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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.

We are developing an adaptive optics system that is based on an array of actuators arranged with subapertures that are equilateral triangles. The wavefront sensor is a video Hartmann sensor that also uses an equilateral array of lenslets. The controller hardware uses a VME bus. The design minimizes the generation of reflected wavefronts higher than first order across each lenslet for large excursions of actuators from positions where the mirror is flat and, thus maximizes the precision of the slopes measured by the Hartmann sensor. The design is also immune to the waffle mode that is present in the reconstructors of adaptive optics systems where actuators are arranged in a square array.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

We have developed an adaptive optics system that corrects up to five waves of 2nd-order and 3M-order aberrations in a high-power laser beam to less than 1/10th wave RMS. The wavefront sensor is a Hartmann sensor with discrete lenses and position-sensitive photodiodes; the deformable mirror uses piezoelectric actuators with feedback from strain gauges bonded to the stacks. The controller hardware uses a VMIE bus. The system removes thermally induced aberrations generated in the master-oscillator-power-amplifier chains of a dye laser, as well as aberrations generated in beam combiners and vacuum isolation windows for average output powers exceeding 1 kW. The system bandwidth is 1 Hz, but higher bandwidths are easily attainable.

A deformable mirror has been designed and built to correct thermally induced aberrations in a medium average power solid-state laser (MPSSL) system. The mirror is capable of correcting typically induced errors of up to two waves in the narrow dimension and up to six waves in the long dimension. It has a clear aperture of 12 mm x 130 mm and serves as one element in the laser cavity. Piezoelectric translators are attached to the mirror through a unique flexure mechanism. This simple mechanical design has proven adequate for controlling low-spatial-frequency aberrations. Although designed specifically for the MPSSL, modifications of this design have application to other optical systems. The mirror design and performance as well as the methodology and techniques used in this development are discussed.

The Nova and Novette alignment systems guide laser pulses through the separate amplifier chains to the target and ensure simultaneous arrival at the target. Alignment tasks accomplished prior to each target shot involve automatic or remote-manual adjustments of ~2000 stepper motors and other actuators for the full 20-beam 3-wavelength Nova system. The sensor packages and individual detectors used for gathering alignment and diagnostic data are modular to minimize the number of different design and fabrication efforts. For example, the output sensor1 comprises separate, but essentially identical, modules for each of the 3 operating wavelengths.

The alignment system for Nova is designed to perform automatically all alignment tasks from the oscillator-to-target chamber center. It is comprised of five subsystems, each utilizing a different sensor package design. All beam alignment data are collected in video form by vidicon, charge-coupled devices (CCD), and charge-injected device (CID) cameras which reside in the sensor packages. Frequently used alignment references are remotely insertable, and the mirrors, lenses, and other components needed to correct alignment errors are installed in motorized mounts so that they too are remotely controlled. Finally, the sensor packages themselves can be placed in any of their several operating modes by external commands from the control system. Table I summarizes the composition of the five subsystems and gives an indication of the magnitude of the control task.

The objective of this laser design is to efficiently extract energy from a low gain, glass zigzag slab in a single, near diffraction-limited laser pulse short enough to be efficiently frequency converted. This task is made difficult because the zigzag slab gain and optical quality are limited by thermally induced stresses resulting from repetitive pulse operation. Efficient extraction in these conditions can be achieved if the laser cavity losses are low to achieve a favorable gain/loss and if the propagation effects are controlled to uniformly illuminate the slab aperture.

The Nova laser, presently under construction at Lawrence Livermore National Laboratory, will be capable of delivering more than 100 kJ of focused energy to an Inertial Confinement Fusion (ICF) target. Operation at the fundamental wavelength of the laser (1.05 Am) and at the second and third harmonic will be possible. This paper will discuss the optical alignment systems and techniques being implemented to align the laser output to the target at these wavelengths prior to each target irradiation. When experiments require conversion of the laser light to wavelengths of 0.53 μm and 0.35 μm prior to target irradiation, this will be accomplished in harmonic conversion crystals located at the beam entrances to the target chamber. The harmonic alignment system will be capable of introducing colinear alignment beams of all three wavelengths into the laser chains at the final spatial filter. The alignment beam at 1.05 μm will be about three cm in diameter and intense enough to align the conversion crystals. Beams at 0.53 μm and 0.35 Am will be expanded by the spatial filter to full aperture (74 cm) and used to illuminate the target and other alignment aids at the target chamber focus. This harmonic illumination system will include viewing capability as well. A final alignment sensor will be located at the target chamber. It will view images of the chamber focal plane at all three wavelengths. In this way, each beam can be aligned at the desired wavelength to produce the focal pattern required for each target irradiation. The design of the major components in the harmonic alignment system will be described, and a typical alignment sequence for alignment to a target will be presented.

The Nova laser, completed in December 1984 at Lawrence Livermore National Laboratory, is being used to conduct inertial confinement fusion experiments. It is capable of focusing more than 100 kJ of energy on small fusion targets. This paper discusses an optical system called the target plane imager that is used during the beam alignment phase of these experiments. The TPI includes a three meter long periscope with a wide field of view, F/3 objective. The telescope relays images of the target focal plane to viewing optics and a video sensor located outside the target chamber. Operation of the system is possible at three wavelengths: 1.05..mu.., 0.527..mu.., and 0.351..mu... These are the three wavelengths at which the ten Nova laser beams can irradiate targets. Both nearfield and farfield images of the ten beams can be viewed with the TPI. This instrument is used to properly align the laser to the target before each target irradiation.

The Nova laser, in operation since December 1984, is capable of irradiating targets with light at 1.05 ..mu..m, 0.53 ..mu..m, and 0.35 ..mu..m. Correct alignment of these harmonic beams uses a system called a target plane imager (TPI). It is a large microscope (four meters long, weighing one thousand kilograms) that relays images from the target chamber center to a video optics module located on the outside of the chamber. Several modes of operation are possible including: near-field viewing and far-field viewing at three magnifications and three wavelengths. In addition, the entire instrument can be scanned in X,Y,Z to examine various planes near chamber center. Performance of this system and its computer controls will be described.

Large-aperture high-power solid-state laser systems for inertial confinement fusion (ICF) research, such as the recently activated Nova system at LLNL, are frequency converted from the 1.053-μm fundamental (1ω) to the second (2ω), third (3ω), and fourth (4ω) harmonic wavelengths for efficient laser-target coupling,1 It is therefore essential that we understand the optical processes that limit efficient frequency conversion. We report here on the implications of recent observations using the Nova laser system of a power-dependent rotation of the plane of polarization of the output beam. This rotation occurs in slightly depolarized regions of the beam and depends in magnitude on the polarization ellipticity and B-integral accumulated in the propagation path subsequent to the depolarizing components. In particular, we find that third harmonic (3ω) conversion with type II phase matching2 is adversely affected, because a polarization orientation error as small as 1° or 2° causes a premature backconversion of the 3ω field to the input 1ω and 2ω fields. We have conducted experiments on Nova to understand the details of dynamic polarization rotation and the implications for second3 and third harmonic, frequency converted, solid-state laser systems. Recent experiments show 65 % conversion of the full aperture 1.053-μm Nova beam to the third harmonic when the output polarization is fixed with an additional polarizing element.