CN117849073A - Method and device for measuring defects and thickness of light-transmitting medium sample based on OCT system - Google Patents
Method and device for measuring defects and thickness of light-transmitting medium sample based on OCT system Download PDFInfo
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Abstract
The invention discloses a light-transmitting medium defect and thickness measuring method and device based on an OCT system. The invention comprises the following steps: a weak coherent optical imaging method, which is used for acquiring OCT signals and OCT tomographic images of a light-transmitting medium sample in a two-dimensional/three-dimensional space; a boundary identification method applied to OCT tomographic image segmentation and defect detection is used for generating each boundary structure mask in an OCT tomographic image according to a sample design value, identifying each boundary of a sample in a mask range and positioning a defect; a method for measuring the optical thickness of a light-transmitting medium based on the light intensity information of OCT signals is used for calculating the optical thickness of a light-transmitting medium sample; the method is applied to OCT defect and thickness measurement, and is used for calculating the physical thickness and positioning defect positions. The invention combines OCT tomographic images and algorithms without adding mechanical leveling devices, and realizes nondestructive and high-precision measurement of defects and thickness of a light-transmitting medium sample.
Description
Technical Field
The invention belongs to a precise measurement method in the field of optical measurement, and particularly relates to a defect and thickness measurement method of a light-transmitting medium sample based on an OCT system.
Background
Precision measurement is a necessary condition for manufacturing high-end optical precision instruments. The defects and thickness measurement of the light-transmitting medium sample are part of the measurement of geometrical parameters of optical elements, and are widely applied to industrial production, such as quality detection of planar and curved glass, thickness measurement of glass plates, thickness measurement of aspherical lenses, thickness control during silicon wafer production and processing, and the like.
Based on the contact detection mode of traditional mechanical tools such as a screw micrometer and a vernier caliper, the surface of a sample to be detected can be damaged, the surface of the sample can only be detected, and the internal defect or thickness of the multilayer medium sample can not be measured. The precision detection technology based on the optical principle is a current mainstream detection mode due to the characteristic of non-contact detection, and mainly comprises transmission detection, reflection detection, interference detection, spectrum detection and other modes.
OCT technology combines the modes of interference detection and spectral detection, and its non-contact, depth-resolved nature has led to extensive research by students in the field of precision measurement. Imaging and analysis of the light-transmitting medium sample are carried out based on the OCT system, so that non-contact and high-precision defect and thickness measurement can be realized, and the method has important significance for geometric parameter measurement of an optical element.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method and a device for measuring the defects and the thicknesses of a transparent medium sample based on an OCT system, which are used for generating a structural mask based on a sample design value to remove multiple scattering to form a first boundary artifact, combining mask positioning defects and boundary positions, using the optical amplitude information of OCT signals as weight to realize the centroid positioning of the defects and the boundaries of the sample, calibrating the axial pixel resolution of the system corresponding to an intensity centroid method, realizing the nondestructive high-precision measurement of the defects and the optical thicknesses among the boundaries of the transparent medium sample, and finally reducing the physical thickness of the sample based on an OCT detection light model and the optical thicknesses of the sample, thereby realizing the nondestructive high-precision detection of the defects and the thicknesses of the transparent medium sample.
The invention is realized by the following technical scheme:
1. OCT system-based defect and thickness measurement method for light-transmitting medium sample
Comprising the following steps: a weak coherent optical imaging method, which is used for acquiring OCT signals and OCT tomographic images of a light-transmitting medium sample in a two-dimensional/three-dimensional space;
a boundary identification method applied to OCT tomographic image segmentation and defect detection is used for generating each boundary structure mask in an OCT tomographic image according to a sample design value, identifying each boundary of a sample in a mask range and positioning a defect;
a method for measuring the optical thickness of a light-transmitting medium based on the light intensity information of OCT signals is used for calculating the optical thickness between each boundary and at the defect of the light-transmitting medium sample according to the OCT signals and OCT tomographic images;
the method is applied to OCT defect and thickness measurement, realizes the method of restoring physical thickness by optical thickness and positions the defect position of the sample, is used for calculating the physical thickness of the light-transmitting medium sample based on the optical thickness between boundaries and at the defect position of the light-transmitting medium sample by utilizing an OCT light detection model, and realizes the defect and thickness measurement of the light-transmitting medium sample.
The weak coherent optical imaging method is used for acquiring OCT signals and OCT tomographic images of a light-transmitting medium sample in a two-dimensional/three-dimensional space, and comprises the following steps:
after carrying out M times of repeated scanning on a region to be detected of a light-transmitting medium sample by utilizing an OCT system, obtaining an OCT signal of the light-transmitting medium sample in a two-dimensional/three-dimensional space;
and carrying out Fourier transformation on the OCT signal in the depth direction to obtain an OCT tomographic image.
After the OCT system is used for repeatedly scanning the light-transmitting medium sample to-be-detected area for M times, the OCT signal of the light-transmitting medium sample in the two-dimensional/three-dimensional space is obtained by adopting one of the following methods:
a time domain OCT imaging method for changing the optical path of a reference arm;
spectral OCT imaging method based on spectrometer airspace light splitting;
swept OCT imaging method based on time domain spectroscopy of swept light source.
The boundary identification method applied to OCT tomographic image segmentation and defect detection comprises the following steps:
selecting an upper boundary structure mask in the OCT tomographic image, identifying the upper boundary position within the upper boundary structure mask range and positioning the defect in the upper boundary;
based on the sample design value, the OCT light detection model and the upper boundary position, generating a structural mask corresponding to the residual boundary, identifying the residual boundary of the sample in the mask range, and positioning the defect of the residual boundary.
