Scanning bright and dark field synchronous interferometry measurement device and measurement method for optical element defects and surface shape

Abstract

The present invention relates to a scanning bright and dark field synchronous interference measurement device and a measurement method for defects and surface shapes of optical elements, and relates to the technical field of surface / sub-surface defect detection of optical elements. The present invention solves the technical problems that the existing imaging method has high requirements on the stability of the device and the environment, the accuracy is greatly affected by the surface quality of the tested piece, and it is not suitable for the situation where the surface roughness is large and the sub-surface defect layer is deep, and there is no technical means for synchronous detection of surface shape and defects. The measuring device includes: a low-coherence light source, a fiber coupler, two reference arms, a detection arm, and a dual-channel spectral detector. The scanning bright and dark field synchronous interference measurement device for defects and surface shapes of optical elements of the present invention can realize synchronous measurement of defects and surface shapes, and can obtain a three-dimensional tomographic image at one time through a low-coherence light source and a scanning galvanometer, and has a certain penetration ability according to the interaction between light and material, and can obtain synchronous detection of surface / sub-surface defects and morphological geometric parameters through an image extraction algorithm.

Description

Technical Field

[0001] The present invention relates to the technical field of optical element surface / sub-surface defect detection, and in particular to a scanning bright and dark field synchronous interference measurement device and a measurement method for optical element defects and surface shapes. Background Art

[0002] Optical components are widely used in aerospace, microelectronics equipment, optical precision measurement, laser optics and many other fields. Surface / subsurface defects of optical components will affect the performance of the optical system. In the manufacturing, cleaning and transportation of optical components, in addition to the exposure of bubbles, improper operation during processing, or damage from external factors, surface / subsurface defects may be formed. The presence of defects will reduce the quality of the beam, damage the film layer, and cause component damage. The performance of advanced optical systems such as space optics, extreme ultraviolet lithography equipment, and microelectronics equipment continues to improve, driving the continuous development of defect detection technology and instruments, and the requirements for defect detection accuracy are also increasing. It is still a technical challenge to achieve high-precision, high-resolution, cross-integration detection of multiple methods, and to manufacture and develop dedicated defect detection equipment.

[0003] Surface defect detection methods are mainly divided into contact and non-contact methods, as shown in Table 1 (Li Mingze et al. Current status and development trend of non-spherical optical surface defect detection technology (invited) [J]. Infrared and Laser Engineering, 2022, 51(9): 20220457.). The contact detection method can achieve a longitudinal resolution of nanometers, but the distance between the contact method and the surface of the component to be tested is too short, which is easy to cause secondary damage, and has the disadvantages of high detection cost and low efficiency. It is difficult to detect large-sized, high-steep complex curved optical components. Among the non-contact detection methods, the optical method is the most widely used. According to the different optical principles used, it can be divided into interference method, diffraction method and scattering method. Among them, the principle of interference method is to detect based on the optical path difference caused by the defect. This method can obtain specific characteristic information of the target defect, but the interference fringes will break at the defect, and the lateral resolution of interference detection is not high. The diffraction method distinguishes the type and size of defects by the difference in diffraction patterns such as scratches and pits, but this method is only suitable for defects with relatively regular shapes. The inspection targets are mainly lenses and circular plane optical components.

[0004] Table 1 Classification and comparison of advantages and disadvantages of defect detection methods

[0005]

[0006] At present, the international community mainly relies on the principle of light scattering to realize defect detection of optical components. According to the way of collecting information and detection principles, it is distinguished, mainly including imaging method and energy method. The former mainly includes visual inspection method, filter imaging method and grazing method, etc., while the latter is mainly based on photothermal detection and light scattering detection method. As the name suggests, the imaging method is to perform imaging detection on defects. It is necessary to use certain lighting methods to converge the real image of the target detection area through a specific imaging light path on the human eye or image sensor. It is distinguished according to the lighting method or imaging method. The energy method is to collect the scattered light after the defect in the component modulates the incident light signal, and judge the presence or absence of defects, or the size and shape of the defects through the intensity value. The energy method mainly analyzes the received signal, mainly including scattered light reception method and spectrum analysis method.

