The refractive
index of LiNbO3 crystals is anisotropic; the Z-axis presents an
extraordinary index ne and the X- and Y-axes present ordinary
indexes no. Light waves propagate in the Z-direction, and the
electric field to be measured is applied in the Ydirection.
When a beam of linearly polarized light whose angle
is 45° to the X-axis enters the LiNbO3 crystal, the light beam
is separated into two eigenmodes: the transverse electric (TE)-
like mode polarized in the X-direction and the transverse
magnetic (TM)-like mode polarized in the Y-direction. The
optical fields of the two modes in the crystal propagate a
distance in the Z-direction.Then the two light beam come
together and unites to be one light beam newly. At last ,the
outputting light goes though an analyzer which has an angle
of -45° and is converted into an electrical signal by the photo
detector. The signal output of the photo detector can be
approximately presented as this following equation: