The initial phase of structured illumination is an important parameter in structured illumination microscopy (SIM). Its estimation accuracy directly affects the reconstruction quality of SIM super-resolution images. However, when the modulation of the system is less than 0.02, the current phase estimation algorithm will cause an estimation error above 0.2 rad. An algorithm based on multi-image correlation processing in frequency domain (MCF) is proposed to solve this problem. Simulation and experimental results show that the MCF algorithm greatly improves both the initial phase estimation accuracy and the reconstruction quality of super-resolution images for low modulation SIM systems with a random phase shift. This means that the MCF algorithm can extend the scope of the SIM technology, especially for low modulation systems or systems lacking precise phase-shifting components.
Laser-induced damage threshold is an important parameter to evaluate the performance of the optical components in high power laser systems. An automated test system is presented to measure multiwavelength laser-induced damage threshold. The presented system can be able to operate the 1-on-1 and R-on-1 methodologies at 1064nm, 532nm, and 355nm. Some solutions are presented to improve the measurement efficiency and the reliability. Experimental results are also provided to confirm the capabilities of the proposed test system.
The optical components employed in high power density solid-state laser for inertial confinement fusion (ICF) must be measured accurately to provide the high resolution measurement neccessary to detect mid-spatial-frequency errors in the wavefront. The use of a Fresnel zone plate (FZP) technique to measure lens transmission wavefront power spectral density (PSD) in mid-frequency domain is disscussed. FZP can provide reference sphere with high-precision, in the meantime greatly shorten air space, thus reduce the effect of vibration and air turbulence, therefore is of great importance for lens transmission wavefront PSD measurement, especially for lens with long focal length used in the ICF facility. To verify the accuracy of the measurement, a comparison experiment of the FZP measurement with results from a Fizeau sphere interferometry method is carried out. Measurement results show excellent agreement, which proves the validity of this method. Finally, measurement uncertainty due to FZP fabrication process is analyed. Analysis of the FZP test showed the overall accuracy of 4.5nm RMS for a sphere lens with 1.5 m focal length and Φ70mm clear aperture. Thus, the measurement accuracy using the proposed FZP method is proved to be very high, FZP can therefore be used to measure lens transmission wavefront PSD accurately.
This work presents the influence of polarization orientation on bulk damage performance of type I doubler KDP crystals under different wavelengths pulses exposure. Pinpoints densities (PPD) and the size distribution of pinpoints are extracted through light scattering pictures. The obtained results strongly indicate that the measured PPD as a function of the fluence are both wavelength and polarization orientation related, while neither fluence nor polarization orientation affect the size distribution of pinpoints. We also find that the bulk damage characteristics can divide into three sorts with respect to the wavelength, suggesting the existence of different species of precursors and different mechanisms responsible for bulk damage initiation in SHG KDP crystals.
The damage morphology information is crucially important for optical components to analysis laser-induced damage resistance. Because of complex configurations, some high-precision and high-resolution techniques have limitations to detect in-line damage sites. Thus, a modified lateral shearing interferometer is proposed to obtain three- dimensional damage morphology information. In the presented method, the original beam passing through damage sites is magnified by a microscope system, and separated by a parallel plate into two sub-beams. In the overlap region of the sub-beams, the interference pattern can be used to extract the damage morphology information. Systematic errors are also eliminated from obtained interferograms before and after pulse laser irradiation, respectively. Experimental results are presented to confirm the feasibility of the proposed lateral shearing interferometer for in-line damage morphology measurement.
Laser trigonometric displacement sensor has characteristics of high efficiency, non-contact and large-scale measurement range, when coupled with scanning system, it can be widely used in profile measurement of complex workpiece surface. But for large steep workpieces, the axis of incident light emitted from the sensor can’t be perpendicular to its surface, accuracy will be largely degraded by this inclination angle. Also, the relationship between error and influencing factors dominated by inclination angle is a nonlinear function. If the influence of measuring distances is taken into account, the relationship becomes a multivariate mapping. So an improved multi-layer BP neural network is proposed to compensate for errors. This paper uses the genetic algorithm to optimize the initialization parameters of the network, while using adaptive training method to optimize the convergence process and adjusting the learning rate, increasing the momentum item to avoid falling into local extreme points. Besides, the laser displacement sensor of Keyence LK-H020 is used to obtain the measurement data and the error was obtained by comparing with a grating ruler with a precision of 10 nm. Based on the simulation and experimental results, the method can reduce the error from 3.8 μm to 0.5 μm when inclination range is from 0° to 8°, and from 7μm to 3 μm when the angle is from 0° to 50°. The results prove effectiveness, generalization and robustness of the algorithm.
The investigation of the influence polarization orientation on damage performance of type I doubler KDP crystals grown by the conventional growth method under under 532nm pulse exposure is carried out in this work. The obtained results point out the pinpoint density (ppd) of polarization parallels the extraordinary axis is around 1.5× less than that of polarization parallels the ordinary axis under the same fluence, although polarization has no influence on size distribution of pinpoints. Meanwhile, crystal inhomogeneity is observed during experiment.
