High-transmittance and high-uniformity geometric phase grating element prepared by utilizing femtosecond pulse near-field normalization, method and application of high-transmittance and high-uniformity geometric phase grating element

Abstract

The invention discloses a high-transmittance and high-uniformity geometric phase grating element prepared through femtosecond pulse near-field normalization, a method and application of the high-transmittance and high-uniformity geometric phase grating element, and belongs to the technical field of laser processing. A novel high-transmittance and high-uniformity birefringence structure is induced, the slow axis direction change of the novel high-transmittance and high-uniformity birefringence structure is controlled, so that the period-adjustable high-efficiency geometric phase grating is prepared, and edge detection is realized by utilizing the light splitting characteristic of the geometric phase grating. According to the invention, an innovative processing technology is applied to optical image edge detection, the limitation of a traditional birefringence structure on transmittance and uniformity is solved, and a solution with better performance and lower cost is provided for the field of optical image edge detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser processing, and in particular relates to a high-transmittance, high-uniformity geometric phase grating element prepared by utilizing near-field normalization of a femtosecond pulse, a method and an application thereof. Background Art

[0002] Birefringent structures are widely used in the field of optical component manufacturing. Edge detection is a fundamental problem in image processing and computer vision. Image edge detection can significantly reduce the amount of calculation while retaining important structural properties and eliminating weakly correlated information. Compared with the traditional bulky edge detection system composed of lenses and spatial filters, the geometric phase element made of birefringent structure has the advantages of easy miniaturization and integration. As an emerging technical means for optical image edge detection. In 2019, the researchers used the traditional birefringent structure based on femtosecond laser-induced in vivo nanogratings to construct a method based on geometric phase metasurfaces, which can perform optical differential calculations on light beams carrying image information and split the beams to achieve edge detection.

[0003] However, the traditional nano-grating birefringence structure has a large overall scattering of the structure voxels due to the coexistence of the positive refractive index change area and the negative refractive index change area, resulting in low transmittance and poor overall uniformity of the voxels in long-range scanning. In this case, the camera's ability to capture effective information is greatly reduced, and the accuracy of the measurement is also reduced simultaneously.

[0004] Therefore, it is urgent to solve the problems of low transmittance and poor uniformity of traditional birefringence structures in order to further improve the imaging effect of edge detection. Summary of the invention

[0005] In view of the above-mentioned defects existing in the prior art, the present invention provides a high-transmittance, high-uniformity geometric phase grating element, method and application thereof prepared by utilizing the near-field normalization of femtosecond pulses. The present invention utilizes the near-field normalization principle of femtosecond laser pulses to perform uniform processing inside fused quartz, inducing a new type of high-transmittance, high-uniformity birefringence structure, controlling the change in the direction of its slow axis to prepare a periodically adjustable high-efficiency geometric phase grating, and utilizing the spectral characteristics of the geometric phase grating to achieve edge detection. The present invention solves the limitations of traditional birefringence structures in transmittance and uniformity by applying innovative processing technology to optical image edge detection, and provides a solution with better performance and lower cost for the field of optical image edge detection.

[0006] The present invention is achieved through the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing a high-transmittance, high-uniformity geometric phase grating element by utilizing the near-field normalization of a femtosecond pulse, which specifically comprises the following steps:

[0008] Step 1: Construction of laser processing optical path;

[0009] The specific steps are as follows: first, the femtosecond laser emitted by the laser passes through the energy control system composed of the electrically controlled half-wave plate HWP and the Glan prism P, and then passes through the polarization control system composed of the Pockels cell and the quarter-wave plate QWP in sequence; then the light beam passes through the beam expansion system composed of the concave lens L1 and the convex lens L2, and then is projected to the front of the processing objective lens through the reflector M1, and finally focused on the inside of the sample to be processed; the LED lighting device is placed on the lower side of the sample stage, so that the white light emitted by the LED lighting device can penetrate the sample to be processed and irradiate the objective lens OL; after the white light is transmitted by the reflector M1, it is reflected again by the reflector M2, and finally focused on the second convex lens L3, and a clear image is formed on the CCD sensor; the CCD sensor is connected to the industrial computer through a data cable to observe the sample processing in real time;

[0010] Step 2: Leveling of the sample stage;

