The point defects exist in the SiO2 thin films can cause high absorption, which is known to be responsible for laser induced damage of the films under high power nanosecond (ns) laser irradiation. Laser conditioning of the film is beneficial to eliminate the film defects and improve the ability of films to resist ultraviolet (UV) ns laser damage. In this article, femtosecond laser is proposed to modify the SiO2 films in the hope of improving the damage resistance of films to UV lasers. After femtosecond laser conditioning, the film properties of ALD SiO2 films were characterized in terms of surface morphology, UV laser damage induced threshold (LIDT) and optical properties. The results show that significant improvement in laser damage resistance is achieved after femtosecond laser conditioning, the LIDT of the 300 nm SiO2 thin film increased from 1.55 J/cm2 to 16.69 J/cm2, and the LIDT of the 600 nm SiO2 thin film increased from 2.01 J/cm2 to 9.46 J/cm2.
Fused silica optical elements have been widely used in high power laser systems because of their good optical and mechanical properties. However, defects such as scratches on the surface/sub-surface will inevitably occur in the process of optical components. It will affect the laser damage threshold. To simulate the effect of the defect on performance of laser radiation, evolution law of the scratches in chemical etching must be predefined. In order to determine the influence of scratch, a surface scratcher used to produce a specific scratches on the surface of the elements, and its morphology was characterized and the damage test was performed. By analyzing the morphology and characteristics of the scratches during chemical processing, and recording their fluorescence effects, it is clear that the width of the scratches increases with the depth of the etching, and the laser damage resistance is gradually improved. At the same time, the initial defects of different characteristics were clarified, the evolution rate in chemical etching was not consistent, and the impact on the ability to resist laser damage was different. In general, scratch defects have severely impaired the resistance of the device to laser damage. When the damage threshold of the component without scratches is about 23J/cm2, the damage threshold of the defect location is only less than 5J/cm2. Through targeted chemical treatment processes, you can increase the threshold of most scratch damage to the level of no scratches.
Based on the ultra-precision grinder, an experimental platform for single abrasive indentation was built to investigate the effect of single abrasive particle on the fused silica optical element in the grinding process. In the paper, the process of crack formation and propagation in the element was analyzed. Two crack growth nuclei were formed on both sides of the indentation point just after indentation. With the increase of the indentation depth, the crack size increased and those two crack growth nuclei joined together, and the longitudinal cracks torn and extended to the interior of the material. Experiments were carried out at different indentation depths, and the variation of force with indentation depth was obtained. The crack morphology was measured by microscopy, and the variation of transverse crack radius and longitudinal crack depth with indentation depth was obtained. The results had important engineering significance for improving the grinding process.
Small scale waviness of aspheric surface inevitably occurs when grinding aspheric surface with grating parallel grinding technology, so aiming at the problem of waviness amplitude and uniformity, this paper theoretically analyses the relationship between grinding processing parameters and aspheric waviness, and designs a single factor experiment to verify the influence of grinding processing parameters on aspheric surface waviness. The processing parameters are determined to minimize the waviness amplitude. Considering the problem of uniformity of waviness, according to the influence of grinding force on uniformity of aspheric waviness in grinding process, down-grinding grating parallel grinding method and up-grinding grating parallel grinding method are used. Experiments verify that down-grinding grating parallel grinding method is the best method to get most uniformity small-scale waviness of aspheric surface. The minimum amplitude is 0.5μm~1.5μm.