[0007] With the increasing maturity of optical sensor devices and the continuous development of computer image technology, digital detection methods represented by microscopic imaging have become an important means in the field of defect quantitative detection with its advantages of non-contact, high resolution, and quantitative evaluation. Laser scattering confocal microscopy uses a scanner to scan the surface of an optical element point by point, and transmits the detected and collected beam signals to a computer for processing. This method can obtain the position and size of surface defects of optical elements, with high imaging accuracy and resolution. It is an effective means to accurately detect surface defects and adapt to ultra-precision optical manufacturing processes. The advantage of laser confocal scanning microscopy technology is that it can detect sub-surface defects of optical elements in real time and in situ. However, the disadvantage is that the use of laser confocal technology has high requirements for the stability of the device and environment; and the accuracy is greatly affected by the surface quality of the tested piece, and has high requirements for surface finish. It is not suitable for situations with large surface roughness and deep sub-surface defect layers. Summary of the invention

[0008] The present invention aims to solve the technical problems that the imaging methods in the prior art have high requirements on the stability of the device and the environment, the accuracy is greatly affected by the surface quality of the tested piece, and is not suitable for situations where the surface roughness is large and the sub-surface defect layer is deep. A scanning bright and dark field synchronous interferometric measurement device and measurement method for optical element defects and surface shapes are provided.

[0009] In order to solve the above technical problems, the technical solutions of the present invention are as follows:

[0010] A scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes, comprising: a low coherence light source, a fiber coupler, a first reference arm, a second reference arm, a detection arm, and a dual-channel spectral detector;

[0011] The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler, the reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm;

[0012] The first reference arm and the second reference arm respectively include: a reference arm collimating lens, achromatic glass, a focusing lens and a reflector in the direction of the optical path; the light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and then return to the original paths;

[0013] The detection arm includes, in order, in the direction of the optical path: a detection arm collimating lens, a spectroscopic element, a scanning mechanism and a scanning objective lens; the detection light beam is focused on different depth layers of the optical element to be measured after passing through the scanning mechanism and the scanning objective lens, so as to realize the lateral two-dimensional surface scanning of the optical element to be measured; the return light after scanning is divided into a bright field light beam and a dark field light beam by the spectroscopic element after passing through the scanning objective lens and the scanning mechanism; the bright field light beam and the dark field light beam are coherently superimposed with the two reference light beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectral detector.

[0014] In the above technical solution, the illumination and detection of the bright field light beam and the dark field light beam are off-axis on the optical element to be measured, and are distributed in parallel on both sides of the central axis, and the off-axis angle range is 1 to 10 degrees.

[0015] In the above technical solution, the light splitting element is: a cylindrical reflector, a center hole reflector, a D-type mirror or a half-reflecting half-mirror.

[0016] In the above technical solution, the scanning mechanism is: a galvanometer or a rotating polygonal mirror with a speed of 10kHz-100kHz.

[0017] In the above technical solution, the light emitted by the low-coherence light source belongs to the spectrum from visible light to near-infrared light.

[0018] In the above technical solution, the spectral bandwidth of the light emitted by the low-coherence light source is: 20nm-100nm.

[0019] In the above technical solution, the power of the light emitted by the low-coherence light source is: less than or equal to 5 mW.

[0020] A measurement method applicable to the above-mentioned scanning bright and dark field synchronous interferometry measurement device for optical element defects and surface shapes comprises the following steps:

[0021] Step 1: The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler. The reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm;

[0022] Step 2: The light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and return along the original paths;

[0023] Step 3: The detection beam is focused on different depth layers of the optical element under test after passing through the scanning mechanism and the scanning objective lens, so as to realize the lateral two-dimensional surface scanning of the optical element under test; the return light after scanning passes through the scanning objective lens and the scanning mechanism, and is divided into a bright field beam and a dark field beam by the beam splitter;

[0024] Step 4: The bright field beam and the dark field beam are coherently superimposed with the two reference beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectral detector;

[0025] Step 5: The light entering the dual-channel spectral detector is collected by the image acquisition card and enters the computer. After data processing, two three-dimensional coherence tomography images of bright field and dark field are obtained. Based on the image analysis, the comprehensive information of the optical element under test is obtained.

[0026] The present invention has the following beneficial effects:

[0027] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention has low cost: only a spectroscopic element, such as a rod mirror, a center hole mirror, a D-type mirror, a half-reflective half-mirror, etc. (not limited to the above types), a dark field coupling lens and an optical fiber array need to be added; the spectroscopic element is used to separate bright field ballistic scattered light and dark field small-angle scattered light; the coupling lens is used to receive dark field small-angle scattered light, and the focal length of the coupling lens is greater than the focal length of the bright field coupling lens, so that dark field scattered light with a larger diameter than the bright field light beam can be received; the light array is used to simultaneously connect bright field and dark field photons to a spectrometer.

[0028] The scanning bright-and-dark-field synchronous interference measurement device for optical element defects and surface shapes of the present invention has a high light scattering energy utilization rate: dark-field scattered photons are additionally received without affecting the reception of traditional bright-field scattered photons.

