Efficient removal of SiO2 contaminated particles from sol-gel SiO2 filmed fused silica surface was achieved by single shot dry laser cleaning and airflow displacement system assisted laser cleaning using a Q switched Nd:YAG pulse laser at 355 nm in wavelength. The experiment result shows that for single-shot laser dry cleaning of sol-gel film fused silica samples under the condition of satisfying the theoretical cleaning threshold, the optimal laser energy density was 2.28 J/cm2, which is different from the process parameters of uncoated fused silica. In the optimal process parameters, single shot laser cleaning had obvious cleaning effect on SiO2 particles with particle diameter larger than 1 μm, and the removal ratio was 64.81%. When the density of particle was too high, the cleaning effect was weakened and the damage to the substrate was caused. The re-attachment of crushed particles also leaded to making the cleaning effect worse. The airflow displacement system assisted laser cleaning method further enhanced the effect of removing the optical surface contaminated particles and the decontamination efficiency was improved to 80.53% at 1.71 J/cm2. Our results demonstrated that airflow displacement system will be useful for single shot dry laser cleaning of SiO2 particles on an optical surface of fused silica.
A type of high-birefringent terahertz (THz) photonic crystal fiber (PCF) with all circle air holes is proposed. The characteristics including birefringence, dispersion, and confinement loss are numerically analyzed in detail by using the finite element methods. Simulation results show that the proposed THz PCFs exhibit high birefringence on the level of 10−2 in the frequency range of 2 to 4 THz, which is realized by the minor position adjustment of air holes in the first ring of the cladding. We believe that the proposed THz PCFs can be fabricated without complications due to their simple structure. In addition, two porous-core THz PCFs are proposed and the birefringence property is investigated.
Subsurface defects of polished fused silica optics are responsible for igniting laser damage in high power laser system. A non destructive measurement technique is developed to detect subsurface photoluminescence defects of fused silica. The fused silica samples polished by different vendors are applied to characterization of subsurface defects and measurement of damage performance. Subsurface photoluminescence defects of fused silica are evaluated by confocal fluorescence microscopy system. Laser induced damage threshold and damage density are measured by 355 nm nanosecond pulse laser. The results show a great differential subsurface quality of fused silica samples. Laser induced damage performance has a good correlation with subsurface defects. This paper shows a new non destructive measurement technique to detect photoluminescence defects on the subsurface layer of polished fused silica. It is very valuable to increasing laser damage performance and improving production-manufacturing engineering of optics.
A novel oxyfluoride glass (OFG) was prepared. The laser induced damage threshold (LIDT) of the novel OFG is 24.9%
higher than fused silica under 355nm nanosecond laser irradiation by R-on-1 procedure. Characterization by optical
microscope and scanning electron microscope shows that the initial damage morphologies of two kind of materials are
significantly different. Experiment results indicate that the novel OFG can be a good candidate component material for
high energy laser applications.
Potassium Dihydrogen Phosphate (KDP) is a non-linear optical material for the Pockelscells and laser frequency
conversion used in inertial confinement fusion (ICF).But the performances are decreased by the presence of defects or
impurities from the growth process. Furthermore, it is difficult to identify them due to their small size and sparse
distribution. Laser conditioning provides an increase for the material damage resistance due to an unknown physical
mechanism resulting from the interaction of sub-damage laser pulses with KDP defects.
In this paper, the crystals first were laser conditioned by raster scanning using Nd:YAG laser at 355nm with pulse
durations of approximately 7 ns. Then we investigated the KDP crystals with different non-destructive optical
diagnostics. We measured in the same time the fluorescence, the photothermal absorption, Raman and FTIR
spectroscopy of the material. We concentrated on the differences of conventionally grown KDP crystals before and after
the laser conditioning. We obtained the different spectra information of KDP crystals.
High–purity fused silica irradiated by UV laser in vacuum with different laser pulse parameters were
studied experimentally. The defects induced by UV laser are investigated by UV absorption, fluorescence
spectra and the structural modifications in the glass matrix are characterized by Raman spectra. Results show
that, for laser fluence below the laser–induced damage threshold (LIDT), irradiation results in the formation of
an absorption band and four defect–related fluorescence (FL) bands, and the intensities of absorption band and
FL bands were increased with laser power and/or number of laser pulses. The optical properties of these point
defects were discussed in detail. Analyzed these spectra, it indicates that the presence of different centers whose
spectral features are modulated by structural disorder typical of the glass matrix.
A series of fused silica surface have been created by reaction ion etching to determine the effect of the contamination
level on surface state and optical performance of the optics. The results show that both impurity elements contamination
and scratches of fused silica surface can be removed dramatically during RIE process. The laser induced damage
threshold is raised by 37.6% when the polishing layer is removed for a thickness of 6μm, and the laser weak absorption
doesn’t increase obviously. The results can provide technique support for improving laser induced damage performance
of fused silica.
