Carolyn L. Weinzapfel

Fabrication of meter-scale laser-resistant mirrors for the National Ignition Facility: a fusion laser

Large-aperture laser-resistant mirrors are required for the construction of the National Ignition Facility, a 1.8 MJ laser. In order to fabricate the 1408 mirrors, a development program was started in 1994 to improve coating quality, manufacturing rate, and lower unit cost. New technologies and metrology tools were scaled to meter size for facilitization in 1999 at Spectra-Physics and the Laboratory of Laser Energetics at the University of Rochester. Pilot production, to fabricate 5-10% of each component, commenced in 2001 and full production rates were achieved in 2002. Coating production will be completed in 2008 with the coating of 460 m2 of high-damage-threshold precision coatings on 100 tons of BK7 glass with yields exceeding 90%.

Development of practical damage-mapping and inspection systems

We have developed and are continuing to refine semi- automated technology for the detection and inspection of surface and bulk defects and damage in large laser optics. Different manifestations of the DAMOCLES system provide an effective and economical means of being able to detect, map and characterize surface and bulk defects which may become precursors of massive damage in optics when subjected to high-fluence laser irradiation. Subsequent morphology and evolution of damage due to laser irradiation can be tracked efficiently. The strength of the Damocles system is that it allows for immediate visual observation of defects in an entire optic, which can range up to 1-meter dimensions, while also being able to provide digital map and magnified images of the defects with resolutions better than 5 micrometers .

Automated damage test facilities for materials development and production optic quality assurance at Lawrence Livermore National Laboratory

The Laser Program at LLNL has developed automated facilities for damage testing optics up to 1 meter in diameter. The system were developed to characterize the statistical distribution of localized damage performance across large- aperture National Ignition Facility optics. Full aperture testing is a key component of the quality assurance program for several of the optical components. The primary damage testing methods used are R:1 mapping and raster scanning. Automation of these test methods was required to meet the optics manufacturing schedule. The automated activities include control and diagnosis of the damage-test laser beam as well as detection and characterization of damage events.

A System for Measuring Defect Induced Beam Modulation on Inertial Confinement Fusion-class Laser Optics

A multi-wavelength laser based system has been constructed to measure defect induced beam modulation (diffraction) from ICF class laser optics. The Nd:YLF-based modulation measurement system (MMS) uses simple beam collimation and imaging to capture diffraction patterns from optical defects onto an 8-bit digital camera at 1053, 527 and 351 nm. The imaging system has a field of view of 4.5 x 2.8 mm2 and is capable of imaging any plane from 0 to 30 cm downstream from the defect. The system is calibrated using a 477 micron chromium dot on glass for which the downstream diffraction patterns were calculated numerically. Under nominal conditions the system can measure maximum peak modulations of approximately 7:1. An image division algorithm is used to calculate the peak modulation from the diffracted and empty field images after the baseline residual light background is subtracted from both. The peak modulation can then be plotted versus downstream position. The system includes a stage capable of holding optics up to 50 pounds with x and y translation of 40 cm and has been used to measure beam modulation due to solgel coating defects, surface digs on KDP crystals, lenslets in bulk fused silica and laser damage sites mitigated with CO2 lasers.

Functional damage thresholds of hafnia/silica coating designs for the NIF laser

Studies into the functional damage thresholds of hafnia/silica thin film coatings for the NIF laser have been conducted on two different-sized substrates: 50-mm-diam test substrates and full-sized (412 x 412 mm) NIF mirror substrates. For both studies, the optics were raster scanned by Q-switched Nd:YAG lasers emitting 1064-nm light with 10- ns pulse lengths. The coatings tested were primarily high reflectors, although polarizing beam splitter and anti- reflective thin films were tested on the small substrates. Tests were performed to find the functional damage threshold: the minimum fluence at which a damaged optic degrades the performance of the NIF laser, or, experimentally, the minimum fluence at which a damaged site begins to grow. Thus, the concern is with finding not only the fluence that caused a pit (for example) but also the fluence at which that pit begins to grow with subsequent laser shots. After a site begins a growth phase, the growth rate is measured as a function of fluence. This provides some information for predicting the optic lifetime for given operating limits of the laser.

