Final Optics Assembly Research Articles

Numerical investigation of the effect of enforced air flow in a closed cavity on temperature distribution of ammonium dihydrogen phosphate crystal with a large aperture

The temperature distribution of a novel Final optics assembly (FOA) made of a square ammonium dihydrogen phosphate (ADP) crystal with a large aperture is examined numerically in this noncritical phase-matching simulation (NCPM). The temperature of the cavity wall is maintained by temperature-controlled, continuously flowing (0.1 m/s) water (313.5 k). The thermal study was first performed numerically inside the cavity with different configurations using the finite volume technique. One fan was placed on one side of the ADP crystal to evaluate the temperature distribution within the cavity and between the crystals. The fan speed is then varied from 10 to 40 rad/s with a step interval of 10 rad/s to evaluate the effect of forced convection. Furthermore, the investigation showed that 20 rad/s is more effective than the other cases. To measure the impact of temperature distribution caused by fan orientation, the fan orientation was adjusted to three different tilt angles (10, 20, and 30 degrees) while the fan rotational speed was maintained at 20 rad/s. The two fans on the side and the two fans on either side of the ADP crystal were also investigated, with different orientations (0, 10, and 20 degrees) and speeds (0, 10, 20, 30, and 40 rad/s). According to the fan speed, direction, number of speeds, and fan locations, the high-temperature spot of ADP crystals can be symmetric, asymmetric, diagonally dominant, center dominant, bottom dominant, or top dominant.

Design and Management of Stray Light for Compact Final Optics Assembly on the High Energy Laser System

In this study, a model is proposed to design and manage the stray light of a compact final optics assembly (FOA) for a high energy laser system. Based on the method we proposed, the high-order stray light can be managed to optimizing the position and angle of the optical elements. A light trap is designed to manage the first-order stray light with high fluence. Applying the method, we provide an experimental demonstration to designing a compact FOA. By comparing the cleaning results with no management testing result, it proves that using the above design and management, it can achieve the great improvement of cleanliness from ISO Class 5 to Class 3, which is significant to improve the output capability of the high energy laser system. In addition, we also verify the stray light by an optical field paper. It demonstrates that the field characteristics and position calculation of stray light are reliable.

Analysis of the relationship between variations in detuning angle and the harmonic generation efficiency of a KDP crystal

The harmonic generation efficiency of a large aperture KDP (KH2PO4) crystal that is used in the Final Optics Assembly of high-energy solid-state lasers is one of the key points to achieve inertial confinement fusion. The variations in the detuning angle of a KDP crystal are achieved by interferometer and calculation. The harmonic generation efficiency is calculated using variations in the detuning angle. The theoretical prediction model of harmonic generation efficiency based on small-signal approximation theory was built. In the experiment, Crystal off-line Debugging System was used to measure the actual harmonic generation efficiency of a KDP crystal. The predictive harmonic generation efficiency of a KDP crystal and the actual efficiency was compared, which concluded that the theoretical prediction model of harmonic generation efficiency was correct. A simple method to predict the harmonic generation efficiency of a KDP crystal was provided. The method can also be applied to other harmonic generation crystals.

Optimization of cleaning technique for mitigating particulate contamination upon final optics assemblies

The cleanliness of the final optics assemblies (FOAs) has become the lynchpin of Inertial Confinement Fusion (ICF). The two-phase flow method is applied to study the motion trajectory of moving aerosol particles in the FOAs. One of efficient clean technology, named purging system with the coupling of low-speed laminar wind and high-speed gas knife to remove particulate pollutants and achieve cleanness of atmospheric environment are designed and analyzed by means of experiments and simulations. Our studies demonstrate that when the exit wind speed of gas knife is more than 40 m/s, the surface temperature unevenness of optical components will certainly exceed 0.2 °C badly reducing the conversion efficiency of FOAs. Investigation of the distance between the outlet of gas knife and the edge of optical lens indicates that the obvious deformation will almost disappear in the region near the outlet of the gas knife when the longitudinal distance is not less than 18 mm. The relationship between the attenuation distance of aerosol particles and the efficiency of purging system is studied in detail. The minimum distance between optical components is elaborately determined to be 170 mm, which realizes the full protection and thorough removal of particulate pollutants on the surface of optical components. Besides, it is thoughtfully settled that backflow at the outlet can be effectively restrained when the height of the backflow chamber is more than 100 mm.