Based on the sample design value, OCT light detection model and upper boundary position, generating a structural mask corresponding to the rest boundary, identifying the rest boundary of the sample in the mask range and positioning the defect of the rest boundary, comprising the following steps:
calculating the included angles between the upper boundary and the x-axis and the y-axis according to the upper boundary of the sample to obtain the OCT light incident angle of a mask for generating a residual boundary structure of the sample, calculating the refraction track of the light according to the OCT light incident angle, calculating the optical path value of the light from the zero optical path surface to each boundary of the sample based on the refraction track of the light, and mapping the optical path value to an OCT tomographic image to obtain the corresponding position S of the residual boundary design value in the OCT tomographic image m ′，S m ' performing up-down expansion of pixels to obtain a structural mask R corresponding to the residual boundary m Boundaries are identified within the mask and defect locations are located.
Calculating an included angle between the upper boundary and the x-axis and between the upper boundary and the y-axis according to the upper boundary of the sample, wherein the included angle comprises:
when the upper boundary of the sample is a plane, fitting the upper boundary plane based on least square to obtain an included angle between the upper boundary of the sample and the x-axis and the y-axis; and when the upper boundary of the sample is a curved surface, calculating an upper boundary point cloud based on the upper boundary and the theoretical pixel resolution of the OCT tomographic image, and registering the upper boundary point cloud with an upper boundary design value point cloud to obtain the included angle between the upper boundary of the sample and the x-axis and the y-axis.
The method for measuring the optical thickness of the light-transmitting medium based on the light intensity information of the OCT signal is used for calculating the optical thickness between each boundary and at the defect of the light-transmitting medium sample according to the OCT signal and the OCT tomographic image, and comprises the following steps:
positioning the centroid position of each boundary and the centroid position of the defect of the light-transmitting medium sample by using the light intensity information of the OCT signal as weight;
and respectively calculating the optical thickness of each boundary or defect of the light-transmitting medium sample according to the centroid position of each boundary or the centroid position of the defect of the light-transmitting medium sample and combining the axial pixel resolution of the OCT system.
The method for locating the centroid position of each boundary and the centroid position of the defect of the light transmission medium sample by using the light intensity information of the OCT signal as the weight comprises the following steps:
taking the maximum value pixel of each boundary or defect of the light-transmitting medium sample in the M OCT tomographic images as a center point of a target position, and fitting the center point of the target position in the M OCT tomographic images and N points corresponding to the center point in front and back by taking light intensity as a weight to obtain the centroid position of each boundary or defect of the light-transmitting medium sample, wherein the formula is as follows:
wherein px is c Centroid position at each boundary or defect of the light-transmitting medium sample; i is the index of pixel values at each boundary or defect of the transparent medium sample; px (px) j (k) For the kth acquisition, indexing the pixel value at position j; i j (k) The light intensity value at position j is indexed for the kth acquisition.
The calculating the optical thickness of each boundary or defect of the transparent medium sample according to the centroid position of each boundary or the centroid position of the defect of the transparent medium sample and combining the axial pixel resolution of the OCT system comprises the following steps:
firstly, moving a displacement table where a light-transmitting medium sample is located, positioning the upper boundary positions of the light-transmitting medium sample before and after movement based on an intensity centroid method, and calibrating to obtain the axial pixel resolution dz of an OCT system;
Δpx＝px′ c1 -px c1
dz＝y/Δpx
wherein px is c1 For the pixel value of the upper boundary of the transparent medium sample obtained by the positioning before the movement, px c1 ' is the pixel value of the upper boundary of the transparent medium sample obtained by positioning after movement, and Deltapx is the light transmittance obtained by positioningThe pixel value of boundary movement on the medium sample, y is the true movement value of the displacement table, and dz is the axial pixel resolution of the OCT system;
then, based on the centroid position of each boundary or defect of the light-transmitting medium sample and the axial pixel resolution dz of the OCT system, calculating the optical thickness of each boundary or defect of the light-transmitting medium sample, wherein the calculation formula is as follows:
d o ＝(px c2 -px c1 )·dz
wherein px is c2 For the centroid position of the upper boundary of the first layer of medium, px c2 Centroid position d for lower boundary of first layer medium o Is the optical thickness between the upper and lower boundaries of the first layer of medium in the light-transmitting medium sample.
The method for realizing optical thickness reduction physical thickness and positioning the defect position of a sample is applied to OCT defect and thickness measurement, and is used for calculating the physical thickness of a light-transmitting medium sample based on the optical thickness between boundaries and at the defect position of the light-transmitting medium sample by utilizing an OCT light detection model, so as to realize the defect and thickness measurement of the light-transmitting medium sample, and comprises the following steps:
calculating the normal vector of the upper boundary plane of the transparent medium sample and the incidence angle of OCT detection light on the upper boundary plane according to the upper boundary of the transparent medium sample;
calculating the transmission direction of light in the light-transmitting medium sample;
based on the OCT detection light principle, calculating and obtaining the physical thickness and defect position of the transparent medium sample by utilizing the normal vector of the upper boundary plane of the transparent medium sample, the incident angle of the OCT detection light on the upper boundary plane, the transmission direction of the light in the transparent medium sample and the optical thickness.
Based on the OCT detection light principle, the physical thickness of the transparent medium sample is obtained by calculating by using the normal vector of the upper boundary plane of the transparent medium sample, the incident angle of OCT detection light on the upper boundary plane, the transmission direction of the light in the transparent medium sample and the optical thickness, and the method comprises the following steps:
the physical thickness d of the light-transmitting medium sample was calculated as follows:
d＝cosθ 2 d o /n g
wherein d o For the optical thickness, θ, of each boundary or defect of a light-transmitting medium sample in OCT space 2 An included angle is formed between the direction of light rays in the transparent medium sample and the normal vector of the upper boundary plane of the transparent medium sample; n is n g To transmit the bulk refractive index of the media sample at the incident optical band.
The light-transmitting medium sample is a light-transmitting medium sample composed of single-layer medium or multi-layer medium.
2. Defect and thickness measuring device of light-transmitting medium sample based on OCT system
Comprising the following steps: the optical coherence tomography device is used for carrying out OCT signal detection and imaging on a region to be detected of the sample; and the one or more signal processors are used for analyzing and processing the OCT signals detected in the sample region to be detected, obtaining the boundary position of the sample region to be detected, measuring the optical path thickness by an intensity centroid method, and restoring the physical thickness to realize the thickness measurement of the light-transmitting medium sample.
