Beam shaping for laser-based adaptive optics in astronomy.

The availability and performance of laser-based adaptive optics (AO) systems are strongly dependent on the power and quality of the laser beam before being projected to the sky. Frequent and time-consuming alignment procedures are usually required in the laser systems with free-space optics to optimize the beam. Despite these procedures, significant distortions of the laser beam have been observed during the first two years of operation of the Gemini South multi-conjugate adaptive optics system (GeMS). A beam shaping concept with two deformable mirrors is investigated in order to provide automated optimization of the laser quality for astronomical AO. This study aims at demonstrating the correction of quasi-static aberrations of the laser, in both amplitude and phase, testing a prototype of this two-deformable mirror concept on GeMS. The paper presents the results of the preparatory study before the experimental phase. An algorithm to control amplitude and phase correction, based on phase retrieval techniques, is presented with a novel unwrapping method. Its performance is assessed via numerical simulations, using aberrations measured at GeMS as reference. The results predict effective amplitude and phase correction of the laser distortions with about 120 actuators per mirror and a separation of 1.4 m between the mirrors. The spot size is estimated to be reduced by up to 15% thanks to the correction. In terms of AO noise level, this has the same benefit as increasing the photon flux by 40%.

Pulsed frequency-shifted feedback laser for laser guide stars: intracavity preamplifier.

Intensive use of laser guide stars with the new generation of extremely large telescopes and hypertelescopes will require the use of more efficient lasers to surmount novel limitations and aberrations. The pulsed frequency-shifted feedback (FSF) laser we have developed overcomes the saturation of sodium atoms and solves the new problems. This work presents a highly efficient solution for operating pulsed FSF lasers. For the first time, an intracavity preamplifier achieves a gain of 10(4) and more than 40 µJ per pulse, with a near-diffraction-limited beam and without amplified spontaneous emission. Endurance tests have shown that good performance is maintained over several hundred hours.

Concept for polychromatic laser guide stars: one-photon excitation of the 4P3/2 level of a sodium atom.

One challenge for polychromatic laser guide stars is to create a sufficiently intense source in the UV. The flux required for the measurement of differential tip-tilt is the main issue that we address. We describe a model that has been validated using on-sky data. We present a method that excites the 4P3/2 sodium level using a one-photon excitation at 330 nm. It is more efficient than the two-photon excitation previously suggested since its power slope flux is 3 x 10(4) photons/s/m2/W instead of 1.3 x 10(3) photons/s/m2/W. This method is very promising both in terms of flux and system simplicity.

Suppression of Rayleigh scattering noise in sodium laser guide stars by hyperfine depolarization of fluorescence.

We propose what we believe is a novel method for enabling the complete suppression of noise due to Rayleigh scattering in sodium laser guide star systems by means of selective discrimination between Rayleigh and fluorescence signals based on polarization properties. We show that, contrary to the nearly 100% polarized Rayleigh scattering, fluorescence from the D(2) sodium line is strongly depolarized under excitation by a modeless laser. This offers the possibility of completely cancelling the effects of the Rayleigh scattering background while preserving the fluorescence signal to about 40% of its maximal value, leading to an improvement of the signal-to-noise ratio by several orders of magnitude. Both theoretical and experimental data confirm this new proposal.