Time stretching Fourier transform optical frequency comb morphology measurement system and method

Abstract

The invention provides a time stretching Fourier transform optical frequency comb morphology measurement system and method. The time stretching Fourier transform optical frequency comb morphology measurement system comprises an optical frequency comb OFC, an interferometer module, a dispersion optical fiber module, a signal detection module and a synchronous control module. An optical frequency comb (OFC) provides an incident light beam for the interferometer; the interferometer module obtains an interference signal of the measured object by adjusting the length of the single-mode optical fiber; the dispersion transmission module performs time stretching on the interference signal through a single-mode optical fiber to generate a time domain interference signal; a signal detection module captures the stretched time domain interference signal, and solves the morphology parameters of the measured object through a time-frequency transformation algorithm; the synchronous control module drives the two-dimensional displacement platform bearing the measured object through the motion control card and triggers synchronous acquisition and storage of three-dimensional shape data of the measured object. The method has enough high precision in morphology measurement, and the detection speed of the method can meet the requirements of industrial real-time detection.

Description

Technical Field

[0001] This application relates to the field of optical measurement technologies, and particularly to a time-stretch Fourier transform optical frequency comb topography measurement system and method. Background Art

[0002] In a solar energy conversion system, monocrystalline silicon solar cells are highly favored by the market due to their relatively high conversion efficiency and long service life. However, the production process of monocrystalline silicon solar cells is complex, and the screen printing of conductive silver paste is very important. Insufficient printing will lead to a significant decrease in conversion efficiency, while excessive printing will cause a significant increase in production costs. Therefore, precise measurement of the silver paste circuit on solar cells has become an important means to evaluate quality and output. Traditional detection methods mainly rely on manual inspection, where workers rely on established standards and common sense to make judgments. This method has low detection efficiency and a high misjudgment rate. With the development of detection technologies, various intelligent detection methods have emerged. Currently, the main methods are visual measurement and optical measurement. Among optical measurement methods, the main ones are infrared light measurement and confocal white light measurement. In these measurement systems, external light sources and detectors are required to obtain images with higher contrast. The measurement systems require higher-quality equipment, more stringent environmental requirements, and stricter sample measurement conditions, and these methods fail to meet the requirements for on-line detection of solar cell silver lines. Currently, there are also many instruments on the market for measuring solar cell silver lines that have the ability to measure their topography, including height, surface roughness, and conductivity. They have high enough accuracy in topography measurement (up to the nanometer level), but the significant limitation of their detection speed cannot meet the needs of industrial real-time detection. Summary of the Invention

[0003] In order to solve this technical problem, this application provides a time-stretch Fourier transform optical frequency comb topography measurement system and method.

[0004] In a first aspect, this application provides a time-stretch Fourier transform optical frequency comb topography measurement system, which includes an optical frequency comb (OFC), an interferometer module, a dispersion fiber module, a signal detection module, and a synchronization control module;

[0005] The optical frequency comb (OFC) is used to provide an incident light beam for the interferometer;

[0006] The interferometer module is used to obtain the interference signal of the object to be measured by adjusting the length of a single-mode fiber;

[0007] The dispersion transmission module is used to perform time stretching on the interference signal through a single-mode fiber to generate a time-domain interference signal;

[0008] The signal detection module is used to capture the stretched time-domain interference signal and calculate the topography parameters of the object to be measured through a time-frequency transformation algorithm;

[0009] The synchronization control module is used to drive the two-dimensional displacement stage carrying the object to be measured through a motion control card, and trigger the synchronous acquisition and storage of the three-dimensional topography data of the object to be measured.

[0010] Further, the interferometer module includes a measurement arm and a reference arm. Among them, the measurement arm is sequentially connected to an erbium-doped fiber amplifier, a circulator CIR, and a collimator COL;

[0011] The incident light beam enters the interferometer module after passing through an optical isolator ISO;

[0012] The incident light beam is split into two paths at the beam splitter of the interferometer module. One path enters the measurement arm, and the other path enters the reference arm;

[0013] The first light beam entering the measurement arm is amplified by an erbium-doped fiber amplifier, and then sequentially passes through the circulator CIR and the collimator COL, and is incident on the object to be measured. The reflected light beam returns along the original path and enters the first port of the coupler OC2 of the interferometer module from the circulator;

[0014] The second light beam entering the reference arm is transmitted through a single-mode fiber SMF1 to the second port of the coupler OC2 of the interferometer, realizing the beam combination of the reference light beam and the measurement light beam;

[0015] By adjusting the length of the single-mode fiber in the interferometer module, the optical path difference between the measurement arm and the reference arm is changed, thereby introducing a time delay. This time delay will cause light of different wavelengths to interfere in space and time, and the interference signal is obtained.

