  We present a theoretical research concerning simultaneous generation of two
terahertz (THz) waves with bulk LiNbO3 and four THz waves with periodically
poled LiNbO3 (PPLN) by coupled optical parametric generation (COPG). First, we
investigate collinear phase matching of COPG generating two orthogonally
polarized THz waves with two types of phase matching of o = e + e and o = e + o
with bulk LiNbO3. The two orthogonally polarized THz waves are generated from
stimulated polariton scattering (SPS) with A1 and E symmetric transverse
optical (TO) modes in bulk LiNbO3, respectively. Then, we find that
perturbations of phase mismatch for o = e + e and o = e + o can be compensated
by a same grating vector of PPLN. As a result, four THz waves are
simultaneously generated with a PPLN crystal and a pump laser. We calculate
third-order nonlinear optical coefficients of o = e + o generating THz waves
from E symmetric TO modes. The intensities of four THz waves are calculated by
solving coupled wave equations. The calculation results demonstrate that the
COPG generating four THz waves have high photon conversion efficiencies.
