Optimization of Sodium Guide Star Return using Polarization and/or Modulation Control

Laser guide stars (LGS) are artificial sources of light produced by laser-induced fluorescence from sodium atoms in the mesosphere between 85 km and 100 km altitude. The fluorescence can be detected on the ground with a telescope and used as a reference for compensating atmospheric aberrations in astronomical and space observations in conjunction with an adaptive optics system. Sodium LGS are vital for the next generation of large ground-based visible-light telescopes. Additional applications include space-debris and object tracking and imaging, ground-to-satellite laser communication, and potentially even interplanetary laser communication. Laser guide stars can also be adapted for atmospheric and remote-sensing applications; in particular, LGS can be used for remote magnetometry by modulating the light beam and observing a resonance when the modulation frequency matches the spin-precession Larmor frequency of sodium atoms in the mesosphere. For any of these applications, an increase in LGS brightness would be desirable; for daytime operation it is essential. This requires experimental and computational research into new LGS methodologies. We propose work to achieve two objectives: Objective 1: Development of enhanced user-friendly LGS modeling software. The open-source LGSBloch package, developed by Rochester Scientific, is currently the primary software serving the adaptive-optics sodium LGS modeling community. However, it could benefit from major improvements in capabilities, computational efficiency, and user-friendliness. We propose to add capabilities to efficiently model pulsed and modulated light (including amplitude and polarization modulation), to model alternative and multi-level pumping schemes, including those in other potentially useful atomic systems (e.g., iron), to integrate over the laser-beam volume, and include atmospheric and mesospheric propagation effects, as well as including a user-friendly graphical user interface to facilitate use by those who are not atomic-physics specialists. Objective 2: Feasibility study of brightness-enhanced laser guide stars based on mesospheric iron. Methodologies for brighter laser guide stars would have civilian and military commercial applications including astronomy, space-object tracking, and laser-based communication. We propose to study the possibility for one such method, namely basing an LGS on mesospheric iron atoms, rather than sodium.  Mesospheric iron may have benefits including brighter daytime operation and polychromatic LGS for tip-tilt correction. The iron-LGS study will include several components: modeling single-color LGS and polychromatic LGS, investigating the benefits of polarization modulation and frequency chirping for LGS brightness, and determining whether a directed-beam iron LGS employing amplified spontaneous emission is possible. Each component of the study will include both computational modeling and auxiliary laboratory experiments for validation.