Hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning

Abstract

The invention discloses a hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning, and belongs to the field of femtosecond laser optical frequency comb technology and hyperspectral digital holographic imaging. The system comprises a mode-locked femtosecond laser, an optical fiber beam splitter, an oscilloscope, a signal generator, a rubidium atomic clock, a frequency counter, a PID proportional-integral-differential controller, a PLO phase-locked oscillator, an MIX mixer, an AMP amplifier, a PD photoelectric detector, a single-mode + dispersion compensation long optical fiber, a polarization controller, an optical fiber collimator, a collimation and beam expansion device, a cubic beam splitter and a short-wave infrared camera CCD. And a computer. Traditional hyperspectral digital holography and femtosecond laser repetition frequency scanning are combined together, measurement errors in traditional hyperspectral digital holography are eliminated in principle, nm-level precision measurement of mm-level objects is achieved, and the device is simple in structure, high in stability and high in measurement precision.

Description

Technical Field

[0001] The present invention belongs to the field of femtosecond laser optical frequency comb technology and hyperspectral digital holographic imaging, and specifically is a hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning. Background Art

[0002] As emerging fields such as artificial intelligence and the Internet of Things develop towards precision, miniaturization, and intelligence, intelligent integrated instrument chip technology is gradually showing an important strategic position. Therefore, it is of great significance to study 3D imaging methods with large measurement depth and high axial resolution during chip production. Digital holography has full-field, non-contact, high-precision and unique phase imaging and numerical focusing capabilities compared to traditional lens imaging, making it a powerful 3D imaging tool and has attracted widespread attention in the fields of label-free living cell imaging, surface topography measurement, industrial inspection, and micro-electromechanical system imaging. In single-wavelength digital holography, the 2π ambiguity of phase wrapping limits the maximum axial range of reflective objects to half the wavelength and the maximum axial range of transparent objects to the full wavelength. Although single-wavelength phase unwrapping operations can eliminate 2π ambiguities, their adoption is limited to continuous surfaces with wavelength-order inhomogeneities.

[0003] In recent years, hyperspectral digital holography has attracted widespread attention because of its ability to solve the 2π ambiguity and noise amplification problems and provide additional spectral information through multi-wavelength layered phase unwrapping. It is a very cutting-edge holography. Traditional hyperspectral digital holography usually mechanically drives an arm to scan the optical path difference between the two arms, and then records the interference pattern at different optical path differences. This method is limited by mechanical vibration, scanning range and accuracy, resulting in a small axial measurement range and low measurement accuracy. The use of mode-locked femtosecond lasers has greatly improved the performance of hyperspectral digital holography. Mode-locked femtosecond lasers provide a large number of narrow combs at the same time at a precisely defined repetition rate, which is superior to traditional tunable continuous wave lasers and continuous spectrum lasers in terms of frequency stability, coherence and locked repetition frequency. This capability is crucial to hyperspectral digital holography, which can improve its measurement accuracy and axial measurement range and solve the drawbacks of traditional hyperspectral digital holography. In addition, although the existing dual-wavelength and multi-wavelength phase unwrapping methods can expand the axial measurement range of digital holography, they are still insufficient in terms of noise suppression capability and robustness. Therefore, in order to improve the measurement accuracy of digital holography, it is urgent to solve the above-mentioned problems. Summary of the invention

[0004] The present invention aims to solve the deficiencies of the above-mentioned prior art and proposes a hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning, so as to greatly simplify the complexity of the imaging system, reduce costs, improve the stability, measurement accuracy and measurement range of the imaging system, and achieve nm-level precision three-dimensional imaging of objects with mm-level height.

[0005] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme:

[0006] The invention discloses a high-spectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning, which comprises: a rubidium atomic clock, a signal generator, a frequency counter, a PLO phase-locked oscillator, a mode-locked femtosecond laser, a MIX mixer, a PID proportional-integral-differential controller, an AMP amplifier, a first optical fiber beam splitter, a PD photodetector, a second optical fiber beam splitter, a polarization controller, a first optical fiber collimator, a first collimating beam expanding device, a sample to be tested, a single-mode+dispersion compensation long optical fiber, a second optical fiber collimator, a second collimating beam expanding device, a cubic beam splitter, a short-wave infrared camera CCD, and a computer;

[0007] The rubidium atomic clock transmits the standard signal generated by itself to the signal generator and the frequency counter respectively. The signal generator receives the standard signal and outputs a sinusoidal signal whose frequency varies with time to the PLO phase-locked oscillator in a scanning state. The PLO phase-locked oscillator phase-locks and amplifies the frequency of the sinusoidal signal by C times to obtain the amplified sinusoidal signal and sends it to the MIX mixer. The scanning range of the signal generator in the scanning state is [q, w]; q represents the lower limit of the repetition frequency, and w represents the upper limit of the repetition frequency.

