Optimization of cw sodium laser guide star efficiency

Abstract

Context. Sodium laser guide stars (LGS) are about to enter a new range of laser powers. Previous theoretical and numerical methods are inadequate for accurate computations of the return flux, hence for the design of the next-generation LGS systems. Aims. We numerically optimize the cw (continuous wave) laser format, in particular, the light polarization and spectrum. Methods. Using Bloch equations, we simulate the mesospheric sodium atoms, including Doppler broadening, saturation, collisional relaxation, Larmor precession, and recoil, taking all 24 sodium hyperfine states into account and 100-300 velocity groups. Results.LGS return flux is limited by "three evils": Larmor precession due to the geomagnetic field, atomic recoil due to radiation pressure, and transition saturation. We study their impact and show that the return flux can be boosted by repumping (simultaneous excitation of the sodium D 2 a and D 2 b lines with 10−20% of the laser power in the latter). Conclusions. We strongly recommend the use of circularly polarized lasers and repumping. As a rule of thumb, the bandwidth of laser radiation in MHz (at each line) should approximately equal the launched laser power in Watts divided by six, assuming a diffraction-limited spot size.

Elastic scattering and rotational excitation of nitrogen molecules by sodium atoms

“…As indicated above, the total cross section is almost independent of j, but the contribution from inelastic scattering decreases with j. The value estimated by Holzlöh-ner et al, 1 who treated Na and N 2 as hard spheres with effective particle radii given by the Van der Waals radii, is σ = 72 × 10 −16 cm 2 , in surprisingly good agreement given the very simple assumptions made in their calculation. However, more detailed models of the velocity redistribution of the sodium atoms following collisions with N 2 can be achieved by using the elastic and inelastic differential cross sections presented in this work over a whole range of energies.…”

“…1 LGS are currently being developed at ground-based telescopes in an effort to improve the use of adaptive optics. This technique requires a reference source (a guide star) in order to efficiently correct the effects of the atmospheric distortion on the light received by telescopes.…”

“…Holzlöhner et al recently simulated the photon return flux by using the optical Bloch equations for the atomic density matrix. 1 They emphasized the importance of including collisional processes between Na and N 2 or O 2 in the simulation, as they modify the velocity of sodium atoms and therefore the a) Electronic mail: jloreau@cfa.harvard.edu.…”

Pressure broadening and shift of theD1line of Ag by He, Ar, and N2

“…We plan to extend our work by measuring broadening and shift due to O 2 , as well as spin depolarization cross-section Ag-O 2 which will serve as a good proxy for the Na-O 2 cross section. The latter is important for modeling and realization of highly effective guide stars widely used in most advanced telescopes [8].…”

“…Our work is motivated, on the one hand, by the ongoing study of silver atoms confined in a cryogenic buffer gas [7], and on the other, possibility of using silver as * kulan@lanl.gov a proxy for the alkali atoms, primarily sodium, important for the laser guide star applications [8] and, more generally, for understanding of the atomic and molecular processes in the atmosphere.…”