A method and system for ranging and line-scan three-dimensional imaging based on microcavity optical frequency comb

Abstract

The present application relates to the field of ranging and three-dimensional imaging technology, in particular to a kind of ranging and line scanning three-dimensional imaging method and system based on microcavity optical frequency comb.The present application uses microcavity optical frequency comb with near-infrared wavelength, so that the influence of stray light in the visible light band of the object surface on the measurement is minimized, the system has strong anti-interference ability, and meets the use scene of engineering measurement.At the same time under the existing technical conditions, there is a mature and stable manufacturing method for microcavity optical frequency comb with near-infrared wavelength.In the system, ranging and three-dimensional imaging of the object surface can be realized by quickly replacing the ranging module and the three-dimensional imaging module, and one set of system can complete two functions.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ranging and three-dimensional imaging technology, in particular to a ranging and line scanning three-dimensional imaging method and system based on microcavity optical frequency comb. BACKGROUND

[0002] Optical frequency comb, simply referred to as "optical frequency comb", is a ruler for measuring frequency and time, which is composed of a series of discrete and strictly equally spaced comb spectrum lines in the frequency spectrum. The optical frequency comb provides a coherent light source with excellent time coherence or spatial coherence. When the object to be measured deforms (or displaces), the interference fringes change, and high-precision three-dimensional measurement can be performed. The optical frequency comb can also be used for precision measurement of relatively large-sized workpieces.

[0003] The biggest advantage of the optical frequency comb is that it can measure the absolute displacement of the object, that is, it does not need to measure the displacement of the object, which causes the interference fringes to change. The microcavity optical frequency comb provides the possibility for chip implementation of the optical frequency comb by using microcavity manufacturing process. The microcavity optical frequency comb has the characteristics of high repetition frequency and wide spectral range, and also has the advantages of small size and chip integration.

[0004] Laser-based three-dimensional object scanning spectroscopy is a non-contact surface mapping with depth information. By combining single-point absolute distance measurement with mechanical scanning, the surface profile of the object to be measured can be accurately provided.

[0005] Since the traditional laser measurement method must measure the displacement of the object, that is, the object deforms, it cannot measure the absolute displacement of the object. Moreover, the traditional optical frequency comb is too large in size, so that the optical measurement instrument manufactured based on the traditional optical frequency comb is also large in size, poor in portability, and difficult to apply in engineering applications where the size of the measurement instrument is required. SUMMARY

[0006] In order to solve the problems of the prior art, the present application provides a ranging and line scanning three-dimensional imaging method and system based on microcavity optical frequency comb. The microcavity optical frequency comb with near-infrared wavelength is used to minimize the influence of stray light in the visible light band on the measurement. The system has strong anti-interference ability and meets the use scenarios of engineering measurement.

[0007] The technical solutions adopted by the present application are as follows:

[0008] A ranging and line scanning three-dimensional imaging method based on microcavity optical frequency comb, comprising a ranging method and a line scanning three-dimensional imaging method, wherein the ranging method comprises the following steps:

[0009] A1. The optical frequency comb emits ultra-short laser pulses;

[0010] A2, splitting the laser pulses emitted by the laser into two paths by a beam splitter, one path as a first measurement light path and the other path as a first reference light path;

[0011] A3, after the two paths of laser light pass through a reference mirror and a target mirror respectively, the two paths of laser light are combined by a beam splitter to obtain a path difference between the two paths of laser light;

[0012] A4, receiving the interference signal by a grating spectrometer and performing sparse processing on the optical frequency comb by using the mode filtering principle of a Fabry-Perot etalon;

[0013] A5, calculating the distance L to be measured by spectrum analysis based on the processed interference distance measurement data of the optical frequency comb;

[0014] The method for linear scanning three-dimensional imaging specifically comprises the following steps:

[0015] B1, emitting ultra-short laser pulses by an optical frequency comb, and placing an object on an object table which can move in parallel along the y direction;

