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 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.
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.
A compact lateral shearing interferometer (LSI) based on circular Modified Hartmann Mask (cMHM) is proposed for the measurement of wavefront aberrations. A cMHM grating consists of a circular apertures amplitude grating and a phase chessboard grating. By choosing the radius of the circular aperture of the amplitude grating to be the first positive root of Bessel function, residual diffraction orders are suppressed. As a result, the diffraction field of cMHM is close to that of the ideal quadriwave lateral shearing interference which only contains ±1 orders in two orthogonal directions. An interferometer adopting cMHM as the diffraction element exhibits a diminished Talbot effect on the detection plane as those adopting the conventional Modified Hartmann Mask (MHM) grating or the improved Randomly Encoded Hybrid Grating (REHG). Compared with the REHG, the cMHM requires non-strict manufacturing process. Numerical simulations shows a better diffraction efficiency compared with that using the conventional MHM grating. In the experiments, the interferograms captured by the cMHM-LSI exhibit the same level of contrast as those by MHM-LSI.
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.
A systematic error calibration method is presented to improve the measurement accuracy of lateral shearing interferometry (LSI). This method is used to remove the most significant errors: geometric optical path difference (OPD) and detector tilt error. Difference fronts in the 0° and 90° directions are used to reconstruct wavefront using difference Zernike polynomial fitting. And difference fronts in the 45° and 135° directions are also used to reconstruct wavefront. The coefficient differences between the reconstructed wavefront are generated from geometric OPD and detector tilt error. The relationship between Zernike coefficient differences and systematic parameters are presented based on shear matrix. Thus, the distance of diffracted light converging point (<i>d</i>) and detector tilt angle can be calculated from the coefficient difference. Based on the calculated <i>d</i> and detector tilt angle, the geometric OPD and detector-tilt induced systematic errors are removed and the measurement accuracy of LSI is improved.
A multipurpose laser damage test facility delivering pulses from 1ns to 20ns and designed to output energy 40 Joule at 351nm is presented. The laser induced damage threshold (LIDT) measurement and test procedure are performed. The original system consist of the online detection system based on the microscopy and an energy detection device based on the scientific grade Charge Coupled Device (CCD) which provides the method to measure the LIDT with high accuracy. This method is an efficient way that allows measuring a small area fluence which the defect exposed. After complete test procedure and data treatment the damage position of the defect has been found. Then we can obtain the local fluence of small area when the damage occurred. This procedure provides a straightforward means of laser-damage threshold obtained from the test method. Damage correlation of measures is discussed in connection with present theoretical understanding of laser damage phenomenon. The damage process in transparent dielectric materials being the results of complex processes involving multi-photon ionization, avalanche ionization, electron-phonon coupling, and thermal effects. Those complex processes lead to the damage on the optical surface. We performed a method to measure the local fluence which defects irradiated with high accurate.
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.
A new systematic error calibration method in lateral shearing interferometry (LSI) is proposed for extreme ultraviolet
lithography. This method is used to remove the most significant errors: geometric optical path difference (OPD) and
detector tilt error. The difference fronts of 0th and ±1st order diffracted waves are used to reconstruct wavefront. The
Zernike coefficients of the reconstructed wavefront are used to calculate the distance among different diffracted light
converging points (d). The difference front of 0th and +1st order diffracted waves is mirrored and added to the difference
front of 0th and –1st order diffracted waves. The sum is used to calculate detector tilt angle. The geometric OPD and
detector-tilt induced systematic errors are removed based on the calculated d and detector tilt angle. Simulations show
that the root-mean-square (RMS) value of the residual systematic error is smaller than 0.1nm. The proposed method can
be used to accurately measure the aberration of EUV optics with large numerical aperture (NA 0.5) in LSI.