Spectral confocal displacement measurement system and measurement method based on heterodyne interference

Abstract

The present invention relates to the field of optical detection, and in particular to a spectral confocal displacement measurement system based on heterodyne interference and a measurement method thereof, wherein a tunable laser emits monochromatic light of different wavelengths, which is divided into two beams of illumination light by a first beam splitter, one beam of illumination light is incident on a beam combiner after being modulated by a first acousto-optic frequency shifter, and the other beam of illumination light is incident on a beam combiner after being modulated by a second acousto-optic frequency shifter, and the two beams of illumination light after being modulated are combined by the beam combiner and incident on a sample through transmission of a second beam splitter and focusing of an objective lens to generate heterodyne interference, forming two light spots with overlapping areas on the sample; the sample reflects the incident light beam, and the reflected light beam is then sequentially transmitted by an objective lens, reflected by a second beam splitter, focused by a third focusing lens, and suppressed by stray light of a third pinhole, and received by a photodetector, and converted into an electrical signal and transmitted to a processor. The present invention can realize the combination of heterodyne interference technology and spectral confocal microscopy technology, and improve the measurement resolution.

Description

Technical Field

[0001] The invention belongs to the technical field of optical detection, and in particular relates to a spectral confocal displacement measurement system based on heterodyne interference and a measurement method thereof. Background Art

[0002] Spectral confocal displacement sensors are widely used in the field of precision measurement, especially in industrial manufacturing, semiconductor processing, microelectronics, precision machining, biomedical imaging and automated detection. Its main applications include high-precision surface morphology detection, thickness measurement, material research, microstructure size measurement, and non-destructive testing of biological samples. In these fields, spectral confocal displacement sensors can provide nanometer-level resolution and achieve non-contact high-precision displacement measurement and surface detection. Since the spectral confocal sensor utilizes the characteristics of the confocal optical system, its measurement principle uses spectral signals to analyze the position information of the sample surface or internal structure, and the light beam can be focused to a specific depth of the sample to achieve high-precision three-dimensional imaging.

[0003] Since white light covers a wide band of wavelengths, the dispersion characteristics of the dispersive lens can be used to focus different wavelengths at different axial positions. Therefore, the measurement of traditional spectral confocal displacement sensors relies on white light sources.

[0004] Heterodyne technology can improve the sensitivity and accuracy of the measurement system by mixing the reference signal with the measurement signal. By introducing heterodyne technology, the measurement system can detect extremely small displacement changes, thereby improving the resolution of sample measurement.

[0005] Since the coherence length of white light is short (usually only a few microns to tens of microns), it seriously affects the detection accuracy and stability of heterodyne signals, and white light sources do not have a single frequency component. Therefore, it is difficult to combine white light with heterodyne signals for effective frequency measurement and demodulation, thus limiting the application of white light in heterodyne measurement. It can be seen that confocal sensors that rely on white light sources cannot use heterodyne technology to improve the resolution of sample measurements. Summary of the invention

[0006] In view of this, the present invention aims to provide a spectral confocal displacement measurement system and a measurement method thereof based on heterodyne interference, so as to solve the problem that the spectral confocal sensor based on white light source cannot be combined with heterodyne technology.

[0007] To achieve the above object, the technical solution created by the present invention is implemented as follows:

[0008] A spectral confocal displacement measurement system based on heterodyne interference comprises a tunable laser, a first beam splitter, a first acousto-optic frequency shifter, a displacement stage, a first plane reflector, a first focusing lens, a first pinhole, a first beam expanding lens, a second plane reflector, a second acousto-optic frequency shifter, a second focusing lens, a second pinhole, a second beam expanding lens, a beam combiner, a second beam splitter, an objective lens, a third focusing lens, a third pinhole, a photodetector and a processor; wherein the first beam splitter is arranged in the light emitting direction of the tunable laser, the first acousto-optic frequency shifter, the displacement stage, the first plane reflector, the first focusing lens, the first pinhole, the first beam expanding lens and the second plane reflector are arranged in sequence on the reflected illumination light path of the first beam splitter, the second acousto-optic frequency shifter, the second focusing lens, the second pinhole and the second beam expanding lens are arranged in sequence on the transmitted illumination light path of the first beam splitter, the first plane reflector is mounted on the displacement stage, the beam combiner is arranged in the direction of the merging of the reflected illumination light path and the transmitted illumination light path of the first beam splitter, and the second plane reflector is arranged in the direction of the merging of the reflected illumination light path and the transmitted illumination light path of the first beam splitter. The beam splitter is arranged in the beam combining direction of the beam combiner, the objective lens is arranged in the transmission light path of the second beam splitter, the third focusing lens is arranged in the reflection light path of the second beam splitter, the photodetector is arranged in the focusing direction of the third focusing lens, and the third pinhole is arranged in the incident direction of the photodetector and close to the photodetector; the tunable laser is used to emit monochromatic light of different wavelengths, the first beam splitter is used to divide the monochromatic light emitted by the tunable laser into transmitted illumination light and reflected illumination light, the first acousto-optic frequency shifter is used to modulate the frequency of the reflected illumination light, and the second acousto-optic frequency shifter is used to modulate the frequency of the transmitted illumination light so that the frequency difference between the reflected illumination light and the transmitted illumination light satisfies the heterodyne interference condition, the frequency-modulated reflected illumination light and the transmitted illumination light are combined at the beam combiner and focused onto the sample through the objective lens, the light beam reflected by the sample is focused onto the photodetector through the third focusing lens, and is transmitted to the processor after photoelectric conversion by the photodetector, and the processor is used to calculate the interference signal to obtain the axial focusing position of the sample.