The optical coherence tomography device adopts one of the following components: a low coherence broadband light source, a point detector; a low coherence broadband light source, an interferometer, and a spectrometer; or comprises a swept broadband light source and interferometer, a point detector.
Compared with the prior art, the invention has the following beneficial effects:
the detection precision of the traditional image edge detection method is influenced by the size of pixels, and is in the micron level, and the invention provides an intensity centroid positioning method based on an OCT system.
The traditional thickness detection method based on the spiral micrometer and the vernier caliper can damage a detected sample, and the thickness detection method based on the optical coherence tomography can realize nondestructive high-precision thickness measurement on the light-transmitting medium sample.
Some optical thickness measuring methods need to level samples based on mechanical structures, increase time and labor cost, and also have the defects of complex light path, complex measuring steps, low measuring speed and the like; in addition, a calculation method from optical thickness reduction to physical thickness is provided, a method for directly measuring thickness based on mechanical structure leveling is replaced, calculation is simple, speed is high, and measurement accuracy of hundred nanometers can be guaranteed. The invention has important engineering application value for measuring the defects and thickness of transparent media such as glass plates, lens groups and the like.
Drawings
FIG. 1 is a schematic illustration of the process of the present invention;
FIG. 2 is a schematic view of the apparatus of the present invention;
FIG. 3 is a schematic diagram of an apparatus according to an exemplary embodiment of the present invention;
FIG. 4 is a block diagram and a schematic structural mask diagram of a lens assembly imaged in accordance with the method of the present invention;
FIG. 5 is a schematic diagram of the principle of intensity centroid method positioning in the method of the present invention;
FIG. 6 is a schematic representation of the results of the method of the present invention for glass sheet thickness measurement;
FIG. 7 is a schematic diagram of OCT light detection model in the method of the present invention;
in the figure: 1. a weak coherent optical imaging method; 1-1, performing repeated scanning on a region to be detected of a light-transmitting medium sample for M times by utilizing an OCT system to obtain an OCT signal of the light-transmitting medium sample in a two-dimensional/three-dimensional space; 1-2, performing Fourier transform on the OCT signal in the depth direction to obtain an OCT tomographic image; 2. a boundary identification method applied to OCT tomographic image segmentation and defect detection; 2-1, framing an upper boundary structure mask in the OCT tomographic image, identifying the upper boundary position in the upper boundary structure mask range and positioning the defect in the upper boundary; 2-2, generating a structural mask corresponding to the rest boundary based on the sample design value, the OCT light detection model and the upper boundary position, identifying the rest boundary of the sample in the mask range, and positioning the defect of the rest boundary; 3. a method for measuring optical thickness of a light-transmitting medium based on light intensity information of OCT signals; 3-1, locating the mass center position of each boundary of the light-transmitting medium sample and the mass center position of the defect by using the light intensity information of the OCT signal as weight; 3-2, respectively calculating the optical thickness of each boundary or defect of the light-transmitting medium sample according to the centroid position of each boundary or the centroid position of the defect of the light-transmitting medium sample and combining the axial pixel resolution of the OCT system; 4. the method is applied to OCT defect and thickness measurement, realizes the reduction of the physical thickness by the optical thickness and positions the defect position of the sample; 4-1, calculating a normal vector of an upper boundary plane of the transparent medium sample and an incident angle of OCT detection light on the upper boundary plane according to the upper boundary of the transparent medium sample; 4-2, calculating the transmission direction of the light in the transparent medium sample; 4-3, calculating and obtaining the physical thickness and the defect position of the transparent medium sample by utilizing the normal vector of the upper boundary plane of the transparent medium sample, the incidence angle of the OCT detection light on the upper boundary plane, the transmission direction of the light in the transparent medium sample and the optical thickness based on the OCT detection light principle; 5. a light source; 6. an optical fiber coupler; 7. a first polarization controller; 8. a first reference arm collimator; 9. a focusing lens; 10: a first reference arm mirror; 11. a first sample arm collimator; 12. OCT scans the imaging device; 13. a sample; 14. a signal detection device; 15. a signal processing and calculating unit; 16. a swept light source with a center wavelength of 1310nm, a bandwidth of 100nm and a sweep rate of 20 kHz; 17. 20: an 80 fiber coupler; 18. a reference arm circulator; 19. a second reference arm collimator; 20. a double cemented lens; 21. a dispersion compensation mirror; 22. a second reference arm mirror; 23. a sample arm ring; 24. a second sample arm collimator; 25. scanning a vibrating mirror; 26. a 54mm focal length field lens; 27. a glass plate; 28. a second polarization controller; 29. 50: a 50 fiber circulator; 30. a balance detector; 31. and a signal processing module.
Detailed Description
The following detailed description of the invention is made in connection with the accompanying drawings, which form a part hereof. It is noted that these descriptions and examples are merely illustrative and are not to be construed as limiting the scope of the invention, which is defined by the appended claims, and any changes based on the claims are intended to be within the scope of the invention.
To facilitate an understanding of embodiments of the invention, the operations are described as multiple discrete operations, but the order of description does not represent the order in which the operations are performed.
The present description uses x-y-z three-dimensional coordinate representation based on spatial direction for the sample measurement space. This description is merely intended to facilitate the discussion and is not intended to restrict the application of embodiments of the present invention. Wherein: the depth z direction is the direction of the optical axis, the x direction is the OCT fast axis scanning direction, the y direction is the OCT slow axis scanning mode, and the x-y plane is the plane vertical to the optical axis.