[0029] The scanning bright and dark field synchronous interference measurement device for optical element defects and surface shapes of the present invention has high contrast: the brightest direct scattered light in the middle of the dark field light beam is blocked, so that the dark field weak light image is displayed, and at the same time has a higher contrast than the bright field.

[0030] The scanning bright and dark field synchronous interference measurement device for optical element defects and surface shapes of the present invention synchronously measures bright field and dark field: the bright field and dark field use the same spectrometer, separate the bright field and dark field by optical path difference, and realize synchronous detection.

[0031] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention has high image quality and strong defect resolution capability: the two images obtained simultaneously are from different angles and have little correlation, so they can be used for speckle elimination and image processing to further improve image quality.

[0032] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention can realize synchronous measurement of defects and surface shapes, obtain three-dimensional tomographic images at one time through a low-coherence light source and a scanning galvanometer, and has a certain penetrating ability based on the interaction between light and materials. Through an image extraction algorithm, synchronous detection of surface / sub-surface defects and morphological geometric parameters can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0034] Figure 1 It is a schematic structural diagram of the scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention.

[0035] Figure 2 Schematic diagram of the principle of a scanning bright and dark field synchronous interferometry measurement device using a D-type reflector as a beam splitting element.

[0036] Figure 3 This is a schematic diagram of the principle of a scanning bright and dark field synchronous interferometry measurement device using a cylindrical reflector as a beam splitting element.

[0037] Figure 4 This is a schematic diagram of the principle of a scanning bright and dark field synchronous interferometry measurement device using a half-reflecting half-mirror as a beam splitting element.

[0038] Figure 5 Schematic diagram of the optical path of the detection arm.

[0039] Figure 6 Schematic diagram of the optical path of the detection arm experiment.

[0040] Figure 7 Schematic diagram of a D-type reflector.

[0041] Figure 8 Schematic diagram of a cylindrical reflector.

[0042] Fig. 9 Schematic diagram of the simultaneous bright and dark field measurement results of optical surfaces and sub-surfaces.

[0043] Fig.10 Schematic diagram of the intensity attenuation curve of the axial point spread function. DETAILED DESCRIPTION

[0044] The inventive concept of the present invention is: the present invention proposes a scanning bright and dark field synchronous interferometric measurement device and a measurement method for defects and surface shapes of optical components, combines the interference method with the scattering imaging method, avoids the problems of broken surface interference fringes, small dynamic range and small aperture through scanning interference imaging, performs interference detection on tiny weak scattered photons on the component surface with high sensitivity, obtains three-dimensional tomographic images through Fourier transform, and improves the recognition accuracy of defects.

[0045] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention measures surface defects of optical glass surfaces, polished silicon carbide surfaces and modified silicon surfaces based on the bright field and dark field detection mechanism. The detection depth reaches the mm level and the resolution is better than 3 μm. The device can also simultaneously obtain the characterization of the optical element surface shape.

[0046] The scanning bright and dark field synchronous interference measurement method of optical element defects and surface shape of the present invention is a scattering imaging method in non-contact, which avoids the shortcomings of contact, non-contact interference method, non-contact diffraction method and non-contact scattered energy method. It inherits the advantages of high resolution and depth information of non-contact interference method through scanning interference tomography, and can detect the defects of optical element surface / sub-surface in real time and in situ by combining laser confocal microscopy method, changing the configuration of only one bright field image in the prior art, and realizing the synchronous detection of bright field and dark field. On the basis of extremely low cost, the present invention enables it to have the ability to detect additional dark field scattered light, and realizes high-sensitivity and high-resolution three-dimensional tomography measurement. Bright and dark field synchronous measurement has the advantages of high sensitivity, high precision, high resolution and high contrast, and can effectively detect the surface / sub-surface defects of optical elements, solve the situation of large surface roughness and deep sub-surface defect layer, and can also extract three-dimensional morphology surface shape based on the obtained three-dimensional tomography image, and obtain the surface shape characterization of optical elements on the basis of defect detection, so as to realize the comprehensive detection of the surface of optical elements, and has the advantages of compact structure, low cost and high speed.

[0047] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention obtains the three-dimensional morphology of the surface and sub-surface of the optical element under test by scanning the optical element at high speed, thereby realizing synchronous detection of surface / sub-surface defects and element morphology.

