Method and device for inhibiting ultraviolet laser damage of fused quartz optical element

Abstract

The present application relates to the field of ultrafast laser technology, in particular to a fused quartz optical element ultraviolet laser damage suppression method and device, the method comprises: obtaining oxygen-18 modified DKDP crystal with oxygen-18 isotopic abundance higher than natural abundance; The oxygen-18 modified DKDP crystal is prepared into a wave plate matching the preset ultraviolet laser wavelength;In the ultraviolet laser path of the laser inertial confinement fusion device, the wave plate is arranged between the linear polarization state output end of the ultraviolet laser and the fused quartz optical element;The ultraviolet laser passes through the linear polarization state output end and the wave plate in turn, forms circularly polarized light and irradiates to the fused quartz optical element.The purpose is to actively suppress the small-scale self-focusing effect of fused quartz, solve the technical problem that the traditional material cannot be used as a large-aperture wave plate due to the excessively high transverse stimulated Raman scattering.

Description

Technical Field

[0001] This invention relates to the field of ultrafast laser technology, specifically to a method and apparatus for suppressing ultraviolet laser damage to fused silica optical elements. Background Technology

[0002] Laser inertial confinement fusion (ICF) is one of the core technological pathways to achieve controlled nuclear fusion. Its principle involves using a high-power laser beam to symmetrically compress and heat a target pellet, bringing it to the high-temperature, high-pressure conditions required for the fusion reaction. In ICF devices, third-harmonic ultraviolet lasers are the mainstream driving light source due to their high energy coupling efficiency and good target absorption. Fused silica, as a key optical element in the transmission and focusing process of third-harmonic lasers, can function as a lens, window, and beam sampler. Its resistance to laser damage directly determines the operational stability and upper limit of laser energy output of the ICF device. Research shows that third-harmonic high-intensity lasers are prone to various forms of damage when transmitted through fused silica, with small-scale self-focusing effects being one of the core causes of irreversible damage to the fused silica. This effect originates from the refractive index gradient caused by uneven intensity distribution within the laser beam. Regions with higher laser energy density locally increase the refractive index of the fused silica, forming a lens-like focusing structure. This further causes the beam to contract, creating extremely high energy density regions within the fused silica, leading to internal filamentary damage and surface defects such as peeling and cracking. To mitigate this effect, research has shown that the small-scale self-focusing effect of circularly polarized lasers in fused silica is significantly weaker than that of linearly polarized lasers. Currently, the third-harmonic generation laser from the third-harmonic crystal of mainstream laser inertial confinement fusion laser drivers is linearly polarized. Therefore, high-performance, large-aperture quarter-wave plates are needed to convert linearly polarized light to circularly polarized light, adapting to the beam aperture requirements of the device.

[0003] Currently, the only optical crystal materials that can meet the requirements for large-aperture growth are potassium dihydrogen phosphate (KDP) and its deuterated product (DKDP). Both can be grown into large-size single crystals by solution cooling and have good optical transmittance in the third harmonic ultraviolet laser band. They are the core optional materials for third harmonic crystals and polarization conversion elements in existing laser inertial confinement fusion devices. However, existing potassium dihydrogen phosphate and deuterated derivative crystals used to prepare large-aperture waveplates have the defect of excessively high transverse stimulated Raman scattering coefficients. When a strong laser is transmitted through these crystals, the laser energy is transferred to the crystal vibration mode through the stimulated Raman scattering process, exciting transversely propagating Raman scattered light. The intensity of this transversely scattered light will rapidly increase to form a strong transverse laser, whose energy density can reach several times the crystal damage threshold, thereby causing irreversible damage such as internal cracks and melting, or even causing the crystal to shatter completely. Even the deuterated potassium dihydrogen phosphate crystals modified by deuteration cannot meet the long-term use requirements of large-aperture quarter-waveplates and cannot withstand the high-flux laser irradiation of laser inertial confinement fusion devices. These two major problems together restrict the improvement of the laser damage resistance of fused silica optical elements and the stable operation of laser inertial confinement fusion devices. Summary of the Invention

[0004] To actively suppress the small-scale self-focusing effect of fused silica and address the technical problem that traditional materials cannot be used as large-aperture waveplates due to excessive transverse stimulated Raman scattering, this invention provides a method and apparatus for suppressing ultraviolet laser damage to fused silica optical elements. The specific technical solution adopted is as follows:

[0005] The first aspect of the present invention provides a method for suppressing ultraviolet laser damage to fused silica optical elements, the method comprising:

[0006] Obtain oxygen-18 modified DKDP crystals with oxygen-18 isotope abundance higher than that of the natural oxygen-18;

[0007] Oxygen-18 modified DKDP crystals were used to prepare waveplates that matched the preset ultraviolet laser wavelength;

[0008] In the ultraviolet laser optical path of the laser inertial confinement fusion device, a waveplate is placed between the linearly polarized output end of the ultraviolet laser and the fused silica optical element.