[0016] B2, splitting the laser pulses emitted by the laser into two paths by a beam splitter, one path as a second measurement light path and the other path as a second reference light path after passing through a beam expander and a one-dimensional dispersion element;

[0017] B3, after the second reference light path passes through a reference mirror and the second measurement light path passes through a beam splitter and is scattered into a physical surface to form a return wave, the two paths of laser light are combined by a beam splitter to obtain a path difference between the two paths of laser light;

[0018] B4, controlling a servo motor to move the object table along the y direction until the linear scanning light scans the entire object;

[0019] B5, receiving the interference signal by a grating spectrometer and performing sparse processing on the optical frequency comb by using the mode filtering principle of a Fabry-Perot etalon;

[0020] B6, calculating the distance L to be measured by spectrum analysis based on the processed interference distance measurement data of the optical frequency comb, so as to obtain three-dimensional imaging of the object.

[0021] Preferably, the optical frequency comb is a microcavity optical frequency comb with a center wavelength in the near-infrared band.

[0022] Preferably, the method for distance measurement specifically comprises the following processes:

[0023] For absolute distance measurement with spectral resolution, the relationship between the intensity of the spectral interference fringes received by the spectrum analyzer and the optical frequency can be expressed as:

[0024] g(v)=s(v)[1+cosφ(v)] (1)

[0025] (1) where s(v) is the comb tooth power spectrum function of the microcavity optical frequency comb, the interference phase can be expressed as:

[0026] φ(v) = 2πva (2)

[0027] where a represents the delay of the optical path, written as the following formula:

[0028] a = 2n(v)l / c (3)

[0029] (3) where n(v) is the refractive index, and c is the speed of light in vacuum;

[0030] The distance to be measured l is calculated from the spectrum, and the Fourier transform of the formula can convert the spectrum to the pseudo-time domain, and the following formula is obtained:

[0031]

[0032] S(τ) is the Fourier transform of s(v), and since the power spectrum density function g(v) is a real function, its Fourier transform is symmetric about τ = 0, and there are three peaks at -a, 0, and a. At this time, the peak value at a can be extracted separately by using a band-pass filter function, and inverse Fourier transform is performed, and the following formula is obtained:

[0033]

[0034] The phase term can be easily obtained from the trigonometric function:

[0035]

[0036] Every 2π radian, the phase term φ(v) will have a phase jump, so the real absolute phase of φ(v) is obtained by unfolding the phase jump of φ(v), and the slope corresponds to the distance to be measured L, which is expressed as:

[0037]

[0038] N = n + (dn / dv)v, where n is the group refractive index of air, and v is a function of the central wavelength of the laser light source;

[0039] Finally, the distance L can be expressed as

[0040]

[0041] Preferably, in the method of line scanning three-dimensional imaging, the intensity information of each comb tooth interference is subjected to Fourier transform, the phase delay of the comb tooth is calculated, for the optical frequency comb, the intensity information of each comb tooth interference in the x direction is subjected to Fourier transform, thereby the phase delay of different positions in the x direction is calculated, and the depth information in the x direction is calculated; after the depth information in the x direction is measured, the servo motor above the guide rail is controlled to control the detector to translate in the y direction, and the scanning is sequentially performed until the scanning of the whole object is completed, and the surface morphology of the object is reconstructed using the existing depth information.

[0042] A ranging and line scanning three-dimensional imaging system based on a microcavity optical frequency comb, comprising a light source module, an interference module, a measurement module and an information acquisition module, the light source module is a microcavity optical frequency comb with a center wavelength in the near-infrared band, providing light pulses; the interference module is to divide the light pulses of the light source module into a measurement light path and a reference light path through a prism, and recombine after reflection by the test module; the measurement module is used for displacement measurement or three-dimensional imaging of an object surface, and the information acquisition module is used for acquiring the light pulses recombined after reflection by the interference module.

[0043] Preferably, in the displacement measurement, the measurement module selects a displacement measurement module with a built-in mirror; in the three-dimensional imaging of the object surface, the measurement module selects a three-dimensional imaging module with a built-in beam expander, a one-dimensional dispersion element and a collimating lens.

