the interference signal is shown, and f represents the corresponding frequency, as shown in fig. 2, wherein (a) is the interference spectrum obtained by performing fast fourier transform on the interference signal in the embodiment of the present invention, and (b) is the relationship between the unwrapped phase and the frequency of the corresponding interference signal.

S3, calculating the optical path difference: optical path difference between reference beam and measuring beamdCan be expressed as:

；（2）

wherein, c =3 × 10⁸m/s, representing the speed of light, and n, representing the refractive index of air, are considered constant 1.00026 during the measurement. After obtaining the slope k of the unwrapped phase, it is possible to calculateObtaining an optical path difference;

s4, morphology characterization: scanning the sample to be measured at intervals of 0.5 mu m on two axial planes, repeating the steps of S1-S3, and obtaining the optical path difference between the reference beam and the measuring beam at different positions of the sample to be measured in two-dimensional direction (x-y)dAnd then drawing the change of the optical path difference in a three-dimensional coordinate system (x-y-d) to obtain the surface distribution conditions of different positions of the sample to be detected, thereby realizing the three-dimensional imaging of the sample to be detected.

Fig. 3 shows a three-dimensional image of a sample to be measured obtained according to an embodiment of the present invention, where (a) is a photo image and (b) is the three-dimensional image obtained according to the embodiment of the present invention.

In summary, the invention provides an apparatus and a method for three-dimensional imaging based on an optical frequency comb interferometry, which can realize three-dimensional topography imaging of a sample to be measured by two-dimensionally scanning optical path differences between a measurement path and a reference path generated at different positions of the sample to be measured, have the advantage of high universality, can realize topography characterization on the sample to be measured in any size, any shape and any thickness, have high measurement accuracy, are simple in measurement operation, are beneficial to equipment integration, and can perform real-time online imaging; these results demonstrate that the present invention will be a powerful tool for three-dimensional imaging.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.