[0009] Furthermore, the focal lengths of the first expanding lens and the second expanding lens are the same, the focal lengths of the first focusing lens and the second focusing lens are the same, the first pinhole is arranged before the front focus of the first expanding lens, and the second pinhole is arranged after the front focus of the second expanding lens.

[0010] Furthermore, the first focusing lens, the first pinhole, the first beam expander lens, and the second plane reflector are sequentially arranged in the reflection direction of the first plane reflector, and the beam combiner is arranged in the merging direction of the second beam expander lens and the second plane reflector.

[0011] Furthermore, the processor includes a light intensity calculation module, a normalization processing module, a Gaussian peak extraction module and a peak position acquisition module; wherein,

[0012] The light intensity calculation module is used to calculate the light intensity of the overlapping area in the interference image ;in,

[0013]

[0014] In the formula, represents the three-dimensional point spread function of the reflected illumination, represents the three-dimensional point spread function of the transmitted illumination, Represents the three-dimensional point spread function of the light reflected from the sample;

[0015] The normalization processing module is , and after normalizing the axial coordinates of the interference image, the axial signal intensity of the interference image is:

[0016] ;

[0017] In the formula, represents the normalized coordinate of the axial coordinate z; The normalized coordinates representing the defocus amount of the two light spots, the axial signal intensity includes three Gaussian peaks, two of which correspond to the two light spots respectively, and the other Gaussian peak corresponds to the overlapping area of ​​the two light spots;

[0018] The Gaussian peak extraction module fits the parameters of each Gaussian peak using the least squares method, constructs a separate Gaussian peak curve based on the parameters of each Gaussian peak, and then extracts three separate Gaussian peaks from the axial signal intensity. , each Gaussian peak It is expressed as:

[0019]

[0020] In the formula, represents the height of the Gaussian peak; represents the position of the Gaussian peak; represents the width of the Gaussian peak, Indicates the lateral position coordinate of the sample;

[0021] The peak position acquisition module is used to obtain the peak position of the Gaussian peak corresponding to the overlapping area of ​​the two light spots. The x-axis coordinate corresponding to the peak position is the axial focusing position of the sample.

[0022] Furthermore, both the first acousto-optic frequency shifter and the second acousto-optic frequency shifter are Bragg diffraction crystals.

[0023] Furthermore, the translation stage is a piezoelectric ceramic translation stage.

[0024] A measurement method implemented by using the above-mentioned spectral confocal displacement measurement system based on heterodyne interference comprises the following steps:

[0025] S1: The tunable laser emits monochromatic light of different wavelengths according to the wavelength modulation function. The monochromatic light is divided into transmitted illumination light and reflected illumination light by the first beam splitter. The reflected illumination light is frequency modulated by the first acousto-optic frequency shifter, and then sequentially reflected by the first plane reflector, focused by the first focusing lens, stray light suppressed by the first pinhole, beam expanded by the first beam expander lens, and reflected by the second plane reflector before being incident on the beam combiner. The transmitted illumination light is frequency modulated by the second acousto-optic frequency shifter, and then sequentially focused by the second focusing lens, stray light suppressed by the second pinhole, and beam expanded by the second beam expander lens before being incident on the beam combiner. After the reflected illumination light and the transmitted illumination light are combined by the beam combiner, they are transmitted by the second beam splitter and focused by the objective lens before being incident on the sample to cause heterodyne interference, forming two light spots with overlapping areas on the sample. The sample reflects the incident light beam, and the reflected light beam is sequentially transmitted by the objective lens, reflected by the second beam splitter, focused by the third focusing lens, and stray light suppressed by the third pinhole before being received by the photodetector and converted into an electrical signal for transmission to the processor.