The method of the invention is shown in fig. 1, and is a weak coherent optical imaging method 1 for high precision positioning, which is used for acquiring OCT signals and OCT tomographic images of a light-transmitting medium sample in a two-dimensional/three-dimensional space, and comprises the following steps:
OCT system light source central wavelength 1310nm, incidence light wave band 1260nm-1360nm, system imaging depth 11mm, balance detector bandwidth 500MHz, acquisition card sampling frequency 500MHz, OCT system is utilized to repeatedly scan the light transmission medium sample to-be-detected area for M times (M=25), then OCT signal 1-1 of the light transmission medium sample in two-dimensional/three-dimensional space is obtained;
after the OCT system is used for repeatedly scanning the light-transmitting medium sample to-be-detected area for M times, the OCT signal 1-1 of the light-transmitting medium sample in the two-dimensional/three-dimensional space is obtained by adopting one of the following methods:
a time domain OCT imaging method for changing the optical path of a reference arm;
spectral OCT imaging method based on spectrometer airspace light splitting;
swept OCT imaging method based on time domain spectroscopy of swept light source.
And carrying out Fourier transformation on the OCT signal in the depth direction to obtain OCT tomographic images 1-2. In this embodiment, a Hanning window function (a Gaussian window may be selected) is added to the OCT signal, short-time fourier transform is performed in the depth direction, so as to implement conversion from the wave number domain (k domain) to the depth domain (z domain), and the absolute value of the signal is taken, where the OCT signal strength is I (k, z):
and displaying the light intensity distribution of the zx section to obtain OCT tomographic images.
A boundary identification method 2 applied to OCT tomographic image segmentation and defect detection, specifically, boundary segmentation of an upper boundary and boundary segmentation of a remaining boundary, for generating boundary structure masks in OCT tomographic images according to sample design values, identifying each boundary of a sample within a mask range and locating defects, comprising:
selecting an upper boundary structure mask in the OCT tomographic image, identifying the upper boundary position within the upper boundary structure mask range, and positioning the defect 2-1 in the upper boundary;
the shape of the upper boundary structure mask can be manually generated according to the outline of the sample to-be-detected area, the upper boundary cuboid mask can be generated when the outline of the sample to-be-detected area is square, and the elliptical cylindrical mask can be generated when the sample to-be-detected area is circular or elliptical.
Based on the sample design value, OCT light detection model and upper boundary position, generating a structural mask corresponding to the rest boundary, identifying the rest boundary of the sample in the mask range and positioning defect 2-2 of the rest boundary. When the light-transmitting medium sample is flat glass, the sample design value is the thickness and the refractive index of the flat glass; when the light-transmitting medium sample is a lens module, the sample design value is aspheric parameters of each surface of the lens module, refractive index and medium thickness value between each surface.
And calculating the included angles between the upper boundary and the x-axis and the y-axis according to the upper boundary of the sample to obtain the OCT light incidence angle of the mask for generating the residual boundary structure of the sample. Specifically, when the upper boundary of the sample is a plane, fitting the upper boundary plane based on least squares to obtain an included angle between the upper boundary of the sample and the x-axis and the y-axis; and when the upper boundary of the sample is a curved surface, calculating an upper boundary point cloud based on the upper boundary and the theoretical pixel resolution of the OCT tomographic image, and registering the upper boundary point cloud with an upper boundary design value point cloud to obtain the included angle between the upper boundary of the sample and the x-axis and the y-axis.
Calculating the refraction track of the light according to the incidence angle of the OCT light, calculating the optical path value of the light from the zero optical path surface to each boundary of the sample based on the refraction track of the light, and mapping the optical path value to the OCT tomographic image to obtain the corresponding position S of the rest boundary design value in the OCT tomographic image m ′，S m ' performing up-down expansion of pixels, in this embodiment, 2 pixels, to obtain a structural mask R corresponding to the remaining boundary m Boundaries are identified within the mask and defect locations are located. Wherein, the mapping formula is as follows:
wherein A is 0 A is zero optical path surface starting point, A q Is A 0 Incident on the intersection of the q-th layer boundary, A m ' is A 0 Corresponding position in OCT tomogram of intersection point of mth layer boundary, n q-1 Refractive index of the medium of the q-1 layer, n 0 Is the refractive index of air.
When boundary and positioning defects are identified in OCT tomographic images combined with a mask, a corresponding defect positioning and boundary identification method can be selected according to the boundary gray level characteristics and the calculation complexity requirements of a sample based on local gray level maximum values, map searching based on a shortest path algorithm, local gradient characteristics based on edge detection operators (Sobel, prewitt, roberts, canny, marr-Hildreteh).
A method 3 for measuring optical thickness of a light transmitting medium based on light intensity information of OCT signals, for calculating optical thickness between boundaries and at defects of a light transmitting medium sample from OCT signals and OCT tomographic images, comprising:
taking the light intensity information of the OCT signal as weight, locating the centroid position of each boundary of the light-transmitting medium sample and the centroid position of the defect 3-1 by using the intensity centroid;
specifically:
taking the maximum value pixel of each boundary or defect of the light-transmitting medium sample in M OCT tomographic images (M=25) as a target position center point, and then taking light intensity as a weight to fit the target position center point and front and back N points (N=1, total (2N+1) M points) corresponding to the center point in the M OCT tomographic images to the intensity centroid positions, thereby obtaining the centroid positions of each boundary or defect of the light-transmitting medium sample, and realizing the sub-pixel positioning of the sample boundary, wherein the formula is as follows:
wherein px is c Calculating the position of the mass center of each boundary or defect of the light-transmitting medium sample, namely the position of the sub-pixel boundary of the intensity mass center; i is the index of pixel values at each boundary or defect of the transparent medium sample; px (px) j (k) For the kth acquisition, indexing the pixel value at position j; i j (k) The light intensity value at position j is indexed for the kth acquisition.
