Method for improving optical storage capacity based on ultraviolet femtosecond laser multi-cycle low-temperature direct writing

Abstract

The invention discloses a method for improving optical storage capacity based on ultraviolet femtosecond laser multi-cycle low-temperature direct writing, and belongs to the technical field of laser processing. Leveling the sample table; designing a random data structure and calculating parameters; preparing a random data structure; reading and characterizing a random data structure; according to the method, firstly, ultraviolet femtosecond laser with the wavelength of 343 nm is used for replacing infrared / visible band laser used in traditional five-dimensional optical storage, secondly, a multi-cycle scanning mode is used for replacing an existing scanning mode, and birefringence modification with the smaller size is induced in quartz glass at low temperature; the tiny birefringence modification is used for realizing data writing and reading of the optical memory with higher storage density; according to the multi-cycle scanning mode, heat accumulation in the machining process is affected by changing single-point one-way acting time, so that the dot spacing is shortened, and the storage capacity is increased.

Description

Technical Field

[0001] The present invention belongs to the field of laser processing technology, and in particular relates to utilizing ultraviolet (UV) femtosecond pulses to write birefringent voxels at low temperatures in multiple cycles, thereby demonstrating high-density multidimensional optical data storage in quartz glass. Background Art

[0002] With the rapid development of information technologies such as artificial intelligence and cloud computing, the total amount of global data has increased rapidly, placing higher demands on data storage technology. Among them, a large amount of cold data that is not frequently accessed but needs to be stored for a long time has always been the focus of attention. As one of the cold data storage methods, optical storage has the advantages of energy saving and offline security, and is considered to be an attractive alternative for future massive information storage. However, the current optical disk storage technology is limited to a storage capacity of about hundreds of GB and a lifespan of several decades, which cannot meet the massive data storage needs. There is an urgent need for a high-density, low-energy, and long-life cold data storage technology.

[0003] Based on the advantages of fast energy deposition and high precision, ultrafast laser writing of metal nanoparticles and dielectrics has opened the door to long-life, high-capacity multiplexed optical data storage. For example, five-dimensional (5D) data storage based on nanogratings has been realized in quartz glass by ultrafast laser writing, with a data capacity of up to hundreds of TB and a seemingly infinite lifetime. Specifically, in addition to the three spatial coordinates, the fourth dimension delay and the fifth dimension slow axis orientation are also utilized to achieve 5D data storage. However, since these nanostructures are written using visible light (VIS) or infrared (IR) femtosecond lasers, the size of the minimum data voxel written is also on the order of hundreds of nanometers, which limits its actual storage capacity to only 1 / 10 of the theoretical storage capacity or even lower. How to reduce the volume of the storage unit in the experiment is a bottleneck problem for increasing the actual storage capacity. According to the Abbe diffraction limit formula, reducing the wavelength of light or increasing the numerical aperture of the processing objective is the basic solution. However, since the bandgap width of short-wave ultraviolet femtosecond lasers is at least half that of commonly used visible light and near-infrared lasers, the thermal effect during the short-wave ultraviolet femtosecond laser processing is too strong, and high-precision processing cannot be achieved. Summary of the invention

[0004] In view of the defects existing in the prior art, the present invention provides a method for improving the optical storage capacity based on ultraviolet femtosecond laser multiple-cycle low-temperature direct writing. The method first uses an ultraviolet (UV) femtosecond laser with a wavelength of 343nm to replace the infrared / visible band laser used in traditional five-dimensional optical storage. Secondly, a multiple-cycle scanning method is used to replace the previous scanning method, and a smaller birefringence modification is induced at low temperature inside the quartz glass to perform high-density multi-dimensional optical data storage.