[0048] The scanning bright and dark field synchronous interference measurement device for optical element defects and surface shapes of the present invention is similar in principle to the Michelson interferometer. Figure 1As shown, the low-coherence light source is a light-emitting diode (LED), and the low-coherence light emitted by the LED enters two reference arms and one detection arm respectively. The wavelength of the light can be selected from visible to near-infrared according to the material of the optical element to be measured, and the spectral bandwidth can be selected according to the resolution requirements, usually 20nm-100nm, and the power of the light is determined by the scattering coefficient of the surface to be measured, usually 5mW or even less. The light emitted by the low-coherence light source is coupled to the optical fiber and divided into a reference beam and a detection beam through a fiber coupler. The reference beam is reflected by the reflector and returned along the original path. The detection beam is focused on different depth layers of the optical element to be measured after passing through the scanning galvanometer. The lateral two-dimensional surface of the optical element is scanned by a galvanometer or a rotating polygonal mirror with a speed of 10kHz-100kHz. The backscattered photons are collected by the same lens, trace their path through the optical fiber, and are recombined with the reverse propagating reference beam in the fiber coupler. The returned light is divided into bright field and dark field, which are coherently superimposed with the two reference beams respectively, and are collected by the image acquisition card after passing through the dual-channel spectral detector and enter the computer. After data processing, two three-dimensional coherence tomography images are obtained simultaneously. Based on the image analysis, the comprehensive information of the optical element under test (surface / sub-surface defects and three-dimensional surface shape) can be obtained. The time of the whole process depends on the density of scanning points, which is usually more than 1024×1024 scanning points. The whole measurement time is completed within 1 minute.

[0049] Unlike previous technologies, the technical solution of the present invention collects single-point interference images of the surface. The one-dimensional depth direction information is obtained at each scanning point through a low-coherence light source. Further horizontal two-dimensional scanning and solution are required to obtain information on surface / subsurface defects and three-dimensional morphology. Two reference arms form a dual Michelson interferometer to interfere with bright field scattered photons and dark field scattered photons respectively, and finally enter a spectrometer through an optical fiber array. The bright field and dark field images are separated by adjusting the length of the reference arm, realizing synchronous imaging of bright field and dark field, improving energy utilization at a very low cost, and obtaining high-contrast dark field images. Speckle suppression can also be performed using bright field and dark field images. In theory, speckle suppression can be √2 times, and dark field images of small-angle scattered light are received synchronously, which improves the utilization rate of OCT energy. The reception of the dark field blocks the strong direct reflected light to obtain a higher imaging contrast. The two bright and dark field images obtained at the same time can further study methods to improve image quality, including the study of speckle elimination and image restoration algorithms.

[0050] The scanning bright and dark field synchronous interference measurement device for optical element defects and surface shape of the present invention belongs to the scattering imaging method in non-contact type, avoids the shortcomings of contact type, non-contact interference method, non-contact diffraction method and non-contact scattering energy method, inherits the advantages of high resolution and depth information of non-contact interference method through scanning interference tomography means, combines the laser confocal microscopy method to detect the defects of the surface / subsurface of the optical element in real time and in situ, changes the configuration of the prior art with only one bright field image, realizes the synchronous detection of bright field and dark field, and enables it to have the ability to detect additional dark field scattered light on the basis of extremely low cost, and realizes high-sensitivity and high-resolution three-dimensional tomography measurement. Bright and dark field synchronous measurement has the advantages of high sensitivity, high precision, high resolution and high contrast, can effectively detect the surface / subsurface defects of optical elements, solves the situation of large surface roughness and deep subsurface defect layer, can also extract three-dimensional morphology surface shape based on the obtained three-dimensional tomography image, and obtains the surface shape characterization of optical elements on the basis of defect detection, realizes the comprehensive detection of the surface of optical elements, and has the advantages of compact structure, low cost and high speed.

[0051] The present invention is described in detail below with reference to the accompanying drawings.

[0052] Embodiment 1

[0053] like Figure 2 As shown, the scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention comprises: an LED low coherence light source, an optical fiber coupler, a first reference arm, a second reference arm, a detection arm and a dual-channel spectrometer;

[0054] The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler, the reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm;

[0055] The first reference arm and the second reference arm respectively include: a reference arm collimating lens, achromatic glass, a focusing lens and a reflector in the direction of the optical path; the light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and then return to the original paths;

[0056] The detection arm includes: a detection arm collimating lens, a D-type reflector (such as Figure 7 As shown), scanning galvanometer and scanning objective lens; Figure 2 and 6As shown in the figure, the detection beam is focused on different depth layers of the optical element under test after passing through the scanning galvanometer and the scanning objective lens, realizing the scanning of the horizontal two-dimensional surface of the optical element under test; the return light after scanning, after passing through the scanning objective lens and the scanning galvanometer, is divided into a bright field beam and a dark field beam by a D-type reflector; the bright field beam and the dark field beam are coherently superimposed with the two reference beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectrometer. The illumination and detection of the bright field beam and the dark field beam are off-axis on the optical element under test, and are distributed in parallel on both sides of the central axis.

