Surface defects,stress evolution,and laser damage enhancement mechanism of fused silica under oxygen-enriched condition

Oxygen ions(O;)were implanted into fused silica at a fixed fluence of 1×10^(17) ions/cm^(2) with different ion energies ranging from 10 ke V to 60 ke V.The surface roughness,optical properties,mechanical properties and laser damage performance of fused silica were investigated to understand the effect of oxygen ion implantation on laser damage resistance of fused silica.The ion implantation accompanied with sputtering effect can passivate the sub-/surface defects to reduce the surface roughness and improve the surface quality slightly.The implanted oxygen ions can combine with the structural defects(ODCs and E′centers)to reduce the defect densities and compensate the loss of oxygen in fused silica surface under laser irradiation.Furthermore,oxygen ion implantation can reduce the Si-O-Si bond angle and densify the surface structure,thus introducing compressive stress in the surface to strengthen the surface of fused silica.Therefore,the laser induced damage threshold of fused silica increases and the damage growth coefficient decreases when ion energy up to30 ke V.However,at higher ion energy,the sputtering effect is weakened and implantation becomes dominant,which leads to the surface roughness increase slightly.In addition,excessive energy aggravates the breaking of Si-O bonds.At the same time,the density of structural defects increases and the compressive stress decreases.These will degrade the laser laser-damage resistance of fused silica.The results indicate that oxygen ion implantation with appropriate ion energy is helpful to improve the damage resistance capability of fused silica components.

Temperature dependences of optical properties,chemical composition,structure,and laser damage in Ta_2O_5 films

Ta2O5 films are prepared by e-beam evaporation with varied deposition temperatures, annealing temperatures, and annealing times. The effects of temperature on the optical properties, chemical composition, structure, and laser- induced damage threshold （LIDT） are systematically investigated. The results show that the increase of deposition temperature decreases the film transmittance slightly, yet annealing below 923 K is beneficial for the transmittance. The XRD analysis reveals that the film is in the amorphous phase when annealed below 873 K and in thehexagonal phase when annealed at 1073 K. While an interesting near-crystalline phase is found when annealed at 923 K. The LIDT increases with the deposition temperature increasing, whereas it increases firstly and then decreases as the annealing temperature increases. In addition, the increase of the annealing time from 4 h to 12 h is favourable to improving the LIDT, which is mainly due to the improvement of the O/Ta ratio. The highest LIDT film is obtained when annealed at 923 K, owing to the lowest density of defect.

Study on Preparation and Ns-Laser Damage of HfO₂Single Layers

The effects of the main parameters of argon flux, oxygen flux and beam voltage on the surface morphology, transmittance spectrum and laser damage of the HfO2 single layers prepared by ion beam sputtering are studied. The HfO2 amorphous single layers have porous surface morphologies. Different processes will cause differences in coatings absorption and surface morphology, which in turn will cause changes in the spectral transmittance curve. The ion beam sputtering HfO2 single layers have high content of argon (4.5% - 8%). The laser damage of HfO2 single layers is related to argon inclusions and non-stoichiometric defects. The changes of argon flux and beam voltage have a greater impact on argon content and O/Hf ratio. When the argon content in the coatings is lower and the O/Hf ratio is higher, the laser damage thresholds of the HfO2 single layers are higher.

Mechanism of micro-waviness induced KH_2PO_4 crystal laser damage and the corresponding vibration source

The low laser induced damage threshold of the KH2PO4 crystal seriously restricts the output power of inertial confinement fusion.The micro-waviness on the KH2PO4 surface processed by single point diamond turning has a significant influence on the damage threshold.In this paper,the influence of micro-waviness on the damage threshold of the KH2PO4 crystal and the chief sources introducing the micro-waviness are analysed based on the combination of the Fourier modal theory and the power spectrum density method.Research results indicate that among the sub-wavinesses with different characteristic spatial frequencies there exists the most dangerous frequency which greatly reduces the damage threshold,although it may not occupy the largest proportion in the original surface.The experimental damage threshold is basically consistent with the theoretical calculation.For the processing parameters used,the leading frequency of micro-waviness which causes the damage threshold to decrease is between 350-1 μm-1 and 30-1 μm-1,especially between 90-1 μm-1 and 200-1 μm-1.Based on the classification study of the time frequencies of microwaviness,we find that the axial vibration of the spindle is the chief source introducing the micro-waviness,nearly all the leading frequencies are related to the practical spindle frequency（about 6.68 Hz,400 r/min） and a special middle frequency（between 1.029 Hz and 1.143 Hz）.