[0009] The ultraviolet laser is sequentially passed through the linearly polarized output end and the waveplate to form circularly polarized light, which then irradiates the fused silica optical element.

[0010] Furthermore, obtaining oxygen-18 modified DKDP crystals with oxygen-18 isotope abundance higher than the natural abundance includes:

[0011] Phosphoric acid, heavy water, and potassium hydroxide enriched with oxygen-18 isotope were used as raw materials for crystal formation.

[0012] The isotopic abundance of oxygen-18 in the raw materials is controlled within the range of 10% to 20%, and the substitution rate of deuterium for hydrogen is not less than 70%.

[0013] Oxygen-18 modified DKDP crystals were grown using a solution cooling method, and the optical uniformity deviation of the grown crystals was less than or equal to 5 × 10⁻⁶. -6 .

[0014] Furthermore, after obtaining oxygen-18 modified DKDP crystals by solution cooling growth, the process also includes:

[0015] Raman spectroscopy was performed on the oxygen-18 modified DKDP crystal obtained by growth to verify the shift in its lattice vibration frequency.

[0016] The transverse stimulated Raman scattering gain coefficient of oxygen-18 modified DKDP crystal was measured using a laser testing platform.

[0017] Furthermore, the oxygen-18 modified DKDP crystal is prepared into a waveplate matching a predetermined ultraviolet laser wavelength, including:

[0018] The crystal orientation of oxygen-18 modified DKDP crystals was determined using X-ray diffraction;

[0019] Based on the refractive index parameters of the oxygen-18 modified DKDP crystal under the preset ultraviolet laser wavelength, the cutting orientation and thickness of the waveplate crystal with a 90° phase delay were determined.

[0020] The crystal blank is cut and optically processed according to the crystal cutting orientation to form an optically parallel planar wafer with a predetermined diameter and thickness.

[0021] Furthermore, the preset ultraviolet laser wavelength is 351nm.

[0022] Furthermore, the fabrication of oxygen-18 modified DKDP crystals into waveplates matching a predetermined ultraviolet laser wavelength also includes:

[0023] Antireflection coatings matching a preset ultraviolet laser wavelength are deposited on the two optical surfaces of an optically parallel planar wafer;

[0024] Using an ultraviolet polarization measurement system, the phase delay of the coated wafer is measured and calibrated at a preset ultraviolet laser wavelength. By adjusting the tilt angle of the wafer in the optical path, the phase delay is calibrated to 90°.

[0025] Furthermore, the waveplate is positioned between the linearly polarized output end of the ultraviolet laser and the fused silica optical element, including:

[0026] The linearly polarized output terminal is the emission terminal of the third harmonic crystal in the laser inertial confinement fusion device;

[0027] The waveplate is fixed in the optical path between the output end of the third harmonic crystal and the input end of the large-aperture fused silica lens by a mounting bracket adapted to the optical path diameter, so that the optical center of the waveplate is coaxial with the optical path center of the ultraviolet laser, and the optical surface of the waveplate is perpendicular to the laser beam.

[0028] Further, the ultraviolet laser is sequentially passed through the linearly polarized output end and the waveplate to form circularly polarized light, which then irradiates the fused silica optical element, including:

[0029] The laser inertial confinement fusion device is activated, outputting a linearly polarized third-harmonic 351nm laser.

[0030] The linearly polarized ultraviolet laser is controlled to pass through a third-harmonic crystal and a waveplate in sequence at a preset power density. After being converted into circularly polarized light, it is transmitted and illuminated to the fused silica optical element in the terminal optical path.

[0031] Furthermore, the method also includes:

[0032] The morphology of the back surface spot of a fused silica optical element under circularly polarized light illumination was monitored using a shadow imaging system.

[0033] The damage rate of the fused silica optical element was statistically analyzed after ultraviolet laser irradiation with a preset power density and number of pulses.

[0034] The second aspect of the present invention provides a device for suppressing ultraviolet laser damage to fused silica optical elements, used to perform the method for suppressing ultraviolet laser damage to fused silica optical elements as described in the first aspect of the present invention, the device comprising:

[0035] The laser inertial confinement fusion device is configured to output linearly polarized third-harmonic 351nm ultraviolet laser.