[0011] The specific steps are as follows: first, fix the sample to be processed on a three-dimensional displacement stage with an adjustment device; the adjustment device of the three-dimensional displacement stage includes an X-axis and a Y-axis in the horizontal direction and a Z-axis in the vertical direction; then, open the light gate in the processing light path, the laser is focused by the high-power objective lens, and the three-dimensional displacement stage is controlled to find the upper left corner of the sample to be processed, and the laser focus is focused on the upper surface of the sample by adjusting the height of the moving stage, and the Z-axis height of the surface at the upper left corner of the sample is recorded at this time; similarly, find the surfaces of the upper right, lower left and lower right of the sample, and record the Z-axis height; adjust the adjustment device of the three-dimensional displacement stage according to the recorded surface Z-axis height data, so that the surfaces at the four corners of the sample are at the same Z-axis height; at this time, the sample stage to be processed has been leveled;

[0012] Step 3: Design and parameter calculation of geometric phase grating elements;

[0013] The specific steps are as follows: first, select the geometric phase grating period Λ, element size and shape that match the actual needs; then, according to the phase gradient formula of the one-dimensional grating In the formula, x and y represent the position coordinates of the rectangular coordinate system on the two-dimensional plane, and the data used to describe the grating phase distribution is generated in the form of a two-dimensional array or matrix; then, through the formula To determine the phase The relationship between the slow axis angle θ of the birefringent structure, where the slow axis angle takes N discrete values ​​in the range of 0-180°, corresponding to N kinds of oriented birefringent structures, to construct a geometric phase grating; finally, through the formula The total phase delay R of the geometric phase grating element is calculated, where λ represents the wavelength of the light source of the detection light path; and the required number of processing layers is determined by dividing the designed total phase delay by the phase delay of a single-layer birefringence structure;

[0014] Step 4: Preparation of uniform linear structure;

[0015] The specific steps are as follows: by adjusting the height of the three-dimensional translation stage, ensure that the femtosecond laser is focused on the predetermined processing area inside the sample; modulate the initial pulse energy, deposit the seed pulse inside the sample, rotate the electrically controlled half-wave plate HWP to adjust the direction of polarized light, and adjust the energy of subsequent pulses, and induce the second point from the initial seed through the principle of near-field normalization, and then use the second point as a seed to induce the third point, and so on; control the scanning speed and exposure time to move the translation stage during pulse radiation to perform line scanning processing; prepare a uniform line structure, the line structure is composed of a birefringent structure; the birefringent structure is a randomly distributed anisotropic nanopore structure;

[0016] Step 5: Femtosecond laser pulse preparation of geometric phase grating;

[0017] On the basis of processing a uniform line structure, the line spacing is set to form a two-dimensional plane structure, and then a geometric phase grating is prepared by processing layer by layer from deep to shallow.

[0018] Furthermore, in step one, the sample to be processed is 7979 glass produced by Corning, with a thickness of 0.1-7 mm; the Pockels cell is a Leysop-1030nm electro-optical modulator produced by the British company Leysop, and the beam expansion system has a beam expansion factor of 1.5-3 times; the objective lens OL used in laser processing is an objective lens produced by New Focus, with a numerical aperture NA = 0.1-0.65, a magnification of 10-60×, and a working distance of 10-14.0 mm; the illumination light source uses a white LED light source.

[0019] Further, in step 4, the central wavelength of the femtosecond laser is 343-1030nm, the pulse width is 100fs-10ps, the repetition frequency is 1kHz-40MHz, the pulse energy is 0.5-0.9μJ, the point spacing Δx is 0.8-2μm, the scanning speed v is 6-10mm / s, and the single point exposure time is

[0020]

[0021] Furthermore, in step four, the number of seed pulses deposited inside the sample is 1-3; the diameter of the femtosecond laser induced spot is continuously adjustable in the range of 200nm-2μm, with an adjustment accuracy of 50nm; the energy of subsequent pulses from the second point onwards is determined by the seed size and is set to 75%-150% of the initial pulse energy; the near-field normalization principle is made to work, and the displacement stage movement distance of the subsequent pulses is within 10-100nm.

[0022] Furthermore, in step five, each layer is processed by scanning line processing according to the phase distribution data in the .csv file. The .csv file is a .csv file that can be recognized and processed by the processing system by converting the designed phase distribution matrix data into MATLAB software, which is used to control the slow axis angle of the birefringent structure and then control its phase.

[0023] In a second aspect, the present invention also provides a high-transmittance, high-uniformity geometric phase grating element prepared using the near-field normalization of femtosecond pulses, which is prepared by the method described in the first aspect.

[0024] In a third aspect, the present invention provides an application of a high-transmittance, high-uniformity geometric phase grating element prepared by utilizing the near-field normalization of femtosecond pulses in terms of edge detection efficiency.