[0016] Further, the dispersion transmission module is specifically used to amplify the interference signal through an erbium-doped fiber amplifier; transmit the amplified signal through a single-mode fiber, and through the dispersion effect, different wavelengths of light generate different time delays when propagating in the fiber, forming a time-domain interference signal.

[0017] Further, the signal detection module is specifically used for a photodetector to obtain the time-domain interference signal, convert the time-domain interference signal into an electrical signal, and capture the electrical signal by a high-speed oscilloscope OSC;

[0018] Extract the information in the time-domain interference pattern through the captured electrical signal;

[0019] Through Fourier transform, convert the information in the time-domain interference signal back to the frequency-domain interference signal to obtain spectral information;

[0020] Calculate the height change of the object to be measured, that is, the Z-axis distance, according to the spectral information.

[0021] Further, the synchronization control module is specifically configured to control and record the movement trajectories of the X-axis and Y-axis through a motion control card, and simultaneously trigger the storage of the measurement data of the Z-axis, so as to achieve synchronous acquisition of the measurement data in three dimensions and ensure the accuracy of surface topography measurement.

[0022] Further, the dispersion transmission module is specifically configured to have a time-stretching scanning rate that is at least equal to the repetition frequency f of the optical frequency comb rep 。

[0023] Further, the dispersion transmission module is specifically configured to obtain the measurement distance through the frequency-domain interference signal;

[0024] Among them, the dynamic measurement range The maximum time delay is

[0025] Therefore, the dynamic measurement range is where ΔF is the bandwidth of the photodetector, D is the dispersion parameter GVD of the dispersion element, ΔF is the bandwidth of the photodetector, and λ is the spectral width of the input pulse.

[0026] Further, the maximum round-trip time of the detection pulse of the first beam relative to the second beam is less than 1 / f rep , where the round-trip time of the detection pulse of the first beam is the time after being reflected from the surface of the object under test and returning to the interferometer.

[0027] In a second aspect, the present invention further provides a time-stretch Fourier transform optical frequency comb topography measurement method, which includes

[0028] The optical frequency comb OFC provides an incident beam for the interferometer;

[0029] The interferometer module obtains the interference signal of the object under test by adjusting the length of the single-mode optical fiber;

[0030] The dispersion transmission module performs time stretching on the interference signal through the single-mode optical fiber to generate a time-domain interference signal;

[0031] The signal detection module captures the stretched time-domain interference signal and calculates the topography parameters of the object under test through a time-frequency transformation algorithm;

[0032] The synchronization control module drives the two-dimensional displacement stage carrying the object under test through a motion control card and triggers the synchronous acquisition and storage of the three-dimensional topography data of the object under test.

[0033] Further, the interferometer module includes a measurement arm and a reference arm. Among them, the measurement arm is sequentially connected to an erbium-doped fiber amplifier, a circulator CIR, and a collimator COL; the method includes:

[0034] The incident light beam enters the interferometer module after passing through the optical isolator ISO;

[0035] The incident light beam is split into two paths at the beam splitter of the interferometer module, one path enters the measurement arm and the other path enters the reference arm;

[0036] The first light beam entering the measurement arm is amplified by an erbium-doped fiber amplifier, then passes through the circulator CIR and the collimator COL in sequence, and is incident on the object to be measured. The reflected light beam returns along the original path and enters the first port of the coupler OC2 of the interferometer module from the circulator;

[0037] The second light beam entering the reference arm is transmitted through the single-mode fiber SMF1 to the second port of the coupler OC2 of the interferometer, realizing the combination of the reference light beam and the measurement light beam;

[0038] By adjusting the length of the single-mode fiber in the interferometer module, the optical path difference between the measurement arm and the reference arm is changed, thereby introducing a time delay. This time delay will cause light of different wavelengths to interfere in space and time, and the interference signal is obtained.

[0039] Furthermore, the dispersion transmission module amplifies the interference signal through an erbium-doped fiber amplifier; the amplified signal is transmitted through a single-mode fiber, and through the dispersion effect, different wavelengths of light generate different time delays when propagating in the fiber, forming a time-domain interference signal.

[0040] Furthermore, the signal detection module obtains the time-domain interference signal through a photodetector, converts the time-domain interference signal into an electrical signal, and the electrical signal is captured by the high-speed oscilloscope OSC;

[0041] The information in the time-domain interference pattern is extracted through the captured electrical signal;

[0042] Through Fourier transform, the information in the time-domain interference signal is converted back into a frequency-domain interference signal to obtain spectral information;

[0043] According to the spectral information, the height change of the object to be measured, that is, the Z-axis distance, is calculated.

[0044] Furthermore, the synchronization control module controls and records the movement trajectories of the X-axis and Y-axis through a motion control card, and simultaneously triggers the storage of the Z-axis measurement data, so as to realize the synchronous acquisition of the measurement data in three dimensions and ensure the accuracy of the surface topography measurement.

