A method for laser conditioning of a nonlinear optical crystal
Patent Information
- Application Number
- CN202311103942.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2023-08-29
- Publication Date
- 2026-09-25
- Estimated Expiration
- 2043-08-29
AI Technical Summary
[0006]综上，目前针对KDP及其同类晶体的激光预处理技术存在处理效率低、处理成本高的问题
[0020]本发明提供了一种基于双光子吸收的非线性光学晶体的激光预处理方法，包括以下步骤：采用激光对非线性光学晶体进行辐照，所述激光的光子能量为Eg/2～Eg，Eg为所述非线性光学晶体的禁带宽度。由于现有技术中采用波长为355nm的激光进行激光预处理，非线性光学晶体的价带电子必须同时吸收三个光子，才能够被电离到导带，是一个三阶非线性过程。而本发明采用激光光子能量为Eg/2～Eg进行预处理，非线性光学晶体的价带电子只需要同时吸收两个光子，就能够被电离到导带，是一个二阶非线性过程。由于双光子吸收的概率比三光子吸收的的概率高数千倍到上万倍，因此，本发明采用激光光子能量为Eg/2～Eg，即采用基于双光子吸收的激光预处理方法比现有的基于三光子吸收的激光预处理的效率有极大的提高。由实施例和对比例的结果表明，本发明采用比355nm弱很多(10倍以上)的266nm的激光长波进行预处理，在激光功率密度只有0.13GW/cm2的条件下，DKDP晶体预处理后零概率体损伤阈值为4.7J/cm2，比预处理前的零概率体损伤阈值2.3J/cm2，提升了120％。同时与现有技术中获得同样预处理效果(体损伤阈值提升1倍左右)需要4～5GW/cm2的激光强度，本发明的激光功率密度显著降低，从而提升了处理效率、降低了预处理成本。而且本发明基于双光子吸收的KDP晶体激光预处理除了具有比三光子吸收激光预处理更高的预处理效率外，还有比三光子吸收激光预处理在提升晶体的体损伤阈值上更大的潜力。
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Figure CN117961262B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of laser preprocessing technology, specifically relating to a laser preprocessing method for nonlinear optical crystals. Background Technology
[0002] Nonlinear artificial crystals are essential optical components in lasers and large-scale high-power laser devices. Potassium dihydrogen phosphate (KDP) nonlinear optical crystals, and other similar crystals with similar lattice structures, such as deuterated potassium dihydrogen phosphate (DKDP) with different deuteration rates and ammonium dihydrogen phosphate (ADP), are currently widely used and important nonlinear artificial crystals. These crystals are typically used as frequency doubling elements or electro-optic switching elements in laser devices and are frequently exposed to high-power laser irradiation. The laser intensity I (unit: W / cm²) irradiating the crystal surface is... 2 The damage threshold is greater than a certain threshold (body damage threshold I). th Subsequently, the laser will cause irreversible damage inside the crystal, causing the nonlinear artificial crystal to lose its basic function, thus rendering the laser unusable. Therefore, the ability of KDP and similar crystals to resist strong laser damage, i.e., the volume damage threshold I... th The size of the value is one of its most important performance indicators.
[0003] To improve the resistance of KDP and similar crystals to strong laser damage, a technique called laser pretreatment has been developed over the past 40 years. The basic approach of this technique is to assume that the bulk damage threshold of untreated KDP and similar crystals is I. th_i If the laser intensity is greater than I th_i Direct laser irradiation of a crystal will cause irreversible bulk damage to the crystal's interior. However, lasers with an intensity less than I can be used. th_i Lasers, for example, with an intensity of 0.5 times that of I... th_i The crystal is irradiated with a certain number of laser pulses, followed by an intensity of I. th_i When the crystal is irradiated with a laser, the crystal no longer suffers damage. This increases the laser-induced damage threshold I. th This method is known as the laser pretreatment technique for KDP and similar crystals. Taking DKDP crystal as an example, the highest level of crystal growth technology currently available produces crystals with Ig... th_i Approximately 4J / cm 2 / 3ns. After pretreatment with a 355nm laser, its I th It can be increased to about 8J / cm 2 / 3ns. In practice, to obtain the best pretreatment effect, through optimization of laser parameters, it has been found that for a 355nm wavelength laser, the pretreatment effect is strongly dependent on the pretreatment laser intensity; the stronger the laser, the better the pretreatment effect. However, the laser cannot be too strong, otherwise it will directly damage the crystal. The laser intensity with the optimal pretreatment effect is approximately 5GW / cm². 2 .
