Studies of atmospheric properties for optical ground-based astronomy and methods to enhance laser guide star adaptive optics performance

Ground-based astronomy suffers from waveform distortion produced by the turbulent atmosphere, which prevents telescopes from reaching diffraction-limited resolution. Modern large telescopes and next generation extremely-large telescopes use or will use adaptive optics systems with laser guide stars to correct for atmospheric wavefront distortion. The first part of the thesis deals with astronomical site testing and the second part with methods for adaptive optics system improvement. Meteorological data for 15 observatory sites were studied. Monthly averages of cloud cover, wind speed at 200 hPa, precipitable water vapour, vertical wind velocity and aerosol index were compared for the sites. The long-term evolution over 45 years of the five atmospheric quantities was investigated. Site testing campaigns characterize potential telescope sites in terms of optical turbulence. Using scintillometers, ground layer turbulence profiles can be measured. For an assessment of sites long-term statistics are needed. Two campaigns for daytime and nighttime turbulence profiling have been started and preliminary results are described. Methods for increasing adaptive optics system performance are studied. Polarization modulation of the adaptive optics laser might be an useful method. Experimental results are presented. The method also enables measurements of the Larmor frequency and the magnetic field strength in the mesosphere. The average Larmor frequency is 260.4 kHz. We found a maximum LGS return flux enhancement of 18% for a pulsed amplitude modulation at Larmor frequency compared to amplitude modulation offset from the Larmor frequency. For polarization modulation at the Larmor frequency a 6% increase in LGS return flux was found over polarization modulation offset by 30-kHz from the Larmor frequency. Adaptive optics system could also benefit from an estimate of the mesospheric sodium density profile. Such profiles can be retrieved by partial amplitude modulation, with pseudo-random binary sequences, of continuous-wave lasers. Results for an experiment at the Large Zenith Telescope in Maple Ridge, and a feasibility study of this method for extremely-large telescopes, are presented. The method could be used on extremely-large telescopes to estimate the sodium density profile with a temporal resolution of a few seconds.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Modeling the response of mesospheric sodium to pulsed-laser excitation

A simulation modeling excitation of the sodium D-2 line by nanosecond time scale pulsed lasers is described. By numerically integrating transition rates in the sodium hyperfine structure, the return flux per sodium atom is predicted as a function of laser power. The simulation should be useful for studies of mesospheric sodium and adaptive optics. Applications include the estimation of sodium column density from lidar return flux, and of laser guide star brightness for different pulsed laser formats. The simulation assumes that the pulse repetition frequency is sufficiently low (smaller than a few kilohertz) that atomic collisions restore local thermodynamic equilibrium between pulses. It is also assumed that the pulse length is short compared to the Larmor precession time scale. The numerical results are well-approximated by a simple analytic model for a three-level atom. The number of emitted photons is found to be primarily dependent on the product of the length of the laser pulse and the energy density. (C) 2017 Optical Society of Americ

Remote sensing of geomagnetic fields and atomic collisions in the mesosphere

Remote sensing of geomagnetic fields in mesosphere is both challenging and interesting to explore the magnetic field structures and atomic collision processes. Here the authors demonstrate an atomic magnetometer that utilizes the Larmor frequency in sodium atoms and operates in kilometers range

Polarization-driven spin precession of mesospheric sodium atoms

We report experimental results on the first on-sky observation of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. The magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 km and 100 km altitude. The experiment was performed at La Palma, and the uncertainty in the measured Larmor frequency ($\approx$260 kHz) corresponded to an error in the geomagnetic field of 0.4 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere