Progress report is made on the development of a multimodal inspection technique for surface/sub-surface defect characterizations of large-aperture optics. A high-speed laser scattering imaging with a sensitivity at 200-nm scale is used for defect discovery of large-aperture optics. The defect discovery provides statistical results of defects of the optics. After that, a multifunctional microscopic method is used for local defect review. A photothermal scanning microscopy is used for specific characterization of absorption defects, and a confocal microscopy is used for characterization of sub-surface defects. The defect review provides information for defect classification. Based on this multimodal technique, an inspection system is also developed and used for defect characterizations of large optics. Data analysis of the output of the system offers insight into the quality control process. In addition, a absorption distribution curve method is also proposed, which provides a new vision for evaluating the quality of large-aperture optics.
It is very challenging to characterize surface/sub-surface defects for large-aperture optics. The first challenge is the conflict between high resolution and large field of view, namely how to reach a good balance between high resolution and high efficiency for detecting various defects with sub-micronover the meter-size surface of optics. The second challenge is how to classify defects accurately, which is very important for determining origin of defects and improving optics quality.In this paper, a multimodal inspection technique for surface/sub-surface defects of large-aperture optics is reported, in which a high speed laser scattering imaging with a sensitivity at 200-nm scale is used for defect discovery of large-aperture optics. The defect discovery provides a statistical result of defects of the optics. After that, a multifunctional microscopic method is used for local defect review. A photo thermal scanning microscopy is used for specific characterization of absorption defects, and a confocal microscopy is used for characterization of sub-surface defects. The defect review provides information for defect classification. Based on this multimodal technique, an inspection system is also developed and used for defect characterization of large optics.
Measurement of the low damage threshold defects in a large aperture fused silica glass is of great significance. Currently, the popular characterization method for detecting the low damage threshold defects is via measuring the optical absorption of a fused silica glass using the surface thermal lensing technology. However the detecting area of a single shot in this method is too small, typically around 10×10 microns, so if a large aperture fused silica glass in the size of 400×400 mm is to be measured, it would take approximate 100000 hours to complete the measurement, which is obviously not acceptable. Here, we report a fast measurement technique for obtaining the low damage threshold defects in a large aperture fused silica glass according to its fingerprint spectrum, for the fused silica optical glass in the size of 400×400 mm, measurement of the low damage threshold defects in the whole surface in several hours is achievable.
The measurement of the spectral diffraction efficiencies of a diffraction grating is essential for improving the manufacturing technique and for assessing the grating’s function in practical applications. The drawback of the currently popular measurement technique is its slow speed due to the hundreds of repetitions of two kinds of time-consuming mechanical movements during the measuring process. This limitation greatly restricts the usage of this technique in dynamic measurement. We present here a motionless and fast measurement technique for obtaining the spectral diffraction efficiencies of a plane grating, effectively eliminating the aforementioned two kinds of mechanical movement. We estimate that the spectral measurement can be achieved on a millisecond timescale. Our motionless and fast measuring technique will find broad applications in dynamic measurement environments and mass industrial testing.
The wavefront of coated optics is one of critical performances. Due to the interference between the coating layers, the measurement results will be totally different if the measurement wavelength is different from the working wavelength. However, all of the commercial interferometers have single measurement wavelength, which can’t treat the optical coatings working at various wavelengths. A wavelength-switchable interferometer (WSI) capable of detecting wavefront information in a wide wavelength range of 488-1064 nm is proposed in this paper. The principle of design and performance of the system are given in detail. Some typical measurement applications, such as reflection plate and optical filters will also be presented.
In this paper, the concept of a mesoscopic method with high-speed and high-sensitivity is proposed for characterization of surface defects for large optics. The technology is a comprehensive integration of laser scattering method and highly sensitive photothermal method. The principle, experimental setup and preliminary measurement results are presented in detail in the paper. A statistical model for evaluation of mapping results of defects is also proposed to show the effectiveness of the comprehensive metrology method. The proposed method can detect non-destructively surface defects with high-speed and high-sensitivity at the mesoscopical level. It is a promising novel tool for mapping defects in meter size optics and hence it can provide clues to eliminate defects during the manufacturing processes and march toward “defect-free” optics.
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