[0005] The present invention is achieved through the following technical solutions:

[0006] The method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing specifically comprises the following steps:

[0007] Step 1: Construction of laser processing optical path;

[0008] The specific steps are as follows: first, the femtosecond laser emitted by the laser passes through an electric attenuator composed of a first electric half-wave plate and a polarization beam splitter prism (PBS), then passes through a beam expansion system composed of a first convex lens L1 and a second convex lens L2, and then is reflected by a first reflector M1, passes through a second electric half-wave plate, and the beam diameter is aligned with the entrance of a galvanometer scanner unit. After the emitted laser beam passes through a four-focal length (4F) system, it is reflected by a second reflector M2 and is focused by an objective lens below the surface of the sample to be processed; an LED lighting device is placed on the lower side of the sample stage so that the white light emitted by the LED lighting device penetrates the sample to be processed and irradiates the objective lens OL; after the white light is transmitted by the second reflector M2, it is reflected again by a third reflector M3, and finally focused on a fourth convex lens L5, and forms a clear image on a CCD sensor; the CCD sensor is connected to an industrial computer via a data cable to observe the sample processing in real time;

[0009] Step 2: Leveling of the sample stage;

[0010] 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 upper right, lower left and lower right surfaces of the sample, and record the Z-axis height; adjust the adjustment device of the three-dimensional displacement stage according to the recorded 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;

[0011] Step 3: Design of random data structure and parameter calculation;

[0012] The specific steps are as follows: First, select the component size that matches the actual needs; according to the Abbe diffraction limit formula Determine the point interval of random data structure processing, where Δx represents the point separation, i.e., the point interval, λ is the wavelength of the light source in the detection light path, and NA is the numerical aperture of the objective lens used in the experiment;

[0013] The birefringence intensity, Δn, is calculated by the formula for the birefringence intensity of the laser-written voxel b =Ret. / l, where Δn brepresents the birefringence intensity, Ret. is the measured delay, l is the length of the structure along the light propagation direction, and then the layer spacing of the random data structure processing is determined by the birefringence intensity;

[0014] Step 4: Preparation of random data structure;

[0015] The specific steps are as follows: during processing, firstly, 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 pulse energy, rotate the electric half-wave plate to adjust the direction of polarized light, and induce a seed structure inside the sample, that is, the starting point of the entire row of data; then, based on the seed structure, import the processing file, perform dot matrix processing, that is, prepare a random data structure; the processing method of the dot matrix is ​​multiple cycle scanning, that is, one pulse is scanned at each point in a single pass, scan n points, and then move the femtosecond laser to the initial point for repeated scanning; the number of scans is N, so that the repetition rate is reduced to 1 / N of the original, and the heat accumulation effect is reduced to 1 / N of the original, and finally a single-layer random data structure with a reduced point spacing is obtained; according to the layer spacing calculated in step three, on the basis of processing the single-layer random data structure, a multi-layer random data structure is prepared by layer-by-layer processing from deep to shallow;

[0016] Step 5: Reading and characterization of random data structures;

[0017] The specific steps are as follows: use a birefringence microscope system to measure the sample, read out the delay and slow axis azimuth, adjust to a suitable objective lens, focus on the random data structure after collecting the background, and take a birefringence image; use a confocal micro-Raman spectrometer to characterize the structural changes in the laser-modified area, focus on the random data structure after collecting the background, and scan with excitation lasers of different wavelengths; use a fiber optic spectrometer to measure the transmission spectrum of the laser-written voxels, focus on the written random data structure after collecting the background, scan, and obtain the corresponding image.

[0018] Furthermore, in step one, the wavelength of the center of the femtosecond laser of the femtosecond laser direct writing processing system is 315-400nm, the pulse width is 100fs-300fs, and the repetition frequency is 0.1MHz-10MHz; the objective lens NA is 0.25-0.95, the magnification is 10×-40×; the single pulse ablation energy is 4-30nJ; and the beam expansion factor of the beam expansion system is 1.5-3 times.

[0019] Furthermore, in step one, the XY dual-axis galvanometer is used to arbitrarily deflect the femtosecond laser beam in the XY plane, thereby achieving the degree of freedom of femtosecond laser direct writing in the XY plane.

[0020] Furthermore, in step 4, the sample is a fused quartz glass sample with a thickness of 1-5 mm.

[0021] Furthermore, in step 4, before writing, the processed text is encoded using the American Standard Code for Information Interchange (ASCII) and then converted into octal system to correspond to the 8 kinds of oriented birefringence structures to generate a processed file.

[0022] Furthermore, in step 4, the dot spacing must be greater than the minimum dot separation, and the minimum value of the dot spacing depends on the readout recognition accuracy, which is in the range of 100-1000nm.

[0023] Furthermore, in step 4, the multiple cycle scanning can reduce the thermal accumulation temperature of a single point by 200-600° C., wherein the value range of n is 10-40, and the value range of N is 5-10.