[0025] Furthermore, the application specifically includes the following steps:

[0026] Step A, constructing a test optical path, placing the prepared geometric phase grating element on the focal plane between the first convex lens L1 and the second convex lens L2 to achieve Fourier transform of the light beam;

[0027] Step B, the laser beam first irradiates the object to be detected, and then passes through the Glan prism P1 and the 4f optical system composed of the first convex lens L1, the geometric phase grating element, and the second convex lens L2 in sequence, wherein the distance between the lenses is twice the focal length f of the lens, and the distance between the object and the first convex lens L1 of the 4f system is the focal length f of the lens; after the linearly polarized light passes through the 4f system, it is divided into left-handed circularly polarized light LCP and right-handed circularly polarized light RCP with horizontal displacement offset, and the overlapping part in the middle appears as linear polarization; the laser further passes through the analyzer P2, which can filter the linear polarized light in the overlapping area in the middle of the image while retaining the circularly polarized light at the edge of the image; finally, the edge information of the object is captured and analyzed by a CCD sensor connected to a computer PC; wherein the distance between the CCD sensor and the second convex lens of the 4f system is the focal length f of the lens.

[0028] Furthermore, the lens of the 4f optical system is a MCX10610 plano-convex lens produced by Shenzhen Lubang Technology Co., Ltd., with a focal length of 10-50 mm; the polarizer is a FLP51-VIS-M thin film linear polarizer produced by Shenzhen Lubang Technology Co., Ltd.

[0029] Furthermore, by changing the period of the geometric phase grating, the angle between the outgoing left-handed circularly polarized light and the right-handed circularly polarized light can be controlled, that is, the horizontal displacement of the two images can be controlled, and then the resolution of image edge detection can be adjusted.

[0030] The principle of the present invention is as follows:

[0031] Mechanism of the new birefringent structure induced by the near-field normalization of femtosecond laser pulses: In the case of line scanning processing using femtosecond laser pulses, it can be seen from the effect of near-field enhancement that when the hole is large, such as a radius of 50nm, the enhancement effect is weak, and only when it is very close to the nanoseed (<1 / 10 radius) will there be a more obvious enhancement, and the width of the enhanced area is <1 / 2 radius; when the seed structure hole is small, such as a radius of 20nm, the near-field enhancement area will be widened (>1 / 3 radius), and the width of the enhanced area is close to or even greater than the diameter. In this case, under the premise that the seed structure has been formed, as long as the point spacing and single pulse energy in the scanning process are properly controlled, uniform processing can be achieved, and finally a birefringent structure with high uniformity and high transmittance is induced. In particular, because the energy is localized in the near field, the near-field propagation distance is ≤1 / 3 wavelength, and there is insufficient energy diffusion, which avoids the generation of positive refractive index change areas and structural scattering, thereby increasing transmittance. The above methods together constitute the principle of near-field normalization, such as Figure 2 As shown;

[0032] Mechanism of preparing geometric phase grating: Since the slow axis direction of the birefringent structure induced by femtosecond laser is always perpendicular to the laser polarization direction, the electro-optic modulator can be used to adjust the polarization direction during scanning and control the slow axis direction of the birefringent structure. The slow axis angle θ and laser polarization at each spatial position can be determined, and then a period-adjustable geometric phase grating with a change in the slow axis direction can be produced.

[0033] The edge detection principle of the geometric phase grating is used: from the grating's spectroscopic characteristics, it can be seen that the grating period Λ can adjust the spectroscopic angle θ, and the relationship between the two is Λsinθ=mλ, where λ is the wavelength of light and m is the diffraction order. After the linearly polarized light irradiates the object and then enters the geometric phase grating, it emits left-handed circularly polarized light and right-handed circularly polarized light with horizontal offset, and the middle part overlaps and appears as linear polarization. Then, the analyzer is used to filter the linearly polarized light in the overlapping area in the middle of the image while retaining the circularly polarized light at the edge of the image to obtain the edge image of the object.

[0034] Principle of improving edge detection efficiency: The new birefringent structure processed by femtosecond laser pulse near-field normalization has high uniformity, and the structure itself has low absorption and scattering of light, and has high transmittance, resulting in high diffraction efficiency and high imaging quality. Therefore, the geometric phase grating element constructed with the new birefringent structure can uniformly enhance the transmission efficiency of the light beam during edge detection, thereby improving edge detection efficiency.

[0035] Compared with the prior art, the advantages of the present invention are as follows:

[0036] (1) The high-transmittance, high-uniformity birefringence structure, method and application thereof prepared by utilizing the near-field normalization of femtosecond pulses of the present invention successfully prepares a high-transmittance birefringence structure by utilizing the near-field normalization principle of femtosecond laser pulses; this structure can effectively enhance the transmission efficiency of the light beam, ensure that the effective information captured by the camera is not weakened, and reduce the system cost.

