  Large telescope's adaptive optics (AO) system requires one or several bright
artificial laser guide stars to improve its sky coverage. The recent advent of
high power sodium laser is perfect for such application. However, besides the
output power, other parameters of the laser also have significant impact on the
brightness of the generated sodium laser guide star mostly in non-linear
relationships. When tuning and optimizing these parameters it is necessary to
tune based on a laser guide star generation performance metric. Although return
photon flux is widely used, variability of atmosphere and sodium layer make it
difficult to compare from site to site even within short time period for the
same site. A new metric, coupling efficiency is adopted in our field tests. In
this paper, we will introduce our method for measuring the coupling efficiency
of a 20W class pulse sodium laser for AO application during field tests that
were conducted during 2013-2015.