[0045] Furthermore, the scanning rate of the time stretching of the dispersion transmission module is at least equal to the repetition frequency f of the optical frequency comb rep .

[0046] Further, the dispersion transmission module is specifically configured to obtain a measured distance through a frequency-domain interference signal;

[0047] Among them, the dynamic measurement range The maximum time delay is

[0048] Therefore, the dynamic measurement range is where ΔF is the bandwidth of the photodetector, D is the dispersion parameter GVD of the dispersion element, ΔF is the bandwidth of the photodetector, and λ is the spectral width of the input pulse.

[0049] Further, the maximum round-trip time of the detection pulse of the first light beam relative to the second light beam is less than 1 / f rep , where the round-trip time of the detection pulse of the first light beam is the time when it returns to the interferometer after being reflected from the surface of the object to be measured.

[0050] A time-stretch Fourier transform optical frequency comb topography measurement system provided by the present application includes an optical frequency comb OFC, an interferometer module, a dispersion fiber module, a signal detection module, and a synchronization control module; the optical frequency comb OFC provides an incident light beam for the interferometer; the interferometer module obtains the interference signal of the object to be measured by adjusting the length of the single-mode fiber; the dispersion transmission module performs time stretching on the interference signal through the single-mode fiber to generate a time-domain interference signal; the signal detection module captures the stretched time-domain interference signal and calculates the topography parameters of the object to be measured through a time-frequency transformation algorithm; the synchronization control module drives a two-dimensional displacement stage carrying the object to be measured through a motion control card and triggers synchronous acquisition and storage of three-dimensional topography data of the object to be measured. The present application has sufficiently high accuracy in topography measurement, and its detection speed can also meet the requirements of industrial real-time detection. Description of the Drawings

[0051] The specification drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0052] Figure 1 is a schematic diagram of the device of a time-stretch Fourier transform optical frequency comb topography measurement system provided by an embodiment of the present application;

[0053] Among them, Figure (a) is the spectrum of the OFC, Figure (b) is the frequency spectrum of the OFC, Figure (c) is the interference pulse on the OSC, Figure (d) is the time-stretch interference pattern on the OSA, and Figure (e) is the control system;

[0054] Figure 2 is the schematic diagram of the frequency-domain interference spectrum in the present application;

[0055] Figure 3It is a schematic diagram of the time-frequency conversion result in this application;

[0056] Figure 4 It is a schematic diagram of the measurement result of the repetition frequency stability of the optical frequency comb in the locked and unlocked states by a frequency meter in this application. Detailed implementation manners

[0057] The present invention will be described in detail below with reference to the drawings and in conjunction with embodiments. It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments may be combined with each other.

[0058] The following detailed descriptions are all exemplary descriptions, aiming to provide further detailed descriptions of the present invention. Unless otherwise specified, all technical terms adopted by the present invention have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. The terms used in the present invention are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present invention.

[0059] The emergence of the optical frequency comb (OFC) provides a new measurement idea for the morphology measurement of solar cell wafers. With its advantages of wide spectrum, narrow linewidth, and frequency lockability, the OFC shows natural advantages in the measurement field. In this invention patent, it is proposed to use the time-stretch dispersive Fourier transform optical frequency comb (TS-DFT-OFC) to measure the morphology of silver paste lines on solar cells online. To a certain extent, this method takes into account speed, accuracy, and real-time performance, showing significant application potential in the mass production inspection of solar cells. It can also be further applied to fields such as precision manufacturing, aerospace, semiconductor industry, and biomedicine.

[0060] The measurement device in this application is as Figure 1 shown. The light source is selected as an optical frequency comb (OFC), with a central wavelength λ of 1566.5 nm, a spectral width Δλ of 18.4 nm, and a repetition frequency f rep of 250.8 MHz, locked to the standard reference of a cesium atomic clock, as Figure 1 (a) and Figure 1(as shown in (b)). The incident light beam passes through an optical isolator (ISO) and then enters a Mach-Zehnder interferometer (MZI). The light beam in the measurement arm is amplified to over 30 mW by an erbium-doped fiber amplifier (EDFA1), and then successively passes through a circulator (CIR) and a collimator (COL), and is incident on the object to be measured. In order to expand the measurement range, the object to be measured is placed on a high-precision two-dimensional displacement stage. The reflected light beam returns along the original path, enters the MZI coupler (OC2) from the circulator. At the same time, the reference light beam is transmitted through a single-mode fiber (SMF1) to another port of OC2 to realize the combination of the reference light beam and the measurement light beam. The length of the single-mode fiber 1 in the interferometer is adjusted to adjust the time delay to generate spectral interference. The interference signal is amplified by an erbium-doped fiber amplifier 2 and then transmitted through a single-mode fiber 2 with a total dispersion of about -132.788 ps 2 . The spectral information is mapped to the time domain based on time-stretch dispersion Fourier transform (TS-DFT) through a data conversion formula, as shown in Figure 1 (c) and Figure 1 (d). The dispersion-stretched optical pulse is transmitted to a high-resolution photodetector (PD) and captured by a high-speed oscilloscope (OSC) for real-time measurement of the Z-axis distance. In order to achieve three-dimensional topography measurement, the signals of the X-axis and Y-axis need to be acquired simultaneously. The industrial control computer uses a motion control card (ETH_NEC_2) to control and record the motion trajectories of the X-axis and Y-axis, and at the same time triggers the storage of the Z-axis measurement data, as shown in Figure 1 (e). The control system ensures the synchronous acquisition of the measurement data in three dimensions to ensure the accuracy of surface topography measurement.