[0037] (2) The new birefringent structure has excellent uniformity, which can ensure the uniformity of light intensity distribution during edge detection, which helps to avoid deviations in detection results and improves the accuracy of edge detection.

[0038] (3) Compared with the conventional bulky system, the geometric phase grating element prepared by the present invention using the novel birefringence structure is easy to miniaturize and integrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0040] Figure 1 It is a schematic diagram of the optical path of a novel geometric phase grating element processed in a fused quartz body by utilizing the near-field normalization of a femtosecond laser according to the present invention;

[0041] In the figure, HWP-electrically controlled half-wave plate, P-Glan prism, Pockels cell-Pockels cell, QWP-quarter wave plate, L1-concave lens, L2-first convex lens, LED-lighting lamp, M1-first reflector, M2-second reflector, three-dimensional translation stage (the adjustment device includes horizontal X-axis, Y-axis and vertical Z-axis), L3-second convex lens, CCD-camera.

[0042] Figure 2 It is a schematic diagram of the principle of near-field normalization when performing line sweeping processing in a fused quartz body using a femtosecond laser according to the present invention;

[0043] Figure 3 Schematic diagram of the effect of the femtosecond laser pulse near-field normalization method on the structural morphology in the present invention;

[0044] Among them, (a) is the processing result without using the principle of near-field normalization of femtosecond pulses;

[0045] (b) is an electron microscope image of line sweeping processing using this principle;

[0046] The results show that uniform lines can be produced by utilizing the principle of near-field normalization of femtosecond pulses.

[0047] Figure 4 This is a schematic diagram of a novel birefringent structure processed in a fused quartz body using near-field normalization of a femtosecond laser according to the present invention, which is a randomly distributed anisotropic nanopore structure. The arrow indicates the polarization direction of light, and the nanopore structure is stretched perpendicular to the polarization direction of light.

[0048] Figure 5 The line shape obtained by using the near-field normalization processing of femtosecond laser pulses of the present invention;

[0049] Figure 6 Schematic diagram of the phase delay R at different horizontal positions of the line processed by near-field normalization of femtosecond laser pulse;

[0050] Figure 7 This is a birefringence microscope image of the femtosecond laser-induced birefringence structure evolving with the energy of a single pulse; the double-headed arrow indicates the polarization direction of the light, the single-headed arrow on the square structure indicates the slow axis direction of the birefringence structure, and the box indicates the processing pulse energy parameter corresponding to the selected new birefringence structure;

[0051] Figure 8 is a schematic diagram of a geometric phase grating;

[0052] Among them, (a) is the phase distribution diagram of the geometric phase grating with a period of 1mm designed by MATLAB software, and the scale value represents the processing value in the suffix .csv file that can be recognized by the processing system;

[0053] (b) is a birefringence microscope image of the geometric phase grating. The arrow indicates the slow axis direction of the birefringence structure, and the diameter is 2 mm.

[0054] Fig. 9 This is the principle diagram of geometric phase grating spectrometry;

[0055] Wherein, linearly polarized light is incident on the geometric phase grating, and the outgoing light is left-handed circularly polarized light and right-handed circularly polarized light;

[0056] Fig.10 This is a diagram to test the spectral effect of geometric phase grating;

[0057] The observed result is that there are only positive and negative first-order diffraction spots, but no 0th-order or other-order diffraction spots; the geometric phase grating has high-efficiency diffraction, and the transmittance of the geometric phase grating element is measured to be 99%.

[0058] Fig.11 A schematic diagram of the test optical path of the edge detection method for preparing a 1mm period geometric phase grating using a novel birefringence structure in a fused quartz body induced by a femtosecond laser according to the present invention;

[0059] In the figure, object-the object to be edge detected, P1-Glan prism, L1-the first convex lens, L2-the second convex lens (L1 and L2 form a 4f system), GPG-geometric phase grating element, P2-polarizer, CCD-camera.

[0060] Fig.12 This is a test diagram of edge detection of a 1mm period geometric phase grating using near-field normalization of a femtosecond laser to induce a birefringence structure in a fused quartz body according to the present invention;

[0061] Among them, (a) is the original image of the object to be detected captured without adding a polarizer; (b) is the image of the edge of the object. DETAILED DESCRIPTION

[0062] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification:

[0063] Example 1

[0064] This embodiment provides a method for preparing a high-transmittance, high-uniformity birefringence structure by utilizing the near-field normalization of femtosecond pulses, which specifically includes the following steps:

[0065] Step 1: Construction of laser processing optical path, such as Figure 1 As shown;

