Biomedical Engineering Reference
In-Depth Information
L
e
1.2°
W
W
e
g
W
e
X
FIGURE 2.22
Schematic representation of modulator structure.
where
λ is the wavelength
g
is the gap between the electrodes
δ is the electric/optical field overlap factor
n
e
is the extraordinary refractive index
r
33
is the material electro-optic coefficient
L
e
is the length
If out of phase, the signal no longer propagates along the waveguide and
radiates away into the LiNbO
3
crystal. If in phase, the light remains in the
guided mode and continues to be transmitted through the device. The inten-
sity-modulated outputs are
Δφ
⎛
⎜
⎞
⎟
2
l
=
l
sin
Crossed - Nichol
(2.4)
0
2
Δ
φ
⎛
⎜
⎞
2
⎟
Mach Zehnder
(2.5)
l
=
l
cos
0
2
where
l
0
= input light intensity. Mach Zehnder type devices generally pro-
vide up to 3× the efficiency of the Crossed-Nichol devices.
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