The design of a line velocity interferometer for any reflector for inertial confinement experiments on the Z-machine

A line VISAR (Velocity Interferometer System for Any Reflector) has been designed and commissioned at the Sandia National Laboratory's Z-machine. The instrument consists of an F/2 collection system, beam transport, and an interferometer table that contains two Mach-Zehnder type interferometers and an eight channel Gated Optical Imaging (GOI) system. The VISAR probe laser operates at the 532 nm wavelength, and the GOI bandpass is 540-600 nm. The output of each interferometer is passed to an optical streak camera with four selectable sweep speeds. The system is designed with three interchangeable optics modules to select a full field of view of 1 mm, 2 mm, or 4 mm. The optical beam transport system connects the target image plane to the interferometers and the gated optical imagers. The target is integrated into a sacrificial final optics assembly that is integral to the transport beamline.

General design and experiment for separated final optics assembly on high energy laser system

Final optics assembly (FOA) is paramount to high energy laser system. It took more than ten years for the National Ignition Facility (NIF) and Laser Megajoule Facility (LMJ) to optimize their FOA and improve output capability. However, the final optical damage limited the output capability of the NIF and LMJ. Contamination is the dominant factor which can dramatically reduce their damage threshold. Accordingly, in this work we propose a separated FOA which mainly locates in the atmosphere and is convenient for contaminants sources management. In addition, we adopts an on-line cleaning system to improve clean status of FOA and achieves longevity lifetime of final optics. The experiment result of the separated FOA demonstrates that even more than one thousand particle contaminants are generated by each laser shot of 6 J/cm2, the on-line cleaning system can take major of them away and maintain cleanliness of separated FOA at ISO Class 3. Our separated FOA can stably operate under a 351-nm laser shot of 6 J/cm2 (21.7 ns shaped-pulse), which is significant to improve the output capability of high energy laser system.

Dual dynamic airflow protection for the removal of fused silica micron particles in the final optics assembly

Micron-sized particles generated during laser-induced event are difficult to control in large optical systems such as final optics assembly (FOA) and have a great impact on load capacity. The relat...

Ultra clean manufacturing techniques for final optics assembly in high-power laser facility

Cleanliness control is one of the most important processes for final optics assembly (FOA) in high-power laser systems. In this paper, an “Ultra Clean Manufacturing (UCM)” concept is put forward for the first time to satisfy the cleanliness requirements for FOA. First, the challenges and requirements for cleanliness in FOA are introduced and the concept of UCM is proposed. Significant surface cleaning technologies for mechanical parts and optics are then expounded. Finally, surface cleanliness control techniques, such as “gas knife” and “laser cleaning”, are developed as online cleaning methods for the surface cleanliness in FOA.

Small-scale self-focusing assessment using PSD Shannon entropy of a 3ω process in a high-power laser facility

A high-power laser facility produces a high power-density laser to drive the target compression. When the facility works at extremely high fluences, the laser may induce small-scale self-focusing, which damages the optics. To ensure safe operation, we propose a method using power spectral density (PSD) entropy to judge the occurrence and assess the development of small-scale self-focusing in the final optics assembly of a high-power laser facility. In this method, Shannon entropy statistics is adopted to analyze the PSD change in the output near-field image. The results show that, as the operating fluence increases, the PSD Shannon entropy curve displays an evident turning point when the small-scale self-focusing begins to develop. PSD Shannon entropy is superior to the intensity entropy method in detecting small-scale self-focusing in this process. It also supplements Shannon entropy applications.

An investigation on the precision mounting process of large-aperture potassium dihydrogen phosphate converters based on the accurate prediction model

Potassium dihydrogen phosphate(KDP) converters are important optics applied in the final optics assembly of inertial confinement fusion(ICF) facilities. The efficiency of the second harmonic generation is greatly affected by the phase matching error, which is induced by microsurface distortion. Firstly, considering the strict requirements of surface distortion and the associated measurement difficulties in the final optics assembly, an effective prediction model based on mechanical analysis is proposed to accurately predict the surface distortion of converters during mounting. Second, through the numerical simulation based on the prediction model, key factors that could have more influence on the surface distortion are analyzed. Then, measurements of the key factors are carried out to further improve the model. Mounting and surface measurement experiment are performed at an optical assembly building, and the results verify the accuracy of the prediction model. Third, because it is difficult to perform a field experiment for a KDP converter under tilted installation attitudes and during the mounting process, the prediction model is employed to analyze more factors that will influence the surface distortion in these conditions, and the analysis results provide helpful suggestions to optimize the mounting process. The results and findings in this article are meaningful for controlling mounting-induced surface distortion of KDP converters within the requirement of 3.2 μm. Moreover, the prediction model will solve the problem of online measurement in ICF facilities.