[0057] Embodiment 2

[0058] like Figure 3 As shown, the scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention comprises: an LED low coherence light source, an optical fiber coupler, a first reference arm, a second reference arm, a detection arm and a dual-channel spectrometer;

[0059] The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler, the reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm;

[0060] The first reference arm and the second reference arm respectively include: a reference arm collimating lens, achromatic glass, a focusing lens and a reflector in the direction of the optical path; the light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and then return to the original paths;

[0061] The detection arm includes: a detection arm collimating lens, a cylindrical reflector (such as Figure 8 As shown), a rotating polygon mirror and a scanning objective lens; Figure 3 and 6 As shown, the detection beam is focused on different depth layers of the optical element under test after passing through the rotating polygon mirror and the scanning objective lens, realizing the scanning of the horizontal two-dimensional surface of the optical element under test; the return light after scanning, after passing through the scanning objective lens and the rotating polygon mirror, is divided into a bright field beam and a dark field beam by a cylindrical reflector; the bright field beam and the dark field beam are coherently superimposed with the two reference beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectrometer. The illumination and detection of the bright field beam and the dark field beam are off-axis on the optical element under test, and are distributed in parallel on both sides of the central axis.

[0062] Embodiment 3

[0063] like Figure 4 As shown, the scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention comprises: an LED low coherence light source, an optical fiber coupler, a first reference arm, a second reference arm, a detection arm and a dual-channel spectrometer;

[0064] The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler, the reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm;

[0065] The first reference arm and the second reference arm respectively include: a reference arm collimating lens, achromatic glass, a focusing lens and a reflector in the direction of the optical path; the light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and then return to the original paths;

[0066] The detection arm includes, in order, in the direction of the optical path: a detection arm collimating lens, a half-reflecting half-mirror, a scanning galvanometer, and a scanning objective lens; Figure 4 and 6 As shown, the detection beam is focused on different depth layers of the optical element under test after passing through the scanning galvanometer and the scanning objective lens, realizing the scanning of the horizontal two-dimensional surface of the optical element under test; the return light after scanning, after passing through the scanning objective lens and the scanning galvanometer, is divided into a bright field beam and a dark field beam by a half-reflecting half-mirror; the bright field beam and the dark field beam are coherently superimposed with the two reference beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectrometer. The illumination and detection of the bright field beam and the dark field beam are off-axis on the optical element under test, and are distributed in parallel on both sides of the central axis.

[0067] The measuring method applicable to the scanning bright and dark field synchronous interferometric measuring device for optical element defects and surface shapes of the present invention comprises the following steps:

[0068] Step 1: The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler. The reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm;

[0069] Step 2: The light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and return along the original paths;

[0070] Step 3: The detection beam is focused on different depth layers of the optical element under test after passing through the scanning mechanism and the scanning objective lens, so as to scan the lateral two-dimensional surface of the optical element under test; the return light after scanning passes through the scanning objective lens and the scanning mechanism, and is divided into a bright field beam and a dark field beam by the beam splitter;

[0071] Step 4: The bright field beam and the dark field beam are coherently superimposed with the two reference beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectral detector;

[0072] Step 5: The light entering the dual-channel spectral detector is collected by the image acquisition card and enters the computer. After data processing, two three-dimensional coherence tomography images of bright field and dark field are obtained. Based on the image analysis, the comprehensive information of the optical element under test is obtained.

[0073] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention is described in detail below.

[0074] 1. System composition

[0075] Scanning bright and dark field synchronous interferometry is a new type of non-contact imaging detection technology. Its principle is based on Michelson interferometry technology. Figure 2-4 As shown. The optical parameters are determined according to the material of the optical element to be measured, including the light source band, bandwidth, beam diameter, imaging scanning field of view, coupler, circulator and the matching spectrometer. This system is a Michelson interferometer structure. The central wavelength of the light source is 850nm and the bandwidth is 50nm. After being split by the coupler, two paths of light enter the two reference arms respectively, and the other path enters the detection arm. The light of the two reference arms provides reference beams for the bright field and the dark field respectively. The light of the detection arm is collimated by a lens into a beam of 2mm diameter. The beam enters the two-dimensional scanning galvanometer through a cylindrical reflector or a middle hole reflector or a D-type mirror or a half-reflective half-mirror, and is irradiated on the surface of the optical element to be measured through a scanning objective lens. The red beam is the bright field; the blue beam is distributed in parallel with the red on both sides of the central axis of the objective lens. It is the additional small-angle scattered photons collected, which is the dark field. The bright field light and the dark field light enter the coupler respectively and interfere with the matching reference beam. The interference signal enters the light array and is finally collected by the spectrometer.