[0061] This application overcomes the limitation of the refresh rate of traditional grating spectrometers and realizes real-time, continuous, single-frame spectral measurement of ultra-short pulse sequences. When an optical pulse propagates through a single-mode fiber (SMF) with a length of L, only considering second-order dispersion at low peak power, the phase can be expressed as where it involves the fiber length L, the group velocity dispersion (GVD) parameter β2, and the central frequency ω0. Assuming that the complex amplitude of the ultra-short pulse is a0(t), the pulse response function caused by transmission through a section of fiber is expressed as h1(t). The measured distance is applied to the measurement arm of the MZI, where the pulse response function of the MZI is represented by h1(t). The result of the pulse electric field can be expressed as:

[0062]

[0063] The photocurrent detected by the photodetector (PD) can be expressed as:

[0064]

[0065] where is the response parameter of the photodetector, aoutput * (t) is the complex conjugate of a output (t), and τ is the time delay between the two arms. Assuming that the optical powers in the probe arm and the reference arm are equal, the time-domain interference signal obtained by the PD can be expressed as:

[0066]

[0067] Meanwhile, the interference signal collected by the spectrometer can be expressed as:

[0068]

[0069] As shown in Equations (3) and (4), the time delay τ can be calculated from the frequency-domain interference fringes and the time-domain interference fringes.

[0070] The scanning rate of the DFT is at least equal to the repetition frequency f of the OFC rep , and the mapping relationship between the time domain and the frequency domain can be expressed as:

[0071]

[0072] When m = 1, β2 is the group velocity dispersion (GVD) parameter, and T(ω) is the time variation caused by the group velocity dispersion GVD. In fact, usually the dispersion parameter D = dβ1 / dλ, and its relationship with β2 can be expressed as The time-domain width of the stretched signal is given by Δτ = |D|z·Δλ, where Δλ is the spectral width of the input pulse. D is the group velocity dispersion parameter GVD of the dispersion element (unit: ps / nm / km), and Δτ is the envelope width of the stretched pulse.

[0073] Due to the influence of high-order fiber dispersion, the time-domain interference signal has a frequency chirping effect, which can be removed by time-frequency transformation, and the measurement distance can be obtained from the frequency-domain interference signal. The dynamic measurement range Z can be expressed as The maximum time delay can be estimated as Therefore, the dynamic measurement range can be expressed as where ΔF is the bandwidth of the photodetector. The maximum round-trip time of the probe pulse relative to the reference pulse should be less than 1 / f rep , that is, the single-pulse period T period .

[0074] The spectral resolution is a key parameter for real-time spectral measurement using TS-DFT, and it is affected by various factors.

[0075] First, the dispersion in the TS-DFT limits the system upper bound, as shown in Equation (6):

[0076]

[0077] In addition, the bandwidth of the detection system composed of a photodetector (PD) and an oscilloscope limits the upper limit of the system, as shown in Equation (7):

[0078]

[0079] Finally, the sampling rate of the oscilloscope is as shown in Equation (8), where F s represents the sampling rate.

[0080]

[0081] The spectral resolution should be the maximum value among the above three formulas. Through calculation, the spectral resolution of this experimental system is approximately 0.39 nm.

[0082] It should be understood that the spectrum can accurately reflect the spectral characteristics of the input signal, including the central wavelength, bandwidth, and spectral intensity distribution. When obtaining the time-domain spectrum through TS-DFT, the time stretching effect will also occur. As time stretches, the dispersion effect of the input signal will be further amplified. To reduce the influence of higher-order dispersion, it is necessary to convert the time-domain spectrum to the frequency-domain spectrum to improve the accuracy of spectral reconstruction.

[0083] First, normalize the intensity axis of the spectrum to eliminate the influence of different measurement intensities and facilitate observing the shapes of the time-domain and frequency-domain spectra.

