Xinda Zhou

Influence of subsurface defects on damage performance of fused silica in ultraviolet laser

Abstract. 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.

Laser-Induced Point Defects in Fused Silica Irradiated by UV Laser in Vacuum

High-purity fused silica irradiated by third harmonic of the Nd:YAG laser in vacuum with different laser pulse parameters was studied experimentally. Laser-induced defects are investigated by UV spectroscopy, and fluorescence spectra and correlated to the structural modifications in the glass matrix through Raman spectroscopy. Results show that, for laser fluence below laser-induced damage threshold (LIDT), the absorbance and intensity of fluorescence bands increase with laser energies and/or number of laser pulses, which indicates that laser-induced defects are enhanced by laser energies and/or number of laser pulses in vacuum. The optical properties of these point defects were discussed in detail.

The effect of RIE-modified surface contamination on opticalperformance of fused silica

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.

Photoluminescence defects on subsurface layer of fused silica and its effects on laser damage performance

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 method for improving the measurement accuracy of lateral shearing interferometry

A systematic error calibration method is presented to improve the measurement accuracy of lateral shearing interferometry (LSI). This method is used to remove the most significant errors: geometric optical path difference (OPD) and detector tilt error. Difference fronts in the 0° and 90° directions are used to reconstruct wavefront using difference Zernike polynomial fitting. And difference fronts in the 45° and 135° directions are also used to reconstruct wavefront. The coefficient differences between the reconstructed wavefront are generated from geometric OPD and detector tilt error. The relationship between Zernike coefficient differences and systematic parameters are presented based on shear matrix. Thus, the distance of diffracted light converging point (d) and detector tilt angle can be calculated from the coefficient difference. Based on the calculated d and detector tilt angle, the geometric OPD and detector-tilt induced systematic errors are removed and the measurement accuracy of LSI is improved.

Preliminary study of the influence of polarization orientation on bulk damage resistances of doubler KDP crystals

The investigation of the influence polarization orientation on damage performance of type I doubler KDP crystals grown by the conventional growth method under under 532nm pulse exposure is carried out in this work. The obtained results point out the pinpoint density (ppd) of polarization parallels the extraordinary axis is around 1.5× less than that of polarization parallels the ordinary axis under the same fluence, although polarization has no influence on size distribution of pinpoints. Meanwhile, crystal inhomogeneity is observed during experiment.

An accurate method for investigation of laser-induced damage of optical component at 351nm

A multipurpose laser damage test facility delivering pulses from 1ns to 20ns and designed to output energy 40 Joule at 351nm is presented. The laser induced damage threshold (LIDT) measurement and test procedure are performed. The original system consist of the online detection system based on the microscopy and an energy detection device based on the scientific grade Charge Coupled Device (CCD) which provides the method to measure the LIDT with high accuracy. This method is an efficient way that allows measuring a small area fluence which the defect exposed. After complete test procedure and data treatment the damage position of the defect has been found. Then we can obtain the local fluence of small area when the damage occurred. This procedure provides a straightforward means of laser-damage threshold obtained from the test method. Damage correlation of measures is discussed in connection with present theoretical understanding of laser damage phenomenon. The damage process in transparent dielectric materials being the results of complex processes involving multi-photon ionization, avalanche ionization, electron-phonon coupling, and thermal effects. Those complex processes lead to the damage on the optical surface. We performed a method to measure the local fluence which defects irradiated with high accurate.

The effect of laser pulse width on laser-induced damage at K9 and UBK7 components surface

In this paper, we investigated the effects of laser pulse width on laser-induced damage. We measured the damage threshold of K9 glass and UBK7 glass optical components at different pulse width, then analysis pulse-width dependence of damage threshold. It is shown that damage threshold at different pulse width conforms to thermal restriction mechanism, Because of cm size laser beam, defect on the optical component surface leads to laser-induced threshold decreased.

Effect of pulse energy and numbers on fused silica surface by ultraviolet laser pulses at 355nm in vacuum

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.

Comparison of fused silica and oxyfluoride glass on laser induced initial damage morphology

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.