According to the centroid position of each boundary or the centroid position of the defect of the light-transmitting medium sample, respectively calculating the optical thickness 3-2 of each boundary or the defect of the light-transmitting medium sample by combining the axial pixel resolution of the OCT system, comprising:
firstly, moving a high-precision displacement table where a light-transmitting medium sample is located, positioning the upper boundary positions of the light-transmitting medium sample before and after moving based on an intensity centroid method, and calibrating to obtain the axial pixel resolution dz of an OCT system;
Δpx＝px′ c1 -px c1
dz＝y/Δpx
wherein px is c1 For the pixel value of the upper boundary of the light-transmitting medium sample obtained by using the intensity centroid method before moving, px c1 ' is the pixel value of the upper boundary of the transparent medium sample obtained by the positioning of the intensity centroid method after the movement, Δpx is the pixel value of the upper boundary movement of the transparent medium sample obtained by the positioning of the intensity centroid method, y is the true movement value of the high-precision displacement table, in this embodiment 100um, dz is the axial pixel resolution of the OCT system.
Then, based on the centroid position of each boundary or defect of the light-transmitting medium sample and the axial pixel resolution dz of the OCT system, calculating the optical thickness of each boundary or defect of the light-transmitting medium sample, wherein the calculation formula is as follows:
d o ＝(px c2 -px c1 )·dz
wherein px is c2 For the centroid position of the upper boundary of the first layer of medium, px c2 Centroid position d for lower boundary of first layer medium o Is the optical thickness between the upper and lower boundaries of the first layer of medium in the light-transmitting medium sample, i.e. the optical thickness of the first layer of medium.
The method for realizing optical thickness reduction physical thickness and positioning a sample defect position 4 is applied to OCT defect and thickness measurement, is used for calculating the physical thickness of a light-transmitting medium sample based on the optical thickness between boundaries and at defects of the light-transmitting medium sample by utilizing an OCT light detection model, and realizes defect and thickness measurement of the light-transmitting medium sample, and comprises the following steps:
according to the upper boundary of the transparent medium sample, calculating the normal vector of the upper boundary plane of the transparent medium sample and the incidence angle 4-1 of OCT detection light rays on the upper boundary plane, comprising:
fitting the upper boundary of the light-transmitting medium sample by using a least square method, calculating an included angle q1 between the plane of the upper boundary of the light-transmitting medium sample and the optical axis in the x direction and an included angle q2 between the plane of the upper boundary and the optical axis in the y direction, and recording as (q 1, q 2), and calculating a normal vector of the plane of the upper boundary of the light-transmitting medium sample based on the included angle q1 between the plane of the upper boundary and the optical axis in the x direction and the included angle q2 between the plane of the upper boundary and the optical axis in the y direction, thereby calculating an included angle between incident light and the normal vector, wherein the calculation formula is as follows:
wherein,is the normal vector of the plane of the upper boundary of the light-transmitting medium sample, +.>For OCT incident ray vector, θ 1 Normal vector to plane of OCT incident ray and upper boundary of light-transmitting medium sample +.>Is included in the plane of the first part; the absolute value is taken.
Calculating the transmission direction 4-2 of the light in the light-transmitting medium sample comprises:
the transmission direction of the light in the transparent medium sample is calculated based on the group refractive index and the incidence angle of the detection light on the upper boundary plane, and the formula is as follows:
sinθ 2 ＝sinθ 1 /n g
wherein θ 2 An included angle is formed between the direction of light rays in the transparent medium sample and the normal vector of the upper boundary plane of the transparent medium sample; θ 1 Normal vector to plane of OCT incident ray and upper boundary of light-transmitting medium sampleIs included in the plane of the first part; lambda (lambda) 0 For OCT incident light center wavelength, 1310nm in this embodiment, n p At wavelength lambda for light-transmitting medium sample 0 Corresponding phase refractive index, n g To transmit the bulk refractive index of the media sample at the incident optical band.
Based on OCT detection light principle, utilize normal vector, OCT detection light's incident angle and light's transmission direction and optical thickness in the printing opacity medium sample of printing opacity medium sample upper boundary plane, calculate and obtain printing opacity medium sample's physical thickness 4-3, include:
the physical thickness d of the light-transmitting medium sample was calculated as follows:
d＝cosθ 2 d o /n g
wherein d o For the optical thickness, θ, of each boundary or defect of a light-transmitting medium sample in OCT space 2 An included angle is formed between the direction of light rays in the transparent medium sample and the normal vector of the upper boundary plane of the transparent medium sample; n is n g To transmit the bulk refractive index of the media sample at the incident optical band.
Light-transmitting medium thickness measurement method based on OCT system measures time calculation: taking sweep OCT system measurement as an example, the measurement time t is only measured by the sweep OCT scanning speed t ss-oct ＝1/f oct And single point repetition number M decision:
t＝M*t ss-oct
taking sweep rate of 20kHz of a sweep OCT system light source and repetition number m=25 single point acquisition as an example:
t＝25/20k＝1.25ms。
FIG. 2 shows an optical coherence tomography instrument for thickness measurement of a light-transmitting medium sample according to the present invention.
The weak coherence interference part of the device is a Michelson interferometer structure and consists of 5-14 parts, including 5 parts and a light source; 6. an optical fiber coupler; 7. a first polarization controller; 8. a first reference arm collimator; 9. a focusing lens; 10: a first reference arm mirror; 11. a first sample arm collimator; 12. OCT scans the imaging device; 13. a sample; 14. a signal detection device; 15. a signal processing and calculating unit;
the upper port of the optical fiber coupler 6 is connected with the broadband light source 5, the lower port is connected with the signal detection device 14 and the signal processing device 15, and the two ends on the right side are respectively connected with the reference arm light path and the sample arm light path; the reference arm light path comprises a first polarization controller 7, a first reference arm collimator 8, a reference arm focusing lens 9 and a first reference arm reflecting mirror 10; the sample arm light path comprises a first sample arm collimator 11, an OCT scanning imaging device 12 and a sample 13; the light source 5 emits light through the light coupler 6, the reference arm light beam emits into the first polarization controller 7, the light beam emits into the focusing lens 9 through the first reference arm collimator 8, and the light beam is focused to the first reference arm reflector 10 and then returned; the sample arm beam is incident on the first sample arm collimator 11, is incident on the sample 13 through the OCT scanning imaging device 12, and the returned sample arm beam interferes with the returned reference arm beam in the optical fiber coupler, is detected by the signal detection device 14, and finally is processed by the signal processing and calculating unit 15 to realize the thickness measurement of the light-transmitting medium.