[0024] Furthermore, in step four, the single-layer and multi-layer random data structures are both composed of birefringent structures; wherein the slow axis angle of each birefringent structure takes 8 discrete values ​​in the range of 0-180°, corresponding to 8 oriented birefringent structures, and random data writing processing under different energies, pulse numbers, and polarization parameters is selected.

[0025] The principle of the present invention to improve the optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing is as follows:

[0026] According to the Abbe diffraction limit formula, the way to increase capacity is to reduce the point spacing, and reducing the point spacing requires a shorter wavelength, but the shorter wavelength brings a greater heat accumulation effect. For short-wavelength lasers, when the repetition frequency is greater than 500kHz, as time increases, the temperature of the sample surface will rise sharply to more than 1000°C, and the glass transition temperature is between 1050-1200°C. The single point accumulates more heat, causing the glass to melt rapidly, thereby destroying the nanostructure, making it impossible to reduce the point spacing. The multiple cycle scanning method mainly affects the heat accumulation of the processing process by changing the single-way action time of the single point. The processing method of the dot matrix is ​​multiple cycle scanning, that is, scanning 1 pulse per point in a single pass, scanning n points, and then moving the femtosecond laser to the initial point for repeated scanning; the number of scans is N, so that the repetition frequency is reduced to 1 / N of the original, and the heat accumulation effect is reduced to 1 / N of the original, and finally a random data structure with reduced point spacing is obtained.

[0027] Compared with the prior art, the advantages of the present invention are as follows:

[0028] The method of improving optical storage capacity based on ultraviolet femtosecond laser multiple-cycle low-temperature direct writing of the present invention uses ultraviolet (UV) femtosecond laser pulses with a wavelength of 343nm to induce smaller birefringence modification inside quartz glass. This tiny birefringence modification is used to achieve data writing and reading of optical storage with higher storage density; the multiple-cycle scanning method affects the heat accumulation of the processing process by changing the single-point single-pass action time, thereby reducing the point spacing and increasing the storage capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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.

[0030] Figure 1 A schematic diagram of the optical path of using ultraviolet (UV) ultrafast laser pulses to write birefringent voxels for high-density multidimensional optical data storage according to the present invention;

[0031] In the figure, PBS-polarization beam splitter, L1-first convex lens, L2-second convex lens, L1 and L2 form a beam expansion system, L3-concave lens, L4-third convex lens, L3 and L4 form a 4f focusing system, L5-fourth convex lens, LED-illumination lamp, M1-first reflector, M2-second reflector, M3-third reflector, three-dimensional translation stage (adjustment device includes horizontal X-axis, Y-axis and vertical Z-axis), CCD-camera;

[0032] Figure 2 This is an image of a birefringence structure written in silicon dioxide using ultraviolet (UV) femtosecond laser pulses in the present invention. The image is taken by a birefringence measurement system, and the arrow indicates the polarization of the laser beam;

[0033] Figure 3 Schematic diagram of data encoding (a) and decoding (b);

[0034] Among them, 8 numbers correspond to 8 colors, corresponding to 8 slow axis orientations;

[0035] Figure 4 To simulate the time-dependent changes in the surface temperature of silicon dioxide under different laser repetition frequencies;

[0036] Among them, (a) is 100kHz, (b) is 500kHz, (c) is 1MHz, and (d) is 5MHz.

[0037] Figure 5This is a schematic diagram of the scanning method of using ultraviolet femtosecond laser multiple cycle low-temperature direct writing processing. The same structure in the figure is processed by ten scanning times;

[0038] Figure 6 (a) is an optical side view of a voxel written by ultraviolet laser, and the purple arrow indicates the propagation direction of the laser beam; (b) is a schematic diagram of the relationship between the average longitudinal length of the written voxel and the number of pulses; (c) is a schematic diagram of the calculated birefringence intensity of the voxel in (b);

[0039] Figure 7 Demonstration of optical data storage by writing an excerpt from the novel "War and Peace" into a quartz glass sample using UV femtosecond laser multi-cycle low-temperature direct writing technology;

[0040] Among them, (a), (b), and (c) are polar coordinate diagrams of the retardation and data voxel azimuth measured from the first layer to the third layer; (d) is taken by the birefringence measurement system;