[0066] The specific steps are as follows: first, the femtosecond laser emitted by the laser passes through the energy control system composed of the electrically controlled half-wave plate HWP and the Glan prism P, and then passes through the polarization control system composed of the Pockels cell and the quarter-wave plate QWP in sequence; then the light beam passes through the beam expansion system composed of the concave lens L1 and the convex lens L2, and then is projected to the front of the processing objective lens through the reflector M1, and finally focused on the inside of the sample to be processed; the LED lighting device is placed on the lower side of the sample stage, so that the white light emitted by the LED lighting device can penetrate the sample to be processed and irradiate the objective lens OL; after the white light is transmitted by the reflector M1, it is reflected again by the reflector M2, and finally focused on the second convex lens L3, and a clear image is formed on the CCD sensor; the CCD sensor is connected to the industrial computer through a data cable to observe the sample processing in real time;

[0067] Step 2: Leveling of the sample stage;

[0068] The specific steps are as follows: first, fix a fused quartz sample with a thickness of 2 mm to be processed onto a three-dimensional translation stage with an adjustment device; the adjustment device of the three-dimensional translation stage includes an X-axis and a Y-axis in the horizontal direction and a Z-axis in the vertical direction; then, open the light gate in the processing light path, the laser is focused by a high-power objective lens, and the three-dimensional translation stage is controlled to find the upper left corner of the sample to be processed, and the laser focus is focused on the upper surface of the sample by adjusting the height of the moving stage, and the Z-axis height of the surface at the upper left corner of the sample is recorded at this time; similarly, find the surfaces of the upper right, lower left and lower right of the sample, and record the Z-axis height; adjust the adjustment device of the three-dimensional translation stage according to the recorded surface Z-axis height data, so that the surfaces at the four corners of the sample are at the same Z-axis height; at this time, the sample stage to be processed has been leveled;

[0069] Step 3: Design and parameter calculation of geometric phase grating elements;

[0070] The specific steps are as follows: First, use MATLAB software to design a geometric phase grating with a period of 1 mm and a circular grating with a diameter of 2 mm. Figure 8 As shown in (a). The phase distribution matrix data designed in MATLAB software is converted into a .csv file that can be recognized and processed by the processing system. In this file, the slow axis angle of the birefringent structure takes 180 discrete values ​​in the range of 0-180°, corresponding to 180 oriented birefringent structures. Then import the processing file with the suffix .csv into the processing control program. The required phase delay is calculated as The number of processing layers is determined by dividing the designed total phase retardation by the phase retardation of the single-layer birefringent structure.

[0071] The .csv file is a .csv file that can be recognized and processed by the processing system by converting the designed phase distribution matrix data into a .csv file through MATLAB software, which is used to control the slow axis angle of the birefringent structure and then control its phase.

[0072] Step 4: Preparation of uniform linear structure;

[0073] The specific steps are as follows: First, by adjusting the height of the moving stage, ensure that the femtosecond laser can be accurately focused on the predetermined processing area inside the sample. The parameters of the femtosecond laser are a pulse width of 300fs, a repetition rate of 200kHz, a scanning speed of 6mm / s, a point spacing of 1μm, and a single-point exposure time of 266μs. It is calculated that the stage moves 30nm for each pulse of the laser, which meets the 10-100nm requirement of near-field modulation. The points are induced by the femtosecond laser, and the points are connected into lines for line scanning. The parameter selection is based on the principle of near-field normalization of femtosecond laser pulses and the interaction mechanism of fused quartz to ensure that the new birefringent structure can be effectively induced. The processed line structure was characterized using a birefringence microscope, and obvious birefringence characteristics were observed, such as Figure 5 As shown in FIG. 1 , the image is obtained by a birefringent microscope, and the arrow indicates the slow axis direction of the novel birefringent structure. Further data analysis is performed on the phase delay R at different horizontal positions, as shown in FIG. Figure 6 As shown, the results indicate that the phase delay of the new birefringent structure processed by near-field normalization of femtosecond laser pulses has almost no fluctuation, that is, it has high uniformity; through the above steps, a uniform line structure is successfully prepared.

[0074] Step 5: Determine the parameters of the geometric phase grating prepared by femtosecond laser pulse;

[0075] On the basis of processing a uniform line structure, the line spacing is set to 1μm, and the line scanning process is performed to form a block structure. The program controls the Pockels cell to select four polarizations as a group, and the processing pulse energies are 0.9μJ, 0.8μJ, 0.7μJ, 0.6μJ, and 0.5μJ respectively. The processed block structure is characterized by a birefringence microscope, and the birefringence structure can be observed, such as Figure 7 The parameter window of the high transmittance birefringence structure was found, and the new birefringence parameter with a suitable phase delay was determined to be a pulse energy of 0.6 μJ.