[0026] S2: The processor calculates the light intensity of the overlapping area in the interference image ;in,

[0027]

[0028] In the formula, represents the three-dimensional point spread function of the reflected illumination, represents the three-dimensional point spread function of the transmitted illumination, Represents the three-dimensional point spread function of the light reflected from the sample;

[0029] S3: Order , and after normalizing the axial coordinates of the interference image, the axial signal intensity of the interference image is:

[0030] ;

[0031] In the formula, represents the normalized coordinate of the axial coordinate z; The normalized coordinates representing the defocus amount of the two light spots, the axial signal intensity includes three Gaussian peaks, two of which correspond to the two light spots respectively, and the other Gaussian peak corresponds to the overlapping area of ​​the two light spots;

[0032] S4: Use the least squares method to fit the parameters of each Gaussian peak, construct a separate Gaussian peak curve based on the parameters of each Gaussian peak, and then extract three separate Gaussian peaks from the axial signal intensity , each Gaussian peak It is expressed as:

[0033]

[0034] In the formula, represents the height of the Gaussian peak; represents the position of the Gaussian peak; represents the width of the Gaussian peak, Indicates the lateral position coordinate of the sample;

[0035] S5: Obtain the peak position of the Gaussian peak corresponding to the overlapping area of ​​the two light spots. The x-axis coordinate corresponding to the peak position is the axial focusing position of the sample.

[0036] Furthermore, the wavelength modulation function is expressed as:

[0037]

[0038] In the formula, express The wavelength of the moment; and Respectively represent the minimum wavelength and maximum wavelength of the scanning range; Indicates the scan cycle.

[0039] Compared with the prior art, the invention can achieve the following beneficial effects:

[0040] The present invention uses a tunable laser to replace the white light source, and simulates the broadband characteristics of the white light spectrum by linearly modulating the single wavelength of the tunable laser. Since the monochromatic light output by the tunable laser has a long coherence length and a single frequency component, it can be combined with heterodyne technology, thereby realizing the organic combination of heterodyne interference technology and spectral confocal microscopy technology, while maintaining high-resolution imaging capabilities, significantly improving the measurement accuracy and sensitivity of the measurement system. In particular, through the precise control of frequency, the measurement system can achieve submicron or even nanometer accuracy in sample imaging and precision measurement, and is suitable for precise detection and analysis under high noise conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation on the present invention. In the drawings:

[0042] Figure 1 It is a structural schematic diagram of a spectral confocal displacement measurement system based on heterodyne interference according to an embodiment of the invention;

[0043] Figure 2 is a schematic diagram of the axial separation of two focused light spots according to an embodiment of the invention;

[0044] Figure 3It is a schematic diagram of the original curve and the separated Gaussian peak curve according to the embodiment of the invention.

[0045] Description of reference numerals:

[0046] 1. Tunable laser; 2. First beam splitter; 3. First acousto-optic frequency shifter; 4. Translation stage; 5. First plane reflector; 6. First focusing lens; 7. First pinhole; 8. First beam expanding lens; 9. Second acousto-optic frequency shifter; 10. Second focusing lens; 11. Second pinhole; 12. Second beam expanding lens; 13. Second plane reflector; 14. Beam combiner; 15. Second beam splitter; 16. Objective lens; 17. Sample; 18. Third focusing lens; 19. Third pinhole; 20. Photodetector. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solution and advantages of the invention more clear, the invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the invention and do not constitute a limitation of the invention.

[0048] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0049] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0050] In the description of the invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the invention can be understood according to specific circumstances.

[0051] In order to solve the technical problem that the white light spectral confocal sensor cannot be combined with heterodyne technology, the overall idea of ​​the technical solution adopted by the present invention is to use a tunable laser instead of a white light source. The tunable laser can dynamically adjust its output wavelength according to time and perform linear scanning within a specified wavelength range (for example, 400nm-700nm). By scanning the entire wavelength range, the broadband characteristics of the white light source can be simulated, while avoiding the problem of non-single frequency components of white light. The laser is more likely to achieve accurate frequency locking, and the monochromatic light output by the tunable laser has higher coherence than white light, which is suitable for combination with heterodyne technology. Therefore, the present invention provides a spectral confocal displacement measurement system and a measurement method thereof that combine a tunable laser with heterodyne interference technology.

[0052] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments.