[0076] 2. System Working Principle

[0077] The essence of this imaging device is a low-coherence detector based on a Michelson interferometer. The system uses a (low-coherence) broadband light source and a fast dual-channel spectrometer to obtain interference information. It mainly includes: a broadband light source, a sample arm, a bright and dark field reference arm, and a broadband spectrometer. Since two sample signals are received, the system needs two reference arms. The light emitted from the light source enters the sample arm and the bright and dark field reference arm after being split. The reference beam entering the reference arm is reflected by the reflector and returns to the original path, and a reference delay is introduced. The detection beam entering the sample arm is focused into the sample, and then returns to the original path after being scattered and reflected by the sample, and interferes with the reference beam. The time delay of the reflection signals of the two arms is related to the structural information inside the sample and will modulate the interference spectrum. The interference spectrum of the bright and dark fields is split by a grating and finally received by a photodetector. The collected interference signal can reconstruct the reflectivity envelope with depth resolution in the focusing direction of the detection beam by Fourier transform. Taking one of the paths as an example, the interference spectrum signal can be expressed as:

[0078]

[0079] Where i is the imaginary part of the complex number, the distance between the sample arm and the end face of the coupler is Z, the corresponding sample back reflection coefficient is a(Z), the distance between the reference arm and the end face of the beam splitter is R, the reference arm back reflection coefficient is a(R), S(k) is the power spectral density of the light source, k is the wave number, ω is the angular frequency 2πf, ν is the speed of light. It can be assumed that the amplitude and phase of the light source are not modulated after entering the reference arm, that is, a(R) = 1. At the same time, the common reference surface of the sample and the reference arm is set at the position of the reference arm reflector, so R = 0, and the distance between each reflective surface inside the sample and the common reference surface is recorded as Z (reference mirror virtual image position), and the simplified interference spectrum signal is obtained:

[0080]

[0081] in, The inverse Fourier transform of A(k) is the axial emissivity distribution a(Z) of the sample.

[0082]

[0083] Among them, Γ(z) is the envelope of the light source autocorrelation function, that is, the inverse transform of the light source power spectrum density S(k). S(k) is a Gaussian curve, and its inverse transform Γ(z) is also a Gaussian curve. The full width at half maximum (FWHM) of Γ(z) becomes the main determinant of the system's axial resolution. The sample information obtained by inverse Fourier transform is not only accompanied by the sample image, but also has related noise such as DC terms and sample autocorrelation terms. It is the DC term at zero optical path z=0. The DC term is the autocorrelation term of the reference arm and is the strongest part of the spectral signal. is the autocorrelation term of each depth information of the sample, which is distributed near the zero optical path and has a relatively small amplitude. The depth information a(z) and a(-z) of the sample are obtained by filtering out the DC term and the sample autocorrelation term. They are a set of mirror images symmetrical relative to the zero optical path. In order to prevent the aliasing phenomenon from occurring, the sample is usually adjusted to one side of the zero optical path during imaging, that is, a z0 offset is introduced relative to the zero optical path. Although the offset can be avoided, it will cause the detection depth of the system to be reduced by half. For the dual-field imaging of this device, by adjusting the optical path difference of the reference arm, that is, introducing different sizes of the z0 offset, the positions of the bright field sample image and the dark field sample image are separated in the z direction, thereby ensuring that two images are collected in one spectrometer. Since the signal-to-noise ratio of the image will decrease with the increase of the introduced offset amplitude, the spectrometer used should be customized to ensure the imaging depth of the dual field. Introducing a full-range spectrometer or swept frequency OCT is a solution. This device uses a customized spectrometer, and the area with 3dB attenuation in the z direction can reach 48% of the spectrometer range.

