Adaptive Optics (AO) based on artificial beacons is the key to achieve high resolution images from large ground-based telescopes. Long pulsed lasers are preferable to create sodium laser guide stars (LGS) as they allow for Rayleigh blanking. However, these lasers may increase the effective light intensity irradiated at the sodium layer, which may lead to transition saturation, and then decline the normalized return flux efficiency. The return flux might be boosted by optical repumping, which could make full use of the advantages of optical pumping without trapping the atoms to the F=1 ground state. In this paper, we study the optical repumping effect by using a small scale long pulsed sodium laser developed in Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences, whose pulse format may be pretty suitable for large telescopes. An electro-optic phase modulator is used to produce 1.713 GHz sidebands from the D_2a center wavelength with the fraction of 20%. As for a vacuum sodium cell at the temperature of 40°C, when the effective laser intensity increases from 4.53×10² W/m² to 6.99×10⁵ W/ m², resonant fluorescence with and without repumping is measured. The result illustrates that the resonant scattering brightness with repumping can be as over 3 times as without it when the light intensity changes between 4.53×10² W/m² to 5 ×104 W/ m². The saturated phenomenon is also observed. This gives direct evidence that repumping could improve the performance of sodium laser guide stars based on TIPC long pulsed lasers. To our knowledge, this is the first experimental demonstration of the repumping effect with the TIPC type long pulsed laser in laboratory.

Date Published: 7 August 2014
PDF: 7 pages
Proc. SPIE 9148, Adaptive Optics Systems IV, 91483T (7 August 2014);

Published in SPIE Proceedings Vol. 9148:
Adaptive Optics Systems IV
Enrico Marchetti; Laird M. Close; Jean-Pierre Véran, Editor(s)