In the research of inertial confinement fusion, laser plasma interaction (LPI) is becoming a key problem that affects ignition. Here, multi-frequency modulation (Multi-FM) smoothing by spectral dispersion (SSD), continuous phase plate (CPP) and polarization smoothing (PS) were experimentally studied and equipped on SG-III laser facility. After using these technologies, the focal spots of SG-III laser facility can be adjusted, controlled and repeated accurately. Experiments on SG-III laser facility indicate when the number of color cycles adopts 1, imposing SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier and the main amplifier with 30-TDL pinhole size. The nonuniformity of the focal spots using Multi-FM SSD, CPP and PS drops to 0.18, comparing to 0.26 with CPP+SSD, and 0.84 with CPP and wedged lens. Polarization smoothing using flat birefringent plate in the convergent beam of final optics assembly (FOA) was studied.
Precise physical experiments place strict requirements on target illumination uniformity in Inertial Confinement Fusion. To obtain a smoother focal spot and suppress transverse SBS in large aperture optics, Multi-FM smoothing by spectral dispersion (SSD) was studied combined with continuous phase plate (CPP) and polarization smoothing (PS). New ways of PS are being developed to improve the laser irradiation uniformity and solve LPI problems in indirect-drive laser fusion. The near field and far field properties of beams using polarization smoothing were studied and compared, including birefringent wedge and polarization control array. As more parameters can be manipulated in a combined beam smoothing scheme, quad beam smoothing was also studies. Simulation results indicate through adjusting dispersion directions of one-dimensional (1-D) SSD beams in a quad, two-dimensional SSD can be obtained. Experiments have been done on SG-III laser facility using CPP and Multi-FM SSD. The research provides some theoretical and experimental basis for the application of CPP, SSD and PS on high-power laser facilities.
KEYWORDS: Photodetectors, Oscilloscopes, High power lasers, Pulsed laser operation, Laser systems engineering, Attenuators, Laser welding, Physics, Ultrafast phenomena, Signal attenuation
Based on the multiple pulses joining, the cascaded photodetection is experimentally researched on high-contrast measurement of ns high-power laser pulse. The ultrafast photodetectors with the saturation characteristics are used. A joining method for multi-pulse waveforms in the nonlinear region is put forward. The experimental results for ns step-pulse at SG-III laser system show the contrast of ~ 400:1 is achieved, in accord with designed contrast value.
For souped-up of Optical Damage Online Inspection system and obtain damage information of the optic exactly, this paper describes an auto focusing method for weak spot which on high signal to noise ratio image in darkroom. We used different calculation strategy to target pixel and adjoining pixel, compute the gray different that on x and y, compared the result of two focusing evaluation function which the gray difference absolute sum function and the gray difference quadratic sum function, select the best calculation strategy and focusing evaluation function. Test results indicate that, the gray difference quadratic sum function can be more exactly in darkroom and under weak light
Multi-FM SSD and CPP was experimentally studied in high fluence and will be equipped on all the beams of SG-III laser facility. The output spectrum of the cascade phase modulators are stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically analyzed. Results indicate inserting a dispersion grating in places with larger beam aperture could alleviate the FM-to- AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments taken on SG-III laser facility indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier and main amplifier with 30-TDL pinhole size. The nonuniformity of the focal spot using Multi-FM SSD and CPP drops to 0.26, comparing to 0.84 only using CPP. The experiments solve some key technical problems using SSD and CPP on SG-III laser facility, and provide a flexible platform for laser-plasma interaction experiments. Combined beam smoothing and polarization smoothing are also analyzed. Simulation results indicate through adjusting dispersion directions of one-dimensional SSD beams in a quad, two-dimensional SSD could be obtained. The near field and far field properties of beams using polarization smoothing were also studied, including birefringent wedge and polarization control plate (PCP). By using PCP, cylindrical vector beams could be obtained. New solutions will be provided to solve the LPI problem encountered in indirect drive laser fusion.
Large aperture optical have high risk of damage when woke on high flux laser. For avoid lethal damages breakdown the expensive large aperture optical, replace the optical that damaged before damage site increase to can’t repaired, we need precision measurement of optical surface damage sites size. The size of the optics which be detected is 400μm ×400μm, and the size of CCD array pixel is 4K×4K which we selected, so pixel resolution only 100μm of the Optical Damage Online Inspection system, it hard to measurement damage sites which size less than 100μm. This paper describes a method of radiometric calibration to measure online optical damage site that greater than 50μm by Optical Damage Online Inspection system. Numerical statement gray on CCD of different size damage sites by select a fixed variable of illumination intensity, shutter and numerical aperture of image-forming system. Fitting a curve with suitable function of gray and actual size, precision measure optical damage sites that greater than 50μm by the curve. Test results indicate that, the deviation less than 20% which measure size and actual size .This method settle problems of micro size damage site hard to measure online under the condition of long working distance and low optical resolution. At present, this method have used on Optical Damage Online Inspection system of high flux laser installation, it important significance for observation damage site size grown and accurately appraise the optical damage.
This paper describes an effective method for the measurements of large aperture sampling grating, the sampling
efficiency the sampling focal length and the sampling angle. By using a 351nm collimation laser source to scan the full
aperture of the sampling grating in every subregion, the diffraction power of “0” order and “+1” order can be obtained
synchronously by two standard integrating balls, And then by calculating the sampling efficiency of this subregion and
splicing the acquisition data, we can get the averaged grating sampling efficiency in the full aperture.Based on this
method, we can effectively eliminate the effect of the output instability of laser source, decrease the uncertainty of test
results. According to the fabrication principle of the sampling grating, measurements for the sampling focal length and
the sampling angle can be performed. Test rersults indicate that this method can be used to measure large aperture
sampling gratings
Introduced a high-precision measurement of measured the super-low reflectivity and small sampling angle. Using
single reflect way measured, and compare with re-swatch. Testing the reflectance of the sampling mirror which be
used on TIL, and analyze the error. Research results indicate, the main factor which affect result is energy detector
error and energy detector linearity. This methods is easy and have high-precision, it can be used to measure the
super-low reflectivity sampling mirror reflectance.
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