[0036] Oxygen-18 modified DKDP crystal waveplates are placed in the ultraviolet laser output optical path of the laser inertial confinement fusion device to convert linearly polarized ultraviolet laser into circularly polarized light.

[0037] A fused silica optical element configured to receive and transmit the circularly polarized light.

[0038] The present invention has the following beneficial effects:

[0039] This invention provides a method for suppressing ultraviolet laser damage to fused silica optical elements. By obtaining oxygen-18 modified DKDP crystals with an oxygen-18 isotope abundance higher than the natural abundance, and by leveraging the alteration of the lattice vibration characteristics of the DKDP crystal by the oxygen-18 isotope, the transverse stimulated Raman scattering coefficient of the crystal is reduced at its source. This overcomes the technical bottleneck that traditional DKDP-type crystals cannot be used as large-aperture waveplates due to their excessively high transverse stimulated Raman scattering coefficient, providing a suitable material for the fabrication of large-aperture polarization conversion elements. Based on this, the modified crystal is fabricated into a waveplate matching a preset ultraviolet laser wavelength and placed in the optical path between the linearly polarized output end of the ultraviolet laser and the fused silica optical element. This converts the linearly polarized laser into circularly polarized light, suppressing the accumulation of refractive index gradients caused by uneven laser intensity distribution within the fused silica, avoiding excessive beam contraction and energy concentration, thereby actively suppressing the small-scale self-focusing effect of the fused silica, reducing irreversible damage such as internal filamentary damage and subsequent surface peeling cracks, and achieving effective protection for the fused silica optical element. This method actively modulates the laser polarization state at the level of damage induction, rather than passively repairing existing damage. It can effectively suppress laser-induced damage such as filamentary damage in the fused silica element and subsequent surface peeling and cracking, improve the laser damage resistance threshold of the fused silica optical element at the terminal of the laser inertial confinement fusion device, extend the service life of the fused silica element, and ensure the laser transmission stability of the terminal optical system. Attached Figure Description

[0040] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a flowchart of a method for suppressing ultraviolet laser damage to fused silica optical elements according to an embodiment of the present invention;

[0042] Figure 2 This is a schematic diagram of the unit cell structure of an oxygen-18 modified DKDP crystal provided in one embodiment of the present invention;

[0043] Figure 3 This is a schematic diagram of the target chamber focusing optical path of the ultraviolet laser damage suppression method for fused silica optical elements provided in an embodiment of the present invention. Detailed Implementation

[0044] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation, structure, features, and effects of a method and apparatus for suppressing ultraviolet laser damage to fused silica optical elements according to the present invention. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0046] The following description, in conjunction with the accompanying drawings, details the specific scheme of the method and apparatus for suppressing ultraviolet laser damage to fused silica optical elements provided by the present invention.

[0047] Please see Figure 1 The diagram illustrates a flowchart of a method for suppressing ultraviolet laser damage to fused silica optical elements according to an embodiment of the present invention, the method comprising:

[0048] Step S100: Obtain oxygen-18 modified DKDP crystals with an oxygen-18 isotope abundance higher than that of the natural source. In this embodiment, deuterated potassium dihydrogen phosphate (DKDP) crystals are artificially synthesized water-soluble functional crystals, hereinafter referred to as DKDP crystals. This step is the material basis for the implementation of this method, and its core lies in the control of the constituent elements of the crystal through isotope engineering. Specifically, the conventional oxygen-16 atoms constituting the phosphate groups in the DKDP crystal are partially replaced with heavier oxygen-18 atoms. This replacement is based on the basic principles of molecular vibration and Raman scattering: increasing the reduced mass of the vibrational group can reduce its intrinsic vibrational frequency, thereby leading to a decrease in its stimulated Raman scattering gain coefficient. Therefore, the purpose of this step is to synthesize a novel optical crystal that combines large-size growth potential with low transverse stimulated Raman scattering (TSRS) characteristics, fundamentally breaking through the bottleneck that traditional DKDP materials cannot be used as substrates for large-aperture waveplates due to their excessively high TSRS coefficient.

[0049] Specifically, please refer to Figure 2 This invention demonstrates the basic unit cell structure of the oxygen-18 modified DKDP crystal, which is a tetragonal crystal system. The basic framework consists of phosphate groups (PO4) and potassium ions (K). + )constitute.