[0008] At the same time, the repetition frequency of the Dth harmonic output by the mode-locked femtosecond laser is also sent to the MIX mixer, so as to obtain a phase difference signal after mixing with the amplified sinusoidal signal;

[0009] The PID proportional-integral-differential controller processes the differential signal, and the obtained output signal is amplified by V times by the AMP amplifier, and then input into the mode-locked femtosecond laser to control the cavity length of the mode-locked femtosecond laser, thereby regulating the repetition frequency of the mode-locked femtosecond laser, and outputting the femtosecond pulse laser after the repetition frequency is regulated;

[0010] After the femtosecond pulse laser passes through the first optical fiber beam splitter, the first femtosecond pulse signal is transmitted to the PD photodetector, and the PD photodetector transmits the first femtosecond pulse signal to the frequency counter, and the frequency counter displays the real-time repetition frequency of the first femtosecond pulse signal according to the standard signal;

[0011] The second femtosecond pulse signal is divided into two paths by the second fiber beam splitter, one path is the measurement fiber light, and the other path is the reference fiber light. After the measurement fiber light passes through the polarization controller and the first fiber collimator in sequence, the measurement fiber light is converted into measurement space light, and then irradiated on the surface of the sample to be measured through the first collimation and expansion device;

[0012] After another reference fiber light passes through the single-mode + dispersion compensation long fiber and the second fiber collimator in sequence, the reference fiber light is converted into reference space light and processed by the second collimating beam expander to obtain expanded reference space light, and the pulse space interval between the expanded reference space light and the measurement space light is calculated and used as the optical path difference, and is adjusted by the repetition frequency;

[0013] The measurement space light reflected from the surface of the sample to be measured and the expanded reference space light pass through the cubic beam splitter, and then are combined and irradiated on the surface of the CCD of the short-wave infrared camera to form a digital hologram that changes with the optical path difference;

[0014] The computer continuously collects digital holograms at different optical path differences on the surface of the short-wave infrared camera and extracts them using Fourier transform spectrum to obtain complex-valued holograms at different wavelengths within the femtosecond pulse laser spectrum range, and then uses the angular spectrum method numerical algorithm to process the complex-valued hologram at each wavelength at a set reconstruction distance to obtain the wavefront amplitude and wrapped phase of the sample to be tested at different wavelengths and form hyperspectral phase information, and then uses the multi-wavelength phase unwrapping algorithm based on the average spectrum and multi-level chain wavelength chain to process the hyperspectral phase information to obtain the surface height map of the sample to be tested, so as to achieve 3D imaging of the sample to be tested.

[0015] The hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning of the present invention is also characterized in that the computer obtains the surface height map of the sample to be measured according to the following steps:

[0016] Step 1: After arranging the S digital holograms with a resolution of M×N under different optical path differences collected by the short-wave infrared camera CCD in the order of collection time, the image intensity signal of any pixel point under different optical path differences is obtained, and an interference signal I with a length of 1×S is formed. , where the Tth digital hologram with a resolution of M×N is recorded as , represents the serial number of any digital hologram, and , x represents the horizontal coordinate of the pixel point in the digital hologram, and , represents the length of the digital hologram, y represents the ordinate of the pixel point in the digital hologram, , represents the width of the digital hologram, Indicates A digital hologram under an optical path difference;

[0017] Step 2, perform Fourier transform on the interference signal I of any pixel point to obtain the amplitude and phase information of any pixel point at 2n light wavelengths, thereby obtaining the amplitude and phase information of M×N pixels in the digital hologram at 2n light wavelengths and forming 2n complex-valued holograms with a resolution of M×N at different light wavelengths, where the complex-valued hologram with a resolution of M×N at the i-th light wavelength is recorded as , represents the i-th light wavelength, i∈[1,2n];

