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

The current automation of image-based alignment of NIF high energy laser beams is providing the capability of executing multiple target shots per day. An important aspect of performing multiple shots in a day is to reduce additional time spent aligning specific beams due to perturbations in those beam images. One such alignment is beam centration through the second and third harmonic generating crystals in the final optics assembly (FOA), which employs two retro-reflecting corner cubes to represent the beam center. The FOA houses the frequency conversion crystals for third harmonic generation as the beams enters the target chamber. 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 centroid detector. This work presents a systematic approach of maintaining FOA corner cube centroid templates so that stable position estimation is applied thereby leading to fast convergence of alignment control loops. In the matched filtering approach, a template is designed based on most recent images taken in the last 60 days. The results show that new filter reduces the divergence of the position estimation of FOA images.

This paper describes the alignment system developed on the Laser Mégajoule facility, allowing to focus the laser beams and to point the plasma diagnostics on the target. After an overview of the main laser components and alignment architecture, we detail some major equipments as the 6 tele-microscopes used to align the target, the continuous phase plate within the final optics assembly, the plasma diagnostic green pointer and the common reference which is the cornerstone of the chamber center alignment. Finally we present some results obtained on the telemicroscope prototype and a photometric prototype of the common reference. The expected performance of the alignment system will also be discussed.

The Project provides for the design, procurement, construction, assembly, installation, and acceptance testing of the National Ignition Facility (NIF), an experimental inertial confinement fusion facility intended to achieve controlled thermonuclear fusion in the laboratory by imploding a small capsule containing a mixture of the hydrogen isotopes, deuterium and tritium. The NIF will be constructed at the Lawrence Livermore National Laboratory (LLNL), Livermore, California as determined by the Record of Decision made on December 19, 1996, as a part of the Stockpile Stewardship and Management Programmatic Environmental Impact Statement (SSM PEIS). Safety: On Saturday April 29, 2000, while preparing the Ringer crane for operation at the NIF site, a mechanical malfunction was observed by the operator. He stopped work and consulted with line management. They agreed with the operator's assessment, and with the Livermore Emergency Duty Officer, implemented a precautionary evacuation of the area around the crane. DOE was notified of the situation. The crane was then placed in a safe condition. A crane maintenance vendor is inspecting the crane and a management team headed by the Beampath Infrastructure System Associate Project Manager is reviewing the documentation, crane history, and repairs to ensure that the crane is fully safe before reuse. Technical Status: The general status of the technologies underlying the NIF Project remains satisfactory. The issues currently being addressed are (1) cleanliness for installation, assembly, and activation of the laser system by Systems Engineering working groups; (2) laser glass, where a second pilot run at both commercial suppliers is expected to confirm the mitigation steps identified in the first pilot run; and (3) operational costs associated with Final Optics Assembly (FOA) optics components, where methods are being developed to mitigate 3 {omega} damage and to resolve beam rotation issues. Schedule: The project completion schedule remains a major concern for the quarter. Completion of the Beampath Infrastructure System (BIS) design and award of the contract for the Integration Management and Installation (IMI) Contractor are on the critical path schedule. The procurement package has been reviewed with DOE Procurement and NNSA Project organizations. DOE HQ approved moving forward with the request for proposal (RFP) for the IMI on April 24, and LLNL procurement has requested the proposal for the BIS IMI Services contract. The construction status of the Conventional Facilities at the end of March 2000 is 87% complete. Cost: The NIF Project Total Project Cost (TPC) is $1.2B. The Project has obligated 78% of the TPC funds. The remaining contingency is $16.7M. Because of schedule delays and projected increases in the design, construction management, assembly, and installation of the system infrastructure, cost growth of the TPC is anticipated and will remain a major concern until the budget rebaseline process is completed. The cost estimates and schedules were reviewed with the Level 1 and 2 Baseline Change Control Boards (BCCB) in preparation for the Energy Systems Acquisition Advisory Board Meetings scheduled in early May.

