Sodium guide star laser for adaptive optics systems

Adaptive optics systems are used on telescopes to remove the effects of turbulence in the atmosphere thereby greatly improving the resolution of astronomical images. Distortion on the wavefronts of light arriving from stars can be analysed and used to provide a correction factor for deformable mirrors in adaptive optics systems. The problem is that there are not enough stars in the sky that are bright enough to analyse accurately and provide this correction factor. A solution to this problem is to create your own stars! This can be done by illuminating sodium atoms in the upper atmosphere with laser light tuned to the sodium resonance near 589nm, creating a laser guide star (LGS).

LGS systems are currently used on several telescopes including the Keck observatory, Gemini-North and at the Starfire optical range, making use of dye and sum frequency lasers. Research into high power 589nm laser sources is still continuing to try and improve laser performance, robustness and affordability. Future 30-100m class “extremely large telescopes” will have more demanding requirements on the laser source being used and at present there is no ideal laser technology available. At the University of Adelaide a new laser system is being developed which aims to produce a high power (50W) 589nm LGS source with a pulse format suitable for use on “extremely large telescopes” with advanced multi-conjugate adaptive optics (MCAO) systems.

Our approach uses sum-frequency generation (SFG) of two mode-locked Nd:YAG lasers to achieve the desired Na wavelength, similar to previous designs. Our major contribution is the application of injection mode-locking as a robust method for simultaneous control of the absolute wavelength, bandwidth and timing for SFG. An additional feature is the use of a novel, efficient resonator involving stretched Q-switching. Together, these two techniques are used to produce a burst of high peak power micro-pulses optimized for efficient SFG, contained in a long macro-pulse, separately optimized to avoid bleaching of the sodium in the upper atmosphere.

Personnel

Collaboration

Publications

• Injection mode-locked guide star laser concept and design verification experiments
  Rutten T.P, Veitch P.J., d’Orgeville C. and Munch J.
  Opt. Express 15, 2369-2374 (2007)