Process parameter optimization is important for improving the lapping effect of optical elements. Based on a temperature-controlled lapping disc, we present a method using fuzzy theory and orthogonal experiment to optimize lapping process parameters. Unlike the theory of optimization target set with a single evaluation criterion, a comprehensive optimization target with multiple evaluation criteria based on a fuzzy algorithm is proposed in this method. The lapping disc temperature, lapping disc rotation speed, and lapping pressure were selected as the optimization process parameters, and the material removal rate (MRR), surface change uniformity (SCU), and fuzzy synthetic index of lapping (FSILAP) were used as the optimization targets to conduct comparative experiments and analysis. The experimental results showed that when using the optimal process parameter combination obtained by the comprehensive index FSILAP as the optimization target, balanced results for MRR and SCU could be obtained. Notably, the result obtained when using a single evaluation criterion as the optimization target was suboptimal. The proposed method is effective and practical for the optimization of lapping process parameters.
Subsurface micro-cracks will be generated during the grinding and polishing processes of optical components. Microcracks have a modulation effect on the laser, thereby reducing the laser damage threshold. The FDTD method is used to simulate the light intensity distribution modulated by micro-crack. By comparing the simulation results of radial crack, parabolic crack and elliptic crack, the modulation mechanism of micro-crack is revealed. The results show that for the crack with the same width and depth, light intensity enhancement factor (LIEF) modulated by radial crack on the rear surface and parabolic crack on the front surface is the largest; LIEF modulated by elliptical crack on the rear surface and radial crack on the front surface is the smallest. In addition, when the crack width-depth ratio is the same, the larger the depth, the higher the LIEF. As the width-depth ratio increases, the LIEF value increases firstly, then decreases, and finally approaches a stable value.
Subsurface defects and contaminations will be generated during the grinding and polishing processes of optical components. Combined modulation is one of the important factors for the laser-induced damages of fused silica. In this paper, by using 2D finite-difference time-domain method, the light intensity distribution modulated by both radial crack and contaminant is studied on front/rear surface, respectively. The results show that the light intensity distribution is significantly affected by the aspect ratio of radial crack and the relative position between radial crack and contaminant. The simulations of the combined modulation on rear surface show that larger LIEFs are generated at certain relative positions compared with those in the single modulation of radial crack or contaminant. Meanwhile, with the increase of distance, the LIEFs are wave-like up and down fluctuations, and gradually tend to stable values. When there is no total internal reflection, the LIEF in contaminant on the crack wall rises significantly with increase of distance, the maximum LIEF occurs when the contaminant is near the intersecting line between radial crack and rear surface. The simulation of the combined modulation on front surface show that the variation of LIEFs in global domain are not very prominent.
Chemical etching is usually utilized to improve the laser damage performance of optical glass by mitigating microcracks, while it inevitably produces some reaction products (RPs). In this paper, two K9 glasses with good quality and two K9 glasses with micro-cracks are etched, statically or dynamically (high-frequency ultrasonic agitation). The morphologies of cracks and RPs are characterized, and the laser-induced damage thresholds (LIDTs) are measured. The results show that with the increase of etching time, the LIDT increases slightly at first and then decreases gradually for the glass with RPs, and the LIDT increases at first and then stabilizes for the glass without RPs. Using finite-difference time-domain method, the light intensities around crack, RP and their combination are simulated, respectively. The results indicate that the light intensity enhancement factor (LIEF) increases at first and then decreases with the decrease of crack aspect ratio, and the LIEF increases with RP radius. The LIEF for the combination is generally larger than that for one crack or one RP, which greatly depends on the relative distance between the crack and the RP. Experimental and simulated results complement each other, revealing the influence mechanism of crack and RP on the LIDT. This work would contribute to improving the LIDT of optical glass by chemical etching.
Chemical etching experiments are carried out to expose and change the morphology of multiple scratches and single scratches in a set of K9 glasses, during which the transmission of some glasses is measured. The results show that brittle scratches have an influence on the transmission, and the transmission decreases at first and then increases obviously with the increase of etching time for the glass with a high density of cracks. The scattered field distribution around trailing indent crack(s) and the transmission of the glass with a scratch etched deeply (adjacent cracks have coalesced with each other) are simulated under different parameters. Using finite-difference time-domain (FDTD) algorithm, the light intensification effect around two single-evolving scratches and two adjacent cracks with different parameters are simulated, respectively. The results indicate that the position with the maximum light intensity changes for the glass with a scratch and its light intensity enhancement factors (LIEFs) will generally decrease as the etching process progresses. Compared with the morphology evolution of a scratch during the etching process, the changing mechanisms of transmission and LIEFs are revealed. This work will contribute to the improvement of service performance of optical glass by chemical etching process.
The subsurface damage (SSD) introduced by manufacturing procedures are the important factors affecting mechanical
and optical properties of the high-precision optical components. In this paper, we investigated the relationships between
the size of SSD’s micro crack and optical properties by studying on the influence of grinding process parameters
(average abrasive size, grinding force, wheel speed, grinding depth) on the optical properties (transmittance, absorbance,
reflectance) of high-precision optical components first, and then determining the micro crack’s size under the grinding
process parameters. Results indicate that with the increase of average abrasive size, grinding force or grinding depth, the
transmittance of high-precision optical components decreases while the absorbance and reflectance increase. However,
as the wheel speed increases, the transmittance of high-precision optical components increases while the absorbance and
the reflectance decrease. Combining these results with the influence of grinding process parameters on the size of micro
crack, it can be found that with the increase of micro crack’s size, the transmittance will decrease, while the absorbance
and the reflectance will increase.
One important approach to characterize full three-dimensional information is to simulate the etching process of a sample with subsurface damage reversely. The simulation starts from the morphology of the sample at a certain time when the subsurface damage can be opened totally. In the etching experiment, it is possible for us to get the surface morphology at any time. The paper presents a finite difference algorithm to simulate the morphology evolution in an etching process and by the finite difference algorithm the morphology of the sample at a specific time can be given. Comparison of the simulated morphology and measured one provides us the clue of improving the finite difference algorithm. In this paper, the accuracy can be calculated through comparing the simulation with experimental result, and the maximum error of subsurface damage will be calculated.
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