[0084] 3. Data Collection Method

[0085] Data acquisition is acquired and processed by the customized software OCTViewer, which mainly generates sawtooth wave drive signals to control the two-dimensional scanning of the scanning galvanometer; synchronously collects the spectral signals of the spectrometer, and the linear array camera runs at a maximum readout rate of 70kHz. The output of the camera is digitized at a sampling rate of 5MS / s per channel using a data acquisition board with 12-bit resolution. The sampled data is continuously transferred to the computer memory. A discrete Fourier transform is performed on each group of 512 data points acquired by the camera to produce an axial depth tomographic image of the sample. GPU acceleration is used to provide real-time visualization of OCT images, so that the quality of the acquired images can be immediately evaluated; the acquired images are saved for later offline image processing and analysis. The scanning range of the sawtooth wave is consistent with the field of view of the imaging system, and the step size satisfies the Nyquist sampling theorem and is less than half of the lateral resolution. These synchronous scanning signals are converted into voltage control waveforms by NIDAQ and then sent to the driver of the two-dimensional galvanometer. Currently, 10Hz can be achieved for three-dimensional imaging of 1024×1024×1024 pixels.

[0086] IV. Experimental Verification

[0087] 1. Construction of scanning bright and dark field synchronous interference tomography system

[0088] All devices in this optical system are commercial devices and do not require custom processing. The light source is SUPERLUM's broadband light source MT-850-HP-I, the fiber coupler, circulator, lens, and reflector are all Thorlabs products, the galvanometer is Thorlabs' GVS002 two-dimensional galvanometer, the driving voltage card is NI6221 from NI, USA, the line array camera is e2v's high-speed line array camera E2V-Octoplus-2K-W4 / EV71YEM4CL2014-BA9, ​​the grating is Wasatch Photonics' WP-HD1800 / 840, and the cylindrical reflector is Edmund's product, number #54-092. Based on the selection of the above devices, optimization is performed through Zemax, and the parameters of the sample arm and the lens in the spectrometer are designed to meet the requirements of fundus imaging. The design results of the sample arm are shown in the figure below. Figure 5 shown.

[0089] 2. Bright and dark field synchronous interference tomography experiment

[0090] Before conducting the experiment, the basic performance of the designed system is measured, including sensitivity and intensity attenuation in the depth direction. The sensitivity measurement uses the following formula:

[0091]

[0092] Among them, ∑N s Represents the total number of photons of mirror-reflected light received by the sample arm, N ref Represents the total number of photons entering the spectrometer from the reference arm, N el represents the electronic noise of the line scan camera (dark current noise, readout noise and discretization noise), N RIN Represents the relative intensity noise of the light intensity in the reference arm, when the number of photons in the reference arm satisfies N RIN =N el According to the above formula, the reference arm energy is 6.7μW, the sample arm mirror reflected light energy is 210μW, and the best sensitivity of this system can be calculated to be 97.1dB. The surface of an optical element is measured, and the measurement results are as follows: Fig. 9 shown.

[0093] The intensity attenuation measurement in the depth direction requires a translation stage. The sample is a reflector. The sample is controlled to move in a certain step length in the depth direction, from the zero optical path position to the maximum range of the spectrometer. Usually the intensity is the highest near the zero optical path. As the sample moves, the farther away from the zero optical path position, the greater the intensity attenuation. Through calibration, the 3dB attenuation of the spectrometer of this system is 48% of the full scale, which meets the requirements of this experiment. The measured axial interference intensity attenuation curve is shown in the figure. Fig.10 shown.

[0094] The step length in the depth direction is 0.12 mm, and there are 24 steps in total. Therefore, from the first point spread function to the last point spread function, the reflector of the sample arm moves a total of 2.88 mm. Fig.10 The full width at half maximum of the point spread function can be calibrated to be 6.1μm, that is, the axial resolution, which is basically consistent with the theoretical value of 5.3μm. Since the axial resolution is only related to the wavelength and light source bandwidth, the resolution of the bright field and the dark field is the same.

[0095] The scanning bright and dark field synchronous interferometric measurement device for defects and surface shapes of optical elements of the present invention has low cost: only a spectroscopic element needs to be added, such as a rod mirror, a center hole mirror, a D-type mirror, a half-reflective half-mirror, etc., not limited to the above-mentioned ones, a dark field coupling lens and an optical fiber array; the spectroscopic element is used to separate bright field ballistic scattered light and dark field small-angle scattered light; the coupling lens is used to receive dark field small-angle scattered light, and the focal length of the coupling lens is greater than that of the bright field coupling lens, so that it can receive dark field scattered light with a larger beam diameter; the light array is used to simultaneously connect bright field and dark field photons to a spectrometer.

[0096] The scanning bright-and-dark-field synchronous interference measurement device for optical element defects and surface shapes of the present invention has a high light scattering energy utilization rate: dark-field scattered photons are additionally received without affecting the reception of traditional bright-field scattered photons.

[0097] The scanning bright and dark field synchronous interference measurement device for optical element defects and surface shapes of the present invention has high contrast: the brightest direct scattered light in the middle of the dark field light beam is blocked, so that the dark field weak light image is displayed, and at the same time has a higher contrast than the bright field.