[0053] like Figure 1As shown, the spectral confocal displacement measurement system based on heterodyne interference provided by the present invention comprises a tunable laser 1, a first beam splitter 2, a first acousto-optic frequency shifter 3, a translation stage 4, a first plane reflector 5, a first focusing lens 6, a first pinhole 7, a first beam expander lens 8, a second acousto-optic frequency shifter 9, a second focusing lens 10, a second pinhole 11, a second beam expander lens 12, a second plane reflector 13, a beam combiner 14, a second beam splitter 15, an objective lens 16, a third focusing lens 18, and a third pinhole 19. 9, a photodetector 20 and a processor (not shown in the figure), a first beam splitter 2 is arranged in the light emitting direction of the tunable laser 1, and the first beam splitter 2 divides the monochromatic light output by the tunable laser 1 into reflected illumination light and transmitted illumination light, the propagation path of the reflected illumination light is the reflected illumination light path, and the propagation path of the transmitted illumination light is the transmitted illumination light path, a first acousto-optic frequency shifter 3, a displacement stage 4, a first plane reflector 5, a first focusing lens 6, a first pinhole 7, a first beam expander lens 8, and a second plane reflector 13 are arranged in sequence The first plane reflector 5 is mounted on a displacement stage 4, and the position of the first plane reflector 5 is adjusted by adjusting the position of the first plane reflector 5. The displacement stage 4 can be a piezoelectric ceramic displacement stage to achieve nanometer-level precise position adjustment of the reflected illumination light. The second acousto-optic frequency shifter 9, the second focusing lens 10, the second pinhole 11, and the second beam expander 12 are sequentially arranged on the transmitted illumination light path. The beam combiner 14 is arranged on the converging direction of the reflected illumination light path and the transmitted illumination light path. The second beam splitter 15 is arranged on the beam combining direction of the beam combiner 14. The objective lens 16 is arranged on the transmitted light path of the second beam splitter 15. The third focusing lens 18 is arranged on the reflected light path of the second beam splitter 15. The photodetector 20 is arranged in the focusing direction of the third focusing lens 18. The third pinhole 19 is arranged in the incident direction of the photodetector 20 and close to the photodetector 20.

[0054] The tunable laser 1 emits monochromatic light of different wavelengths according to the wavelength modulation function. The monochromatic light is divided into transmitted illumination light and reflected illumination light by the first beam splitter 2. The reflected illumination light is frequency modulated by the first acousto-optic frequency shifter 3, and then sequentially reflected by the first plane reflector 5, focused by the first focusing lens 6, stray light suppressed by the first pinhole 7, beam expanded by the first beam expander 8, and reflected by the second plane reflector 13 to enter the beam combiner 14. The transmitted illumination light is frequency modulated by the second acousto-optic frequency shifter 9, and then sequentially focused by the second focusing lens 10, stray light suppressed by the second pinhole 11, and stray light suppressed by the second beam expander 8. The expanded beam of light 12 is incident on the beam combiner 14. The reflected illumination light and the transmitted illumination light are coaxial after being combined by the beam combiner 14, and the two beams of light are spatially overlapped. The two beams of light after spatial overlap are transmitted by the second beam splitter 15 and focused by the objective lens 16 and are incident on the sample 17 to cause heterodyne interference, forming two light spots with overlapping areas on the sample. The sample 17 reflects the incident light beam, and the reflected light beam is successively transmitted by the objective lens 16, reflected by the second beam splitter 15, focused by the third focusing lens 18, and the stray light suppressed by the third pinhole 19 is received by the photodetector 20 and converted into an electrical signal and transmitted to the processor.

[0055] The tunable laser 1 modulates and outputs monochromatic light of different wavelengths according to the wavelength modulation function, and the monochromatic light of different wavelengths will change linearly in a predetermined time sequence (for example, continuous scanning of 400nm-700nm), thereby achieving accurate measurement of the sample 17. The tunable laser 1 outputs specific monochromatic light in different time periods, and linearly modulates the wavelength modulation function to simulate broadband white light, with a frequency of hundreds of Hz or even kHz, thereby achieving continuous scanning from 400nm-700nm in a short time.

[0056] The wavelength modulation function is a linear function:

[0057]

[0058] In the formula, express The wavelength output by the laser 1 can be tuned at any time; Indicates the minimum wavelength of the scanning range, is the maximum wavelength of the scanning range; Indicates a complete scan cycle.

[0059] The first acousto-optic frequency shifter 3 and the second acousto-optic frequency shifter 9 modulate the frequency of the light beam through the internal acousto-optic diffraction effect. The first acousto-optic frequency shifter 3 and the second acousto-optic frequency shifter 9 both use Bragg diffraction crystals, and the frequency change of the light beam originates from the oscillation of the sound wave in the Bragg diffraction crystal, and the frequency shift is usually in the order of several megahertz. This frequency change is directly related to the oscillation frequency of the sound wave, thereby achieving precise modulation of the frequency of the light beam. In order to achieve a precise frequency difference between the transmitted illumination light and the reflected illumination light, the present invention uses two independent Bragg diffraction crystals so that the frequency difference between the transmitted illumination light and the reflected illumination light can be controlled at the order of several hertz to meet the requirements of heterodyne interference.