[0044] The technical scheme provided by the application has the beneficial effects that:

[0045] Compared with the traditional method, the microcavity optical frequency comb is innovatively used in the application. The microcavity optical frequency comb has a smaller volume, so that the miniaturization and integration of the measurement system are higher. Within the error allowable range, the single optical frequency comb line scanning three-dimensional imaging system based on the microcavity optical frequency comb is smaller. In the application, the microcavity optical frequency comb with a wavelength in the near-infrared band is used, so that the influence of stray light in the visible light band on the measurement of the object surface is minimized, the system has strong anti-interference ability, and meets the use scenarios of engineering measurement. Meanwhile, under the existing technical conditions, the microcavity optical frequency comb with a wavelength in the near-infrared band has a mature and stable manufacturing method. In the system, the ranging and three-dimensional imaging of the object surface can be realized by quickly replacing the ranging module and the three-dimensional imaging module, and one set of system can complete two functions. BRIEF DESCRIPTION OF DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0047] Figure 1 Absolute displacement measurement optical path diagram in a ranging and line scanning three-dimensional imaging method and system based on microcavity optical frequency comb of the present application;

[0048] Figure 2 Three-dimensional imaging optical path diagram in a ranging and line scanning three-dimensional imaging method and system based on microcavity optical frequency comb of the present application;

[0049] Figure 3 System modular structure diagram in a ranging and line scanning three-dimensional imaging method and system based on microcavity optical frequency comb of the present application. DETAILED DESCRIPTION

[0050] In order to make the objects, technical solutions and advantages of the present application more clear, the embodiments of the present application will be further described in detail below with reference to the drawings.

[0051] Embodiment one

[0052] Figure 1 The principle of the measurement technology in the ranging application is shown.

[0053] An interference method using a Michelson-like interferometer is adopted for measurement, and a microcavity optical frequency comb with a center wavelength in the near-infrared band is selected. First, the light pulses emitted by the microcavity optical frequency comb in the near-infrared band are split using a beam splitter, one of which is used as a measurement optical path, that is, an object light, and the other is used as a reference optical path. When performing absolute displacement measurement, a mirror M2 is placed on the surface of the object. After the light is split, the object light passes through the objective lens M2, and the reference light is reflected by the mirror M1. The two lights are recombined at the beam splitter. The target distance is the optical path difference between the two arms of the interferometer.

[0054] From the frequency domain, each component of the optical frequency comb (each comb tooth independently completes the interference, when the beams are combined, the frequency comb contains the self-coherence information of each comb tooth), due to the different frequencies of each comb tooth, the grating spectrometer receives the interference signal, and the light and dark stripes can be observed in the optical frequency domain. The interval between the stripes carries the distance information of the target to be measured. Due to the limited spectral resolution of the grating spectrometer, the mode filtering principle of the Fabry-Perot etalon (FPE) is used to pre-process the optical frequency comb, that is, the spectral filtering method is used to increase the interval between adjacent comb teeth, so that each pixel of the linear array photodetector of the optical spectrum analyzer receives the interference intensity information of a frequency comb tooth.

[0055] For absolute distance measurement of spectral resolution, the relationship between the intensity of the spectral interference fringe received by the optical spectrum analyzer and the optical frequency can be expressed as:

[0056] g(v)=s(v)[1+cosφ(v)] (1)

[0057] In the formula, s(v) is the comb tooth power spectrum function of the microcavity optical frequency comb, and the interference phase can be expressed as:

[0058] φ(v)=2πvα (2)

[0059] Wherein, α represents the delay of the optical path, which is written as the following formula:

[0060] α=2n(v)l / c (3)

[0061] In formula (3), n(v) is the refractive index, and c is the speed of light in vacuum.