According to the different detection modes of the OCT signal, a thickness measuring device for a light-transmitting medium sample based on the OCT system shown in fig. 2 includes:
1. an optical coherence tomography apparatus for time domain spectroscopy. The light source 5 adopts broadband low-coherence light, the first reference arm reflecting mirror translates back and forth along the optical axis direction of the reference arm, and the signal detector 14 is a point detector. The change of the optical path length of the reference arm is realized by the position of the first reference arm reflecting mirror 10, and the OCT signals generated by the two arms are collected by the point detector 14, so that the low-coherence interference detection of the scattering signals in the z direction of a certain spatial depth is realized, and a depth-resolved OCT sampling signal is obtained.
2. An optical coherence tomography device for spectral domain space spectroscopy. The light source 5 is a low coherence broadband light source and the signal detection device 14 is a spectrometer. The high-speed linear array camera in the spectrometer is utilized to carry out spectral space light splitting on the OCT signal, so that simultaneous detection on the depth direction of the OCT signal of the sample is realized, the conversion of the OCT signal from the spectral space to the depth space is reduced by utilizing a Fourier transform method, and the parallel detection on the backward scattering signal in the depth direction of the sample is realized, thereby obtaining the depth-resolved OCT sampling signal.
3. An optical coherence tomography device for time domain light splitting of a sweep frequency light source. The light source 5 is a sweep broadband light source, the signal detection device 14 is a point detector, the point detector is used for carrying out time-sharing recording on the low-coherence interference spectrum emitted by the sweep broadband light source, the interference spectrum signal is analyzed by a Fourier transform method, and parallel detection of the backward scattering signal in the depth direction of the sample is realized, so that the depth-resolved OCT sampling signal is obtained.
For the different signal detection devices, the thickness measurement method of the light-transmitting medium shown in fig. 1 can be combined respectively, and the thickness measurement of the sample to be measured can be realized based on the light intensity information of the OCT signal of the region to be measured of the glass plate.
Fig. 3 illustrates an exemplary embodiment of the present disclosure. The thickness device of the light-transmitting medium sample based on the OCT system comprises a central wavelength 1310nm, a bandwidth 100nm and sweep frequency speed 20kHz sweep frequency light sources 16 and 20:80 optical fiber coupler 17, reference arm ring 18, second reference arm collimator 19, doublet 20, dispersion compensation mirror 21, second reference arm mirror 22, second sample arm ring 23, second sample arm collimator 24, field lens 25 of focal length 54mm, scanning galvanometer 26, glass plate 27, polarization controller 28, 50: a fiber coupler 29, a balance detector 30 and a signal processing module 31.
20: the left end port of the 80 optical fiber coupler 17 is connected with the sweep frequency light source 16, the other port is idle, and the right end is respectively connected with the reference arm light path and the sample arm light path; the reference arm light path comprises a reference arm optical fiber circulator 18, a second reference arm collimator 19, a double-cemented lens 20, a dispersion compensation mirror 21 and a second reference arm reflecting mirror 22; the reference arm optical circulator 18, the entrance end connects the right side of the optical fiber coupler 17 to a part, the exit end of the reference arm optical circulator 18 is branched and connected with the second reference arm collimator 19, the second reference arm collimator 20 collimates the light beam to the reference arm double-cemented lens 20, connect the dispersion compensating mirror 21 to focus the light beam and reflect the light to form the reference arm light path through the second reference arm reflecting mirror 22; the sample arm light path comprises a sample arm light circulator 23, a second sample arm collimator 24, a field lens 25 with a focal length of 54mm, a scanning galvanometer 26 and a glass plate 27; the inlet end of the sample arm optical circulator 23 is connected with a branch on the right side of the optical fiber coupler 17, the branch on the outlet end of the second sample arm optical circulator 23 is connected with a sample arm collimator 24, the second sample arm collimator 24 collimates light beams to a field lens 25 with a focal length of 54mm, and the focused light beams reflect light through a glass plate 26 to form a sample arm light path; the reference arm optical circulator 18 has an outlet end connected to the inlet end of the polarization controller 28 and an outlet end of the second sample arm optical fiber circulator 29 are connected to 50:50 two inlet ends of fiber optic coupler 29, 50: and 50, two outlet ends of the optical fiber coupler 29 are connected with the inlet ends of the balance detector 30, and the outlet ends of the balance detector 30 are connected with the signal processing module 31.
Probe light emitted by swept light source 16 is transmitted through 20: the 80 optical fiber coupler 17 is divided into two beams, one beam enters a reference arm part of the device through the reference arm optical fiber circulator 18, and the beam is focused by the second reference arm collimating lens 19, the reference arm double-gluing lens 20 and the dispersion compensating lens 21 in sequence to be reflected through the second reference arm reflecting lens 22 to form a reference arm optical path; the other beam of light emitted by the optical fiber coupler 17 is emitted by the second sample arm optical fiber circulator 23, enters the second sample arm optical fiber collimator 24, changes the direction of the beam of light by the scanning galvanometer 25, focuses the beam of the sample arm to the glass plate 27 by the field lens 26 with the focal length of 54mm, and reflects the beam of light by the glass plate 27 to form a sample arm light path; the sample arm return light exits through the second sample arm fiber optic circulator 23 into 50:50 optical fiber coupler 29, while reference arm return light enters first polarization controller 28 via reference arm optical fiber circulator 18 to adjust the polarization state, ensuring that sample arm and reference arm polarization states are the same, reference arm beam enters 50: the optical fiber coupler 29 interferes with the sample arm beam, and the interference light is converted into an electric signal by the optical signal through the balance detector 30 and is received by the signal processing module 31 to realize high-precision rapid distance measurement.