[0060] The first focusing lens 6 has the same focal length as the second focusing lens 10, the first beam expander lens 8 has the same focal length as the second beam expander lens 12, the first pinhole 7 is arranged at the rear focal position of the first focusing lens 6, and the position of the first beam expander lens 8 is adjusted so that the first pinhole 7 is located before the front focal point of the first beam expander lens 8, and the second pinhole 11 is arranged at the rear focal position of the second focusing lens 10, and the position of the second beam expander lens 12 is adjusted so that the second pinhole 11 is located after the front focal point of the second beam expander lens 12, that is, the distance between the first pinhole 7 and the first beam expander lens 8 is greater than the distance between the second pinhole 11 and the second beam expander lens 12.

[0061] The first focusing lens 6 focuses the reflected illumination light onto the first pinhole 7, and the second focusing lens 10 focuses the transmitted illumination light onto the second pinhole 11. The reflected illumination light at the first pinhole 7 can be regarded as a point light source, and the transmitted illumination light at the second pinhole 11 can be regarded as another point light source. By adjusting the positions of the first beam expander lens 8 and the second beam expander lens 12, one point light source is located at a position +∆z before the focus of the first beam expander lens 8, and the other point light source is located at a position -∆z after the focus of the second beam expander lens 12.

[0062] If the first beam expander lens 8 and the second beam expander lens 12 are regarded as the same beam expander lens, the first pinhole 7 is closer to the beam expander lens than the second pinhole 11. After passing through the beam expander lens, the reflected light will be more divergent than the parallel light, and the transmitted light will be more convergent than the parallel light.

[0063] Two point light sources are imaged at the near and far ends of the objective lens 16, and the two point light sources are focused at different depths of the sample 17. Under the wavelength of monochromatic light, two point light sources with different frequencies and at non-focal points are separated in the axial direction, forming two foci from the same light source but with different frequencies. When the Airy disks of the two foci are separated by a certain distance in the axial direction, there is a small overlapping area between the two Airy disks. Since the frequencies of the point light sources that generate the two foci are different, the two Airy disks in the overlapping area will undergo heterodyne interference, generating a beat frequency signal.

[0064] By adjusting the position of the first plane reflector 5 through the translation stage 4 , the overlapping area of ​​the two Airy disks can be moved on the surface of the sample 17 , thereby achieving axial scanning of different positions of the sample 17 .

[0065] Each beam of monochromatic light will form two Airy disks with different focusing depths on the sample 17. Through linear modulation of the wavelength modulation function, multiple groups of Airy disks with overlapping areas can be obtained.

[0066] like Figure 2 As shown, A and B are the areas of two Airy disks respectively, and C is the overlapping area of ​​the two Airy disks. The light energy of area C is higher than the light energy of areas A and B. The present invention only uses the light energy of area C as the imaging area for heterodyne interference imaging.

[0067] The processor includes a light intensity calculation module, a normalization processing module, a Gaussian peak extraction module and a peak position acquisition module; wherein,

[0068] The light intensity calculation module is used to calculate the light intensity of the overlapping area in the interference image ;in,

[0069]

[0070] In the formula, represents the three-dimensional point spread function of the reflected illumination, represents the three-dimensional point spread function of the transmitted illumination, Represents the three-dimensional point spread function of the light reflected from the sample;

[0071] The normalization processing module is , and after normalizing the axial coordinates of the interference image, the axial signal intensity of the interference image is:

[0072] .

[0073] In the formula, represents the normalized coordinate of the axial coordinate z; The normalized coordinates representing the defocus amount of the two light spots, the axial signal intensity includes three Gaussian peaks, two of which correspond to the two light spots respectively, and the other Gaussian peak corresponds to the overlapping area of ​​the two light spots;

[0074] The Gaussian peak extraction module fits the parameters of each Gaussian peak using the least squares method, constructs a separate Gaussian peak curve based on the parameters of each Gaussian peak, and then extracts three separate Gaussian peaks from the axial signal intensity. , each Gaussian peak It is expressed as:

[0075]

[0076] In the formula, represents the height of the Gaussian peak; represents the position of the Gaussian peak; represents the width of the Gaussian peak, Indicates the lateral position coordinate of the sample;

[0077] The peak position acquisition module is used to acquire the peak position of the Gaussian peak corresponding to the overlapping area of ​​the two light spots. The x-axis coordinate corresponding to the peak position is the axial focusing position of the sample 17.

