Characterization of artificial guide stars generated in the mesospheric sodium layer with a sum-frequency laser

Abstract

We generated resonant-fluorescent artificial guide stars in the Earth’s mesospheric sodium layer with an 840-Hz, 10-mJ sum-frequency laser operating at 589 nm and have conducted experiments to characterize the guide stars for use as beacons for atmospheric compensation with adaptive optics. We made high-resolution images with a 1.5-m telescope and CCD camera with exposures from 0.1 to 10.1 s of spots as small as 1.64 times the diffraction limit. We also used an intensified charge-injection-device camera to record live video at frame rates of 30 Hz. Using a photomultiplier tube with both the 1.5-m telescope and a 14-in. telescope, we collected temporal data and calculated the sodium-layer column densities of 2–3 × 103/m2 from long pulse measurements. We have observed the phenomenon of optical pumping of the sodium layer by propagating circularly polarized light at peak power densities incident on the layer of 1.0 mW/cm2. Propagating circularly polarized light gives a 41–48% increase in fluorescent return signal over linearly polarized light. Artificial guide stars are suitable for atmospheric compensation with the use of adaptive optics.