Chapter 1: Description
Page 2
Figure 1 Output Polarization
Chapter 1
Description
1.1. Overview
The electro-optic amplitude modulator (EO-AM) is a Pockels cell type modulator consisting of two lithium niobate
crystals packaged in a compact housing with an RF input connector. Voltage applied across the crystal structure
induces change in the indices of
refraction (both ordinary and
extraordinary), leading to an electric field
dependent birefringence. An optical wave
(with polarization components on both the
ordinary and extraordinary axes) will
experience a change in polarization state
after traversing the crystal, from the
relative phase delay
between
the
orthogonal components. The electro-optic
crystal acts as a variable waveplate with
retardance linearly dependent on the
applied voltage. Figure 1, below, shows
the polarization state emerging from the
electro-optic crystal as a function of
applied voltage when the input
polarization state is linear at 45
°
relative
to the ordinary and extraordinary axes.
The voltage required to produce a
retardance of
π
radians is called the
halfwave voltage or simply V
π
. For an
optical input linearly polarized at 45
°
,
applying a halfwave voltage rotates the
polarization by 90
°
. When the output
beam is then passed through a linear
polarizer, the resulting intensity can be
rapidly modulated from maximum
intensity (output linearly polarized along the polarizer transmission axis) to minimum intensity (output polarized
perpendicular to the polarizer transmission axis) by changing the voltage applied to the crystal from 0 volts to V
π
.
Note
: Regarding static birefringence, lithium nobiate is birefringent even without an electric field applied. This
static birefringence will induce a change in polarization without an electric field applied. In addition, this static
birefringence is temperature dependent leading to variations in the output polarization with slight changes in
temperature. To minimize this effect, the modulator is constructed of two matched lithium niobate crystals rotated
90
°
with respect to each other. The temperature dependent static birefringence in the first crystal is canceled by
the temperature dependent static birefringence in the second. In this manner, the temperature sensitivity of the
modulator is generally less than 1 mrad retardance per
°
C.
Applied Voltage
π/2
π
π/4
3π/4
R
et
a
rd
anc
e
Output
Polarization
n
n
E
V