[0084] Secondly, establish the time-frequency mapping relationship. During the time stretching process, different time sampling points correspond to different spectral wavelengths. The wavelength range corresponding to the time-domain spectrum can be calculated by Δτ = |D|LΔλ. To obtain a more accurate time-frequency mapping relationship, 9 groups of frequency-domain interference spectrograms with an interval of 100 μm can be collected, and the peaks of the interference fringes are respectively subjected to Gaussian fitting, as Figure 2 shown in the frequency-domain interference spectrum. The blue stripe pattern is the interference pattern at different positions, and the red curve is the Gaussian fitting curve. The wavelength center can be obtained through calculation, where x 0i is the central value corresponding to the full width at half maximum (FWHM) of the Gaussian fitting curves of the 9 groups of interference spectra. The calculated wavelength center λ0 is 1561.38 nm. Finally, complete the time-frequency conversion according to the calculated parameters. One set of experimental results is as Figure 3 (a) Schematic diagram of the time-frequency conversion result shown, where yellow and blue are the spectra in the time domain and frequency domain respectively. It can be seen from the figure that the time-domain and frequency-domain interference patterns are slightly different, and this small difference comes from the nonlinear effect of the optical fiber during the TS-DFT transmission process.

[0085] During the stretching process, the frequency changes linearly with time under the dispersion effect. In fact, the spectral variation of the signal in the time domain is as follows,

[0086]

[0087] where A is the slow-varying amplitude of the pulse envelope, T is the time variable in the reference frame propagating at the group velocity (GVD), and γ is the nonlinear parameter. The right side of Equation (9) describes the fiber loss, dispersion effect, and nonlinear effect when the optical pulse propagates in the optical fiber. The influence of dispersion and nonlinear effects on the evolution of the pulse in the optical fiber can be evaluated by the dispersion length L D and the nonlinear length L NL which are specifically expressed as:

[0088]

[0089] where T0 represents the initial width of the incident pulse, and P0 is the peak power of the incident pulse. It is found through calculation that the spectral variation in the time domain is affected by the nonlinear effect, which is the main cause of the time-frequency conversion error. To recover the detected spectrum, a reliable system dispersion calibration method is proposed to eliminate the nonlinear effect. Using 9 sets of spectral variations in the frequency domain and their corresponding spectral variations in the time domain, the average spectral width Δτ a in the frequency domain and the average spectral width Δλ a in the time domain are calculated. Define the calibration parameter D a = Δτ a / Δλ a . After calibration, the value of D a is 1.046×104. Applying this D a to the time-frequency conversion process can effectively eliminate the nonlinear effect. As shown in Figure 3 (b), the pink color represents the original spectral variation in the time domain, and the blue color represents the spectral variation in the time domain after eliminating the nonlinear effect. By comparing the two sets of spectra (one set retains the nonlinear effect, and the other set has eliminated the nonlinear effect), it can be clearly observed that the spectrum after eliminating the nonlinear effect shows a certain degree of convergence. The experimental results are in good agreement with the theoretical calculation results of Equation (9), verifying that this method can meet the measurement requirements.

[0090] First, the influence of the repetition frequency being locked or unlocked on the measurement accuracy is repeated. The stability of the repetition frequency of the optical frequency comb in the locked and unlocked states is tested by a frequency meter, and the measurement results are as shown in Figure 4 , where (a) and (b) respectively show the variation range of the repetition frequency under locked and unlocked conditions within 20 minutes. In the locked state, the maximum variation range is 0.5422 Hz, while in the unlocked state, it is 73.8334 Hz. The results show that when the repetition frequency state changes from locked to unlocked, its drift range will expand from the sub-hertz level to nearly 100 Hz.

[0091] Assume that the time-domain interference spectrum has been fully stretched and its period is T = 1 / f rep . The system calibration parameter is denoted as D a . According to the principle of TS-DFT, the following formula (11) can be obtained:

[0092]

[0093] where f0 is the starting frequency of the frequency-domain interference spectrum and f1 is the ending frequency of the frequency-domain interference spectrum. According to the FFT principle and the peak interval method, the measured time-domain resolution is shown in Equation (12):

[0094]

[0095] where M and N represent the number of sampling points and the number of calculations of FFT respectively. Substitute Equation (11) into Equation (12), and combine with the speed of light c, the measured length resolution L S can be expressed as:

[0096]

[0097] Define the change in f rep as Δf, and ignore the changes in the starting and ending frequencies of the interference spectrum, then the change in the measured value ΔL S can be expressed as:

[0098]

[0099] Define M = N = 2001, Δf = 100Hz, and 100Hz can be considered as the maximum amount of change in the repetition frequency of the system light source. Then, substitute all the parameters in this system into Equation (14), and the change in the measured value ΔL S is approximately 0.1μm. To further reduce the influence, define N = 10001 or larger. At this time, the error can be controlled within 0.01μm, but this will inevitably increase the computational load of the system. The measurement accuracy of the system is also affected by many other factors such as the carrier-envelope offset frequency, group refractive index, vibration, and time jitter. In actual comparative experiments, other conditions should be kept as consistent as possible.