In ultraviolet pulse laser, damage performance of fused silica optics is directly dependent on the absorptive impurities and scratches in subsurface, which are induced by mechanical polishing. In the research about influence of subsurface defects on damage performance, a series of fused silica surfaces with various impurity concentrations and scratch structures were created by hydrofluoric (HF) acid solution etching. Time of Flight secondary ion mass spectrometry and scanning probe microprobe revealed that with increasing etching depth, impurity concentrations in subsurface layers are decreased, the scratch structures become smoother and the diameter:depth ratio is increased. Damage performance test with 355-nm pulse laser showed that when 600 nm subsurface thickness is removed by HF acid etching, laser-induced damage threshold of fused silica is raised by 40 percent and damage density is decreased by over one order of magnitude. Laser weak absorption was tested to explain the cause of impurity elements impacting damage performance, field enhancement caused by change of scratch structures was calculated by finite difference time domain simulation, and the calculated results are in accord with the damage test results.
Measurements of birefringence induced in K9 and fused silica specimens by cracks produced by 1064 nm Nd∶YAG laser have been presented. The Birefringence data is converted into the units of stress, thus permitting the estimation of residual stress near crack. The intensity of residual stress in K9 glass is larger than that in fused silica under the same condition. The similarity of residual stress distribution along the y-axis reveals that the nature of shock wave transmission in optical materials under 1064 nm laser irradiation is the same with each other. The value of residual stress can be influenced by laser parameters and characterization of optical material. Simulation based on a theoretical model giving the residual stress field around a crack is developed for comparison with experiment results. The probability of initial damage and the direction of the energy dissipation in cracks determine the residual stress distribution. The thermal stress coupling enlarges the asymmetry of residual stress distribution. Residual stress in optical material has a strong effect on fracture and should be taken into account in any formulation that involves the enhanced damage resistance of optical components used in laser induced damage experiments.
We report the experiment results of the study on dust pollution initiated damage in optical
component of the ICF facility and evaluate the influence of this kind damage on other optical component. Dust caused by laser-induced damage in optical component in vacuum environment at the high power laser facility was collected with fused silica flat optics. The transmission change of the dust polluted optics was observed and analyzed. The damage probability of the dust polluted optics was tested by s-on-1 method using pulsed Nd-YAG solid laser. Results showed that dust polluted sol-gel anti-reflection film coated fused silica optics exhibited lower transmission and higher damage probability than the naked fused silica flat optics. Besides, the dust particle on the input surface will cause severer damage than on the output surface.
Laser damage performance of K9 glass and fused silica glass were tested respectively at same experimental
condition with 1064 nm nanosecond laser. The initial damage threshold (IDT), the damage growth threshold (DGT)
and the damage growth laws of the two optics glass were investigated comparatively. The results show that the
damage growth behavior of the two glasses are quite different, for example, the lower damage growth threshold
and the higher damage growth coefficient for K9 glass, which can attribute to the difference of the material's
damage morphology, optical absorption, residual stress near damage site between the two optics glass. The
research is very important to choose transparent optical material applied in high power laser.
The dynamics of 355-nm laser ablation on fused silica were studied by instantaneous scattering pulse
measurement and a time-resolved shadowgraph imaging. The sharp increase of scattered light of pumped
pulse is assumed to be the damage precursor, therefore, the damage start nearly at the peak of the
pumped pulse. The plasmas flash due to ion-electron recombination occurred about 21ns after the
peak of pumped pulses. The propagating shock wave and ejected material to the air were imaged by
shadowgraphic technology. The damage process of fused silica under UV laser ablation was also
discussed.
As a key specification of the beam quality control, the profile of the optics requires hi-precise testing, which is indispensable during the R&D of high power laser drivers. Currently, the commercial interferometers provided by such companies as Veeco , Zygo and 4D in US and Fuji in Japanese are widely used in profile testing of optics. However, during our practice, a fairly good accordance can not be found after a series of the profile tests of the optics. Generally, there’re certain fluctuations or even remarkable difference among the testing results occurring in the testing results compared with the design specifications. In order to improve the existing poor accordance of the testing results of profile specifications of the optics, and in accordance with our repeated experiments, the Main factors that affect the profile testing are presented respectively, as well as their corresponding impacts upon the profile based on the clarified evaluation parameters and unified testing principles, which are of significant reference value to standardize the testing method of optics profile and to strictly control the optics quality.
We are now constructing a technical integration experiment line (TIL) at CAEP, which is the prototype facility of Shenguang III laser fusion driver. Currently, many important results have been obtained on the first integrated beam line, which established a sound foundation for Shenguang III engineering design.
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