[0041] Figure 8 This is a diagram of the structural changes in the laser-modified area characterized by confocal micro-Raman spectroscopy; the two curves are the Raman spectra of the laser-written voxel and the original quartz glass;

[0042] Among them, (a) is the excitation wavelength of 325nm, (b) is the excitation wavelength of 514nm;

[0043] Fig. 9 The transmission spectrum of random data structures with different numbers of layers processed in silica is measured using a fiber optic spectrometer. DETAILED DESCRIPTION

[0044] 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:

[0045] Example 1

[0046] Depend on Figure 1 It can be seen that the femtosecond laser direct writing processing system used in this embodiment is as follows: the laser emits 343nm femtosecond laser, which is continuously controlled by the pulse energy system through an electric attenuator composed of an electric half-wave plate and a polarization beam splitter prism (PBS). Subsequently, the beam expansion system composed of two convex lenses L1 and L2 aligns the beam diameter from, with the entrance diameter of the galvanometer scanner unit. The polarization direction of the linearly polarized writing laser beam is adjusted by an electric half-wave plate placed in front of the galvanometer scanner. The galvanometer scanner accurately positions the laser beam in the XY plane, parallel to the upper surface of the sample. After the outgoing laser beam passes through the four-focal length (4F) system, it passes through a reflector and reaches the objective lens for focusing.

[0047] This embodiment provides a method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing, which specifically includes the following steps:

[0048] Step 1: Leveling of the sample stage;

[0049] 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 upper right, lower left and lower right surfaces 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;

[0050] Step 2: Design of random data structure and parameter calculation;

[0051] First, select the component size that matches the actual needs; the femtosecond laser wavelength is selected as 343nm, the pulse width is selected as 280fs, the repetition frequency is selected as 200kHz, the numerical aperture of the objective lens is selected as 0.95, the pulse energy is selected as 11.2nJ, and the number of pulses is 40-320; according to the Abbe diffraction limit formula In the formula, Δx represents the point separation, i.e., the point interval, λ represents the wavelength of the light source in the detection light path, NA is the numerical aperture of the objective lens used in the experiment, and the point interval Δx is selected to be 0.7 μm;

[0052] Depend on Figure 2 It can be seen that the slow axis angle takes 8 discrete values ​​in the range of 0-180°, corresponding to 8 oriented birefringence structures. The 8 colors measured in the birefringence system correspond to the corresponding 8 orientations. Different pulse numbers and polarizations are selected for random data writing processing.

[0053] Step 3: Preparation of random data structure;

[0054] Depend on Figure 3 It can be seen that before writing, the text is encoded using the American Standard Code for Information Interchange (ASCII), and then converted into octal, corresponding to the 8 types of oriented birefringence structures, to generate a processing file;

[0055] Depend on Figure 4It is known that for short-wavelength lasers, the surface temperature of the sample will increase to varying degrees with time at different repetition frequencies. When the repetition frequency is 100kHz, the surface temperature of the sample remains below 1000℃; when the repetition frequency is 500kHz, the surface temperature of the sample will rise sharply and exceed 1000℃ with time; when the repetition frequency is 1MHz, the surface temperature of the sample will rise sharply and exceed 1500℃ with time; when the repetition frequency is 5MHz, the surface temperature of the sample reaches 2500℃ in just 2.5μs; and the glass transition temperature is between 1050-1200℃, and more heat accumulates at a single point, causing the glass to melt rapidly and destroy the nanostructure, making it impossible to reduce the point spacing.

[0056] Therefore, by Figure 5 It can be seen that the processing method of the random data structure adopts multiple cycle scanning, that is, one pulse is scanned at each point in a single pass, n points are scanned, and then the femtosecond laser is moved to the initial point for repeated scanning; the number of scans is N, so that the repetition rate is reduced to 1 / N of the original, and the heat accumulation effect is reduced to 1 / N of the original, and finally a single-layer random data structure with a reduced point spacing can be obtained.