[0100] In addition, this application also provides a time-stretched Fourier transform optical frequency comb topography measurement system, which includes an optical frequency comb OFC, an interferometer module, a dispersion fiber module, a signal detection module, and a synchronization control module;

[0101] The optical frequency comb OFC is used to provide an incident light beam for the interferometer;

[0102] The interferometer module is used to obtain the interference signal of the object to be measured by adjusting the length of the single-mode fiber;

[0103] The dispersion transmission module is used to perform time stretching on the interference signal through a single-mode optical fiber to generate a time-domain interference signal;

[0104] The signal detection module is used to capture the stretched time-domain interference signal and solve the morphological parameters of the object to be measured through a time-frequency transformation algorithm;

[0105] The synchronization control module is used to drive the two-dimensional displacement stage carrying the object to be measured through a motion control card and trigger the synchronous acquisition and storage of the three-dimensional morphological data of the object to be measured.

[0106] Further, the interferometer module includes a measurement arm and a reference arm. Among them, the measurement arm is sequentially connected to an erbium-doped fiber amplifier, a circulator CIR, and a collimator COL;

[0107] The incident light beam enters the interferometer module after passing through an optical isolator ISO;

[0108] The incident light beam is split into two paths at the beam splitter of the interferometer module. One path enters the measurement arm, and the other path enters the reference arm;

[0109] The first light beam entering the measurement arm is amplified by an erbium-doped fiber amplifier, then sequentially passes through the circulator CIR and the collimator COL, and is incident on the object to be measured. The reflected light beam returns along the original path and enters the first port of the coupler OC2 of the interferometer module from the circulator;

[0110] The second light beam entering the reference arm is transmitted to the second port of the coupler OC2 of the interferometer through a single-mode optical fiber SMF1 to realize the beam combination of the reference light beam and the measurement light beam;

[0111] By adjusting the length of the single-mode optical fiber in the interferometer module, the optical path difference between the measurement arm and the reference arm is changed, thereby introducing a time delay. This time delay will cause light of different wavelengths to interfere in space and time, and the interference signal is obtained.

[0112] Further, the dispersion transmission module is specifically used to amplify the interference signal through an erbium-doped fiber amplifier; the amplified signal is transmitted through a single-mode optical fiber, and through the dispersion effect, different wavelengths of light generate different time delays when propagating in the optical fiber, forming a time-domain interference signal.

[0113] Further, the signal detection module is specifically used for a photodetector to obtain the time-domain interference signal, convert the time-domain interference signal into an electrical signal, and the electrical signal is captured by a high-speed oscilloscope OSC;

[0114] Extract the information in the time-domain interference pattern from the captured electrical signal;

[0115] Convert the information in the time-domain interference signal back to the frequency-domain interference signal through Fourier transform to obtain spectral information;

[0116] Calculate the height change of the object under test, i.e., the Z-axis distance, based on the spectral information.

[0117] Furthermore, the synchronization control module is specifically configured to control and record the movement trajectories of the X-axis and Y-axis through a motion control card, and at the same time trigger the storage of the Z-axis measurement data, so as to synchronously collect the measurement data in three dimensions to ensure the accuracy of surface topography measurement.

[0118] Furthermore, the dispersion transmission module is specifically configured to have a time-stretching scanning rate that is at least equal to the repetition frequency f of the optical frequency comb rep 。

[0119] Furthermore, the dispersion transmission module is specifically configured to obtain the measurement distance through the frequency-domain interference signal;

[0120] Among them, the dynamic measurement range The maximum time delay is

[0121] Therefore, the dynamic measurement range is where ΔF is the bandwidth of the photodetector, D is the dispersion parameter GVD of the dispersion element, ΔF is the bandwidth of the photodetector, and λ is the spectral width of the input pulse.

[0122] Furthermore, the maximum round-trip time of the detection pulse of the first beam relative to the second beam is less than 1 / f rep , where the round-trip time of the detection pulse of the first beam is the time to return to the interferometer after reflection from the surface of the object under test.

[0123] In addition, the present invention also provides a time-stretching Fourier transform optical frequency comb topography measurement method, which includes providing an incident beam for the interferometer by an optical frequency comb OFC;

[0124] The interferometer module obtains the interference signal of the object under test by adjusting the length of the single-mode fiber;

[0125] The dispersion transmission module time-stretches the interference signal through the single-mode fiber to generate a time-domain interference signal;

[0126] The signal detection module captures the stretched time-domain interference signal and calculates the topography parameters of the object under test through a time-frequency transformation algorithm;

[0127] The synchronization control module drives the two-dimensional displacement stage carrying the object to be measured through a motion control card, and triggers the synchronous acquisition and storage of the three-dimensional topography data of the object to be measured.