[0062] The distance l to be measured is calculated from the spectrum, and the Fourier transform of the formula can be performed to obtain the pseudo-time domain from the spectrum

[0063]

[0064] S(τ) is the Fourier transform of s(v). Since the power spectral density function g(v) is a real function, its Fourier transform is symmetric about τ=0, and there are three peaks at-α, 0 and α. At this time, the peak value at α can be extracted separately by using a band-pass filter function, and the inverse Fourier transform is performed, and then the following formula is obtained:

[0065]

[0066] The phase term can be easily obtained from the trigonometric function

[0067]

[0068] Every 2π radian, the phase term φ(v) will have a phase jump, so the real absolute phase of φ(v) is obtained by unfolding the phase jump of φ(v), and the slope corresponds to the distance L to be measured, expressed as:

[0069]

[0070] N = n + (dn / dv)v. Here, n is the group refractive index of air, and v is a function of the central wavelength of the laser light source.

[0071] The distance L can be expressed as

[0072]

[0073] Here, the central wavelength of the microcavity optical frequency comb is selected to be in the near-infrared band.

[0074] When performing three-dimensional imaging scanning, the reflecting mirror of the measured object surface is replaced by a one-dimensional dispersive element (composed of a beam expander and a grating), which disperses the comb teeth of the optical frequency comb and reflects the measured object surface (by default), and each comb tooth carries the object surface information echo reflection, and converges at the light splitting prism, and each comb tooth of the object light and the comb tooth of the reference light respectively occurs interference. And again use the Fabry-Perot device to increase the interval between the comb teeth after diffraction, so that each pixel of the line array photoelectric detector of the optical spectrum analyzer receives the interference intensity information of a frequency comb tooth.

[0075] According to the intensity information of each comb tooth interference, the phase delay of the comb tooth can be calculated. For the optical frequency comb, the frequency of the optical frequency comb comb tooth has a corresponding relationship with the x-direction position. Thus, the phase delay of different positions in the x-direction can be calculated, and the depth information in the x-direction can be calculated.

[0076] After completing the depth information measurement in the x-direction, the servo motor above the guide rail is controlled to control the probe to translate in the y-direction, and the scanning is sequentially performed until the scanning of the entire object is completed, and the existing depth information is used to reconstruct the surface morphology of the object.

[0077] As shown in FIG. 1, the optical frequency comb is used to measure the distance L to be measured. Figure 3As shown, the embodiment also provides a ranging and line scanning three-dimensional imaging system based on microcavity optical frequency comb, the whole system is divided into four modules, light source module, interference module, measurement module and information acquisition module, small size optical components are used in each module, thereby meeting the miniaturization and chip needs, when displacement measurement is carried out, the measurement module selects the displacement measurement module with a mirror inside. When three-dimensional imaging of the object surface is carried out, the test module selects the three-dimensional imaging module with a beam expander, a one-dimensional dispersion element and a collimating lens, through the modular and integrated design of the whole system, two kinds of measurement are completed by using a set of system.