[0050] The specific atomic identifiers are as follows: P atoms represent phosphorus atoms at the center of phosphate groups; K atoms represent potassium ions filling the interstitial spaces in the crystal lattice;¹ 6 The O atom represents the conventional oxygen-16 atom, and in unmodified DKDP crystals, the phosphate group is entirely composed of this atom;¹ 8 The O atom represents the heavier oxygen-18 isotope introduced in this invention. Its position in the crystal lattice is shown in the diagram, visually illustrating the effect on a portion¹ 6 The substitution of O atoms; H atoms represent hydrogen atoms; D atoms represent deuterium atoms, an isotope of hydrogen, shown in the diagram in the OD bond; this diagram does not represent a completely deuterated structure. 8 The lattice is composed of O, but it schematically represents¹ 8 O isotope atoms are randomly substituted in the DKDP crystal lattice. This substitution occurs in the phosphate anion groups of potassium dideuterium phosphate crystals. In conventional DKDP crystals, all oxygen atoms are ¹ 6 O. This invention utilizes a material preparation process to incorporate a certain proportion of ¹ 6 O atoms are replaced with larger ones¹ 8 The oxygen atom forms an oxygen-18 modified PO4 group.¹ 8 O Replacement¹ 6 O directly increases the reduced mass of the PO4 group, the core vibrational unit. According to molecular vibration theory, under the premise that the bond force constant remains essentially unchanged, the increase in the reduced mass of the vibrational group will lead to a decrease in its intrinsic vibrational frequency. The decrease in vibrational frequency means a reduction in the Raman scattering cross section, resulting in a significant decrease in the TSRS gain coefficient. This invention modifies specific atomic sites in traditional DKDP crystals through isotopic engineering, while maintaining the crystal's original excellent macroscopic properties such as large-size growth capability, optical uniformity, and ultraviolet transmittance. At the same time, it changes its optical nonlinear response characteristics from the root of microscopic lattice dynamics, giving it a lower TSRS coefficient, thus meeting the requirements as a core material for large-aperture, high-load quarter-wave plates in laser inertial confinement fusion devices.

[0051] Step S100 specifically includes:

[0052] Step S110: Phosphoric acid, heavy water, and potassium hydroxide enriched with oxygen-18 isotopes are used as crystal raw materials; oxygen-18 isotope-enriched phosphoric acid is selected as the phosphorus and oxygen source. Heavy water is used as the deuterium source and solvent, and potassium hydroxide as the potassium source. To ensure the optical quality of the final crystal and avoid the introduction of light absorption or scattering centers by impurities, the chemical purity of all starting materials must be no less than 99.999%, and the total content of impurities such as key metal ions must be controlled to no more than one part per million. This embodiment uses such ultra-high purity raw materials enriched with specific isotopes, laying the chemical foundation for the growth of high-performance oxygen-18 modified DKDP single crystals.

[0053] Step S120: To effectively suppress the TSRS coefficient while considering material costs, the raw material ratio needs to be precisely calculated and weighed before crystal growth. In this embodiment, by adjusting the ratio of oxygen-18-enriched phosphoric acid to conventional phosphoric acid, the target value of the percentage abundance of oxygen-18 isotopes in the planned crystal growth is set within the range of 10% to 20%. Simultaneously, to ensure the crystal possesses good ultraviolet transmittance and stable physicochemical properties, the substitution rate of deuterium atoms for hydrogen atoms in the crystal needs to be controlled to be no less than 70%. This step achieves predetermined control of the intrinsic properties of the crystal by pre-preparing a growth solution with a specific isotopic composition and degree of deuteration.

[0054] Step S130: Oxygen-18 modified DKDP crystals are grown by solution cooling method, and the optical uniformity deviation of the grown crystals is less than or equal to 5 × 10⁻⁶. -6 Specifically, a solution cooling method, proven through long-term practice, is employed for single crystal growth. The growth solution prepared in step S120 is placed in a precisely temperature-controlled growth tank. By slowly lowering the solution temperature, the crystal spontaneously nucleates and gradually grows along a specific crystal orientation in the supersaturated solution. The core process objective of the entire growth process is to obtain single crystals with high optical uniformity. To achieve this, it is necessary to strictly control the stability of the temperature field and the solution convection state to eliminate defects such as growth streaks and inclusions, ensuring that the optical uniformity deviation of the grown oxygen-18 modified DKDP single crystal does not exceed 5 × 10⁻⁶. -6 This is to meet the requirements of material uniformity for subsequent processing of large-diameter, high-precision wave plates.

[0055] In some embodiments, after obtaining oxygen-18 modified DKDP crystals by solution cooling, the process further includes:

[0056] Step S140: Raman spectroscopy was performed on the oxygen-18 modified DKDP crystal to verify the shift in its lattice vibration frequency. After the crystal growth was completed, by analyzing the Raman spectrum peak positions and comparing them with the standard spectrum of conventional DKDP crystal, the shift of the characteristic vibration peak of the phosphate group to the lower wavenumber direction due to the introduction of oxygen-18, i.e., the redshift phenomenon, can be directly observed. The reduction in lattice vibration frequency can be confirmed from the spectroscopic perspective.