[0078] The above content describes in detail the spectral confocal displacement measurement system based on heterodyne interference provided by the embodiment of the present invention. Corresponding to the measurement system, the embodiment of the present invention also provides a measurement method implemented by using the measurement system, which specifically includes the following steps:

[0079] S1: The tunable laser emits monochromatic light of different wavelengths according to the wavelength modulation function. The monochromatic light is divided into transmitted illumination light and reflected illumination light by the first beam splitter. The reflected illumination light is frequency modulated by the first acousto-optic frequency shifter, and then sequentially reflected by the first plane reflector, focused by the first focusing lens, stray light suppressed by the first pinhole, beam expanded by the first beam expander lens, and reflected by the second plane reflector to enter the beam combiner. The transmitted illumination light is frequency modulated by the second acousto-optic frequency shifter, and then sequentially focused by the second focusing lens, stray light suppressed by the second pinhole, and beam expanded by the second beam expander lens to enter the beam combiner. The reflected illumination light and the transmitted illumination light are combined by the beam combiner, and then transmitted by the second beam splitter and focused by the objective lens to enter the sample to cause heterodyne interference, forming two light spots with overlapping areas on the sample. The sample reflects the incident light beam, and the reflected light beam is sequentially transmitted by the objective lens, reflected by the second beam splitter, focused by the third focusing lens, and stray light suppressed by the third pinhole to be received by the photodetector, and converted into an electrical signal and transmitted to the processor.

[0080] S2: The processor calculates the light intensity of the overlapping area in the interference image ;in,

[0081]

[0082] In the formula, represents the three-dimensional point spread function of the reflected illumination, represents the three-dimensional point spread function of the transmitted illumination, Represents the three-dimensional point spread function of the light reflected from the sample.

[0083] The illumination light field incident on the sample is the sum of the light fields of the two point light sources. It can be expressed as:

[0084] and Represent two point light sources respectively. represents the coordinates of the focal points of the first focusing lens and the second focusing lens, is the Dirac function, Used to represent pulse light source, z represents the axial coordinate, and The focal positions of the first beam expander lens and the second beam expander lens are used to distinguish the two. , located behind the focus is , represents the position in object space, represents the three-dimensional point spread function of the illumination light field, Indicates the magnification. It represents the integral with the front focal points of the first focusing lens and the second focusing lens as the starting coordinates, i and j represent the imaginary units in the complex numbers, w1 represents the frequency of the light beam passing through the first focusing lens, the first pinhole and the first beam expanding lens, w2 represents the frequency of the light beam passing through the second focusing lens, the second pinhole and the second beam expanding lens, and k represents the wave number of the light.

[0085] According to the superposition principle of light waves, the three-dimensional intensity distribution of the imaging in the overlapping area of ​​two light waves in space It can be expressed as:

[0086]

[0087] In the formula, represents the position in image space, represents the frequency difference between two point light sources, Represents the light field between two point light sources reflected by the sample to the photodetector.

[0088] If the axial offset between the first pinhole and the second pinhole is infinitesimal, then according to and Derive the total light intensity in the interference overlap region , which can be expressed as:

[0089]

[0090] S3: Order , and after normalizing the axial coordinates of the interference image, the axial signal intensity of the interference image is:

[0091] .

[0092] In the formula, represents the normalized coordinate of the axial coordinate z; The normalized coordinates representing the defocus amounts of the two light spots.

[0093] Since the two point light sources are defocused during imaging, the axial signal intensity includes three Gaussian peaks, two of which correspond to two Airy disks, and the other Gaussian peak corresponds to the overlapping area of ​​the two Airy disks.

[0094] S4: Use the least squares method to fit the parameters of each Gaussian peak, construct a separate Gaussian peak curve based on the parameters of each Gaussian peak, and then extract three separate Gaussian peaks from the axial signal intensity , each Gaussian peak It is expressed as:

[0095]

[0096] In the formula, represents the height of the Gaussian peak; represents the position of the Gaussian peak; represents the width of the Gaussian peak, Represents the lateral position coordinate of the sample.

[0097] The original curve corresponding to the axial signal intensity consists of three Gaussian peaks, denoted as , its mathematical expression is:

[0098] .

[0099] in, and represents the Gaussian peaks of the two Airy disks (Gaussian fitting curves), The Gaussian peaks (Gaussian fitting curve) representing the overlapping area of ​​two Airy disks, each with different height and width, and with some overlap.