[0078] The above merely illustrates the preferred embodiment of the present application, and is not intended to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for ranging and line-scanning three-dimensional imaging based on a microcavity optical frequency comb, comprising a ranging method and a line-scanning three-dimensional imaging method, wherein, The distance measurement method includes the following steps: A1. The optical frequency comb emits ultrashort laser pulses; A2. Use a beam splitter to split the laser pulse emitted by the laser into two paths, one as the first measurement optical path and the other as the first reference optical path. A3. The two laser paths, the first reference optical path and the first measurement optical path, are reflected by the reference mirror and the target mirror respectively, and then re-combined in the beam splitter to obtain the optical path difference between the two paths. A4. Use a grating spectrometer to receive interference signals and use the mode filtering principle of the Fabry-Perot etalon to sparsify the optical frequency comb. A5. By processing the interferometric ranging data of the optical frequency comb, the distance L to be measured is calculated from the spectrum; Specifically, the process includes the following: For absolute distance measurements with spectral resolution, the relationship between the intensity of the spectral interference fringes received by the spectrometer and the optical frequency is expressed as: （1） (1) In the formula, s(v) is the power spectrum function of the microcavity optical frequency comb, and the interference phase is expressed as: （2） in, The optical path delay is expressed by the following formula: （3） (3) In the formula, n( v） For refractive index, c The speed of light in a vacuum; The distance l to be measured is calculated from the spectrum. A Fourier transform is performed on the formula to convert the spectrum to the pseudo-time domain, yielding: （4） S(τ) is the Fourier transform of s(ν). Since the power spectral density function g(v) is a real function, its Fourier transform is symmetric about τ=0, exhibiting three peaks at positions −α, 0, and α. Using a bandpass filter to extract the peak at α and performing an inverse Fourier transform, we obtain: （5） Its phase term can be easily obtained from trigonometric functions: （6） Phase term every 2π radians A phase transition will occur, therefore, Phase abrupt change fold unfolding yields The true absolute phase, whose slope corresponds to the distance L to be measured, is expressed as: （7） Here, n is the group refractive index of air, and v is a function of the center wavelength of the laser source; Finally, the distance L is represented as （8）； The line-scan three-dimensional imaging method specifically includes the following steps: B1. The optical frequency comb emits ultrashort laser pulses and places the object on a platform that moves parallel to the y-direction. B2. The laser pulse emitted by the laser is split into two paths using a beam splitter. The first path serves as the second measurement optical path, and the second path, after passing through a beam expander and a one-dimensional dispersive element, serves as the second reference optical path. B3. The second reference optical path is reflected by the reference mirror, and the second measurement optical path is diffused into the physical surface by the combing prism and forms an echo. The two lasers are then recombined in the beam splitter to obtain the optical path difference between the two paths. B4. Control the servo motor to move the stage along the y-direction until the line scan light scans the entire object; B5. Use a grating spectrometer to receive interference signals and use the mode filtering principle of the Fabry-Perot etalon to sparsify the optical frequency comb. B6. By processing the interferometric ranging data of the optical frequency comb, the distance L to be measured is calculated from the spectrum, thereby obtaining a three-dimensional image of the object.

2. The method for ranging and line-scan three-dimensional imaging based on a microcavity optical frequency comb according to claim 1, characterized in that, The optical frequency comb is selected from microcavity optical frequency combs with a center wavelength located in the near-infrared band.

3. The method for ranging and line-scan three-dimensional imaging based on a microcavity optical frequency comb according to claim 1, characterized in that, In the described line-scan 3D imaging method, a Fourier transform is performed based on the intensity information of interference at each comb tooth to calculate the phase delay of that comb tooth. For an optical frequency comb, a Fourier transform is performed based on the intensity information of interference at each comb tooth in the x-direction to calculate the phase delay at different positions in the x-direction, and then the depth information in the x-direction is calculated. After completing the depth information measurement in the x-direction, the detector is controlled to translate along the y-direction by controlling the servo motor above the guide rail, and scanning is performed sequentially until the entire object is scanned. The surface morphology of the object is then reconstructed in 3D using the existing depth information.

4. A system for ranging and line-scan three-dimensional imaging based on a microcavity optical frequency comb according to claim 1, comprising a light source module, an interference module, a measurement module, and an information acquisition module, wherein the light source module is a microcavity optical frequency comb with a center wavelength in the near-infrared band to provide light pulses; the interference module uses a beam splitter to divide the light pulses from the light source module into a measurement optical path and a reference optical path, and after reflection by a test module, they are recombined; the measurement module is used for displacement measurement or three-dimensional imaging of an object surface, and the information acquisition module is used to acquire the light pulses recombined after reflection by the interference module.

5. The system for ranging and line-scan three-dimensional imaging based on a microcavity optical frequency comb according to claim 4, characterized in that, When performing displacement measurement, the measurement module selected is a displacement measurement module with an internal reflector; when performing three-dimensional imaging of the object surface, the measurement module selected is a three-dimensional imaging module equipped with a beam expander, a one-dimensional dispersive element, and a collimating lens.

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