We present a method for measuring the polarization extinction ratio of a prism with the heterodyne-detecting. The key of this method is converting the intensity difference of more than one million times between vertical and parallel polarized transmitted light to almost equal level, so as to overcome the difficulty of measuring the intensity of both vertical and parallel components in the linear response range of a normal photodetector without attenuation. Using this method, we have realized the measurement of a polarizing Glan-Taylor prism with extinction ratio of 60.4dB.
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.
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.
Z-scan technology is a popular experimental technique for determining the nonlinear refractive index of the material. However, it encounters a great difficulty in measuring the weak nonlinear material like fused silica which is about two orders of magnitude below the nonlinear refractive index of most of the materials studied with the nanosecond and picosecond Z-scan methods. In this case, the change of refractive index introduced by accumulation of thermal effects cannot be neglected. In order to have a reliable measurement of the nonlinear refractive index, a metrology bench based on the femtosecond Z-scan technology is developed. The intensity modulation component and the differential measurement system are applied to guarantee the accuracy of the measuring system. Based on the femtosecond Z-scan theory, the femtosecond laser Z-scan technique is performed on fused silica, and the nonlinear refractive index of Fused silica is determined to be 9.2039×10-14esu for 800nm, 37fs pulse duration at I0=50GW/cm2 with a good repeatability of 6.7%.
With the split-step-Fourier-transform method for solving the nonlinear paraxial wave equation, the intensity distribution of the light field when the pits diameter or depth change is obtained by using numerical simulation, include the intensity distribution inside optical element, the beam near-field, the different distances behind the element and the beam far-field. Results show that with the increase of pits diameter or depth, the light field peak intensity and the contrast inside of element corresponding enhancement. The contrast of the intensity distribution of the rear surface of the element will increase slightly. The peak intensity produced by a specific location element downstream of thermal effect will continue to increase, the damage probability in optics placed here is greatly increased. For the intensity distribution of the far-field, increase the pitting diameter or depth will cause the focal spot intensity distribution changes, and the energy of the spectrum center region increase constantly. This work provide a basis for quantitative design and inspection for pitting defects, which provides a reference for the design of optical path arrangement.
Parallelism is an important parameter of plane glass in high power solid-stat lasers. In this paper, a method for parallelism measurement is proposed. This method is based on total least squares(TLS) to fit wavefront into plane, the parallelism can be calculated by the included angle between planes. Furthermore, with tri-dimensional ray-tracing method, larger aperture plane glass can be measured with smaller interferometer at oblique incidence. A comparison test about parallelism measured at 0° and oblique incidence is set up to verify the accuracy of this method.
Interferometry is a high-precision method to test the transmitted wavefront error of lens. However, the radiuses of curvature of some part of lens are too small to produce interference with other optical surface. The resulting interference ring is parasitic in the measured wavefront. The influence of the phase and position of parasitic interference ring on the measured wavefront are analyzed in this paper. Analysis results shows that the existence of parasitic interference ring seriously affects the three-dimensional shape of the measured wavefront. To reduce its influence, a new wavelet filtering method is demonstrated and discussed. By subtract the wavelet decomposition vector of quadratic sum of XY direction gradient from the wavelet coefficients of the measured wavefront, the wavelet reconstruction decomposition vector of the real wavefront are obtained. The whole process is described in detail, and the simulation experiments are done. Compared with the median filtering and mean filtering, the result of the wavelet filtering based on the gradient wavefont is closer to the truth wavefront. This method effectively improves the wavefront filtering effect when the parasitic interference ring exists, and advantageous to guide higher quality lens processing.
The focal length is one of the important parameters in the optical element, and the high precision measurement of the focal length has become a key problem in the processing and use of the optical element. The laser differential confocal length measurement system is introduced, and the uncertainty of the two sets of focal length measurement is evaluated. The relative error (K = 2) is better than 4.77×10-5, and the relative error is 0.00025%.