[0018] Step 3: Use the angular spectrum method to analyze the i-th light wavelength The complex-valued hologram under the condition is reconstructed to obtain the i-th light wavelength The complex amplitude of the wavefront on the surface of the object below ; represents the horizontal coordinate on the surface of the object to be measured, , Represents the ordinate on the surface of the object to be measured, ;

[0019] Step 4: Use equations (3) and (4) to obtain the wrapping phase and amplitude at different wavelengths:

[0020] (3)

[0021] (4)

[0022] In formula (3) and formula (4), is the i-th light wavelength The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapped phase; arctan represents the inverse tangent function; Im represents the imaginary part of a complex number; Re represents the real part of a complex number, is the i-th light wavelength The coordinates of the surface of the object to be measured are (x o ,y o ) is the amplitude, abs is the absolute value function;

[0023] Step 5: Use a multi-wavelength phase unwrapping algorithm based on average spectrum and multi-level wavelength chain to The wrapping phase Perform phase unwrapping to obtain the first optical wavelength Lower phase wrapping ambiguity number ;

[0024] Step 6: Use equation (9) to reconstruct the coordinates of the surface of the object to be measured as (x o ,y o )’s height distribution:

[0025] (9).

[0026] Furthermore, the step 3 comprises:

[0027] Step 3.1: According to the scalar diffraction theory, use formula (1) to construct the i-th light wavelength The coordinates of the complex amplitude of the object light wave in the frequency domain (f x ,f y ) is the angular spectrum diffraction transfer function :

[0028] (1)

[0029] In formula (1), (f x ,f y ) is the coordinate of the complex amplitude of the object light wave in the frequency domain, , , and are respectively the length and width of the CCD format of the short-wave infrared camera, is the set reconstruction distance;

[0030] Step 3.2: Use formula (2) to get the i-th light wavelength The coordinates on the surface of the object to be measured are (x o ,y o ) of the complex amplitude of the object light wave :

[0031] (2)

[0032] In formula (2), represents the i-th light wavelength The complex-valued hologram with coordinates (x, y) in the spatial domain below, FFT is the fast Fourier transform; is the inverse fast Fourier transform.

[0033] Further, the step 5 comprises:

[0034] Step 5.1: Use equation (5) to obtain the coordinates of the object surface reconstructed by the average dual wavelength as (x o ,y o )'s height distribution :

[0035] (5)

[0036] In formula (5), Indicates the 2i-1th light wavelength The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapping phase, Indicates the 2ith wavelength of light The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapping phase, is the synthetic wavelength, and ,and ;

[0037] Step 5.2, use formula (6) to get the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o ) :

[0038] (6)

[0039] In formula (6), For rounding operation, Indicates the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o ), and , ;

[0040] Step 5.3, use equations (7) and (8) to get the first light wavelength The coordinates of the surface of the object to be measured are (x o ,y o ) :

[0041] (7)

[0042] (8)

[0043] In formula (6), Indicates the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o )’s height distribution.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] 1. The present invention applies femtosecond laser repetition frequency scanning to hyperspectral digital holography. By scanning the repetition frequency and introducing a long optical fiber, the optical path difference between the scanning reference light and the object light is realized, and the signal source is referenced to the rubidium atomic clock, thereby achieving accurate tracing of the repetition frequency. In principle, it solves the problems of traditional hyperspectral digital holography methods being limited by mechanical vibration, scanning range and accuracy, resulting in a small axial measurement range and low measurement accuracy.

[0046] 2. The multi-wavelength phase unwrapping algorithm based on average spectrum and multi-level wavelength chain proposed in the present invention adopts the method of averaging multiple pairs of dual-wavelength synthetic phases and unwrapping them at the optical wavelength, which solves the problems of insufficient noise suppression ability and robustness of existing dual-wavelength and multi-wavelength phase unwrapping methods.