Fig. 4 shows an OCT tomographic image and a structural mask for imaging a lens group by the method of the present invention, where (a) of fig. 4 is an OCT tomographic image for imaging a lens group, (b) of fig. 4 is a structural mask position generated corresponding to each boundary of a lens, and white arrows in (a) of fig. 4 are true boundary positions in the OCT tomographic image, and the structural mask prevents multiple scattering artifacts of light at each boundary of a lens from affecting boundary identification.
Fig. 5 is a schematic diagram of the method for locating the centroid of intensity in the method of the present invention, for example, the sample is repeatedly scanned M times, the actual sampling point at the upper boundary of the sample is Z (i), the light amplitude information is used as the weight, Z (i), Z (i-1) and Z (i+1) are used as the weights based on N points before and after the sample, namely M (2n+1) points, (m=25 and n=1), and the upper boundary pixel position of the sample where the centroid of intensity is located is obtained by fitting the three points.
Fig. 6 shows the results of the imaging and thickness measurement of a glass plate using the method of the present invention, and fig. 6 (a) is a block diagram obtained by performing m=25 repeated scans on a glass plate based on the OCT system; three gray scale highlighting lines are Auto-contrast (AC) and system reference lines for each border of the glass plate; fig. 6 (b) shows the light intensity information distribution in the depth direction of a single point of the glass plate, S1 is the upper boundary of the glass plate, S2 is the lower boundary of the glass plate, AC is the OCT system reference line, and the light intensities of S1, S2 and AC are all strong; fig. 6 (c) is a measurement result of measuring the upper boundary, the lower boundary and the reference line of the glass plate by an intensity centroid method, and the experiment is repeated 512 times, and three times of standard deviation of the measurement result of 512 times is stabilized at the hundred-nm level, so that high-precision measurement of the hundred-nm level can be realized; fig. 6 (d) shows the optical path result of measuring the thickness of the glass plate by using the intensity centroid method, and the repeated experiment is carried out 512 times, wherein the three-time standard deviation of the measurement result of 512 times is less than 200nm, and the high-precision measurement of hundred nanometers can be realized; fig. 6 (e) is a histogram of measurement result distribution of 512 times of repeated experiments, the measurement result distribution is concentrated, the triple standard deviation is less than 200nm, and high-precision measurement of hundred nanometers can be realized.
Fig. 7 shows a schematic representation of the method of the present invention based on the principle of restoring thickness by OCT photothermographic models. In the OCT light detection model, the optical path value of the incident light trajectory from the zero optical path surface is recorded in a straight line form in the detector. Namely: a is that 0 The plane of the dotted line is the OCT interference zero optical path plane,for incident light direction, θ 1 For incident light and first side S of the sample 1 Normal vector included angle, actual ray trace +.>In OCT with +.>Presentation (S)>Sample plate thickness-> Calculating optical path difference of each boundary of flat plate in OCT space based on intensity centroid method Corresponding refractive index relation, restore->Obtaining the corresponding refraction angle theta 2 Obtaining the calculation result of the thickness of the flat plate, namely d=d o *cosθ 2 /n。
The experimental results fully illustrate that: the invention relates to a light-transmitting medium sample thickness measuring method and device based on OCT, which adopts the intensity mass center to position the sample boundary and restores the sample thickness based on an OCT light detection model. The non-destructive high-precision measurement of the thickness of the transparent medium sample can be realized, the measurement time is short, the measurement precision is in the order of hundred nanometers, and the method has important significance for the thickness measurement and the distance measurement of an actual optical element.
Finally, it should be noted that the above-mentioned embodiments and descriptions are only illustrative of the technical solution of the present invention and are not limiting. It will be understood by those skilled in the art that various modifications and equivalent substitutions may be made to the present invention without departing from the spirit and scope of the present invention as defined in the appended claims.
Claims (10)
1. A method for measuring defects and thickness of a light-transmitting medium sample based on an OCT system, comprising:
a weak coherent optical imaging method (1) for acquiring OCT signals and OCT tomographic images of a light-transmitting medium sample in two-dimensional/three-dimensional space;
a boundary identification method (2) applied to OCT tomographic image segmentation and defect detection is used for generating each boundary structure mask in an OCT tomographic image according to a sample design value, identifying each boundary of a sample in a mask range and positioning a defect;
a method (3) for measuring the optical thickness of a light-transmitting medium based on the light intensity information of OCT signals, for calculating the optical thickness between the boundaries and at the defects of a light-transmitting medium sample from the OCT signals and OCT tomographic images;
the method is applied to OCT defect and thickness measurement, realizes the reduction of physical thickness by optical thickness and positions the defect position (4) of the sample, and is used for calculating the physical thickness of the light-transmitting medium sample based on the optical thickness between boundaries and at the defect position of the light-transmitting medium sample by utilizing an OCT light detection model so as to realize the defect and thickness measurement of the light-transmitting medium sample.
2. The method for measuring the defects and the thickness of a transparent medium sample based on an OCT system according to claim 1, wherein said boundary recognition method (2) applied to OCT tomographic image segmentation and defect detection comprises:
selecting an upper boundary structure mask in the OCT tomographic image, identifying an upper boundary position within the upper boundary structure mask and locating a defect (2-1) in the upper boundary;
based on the sample design value, OCT light detection model and upper boundary position, generating a structural mask corresponding to the rest boundary, identifying the rest boundary of the sample in the mask range and positioning the defect (2-2) of the rest boundary.
3. The method for measuring defects and thickness of a transparent medium sample based on an OCT system according to claim 2, wherein the generating a structural mask corresponding to the remaining boundary based on the sample design value, the OCT light detection model and the upper boundary position, and identifying the remaining boundary of the sample and locating the defects (2-2) of the remaining boundary within the mask range, comprises:
calculating the included angles between the upper boundary and the x-axis and the y-axis according to the upper boundary of the sample to obtain the OCT light incident angle of a mask for generating a residual boundary structure of the sample, calculating the refraction track of the light according to the OCT light incident angle, calculating the optical path value of the light from the zero optical path surface to each boundary of the sample based on the refraction track of the light, and mapping the optical path value to an OCT tomographic image to obtain the corresponding position S of the residual boundary design value in the OCT tomographic image m ′，S m ' performing up-down expansion of pixels to obtain a structural mask R corresponding to the residual boundary m Boundaries are identified within the mask and defect locations are located.