[0076] Step 6: Femtosecond laser pulse preparation of geometric phase grating;

[0077] On the basis of processing a uniform line structure, the line spacing is set to form a two-dimensional plane structure, and then a geometric phase grating is prepared by processing layer by layer from deep to shallow.

[0078] After processing, a birefringence microscope is used to test the processing quality to ensure that the phase distribution and phase delay meet the design requirements. The birefringence microscope image is as follows: Figure 8 as shown in (b).

[0079] In this embodiment, the sample to be processed is 7979 glass produced by Corning, with a thickness of 0.1-7 mm; the Pockels cell is a Leysop-1030nm electro-optical modulator produced by the British company Leysop, and the beam expansion system has a beam expansion multiple of 1.5-3 times. The objective lens OL used in laser processing is an objective lens produced by New Focus, with a numerical aperture NA=0.16, a magnification of 10×, and a working distance of 14.0 mm; the illumination light source uses a white LED light source.

[0080] Example 2

[0081] This embodiment provides an application of a high-transmittance, high-uniformity birefringence structure prepared by utilizing near-field normalization of femtosecond pulses in edge detection efficiency, and specifically includes the following steps:

[0082] Step A, testing the efficiency of the geometric phase grating;

[0083] The test optical path uses a laser with a wavelength of 532nm. The test process first allows the laser to pass through a Glan prism and then enter a geometric phase grating. The grating's splitting principle is as follows Fig. 9 As shown in the figure, the outgoing light beam is divided into left-handed circularly polarized light and right-handed circularly polarized light with horizontal displacement. Fig.10 As shown in the figure, the observed result is that there are only positive and negative first-order diffraction spots, but no 0th-order and other-order diffraction spots. This phenomenon proves that the geometric phase grating has a highly efficient splitting ability and can effectively decompose the incident light beam into the required polarization state while suppressing unnecessary diffraction orders.

[0084] Step B: edge detection;

[0085] The specific steps are: Fig.11 As shown, the laser beam first irradiates the object to be detected, and then passes through the Glan prism P1 and the 4f optical system composed of two convex lenses L1 and L2 in sequence, wherein the distance between the lenses is twice the focal length f of the lens, and the distance between the object and the first convex lens L1 of the 4f system is the focal length f of the lens; the geometric phase grating element prepared in step A is placed on the focal plane between the first and second lenses to realize the Fourier transform of the light beam; after the linearly polarized light passes through the 4f system, it is divided into left-handed circularly polarized light LCP and right-handed circularly polarized light RCP with horizontal displacement offset, and the overlapping part in the middle thereof appears as linear polarization; the laser further passes through the analyzer P2, which can filter the linearly polarized light in the overlapping area in the middle of the image while retaining the circularly polarized light at the edge of the image; finally, the edge information of the object is captured and analyzed by a CCD sensor connected to a computer PC; wherein the distance between the CCD sensor and the second convex lens of the 4f system is the focal length f of the lens.

[0086] In this embodiment, the lens of the 4f optical system is a MCX10610 plano-convex lens produced by Shenzhen Lubang Technology Co., Ltd., with a focal length of 10-50 mm; the polarizer is a FLP51-VIS-M thin film linear polarizer produced by Shenzhen Lubang Technology Co., Ltd.

[0087] By changing the period of the geometric phase grating, the angle between the outgoing left-handed circularly polarized light and the right-handed circularly polarized light can be controlled, that is, the horizontal displacement of the two images can be controlled, and then the resolution of image edge detection can be adjusted.

[0088] The birefringence microscope is a BX53 optical microscope produced by Olympus equipped with an Oosight imaging system; the specific method for determining the new birefringence structure is to utilize the characteristic that the structure has a transmittance of up to 99% in the visible light range. Specifically, the structure cannot be seen under an optical microscope, but can be seen under a birefringence microscope for judgment.

[0089] Without adding a polarizer, the original image of the object to be detected is first obtained, such as Fig.12 As shown in (a); insert a polarizer in the optical path to filter out the linear polarized light in the overlapping part of the left-hand circular polarization (LCP) and right-hand circular polarization (RCP) images, leaving the circular polarization light component at the edge, thereby obtaining the image of the edge of the object, as shown in FIG. Fig.12 (b) as shown.

