  Traditional methods for generating orbital angular momentum (OAM) light
include holographic diffraction gratings, vortex phase plate and spatial light
modulator. In this article, we report a new method for high efficient OAM light
generation. By pumping an external cavity contains a quasi phase matching
nonlinear crystal with a fundamental OAM carrying light and properly aligning
the cavity, mode matching between the pump light and the cavitys higher order
Laguerre-Gaussian (LG) mode is achieved, conversion efficiency up to 10.3
percentage have been obtained. We have demonstrated that the cavity can stably
operate at its higher order LG mode just as Gaussian mode for the first time.
The SHG light possesses a doubled OAM value with respect to the pump light. The
parameters that affect the beam quality and conversion efficiency are discussed
in detail. Our work opens a brand new field in laser optics, and makes the
first step toward high efficiency OAM light processing.