[0004] The paper "Offline Sub-nanosecond Laser Pretreatment Technology for Large-Aperture Potassium Dihydrogen Deuteride Phosphate Crystals" (Liu Zhichao et al., Acta Physica Sinica, Vol. 70, No. 7 (2021), 074208) discloses a laser pretreatment technique for KDP / DKDP crystals using a third-harmonic laser (wavelength 355 nm) output from an Nd:YAG laser. The experimental parameters for pretreatment are: pulse width of the pulsed laser acting on the DKDP crystal to be pretreated is 0.5 nanoseconds, spot diameter is 0.68 mm, wavelength is 355 nm, and maximum laser flux for pretreatment is 2 J / cm². 2 Maximum laser power density 4GW / cm² 2 Sub-nanosecond laser pretreatment can increase the zero-probability bulk damage threshold of DKDP crystal by about 1 time.
[0005] However, due to the limited output laser energy of pulsed lasers, in order to achieve the 4-5 GW / cm² required for preprocessing... 2 The laser intensity can only reduce the laser spot size, making the spot diameter less than 1 mm (0.68 mm as reported in the literature above). Considering the maximum repetition frequency of the laser pulse, in terms of processing efficiency, current laser pretreatment can only process approximately 20 cm² of crystal surface area per hour. 2 .
[0006] In summary, current laser pretreatment technologies for KDP and similar crystals suffer from low processing efficiency and high processing costs. Summary of the Invention
[0007] The purpose of this invention is to provide a laser preprocessing method for nonlinear optical crystals based on two-photon absorption. The laser preprocessing method provided by this invention can significantly and efficiently improve the damage threshold of the nonlinear optical crystal laser body and reduce the cost of preprocessing.
[0008] To achieve the above objectives, the present invention provides the following technical solution:
[0009] This invention provides a laser preprocessing method based on a two-photon absorption nonlinear optical crystal, comprising the following steps:
[0010] A nonlinear optical crystal is irradiated with a laser, wherein the photon energy of the laser is Eg / 2 to Eg, and Eg is the bandgap width of the nonlinear optical crystal.
[0011] Preferably, the wavelength of the laser is 165–305 nm.
[0012] Preferably, the wavelength of the laser is 266nm.
[0013] Preferably, the laser is generated by a laser, which is a solid-state laser or an excimer laser.
[0014] Preferably, the nonlinear optical crystal comprises potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate, or ammonium dihydrogen phosphate.
[0015] Preferably, the pulse width of the laser is 0.3 to 40 ns or 100 fs to 100 ps.
[0016] Preferably, the laser spot diameter is 0.05 to 5 mm.
[0017] Preferably, the power density of the laser is 0.2 to 1.5 times the volume damage threshold of the nonlinear optical crystal, and the volume damage threshold of the nonlinear optical crystal is the volume damage threshold corresponding to the wavelength of the pre-processed laser.
[0018] Preferably, the number of laser pulses is 90-95% of the saturation pulse number.
[0019] Preferably, the nonlinear optical crystal is potassium dihydrogen phosphate; the laser wavelength is 266 nm, the pulse width is 10 ns, the spot diameter is 1 mm, and the power density is 0.13 GW / cm². 2 .