[0057] During processing, firstly, the height of the three-dimensional translation stage is adjusted to ensure that the femtosecond laser is focused on the predetermined processing area inside the sample; the pulse energy is modulated, and the electric half-wave plate HWP is rotated to adjust the direction of polarized light, so as to induce a seed structure inside the sample, i.e., the starting point of the entire row of data; then, based on the seed structure, dot matrix processing is performed, i.e., a random data structure is prepared, and the random data structure is composed of a birefringent structure;

[0058] Depend on Figure 6 It can be seen that (a) is the optical side view image of the voxel written by ultraviolet laser, the writing pulse energy is 19.6nJ, the pulse number is 320, and the purple arrow indicates the propagation direction of the laser beam; (b) is the relationship between the average longitudinal length of the voxel written with pulse energy of 7nJ-22nJ and the pulse number; the formula for estimating the birefringence of the laser-written voxel is Δn b =Ret. / l, where Δn b is the birefringence intensity, Ret. is the measured delay, l is the structure length along the direction of light propagation, (c) is the calculated birefringence intensity of the voxel in (b), and under low energy pulses (Ep=7nJ), the birefringence intensity of the laser-written voxel increases with the increase of the pulse number; the image is used to determine the appropriate layer spacing under different processing parameters; on the basis of processing a single-layer random data structure, a multi-layer random data structure is prepared by layer-by-layer processing from deep to shallow.

[0059] Step 4: Reading and characterization of random data structures;

[0060] Use the birefringence measurement system to quantitatively check the birefringence of laser-induced voxels, especially its delay and slow axis azimuth. Adjust to the appropriate objective lens, focus on the random data structure after collecting the background, take the birefringence image, and import the data into Origin software for analysis;

[0061] like Figure 7 It can be seen that (d) image is a demonstration of optical data storage in which an excerpt from the novel "War and Peace" is written into a quartz glass sample by ultraviolet femtosecond laser multiple-cycle low-temperature direct writing technology; the data is written in three layers with a point spacing of 0.7μm, (a), (b), and (c) are polar coordinate diagrams of the delay and data voxel azimuth measured from the first layer to the third layer with layer spacings of 9.6μm, 6.6μm, and 4.8μm, respectively. The data readout accuracy of the three layers is always 100%, which confirms the practical feasibility of ultraviolet femtosecond laser multiple-cycle low-temperature direct writing technology in optical data storage applications;

[0062] A confocal micro-Raman spectrometer was used to characterize the structural changes in the laser-modified area. After collecting the background, the focus was on the random data structure and scanned using excitation light of different wavelengths. The obtained data was processed using Origin software;

[0063] like Figure 8 It can be seen that the images are Raman spectra of laser-written voxels and original quartz glass; the excitation wavelengths are (a) 325nm and (b) 514nm, respectively. -1 No obvious peak difference is observed at (a), which can be attributed to the photobleaching effect caused by hundreds of UV femtosecond laser pulses. In (b), it can be noted that the peak at 3700 cm -1 There is a difference in the peak value at , that is, the intensity of the peak in the original glass is higher than the peak intensity in the laser-written structure, indicating that the high concentration of hydroxyl groups in the CVD quartz glass has been destroyed by the ultraviolet femtosecond laser pulse, that is, modified;

[0064] The transmission spectrum of the laser-written voxels was measured using a fiber optic spectrometer. After collecting the background, the random data structure was focused and scanned. The obtained data was processed using Origin software;

[0065] like Fig. 9 As can be seen, the image is the optical transmission spectrum of UV laser written voxels of different layers (1, 3 and 5 layers) with a point spacing of 0.7μm, using the original silica glass as a reference. The transmittance of the 5-layer voxel at 550nm is about 97.5%. This value is lower than the X-type data voxel based on anisotropic nanoholes (>99% at 100 layers), but is close to the transmittance of nano-thin layered voxels written with 515nm laser, which means that data voxels, that is, random data structures, can be written by UV laser pulses and retrieved with reasonable accuracy.

[0066] In this embodiment, the point spacing is reduced to 0.7μm and the layer spacing is reduced to 4.8μm, indicating that optical data storage of up to 7.5TB / disk can be achieved in silicon glass through ultraviolet femtosecond laser multiple-cycle low-temperature direct writing technology, which is higher than the 1.7TB reported by the existing technology using long wavelength processing or the several TB reported by Microsoft, and the readout accuracy can reach 100%, proving that the method is practical and effective.

[0067] 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.

[0068] 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.