[0057] Step S150: The transverse stimulated Raman scattering (TSRS) gain coefficient of the oxygen-18 modified DKDP crystal is measured using a laser testing platform. Specifically, a high-power laser testing platform is constructed to quantitatively measure the TSRS gain coefficient of the oxygen-18 modified DKDP crystal. By comparing the measurement results with those of a conventional DKDP crystal of the same specifications and under the same testing conditions, it is confirmed that the TSRS gain coefficient of the modified crystal is significantly reduced. This experimental data verifies its feasibility and superiority as a low-TSRS large-aperture waveplate material, providing crucial performance data support for subsequent applications.

[0058] Step S100, through systematic material design, controlled crystal growth, and verification characterization, achieved a targeted improvement in the intrinsic optical nonlinear properties of DKDP crystals. A modified DKDP crystal material with high oxygen-18 isotope abundance was prepared. This material, while inheriting the excellent large-size solution growth capability and good ultraviolet transmittance of traditional DKDP crystals, fundamentally and significantly reduces its transverse stimulated Raman scattering coefficient.

[0059] Step S200: Prepare an oxygen-18 modified DKDP crystal into a waveplate that matches the preset ultraviolet laser wavelength;

[0060] Step S200 specifically includes:

[0061] Step S210: Determine the crystal orientation of the oxygen-18 modified DKDP crystal using X-ray diffraction. X-ray diffraction is used to analyze the crystal sample. When X-rays irradiate the crystal, the periodically arranged atomic lattice inside diffracts the X-rays, forming a characteristic diffraction pattern. By analyzing this pattern and comparing it with the standard crystallographic database for DKDP crystals, the crystal axis orientation of the crystal can be accurately determined, finding the specific crystal orientation related to the crystal's optical principal axis. The crystal orientation directly corresponds to the crystal's optical axis, ensuring that the optical fast and slow axes of the waveplate are correctly aligned with the intrinsic optical direction of the crystal during subsequent cutting and processing, thereby guaranteeing that the waveplate can produce accurate and stable phase delay.

[0062] Step S220: Based on the refractive index parameters of the oxygen-18 modified DKDP crystal at a preset ultraviolet laser wavelength, determine the crystal cutting orientation and thickness for a 90° phase retardation matching the waveplate; the preset ultraviolet laser wavelength is 351 nm. Based on the crystal orientation determined in step S210 and the refractive index parameters of the oxygen-18 modified DKDP crystal measured for the preset ultraviolet laser wavelength, perform theoretical calculations. According to the principle of waveplate phase retardation, i.e., the phase difference caused by the different refractive indices of light waves passing through a crystal along different polarization directions, calculate the physical crystal thickness required to achieve a 90° phase difference (i.e., a quarter wavelength). Simultaneously, combined with the crystal optical axis orientation, determine the optimal crystal cutting plane orientation, such as a Z-cut perpendicular to the optical axis or an X-cut parallel to a specific crystal axis.

[0063] Step S230: The crystal blank is cut and optically processed according to the crystal cutting orientation to form an optically parallel planar wafer with a predetermined aperture and thickness. First, the blank is cut along the designed crystal cutting orientation using an internal circular cutter or a wire saw to obtain a wafer with a thickness and aperture close to the target. Subsequently, the wafer is ground and polished to process it into an optically parallel planar wafer with two highly parallel surfaces and a smooth surface.

[0064] In some embodiments, the preparation of an oxygen-18 modified DKDP crystal as a waveplate matching a preset ultraviolet laser wavelength further includes:

[0065] Step S240: Deposit antireflection films matching the preset ultraviolet laser wavelength on the two optical surfaces of the optically parallel planar wafer; using physical vapor deposition or coating processes, deposit antireflection films optimized for the preset ultraviolet laser wavelength on the two light-transmitting surfaces of the optically parallel planar wafer obtained in step S230. The antireflection film is a single-layer or multi-layer SiO2 thin film structure adapted to the 351nm ultraviolet band. Utilizing the principle of optical interference cancellation, the reflected light of a specific wavelength on the film surface cancels each other out, thereby reducing the reflectivity of each surface to an extremely low level; thus improving the transmittance of the waveplate, reducing energy loss and the risk of secondary interference and damage that may be caused by reflected light.