4. The method for measuring defects and thicknesses of a light-transmitting medium sample based on an OCT system according to claim 1, wherein the method (3) for measuring the optical thickness of the light-transmitting medium based on the light intensity information of the OCT signal, for calculating the optical thickness between boundaries and at defects of the light-transmitting medium sample based on the OCT signal and the OCT tomographic image, comprises:
positioning the centroid position of each boundary of the light-transmitting medium sample and the centroid position (3-1) of the defect by using the light intensity information of the OCT signal as weight;
and respectively calculating the optical thickness (3-2) of each boundary or defect of the light-transmitting medium sample according to the centroid position of each boundary or the centroid position of the defect of the light-transmitting medium sample and combining the axial pixel resolution of the OCT system.
5. The method for measuring the defect and thickness of a transparent medium sample based on an OCT system according to claim 4, wherein locating the centroid position of each boundary of the transparent medium sample and the centroid position (3-1) of the defect using the intensity information of the OCT signal as a weight, comprises:
taking the maximum value pixel of each boundary or defect of the light-transmitting medium sample in the M OCT tomographic images as a center point of a target position, and fitting the center point of the target position in the M OCT tomographic images and N points corresponding to the center point in front and back by taking light intensity as a weight to obtain the centroid position of each boundary or defect of the light-transmitting medium sample, wherein the formula is as follows:
wherein px is c Centroid position at each boundary or defect of the light-transmitting medium sample; i is the index of pixel values at each boundary or defect of the transparent medium sample; px (px) j (k) For the kth acquisition, indexing the pixel value at position j; i j (k) The light intensity value at position j is indexed for the kth acquisition.
6. The method for measuring the defect and the thickness of the transparent medium sample based on the OCT system according to claim 4, wherein the calculating the optical thickness (3-2) of each boundary or defect of the transparent medium sample according to the centroid position of each boundary or the centroid position of the defect of the transparent medium sample and the axial pixel resolution of the OCT system comprises the following steps:
firstly, moving a displacement table where a light-transmitting medium sample is located, positioning the upper boundary positions of the light-transmitting medium sample before and after movement based on an intensity centroid method, and calibrating to obtain axial pixel resolution dx of an OCT system;
Δpx＝px′ c1 -px c1
dz＝y/Δpx
wherein px is c1 For the pixel value of the upper boundary of the transparent medium sample obtained by the positioning before the movement, px c1 ' is the pixel value of the upper boundary of the transparent medium sample obtained by positioning after moving, deltapx is the pixel value of the upper boundary movement of the transparent medium sample obtained by positioning, y is the true movement value of the displacement table, and dz is the axial pixel resolution of the OCT system;
then, based on the centroid position of each boundary or defect of the light-transmitting medium sample and the axial pixel resolution dz of the OCT system, calculating the optical thickness of each boundary or defect of the light-transmitting medium sample, wherein the calculation formula is as follows:
d o ＝(px c2 -px c1 )·dz
wherein px is c2 For the centroid position of the upper boundary of the first layer of medium, px c2 Centroid position d for lower boundary of first layer medium o Is the optical thickness between the upper and lower boundaries of the first layer of medium in the light-transmitting medium sample.
7. The method for measuring defects and thicknesses of a light-transmitting medium sample based on an OCT system according to claim 1, wherein the method for implementing a method for reducing physical thickness by optical thickness and locating a sample defect position (4) for calculating physical thickness of a light-transmitting medium sample based on optical thickness between boundaries and at defects of the light-transmitting medium sample by using an OCT light detection model is applied to OCT defects and thicknesses measurement, and comprises:
calculating the normal vector of the upper boundary plane of the transparent medium sample and the incidence angle (4-1) of OCT detection light on the upper boundary plane according to the upper boundary of the transparent medium sample;
calculating the transmission direction (4-2) of the light in the light-transmitting medium sample;
based on the OCT detection light principle, calculating and obtaining the physical thickness and defect position (4-3) of the transparent medium sample by using the normal vector of the upper boundary plane of the transparent medium sample, the incident angle of the OCT detection light on the upper boundary plane, the transmission direction of the light in the transparent medium sample and the optical thickness.
8. The method for measuring defects and thickness of a transparent medium sample based on an OCT system according to claim 7, wherein the calculating to obtain the physical thickness (4-3) of the transparent medium sample based on the OCT probe light principle using a normal vector of an upper boundary plane of the transparent medium sample, an incident angle of an OCT probe light on the upper boundary plane, a transmission direction of the light in the transparent medium sample, and an optical thickness includes:
the physical thickness d of the light-transmitting medium sample was calculated as follows:
d＝cosθ 2 d o /n g
wherein d o For the optical thickness, θ, of each boundary or defect of a light-transmitting medium sample in OCT space 2 An included angle is formed between the direction of light rays in the transparent medium sample and the normal vector of the upper boundary plane of the transparent medium sample; n is n g To transmit the bulk refractive index of the media sample at the incident optical band.
9. The method for measuring defects and thickness of a light-transmitting medium sample based on an OCT system according to claim 1, wherein the light-transmitting medium sample is a single-layer medium or a multi-layer medium.
10. A defect and thickness measurement device for an OCT-based light-transmitting medium sample for performing the method of any one of claims 1 to 9, comprising: the optical coherence tomography device is used for carrying out OCT signal detection and imaging on a region to be detected of the sample; and the one or more signal processors are used for analyzing and processing the OCT signals detected in the sample region to be detected, obtaining the boundary position of the sample region to be detected, measuring the optical path thickness by an intensity centroid method, and restoring the physical thickness to realize the thickness measurement of the light-transmitting medium sample.
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