The objective of the research is to experimentally investigate multi-conjugate adaptive optics for compensating strong atmospheric turbulence in laser beam propagation. By using multiple wavefront sensors and deformable mirrors placed at multiple conjugate planes along the beam propagation path, multi-conjugate adaptive optics allows for the simultaneous compensation of both the amplitude and phase of the propagating laser beam. This paper provides a description of the laboratory multi-conjugate adaptive optics testbed developed at the Naval Postgraduate School and presents current experimental results on the compensation of laboratory-simulated atmospheric turbulence using multi-conjugate adaptive optics.
This paper presents NPS Sparse Aperture Testbed development to demonstrate sparse aperture concept. The testbed has a sparse aperture array consisting of three 2-inch diameter F/20 spherical mirrors. Each mirror is connected to a separate 6 axis ThorLabs stage to simulate motion in satellite formation. The beam combining platform employs three 1-inch diameter flat mirrors mounted on the correcting PI tip/tilt/piston actuated stages to provide coherent beam combining capabilities. For metrology, the system uses Zygo 9 single-axis displacement measuring interferometers. Transformation matrix between the aperture motion measured by the metrology and required motion by the correcting piston and tip/tilt stages was determined. The mirrors were moved continuously representing satellite formation errors and the motion was measured by laser metrology and using transformation matrix, the correcting stages were commanded, resulting in coherent combination of images from three apertures.
The Naval Postgraduate School’s segmented mirror telescope (SMT) was developed using prototype silicon carbide active hybrid mirror technology to demonstrate lower cost and rapid manufacture of primary mirror segments for a space telescope. The developmental mirror segments used too few actuators limiting the ability to adequately correct the surface figure error. To address the unintended shortfall of the developmental mirrors, a deformable mirror is added to the SMT and control techniques are developed. The control techniques are similar to woofer-tweeter adaptive optics, where the SMT segment represents the woofer and the deformable mirror represents the tweeter. The optical design of an SMT woofer-tweeter system is presented, and the impacts of field angle magnification on the placement and size of the deformable mirror are analyzed. A space telescope woofer-tweeter wavefront control technique is proposed using a global influence matrix and closed-loop constrained minimization controller. The control technique simultaneously manipulates the woofer and tweeter mirrors. Simulation and experimental results demonstrate a significant improvement in wavefront error of the primary mirror and the control technique shows significant wavefront error improvement compared to sequentially controlling the woofer and tweeter mirrors.
High development cost is a challenge for space telescopes and imaging satellites. One of the primary reasons for this high cost is the development of the primary mirror, which must meet diffraction limit surface figure requirements. Recent efforts to develop lower cost, lightweight, replicable primary mirrors include development of silicon carbide actuated hybrid mirrors and carbon fiber mirrors. The silicon carbide actuated hybrid mirrors at the Naval Postgraduate School do not meet the surface quality required for an optical telescope due to high spatial frequency residual surface errors. A technique under investigation at the Naval Postgraduate School is to correct the residual surface figure error using a deformable mirror in the optical path. We present a closed loop feedback gradient controller to actively control a SMT active segment and an additional deformable mirror to reduce residual wavefront error. The simulations and experimental results show that the gradient controller reduces the residual wavefront error more than an integral controller.
Future large aperture space telescopes may use lightweight correctable active mirrors. The Naval Postgraduate School’s Segmented Mirror Telescope (SMT) test bed uses 1-meter silicon carbide (SiC) active mirror segments to form a sixsegment deployable 3-meter telescope. The active segments suffer from residual surface errors after a correction is applied. A deformable mirror is added at the SMT pupil plane to improve this residual error. The large active SMT segment represents the woofer, and a small continuous micro-electro-mechanical system (MEMS) deformable mirror represents the tweeter. A global influence matrix and closed loop constrained least squares controller command the active segment and additional deformable mirror as a single device. An interferometer measures the surface error and provides feedback to the controller. Simulation and experimental results demonstrate a significant improvement in wavefront error compared to a 2-step sequential woofer-tweeter constrained least squares control approach.
Precision laser beam pointing is a key technology in High Energy Laser systems. In this paper, a laboratory High Energy
Laser testbed developed at the Naval Postgraduate School is introduced. System identification is performed and a
mathematical model is constructed to estimate system performance. New beam pointing control algorithms are designed
based on this mathematical model. It is shown in both computer simulation and experiment that the adaptive filter
algorithm can improve the pointing performance of the system.
Future space based deployable telescopes will be subject to non-atmospheric disturbances. Jitter and optical
misalignment on a spacecraft can be caused by mechanical noise of the spacecraft, and settling after maneuvers. The
introduction of optical misalignment and jitter can reduce the performance of an optical system resulting in pointing
error and contributing to higher order aberrations. Adaptive optics can be used to control jitter and higher order
aberrations in an optical system. In this paper, wavefront control methods for the Naval Postgraduate School adaptive
optics testbed are developed. The focus is on removing structural noise from the flexible optical surface using discrete
time proportional integral control with second order filters. Experiments using the adaptive optics testbed successfully
demonstrate wavefront control methods, including a combined iterative feedback and gradient control technique. This
control technique results in a three time improvement in RMS wavefront error over the individual controllers correcting
from a biased mirror position. Second order discrete time notch filters are also used to remove induced low frequency
actuator and sensor noise at 2Hz. Additionally a 2 Hz structural disturbance is simulated on a Micromachined
Membrane Deformable Mirror and removed using discrete time notch filters combined with an iterative closed loop
feedback controller, showing a 36 time improvement in RMS wavefront error over the iterative closed loop feedback
alone.
The purpose of the research is to develop acquisition, tracking, and pointing technologies for the Bifocal Relay Mirror
Spacecraft and verify these technologies with the experimental test-bed. Because of the stringent accuracy requirement
of the laser beam and the agile maneuverability requirement, significant research is needed to develop acquisition,
tracking, and pointing technologies for the Bifocal Relay Mirror Spacecraft. In this paper, development of the Bifocal
Relay Mirror Spacecraft experimental test-bed is presented in detail. The current operational results are also presented
including precision attitude control of the spacecraft for fine tracking and pointing.
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