Charles D. Swift

Real-time wavefront correction system using a zonal deformable mirror and a Hartmann sensor

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.

Deformable mirror for zigzag solid-state lasers

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.

Wavefront correction system based on an equilateral triangular arrangement of actuators

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.

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.

Target Plane Imaging System For The Nova Laser

The Nova laser, in operation since December 1984, is capable of irradiating targets with light at 1.05 µm, 0.53 µm, and 0.35 µm. 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.

Three Wavelength Optical Alignment Of The Nova Laser

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.

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.

A Target Plane Imager For Inertial Confinement Fusion

The Nova laser, completed in December 1984 at Lawrence Livermore National Laboratory, is being used to conduct inertial confinement fusion experiments.1 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 (TPI) that is used during the beam alignment phase of these experiments.2 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: 1.05μ, 0.527μ, 0.351μ. 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.