Effect of the geomagnetic field onthe intensity of sodium laser guide stars

摘要

In calculating the return flux of sodium laser guide stars (LGS), often only the best return conditions are assumed,relying on strong optical pumping with a circularly polarized laser. However, one can obtain this optimal return onlyalong one specific laser orientation on the sky, depending on the direction of the geomagnetic field. Sodium atomsprecess around magnetic field lines which cycles the magnetic quantum number, reducing the effectiveness of opticalpumping. The risk is hence for the system designer to overestimate the effective return flux for a given laser power. In this paper we present calculations for the effect of the geomagnetic field on the return flux. Optical pumping actuallyproduces little improvement in most observing directions. In addition, we find that at many observatory locations, thedirection of optimal return flux is very close to the horizon, often below the observing limit.