[0066] Step S250: Using an ultraviolet polarization measurement system, the phase retardation of the coated wafer is measured and calibrated at a preset ultraviolet laser wavelength. By adjusting the tilt angle of the wafer in the optical path, the phase retardation is calibrated to 90°. The ultraviolet polarization measurement system is used to inspect the coated wafer at the target application wavelength. This system emits probe light with a known polarization state. By analyzing the change in polarization state of the emitted light after passing through the waveplate, the actual phase retardation of the waveplate can be accurately calculated. Due to processing errors, the initial phase retardation may not be precisely 90°. In this case, by placing the waveplate on a precision adjustment frame with a finely adjustable angle and changing its tilt angle relative to the incident laser, the effective phase retardation can be corrected using the small influence of the angle on the optical path until the phase retardation is precisely calibrated to 90°, ensuring the waveplate's accurate function as a quarter-wave plate.

[0067] Step S300: In the ultraviolet laser optical path of the laser inertial confinement fusion device, a waveplate is placed between the linear polarization output end of the ultraviolet laser and the fused silica optical element;

[0068] Step S300 specifically includes:

[0069] Step S310: The linearly polarized output terminal is the emission terminal of the third-harmonic crystal in the laser inertial confinement fusion device. In a typical laser inertial confinement fusion actuator, a third-harmonic converter made of potassium dihydrogen phosphate or potassium dideuterium phosphate crystal is used to obtain third-harmonic ultraviolet laser with high target absorption efficiency. The converter outputs 351nm laser in a linearly polarized state.

[0070] Step S320: Fix the waveplate in the optical path between the output end of the third harmonic crystal and the input end of the large-aperture fused silica lens using a mounting bracket adapted to the optical path aperture, ensuring that the optical center of the waveplate is coaxial with the optical path center of the ultraviolet laser, and that the optical surface of the waveplate is perpendicular to the laser beam. First, a mounting bracket matching the entire beam aperture needs to be designed, and the large-aperture oxygen-18 modified DKDP waveplate prepared in step S200 is mounted on the bracket.

[0071] Step S400: The ultraviolet laser is sequentially passed through the linearly polarized output end and the waveplate to form circularly polarized light and irradiate the fused silica optical element; by using circularly polarized light, the threshold power for filamentation in the fused silica is increased, making it difficult for filamentation, a key process that causes damage to the in vivo and back surface, to occur under the same flux.

[0072] Step S400 specifically includes:

[0073] Step S410: Start the laser inertial confinement fusion device and output linearly polarized third-harmonic 351nm laser;

[0074] Step S420: The linearly polarized ultraviolet laser is controlled to pass sequentially through a third-harmonic crystal and a waveplate at a preset power density. After being converted into circularly polarized light, it is transmitted and illuminates the fused silica optical element in the terminal optical path; see also... Figure 3 As shown, the ICF driver is activated, allowing the generated 351nm linearly polarized ultraviolet laser to enter the oxygen-18 modified DKDP waveplate integrated into the optical path. When the linearly polarized light is incident at a specific azimuth angle and passes through the waveplate, the phase delay generated by the crystal for the two orthogonal polarization components of the light wave is precisely designed to be 90°. The two components of the outgoing light combine to form an electric field vector whose endpoint rotates circularly over time, thus converting it into circularly polarized light. This circularly polarized light continues to propagate in the terminal optical path and eventually illuminates the fused silica optical element. By changing the laser polarization state acting on the fused silica from linear to circular polarization, the effective nonlinear coefficient of the beam's self-focusing in the medium is physically reduced, thereby actively increasing the laser power threshold that causes optical filamentation damage. This invention only requires inserting a large-aperture 1 / 4-inch waveplate upstream of the optical path to benefit all downstream fused silica optical elements. This method simplifies the process and reduces the maintenance cost and complexity of the entire system. This invention involves a one-time investment in a waveplate, providing long-term benefits without the need for expensive post-processing of each component. It is also a purely optical physics solution that does not involve hazardous chemicals or high-energy particle bombardment. Furthermore, based on mature crystal growth and optical processing technologies, its performance is stable and predictable.

[0075] In some embodiments, the method further includes:

[0076] Step S430: Monitor the morphology of the back surface of the fused silica optical element under circularly polarized light illumination using a shadow imaging system. Specifically, during the illumination of the fused silica optical element by a circularly polarized laser, a shadow imaging system can be used to monitor its back surface area in real time. The principle is that a probe beam of a different wavelength, synchronized with the main laser, sweeps across the back surface of the fused silica element from the side. If the main laser forms high-density plasma filaments (i.e., filamentation) inside the fused silica due to a strong self-focusing effect, this plasma will significantly scatter or refract the probe light, thus exhibiting specific dark lines or distortions in the shadow image. By observing and analyzing whether such characteristic patterns appear in the obtained shadow image, it can be determined whether filamentation has occurred inside the fused silica element, thereby verifying the suppression effect of circularly polarized light on self-focusing.