Standing wave field distributions in three classic types of graded-index antireflection coatings are studied. These graded-index antireflection coatings are designed at wavelengths from 200 nm to 1200 nm, which is the working wavelength range of high energy laser system for inertial-fusion research. The standing wave field distributions in these coatings are obtained by the numerical calculation of electromagnetic wave equation. We find that standing wave field distributions in these three graded-index anti-reflection coatings are quite different. For the coating with linear index distribution, intensity of standing wave field decreases periodically from surface to substrate with narrow oscillation range and the period is proportional to the incident wavelength. For the coating with exponential index distribution, intensity of standing wave field decreases periodically from surface to substrate with large oscillation range and the period is also proportional to the incident wavelength. Finally, for the coating with polynomial index, intensity of standing wave field is quickly falling down from surface to substrate without an obvious oscillation. We find that the intensity of standing wave field in the interface between coating and substrate for linear index, exponential index and polynomial index are about 0.7, 0.9 and 0.7, respectively. Our results indicate that the distributions of standing wave field in linear index coating and polynomial index coating are better than that in exponential index coating for the application in high energy laser system. Moreover, we find that the transmittance of linear index coating and polynomial index coating are also better than exponential index coating at the designed wavelength range. Present simulation results are useful for the design and application of graded-index antireflection coating in high energy laser system.

Evolution of physical and mechanical properties of fused silica after subsurface damage layer removal by RIE treatment

Estimating the vulnerability is a key challenge for plasma diagnostics designed to operate in radiative background associated with megajoule class laser facilities. Since DT shots at OMEGA laser facility reproduce the perturbing source expected during the first 100 nanoseconds of a typical DT shot realized at National Ignition Facility (NIF) and Laser MegaJoule facility (LMJ), vulnerability of diagnostic elements such as optical relays or optical analyzers were experimentally studied and, if necessary, hardening approaches have been initiated to authorize their use at higher radiative constraints. Other facilities such as nuclear reactor or accelerator have been also used to estimate vulnerability issues as radiation induced emission of glasses or damage in multilayer coatings.

In the final optics assembly of Inertial Confinement Fusion (ICF) driver, Diffractive Optical Elements (DOEs) are applied to achieve some important functions, such as harmonic wave separation, beam sampling, beam smoothing and pulse compression etc. However, in order to optimize the system structure, decrease the energy loss and avoid the damage of laser induction or self-focusing effect, the number of elements used in the ICF system, especially in the final optics assembly, should be minimized. The multiple exposure method has been proposed, for this purpose, to fabricate BSG and CSG on one surface of a silica plate. But the multiple etch processes utilized in this method is complex and will introduce large alignment error. Error diffusion method that based on pulse-density modulation has been widely used in signal processing and computer generated hologram (CGH). In this paper, according to error diffusion method in CGH and partial coherent imaging theory, we present a new method to design coding mask of combine CSG-BSG element with error diffusion method. With the designed mask, only one exposure process is needed in fabricating combined element, which will greatly reduce the fabrication difficulty and avoid the alignment error introduced by multiple etch processes. We illustrate the designed coding mask for CSG-BSG element with this method and compare the intensity distribution of the spatial image in partial coherent imaging system with desired relief.

The main working mode of high energy laser system is to focus the transmitting laser beam on target under the condition that the target is tracked in closed loop by fine tracking module, so that it can be damaged or invalidated. Therefore, in order to achieve this effect, an optical device of common aperture is designed for high energy laser system. The emitting system of this device consists of an off-axis two-trans primary telescope module, a Galileo transmission telescope module used in focusing and a beam feeding module composed of dichroic mirror, fast mirror and other optical elements; at the same time, the receiving system of this device consists of the same off-axis two-trans primary telescope module, an imaging module used in fine tracking and the same beam feeding module. Taking incoherent combination laser in space as incident ray, we use optical design software in both sequential mode and non-sequential mode to design and simulate this device. The simulation results show that the distribution of spot at 0.5km~5km is obtained after the laser is modulated by different focusing quantities in the focusing telescope module, and the RMS value of laser wave front is better than λ/20 for emitting system. In addition, the performance of an imaging optical system composed of the primary telescope module and the fine tracking imaging module approach the diffraction limit after optimizing, and the system transfer function is greater than 0.6 at 70lp/mm. Such results in this paper confirm that the structure of this optical transceiver possesses reasonable structure and reliable performance, which meets the requirements of engineering application for high energy laser system.