[0047] 3. The present invention achieves an axial measurement range of mm and a measurement accuracy of nm, improving the measurement accuracy of digital holography with a large axial measurement range. The device of the present invention has a simple structure, high stability, high measurement accuracy, good synchronization, automatic acquisition, and short measurement time. It is expected to be applied in the fields of semiconductor nondestructive testing, high-precision micromechanical measurement, and intelligent integrated instrument chip technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a schematic diagram of a hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning of the present invention;

[0049] Figure 2 This is a flow chart of a multi-wavelength phase unwrapping algorithm based on average spectrum and multi-level chain wavelength chain of the present invention;

[0050] Figure 3 A three-dimensional surface height map of an initial object of the present invention;

[0051] Figure 4 A hyperspectral digital hologram collected by the hyperspectral digital holographic three-dimensional imaging system of the present invention;

[0052] Figure 5 The specific reconstruction process of the hyperspectral digital hologram of the present invention;

[0053] Figure 6 The present invention adopts a multi-wavelength phase unwrapping algorithm based on average spectrum and multi-level chain wavelength chain to reconstruct the object surface height map and its error map;

[0054] Numbers in the figure: 1 rubidium atomic clock, 2 signal generator, 3 frequency counter, 4 PLO phase-locked oscillator, 5 mode-locked femtosecond laser, 6 MIX mixer, 7 PID proportional-integral-differential controller, 8 AMP amplifier, 9 the first fiber beam splitter, 10 PD photodetector, 11 the second fiber beam splitter, 12 polarization controller, 13 the first fiber collimator, 14 the first collimation and expansion device, 15 sample to be tested, 16 single-mode + dispersion compensation long optical fiber, 17 the second fiber collimator, 18 the second collimation and expansion device, 19 cubic beam splitter, 20 short-wave infrared camera CCD, 21 computer. DETAILED DESCRIPTION

[0055] In this embodiment, a hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning is provided. Figure 1 As shown, it includes: a rubidium atomic clock 1, a signal generator 2, a frequency counter 3, a PLO phase-locked oscillator 4, a mode-locked femtosecond laser 5, a MIX mixer 6, a PID proportional-integral-differential controller 7, an AMP amplifier 8, a first optical fiber beam splitter 9, a PD photodetector 10, a second optical fiber beam splitter 11, a polarization controller 12, a first optical fiber collimator 13, a first collimating and expanding device 14, a sample to be tested 15, a single-mode+dispersion compensation long optical fiber 16, a second optical fiber collimator 17, a second collimating and expanding device 18, a cubic beam splitter 19, a short-wave infrared camera CCD 20, and a computer 21.

[0056] The mode-locked femtosecond laser 5 emits a femtosecond pulse laser with a central wavelength of 1560nm and a bandwidth of 40nm. The repetition frequency of the femtosecond pulse laser is 100±2MHz and the power is 1mw.

[0057] The rubidium atomic clock 1 inputs the standard sinusoidal signal with a frequency of 10 MHz generated by itself into the signal generator 2 and the frequency counter 3 respectively. The signal generator 2 receives the standard signal and outputs a sinusoidal signal with a frequency varying with time to the PLO phase-locked oscillator 4 in a scanning state with a scanning range of 9.9998 MHz to 10.0002 MHz and a sweep time of 40 seconds; the PLO phase-locked oscillator 4 phase-locks the frequency of the sinusoidal signal and amplifies it by 100 times to obtain the amplified sinusoidal signal and send it to the MIX mixer 6.

[0058] At the same time, the repetition frequency of the 10th harmonic output by the mode-locked femtosecond laser 5 is also sent to the MIX mixer 6, so as to obtain a phase difference signal after mixing with the amplified sinusoidal signal.

[0059] The PID proportional-integral-differential controller 7 processes the differential signal, and the obtained output signal is amplified by V=10 times through the AMP amplifier 8, and then input into the mode-locked femtosecond laser 5 for controlling the cavity length of the mode-locked femtosecond laser 5, thereby accurately sweeping and regulating the repetition frequency of the mode-locked femtosecond laser 5 from 99.998MHz to 100.002MHz within a scanning time of 40 seconds, and outputting the femtosecond pulse laser after the repetition frequency is regulated.

[0060] After the femtosecond pulse laser passes through the first optical fiber beam splitter 9, the first femtosecond pulse signal of 0.1mw is transmitted to the PD photodetector 10, and the PD photodetector transmits the first femtosecond pulse signal to the frequency counter 3, and the frequency counter 3 displays the real-time repetition frequency of the first femtosecond pulse signal within the scanning time of 40 seconds according to the standard signal; wherein the central wavelength of the first optical fiber beam splitter 9 is 1560nm; the working wavelength of the PD photodetector 10 is 1200nm~1700nm, and the frequency is 250MHz.