Thanks to the advantages of improving the focusing precision and reducing the energy loss in the beam focusing, complex and off-axis aspheric mirrors are widely used in the field of aviation, aerospace, national defense and other large optical systems. Ultra-precision grinding is an important technology to manufacturing large aperture aspheric optics in enormous quantities. In order to fabricate large aperture aspheric optics high efficiently and precisely, several key technologies when parallel grinding were proposed in this article. First, the computer aided programming system was developed, which could compute the coordinates of aspheric surface and diamond wheel when grinding and generate the CNC programs automatically, which can be directly executed by the grinder. On the premise of waviness controlling, the raster grinding trajectory was optimized to improve the material removal efficiency. To acquire the radius and form error of diamond wheel, the measurement of diamond wheel based on corkscrew spin trajectory was proposed, which could detect the 3-D geometric morphology of wheel. By precision tool setting using displacement sensor, the definitive position between wheel and element was established, which avoided the error correction in subsequent grinding process. Through on-machine measurement using non-contact displacement sensor, the 3-D form error of optics was acquired, which was combined with the theoretical coordinates of aspheric to compensation grinding. In the end the grinding experiment was carried out. The material removal rate of rough grinding, semi-fine grinding and fine grinding were about 520mm3/s, 26 mm3/s and 1.6 mm3/s, respectively. The P-V of form error after fine grinding was about 3.21μm. The destination of highly active and ultra-precision grinding of large aperture and complex aspheric lens was achieved.
As the advantages of high forming accuracy, fast material removal efficiency and slight machining defects, the ultraprecision grinding using micro-powder diamond wheel has been widely applied to the processing of large aperture and complex surface optical elements. Due to the brittleness and hardness of optical materials, micro-powder diamond wheel is easy to wear during grinding process, which affects the surface roughness and depth of sub-surface damage layer of components. In order to accurately characterize the wear state of diamond wheel in the grinding process, a method based on in-situ micro-observation of grinding wheel and abrasive particle image contour recognition was proposed to detect the diamond wheel. First, based on the grinding experiments, the surface micromorphology of grinding wheel was acquired by in-situ microscopic observation, and the wear forms of the grinding wheel were analyzed. Then the average distribution density of wear particles and average wear area were taken as the evaluation parameters of the wear state of the wheel. After outstanding the edge profile of abrasive particles by Laplacian enhancement operator and binary processing, the edge profiles of wear particles were extracted out. And by calculating the number and projection area of each wear abrasive particles, the average distribution density of wear particles and the average wear area in the measured region on the surface of grinding wheel were obtained. At the end, the wear state of resin bonded diamond grinding wheel used for grinding fused silica optics was tested. The experimental results showed that the diamond wheel states of initial wear stage and steady wear stage were accurately identified by the parameters of distribution density of wear abrasive particles and average wear area.
Chemical mechanical polishing (CMP) is the most important process for global planarization. The micro material removal and planarization of the optical surface is a complicated process, and the surface shape of optics is effected by kinematics, pressure, and chemical conditions. Moreover, it is a remarkable fact that the distribution characterization of polishing particles also has an important effect on material removal uniformity, especially for leather pad and Tin polishing lap. Large optics were always polished to a convex shape for the low density of valid abrasives in optic center. The porosity and grooves distribution of pad plays a major role in slurry delivering. The novel model of contact and material removal is presented in which pad characterization, and polishing particles delivery and distribution effects are included. With the modified pad asperity and optimized grooves, the particles have been inclined towards well-distributed, and experiments validated that the optic figure is significantly promoted.
Publisher’s Note: This paper, originally published on 8 July 2019, was replaced with a corrected/revised version on 13 August 2019. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
The application of fused silica in the field of High power laser requires that the formation of sub-surface damage be reduced in the process of grinding and polishing. Subsurface damage is unavoidable in traditional processing methods. Laser smoothing, as a non-contact polishing method, has attracted more and more attention in the surface treatment of fused silica. Laser smoothing is capable of producing smooth surface without incurring serious mechanical defects. Thus it is employed to polish fused silica in the hope of reducing mechanical defects on the optical components. In this paper, aiming at the ground surface of fused silica, the characteristics of mid-far infrared laser treatment and modification are studied. The surface smoothness under different laser power are studied, and the optimal laser power and action time for laser smoothing are obtained. This technology can reduce the ground surface roughness from above 100 nanometer to nanometer.