分离式终端光学系统的鬼像分析

分离式终端光学系统的鬼像分析

Cleaning State of the Loop Case for Optical Crystal Module in Final Optics Assembly

In order to solve the online clean maintenance problem of the optical frequency-doubling crystal module in the final optics assembly, a crystal loop case is developed for better controlling the clean maintenance process. The study presents a detailed research on the cleanliness of the crystal loop case during the online maintenance process and the installation sequence of the crystal modules in the crystal. It first established a fluid simulation model of the crystal loop case system according to the online maintenance process and then analyzed the isolation effect of inlet velocity on the polluted air in view of the cleaning effect of the crystal loop case. Based on the simulation principle of gas–solid two-phase flow and the tracking results of the solid particle contamination, the range of air inflow is given for achieving the best cleaning status of the loop case. The optimal sequence of crystal module is explicit for its installation in the crystal loop case of frequency-doubling crystal module. The experimental setup has been built to examine the cleaning state of the crystal loop case, and the simulation result has been validated. The research on the cleanliness of crystal loop case can provide a useful reference for the ultra-clean manufacturing of line replaceable units and the closed loop control of cleaning in high-power laser facility.

Measurement of chlorine concentration on steel surfaces via fiber-optic laser-induced breakdown spectroscopy in double-pulse configuration

The corrosive environment provided by chlorine ions on the welds of stainless steel dry cask storage canisters for used nuclear fuel may contribute to the occurrence of stress corrosion cracking. We demonstrate the use of fiber-optic laser-induced breakdown spectroscopy (FOLIBS) in the double-pulse (DP) configuration for high-sensitivity, remote measurement of the surface concentrations of chlorine compatible in constrained space and challenging environment characteristic for dry cask storage systems. Chlorine surface concentrations as low as 5mg/m2 have been detected and quantified by use of a laboratory-based and a fieldable DP FOLIBS setup with the calibration curve approach. The compact final optics assembly in the fieldable setup is interfaced via two 25-m long optical fibers for high-power laser pulse delivery and plasma emission collection and can be readily integrated into a multi-sensor robotic delivery system for in-situ inspection of dry cask storage systems.

Design and performance of final optics assembly in SG-II Upgrade laser facility

In high power laser facility for inertial confinement fusion research, final optics assembly (FOA) plays a critical role in the frequency conversion, beam focusing, color separation, beam sampling and debris shielding. The design and performance of FOA in SG-II Upgrade laser facility are mainly introduced here. Due to the limited space and short focal length, a coaxial aspheric wedged focus lens is designed and applied in the FOA configuration. Then the ghost image analysis, the focus characteristic analysis, the B integral control design and the optomechanical design are carried out in the FOA design phase. In order to ensure the FOA performance, two key technologies are developed including measurement and adjustment technique of the wedged focus lens and the stray light management technique based on ground glass. Experimental results show that the design specifications including laser fluence, frequency conversion efficiency and perforation efficiency of the focus spot have been achieved, which meet the requirements of physical experiments well.

Centroid stabilization for laser alignment to corner cubes: designing a matched filter

Automation of image-based alignment of National Ignition Facility high energy laser beams is providing the capability of executing multiple target shots per day. One important alignment is beam centration through the second and third harmonic generating crystals in the final optics assembly (FOA), which employs two retroreflecting corner cubes as centering references for each beam. Beam-to-beam variations and systematic beam changes over time in the FOA corner cube images can lead to a reduction in accuracy as well as increased convergence durations for the template-based position detector. A systematic approach is described that maintains FOA corner cube templates and guarantees stable position estimation.

Mitigation of beam sampling grating damage induced by upstream flaws in the final optics assembly

The high fluence performance of high-power laser systems is set by optical damage, especially in the final optics assembly (FOA). The flaws on the frequency converter surface can cause optical intensity intensification and, therefore, damage the downstream optical elements, such as the beam sampling grating (BSG), which is an important component in the FOA. Mitigation of BSG damage caused by flaws is discussed. Physical models are established to simulate the optical field enhancement on BSG modulated by the upstream flaw, considering both the linear and nonlinear propagation effects. Numerical calculations suggest that it is important to place the BSG in a properly selected position to mitigate the laser-induced damage. Furthermore, strict controls of flaw size, modulation depth, distance between frequency converter and focusing lens, and the thickness of the focusing lens are also significant to mitigate the BSG damage. The results obtained could also give some suggestions for damage mitigation of optical components and the layout design of the final optics assembly.

