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Abstract
We present results of on-sky tests performed in the summer of 2013 to characterize the performance of a prototype high-power pulsed laser for adaptive optics. The laser operates at a pulse repetition rate (PRR) of 600–800[Formula: see text]Hz, with a 6% duty cycle. Its coupling efficiency was found to be, in the best test case (using 18[Formula: see text]W of transmitted power), [Formula: see text] photons s[Formula: see text] sr[Formula: see text] atom[Formula: see text] W[Formula: see text] m2 when circular polarization was employed and [Formula: see text] photons s[Formula: see text] sr[Formula: see text] atom[Formula: see text] W[Formula: see text] m2 with linear polarization. No improvement was seen when D[Formula: see text] repumping was used, but this is likely due to the relatively large laser guide star (LGS) diameter, typically 10 arcsec or more, which resulted in low irradiance levels. Strong relaxation oscillations were present in the laser output, which have the effect of reducing the coupling efficiency. To better understand the results, a physical modeling was performed using the measured pulse profiles and parameters specific to these tests. The model results, for a 10 arcsec angular size LGS spot, agree well with the observations. When extrapolating the physical model for a sub-arcsecond angular size LGS (typical of what is needed for a successful astronomical guide star), the model predicts that this laser would have a coupling efficiency of 130 photons s[Formula: see text] sr[Formula: see text] atom[Formula: see text] W[Formula: see text] m2, using circular polarization and D[Formula: see text] repumping, for a LGS diameter of 0.6 arcsec Full Width at Half Maximum (FWHM), and free of relaxation oscillations in the 589 nm laser light.
Highlights
The use of lasers to create articial reference stars by means of resonance °uorescence from sodium atoms in the Earth's mesosphere is a well-established technique used in adaptive optics (Roy & Labeyrie, 1985; Jeys, 1991; Fugate et al, 1991; Happer et al, 1994)
We report on optical tests of a pulsed sodium laser, developed by the Technical Institute of Physics and Chemistry (TIPC) in collaboration with the National Astronomical Observatories of China, both sponsored by the Chinese Academy of Sciences, which is currently being considered for therst-light adaptive-optics system of the Thirty Meter Telescope (TMT) (Ellerbroek et al, 2012, 2013)
The irradiance was estimated by multiplying the average laser power within each pulse by the beam transfer optics (BTO)/laser launch telescope (LLT) throughput and atmospheric transparency, dividing by the product of the laser beam area in the mesosphere
Summary
The use of lasers to create articial reference stars by means of resonance °uorescence from sodium atoms in the Earth's mesosphere is a well-established technique used in adaptive optics (Roy & Labeyrie, 1985; Jeys, 1991; Fugate et al, 1991; Happer et al, 1994). We report on optical tests of a pulsed sodium laser, developed by the Technical Institute of Physics and Chemistry (TIPC) in collaboration with the National Astronomical Observatories of China, both sponsored by the Chinese Academy of Sciences, which is currently being considered for therst-light adaptive-optics system of the Thirty Meter Telescope (TMT) (Ellerbroek et al, 2012, 2013). This solid-state laser, employing sum-frequency Nd:YAG technology, has a repetition rate of up to 800 pulses per second and a duty cycle in the range 6%–10%.
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