[0077] Step S440: Statistically determine the damage incidence rate of the fused silica optical element after ultraviolet laser irradiation with a preset power density and number of pulses. Specifically, use surface defect detection equipment such as a Nomarski differential interferometry phase-contrast microscope or a dark-field scattering microscope to inspect the surface and subsurface of the irradiated fused silica optical element. Record the number, size, and distribution of laser-induced pits, cracks, or modified regions, and calculate the probability of damage occurrence, i.e., the damage incidence rate. By comparing this data with the damage incidence rate of a control group element irradiated with the original linearly polarized light under the same laser parameters but without using the waveplate of this invention, the effectiveness of the method of this invention in improving the laser-induced damage threshold of fused silica elements can be quantitatively confirmed statistically.

[0078] Step S400 efficiently and stably converts the high-power linearly polarized ultraviolet laser output from the laser inertial confinement fusion device into a circularly polarized state. This active control over the intrinsic properties of the beam directly intervenes in the physical mechanism of the interaction between the laser and the fused silica material. By increasing the threshold of nonlinear filamentation, it fundamentally suppresses the small-scale self-focusing effect that leads to damage within the element and on its back surface. This transforms the waveplate based on oxygen-18 modified DKDP crystal from a static optical element into an active protection unit that dynamically and continuously provides universal protection for downstream fused silica optical elements, ultimately achieving the goal of improving the load capacity and operational reliability of the final optical system.

[0079] In summary, the ultraviolet laser damage suppression method for fused silica optical elements provided by this invention obtains oxygen-18 modified DKDP crystals with an oxygen-18 isotope abundance higher than that of natural crystals. By leveraging the alteration of the lattice vibration characteristics of the DKDP crystal by the oxygen-18 isotope, the transverse stimulated Raman scattering coefficient of the crystal is reduced at its source. This overcomes the technical bottleneck that traditional DKDP-type crystals cannot be used as large-aperture waveplates due to their excessively high transverse stimulated Raman scattering coefficient, providing a suitable material for the fabrication of large-aperture polarization conversion elements. Based on this, the modified crystal is fabricated into a waveplate matching a preset ultraviolet laser wavelength and placed in the optical path between the linearly polarized output end of the ultraviolet laser and the fused silica optical element. This converts the linearly polarized laser into circularly polarized light, suppressing the accumulation of refractive index gradients caused by uneven laser intensity distribution within the fused silica, avoiding excessive beam contraction and energy concentration, thereby actively suppressing the small-scale self-focusing effect of the fused silica, reducing irreversible damage such as internal filamentary damage and subsequent surface peeling cracks, and achieving effective protection for the fused silica optical element. This method actively modulates the laser polarization state at the level of damage induction, rather than passively repairing existing damage. It can effectively suppress laser-induced damage such as filamentary damage in the fused silica element and subsequent surface peeling and cracking, improve the laser damage resistance threshold of the fused silica optical element at the terminal of the laser inertial confinement fusion device, extend the service life of the fused silica element, and ensure the laser transmission stability of the terminal optical system.

[0080] Please see Figure 2 The diagram illustrates a structural schematic of a fused silica optical element ultraviolet laser damage suppression device according to an embodiment of the present invention, the device comprising:

[0081] The laser inertial confinement fusion device is configured to output linearly polarized third-harmonic 351nm ultraviolet laser.

[0082] Oxygen-18 modified DKDP crystal waveplates are placed in the ultraviolet laser output optical path of the laser inertial confinement fusion device to convert linearly polarized ultraviolet laser into circularly polarized light.

[0083] A fused silica optical element configured to receive and transmit the circularly polarized light.

[0084] This invention aims to provide a system-level solution. By inserting a large-aperture quarter-wave plate made of a new crystal upstream, the polarization state of the entire beam is converted to circular polarization, which can universally improve the damage resistance of all downstream fused silica optical elements. This method eliminates the need for complex post-processing of each expensive and delicate fused silica element, simplifies the process, improves system reliability, and ultimately helps to enhance the output capacity and operating efficiency of ICF devices.