[0098] The scanning bright and dark field synchronous interference measurement device for optical element defects and surface shapes of the present invention synchronously measures bright field and dark field: the bright field and dark field use the same spectrometer, separate the bright field and dark field by optical path difference, and realize synchronous detection.

[0099] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention has high image quality and strong defect resolution capability: the two images obtained simultaneously are from different angles and have little correlation, so they can be used for speckle elimination and image processing to further improve image quality.

[0100] The scanning bright and dark field synchronous interferometric measurement device for optical element defects and surface shapes of the present invention can realize synchronous measurement of defects and surface shapes, obtain three-dimensional tomographic images at one time through a low-coherence light source and a scanning galvanometer, and has a certain penetrating ability based on the interaction between light and materials. Through an image extraction algorithm, synchronous detection of surface / sub-surface defects and morphological geometric parameters can be obtained.

[0101] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A scanning bright and dark field synchronous interferometry measurement device for optical element defects and surface shapes, characterized in that: include: A low coherence light source, a fiber coupler, a first reference arm, a second reference arm, a detection arm, and a dual-channel spectral detector; The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler, the reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm; The first reference arm and the second reference arm respectively include: a reference arm collimating lens, achromatic glass, a focusing lens and a reflector in the direction of the optical path; the light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and then return to the original paths; The detection arm comprises, in order, in the direction of the optical path: a detection arm collimating lens, a spectroscopic element, a scanning mechanism and a scanning objective lens; the detection light beam is focused on different depth layers of the optical element to be measured after passing through the scanning mechanism and the scanning objective lens, so as to realize the lateral two-dimensional surface scanning of the optical element to be measured; the return light after scanning is divided into a bright field light beam and a dark field light beam by the spectroscopic element after passing through the scanning objective lens and the scanning mechanism; the bright field light beam and the dark field light beam are coherently superimposed with the two reference light beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectral detector; The illumination and detection of the bright field light beam and the dark field light beam are off-axis on the optical element to be measured, and are distributed in parallel on both sides of the central axis, and the off-axis angle thereof is in the range of 1 to 10 degrees.

2. The scanning bright and dark field synchronous interferometry measuring device for optical element defects and surface shapes according to claim 1, characterized in that: The light splitting element is: a cylindrical reflector, a central hole reflector, a D-shaped mirror or a half-reflecting half-mirror.

3. The scanning bright and dark field synchronous interferometry measuring device for optical element defects and surface shapes according to claim 1, characterized in that: The scanning mechanism is a galvanometer or a rotating polygonal mirror with a speed of 10kHz-100kHz.

4. The scanning bright and dark field synchronous interferometry measuring device for optical element defects and surface shapes according to claim 1, characterized in that: The light emitted by the low-coherence light source belongs to the spectrum from visible light to near-infrared light.

5. The scanning bright and dark field synchronous interferometry measuring device for optical element defects and surface shapes according to claim 1, characterized in that: The spectral bandwidth of the light emitted by the low coherence light source is 20nm-100nm.

6. The scanning bright and dark field synchronous interferometry measuring device for optical element defects and surface shapes according to claim 1, characterized in that: The power of the light emitted by the low-coherence light source is less than or equal to 5 mW.

7. A measurement method applicable to the scanning bright and dark field synchronous interferometry measurement device for optical element defects and surface shapes as claimed in any one of claims 1 to 6, characterized in that: The following steps are involved: Step 1: The light emitted by the low-coherence light source is divided into a reference beam and a detection beam through a fiber coupler. The reference beam enters the first reference arm and the second reference arm respectively, and the detection beam enters the detection arm; Step 2: The light beams entering the first reference arm and the second reference arm are respectively reflected by the reflectors and return along the original paths; Step 3: The detection beam is focused on different depth layers of the optical element under test after passing through the scanning mechanism and the scanning objective lens, so as to realize the lateral two-dimensional surface scanning of the optical element under test; the return light after scanning passes through the scanning objective lens and the scanning mechanism, and is divided into a bright field beam and a dark field beam by the beam splitter; Step 4: The bright field beam and the dark field beam are coherently superimposed with the two reference beams returned by the first reference arm and the second reference arm after being reflected by the reflector, and then enter the dual-channel spectral detector; Step 5: The light entering the dual-channel spectral detector is collected by the image acquisition card and enters the computer. After data processing, two three-dimensional coherence tomography images of bright field and dark field are obtained. Based on the image analysis, the comprehensive information of the optical element under test is obtained.

Images