[0020] This invention provides a laser pretreatment method for nonlinear optical crystals based on two-photon absorption, comprising the following steps: irradiating the nonlinear optical crystal with a laser, wherein the photon energy of the laser is Eg / 2 to Eg, where Eg is the bandgap of the nonlinear optical crystal. In existing technologies, laser pretreatment using a 355nm wavelength laser requires the valence band electrons of the nonlinear optical crystal to absorb three photons simultaneously to be ionized to the conduction band, which is a third-order nonlinear process. However, this invention uses a laser photon energy of Eg / 2 to Eg for pretreatment, requiring only the absorption of two photons simultaneously for the valence band electrons to be ionized to the conduction band, which is a second-order nonlinear process. Since the probability of two-photon absorption is thousands to tens of thousands of times higher than that of three-photon absorption, this invention, using a laser photon energy of Eg / 2 to Eg, significantly improves the efficiency compared to existing laser pretreatment methods based on three-photon absorption. The results from the examples and comparative examples show that the present invention uses a 266nm laser wavelength, which is much weaker than 355nm (more than 10 times weaker), for preprocessing, at a laser power density of only 0.13GW / cm². 2 Under these conditions, the zero-probability volume damage threshold after DKDP crystal pretreatment is 4.7 J / cm. 2 Compared to the zero-probability volumetric damage threshold of 2.3 J / cm before pretreatment. 2 This represents a 120% improvement. Simultaneously, achieving the same pretreatment effect (approximately doubling the volumetric damage threshold) with existing technologies requires 4–5 GW / cm³. 2 The laser power density of this invention is significantly reduced, thereby improving processing efficiency and reducing preprocessing costs. Furthermore, the two-photon absorption-based KDP crystal laser preprocessing of this invention not only has higher preprocessing efficiency than three-photon absorption laser preprocessing, but also has greater potential in improving the bulk damage threshold of the crystal. Attached Figure Description
[0021] Figure 1 This is a schematic diagram of the process for DKDP crystal laser preprocessing based on two-photon absorption provided in an embodiment of the present invention;
[0022] Figure 2 A schematic diagram of the experimental optical path system for preprocessing DKDP crystal frequency-doubled laser based on two-photon absorption provided in an embodiment of the present invention;
[0023] Figure 3 A schematic diagram of the bulk damage probability curve of a crystal measured by a small-aperture laser and the optical path system for third-harmonic laser preprocessing;
[0024] Figure 4 The electron paramagnetic resonance spectra of the DKDP crystal before and after laser pretreatment in Example 1 are shown.
[0025] Figure 5 The graph shows the measurement results of the DKDP crystal damage probability curves before and after laser pretreatment in Example 1 and Comparative Example 1. Detailed Implementation
[0026] This invention provides a laser preprocessing method based on a two-photon absorption nonlinear optical crystal, comprising the following steps:
[0027] A nonlinear optical crystal is irradiated with a laser, wherein the photon energy of the laser is Eg / 2 to Eg, and Eg is the bandgap width of the nonlinear optical crystal.
[0028] In this invention, unless otherwise specified, all raw materials / components used in the preparation are commercially available products well known to those skilled in the art.
[0029] Based on the physical phenomenon of two-photon absorption in crystals, this invention pre-processes nonlinear optical crystals, including potassium dihydrogen phosphate (KDP) nonlinear optical crystals and similar crystals with lattice structures similar to KDP; the similar crystals preferably include deuterated potassium dihydrogen phosphate (DKDP) or ammonium dihydrogen phosphate (ADP), etc., which can significantly and efficiently improve the laser damage threshold of these crystals.
[0030] This invention does not specify a particular method for preparing the nonlinear optical crystal; any artificial nonlinear optical crystal, known to those skilled in the art, obtained through artificial growth followed by annealing, can be used. In this invention, prior to irradiation, the nonlinear optical crystal is preferably cut and surface-polished sequentially. The specific implementation of the cutting and surface polishing is not particularly important. The cutting is preferably performed according to the direction and size required for the use of the nonlinear optical crystal. The surface of the nonlinear optical crystal obtained after surface polishing preferably achieves an optical-grade finish.
[0031] In this invention, the nonlinear optical crystal preferably comprises potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP), or ammonium dihydrogen phosphate (ADP). In this invention, the deuteration rate of the DKDP is preferably 5-98%. In a specific embodiment of this invention, the laser pretreatment method is described in detail using DKDP as an example. The deuteration rate of the DKDP crystal is 70%. The DKDP crystal is an insulator with a large band gap (Eg). The band gap of the DKDP crystal is 7.5-9 eV.