[0100] Since there are overlaps between the three Gaussian peaks, the first Gaussian peak ( Figure 3 The red dashed line in the middle), the second Gaussian peak ( Figure 3 The green dashed line in the middle) and the third Gaussian peak ( Figure 3 The superposition effect of the overlapping area between the two peaks (purple dashed line in the figure) will cause the peak base to be raised, making it difficult to directly identify each individual Gaussian peak. To this end, the present invention uses least squares fitting to estimate the parameters of each Gaussian peak, and the fitting function is:

[0101] = .

[0102] The parameters of each Gaussian peak are obtained by fitting the above fitting function, so that the specific information of each Gaussian peak can be extracted from the parameters of each Gaussian peak, including the peak position, peak height and peak width. This information is used to separate each Gaussian peak in the original curve.

[0103] In addition, since the overlapping effect of Gaussian peaks will cause padding, the data of the remaining peaks can be obtained by gradually subtracting the fitted peaks. This process can be carried out by the following steps: first, a separate curve of the first Gaussian peak is constructed using the fitting parameters; second, the first Gaussian peak is subtracted from the original curve to obtain the superimposed curve of the remaining two Gaussian peaks, and the above process is repeated until all Gaussian peaks are separated.

[0104] S5: Obtain the peak position of the Gaussian peak corresponding to the overlapping area of ​​the two light spots. The x-axis coordinate corresponding to the peak position is the axial focusing position of the sample.

[0105] Figure 3 middle, The vertex position of the corresponding Gaussian fitting curve is the peak position of the Gaussian peak, and the x-axis coordinate corresponding to the vertex position is the axial focusing position of the sample.

[0106] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0107] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A spectral confocal displacement measurement system based on heterodyne interferometry, characterized in that: The device comprises a tunable laser, a first beam splitter, a first acousto-optic frequency shifter, a translation stage, a first plane reflector, a first focusing lens, a first pinhole, a first beam expanding lens, a second plane reflector, a second acousto-optic frequency shifter, a second focusing lens, a second pinhole, a second beam expanding lens, a beam combiner, a second beam splitter, an objective lens, a third focusing lens, a third pinhole, a photodetector and a processor; wherein, The first beam splitter is arranged in the light emitting direction of the tunable laser, the first acousto-optic frequency shifter, the translation stage, the first plane reflector, the first focusing lens, the first pinhole, the first beam expanding lens, and the second plane reflector are arranged in sequence on the reflected illumination light path of the first beam splitter, the second acousto-optic frequency shifter, the second focusing lens, the second pinhole, and the second beam expanding lens are arranged in sequence on the transmitted illumination light path of the first beam splitter, the first plane reflector is mounted on the translation stage, the beam combiner is arranged in the merging direction of the reflected illumination light path and the transmitted illumination light path of the first beam splitter, the second beam splitter is arranged in the beam combining direction of the beam combiner, the objective lens is arranged on the transmitted light path of the second beam splitter, the third focusing lens is arranged on the reflected light path of the second beam splitter, the photodetector is arranged in the focusing direction of the third focusing lens, and the third pinhole is arranged in the incident direction of the photodetector and close to the photodetector; The tunable laser is used to emit monochromatic light of different wavelengths. The first beam splitter is used to split the monochromatic light emitted by the tunable laser into transmitted illumination light and reflected illumination light. The first acousto-optic frequency shifter is used to modulate the frequency of the reflected illumination light. The second acousto-optic frequency shifter is used to modulate the frequency of the transmitted illumination light so that the frequency difference between the reflected illumination light and the transmitted illumination light satisfies the heterodyne interference condition. The frequency-modulated reflected illumination light and the transmitted illumination light are combined at a beam combiner and focused onto a sample through an objective lens. The light beam reflected by the sample is focused onto a photodetector through a third focusing lens, and is transmitted to a processor after photoelectric conversion by the photodetector. The processor is used to calculate the interference signal to obtain the axial focusing position of the sample.

2. The spectral confocal displacement measurement system based on heterodyne interference according to claim 1, characterized in that: The first beam expander lens and the second beam expander lens have the same focal length, the first focusing lens and the second focusing lens have the same focal length, the first pinhole is arranged before the front focus of the first beam expander lens, and the second pinhole is arranged after the front focus of the second beam expander lens.

3. The spectral confocal displacement measurement system based on heterodyne interference according to claim 1, characterized in that: The first focusing lens, the first pinhole, the first beam expander lens, and the second plane reflector are sequentially arranged in the reflection direction of the first plane reflector, and the beam combiner is arranged in the merging direction of the second beam expander lens and the second plane reflector.