[0061] The second femtosecond pulse signal of 0.9mw is divided into two paths through the second fiber optic beam splitter 11, one path is the measurement fiber light of 0.6mw, and the other path is the reference fiber light of 0.3mw. After the measurement fiber light passes through the polarization controller 12 and the first fiber collimator 13 in turn, the measurement fiber light is converted into measurement space light and irradiated on the surface of the sample to be measured 15 through the first collimating beam expander 14; wherein the polarization state of the measurement fiber light is adjusted by adjusting the angles of the three paddles of the polarization controller 12, the power of the measurement fiber light is changed, and the interference contrast of the digital hologram is thereby improved; wherein the central wavelength of the second fiber optic beam splitter 11 is 1560nm; wherein the working wavelength of the first fiber optic collimator 13 is 1200nm~1700nm, and the output spot diameter is 5mm; wherein the magnification of the first collimating beam expander 14 is 3 times; wherein the sample to be measured 15 is a stepped object with a height of 1mm, and its surface height distribution diagram is as shown in Figure 3 shown.

[0062] Another reference fiber light passes through the single-mode + dispersion-compensating long fiber 16 and the second fiber collimator 17 in sequence, and then the reference fiber light is converted into reference space light and passes through the second collimating and expanding device 18 to obtain the expanded reference space light, wherein the single-mode + dispersion-compensating long fiber 16 is used to ensure that the total dispersion of the reference fiber light during the transmission process of the single-mode + dispersion-compensating long fiber 16 is basically 0, and introduce an optical path difference, which is defined as the pulse space interval between the expanded reference space light and the measurement space light. The optical path difference changes with the repetition frequency of the femtosecond pulse signal when the signal generator 2 is in the scanning state; wherein the optical path length of the single-mode + dispersion-compensating long fiber 16 is 60m; wherein the range of the optical path difference is -1.2mm~1.2mm; wherein the working wavelength of the second fiber collimator 17 is 1200nm~1700nm, and the output spot diameter is 5mm; wherein the magnification of the second collimating and expanding device 18 is 3 times.

[0063] After the measurement space light reflected from the surface 15 of the sample to be measured and the expanded reference space light pass through the cubic beam splitter 19, they are combined and irradiated on the surface of the short-wave infrared camera CCD20. Since there is an optical path difference between the measurement space light and the expanded reference space light, a digital hologram that changes with the optical path difference is formed on the surface of the short-wave infrared camera CCD20; the size of the cubic beam splitter 19 is 50mm×50mm×50mm, and the working wavelength is 1200nm~1700nm; the digital hologram is formed by the interference of the reflected measurement space light and the expanded reference space light on the surface of the short-wave infrared camera CCD20.

[0064] In this embodiment, the surface height map of the sample 15 to be tested is obtained according to the following steps:

[0065] Step 1, using the host computer software in the computer 21 to simultaneously control the signal generator 2 and the short-wave infrared camera CCD21, when the signal generator 2 starts scanning, the short-wave infrared camera CCD20 starts to continuously collect 12,000 holograms under different optical path differences within 40 seconds and store them in the computer 21. After arranging the 12,000 digital holograms with a resolution of 300×300 pixels under different optical path differences collected by the short-wave infrared camera CCD20 within a scanning time of 40 seconds in the order of collection time, the image intensity signal of any pixel point under different optical path differences is obtained and forms an interference signal I with a length of 1×12000. ; Among them, the 10000th digital hologram with a resolution of 300×300Pixel is recorded as , represents the serial number of any digital hologram, and , represents the horizontal coordinate of the pixel point in the digital hologram, and , represents the length of the digital hologram, Represents the vertical coordinate of the pixel point in the digital hologram, , represents the width of the digital hologram, Indicates A digital hologram under an optical path difference; the 12,000 digital holograms collected by the short-wave infrared camera CCD20 in 40 seconds are as follows Figure 4 As shown; the intensity diagram of the interference signal I is as follows Figure 5 As shown in (a) in the figure; in order to demonstrate that the phase unwrapping algorithm we proposed has strong noise suppression ability and robustness, Gaussian white noise with a mean of 0 and a standard deviation of 0.015rad is added to each of the 12,000 holograms.