Improvement of laser damage thresholds of fused silica by ultrasonic-assisted hydrofluoric acid etching

Polished fused silica samples were etched for different durations by using hydrofluoric（HF） acid solution with HF concentrations in an ultrasonic field. Surface and subsurface polishing residues and molecular structure parameters before and after the etching process were characterized by using a fluorescence microscope and infrared（IR） spectrometer, respectively. The laser induced damage thresholds（LIDTs） of the samples were measured by using pulsed nanosecond laser with wavelength of 355 nm. The results showed that surface and subsurface polishing residues can be effectively reduced by the acid etching process, and the LIDTs of fused silica are significantly improved. The etching effects increased with the increase of the HF concentration from 5 wt.% to 40 wt.%. The amount of polishing residues decreased with the increase of the etching duration and then kept stable. Simultaneously, with the increase of the etching time, the mechanical strength and molecular structure were improved.

Vulnerability assessment of UAV engine to laser based on improved shotline method

Laser anti-drone technology is entering the sequence of actual combat,and it is necessary to consider the vulnerability of typical functional parts of UAVs.Since the concept of"vulnerability"was proposed,a variety of analysis programs for battlefield targets to traditional weapons have been developed,but a comprehensive assessment methodology for targets'vulnerability to laser is still missing.Based on the shotline method,this paper proposes a method that equates laser beam to shotline array,an efficient vulnerability analysis program of target to laser is established by this method,and the program includes the circuit board and the wire into the vulnerability analysis category,which improves the precision of the vulnerability analysis.Taking the UAV engine part as the target of vulnerability analysis,combine with the"life-death unit method"to calculate the laser penetration rate of various materials of the UAV,and the influence of laser weapon system parameters and striking orientation on the killing probability is quantified after introducing the penetration rate into the vulnerability analysis program.The quantitative analysis method proposed in this paper has certain general expansibility,which can provide a fresh idea for the vulnerability analysis of other targets to laser.

Postprocessing treatments to improve the laser damage resistance of fused silica optical surfaces and SiO_2 coatings

The combination of deep wet etching and a magneto-rheological finishing （MRF） process is investigated to simultaneously improve laser damage resistance of a fused-silica surface at 355 nm. The subsequently deposited SiO2 coatings are researched to clarify the impact of substrate finishing technology on the coatings. It is revealed that a deep removal proceeding from the single side or double side had a significant impact on the laser-induced damage threshold （LIDT） of the fused silica, especially for the rear surface. After the deep etching, the MRF process that followed does not actually increase the LIDT, but it does ameliorate the surface qualities without additional LIDT degradation. The combination guarantee both the integrity of the surface＇s finish and the laser damage resistance of the fused silica and subsequent SiO2 coatings.

Modified porous silica antireflective coatings with laser damage resistance for Ti:sapphire

Porous SiO2 antireflective (AR) coatings are prepared from the colloidal silica solution modified with methyltriethoxysilane (MTES) based on the sol-gel route. The viscosity of modified silica suspensions changes but their stability keeps when MTES is introduced. The refractive indices of modified coatings vary little after bake treatment from 100 to 150℃. The modified silica coatings on Ti:sapphire crystal, owning good homogeneity, display prominent antireflective effect within the laser output waveband (750-850 nm) of Ti:sapphire lasers, with average transmission above 98.6%, and own laser induced damage thresholds (LIDTs) of more than 2.2 J/cm2 at 800 nm with the pulse duration of 300 ps.

Influence of coating material on laser damage threshold of TiO_2 films

The optical property, structure, surface properties （roughness and defect density） and laser-induced damage threshold （LIDT） of TiO2 films deposited by electronic beam （EB） evaporation of TiO2 （rutile）, TiO2 （anatase） and TiO2 ＋ Ta205 composite materials are comparatively studied. All films show the polycrystalline anatase TiO2 structure. The loose sintering state and phase transformation during evaporating TiO2 anatase slice lead to the high surface defect density, roughness and extinction coefficient, and low LIDT of films. The TiO2 ＋ Ta205 composite films have the lowest extinction coefficient and the highest LIDT among all samples investigated. Guidance of selecting materials for high LIDT laser mirrors is given.