[0032] In this invention, the laser is generated by a laser, preferably a solid-state laser or an excimer laser, more preferably a solid-state laser. Compared to excimer lasers, solid-state lasers are more environmentally friendly. This invention preferably uses a solid-state laser for laser pretreatment of nonlinear optical crystals, resulting in a green and environmentally friendly process suitable for widespread industrial application. In a specific embodiment of this invention, the laser used for laser pretreatment is an Nd:YAG laser. The pulse width of the Nd:YAG laser is nanosecond to sub-nanosecond, enabling high-efficiency output of fourth-harmonic laser light.
[0033] In this invention, the photon energy (hν) of the laser is Eg / 2 to Eg. Laser photons with photon energies of Eg / 2 to Eg can ionize valence band electrons in the nonlinear optical crystal to the conduction band of the crystal through a two-photon absorption process. By subjecting the nonlinear optical crystal to a certain number of laser pulses at a laser intensity below its bulk damage threshold, a significant and efficient improvement in the laser bulk damage threshold of the nonlinear optical crystal is achieved.
[0034] In this invention, the wavelength of the laser is preferably 165–305 nm, more preferably 175–295 nm, and even more preferably 205–275 nm. In a specific embodiment of this invention, the wavelength of the laser is 266 nm. This invention uses a fourth-harmonic generation laser with a wavelength of 266 nm and a photon energy of 4.66 eV, which is between 3.75 and 7.5 eV, meeting the photon energy requirements for laser preprocessing of DKDP crystals based on two-photon absorption.
[0035] In this invention, the pulse width of the laser is preferably 0.3–40 ns or 100 fs–100 ps, more preferably 5–40 ns or 150 fs–80 ps, and even more preferably 15–30 ns or 500 fs–50 ps. The spot diameter of the laser is preferably 0.05–5 mm, more preferably 0.1–5 mm, and even more preferably 1–5 mm. The power density of the laser is preferably 0.2–1.5 times the volume damage threshold of the nonlinear optical crystal, more preferably 0.3–1.5 times the volume damage threshold of the nonlinear optical crystal, even more preferably 0.4–1.5 times the volume damage threshold of the nonlinear optical crystal, and most preferably 0.5–1.5 times the volume damage threshold of the nonlinear optical crystal. The volume damage threshold of the nonlinear optical crystal is the volume damage threshold corresponding to the wavelength of the preprocessed laser. In this invention, the volume damage threshold of the nonlinear optical crystal to be preprocessed varies with the wavelength of the preprocessing laser. In this invention, the power density of the laser is preferably 0.2 to 1.5 times the volume damage threshold of the nonlinear optical crystal. The volume damage threshold mentioned here means the volume damage threshold of the nonlinear optical crystal corresponding to the wavelength of the preprocessing laser.
[0036] In a specific embodiment of the present invention, the power density of the laser is preferably 0.565 times the bulk damage threshold of the nonlinear optical crystal. In this invention, the bulk damage threshold of the nonlinear optical crystal is the maximum laser power density (in W / cm²) at which irreversible damage is caused to the nonlinear optical crystal by laser irradiation before pretreatment. 2 Preferably, before performing the laser preprocessing, the nonlinear optical crystal to be preprocessed is evaluated and calculated to obtain its volume damage threshold. The number of laser pulses is preferably 90-95% of the saturation pulse number, more preferably 95%. In this invention, during the laser preprocessing process, as the number of laser pulses increases, the volume damage threshold of the nonlinear optical crystal gradually increases. However, after the number of laser pulses reaches a certain value, further increasing the number of laser pulses will no longer increase the volume damage threshold of the nonlinear optical crystal, reaching saturation. This invention defines the number of laser pulses at which the volume damage threshold of the nonlinear optical crystal no longer increases with the increase of the number of laser pulses during laser preprocessing as the saturation pulse number.
[0037] In a specific embodiment of the present invention, the nonlinear optical crystal is potassium dihydrogen phosphate; the laser wavelength is 266 nm, the pulse width is 10 ns, the spot diameter is 1 mm, and the power density is 0.13 GW / cm². 2 The number of pulses is 5. The number of pulses refers to the number of laser pulses received at the same location by the nonlinear optical crystal. A pulse count of 5 indicates that the nonlinear optical crystal receives 5 laser pulses at the same location.
