Wavelength tuning phase-shifting interference method assisted by element absorption spectral line and resonant cavity

Abstract

The invention discloses a wavelength tuning phase-shifting interference method assisted by element absorption spectral lines and a resonant cavity. The wavelength tuning phase-shifting interference method comprises the following steps: establishing a system comprising a Fizeau interferometer, an external cavity tunable laser, an element absorption cell and the resonant cavity; phase-shifting interference measurement is carried out by using a sweep-frequency light source in cooperation with a Fizeau interferometer, and the change of an interference image is recorded by a high-speed CMOS camera; recording the spectral line of the light source scanning through the rubidium absorption spectral line and the resonant cavity; and calibrating the relationship between the scanning time and the scanning frequency according to the measurement spectral line, determining the phase change quantity according to the wavelength, selecting an interferogram corresponding to the phase change quantity according to the requirement of a phase shift algorithm, and calculating to realize high-precision detection of the surface type. According to the method, the phase shift is determined through high-precision wavelength determination, and the detection precision is improved. Meanwhile, due to high-precision wavelength resolution, the method can be applied to wavelength tuning phase-shifting interference with an ultra-long detection distance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical high-precision surface profile detection, and particularly relates to a wavelength tuning phase-shifting interference method assisted by element absorption spectrum and resonant cavity. BACKGROUND

[0002] Phase-shifting interference is a method for measuring the phase difference between two beams of light by using phase-shifting technology, which can be used for surface profile detection, and further realize high-precision surface profile detection of optical devices such as mirrors and gratings. The phase-shifting interferometer has the advantages of high precision, non-contact, high sensitivity, wide measurement range, high repeatability and the like in surface profile detection.

[0003] There are various ways to generate phase shifts in phase-shifting interference technology, such as moving the measured object by using piezoelectric ceramics, rotating the polarizer, changing the wavelength of the laser, etc. The method of changing the wavelength to achieve phase shift is called wavelength tuning phase-shifting interference. In the process of wavelength tuning phase shift, all parts of the interferometer do not need to move. Compared with the piezoelectric ceramic phase shift method, the wavelength phase shift technology reduces the vibration error caused by pushing the reference mirror and the influence of air turbulence, and has the potential to realize higher precision detection.

[0004] However, there is still a key problem in using wavelength tuning phase-shifting interferometer for high-precision detection. The detection precision of phase-shifting interference technology mainly depends on the accuracy of obtaining the phase shift amount, and the currently used wavelength tuning light sources such as external cavity lasers and tunable laser diodes all face the problem of being unable to accurately determine the wavelength and thus being unable to accurately determine the phase shift amount. SUMMARY

[0005] The present application relates to the field of optical high-precision surface profile detection, and particularly relates to a wavelength tuning phase-shifting interference method assisted by element absorption spectrum and resonant cavity.

[0006] Technical scheme: In order to achieve the purpose of the present application, the technical scheme adopted by the present application is as follows: a wavelength tuning phase-shifting interference method assisted by element absorption spectrum and resonant cavity, comprising the following steps:

[0007] S1, collecting the interference pattern, element absorption spectrum and resonant cavity transmission spectrum at each time in the laser frequency scanning process, and recording the corresponding scanning time;

[0008] S2, calibrating the frequency at each time in the frequency scanning according to the element absorption spectrum and the resonant cavity transmission spectrum, and calculating the phase change amount;

[0009] S3, setting the phase change amount at a certain time as the reference 0 point, selecting the corresponding phase shift amount interference pattern required by the phase-shifting algorithm, and calculating to obtain the surface profile distribution.

[0010] Furthermore, S2 includes:

[0011] S2.1 Identify element absorption peaks based on element absorption spectral lines, determine the scanning time corresponding to the peak value, and match the element absorption peak frequency value with the time.

[0012] S2.2 Find the resonant cavity transmission peak closest to the element absorption peak based on the resonant cavity transmission spectrum, and obtain its frequency by performing local polynomial (third degree or lower) fitting; obtain the frequencies of all transmission peaks based on the assumption that the resonant cavity transmission peaks are evenly spaced.

[0013] S2.3. Based on the transmission peak frequencies of all resonant cavities and their corresponding time relationships, spline fitting is used to obtain the relationship between laser frequency and time.

[0014] Furthermore, in S2, a certain moment is selected as the position of 0 phase change, and the phase change at different moments is calculated according to formula (1);

[0015]

[0016] in Where h is the phase change, c is the detection distance, f0 is the initial frequency value, and f is the speed of light. t Let t be the frequency value at time t.

