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When
an electric field (E) is applied to an electro-optic (E-O)
crystal, the refractive index of E-O crystal will change linearly
to electric field. The phenomenon is called linear electro-optic
effect. For KD*P crystal, for example, the change of the refractive
index (Dn) is Dn = 0.5n3or63E
if both the directions of light propagation and electric field
are along the z-axis, where no is refractive index
without electric field and r63 is electro-optic
coefficient of KD*P.
If
a linearly polarized light passes through an E-O crystal,
the phase retardation (G) will be
induced by Dn to G = 2pDnL, where
L is crystal length, for KD*P, again as an example, G = pLn3or63E/l. It is
clear that the phase of light will change together with electric
field (E). This is called electro-optic phase modulation.
If two crossed polarizers are placed at input and output ends
of E-O crystal separately, the output intensity of light will
be I = I0sin2(G/2), where
I0 is input intensity. That means the intensity
or amplitude of light can also be modulated by electric field.
This is called amplitude modulation.
There
are two kinds of E-O modulations. One is longitudinal E-O
modulation if the directions of electric field and light propagation
are the same. The KDP isomorphic crystals are normally used
in this scheme. If the directions of electric field and light
propagation are perpendicular, it is called transverse E-O
modulation. The LiNbO3, MgO:LiNbO3,
ZnO:LiNbO3, BBO and KTP crystals are usually employed
in this scheme.
The
half-wave voltage (Vp)
is defined as the voltage at G
= p,
for example, Vp=l/(2no3r63)for
KD*P and Vp=ld/(2no3r22L)
for LiNbO3, where lis
light wavelength and d is the distance between the electrodes.
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