  We experimentally demonstrate the orbital angular momentum (OAM) conversion
by the coupled nonlinear optical processes in a quasi-periodically poled LiTaO3
crystal. In such crystal, third-harmonic generation (THG) is realized by the
coupled second-harmonic generation (SHG) and sum-frequency generation (SFG)
processes, i.e., SHG is dependent on SFG and vice versa. The OAMs of the
interacting waves are proved to be conserved in such coupled nonlinear optical
processes. As increasing the input OAM in the experiment, the conversion
efficiency decreases because of the reduced fundamental power intensity. Our
results provide better understanding for the OAM conversions, which can be used
to efficiently produce an optical OAM state at a short wavelength.
