Atmospheric turbulence over long horizontal paths perturbs phase and can also cause severe intensity scintillation in the pupil of an optical communications receiver, which limits the data rate over which intensity-based modulation schemes can operate. The feasibility of using low-order adaptive optics by applying phase-only corrections over horizontal propagation paths is investigated. A Shack-Hartmann wave-front sensor was built and data were gathered on paths 1 m above ground and between a 1- and 2.5-km range. Both intensity fluctuations and optical path fluctuation statistics were gathered within a single frame, and the wave-front reconstructor was modified to allow for scintillated data. The temporal power spectral density for various Zernike polynomial modes was used to determine the effects of the expected corrections by adaptive optics. The slopes of the inertial subrange of turbulence were found to be less than predicted by Kolmogorov theory with an infinite outer scale, and the distribution of variance explained by increasing order was also found to be different. Statistical analysis of these data in the 1-km range indicates that at communications wavelengths of 1.3 mum, a significant improvement in transmitted beam quality could be expected most of the time, to a performance of 10% Strehl ratio or better.
The James Webb Space Telescope (JWST) Coarse Phase Sensor utilizes Dispersed Hartmann Sensing (DHS) 1 to measure the inter-segment piston errors of the primary mirror. The DHS technique was tested on the Keck Telescope. Two DHS optical components were built to mate with the Keck optical and mechanical interfaces. DHS images were acquired using 20 different primary mirror configurations. The mirror configurations consisted of random segment pistons applied to 18 of the 36 segments. The inter-segment piston errors ranged from phased (approximately 0 µm) to as large as ±25 µm. Two broadband exposures were taken for each primary mirror configuration: one for the DHS component situated at 0°, and one for the DHS component situated at 60°. Finally, a "closed-loop" DHS sensing and control experiment was performed. Sensing algorithms developed by both Adaptive Optics Associates (AOA) and the Jet Propulsion Laboratory (JPL) 2 were applied to the collected DHS images. The inter-segment piston errors determined by the AOA and JPL algorithms were compared to the actual piston steps. The data clearly demonstrates that the DHS works quite well as an estimator of segment-to-segment piston errors using stellar sources.
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