[0090] The results show that this implementation utilizes the near-field enhanced normalization effect of the femtosecond pulse in the seed modified area. Under the technology of reasonably adjusting the scanning speed and scanning power change, the continuous modification caused by the near-field enhancement makes the energy localized in the newly formed nanostructure, forming anisotropic nanostructures. In addition, because the near-field modulation effect size is at the nanometer level and does not introduce scattering effects, the high transmittance and high uniformity birefringence structure prepared has good accuracy in edge detection efficiency; in comparison, under non-near-field normalization conditions, the non-new birefringence structure is induced. Because energy control is usually not achieved, and the spacing is rarely controlled within the range of near-field enhancement, the structure is wildly enlarged to the micrometer level, and the scattering directly changes from Rayleigh scattering to Mie scattering, resulting in low transmittance and poor uniformity. When used as a device, the strong scattered light will cause uneven light intensity distribution in the edge detection process, bringing strong speckle, especially when detecting objects with complex shapes, so that the edge information cannot be accurately captured and reconstructed or false information is generated, reducing the accuracy of edge detection.

[0091] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0092] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0093] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for preparing a high-transmittance, high-uniformity geometric phase grating element using near-field normalization of femtosecond pulses, characterized in that: The specific steps include: Step 1: Construction of laser processing optical path; The specific steps are as follows: first, the femtosecond laser emitted by the laser passes through the energy control system composed of the electrically controlled half-wave plate HWP and the Glan prism P, and then passes through the polarization control system composed of the Pockels cell and the quarter-wave plate QWP; then the light beam passes through the beam expansion system composed of the concave lens L1 and the convex lens L2, and then is projected to the processing objective lens through the reflector M1, and finally focused into the inside of the sample to be processed; The LED lighting device is placed on the lower side of the sample stage, so that the white light emitted by the LED lighting device can penetrate the sample to be processed and illuminate the objective lens OL; the white light is transmitted by the reflector M1, and then reflected again by the reflector M2, and finally focused on the second convex lens L3, and forms a clear image on the CCD sensor; the CCD sensor is connected to the industrial computer through a data cable to observe the sample processing status in real time; Step 2: Leveling of the sample stage; The specific steps are as follows: first, fix the sample to be processed on a three-dimensional displacement stage with an adjustment device; the adjustment device of the three-dimensional displacement stage includes an X-axis and a Y-axis in the horizontal direction and a Z-axis in the vertical direction; then, open the light gate in the processing light path, the laser is focused by the high-power objective lens, and the three-dimensional displacement stage is controlled to find the upper left corner of the sample to be processed, and the laser focus is focused on the upper surface of the sample by adjusting the height of the moving stage, and the Z-axis height of the surface at the upper left corner of the sample is recorded at this time; similarly, find the surfaces of the upper right, lower left and lower right of the sample, and record the Z-axis height; adjust the adjustment device of the three-dimensional displacement stage according to the recorded surface Z-axis height data, so that the surfaces at the four corners of the sample are at the same Z-axis height; at this time, the sample stage to be processed has been leveled; Step 3: Design and parameter calculation of geometric phase grating elements; The specific steps are as follows: first, select the geometric phase grating period Λ, element size and shape that match the actual needs; then, according to the phase gradient formula of the one-dimensional grating In the formula, x and y represent the position coordinates of the rectangular coordinate system on the two-dimensional plane, and the data used to describe the grating phase distribution is generated in the form of a two-dimensional array or matrix; then, through the formula To determine the phase The relationship between the slow axis angle θ of the birefringent structure, where the slow axis angle takes N discrete values ​​in the range of 0-180°, corresponding to N kinds of oriented birefringent structures, to construct a geometric phase grating; finally, through the formula The total phase delay R of the geometric phase grating element is calculated, where λ represents the wavelength of the light source of the detection light path; and the required number of processing layers is determined by dividing the designed total phase delay by the phase delay of a single-layer birefringence structure; Step 4: Preparation of uniform linear structure; The specific steps are as follows: by adjusting the height of the three-dimensional translation stage, ensure that the femtosecond laser is focused on the predetermined processing area inside the sample; modulate the initial pulse energy, deposit the seed pulse inside the sample, rotate the electrically controlled half-wave plate HWP to adjust the direction of polarized light, and adjust the energy of subsequent pulses, and induce the second point from the initial seed through the principle of near-field normalization, and then use the second point as a seed to induce the third point, and so on; control the scanning speed and exposure time to move the translation stage during pulse radiation to perform line scanning processing; prepare a uniform line structure, the line structure is composed of a birefringent structure; the birefringent structure is a randomly distributed anisotropic nanopore structure; Step 5: Femtosecond laser pulse preparation of geometric phase grating; On the basis of processing a uniform line structure, the line spacing is set to form a two-dimensional plane structure, and then a geometric phase grating is prepared by processing layer by layer from deep to shallow.