In order to improve the overall efficiency of machining and the defect quality of large aperture fused silica optics, it’s necessary to strictly control the crack defects and their depth uniformity in the first grinding process. Firstly, the morphology of three typical defects affecting the uniformity of crack depth was analyzed, which were scattered superficial sand holes, continuous linear defects and dotted linear defects. Then according to the morphology, the causes were investigated and the control techniques were proposed. The scattered superficial sand holes were caused by the large size glass powder, the diamond particles dropped from the grinding wheel and other foreign body impurities in the grinding fluid, which had been squeezed into the surface of the element by the grinding wheel. Through the clean filtration of grinding fluid, the quantity of such defects could be effectively reduced. The continuous linear defects were caused by stress concentration at the two sharp edges of the grinding wheel. Arcing the two sides of wheel could reduce the machining stress at the edges and avoid the occurrence of continuous linear defects. The dotted linear defects were caused by the stress concentration of the grooves on the surface of the wheel during grinding. After optimizing the dressing parameters to reduce the dressing force and avoid the grains of dressing wheel being embedded into the grinding wheel surface, all the grooves on the wheel surface and all the dashed line defects on the element surface disappeared practically. At the last, after integrating all the above control techniques, the grinding experiment of large scale fused silica optics was carried out. The scattered superficial sand holes, continuous linear defects and dotted linear defects on the surface of element after grinding were obviously alleviated. The crack depth of the whole aperture was between 4μm and 7μm. All the results indicated that the control techniques were efficacious.
For the purpose of ultra-precision grinding large scale and complex off-axis aspheric optics effectively and automatically, computer-aided NC programming system was developed in this article. First the mathematical model of aspheric parallel grinding was analyzed, and the manufacture process of aspheric grinding is designed. Then the system architecture was established, which included initial grinding module, on-machine measuring module and error compensation grinding module. After inputting process and aspheric parameters, the system could calculate the grinding wheel X/Z/Y coordinates precisely and simulate the grinding pathway automatically, and then create grinding CNC program, which could control the grinding wheel to move along the aspheric surface. And the on-machine measurement CNC program was created to acquire the form error by displacement sensor. By combining the form error with the aspheric surface coordinates, the grinding wheel coordinates could be calculated and compensation machining CNC program was created. Using this system to manufacturing one large scale and off-axis aspheric optics, the PV of final form error was below 3.0μm, and the RMS was below 0.5μm.
Spiral phase plate (SPP) for generating vortex hollow beams has high efficiency in various applications. However, it is difficult to obtain an ideal spiral phase plate because of its continuous-varying helical phase and discontinued phase step. This paper describes the demonstration of continuous spiral phase plate using filter methods. The numerical simulations indicate that different filter method including spatial domain filter, frequency domain filter has unique impact on surface topography of SPP and optical vortex characteristics. The experimental results reveal that the spatial Gaussian filter method for smoothing SPP is suitable for Computer Controlled Optical Surfacing (CCOS) technique and obtains good optical properties.
The flatness and parallelism error of large scale and ultra thin optics have an important influence on the subsequent polishing efficiency and accuracy. In order to realize the high precision grinding of those ductile elements, the low deformation vacuum chuck was designed first, which was used for clamping the optics with high supporting rigidity in the full aperture. Then the optics was planar grinded under vacuum adsorption. After machining, the vacuum system was turned off. The form error of optics was on-machine measured using displacement sensor after elastic restitution. The flatness would be convergenced with high accuracy by compensation machining, whose trajectories were integrated with the measurement result. For purpose of getting high parallelism, the optics was turned over and compensation grinded using the form error of vacuum chuck. Finally, the grinding experiment of large scale and ultra thin fused silica optics with aperture of 430mm×430mm×10mm was performed. The best P-V flatness of optics was below 3 μm, and parallelism was below 3 ″. This machining technique has applied in batch grinding of large scale and ultra thin optics.
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