[0038] In a specific embodiment of the present invention, a schematic diagram of the laser preprocessing flow of the nonlinear optical crystal is shown below. Figure 1 As shown, the schematic diagram of the optical path system is as follows: Figure 2 As shown. Figure 2 As shown, the optical path system used in the laser preprocessing system provided by this invention consists of a laser and an optical path transmission and adjustment system, wherein: the laser is an Nd:YAG laser; the optical path transmission and adjustment system comprises, in sequence, a beam splitter, a 532 high-reflection mirror, a triple beam shrinking system, a quadruple frequency-harmonic crystal, a prism, a convex lens, and a reflecting mirror. In a specific embodiment of this invention, as... Figure 2 As shown, the 266nm ultraviolet laser output from the Nd:YAG laser is focused by a long-focal-length converging lens and incident on the nonlinear optical crystal to be preprocessed. Typically, the area of the crystal to be preprocessed is much larger than the laser spot; therefore, this invention preferably uses laser spot scanning to ensure uniform preprocessing of the entire crystal.
[0039] In this invention, the selection of laser photon energy is key to significantly and efficiently improving the damage threshold of the nonlinear optical crystal laser. In existing technologies, when using a third-harmonic laser with a wavelength of 355 nm for laser pretreatment, the valence band electrons of the nonlinear optical crystal must simultaneously absorb three photons to be ionized to the conduction band, which is a third-order nonlinear process. This invention uses a fourth-harmonic laser with a wavelength of 266 nm for laser pretreatment, where the valence band electrons of the nonlinear optical crystal only need to simultaneously absorb two photons to be ionized to the conduction band, which is a second-order nonlinear process. The probability of two-photon absorption is thousands to tens of thousands of times higher than the probability of three-photon absorption. Therefore, the laser pretreatment technology based on two-photon absorption in this invention will significantly improve efficiency compared to existing laser pretreatment based on three-photon absorption.
[0040] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.
[0041] Example 1
[0042] according to Figure 1 The laser preprocessing flowchart described above adopts... Figure 2 The optical path diagram shown illustrates laser preprocessing of a DKDP crystal (70% deuteration). The specific processing steps include:
[0043] A piece of artificially grown DKDP crystal, after undergoing a thermal annealing process, is cut according to the orientation and size required for its application. The cut crystal is then polished to achieve an optical-grade finish.
[0044] A 266nm ultraviolet laser output from an Nd:YAG laser is focused by a long-focal-length converging lens and incident on the DKDP crystal to be pretreated. The entire crystal is uniformly pretreated by scanning the laser spot. The laser parameters during the pretreatment process are: wavelength: 266nm, spot diameter: 1mm, power density: 0.13GW / cm². 2 The pulse count is 5 (the DKDP crystal receives 5 laser pulses at the same location), pulse width is 10 ns, and flux is 1.3 J / cm². 2 .
[0045] Example 1: The relative number of intrinsic point defects in DKDP crystals before and after laser pretreatment was measured using electron paramagnetic resonance (EPR) spectroscopy. The microwave frequency of the spectrometer was 9.4 GHz, and the sample temperature was 30 K during measurement. The obtained EPR spectra are shown below. Figure 4 As shown. Figure 4The black curve represents the spectrum of the crystal without laser pretreatment, while the red curve represents the spectrum of the crystal after laser pretreatment. From... Figure 4 As can be seen, the vacuolar "point defects" [H2PO4] are directly related to the laser body damage threshold. 0 The quantity decreased to only 60% of the quantity before laser pretreatment.
[0046] Comparative Example 1
[0047] according to Figure 1 The laser preprocessing flowchart described above adopts... Figure 3 The optical path diagram shown illustrates the laser preprocessing of the DKDP crystal, in which... Figure 3 The optical path diagram shown is used both for measuring the damage probability curve and for 355nm laser preprocessing. The specific processing steps include:
[0048] A piece of artificially grown DKDP crystal, after undergoing a thermal annealing process, is cut according to the orientation and size required for its application. The cut crystal is then polished to achieve an optical-grade finish.
[0049] A 355nm ultraviolet laser output from an Nd:YAG laser is focused by a long-focal-length converging lens and incident on the DKDP crystal to be pretreated. The entire crystal is uniformly pretreated by scanning the laser spot. The laser parameters during the pretreatment process are: wavelength: 355nm, spot diameter: 1mm, power density: 0.1GW / cm². 2 The pulse count is 5 (the DKDP crystal receives 5 laser pulses at the same location), pulse width is 10 ns, and flux is 1.0 J / cm². 2 .