2. The method for preparing a high-transmittance, high-uniformity geometric phase grating element using near-field normalization of femtosecond pulses as claimed in claim 1, characterized in that: In step 1, the sample to be processed is 7979 glass produced by Corning, with a thickness of 0.1-7 mm; the Pockels cell is a Leysop-1030nm electro-optical modulator produced by the British company Leysop, and the beam expansion system has a beam expansion factor of 1.5-3 times; the objective lens OL used in laser processing is an objective lens produced by New Focus, with a numerical aperture NA=0.1-0.65, a magnification of 10-60×, and a working distance of 10-14.0 mm; the illumination light source uses a white LED light source.

3. The method for preparing a high-transmittance, high-uniformity geometric phase grating element using near-field normalization of femtosecond pulses as claimed in claim 1, characterized in that: In step 4, the central wavelength of the femtosecond laser is 343-1030nm, the pulse width is 100fs-10ps, the repetition frequency is 1kHz-40MHz, the pulse energy is 0.5-0.9μJ, the point spacing Δx is 0.8-2μm, the scanning speed v is 6-10mm / s, and the single point exposure time is 4. The method for preparing a high-transmittance, high-uniformity geometric phase grating element using near-field normalization of femtosecond pulses as claimed in claim 1, characterized in that: In step 4, the number of seed pulses deposited inside the sample is 1-3; the diameter of the femtosecond laser induced spot is continuously adjustable in the range of 200nm-2μm, with an adjustment accuracy of 50nm; the energy of subsequent pulses from the second point onwards is determined by the seed size and is set to 75%-150% of the initial pulse energy; the near-field normalization principle is made to work, and the displacement stage movement distance of the subsequent pulses is within 10-100nm.

5. The method for preparing a high-transmittance, high-uniformity geometric phase grating element using near-field normalization of femtosecond pulses as claimed in claim 1, characterized in that: In step five, each layer is processed by scanning line according to the phase distribution data in the .csv file. The .csv file is a .csv file that can be recognized and processed by the processing system by converting the designed phase distribution matrix data into MATLAB software, which is used to control the slow axis angle of the birefringent structure and then control its phase.

6. A high-transmittance, high-uniformity geometric phase grating element prepared by utilizing the near-field normalization of femtosecond pulses, characterized in that: The method is prepared by the method according to any one of claims 1 to 5.

7. Application of the high-transmittance, high-uniformity geometric phase grating element prepared by utilizing the near-field normalization of femtosecond pulses as claimed in claim 6 in edge detection efficiency.

8. The application of the high transmittance and high uniformity geometric phase grating element prepared by utilizing the near-field normalization of femtosecond pulses in edge detection efficiency as claimed in claim 7, characterized in that: The application specifically comprises the following steps: Step A, constructing a test optical path, placing the prepared geometric phase grating element on the focal plane between the first convex lens L1 and the second convex lens L2 to achieve Fourier transform of the light beam; Step B, the laser beam first irradiates the object to be detected, and then passes through the Glan prism P1 and the 4f optical system composed of the first convex lens L1, the geometric phase grating element, and the second convex lens L2 in sequence, wherein the distance between the lenses is twice the focal length f of the lens, and the distance between the object and the first convex lens L1 of the 4f system is the focal length f of the lens; after the linearly polarized light passes through the 4f system, it is divided into left-handed circularly polarized light LCP and right-handed circularly polarized light RCP with horizontal displacement offset, and the overlapping part in the middle appears as linear polarization; the laser further passes through the analyzer P2, which can filter the linear polarized light in the overlapping area in the middle of the image while retaining the circularly polarized light at the edge of the image; finally, the edge information of the object is captured and analyzed by a CCD sensor connected to a computer PC; wherein the distance between the CCD sensor and the second convex lens of the 4f system is the focal length f of the lens.

9. The application of the high transmittance and high uniformity geometric phase grating element prepared by utilizing the near-field normalization of femtosecond pulses in edge detection efficiency as claimed in claim 8, characterized in that: The lens of the 4f optical system is a MCX10610 plano-convex lens produced by Shenzhen Lubang Technology Co., Ltd., with a focal length of 10-50 mm; the polarizer is a FLP51-VIS-M thin film linear polarizer produced by Shenzhen Lubang Technology Co., Ltd.

10. The application of the high transmittance and high uniformity geometric phase grating element prepared by utilizing the near-field normalization of femtosecond pulses in edge detection efficiency as claimed in claim 8, characterized in that: By changing the period of the geometric phase grating, the angle between the outgoing left-handed circularly polarized light and the right-handed circularly polarized light can be controlled, that is, the horizontal displacement of the two images can be controlled, and then the resolution of image edge detection can be adjusted.

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