The giant laser device used in inertial confined fusion (ICF) experimental research is the largest optical engineering ever built by humans. It requires thousands of large-diameter optical components, especially for optical components with diagonal dimensions close to or exceeding 1 meter, which leads to the manufacturing is extremely difficult. Wavefront characteristics are one of the key parameters of meter-size optical components. Since different degrees of wavefront error are introduced during the material preparation, manufacturing and coating processes, it requires precise measurement and precise control throughout the manufacturing process. In this paper, the research work on key problems such as measurement accuracy, measurement aperture and measurement efficiency in the wavefront error detection of meter-size optical components is carried out and summarized.
The deformation of the φ600mm reference mirror under gravity cannot be ignored when measuring optical components with a φ600mm interferometer. Therefore, it is necessary to choose a reasonable support and rotation mechanism to adjust the reference mirror of the interferometer in order to meet the accuracy requirements. In this paper, two mechanical structures for supporting and rotating the φ600mm reference mirror are analyzed. One is using the roller slings to provide tension, and the conveyor belt drives the reference mirror to rotate by friction; while the other one is using glue to connect the silica gel and the edge of the reference mirror where silica gel is fixed on the metal frame. The rotation of the reference mirror can be realized by manually rotating the frame. In order to analyze the influence of the two mechanisms on the mirror under working conditions, a three-dimensional model of the φ600mm reference mirror with its supporting system was established using the finite element analysis software ABAQUS, and the contact conditions and boundary conditions were reasonably applied to the reference mirror. Reference mirror is analyzed in both statical and dynamical states. The changes of internal stress and surface shape are compared when the reference mirror is supported and rotated on both mechanisms. The advantages of different mechanisms are elaborated, and the parameters of mechanisms are optimized theoretically. Analysis results can provide a useful guide for the support of the reference mirror and the adjustment of the φ600mm large aperture interferometer.
Existing absolute measurement methods are difficult to avoid the replacement and rotation of the large aperture interferometer’s reference flat, and it is impossible to achieve in-situ absolute measurement on the basis of keeping the reference flat of the interferometer not rotating and replacing each other. In this paper, a test method based on oblique incidence is practically implemented in the interferometric measurement process, and an auxiliary rotating device for large aperture flat is designed. Three sets of wavefront data are achieved through cavity interference measurement with a Fizeau interferometer and one oblique incidence measurement. An iterative algorithm is applied to retrieve the absolute surface shape of the test flat. The absolute measurement experiment is carried out on the 600-mm aperture interferometer produced by Zygo. The experimental results show that the absolute surface error of the reference flat measured according to the proposed method is 2.0 nm (RMS), differing from the absolute measurement results based on the Zygo approach.
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.
Phase step plate has the property of step function, and it can be used to measure the interferometer system transfer function (ITF), which is used to objectively evaluate interferometer spatial frequency response characteristics. The traditional fabrication method of step plate is lithographic exposure combined with reactive ion beam etching method, which is costive, and not suitable for fabricating large diameter step plate. In this paper, a new fabrication method of step plate based on blade-mask coating technology is proposed. A step plate of 100mm diameter and 103.5nm step height is successfully fabricated, and the surface roughness, homogeneity, steepness of the step plate are satisfied for measurement requirements. Furthermore, the coated step plate is used to measure the ITF compared with an etched step plate, and the measurement results of two plates differ by no more than 5%, which shows a good consistency. Thus, the reliability of coated step plate for ITF measurements is proved.
KEYWORDS: Interferometry, Optical components, 3D image processing, Detection and tracking algorithms, Optics manufacturing, High power lasers, Defect detection, Laser systems engineering, Computer simulations, 3D metrology
Surface defects on large-aperture optical elements are one of the key factors to restrain the improvement of performance of high-power laser systems. Detecting and characterizing accurately these defects are an important basis for optical elements manufacturing and system performance evaluation. Based on the principle of transient interferometry, a measurement setup used to detect the three-dimensional features of surface defects is reported in this paper. We also propose a template matching and stitching algorithm for interferometric images obtained by the system: Firstly, determining whether the interference images have enough features to complete the stitching work; secondly, selecting the stitching template in the area where the template image needed to be stitched; Finally, searching for the template in the stitched image, and calculating the relative offset of these two images according to the relative position of the template to complete the stitching work. Through simulation analysis and actual image obtained by the measurement system verification, the proposed algorithm has the advantages of good noise resistance and high mosaic accuracy. Moreover, it has a positive significance for the study of transient interferometry detection.
