On-Sky Tests of a High-Power Pulsed Laser for Sodium Laser Guide Star Adaptive Optics

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 Hz, with a 6% duty cycle. Its coupling efficiency was found to be, in the best test case (using 18 W of transmitted power), 231 +/- 14 photons s(-1) sr(-1) atom(-1) W-1 m(-2) when circular polarization was employed and 167 +/- 17 photons s(-1) sr atom(-1) m(2) with linear polarization. No improvement was seen when D-2b 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-1 atom(-1) W-1 W-1 m(2), using circular polarization and D25 repumping, for a LGS diameter of 0.6 arcsec Full Width at Half Maximum (FWHM), and free of relaxation oscillations in the 589 run laser light.