The characteristics of broadband transmission, environmental durability, and laser damage resistance are critical for silica glass exit aperture windows for their use in kW-level, high energy laser systems. The use of conventional antireflective (AR) coatings on windows for high energy lasers operating in the near infrared is impacted by laser induced damage that occurs under high power irradiation as well as the potential for delamination in operational environments. Novel methods for fabricating antireflective surface structures (ARSS) directly on optics have resulted not only in reduced reflection loss, but also in other advantages to AR coatings as well. The ARSS approach involves sub-wavelength surface structures fabricated directly into the actual surface of the window, eliminating the need for a coating of extraneous materials. We will report on results for ARSS fabricated on silica glass windows. Recently we have reported broadband, low reflectance (< 0.02% at 1 µm) for silica glass windows with random ARSS, fabricated using reactive ion etching. These windows have shown remarkably high laser damage thresholds of 100 J/cm2 at 1.06 µm, which is 5x the threshold measured for a conventional AR coating. We will also present results for MILSPEC durability tests on silica windows, both with and without ARSS, for rain and sand erosion as well as salt fog testing, conducted at a government facility. We will also report on scale up of ARSS on silica windows of large sizes (33 cm), making them practical for system implementation.

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.

This paper discusses plans and initial results of efforts to characterize optical coatings, coating materials and optical substrates planned for use in the Space Based Laser (SBL) Integrated Flight Experiment (IPX) for their endurance in the low earth orbit (LEO) space environment. High energy laser (HEL) systems such as IPX place very challenging requirements on their constituent optics, and the degradation due to the natural LEO environment creates additional risk to system performance. The SBL program has chosen to mitigate this risk by use of ground and flight experiments. The flight experiment is participation in the collaborative experiment, Materials International Space Station Experiment (MISSE), a program funded by NASA and the U.S. Air Force Research Laboratory. The ground experiments are to use laboratory facilities to simulate the atomic oxygen exposure of the LEO environment. This paper begins with a discussion of the LEO environment and the potential effects on IPX optical components and materials. The next section gives an overview of the IPX and the optical components that make up each of the program elements. Next, an overview of the planned ground experimentation and participation in the MISSE collaboration is given. The final section gives an example of laboratory measurements madeon the samples with SBL legacy as a preview to the results expected from the flight experiment. The first flight of MISSE samples to Space Station Alpha is Summer 2001, with a return to Earth in June 2002. The Low Earth Orbit Environment and the Optics Challenge to IPX High energy laser (HEL) systems require extreme performance from their optics in order to meet system goals. Typically, HEL optics are stressing for high damage resistance, high reflectance, low absorptance, scatter and aberration along with predictable spatial uniformity. Producing such optics has been a main concern of the HEL community since the discovery of the laser. Usually, just building the optics is challenge enough. The space based laser (SBL) integrated flight experiment (IPX) program must make these optics and have them meet mission requirements in the challenging environs of low earth orbit (LEO). The LEO environment can be characterized by exposure to the neutral thermosphere (ambient atmosphere), charged plasma, the thermal environment (including diurnal cycles), exposure to solar radiation, debris/micrometeorites and ionizing radiation. The neutral thermosphere, is the atmospheric layer above an altitude of approximately 100 km. At an altitude of 80 kilometers or greater, solar ultra-violet (UV) bombardment of the tenuous atmosphere initiates chemical reactions and breaks down molecular species to their atomic components. In this region of the atmosphere, the concentration of individual species is essentially determined by weight segregation. At the expected altitude of IPX, ~480 km, the predominant species is atomic oxygen (AO). The abundance of neutral species at altitude is shown in Figure 1.

A variety of 1064-nm laser conditioning methods were investigated to find the optimum method for production of large aperture (0.25 m2) multilayer coatings for the National Ignition Facility and Laser MegaJoule. Two conditioning methods were explored, multi-step and single-step on two different laser systems. Off-line conditioning was done on PLATO, a small beam diameter ('-1 mm) raster scanning system. On-line conditioning was done on Beamlet, a single beam prototype of the National Ignition Facility with a large rectangular beam (35 cm x 35 cm). Concurrent with this work, coating process development for low-defect density high damage threshold coatings was realized by switching from hafnia to hafnium starting materials. The results of this study suggest that single-step raster-scan off-line laser conditioning is an effective method to raise the damage threshold of multilayer mirrors deposited from hafnium and silica by reactive electron beam deposition.

