In this paper, we present a method for measuring the focal length of large aperture lenses using reflective computergenerated hologram (CGH). The CGH is designed to emulate the properties of a large aperture retrosphere through diffraction when performing null test of a lens with an interferometer. To validate this test approach, we designed and fabricated a 450 mm × 450 mm reflective CGH specifically for testing the 440 mm × 440 mm spatial filter lenses with a focal length of 31984.222 mm (@ 632.8 nm). Experiments and error analysis were carried out. The results show that the CGH test approach features high accuracy and good repeatability.
In this paper, we present a method of using computer-generated hologram (CGH) to measure the mid-spatial frequency error of large aperture lenses. To validate this test approach, we designed and fabricated a 450 mm × 450 mm reflective CGH for testing the 440 mm × 440 mm spatial filter lens with a focal length of 32500 mm. In our experiment, both the 0th and 1st order diffraction wavefront of CGH were measured, and the 0th order diffraction wavefront was used to calibrate the substrate error. The mid-spatial frequency error caused by the CGH fabrication errors were evaluated using the binary linear grating model and power spectral density theory (PSD). Experimental results and error analysis indicate that the measurement accuracy of PSD1 is ~0.9 nm RMS, which means the CGH test approach can be used to measure the mid-spatial frequency error of large aperture lenses.
We present a method of using computer-generated hologram (CGH) to measure the radius of curvature of large
aperture long-focal-length lens. In this method, a large aperture transmission CGH is used as a transmission sphere to
generate the test and reference wavefronts by means of diffraction. To verify the feasiblity of this method, a 450 mm ×
450 mm transmission CGH is designed and fabricated for measuring the radius of 440 mm × 440 mm spatial filter lens.
Experimental results and error analysis show that the CGH test method features high accuracy and good repeatability.
Automatic measurement of single points schema by coordinate measuring machine(CMM) is used to measure the Ultra-Long curvature radius of spherical optical element. The removal quantity of each measuring point can be calculated through contrasting the measure value and the theoretical value. A removal model of spherical optical element polishing is established based on Preston equation, and the required machining parameters are predicted by removal simulation in MATLAB. A processing test on a fused silicon with an aperture of 440mm×440mm was performed and the result shows that the model is effective in Ultra-Long curvature radius control of spherical optical element during full aperture polishing.
This article mainly take the research in controlling the parameters in the full aperture fine grinding and polishing stage of the 350mm×300mm×10mm size sapphire window. In the period of full aperture fine grinding period ,by adjusting the grinding parameters, the whole aperter parallel error of the sapphire window conversed to the level of below 3".In the period of full aperture polishing period , by adjusting the polishing parameters including the vaccum adsorbing parameterm, with those measures the wavefront error conversed to the PV value of 2λ(λ=632.8nm), the whole aperture parallel error of the sapphire window conversed to the level of below 3", and the inside arbitrary Φ100mm aperture’s parallel error also conversed to the level of below 3", the roughness of the polishing face attained to the value Rq≤2nm.Through the technical research in the full aperter processing of the sapphire window , the wavefront error, the parallel error and the surface roughness are well controlled which provide the fine results import in the fine sub-aperture polishing period.
The in-situ monitoring of subsurface defects and laser damages initiation using high resolution on-line microscope is performed on medium aperture fused silica optics manufactured by different procedures to investigate the specific damage precursors. The digital camera, Nomarski microscope and white light interferometer are used to characterize the subsurface defects. With shallow HF etching depth, the laser induced damages are mostly initiated on indents or invisible defects under the fluence of 8~10 J/cm2@355nm. The laser induced damages initiated on indents is gradually decreased with the increased etching depth and the laser induced damage density is also decreased. Besides, decrease of the indents by optimizing the polishing process could also make the laser induced damage density sharply decrease. These results prove that the indents are important damage precursors and the laser induced damage performance of fused silica optics could be substantially improved by decreasing the indents or deep HF etching.
HF-based etching has been an effective method to increase the laser induced damage thresholds (LIDTS) of fused silica optics. In this research, the effect of etching parameters on the surface quality and laser damage performance of fused silica with Megasonic-assisted HF acid etching has been investigated systematically. The fused silica samples (50mm in diameter and 5mm thick) were maufactured through the conventional grinding and chemical mechanical polishing process, these processed samples are etched with different etching parameters. Our results show that the frequency and distribution of megasonic field will bring great effect on the surface quality of optics. The LIDTS were measured by 1- on-1 mode, results showed that the 1.3MHz megasonic field and a certain amount of etching depth will benefit the laser damage performance of fused silica optics.
Advanced mitigation process (AMP) has been identified as a most effective method to improve the laser induced damage threshold of the fused silica optics used in the large laser facility, and as the most important sub-process HF etching with multi-frequency megasonic agitation plays a decisive role to improve the damage threshold of fused silica. But because of the physical characteristics of megasonic, the etched surface is apt to generate striated haze which not only modulates the optical field, but also reduces the damage threshold significantly. In this paper, the generating mechanism of the striated haze is discussed, and both of the uneven acoustic field distribution and the optics' redeposited substance are recognized as the primary factor resulted this phenomenon, ultimately based on these discussions a slight swing when etching is proposed to eliminate this phenomenon.
Subsurface damage (SSD) has been identified as a main initiator of laser-induced damage in fused silica, and the most of SSD is produced during grinding process. The distribution and morphology of SSD in fused silica samples ground with loose abrasive are investigated by magneto-rheological finishing (MRF) dimpling and buffered oxide etch (BOE) etching method. The results demonstrate that the SSD depth is most responsive to the loose abrasive size and the BOE etching is good for removing the SSD. Based on these results, an efficient grinding technique combined with BOE etching is proposed to reduce the SSD of fused silica, and the damage threshold is obviously improved by this routine as a result.
Conventional used ceria polishing would induce both of Ce contaminants and subsurface damages, which mainly restricts the laser induced damage resistance of fused silica optics. To control the near surface defects, nanometer sized colloidal silica are used to polish fused silica optics after the normal ceria polishing process. Then the contaminant elements and subsurface damages of the polished samples were analyzed by secondary ion mass spectrometry and Nomarski microscopy. It reveals that ceria polishing would introduce lots of subsurface damages whereas colloidal silica polishing induces much fewer subsurface damages especially no fracture induced severe subsurface damages. The laser damage tests reveal that subsequent colloidal silica polishing of the ceria pre-polished samples could gradually eliminate the ceria polishing induced subsurface damages and lower the laser induced damage density accordingly with the increased polishing time. But unlike the damage density, only the severe subsurface damages are totally eliminated could the damage threshold be substantially improved. These results incline to indicate that the subsurface damages have great influence on the laser induced damage density and the fracture related severe subsurface damages will greatly restrict the damage threshold in polished optics.
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