Z-scan technology is an experimental technique for determining the nonlinear refractive index based on the principle of transformation of phase distortion to amplitude distortion when a laser beam propagates through a nonlinear material. For most of the Z-scan system based on the nanosecond or picosecond laser, the accumulation of thermal effects becomes a big problem in nonlinear refractive index measurement especially for the nonlinear materials such as fused quartz and neodymium glass which have a weak nonlinear refractive effect. To overcome this problem, a system for determining the nonlinear refractive index of optical materials based on the femtosecond laser Z-scan technology is presented. Using this system, the nonlinear refractive index of the fused quartz is investigated.
In this paper, a method is introduced to test wavefront aberration of parallel optical plate wavefront, which based on
Fourier transform. When testing the wavefront aberration of parallel optical plate, there is multiple-surface interference
which phase-shifting interferometry cannot handle with. With the help of wavelength-modulation phase-shifting
interferometry and Fourier transform, this method can isolate the multiple-surface interference fringes in frequency
domain. This paper expound the principle of the method, and analyze the key parameter of the testing method: the
suitable cavity/thickness ratio and sampling frame number. A simulation experiment is set up to verify the accuracy of
this method, then the comparison test with traditional method shows that the relative deviation between the two methods
is ≤5%.
A simple method for focal length measurement based on image processing is demonstrated and discussed. The collimated beam, detector, motorized translation stage and computer make up of this test system. The two spots pass through the tested lens is accepted by detector, which is transferred twice by motorized translation stage. By acquired the difference of two spots by image processing, the focal length of the tested lens can be gained. The error sources in the measurement are analyzed. Then the results of experiment show that the relative error was 0.1%. This method can be used in workshop and labs for its convenience and low cost.
For multilayer films system, in order to obtain the thickness and surface profile in each layer of thin film, a method to measure the 3D morphology of a multilayer films system based on scanning white light interferometer has been proposed in this article. At first, the mathematical relationship between reflection phase and thickness of each film layer has been obtained by using the electromagnetic field boundary conditions. Then, a nonlinear least square algorithm has been used to fit the reflection phase which had been found through a scanning white light interferometer, in this way the linear and nonlinear terms of the reflection phase have been separated, which made it possible to measure top-layer surface profile and thickness of each thin film layer respectively and avoided the interference with each other, because the linear term is related to the top layer’s surface profile but the nonlinear term is correlated to the thickness of each film layer in multilayer thin films system. Thus, the three-dimensional morphology of multilayer thin films system could be reconstructed. Experimental results showed this method was effective in the three-dimensional morphology measurement for multilayer thin films. And the measurement could be completed just using the existing commercial scanning white light interferometer, as a consequence the measurement cost is low, and the operation will be quite simple.
In many long focal distance lens focal length detection method, laser confocal combined focal length measurement with
ultra high focusing accuracy, more and more attention and application. The accurate measurement of the distance
between the lens focus and the focus in the measurement process is the key to improve the accuracy of the whole
measurement. By repeating the measurement of focal distance under different environmental conditions, the results of
repeated measurements of influenced by the environmental conditions. The results show that air disturbance is the
measurement repeatability factors having the greatest impact, for better use of the equipment and in the equipment
development research foundation laid the foundation.
Reflection Z-scan technique allows the measurements of optical nonlinearities of highly absorbing media and surface of transparent media, when transmission Z-scan can not be used. However, Reflection Z-scan needs multiple measurements under strong laser pulse excitation in the scanning process. This can induce damage in the sample in some cases. In this paper, a Non-scanning Reflection Technique (NRT) for measurement of optical nonlinearities is presented to overcome this drawback. Both the nonlinear refraction index and nonlinear absorption coefficient can be determined by measuring the reflection in combination of variable attenuator and an aperture. Based on the Fresnel theory, a theoretical analysis of Non-scanning Reflection Technique (NRT) demonstrates the feasibility of this approach is given and a general expression for the normalized reflectance is derived. In order to illustrate our analytical results, we performed a numerical simulation of the normalized reflectance. Besides, retardance and size of the induced phase plate also make contributions to the normalized reflectance. Moreover, this technique shows a higher sensitivity property compared with traditional reflection Z-scan method.