[0017] Furthermore, S3 uses a four-step phase-shifting method to calculate the phase shift distribution, as follows:

[0018] Interferograms were selected at phase shifts of 0, π / 2, π, and 3π / 2, respectively. The interferogram intensities at these times were I0, I1, I2, and I3, respectively. The phase shift distribution was calculated using the following formula:

[0019]

[0020] Where Φ is the phase difference between the reference plane and the plane to be measured.

[0021] The present invention also provides a wavelength-tuned phase-shifting interferometry system using element absorption spectral lines and a resonant cavity, including a light source, an interferometer, an element absorption cell, a resonant cavity, and a data acquisition card;

[0022] The optical path connection of the system is as follows: the light source is a wavelength-tunable laser. After the laser is emitted, it is first split into three paths by an optical fiber splitter, which then pass through an interferometer, an element absorption cell, and a resonant cavity, and is received by a photodetector.

[0023] The circuit connections in the system are as follows: the laser controller is connected to the terminal; the element absorption cell is connected to the element absorption cell controller, the element absorption cell controller and the resonant cavity are respectively connected to the data acquisition card through photodetectors, and the data acquisition card is connected to the terminal; the camera in the interferometer is connected to the terminal.

[0024] Preferably, the interferometer is a Fizeau-type interferometer. The resonant cavity is a 100MHz free spectral range Fabry-Perot cavity.

[0025] Beneficial effects: Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0026] This invention provides a wavelength-tuned phase-shifting interferometry method using elemental absorption spectral lines and a resonant cavity. It employs elemental spectral lines and the transmission spectral lines of a resonant cavity with a free spectral range of hundreds of MHz to perform high-precision calibration of the swept-frequency laser wavelength. Phase-shifting interferometry measurements are then performed on the corresponding interferogram selected according to the determined wavelength. This method can determine the phase shift corresponding to different interferograms with high precision without the need for algorithms, enabling high-precision detection.

[0027] This invention can obtain accurate phase shifts at various times and allows for flexible selection of phase-shifting interferometry algorithms (various fixed-step and equal-step algorithms). By using elemental absorption spectral lines and resonant cavity calibration of the swept-frequency laser frequency change, the wavelength and corresponding phase change at each time point are obtained with high precision, achieving high-precision detection. This method improves detection accuracy by determining the phase shift through high-precision wavelength determination. Furthermore, due to its high wavelength resolution, this method can be applied to wavelength-tuned phase-shifting interferometry with ultra-long detection distances. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a wavelength-tuned phase-shifting interference system assisted by element absorption spectral lines.

[0029] Figure 2 It is a flowchart of the method.

[0030] Explanation of reference numerals in the attached figures: 1 terminal, 2 laser controller, 3 external cavity laser head, 4 fiber optic beam splitter, 5 element absorption cell, 6 element absorption cell controller, 7 resonant cavity, 8 photodetector, 9 Fizeau interferometer, 10 collimating lens, 11 beam splitter, 12 reference mirror, 13 surface under test, 14 lens and camera, 15 data acquisition card. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0032] The present invention describes a wavelength-tuned phase-shifting interference system using elemental absorption spectral lines and a resonant cavity for assistance, such as... Figure 1As shown, the system includes an external cavity laser head 3 (light source), a laser controller 2, a fiber optic beam splitter 4, an element absorption cell 5, an element absorption cell controller 6, a resonant cavity 7, a photodetector 8, a data acquisition card 15, a lens and camera 14 (surface detector), a reference mirror 12, a beam splitter 11, and a collimating lens 10. The light source in the system is an external cavity laser. After laser emission, it is split into three paths by the fiber optic beam splitter 4, passing through the element absorption cell 5, the resonant cavity 7, and the Fizeau interferometer 9, respectively, and detected by the corresponding photodetectors. The data is then transmitted to terminal 1 via the data acquisition card 15. In the Fizeau interferometer 9, the fiber-emitting light first passes through the collimating lens 10 to become parallel light of a certain size, then passes through the reference mirror 12 and the surface under test 13 and is reflected back. The reflected light passes through the beam splitter 11 and is recorded by the lens and high-speed CMOS camera at 150 frames per second, and the results are transmitted back to terminal 1.

