Dependence of sodium laser guide star photon return on the geomagnetic field

Abstract

Aims. The efficiency of optical pumping that increases the backscatter emission of mesospheric sodium atoms in continuous wave (cw) laser guide stars (LGSs) can be significantly reduced and, in the worst case, eliminated by the action of the geomagnetic field. Our goal is to present an estimation of this effect for several telescope sites. Methods. Sodium atoms precess around magnetic field lines that cycle the magnetic quantum number, reducing the effectiveness of optical pumping. Our method is based on calculating the sodium magnetic sublevel populations in the presence of the geomagnetic field and on experimental measurements of radiance return from sodium LGS conducted at the Starfire optical range (SOR). Results. We propose a relatively simple semi-empirical formula for estimating the effect of the geomagnetic field on enhancing the LGSs photon return due to optical pumping with a circularly polarized cw single-frequency laser beam. Starting from the good agreement between our calculations and the experimental measurements for the geomagnetic field effect, and in order to more realistically estimate the sodium LGSs photon return, we introduce the effect of the distance to the mesospheric sodium layer and the atmospheric attenuation. The combined effect of these three factors is calculated for several telescope sites. Conclusions. In calculating the return flux of LGSs, only the best return conditions are often assumed, relying on strong optical pumping with circularly polarized lasers. However, one can only obtain this optimal return along one specific laser orientation on the sky, where the geomagnetic field lines are parallel to the laser beam. For most of the telescopes, the optimum can be obtained at telescope orientations beyond the observation limit. For the telescopes located close to the geomagnetic pole, the benefit of the optical pumping is much more important than for telescopes located close to the geomagnetic equator.

Statistics of the sodium layer parameters at low geographic latitude and its impact on adaptive-optics sodium laser guide star characteristics

“…The expression of this parameter is given by where F R is the specific return photon flux to the ground per watt of launched laser power, where s ce is the coupling efficiency of light at 589 nm to the sodium atoms (unitphotons × m 2 /s/W/atom), C Na is the vertical column density of sodium, sec(ζ) is the secant of the zenith angle ζ (the airmass), T atmo is the single-pass transmission of the atmosphere at 589 nm at zenith, and H Na is the vertical distance from the telescope to the sodium layer centroid. We note that s ce and thus F R strongly (and to some extent nonlinearly) depend on laser power, laser spectrum, laser polarization, and angle between laser propagation direction and the magnetic field (Moussaoui et al 2008(Moussaoui et al , 2009, in addition to pulse format if any.…”

Characterization of the sodium layer at Cerro Pachón, and impact on laser guide star performance

“…With the increasing use of sodium LGS in astronomical AO, many studies have been done in the past years to characterize more precisely the properties of the sodium layer in the light of AO requirements. Models have been developed to predict the sodium abundance and expected photon return for different sites (Moussaoui et al 2009;Holzlohner et al 2010). The temporal behavior of sodium profiles, and the variations of the mean altitude have also been studied in detail (Herriot et al 2006;Pfrommer & Hickson 2010).…”

Gemini South multi-conjugate adaptive optics (GeMS) laser guide star facility on-sky performance results

“…A factor of two discrepancy between the sodium return per Watt specification and achieved results can partially be explained by what has been learned since 2001 about optimal, and conversely sub-optimal, laser formats 19,20,21 . The large ~2GHz spectral bandwidth of the GS laser system is partly responsible for this discrepancy.…”