[0128] Further, the interferometer module includes a measurement arm and a reference arm. Among them, the measurement arm is sequentially connected to an erbium-doped fiber amplifier, a circulator CIR, and a collimator COL; the method includes:

[0129] The incident light beam enters the interferometer module after passing through an optical isolator ISO;

[0130] The incident light beam is split into two paths at the beam splitter of the interferometer module, one path enters the measurement arm, and the other path enters the reference arm;

[0131] The first light beam entering the measurement arm is amplified by an erbium-doped fiber amplifier, then sequentially passes through the circulator CIR and the collimator COL, and is incident on the object to be measured. The reflected light beam returns along the original path and enters the first port of the coupler OC2 of the interferometer module from the circulator;

[0132] The second light beam entering the reference arm is transmitted through a single-mode fiber SMF1 to the second port of the coupler OC2 of the interferometer, realizing the beam combination of the reference light beam and the measurement light beam;

[0133] By adjusting the length of the single-mode fiber in the interferometer module, the optical path difference between the measurement arm and the reference arm is changed, thereby introducing a time delay. This time delay will cause light of different wavelengths to interfere in space and time, and the interference signal is obtained.

[0134] Further, the dispersion transmission module amplifies the interference signal through an erbium-doped fiber amplifier; the amplified signal is transmitted through a single-mode fiber, and through the dispersion effect, different wavelengths of light generate different time delays when propagating in the fiber, forming a time-domain interference signal.

[0135] Further, the signal detection module obtains the time-domain interference signal through a photodetector, converts the time-domain interference signal into an electrical signal, and the electrical signal is captured by a high-speed oscilloscope OSC;

[0136] The information in the time-domain interference pattern is extracted through the captured electrical signal;

[0137] Through Fourier transform, the information in the time-domain interference signal is converted back into a frequency-domain interference signal to obtain spectral information;

[0138] According to the spectral information, the height change of the object to be measured, that is, the Z-axis distance, is calculated.

[0139] Further, the synchronization control module controls and records the movement trajectories of the X-axis and Y-axis through a motion control card, and simultaneously triggers the storage of the measurement data of the Z-axis, so as to enable the synchronous acquisition of the measurement data in three dimensions and ensure the accuracy of surface topography measurement.

[0140] Further, the scanning rate of the time stretching of the dispersion transmission module is at least equal to the repetition frequency f of the optical frequency comb rep 。

[0141] Further, the dispersion transmission module is specifically configured to obtain the measurement distance through the frequency-domain interference signal;

[0142] Among them, the dynamic measurement range The maximum time delay is

[0143] Therefore, the dynamic measurement range is Where ΔF is the bandwidth of the photodetector, D is the dispersion parameter GVD of the dispersion element, ΔF is the bandwidth of the photodetector, and λ is the spectral width of the input pulse.

[0144] Further, the maximum round-trip time of the detection pulse of the first beam relative to the second beam is less than 1 / f rep , where the round-trip time of the detection pulse of the first beam is the time when it returns to the interferometer after being reflected from the surface of the object to be measured.

[0145] A time-stretch Fourier transform optical frequency comb topography measurement system provided by the present application includes an optical frequency comb OFC, an interferometer module, a dispersion fiber module, a signal detection module, and a synchronization control module; the optical frequency comb OFC provides an incident beam for the interferometer; the interferometer module obtains the interference signal of the object to be measured by adjusting the length of the single-mode fiber; the dispersion transmission module performs time stretching on the interference signal through the single-mode fiber to generate a time-domain interference signal; the signal detection module captures the stretched time-domain interference signal and solves the topography parameters of the object to be measured through a time-frequency transformation algorithm; the synchronization control module drives a two-dimensional displacement stage carrying the object to be measured through a motion control card and triggers the synchronous acquisition and storage of the three-dimensional topography data of the object to be measured. The present application has a sufficiently high accuracy in topography measurement, and its detection speed can also meet the requirements of industrial real-time detection.

[0146] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the specific implementation manners of the present invention can still be modified or equivalently replaced, and any modification or equivalent replacement without departing from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A time-stretched Fourier transform optical frequency comb topography measurement system, characterized in that, The system includes an optical frequency comb (OFC), an interferometer module, a dispersion fiber module, a signal detection module, and a synchronization control module; The optical frequency comb (OFC) is used to provide an incident light beam for the interferometer; The interferometer module is used to obtain the interference signal of the object to be measured by adjusting the length of the single-mode fiber; The dispersion transmission module is used to perform time stretching on the interference signal through a single-mode fiber to generate a time-domain interference signal; The signal detection module is used to capture the stretched time-domain interference signal and calculate the morphological parameters of the object to be measured through a time-frequency transformation algorithm; The synchronization control module is used to drive a two-dimensional displacement stage carrying the object to be measured through a motion control card and trigger the synchronous acquisition and storage of the three-dimensional morphological data of the object to be measured.