In this paper, a test method based on oblique incidence is practically implemented in the interferometric measurement process. Three sets of wavefront data are achieved through cavity interference measurement with a Fizeau interferometer and one oblique incidence measurement. An iterative algorithm is applied to retrieve the matrix of transmission flatness and reference flatness. The new method can not only calibrate the reference flat error of large aperture interferometer, but also provide the absolute measurement method for large rectangular optical components applied in high power laser systems.
A common way to test high-quality aspherical lenses is to use a measurement system based on a set of null corrector and
a laser interferometer. The null corrector can either be a combination of spherical lenses or be a computer generated
hologram (CGH), which compensates the aspheric wave-front being tested. However, the null optics can’t be repeatedly
used once the shape of tested optics changes. Alternative active null correctors have been proposed based on dynamic
phase modulator devices. A typical dynamic phase modulator is liquid crystal spatial light modulator (LCSLM), which
can spatially change the refractive index of the liquid crystal and thus modify the phase of the input wave-front. Even
though the measurement method based on LCSLM and laser interferometer has been proposed and demonstrated for
optical testing several years ago, it still can’t be used in the high quality measurement process due to its limited accuracy.
In this paper, we systematically study the factors such as LCSLM structure parameters, encoding error and laser
interferometer performance, which significantly affect the measurement accuracy. Some solutions will be proposed in
order to improve the measurement accuracy based on LCSLM and laser interferometer.
The multi-object broadband imaging echellette (MOBIE) is the seeing-limited, visible-wavelength imaging multi-object spectrograph (MOS) planned for first-light use on the thirty meter telescope (TMT). The current MOBIE optical design provides two color channels, spanning the 310nm–550nm and 550nm-1000nm passbands. The involved large optics includes an atmospheric dispersion corrector (ADC) prism (1.4m in diameter), a collimator (1.7mx1.0m), a dichroic(680 mm x500 mm x 30 mm), a red folding mirror and two corrector lenses(570mm in diameter) for different channels. In the past two years, Shanghai Institute of Optics and Fine Mechanics (SIOM) has been included in the preliminary study of folding mirror sub-system in MOBIE, especially the study on the large optics manufacture techniques. The research progress of these large optics will be reviewed in this paper. The influence of optical quality of the large optics on the MOBIE is analyzed in order to define the specifications of the large optics. The manufacture methods are designed for different large optics. In order to testify the effectiveness of the manufacture methods, some samples have been processed and the final performance including wavefront error and spectral properties are tested. Finally, the future work including remaining problems and possible solutions are introduced.
With the unprecedented developments of the intense laser and aerospace projects', the interferometer is widely used in detecting middle frequency indicators of the optical elements, which put forward very high request towards the interferometer system transfer function (ITF). Conventionally, the ITF is measured by comparing the power spectra of known phase objects such as high-quality phase step. However, the fabrication of phase step is complex and high-cost, especially in the measurement of large-aperture interferometer. In this paper, a new fringe method is proposed to measure the ITF without additional objects. The frequency was changed by adjusting the number of fringes, and the normalized transfer function value was measured at different frequencies. The ITF value measured by fringe method was consistent with the traditional phase step method, which confirms the feasibility of proposed method. Moreover, the measurement error caused by defocus was analyzed. The proposed method does not require the preparation of a step artifact, which greatly reduces the test cost, and is of great significance to the ITF measurement of large aperture interferometer.
In this paper, we extended the measurement method of mid-frequency wavefront error to spherical and aspherical
components. Some influence factors like the system error of interferometer, environment variation, surface parallelism of
optics, polarizations of interferometer illumination, and the wavefront distribution of optics are analyzed in detail. Some
optimized measurement strategies for different kinds of optics are suggested. Finally, experimental measurements on flat
optics, spherical and aspherical optics are performed to testify the suggested measurement methods, respectively.
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