[0033] The signal transmission relationship of the system is as follows: the terminal transmits the control signal to the laser; the laser is divided into three paths, passing through the interferometer, the element absorption cell, and the resonant cavity respectively; the photodetector in the element absorption cell assembly, the photodetector after the resonant cavity, and the camera in the interferometer respectively transmit the detection results to the terminal.

[0034] In this embodiment, the system is used to detect the surface shape of the surface under test. The system uses a rubidium absorption cell, a fiber Fabry-Perot resonator with a free spectral range of 100 MHz, and a Fizeau interferometer. The photodetector integrated into the rubidium absorption cell, the photodetector after the resonator, and the camera ensure that the system can acquire the rubidium D2 absorption spectrum, the Fabry-Perot cavity transmission peak, and the interferogram at different times during a single laser frequency scan. The free spectral range of the resonator in the system is 100 MHz and below, and the sufficiently dense peaks can effectively fit the frequency sweep nonlinearity of the external cavity tunable laser.

[0035] When the system starts working, the laser scans at a non-mode-hopping frequency within the 100 GHz range. A fiber optic splitter divides the laser into three paths, which pass through a rubidium absorption cell, a resonant cavity, and a Fizeau interferometer, respectively. The rubidium absorption cell module has a built-in photodetector; the detection result is transmitted to the data acquisition card via the controller. The light after passing through the resonant cavity is detected by the photodetector and transmitted to the data acquisition card. The data acquisition card then transmits these two detection data to the terminal.

[0036] In a Fizeau interferometer, the light emitted from the optical fiber first passes through a collimating mirror to become parallel light of a certain size, then passes through a reference mirror and the surface under test and is reflected back. The reflected light passes through a beam splitter and is recorded by a lens and a high-speed CMOS camera at 150 frames per second, and the results are transmitted back to the terminal.

[0037] After obtaining the above detection results, as Figure 2As shown, the absolute values ​​of the laser wavelength at these moments are obtained based on the time corresponding to the rubidium absorption peak. The frequency value of the nearest Fabry-Perot transmission peak is obtained from these absolute frequency values. Based on the assumption that the free spectral range of the Fabry-Perot cavity is uniform within the swept frequency range, the frequency values ​​of the transmission peaks of all resonant cavities are obtained. The relationship between the laser frequency and time is then fitted using these values.

[0038] The phase change at different times is calculated based on the laser frequency and actual detection distance. A random time is selected as the reference time, with a phase change of 0. The phase change at each of these times is then calculated. For example, in a four-step phase shift method, interferograms with phase changes of 0, π / 2, π, and 3π / 2 are selected. The phase difference between the test surface and the reference standard surface is calculated using the phase shift method. Since the surface shape of the standard plane is known, the surface shape distribution can be calculated from this phase difference.

[0039] This invention provides a wavelength-tuned phase-shifting interference method using elemental absorption spectral lines and a resonant cavity, comprising the following steps:

[0040] S1. The interferogram, element absorption spectrum, and resonant cavity transmission spectrum are collected at each moment during a single laser frequency scan using the camera, the photodetector built into the element absorption cell, and the photodetector behind the resonant cavity.

[0041] S1.1 First, the laser frequency changes with time, and the absorption spectrum of the element is detected over time; the element absorption peak is identified based on the element absorption spectrum, the scanning time corresponding to the peak value is determined, and the absorption peak frequency value and time are matched one by one.

[0042] S1.2 Find the resonant cavity transmission peak closest to the element absorption peak, and use the frequency and time correspondence of the element absorption peaks near these two peaks to perform linear fitting to obtain the local laser frequency change relationship with time. Based on this relationship, obtain the laser frequency at the scanning time corresponding to the transmission peak of the selected resonant cavity. Based on the assumption that the frequencies corresponding to the transmission peaks of these two resonant cavities and the frequency interval of the resonant cavity peaks are equal, obtain the laser frequency at the time corresponding to all transmission peaks.

[0043] S1.3. Based on the frequency at the transmission peak of all resonant cavities and the corresponding time relationship, spline fitting is used to obtain the relationship between the laser frequency and time during this scan.

[0044] Since the laser frequency does not change linearly with time, it is obtained by fitting the elemental spectral lines (which provide known frequency values) and the dense spectral lines of the resonant cavity (which extend the known frequency relationship to the entire scanning range).

[0045] S2. Calculate the phase change based on the scanning laser frequency obtained from the fitting.