2. The system according to claim 1, wherein The interferometer module includes a measurement arm and a reference arm. Among them, the measurement arm is sequentially connected to an erbium-doped fiber amplifier, a circulator (CIR), and a collimator (COL); The incident light beam enters the interferometer module after passing through an optical isolator (ISO); The incident light beam is split into two paths at the beam splitter of the interferometer module. One path enters the measurement arm, and the other path enters the reference arm; The first light beam entering the measurement arm is amplified by an erbium-doped fiber amplifier, then sequentially passes through the circulator (CIR) and the collimator (COL), and is incident on the object to be measured. The reflected light beam returns along the original path and enters the first port of the coupler (OC2) of the interferometer module from the circulator; The second light beam entering the reference arm is transmitted through a single-mode fiber (SMF1) to the second port of the coupler (OC2) of the interferometer to realize the beam combination of the reference light beam and the measurement light beam; By adjusting the length of the single-mode fiber in the interferometer module, the optical path difference between the measurement arm and the reference arm is changed, thereby introducing a time delay. This time delay will cause light of different wavelengths to interfere in space and time, and the interference signal is obtained.

3. The system according to claim 1, wherein, The dispersion transmission module is specifically used to amplify the interference signal through an erbium-doped fiber amplifier; transmit the amplified signal through a single-mode fiber, and through the dispersion effect, different wavelengths of light generate different time delays when propagating in the fiber, forming a time-domain interference signal.

4. The system according to claim 1, characterized in that, The signal detection module is specifically used to obtain the time-domain interference signal by a photodetector, convert the time-domain interference signal into an electrical signal, and capture the electrical signal by a high-speed oscilloscope (OSC); Extract the information in the time-domain interference pattern through the captured electrical signal; Convert the information in the time-domain interference signal back to a frequency-domain interference signal through Fourier transform to obtain spectral information; Calculate the height change of the object to be measured, that is, the Z-axis distance, according to the spectral information.

5. The system according to claim 1, wherein, The synchronization control module is specifically used to control and record the movement trajectories of the X-axis and Y-axis through a motion control card, and at the same time trigger the storage of the Z-axis measurement data, so as to perform synchronous acquisition of the measurement data in three dimensions to ensure the accuracy of the surface morphology measurement.

6. The system according to claim 4, wherein The dispersion transmission module is specifically configured such that the scanning rate for time stretching is at least equal to the repetition frequency f_rep of the optical frequency comb.

7. The system according to claim 6, wherein The dispersion transmission module is specifically configured to obtain the measured distance through the frequency-domain interference signal; wherein, the dynamic measurement range Z = (cτ_max) / 2, and the maximum time delay is τ_max = (DLλ^2ΔF) / c; Therefore, the dynamic measurement range is Z = (DLλ^2ΔF) / 2, where ΔF is the bandwidth of the photodetector, D is the GVD of the dispersion element, ΔF is the bandwidth of the photodetector, and λ is the spectral width of the input pulse.

8. The system according to claim 2, characterized in that, The maximum round-trip time of the detection pulse of the first beam relative to the second beam is less than 1 / f_rep, that is, T_period is the single-pulse period, where the round-trip time of the detection pulse of the first beam is the time after reflection from the surface of the object to be measured and then returned to the interferometer.

9. A method for measuring the morphology of a time-stretched Fourier transform optical frequency comb, characterized in that, including The optical frequency comb OFC provides the incident beam for the interferometer; The interferometer module obtains the interference signal of the object to be measured by adjusting the length of the single-mode optical fiber; The dispersion transmission module performs time stretching on the interference signal through the single-mode optical fiber to generate a time-domain interference signal; The signal detection module captures the stretched time-domain interference signal and calculates the shape parameters of the object to be measured through the time-frequency transformation algorithm; The synchronization control module drives the two-dimensional displacement stage carrying the object to be measured through a motion control card and triggers the synchronous acquisition and storage of the three-dimensional shape data of the object to be measured.

10. The method according to claim 9, characterized in that, The interferometer module includes a measurement arm and a reference arm. Among them, the measurement arm is sequentially connected to an erbium-doped fiber amplifier, a circulator CIR, and a collimator COL; the method includes: The incident beam enters the interferometer module after passing through the optical isolator ISO; The incident beam is split into two paths at the beam splitter of the interferometer module, one path enters the measurement arm, and the other path enters the reference arm; The first beam entering the measurement arm is amplified by the erbium-doped fiber amplifier, then sequentially passes through the circulator CIR and the collimator COL, and is incident on the object to be measured. The reflected beam returns along the original path and enters the first port of the coupler OC2 of the interferometer module from the circulator; The second beam entering the reference arm is transmitted through the single-mode optical fiber SMF1 to the second port of the coupler OC2 of the interferometer to achieve the combination of the reference beam and the measurement beam; By adjusting the length of the single-mode optical fiber in the interferometer module, the optical path difference between the measurement arm and the reference arm is changed, thereby introducing a time delay. This time delay will cause the light of different wavelengths to interfere in space and time, and the interference signal is obtained.

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