Noncritically phase-matched fourth-harmonic generation of Nd:glass lasers and design of final optics assembly

The noncritically phase-matched (NCPM) fourth-harmonic generation (FHG) with partially deuterated dihydrogen phosphate (KD*P) crystal at an Nd:glass laser radiation wavelength of 1053.1nm has been confirmed. NCPM FHG has been achieved in 70% and 65% deuterated KD*P crystal at the temperature of 17.7°C and 29.3°C, respectively. The angular acceptance of 70% and 65% deuterated KD*P crystals fixed at their NCPM temperature were measured, which were 53 and 55mrad, respectively. The application of the NCPM FHG in a high-power laser facility for inertial confinement fusion is also discussed. Based on the theoretical analysis, the NCPM KD*P can be placed after the focus lens; thus, the laser-induced damage of a fused-silica lens at ultraviolet can be avoided.

Research of beam conditioning technologies using continuous phase plate, Multi-FM smoothing by spectral dispersion and polarization smoothing

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 implemented on the SG-III laser facility. After using these techniques, the far field distribution of SG-Ⅲ laser facility can be adjusted, controlled and repeated accurately. The output spectrums of the cascade phase modulators used for Multi-FM SSD were stable and the FM-to-AM effect can be restrained. Experiments on SG-III laser facility indicate that 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 preamplifier and main amplifiers 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, 0.57 with CPP+PS and 0.84 with only CPP and wedged lens. Polarization smoothing using flat birefringent plate in the convergent beam of final optics assembly (FOA) was studied. The PS plates were manufactured and equipped on SG-III laser facility for LPI research. Combined beam smoothing and polarization manipulation were also studied to solve the LPI problem. Results indicate that through adjusting dispersion directions of SSD beams in a quad, two dimensional SSD can be obtained. Using polarization control plate (PCP), polarization on the near field and far field can be manipulated, providing new method to solve LPI problem in indirect drive laser fusion.

Studies on design of 351 nm focal plane diagnostic system prototype and focusing characteristic of SGII-upgraded facility at half achievable energy performance.

In order to obtain the intensity distribution of a 351 nm focal spot and smoothing by spectral dispersion (SSD) focal plane profile of a SGII-upgraded facility, a type of off-axis imaging system with three spherical mirrors, suitable for a finite distance source point to be imaged near the diffraction limit has been designed. The quality factor of the image system is 1.6 times of the diffraction limit tested by a 1053 nm point source. Because of the absence of a 351 nm point source, we can use a Collins diffraction imaging integral with respect to λ=351 nm, corresponding to a quality factor that is 3.8 times the diffraction limit at 351 nm. The calibration results show that at least the range of ±10 mrad of view field angle and ±50 mm along the axial direction around the optimum object distance can be satisfied with near diffraction limited image that is consistent with the design value. Using this image system, the No. 2 beam of the SGII-upgraded facility has been tested. The test result of the focal spot of final optics assembly (FOA) at 351 nm indicates that about 80% of energy is encompassed in 14.1 times the diffraction limit, while the output energy of the No. 2 beam is 908 J at 1053 nm. According to convolution theorem, the true value of a 351 nm focal spot of FOA is about 12 times the diffraction limit because of the influence of the quality factor. Further experimental studies indicate that the RMS value along the smoothing direction is less than 15.98% in the SSD spot test experiment. Computer simulations show that the quality factor of the image system used in the experiment has almost no effect on the SSD focal spot test. The image system can remarkably distort the SSD focal spot distribution under the circumstance of the quality factor 15 times worse than the diffraction limit. The distorted image shows a steep slope in the contour of the SSD focal spot along the smoothing direction that otherwise has a relatively flat top region around the focal spot center.

Fundamental-frequency-absorbed oxyfluoride glass in a high-power laser.

A high-power third-harmonic laser faces challenges in the filtering remnant unconverted fundamental frequency, which is from the frequency converting crystal. In this work, a novel fundamental-frequency-absorbed oxyfluoride glass has been prepared, which provides a possible option to solve the problem. By being doped with Fe2+ ion, the glass shows strong absorption property at 1053 nm, and the glass's transmittances at 351 and 1053 nm are stable with changing the laser power or increasing the irradiation times under high-power laser irradiation. Meanwhile, the laser-induced damage threshold of the glass is 12.5 J/cm2 at 351 nm, which is two times higher than that of fused silica whose threshold is 6.2 J/cm2 in the same testing condition. The glass also exhibits a higher laser-induced damage threshold as well as 36.6 J/cm2 at its absorption wavelength of 1053 nm. The results indicate that this glass is promising as a color-separation optic, thus allowing a novel design for the final optics assembly in an inertial confinement fusion laser system.