[0046] Select a certain moment as the position of 0 phase change, and calculate the phase change at different moments according to formula (1);

[0047]

[0048] in The phase change is given by h, the distance between the reference plane and the plane under test (detection distance), c, the speed of light, f0, and the initial frequency value. t Let t be the frequency value at time t.

[0049] S3. Randomly select a certain moment as the phase change amount 0. According to formula (1), the scanning time corresponding to the required phase change amount can be determined. Select the interferograms collected at these corresponding moments and calculate the surface shape distribution of the surface to be tested according to the algorithm.

[0050] In this embodiment, a four-step phase shift is used. Interference patterns are selected at phase shift amounts of 0, π / 2, π, and 3π / 2, respectively. The interference pattern intensities at these times are I0, I1, I2, and I3, respectively. The phase shift distribution can be calculated using the following formula:

[0051]

[0052] Where Φ is the phase difference between the reference plane and the plane to be measured. Other phase shift algorithms are also applicable to this method; it is only necessary to select the interferogram at the moment of phase shift required by the algorithm.

Claims

1. A wavelength-tuned phase-shifting interferometry method using an elemental absorption line and a resonant cavity assistance, characterized by, It comprises the following steps: S1, collecting the interference pattern, element absorption spectrum line and resonant cavity transmission spectrum line at each time in the laser frequency scanning process, and recording the corresponding scanning time; S2, calibrating the frequency at each time in the frequency scanning in S1 according to the element absorption spectrum line and the resonant cavity transmission spectrum line, and calculating the phase change; S3, setting a certain time as the reference 0 point of the phase change, selecting the corresponding phase shift interference pattern required by the phase shift algorithm, and calculating to obtain the surface profile distribution.

2. A wavelength-tuned phase-shifting interferometry method using an element absorption line and a resonant cavity assistance according to claim 1, wherein S2 It comprises: S2.1, identifying the element absorption peak according to the element absorption spectrum line, determining the scanning time corresponding to the peak value, and corresponding the element absorption peak frequency value and time; S2.2, finding the resonant cavity transmission peak value closest to the element absorption peak according to the resonant cavity transmission spectrum line, and obtaining its frequency by local polynomial fitting; according to the equal interval assumption of resonant cavity transmission peak value, obtaining all transmission peak frequencies; S2.3, according to the relationship between all resonant cavity transmission peak frequencies and corresponding time, using spline fitting to obtain the relationship between laser frequency and time.

3. The wavelength-tunable phase-shifting interferometry method using an element absorption spectral line and a resonant cavity auxiliary according to claim 1, characterized in that, In S2, a certain time is selected as the 0 position of the phase change, and the phase change at different times is calculated according to formula (1); wherein is the phase change, h is the probe distance, c is the speed of light, f0is the starting point frequency value, f t is the frequency value at time t.

4. A wavelength-tunable phase-shifting interferometry method using an element absorption spectral line and a resonant cavity auxiliary according to any one of claims 1-3, characterized in that, S3 adopts four-step phase shift method to calculate the phase shift distribution, which is as follows: Select the interference pattern when the phase shift is 0, π / 2, π and 3π / 2 respectively, and the interference pattern intensity at these times is I0, I1, I2 and I3 respectively. The phase shift distribution is calculated by the following formula: Where Φ is the phase difference between the reference surface and the surface to be measured.

5. A wavelength-tunable phase-shifting interferometry system using an elemental absorption line and a cavity-assisted wavelength tuning, characterized in that, It comprises a light source, an interferometer, an element absorption cell, a resonant cavity and a data acquisition card; The optical path connection relationship of the system is that the light source is a wavelength tunable laser, and after the laser is emitted, it is first divided into three paths by a fiber optic splitter, and then passes through the interferometer, the element absorption cell and the resonant cavity respectively and is received by a photodetector; The circuit connection relationship in the system is that the laser controller is connected to the terminal; the element absorption cell is connected to the element absorption cell controller, and the element absorption cell controller and the resonant cavity are connected to the data acquisition card through the photodetector, and the data acquisition card is connected to the terminal; the camera in the interferometer is connected to the terminal.

6. A wavelength-tunable phase-shifting interferometer system using an element absorption line and a resonant cavity according to claim 5, wherein, The interferometer adopts Fizeau interferometer.

7. A wavelength-tunable phase-shifting interferometer system using an elemental absorption spectral line and a resonant cavity according to claim 5 or 6, characterized in that, The resonant cavity adopts a hundred MHz free spectral range Fabry-Perot cavity.

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