Research on the mitigation of redeposition defects on the fused silica surface during wet etching process.

The laser-induced damage of ultraviolet fused silica optics is a critical factor that limits the performance enhancement of high-power laser facility. Currently, wet etching technology based on hydrofluoric acid (HF) can effectively eliminate absorbing impurities and subsurface defects, thereby significantly enhancing the damage resistance of fused silica optics. However, with an increase in the operating fluence, the redeposition defects generated during wet etching gradually become the primary bottleneck that restricts its performance improvement. The composition and morphology of redeposition defects were initially identified in this study, followed by an elucidation of their formation mechanism. A mitigation strategy was then proposed, which combines a reduction in the generation of precipitation with an acceleration of the precipitation dissolution process. Additionally, we systematically investigated the influence of various process parameters such as extrinsic impurity, etching depth, and megasonic excitation on the mitigation of deposition defects. Furthermore, a novel multiple-step dynamic etching method was developed. Through comprehensive characterization techniques, it has been confirmed that this new etching process not only effectively mitigate redeposition defects under low fluence conditions but also exhibits significant inhibition effects on high fluence precursors. Consequently, it significantly enhances the laser damage resistance performance of fused silica optics.

Measurement error estimation and calibration for on-line detection phase variations in the laser-induced damage growth features of optical components.

The amplitude and phase distributions of reflection or transmission light are locally disturbed by damage regions. The damage state of optical components under successive laser irradiation can be evaluated according to the phase variation of a transmitted beam. The measurement accuracy of phase information related to the phase-unwrapping method is a critical factor for evaluating the damage state. This study analyzes and compares the performance of two important phase-unwrapping methods for detecting on-line large damage sites based on optimal modified lateral shearing interferometry. Meanwhile, the system stability and the measurement repeatability are also validated according to the retrieved phase pattern, which is beneficial to estimating the measurement accuracy. Experimental results of optical films are also presented and discussed to verify the feasibility of the estimation method utilized and the recommended phase-unwrapping method for on-line detection of the laser-induced damage growth features.

Detection of laser-induced bulk damage in optical crystals by swept-source optical coherence tomography.

An approach to characterize laser-induced bulk damage in optical crystal materials was demonstrated. With a homemade swept-source optical coherence tomography (SS-OCT) system, we obtained three-dimensional images of the bulk damage produced by laser pulses with wavelength of 351 nm, pulse width of 5 ns, beam diameter of 5.5 mm and fluences from 4.56 J/cm2 to 9.95 J/cm2 in Potassium Dihydrogen Phosphate (KDP) crystal. Algorithms based on three-dimensional OCT images were specially designed to count and locate bulk damage pinpoints in KDP crystal, obtaining their equivalent diameter distribution and pinpoint density caused by different fluences. It is demonstrated that the characteristics of bulk damage detected by SS-OCT are similar to those obtained by available approaches. The rapid three-dimensional imaging by SS-OCT provides a new approach of detecting laser-induced bulk damage.