Automated damage diagnostic system for laser damage threshold tests

An automated damage diagnostic system for collecting plasma flash is developed to diagnose damage in a laser-induced damage threshold（LIDT） test system.Experiment is done to verify the accuracy of this system and analyze the relationship between the plasma signals and the damage morphologies.The results obtained by the system are found to be in excellent agreement with those obtained by the much laborious method of Normaski microscope.Results show that plasma signals above 1 V correspond to the damage morphology of surface discolorations with or without pits in their centers,and plasma signals below or just around 1 V correspond to the damage morphology of pits.The misdiagnosis is attributed to contaminations and air breakdown.

Femtosecond laser damage of broadband pulse compression gratings

The fabricated gratings used for an 800-nm compressed laser pulse have more than 90% di?raction e?ciency in the –1st order for TE polarization within 760–860 nm, and the maximum value is 94.3%. The laserinduced damage threshold(LIDT) of the gratings increases from 0.53 to 0.75 J/cm 2 in the normal beam in a pulse width τ of 40–100 fs. The LIDT of the gratings is observed a τ 0. 25 scaling in the pulse width region. The damage morphologies of the gratings indicate that the initial damage of the gratings locates at the grating lines, a position that coincides with that of the electric field maximum.

High laser damage threshold LiNa_(5)Mo_(9)O_(30) prism:for visible to mid-infrared range

In this study,an excellent polarization optical crystal LiNa_(5)Mo_(9)O_(30) with wide transmission range and high laser damage threshold was researched in detail.The laser damage threshold of the LiNa_(5)Mo_(9)O_(30) crystal was measured to be 2.64GW/cm^(2),which was the highest among polarized optical crystals.The birefringence in the range of 0.435-5μm was larger than 0.14,while the wedge angle between 31.94°and 32.12°would satisfy the application in this waveband.The extinction ratio of the fabricated prism with the wedge angel of 31.09°was larger than 15,000:1.The results show that the LiNa_(5)Mo_(9)O_(30) prism is an excellent polarization device,especially in the mid-infrared range and high-power applications.

Research progress on mid-IR nonlinear optical crystals with high laser damage threshold in China

Nonlinear optical(NLO)crystals have been playing an increasingly important role in laser science and technology.The NLO crystals used in the middle infrared(mid-IR)region,compared with the NLO crystals in the other wavelength regions,are still not good enough for the application of high-energy laser.The main defect is that their laser damage thresholds(LDT)are low.Chinese scientists have made a lot of important contributions to the UV and visible NLO crystals.In the last decade,they also did a lot of work on the mid-IR NLO materials.The main purpose of these researches is to increase the LDT and simultaneously balance the other properties.This paper presents a brief summary of their research progress in this topic on three types of materials:chalcogenides,oxides,and halides.The emphasis is put on the design strategy and quality control of the crystals.

Preparation and laser induced damage properties of rare earth doped BAS glass

Rare earth doped B2O3-Al2O3-SiO2 glass （RExBAS, x=5, 10, 20; RE=La, Sm） were prepared by solid state reaction method. Optical transmission spectra of such glass were characterized by ultraviolet spectrometers, and 1064 nm laser induced damage performance was investigated through the method of ＂1-on-1＂. The results indicated that there was a strong absorptive peak near 1064 nm in SmxBAS glass, the peak was enhanced with increasing x. While LaxBAS glass was transparent to 1064 nm laser, at the same time, the results of laser induced damage showed that the anti-laser induced damage performance of such glass was strengthened with the addition of rare earth oxide. Furthermore, the laser induced damage threshold （LIDT） of SmxBAS glass was significantly higher than that of LaxBAS glass. Consequently, Sm^3＋ doping was favor in the improvement of anti-laser induced damage performance for BAS glass.

Fiber fuse behavior in kW-level continuous-wave double-clad field laser

In this study, original experimental data for fiber fuse in kW-level continuous-wave （CW） high power double-clad fiber （DCF） laser are reported. The propagating velocity of the fuse is 9.68 m/s in a 3.1-kW Yb-doped DCF laser. Three other cases in Yb-doped DCF are also observed. We think that the ignition of fiber fuse is caused by thermal mechanism, and the formation of bullet-shaped tracks is attributed to the optical discharge and temperature gradient. The inducements of initial fuse and formation of bullet-shaped voids are analyzed. This investigation of fiber fuse helps better understand the fiber fuse behavior, in order to avoid the catastrophic destruction caused by fiber fuse in high power fiber laser.

