Return flux budget of polychromatic laser guide stars

Abstract

The polychromatic laser guide star (PLGS) is one of the solutions proposed to extend the sky coverage by large telescopes to 100% by enabling a complete knowledge of all perturbation orders of the wavefront. The knowledge of the tip-tilt is deduced from the monitoring of the chromatic components of the PLGS, from 330 nm to the visible or near infrared. Here we study the original scheme to create the PLGS by resonant excitation of the mesospheric sodium by two pulsed lasers (tens of kilohertz repetition rate, tens of watts average power, tens of nanoseconds pulse duration), at 589 and 569 nm, respectively. The efficiency of this process is investigated numerically by means of both Bloch equation and rate equation models. The influence of numerous laser parameters is studied. In the best case, having optimized all laser parameters, the return flux at 330 nm should not exceed 7x10(4) photons/s/m2 for 2x18 W laser average power at the mesosphere. This maximum is obtained for a modeless laser whose spot diameter corresponds to 4 times the diffraction limit. For a diffraction-limited spot, the return flux falls down to 4x10(4)photons/s/m2.

Pulsed cooperative backward emissions from non-degenerate atomic transitions in sodium

“…To overcome these specific limitations, socalled polychromatic LGS has emerged [19]. The concept of polychromatic LGS is to utilize either single-photon (at 330 nm [20]) or nondegenerate two-photon (at 569 and 589 nm [21] or at 1140 and 589 nm [18]) excitations of Na atoms and detect cascade fluorescence at different wavelengths, including 589 nm. In particular, the pulsed bichromatic LGS scheme proposed in [18] uses nondegenerate two-photon excitation, whereas a resonant degenerate two-photon excitation is used in the present work.…”

“…The conventional approach for LGS is based on resonance fluorescence with the Na D2line (589 nm) [18][19][20]. However, there are several fundamental limits [21], including the saturation effect and the limited number of return photons, to name two. To overcome the saturation effect, polychromatic LGS has been extensively studied [18][19][20][21].…”

“…However, there are several fundamental limits [21], including the saturation effect and the limited number of return photons, to name two. To overcome the saturation effect, polychromatic LGS has been extensively studied [18][19][20][21]. In spite of some scalability limitations, the present research may have possible applications in polychromatic LGS technology.…”

Optimization of cw sodium laser guide star efficiency

“…BEACON neglects atomic collisions and recoil. Guillet de Chatellus (2008) reports that it requires on the order of 24 h per run on a 2.6 GHz processor, and that the agreement with a pure rate-equation model can be good in certain cases.…”

Towards laser guide stars for multi-aperture interferometry: an application to the hypertelescope

“…A detailed simulation of sodiumlayer physics to calculate the number of return photons for different types of lasers is beyond the scope of this paper. Below we provide order-of-magnitude estimates in view of the simulations made by (D'Orgeville et al 2000) and (Guillet de Chatellus et al 2008) which include saturation effects.…”