[0069] 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 improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing, 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 an electric attenuator composed of a first electric half-wave plate and a polarization beam splitter prism, then passes through a beam expansion system composed of a first convex lens L1 and a second convex lens L2, and then is reflected by a first reflector M1, passes through a second electric half-wave plate, and the beam diameter is aligned with the entrance of a galvanometer scanner unit, and after the emitted laser beam passes through a four-focal length system, it is reflected by a second reflector M2 and is focused by an objective lens below the surface 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 second reflector M2, and then reflected again by the third reflector M3, and finally focused on the fourth convex lens L5, 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 upper right, lower left and lower right surfaces of the sample, and record the Z-axis height; adjust the adjustment device of the three-dimensional displacement stage according to the recorded 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 of random data structure and parameter calculation; The specific steps are as follows: First, select the component size that matches the actual needs; according to the Abbe diffraction limit formula Determine the point interval of random data structure processing, where Δx represents the point separation, i.e., the point interval, λ is the wavelength of the light source in the detection light path, and NA is the numerical aperture of the objective lens used in the experiment; The birefringence intensity, Δn, is calculated by the formula for the birefringence intensity of the laser-written voxel b =Ret. / l, where Δn b represents the birefringence intensity, Ret. is the measured delay, l is the length of the structure along the light propagation direction, and then the layer spacing of the random data structure processing is determined by the birefringence intensity; Step 4: Preparation of random data structure; The specific steps are as follows: during processing, firstly, 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 pulse energy, rotate the electric half-wave plate to adjust the direction of polarized light, and induce a seed structure inside the sample, that is, the starting point of the entire row of data; then, based on the seed structure, import the processing file, perform dot matrix processing, that is, prepare a random data structure; the processing method of the dot matrix is ​​multiple cycle scanning, that is, one pulse is scanned at each point in a single pass, scan n points, and then move the femtosecond laser to the initial point for repeated scanning; the number of scans is N, so that the repetition rate is reduced to 1 / N of the original, and the heat accumulation effect is reduced to 1 / N of the original, and finally a single-layer random data structure with a reduced point spacing is obtained; according to the layer spacing calculated in step three, on the basis of processing the single-layer random data structure, a multi-layer random data structure is prepared by layer-by-layer processing from deep to shallow; Step 5: Reading and characterization of random data structures; The specific steps are as follows: use a birefringence microscope system to measure the sample, read out the delay and slow axis azimuth, adjust to a suitable objective lens, focus on the random data structure after collecting the background, and take a birefringence image; use a confocal micro-Raman spectrometer to characterize the structural changes in the laser-modified area, focus on the random data structure after collecting the background, and scan with excitation lasers of different wavelengths; use a fiber optic spectrometer to measure the transmission spectrum of the laser-written voxels, focus on the written random data structure after collecting the background, scan, and obtain the corresponding image.

2. The method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing as claimed in claim 1, characterized in that: In step 1, the wavelength of the femtosecond laser center of the femtosecond laser direct writing processing system is 315-400nm, the pulse width is 100fs-300fs, and the repetition frequency is 0.1MHz-10 MHz; the objective lens NA is 0.25-0.95, the magnification is 10×-40×; the single pulse ablation energy is 4-30nJ; the beam expansion factor of the beam expansion system is 1.5-3 times.

3. The method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing as claimed in claim 1, characterized in that: In step 4, the sample is a fused quartz glass sample with a thickness of 1-5 mm.

4. The method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing as claimed in claim 1, characterized in that: In step 4, before writing, the processed text is encoded using the American Standard Code for Information Interchange, and then converted into octal, corresponding to the 8 kinds of oriented birefringence structures, to generate a processing file.

5. The method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing as claimed in claim 1, characterized in that: In step 4, the dot spacing is greater than the minimum dot separation, and the minimum value of the dot spacing depends on the readout recognition accuracy and is in the range of 100-1000 nm.

6. The method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing as claimed in claim 1, characterized in that: The multiple cycle scanning can reduce the heat accumulation temperature of a single point by 200-600° C., wherein the value range of n is 10-40, and the value range of N is 5-10.

7. The method for improving optical storage capacity based on ultraviolet femtosecond laser multiple cycle low temperature direct writing as claimed in claim 1, characterized in that: In step 4, the single-layer and multi-layer random data structures are both composed of birefringent structures; wherein the slow axis angle of each birefringent structure takes 8 discrete values ​​in the range of 0-180°, corresponding to 8 oriented birefringent structures, and random data writing processing under different energies, pulse numbers, and polarization parameters is selected.

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