Enhanced laser induced damage threshold of dielectric antireflection coatings by the introduction of one interfacial layer

A new method for increasing laser induced damage threshold （LIDT） of dielectric antireflection （AR） coating is proposed. Compared with AR film stack of H2.5L （H：HfO2, L：SiO2） on BK7 substrate, SiO2 interracial layer with four quarter wavelength optical thickness （QWOT） is deposited on the substrate before the preparation of H2.5L film. It is found that the introduction of SiO2 interfacial layer with a certain thickness is effective and flexible to increase the LIDT of dielectric AR coatings. The measured LIDT is enhanced by about 50%, while remaining the low reflectivity with less than 0.09% at the center wavelength of 1064 nm. Detailed mechanisms of the LIDT enhancement are discussed.

Contamination process and laser-induced damage of HfO_2/SiO_2 coatings in vacuum

The performances of HfO2/SiO2 single- and multi-layer coatings in vacuum influenced by contamination are studied. The surface morphology, the transmittance spectrum, and the laser-induced damage threshold are investigated. The results show that the contamination in vacuum mainly comes from the vacuum system and the contamination process is different for the HfO2 and SiO2 films. The laser-induced damage experiments at 1064 nm in vacuum show that the damage resistance of the coatings will decrease largely due to the organic contamination.

Laser-conditioning mechanism of ZrO_2/SiO_2 HR films with fitting damage probability curves of laser-induced damage

The method of fitting damage probability curves of laser-induced damage is introduced to investigate the laser-conditioning mechanism of ZrO2/SiO2 high reflection（HR） films.The laser-induced damage thresholds（LIDTs） of the sample are tested before and after the laser-conditioning scanning process.The parameters of the defects are obtained through the fitting process of the damage probability curve.It can be concluded that the roles of laser conditioning include two aspects：removing defects with lower threshold and producing new defects with higher threshold.The effect of laser conditioning is dependent on the competition of these two aspects.

High-power electro-optic switch technology based on novel transparent ceramic

A novel high-power polarization-independent electro-optic switch technology based on a reciprocal structtire Sagnac interferometer and a transparent quadratic electro-optic ceramic is proposed and analyzed theoretically and experimentally. The electro-optic ceramic is used as a phase retarder for the clockwise and counter-clockwise polarized light, and their po- larization directions are adjusted to their orthogonal positions by using two half-wave plates. The output light then becomes polarization-independent with respect to the polarization direction of the input light. The switch characteristics, including splitter ratios and polarization states, are theoretically analyzed and simulated in detail by the matrix multiplication method. An experimental setup is built to verify the analysis and experimental results. A new component ceramic is used and a non- polarizing cube beam splitter （NPBS） replaces the beam splitter （BS） to lower the ON/OFF voltage to 305 V and improve the extinction ratio by 2 dB. Finally, the laser-induced damage threshold for the proposed switch is measured and discussed. It is believed that potential applications of this novel polarization-independent electro-optic switch technology will be wide, especially for ultrafast high-power laser systems.

Comparison of laser-induced damage in Ta_2O_5 and Nb_2O_5 single-layer films and high reflectors

Ta2O5 and Nb2O5 films are deposited on BK7 glass substrates using an electron beam evaporation method and are annealed at 673 K in the air. In this letter, comparative studies of the optical transmittance, microstructure, chemical composition, optical absorption, and laser-induced damage threshold （LIDT） of the two films are conducted. Findings indicate that the substoichiometric defect is very harmful to the laser damage resistance of Ta2O5 and Nb2O5 films. The decrease of absorption improves the LIDT in films deposited by the same material. However, although the absorption of the Ta2O5 single layer is less than that of the Nb2O5 single layer, the LIDT of the former is lower than that of the latter. High-reflective （HR） coatings have a higher LIDT than single layers due to the thermal dissipation of the SiO2 layers and the decreased electric field intensity （EFI）. In addition, the Nb2O5 HR coating achieves the highest LIDT at 25.6 J/cm^2 in both single layers and HR coatings.