[0066] Step 2, perform Fourier transform on the interference signal I of any pixel point to obtain the amplitude and phase information of any pixel point under 20 light wavelengths; Figure 5 (b) in the figure shows the high-spectral amplitude information obtained after Fourier transform of the interference signal I; the Fourier transform operation is repeated for the interference signal I of each pixel point, thereby obtaining the amplitude and phase information of 300×300 pixels in the digital hologram at 40 light wavelengths and forming 40 complex-valued holograms with a resolution of 300×300 pixels at different light wavelengths. ;in, .

[0067] Step 3, reconstructing the complex-valued holograms at 40 light wavelengths using an angular spectrum method to obtain the wavefront complex amplitudes of the object surface at 40 light wavelengths;

[0068] Step 3.1: According to the scalar diffraction theory, use formula (1) to construct the i-th light wavelength The coordinates of the complex amplitude of the object light wave in the frequency domain (f x ,f y ) is the angular spectrum diffraction transfer function :

[0069] (1)

[0070] In formula (1), (f x ,f y ) is the coordinate of the complex amplitude of the object light wave in the frequency domain, , , and are the length and width of the short-wave infrared camera CCD (20) format, is the set reconstruction distance, where z=10 cm, , ;

[0071] Step 3.2: Use formula (2) to get the i-th light wavelength The coordinates on the surface of the object to be measured are (x o ,y o ) of the complex amplitude of the object light wave :

[0072] (2)

[0073] In formula (2), represents the i-th light wavelength The complex-valued hologram with coordinates (x, y) in the spatial domain below, FFT is the fast Fourier transform; is the inverse fast Fourier transform, represents the horizontal coordinate on the surface of the object to be measured, and , represents the ordinate on the surface of the object to be measured, and ;

[0074] Step 4: Use equations (3) and (4) to obtain the wrapping phase and amplitude at different wavelengths:

[0075] (3)

[0076] (4)

[0077] in, The wavelength of light The coordinates (x o ,y o ). arctan represents the inverse tangent function; Im represents the imaginary part of the complex number; Re represents the real part of the complex number. Due to the limitation of the inverse tangent function, the obtained phase is wrapped in middle; The wavelength of light The coordinates (x o ,y o ) is the amplitude of the vibration, abs is the absolute value function; Figure 5 (c) shows that at the 20th wavelength (i.e. The wrapped phase diagram at .

[0078] Step 5: Use a multi-wavelength phase unwrapping algorithm based on average spectrum and multi-level wavelength chain to The wrapping phase Perform phase unwrapping to obtain the first optical wavelength Lower phase wrapping ambiguity number ; Algorithm flow chart as follows Figure 2 shown.

[0079] Step 5.1: Use equation (5) to obtain the coordinates of the object surface reconstructed by the average dual wavelength as (x o ,y o )'s height distribution :

[0080] (5)

[0081] In formula (5), Indicates the 2i-1th wavelength of light The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapping phase, Indicates the 2ith wavelength of light The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapping phase, is the synthetic wavelength, and ,and ;

[0082] Step 5.2, use formula (6) to get the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o ) :

[0083] (6)

[0084] In formula (6), For rounding operation, Indicates the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o ), and , ;

[0085] Step 5.3, use equations (7) and (8) to get the first light wavelength The coordinates of the surface of the object to be measured are (x o ,y o ) :

[0086] (7)

[0087] (8)

[0088] In formula (6), Indicates the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o )’s height distribution;

[0089] Step 6: Use equation (9) to reconstruct the coordinates of the surface of the object to be measured as (x o ,y o )'s height distribution :

[0090] (9).

[0091] In this embodiment, an electronic device includes a memory and a processor, wherein the memory is used to store a program that supports the processor to execute the above method, and the processor is configured to execute the program stored in the memory.

[0092] In this embodiment, a computer-readable storage medium stores a computer program on the computer-readable storage medium, and the computer program executes the steps of the above method when executed by a processor.

[0093] By comparing with the surface height distribution of the initial object, this example correctly reconstructs the stepped objects with a height of 100 to 1000 microns, such as Figure 6 (a) in Figure 1, and when Gaussian phase white noise with a mean of 0 and a standard deviation of 0.015 is added, the root mean square error of the reconstructed object is only 6.3nm, and the maximum error is 10nm. The error distribution diagram is shown in Figure 6 As shown in (b) in .

