A program to compute the two-step excitation of mesospheric sodium atoms for the Polychromatic Laser Guide Star Project

Bellanger, Véronique; Courcelle, Arnaud; Petit, A.

doi:10.1016/j.cpc.2004.06.089

Cited by 8 publications

(8 citation statements)

References 6 publications

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“…There are also approaches that mix aspects of Bloch and rate-equation codes, such as BEACON, which has been adapted to model two-step sodium excitation for polychromatic LGS, (Bellanger 2004). BEACON neglects atomic collisions and recoil.…”

Section: Introductionmentioning

confidence: 99%

Optimization of cw sodium laser guide star efficiency

Holzlöhner

Rochester

Calia

et al. 2010

A&A

104

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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.

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Section: Introductionmentioning

confidence: 99%

Optimization of cw sodium laser guide star efficiency

Holzlöhner

Rochester

Calia

et al. 2010

A&A

104

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“…2×15 W and 4×10 9 atoms/cm 2 ) one get a result which is very close to our calculation (point 1). Again in the two photon excitation scheme, it has been clearly shown 16 that the flux at 330 nm is 100 times weaker that the returned flux on the D 2 line (see figure 3 of reference 16). The consequence is that, because the measured D 2 flux at Keck II, Gemini and VLT/ESO is about 10 6 photons/s/m 2 with similar laser power, the two photons excitation scheme cannot produce a flux at 330 nm larger than about 10 4 photons/s/m 2 .…”

Section: Discussionmentioning

confidence: 92%

Polychromatic laser guide star using a single laser at 330 nm

et al. 2006

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International audienceThe differential atmospheric tip-tilt can be measured using a Polychromatic Laser Guide Star. A two photon excitation has been proposed. It consists in exciting the 4D 5/2 level of mesospheric sodium atoms with two identical lasers operating at 589 nm and 569 nm. With two modeless lasers of 2×15W at the mesosphere level, this scheme will produce a returned flux at 330 nm of about 4×10 4 photons/s/m2. Thanks to our modeless laser, we propose a new method which consists in exciting directly the 4P 3/2 sodium level with one photon excitation, using a single laser operating at 330 nm. This solution was previously rejected probably because of strong saturation problems using single longitudinal mode lasers. We show that 1 W modeless laser at 330 nm can produce the same returned flux at 330 nm. This solution will save at least 400 k€ of equipment. Moreover, our new method is very promising in terms of simplicity but also in terms of flux because the returned flux above will probably be not sufficient for getting a good Strehl ratio. We propose very efficient solid state laser systems for the production of tens of watts at 330 nm

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“…The excitation of the mesospheric sodium atoms and the resulting backscattered flux are computed by using the BEACON code [1]. This code uses the optical Bloch equations to compute the number of photons emitted from the mesosphere by fluorescence.…”

Section: Sodium Excitation In Mesospherementioning

confidence: 99%

“…The modeled twin laser beam is launched with a three aperture interferometer that produces an interferometric pattern in the mesosphere after the passage through a Kolmogorov phase screen. The backscattered flux is computed with an optical Bloch equations code [1]. The result of this excitation is an emission at different wavelengths.…”

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confidence: 99%

End-to-end model for the Polychromatic Laser Guide Star project (ELP-OA)

Meilard

Foy

Tallon

et al. 2010

1st AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes

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Abstract. In order to optimize parameters for the ELP-OA project, we have developped an end-to-end model from the laser emission to the computation of the tip-tilt Strehl ratio. ELP-OA aims at measuring the wavefront tip-tilt from the laser guide star alone, without any natural source. It relies on 2-photon excitation of mesospheric sodium, achieved by two laser chains (at 569nm and 589nm) delivering 34W average power each. The modeled twin laser beam is launched with a three aperture interferometer that produces an interferometric pattern in the mesosphere after the passage through a Kolmogorov phase screen. The backscattered flux is computed with an optical Bloch equations code [1]. The result of this excitation is an emission at different wavelengths. The wavelengths retained are 330nm, 569nm and 589nm. An image of this polychromatic LGS (PLGS) is computed, as it will be observed at the 1.52m telescope at the Observatoire de Haute-Provence (OHP), through a Kolmogorov phase screen again. Then we simulate the adaptive optics device (adapted from ONERA's BOA AO) at the telescope except for tip-tilt. Air refraction index variations due to atmospheric turbulence create a dispersion between 330nm and 569nm, and the tip-tilt value is derived from this dispersion. This dispersion between 330nm and 569nm is measured using a correlation method and a sub-pixel interpolation. The calculation of the relative position between two simultaneous images at 330nm and 569nm gives a position error, from which the Strehl ratio for tip-tilt is deduced. At this time, we have got Strehl ratios up to approximatively 35% at 550nm, for tilt corrected r 0 = 10cm and a projector baseline of 70cm.

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A program to compute the two-step excitation of mesospheric sodium atoms for the Polychromatic Laser Guide Star Project

Cited by 8 publications

References 6 publications

Optimization of cw sodium laser guide star efficiency

Optimization of cw sodium laser guide star efficiency

Polychromatic laser guide star using a single laser at 330 nm

End-to-end model for the Polychromatic Laser Guide Star project (ELP-OA)

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