The computation time of wavefront reconstruction is decreased by sampling the difference fronts in the present study. The wavefront can be reconstructed with high accuracy up to 64 Zernike terms with only 32×32 sampled pixels. Furthermore, the computational efficiency can be improved by a factor of more than 1000, and the measurement efficiency of lateral shearing interferometry is improved. The influence of the terms used to reconstruct the wavefront, the grid size of the test wavefront, the shear ratio, and the random noise on the reconstruction accuracy is analyzed and compared, when the difference fronts are sampled with different grid sizes. Numerical simulations and experiments show that the relative reconstruction error is <5% if the grid size of the sampled difference fronts is more than four times the radial order of difference Zernike polynomials with a reasonable noise level and shear ratio.
Computer generated holograms (CGHs) are state-of-the-art components in optical systems, and are widely used in combination with standard Fizeau interferometers. The primary role of the CGHs is to generate reference wavefront with any desired shape. A method of interferometrically measuring large convex lens with CGHs is adopted, and the results from a set of experiments that demonstrate the accuracy and simplicity of performing the holographic test are presented. A direct comparison of the CGH measurement with results from a compensation method shows excellent agreement. Finally, measurement uncertainty due to substrate error and hologram fabrication processes is analyzed.
A new reconstruction algorithm for absolute shape calibration in two-flat test is proposed. The shift-rotation method is applied to absolute shape calibration in two-flat test. Relying on the decomposition of the reconstructed shapes into rotationally asymmetric and symmetric components, an iteration algorithm is presented to reconstruct the rotationally asymmetric components, and Zernike polynomial fitting algorithm is used to calculate rotationally symmetric components. Compared to the traditional algorithms, the proposed algorithm has the characteristics of considerable accuracy and less computational effort. A simulation experiment proves the validity of the presented algorithm.
The paper will describe an automated subaperture stitching interferometry for large plano surface based on relevant algorithm, which restruct the whole surface without recording the position of every subaperture. Both correction and data fusion algorithm are used to minimize the stitching error.
Zernike Polynomial fitting method is an effective way to reconstruct absolute surface for three-flat test. However, the Zernike circle polynomials are not orthogonal over the circular area, hence they are not suitable for square flat. We present an absolute testing of a square flat with Legendre polynomial fitting method, which relies on calculating the coefficients of the Legendre terms by least-square fitting method. To obtain the three-dimensional surface data, one additional measurement that rotate the test flat through 90° should be introduced. The formulas are derived theoretically in detail, and validity has been proved by simulated experiment. Vertical profiles of the three surfaces are compared with the measurement results obtained by three-flat test. Good agreement validates our method.
A new technique for precise wavefront measurement of lens with a hologram is presented. In diffraction, the Fresnel-zone plate hologram emulates the reflective properties of a spherical mirror for use during transmission null tests of an optic by use of a phase-shifting interferometer. Experiment shows that the Fresnel-zone hologram method result is quite similar with that of the traditional interferometry testing method, in which retroreflecting spherical surfaces are used as the reference. The benefit of this methodology is the higher degree of precision at lower cost of manufacturing the reflecting hologram, compared with retrospheres capable of delivering similar precision. This technique is widely applicable and is particularly useful for measuring long focus lens.
An iterative algorithm has been successfully used to process data from the three-flat test. On the basis of the iterative algorithm proposed by Vannoni, which is much faster and more effective than the Zernike polynomial fitting method, an improved algorithm is presented. By optimizing the iterative steps and removing the scaling factors, the surface shape can be easily computed in a few iterations. The validity of the method is proved by computer simulation, and the interpolation error and principle error are analyzed.
Residual stress birefringence in crystal will affect frequency conversion efficiency and beam quality. In this paper the
distribution characteristics of inherent stress birefringence in crystal is analyzed, through delicate adjustment the optical
axis is oriented and qualitative results obtained for KDP crystals are presented and discussed by imaging digital stress
measurement instrument, and the stress gradient distribution is calculated, also the effect of deviation from optical axis
on the measured stress distribution results is discussed.
A simple method which can be used to map mid-spatial scale surface irregularities with high signal noise ratio is described. Two major sources of errors are analyzed and removed. One is the contributions of small-scale irregularities of the reference surface, which are subtracted by shifting the test surface laterally by a distance. The other is the spurious response of CCD, which is removed by interpolation function. The presented method is verified by simulations and experiments. It shows that it can measure mid-spatial scale surface irregularities exactly and smaller scale surface irregularities can be obtained by making measurement for a series of the lateral shifting values corresponding to one-half of the pixel space on CCD.
A 100-mm-aperture high lateral resolution interferometer has been developed in Fine Optical Engineering Research Center (FOERC), which is applied to the measurement of spatial frequencies of up to 2.5 lines/mm over a 100mm field of view. The system transfer function of the interferometer is greater than 60% at near half the Nyquist frequency. To demonstrate the performance of this high lateral resolution interferometer, theoretical errors of the system are thoroughly analyzed and the design implementation is carefully studied, such as light source, wave front slope, tolerance analysis, CCD sample and so on.
A 500-mm-aperture wavelength-tuning phase-shifting interferometer has been developed in FOERC applied to the measurement of large optics. The optical and mechanical design and the calibration technique of the phase shifter are described in detail. Test results show that Peak-to-Valley value smaller than 63nm of interference cavity is achieved.
A 500-mm-aperture wavelength-tuning phase-shifting interferometer has been developed in FOERC applied to the measurement of large optics. Also it can switches to a smaller 130-mm-aperture. We describes in detail the optical and mechanical design as well as calibration technique of phase shifter and phase-shifting algorithm design. A Zygo 4 inch standard reflective flat is used to evaluate the accuracy and repeatability of our wavelength-tuning phase-shifting system.