[0094] In summary, the present invention applies femtosecond laser repetition frequency scanning to hyperspectral digital holography, which solves the problems that traditional hyperspectral digital holography methods are limited by mechanical vibration, scanning range and accuracy, resulting in a small axial measurement range and low measurement accuracy; secondly, the proposed multi-wavelength phase unwrapping algorithm based on average spectrum and multi-level chain wavelength chain solves the problems of insufficient noise suppression ability and robustness of existing dual-wavelength and multi-wavelength phase unwrapping methods; ultimately, an axial measurement range of mm and a measurement accuracy of nm can be achieved.

Claims

1. A hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning, characterized in that: include: Rubidium atomic clock (1), signal generator (2), frequency counter (3), PLO phase-locked oscillator (4), mode-locked femtosecond laser (5), MIX mixer (6), PID proportional-integral-differential controller (7), AMP amplifier (8), first optical fiber beam splitter (9), PD photodetector (10), second optical fiber beam splitter (11), polarization controller (12), first optical fiber collimator (13), first collimation and expansion device (14), sample to be tested (15), single-mode + dispersion compensation long optical fiber (16), second optical fiber collimator (17), second collimation and expansion device (18), cube beam splitter (19), short-wave infrared camera CCD (20), computer (21); The rubidium atomic clock (1) transmits the standard signal generated by itself to the signal generator (2) and the frequency counter (3) respectively; the signal generator (2) receives the standard signal and outputs a sinusoidal signal whose frequency varies with time to the PLO phase-locked oscillator (4) in a scanning state; the PLO phase-locked oscillator (4) phase-locks the frequency of the sinusoidal signal and amplifies it by C times to obtain the amplified sinusoidal signal and sends it to the MIX mixer (6); wherein the scanning range of the signal generator (2) in the scanning state is [q, w]; q represents the lower limit of the repetition frequency, and w represents the upper limit of the repetition frequency; At the same time, the repetition frequency of the Dth harmonic output by the mode-locked femtosecond laser (5) is also sent to the MIX mixer (6), so as to obtain a phase difference signal after mixing with the amplified sinusoidal signal; The PID proportional-integral-differential controller (7) processes the differential signal, and the obtained output signal is amplified by V times by the AMP amplifier (8), and then input into the mode-locked femtosecond laser (5) to control the cavity length of the mode-locked femtosecond laser (5), thereby regulating the repetition frequency of the mode-locked femtosecond laser (5), and outputting the femtosecond pulse laser after the repetition frequency is regulated; After the femtosecond pulse laser passes through the first optical fiber beam splitter (9), the first femtosecond pulse signal is transmitted to the PD photodetector (10), and the PD photodetector transmits the first femtosecond pulse signal to the frequency counter (3), and the frequency counter (3) displays the real-time repetition frequency of the first femtosecond pulse signal according to the standard signal; The second femtosecond pulse signal is divided into two paths through the second fiber beam splitter (11), one path being the measurement fiber light and the other being the reference fiber light. After the measurement fiber light passes through the polarization controller (12) and the first fiber collimator (13) in sequence, the measurement fiber light is converted into measurement space light and irradiated onto the surface of the sample to be measured (15) through the first collimating beam expander (14); After another reference optical fiber light passes through the single-mode + dispersion compensation long optical fiber (16) and the second optical fiber collimator (17) in sequence, the reference optical fiber light is converted into reference spatial light and processed by the second collimating beam expander (18) to obtain expanded reference spatial light, and the pulse spatial interval between the expanded reference spatial light and the measurement spatial light is calculated and used as the optical path difference, and is adjusted by the repetition frequency; After the measurement space light reflected from the surface (15) of the sample to be measured and the expanded reference space light pass through the cubic beam splitter (19), they are combined and irradiated on the surface of the short-wave infrared camera CCD (20) to form a digital hologram that changes with the optical path difference; The computer (21) continuously collects digital holograms at different optical path differences on the surface of the short-wave infrared camera (20) and extracts them using Fourier transform spectrum to obtain complex-valued holograms at different wavelengths within the femtosecond pulse laser spectrum range. The complex-valued holograms at each wavelength are then processed at a set reconstruction distance using an angular spectrum method numerical algorithm to obtain the wavefront amplitude and wrapped phase of the sample to be tested (16) at different wavelengths and form hyperspectral phase information. The hyperspectral phase information is then processed using a multi-wavelength phase unwrapping algorithm based on an average spectrum and a multi-level chain wavelength chain to obtain a surface height map of the sample to be tested (16), thereby achieving 3D imaging of the sample to be tested (15).