[0050] Example 1 and Comparative Example 1 show DKDP crystals before and after laser pretreatment according to... Figure 3 The optical path diagram of the bulk damage probability curve of the small-aperture laser measuring crystal is shown for testing. The test results are as follows: Figure 5 As shown. Figure 5 The figures show the damage probability curves of the DKDP crystal before and after laser pretreatment obtained in Example 1 and Comparative Example 1. Figure 5 It can be concluded that the zero-probability volume damage threshold of the DKDP crystal before laser pretreatment is 2.3 J / cm. 2 The zero-probability volume damage threshold of the DKDP crystal after the fourth harmonic pretreatment in Example 1 is 4.7 J / cm. 2 The zero-probability volume damage threshold of the DKDP crystal after third harmonic pretreatment in Comparative Example 1 is 2.7 J / cm². 2Comparing the results of Example 1 and Comparative Example 1, it can be seen that: when the laser pulse width and laser power (intensity) are the same, but the laser wavelength is different, the zero-probability volume damage threshold of the 266nm laser pretreatment based on two-photon absorption in Example 1 is improved by 120%, while the zero-probability volume damage threshold of the 355nm laser pretreatment based on three-photon absorption in Comparative Example 1 is only improved by 20%. Figure 5 This demonstrates the effectiveness of the efficient laser preprocessing method based on two-photon absorption nonlinear optical crystals provided in this invention. Furthermore, this embodiment uses a laser wavelength of 266nm, which is significantly weaker than 355nm (more than 10 times weaker), for preprocessing, achieving a laser power density of only 0.13GW / cm². 2 Under these conditions, the zero-probability volume damage threshold after DKDP crystal pretreatment was observed to be 4.7 J / cm. 2 Compared to the zero-probability volumetric damage threshold of 2.3 J / cm before pretreatment. 2 The improvement is 120%, and compared to Comparative Example 1, the pretreatment method provided in Embodiment 1 of this invention significantly improves the bulk damage threshold of the crystal. Moreover, to achieve the same pretreatment effect (approximately doubling the bulk damage threshold) as in the prior art, this invention uses a 266nm laser with a laser power density of 0.13GW / cm². 2 It only requires a fraction of the wavelength of a 355nm laser (4-5 GW / cm²). 2 This improves laser pretreatment efficiency and reduces pretreatment costs. Finally, the two-photon absorption-based KDP crystal laser pretreatment of this invention, besides having higher pretreatment efficiency than three-photon absorption laser pretreatment, also has greater potential in increasing the bulk damage threshold of the crystal. This potential lies in the ability to obtain higher conduction band electron density when using two-photon absorption laser pretreatment.
[0051] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.
Claims
1. A laser preprocessing method for a nonlinear optical crystal, characterized in that, Includes the following steps: A nonlinear optical crystal is irradiated with a laser. The nonlinear optical crystal is any one of potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate, and ammonium dihydrogen phosphate. The wavelength of the laser is 266 nm, and the photon energy of the laser is Eg / 2 to Eg, where Eg is the bandgap of the nonlinear optical crystal.
2. The laser pretreatment method according to claim 1, characterized in that, The laser is generated by a laser, which can be a solid-state laser or an excimer laser.
3. The laser pretreatment method according to claim 1, characterized in that, The pulse width of the laser is 0.3~40ns or 100fs~100ps.
4. The laser pretreatment method according to claim 1, characterized in that, The laser spot diameter is 0.05~5mm.
5. The laser pretreatment method according to claim 1, characterized in that, The power density of the laser is 0.2 to 1.5 times the volume damage threshold of the nonlinear optical crystal, and the volume damage threshold of the nonlinear optical crystal is the volume damage threshold corresponding to the wavelength of the pre-processed laser.
6. The laser pretreatment method according to claim 1, characterized in that, The number of laser pulses is 90-95% of the saturation pulse number.
7. The laser pretreatment method according to claim 1, characterized in that, The pulse width is 10 ns, the spot diameter is 1 mm, and the power density is 0.13 GW / cm². 2 .