[0085] It should be noted that the order of the above embodiments of the present invention is merely for descriptive purposes and does not represent the superiority or inferiority of the embodiments. The processes depicted in the accompanying drawings do not necessarily require a specific or sequential order to achieve the desired result. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0086] The various embodiments in this specification are described in a progressive manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

Claims

1. A method for suppressing ultraviolet laser damage to fused silica optical elements, characterized in that, The method includes: Obtaining oxygen-18 modified DKDP crystals with oxygen-18 isotope abundance higher than the natural abundance includes: Phosphoric acid, heavy water, and potassium hydroxide enriched with oxygen-18 isotope were used as raw materials for crystal formation. The isotopic abundance of oxygen-18 in the raw materials is controlled within the range of 10% to 20%, and the substitution rate of deuterium for hydrogen is not less than 70%. With an optical uniformity deviation of less than or equal to 5 × 10 -6 To achieve this goal, oxygen-18 modified DKDP crystals were grown using a solution cooling method. Raman spectroscopy was performed on the oxygen-18 modified DKDP crystal obtained by growth to verify the shift in its lattice vibration frequency. The transverse stimulated Raman scattering gain coefficient of oxygen-18 modified DKDP crystal was measured using a laser testing platform. Oxygen-18 modified DKDP crystals are used to prepare waveplates that match a predetermined ultraviolet laser wavelength; including: The crystal orientation of oxygen-18 modified DKDP crystals was determined using X-ray diffraction; Based on the refractive index parameters of the oxygen-18 modified DKDP crystal under the preset ultraviolet laser wavelength, the cutting orientation and thickness of the waveplate crystal with a 90° phase delay were determined. The crystal blank is cut and optically processed according to the crystal cutting orientation to form an optically parallel planar wafer with a predetermined diameter and thickness. In the ultraviolet laser optical path of the laser inertial confinement fusion device, a waveplate is placed between the linearly polarized output end of the ultraviolet laser and the fused silica optical element. The ultraviolet laser is sequentially passed through the linearly polarized output end and the waveplate to form circularly polarized light, which then irradiates the fused silica optical element.

2. The method for suppressing ultraviolet laser damage to fused silica optical elements as described in claim 1, characterized in that, The preset ultraviolet laser wavelength is 351nm.

3. The method for suppressing ultraviolet laser damage to fused silica optical elements as described in claim 2, characterized in that, The fabrication of a waveplate from oxygen-18 modified DKDP crystal to match a predetermined ultraviolet laser wavelength also includes: Antireflection coatings matching a preset ultraviolet laser wavelength are deposited on the two optical surfaces of an optically parallel planar wafer; Using an ultraviolet polarization measurement system, the phase delay of the coated wafer is measured and calibrated at a preset ultraviolet laser wavelength. By adjusting the tilt angle of the wafer in the optical path, the phase delay is calibrated to 90°.

4. The method for suppressing ultraviolet laser damage to fused silica optical elements as described in any one of claims 1 to 3, characterized in that, Placing a waveplate between the linearly polarized output end of the ultraviolet laser and the fused silica optical element includes: The linearly polarized output terminal is the emission terminal of the third harmonic crystal in the laser inertial confinement fusion device; The waveplate is fixed in the optical path between the output end of the third harmonic crystal and the input end of the large-aperture fused silica lens by a mounting bracket adapted to the optical path diameter, so that the optical center of the waveplate is coaxial with the optical path center of the ultraviolet laser, and the optical surface of the waveplate is perpendicular to the laser beam.

5. The method for suppressing ultraviolet laser damage to fused silica optical elements as described in claim 4, characterized in that, The ultraviolet laser is sequentially passed through the linearly polarized output end and the waveplate to form circularly polarized light, which then illuminates the fused silica optical element, including: The laser inertial confinement fusion device is activated, outputting a linearly polarized third-harmonic 351nm laser. The linearly polarized ultraviolet laser is controlled to pass through a third-harmonic crystal and a waveplate in sequence at a preset power density. After being converted into circularly polarized light, it is transmitted and illuminated to the fused silica optical element in the terminal optical path.

6. The method for suppressing ultraviolet laser damage to fused silica optical elements as described in claim 5, characterized in that, The method further includes: The morphology of the back surface spot of a fused silica optical element under circularly polarized light illumination was monitored using a shadow imaging system. The damage rate of the fused silica optical element was statistically analyzed after ultraviolet laser irradiation with a preset power density and number of pulses.

7. A device for suppressing ultraviolet laser damage to fused silica optical elements, characterized in that, The apparatus for performing the ultraviolet laser damage suppression method for fused silica optical elements according to any one of claims 1 to 6, the apparatus comprising: The laser inertial confinement fusion device is configured to output linearly polarized third-harmonic 351nm ultraviolet laser. Oxygen-18 modified DKDP crystal waveplates are placed in the ultraviolet laser output optical path of the laser inertial confinement fusion device to convert linearly polarized ultraviolet laser into circularly polarized light. A fused silica optical element configured to receive and transmit the circularly polarized light.

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