Multiple surfaces transform interferometery is a preferred technology for surface profile and index homogeneity measurement using a Fourier based analysis method combined with phase-shifting interferometer. As a four-surface cavity for example, the surface form and index inhomogeneity of the parallel plate are deduced by extracting the information from the corresponding interference frequency. The errors of surface form and index homogeneity are simultaneously simulated and analyzed with different sampling buckets. The results show the feasibility and high precision of this approach compared with traditional methods.
Three-flat test can separate the reference surface error from the test part surface in the surface measurement by interferometry. The solution based on mirror symmetry of three-flat test is compact and highly accurate. In practice, the error will be introduced when the position of three flats are misalignment or the relative rotation angle are not accurate. The influence of rotation angle error are simulated and discussed. Then the experiment was carried out on three reference flats and the flat surface profiles were derived by mirror symmetry method. The experiment results show good agreement and the difference is in a nanometer level.
As the development of high powered laser system, the measuring accuracy of optic’s transmissivity need to be improved in order to guarantee all the optics’ quality. This paper analyses all the factors which largely influence the testing accuracy of optics transmissivity when using the spectralphotometer. Experiments are carried out to verify all the deduced results. The results show that the accuracy of wavelength, error of incident angle and divergence of measuring beam would influence the testing result of transmissivity, and the divergence of measuring beam would contrubite to the largest error. Therefore, the testing accuracy could be greatly increased by decreasing the divergence angle of measuring beam.
A scanning measurement system is constructed to measure the transmissivity of each site in large aperture optic components. This system enlarges the beam diameter of light source, and uses a computer-controlled scanning system to measure the transmissivity of optics. The experiments show that the testing accuracy and repeatability of this system both are better than ±0.1%.
In this paper, the measurement method of long focal length in a Fizeau interferometer with combination lenses is stated. According to the principle of geometry optics, two focal length calculation formulae are deduced for the measurement method. The optimization of measurement parameter values is analyzed, and the optimized values of measurement parameters are selected. By analyzing the measurement error of the focal length needed to be measured, the permitted error ranges of measurement parameters are obtained for the optimization of parameters so as to meet the requirement of measurement accuracy of focal length (±0.1%). By means of analysis, it is obtained that the measurement method can meet the requirement (±0.1%) of measurement accuracy of long focal length of high power laser Facility.
A peak power of 286-TW Ti:sapphire laser facility referred to as SILEX-I was
successfully built at China Academy of Engineering Physics, for a pulse duration of 30 fs in
a three-stage Ti:sapphire amplifier chain based on chirped-pulse amplification. The beam
have a wavefront distortion of 0.63μm PV and 0.09μm RMS, and the focal spot with an
f/2.2 OAP is 5.7μm, to our knowledge, this is the best far field obtained for high-power
ultra-short pulse laser systems with no deformable mirror wavefront correction. The
peak focused intensity of ~1021W /cm2 were expected.
As a key specification of the beam quality control, the profile of the optics requires hi-precise testing, which is indispensable during the R&D of high power laser drivers. Currently, the commercial interferometers provided by such companies as Veeco , Zygo and 4D in US and Fuji in Japanese are widely used in profile testing of optics. However, during our practice, a fairly good accordance can not be found after a series of the profile tests of the optics. Generally, there’re certain fluctuations or even remarkable difference among the testing results occurring in the testing results compared with the design specifications. In order to improve the existing poor accordance of the testing results of profile specifications of the optics, and in accordance with our repeated experiments, the Main factors that affect the profile testing are presented respectively, as well as their corresponding impacts upon the profile based on the clarified evaluation parameters and unified testing principles, which are of significant reference value to standardize the testing method of optics profile and to strictly control the optics quality.
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