Photothermal multi-pixel imaging microscope

Photothermal microscopy is a useful nondestructive tool for the identification of fluence-limiting defects in optical coatings. Traditional photothermal microscopes are single-pixel detection devices. Samples are scanned under the microscope to generate a defect map. For high-resolution images, scan times can be quite long (1 mm2 per hour). Single-pixel detection has geen used traditionally because of the ease in separating the laser-induced topographical change due to defect absorption from the defect surface topography. This is accomplished by using standard chopper and lock-in amplifier techniques to remove the DC signal. Multi-pixel photothermal microscopy is now possible by utilizing an optical lock-in technique. This eliminates the lock-in amplifier and enables the use of a CCD camera with an optical lock in for each pixel. With this technique, the data acquisition speed can be increased by orders of magnitude depending on laser power, beam size, and pixel density.

Vendor-based laser damage metrology equipment supporting the National Ignition Facility

A sizeable laser damage metrology effort is required as part of optics production and installation for the 192 beam NIF laser. The large quantities, high damage thresholds, and large apertures of polished and coated optics necessitates vendor-based metrology equipment to assure component quality during production. This equipment must be optimized to provide the required information as rapidly as possible with limited operator experience. This equipment must be optimized to provide the required information as rapidly as possible with limited operator experience. The damage metrology tools include: 1) platinum inclusion damage test system for laser amplifier slabs, 2) laser conditioning stations for mirrors and polarizers, and 3) mapping and damage testing stations for UV transmissive optics. Each system includes a commercial Nd:YAG laser, a translation stage of the optics, and diagnostics to evaluate damage. The scanning parameters, optical layout, and diagnostics vary with the test fluence required and the damage morphologies expected. This paper describes the technical objectives and milestones involved in fulfilling these metrology requirements at multiple vendors.

Zonal deformable mirror for laser wavefront control

We have developed a zonal deformable mirror that controls the wavefront of a high average power visible laser beam used for isotope separation. The mirror corrects greater than five waves of astigmatism, power, or random second order aberrations to 1/20 wave rms. Sufficient resolution is achieved to correct third order aberrations as well. A monolithic glass substrate with dimensions 77 mm X 121 mm X 10 mm is used in this design. Twenty-five actuator attachment members are incorporated into the shape of the back side of the substrate. Piezoelectric translators (PZTs) attached in a rectangular array deform the continuous substrate to the proper conjugate shape. The PZTs are attached through flexures designed to be compressionally stiff and laterally soft. In this way the intended PZT displacement is transmitted efficiently to the substrate while isolating both the mirror and the PZTs from undesirable lateral loads. Mirror parameters were determined from elastic mechanical beam approximations. Finite element analysis was used to verify performance prior to prototyping. A Hartmann sensor controls the mirror in a closed loop adaptive system. The system description is covered in a companion paper. This paper describes the mirror design and presents performance data.

Adaptive optics package designed for astronomical use with a laser guide star tuned to an absorption line of atomic sodium

We present the design and implementation of a very compact adaptive optics system that senses the return light from a sodium guide-star and controls a deformable mirror and a pointing mirror to compensate atmospheric perturbations in the wavefront. The deformable mirror has 19 electrostrictive actuators and triangular subapertures. The wavefront sensor is a Hartmann sensor with lenslets on triangular centers. The high-bandwidth steering mirror assembly incorporates an analog controller that samples the tilt with an avalanche photodiode quad cell. An f/25 imaging leg focuses the light into a science camera that can either obtain long-exposure images or speckle data. In laboratory tests overall Strehl ratios were improved by a factor of 3 when a mylar sheet was used as an aberrator. The crossover frequency at unity gain is 30 Hz.