Atmospheric Pressure Plasma Jet (APPJ) is a promising technology for precision machining of optical elements due to its high efficiency, free damage and lower cost, it has been developed as a promising technique for advanced optical fabrication in high energy laser systems, space optics and large telescope projects. Nowadays the challenge faced by the plasma technology application is the effect of plasma temperature on removal rate and surface accuracy. In the plasma processing, the plasma itself not only acts as the supplier of reactive species but also acts as a local heat source. Based on the pure chemical reaction mechanism the material removal rate, the removal rate strongly depends on the workpiece surface temperature, and the temperature fluctuations originating from the plasma jet lead to a nonlinear etching behavior. The thermal effect caused by high temperature and high enthalpy of plasma jet restricts the high-precision machining of optical components. In this paper the heat transfer process between plasma jet and substrate is established and the simulation analysis, experiment research also conducted. Based on the energy balance in the generalized lowtemperature plasma processing, the suitable boundary conditions are selected and the heat transfer model in the jet processing is established. The heat flux density on the surface of the components under different machining conditions is acquired by conducting a steady-state temperature measurement experiment. An iterative method is proposed to fully determine the thermal boundary condition, the heat transfer model between plasma jet and substrate is established. And the heating and removal experimental studies are carried out to verify the proposed heat transfer model and thermal effect on material removal is analyzed for plasma processing.

3D Meta-Optics are optical components that are based on the engineering of the electromagnetic fields in 3D dielectric structures. The results of which will provide a class of transformational optical components that can be integrated at all levels throughout a High Energy Laser system. This paper will address a number of optical components based on 2D and 3D micro and nano-scale structures and their performance when exposed to high power lasers. Specifically, results will be presented for 1550 nm and 2000 nm spectral bands and power densities greater than100 kW/cm².

Laser facility such as Megajoule Laser dedicated to laser-matter interaction including inertial fusion need pre-amplification modules (PAM) which must respect a high beam quality. The actual Nd:Phosphate is used in high energy laser system because of its capacity to be produced in big size. The current PAM work at a repetition rate of 1 shot/5 min limited by a low thermal conductivity of the Phosphate glass. However, it would be interesting to increase the shot rate for alignment and diagnostics purposes. Therefore we propose to change this amplification material by some Nd:crystal in the PAM with a higher thermal conductivity and working at 1053 nm to match the power chain wavelength. For long time Nd: CaF2 has been abandoned because of quenching between Nd ions. The lutetium is “buffer” ion used to break Nd clusters and allow high emission cross section. The Nd Lu:CaF2 thermal conductivity is ten times higher than actual Nd:Phosphate and would permit to achieve a repetition rate at 10 Hz. Nevertheless, this material must fulfil the beam specifications to be integrated in the actual amplification chain. We report a characterization of the thermal induced effects on a parallelepiped rod pumped transversally by laser diodes. The pomp-probe beam configuration contains laser diodes emitting at 797 nm with a fluency of 13 J/cm2 and a probe beam passing through the rod at 1053 nm We study the spatially resolved induced birefringence under a mono-shot pump or variable repetition rates. The experimental setup is composed with a cross-rotating polarizer-analyzer and a camera that measures the intensity signal transmitted by the analyzer. A post numerical analysis consists in fitting the intensity signal transmitted for several polarizer-analyzer angles all over the camera picture. Hence the birefringence can be determined spatially at the end front of the rod. These measures are resolved in time to compare the relaxation behaviour of these two materials. Then we simulate the experiment setup with COMSOL software that includes the thermal and mechanic multiphysics interaction. The objective is to assess physical effects we cannot determine by measures like the mechanical stress induced at the origin of the birefringence pattern. We numerically solve the thermal equation. The thermal source defined must fit the experimental pump geometry and time mono-shot pulse rate or variable repetition rates. We take in account of the Beer-Lambert’s absorption law and supergaussian profile for geometry and time definition. Then we use Hooke’s law in general case for free-elastic material linked to the thermal distribution to deduce the stress and strain induced in the material. The induced birefringence is directly associated to the piezo-optic tensor and the stress material. The stress tensor COMSOL computes allow reconstructing the Jones matrices throughout the rod and thus the spatial birefringence. The numerical spatially and time resolved birefringence is in good agreement with experimental measures. This numerical model allows us to optimize the spatial geometry of cooling in transverse pumping as in longitudinal pumping in thick disk amplifier. CPER#16004205 FEDER#2663710