2. The hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning according to claim 1, characterized in that: The computer (21) obtains the surface height map of the sample to be tested (15) according to the following steps: Step 1, after arranging the S digital holograms with a resolution of M×N under different optical path differences collected by the short-wave infrared camera CCD (20) in the order of collection time, the image intensity signal of any pixel point under different optical path differences is obtained, and an interference signal I with a length of 1×S is formed. , where the Tth digital hologram with a resolution of M×N is recorded as , represents the serial number of any digital hologram, and , x represents the horizontal coordinate of the pixel point in the digital hologram, and , represents the length of the digital hologram, y represents the ordinate of the pixel point in the digital hologram, , represents the width of the digital hologram, Indicates A digital hologram under an optical path difference; Step 2, perform Fourier transform on the interference signal I of any pixel point to obtain the amplitude and phase information of any pixel point at 2n light wavelengths, thereby obtaining the amplitude and phase information of M×N pixels in the digital hologram at 2n light wavelengths and forming 2n complex-valued holograms with a resolution of M×N at different light wavelengths, where the complex-valued hologram with a resolution of M×N at the i-th light wavelength is recorded as , represents the i-th light wavelength, i∈[1,2n]; Step 3: Use the angular spectrum method to analyze the i-th light wavelength The complex-valued hologram under the condition is reconstructed to obtain the i-th light wavelength The complex amplitude of the wavefront on the surface of the object below ; represents the horizontal coordinate on the surface of the object to be measured, , Represents the ordinate on the surface of the object to be measured, ; Step 4: Use equations (3) and (4) to obtain the wrapping phase and amplitude at different wavelengths: (3) (4) In formula (3) and formula (4), is the i-th light wavelength The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapped phase; arctan represents the inverse tangent function; Im represents the imaginary part of a complex number; Re represents the real part of a complex number, is the i-th light wavelength The coordinates of the surface of the object to be measured are (x o ,y o ) is the amplitude, abs is the absolute value function; Step 5: Use a multi-wavelength phase unwrapping algorithm based on average spectrum and multi-level wavelength chain to The wrapping phase Perform phase unwrapping to obtain the first optical wavelength Lower phase wrapping ambiguity number ; Step 6: Use equation (9) to reconstruct the coordinates of the surface of the object to be measured as (x o ,y o )’s height distribution: (9)。 3. The hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning according to claim 2, characterized in that: The step 3 comprises: Step 3.1: According to the scalar diffraction theory, use formula (1) to construct the i-th light wavelength The coordinates of the complex amplitude of the object light wave in the frequency domain (f x ,f y ) is the angular spectrum diffraction transfer function : (1) In formula (1), (f x ,f y ) is the coordinate of the complex amplitude of the object light wave in the frequency domain, , , and are respectively the length and width of the short-wave infrared camera CCD (20), is the set reconstruction distance; Step 3.2: Use formula (2) to get the i-th light wavelength The coordinates on the surface of the object to be measured are (x o ,y o ) of the complex amplitude of the object light wave : (2) In formula (2), represents the i-th light wavelength The complex-valued hologram with coordinates (x, y) in the spatial domain below, FFT is the fast Fourier transform; is the inverse fast Fourier transform.

4. The hyperspectral digital holographic three-dimensional imaging system based on femtosecond laser repetition frequency scanning according to claim 3, characterized in that: The step 5 comprises: Step 5.1: Use equation (5) to obtain the coordinates of the object surface reconstructed by the average dual wavelength as (x o ,y o )'s height distribution : (5) In formula (5), Indicates the 2i-1th light wavelength The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapping phase, Indicates the 2ith wavelength of light The coordinates of the surface of the object to be measured are (x o ,y o )’s wrapping phase, is the synthetic wavelength, and ,and ; Step 5.2, use formula (6) to get the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o ) : (6) In formula (6), For rounding operation, Indicates the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o ), and , ; Step 5.3, use equations (7) and (8) to get the first wavelength of light: The coordinates of the surface of the object to be measured are (x o ,y o ) : (7) (8) In formula (6), Indicates the second synthetic wavelength The coordinates of the surface of the object to be measured are (x o ,y o )’s height distribution.

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