4. The spectral confocal displacement measurement system based on heterodyne interference according to claim 1, characterized in that: The processor includes a light intensity calculation module, a normalization processing module, a Gaussian peak extraction module and a peak position acquisition module; wherein, The light intensity calculation module is used to calculate the light intensity of the overlapping area in the interference image ;in, In the formula, represents the three-dimensional point spread function of the reflected illumination, represents the three-dimensional point spread function of the transmitted illumination, Represents the three-dimensional point spread function of the light reflected from the sample; The normalization processing module is , and after normalizing the axial coordinates of the interference image, the axial signal intensity of the interference image is: ； In the formula, represents the normalized coordinate of the axial coordinate z; The normalized coordinates representing the defocus amount of the two light spots, the axial signal intensity includes three Gaussian peaks, two of which correspond to the two light spots respectively, and the other Gaussian peak corresponds to the overlapping area of ​​the two light spots; The Gaussian peak extraction module fits the parameters of each Gaussian peak using the least squares method, constructs a separate Gaussian peak curve based on the parameters of each Gaussian peak, and then extracts three separate Gaussian peaks from the axial signal intensity. , each Gaussian peak It is expressed as: In the formula, represents the height of the Gaussian peak; represents the position of the Gaussian peak; represents the width of the Gaussian peak, Indicates the lateral position coordinate of the sample; The peak position acquisition module is used to obtain the peak position of the Gaussian peak corresponding to the overlapping area of ​​the two light spots. The x-axis coordinate corresponding to the peak position is the axial focusing position of the sample.

5. The spectral confocal displacement measurement system based on heterodyne interference according to claim 1, characterized in that: The first acousto-optic frequency shifter and the second acousto-optic frequency shifter are both Bragg diffraction crystals.

6. The spectral confocal displacement measurement system based on heterodyne interference according to claim 1, characterized in that: The translation stage is a piezoelectric ceramic translation stage.

7. A measurement method implemented by using the spectral confocal displacement measurement system based on heterodyne interference according to any one of claims 1 to 6, characterized in that: The steps include: S1: The tunable laser emits monochromatic light of different wavelengths according to the wavelength modulation function. The monochromatic light is divided into transmitted illumination light and reflected illumination light by the first beam splitter. The reflected illumination light is frequency modulated by the first acousto-optic frequency shifter, and then sequentially reflected by the first plane reflector, focused by the first focusing lens, stray light suppressed by the first pinhole, beam expanded by the first beam expander lens, and reflected by the second plane reflector before being incident on the beam combiner. The transmitted illumination light is frequency modulated by the second acousto-optic frequency shifter, and then sequentially focused by the second focusing lens, stray light suppressed by the second pinhole, and beam expanded by the second beam expander lens before being incident on the beam combiner. After the reflected illumination light and the transmitted illumination light are combined by the beam combiner, they are transmitted by the second beam splitter and focused by the objective lens before being incident on the sample to cause heterodyne interference, forming two light spots with overlapping areas on the sample. The sample reflects the incident light beam, and the reflected light beam is sequentially transmitted by the objective lens, reflected by the second beam splitter, focused by the third focusing lens, and stray light suppressed by the third pinhole before being received by the photodetector and converted into an electrical signal for transmission to the processor. S2: The processor calculates the light intensity of the overlapping area in the interference image ;in, In the formula, represents the three-dimensional point spread function of the reflected illumination, represents the three-dimensional point spread function of the transmitted illumination, Represents the three-dimensional point spread function of the light reflected from the sample; S3: Order , and after normalizing the axial coordinates of the interference image, the axial signal intensity of the interference image is: ； In the formula, represents the normalized coordinate of the axial coordinate z; The normalized coordinates representing the defocus amount of the two light spots, the axial signal intensity includes three Gaussian peaks, two of which correspond to the two light spots respectively, and the other Gaussian peak corresponds to the overlapping area of ​​the two light spots; S4: Use the least squares method to fit the parameters of each Gaussian peak, construct a separate Gaussian peak curve based on the parameters of each Gaussian peak, and then extract three separate Gaussian peaks from the axial signal intensity , each Gaussian peak It is expressed as: In the formula, represents the height of the Gaussian peak; represents the position of the Gaussian peak; represents the width of the Gaussian peak, Indicates the lateral position coordinate of the sample; S5: Obtain the peak position of the Gaussian peak corresponding to the overlapping area of ​​the two light spots. The x-axis coordinate corresponding to the peak position is the axial focusing position of the sample.

8. The measuring method according to claim 7, characterized in that: The wavelength modulation function is expressed as: In the formula, express The wavelength of the moment; and Respectively represent the minimum wavelength and maximum wavelength of the scanning range; Indicates the scan cycle.

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